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Some Problems with Partial Discharge Measurement in On-line Mode

K. Zalis, L. Beranova, L. Prskavec and R. TeminovaCzech Technical University in Prague, Faculty of Electrical Engineering, Department of ElectroenergeticsTechnicka 2, CZ- 16627 Prague 6, Czech RepublicPhone: +420-2-2435 2369, fax: +420-2-3333 7556

E-mail: {zalis, beranol, xprskave, teminor}@ fel.cvut.cz

Abstract This paper deals with the problems

discussing the transition from off-line diagnostic methods

to on-line ones. Based on the experience with commercial

partial discharge measuring equipment a new digital

system for the evaluation of partial discharge

measurement including software and hardware facilities

has been developed at the Czech Technical University in

Prague. Problems with the elimination of disturbances,recognition of patterns of partial discharges and

calibration of partial discharge equipment and measuring

circuit are also discussed.

Index Terms Dielectric diagnostics, measurement, on-

line measurement, partial discharges.

I.INTRODUCTION

Diagnostic methods are usually used for thedetermination of actual state of high voltage (HV)insulating systems, for the estimation of their residuallifetime, their behavior estimation and the risk

assessment in the future operation [1]. Diagnostics ofHV insulating systems in off-line mode, i.e. during the

putout period or overhauling of the machine, is workedup sufficiently and it is broadly executed. However,

both the price and power of the newly installed HVequipment in the power engineering branch grow up,and that is why the operators attention is focussed onthe operational reliability of their equipment at first. Thetendency of all operators is to monitor the state of theirequipment continuously, i.e. using on-line methods.

However, the application of some classical methodsfor on-line diagnostics is inappropriate, sometimes evenimpracticable (e.g. direct current methods, loss factormeasurement, overvoltage tests). On the other hand,suitable methods for on-line diagnostics are methods forthe observation of a discharge activity, which areusually based on the monitoring of secondary effectsaccompanying partial discharges (PD) in dielectricmaterials ([2], [3]). One of these PD methods,applicable on HV grounded objects, is the galvanicmethod with parallel connection of the HV couplingcapacitor and the measuring impedance with a lowpassfilter. This method is broadly expanded due to its high

sensitivity, a good resolution of individual types of PDimpulses, and due to the fact that its using is not limitedin the capacity of measured object or the quantity ofused testing voltage. The advantage of this method isalso in the possibility in using it directly during themachine operation by means of permanently installed

probes.In the transition process from off-line diagnostics toon-line one (monitoring) it is not possible to take overoriginal methodologies of the evaluation of diagnostic

parameters automatically. Some of diagnosticparameters of off-line diagnostics can not be measuredin on-line diagnostics, some lose their sense and, on theother hand, it is necessary to develop new on-linediagnostic methodologies regarding of new conditions.For example, in an off-line PD measurement, theevaluation methodology is based on the dependence of

basic diagnostic parameters (apparent charge, PDcurrent, PD frequency) on applied testing voltage. In on-

line measurement, the value of voltage is constant, butnew dependencies appear, e.g. changes of basicdiagnostic parameters in operational time. That is whyis necessary to develop new methodologies based on themonitoring of time shift of diagnostic parameters.Recently, some suspects about calibration processaccuracy in the case of measurement of large capacityobjects have appeared in technical community.Although these problems are not still satisfactorilysolved, in this case the transition process from the off-line diagnostics to the on-line one offer a quick and non-troubleshooting solution.

As regards the evaluation of a diagnostic

measurement, the quality of the evaluation and thereproducibility of results are stigmatized by relativelycomplicated methodologies and complicated (frequentlyartificially made) diagnostic parameters, what leads tothe necessity of the consultation of top human expertsfor the high-quality evaluation. However, thecomplexity of the decision-making mechanisms(frequently on the edge of the intuitive decision-making) leads very often to the ambiguous or oppositeevaluation of the actual state of the tested machine and

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The proper detection and digitization of the input data(measured values of diagnostic PD parameters) is

performed in the measuring unit, where the PD impulsesenter. These impulses are detected on the classicalmeasuring impedance. Like in case of the classical PD

measurement, the surface of the individual current PDimpulse is converted into a voltage value in a standardcapacitor, which is then discharged in a dischargingcircuit. In contrast to classical PD devices, thedischarging time is set by a built-in digital clock in thiscase, which is advantageous in exact countdown of thedischarging time and in the possibility of its furtherdigital processing. The discharging circuit wasdeveloped and set in such a way that the dischargingtime of the maximal charged standard capacitor (in caseof the maximal value of the current impulse in the inputamplifier), including resetting, should not take longertime than 50 s (it is adequate to 256 levels in a digital

form). It has a sufficient accuracy for reading theapparent charge value as well as it has a sufficientlyhigh speed for processing PD signals (to the limit 200signals during the one period of supply voltage, i.e.during 20 ms). A phase shift of the PD impulses isdistinguished with the accuracy 1.8 el., which issufficient enough.

