Significance 

Part 4 – Propagation and coupling of surges 

This paper is only indirectly related to the topic of surge propagation in the context of surgeprotective devices, but is included here as it might be a gateway to further information on pulse 

propagation – the basic technique used in the investigation summarized by this paper 

Filename: PD in cables 
Conference Record of the 1990 IEEE International Symposium on Electrical Insulation, Toronto, Canada, June 3-6, 1990 

PARTIAL DISCHARGES IN LOW-VOLTAGE CABLES 

J.P. Steiner 

Biddle Instruments 

Blue Bell, PA 19422 

ABSTRACT-Testing of high voltage apparatus for partial discharges has long been recognized as an important part of quality 

control for these devices. Recently, interest has been focused on 

methods for testing low voltage cables to determine their integrity 

under adverse operating conditions such as a loss of coolant accident. A new method, utilizing partial discharges, is presented 

which has the potential for locating breaches in the insulat,ion of 

in situ, low voltage, multi-conductor cables. 

INTRODUCTION 

Cables used in power plants are selected on the basis of qualification tests [l] which provide a sufficient degree of confidence 

in the ability of the cables to maintain operational readiness for 

a well-defined "life" duration. As the cables age under the various environmental stresses prevailing in a power plant, nuclear 

plants in particular, the question arises of how much "residual 

life" exists in the cables as they approach their rated, qualified 

life. There is considerable interest in assessing this readiness, and 

a number of investigators have proposed various test methods [2]. 

In particular, a method that could allow nondestructive, in situ 

assessment would be extremely valuable. The detection of partial discharges at identifiable sites along a cable offers a method 

for assessing the likelihood of a fault to occur during a loss of 

coolant accident on a cable containing an incipient defect. Such 

an incipient defect might be a breach of the insulation that would 

remain undetected under normal operating conditions. 

Low-voltage cables, in contrast with cables designed for highvoltage service, arc not provided with an insulation structure 

aimed at making them free from partial discharges under modelate overvoltage conditions such as the application of a test voltage. Therefore, the occurrence of a partial discharge at an incipient defect site has to be differentiated from tlie background of the 

expected partial discharges that will occur over the length of the 

cable in the absence of any significant defcct. The test method 

described in this paper offers the opportunity to make this differentiation and locate the discharge sites associated with insulation 

breaches. Three representative types of cables in which artificial 

defects had been created were subjected to the test; every defect 

was successfully located. 

PRINCIPLE OF THE METHOD 

Partial discharges will occur along the entire length of a cable 

when voltages greater than the partial discharge inception voltage 

are applied. To develop a strategy for analyzing experimental 

data from a cable that also contains defects resulting in partial 

discharges at specific sites, several assumptions need to be made. 

The first is that there are no preferred partial discharge sites in 

the cable under test. Therefore, it would be expected that, ideally 

and in the absence of any defect, the partial discharges would be 

uniformly distributed along the length of the cable. 

F.D. Martzloff 

National Institute 

of Standards and Technology 

Gaithersburg, MD 20899 

The second assumption is that, in the absence of a defect, the 

partial discharge pulses occur as random events in time, i.e., the 

observed process is a marked random point process [3]. Using 

these assumptions causes the observed partial discharge process 

to resemble, asymptotically, a Poisson shot noise process [3,4], 

with pulses emanating from all positions along the cable. If a 

sample of data werc analyzed by calculating a histogram of the 

positions from which each pulse in the sample emanated, then, 

using this model, the histogram would correspond to a uniform 

distribution. 

The third assumption is that amplitudes of the pulses, expressed 

in units of charge at each partial discharge site, are gammadistributed [4] with the parameters of the probability density 

being independently and identically distributed. The observed 

probability density of the charge will then have the form 

where p(q) is the probability density of the observed charge in 

the entire cable, p(q/z, w) is a conditional probability density of 

the charge at a single site, p(z) is the probability density of the 

locations of the partial discharge sites along the cable and p(w) 

is the probability density of the parameter w, such as the degree of imperfection in the insulation (local field enhancements) 

that control the most probable discharge amplitude q at that 

site. By assu~nption, there is no preferred site and experimental evidence indicates that the physic~l behavior at each site is 

similar. This suggests that the superposition of these infinitesimal processes will yield a seemingly well behaved, predominantly 

unimodal probability dcnsity for the charge and this has been 

observed experimentally. 

Now consider the modcl for a defect. The goal of this measurement technique is to detect an insulation flaw which is defined, 

for these purposes, as a conlplete crack through the insulation to 

the conductor. Partial discharge in electrode arrangements that 

consist of a conductor-conductor interface or conductor-dielectric 

interface is often more active than when the electrodes are a 

dielectric-dielectric interface. The implication is that tlie partialdischarge sites at the flaws will be more active than the sites in 

the rest of the cable and consequently cause significant deviations 

from the ideal model. 

