2011 International Conference on Electrical Engineering and Informatics 17-19 July 2011, Bandung, Indonesia

Evaluation of 20 kV Cable Diagnostic Method in Field Condition

N. Fauziah1, B.S. Munir2 and E. Sinambela3 PT PLN (Persero), Jakarta, Indonesia

1nurul.fauziah@pln.co.id 2buyungsm@pln.co.id

3elpis.sinambela@pln.co.id

Abstract— The increased use of cable systems in power distribution in urban areas lead to the need for more reliable cable system. To ensure the reliability of the cable system, a diagnostic system that can assess the cable system in field conditions is needed. Partial Discharge (PD) and Tan Delta (TD) are the two parameters that can be measured to diagnose the condition of cable system in the field. Based on the measurement of these parameters, the condition of cable system can be determined whether the cable is in good condition, needs attention or in bad condition. In this paper the methods of PD and TD field measurement on 20 kV cable systems are described. Furthermore, the method of analysis using data obtained from the measurement of PD and TD is also described. PD measurement in this study is performed using damped AC voltage source (DAC), while TD measurement is carried out by using Very Low Frequency voltage source (VLF). In the end an assessment scheme is proposed for cable diagnostic that can be served as a guideline for Condition Based Maintenance (CBM) of cable system. This scheme will increase not only the reliability of cable system but also the performance of power distribution network. Keywords— Partial Discharge, Tan Delta, Cable Diagnostic.

I. INTRODUCTION Several diagnostic tools have been developed to assess the

underground cable periodically. This assessment is required to have a high reliability of cable system. Two of these diagnoses tools are Partial Discharge (PD) and Tan Delta (TD) measurement. PD Diagnostic is non-destructive method to perform predictive maintenance since PD is a sensitive symptom of weak spots in high voltage cable insulation which can cause failure in the cable system. However, the PD diagnostic requires an expert and a long time to analyse the results. Meanwhile, TD test is a diagnostic test that indicates the level of degradation of insulation of a cable system. The test is performed to predict the remaining life expectancy of the cable and it also useful to determine whether another test might be useful. Although analysing the results of TD measurements require only a short time but this measurement method cannot provide information about the overall condition of the cable system.

This paper discusses the diagnostic method of 20 kV cables using the PD and TD measurement that can be used in the field and the analysis method using the data resulted from

those measurements. Ultimately a scheme is proposed to assess the cable system quickly, sustainable and comprehensive. It is expected that this scheme can be used as guidelines in conducting diagnostic of new and old cable in the field to overcome the shortcomings of each measurement method by combining the advantage of both methods.

II. PD MEASUREMENT METHOD PD in a power cable mostly occurs in defect of the

insulation cable or in the accessories of the cable system. PD is considered as one of indication of possible discharging weak spots in cable insulation that may eventually lead to failure in the cable system. The detection, location and recognition of PD at an early stage of possible insulation failure are great importance for maintenance purposes [1]. Detection in Power Cable systems can be performed off-line or on-line. However in this study, the PD measurement is done off-line and in the field using a Damped AC (DAC) voltage source which is applied to the cable diagnostic technology, Oscillating Wave Test System (OWTS). This system is used to energize, to measure and to localize the position of PD source in the cable. In this method, DAC voltage is used to energize a cable system in frequency range 50 Hz-1.5 kHz. DAC voltage is generated using RLC loop circuit which is created after the cable is charged with DC power supply and disconnected very quickly. The schematic diagram of this system can be seen in Fig. 1.

The goal of PD diagnostic in the power cable system is to determine the condition of the cable system based on parameters that is obtained from the measurement. Several parameters (PD properties) which are obtained from PD diagnostic can be seen in Table 1.

The first stage of PD measurements using DAC voltage is the calibration. This is a single method to verify the measurement system is functioning correctly and to check the sensitivity of the measurement system to detect PD. The second stage is PD measurement which its complete procedure can be seen in Fig. 2. All relevant information obtained from a PD measurement on cable system should be collected and this data collection is called fingerprint of a power cable.