Only two diagnostic parameters, an apparent chargeand phase shift of each impulse, are processed. Thesetwo data (information about every PD impulse) of10 periods of the supply voltage are saved in thememory data block of the measuring unit and, after therequest from the computer, are, with the help of a

standard serial RS232 line, transferred into thecomputer, where a special software processes themfurther. The central computer automatically controls thegain of the amplifier of the measuring unit, also viaRS232 line.

After the value digitization of diagnostic PDparameters, the crux of the further activity lies in thesoftware data processing by a special software [12].Before a proper evaluation of a PD activity, thestatistical processing is applied on measured data forremoving both random data and characteristicdisturbances (radio interference, thyristor disturbances,etc.). Cleaned up data are further processed andmodified for the input into the expert systems.

The evaluation of the diagnostic parameters andmonitoring of the insulation system in operation are

performed not only by standard classical methods(according to the criteria values and alarm systems), butalso by the expert systems with the elements of artificialintelligence, which enable to include the experience ofthe human experts in this branch as well.

The developed evaluating system also uses twoindependent expert systems for proceeding measured

PD data. These expert systems work simultaneously andspecial software controls their coordination. The rule-

based expert system performs the amplitude analysis ofPD impulses to specify the extent of the damage of theinsulating system. The neural expert system (a neural

network) has better ability of the abstraction, andtherefore it is used for the phase analysis of PDimpulses (the recognition of PD patterns), which enablenot only to specify the kind of PD activity, but even tolocalize PD resources.

The outputs and visualization are performed user-friendly; i.e. all results are displayed on a virtual panelof a standard measuring device. Except of the classicalvisualization of PD impulses within one period of thesupply voltage (the visualization of PD impulses on asine curve, an ellipse or on an abscissa), the results ofevaluations the by expert systems, the mode of datafiltering, and the results of statistical processing, the

actual levels of diagnostic parameters, levels and thealarm state activity are also continuously displayed onthe monitor screen.

III.ELIMINATION OF DISTURBANCES

In case of the PD measurement we usually want tofind the magnitude of partial discharges, especially wemade an integration of the current impulses. However,great problems in the evaluations of PD data are invarious sorts of interferences.

We can divide disturbing signals into internaldisturbing signals and external ones. The internal

interferences are dependent on the actual voltage of themeasured subject; external interferences are notdependent on it.

A lot of disturbing signals we can eliminate by usingof a suitable circuit modification. E.g. the four-capacity

bridge is usually used for small capacitance objects dueto its technical simplicity. We can also convert the PDsignal into digital (discrete) form.

The staff of the High Voltage Laboratory of theCzech Technical University in Prague, Faculty ofElectrical Engineering, is interested in problems withinterference for several years, particularly in the frameof the development of the new digital PD measuring

equipment. This equipment uses special software toeliminate the interferences. It is then possible torecognize and delete disturbing signals from processingdata sets.

We developed several algorithms for the eliminationof disturbing signals of digitized PD data [13]. Mainly,we can eliminate randomize signals and thyristordisturbances. The program applies math theorems to allmeasured samples; it is for 2000 data from ten sets(periods).

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For the detection of randomize interferences we usethe following theorem:

knqqqn

immmimm

=1)1/)/(( 20;3n ,

where qmi is a maximal value from i-set, qmm is a

maximal value from all sets, n is a number of sets, k is aconstant and k2;10.

For the detection of thyristor interferences, we usetwo theorems: At first, we find six highest chargemagnitudes in each set and we sort them (from m1 tom6). Each of these elements has the own chargemagnitude qmi and the own phase shift mi. Thyristorinterference impulses must perform two followingconditions :

Amplitude condition:

,( ) knqqqn

i imii

imi

= ==)6/)/(()6/)((

1

6

1

6

1

where n is a number of sets (n = 200), qmi is an i-element from the six highest values of the actual set, qiis an i-element of the set, k is a constant andk2;10.

Time condition:

k+ 6n

-- mi1mi ,

where n is a number of sets (n = 200), mi+1 is aposition of an (i+1) element, mi is a position of an i-element, k is a constant and k1;5.

