The various factors influencing the VFTO levels in 500kV and. 750kV GIS have been discussed by developing the simulink models which

Transcription

1 185 CHAPTER 7 CONCLUSIONS 7.0 VFTOS IN 500KV AND 750KV GIS The various factors influencing the VFTO levels in 500kV and 750kV GIS have been discussed by developing the simulink models which are obtained from the equivalent models of each GIS component. The models have been run for the different switching conditions and the results are analyzed. The following conclusions have been made from the above study: 1. The values of the VFTO s at different locations such as V14S, V15S, V17S, V11UA, V12UA, V13UA, VTR1, VTR3, VTR4, and VTR6 have been measured for the disconnector switch is opened/closed at two locations i.e. DS-50543,DS The voltage levels calculated at all of these locations is less than 2 p.u. 2. From the above results, it can be concluded that the overvoltages has high frequency oscillations and may cause harm to electric equipment with windings. 3. It can be concluded that as increasing the supply voltage from 500 kv to 750 kv the VFTO levels also increased. 4. As the residual charges are increased from -1.0 p.u. to 0.5 p.u., the magnitude of VFTO levels i.e. voltage to ground of bus-bars [V14S, V15S, V17S] and voltage to ground of transformers [VTR1, VTR3, VTR4, VTR6] are decreased in both 500 kv and 750 kv GIS.

2 As the spark resistance is increased from 0.1 to 200Ω, the magnitude of VFTO levels i.e. voltage to ground of bus-bars [V14S, V15S, V17S] and voltage to ground of transformers [VTR1, VTR3, VTR4, VTR6] are decreased in both 500 kv and 750 kv GIS. 6. As the entrance capacitance of transformer is changed from 5000 to pf, the magnitude of VFTO levels i.e. voltage to ground of transformers [VTR1, VTR3, VTR4, VTR6] are decreased in both 500 kv and 750 kv GIS. 7.1 VOLTAGE DISTRIBUTION ACROSS 420 KV SF6 / AIR CAPACITIVELY GRADED BUSHING The effect of VFTO generated during switching operation or earth fault on 420 KV SF6 / air capacitive graded bushing is evaluated. The 420 KV SF6 / air capacitive graded bushing has been represented by considering the physical and electrical equivalent structure. The total number of layers is divided into number of equivalent layers depending on the type of modelling circuit. Further, each equivalent layer of the bushing has been divided into number of sections depending on its length. The voltage distribution across the equivalent layers of the bushing is calculated by applying a unit step pulse of rise time ranging from 1 to 20 ns. Further, observations are made when the first modelling circuit is terminated with an overhead transmission line of surge impedance, 350 ohms. From the above studies, the following conclusions are made:

3 The voltage distribution across layers of 420 KV SF6 / air capacitive graded bushing is highly non-linear for VFTO pulses of rise time less than 5ns. The critical rise time of pulse increases with rated voltage. 2. The voltage distribution across layers of 420 KV SF6 / air capacitive graded bushing has been estimated accurately by considering ten equivalent layers. 3. The voltage distribution across layers of bushing changes from one position to another, because the travelling time for VFTO pulse along the conductor of bushing is comparable to the rise time of the VFTO pulse. 4. The maximum voltage drop across the first equivalent layer of 420 KV SF6 / air capacitive graded bushing, when 1ns rise time pulse applied, is times more than the normal a.c voltage distribution. 5. The rise time of VFTO pulse increases while it travels through the capacitive graded bushing. 6. The maximum voltage drop across the equivalent layers of the bushing changes with the terminal equipment of the bushing. 7.2 VOLTAGE LEVELS ACROSS INSULATING FLANGE An electrical equivalent circuit of GIS with insulating flange has been developed for the prediction of the electromagnetic phenomena in the insulating flange, enclosure in gas insulated substation (GIS). The transients coupled to the insulating flange and enclosure during switching operations may result to malfunctions such as formation of

4 188 sparks generated in primary/secondary equipments in GIS. The transients coupled to the control and protection equipment depends on the characteristics of very fast transient over voltages (VFTO) generated in GIS during switching operations. Insulating flange simulated by considering different parameters such as height of the flange, length of the flange, copper strips and capacitors connected across flange as well as copper strips and capacitors connected across enclosure for VFTO suppression across flange for prevention of spark generation. From the above analysis, the following conclusions are made: 1.Transient voltages across insulating flange suppressed by capacitors is more effective than copper strips because it can be capable of reducing the stray inductance and also it provides less grounding impedance between two sides of flange in order to reduce the transient voltages. 2. Copper strips or shunting bars across the two metallic parts of the flange can be used to reduce the transient voltages by decreasing the external ground impedance and also it can be capable to reduce stray inductance significantly by greater number of parallel copper strips connection across flange and also it controls the power frequency current which provides greater protection. 3. Suppression of transient voltages across enclosure is more effective than flange by the greater number of copper strips or capacitors connected across enclosure.

