On-site PD Diagnosis of Transmission Power Cables

Edward Gulski1, Johan J. Smit1, Sander Meijer1 Paul P. Seitz2, E.R.S. Groot3

1 Delft University of Technology, Mekelweg 4, 2628 CD Delft, The Netherlands 2 Seitz Instruments AG, Mellingerstrasse 12, 5443 Niederrohrdorf, Switzerland

3 Nuon Tecno, Voltastraat 2, 1800 AJ Alkmaar, The Netherlands

Abstract- Condition assessment of HV assets is one of the issues of asset management in power utility business. In particular, due to there importance in the transmission network is the knowledge about the starting conditions during after-laying as well as the actual condition of HV power cable sections during operation after several years of service very important. With regard to partial discharge (PD) processes in transmission power cables there is a need for advanced, sensitive and economical attractive tools suitable for non-destructive PD diagnosis on-site: the after-laying testing as well as the service diagnosis. In an international co-operation, based on utility experiences and laboratory investigations as obtained for PD diagnosis of distribution power cables using damped AC voltages, a complete new method of PD detection and localization for transmission power cables up to 250kV has been developed in 2003 and in field use for already three years. In this contribution based on field experiences on power cable system up to 220kV the test procedure and the data interpretation are discussed.

I. INTRODUCTION From the utility point of view the reliability and availability of connections of the transmission network are very important. In particular, the classification of cable condition to support the Asset Management decisions abut maintenance policy is the major goal of on-site testing of service aged HV cables, figure 1. It is known, that the insulation failures in a cable network may be caused by lower dielectric strength due to aging processes and by internal defects in the insulation system. - To reduce the failure by internal defects, on-site cable

diagnostics can be applied based on quantities related to insulation degradation, as partial discharges.

- To reduce the failures by the aging of the impregnated insulation different types of diagnostic methods are in use to assets the bulk properties of insulation e.g. by the measurement of dielectric losses.

As a result, the interpretation can be done based on criteria for each diagnostic. All results together are used for a classification of the cable into four possible categories, see table 1. It follows from this table that based on information which is provided during periodic or condition based inspection the actual condition of a cable section can be used to plan the necessary maintenance activities and to determine the reliability of this particular section in the total network configuration.

II. POWER CABLES DIAGNOSTIC TOOLS

DIFFERENT HV POWER CABEL TYPES AND INTRESSTING INFORMATION ABOUT THEIR INSULATION CONDITION.

Unlike voltage testing, measurements of the dielectric properties are a valuable indicator for the quality level of the cable insulation. As a result, nowadays with regard to HV power cables no standardized procedures are developed for condition assessment of service aged systems. On the one hand this is due to the fact that no one degradation mechanism is responsible for a possible insulation system failure. On the other hand, with regard to on-site testing there is no standardized equipment or tool available to energize and to diagnose the system on-site in a non-destructive and sensitive way.

TABLE I

In table 1 an example is shown of different HV power cables up to 150kV level which may occur in a utility network and the information which would be of interest for insulation condition assessment. In particular, collection these data of service aged power cable section will have a direct relation to the average qualitative level of the insulation at the moment of measurement and can thus be applied as a trend- or fingerprint measurement.

Cable type Insulation status 50kV XLPE PD activity in cable accessories:

terminations, joints 50kV Oil filled General status of impregnation and insulation

degradation by e.g. dielectric losses, PD’s 50kV Mass insulated PD level in insulation and accessories

indication of degradation 150kV XLPE PD activity in cable accessories:

terminations, joints 150kV Oil filled General status of impregnation and insulation

degradation by e.g. dielectric losses, PD’s 150kV Gas pressured General status of impregnation and insulation

degradation by e.g. dielectric losses, PD’s

III. PD DIAGNOSTICS

In the last decade more than 10 different methods have been introduced to evaluate the insulation condition of HV power cables [2]. As a result, with regard to long and short term condition assessment two of them have got high importance:

Conference Record of the 2006 IEEE International Symposium on Electrical Insulation

1-4244-0333-2/06/$20.00 ©2006 IEEE. 93

TABLE 2 PD QUANTIEIS AND THEIR DIAGNOSTICS INPORTANCE

Partial discharges are an indication for weak spots in a cable connection. In order to run the measurement partial discharges are ignited in the cable insulation or joints by the application of a test voltage [4]. Due to the physical character of discharge occurrence, the interpretation of the results is crucial for obtaining right picture about the discharging defect in the cable insulation, see table 2. It follows from this table that to obtain a complete picture of PD activity in a power cable several PD quantities have to be taken in to account and evaluate on their behavior. Based on such information knowledge rules can be developed to support condition status and maintenance decision processes. Moreover, for utilities interested in applying PD diagnostics for condition assessment of their power cable networks, a number of technical and economical aspects play an important role the diagnostic choices: Voltage type: equivalence in PD inception processes among different voltage stresses for solid insulating materials; Non-destructiveness: non-destructiveness of voltage stress during the diagnosis; IEC 60270 conformity: in the case of measuring the PD quantity apparent charge of PD pulses in [pC] and [nC] the PD detection methods applied has to fulfill the recommendation of IEC 60270; Sensitivity: immunity for on-site interferences and the level of system background noise; Analysis: possibility to generate advanced diagnostic information to support diagnostic knowledge rules; Efficiency: investment costs, maintenance costs, transportability and operation of the method in different field circumstances.