A new way in the evaluation of PD signals is themathematical analysis, which makes possible a betterdata processing. According to the frequency spectrum

of the signal we can recognize steady signals andunsteady signals. In case of steady signals (stochasticimmediate broadband signals) we are interested in thefrequency spectrum only. It prefers Fourier Transform(FT), which transform a view of the signal from time-

based region to a frequency-based one. Mathematically:

,=

dtetxfX tj)()(

where tis time, fis frequency, = 2fand 1=j ,xrepresents signal in the time region and X representssignal in the frequency region. The signal does notchange in time.

In the case of the unsteady signals, the frequency isvariable. We must adapt FT to analyze signal in thesmall section of the time only. A this technique is calledthe windowing of the signal, Short-Time FourierTransform (STFT). Mathematically:

, =

dtetwtxfSTFT tj )]()([),(

where w is a window size function and is a timeshift. STFT represents a sort of compromise between

the time- and frequency-based views of a signal. Theprecision is determined by the size of the window.

The Wavelet Transform (WT) does not process ofsignal in the time-frequency region as STFT, but it

processes it in the time-scale region. One of the main

advantages of this processing is the fact, that it ispossible to perform a local analysis. That is, to analyze alocalized area of a larger signal. In comparison with theSTFT the window size could be changed duringanalysis. Mathematically:

,( ) =

dttstfsC ,,)(),(

where Care coefficients of CWT, sis scale, tis time,is time shift and represents a wavelet function.

The wavelet functions are unstable signals (wavelets)and the coefficients represent their modifications. Weuse several families of wavelets (Haar, Daubechies,Biorthogfonal, Symlets and the others). Their variabilityis very advantageous for the processing of unsteadysignal, e.g. for evaluation of PD signals. It could detectsignal changes and other variability. That is why theWT is the best for PD signal analysis.

IV.RECOGNITION OF PDPATTERNS

The neural network [15] based on the empty expertsystem named Neurex 5.1 was developed for therecognition PD-patterns. This neural network wastrained by means of the software generator named GCV(PD-generator). Two kinds of training set were created

by means of the GCV. The first one was the training setwith fixed values and the second one was with intervalvalues of input parameters. Both training sets have eightfinal elements in the output layer, according to the basicPD-patterns. We developed three versions of neuralnetwork and six neural networks for training (threeversions with two kinds of training sets).

Testing set with sixteen elements was also created bythe GCV. None element from the training set wasaccepted in the testing set.

After training procedure, all developed neuralnetworks were tested. Four training sets was acceptable,two training sets (A-A and B-A) were unacceptable for

the neural network in the consultation mode. VariantsA, B and C are different by the count of neurons ininner layers. Variant marked A-B has 25 neurons ininner layer and it has the training set marked B (fixedvalues). Training set A is with interval values. Variant Bhas 100-25 and C has 100 neurons in the inner layer. Allour developed neural networks have three layers only.

After testing procedure we selected variant C-B forthe using in the neuron expert system, because it has the

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best testing results. Training set A has not so acceptablefor this purpose.

Measuring data was then put into graph (see Fig. 1 and2), where we can compare characteristics of testedcalibrators.

V.PDCALIBRATIONCalibration levels of 5000 and 10000 pC

0

2000

4000

6000

8000

10000

12000

010

20

50

100

200

500

10

00

20

00

50

00

100

00

200

00

Loading capacitance [pF]

Calibration

charge[pC] G6-6

G6-7

G3

TETTEX

PD measurement is the comparative measurementand that is why that result of whole measurementdepends considerably on the calibration of measuringdevices and measuring circuits. Nevertheless at themoment calibration is unjustly underrated in practice.

The newly released IEC 60270 introduces many newrequirements for PD measuring systems and relatedcalibrators. Calibrator consists of generator producing

pulses of voltage of the amplitude U0 connected inseries with capacitor C0. Charge of calibration pulses q0then equals q0=U0C0.

Accuracy of measurement depends on the accuracy ofcalibrators and we have to carry out operational test.

These tests have to be carried out because ofdetermination and keeping of characteristics ofcalibrators.

Fig. 2. Calibration levels of 5000 and 10000 pC.

Alternative method: The output of calibrator is loadedby the resistance Rm. The output voltage um(t) measuredon Rm is monitored by the digital scope. This scope musthave the bandwidth greater or equal than 50 MHz. Thevalue of Rm should be chosen between 50 and 200 .

Operational test from the newly released IEC 60270consists of: Charge of calibrator q0

for all setting of calibrator,with an uncertainty within 5% (or 1 pC whicheverthe greater).

