1310 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 23, NO. 3, JULY 2008 Impact of Waveform Distorting Fault Current Limiters on Previously Installed Overcurrent Relays Yan Pan, Student Member, IEEE, Michael Steurer, Senior Member, IEEE, Thomas L. Baldwin, Senior Member, IEEE, and Peter G. McLaren, Fellow, IEEE Abstract—This paper investigates in detail the impacts of dis- torted current waveforms, produced by certain types of fault cur- rent limiters on time-overcurrent protection relays. A thyristor- based solid-state fault current limiter is chosen as representative of such a device for a case study which investigates its effects on two coordinated protection relays. A detailed software model of the current limiter has been developed and implemented on the real-time digital simulator platform, modeling a typical distribu- tion system. Relay models are used to obtain initial results, which are later validated by an actual protective relay connected in a hardware-in-the-loop simulation setup. The results illustrate the increase of relay tripping times due to severe current limitation caused by the fixed firing angle control of the current limiter. It is revealed that different current measurement principles employed by the relays, such as fundamental, peak, or true rms, can lead to miscoordination due to the distorted fault current waveform. It is demonstrated that these undesirable effects can be mitigated by employing appropriate control strategies for the firing angle in the current limiter. Index Terms—Distribution system, fault current limiter, grading margin, protection relay, real-time simulation. I. INTRODUCTION I N recent years, interest in new fault current limiter (FCL) technologies has increased significantly as the need for these devices has noticeably grown [1]. While many publications dis- cuss the various FCL technologies [2]–[5] only a few discuss the impact of these devices on the system [6]–[8]. Even fewer address the impact of these devices on protection systems [9], [10]. The only international body currently addressing this issue is the CIGRE WGa3.16 [11]. Amongst the many new FCL technologies investigated today, most will cause a distorted (i.e., nonsinusoidal) fault current. Only a few, which temporarily insert passive elements, such as resistive or inductive impedances into the circuit, cause nondis- torted fault currents. An example of the first type is a thyristor- based phase-controlled circuit, and an example of the second type is the resistive-type superconducting FCL [12]. Interest- ingly, this current distortion is not widely discussed, although Manuscript received January 16, 2007; revised August 17, 2007. This work was supported part by the U.S. Department of Energy, Office of Electricity De- livery and Energy Reliability, under Award DE FG02 05CH11292. Paper no. TPWRD-00021-2007. The authors are with the Center for Advanced Power System at Florida State University, Tallahassee, FL 32310 USA (e-mail: pan@caps.fsu.edu; steurer@caps.fsu.edu; baldwin@caps.fsu.edu; mclaren@caps.fsu.edu). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TPWRD.2008.919170 Fig. 1. Principle of the diode-bridge FCL with dc biased coil. significantly distorted fault currents are expected to affects the protection system. This paper examines in detail the impact of distorted fault currents caused by certain FCL technologies, such as the bridge-type FCL [13], the saturated iron core-type FCL [14], and the solid-state FCL (SSFCL) [15]. The effects of these FCLs not only include reducing the fault current amplitude but also causing waveform distortion. It is important to carefully examine the impacts such FCLs may have with respect to protection relay functionalities. The three FCLs mentioned before introduce different types of current distortions. This is further discussed in the next sec- tion. For the distorted current waveforms, the general responses of relay measurement techniques are analyzed. A case study of the thyristor-based SSFCL is investigated as a representative ex- ample. The SSFCL is inserted into a typical distribution system and its impact on coordinated overcurrent relays is tested. The distribution system, the SSFCL circuit, the overcurrent relays and the control logic are all modeled and simulated in sufficient detail in the real-time digital simulator (RTDS) [16]. To validate the results, a realistic application scenario, using real-time hardware-in-the-loop (HIL) simulation is performed by connecting a commercially available overcurrent protection relay to the power system simulator. II. CURRENT WAVEFORM DISTORTING FCLS A. Diode Bridge FCL With DC Biased Coil Due to its specific characteristics, a diode-bridge FCL with dc biased coil will cause current waveform distortions as soon as the FCL becomes active. “Active” In this context means that the FCL limits the current in the circuit. In its basic concept, as shown in Fig. 1, this FCL consists of four diodes, one dc voltage source and one inductor of significantly large impedance to limit the fault current effectively [13]. Variants of this concept employ thyristors instead of diodes and are known by the term fault current controller (FCC) [17]. 0885-8977/$25.00 © 2008 IEEE