IV. ON-SITE TESTING

For the on-site detection of PD related defects in power cables, it is necessary to energize the disconnected cable sample for the ignition of the PD sources. The detection equipment is therefore directly connected to the cable conductors (or through the switchgear). In this way, the different phases of the cable circuit can be energized and the PD pulses can be detected and analyzed.

PD Quantity Diagnostic Aspects PD inception voltage PDIV inversely related to PD degradation

processes PD pulse magnitudes versus voltage applied e.g. at U0, 1.7U0 and 2U0,

recognition of defect severity; e.g. size of the internal defect

PD patterns recognition of defect type; e.g. cavity(s), surface discharges, treeing

PD mappings localization of PD activity; e.g. accessories, cable insulation

PD intensity directly related to defect severity and degradation processes

The capacitive power P needed to stress on-site the cable insulation is determined by 2Π•f•Ccable•U2

test where f is the test voltage frequency, Ccable is the cable capacitance and Utest, is the test voltage level. In order to decrease the capacitive power demands for energizing cables as compared to 50 Hz test voltages, different energizing methods using specific voltage shaped and frequencies have been introduced for PD diagnostics nowadays [3]. Till now, due to external disturbances, are the methods as mentioned in [3] difficult applicable in the field Moreover, on-site resonant test sets use tuned frequency devices which produce very high level of noises. Therefore, using a conventional PD measuring circuit according to IEC 60270, dedicated noise suppression techniques have to be used to obtain sensitive PD detection. Also the use of on-line PD analyzing techniques (used for after laying tests of XLPE cable accessories) has his limitations. In particular, this technique is limited to detect in cable accessories the PD activity at U0 voltage level only. For this purpose the cables accessories have to be equipped with dedicated HF sensors equipped. As a result no information can be obtained about the PD behavior at different voltages and the no insight is given into the cable insulation degradation. Considering the above mentioned situation in the field of on-site PD detection for HV power cables it can be concluded that there is still a structural need to explore the possibility of on-site PD diagnosis for HV cables.

Dam

ped

AC

volta

ge

Partial discharges: PDIV, PD level, PD location

Dielectric losses

Dam

ped

AC

volta

ge

Partial discharges: PDIV, PD level, PD location

Dielectric losses

Fig. 2 DAC voltage and diagnostic parameters which can be obtained during a measurement.

Fig. 1 PD testing of a 150kV XLPE power cable. In foreground the

complete installation of OWTS 250 kV (in blue).

Therefore on the basis of 4 years of systematic field experiences with PD diagnosis of distribution power cables [1, 4, 5] as well as on the base of 2 years of fundamental research the authors discuss in this contribution a new solution for PD diagnosis of transmission power cables. This solution covers the on-site application of damped AC (DAC) voltages in the

94

range of 50Hz up to 500Hz to energize HV cables and to detect and to locate discharging sites. PD vs Ut

0

2000

4000

6000

8000

10000

12000

14000

0.5 0.7 1 1.3 1.5 1.7

Meetspanning Uo

On

tlad

ing

sniv

eau

(p

C)

Fase L1

Fase L2

Fase L3

PD vs Ut

0

2000

4000

6000

8000

10000

12000

14000

0.5 0.7 1 1.3 1.5 1.7

Meetspanning Uo

On

tlad

ing

sniv

eau

(p

C)

Fase L1

Fase L2

Fase L3

The off-line PD diagnostic tool as used for the investigations in this paper is based on external energizing of a cable circuit by damped AC voltages with a voltage source up to 250kV. Originally, the DAC voltages are introduced as a cost-effective withstand-voltage test for XLPE-insulated HV cables

[6, 7]. Nowadays, DAC voltages are more and more used for non-destructive PD diagnosis of distribution power cables [2, 4, 5, 8, 9]. One of the methods using DAC voltages for detection and localization of PD in cables is known as Oscillating Wave Test System (OWTS), see figure 2 [4,5]. For the generation of damped AC voltages, the power demand is low due to the charging the cable capacitance with an continuously increasing HV stress, after which the cable capacitance is switched in series with large inductance, resulting in an oscillating voltage wave with a frequency comparable to power frequencies. Due to the fact that for the above described technique:

Dielectric losses during DAC voltages

PD during DAC voltages

Time →

Voltage ↑

Dielectric losses during DAC voltages

PD during DAC voltages

Time →

Voltage ↑

Fig. 3 Voltage withstand diagnosis (voltage increase between 1xU0 and 2xU0) of a 644m long XLPE power cable using DAC voltages. This

example shows that the strong increase of PD activity in a termination is also reflected in the increase of the dielectric losses.