Charge q generated by the calibrator then is:

( ) ( ) dttuR

dttiq mm

==1

, Rise time trof voltage U0, with an uncertainty within10%. Rise time must be less than 60 ns (tr< 60 ns).

where i(t) is current impulse generated by thecalibrator and um(t)is voltage impulse measured by thescope.

Following calibrators were tested: Calibrator TETTEX, type 9216, range 10 pC

10 000 pC.In the High Voltage Laboratory of CTU FEE in

Prague we measured the characteristics of commercialcalibrators by using the alternative method [15]. Thedigital scope LeCroy 9350 with bandwidth 500 MHzwas used, which satisfied SN EN 60270. The values ofRmused for loading of the calibrators output were 5000,1000, 500, 200, 100, 50, 20, 10 and 5 .

Calibrator developed in the Research Laboratory ofthe CTU, Faculty of Electrical Engineering inPodebrady, type G6-6 (1993), range 5 pC 10 000 pC.

Calibrator developed in the Research Laboratory ofthe CTU, Faculty of Electrical Engineering inPodebrady, type G6-8 (1998), range 10 pC 25 000

pC, impulse frequency 50 Hz, 100 Hz, 1 Hz or5 kHz switchable.

ACKNOWLEDGMENT

Calibration levels of 500 and 1000 pC

0

200

400

600

800

1000

1200

010

20

50

100

200

500

1

000

2

000

5

000

10

000

20

000

Loading capacitance [pF]

Calibration

charge

[pC]

G6-6

G6-7

G3

TETTEX

BIDDLE

This research is financially supported by theDepartment of Electroenergetics of the CTU and by theGrant Agency of the Czech Republic (grant No.102/02/0105).

REFERENCES

[1] A. andrik and J. Zlatovsk, Diagnostics andconstruction of the insulating system of electricalmachines, Proceedings of the 12th InternationalConference Dielectric and Insulating Systems inElectrical Engineering DISEE 98, 1998, Bratislava andast-Pla (Slovakia), pp. 62-65.

Fig. 1. Calibration levels of 500 and 1000 pC.

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[2] D. Knig and Y.N. Rao Y.N.: Partial discharges inelectrical power apparatus, VDE-Verlag GmbH, Berlinand Offenbach, 1993.

[3] K. Marton, Imperfect point in insulating system as aradiator of electromagnetic signal, Proceedings of the12th International Conference Dielectric and InsulatingSystems in Electrical Engineering DISEE 98, Bratislavaand ast-Pla (Slovakia), 1998, pp. 30-33.

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[5] R. Cimbala, Utilization of pattern recognitionprinciples for diagnostic purposes, Journal of ElectricalEngineering (Slovakia), Vol. 45, No. 8, 1994, pp. 315-317.

[6] E. Gulski and F.H. Kreuger, Computer-aidedrecognition of discharge sources, IEEE Trans. ofElectrical Insulation, Vol. 27, 1992, No 1, pp. 82-92.

[7] F.H. Kreuger, E. Gulski and A. Krivda, Classificationof partial discharges, IEEE Trans. on ElectricalInsulation, Vol. 28, 1993, pp. 917-931.

[8] I. Kolcunov, Diagnostics of high voltage and extrahigh voltage systems by partial discharge method, FEITU Koice, 1999.

[9] I. Kolcunov and I. Krk, Types of partial dischargesand their recognition in insulation of high voltagerotating machines, Proceedings of the ScientificConference, Oradea (Romania), 1997, pp. 67-70.

[10] J. ha and K. Zli, Charge calibrator G 6-8, CTUFEE, Prague, 1999.

[11] Patent No. 284558 Final stage of Partial dischargecalibrator. 1997, CZ.

[12] J. Karas, Evaluation System for Partial DischargeMeasurement, Proceedings of the 3rd InternationalStudent Conference on Electrical Engineering POSTER99, Prague, 1999, ref. EE1.

[13] K. Zli and L. Beranov, Sort of disturbancesinfluencing partial discharge measurement and theirelimination, Proceedings of the DIAGNOSTIKA 01Conference, 2001, Netiny (Czech Republic), pp. 57-60.

[14] K. Zli and L. Prskavec, Mathematical models ofpartial discharge patterns, Proceedings of theDIAGNOSTIKA 01 Conference, 2001, Netiny (CzechRepublic), pp. 61-64.

[15] K. Zli and R. Tmnov, Calibrators for partialdischarge measurement, Proceedings of the DIAG-NOSTIKA 01 Conference, 2001, Netiny (CzechRepublic), pp. 65-67.