JPL Publication 86-22 DOEtET 29372-5

Distribution Category UC-97b

Power System Applications

of Fiber Optic Sensors

A. R. Johnston

S. P. Jackson

H. Kirkham

C. Yeh

June 1986

Prepared for

Office of Energy Storage and Distribution

U.S. Department of Energy

Through an Agreement with

National Aeronautics and Space Administration

by

Jet Propulsion Laboratory

California Institute of Technology

Pasadena, California
Prepared by the Jet Propulsion Laboratory, California Institute of Technology,

for the U.S. Department of Energy through an agreement with the National

Aeronautics and Space Administration.

This report was prepared as an account of work sponsored by an agency of the

United States Government. Neither the United States Government nor any

agency thereof, nor any of their employees, makes any warranty, express or

implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process

disclosed, or represents thai its use would not infringe privately owned rights.

Reference herein to any specific commercial product, process, or service by trade

name, trademark, manufacturer, or otherwise, does not necessarily constitute or

imply its endorsement, recommendation, or favoring by the United States

Government or any agency thereof. The views and opinions of authors

expressed herein do not necessarily slate or reflect those of the United States

Government or any agency thereof.

This publication reports on work performed under NASA Task RE-152, Amendment 203 and sponsored through DOE/NASA Interagency Agreement No.

DE-AI01-79ET 29372 (Mod. AO09).
_S_A_

This document is a progress report of work done in 1985 on the Communications

and Control for Electric Power Systems Project at the 3et Propulsion

Laboratory. Three topics are covered: Electric Field Measurement, Fiber Optic

Temperature Sensing, and Optical Power Transfe_

Work was done on the measurement of ac and dc electric fields. A prototype

sensor for measuring alternating flelds was made using a very slmple

electroscope approac_ An electronic field mill sensor for dc fields was made

using a fiber optic readout, so that the entire probe could be operated

isolated from 8roun&

_he=e are several instances in which more precise knowledge of the temperature

of electrical power apparatus would be useful This report describes a number

of methods whereby the distributed temperature profile can be obtained using a

fiber optic senso_

The ability to energize electronics by means of an optical fiber has the

advantage that electrical isolation is maintained at low cost In order to

accomplish this, it is necessary to convert the light energy into electrical

form by means of photovoltaic cells. JPL has developed an array of PV cells

in gallium arsenide specifically for this purpose. This work is describe&

iii
ACKNOWLEI_MENTS

_xe research described in this report was carried out at the Jet P_opulsion

Laboratory (JPL), California Institute of Technology, and was sponsored by the

U.S. Department of Energy, Office of Energy Storage and Distribution, through an

agreement with NASA.

The authors thank Ken Klein of DOE's Office of Energy Storage and Distribution

for his continued help and encouragement_ We also thank Luclano Zafanella and

the staff of General Electric's High Voltage Transmission Research Facility for

their time and assistance. In particular, we thank Gary _ohnson and Bill

McGonigle for their assistance in preparing for and conducting the tests at the

High Voltage Transmission Research Facility. We also acknowledge the

contribution of Bjorn Eng at _FL in the design and construction of the prototype

dc field sensor, as well as the help of Keith Sheu (also at JPL) in assembllng

the electronlcs.

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TABLE OF CONTENTS

Xe Introduction .......................... 1-1

1.1 Power System Sensors ................... l-1

0 Electric Field Measurement ................... 2-1

2.1 Introduction ....................... 2-1

2.2 AC Field Measurement ................... 2-3

2.3 DC Field Measurement ................... 2-9

2.3.1 Theoretical Background ............... 2-9

2.3.2 Sensor Design ................... 2-15

2.3.3 Position-Reference Encoding ............ 2-18

2.3.4 The Optical Link .................. 2-18

2.3.5 Laboratory l_valuation ............... 2-18

2.3.6 Field Tests .................... 2-23

2.3.7 Self-Charging Experhnents ............. 2-31

2.3.7.1 Experimental Test Arrangement ....... 2-31

2.3.7.2 Paraueter Measurements .......... 2-33

2.3.8 Conclusions .................... 2-39

. Fiber Optic Temperature Sensing ................ 3-1

3.1 Applications ....................... 3-1

3.1.1 Overhead Phase Conductor Temperature Measurement . . 3-1

3.1.2 Underground Cable Temperature Measurement ..... 3-2

3.1.3 Power System Equipment Temperature Monitoring . . . 3-2

3.2 Methods of Measuring Temperature Distribution ....... 3-3

3.2.1 Temperature Dependent Birefringence ........ 3-5

3.2.2 Temperature Dependent Loss ............. 3-5

3.2.3 Coupling Variations with Temperature ........ 3-7

3.2.4 Temperature Dependent Optical Index Ratio ..... 3-7

3.2.5 Temperature Dependence of Refractive Index ..... 3-11

3.2.6 Point Sensing for Distributed Measurement ..... 3-11

3.2.7 Distance Reached by a Lossy Distributed Sensor . . . 3-14

. Optical Power Transfer ..................... 4-1

4.1 Background ........................ 4-1

4.2 Device Design Parameters ................. 4-5

4.2.1 ]he GaAs starting wafers .............. 4-5

4.2.2 Light Capture Area and Conversion Efficiencies . . . 4-7

4.2.3 Device Fabrication ................. 4-14

4.2.4 Teat Results .................... 4-16

4.3 Future Work ........................ 4-16

5, Conclusions .......................... 5-1

REFI_qCES ................................ 6-1

V
FIGURES

2-1 Examples of Field Mill Designs 2-2

2-2 Laboratory Model of Fused Silicon Fiber Electroscope with

Metallized Fiber 2-4

2-3 Photograph of the Fiber Electroscope as Built ............ 2-6

2-4 Input-Output Curve for an All-Dielectric Test Model of the Fiber

Elec troscope . 2-7

2-5 An All-Dielectrlc Configuration of the Fiber Electroscope Sensor . . 2-8

2-6 Geometry of Split-Cylinder Sensor Probe ............... 2-10

2-7 Fractional Change in Sensor Output Induced by a Nearby Conducting

Surface for Full Ion Current Charging ................ 2-14

2-8 Photograph of the Prototype Sensor with One-Half Cylindrical Shell

Removed to Show Internal Parts 2-16

2-9 Functional Block Diagram of the Sensor Electronics ......... 2-17

2-10a Schematic of DC Field Sensor Probe ................. 2-19

2-10b Schematic of DC Field Sensor Receiver ............... 2-20

2-11 Typlcal AC Output as Seen at the Receiver .............. 2-21

2-12 Output of Sensor at Rotational Frequencies Corresponding to Speeds

between 600 and 4200 rvn ..................... 2-22

2-13 Calibration Curve Obtained in BVTRF Test Cage ............ 2-24

2-14 Electric Field Vectors Measured Near a Test Subject ......... 2-26

2-15 Measurement of Electric Field in the Vicinity of a Person ...... 2-27

2-16 Electric Field Versus Time Measured Under DC Conductor, Ccmpared to

IIVTRF Data • • • • • • • • e • • • • • • • • • • • • • • • • • • • • 2-29

2-17 Measured Field Values Along s Line Crossing Pemanent

Instrumentation 0.6 m Above Ground .................. 2-30

2-18 Field Cage and Test Bench Arrangement ................ 2-32

2-19 Measured Sensor Output as a Function of Electric Field ....... 2-34

2-20 Example ShowingArrangement Used to Produce an Excess Voltage

of +1027 V on the Sensor ...................... 2-35

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2-21 SensorOutputvs. SensorExcessVoltage fo_ Various Ratios of

d to r. E Field Constant at 30 kV/m ................ 2-36

2-22 I_asured Change in Sensor Output as a Function of Distance to

Screen, Sensor Uncharsed ...................... 2-37

2-23 Measured Change in Sensor Output as a Function of Distance to

Screen Sensor Fully Charsed (-2.64 kV) ............... 2-38

3-1 Optical Time Domain Reflectometry

a) Optical Time-domain Reflectometer ............... 3-4

b) OTDR display .......................... 3-4

3-2 Birefringence Sensins ........................ 3-6

3-3 Temperature-Dependent J/icrobending Loss ............... 3-8

3 4 Tes_rature-i)ependent Couplinl 3-9

3-5 Temperature-Dependent Optical Index Ratio .............. 3-10

3-6 Temperature-Dependent Refractive Index ............... 3-12

3-7 s. Separate Sensors Using Optical Fiber as Communication Channel.. 3-13

b. Separate Sensors Spaced Within the Resolution of the (YIDR . . . 3-13

4-1 Block Diasram of Optical Power Transfer System ........... 4-2

4-2 Electrical and Physical Arrangenent of 32-Diode Array ........ 4-3

4-3 Sequence of Rvents in Producing Gallium Arsenide Diode Array .... 4-4

4-4 Diode Array Layout and Mounting Arrangements ............ 4-6

4-5 Gallinu Arsentde Wafer Dicing, showing Sawed l_lge .......... 4-8

4-6 Two Views of a Laser-Scribed Gallium Arsenide Wafer ......... 4-9

4-7 Edge of Galli nm Arsenide Wafer Scribed Sider-to-Side ......... 4-10

4-8 Two Views of a Diamond-scribed Gallium Arsenide Wafer ........ 4-11

4-9 Two Views of a Gallinm Arsenide Wafer Cut with a Diamond Saw .... 4-12

4-10 Photodiode Array Mounting Arrangement on Fiber Connector ...... 4-13

4-11 Photograph of 32-Diode Array .................... 4-15

vii
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SECTION 1

INTRODUCTION

The word optic comes from the greek o?r_Ko_, meaning eye. As a noun the word

has traditionally been applied to any element of an optical system, but in

particular it has been the collective noun for items such as lenses and

mirrors.

One thing that such optics had in common was that they were much wider than

their lengt_ Uncounted numbers of independent rays of light can be made to

pass through a small lens, for example, and a mirror reflects any light

impinging upon it, regardless of the direction the light is coming from. This

feature of optics, the aspect rati_ changed in the 1970s, and particularly in

the 1980s, with the advent of fiber optics, optical components whose lengths

are millions of times their width. In these components, the light can only be

inserted into the optical element from near the axis of the fiber. Once it is

coupled into the optic, light emerges from the other end almost unaltered by

physical interactions between the fiber and the medium through which it

passes. Modulated_light (or light which is switched on and off rapidly) can

be made to pass through considerable lengths of fiber without degradation.

This fact made optical fibers'extremely attractive in the telecommunications

industry, which has been quick to demonstrate the superiority of optical

fibers as transmitters of information.

Not only are optical fibers immune to their physical and chemical environment,

they are also relatively immune to the electromagnetic envtronmenL This

feature makes them of considerable interest in the electric power industry.

Since 1983, The Communications and Control for Electric Power Systems Task at

the 3et Propulsion Laboratory (JPL), funded by the Department of Energy (DOE),

has been investigating the application of optical fibers to power systems. A

good deal of that effort has been aimed at using fiber optics as sensors in

power systems.

This report is a summary of the work done at JPL in this area over the last

twelve months.

1.1 POWER SYSTEM S_NSORS

Klectrtc power systems comprise some of the largest and most complicated

systems in the world: They are widespread both geographically and

hierarchically, that is to say they cover large areas of land and tnclnde

wlth/n a given area elements of generation, transmission and distribution.

