Real Time Monitoring of SF6 Gas Pressure for Optimization Point on Wave Switching of SF6 Circuit Breaker

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1 Real Time Monitoring of SF6 Gas Pressure for Optimization Point on Wave Switching of SF6 Circuit Breaker Ashish Maheshwari 1, Sunil Kumar Singla 2 1 PG Scholar, EIE Department, Thapar University Patiala, India 2 Assistant Professor, EIE Department, Thapar University Patiala, India Abstract This paper introduce the real time analysis of sf6 gas pressure for optimizing point on wave switching of sf6 circuit breaker. Circuit Breaker plays an important role in today s growing Indian economy in power systems. It provides protection to transmission equipment incorporated in transmission networks. SF6 Circuit Breaker is very important equipment in Power Systems which is used for up to 1200 kv because of its excellent performance. SF6 Gas plays a vital role to operate the Breaker. Also with the help of SF6 gas contact life of circuit breaker is increased. This paper describes the Operation of SF6 Circuit Breaker in real time due to variation in SF6 gas pressure as SF6 gas is used for arc quenching in the Circuit Breaker to obtain the Point on wave Switching and stress free operation. The paper also describes the Compensation of SF6 Gas Pressure on Point on wave switching method used for Controlled operation of Circuit Breaker. Keywords Circuit Breaker, Sf6 gas, Controlled Switching, Compensation I. INTRODUCTION SF6 has been used as arc quenching and insulating medium for high voltage switchgear systems. The favourable electro technical, chemical and physical characteristics of the gas have considerably influenced the development of the switchgear technology. SF6 is an alternative to other conventional insulating and quenching media such as e.g. oil, and air. The use of SF6 gas considerably increases, in some applications, the efficient utilization of resources in energy transmission and distribution with respect to technology, finances and personnel. At the same time SF6 in comparison to oil reduces the risk of hazard (e.g. Fire, explosion) to personnel and the environment. An overall evaluation considering all ecological, economic, safety and technological aspects has proven that SF6 is still an excellent choice as insulating medium. The existing SF6 technology in the field of energy transmission and distribution is the result of decades of optimization and contributes essentially to the further development of the economically efficient power distribution [1]. The point on wave switching for circuit breakers is installed with an electronic device that monitors the health of the circuit breaker and takes in the command from the user in order to CLOSE or OPEN the breaker on the point of wave of a reference voltage or current. The point on wave is also called as synchronous switching or controlled switching [1] [2]. II. SF6 CIRCUIT BREAKER SF6 circuit breaker is getting a major position in the field of circuit breakers because of its excellent performance. Some of the outstanding properties of SF6 making it desirable to use in power applications such as high dielectric strength, unique arc-quenching ability, excellent thermal stability and good thermal conductivity. The SF6 CB is also dependent on the pressure of two gases Air and SF6. Based on the pressure its mechanism requires certain amount time for actuation. Furthermore the switching phenomenon is largely based on the angles at which the phases are CLOSE or OPEN. By this we mean that sequence and angles of switching of phases for a specific load is important. In this for capacitive load close switching is done on the voltage peak when the current is zero per phase. New applications are being researched that adapts to different angles dynamically based on the load type. Thus the overall circuit breaker for point on wave switching is largely dependent on parameters such as DC voltage to coil, SF6 pressure, Air Pressure, load type, frequency of the reference phase, in this we take R-Phase as a reference [4]. III. CONTROLLED SWITCHING Controlled switching is used for elimination of harmful electrical transients upon planned switching of mainly capacitor banks, shunt reactors and power transformers. The method is also gaining acceptance for re-energizing of EHV transmission lines, and replacing traditional pre-insertion resistors. A key aspect of all controlled switching applications is the precision achieved during making and breaking. Optimization of the switching instant minimizes number of re-ignitions and the corresponding over voltages. Uncontrolled making or breaking can lead to increased stress on the equipment; closing resistors and reactors that were previously used to reduce these stresses may no longer be required. Controlled switching offers an economical alternative. By taking current values into consideration, the control unit optimizes the switching operations of the circuit breaker with reference to current or voltage phase [5]. ISSN: Page 2909

