Measurement, Modeling and Simulation of Capacitor Bank Switching Transients

Transcription

1 Measurement, Modeling and Simulation of Capacitor Bank Switching Transients Mirza Softić*, Amir Tokić**, Ivo Uglešić*** *Kreka - Dubrave e, Dubrave, Bosnia and Herzegovina ( softic_mirza@yahoo.com). ** Faculty of Electrical Engineering, University of Tuzla, Tuzla, Bosnia and Herzegovina ( amir.tokic@untz.ba) *** Faculty of Electrical Engineering and Computing, University of Zagreb, Zagreb, Croatia ( ivo.uglesic@fer.hr) Abstract: This paper presents the results of experimental and simulated investigations of electromagnetic transient phenomena during energizing of industry. Experimental and simulated investigations based on the electrical network model having the nominal voltage of 6 kv are carried out. In addition, sensitive analyses of characteristic impact factors are performed. It is shown how capacitor banks switching transients influence a degradation of the power quality in electrical distribution system. Keywords: Power quality,, switching transients, overcurrents, overvoltages. 1. INTRODUCTION Power quality is a topic of constant study as problem inherent to it can lead to economical losses, mainly in industrial processes. Although many factors influence the power quality, the paper presented here focuses on electromagnetic switching transients originating from capacitor bank switching in typical mining distribution systems, Adams et al. (1998), Bollen et al. (2006), Grebe (1996), McCoy et al. (1994). Two main advantages of connecting are: improvement of the network s voltage profile and reducing the network s losses. In general, these capacitors are not connected all of the time, since the network loads are changing with time according to certain load curves. Hence, they may be switched on and off several times during a typical day. These switching actions will be accompanied by low or medium frequency of electromagnetic transient voltages and currents which may have an influence on sensitive electrical equipment connected in local networks. The switching provokes transient overvoltages that theoretically can reach peak phase-to-phase values of 2.0 p.u., Saided (2004). Generally, the frequency of switching transients is below 2 khz. Other factors that affect amplification of the transient voltages during the banks switching should also be mentioned: size of the switched, short circuit capacity at the location, rated power of the distribution transformer and characteristics of the connected loads. transient during energizing of industry is calculated by the equation ( L S >> L) : L s inductance of system C capacity of L inductance of. 2. EXPERIMENTAL MEASUREMENTS Characteristics of electromagnetic transients originating from industry switching are studied in this paper. Moreover, factors that influence the intensity of such transients are investigated in order to identify the conditions in which these effects can be undermined. It should be pointed out that the electrical network which represents a real-life feeder of the typical 6 kv mining distribution system is investigated in this paper. The feeder supplies mining loads (total power is approximately 2.5 MVA) with installed three phase, star connected, (rated power 500 kvar), Fig. 1. The results of experimental investigations of switching transients are presented in this chapter. Current waveforms and phase to phase voltage waveforms are measured during energizing of three phase. f = 2π 1 ( L + L) C 2π L C S 1 S (1) where are: Fig. 1. Simplified configuration of 6 kv electrical network

2 Phase current waveforms and phase to phase voltage waveforms during energizing of 500 kvar at the 6 kv electrical network with isolated neutral point are presented in Fig. 2 and Fig MODELING AND SIMULATIONS Simulations of electromagnetic transient phenomena during energizing of are carried out in electrical networks of 6 kv. An equivalent three phase electrical model is implemented within the MATLAB/Simulink software. Here are being modelled: power system equivalent, industrial loads, distribution transformer, breaker, supply cable, equivalent and protection inductors. A stiff differential equation system, describing the behaviour of three phase switching transient, is solved by using the numerical method of L stable backward differentiation formulas (BDFs), Tokić et al. (2005). The results of simulating phase current waveforms and phase to phase voltage waveforms during energizing of three phase 500 kvar in electrical networks of 6 kv with isolated neutral point are presented in Fig. 4 and Fig. 5. Fig. 2. Measured phase currents during energizing of Currents [simulation] I1 I2 I3 400 Current [A] Time [sec] Fig. 4. Simulated phase currents during energizing of Fig. 3. Measured phase to phase voltages during energizing of 1.5 x Voltages [simulation] U12 U23 U31 Values of characteristic parameters of phase transient currents and phase to phase transient voltage waveforms (amplitude, duration and frequency) obtained as a result of experimental measurements during energizing of a 500 kvar three phase at the 6 kv mining electrical network with isolated neutral point are presented in Table 1. Table 1. Characteristics of transient during energizing of a 500 kvar three phase (measurements) 266/ / / Voltage [V] Time [sec] Fig. 5. Simulated phase to phase voltages during energizing of Values of characteristic parameters of phase transient currents and phase to phase transient voltage waveforms (amplitude, duration and frequency) resulting from the simulation during switching/energizing of a 500 kvar three phase at the typical 6 kv mining electrical network with isolated neutral point are presented in Table 2.