They are wldely interconnected and contain, from an analysls point of view,

large numbers of nodes. The calculation of the state of an entire power

system at any glee instant is a formldable task, which even today requires

several seconds of computer time on the very largest and fastest computers,

The calculations begin with measured data. In the case of a typical power

system, data on many hundreds or even thousands of parameters will be

collected by the Supervisory Control and Data Acquisition (SCADA) system and

1-1
relayed back to the energy control center. _he measured data will typically

include voltage and current at a large number of buses in the transmission

system, a smaller number in the subtransmission system, and perhaps some of

the distribution system parameters, as well as data concerning the status of

the generators in the system. (One might note in passing that the unknown in

the equation which describes the power system state is the load on the power

system. )

_he fact that there are so many sensed values in a power system has led to a

considerable amount of interest in the development of low cost sensors based

on fiber optics. Several companies, particularly in 3apan, have been active

in developing sensors for the primary electrical quantities (voltage and

current) for ranges which are realistic in power systems, thousands of volts

and thousands of crops.

:=

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At the time of this writing, fiber optic power system sensors have developed

to a point where they are competitive with some of the conventional

approaches, using, for example, voltage transformers and current transformers.

However, they have not yet shown themselves to have a clear performance or

economic advantage. _'nis will change, nonetheless, since magnetic and

capacitive sensors have been developed over many decades to a point of

considerable refinement, whereas the optical techniques are just beginning to

be develope&

1_ne decision was made early on in the fiber optic sensors work of the

Communications and Control Task at 3PL to concentrate on developing a new

measurement technique, rather than the refinement of a method of whose

practicality had already been shown Be measurement of electric fields in

air was chosen for the initial sensor efforts.

While the electric fields caused by power lines can be measured in the plane

of the ground, their measurement outside of that plane is difficult. _he

development of a suitable sensor to accomplish this measurement has provided

some interesting challenges.

Sensors for measuring alternating electric fields outside the ground plane had

previously been demonstrated. These instruments consisted of a current

measuring device placed between the electrodes of a large capacitor, measuring

the displacement current between the electrodes, in such a way that the

investigator could read the instrument when it was held a meter or two from

him at the end of a fiberglass pole. There was typlcally no way to record

the intensity of the electric field other than for the investigator to note

the reading of the instrument.

A fairly simple modification of this principle would be to encode the data

into optical form and insert it into an optical fiber which runs along the

fiberglass pole. In this way the information can be read from the instrument

by a data acquisition system without perturbing the electric field any more

than it already is.

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The measurement of direct electric fields can, in principle, be done the same

way provided that the capacitor electrodes rotate (there is otherwise no

displacement current in a nonalternating field).

Most of the effort of the sensor work on the Communications and Control Task

during 1984-1985 has been devoted to the measurement of direct electric

fields. Consequentl_ the largest part of this report, Section 2, is devoted

to describing that work. An alternating electric field sensor was also

developed during this period, and is described in Section 2.

Section 3 describes several ways of using fiber optics to measure temperature

profile (temperature as a function of distance). The knowledge Bained from

such a measurement could be useful in both the design and the operation of

a variety of electrical apparatus, such as cables and transfomers.

The development of optical diodes for the transfer of small amounts of

electric power over fibers is described in Section 4. Diodes such as those

described could be used to provide power for electronics used in measurements

in parts of the power system that are relatively inaccessible, such as in a

high-voltage bus. The diodes being developed at _PL are made of gallium

arsen/de, a semiconductor capable of higher efficiency conversion than the

more usual silicon when made into the small sizes required for this kind of

application.

1-3
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SECTION 2

ELEC'rRIC FIELD HEASUREMENT

2.1 INTRODUCTION

The magnitude of the electric field under a transmission line, or in a

substation, is of considerable interest because of possible environmental

impact. In the absence of ion current the field may be calculated if the

geometry is fairly simple, but for even a moderately complex geometry the

calculation can become difficult and expensive to carry out. Dnder a high

voltage dc transmission line in corona, measurement is the only practical way

to determine the field.

Existing electric field sensors typically have conductive electrodes and some

form of cabling connecting them to the measurement electronics. This

arrangement can cause considerable distortion of the unknown field if the

measurement is to be made above the ground plane. In one implementation for

ac measurements (Deno and Zaffanella, 19/5) the sensor is self-contained, and

furnished with a scale large enough that an operator a meter or two away can

read it visually. The device is calibrated in a uniform field in a large test

chamber. However, the probe is relatively large, and it distorts the field

nearby, introducing an error if the field is non-uniform (as it would be near

another conductor.) There is clearly some advantage to a small, alldielectric sensor.

In other approaches, Feser and Pfaff (1984) described a small (4.0 cm

diameter) probe connected to measurement electronics by fiber optics for

electrical tsolatiov_ Earlier experiments along the same line were reported

by Wilhelmy (1972). Misakian and his co-workers (1978) described an even

smaller spherical probe (1_ cm diameter), although it was connected to its

electronics with copper wires which had to be placed along an equipotential

surface of the measured field.

These instruments were all intended for measurement of ac or transient fields.

However, the work of Feser and Pfaff, and especially Misakian, has

demonstrated that small probe dimensions are needed for field measurements

within a few centimeters of irregularly shaped, conductive objects.

For the measurement of the field under a direct-current line, the induced

charge on the probe must be modulated in order to make it measureable.

Previously reported instruments intended for dc field measurement have been

designed with rotating or vibrating electrodes to modulate periodically the

acquired charge. The resulting device is then known as a field mill or a

vibrating plate electrometer, respectively. Comparative tests of a number of

these instruments under adc line have been reported in the literature

(Comber, ][otter and McKuight, 1983). All of the instruments described operate

with part of the system grounded, and must be mounted in the ground plane.

Examples of field mill designs are shown in Figare 2-1.

This report describes the work done at YPL on the design of an all-dielectric

ac field meter, and an isolated-probe dc field sensor. Both sensors use fiber

optics, but in quite different ways.

2-1
FIXED, GROUNDED

SHIELD ELECTRODE

ROTATING SENSOR

ELECTRODE

,so,..AT_O--_I,_'_ I L

CONDUCTIVE SHAFT--_ _"""""""""__

GROUNDED PLATE

__ SIGNAL OUTPUT

BRUSH

a) WITH ROTATING SENSOR ELECTRODE

_ RoTA:T 'NG, 'GROUNDED

L _ _" SH!ELDELECTRODE '

___ _ _,×_o_o_

_-"_-_ EL;CTuRNOD%ED PLATE

GROUNDED _ [ _ ; SIGNAL OUTPUT

SHAFT _ _ - -

b) WITH FIXED SENSOR ELECTRODE

Fign_e 2-1. Exa-ple$ of Field Mill Designs

2-2
2.2 AC FIELD MEASUREMENT

A compact electroscope for measurement of ac fields was investigated. A

number of feasibility models were fabricated, and test data were obtained. _he

basic principle involved was to detect a field-induced force on a quartz fiber

or fibers by measuring their flexure optically. The fibers used in the

electroscope were optical fibers with core and cladding, which could readily

be connected to a monitoring instrument via cables containing similar fibers.

The basic idea was described in an earlier report (Kirkham, Johnston, Lutes,

Daud, and [lyland, 1984).

There are a number of attributes, both good and bad, that distinguish the

quartz fiber electroscope from other devices for field measurement. On the

good side, the device could be made very small: a package on the order of 5 mm

in diameter and 20 mm long should be quite practicable. No power is required

at the device, it is completely passive. It is possible, as will be described

later, to build the sensor of dielectric materials only. No conductors need

be present. This is attractive in applications involving measurement of very

high fields where corona effects or air breakdown at structural

discontinuities must be guarded against.

On the negative side, the device is inherently non-llnear, and is sensitive to

acceleration. The latter point, acceleration or g-sensitivity, is strongly

dependent on the electroscope geometry. A single cantilevered fiber used as

the sensitive element is clearly g-sensitlve. Experimental tests showed that

18 could produce an output corresponding to a significant fraction of the full

scale deflection. On the other hand, it is possible to compensate for

acceleration-induced response by building a two-fiber electroscope such that

both fibers deflect together, eliminating significant relative motion under gforces, but deflect oppositely due to electric field-induced forces.

Compensation of g-forces or vibration effects were not investigated

experimentally. A final factor that is not necessarily emphasized is that the

device responds inherently to E2, which makes it blind to the sign of a field.

It will measure fields in either direction along its sensitive direction,

consistent with its being considered an ac sensor. In fact, the device can

measure high frequency or RF fields as well as those at power frequency, which

may suggest other applications.

In order to facilitate building of test models of the electroscope by a

skilled technlcla_ but one untrained in techniques for making fused silica, a

design was worked out which was based on building the device up from

microscope slides and glass spacers cut from microscope slides. IN curing

cement was used for assembly.

The configuration is illustrated in Figure 2-2. A conductive coating was

placed on the fiber itself and on a very small area of the mounting slide

immediately under the fiber. The sensitive field direction is perpendicular

to the microscope slide base.

2-3
TWO

FIBERCABLE

SPACER.

MOVING

METALLIZED FIBER

;PACER

GLASS SUBSTRATE

METALLIZED TRACK

FIXED FIBER

Figure 2-2. Laboratory Model of Fused Silica Fiber

Elec troscope with lie tall ized Fiber

2-4
The photograph shownin Figure 2-3 shows the feasibility model, with the

sensor mounted in a lucite tube together with the accessing fiber optic cable

and a readout box. No effort was made to miniaturize this model. A

representative curve of output versus applied field is shown in Figure 2-4.

An alternative design is shown in Figure 2-5. In this configuration, the

sensing fiber is placed at the end of a gap or slot between two field-shaplng

pieces of dielectric. The field-induced force arises as a result of locating

the sensing fiber in a strong field gradient created by the dielectric slot.

No metal or other conductor is used in the device. Several test models of

this all-dielectric configuration were built, and they all functioned well.

These sensors are best suited to measurement of large fields, 100 kV/m or

more, but they can, with difficulty, be scaled for lower fields, down to

perhaps 10 kV/m.

A number of applications for these unique sensors were discussed with other

workers in the power industry. Examples are briefly described below,

(i) It should be possible to monitor the state of very large circuit

breakers using variations in field near the gap. This field is

dependent on the position of the contacts. It would not be possible

to directly measure contact position with any conventional devices

because of the potentials involved.

(2)

(3)

A 'hot-stick _ with a very simple and robust sensor encapsulated in

it would be a valuable safety tool for service personnel. The

sensor could detect a live conductor on approach and trigger an

alarm signal.

A simple dielectric device that could be manipulated in proximity to

a long insulator string could be used to detect a shorted module in

the strin_ Multiple failures of this type could result in a failure

of the entire strin_ Failure (tpnncture j) detection of this kind

could improve insulator string performance.

A simple test of this idea was conducted at General Electric's High

Voltage Transmission Research Facility (HVTRF) in Pittsfield,

Massachusetts during a visit there last year by JPL personnel.

Detection of a deliberately shorted element in a short insulator was

readily done.

2-5
ORIGINAL PAGE

.OF,,'_R QUALrTY

,i,n, ...... " ....... iPrilI'l III II ........................... 11T11-T

"!rlr Fr ,7_7,,:1 ....

lr r=_

lZ:l,.g'_.e 2.-3. Photo_aph of the Fiber Electroscope as Built

(the microscope slide base is mounted on a larger

dielectri¢ Iountin$ board)

2-6
1.0

0.9

0.8

0.7

i 0.6

0.5

0.4

0.3

0.2 I I I I I I I I 1 I I I

0 1 2 3 4 5 6 7 8 9 10 11 12

FIELD STRENGTH, kV/cm

Figure 2-4. Input-0utput Curve for an All-Dielectric Test Model

of the Fiber Electroscope

2-7
SPACER

DIELECTRIC ELECTRODE

SPACER

GLASS SUBSTRATE

DIELECTRIC ELECTRODE

Figusr.o 2-5. An All-Dielectric Con£1guration of the Fibex Electroscope

Sensor (No Metal or Conductive Coatings are used)

2-8
:2.3 DC FIELD MBASUREMENT

In contrast to the situation under an ac transmission line, ion current is a

characteristic part of the environment under a High Voltage Direct

Current (HVDC) line.

