Ehv Gis Substation

Abstract— One of the primary requirements in the planning and design of the power system is reliability. The major cause of power system interruption is due to insulation failure resulting from over voltages in the power system. The objective of this paper is to simulate an EHV-GIS substation and to study what is effect of the generated VFT (very fast transient) in EHV-GIS system. The system has been modeled and simulated using PSCAD 4.2.1.

I. INTRODUCTION

Gas insulated substations (GIS) have found an increasingly wide range of application during the last three decades because of their remarkable advantages over their conventional counterparts. These advantages include compactness, freedom from environmental influences, high reliability and minimum maintenance, excellent protection against atmospheric over voltages and flexibility in choice of arrangement and for future expansion. The development of GIS technology has made it competitive and, in many cases, preferable in techno- economic comparison with the open-air type substation. In spite of these merits, GIS has its unique problems, among which is the susceptibility of the insulation system under very fast transients (VFT) caused by disconnector operation [1].

For accurate and reliable study of the transients it is essential to replicate the system corresponding to frequency range in which transients occur. Very fast transients belong to highest frequency range. This means that the model used should be suitable for the frequencies 100 kHz to 50 MHz [2]. Main subject of very fast transients are effects in Gas insulated substations (GIS). Modeling and effects of GIS are therefore main subjects of this paper. This paper summarizes the results obtained with the help of computer simulation.

II. MODELLING OF EHV-GIS SUBSTATION The quality of the simulation depends on the quality of the model of each individual GIS component. In order to achieve reasonable results even for longer time periods of some micro-seconds or for very complex GIS structures highly accurate models for each of the internal equipment and also for components connected to the GIS are necessary. Due to traveling wave nature of VFT, modeling of GIS components make use of electrical equivalent circuits composed of lumped elements and distributed parameters lines. At very high frequencies, the skin losses can produce a noticeable

Amit Kumar (e-mail: [email protected]) and Mahesk K.

Mishra (e-mail: [email protected]) are with the department of Electrical Engineering, Indian Institute of Technology Madras, Chennai, India.

attenuation. However due to geometrical structure of GIS and the enclosure material, skin losses are usually neglected [3]. Very fast electromagnetic transients caused by switching operations in gas insulated substations (GIS) cannot be calculated if conventional techniques of modeling and simulations are used. Choice and adjustments of the most suitable models as well as determinations of their limitations in digital simulations of very fast transients in GIS are very important. Results obtained from very extensive field tests as well as from digital simulations for different types of GIS have been used for development of more accurate models for GIS components and GIS as whole. A comparison between field test and calculation results enables development of a model which takes into account the most decisive physical phenomena inherent to the very fast transients in the GIS. Modeling can be divided into different parts which as a whole make the complete system [2] - [6]. Equivalent circuits of various GIS components have been given in Table I. Busbar can be modeled as the lossless distributed parameter transmission line with an impedance of 95 . Spacer can be represented by a capacitance of value exceeding 20 pF. For elbows a capacitor of value 6 pF can be used. The modeling of closed switch and disconnector can be done using distributed parameter lines with surge impedance of 42 has been used. Open disconnector is represented by a capacitor of value 4 pF. Bushing has been modeled using a short transmission line in series with a capacitor. The surge impedance of the line is 126

and capacitor has been taken of any value greater than 10 pF. A fixed resistance in series with a variable resistance represents the modeling of spark resistance. Capacitive voltage transformer is represented by a capacitor. Aerial line is modeled using the surge impedance of line.

III. SIMULATION STUDY OF EHV-GIS SUBSTATION For the purpose of modeling and simulation a 420KV GIS has been taken. It consists of one incoming line feeder bay and one outgoing feeder with transformer. Block diagram of this system is shown in Fig.1. The system used consists of a double bus bar with sectionalizers. According to their internal design all parts of GIS has been represented thoroughly by line sections with the corresponding surge impedance and traveling time and by lumped capacitance for spacers and additional capacitance caused by shielding devices [5]. The behavior of the spark in the disconnect switch during closing operation is represented by a fixed resistance of Rf =0.5 ohm in series with an exponentially decreasing resistance R=R0e-t/T,R0 = 1012 and T=1 ns resulting in a time duration of voltage breakdown of about 10 ns. After the successful modeling of the GIS substations voltages are measured at different points as Em1 to Em5. Em1 shows the voltage stress at connection

