Electronic current transducer for high voltage transmission lines

An electronic current transducer for high voltage transmission lines including a fast analog data channel for providing effective relaying information which is stablized by a slow, highly accurate, channel which is insensitive to changes in gain. In addition for dc transmission lines where polarity may reverse, offsetting or biasing allows for bipolar measurement.

BACKGROUND OF THE INVENTION 
In high voltage ac and dc power transmission lines it is necessary to 
deliver at ground potential an accurate value or replica of the amplitude 
and phase of the line current. This is then used for operating a variety 
of sophisticated relays that protect the line and make measurements of 
faults. It is also used for measuring the amount of ac power that the line 
is delivering to the customer (dc metering is not yet available). The 
foregoing applications require fast response and high accuracy. 
Magnetic core current transformers have been used for the above tasks but 
at line voltage of 500 kv--ac and 250 kv-dc and above conventional ac 
current transformers and dc magnetic amplifiers are very heavy and 
expensive. 
However, there have been a number of efforts to develop an electronic 
current transducer or transformer in which the required information is 
measured at line potential and transmitted to ground on a modulated beam 
of light or via radio waves. In a typical system a high speed electronic 
(analog or digital) converter at line potential converts the instantaneous 
current amplitude to a low level signal which is used to modulate a 
light-emitting diode. The modulated light is transmitted by a long fiber 
optic light guide to a ground unit where the modulated light is detected 
and converted back to an analog waveform duplicating the current waveform 
on the line. Some of the serious problems with such a system are that it 
is very difficult and expensive to combine the required wide dynamic range 
(e.g., 10,000), the fast sampling rate (2 msec. response time), and the 
accuracy (0.3% over a -40.degree. to 50.degree. C. temperature range). 
Another technique uses Faraday Rotation where a beam of light, for example 
from a laser, is sent up through a hollow insulator from the ground, 
modified by the magnetic field of the current by polarization, and is 
returned to ground where this rotation is sensed. The foregoing techniques 
are illustrated in Hermstein U.S. Pat. No. 3,681,688 and Heintz U.S. Pat. 
No. 3,492,574. Hermstein improves accuracy as illustrated in FIG. 3 of his 
patent by the use of a second laser channel for temperature compensation. 
Heintz has a two channel fiber optic system: one channel carries pulse 
length information related to the magnitude of the line current and a 
second channel polarity. 
Yet another problem in the prior art, especially with the advent of high 
voltage dc transmission grids, is that the dc current may flow in either 
direction in some of the branches. Then in addition to the above accuracy 
and response requirements a line current sensor must have the capability 
of measuring bipolar currents. This is especially difficult where light 
transmitting channels such as fiber optics are used since there can be no 
"negative light." 
OBJECT AND SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an improved electronic 
current transformer apparatus for measuring the line current of a high 
voltage transmission line. 
In accordance with the above object there is provided an electronic current 
transducer apparatus for measuring the line current of a high voltage 
transmission line comprising an analog fast channel having means for 
transmitting line current data in continuous form with a relatively 
unstable gain characteristic. A parallel slow channel transmits line 
current data in discrete form with a relatively stable gain characteristic 
but with relatively narrow dynamic range and frequency response. The fast 
channel data is calibrated with the slow channel data to produce a gain 
stabilized replica of the line current.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, the high voltage transmission line 10 is illustrated 
as having a current I(t). Connected to this transmission line at 
substantially line potential is a line portion 11 which is connected to a 
ground portion 12, which is substantially at ground potential, by a pair 
of fiber optic light guides 13 and 14. In practice, such guides would be 
several meters in length. The actual current I(t) is sensed by both a fast 
channel 16 and a slow channel 17. 
Fast channel 16 includes a fast sensing unit 18 which senses the line 
current by either a simple current transformer in the case of ac or a 
resistor shunt in the case of a dc line. It includes a modulator which 
modulates light from a light source in, for example, an amplitude 
modulation mode and transmits line current data on fiber optic light guide 
13. This is detected by fast detector 19 which produces an output voltage 
of V.sub.0 which is a direct function of I(t). Fast channel 16 is an 
analog system (that is, one having no analog to digital conversion) and is 
by its nature continuous; namely, in any short time interval the output 
voltage V.sub.0 is proportional to line current. Such a fast system 
exhibits good frequency response in that when a "step" occurs in I(t) the 
V.sub.0 must follow rapidly. V.sub.0 is accurately proportional to I(t) 
within a few msecs. Also the system operates over a wide dynamic range, 
for example 0.01 to 57 p.u., where one per unit (p.u.) is the rated line 
current under normal conditions. Thus 57 P.u. would be a relatively high 
fault current and 0.01 p.u. would be almost no load. 