The basic idea of the nleasurement technique is to identify whether 

the data significantly deviate from t,he behavior defined by cquation (1). In this case, equation (1) is modifird by adding a term 

in which p(w) is replaced by p(wlz) 

where w depends on z and is highly localized to particular points 

z,. .4dmittcdly, thc model is a gross idcalization of the cxprcted 

U.S. Government work not protected by U.S. copyright 
Figure 1. Block diagram of partial discharge detection and 

recording. 

behavior, however, the general concept has merit if the defects do 

behave differently from the normal background partial discharge 

in the cable. In practice, there will be naturally occurring, preferred partial discharge sites that are not flaws but cause variations in the expected background count rates. In fact two such 

preferred locations have already been identified, the ends of the 

cable. 

EXPERIMENTAL APPARATUS 

The experimental apparatus implemented is a general purpose 

system designed for acquiring broadband partial discharge waveforms and processing the acquired signals as shown in the block 

diagram of Figure 1. The key elements are the high-voltage system, the coupling circuit and the data acquisition and analysis 

system. The high-voltage system used to apply the necessary 

voltages to the cables under test consists of a 60 Hz high-voltage 

transformer with appropriate noise filtering. Measurements on 

the cables are made at voltage levels exceeding the partial discharge inception voltage by 10, 20, and 40 percent. The inception 

level is defined as that voltage which causes at least one partial 

discharge pulse above a specified charge level q per second. 

The partial discharge signals are coupled from the cable under 

test to the measurement system through a coaxially mounted, 

1 nF capacitor. The capacitor is the first element in a high-pass 

filter having a lower cutoff frequency of 30 kHz and a 1000 0 input impedance. Initial testing with sensitivities ranging down to 

0.05pC did not produce useful results because the partial discharge magnitudes at inception are in excess of 10 pC. Since the 

predominant partial discharge levels are greater than 10pC, an 

attenuator and input protection are included in the circuit which 

limits the input sensitivity to 1 pC. The capacitance of the input 

protection limits the bandwidth of the system to approximately 

10 MHz. This input bandwidth is adequate for these experiments 

since the cables under test are lossy and limit the usable bandwidth to within this range. A variable attenuator is included 

between the input preamplifier and the gain block of the system 

to provide amplitude scaling which prevents saturation of the 

amplifier for larger partial discharge amplitudes. 

Partial discharge pulses are recorded using a LeCroy 9400A1 digital oscilloscope which has an 8-bit digitizer that can sample 32 000 

'The identification of commercial materials and their sources is made to 

describe the experiment adequately. In no case does this identification imply 

rcommendation by the National Institute of Standards and Technology, nor 

does it imply that the instrument is the best available. 

points at a 100 MHz rate. The advantage of using this oscilloscope 

is its ability to segment its 32 kB acquisition memory so that it 

can acquire up to 250 separate waveforms at its maximum sampling rate before having to transfer them to a computer over an 

IEEE 488 interface. The computer is a PC-AT class computer. 

The ASYST' scientific software package is used for acquisition 

control and data analysis. Up to 20000 waveforms can be collected before the available disk space limits the computer's ability to process the data; typically 5000 waveforms are collected. 

Active data collection occurrs only during a few seconds of the 

voltage application to the cable under test, however, voltage is 

typically applied for 8 minutes since the high voltage system is 

not under control of the computer and time is needed to archive 

the data. Since the system uses a computer with only moderate 

computational ability, all processing has to be performed off line. 

MEASUREMENTS 

Two fundamental estimates are made from each recorded waveform: an estimate of the charge, and an estimate of the location 

of the partial discharge site in the cable. It would be desirable to 

calculate estimates for all detectable pulses; however, since the 

measurement system is general purpose, trade-offs for cost, speed, 

and data handling capabilities have been made. With these constraints, only a limited number of waveforms can be collected and 

analyzed. 

The method used to calculate the charge in the partial discharge 

waveform is the same as that used in [4] and has been shown to be 

more accurate than the usual measurement of the peak value of 

the pulse. The technique uses the pulse energy derived from the 

waveform to develop an estimate of the charge and assumes that 

the measured partial discharge waveform is similar to calibration 

pulses injected into the cable. The calibration pulses are generated by applying a fast-rise step of known amplitude through a 

known capacitance into the cable under test. Further corrections 

in the estimates to account for the cable attenuation were not 

deemed necessary. 

