High frequency signal propagation characteristics in ...

High frequency signal propagation characteristics in high voltage power cables H. N. O1,

T. R. Blackburn1,

B. T. Phung1,

1

School of Electrical Engineering and Telecommunication, The University New South Wales, Australia

M. Vakilian2, 2

H. Zhang1,

M. S. Naderi2

Department of Electrical Engineering Sharif University of Technology, Iran

[email protected] Abstract Partial discharge (PD) measurement and location has become an essential part of insulation condition monitoring of high voltage cables, particularly of the XLPE insulated type. However, such PD detection and location techniques require the monitoring of very high frequency pulses as they propagate through the cable. Such pulses will suffer attenuation due to lossy cable insulation and this will then complicate measurement and location of the PD source. This paper presents results of an investigation of high frequency signal propagation in medium voltage (11kV) XLPE power cables. The aim of the work is to elucidate the detailed propagation characteristics for application to PD monitoring. In particular, the effect of the semiconductor stress-grading screens in the cable is of some interest as it can have some significant effect on propagation of high frequency pulses. Experimental tests have been performed on single core XLPE cable in the laboratory. In addition, theoretical work has been performed which aims to develop a reliable and accurate model for the high frequency propagation characteristics of the power cable. This paper introduces an extended version of a frequency-dependent cable model available in ATPDraw. The work reported in the paper compares the experimental results obtained with the high frequency cable model which extends the ATP version to include the semi-conducting layers. The comparison found good agreement between the measured and predicted attenuation factors and velocity of propagation. It was found that the presence of the semi-conducting layers has significant effect on the propagation characteristics of such cables. Keywords: ATP, line model, partial discharge, power cables, signal propagation

1.

INTRODUCTION

Modern power cables are suffering increased loads present an increased problem to their insulation viability. In particular, to the generation of partial discharge activity that can degrade the insulation. Partial discharge (PD) activity in power cables is caused by a number of possible defects, such as voids and water trees that have progressed to the electrical tree stage in cable insulation [1]. The PDs will gradually degrade the insulation

material, eventually leading to full and final insulation breakdown. While the older style paper-insulated cables are able to tolerate PDs of moderate level for long periods, modern cross linked polyethylene (XLPE) power cables are very sensitive to any level of partial discharge activity. Because XLPE cables are increasingly being used due to a number of technical and economical advantages, it is imperative that they not be subject to PD activity for any length of time. This requires monitoring of PD activity and this in turn requires knowledge of the PD propagation characteristics in such cables. Partial discharges in the solid insulation of the cables and their accessories can generate propagating signals with frequencies up to several hundred MHz and this thus requires a valid model of the cable for such frequencies. The ability to detect and locate PD sources along the cable length is limited by the high frequency attenuation and dielectric loss of the PD pulse signal as it propagates through the cable. In addition the wave shape of the original PD pulse must be resurrected to enable an analysis of the ageing effect of the cable from the true PD wave shape as it was when emitted at the PD site. The shape of the PD pulse, particularly in the time domain, is a good means of identifying insulation change. Thus, in order to detect the PD, it is necessary to understand the PD propagation characteristics and mechanism of high frequency propagation in the cable. It is important to develop a cable model to predetermine the characteristics of PD propagation correctly. ATPDraw is used in this paper. This is a graphical preprocessor to the ATP-EMTP (Electro-Magnetic Transients Program [3]) on the MS Windows platform and ATPDraw has a wide range of theoretical models. ATPDraw supports cable modeling by the user in a friendly way. Users just need to specify the geometrical arrangement and the electrical data for each separate layer of the cable, and then ATPDraw calls on the Line Constants routine of ATP to obtain the distributed line parameters [4]. However there are some limitations to such models for cables in ATPDraw. Specifically, they do not enable incorporation of any stress-grading semi-conducting layers in the model. Such semi-conducting layers are always present in such cables and they can have

significant impact on the high frequency characteristics of cables. Thus the model must be modified to incorporate such entities in the cable simulation.

conducting layer can be modeled as an additional contribution to the per unit length admittance [6].