The ions of interest are produced by corona processes at the conductor

surface, and accelerated away from the conductor by the electric field. The

density of the atmosphere is such that the subsequent motion of the ions is

dominated by collisions with atmospheric molecules, and an ion of given size

and charge will acquire an effective velocity determined only by the field.

Such motion is usually described in terms of ion mobility. Ions may be

thought of as drifting along field lines, at a velocity given by the product

of the field and the mobility. Any ion current present will flow into a

grounded instrument, or will deposit charge an electrically isolated

instrument. As a result, the instrument must be capable of distinguishing

between field-induced charge and ion current. With field mills mounted in the

ground plane, the two modulated current components are in phase quadrature,

and can be separated by phase sensitive detection (Maruvada, Dallaire, and

Pedneault, 1983). However, the same technique, when applied to a grounded

rotating cylinder field mill (Maruvada et al., 1983) required two sensors

rotating at different speeds to separate the ion current component from the

field-induced output. The device in question had two significant drawbacks.

It was large (over a meter in length), and it was grounded.

The sensor described in this report is of the rotating cylindrical field mill

type, but it is quite small, less than 20 cm long and 10 cm in diameter.

Further, it utilizes a fiber-optic readout to isolate the probe electrically

from ground. Therefore, no ion current can flow to the probe in the steady

state. Instead, it will acquire a charge sufficient to prevent additional ion

current from flowing to it. The effects of self-charging due to ion current

are discussed briefly in the following section.

As noted above, instruments capable of measuring electric field at space

potential have been available for some time for ac use. Similar dc

instruments suitable for use in the field have been described in the

literature, but are not routinely made. Free space dc measurements have been

made in the laboratory using a Pockels crystal (Hikada and Fujita, 1982). The

availability of measurements of dc electric field at space potential will make

possible better evaluation of the field effects due to dc lines, and put dc

measurements on an equal footing with ac measurements.

2.3.1 Theoretical Background

The principles involved in the design of our electric field sensor can be

understood with respect to Figure 2-6. The external electric field is

determined by measurement of the displacement current between two shorted,

semi-cylindrical rotating electrodes. In the figure, the radius of the

cylindrical electrodes is r, and the split between the two halves is at angle

with the x axis. A uniform field Eo is directed along z and the axis of the

cylinder is along y. Under these conditions, the surface charge density a

2-9
Z

v

Fig_e 2--6. Ge_etry of _lit-Cylimler ;ensor Probe

2-10
on the cylinder at azimuthal position 6 is easily derived, and is

given by

o = 2 s E sln0 (2-1)

o o

The total charge on the cylinder is assumed to be zero, which means that its

potential is set at the value existing at the location of its axis before it

is introduced into the field.

By integrating over one half of the cylinder, the total charge on one of the

electrodes can be evaluated as a function of the electrode orientation angle

= wt. Differentiation with respect to time yields the current i c in a wire

connecting the two half cylinders of length 1:

i = 4 s E I r w sin _t (2-2)

c o o

Therefore, if the rotating sensor is placed in a uniform field far from any

conducting surface, its output is sinusoidal, with amplitude proportional to

the field Eo. The current i c is measured electronically and telemetered as a

measure of the field Eo.

A similar analysis, applied to a split sphere rotating about an axis through

the split, yielded a short circuit current i s of

i = 3n s E r2_ cos _t (2-3)

s o o

This result is different by a constant factor from the cylindrical case

(is/i c ~ 1,2 for I = 2r), indicating that the finite length of the actual

sensor probe does not affect the foregoing argument significantly.

If the sensor is subjected to an ion drift current, it will acquire a charge.

The effect of this charge can be estimated by superposin8 the field from a

charged conducting cylinder and the previously discussed uncharged cylinder

immersed in a uniform field.

The field component resulting from the charged cylinder is independent of O in

the absence of nearby conductors, and, therefore, can make no contribution to

the measured field-induced current i c. Consequently, the sensor will not

respond to a steady-state self-charge if it is far from any conducting

surfaces.

The error introduced by self-charging when the sensor is near a conducting

surface can be estimated using the following argument. Assume that the sensor

is located in a semi-infinite region of uniform field bounded by a ground

plane, with the sensor axis at distance d from the ground plane, and that an

ion drift current flows toward the ground plane. The sensor, assumed to be an

infinite cylinder, will intercept a small portion of the ion current,

acquiring a charge (_ per unit length.

2-11
Under these conditions, the field in the neighborhood of the sensor can be

specified approximately as the superposttion of three components, the unifor_

field (as perturbed by the presence of the uncharged conducting sensor

cylinder), the field of a line charge Q on the sensor axis, and the field of

an image line charge -Q reflected in the ground plane at a distance 2d from

the sensor axis. Additional secondary images resulting from the presence of

the conducting cylinder in the field of the image -Q are neglected.

As we have seen, the field of the line charge on the sensor axis is not

detected by the sensor. However, the field component from the image charge

would be detected, and gives an approximate estimate of the error if the

distance d is large.

In order to estimate the magnitude of the line charge, Oj note that ion

current flow into the sensor will stop when the field component at its surface

due to the self charge Q exactly cancels the field due to the external uniform

field at points on the cylinder opposite the ground plane (0 = n/2 in

Figure 2-6). When this is true, there no longer is a field component at any

point on the surface of the cylinder causing an ion drift toward the cylinder.

Since the field on the top surface of the cylinder at 0 = n/2 in a uniform

field Eo is 2 Fo, the condition for charge collection to stop is

2 E = 0 (2-4)

o 2_e r

O

It follows that the magnitude of the field Ei at the axis of the sensor due to

the image charge is

o (2-s)

Ei = 2nSo(2d)

and the error caused by self-charging obtained by combining equations 4 and 5

is

Ri r

E d

o

for r/d (( 1 (2-6)

Again. an analogous estimate was carried out assuming a spherical sensor

instead of a long cylinder, with the result: " ''

Ei 3 r s

E-- ~ 4 d 2 for r/d << 1 (2-7)

O

These two estimates set limits on the error expected for the actual sensor

geometry. Note that the result does not depend on the ion current density.

2-12
It assumesonly that the sensor charges fully, satisfying the surface field

condition (Equation2-4). If full charging does not occur, the error will be

proportionately less.

The above expressions are not accurate if r is not much smaller than d.

Although exact analytical expressions in terms of a series can be derived

using the image method, the slow convergence of the resultant series prompted

us to find a numerical means of solving this problem. Using the finiteelement technique, a numarical method was used to find the potential V(x,y)

everywhere within a given enclosed region, provided that the potentials on the

bounding surfaces are known This approach was applied to the problem of an

infinite cylinder near a ground plane.

Because of symmetry, only half of the structure needs to be included. This

region was divided into 260 x 130 equally spaced subregions. Given V1, V2 and

V3 on the bounding surfaces, respectively, the ground plane, the cylinder and

a plane surface parallel to the ground plane but far enough from the cylindar

not to influence the results, the potential at each grid point was calculated.

Knowing the voltage everywhere, the electric field can readily be calculated,

and the resultant charge on the surface of the cylinder is proportional to the

normal component of the electric field Er there. Summing over all the charges

on the upper half of the cylinder yields the total charge Qu and summing over

all the charges on the lower half yields QI" The quantity Qa = (0_ - Q1)/2 is

taken as a measure of the sensor output, since the charge distribution is not

symmetrical when the sensor is near the ground plane.

The fractional change in Qa' AQa/O_o = (Cla - Qao)/(_ao is plotted in Figure 2-7

as a function of d/r. Qa A is the calculated value of (t a calculated

for d/r }} I, that is, with _e sensor in free space.

The quantities AQa/Qao and EI/E o (Equations 2-6 and 2-7) are directly

comparable, and are a measure of the change in sensor output due to the

presence of the ground plane. The condition for charge equilibrium in the

presence of ion current (Equation 2-4), was imposed in calculating AO_/Qao.

It can be seen that a significant change in sensor output can be produced by a

nearby conductor. However, these results indicate that if the sensor is kept

at a distance greater than approximately five times its own radius from the

nearest surface, the error will be less than 10_. In addition, estimates made

using a two dimensional cylindrical model are upper limits on the perturbation

that would be seen by a sensor probe of finite length. If the sensor is not

much closer to a surface than its own length, then the perturbations will be

smaller than the two dimensional model predicts.

The response of similar probes in nonuniform fields has been treated by other

authors (Wilhelay, 1972; tiisakian et al, 1978). Their results show that the

sensor response indicates the field at the location of the center of the probe

before the probe is introduced. Errors of the order of I_ or less were

predicted when the undisturbed field varies by tens of percent over the volume

of the probe. It can be concluded that the dominant error source is image

fields resulting from self-charging when near a conducting surface.

2-13
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RATIO OF DISTANCE OF CYLINDER TO GROUND TO

RADIUS OF CYLINDER, d/a

Fractional Change in Sensor Output Induced _a

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2-14

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2.3.2 Sensor Design

The overall instrument consists of two parts: (1) the sensor probe, which is

self-contained and isolated from ground, and (2) a set of readout electronics,

providing an output suitable fox use by a data acquisition system. The two

parts are connected by a conventional 100 micrometer diameter core multimode

fiber optic cable. .The overall cable diameter including strength members is

about 3 mm. ...... -_ ";

The unconventional aspects of the instrument are located in the sensor probe,

which will be briefly described. A stationary tube supports the rotating

sensing head, and carries a small battery powered dc motor to drive the

surrounding rotating element. The rotating element contains an electronics

package, powered by separate watch-type batteries, which measures both the

displacement current between the two electrodes and the position of the

rotating part of the sensor with respect to the stationary part. This

information is encoded in terms of pulse repetition frequency and used to

drive an LED transmitter.

A rotary optical coupler transfers information from the rotating electronics

into a non-rotating fiber cable termination mounted on the axis of the

supporting tube. No use is made of slip rings or rotary transformers.

The entire sensor, including its drive motor and battery, the rotating

electronics and its battery, and the fiber-optic coupler, is housed inside the

split cylindrical sensing electrode pair, as shown in Figure 2-8. No

particular effort was made, for this prototype, to produce a miniature

instrument. The sensor is about 8 cm in diameter and 20 cm long. It is

intended, in subsequent phases of development, to reduce the sensor size by

about an order of magnitude in its linear dimensions. Such a sensor should

enable free-body field measurements to be made of fields which vary

significantly over distances of only a few centimeters, providing data on the

magnitude of electric fields after they have been perturbed by the presence of

an animal or a man.

The block diagram shown in Figure 2-9 shows the principle of operation. The

externally mounted cylindrical electrodes are connected to an operational

amplifier configured in a charge measurement (integrating) mode. When the

sensor rotates in a dc field, a current is induced between the two halfcylinders which is proportional to the rotational frequency as well as the

field, as shown in Equation 2-2. This frequency-dependence is removed by

the capacitive feedback element in the amplifier, resulting in a periodic

output signal which is independent of rotational speed. This signal, in turn,

modulates a voltage-controlled oscillator (VCO). Position-reference

information is then combined with the VCO output in the encoding logic block,

which creates a coded train of short pulses for optical t_ansmissio_ The

optical link transfers the pulse train to the readout electronics where the

field amplitude and position reference signals are reconstructed and measured.