E9 - 3

978-1-4577-0752-0/11/$26.00 ©2011 IEEE

HV

DC

Su

pply

Sw

itch

Mea

sure

men

t D

evic

e

Fig. 1 Schematic diagram of PD measurement system using DAC voltage source

TABLE I

PD PROPERTIES OBTAINED FROM DAC MEASUREMENT

PD Properties Description PD inception Voltage (PDIV)

The applied voltage at which repetitive PD are first observed in the test object [2]

PD Extinction Voltage (PDEV)

The voltage at which the internal PDs stop occurring [3]

PD Magnitude at ~V

The specific magnitude of the apparent charge q [2]

PD Pattern Representation of the appearing PD as a function of the phase angle Фi of the applied voltage test [3]

PD Mapping The representation of PD location along the length of the cable using time domain reflectometry (TDR)

PD Occurrence Frequency

Number of PD events in the specific location in the mapping at one DAC voltage

III. TD MEASUREMENT METHOD TD, also called the shift in angle or dissipation factor, is the

parameters of dielectric materials that show the electromagnetic energy dissipation properties of dielectric materials. This term refers to the angle on the complex plane between the resistive component of the electromagnetic field and its reactive components. In addition, TD measurement on a cable system is a method to test the quality of the cable insulation. This method is done to estimate the remaining life of cables and to set priorities of cable replacement.

Fig. 2 PD measurement of cable system procedure

Test Cable

t

U

Computer

Fig. 3 Schematic diagram of TD measurement system using VLF voltage source

In this study the TD measurement is conducted using Very

Low Frequency (VLF) voltage source. VLF is an off-line cable diagnostic method that generates AC voltages and currents at a frequency of 0.01 Hz - 0.1 Hz. VLF equipment is usually used for fail test of cable system. However in this study, it is used to perform off-line TD measurement of cable system since AC voltage testing is the best way to ensure the integrity of the cable. The schematic diagram of TD measurement system can be seen in Fig. 3.

The TD measurement is performed by applying VLF voltage from nominal voltage until 2 times nominal voltages. The complete TD measurement flowchart procedure can be seen in Fig. 4.

IV. ANALYSIS METHOD The analysis can be done by using the fingerprint of the

cable system acquired from the PD measurement. Later the decision regarding the cable system is made based on the result of this analysis which is divided the cable system condition into good, needed attention and bad condition. One of the properties of this fingerprint is PD magnitude which is for a new PILC cable, PD level up to 500 pC is accepted and for new XLPE cable, PD level < 50 pC is accepted. For the aged PILC cable, PD level up to 2000 pC is accepted and for aged XLPE cable, PD level < 50 pC is accepted [4]. If PD magnitude is high while both PDIV and PDEV are above nominal voltage, then the cable is considered in needed attention whereas if one of PDIV or PDEV is under nominal voltage then PD has to be located. If PD is located spread along the cable length then cable is in bad condition whereas if PD is only occurred in the accessories then the decision maker has to decide whether to replace the cable system or to make a schedule for another assessment in the future. Analysis process flow diagram can be seen in Fig. 5.

In addition to the PD measurements, the analysis could also carry out using TD measurement. If the cable insulation is in good condition, TD will change slightly when the applied voltage increases. Capacitance and resistive loss will be the same if the cable is given a nominal voltage or two times nominal voltage. If the cable has been contaminated, the capacitive/resistive isolation will be changed and the TD value will be higher at higher voltage as being shown by the work of [5].

By comparing the results of some measurement, the cable condition can be categorized as good, needs attention or bad condition. TD analysis flow chart is very simple and can be seen in Fig. 6.