3 Table 2. Characteristics of transient during energizing of a 500 kvar three phase (simulations) 285/ / / Waveforms of phase currents and phase to phase voltages obtained as a result of experimental measurements and simulations during energizing of a 500 kvar at the 6 kv network are in very good agreement. It should be noted there are no many papers that show both measured and simulated three phase currents and voltages during industry energizing. Fig. 6. Simulated phase currents during energizing of 4. SENSITIVE ANALYSIS OF THE SYSTEM PARAMETERS Based on an implemented equivalent model, different simulation scenarios of system parameters are investigated. This chapter focuses on the sensitive analysis of influential parameters within a three phase system by observing characteristic parameters of phase transient currents and phase to phase transient voltage waveforms (amplitude, duration, frequency). Influential parameters on a three-phase system are: initial conditions, impedance of system, consumers characteristics, capacity of and moment of circuit breaker switching. The important parameters of a three-phase system had the following values: peak of system voltage/per phase: U m = 8000 V Fig. 7. Simulated phase to phase voltages during energizing of Second case: Reducing the system impedance twice in the equivalent three-phase electrical simulation model. in Fig. 8 and Fig. 9. initial conditions (residual voltage of at the moment of circuit breaker switching): U res = 1 kv impedance of system: R = Ω, L = 0.55 mh consumers characteristics: S = 2.5 MVA capacity of /per phase: C = 36.5 µf moment of circuit breaker switching: T 0 = ms First case: Disregarding the residual voltage in the equivalent three-phase electrical simulation model. in Fig. 6 and Fig. 7. Fig. 8. Simulated phase currents during energizing of capacitor bank

4 in Fig. 12 and Fig. 13. Fig. 9. Simulated phase to phase voltages during energizing of Third case: Increasing the industrial load twice in the equivalent three-phase electrical simulation model. in Fig. 10 and Fig. 11. Fig. 12. Simulated phase currents during energizing of Fig. 10. Simulated phase currents during energizing of Fig. 13. Simulated phase to phase voltages during energizing of Fifth case: Moving the moment of circuit breaker switching in phase 2 for 0.1 ms (pole asynchronism of circuit breaker switching) in the equivalent three-phase simulation model. in Fig. 14 and Fig. 15. Fig. 11. Simulated phase to phase voltages during energizing of capacitor bank Fourth case: Increasing the capacity of capacitor bank twice in the equivalent three-phase electrical simulation model. Fig. 14. Simulated phase currents during energizing of

5 Fig. 15. Simulated phase to phase voltages during energizing of The results of investigation change the initial conditions (residual voltage), serial system impedance, consumers characteristics, capacity of and pole moments of circuit breaker switching into the values of characteristic parameters of phase transient currents and phase to phase transient voltage waveforms, as presented in Table 3. Table 3. The results of investigation change the system parameters: different scenarios (a) S = 500 kvar 285/ / / (b) U res = 0 V 185/ / / (c) z = z/2 V 383/ / / (d) S = 2S 260/ / / (e) C = 2C 450/ / / (f) T breaker asynchronism 2060/ / / The collected simulation results, for different electrical system scenarios, are shown in Table 3. It can be concluded that the last case represents the most critical case where the worst-case transient phenomena in terms of amplitude overvoltages and overcurrents transients occur. High values of amplitude overvoltages and overcurrents transients result from induced waveforms of voltages in phase 1 and phase 3, generated by the waveform of voltage in phase 2 at the moment of three phase circuit breaker switching. In order to reduce overvoltages and overcurrent transient peak values in three phase system, the following measures can be applied: preresistors/inductors adding, fixed inductors (reactors) and applying of controlled (intelligent) switching. A fixed inductor (reactor) of rated inductance 125 µh can be applied in real life to reduce the overvoltages and overcurrent transients during energizing of a 500 kvar three phase. Practical experience have showed to be very effective in case of electromagnetic transients mitigation generated by energizing of a 500 kvar three phase capacitor banks at the typical 6 kv mining electrical network.

6 5. CONCLUSIONS Based on the results of experimental measurements and simulations of electromagnetic transient phenomena during energizing of three phase industry, the amplitude of overvoltages occurs in phase to phase voltage U 23 and approaches the value 2U max, whereas the amplitude of overcurrents occurs in phase current I 3, that is times greater than the current amplitude of a in the steady state I s. electromagnetic transient phenomena during energizing of three phase industry is approximately 8 ms, that is less than one time period of the system. electromagnetic transient phenomena during energizing of three phase industry is 1124 Hz. On the basis of characteristic parameters values of phase transient currents and phase to phase transient voltage waveforms (amplitude, duration and frequency) obtained as a result of experimental measurements and simulations during energizing of a 500 kvar three phase at the typical 6 kv mining electrical network with isolated neutral point, it can be concluded that the transient phenomena are classified as medium frequency electromagnetic transients. REFERENCES Adams, R.A. and Middlekauff, S.W. (1998). Solving customer power quality problems due to voltage magifications. IEEE Transaction on Power Delivery, volume 13 (number 4), Bollen, M.H. and Gu, I. Y. (2006). Signal processing of power quality disturbances, IEEE Press, Wiley, New York. Grebe, T.E. (1996). Application of distribution system and their impact on power quality. IEEE Transaction on Industry Applications, volume 32 (number 3), McCoy, C.E. and Floryancic B.L. (1994). Characteristics and measurements of capacitor switching at medium voltage distribution level. IEEE Transaction on Industry Applications, volume 30 (number 6), Saided, M.M. (2004). Capacitor switching transients: analysis and proposed technique for identifying capacitor size and location. IEEE Transaction on Power Delivery, volume 19 (number 2), Tokić, A., Madžarević V. And Uglešić I. (2005). Numerical calculations of three-phase transformer transients. IEEE Transaction on Power Delivery, volume 20 (number 4),