2-15
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2-16
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2-17
2.3.3 Position-Reference Encoding

The position-reference signal is needed in order to obtain the angle of the

electric field vector as well as its magnitude. In this design, a

phototransistor on the rotating part of the sensor passes in front of a

stationary light-emitting diode (LED) once every revolution, sending a

signal to the encoder to mark the reference position. The optical

transmitter normally generates a short pulse (0.25 ps) at each positive

transition of the VC0 output. When the slgnal from the position-reference

phototranslstor is received, the next optical pu_se is replaced by two

pulses in close succession The addition of the reference pulses to the

data-pulse train does not disturb the amplitude signal, and gives an average

angle resolution of 1.5 degrees (at 2400 rpm).

2.3.4 The Optical Link

The optical link consists of an LED transmitter, an appropriate length of

fiber-optic cable, and a receiver module with a built-in amplifier. A

standard fiber-optic connector was modified to provide the rotary

coupling between the sensor and the cable. The encoding format, a very low

duty-cycle pulse frequency modulation, was chosen to conserve battery power,

with the object of increasing the duration of operation of the instrument.

A schematic of the sensor head and the receiver system is shown in

Figure 2-10. Figure 2-11 shows the sinusoidal output, as seen at the

demodulator output of the receiver for adc field at the sensor head of

30 kV/L

2.3.5 Laboratory Evaluation

Laboratory evaluation of the sensor gave the following results:

(i) Direct fields ranging from 70 Vim to 50 kVlm, a dynamic range of

roughly 103, were measured using an ac voltmeter to read the output

voltage. Within the above range the sensor was linear to

within 2_.

(2) The measured ac output voltage was independent of frequency (within

I_) over a range corresponding to sensor speeds between 600 and

4200 rpm, as shown in Figure 2-12. _ith a new 9 V battery, the

sensor rotates at approximately 2400 rpL

(3) The current drain of the rotating circuitry was less than 1.4 mA at

6 _ This was measured with the encoder driving the LED with

40 mA pulses at a center frequency of 10 kHz, giving an optical

power margin of 6 dl_ The LED accounted for less than I0_ of the

total current drain The pulse-coded fiber opti_ data readout does

not dominate the power requirement of the rotating circuitry. The

present power drain is approximately 8 mW, but an optimized design

could reduce it significantl_

Taken together, the results of the laboratory tests show that the sensor is

capable of measuring the field-induced charge on its electrodes within ~2k

2-18
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2-19
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ORIGINAL PAGI_ IS

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2-21
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to Speeds between 600 and 4200 rim

2-22
2.3.6 FIELD TESTS

The prototype sensor was taken to General Electric*s High Voltage Transmission

Research Facility (IlV_F) near Pittsfield, Massachusetts, for evaluation in a

more realistic environment using facilities described in earlier reports

(Comber and Johnson, 1982; Comber et al., 1983). Three types of measurements

were undertaken: (1) sensor response in a large test chamber used for field

calibration, (2) a field survey under a --ipolar dc transmission line, and (3)

demonstrations of the kind of measurements possible only with an isolated

sensor: measurements of the field above the permanent equipment at the

measurement location, and around a person standing directly under the line.

The results of these initial measurements are given in the following

paragraphs.

As was pointed out earlier, the environment around a full-scale transmission

line is characterized by space-charge and ion current, as well as the electric

field. _he space-charge significantly modifies the electric field, and the

magnitude of the space-charge field varies with time and weather conditions.

Of more concern to free space field sensing, the ion current will deposit

charge on any electrically isolated object, including the sensor and its

supporting dielectric pole. The effect of the collected charge on the field

measurement must be understood in order to make dependable and accurate field

measurements. As will be seen, the data obtained in the test cage do show a

response which is thought to be due to self-charging, although its magnitude

is not large.

The sensor response was determined in a planar test cage which had a 92 cm

spacing between the ground plane and the energized electrode. The cage was

cylindrical, approximately 3 m in diameter (Comber et al., 1983) and had two

peripheral grading rings to minimize the effect of fringing. The sensor was

placed on the axis of the cage approximately 30 cm above the ground plane.

For this measurement, and for those described below, the sensor was supported

by a fiberglass pole 2.5 cm in diameter and 3 m in length. The pole was

oriented along the projected axis of the rotating sensor, although there was a

small (~ 2 cm) offset between the axis of the pole and the axis of the sensor.

The surface resistivity of the pole was not known.

The calibration curve obtained in the cage is shown in Figure 2-13. The

applied field, shown on the abscissa, is based on measurement of voltage and

electrode spacing. The plot represents one excursion from zero field to

maximum and return. The indication from a separate field probe (Monroe

Electronics) located in the ground plane of the test cage is consistent with

the small hysteresis loop seen in Figure 2-13 and the droop of the curve below

a linear response being due to self-charging of the sensor probe, or the pole

used to support it. The magnitude of the perturbation averaged 3_,. No

attempt was made to monitor field direction in the test cage.

The second set of data, shown in Figure 2-14, was obtained in order to

illustrate the type of field-mapping above the ground plane which is possible

only with an electrically isolated probe.

2-23
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Fisur e 2-13. Calibration Oarve Obtained in HVTRF Test Cage

2-24
Field measurements were made around a person while he was standing directly

under the conductor of the short test line at the IlVIRF. 1]ae fiber glass pole

was counterbalanced (for convenience) and the field readings were noted on a

large Polaroid photograph which included the test subject. The sensor is

shown in Figure 2-15, held by Dr. Gary _ohnson of HV]RF. The test subject is

Bj6rn Eng of 3PI_ The conductor was charged to -200 kV dc for this

experiment, which yielded an average field at ground level directly under the

conductor of 7.7 kV/m, and an ion current density varying between 0.5 and

5.0 nA/m 2 as measured by the IIV]RF data system located in the ground plane.

The prototype sensor measured 6.2 kV/m at 1/2 m above the ground surface

without the human subject being there.

In the figure, the length of each arrow indicates the magnitude of the field

as measured at the location of the arrowhead. The sensor, attached to the 3 m

fiberglass pole, was held stationary for a short period of time (~ 10 sec) at

each position indicated in the figure. One of the investigators recorded the

output of the sensor, while another noted the location of the sensor. To

facilitate estimating the sensor position, a large-format instant photograph

was made, and a grid was drawn on it, The position of the sensor probe was

noted in terms of grid coordinates by reference to the photograph. The uncertainty of the position was roughly + 5 cm. Figure 2-14 was later produced

from this photograph, by combining the appropriate position, amplitude, and

direction information Field direction was measured vianally on an oscilloscope (by a third investigator), using the phase of the angular position

reference signal transmitted from the rotating sensor head. The direction

measured without the person being present was assumed to be vertical and used

to calibrate the position output. The accuracy of field direction was roughly

+ 20 o. Instrumentation for precise, direct readout of field angle has subsequently been completed. The observed field at a location 20 cm above the head

was 20.8 kV/m, 3.4 times the undisturbed field at ground level

For comparison to the measured values, field intensification over a conducting

prolate ellipsoid, placed with its axis perpendicular to a conducting ground

plane and its center on the ground plane, was calculated as a function of the

aspect ratio of the ellipsoid and the height above the ellipsoid surface. On

the top surface of the ellipsoid, corresponding to the top of the head, a

field intensification by a factor of 23 was obtained for an aspect ratio

(metal-major axls/seml-mlnor axis) of 6, and 13 for an aspect ratio of 4.

These results compare to a factor of. 12 to 14 at the forehead reported by

Silva et. al., (1985) and a factor of 18 reported by Kaune and Phillips

(1980), both for ac fields. However, the calculated field intensification on

axis, at a distance of 20 cm above the top surface of an ellipsoid with semimajor axis 2 m, was between 3 and 3.5, and not sensitive to aspect ratio, in

agreement with observation.

The field intensity observed over the head was reduced to 9 kV/m when the

subject was wearing a plastic hard-hat, and with no other change. This

reduction is thought to be a result of charge deposited on the dielectric

helmet. It appears likely that the electric field around a human subject can

be modified by charge deposited on the hair and clothing by a de ion current.

The presence of charge on the hair was clearly observed on several of the

investigators during the course of these measurements.

2-25
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Figure 2-14.

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2-26
ORIGINAL PAGE IS

OF pOeR QUALIFY

Figure 2-15. Measurment of Eleotrte Field in the

Vicinity of a Person

2-27
]he third set of data, shownin Figure 2-16, shows the response of the sensor

as a function of time, at a fixed location directly under the dc conductor and

0.6 m above the 8round_su_rf_ce. The conductor voltage was changed

from -200 kV to -500 kV during the interval between the one minute and the two

minute marks in the figure, and remained constant after that.

_he field at ground level as measured by the HV]RF instrumentation is also

shown in the figure for comparison. The ion current, also measured by the

IlV_RF instruments, varied between roughly 20 and 170 nA/m 2 during the period

of measurement, the changes being due to a light and variable wind transverse

to the test line. The magnitude of the ion current that would have been

collected and measured by the field probe, if it had been grounded, was about

three orders of magnitude smaller than the current induced by the electric

field. O, anges in the field with time, as measured with the two instruments,

correlate well The prototype sensor yielded a lower field value by about

15_, on the average.

Similar observations were made by Maruvada et a1.,(1983) using two

independently calibrated instruments, one a ground plane field mill, and the

other a large split cylinder probe. He also observed a lower field, by

approximately the same amount under the same test line at the IIV_J_, although

anothez comparison involving severai instzuments-_ows no similar systematic

difference for all the instzt, ments (Comber et ai., i983).

A number of possible error sources were examined durin $ analysis of the data,

but none were found to be i arge enough to explain the observed difference.

These included error due to self-charging of our prototype sensor, either in

the calibration cage or under the test line, charge accumulation or leakage

along the flberglass supporting pole, and instrumentation errors.

To calibrate the HVTRF fixed-location field meters, a vibratin 8 plate

electrometer, mounted in a 1 m2 _pl_-a-te, was first calibrate_d in the test cage

and then Piaced on the ground adjacent to. the fixe d iinstr_enta_on, where i£

was used as a transfer standard. It is felt that the 15_ difference between

the prototype sensor and the HV_RF instruments may represent a nonuniformity

of the field near the ground surface in the measurement area, inclnding the

temporary location of the transfer standard. To illustrate this explanation,

Figure 2-17 shows the field intensity as measured 0.6 m above the ground

surface along a line parallel to the HVDC conductor and crossing the line of

instrumentation.

The horizontal scale in the figure is approximate, but the relative location

of the elements of the HVIRF instrumentation is correctly noted. Variations

in the electric field of the required magnitude are seen, especially over the

slightly raised ion current sensing plates, as expected. These variations

cannot be due to self-charging or calibration errors in the isolated-probe

sensor. However, if the effective_ground plane surrounding the transfer

standard were depressed below the 1 m_ aluminum plate in which it was mounted,

or if a local depression of the effective ground existed under our prototype

sensor, distortion could occur that would explain the discrepancy.

2-28
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Figure 2-16. Electric Field Versus Time Measured Under DC Conductor,

Compared to HVIRF Data

2-29
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Figure 2-17. Measured Field Values Along a Line Crossln8 Permanent

Instrn_entatlon, 0.6 u Above 6round

2-30
2.3.7 SELF-CHARGING EXPERIMENTS

A detailed investigation, both analytical and experimental, of the effects of

self-charging of the sensor is underway, with the goal of understanding

quantitatively the errors induced b_ charge accumulation in a space-charge

environment. - "

The space a_ormd high-voltage conductors where field measurement is likely to

be of interest normally conducts a significant ion current. The sensor is

electrically isolated in order to allow field measurements far from the ground

plane. This combination of an isolated sensor subjected to space charge

current has not been previously analyzed.