V. PROPOSED CABLE ASSESSMENT SCHEME Considering cable breakdown is permanent so it is expected

that such unplanned interruption can be avoided or predicted earlier. Therefore a new scheme for assessing a cable system is proposed to identify the cable failure earlier and repair the cable before the breakdown actually happens.

Fig. 4 TD measurement of cable system procedure

Fig. 5 PD analysis of cable system flow chart

TD measurement is proposed to be performed routinely. If cable is found in bad conditions, then the applied voltage is increased until the breakdown of cable insulation is occurred. This is performed to avoid the cable breakdown at an unexpected time. After the insulation failure is localized and repaired then TD measurement was repeated.

If a bad cable condition is founded after the repair or not long after the repair performed, PD measurement is conducted to determine the condition of cable as a whole. The result of this PD measurement is used as a basis for replacing the cable system or scheduling another assessment. Flow chart of the proposed scheme can be seen in Fig. 7.

TD MEASUREMENT

IF CABLE IN BAD

CONDITION

TEST CABLE UNTIL IT FAILED

DETECT THE FAILURE AND FIX

IT

TD MEASUREMENT

PD MEASUREMENT

SUGGEST THE CHANGE OF THE

CABLE

SCHEDULE THE NEXT TD

MEASUREMENT

ANALYSIS THE CABLE AND

SCHEDULE THE NEXT TD

MEASUREMENT

CABLE ASSESMENT

END OF ASSESSMENT

A

A

Y

Y

Y

N

N

N

IF CABLE IN BAD CONDITION

SCHEDULE THE NEXT TD

MEASUREMENT

IF CABLE IN BAD

CONDITION

Fig. 7 Proposed flow chart of cable systems assessment

VI. CONCLUSIONS In this study, PD and TD measurement of cable system

method is reviewed. Fingerprint or data collection of cable system produced by PD measurement such as the level of PD, PDIV, PDEV and the processed data either frequency PD or PD mapping can be used to determine the insulation condition of a cable system. So the cable can be categorized as good cable, needed attention or bad. However PD data analysis is very difficult for a field operator to perform so that an expert is required. Moreover it takes a long time to analyse the data from PD measurement.

TD measurement of cable system can determine the quality of insulation of a cable system since the cable that has been contaminated will change the nature of capacitive or resistive cable insulation. On a good cable TD value will not change for each voltage is applied. But the value of TD cable will be higher at higher voltage if cable has been contaminated. By comparing the results of several TD measurements, the cable can be categorized as good, needs attention or bad condition. However TD measurement cannot determine the location of cable damage and give an assessment of the quality of the insulation at each point throughout the cable length.

Therefore a cable assessment scheme is proposed by combining advantages of PD and TD measurement. Whereas TD measurement is conducted regularly depending on the

Fig. 6 TD analysis of cable system flow chart

condition of the cable system and PD measurement is conducted when the cable system is in bad condition repeatedly after TD measurement procedure.

REFERENCES [1] P. van der Wielen, “On-line detection and location of partial discharges

in medium-voltage power cable,” Ph.D. dissertation, TU Eindhoven, 2005.

[2] B. Quak, “Information strategy for decision support in maintaining high voltage infrastructures,” Ph.D. dissertation, TU Delft, 2007.

[3] F. Wester, “Condition assessment of power cables using partial discharge diagnosis at damped ac voltages,” Ph.D. dissertation, TU Delft, 2004.

[4] S. Brettschneider, E. Lemke, J. Hinkle, and M. Schneider, “Recent field experience in pd assessment of power cables using oscillating voltage waveforms,” in Electrical Insulation, 2002. Conference Record of the 2002 IEEE International Symposium on, apr 2002, pp. 546 – 552.

[5] M. Kuschel, R. Plath, I. Stepputat, and W. Kalkner, “Diagnostic techniques for service-aged xlpe-insulated medium voltage cables,” in 4th International Conference on Insulated Power Cables (JICABLE’95), 1995, pp. 504–508.