3. 2.

THEORETICAL MODELS OF CABLE

The power cable has a similar circuit model to low voltage coaxial cables used for signal transmission, but the parameters of power cables are more difficult to compute accurately. The propagation constant ( γ ) of any distributed parameter cable is given by:

γ =

yz

and the characteristic (or surge) impedance (Z) is: Z=

y/z

Where z is the series impedance per unit length and y is the shunt admittance per unit length of the cable: z (ω ) = R (ω ) + jω L (ω ) y (ω ) = G (ω ) + jω C (ω ) Where R, L, C, G are the series resistance, the series inductance, the shunt capacitance and the conductance per unit length of the cable system. For multi conductor cables, these quantities are n by n matrices where n is the number of parallel conductors of the cable system. These quantities are frequency-dependent. For example, skin effect will alter R and L and dielectric loss will alter C and G. We define γ =

yz = α + jβ

Where the real part of γ , the attenuation coefficient α, is the signal attenuation per unit length of the cable while the imaginary part, the phase constant β, is the phase shift per unit length of cable. Both α and β depend on the cable dimensions and on the cable material properties and therefore vary from cable to cable. The velocity of propagation, v, of a signal at a frequency ω is v = ω / β . If the insulation is ideal (lossless) with dielectric dissipation factor = 0, then α = 0 = G, and zy = LC = µ0• and the propagation speed, v = (µ0•)-1/2 [=c (the velocity of light)]. µ0= 4• x 10-7 h/m and • is the permittivity of the insulation dielectric (ε = εoεr) [5]. In XLPE cables, semi-conducting layers are applied between the inner and outer conductors and the insulation material to enable smoothing of the electric field that helps to limit localized stress on the insulation due to conductor stranding. The influence of the semi-

SPECIFICATION

The power cables used in the experimental were 11 kV aluminum-conductor XLPE-insulated single phase cables. The structure includes XLPE insulation sandwiched in the annular between the two semiconducting tape layers. In order to avoid any voids in the cable that may lead to any PD activity, these semi-con layers are essential and provide a good and smooth interface between the insulation and the conductors. The semi-conducting layers are usually made of semiconducting material such as polyethylene or ethylene copolymer mixed with conductive carbon black [4]. The semi-conducting layers influence the propagation characteristics. The center conductor consists usually of a number of strands of Aluminum or Copper wire sufficient to provide good conductivity. The outer conductor of the second semi-conducting layer may take several forms. It may be a copper tape wrap or a heavier copper strap. All of these details have at least small effects on the propagation characteristics and need to be considered in the cable model [7]. Works by Weeks [5] and Mugala [7] have shown that the semi-conducting layer have a dominant role on the propagation characteristics at high frequencies, propagation velocity and attenuation especially.

4.

THE SETUP OF CABLE MEASUREMENTS

Two cables were employed in the measurements and they were both 11kV single core XLPE cables. Cables A and B had lengths of 136 metres and 45 metres respectively. Three types of measurements were performed for the comparison with the cable model predictions. The first test used a high frequency sinusoidal voltage source to provide variable frequency signals applied to the cable. The test setup is shown in Figure 1. The signal was applied between the conductor at the left side (the sending end) and the (earthed) cable sheath. The other end of the cable (the receiving end) has the conductor short-circuited to the sheath (earth). The measurement of the sending and receiving end signals was done using two commercial high frequency current transformers (HF-CTs). PD monitoring normally uses such HF-CTs to monitor current pulses associated with the PDs in the sheath-earth connection. The first test was simply a measure of attenuation degree and propagation velocity in the cables. The second test involved a measurement of the voltage response on the outer and inner semi-conducting layers of the cable. As the layers represent a capacitivelycoupled connection, their voltage will be related to the signal voltage level. The purpose of this test was to determine the viability of using the semi-conducting layers as capacitive probes to detect PD signals. This test used the sinusoidal voltage source with a short circuit at the receiving end.