If the flaw is detectable, then the histogram of the charge will 

no longer be unimodal. Using this fact, the measurement can be 

enhanced by setting a lower discrimination level so that the instrument measures only those partial discharges which appear to 

be anomalous. The lower discrimination level is also referred to as 

the trigger level, the level which the partial discharge pulse must 

exceed to initiate the recording cycle of the digital oscilloscope. 

The basic principle used to determine the location of the partial 

discharge can be understood by referring to Figure 2. When a 

partial discharge pulse originates at some point interior to a cable 

it splits evenly into two pulses which travel in opposite directions. 

One pulse, referred to as the direct pulse, proceeds toward the 

detector end arriving at time t,. The other pulse proceeds toward 

the open end of the cable and is reflected from the open circuit 

back toward the detector end, arriving at the detector at time 

t2. Knowing the velocity of propagation, v, for the cable, the 

location, e, is given by 

where the location is referenced from the open circuit end of the 

cable. 

To save time in processing, a simple but accurate method is used 

to calculate the locations. The technique applies an approximate 

matched filter to the recorded plses whose shape resembles a 
DISCHARGE 

DETECTOR SITE 

EN0 -A- OPEN 

END 

Figure 2. Position versus time diagram of direct pulse and reflected pulse propagation. 

partial discharge pulse. In effect, this assumed pulse is crosscorrelated with the pulses in the measured data. The time delay 

between the two largest pulses in the waveform is determined 

and converted into a location using a measured propagation velocity. More accurate delay estimators are not justified since the 

increased computational effort does not provide enough increase 

in accuracy. When the two largest pulses in the data record are 

from two distinct discharge sites, the estimate is referred to as a 

phantom delay. The probability of calculating a phantom delay 

is small at the voltage levels used, with less than 1 percent of the 

estimates being phantoms. These phantoms are not a problem 

since the time delay between the pulses is random and does not 

lead to a count rate that favors any particular site. A limitation 

of this technique is the existence of a measurement blind spot in 

the cable: when the arrival times of the direct pulse and reflected 

pulse are nearly the same, it becomes difficult to differentiate 

between a single pulse and two closely spaced pulses. Without 

special, computationally intensive processing, these sites all appear to be at a distance range near the end of the cable, in effect 

creating a blind spot. 

TEST RESULTS 

In this case, the measurement technique is intended for use in 

situ on 600V class nuclear power plant cables and so representative cable types were used in the experiments. These cables 

can be grouped into three categories: shielded cables, unshielded 

multiconductor cables, and unshielded single conductor cables. 

The following samples were used in a series of experiments: 

An unaged, unshielded, two conductor, #14 AWG, 600V 

class, neoprene jacketed cable of the same type used for 

control cabling in nuclear power plants. 

An unaged, 50ohm, coaxial cable of the same type used for 

instrumentation cabling in nuclear power plants. 

An aged, unshielded, 15 conductor, #18 AWG cable of the 

same type used for instrumentation cabling in nuclear power 

plants. 

No experiments were conducted on unshielded single conductor 

cables since the techniques to be discussed require that the cable 

have some transmission line properties; a single conductor, in the 

absence of ground plane, does not. 

Vertical: 

Number of pairs 250, 

counted per bin of 

the histogram 1 

150. 

Horizontal: 

Distance in feet 58.8 

from the open end 

Note the scattering of counts over the length of the cable 

Figure 3. Histogram of delay times between the two major 

pulses in each event of a 5000-element record containing true 

pairs and random pairs. 

Vertical: 

Positive PD 

Numberof 2r.o-C 1 \ 

pulses per 

bin of the 8.99 

histogram ' 79'.0 ' 21'9. ' 35'9. ' 490. ' 639.. * 

Horizontal: 

Pulse charge 

in pC 

Figure 4. Histogram of the distribution of partial discharge 

pulses (PD) as a function of their charge transfer magnitude 

( "pulse height"). 

Tests were performed on 100-m (330ft) length of unaged, twoconductor cable with artificial defects introduced at three locations. A small portion of the jacket was removed at three locations and the insulation was carefully pierced through to the conductor using a #60 twist drill in two of the locations; a small razor 

knife was used to slit the wire insulation in the third. In neither 

case was any of the insulation removed; only a break was created 

from the conductor surface to the surrounding atmosphere. The 

partial discharge inception voltage of the cable was 4500V and 

measurements were made at voltages levels 10, 20, and 40 percent above inception. With a small lower discrimination level, 

the low-level background partial discharges have dominant count 

rates indicating that the partial discharges are distributed along 

the length of the cable as shown in Figure 3. If the lower discrimination level is increased, then the count rates of the partial 

discharges from the defect sites dominate the background count 

rate as indicated by the charge histograms of figure 4; note that 

the histograms are not unimodal. The locations of the individual damage sites were found using those delay values which 

corresponded to data near the peaks of the charge histograms. 