Fig. 4 PD evaluation of 50kV, 6km long power cable. From the PD analysis phase blue shows high PD activity as compared to other phases and the reference norms. Using PD mapping analysis of the phase blue

the PD source has been located in the near cable termination.

- the frequency of damped AC voltages is in the range of power frequency of acceptable HV test systems,

- a number of power cycles is applied to the cable sample provide ignition of PD sources in similar ways as compared to operating conditions,

the PD activity can be on-site measured with multiple undisturbed sinusoidal voltage cycles. As the DAC frequency represents the AC power line frequency, the measurement bandwidth of the PD detection circuit is chosen in accordance with IEC 60270 recommendations. The PD activity signals, ignited during one or more oscillating voltage waves, are detected by the system, which can process the signals for two purposes: 1) A phase-resolved PD pattern can be resolved from multiple DAC sequences. In this way, patterns can be obtained which

are similar to those recognized under 50(60) Hz conditions (figures 2, 5). 2) Single PD pulses can be analyzed for original location by using traveling wave analysis. Statistical evaluation of PD signals obtained after several oscillating waves can be used to evaluate the location of discharge sites in the power cable. A PD mapping is created, which shows the distribution of the detected PD in a cable circuit, as a function of the magnitude or the intensity, see figure 4. As shown in figure 3 the OWTS HV 250 system can provide information about dielectric losses in the test object. Using DAC voltages the dielectric losses can be derived from the decay characteristics of the oscillating voltage wave. In this way the dielectric losses as measured at different voltage levels can be used to evaluate the insulation condition of power cables. Figure 5 shows an example diagnosis of a service aged (mass-insulated) 50kV power cable. It follows from this figure that phases L2 and L3 show higher PD activity as phase L1 but due to the fact that the reference norm of 5.000pC has not been crossed and there is no significant concentration on single site in the cable insulation (see figure 6) the condition of this cables section is judged as good.

95

Fig. 5 PD evaluation of (mass-insulated) 50kV, 6.2 km long power cable. From the PD analysis phases L2 and L3 show high PD

activity as compared to phase L1.

V. CONLUSIONS

In this paper new method OWTS HV 250 for advanced and non-destructive condition assessment of HV power cables has been shown. Based on 2 years of systematic field experiments on different types of power cables up to 220kV the following conclusions can be made:

1. This method is applicable for PD measurements as part of

an after-laying test or as inspection during service life. 2. New technology to generate on-site test voltages is

suitable to detect, locate partial discharges and to determine dielectric losses is described.

3. The use of damped AC voltages for HV power cables is demonstrated.

4. To obtain complete picture of the PD processes the diagnosis has to be done at voltage up to 2U0 is important.

5. TDR analysis of single PD pulses provides information about the origin of PD site in the power cable.

6. Based on field experiences knowledge rules can be determined to support the maintenance decision process.

Since one year this method is successfully used in the field and therefore in the next time intensive and systematic field measurements on power cables in European transmission grid will be performed to develop knowledge rules for condition assessment of HV power cable systems.

REFERENCES

[1] E. Gulski, F.J. Wester, W. Boone, N. van Schaik, E.F. Steennis, E.R.S. Groot, J. Pellis, B.J. Grotenhuis, ‘’Knowledge Rules Support for CBM of Power Cable Circuits’’ Cigre Paris 2002, SC 15 paper 104

[2] E. Gulski, F.J. Wester, Ph. Wester, E.R.S. Groot, J. W. van Doeland, ‘’Condition Assessment of High Voltage Power Cables. ’ Cigre Paris 2004, SC D1 paper D1.306

[3] W. Hauschild, W. Schufft, R. Plath, K. Polster, "The Technique of AC On-Site Testing of HV Cables by Frequency-Tuned Resonant Test Systems", CIGRE 2002, paper 33-304.

[4] E. Gulski, F.J. Wester, J.J. Smit, P.N. Seitz and M. Turner “Advanced PD diagnostic of MV power cable system using oscillating wave test system”, IEEE Electrical Insulation Magazine, 16, 2, 2000, p. 17-25

[5] F. J. Wester, E. Gulski, J.J. Smit, ‘’CBM of MV power cable systems on the base of advanced PD diagnosis’’, presented at the 16th CIRED 2001, Amsterdam, The Netherlands, paper No. 1.29 Vol. 16, No. 2, pp. 17-25, 2000

[6] F. Farneti, F. Ombello, E. Bertani and W. Mosca, “Generation of oscillating waves for after-laying test of HV extruded cable links”, CIGRE Session Paris (1990) paper 21-10.

[7] Lefèvre, W. Legros and W. Salvador. “Dielectric test with oscillating discharge on synthetic insulation cables”, CIRED 1989, Paris, France, pp.270-273.

Fig. 6 PD location mapping as generated for the power cable tested in figure 5.

[8] S. Brettschneider, E. Lemke, J.L. Hinkle, M. Scheider, Recent Field Experiments in PD assessment of Power Cables, Using Oscillating Voltage Waveforms, proceedings of IEEE ISEI, 2002 p. 546-552

[9] F.H. Kreuger, Industrial High DC Voltage, Delft University Press, 1995

96