EFFECT OF SFCL ON DISTRIBUTION POWER QUALITY

Transcription

1 EFFECT OF SFCL ON DISTRIBUTION POWER QUALITY by Shahram Najafi, Prof. Vijay K. Sood, Ahmed Hosny University of Ontario Institute of Technology, Oshawa, Ontario Electrical Power and Energy Conference (London-Canada) October

2 Outline Background Faults and High Currents SFCL as a Potential Solution Power Quality Voltage Sag Harmonic Distortion Case Study Model and Conditions Simulation Results Different Fault Types & SFCL Performance Switching Distortion Conclusion 2

3 Background Continuous growth of demand resulted in higher fault current levels An electrical fault occurs when current flows through an abnormal or unintended path Mitigation of fault current levels using newer technology 3

4 Potential Solution Responds to faults instantaneously, improves power quality, occupies less space, have less power losses in comparison to conventional fault current limiters Extends the life of many protective devices, more reliable and allows the usage of existing switchers and circuit breakers SFCL Operation principals and controller 4

5 SFCL Operation Principal Consisting of two or more parallel connected circuit branches RC snubber circuit is connected across each power electronic switch for mitigating the effect rate-of-change of (di/dt) during the switching ON instant The SFCL is required to have low impedance under normal conditions but to have high impedance under fault conditions The speed of the intervention must be high enough 5

6 SFCL Controller In normal operating conditions, the control scheme always triggers the IGBT ON, since modulated output wave (Vm) is higher than the sawtooth waveform (ST) Vm > ST On detection of a fault, the conducting IGBT is switched OFF and the fault current is diverted to the limiting impedance since modulated output wave (Vm) is less than the sawtooth waveform (SW) Vm < ST 6

7 System Parameters & Conditions PARAMETERS Utility Voltage VALUE 115 kv Utility Voltage Source Impedance 1% Transformer #1 Voltage 115:15 kv Transformer#1 Power & Impedance 25MVA & 4.5% Transmission Line Z 1 = j0.909 Ω, Z 0 = j4.317 Ω Transformer #2 Voltage 15:4.16 kv Transformer#2 Impedance 20MVA & 9% Load Rating 10MW, 4.16 kv & 0.92 PF lagging Initiation of fault at sec and fault lasts for 0.05 sec (3 cycle), and then cleared at sec 7

8 ONE-LINE DIAGRAM OF DISTRIBUTION NETWORK USING SFCL 8

9 SFCL Reference Current The selection of the reference current is limited to the pick-up current of over-current relays, and the maximum current interrupting capability of the IGBT And accordingly limiting impedance Zlim will be designed Using Kirchhoff s voltage law, the line current through the SFCL is given by, I lim = V Z utility T lim...(1) 9

10 SFCL Limiting Impedance Z T-lim is the total impedance from utility point up to the load including the point of fault Z Z + Z + [( Z + Z 2) // Z T lim = lim Trnsf 1+ utility load Transf fault ]...(2) Where, Z Trnsf1+utility is cumulative impedance of transformer# 1 and the utility, Z Transf2 is transformer# 2 impedance, Z load is the load impedance, Z fault is the fault impedance (in this paper the fault is assumed to be bolted with zero impedance) 10

11 SFCL Limiting Impedance Cont. But, I lim is the desired limited maximum faulted current value during fault I lim = I ratd *(desired limited value) where, I ratd is the rated current value and desired limited value is represented by a number multiplied by the pu rated current value So, the limiting impedance that will result in desired limited fault current can be calculated as: Z lim = ZT lim { ZTrnsf 1+ utility + [( Zload + ZTransf 2 ) // Z fault...(3) ]}...(4) 11

12 Line Currents: Without & With Limiter Phase a current waveform for symmetrical fault at bus B without SFCL; fault inception time = 16.67ms, t = 10µs, Z fault =0 Phase a line current waveform for symmetrical fault at bus-bar B using SFCL; fault inception time = ms, t = 10µs, Z fault =0 12

13 Current Waveforms Through SFCL A three-phase to ground (abc-g) fault with zero fault impedance is simulated at bus-bar B, just after the SFCL The Simulated current waveforms through the SFCL components are presented I IGBT I ZnO I D1-D3 I LL I D2-D4 13

14 Distortion in Voltage Waveform Voltage waveform for symmetrical fault at bus-bar B with the SFCL; fault inception time = 16.67ms, Z fault =0 With SFCL Without SFCL Voltage magnitude for symmetrical fault) at bus-bar B with/without the SFCL; fault inception time = 33.33ms, Z fault =0 14

15 Current and Voltage Distortion Although the SFCL provides the desirable current limiting function, it exhibits harmonic characteristics that need to be carefully studied The Total Harmonic Distortion (THD) in the current wave is calculated as 40.11% and that in the voltage is 125% Per-unit frequency spectrum for phase a current in case of symmetrical fault at bus-bar B, Imax = 3.55 ka Per-unit frequency spectrum for phase a voltage in case of a symmetrical fault at bus-bar B, Vmax=10.59 kv 15

16 A Single-Phase to Ground (a-g) Fault, Z fault =0 Single phase to ground (the most frequent of occurrence fault in power system) is also shown below Current waveforms through the SFCL for line-to-ground (a-g) fault at bus-bar B Voltage waveforms for line-to-ground (a-g) fault at bus-bar B Voltage waveforms for line-to-ground (a-g) fault at bus-bar C, fewer switching actions & lower magnitudes 16

17 Conclusions The SFCL has been used and implemented in this paper using EMTP program to study the impact of SFCL SFCL effectively suppressed the fault voltage and mitigated fault current which decrease the short circuit stress on the network Analyzing transient behavior of the semiconductor switch assist to improve power quality, to decrease energy dissipation and to reduce the stress on system equipment 17

18 Conclusions Cont. The SFCL however exhibits harmonic generation due to the switching of the IGBT Some alternative control circuits to alleviate this problem are under investigation In future work, the coordination of the SCFL and the existing circuit breaker elements in the studied power system will also be investigated 18

19 THANK YOU QUESTIONS? 19