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Modelling of 420kV Gas Insulated Substation For Measurement of Very Fast Transient OvervoltagesK.Abhishek, D. Deepak Sai, V.Kranthi JNTU college of engineering,HYDERABAD

AbstractThe range of application of Gas Insulated Substations (GIS) has expanded year by year, due to its numerous merits, such as large miniaturizing effects, good anti-contamination characteristics, easy maintenance and good environmental adaptability. In our country a few GIS units have been in operation and large numbers of units are under various stages of installation. Although, GIS have been in operation for several years, some of the problems need attention. These problems include, generation of overvoltage during disconnector operation, line to enclosure faults. These over voltages are caused in two ways, viz., due to switching operations of circuit breaker/disconnect switch and line to enclosure faults. In the project work, very fast transient over voltages are generated due to switching operations have been analysed and presented. . During closing as the contacts approach, the electric field between them will rise until sparking occurs. Each re-striking generates transient over voltage with different levels of magnitude. Since these transients have travelling wave behaviour, they travel to the external systems through enclosures, bushings, cable joints etc., and cause damage to the outside equipment, also secondary breakdowns may lead to electromagnetic interference.

Simulation models and equivalent circuits are developed for the components of GIS and the very fast transient voltages are calculated for 420 kV input voltage by using the PSPICE software. The fast transient over voltage obtained due to switching operations and faults with fixed and variable arc resistance for different lengths of GIS. The values are also calculated with the assumed load. Analysis has also been made to suppress these over voltages by inserting a resistor of appropriate value during switching.

Keywords Gas Insulated Substation (GIS), Transient over voltages, switching operations, line faults and Suppression resistance.I. Introduction

Very Fast Transients (VFT) belong to highest frequency range of transients in power systems. These transients are originated within a Gas Insulated Substation (GIS) at any time there is an instantaneous change in voltage. This generally occurs as the result of the opening or closing of a disconnect switch.During the operation of the DS, a small capacitor connecting to the breaker will be switched. The velocity of DS contacts is small (generally more than 0.6s), before the completely switching, the arc reignition or prestrike occurs, which is the main cause of VFTO.

When the switch closes, the distance between two contacts of the breaker reduces, the first reignition of arc occurs when the voltage across contacts exceeds the breakdown voltage of the gas insulation. The potential of the bus will increase to the source voltage. When the high frequency current turn to zero, the bus is disconnected form the source and the potential of the bus will be kept if the leakage of electric charge is neglected. The same process happens when the voltage across contacts exceeds the breakdown voltage of the gas insulation for a second time. The arc reignites repeatedly and more frequently until the two contacts totally contact. When the switch opens, the same process of arc reignition occurs. Differently it happens less frequently with the separation of the contacts. The arc reignition can result a high frequency oscillation of the current and voltage. The maximum magnitude of the transient over voltage is 2.0 p.u. this value is largely dependent upon the level of trapped charge on the GIS bus bar existing at the time of the re-strike. The amplitude of trapped charge is strongly influenced by the asymmetry of the inter-contact break down voltage occurring on the fixed and mobbing contacts of the dis-connector. This paper is aimed at calculating magnitude of fast transient over voltages in GIS due to Switching Operations and Line-to Enclosure faults by suitably modelling a typical GIS system.The main problems associated with the VFTOs:

1. Flashover to ground at the dis-connector switch contacts.

2.Failure of electronic controlled circuits connected to GIS, because of electromagnetic interference of VFTO.

3. Dielectric Strength is reduced under VFTO, if non uniform electric field is formed by the particles (mainly metallic).

4. Effect on components such as bushing and transformer.

5. Transient enclosure voltage on external surface of the earth and may cause flashover to nearby grounded objects.

II. Modelling of 420kv gis systemA GIS system comprising of an Input Cable, Spacer, Dis-connector Switch, Bus bar of 5mts length and load has been considered for modelling into electrical network and analysis. The Fast Transient Over voltage waveform generated during Closing and Opening operation of Dis-connector Switch and Line-to-Enclosure faults has been considered for calculations. Spacers are simulated by lumped Capacitance. The Inductance of the bus duct is calculated from the diameters of Conductor and Enclosure. Capacitances are calculated on the basis of actual diameters of inner and outer cylinders of central conductor and outer enclosure. Cone Insulators used for supporting inner conductor against outer enclosure are assumed to be disk type for approximate calculation of spacer capacitance. the bus duct can be modelled as a series of Pi-network or as sequence parameters. However in this model, it is considered as distributed Pi-network. The Schematic Diagram of a Typical Gas Insulated System (GIS) is shown in below figure 1.