_he parameters of interest are, first, the influence of a conducting surface

near the sensor, and second, the potential of the sensor itself, or more

precisely, the potential difference between the sensor and the free space

potential at its location. This voltage difference will be referred to as the

excess voltage of the sensor, l_e dc sensor described above was placed in a

specially configured test cage for experimental measurements, and theoretical

calculations to confirm the results are still in progress.

2.3.7.1 Experilental Test Arrangement

Before the dc sensor could be evaluated for the effects of self-charging, a

means of producing a miform dc electric field with negligible space charge

was needed. A test cage arrangement similar to those used elsewhere to

calibrate ac field strength meters was designed and constructed. This

arrangement consists of two parallel electzodes of equal size separated and

supported by dielectric rods.

The dimensions of the test cage and clearance distance between the cage and

surrounding conductors was determined after taking into account published

material by Takuma, Kawamoto and Sunga (1985) which analyzed field variations

within a test cage. The dimensions selected for our test cage are expected to

keep distortion to within 2% at the sensor location.

The field cage was made from two 6 ft by 6 ft aluminum screens, separated by

one meten This assembly was supported one meter off the ground, and at least

one meter away from any surrounding conducton Additional dielectric supports

outside the cage were used to prevent the screens from sagging, since

significant deviations from a uniform field would otherwise occu_ Sharp

edges and corners of the screens were coated with silicone rubber to eliminate

Corona.

The upper screen was connected to a variable negative $0 kV dc supply via a

feed line parallel to and in the same plane as the screen. The lower screen

was similarly connected to a variable reversible polarity 30 kV dc supply.

Screen potentials were carefully monitored using separate high voltage probes.

With both supplies adjusted to their maximum output, corresponding to a

70 kV/m field, no noticeable corona was observed. The test cage and the

sensor mounted in it are shown in a photograph, Figure 2-18.

2-31
ORIGINAL PAGE iS

OF POeR (_AJALrTY

Figure 2-18. Field Case and Test Benoh Arrangeeent

(Inset Shows Sensor Without Tubes)

2-32
A special test bench, seen in Figure 2-18, was constructed to support and

manipulate not only the sensor by itself, using its fiberglass support pole,

but also to hold two stationary 6 ft aluminum tubes extending from each end of

the sensor. These tubes, which have the same diameter as the sensor, were

adjusted to be coaxial and within 3 mm of each end of the sensor to

approximate the Infin/te cylinder geometry treated in the analysis.

2.3.7.2 Parameter Measurements

Prior to the self-charging experiments, a ltnearity test of the sensor output

versus electric field was made. The sensor was positioned in the center of

the cage (without the tubes) while its output was measured for fields ranging

from 1 to 60 kV/u_ Within the above range the sensor was linear to within 1%

as shown in Figure 2-19.

With the cylindrical tubes added to the arrangement, measurements were taken

with the axis of the sensor at different distances above the bottom screen,

ranging from a minimum of 1_ sensor radii to a maximum of 13.1 radii at the

center of the cage. During these measurements, the extension tubes and the

sensor were both grounded to ensure that its potential was known. The

connection to the circuit ground on the rotating electronics package was made

by means of a brush and slip-ring assembly.

While maintaining the field in the cage at 30 kV/m +1% the variable supplies

were manipulated to create different excess voltages at the sensor.

Figure 2-20 shows how one such excess voltage was produced.

Figure 2-21 shows the observed variations in sensor output as a function of

excess voltage and distance from the lower plane of the cage. In the case

where d/r is 12, the gap between sensor and conducting plane is only 02r.

As can be expected in this case, measurement error due to field distortion

between sensor and plate is large. In the case where d/r is 5:25, the surface

of the sensor is 16 cm above the conductor (or about 6 inches) and the error

has been reduced to less than 10_, even with excess voltages as great as 2kV.

From the data above, graphs were made which plot fractional change in sensor

output as a function of d/r, both for an uncharged sensor (excess voltage

zero) and for a sensor charged to an excess voltage of -2.64 kV. An excess

voltage of -2.64 kV was determined through preliminary finite element

calculations to be the equilibrium potential to which the sensor would charge

in this field if subjected to a space-charge current. Figure 2-22 and

Figure 2-23 show some of our measurements.

The entire experimental procedure described above was repeated with the

extension tubes removed. The only observed difference was an increase in

scale factor of approximately 25_ when the extension tubes were removed. The

two factors creating this increase are field fringing in an axial direction at

each end of the sensor, and the effect of the curved anti-corona shells on the

end of each electrode. Both of these effects increase the effective area of

the electrodes. The magnitude of the observed increase in scale factor is in

agreement with an analytical estimate.

2-33
400

5

_ 300

_ 200

100

01 5 10 15 20 25 30 35 40 45 50 55 60

ELECTRIC FIELD, kV/m

Figure 2-19. Measured Sensor Output as a Function of Electric Field

2-34
UPPER SCREEN V 1 = -26 kV

,_ =J N1

Ir

SENSOR

AXES --_

_ FOR d/r=2.6

r = cm

t V2 = +4 kV

E- Vl ,_-V2 _ -26kVlm- (+4 kV) -- 30kV,/m

VEXCEss1⁄4V 2- _E.d] :+4kV- [30kV/m' 0.991m] :+1027V

Figure 2-20. Example Showing Arrangement Used to Produce an Excess

Voltage of +1027 V on the Sensor

2-35
4OO

300

100

cl/r = 1.2

2.62

5.25

13.12

-2000

I I I I

-1000 0 +1000 +2000

VEXCESS, VOLTS

Figure 2-21. Sensor Output vs Sensor Excess Voltase for Various

Ratios of d to r, E Field Constant at 30 kV/:

2-36
I

60%

5O%

0%1 2 3 4 8 9 10 11 12

RATIO OF DISTANCE OF SENSOR TO SCREEN

TO RADIUS OF SENSOR_ d/r

14

Figure 2-22. Measured Chanse in Sensor Output as a Function of

Distance to Screen, Sensor Uncharged

2-37
300%

o

o

=)

0

0

Z

1.1.1

Z

m

I,,LI

0

Z

<

"r"

u

200%

150%

100%

50%

0%1 2 3 4 5 6 7 8 9 10 11 12

RATIO oF DISTANCE OF SENSOR TO SCREEN TO

RADIUS OF SENSOR, d/r

Figure 2-23. Measured Change in Sensor Output as a Function of

Distance to Screen, Sensor Fully Charged (-2.64 kV)

2-38

13 14
2.3.8 CONCLUSIONS

Initial laboratory and field experiments have shown that electric field

measurements in free space above the ground plane can be readily made with an

electrically isolated probe which is small and portable. Experimental and

analytical work to date indicates that readout of induced charge with 2_

accuracy will be readily achievable, and that image fields due to selfcharging in the presence of nearby conductogs will not unduly limit accuracy.

In addition, it was possible to measure the electric field vector, that is,

both magnitude and direction of an unknown field, at locations above the

ground plane with the sensor.

It should be noted that the same sensor has been used to measure ac (60 Hz)

fields simply by stopping the spin and reading out the 60 Hz component of the

output signal. Field direction for an ac field could be determined from the

internal position reference with appropriate instrumentation using, for

example, a spin rate small compared to 60 Hz (say, 80 rpm). Experimental

results obtained in ac fields will be reported on later.

Further work on this sensor will focus on refinement of the sensor design,

with the goal of reducing the size and easing the requirement for frequent

battery change. Analysis indicates that the induced charge can be detected

with adequate accuracy on an electrode area as small as 1 cm 2 with good

electrometer amplifier desif_ For example, referring to Equation 2, with

applied field Eo = 100 V/m, a = 0.5 cm, 1 = 2 cm, and w corresponding to

20,000 rpm, the field-induced current i c is I hA, well within the reach of

good semiconductor electrometer amplifier design.

2-39
_LEE

LL
SECTION 3

FIBER OPTIC TEMPERATURE SENSING

One area where fiber optics can make possible a power system measurement that

has not previously been made is the distributed sensing of temperature in

electrical apparatus. There are three areas withinpower systems where such a

measurement would be of considerable value: overhead phase conductors,

underground cables, and station equipment. Each of these areas w illbe

discussed in turn

3.1 APPLICATIONS

3.1.1 Overhead Phase Conductor Temperature Measurement

Since any electrlcal conductor at normal temperatures has a finite resistance,

the current that it transmits causes it to dissipate heah The temperature of

a heavily loaded phase conductor varies according to the wind speed and

dlrectio_ and according to the current that it is carrying. Because of this,

it probably varies from span to span The hottest span in the llne (wherever

that may be) determines the thermal rating for the line. Since the hottest

span is not, in general, known during operation of the line, the thermal

rating is based on a statistically developed table of line capacity against

ambient temperature. This is, of necessit_ somewhat conservative.

If a fiber optic cable could be installed so as to measure the temperature

along the phase conductor, it would be possible to develop a dynamic thermal

rating system. A system of this sort would allow the line ampacity to be

determined accurately based on measurement. This would result in improved

utilization of equipment, and, therefore, of the capital investment associated

with the equipmen_

To be of maximum use, the fiber optic cable has to be installed on existing

conductors. This can be accomplished by placing the fiber optic cable around

the existing conductor, probably within a tube of plastic. The relation

between the temperature seen by the fiber and the temperature inside the phase

conductor would have to be determined carefully.

The use of small objects wrapped around the larger phase conductor in this way

has been likened to the action of a aerodynamic spoiler, and is said to have

advantages in terms of controlling aeolian line vibration and ice galloping.

Overhead lines frequently go for distances in the order of 50 kilometers

without being tapped, and, consequently, a temperature measurement system must

be able to reach distances in this order of magnitude. It will be seen later

that this tends to limit the number of ways in which temperature can be

measured.

3-1
3.1.2 Underground Cable Temperature Measurement

Knowledge of the temperature distribution along an underground cable offers

potentially the same benefits as it does in the case of an overhead conductor.

The circuit can be operated closer to its thermal limit in the steady state,

and emergency operation at higher temperatures may be possible, since the

ability of an underground cable to carry an overload is strongly dependent on

the temperature when the overload begins. Unlike the case of an overhead

conductor, however, which will sag as it gets hot, the underground cable is

supported along its length, and cannot move at all. However, operation at

high temperatures is knows to degrade the insulation and shorten its life.

Under present circumstances, operation of cables is perhaps even more

conservative than overhead lines.

There is considerable interest in know in 8 the temperature profile of pipe-type

cables, in which the three phase conductors separately are inserted into a

pipe filled with dielectric fluid (oil). A know ledge of the temperature

profile along the line will permit the evaluation of the effectiveness of

attempts to improve the cooling by circulating the oil, or other measures.

Normaliy plpe-type cables are spliced at distances not in excess of two

kilometers. Consequently, a system of temperature measurement might only have

to reach that distance.

3.1.3 Power System Equipment Temperature Monitoring

There are a number of potential applications for fiber optic temperature

monitoring inside power system equipment. Of particular interest are

measurements which will enable the determination of winding hot spots in

transformers and reactors, and the temperature distribution of surge

arrestors.

In new transformers, the fiber could be integrated with the winding or

incorporated in the winding duct spacer. In existing units, the fiber could

be threaded through the cooling ducts or placed on the outside of the winding.

The exact location of the fiber would also influence whether the fiber was

exposed to an electrlcal stress as well as temperature effects. This in turn

might affect the method chosen for the measurement.