2 IV. POINT ON WAVE SWITCHING (POWS) Point on wave switching (POWS) is a method to eliminate harmful transients with time controlled switching operations. Closing or Opening commands to circuit breaker are delayed in such a way that the closing or the Opening of contact will occur at optimum time related to phase angle. With the help of POWS we can increase the performance and life of the circuit breaker. The following (Figure 1) example illustrates the general operating principle of a system, for energizing of a capacitor bank. In order to avoid switching transients, the making instant in this case shall be at voltage zero. For simplicity, only a single phase is considered [4]. A. Closing Time Figure. 1 Point on Wave Switching Block Diagram [4] V. PARAMETERS IN POWS [4] Time from energizing the closing coil until contact touches the circuit breaker. B. Opening Time Time from energizing the opening coil until contact separation occurs. C. Make/Break Time Time for energizing the closing coil until current starts flowing in the main circuit; Adaptation control adjusts the making instant. D. Pre-arcing Time Time from start of current flow in the main circuit until contact touch. Pre-arcing time = Closing time - Make time E. Arcing Time Time from contact separation until contact separation occurs. F. RDDS The rate of decrease of dielectric strength, circuit breaker characteristics that describe the rate of fall of the voltage withstand at closing of a circuit breaker. G. RRDS Rate of rise of dielectric strength, Circuit breaker characteristic that describes the rate of rise of the voltage withstand at the opening of a circuit breaker. The value of the RRDS defines the minimum arcing time needed for re-ignition free interruption of inductive loads [6]. VI. BENEFITS OF POWS The major benefit of point on wave switching is the reduction of switching transients, with an associated reduction of stresses in the system and its components. Decreased transient stresses, e.g. on a power transformer, will have a positive impact on the ageing of the equipment. Therefore controlled switching may be considered a valuable tool contributing to maximize the useful service life of expensive equipment [7]. In addition, the controlled circuit breaker itself will experience reduced energization currents, which will result in reduced interrupter wear. Optimizing the instant of contact parting also means that life-limiting restrike or Regnition phenomena are avoided or reduced in severity. For capacitor bank, filter bank and shunt reactor applications, the number of load switching operations can therefore typically be doubled before a scheduled overhaul is required, compared to uncontrolled operation [7]. VII. CONTROL SYSTEM The purpose of the Control system is to provide controlled (synchronous) switching commands to the OPEN and CLOSE coils of the circuit breaker. Successful controlled switching reduces the mechanical and electromagnetic stresses endured during normal switching operations by reducing the inrush currents upon closing and re-ignition currents during the opening. A further benefit is that the insertion resistors, as commonly found in air-blast breakers, can be done away with as well as their costly recurring maintenance requirements. Given the well documented effect of fluctuations in operating variables (temperature, pressure, auxiliary voltage, etc.) on the operating time of the breaker, compensation is built into the calculation algorithm, ensuring optimum performance over a wide range. Due to differences in operating times between phases, the system is designed to compute and send phase segregated switching commands to the breaker. Monitoring of inrush and re-ignition currents confirms successful synchronous switching of the breaker. The operating principles use the zero-crossings of a sinusoidal signal; a voltage signal will be used as a reference prior to closing and a current or voltage signal can be used prior to opening. When the breaker characteristics are identical for each phase, then a constant time delay (T/6) exists between synchronous commands in each phase, for example in the R, Y, B sequence (1/3 of a cycle overall). For ungrounded capacitor bank switching, simultaneous switching of the two leading phases is followed by a quarter cycle delay in the switching of the third phase. As soon as a CLOSE command is received, it is blocked and a time delay is initiated, awaiting the first zero-crossing of ISSN: Page 2910

3 the voltage. Should no zero-crossing be detected within the preset time delay, synchronous switching is aborted and a 3- phase random CLOSE is executed. Should a zero-crossing occur within the preset delay and no further failures are detected, synchronous CLOSE commands to each phase are issued after the computed time delays. The computed time delays are measured from the reference zero-crossing and compensated for variable effects and variations in the breaker characteristics of each phase. Sequencing of individual phase is monitored to insure proper sequencing at the breaker coils. Sequencing of individual phase commands is monitored. Output control circuits are monitored for timing analysis, if a sequence error is detected, synchronous control is aborted and a 3-phase random control switching is executed. Following Figures 2 and 3 shows the opening operation and closing operation with a control system respectively. The functional design of the OPEN operation is the same as that for CLOSE with the variation that a current or a voltage zero-crossing can be used as the reference signal and that reignited currents are monitored [6] [8]. Figure.2 Opening operation of circuit breaker with a control system [6] 1 - Current zero crossing 2 - Phase R current 3 - Open command input 4 - Open output (power output to open CB coil) 5 - Breaker Contacts Ty - Delay to achieve breaker synchronization (calculated) Topen - Mechanical breaker opening time (predicted) Tarc - Arcing delay (planned and Para metered) Figure.3 Closing operation of circuit breaker with a control system [6] 1 - Voltage zero crossing 2 - Phase Neutral Voltage 3 - Close command input 4 - Close output (power output to close CB coil) 5 - Breaker Contacts Tx - Delay to achieve breaker synchronization (calculated) Tferm - Mechanical breaker closing time (predicted) Tdel - Pre-arcing delay (planned and Para metered) VIII. COMPENSATION IN CONTROL SYSTEM When the circuit breaker has a well-known variation in its behaviour, with variations in external conditions, corrections for these can be made. Verification of systematic mechanical variations depending upon varying ambient conditions like influence of idle time, change of temperature, change of coil voltage, change of air pressure and change of SF6 gas pressure. Variations upon an ambient parameter variation can be compensated for by suitable transducers. The need for compensation will depend on the degree of variation and the actual operating conditions. For frequent operation adaptation control may be good enough to gradually take small variations into account. The system can compensate the circuit breaker s expected operating time for variations in temperature, pressures, SF6 gas pressure and auxiliary voltage supply. In this paper we only discuss SF6 gas pressure compensation for real time monitoring of sf6 gas pressure for optimization point on wave switching of sf6 circuit breaker [2]. IX. SF6 GAS COMPENSATION SF6 gas is the ideal media for the arc interruption and dielectric strength. This compensation comes into action during the closing operation; at close time SF6 gas rated pressure is 7 Kg/cm² at C temperature. Real time monitoring of SF6 gas pressure is very important for the compensation provided in the system. In table I we provide the sf6 gas compensation with the help of windows based software in this we set breaker close time of 80 msec for each phase (R, Y, B) and sf6 gas pressure is varied. In this the ideal operation is performed when sf6 gas pressure in 7 kg/cm² at C temperature As the temperature increases the sf6 gas pressure should be decreased [9]. TABLE I SF6 GAS PRESSURE COMPENSATION FOR BREAKER CLOSE TIME sf6 (kg/cm²) Breaker CLOSE time R (msec) Y (msec) B (msec) X. RESULTS OF SF6 GAS COMPENSATION ISSN: Page 2911

