Modular based design for a small to medium scale toroidal type SMES

A small to medium scale SMES (superconducting magnetic energy storage) is one of the most attractive devices as a controller in electric power systems. This paper proposes one design strategy for a small to medium scale toroidal type SMES based on the idea of a modular structure of a superconducting toroidal coil and a power conditioning system. The idea of the modular structure, an example of design of SMES using the proposed idea, and characteristics of the designed system are presented. The power conditioning system using a voltage-source type ac/dc converter circuit and a chopper circuit is presented for this structure     

Transient modeling and simulation of a SMES coil and the powerelectronics interface

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     

30-MJ superconducting magnetic energy storage system for electric utility transmission stabilization

A superconducting magnetic energy storage (SMES) system has been built to damp power oscillations on the Western U.S. Power System, particularly on the Pacific AC Intertie that is used to transmit power from the Northwest to southern California. The 30-MJ superconducting inductor that stores energy for this purpose is contained in a nonconducting dewar and is supported by a helium refrigerator and a gas-handling system mounted on trailers. Energy flows in and out of the inductor at frequencies from 0.1 to 1.0 Hz with power amplitudes up to 11 MW. The principal oscillation to be damped has a characteristic frequency of 0.35 Hz. The superconducting coil maximum current is 5 kA with terminal voltages up to 2.2 kV. The coil interfaces with the Bonneville Power Administration 13.8-kV bus at the Tacoma Substation through a converter and transformers. The system can be operated with the converter either in parallel-bridge mode or for constant VAR control with the bridges in buck-boost mode. The program for the design, fabrication, installation, and the preliminary experimental operation of the system is reviewed.     

Superconducting magnetic energy storage

Superconducting magnetic energy storage (SMES) is unique among the technologies proposed for diurnal energy storage for the electric utilities in that there is no conversion of the electrical energy, which is stored directly as a circulating current in a large superconducting magnet, into another energy form such as mechanical, thermal, or chemical. Thus one advantage of SMES is the inherent high storage efficiency that is possible because energy conversion processes are avoided. The actual round-trip efficiency of a large unit is expected to be 90 percent or greater. The fast response (< 100 ms) of the system to power demand means that a diurnal storage unit can also function as a swing generator or provide system stabilization. The major components of a SMES system are a large superconducting coil cooled by liquid helium, an ac-to-dc convertor, and a refrigerator that maintains the temperature of the helium coolant. This paper describes the design and functions of these and other components of an engineering reference design for a 1-GWh SMES unit. Also included is a sketch of the historical development of superconductivity, which was first discovered in 1911, and SMES, which was first proposed as a method of diurnal storage in 1969.     

400 MW SMES power conditioning system development and simulation

A conceptual design for a 22 MWh superconducting magnetic energy storage (SMES) system engineering test model (ETM) was developed. The objectives of the SMES-ETM are to demonstrate the feasibility of using a SMES system to perform load-leveling and system stabilization for commercial utilities and to supply 400 MW power pulses for ground-based defense systems. The performance requirements and configuration of the proposed 22 MWh SMES-ETM and its power conditioning system are presented. The power conditioning system consists of a DC-DC chopper linked to a GTO-based voltage source converter interfacing the superconducting energy storage coil to the AC power system. The SMES system operation in the charging and discharging modes is described and the results of digital simulations demonstrating the feasibility of the proposed power conditioning system and exploring its overall behavior under normal and fault conditions are prevented     

Development of a Conduction-Cooled HTS SMES

A 35-kJ/7-kW conduction-cooled high-temperature superconductor (HTS) superconducting magnetic energy storage (SMES) is developed. This paper presents the configuration of the HTS SMES and a part of important test results. The magnet of the SMES is a solenoid-type coil made of Bi-2223/Ag tapes, and cooled to about 20 K by conduction-cooled method. The SMES is assembled in the Electric Power System Dynamic Simulation Laboratory, Huazhong University of Science and Technology. A series of tests are successfully carried out to evaluate the performance of the SMES, including the current-carrying ability of the magnet, the active and reactive power exchange capability between the SMES and a simulated power system, as well as the power-compensating effect of the SMES to the simulated power system after short-circuit fault, etc.     