The distance that would need to be monitored for an understanding of the

temperature distribution inside a piece of power system equipment is the

shortest of the three applications considered here. It would probably not

exceed a few tens of meters even if the fiber were wrapped around the

transformer winding a few times.

i

!

3-2
3.2 METHODS OF MEASURING TEMPERATURE DISTRIBUTION

In order to measure any parameter (including temperature) as a function of

distance, some form of reflectometer is needed. A reflectometer can be

thought of as a kind of radar operating down the optical fiber. Radar of the

kind used at airports operates in free space. The velocity of radio waves in

free space is a constant. It is therefore possible to calibrate the radar

reflections in terms of their distance from the transmitter. The reflection

of a pulse a certain time after it was transmitted yields directly the

distance to the object which caused the reflectio_

In cables, whether optical or radio type cables, the velocity of transmission

is lower than the velocity of light in free space, and the calibration in

terms of distance is more difficult, although, in principle, if all the

parameters of the cable are known, it can be done. For this reason,

reflectometers operating in cables are conventionally referred to as timedomain reflectometers. In fiber optics, since the pulses are optical rather

than of radio wavelengths, the device used is known as an optical time-domain

reflectometer.

An optical pulse launched into a fiber will be transmitted through it almost

unchanged for the entire length of the fiber. It will be slightly changed,

however. At any particular position there will be some reflection of the

energy in the pulse. Some of the energy is scattered by the molecules of the

glass, a phenomenon known as Rayleigh scatterin_ (The energy returning to

the sending end in this manner is said to be backscattere&) This effect,

while small, does mean that the pulse is slightly attenuated as it proceeds

down the fiber. The curve of reflected power (measured by the OTDR from the

backscattered energy) as a function of time (or distance) as seen on the

display of the OTDR will, therefore, inevitably show some droop. Any

discontinuity in the fiber will show up as an excess loss over and above this

normal droop. In this way, it is possible to test the quality of splices and

connectors in fiber optic systems, an application in which the OTDR is

invalunble. Figure 3-1 shows an OTDR system and its output from a sample

cable.

Another way in which the pulse lannched into the fiber is changed during its

transmission is its tendency to spread out due to the various kinds of

dispersion occurring in the fiber. (See Kirkham et al. (1984) for a detailed

explanation of dispersion.) In an ordinary telecommunications application,

dispersion limits the bandwidth-distance product of the fiber, that is to say,

it provides an upper limit on either the distance or the bandwidth for which

the fiber can be used.

For our application in measuring the position of a hot spot, dispersion will

limit the accuracy with which the position of any discontinuity in the fiber

can be determined. Based on the approximation that in free space light

travels about one foot in one nanosecond, one can develop the rule of thumb

that a pulse which is spread to a microsecond in width in an optical fiber

cannot give the location of any discontinuity to an accuracy better than a few

hundred feet.

3-3
OPTICAL

PULSE

SOURCE

OPTICAL

RECEIVER

OPTICAL

. >>

CONNECTOR

ELECTRICAL SYNCHRONIZING

SIGNAL L!

DISPLAY

OPTICAL FIBER UNDER TEST l

a) Optlcal Time-domain Reflectometer

CONNECTOR

REFLECTED

POWER

LIGHT PULSE

LAUNCHED

INTO FIBER

FUSION

SPLICE

COUPLER FEEDTHROUGH

I I

BACKSCATTER

LOSS IN SPLICE

REFLECTION FROM CONNECTOR

LOSS IN CONNECTOR

LOSS IN MICROBEND

FRESNEL END

REFLECTION

MICROBEND

FIBER

END

DISTANC_

LAUNCH ........

b) OTDR display

Figure 3-1. Optical Time Domain Reflectometry

SYSTEM NOISE

TIME

:3-4
To obtain the temperature profile of a fiber, the OTDR display can be used to

show temperature as a function of distance, rather than loss as a function of

distance. To do this, the temperature must be related to the loss, or to some

form of reflectivity, in some known way. The OTDR display could then be

calibrated directly in terms of temperature. Temperature resolution is set by

the ability of the OTDR to distinguish between two values of loss. A

reasonable value here is 0.1 dB.

Distance resolution is a function of the width of the pulse launched into the

fiber, and the dispersion. Since the effects of dispersion increase with

distance, the resolution will deteriorate with 'reach' for a given OTDR. For

a 2 km reach, a resolution of 10 nsec (about 3 m) should be possible,

depending on the fiber.

Distributed temperature can be sensed through a number of possible mechanisms

using an optical time-domain reflectometer. These include temperature

dependence of birefringence, temperature dependence of losses, coupling

variations with temperature, temperature dependence of core-cladding index

ratio (or numerical aperture), optical effects in temperature sensitive

compounds (particularly organic compounds), and simple temperature dependence

of optical index. Each of these approaches will be discussed in turn.

3.2.1 Temperature Dependent Birefringence

Certain materials exhibit a different refractive index for two orthogonal

polarizations of light. Materials exhibiting this property are said to be

birefringent. Normally, one considers two orthogonal linear polarizations,

one exhibiting the fastest propagation velocity in the material and the other

(orthogonal to it) exhibiting the slowest propagation velocity. If the

birefringence can be made temperature dependent, then, in principle, the

temperature can be sensed along the fiber. The different transmission

velocities of the two orthogonal components will result in their having a

phase relation which varies as a function of temperature. Analysis of the

reflected information for a sensor of this type will be a little more

complicated than for a straightforward optical time-domain reflectometer.

However, the equipment shown in Figure 3-2 can probably be arranged to provide

a readout of temperature as a function of distance.

Until recently, fibers which preserved the state of polarization were not

available. While they are currently being manufactured, the temperature

dependence of the birefringence has not been investigated. Therefore, while

this approach, at least in concept, can give a temperature distribution

sensor, it must be regarded as somewhat high risk at this point.

3.2,2 Temperature Dependent Loss

Normally, the glass used in making a fiber is extremely pure, because

impurities have been found to cause loss of optical power. By deliberately

doping the glass with a suitable material, however, it is possible to

construct a fiber in which the loss varies as a function of temperature. This

may be the simplest method of measuring temperature, and may require the least

3-5
POLARIZED

SOURCE

i,

BEAMSPLITTER

POLARIZATION I [

ANALYZING

DETECTOR J t

PROCESSOR ]

PHASE RELATION OF ORTHOGONAL

POLARIZATIONS DEPENDS ON

BIREFRiNGENCE

Figure 3-2. Birefringence Sensing

3-6
development. The transition metals, such as Cu, Fe, Ni, Co, and Cr have ions

that will absorb photons at the visible and infra-red wavelengths of interest.

Doping with one of these materiels would seem like a reasonable place to

start.

As explained in an earlier JPL report (Kirkham et al., 1984) microbendlng

represents an excess loss in many fibers. In some implementations of

microbending sensors, the bare fiber stripped of buffer is deliberately

exposed to pressures from a micro-anvil which causes the loss. In the

anticipated implementation of a temperature dependent microbending loss

sensor, the buffer remains on the fiber and is deliberately made to have

different physical temperature sensitivities than the fiber itself. As a

result of this, a temperature change in the buffer/fiber system induces a

mechanical force on the fiber which results in the required excess loss. The

arrangement is shown in Figure 3-3.

While excess loss of this type has been observed, for example, in some of the

early communications fibers, its mechanism is not well understood and has not

been investigated thoroughly. It might be fair to say that the effect was

observed as a nuisance in the early days of communications fibers. Subsequent

work done on it was aimed at reducing or ellmlnatlng its effects.

3.2.3 Coupling Variations With Temperature

If two fibers are run in close proximity for a considerable length, some of

the energy initially launched into one fiber will be coupled into the other.

By choosing the appropriate physical construction, the degree of coupling can

be made temperature dependent. The implementation then becomes a variation of

the basic optical time-domain reflectometer in that the energy is launched

into one fiber and the reflections are received from another.

It is possible to implement the arrangement in a number of ways, one of which

is shown in Figure 3-4. For example, a multicore fiber can be constructed, or

two separate fibers can be encased in the appropriate polymer buffer, or other

combinations of materials can be use&

3.2.4 Temperature Dependent Optlcal Index Ratio

The efficiency of an optical fiber depends on the ratio of the refractive

indices of the core and the claddin 8 material Normally, these two parameters

differ only by a few percent. If the refractive index of either the core or

the cladding is made temperature dependent, as in Figure 3-5, then the ratio

between the two, and hence the numerical aperture of the fiber, or efficiency

of transmission, will be made strongly sensitive to temperature. This

variation can be picked up by an optical time-domain reflectometer at the

sending end. Variations in the received power may also be useful in

indicating peak temperatures.

Some earlier work on this approach has been done by Fox (1983) at Oak Ridge

National Laboratory, using an organic liqnid core.

3-7
POLYMER BUFFER

/

•TEMPERATURE

DEPENDENT

FORCE

FIBER

Figure 3-3. Temperature-Dependent Kt©robending Loss

3-8

=

L
\

FIBER 1

v/

FIBER 2

Figure 3-4. Temperature-Dependent Coupling

3-9
SILICA

FIBER CORE

POLYMER _

CLADDING f

LIQUID CORE

f SILICA CLAD

Figure 3-5. Temperature-Dependent Optical Index Ratio

3-10
3.2.5 Temperature Dependence of Refractive Index

In all the previous applications of fiber-based temperature sensings the

distance was estimated on the basis of time. It is assumed in these

measurements that the propagation velocity through the fiber is known, or can

be estimated, from the refractive index. An alternative implementation is

possible if the distance is calibrated, for example, by putting a small notch

or other discontinuity in the fiber, say every hundred meters. Then the

discontinuities can be used to calibrate the propagation velocity or the

change in refractive index. In an implementation based on this approach, the

transit time between marks which were known to be equidistant would be used to

give an indication of temperature. Figure 3-6 shows the arrangement.

3.2.6 Point Sensing for Distributed Measurement

There are two other ways in which an optical fiber can be used to give an

indication of temperature distribution. First, a number of discrete sensors

can be separately interrogated by means of an optical fiber (without using an

optical time-domain reflectometer). This is shown in Figure 3-7a.

Alternatively, sensors can be distributed down an optical fiber as in

Figure 3-7b, giving the same effect as the previously described distributed

methods.

Implementation of the first approach amounts to using the optical fiber only

as a communications channel. Each sensor along the fiber can be addressed

separately and the resolution obtained can be arbitrarily defined. Using this

approach, sensors could be located close together where it was thought that a

hot spot may occur, and far apart elsewhere.

Using the second approach, if the sensors are located close together, they

cannot be individually resolved with a normal OTDR. The fiber optic then

appears to the OTDR exactly the same as a temperature sensitive fiber. This

approach has the advantage that a large number of physical effects can be used

to detect the temperature change.

3-11
I OTDR

FIBER, WITH EVENLY SPACED NOTCHES

(a) PHYSICAL ARRANGEMENT

REFLECTED

POWER

REFLECTED

POWER

(b) OTDR DISPLAY WITH FIBER AT

UNIFORM TEMPERATURE

(c)OTDR DISPLAY WITH CENTER SECTION

AT LOWER REFRACTIVE INDEX

Fisur e 3-6. Temperature-Dependent Refractive Index

3-12
DATA

FIBER OPTIC DATA

SYSTEM

COUPLEI

SENSOR SEPARATION

ARBITRARY

Figure 3-7a. Separate Sensors Using Optical

Fiber as Communication Channel

FIBER

I OTDR I-- LENGTHOF -- _

OPTICAL PULSE _.

