Presented by:
Manoharan.S.

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Mitigation of Power Quality Issues
(Transient suppression by SMES system)
Abstract — This paper presents the modeling and simulation results of a superconducting magnetic energy storage (SMES) system for power transmission applications. This is the largest SMES coil ever built for power utility applications and has the following unique design characteristics: 50 MW (96 MW peak), 100 MJ, 24 kV dc interface. As a consequence of the high-power and high-voltage interface, special care needs to be taken with overvoltages that can stress the insulation of the SMES coil, especially in its cryogenic operating environment. The transient overvoltages impressed on the SMES coil are the focus of this investigation. Suppression methods were also studied to minimize transients. The simulation is based on detailed coil and multiphase gate turn-off (GTO)-based chopper models. The study was performed to assist in the design of the SMES coil insulation, transient protection, and the power electronics specification and interface requirements.
I. INTRODUCTION
OWER systems have been experiencing dramatic changes in electric power generation, transmission, distribution,
and end-user facilities. Continuing electric load growth and
growing power transfer in a largely interconnected network lead to complex and less secure power system operation. Certain factors such as technical, economical, environmental, and governmental regulation constraints put a limitation on power-system planning and operation. Recent developments and advances in both superconducting and power electronics technology have made the application of superconducting magnetic energy storage (SMES) systems a viable choice to solve some of the problems experienced in power systems.
SMES is a technology that has the potential to bring essential functional characteristics to the utility transmission and distribution systems. A SMES system consists of a su- perconducting coil, the cryogenic system, and the power conversion or conditioning system with control and protection functions. Because of its fast response to power demand and high-efficiency features, it has the capability of providing: 1) frequency support (spinning reserve) during loss of generation; 2) transient and dynamic stability by damping transmission line oscillations; 3) dynamic voltage support; and 4) automatic generation control,thus enhancing security, reliability, power quality, and transmission capacity. SMES systems have received considerable attention by electric utilities and government due to their attractive performance characteristics and potential benefits.
The purpose of this study is to investigate the electromagnetic transient interactions between a superconducting coil and the power electronics interface for a flexible ac transmission systems (FACTS) application. Understanding the transient phenomena associated with an SMES system is essential in this investigation. Transient overvoltages can endanger the insulation of a superconducting coil, especially in its cryogenic operating environment, where the insulation characteristics are different from that at normal conditions. The transients may originate from normal or abnormal SMES switching operations and/or faults or lightning and switching surges from the ac and dc systems. They usually take place for a very short time as compared to the steady state, but have the potential to stress the coil insulation. The understanding of the possible transient overvoltages the SMES coil will be subjected to is essential in the design of its insulation and transient suppression schemes. The high-power and high-voltage dc interface of this particular design poses significant design challenges, which need to be well understood and adequate solutions need to be proposed. The transients associated with a SMES coil, which is interfaced with a gate turn-off (GTO) thyristor-based chopper, were simulated using an electromagnetic transient program EMTDCTM (electromagnetic transients for dc). The fol- lowing transient suppression schemes were investigated to minimize the transient overvoltages: 1) adding filtering/surge capacitor; 2) adding metal oxide varistor (MOV) elements;
3) Changing the current sharing inductances. In addition, grounding resistors are used to reduce the terminal to ground voltage stress
Section II gives an overview of a general SMES system and transient modeling and simulation concerns. The SMES coil model will be described in Sections III. The transient simulation and suppression scheme study results will be given in Sections IV. The last section will summarize the results of this study.
II. OVERVIEW ON SMES AND TRANSIENT SIMULATION CONCERNS
A SMES system connected to a power system consists of a superconducting inductor (the SMES coil), a cryogenic system, and a power conditioning system (PCS) with control and protection functions. The power conditioning system is also referred to as the power electronics interface of the SMES coil. Fig. 1 shows the general structure of a SMES system.
The SMES coil is charged or discharged by making voltage across the coil— positive or negative. The SMES system enters a standby mode operation when the average is zero and the average coil current is constant.
The PCS transfers energy into or out of the SMES coil on commands to control real and reactive power flow. Several different types of PCS have been developed or proposed. A double six-pulsed thyristor Graetz Bridge was the first applied. One of its limitations is that it cannot provide simultaneous real and reactive power control. Two GTO-based PCS’s were proposed for an engineering test SMES model. The one with two six-pulse current source inverters (CSI’s) can provide independent power control in a narrow range, but may cause serious harmonic generation. The second one is a voltage source inverter that consists of an ac/dc converter, a dc page link capacitor, and a multiphase chopper. This latter type has received more attention in SMES applications due to its capability of generating reactive power independent of the coil current.
The design of SMES coil insulation and protection systems
plays an important role in avoiding SMES coil failures. A SMES coil is subjected to transients produced by the switching
operations of semiconductor devices in the PCS as well as fault, lightning, and switching surges coming from the ac system. A SMES coil may experience unexpected failure if the behaviors of the transient overvoltages associated with the coil are not well understood.
Most power system transients are considered oscillatory in nature where the frequency range is between 1 Hz–1
MHz. For low-frequency range simulation (from dc up to several kilohertz), general models in electromagnetic transient simulation programs such as EMTPTM, EMTDCTM can be used. For high-frequency transient studies such as the study of impulse propagation in a component, a more detailed modeling of the component is necessary.
III. MODELING OF THE SMES COIL
The structure of the SMES coil is illustrated in Fig. 2(a). The entire SMES coil has a height/ width ratio of 3.66 m (144 in)/1.53 m (60 in) made of 48 double pancakes.
Each double pancake has 40 turns. In order to reduce the computational burden, an equivalent circuit of the coil wa
represented by a six-segment model comprised of self induc- tances, mutual couplings, ac loss resistances, and series and shunt capacitances, as shown in Fig. 2(b). Including the mutual couplings between segments was to obtain more accurate frequency and voltage response. The coil parameters ( and ) were computed for each turn and then approximated to the segment level. The formulas developed by Miki et al. and Lyle are used to compute the self inductances and mutual inductances between turns, respectively. The computations of series and shunt capacitances were based on the parallel-plate model.
Frequency scan analysis was performed to predict resonant frequencies. The result showing the magnitude of the coil terminal voltage versus frequency is given in Fig. 3. As can be seen, the coil has several resonance frequencies, parallel resonances at frequencies around 60Hz, 400Hz, 890Hz, and series resonances at 280Hz, 830Hz, which can lead to magni- fication of transients. Since the SMES coil has a rather high inductance of 12.5H, the resonance frequencies of the coil are relatively low.