Suitable fast channel systems in addition to an amplitude modulated light 
beam are Faraday Rotation systems well known in the art or a modulated 
solid state transmitter such as, for example, using a Gunn diode modulated 
by the function I(t) and where the resultant microwave energy is 
transmitted to ground potential either in free space or down a dielectric 
wave guide. 
However, as discussed in the description of the prior art, all of the 
foregoing are fast systems which have a relatively unstable gain 
characteristic especially at the system limits of, for example, 0.01 p.u. 
and 57 p.u. In other words, there is a tradeoff between speed and system 
accuracy. 
To provide such accuracy, a slow channel 17 is provided. This channel 
includes current transformer T.sub.1 with an output voltage of V.sub.1 
which is a replica of the current waveform I(t) on the line 10. V.sub.1 is 
appled to a limiter-rectifier-filter circuit 21 which produces a slowly 
varying dc voltage U.sub.1. Normally, because of the filtering, the time 
response of U.sub.1 to a step change is several seconds; e.g. 6. The 
voltage U.sub.1 is periodically measured, for example once per second, by 
an analog to digital converter 22 powered by a line transformer T.sub.2. 
It has a digital pulse output D.sub.1 which modulates a light emitting 
diode 23. The resultant digital light pulses are transmitted by a fiber 
optic channel 14 to photo-detector 24 which again reproduces the D.sub.1 
pulses and couples them to a digital comparator 26. 
Slow channel 17, which because of its analog to digital conversion provides 
line current data in a discrete form, has an inherently relatively stable 
gain characteristic; but at the sacrifice of a relatively narrow dynamic 
range and frequency response. Where data is in discrete form, there is 
necessarily a lowered frequency response because with such data, it must 
be sampled over a sample time interval of, for example, 100 msecs. in 
order to produce any usable information. 
Now referring in greater detail to ground portion 12, the fast detector 19 
is connected to a potentiometer R.sub.1, R.sub.2, with a movable contact 
27 which provides a variable output voltage V.sub.2, a replica of I(t). 
However, it has an amplitude that is a fraction of V.sub.0 which is for 
example 80%. V.sub.2 drives a limiter-rectifier-filter circuit 28 similar 
to circuit 21 (viz., the filter portion has a time constant of 6 seconds 
also) and provides a dc voltage U.sub.2 which drives analog to digital 
converter 29 to produce a digital signal D.sub.2 which is connected to 
comparator 26. Comparator 26 calculates difference in values between 
D.sub.1 and D.sub.2 and integrates this difference over a short period of 
time, for example, five minutes. A motor control output on line 31 drives 
a motor 32 which moves the contact 27 in a direction so as to reduce the 
error signal to effectively make V.sub.2 equal to V.sub.1. If the 
integrated error signal is above a predetermined level, alarm line 33 is 
actuated. This might be caused, for example, by a malfunction in the slow 
channel, the fast channel, or of motor 32. 
In summary, slow channel 17 which is relatively accurate therefore 
calibrates the fast channel 16 whose output information V.sub.2 may be 
used for rapid system control. 
The system as described in FIG. 1 has several advantages: 
1. Design of the fast channel system is greatly simplified by the fact that 
it is allowed to have a slow drift in gain. 
2. Since the sampling rate of the slow channel 17 is relatively low, it may 
have a fairly simplified design. In contrast, in order to provide a fast 
response channel of this type (with an A/D converter) a sampling rate of 
at least 1,000 per second would be necessary and would require more costly 
components and much more bandwidth than would an analog system. 
3. The design of the analog to digital converters is simplified since they 
need not operate over a wide dynamic range. 
4. There is a system symmetry in that the same analog to digital converter 
and limiter-rectifier-filter units are used both in the line portion 11 
and ground portion 12. Thus temperature sensitivity tends to cancel out. 
FIG. 2 illustrates a typical limiter-rectifier-filter of unit 21 or 28. 
FIG. 3 is another embodiment of the invention which is especially useful 
for high voltage dc transmission lines, such as line 40 shown. Here line 
current shunts 41 are utilized. The shunts feed a slow channel 42 and a 
fast channel 43. Fast channel 43 includes a linear isolator 44 which 
amplitude modulates light transmitted over a fiber optic guide 45 to 
produce a signal output from photo-detector 46. Slow channel 42 includes 
an amplifier 47 which drives an analog to frequency converter 48 or analog 
to digital (A/D) converter more generally. It has an output such as shown 
by the waveform 49. Converter 48 drives a light-emitting diode (LED) unit 
51 to provide digital data over fiber optic light guide 52 which is sensed 
by a photo-detector 53. All of the foregoing is contained by a line unit 
54, except for photo-detectors 46 and 53 which are included in ground unit 
56. 
The output of photo-detector 53 is converted to analog form by a digital to 
analog converter 57 which analog output is filtered by a low pass filter 
58 and coupled to a comparator 59. The other input to comparator 59 is on 
line 61 which is from the fast channel 43 via a fast amplifier 62 
connected to photo-detector 46, a multiplier 63 and a low pass filter 64. 