Tests on a 200 kA cable-in-conduit conductor for SMES application

This paper describes the tests conducted to characterize the performance of the 200 kA conductor used in the Bechtel Team's SMES (superconducting magnetic energy storage) design. The CICC-type conductor was tested at operating temperature (1.8 K) and field by placing the sample in the bore of a dipole magnet and simultaneously ramping the current and the background field to simulate operation along the load line. The test sample and its return cables were the secondary of a superconducting transformer rated at 300 kA. Two test series were conducted with slightly different samples: one in which the cable had been soldered, the other without the solder. In both cases, the first quench along the load line occurred at 110% of the rated operating current. The training behavior and the observed critical current curve were slightly different for the samples. The tests validated the conductor design for application in the SMES coil, and also yielded a world record in current through a superconductor, reaching 303 kA along the 4.5 tesla load line     

Development of a protection system for an experimental SMES device

The development and testing of a quench detection and protection system for a small, liquid helium cooled, superconducting coil are described. The coil is being used as part of an experimental superconducting magnetic energy storage (SMES) device for power system applications. Special requirements are placed on the detection and protection system of such a coil as rapid energy exchanges are frequent. The detection signal is derived from a combination of the main coil voltage and a search coil voltage. The protection method is by way of an external parallel resistor. The problem of opening a circuit breaker in series with the coil is analyzed. Actual quench recordings are used to demonstrate the effectiveness of the system. The selection of the dump resistor value is considered     

A preliminary study of mechanical switch using high temperature superconducting material

A persistent current switch (PCS) is a key component for superconducting magnetic energy storage (SMES) system because it is used to switch a superconducting coil from storage mode to transfer mode. In previous research on a mechanical PCS for SMES, a metallic superconductor made of Nb and NbTi is used as a switching material. However, recently the research of SMES coil of a high-temperature superconductor (HTS) is widely carried out, in which a PCS is also expected to work in liquid nitrogen temperature. In this paper, yttrium barium copper oxide (YBCO) bulk of HTS was proposed as a switch material, and the test on the fundamental characteristics of a mechanical contact between two YBCO bulks was carried out. As a result, a switching phenomenon between low and high resistance ranges is observed at a threshold current value in the V-I characteristics of the YBCO contact. This phenomenon is also observed after the break and make action is carried out in liquid nitrogen. It means a possibility of a superconducting mechanical switch using YBCO bulks. Furthermore, the tests indicate that a certain amount of current needs to flow before the switching phenomenon is obtained.     

Theoretical studies and experimental results of a SMES used in apulsed current supply

A superconducting magnet energy storage (SMES) can be used as a pulsed power supply. A superconducting coil stores energy without electrical losses and this energy can be recovered through a second wire on which the charge (electromagnetic launcher, for example) is linked. The design of such an apparatus needs to solve simultaneously thermal, magnetic, and electric equations. We proposed a three-dimensional finite difference method to solve these coupled problems. This tool enables us to describe resistive zones of expansion in thick coils during a quench and to predict the duration and the efficiency of the discharge. Moreover, it indicates if the coil is prevented from an excessive temperature increase. Then, a probative device is described and experimental results are compared with theoretical ones     

Electric power applications of superconductivity

The development of superconducting systems for electric power is driven by the promise of improved efficiency, smaller size, and reduced weight as compared to existing technologies and by the possibility of new applications. Superconducting power components can also contribute to improved power quality and increased system reliability. This paper addresses historical developments and technology status of four superconducting power applications: cables, superconducting magnetic energy storage (SMES), fault-current limiters, and transformers. Today, SMES is the only fully functional superconducting system and it has seen only limited use at grid power levels. A few model or demonstration units exist for each of the other three applications. Superconductivity faces several hurdles on the path to widespread use. Perhaps the most important is the need for operating voltages of 100 kV or more. Though progress in this and other areas has been rapid, considerable development is needed before superconducting devices perform reliably in the utility environment. As a result, today, most initial installations are aimed at niche applications and will be installed where space is limited, where power demands are increasing over existing corridors, and/or where initial development costs can be offset by enhanced power grid performance.     