In the final test, a standard PD calibrating signal source was used to inject a simulated PD pulse into the cable to simulate the practical situation. The sending and receiving end current signals were monitored with the HF-CTs to determine the real attenuation level of the signals. The receiving end was short-circuited again.

conducting layers decrease the electric field between the conductor and insulation or between the insulation and the screen bed [7] and provide a useful voltage monitor. The semi-conducting layers have an effect on the propagation characteristics of the cable in terms of the attenuation and velocity. In this section, voltage sensors are used and are connected to the semi-conducting layers at the sending and receiving end as shown in Figure 1. Figure 3 shows the attenuation versus frequency as measured on the inner semi-conducting layer and outer semi-conducting layer of the 136 m XLPE cable (Cable A) and also the outer semi-conducting layer of the 45 m XLPE cable (Cable B). The results show that the receiving end signal becomes more attenuated as the frequency increases.

5.

TEST RESULTS

5.1

HF S INUSOIDAL SOURCE TEST

The measurement test on the XLPE cables used a high frequency signal generator (variable from 0.1 to 100 MHz with a sinusoidal waveform) and high frequency measurement current transformers. The variation of source voltage was effectively negligible [8]. A typical plot of signal attenuation versus frequency is shown in Figure 2.

Figure 4 shows the inner and outer semi-conducting layer voltage for the cables compared with the output level of the current transformer at the receiving end (T2) versus frequency. The voltage of the semi-conducting layers has greater sensitivity than the current transformer monitors and the outer semi-conducting later has higher sensitivity with respect to the inner semi-conducting layer. In addition the results show that the semiconducting voltage values are increasing as frequency increasing to the test limit of 40 M Hz. 4 3.5 3 Attenuation

Figure 1: The measurement setup. 1. Digital function generator or PD calibration source. 2. Inner semi-conducting layer, 3. Outer semi-con layer, 4. High Frequency Current Transformer (HF-CT), 5. Digital oscilloscope, 6. Computer.

3. 1. 2.

2.5 2 1.5

1. Outer semi-con layer (136 m) 2. Inner semi-con layer (136 m) 3. Outer semi-con layer (45 m)

1

As expected the result shows that the receiving end CT signal is more attenuated as frequency increases. The signal propagation through the cables depend more on the frequency than on the length of cable [2]. The degree of attenuation may be affected also by temperature and by the pressure applied on the semi-conducting layers [7]. It will also be affected by the loss factor and permittivity of the insulation material.

0.5 0 0.1

1

10

Figure 3: Attenuation versus frequency on the inner and outer semi-conducting layers (136 m and 45 m XLPE cable).

6 5

Attenuation

4 3 2 1 0 0.1

1

10

100

Freqeuncy (M Hz)

Figure 2: Signal attenuation versus frequency: 136 metre XLPE cables.

5.2

VOLTAGE MEASUREMENTS ON SEMI-

CONDUCTING LAYERS

The second test looks at the semi-conducting layer response to the signals on the cables. The semi-

100

Frequency (M Hz)

Figure 4: Semi-conducting layer voltage at the receiving end versus frequency on outer semiconducting layer and comparison with HFCT measurements. From left: Cable B (outer semi-con), Cable A (outer), Cable A (inner), Cable B (CT at rec end), Cable A (CT at rec end).

5.3

PARTIAL DISCHARGE CALIBRATION SIGNAL MEASUREMENT Using the setup shown in Figure 1, a PD calibrator voltage signal, simulating a 1000pC partial discharge was applied to the sending end of the cables with the receiving end short circuited. The current pulse of the calibrator is shown in Figure 5. The calibrator signal is attenuated, as expected during propagation along the cable. From the measurements, the ratio of I receiving end / I sending end is calculated to be 1.028 which is more than 1, as expected because of the termination short circuit. For a loss-less cable the ratio would be 2 giving a current doubling at the short circuit. The travel time of signal over the 136 metres is 923 nano-seconds. The propagation speed from measurement [9] is then: v=

l t

Where l is length of cable, t is the measured travel time. Therefore the propagation velocity on this cable is v = 136 / (923 x 10-9) = 1.47 x 108 m/sec which is less than the speed of light, 3 x 108 m/s, as expected for such insulation. In Figure 5 shows the sending end pulse while Fig 6 shows the receiving end pulse (the left hand negative pulse). The wave shape at the receiving end is deformed due to the attenuation and dispersion of the cable. (The positive pulse seen at about 1.7 microseconds is the pulse reflected back from the (high impedance) source after two further transits of the cable length).