TO OTHER

SENSING REGIONS

Figure 3-7b. Separate Sensors Spaced Within the

Resolution of the O_

3-13
3.2.7 Distance Reached by a Lossy Distributed Sensor

In this section the capability of a fiber with temperature-dependent loss to

measure a temperature change and its location is estimated from first

principles. Signal attenuation along a fiber is usually expressed in decibels

per unit length, so that the actual input and output power do not have to be

specifie& Thus, if a is the attenuation per unit length,

Pin

a = 10 lo8

Pout

(3-1)

The total loss A over a length X of fiber is

A = ;adx = i0 ;log Pl---n dx

o o Pout

(3-2)

It can be seen that if the loss is constant along the fiber, a length (of

fiber) can be calculated at which the loss equals some particular value. This

can be used to calculate usable fiber lengths for a particular system

signal/noise rati_ If the loss is not constant, the situation becomes more

complica ted.

If the incremental loss a is made temperature dependent, i.e., if a = a(T),

then the loss down a fiber of arbitrary temperature distribution T(x)

A = la(z) dx

0

(3-3)

can be found by substitutin 8 a(T) into T(x). There is experimental evidence

for some rather simple functions a(T), such as a = kT or a = kTI/2 (depending

on the material). To see the effect on maximum length, let us assume that the

loss is of the form a = kT so that a(x) = kT(x). For this case,

A = k _T(x) dz

O

and the solution requires only a specification of T(x).

we obtain the simple result that

(3-4)

With constant T(z),

k = kXT (3-5)

In practice, OTDR technology is limited in the total loss which can be covered

to a value which we will assume to be 30 dB. This means that, for a fiber

with positive temperature coefficient of loss, the distance (X), the

3-14
temperature (T), and the sensitivity (k) must be chosen together. Some

compromise will be inevitable in most applications.

If the required distance is 2 kin, and the temperature range of interest is

above 60 °C (333K), the sensitivity is limited to

k = 30 dB (3-6)

2 km x 333

= 0.045 dB/km oC

Such a low temperature coefficient of loss would result in a system of low

resolution. A 10°C change in temperature for 100 m would produce less than

0.05 dB change in loss. This would be just discernible with *ordinary'

equipment.

One solution is to produce a fiber with a low loss at the lowest temperature

of interest, and a greater sensitivity. (This approach might require some

exotic material used in the point-sensing approach described above.

Nevertheless, it should be workable.)

Suppose, for example, that a fiber is produced that has low loss (say 2 dB/km)

at the low end of the temperature range of interest (below 60°C), and a

sensitivity of 2 dB/km °C above that temperature.

Suppose that most of a 2-kin cable is at 64°C, but a 100 m section near the

middle is at 80°C. The total loss can be calculated by modifying

Equation 3-5. The two temperature regimes can be considered separately, and

their losses added. For each section, a loss rate can be calculated from the

basic loss (2 dB/km), the sensitivity and the temperature. Thus

A ffi 1.9 km • 10 dB/km + 0.1 km• 42 dB/]m (3-7)

= 23.2 dB

Since the total loss is comfortably below the value set by the O_DR limit of

sensitivity, a temperature profile could be obtained.

Suppose, now, that the hot-spot were doubled in length. The total loss is

then

A = 1.8 ]an x 10 dB/km + 0.2 km x 42 dB/km

- 26.4 dB

which is still a usable value, although it is getting close to the limit.

(3-8)

3-15
If the fiber temperature increased by l°C over its entire length, the loss is

given by

A = 1.8 km• 12 dB/km + 0.2 km • 44 dB/kn (3-9)

= 30.4 dB

This value exceeds the capability of the _ which therefore would not be

able to provide infomation about the last 200 m (0.4 dB) of the cable.

If the basic temperature and the hot-spot range were known in advance with

sufficient precision, a system like this could be made. To achieve the

desired resolution, computerized data-taking would be necessary, and the OTDR

could be operated many thousands of times to obtain averaged results.

One way in which the system could function would be for the computer to

present the results as temperature (computed from loss data) as a frmction of

distance at increments set by the system resolution. If a horizontal

resolution of 10 m was achieved, and a loss resolution of 0.1 dB, the

temperature resolution would seem to be no better than 5°C. (This result is

based on a sensitivity of 2 dB/km/OC.) Temperature and distance resolution

can be traded off: either can be improved at the expense of the other. It

seems likely that a system of this kind could be arranged to have a loss

resolution of 0.01 dB, by means of suitable data processing. The overall

temperature profile could then be obtained with a resolution of 10 m and

0._oc, or 5 m and leC. This should satisfy all reasonable requlrenentL

3-16
SECTION 4

OPTICAL POWER TRANSFER

4.1 BACKGROUND

With the development of efficient optical to electrical converters, optically

transmitted power could be used to energize electronlcs for power system

measurements. A system powered by optical energy would avoid the need to

replenish batteries, or to extract power from power lines. Some work was done

on component development at JPL during 1983 and 1984, and is reported

elsewhere (Kirkham et al., 1984).

The first experiments performed involved transmitting light over a fiber optic

cable, then converting the light into usable electric power using silicon

photodiodes. These experiments were performed at JPL using 850 nm wavelength

light. The simple diodes used produced only about 0.5 volts (open circuit).

This voltage is too low to be useful. Therefore, an array of series connected

diodes is neede& Figure 4-1 shows the entire optical power transfer system

in block diagram form.

Silicon was chosen in the first experiments because of its availability and

easy processing. Unfortunately, unless it is very thick silicon is not very

efficient at converting light at 850 nm to electric energy. If the sample is

thick, it would be very difficult to produce a small diode array using integrated circuit techniques. Gallium arsenide (GaAs) is much more efficient at

this wavelengt_ thereby allowing thin layers of material to be use& GaAs

was selected for the second set of experiments.

The work done in 1985 was aimed at producing a photodiode array consisting of

32 AlGaAs-GaAs diodes internally connected as four strings of 8 diodes (see

Figure 4-2). Although the conversion efficiency for GaAs is much 8reate_ than

silicon at this wavelength, GaAs has the disadvantage that it is much more

difficult to process. GaAs was still considered the better choice because it

requires much less material than silicon, making the final array in GaAs

smaller.

The diode array was designed using standard integrated circuit design rules.

In order to hold the series resistance to a minimum, the diodes were designed

as short, vide rectangles. This design resulted in some problems with mask

alignment during fabricatioL A standard glass photomask set was produced for

the photolithographic process.

The diodes were produced using Metal Organic Chemical Vapor Deposition (MOCVD)

techniques as follows. A semi-insulating GaAs wafer was used as the substrate

that held the individual diodes, and isolated them from each other. A thick

layer of n-type GaAs was deposited on this layer, followed by a thinner layer

of p-type GaAs. These two layers formed the p-n junction photodiode where the

light-to-current conversion takes place. A thin layer of p-type A1GaAs was

then deposited as a surface passlvation layer. Figure 4-3 shows this sequence

of events.

4-1
LIGHT

SOURCE

(LASER)

POWER

INPUT

FIBER

OPTIC

CABLE

OPTICALLY _ ÷V

COUPLER _ DRIVEN POWER _ COMMON

SOURCE _ - V

LIGHT TO VOLTAGE

CONVERTER

POWER

OUTPUT

Figure 4-1. Block Diagram of Optical Power Transfer Syste=

4-2
LIGHT

PHOTODIODEARRAYI- +l PHOTODIODEARRAY

PHOTODIODE

ARRAY ]- _1°°'1-

+V

COMMON

-V

+V

PHOTODIODE ARRAY #4 PHOTODIODE ARRAY #2

PHOTODIODE ARRAY #3 PHOTODIODE ARRAY # 1

COMMON

-V

Figure 4-2. Electrical and Physical Arrangement of 32-Diode Array

4-3
P

__j

i

SEMI-INSULATING n

a) SEMI-INSULATING SUBSTRATE WITH

EPITAXtAL n and p LAYERS

I

d) OXIDE LAYER SPUTTERED OVER SURFACE

b) ETCHED MESA

l/, _/, t

e) MOST OF OXIDE LAYER REMOVED

f

c)

I I I '

\ /

SECOND ETCH EXPOSES n LAYER f) SILVER DEPOSITED AS INTERCONNECTION

Fisure 4-3. Sequence of Events in l_coducins Gallium A.rsenide Diode Array

4-4
The samples were then etched into individual mesas, each mesa being a diode.

A layer of silicon dioxide was deposited on the diode mesas (by RF sputtering)

and then etched to remove all the oxide except that which covered one of the

sides of mesa. This oxide was used to prevent shorting out a diode when the

interconnecting metal which formed the series connection between the diodes

was applied. The final step was to evaporate silver on the sample, and etch

to create the necessary Interconnections. After the processing was completed

and the array was diced from the substrate, the sample was mounted in a TO-5

header. The overall array is 0A inch by 0.1 inch

Several major problems were faced in the design of this diode array. Problems

that were solved included: precise etching of the GaAs-A1GaAs (both in terms

of area and depth), and deposition of the silicon dioxide layer, which had to

be deposited over a step.

A cross-sectional view of the diode array is shown in Figure 4-3. The diode

array layout and mounting arrangements are shown in Figure 4-4.

4.2 DEYI(Z DESIGN PARAMETERS

In designing the first of these photodiode arrays, the problem was divided

into three parts. Each part was considered separately, although there was

considerable interaction between them. The three parts were: I) the

multilayer GaAs starting wafers, including the p-n junction and passivation

layer, 2) the light capture area, and 3) the device conversion efficiency.

Each of these is discussed below.

4.2.1 The GaAs starting wafers.

The technical approach to solving this problem was based on performing a set

of experiments to determine the optimum growth conditions needed to produce

the required resistivity, dopant impurity type, and layer thickness.

A matrix array of test growths were performed. Hall mobility measurements

were made on these test samples to determine the optimum growth conditions.

Hall measurements were used to determine sample resistivity and impurity

levels, in addition to mobility. The mobility data gave a good indication of

the purity of a grown layer. Therefore, the optimum conditions were defined

as those conditions where the highest Hall mobility was obtained. Test

growths to determine the optimum values were made using the JPL-built MOCVD

reactor over a temperature range of 675 ° to 700 ° C, growth rates over the

range of 40 to 200 um per minute, d_pant concentrations for Si (n-type) layers

in the range of 2 x I0 "q to 5 x I0 cm-_, and film layer thicknesses over the

range from i0 nm to 3 pL

For the JPL-built MOCVI) system, the optimum growth temperature was found to be

700 ° C. At this temperature, a single undopod layer of GaAs had a Hall

mobility of 60,000 at 60K. Therefore, 700 ° C was used to produce the GaAs

layers for the photodiode array. In addition, the layers were doped with an

impurity element. The tests indicated that layers as thick as 3 pm could

easily be grow_ For the photodlodes, thicknesses of 1.6 to 2.0 _m are needed

with doping levels as high as 1018 cm-3.

4-5
PHOTODIODE _

ARRAY # 1 -_

PHOTODIODE

ARRAY #3

DIE (SUBSTRATE)

 PHOTOD

ARRAY # 2

PHOTODIODE •

.,_LU_,_T_D _

Figure 4-4. Diode Array Layout and Mountin 8 Arrangements

4-6
The tests indicated that there was no problem making thick, heavily doped

n-type layers of GaAs. However, problems were encountered when attempting to

produce p-type layers. At this time, zinc-doped p-type layers with thickness

of around 2 jnn have not been consistently produced using the JPL MOCVD reactor

because of doping gas problems. It is planned to try the Liquid Phase

Epitaxial (LPE) reactor to grow the p-type layer. The LPE reactor has been

successful in the past at growing p-tyl_ layers. Meanwhile, a new tank of

zinc doping gas will be purchased for the JPL MOCVI).

