Superconducting Fault Current Limiter THE EFFECT OF SFCL ON ELECTRIC POWER GRID WITH WIND-TURBINE GENERATION SYSTEM Superconducting Fault Current Limiter SFCL

OVERVIEW Introduction. Why SFCL? Modelling of SFCL. Doubly-Fed Induction Generator (DFIG) Characteristics. System configurations for two case studies. SFCL performance. Effect of WTGs on power grid. Effect of SFCL on OCR operation. Conclusion

INTRODUCTION With the increase of electricity demand and change of concerning environment, the capabilities of renewable energy generation systems are being expanded. Renewable energy sources considered as clean and prospective energy sources of the future world. 12% of world’s electricity is generated from wind power. The wind-turbine generation system (WTGs) is a representative renewable energy system. Does not cause pollution problem. Lowest maintenance cost. WTGs increases the level of short-circuit current during a fault in a distribution system. A high temperature superconducting fault current limiter (SFCL) can be solution to reduce the level of short-circuit current during fault.

Why SFCL? Superconducting Fault Current Limiter (SFCL): Reduces the peak value of fault current. Improves the transient stability of the power system. Provides the system effective damping for low- frequency oscillations. Causes no power loss in steady-state condition.

Connection of SFCL to an electric power grid: Optimal place to install the SFCL. Optimal resistive value of the SFCL occurred in series with a transmission line during a short circuit fault. Potential protection-coordination problem with other existing protective devices such as circuit breakers.

Modelling Of SFCL Modelling of a resistive (non-inductive winding) SFCL. Consists of: Stabilizer resistance of the n-th unit, Rns. Superconductor resistance of the n-th unit, Rnc(t) connected with Rns in parallel. Coil inductance of the n-th unit, Ln.

Structure of a resistive SFCL:

Normal steady state condition, the values of Rnc(t) and Rns(t) are normally zero. Total resistance of parallel connection becomes zero in steady state condition. Total resistance (Rsfcl) of the SFCL during fault depends on total number of units in series. Value of Ln determined by the wound coils. Coil wound to have very small inductivity.

Expression to describe quenching and recovery characteristics: Rm - Maximum resistance of superconducting coil in the normal state Tsc - Time constant of transition from superconducting state to normal state (Tsc = 1ms)

Doubly-Fed Induction Generator (DFIG) Induction generator widely used as wind generators Brushless and rugged construction. Low cost. Maintenance free. Operational Simplicity.

Two types of wind generator topologies : Fixed speed wind generator. Variable speed wind generator. DFIG mostly used as variable speed wind generator. DFIG wind turbines are based on wound-rotor induction machines where rotor circuit is fed through back-to-back voltage source converters.

General control scheme for the DFIG system in Power Factory:

DFIG generator model is a built-in model which integrates the induction machine and rotor-side converter (RSC). DFIG and RSC modelled in rotor reference frame (RRF) rotating at generator speed. RSC controller operates in a stator flux-oriented reference frame (SFRF) rotating at synchronous speed. RSC control modifies the active (P) and reactive (Q) power by regulating the q- and d-axis rotor currents.

Equations for modelling DFIG : - flux linkage - base angular speed - stator electrical angular speed subscripts, d and q represent the direct and quadrature axis. subscripts, r and s denote the rotor and stator quantities.

Active and reactive powers transferred to stator, can be computed by :

System Configurations For Two Case Studies Single-Machine Infinite Bus System (SMIB) The SMIB system with the DFIG based WTGS

System consists of one DFIG and a transmission system connected to an infinite bus. SFCL is located between the DFIG and the infinite bus. Power scale of wind farm by the DFIG is 9MW. Capacitive bank is used. Wind speed is fixed as 10 m/s. Rsfcl of 0.2 used in normal stage of SFCL. High value not accepted. Rsfcl reduces the level of short circuit current.

Proper value of SFCL will guarantee enough time intervals for adequate protective coordination between multiple OCRs. Too small value of Rsfcl : SFCL cannot provide desirable damping performance for low- frequency oscillations during a fault.

IEEE Benchmarked four-machine two-area test system System consists of AREA 1 and AREA 2 linked together by two 230kv transmission line of lengths 220km. Each area equipped with two identical synchronous generators of 20 kv/900 MVA. Each generator produces active power of 700 MW. Active power provided from G1 and G2 mostly consumed by LOAD1. Remaining transferred from AREA 1 to AREA 2.