VFT Study for EHV-GIS SubstationAmit Kumar and Mahesh K. Mishra, Member, IEEE

Fifteenth National Power Systems Conference (NPSC), IIT Bombay, December 2008

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inside the transformer between bushing and transformer winding. Em2 shows the high frequency oscillations at the GIS side of transformer bushing. Em3 shows the voltage oscillations at the end of busbar ss2. Em4 shows the voltage at oscillations at busbar ss1 and Em5 shows the voltage oscillations in the transformer feeder at the end of closed disconnector. This system has been modeled using the electromagnetic transient program PSCAD 4.2.1 and will be used for the further study. The actual simulated system has been given in Appendix A.

TABLE IEQUIVALENT CIRCUIT OF GIS COMPONENTS

Component Equivalent circuit

Notes

Bus bar Loss free distributed parameter transmission line of Z=95 ohm

Spacer C > 20 pF

Elbows C= 6pF

Surge arrester

Closed switch Short transmission line with surge impedance = 42 ohm

Open switch n= no. of breaking chambers

ClosedDisconnector

Short transmission line with surge impedance = 42 ohm

OpenDisconnector

Capacitor with C=4 pF

Disconnector during sparking

r=r(t) ; R=0.5 ohm, c=1μF

Bushing(capacitive type)

n =number of equivalent shields

Bushing (gas filled)

C >10pF , Zs = 126 ohm

Current transformer

Can be represented by a surge impedance only , sometimes negligible

Capacitivevoltagetransformer

Value depends on the CVT used

Earth connection

Aerial line r =surge impedance

IV. RESULTS Simulation results of the VFT studies have been taken in three different cases. The time step for simulation is 0.01ms In case 1, normal lengths of the system given in appendix A have been used, In case 2, the length of busbar has been changed to the double of the given length and In case 3, lengths of the disconnector and bushing have been changed.

Case 1: There are five points at which VFT over voltages have been measured. These VFT Over voltages are given by Em1,

Em2, Em3, Em4, and Em5. These points have been shown in appendix B.

Incoming Line

DS- Disconnector

CB-Circuit Breaker

BusbarSS1

BusbarSS2

Load

Fig. 1. Block diagram of the system used for the simulation

Gas to ai r Bushing

DS

DS DS

CB

DS

DS DS DS

CB

DS

Gas to oil Bushing

Transformer


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Fig. 2 (a). VFT over voltage Em1

Fig. 2 (b). Enlarged version of the VFT over voltage Em1

Fig. 3 (a). VFT Over voltage Em2









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The voltage stress Em1 measured at the connection inside the transformer between bushing and the transformer winding is shown in Fig. 2 (a) and (b). The voltage stress Em2 which represents the high frequency oscillations at the GIS side of transformer bushing is shown in Fig. 3 (a) and (b). The voltage stress Em3 measured at the end of switched busbar can be seen in Fig. 4 (a) and (b). The voltage stress Em4 measured at the switched busbar is shown in Fig. 5 (a) and (b). The voltage stress Em5 measured at the disconnector end of the system is shown in Fig. 6 (a) and (b). The transient frequency of the very fast transients measured is 1 MHz at each location in this case. Results obtained in this case are given in Table II.

TABLE IIVALUES OF THE VFTO AT DIFFERENT POINTS IN GIS

Basevalue

VFTO VFT MagnitudE

Transientfrequency

PU values

420 Em1 610 1 1.45 420 Em2 600 1 1.43 420 Em3 650 1 1.55

420 Em4 450 1 1.07

420 Em5 600 1 1.43

Case 2: In this case the length of the busbar has been changed to double of the given length and GIS components have same values as discussed. The results obtained in this case are given in Table III.