Current (mA)

Another cable model is only included conductor, insulation and sheath (without Inner and outer semiconducting layers). Table 1 is shown the simulations results of this cable model (propagation velocity and attenuation) when different permittivity of insulation is applied. Permittivity Insulation 2.2 2.5

3

-0.1

In ATPDraw, Marti model of the cable can only allow a three layer configuration to be used in the model: the conductor, the sheath and the armor. Since the ATPDraw cable model does not have the capability of modeling the inner and outer semi-conducting layers, it had to be modified to allow them to be incorporated. In order to develop a full XLPE cable model, the original sheath layer was employed as the inner semi-conducting layer, the armor layer is substituted for the outer semiconducting layer and between them is the insulation layer. The metal sheath was included by using an enclosing pipe in a pipe-type cable model in the ATPDraw.

of

Propagation velocity

The ratio of I receiving end / I sending end

7

-0.3

power cable insulation at such high frequencies. Moreover, it is important to model the frequency dependence of power cables accurately in order to minimize the cost of construction. In this paper, the simulation work used the ATPDraw software as a basis to develop the existing J-Marti models for power cables to simulate the PD propagation.

-1

0.1

0.3

0.5

2.016 x 108 m/s 1.86 x 108 m/s

1.57 1.54

Table 1: Simulation results of different permittivity of insulation.

0.7

-5 -9

7.1

S IMULATION RESULT

-13 Second (us)

Figure 5: The measured result at the sending end of 136 m XLPE cable. 8

Current (mA)

4 0 -0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

-4 -8 -12 Second (us)

Figure 6: The measured result at the receiving end of 136 m XLPE cable.

6.

SIMULATION MODEL FOR XLPE CABLE

The problem that arises in PD propagation is that there are no distributed parameter models available for such

In this section the new modified XLPE cable models were employed for simulation purposes and their calculated responses are compared with the measurement results on the real cables. The cable model results demonstrated that XLPE cable can in fact be fully modeled, including the conductor, two semi-conducting layers, the XLPE-insulation and the sheath. From the simulation results of the 136 m XLPE cable shown in Figure 7, the travel time is 866 nano-seconds and the ratio of I receiving end / I sending end is 1.04. The pulse at left is the sending end pulse and the second pulse is the receiving end pulse.

0.3 [mA]

sending end

Traveling speed (m /sec)

1.04

866ns

1.53 x108

Measurement:

1.028

923ns

1.47 x108

Error:

1%

6.5%

4%

Simulation:

1.4

265ns

1.69 x108

Measurement:

1.33

277ns

1.62 x108

Error:

5%

4.5%

4.1%

Length of XLPE cable: 136 m Simulation:

-0.1

-0.3

1

-0.5

2 -0.7

-0.9 0.0

0.2

0.4

0.6

0.8

1: Sending end current

1.0

1.2

1.4

[us]

1.6

45m

2: Receiving end current

Figure 7: The simulated result at the sending end and receiving end of 136 m XLPE cable.

Ratio of I receiving end / I

Travel time (s)

0.1

Table 2: Brief comparison of measurement simulation results for cal. PD signal of 1000 pC.

Comparing the simulation result and measurement result, there is 1% error in the ratio and 6.5% error in the travel time. The high travel time error may be due to the distortion of the pulse affecting the rise time somewhat. Further, the ATPDraw assumes that all the common earths are perfectly grounds. However, in the practical situation, a perfect ground is not realisable. This may affect the timing error also. Figure 8 shows the simulation of other cable, the 45 metre Cable B.

1 2

1: Sending end current

2: Receiving end current

Figure 8: The simulated result at the sending end and receiving end of 45 m XLPE cable.