After test structures (wafers) were made, the wafers were etched using

standard photolithographic techniques. At first, some problems were

encountered. The p_oblems were caused mainly by equipment limitations, and

were most acute when using the mask alignar. These problems were eventuaUy

solved by using a newer mask aligner with a higher resolution microscope.

Additional tests were made to determine the best method of dicing (cutting)

the completed devices from the substrate wafer. This is always a problem when

designing integrated circuits, especially when the circuits are made with

GaAs. Each of the common methods of dicing was evaluated. _hese methods

included: 1) laser scribing, 2) diamond scribing, and 3) diamond sawing.

The results of these tests are given in Figures 4-5 through 4-9.

The damage seen on the cut edges in these photographs (Figures 4-5 through

4-9) probably propagates into the material. Therefore, the minimum distance

from the edge (or scribe line) to the circuit is dependent on this damage.

Using the method which produces the least amount of damage will allow a

maximum of the die (chip) area to be used for the active circuits. Diamond

scribing was determined to create the least amount of damage to the wafer, and

therefore will be used as the dicing method.

In order to passivate the surface region of the photodiodes, AIGaAs was

epttaxially grown on top of the p-n junction. This AIGaAs layer is basically

transparent to 850 nm light, and helps to reduce surface recombination in the

p-layer material, which decreases the overall efficiency of the photodiode.

4.2.2 Light Capture Area and Conversion Efficlenolea

The size of the photodiode array was determined by the light source

arrangement to be used. In this case, the light source feeding the photodlode

array was a fiber optic cable, l_igure 4-10 shows how the fiber optic cable

feeds light to the photodiode array. A standard TO-$ IC header was chosen to

be the device mount since it fits easily into a fiber optic connector. With

the fiber not physically contacting the TO-5 header, but spaced away by a few

millimeters, a fiber should be able to project light over most of the TO-5

device mounting area.

4-7
GaAs

VIEWING

ANGLE

DIRECT SAWED EDGE

OBLIQUE _ _,

POLISHED

SURFACE

Figure 4-5. Gallium Arsenlde Wafer Dicing, Showing Sawed Edge

Subsequent Scanning Electron Kicroscope (Sin/) photographs of

diced wafers show the edge (or the corner) by either direct or

oblique-angle view

4-8
ORIGINAL PAGE IS

OF POeR QUALITY

SEM HEADER

LASER

SCRIBED

SURFACE

POLISHED

SURFACE

SCRIBED

SURFACE

Part of the polished surface of the wafer can be seen in

(a): this is an oblique view. View (b) has $ times the

magnification of (a). Had the material been silicon the

roughness of the surface would have disappeared because of local

melting.

Figure 4-6. Two Views of a Laser-Scribed Gallium Arsenide Wafer

4-9
ORIGINAL PAGE IS

OF. POeR QUALITY

POLISHED

__ SURFACE

__

/-S_ CRIBED

SURFACE

i

i

Surface ¢_acks can be seen propagatin S from the scribed surface

into the polished surface of the valet.

Figure 4-7. Edge of Gallium Arsenide Wafer Scribed Side-to-Side

4-10
ORIGINAL PAGE IS

POeR QUALITY

SEM HEADER

EDGE

POLISHED

SURFACE

WAFER

EDGE

POLISHED

SURFACE

In this kind of dicing, only the corner of the wafer is scribed by

the diamond. The wafer is then broken apart along a cleavage

plane of the crystal. In (a) the damage caused by the scribe can

be seen as surface cracks propagating into the photograph from the

left. Further away from the scribed region, in (b), the edge can

be seen to be smooth, and the corner between the edge and the

polished surface is sharp.

Figure 4-8. 1_o Views of a Diamond-Scribed Gallium Arsenlde Wafer

4-11
SAWED

SURFACE

-- POLISHED

SURFACE

SAWED

SURFACE

POLISHED

SURFACE

In (a) the cut surface seems to be smooth, but with more masnification,

as in (b), roughness can be seen, and the corner with the polished

surface is not sharp. Further, this method of dicing requires large

separation between the circuits integrated on the wafer.

Figure 4-9. Two Views of a Gallium Arsenlde Wafer Cut with a Diamond Saw

4-12 CRIGINAL PAGE IS

OF POOR QUALrrY
WIRES--_

FIBER OPTIC

DEVICE . . _ . . _ J CABLE CONNECTOR

LIGHT

FIBER OPTIC

TO-5 CABLE

HEADER

Figure 4-10. Photodiode Array Mountin s Arrangement on Fiber Connector

4-13
The largest square device that can be mounted in a TO-5 header is O_ inches

on a side. However, the photodiode array cannot be built all the way to the

edge of the available area. Aside from the need for bonding pads (for

connection to the outside), the act of cutting the circuit from the die

damages the material at the edge. The array is therefore smaller than the

available 0.2 by 02 inch ares. The prototype design is approximately 0.07

inches on a side. A square of this size has a diagonal of 0.1 inch, so that

0.1 inch becomes the diameter of the smallest circle which will completely

enclose the phototype array. Because the array is square, and the light

emerging from a fiber can be assumed to illuminate a circular area, there is

an effective loss of about 365. Only 645 of the light in the circle is on

the inscribed square. Once the lighted area is defined, an approximation of

the device performance can be made.

It would be conservative to estimate that the conversion efficiency of a GaAs

photodiode is equal to that of a typical silicon photodiode, such as those

produced earlier as part of this project. A typical silicon photodiode has

about 145 conversion efficiency. A higher conversion efficiency is expected

for the GaPs photodiode array. A typical GaAs F-n junction cell conversion

efficiency for monochromatic light is about 405.

The light source will be assumed to project 100 mW onto the device. With

100 mW illuminating the circular region within which the square array is

mounted, the active circuit area of the photodiode array sees 64 roW. Thus,

the power available to be converted is 64 roW. If the array has 405 efficiency

then 25 mW is delivered by the array. (These estimates do not account for any

series resistance of the arrays, nor do they account for any degradation of

performance when the device heats up.) The Woverall r efficiency of the system

is thus on the order of 25_

If the cells each produce about 0.75 volt (a very conservative estimate for

GaAs), then the current per cell is 1 mA. It should be possible to produce

+_ 12 volts at 1 mA in a 32 diode array. This would power a reasonebly large

amount of electronics.

4.2.3 Device Fabrication.

As stated earlier in this report, all photodiodes will be produced using MOCVD

or LPE techniques. These techniques have existed for about i0 years, but are

still considered experimental. Standard GaAs IC etching techniques used with

standard photolithography are used to produce the arrays. An actual etched

array is shown in Figure 4-11.

4-14
ORiGiNAL PAGE IS

OF POOR QUALITY

_Z

Figure 4-11o Photograph of 32-Diode Array

4-15
4.2.4 Test Results.

At this writing there are no test results from completed arrays. As stated

earlier, problems with producing a 'good' p-layer, and alignment problems,

have prevented making workable devices. Tests to determine the optimum grQwth

conditions, and find solutions to the problems of p-layer growth, were

performed, and results were presented earlier in this _eport.

4.3 Future Work.

After a workable device (array) is-produ-ced, and I-V curves made to determi_-

co-nver-Sl-on e_iciency, stu idles w-ii-i--be_-c-_ducted to_ dete_mi-ne--_he effects of

heating_ue to device dissipation), _and to determine if the light level can

be safel-yincreased. Further, the questlon o_ l_etime and reliability of

these devices will require study when a workable design has been established

4-16
This report has described work done at JPL on the Communications and Control

for Electric Power Systems Project. The report reflects the emphasis placed

by the project on the measurement of dc electric fields and the transfer of

power over optical fibers.

At the time that this report was being finalized, .tests were being carried out

on the prototype of a miniaturized dc field sensor, This sensor is about a

quarter of the diameter and half the length of the dc field probe described in

this report. The electronics are packaged in a hybrid assembly on a ceramic

substrate contained in a 0.6 inch diameter transistor can. Drive for the

rotating components is by a modified dental-drill air turbine. The on-board

electronics are powered by watch batteries, which are turned on by a

centrifugally actuated switch at about 20,000 RPM. The electrodes are

0.75 inch in diameter and 1.5 inch long. Testing and evaluation of this

instrument will continue in the coming year. The design, construction, and

test results will be described in a separate report.

Gallium arse_ide diodes have been produced singly (rather than in arrays)

since this report was assemble& The first application of these diodes is

planned to be in an ac field probe housed completely within a 2 cm sphere.

These diodes and this field meter will also be described separately.

This report describes several methods of sensing distributed temperature. At

the time of writing none of these have been experimentally teste& It is

hoped to do so by 1987.

5-1

REFERENCES

Comber, M. G., and Johnson, _ B. (1982, July). HVDC Field and Ion Effects at

Project U[N: Results of Electric Field and Ion Current Measurements. IEEE

Trans. Power Apparatus and Systems, PAS-101 (7), pp. 1998-2006.

Comber, M. G., Kotter, R, and McKnight, R. (1983, November). Experlmental

Evaluation of Instruments for Measuring DC Transmission Line Electric Fields

and Ion Currents. IEEE Trans. Power Apparatus and Systems, PAS-102 (11)

pp. 3549-3557.

Deno, D. W. and Zaffanella, L Fo (1975). Electrostatic Effects of Overhead

Transmission Lines and Stations. In Transmission Line Reference Book

345 kV and Above, Electric Power Research Institute, Pale Alto, CA, 1975.

Feser, K. and Pfaff, W. (1984, October). A Potential Free Spherical Sensor

for the Measurement of Transient Electric Fields. IEEE Trans. Power Apparatus

and Systems, PAS-103 (10), pp. 2904-2911.

Fox, Ih (1983). Measurement of Peak Temperature along an Optical Fiber.

Appl. Opt. 22 (7), pp. 967-969.

Bidaka, K. and Fujita, B. (1982, July). A New Method of Electric Field

Measurements in Corona Discharge Using Pockels Device. J. AppL Phys. 53 (9),

pp. 5999-6003.

Kaune, W. T. and Phillips, R. D. (1980). Comparison of the Coupling of

Grounded Humans, Swine and Rats to Vertical 60 Hz Electric Fields.

Bioelectromagnetics, 1_, 117

Kirkham, _, Johnston, & _, Lutes, G., Baud, T., and Hyland, S. Power

SystemAvvlications of Fiber Ovtics. Jet Propulsion Laboratory, Publication

No. 84-28.

Maruvada, P. S., Dallaire, IL D., and Pedneault, R. (1983, March).

Development of Field Mill Instruments for Ground Level and above-ground

Electric Field Measurement under HVDC Transmission Lines. IEEE Trans. Power

Apparatus and Systems, PP_I02 (3), pp. 738-744.

Misakian, M.,S getter, F. R., and Kahler, IL (1978, July). Miniature ELF

Electric Field Probe. Rev. Sci Instr. 49 (7), pp. 933-935.

Silva, t&, Zaffanella, L, and Bummon, N. (1985, July). An Activity Systems

Model for Estimating Human Exposure to 60 Hz Electric Fields. IF_J_ Trans.

Power Apparatus and Systems, PAS-104 (7), pp. 1923-1929.

Takuma, T., Kawamoto, T, and Sunaga, Y. (1985, February). Analysis of

Calibration Arrangements for AC Field Strength Meters. IEEE Trans. Power

Apparatus and Systems, PAS-104 (2), pp. 489-496.

6-1
Wilhelmy, W.(1ff72). Floating Potential Field Strength Measurement in a Space

Charge Free, Periodically Time Dependent and Transient Electric Field.

Dissertation, University of Stuttgart. Available in English from University

Microfilms, Ann Arbor, Michigan

6-2