Analysis and Mitigation of Shunt Capacitor Bank Switching Transients on 132 kV Grid Station, Qasimabad Hyderabad

Analysis and Mitigation of Shunt Capacitor Bank Switching Transients on 132 kV Grid Station, Qasimabad Hyderabad SUNNY KATYARA*, ASHFAQUE AHMED HASHMANI**, AND BHAWANI SHANKAR CHOWDHRY*** RECEIVED ON 1811.2014 ACCEPTED ON 17.03.2015 ABSTRACT In this paper analysis and mitigation methods of capacitor bank switching transients on 132KV Grid station, Qasimabad Hyderabad are simulated through the MATLAB software (Matrix Laboratory). Analysis of transients with and without capacitor bank is made. Mathematical measurements of quantities such as transient voltages and inrush currents for each case are discussed. Reasons for these transients, their impact on utility and customer systems and their mitigation are provided. Key Words: Capacitor Banks, Switching Transients, Capacitor Inrush Current, Pre-Insertion Resistor, Current Limiting Reactor, Surge Arrestor, MATLAB. 1. INTRODUCTION I n power system, transients have bad impact on its reliability and may cause damage to or malfunctioning of major equipments. The source of these transients may be switching operations, lightning strikes or failure of equipment. When shunt capacitor bank is to be switched on live network, high frequency and high magnitude transients may occur. Usually capacitor banks are installed at feeder circuit to improve power factor and voltage profile. Normally, these capacitor banks are not connected all the time but switched on and off many times during the day because the load on power system changing with time according to certain load curves. These switching actions will be accompanied by transient currents, when a capacitor bank is closed on energized circuit [1]. Actually switching of capacitors produces more severe transients as compared to energisation of load or cable [2]. When an uncharged capacitor is switched on to network, the system voltage will reduce, as it starts taking energy from network instead of supplying. Hence severe transients will take place when uncharged capacitor is being switched on to network at peak voltage. The system voltage will shoot up by the magnitude equal to the difference between the system voltage and the voltage of capacitor bank when a charged capacitor is switched on [3-4]. Theoretically, it has been observed that these transients would lead to peak amplitude of 2 pu (per unit) but due to inherent damping present in circuit, these transients are limited below this value. The initial peak of transients usually during sub-transient period is the most dangerous one [5]. However effects of these transients are not damaging enough to cause failure of line equipments, but can act as catalyst to affect the most sensitive equipments of the system and may produce un-necessary tripping of equipments [6]. In this paper we are presenting Simulink analysis and mitigation of capacitor switching transients on the * Post-Graduate Student, ** Professor, and *** Meritorious Professor, Institute of Information & Communication Technologies, Mehran University of Engineering & Technology, Jamshoro Mehran University Research Journal of Engineering & Technology, Volume 34, No. 3, July, 2015 [ISSN 0254-7821] 291 Analysis and Mitigation of Shunt Capacitor Bank Switching Transients on 132 kV Grid Station, Qasimabad Hyderabad distribution feeder connected to 40 MVA transformer at 132 kV grid station Qasimabad Hyderabad. Capacitor banks are placed before feeder circuit and initially energized at peak voltage of the circuit at grid station and transients in current and voltage near the bank are analyzed. Now in order to remove these transients different mitigation techniques are to be adopted and there results have been included too in this paper and finally comparison is made on techniques that which is most efficient one. 2. BASIC CONCEPT CONCERNING ENERGIZATION OF CAPACITORS Fig. 1 represents distribution network which provides conceptual introduction about the capacitor switching transients. R 1 and L 1 represent the resistance and inductance of source. As shown in Fig. 1 capacitors C1 and C2 are fed by circuit breaker CB4. S1 and S2 represent switches used to take in and out the capacitors C1 and C2. Inductance of feeder between C1 and C2 is represented by LB. Total impedance of feeder and distribution transformer is represented by the combination of R2 and L2. Circuit breaker CB3 is used here to interrupt the ground fault at some distance on the feeder. When switches S1 and S2 close at any instant to energize capacitors, voltage at feeder to which CB4 is connected collapses to the voltage on the capacitors and produce oscillations. When these oscillations are damped out, feeder voltage would return to its normal value again. But if the capacitors are energized at peak value of voltage, FIG. 1. NETWORK USED TO ILLUSTRATE CAPACITOR SWITCHING TRANSIENTS which is typically about 1.5-1.8 pu of phase to ground voltage, large transients with frequency in range of 300800 Hz are produced initially. The frequency of these transients is determined by source inductance L1 and capacitance of capacitors connected [2]. Actually capacitors are connected in the form of three phase large banks and breaker closes all three phases of a capacitor bank simultaneously, so voltage at one of the phase is always at peak [6-7]. 3. REDUCTION OF CAPACITOR SWITCHINGTRANSIENTS Many topologies and techniques are available to mitigate transients such as; use of pre-insertion resistors, inrush limiting reactors and surge arrester connected across the capacitor bank. Each technique results in a different manner to reduce the switching transients. 3.1 Current Limiting Reactors Here reactor of suitable rating is used in series with the capacitor bank. Due to use of this reactor the surge impedance of the circuit increases and hence the peak of inrush current is reduced. Since the inductor opposes the change in current so current cannot change instantly, therefore elevated frequency components of transient are restricted and the effect of these inrush transients current is condensed [2,7]. 3.2 Pre-Insertion Resistors In this method, switched resistors are used in series with capacitor banks. Here the switch will introduce resistors initially into network and then make contact with capacitor bank. Due to the use of these resistors, overall losses in the circuit are increased which facilitate to decrease the peak value of transients in the voltage and current. The time at which pre-insertion resistors are introduced into circuit is about one fourth of supply frequency i.e. 50 Hz. This helps to decrease the initial peak which is most damaging one of transients [7]. Mehran University Research Journal of Engineering & Technology, Volume 34, No. 3, July, 2015 [ISSN 0254-7821] 292 Analysis and Mitigation of Shunt Capacitor Bank Switching Transients on 132 kV Grid Station, Qasimabad Hyderabad 3.3 Surge Arrester Gap type surge arresters with a series non-linear resistor can be used along with capacit (...truncated)