IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 19, NO. 3, JULY2004 1267 SMES for Protection of Distributed Critical Loads M. V. Aware and D. Sutanto, Senior Member, IEEE Abstract—This paper proposes the use of a micro high-tem- perature superconducting magnetic energy storage system (HTS- SMES) to support critical industrial loads connected to the cus- tomer’s 11 kV load bus with a ride-through capability of around 20 cycles. With the advances of power electronics and digital signal processing technology, such a system can also be utilized to improve the power quality of the power system. A novel control method- ology is proposed to regulate the SMES discharge to extend the support time to critical loads during a short-term disturbance in the distribution network. Using a P-Q diagram, the analysis of the power flow from the energy storage system to the power system is presented. The proposed hysteresis controller with SMES in- creases the operating area in P-Q plane as compared to the con- ventional d-q controlled scheme used in the literature. This allows the system capability to be utilized to its maximum thermal limits. The scheme also has the capability to control the real and reactive power flow through the converter using the proposed hysteresis current control scheme. This provides the capability to optimize the available energy storage by using load priority scheduling. The detailed three-phase system simulation is carried out using realistic models of the power electronics components used in the converters. Laboratory test results on the prototype are presented to validate the proposed control scheme. Index Terms—Critical loads, hysteresis control, power quality, superconducting magnetic energy storage (SMES). NOMENCLATURE C DC link capacitor (in farads). CB1, CB2 Circuit breakers 1 and 2. Total energy storage in SMES coil (in joules). Deliverable energy in SMES coil (in joules). Minimum coil current at the end of discharge (in amps). Nominal current in the SMES coil (in amps). Controlled ac source current (in amps). Three phase inverter currents (in amps). Coil reference current (in amps). Three phase source currents (in amps). Load current (in amps). L SMES coil inductance (in henry). Source inductance (in henry). Inverter interfacing inductance (in henry). Modulation index Manuscript received December 28, 2002. This work was supported by the RGC of Hong Kong (Project code: PolyU 5114/01E) and the Hong Kong Poly- technic University. The authors are with the Department of Electrical Engineering, Hong Kong Polytechnic University, Kowloon, Hong Kong (e-mail: eemaware@polyu.edu.hk; eesutant@polyu.edu.hk). Digital Object Identifier 10.1109/TPWRD.2003.823176 Total load (in watts). Critical load (in watts). Noncritical load (in watts). Real power of source, inverter, and load (in watts). Reactive power of source, inverter, and load (in var.) Inverter interfacing resistance (in ohms). Total power of source, inverter and load (in volt amperes). Switching signals for inverter. Switching signals for chopper. Protection time (in seconds). AC source voltage varying with time (in volts). AC source voltage of three phase a, b, c (in volts). Inverter output voltage (in volts). Inverter output voltages of three phases a, b, c (in volts). DC link voltage (in volts). DC reference voltage (in volts). V Voltage across the SMES coil (in volts). Depth of discharge. I. INTRODUCTION I N many industrial plants, some loads are very critical and any short-term power losses or supply voltage disturbance could result in significant financial losses. To overcome this, diesel generators or uninterruptible power supplies are used to support the critical loads. However diesel generators have the disadvantage of slow response time during startup and the unin- terruptible power supplies suffers from the dynamics of the bat- teries and the associated controllers [1], [2]. An attractive and a potentially cost-effective option is to use the high-temperature superconductor magnetic energy storage (HTS-SMES) system to provide a short-term buffer during the disturbance. In order to be commercially competitive with other power quality solu- tions, the HTS-SMES must be optimized in its operation both from the system point of view and the criticality of the load. Its life cycle cost should also be minimized. At substations, the typical fault clearing time is around 6 to 20 cycles [3]. An HTS-SMES system with protection time (ride through capability) of 1 s can be sufficient for the protection of all critical loads connected to a distribution bus. A micro HTS-SMES system is capable of supplying such a short-term premium power that can protect critical loads from more than 95% of credible events, which would otherwise interrupt the normal operation of the equipment. A typical integrated HTS-SMES unit connected to a load bus is shown in Fig. 1. 0885-8977/04$20.00 © 2004 IEEE