IEEE benchmark four machine two-area system with the WTGs

SFCL located next to Wind Turbine Generation System (WTGs) based on DFIG of 9 MW connected to bus 4 in AREA 2. A 100 ms three-phase short-circuit current applied to bus 4 at 0.1s, performance of SFCL to improve low-frequency oscillation damping is evaluated. Wind speed is fixed as 10 m/s. Resistance of Rsfcl is 30 in normal stage of SFCL.

SFCL Performance Case study on SMIB system Responses of rotor speed, terminal voltage, and output active power of the WTGs when a 100 ms three-phase short-circuit applied to infinite bus at 0.1 s.

SMIB system without SFCL : Rotor speed increased by the fault and settles slowly after clearing the fault. Terminal voltage drops from 1 pu to 0.4 pu Output active power response shows large oscillations from 2 pu to 1.2 pu. a-phase current of WTGs increases to 2.34 pu at its maximum.

SMIB system with SFCL : Variations of output active power dramatically reduced and rapidly damped. Power scale of WTGs can be extended by the SFCL. a-phase current almost same as that of steady state operation.

Response of a-phase current of WTGs

Case study on Multi-Machine Power system Responses of rotor speed, terminal voltage, and output active power of the WTGs when a 100 ms three- phase short-circuit applied to bus 4 at 0.1 s.

System without SFCL : Rotor speed increases up to 1.0035 pu at its maximum. Takes very long time to restore its original steady-state value. Transient variations for the terminal voltage and active power responses of WTGs are more severe . Peak value of a-phase current increases up to 2.2 pu after the fault.

System with SFCL : Peak value of a-phase current is 0.51 pu. SFCL extends the scale of WTGs in multi-machine power systems.

Response of a-phase current of WTGs

Effect Of WTGs On Power Grid WTGs increase the level of short-circuit current if there are no any protective devices. Response of a-phase current with and without WTGs for first case study

Response of a-phase current flowing from bus 4 for the second case study

Peak value of the fault current from 0. 1 s to 0 Peak value of the fault current from 0.1 s to 0.2 s without WTGs is less than that with WTGs for both case studies. System more sensitive to fault.

Effect Of SFCL On Overcurrent Relay Operation Overcurrent relay (OCR) is a protective relay. OCRs must guarantee : Fast operation Reliability Selectivity OCRs has inverse time characteristics. OCRs has specific rated short-circuit current.

(a) Distribution system with the DG, the SFCL, and two OCRs (b) Operation characteristics of OCR-1 and OCR-2 according to the alteration of the power system

Multiple OCRs have required coordination time in steady-state condition. If short circuit current increases over a rated value of OCR-2, results in its malfunction. To solve the problem : SFCL is applied with the distribution generation. SFCL reduce the level of short-circuit current during a fault. Coordination time of OCRs is restored. OCRs can operate properly.

CONCLUSION SFCL provides quick system protection during a severe fault. Effectiveness of SFCL as protective device is verified with several case studies. Simulation results showed that SFCL reduces the level of short-circuit current which is increased by WTGs. Provides main advantage to avoid changing the ratings of multiple protective devices.

REFERENCES S. M. Muyeen, R. Takahashi, M. H. Ali, T. Murata, and J. Tamura, “Transient stability augmentation of power system including wind farms by using ECS,” IEEE Trans. Power Syst., vol. 23, no. 3, pp. 1179–1187, Aug. 2008. M. Kayikci and J. V. Milanovic, “Assessing transient response of DFIG-based wind plants-the influence of model simplifications and parameters,” IEEE Trans. Power Syst., vol. 23, no. 2, pp. 545–554, May 2008. L. Ye, M. Majoros, T. Coombs, and A. M. Campbell, “System studies of the superconducting fault current limiter in electrical distribution grid,” IEEE Trans. Appl. Supercond., vol. 17, no. 1, pp. 2339–2342, Jun. 2007. B. C. Sung, D. K. Park, J.-W. Park, and T. K. Ko, “Study on optimal location of a resistive SFCL applied to an electric power grid,” IEEE Trans. Appl. Supercond., vol. 19, no. 3, pp. 2048–2052, Jun. 2009.