Fig. 1 Schematic Diagram of a Typical GISA. Calculation of Parameters1. Calculation of InductanceThe inductance of the bus duct can be calculated by using the formula given below: Where r1, r2, r3, r4, are the radii of the conductors in the order of decreasing magnitude and l is the length of the section.

Fig. 2 Cross Section of Typical GIS System

2. Calculation of CapacitanceThe Capacitance is calculated with the assumption that the conductors are Cylindrical. Capacitance is calculated by using

the standard formulae given below:

Whereo = 8.854 * 10-12, r = 1b = Outer Cylinder Radiusa = Inner Cylinder Radiusl = Length of the Section

3. MODELLING DIAGRAM OF 420KV GIS

Fig. 3 Modeling Diagram Of 420kv GISIII. EQUIVALENT CIRCUIT FOR 245KV GISThe bus duct is divided into three sections of length 2.5mts, 1.5mts, and 1.0mts respectively from load side. The GIS bushing is represented by a capacitance of 200pf. A Fixed Resistance of 2ohms of the spark channel is connected in series with the circuit breaker. The equivalent circuit is shown in figure 4.

Fig 4 Single - Phase Equivalent Circuit For 5mts Length GIS Due to Switching Operation

Due to trapped charge some voltage remains on the floating section which can create severe conditions because the first re-strike can occur at the peak of power frequency voltage giving a voltage of 2. 0 p.u. On re-strike the voltages on each side will collapse initially zero and hence creating two 1.0 p.u voltage steps of opposite polarities. In this, it is assumed that re-striking is created at 1.0 p.u and -1.0p.u respectively on either side of dis-connector Switch (DS). The transients due to different switching operations are observed.

Fig 5 Transient Voltage Waveform during Closing of CB for 5mts GIS

Fig 6 Transient voltage waveform during opening operation of CB for 5mts GIS

Fig 7 Transient voltage waveform during restrike operation of CB for 5mts GISTransient voltage waveform for load of 1 and 4 are also measured.

Fig 9 Single-Phase Equivalent Circuit for 10mts Length GIS

Due to Switching Operation

Fig 10 Transient Voltage Waveform during Closing Operation of CB for 10mts GIS

Fig 11 Transient voltage waveform during opening operation of CB for 10mts GIS

Fig 12 Transient voltage waveform during restrike operation of CB for 10mts GISTransient voltage waveform for load of 1 and 4 are also measured.

IV. TRANSIENTS DUE TO SWITCHINGOPERATION

1 EQUIVALENT CIRCUIT FOR 5MTS LENGTH:

Fig 13 5mts Length GIS With Variable Arc Resistance Due

To Switching Operation

Fig 14 Transient Voltage Waveform during Closing Operation

of CB for 5mts GIS, Wit Variable Arc Resistance

Fig 15 Transient Voltage Waveform during Opening Operation of CB for 5mts GIS, With Variable ArcResistance

Fig 16 Transient Voltage Waveform during Second Re-Strike Of CB for 5mts GIS, With Variable Arc Resistance

2 EQUIVALENT CIRCUITS FOR 10MTS LENGTH:

Fig 17 10mts length GIS with Variable Arc resistance due to switching operations

Fig 18 Transient Voltage Waveform during Closing

Operation of CB for 10mts GIS, With Variable Arc Resistance

Fig 19 Transient Voltage Waveform during Opening

Operation of CB for 10mts GIS, With Variable Arc Resistance

Fig 20 Transient Voltage Waveform during Second Re Strike For 10mts GIS, With Variable Arc

Resistance

V. SUPPRESSION OF OVERVOLTAGESThe fast transient over voltages during switching operation and faults can cause damage to the system equipment. Hence it is advisable to suppress these over voltages for protection of equipments. One of the methods of suppressing these over voltages is by insertion of resistance during switching. Generally a Resistor of 500 is used for this purpose. In this analysis, a resistor of 500 is connected in parallel with the circuit breaker and a switch is connected in series with the resistor. The analysis and comparison for 250,750 and 1000 are also done in the project. The transient over voltages are suppressed only if the current during contact operation flows through the resistor. The switch connected in series with the resistor is closed at the time maximum voltage is obtained during second re-strike.