TABLE IIIVALUES OF THE VFTO AT DIFFERENT POINTS IN GIS SYSTEM FOR

CHANGED BUSBAR LENGTHBasevalue

VFTO VFT MagnitudE

Transientfrequency

PU values

420 Em1 564 1 1.34 420 Em2 560 1 1.33 420 Em3 735 1.6 1.75

420 Em4 481 1.4 1.15

420 Em5 600 1 1.43

From Table III we can see that when the lengths of busbars have been doubled, the VFT magnitudes and transient frequencies, Em3 and Em4 have gone up, while at the transformer end the VFT magnitudes Em1 and Em2 have come down. There is no change observed in transient frequencies at these points.

Case 3: In this case the lengths of disconnector and bushing have been changed, while other components have same lengths as discussed. The results obtained in this case are given in Table IV.

TABLE IVVALUES OF THE VFTO AT DIFFERENT POINTS IN GIS SYSTEM FOR

CHANGED DISCONNECTOR AND BUSHING LENGTHSBasevalue

VFTO VFT MagnitudE

TransientFrequency

PU values

420 Em1 617 1 1.47 420 Em2 594 1 1.41 420 Em3 622 1 1.48 420 Em4 454 1 1.08

420 Em5 600 1 1.43

From the Table IV, we observe that with the change in disconnector and bushing length there has not been any major change in magnitudes at transformer end. The VFT magnitudes Em1 and Em2 are almost same as in case 1. The VFT magnitudes at busbars Em3 and Em4 are also almost same. From the above results we can also observe that has not been any change in transient frequency at any of these five points.

V. CONCLUSIONSFrom the results obtained in all the cases we can observe that in case 1 where the length of the lines and components are taken as given in appendix A, the values of the VFT over voltages measured at different locations are Em1, Em2, Em3, Em4 and Em5. The voltage stress measured at all of these locations is less than 2 pu which means the system is protected from the damage caused by the VFT over voltage. In this case the transient frequency is 1 MHz at each of the measuring location. In case 2, where the length of GIS busbar has been changed, the magnitude is decreased for Em1 and Em2 but frequency remains unchanged. There is no change in the VFT magnitude Em5 and transient frequency at the disconnector end. But at the busbar ends the magnitude and transient have increased in this case. In case 3, where the lengths of disconnector and busing have been changed, the VFT magnitudes are almost same as in case 1. There has not been any change in the transient frequency at any of the location in this case. In case 2 and case 3 also the VFT magnitudes are less than 2 pu at each location, so system is protected from any kind of damage due to generation of the VFT. From these 3 cases we can conclude that change in the lengths of disconnector and bushing does not affect the VFT magnitude and transient frequency, but the change in busbar length affects the VFT magnitude and transient frequency. So busbar is the critical component in VFT study for EHV-GIS substation. From the results we can also see that the obtained are very consistent, so this model can also be used for the VFT study in any GIS system.

APPENDIX A A. 1- Details of simulation data for transformer feeder

Branch Length Z ( ) C (pF) 1-2 1.28 95 --

2-3 0.75 42 -- 3-4 -- -- 4 4-5 0.30 42 -- 5-6 0.45 95 -- 6-7 0.45 95 -- 7-8 0.15 42 -- 7-9 0.75 42 -- 9-10 0.15 42 --

10-11 0.30 42 -- 11-12 1.10 95 -- 12-13 1.15 95 -- 13-14 1.20 95 -- 13-15 0.75 42 --


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15-16 -- -- 4 16-17 0.30 42 -- 17-18 0.65 95 -- 18-19 0.40 42 -- 19-20 1.10 42 -- 20-21 0.30 95 -- 21-22 0.70 95 -- 22-23 1.10 66 -- 23-24 1.15 66 -- 24-25 0.70 95 -- 8-25 2.30 95 --

17-26 0.80 95 -- 26-27 0.90 72 -- 27-28 0.80 95 -- 28-29 0.90 95 -- 29-30 1.50 95 -- 30-31 0.80 95 -- 31-32 0.60 95 -- 32-33 0.60 25 -- 31-34 11 25 -- 34-35* 2.30 126 --