5 Suppression elements such as copper strips or capacitors across flange may not suppress enclosure transient voltages. VFTO suppression scheme must prevent the formation of sparks generated by the fast transients and control the flow of transient currents. Whatever may be the scheme is used to suppress transients; the important factors are the reduction of stray inductance and decreasing the external ground impedance by the use of greater number of components in parallel with the flange or enclosure 7.3 VOLTAGE LEVELS ACROSS INSTRUMENT TRANSFORMERS An equivalent electrical circuit is developed for the prediction of the electromagnetic phenomena in the secondary circuits of current transformer (CT) and potential transformer (PT) in a gas insulated substation (GIS). The transients coupled to the low voltage electronic circuits during switching operations may result to mal-functioning of primary/secondary equipments in GIS. The transients coupled to the control and protection equipment depend on the characteristics of very fast transient overvoltages (VFTO) generated in GIS during switching operations. 1) The instrument transformers i.e., CT and PT are simulated by considering different parameters such as capacitance between windings, capacitance between winding and ground, inductance of windings, length of the control wiring and burden. From the analysis, it is evident that the

6 190 transient voltages are possible in the order of 1 to 4 kv across the load terminals of the CT/PT depending on the type of the control circuit. 2) The transient voltages coupled to the potential transformer control circuit are more than that to the current transformer control circuit. As the length of the control wiring of the secondary circuitry of CT/PT increased, the transient voltages across the load terminals are also increased significantly. Thus the length of the control circuit shall be limited to as low as possible. 3) The transient voltages coupled to the secondary circuit of CT and PT may not change significantly due to increase in the length of the GIS section. The effect of high voltage capacitance of current transformer (C 1) on transient voltages coupled to the secondary circuit is considerable compared to the effect of winding capacitance (C 2). Finally, the transient voltages coupled to the secondary circuit of instrument transformers depend on the transient response of the corresponding burden circuit. 7.4 SCOPE OF THE FUTURE WORK The present work was modeled for 500 kv and 750 kv Gas Insulated substations by using Matlab software. It can also be extended by increasing the supply voltage to 800 kv and 1000 kv and can also be extended by adding the protection devices such as opening and closing resistor, metal oxide arrester, R-C filter, and ferrite rings which are used to reduce the harmful effects of the VFTO. The effect of VFTO s in the Gas

7 191 Insulated Substation can also be analyzed by using Pspice and EMTP software. For simplicity, in the present work bushing has been represented by considering the equivalent Π sections. For complex waveforms like VFTO UHV substations, it is necessary to extend the present model to distributed parameters representation (equivalent surge impedance).the suppression of VFTOs across insulating flange can be studied for different ratings of GIS by using EMTP software. The analysis of FTOs across secondary circuits of current transformer and potential transformer can be studied for different ratings of GIS by using EMTP software.

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11 195 transaction on power delivery, IEEE transaction on power delivery No 4 October [21] YoshibuminYamagata, Yasuhiko Nakada, of Tokyo electrical power Co., Japan, Very fast transients in 1000KV Gas Insulated Switchgear IEEE Transactions on power delivery, [22] V. Vinod Kumar, M.Joy Thomas and M.S.Naidu, Influence of switching conditions on the VFTO magnitudes in a GIS IEEE transactions on power delivery volume: 16 No: 4, October [23] Mr. Y. Shibuya, S.Fujita and T.Shimomura Effects of very fast transient overvoltages on transformer. IEE proceedings Transmission & Distribution volume: 146, No. 4, July [24]. Working Group 33/13-09, Very Fast Transient Phenomenon Associated With Gas Insulated Substations, CIGRE, [25].Grover Frederick W., Inductance Calculations, Dover Publications New York, [26].Ragaller K. ( Ed.): Surges in High Voltage Networks, (plenum press, London, 1980), pp, [27].H.W. Dommel, Non Linear and time varying elements in digital simulation of Electromagnetic Transients, IEEE Transactions on power Apparatus and systems, vol.pas-90, PP ,1971.

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13 197 List of publications: 1. Estimation of Very Fast Transient Over voltages on Bushing connected in GIS, International Journal of Engineering Science and Technology, Vol.4, No.02, Feb2012, pp Analysis of Very Fast Transient Over voltages in 750 kv Gas Insulated substation, International Journal of Advances in Science and Technology, Vol.4, No.1, Mar Calculation of Transient Voltages Coupled to Instrument Transformers Secondary circuitry Ina GIS during Switching Operations, 15 th International Symposium on High Voltage Engineering, University of Ljubljana, Elektroinstitut Milan Vidmar, Ljubljana, Slovenia, Aug 27-31, 2007, T Effect of Very Fast Transient over Voltages in UHV Gas Insulated Substation, International conference of IJAS, UNLV, LASVEGAS, th March, Effect of very fast Transient Over Voltage on 420 KV SF6 / Air Capacitively Graded Bushing Connected in GIS, IEEMA Journal, July, 2012.

14 Suppression of Very Fast Transient Over Voltages across Insulating flange of UHV GIS, i-manager s Journal of Electrical Engineering, April-June 2012 Edition.