Each of the damage sites was correctly identified as can be seen 

in figures 5, 6 and 7. Other measurements were made on a 

20m (66ft) length of coaxial cable with artificial damage inflicted 

by slightly abrading the shield. The partial discharge inception 

voltage was 5200V. In this case, the defect was immediately 
Vertical: 

Number of pairs 200. /I 

counted per bin of 

the histogram 

12, t II I 

Horizontal: 

Distance in feet 48.8 -- 

from the open end 

4d.e ' iia. ' 2d0. ' ?ah. t 

Note the emergence of a defect signal at 70.2 feet , location of an actual defect. 

Figure 5. Histogram of time delays for 5000 V and pruned data. 

Vertical: 

Number of pairs 

counted per bin of 

the histogram lZas 

Horizontal: 

Distance in feet 

from the open end 

Note the emergence of a defect signal at 34 feet (actual defect), 

with minor indications at 70 feet (already detected in Figure 5), 

and at 178 feet (no known defect at that point). 

Figure 6. Histogram of time delays at 5200V and pruned data. 

Note the emergence of a defect signal at 240 feet (actual defect), 

with substantial signal at 70 feet (already detected in Figure 6), 

small signal at 34 feet (as in Figure 5), and new signal at 280 feet. 

Vertical: 12s. 

Number of pairs 

counted per bin of 

the histogram 

Horizontal: 

Distance in feet .a 

from the open end 

Figure 7. Histogram of time delays at 6000 V and pruned data. 

C 

-- 

-- 

-- 

identified because the partial discharge activity at the damage 

site was much greater than the background partial discharges. 

Similar results were also obtained while measuring a 12-m (40 ft) 

length of aged multiconductor cable containing an artificial defect. The artificial defect was created by piercing the insulation 

on one of the 15 conductors with a small razor knife, similar to 

the slit described above. The ~artial discharge inception voltage 

was 2300V. Similar to the previous case, the partial discharge 

activity at the damage site was greater than the background of 

the rest of the cable allowing easy identification of the flaw. 

+ *',*,a ' ,*.e ' ,Sb. , Zlb. ' 27 #,3888- 

CONCLUSIONS 

The test method based on partial discharge detection using signal 

processing makes possible the identification and location of insulation breaches, undetectable under normal service conditions, 

that may become a fault under additional environmental stresses, 

or may progress into a fault under further aging. 

Tests performed on three representative types of low-voltage cables have demonstrated that the method can differentiate between signals emanating at incipient defect sites and the background of partial discharges occurring over the length of the cable. 

In order to induce partial discharges, the test voltage must clearly 

be raised above the inception voltage. Depending on the cable 

structure, this test voltage will be on the order of a few thousand 

volts, a subject of possible concern, but not a fatal limitation 

for a test method offering the reward of definite identification of 

defects that could become fault locations. 

The demonstration was obtained with a general-purpose PC-type 

computer, using a commercial software package augmented by 

moderately complex custom-designed software. In a more dedicated system, with further software enhancement, additional benefits are possible, such as the detection of incipient defects with 

only brief exposure of the insulation to the test voltage, automated scanning of the data for most significant content, and optimization of the test procedure into an expert system approach. 

ACKNOWLEDGEMENTS 

The authors acknowledge the contributions of S. Clevenger and 

D.R. Flach in the development and implementation of the menudriven software used in the application of the principle of the 

method. The signal processing concepts were developed by J.P. 

Steiner as described in Reference 4 and implemented prior to 

his new position with Biddle Instruments. The implementation 

and demonstration tests were performed by J.P. Steiner and F.D. 

Martzloff with funding provided by the USNRC Nuclear Plant 

Aging Research Program. 

REFERENCES 

[I] IEEE Std 383-1974 Standard for Type Test of Class 1E Electrical Cables, Field Splices, and Connection for Nuclear Power Generating Stations. 

(21 Proceedings: Workshop on Power Plant Cable Condition Monitoring 

EPRI EL/NP/CS-5914-5R, July 1988. 

[3] Snyder, D.L., Random Point Processes, John Wiley and Sons, 

New York NY, 1975. 

[4] Steiner, J.P. "Digital Measurement of Partial Dischargen, Ph.D. Thesis, 

Purdue University, W. Lafayette IN, May 1988. 

[5] Steiner, J.P., Weeks, W.L. and Furgason, E.S., UDigital Estimation of 

Partial Dischargen, 1987 Annual Report, Conference on Electrical Insulation and Dielectric Phenomena, Gaithersburg MD, October 1987.