Fig 21 GIS of 5mts Length with Fixed Arc Resistance during Suppression

Fig 22 Suppression of overvoltage for 5mts length GIS with fixed resistance

Fig 23 GIS of 5mts Length with Variable Arc Resistance during Suppression

Fig 24 GIS of 5mts length with Variable Arc Resistance during suppression

Fig 25 GIS of 10mts Length with Fixed Arc Resistance during Suppression

Fig 26 Suppression of over voltages for 10mts GIS, with Fixed Arc Resistance

Fig 27 GIS Of 10mts Length with Variable Arc Resistance during Suppression

Fig 28 Suppression Of Over Voltages for 10mts GIS, With Variable Arc ResistanceVI. RESULTS AND SUMMARYBy the switching operations and line-to-enclosure faults in a Gas Insulated Substation (GIS) leads to Very Fast TransientOver voltages (VFTO), these VFTOs stress the equipments in GIS and reduces the reliability of the switchgear equipment. For knowing the maximum values of VFTO, the PSPICE software is used and a simulation is carried out by designing suitable equivalent circuits and its models are developed. The main advantage of such models is used to enable the transient analysis in GIS. The variable arc resistance is calculated by using the Toeplers formula. The inductance of the bus bar is found out from the diameters of conductors and enclosure. The bus capacitance is calculated using formula for concentric cylinders. The entire bus length is modeled as distributed pi-network. The transients due to switching operations and line to enclosure faults with fixed arc resistance for different lengths of GIS are found. Transients are calculated along with load and it was observed that the transients obtained in 5mts length GIS will effect the system more than that obtained in 10mts length GIS. As the distance between the fault point and load increases during fault analysis the magnitudes and rise times of the transients also increases. Transients are also calculated due to switching operations and faults with variable arc resistance for different lengths of GIS were found. It was observed that the transients due to variable arc resistance give lower value of peak voltages than that obtained in with fixed arc resistance. The transient over voltages caused by a switching operation for different lengths of GIS was suppressed by choosing appropriate value of resistance connected in series with the switch.

Table 1. Comparison of Transients Due To Switching Operations for 5mts Length GIS With Fixed and Variable Arc Resistance

With Fixed ArcWith Variable Arc

ResistanceResistance

RiseRise

Mode of operationVoltageVoltage

timetime

(p.u)(p.u)

(ns)(ns)

opening3.1403.240

closing2.6201.440

Second Restrike2.9503.110

suppression2.2401.610

Table 2. Comparison of Transients Due To Switching Operations for 10mts Length GIS With Fixed and Variable Arc Resistance

With Fixed ArcWith Variable Arc

ResistanceResistance

Mode of

RiseRise

operationVoltageVoltage

timetime

(p.u)(p.u)

(ns)(ns)

opening3.01002.8100

closing1.6501.4120

Second3.0101.9100

Restrike

suppression2.21001.6 100

VI. CONCLUSIONThe fast transient over voltages are obtained due to switching operations and short-circuit faults are studied. The transients are calculated initially with fixed arc resistance and then variable arc resistance. The variable arc resistance is calculated by using Toeplers formulae. Transients along with load and without load are also estimated.At the end, these transients over voltages are reduced by connecting suitable resistor during switching operation. From the above studies the following observations are made.1) It was observed that the transients obtained due to switching operations and faults in 5mts length GIS will affect the system more than that obtained in 10mts length GIS.

2) It was also found that during fault analysis, as the distance increases, the magnitude of transients also increases. However, when load is connected, these do not follow a definite pattern.

3) When load is connected at the open end of GIS, the peak voltages and rise times that are obtained due to short-circuit do not follow a particular pattern. This may be due to variation in damping due to combined effect of circuit and load parameters. 4) For any length of GIS it was found that transients due to variable arc resistance give lower value of peak voltages than that obtained with fixed arc resistance.

5) In case of 5mt and 10mt length GIS, the transients obtained due to short-circuit are more severe than the switching operation.

6) The transients obtained during opening operation of the circuit breaker with fixed and variable arc resistance are calculated and found that the difference between peak values in both the cases is significant and that higher peaks are obtained when

fixed arc resistance is used.Transients obtained during closing operation of the circuit breaker, have no significant change in magnitude of voltages.

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[2] Working Group 33/13-09 (1988), Very Fast Transient Phenomenon Associated with Gas Insulated Substations,CIGRE.[3] B.P. Singh, G.B.D. Varaprasad, K. Uday Kumar, EMTP simulation of Fast Transients in Gas Insulated Switchgear.

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IEEE Trans. on PD, Vol. 4, No. 1, January 1989, pp. 223-233.

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[7] K.Usha, M.Zeenath Ameena and S.Usa, Suppression of VFTO and VFTC in GIS, International Journal of Electrical Engineering, ISSN 0974-2154 Volume 4, Number (8),2011