7.2

V ERIFICATION OF PD SIMULATION RESPONSES AND MODELS A summary of the results of the various lengths of cable are shown in Table 2. The errors in the 136 and 45 metre lengths are acceptable in the circumstances. The 45 metre cable had a very thin inner semi-conducting layer and this caused difficultly in modeling the cable in ATPDraw since the thickness was not precisely known and this may generate some error. Compare the results of Table 1 and 2, the semi-conducting layers have the significant effect on the propagation velocity and attenuation at high frequency.

8.

and

CONCLUSION

This paper has shown that the XLPE cable can be fully and accurately modeled in ATPDraw and that the inclusion of the semi-conducting layer influence is very important in the modeling. The semi-conducting layers affect signal propagation and must be taken into account in any PD analysis work. The match of the frequency dependent cable model response with measured data demonstrates the validity of the ATP frequency dependent cable models for PD propagation study. However, it must be noted that practical realities such as earth resistances, proximity of other cables, temperature and semi-conducting layer pressure may also affect the results. In addition this paper presents results of high frequency signal response, using PD simulations, in two different lengths of XLPE cables and their semi-conducting layers. The high frequency response results show that attenuation of the HF-CTs based measurements is increasing as frequency increases. However when monitoring the semi-conducting layer voltage, it was found that this increased as frequency increased, as would be expected from a capacitively coupled sensor. This indicates that use of the semi-conducting layer can have a high sensitivity to partial discharge monitoring and needs to be considered. The simulations in ATPDraw are carried out using circuit theory approximations. However, when the device structure is complex, such as in lossy dielectric cables, simulation through circuit theory approximation becomes difficult or impossible because of the distributed parameters and losses.

REFERENCES Clearly, the simulation model gives good approximations of the test results. This is despite neglect of the dielectric loss which has less effect on attenuation than the semi-conducting layers. This is in agreement with the discussion in [5].

[1] Blackburn T.R., James R.E., Phung B.T. and Liu Z., “Partial discharge characteristics in polymeric cable accessories”, 2001 International Symposium on Electrical Insulating Materials, Japan, pages: 532-535, 2001.

[2] H. N. O, M. Vakilian, T.R. Blackburn, B.T. Phung, H. Zhang and M. S. Naderi “Propagation of high frequency and partial discharge pulses in PILC cable”. In Proc. 14th International Symposium on High Voltage Engineering (ISH’05), Beijing, China, 2005. [3] Electro-Magnetic Transients Program theory book, Chapter 5-underground cables pages 150-192, BPA. [4] Prikler L., “Be Careful when Using the Nominal •line Model (short line equivalent) in ATPDraw”, 2003, 01’ Aug-EEUG news, pages 33-35. [5] W. L. Weeks and Y. M. Diao, ‘‘Wave Propagation Characteristics in Underground Power Cable’’, IEEE Trans. Power App. Syst., Vol. 103, No. 10, pp. 2816_2826, October 1984. [6] P. A. A. F. Wouters, W. J. S. Bollen, P. C. T. v. d. Laan, and E. F. Steennis, “Lokalisatie van partiele ontladingen in XLPE kabels”, Tech. Rep. EHC/RAP/91/011, Eindhoven University of Technology, Eindhoven, The Netherlands, 1991. [7] G. Mugala, R. Eriksson, U. Gafvert and P. Petterson, “Measurement technique for high frequency characterization of semiconducting materials in extruded cables” Dielectrics and Electrical Insulation, IEEE Transactions, Volume: 11, Issue: 3, Jun 2004, Pages: 471 – 480 [8] M. Vakilian, T.R. Blackburn, and B.T. Phung, “Evaluation of over-voltage surges and PD propagation on XLPE cables”, Australasian Universities Power Engineering Conference (AUPEC’04), Brisbane, 26-29 Sept. 2004. [9] M. Vakilian, T.R. Blackburn, B.T.Phung and H. Zhang, “Investigation of PD signal propagation characteristics in XLPE Cables”. PowerCon, Singapore, 2004