2-E -- -- 10 5-E -- -- 14

6-E -- -- 10 8-E -- -- 10 11-E -- -- 24 13-E -- -- 10 17-E -- -- 10 22-E -- -- 10 23-E -- -- 32 24-E -- -- 32 25-E -- -- 32 26-E -- -- 10 27-E -- -- 10 28-E -- -- 16 29-E -- -- 10 30-E -- -- 10 31-E -- -- 10 33-E -- -- 10 34-E -- -- 14

(*) special bushing arrangement A. 2- Details of simulation data for busbar

Branch Length Z ( ) C(pF) 1-2 1.8 157 -- 2-3 2.7 157 -- 3-4 5.6 157 -- 4-5 0.90 157 -- 5-6 0.85 104 -- 6-7 0.15 104 -- 7-8 0.20 104 -- 7-9 1.80 157 -- 9-10 7.4 157 --

10-11 1.8 157 -- 1-E -- -- 12 2-E -- -- 3 3-E -- -- 3 4-E -- -- 3 8-E -- -- 3 9-E -- -- 3 10-E -- -- 3

11-E -- -- 12

A. 3- Details of simulation data for transformer feeder Branch Length Z ( ) C (pF)

1-2 1.20 95 -- 2-3 0.75 42 -- 3-4 0.15 42 -- 4-5 0.30 42 -- 5-6 0.45 95 -- 6-7 0.45 95 -- 7-8 0.15 42 -- 7-9 0.75 42 --

9-10 -- -- 4 10-11 0.30 42 -- 11-12 1.10 95 -- 12-13 1.15 95 -- 13-14 1.20 95 -- 13-15 0.75 42 --

15-16* -- -- 4 16-17 0.30 42 -- 17-18 0.65 95 -- 18-19 0.40 42 -- 19-20 1.10 42 -- 20-21 0.30 95 -- 21-22 0.70 95 -- 22-23 1.10 66 -- 23-24 1.15 66 -- 24-25 0.70 95 -- 8-25 2.30 95 -- 17-26 0.80 95 -- 26-27 0.90 72 -- 27-28 0.60 95 -- 28-29 0.60 25 -- 27-30 4 95 -- 30-31 0.15 95 -- 31-32 0.1.5 95 --

32-33** 4.9 95 -- 2-E -- -- 10

5-E -- -- 14 6-E -- -- 10 8-E -- -- 10

11-E -- -- 24 17-E -- -- 10 21-E -- -- 10 22-E -- -- 10 23-E -- -- 32 24-E -- -- 32 25-E -- -- 32 26-E -- -- 10 27-E -- -- 10 28-E -- -- 10 29-E -- -- 10 30-E -- -- 16 31-E -- -- 16

(*) by-pass disconnector switch (**) special bushing arrangement

Note—These values given in appendix A are used in simulation purpose for different sections of the circuit.


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APPENDIX B

Actual simulated model

Fig.7. Simulated Circuit

REFRENCES [1] C.Y. lui and J. Hiley, “Computational study of very fast transients in GIS with special reference to effects of trapped charge and rise time on over-voltage amplitude,” IEE Proc.-Genre. Transom. Distrib., vol. 141, no. 5, pp. 485-490, September 1994. [2] IEEE TF on Very Fast Transients (D. Povh, Chairman), "Modeling and analysis guidelines for very fast transients," IEEE Trans. on Power Delivery,vol. 11, no. 4, pp. 2028-2035, October 1996.

[3] Hirohiko Yatsuzuka, Manabu Takamoto, Masaharu Matsuyama, “New 550-kV Gas-insulated Switchgear,” Hitachi Review vol. 48, no. 5, pp. 285-289,1999. [4] ABB-Insulation Co-ordination Maharani bag, New Delhi, 2007. [6] Z. Haznadar, S. Cariimamovid, R. Mahmutdehajid, “More accurate modeling of gas insulated substation components in digital simulations of Very fast transients,” Transactions on Power Delivery, vol. 7 no. 1, pp. 434-441, January 1992. [7] M. Mohan Rao, M. Joy Thomas, and B. P. Singh, “Frequency characteristics of very fast transient currents in a 245 KV GIS,” IEEE Transaction on Power delivery, vol. 20, no. 4, pp. 2450-2457, October 2005.


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Ehv Gis Substation