Multiplier 63 is in turn driven by the error output of comparator 59 so 
that it multiplies the fast channel signal level to cause a comparison 
with the slow channel calibrating signal. The resultant gain stabilized 
output is thus on line 65. 
FIG. 4 shows line unit 54 in greater detail. The input from the line shunt 
41 is actually connected to a common operational amplifier 47 which 
supplies both slow channel 42 and fast channel 43. Operational amplifier 
47 has a voltage offset applied to it by a bias injection circuit which 
includes a Zener diode 67, a positive 15 volt input 68 connected to diode 
57 by a resistor 69, and a common series resistor 71 connecting the bias 
voltage source to the inverting input of operational amplifier 47. Thus an 
offset voltage is applied to the dc input voltage from shunt 41. 
Such bias level offset is illustrated in FIG. 6 as a +5 p.u. level. The 
actual input from the shunt 41 is shown by the waveform 72. If a reduction 
in line current such as indicated at 73 occurs, the overall signal would 
still remain positive; that is, above zero amplitude. In comparison, with 
a bias level offset as illustrated by signal 72', a reduction in line 
current 73' will produce a negative signal would could not effectively be 
transmitted by the optical system. Such offset signal which appears at the 
output 75 of operational amplifier 47 of the slow channel 42 drives the 
voltage to frequency converter 48. The amplitude modulated signal on fiber 
optic light guide 52 is produced by a light emitting diode 77 driven by a 
transistor switch 78 having its base input connected to converter 48. With 
respect to fast channel 43 the offset line current on line 75 is amplified 
in amplifier 78 which drives the base of the transistor 79 which has its 
collector connected to light emitting diode 81. Diode 81 is coupled to 
light guide 45 with the light guide also being coupled to a photodiode 
detector 82. 
The circuit consisting of LED 81, photodiode 82 and amplifiers 78 and 79 
with their associated capacitors, resistors and fiber optic couplers 
constitutes a distortion correction means for forcing the light output of 
the LED 81 to be a linear function of the input driving signal. This 
ensures an accurate reproduction of the line current signal at the ground 
end of the light pipe 45. The corrective action is based on the high 
linearity between light input and current output inherent in high quality 
photodiodes. The light output of the LED 81 energizes the correction 
photodiode 82 and also sends a portion of its light down to the ground 
unit by means of the split fiber optic. The correction circuit compares 
the electrical signal output of the photodiode against the electrical 
signal input, and through the negative feedback connection of amplifiers 
78 and 79, the LED distortion is eliminated. 
The line current data on light guides 45 and 52 is received by ground unit 
56 as illustrated in FIG. 5. In the slow channel 42 photo-detector 53 
receives the discrete light pulses and processes them in the cathode 
follower type circuit 84 which drives the frequency to voltage converter 
57 which is equivalent to but opposite in operation to voltage to 
frequency converter 48 in FIG. 4. Converter 57 has its output filtered by 
low pass filter 58 which is connected to a comparator 59. 
In fast channel 43 photodetector 46 senses the continuous analog type data 
signal which is processed by operational amplifier 62 to drive multiplier 
63. The output of multiplier 63 on line 65 is filtered by a low pass 
filter 64, as is illustrated in FIG. 3, and is the other output 61 of 
comparator 59 whose output drives the Xin input of multiplier 63. This 
provides in effect an automatic gain control action where temperature and 
aging effects are cancelled out. Thus the slow channel 42 effectively 
calibrates fast channel 43. In addition line 65 is also connected to a 
biasing circuit indicated by the dashed line 86 to remove the previously 
added offset voltage. It includes an operational amplifier 87 and a Zener 
diode biasing circuit 88 with the Zener diode 89 of course connected in 
reverse polarity compared to diode 67 of FIG. 4. Appropriate resistor and 
capacitor values are indicates. Thus the output of amplifier 87 is a gain 
stabilized replica of line current. 
In summary, an improved electronic current transducer for high voltage 
transmission lines has been provided. The low pass filters in the two 
channels reduce the effect of any fast changes in signal, so that only 
slowly varying signals will in general affect the operation of the 
calibration circuits. If the filter networks differ drastically in time 
constant (e.g., by 2 to 1) the high gain of the correction amplifiers will 
pick up the imbalance relatively quickly and will cause false correction 
signals to appear which produce errors in the output level during 
relatively fast transients. 
On the other hand, as provided by the present invention if the two filters 
have exactly equal time constants, the wave shapes at each filter output 
will be identical, and no transient imbalance will appear. If a steady 
state imbalance is present, however, the system will respond and will 
correct the gain slowly to compensate errors due to slow disturbing 
factors such as temperature sensitivity and aging.