4 SF6 circuit breaker gives the best results during closing and opening that s why they are widely used in the power system The best about this circuit breaker is SF6 gas which quenches the arc in very less time among all other circuit breaker. Table II describes the final results we obtained when we set the coil voltage at 110 Volts and we varied the close angle i e and and as shown in Figure and 3, the error is very less i e when our closing angle is breaker closing pulse is at voltage peak and when our angle is breaker closing pulse is at voltage zero. In this we take R- Phase for reference. DC1 Voltage TABLE III SF6 GAS PRESSURE COMPENSATION SF6 Pressure R (kg/cm²) R Y B Expected Time (msec) Observed Time (msec) Error Figure. 5 R-Phase wave form when closing angle is 0 As seen from the Figure 4, when we set the closing angle at 90 degrees we get close command at the voltage peak which shows its 90 degrees. Same case in Figure 5 angle is changed from 90 degree to 0 degree and we get close command exact at 0 degrees. This shows Sf6 gas pressure compensation comes into action when we perform closing operation and gives accurate results even when we have less pressure In Figure 6, it can be seen that the expected time and observed time are very close to each other so the error is reduced to a minimum. Thus, after giving the close command to the circuit breaker so it will be closed at the expected time. Figure 4 and 5 gives the resulted wave form of the Sf6 gas pressure compensation. Fig. 6 R-Phase Close Time Characteristics with SF6 gas compensation Figure. 4 R- Phase waveform when closing angle is 90 XI. CONCLUSIONS This paper presents a method for setting the close and open targets in a controlled switching system based on sf6 gas pressure compensation which is provided with the help of windows based configuration software. The paper also highlights the importance of the sf6 gas pressure compensation as sf6 gas pressure is not in a healthy state, we cannot achieve point on wave switching. To achieve the point on wave switching in circuit breakers there are various types of compensations like idle time compensation, coil voltage compensation, temperature compensation etc. which are not described in this paper. This paper is only meant for the sf6 gas pressure compensation. Furthermore, it was confirmed, in connection with the variations in closing or opening time in response to the operating conditions, that the operating characteristics could be stably compensated over a long period by using the characteristics obtained from the factory test. ISSN: Page 2912

5 ACKNOWLEDGMENT This work is supported by the CG Global R&D Centre Crompton Greaves Ltd. The grateful acknowledgement is made to the contributions of Upendra Kambli, Neelabh Trivedi, Mayank Shukla, Rohit Jain, Rohit Arora and Roshan Joseph for giving their support in the development and testing of the work described in this paper. REFERENCES [1] F Jun Liu; Huang, G.M.; Zhiying Ma; Geng, Yingsan, "A Novel Smart High-Voltage Circuit Breaker for Smart Grid Applications," Smart Grid, IEEE Transactions on, vol.2, no.2, pp.254,264, June 2011 [2] Namjoshi Y, Auto-Calibration of a Sf6 Circuit Breaker for Point-onwave Switching an Application of Signal processing to Switchgear, International Conference on Electrical and Electronics Engineering, Goa, pp , 2013 [3] Sharma S ; Bhardwaj H, How to maintain sf6 circuit breaker, International Journal of Scientific Research Engineering &Technology, vol. 1, pp March 2012 [4] Condition monitoring of SF6 circuit breakers. by U. Akesson etal., CIGRE 1992 Session, Paper , 8 pages [5] Tsudata, H Controlled Switching System for Capacitor Bank and Transformer Switching In Electrical Engineering [6] ABB, Controlled Switching with Switchsync, Presentation manual Edition [7] H Ito Controlled Switching Technologies, State-of-the-Art, Transmission and Distribution Conference and Exhibition 2002: Asia Pacific. IEEE/PES, Pages , Vol. 2, 2002 [8] Controlled switching technologies, state-of-the-art, Ito, H.; Transmission and Distribution Conference and Exhibition 2002: Asia Pacific. IEEE/PES, Volume: 2, Publication Year: 2002, Page(s): vol.2 [9] Catalogue publication 1HSM en Controlled Switching, Buyer s Guide, Application Guide Edition , ISSN: Page 2913