Control strategies for multiple parallel current-source convertersof SMES system

Multiple structured current-source converters are applied for a superconducting magnetic energy storage (SMES) system. Suitable control method and control block diagram are proposed by taking much use of the advantage of this multiple structure. A multi-modular pulse width modulation (PWM) control strategy of current-source converters for the purpose of higher efficiency and less harmonic distortion is developed with an idea for practical application. A design of a multi-reduced instruction set computer (RISC) controller is presented to put the proposed control blocks into practice. The results of the digital simulation by Electromagnetic Transients Program (EMTP) and the experimental results of the test module are provided to validate the proposed control method of SMES power conditioning     

Neural Network and Bandless Hysteresis Approach to Control Switched Capacitor Active Power Filter for Reduction of Harmonics

This paper proposes a combination of neural network and a bandless hysteresis controller, for a switched capacitor active power filter (SCAPF), to improve line power factor and to reduce line current harmonics. The proposed active power filter controller forces the supply current to be sinusoidal, in phase with line voltage, and has low current harmonics. Two main controls are proposed for it: neural network detection of harmonics and bandless digital hysteresis switching algorithm. A mathematical algorithm and a suitable learning rate determine the filter's optimal operation. A digital signal controller (TMS320F2812) verifies the proposed SCAPF, implementing the neural network and bandless hysteresis algorithms. A laboratory SCAPF system is built to test its feasibility. Simulation and experimental results are provided to verify performance of the proposed SCAPF system.     

Comparative analysis of fixed and sinusoidal band hysteresiscurrent controllers for voltage source inverters

A hysteresis controller with a sinusoidal band for current regulation is described. The behavior of the conventional fixed-band controller and the proposed sinusoidal band controller has been thoroughly studied. Simulation results demonstrate that with no lockout (permitting a very high switching frequency) the current waveform can be confined within the desired hysteresis bands. At low lockout frequencies the current is not confined within the hysteresis bands and both fixed and sinusoidal band controllers give a high ripple. The study also shows that with a reasonable lockout frequency, a sinusoidal band control results in a reduced ripple and lower harmonic content. However, the switching frequency is higher with the sinusoidal bands. This should not be a major concern with the availability of fast switching devices that allow higher lockout frequencies     

Experimental design of a fuzzy controller for improving power factor of boost rectifier

This article presents the design and the implementation of dSPACE DS1104 controller board-based PI and fuzzy logic peak current-mode controllers in the voltage loop and two controllers in the current loop based first on a standard fixed hysteresis band control, followed by a variable hysteresis band control to achieve constant switching frequency for a single-phase active power factor corrector in the continuous conduction mode. All these controllers have been verified via simulation in Simulink and a real-time implementation is performed on an experimental test bench utilising a rapid prototyping tool. The controllers are experimentally compared for steady-state performance and transient response. It is shown that the PI and fuzzy logic controllers give a superior steady-state performance, whereas the fuzzy logic inference based controller can achieve better dynamic response than its PI counterpart under large load disturbance and plant uncertainties. Furthermore, the variable hysteresis band control in the current loop gives a low total harmonic distortion of the input current compared to a standard fixed hysteresis band control.     

A family of converters for UPS production burn-in energy recovery

This paper introduces a family of power converters for power recycling during the burn-in test of synchronized uninterruptible power supplies (UPSs) with sinusoidal output voltage. The use of the power recycler to replace the resistor load bank in the UPSs burn-in test causes great energy savings, and the optimized use of electrical energy contributes in reducing the final cost of the product. The main feature of the new circuits is their ability to draw from the UPS and to inject into the utility-grid currents with low total harmonic distortion (THD) and high power factor (PF). The new circuits operate at constant frequency and are regulated by conventional pulse width modulation (PWM) using dedicated PWM and PF controller integrated circuits developed for power supplies. Circuit operation, mathematical analysis, design example, and experimental results for discontinuous current mode (DCM) and continuous current mode (CCM) operation are provided in this paper     

Quench Protection System for the Superconducting Coil of the KSTAR Tokamak

This paper presents two-stage high-power mechanical-thyristor switches developed as 40-kA direct current (dc) circuit breakers for the superconducting coil energy dump system of Korea superconducting Tokamak advanced research (KSTAR). An advanced switching system used in the quench protection should allow multishot mode of operation without maintenance and long lifetime with arcless dc current commutation. The system is designed to combine the advantages of the both mechanical and solid-state power switches. The mechanical stage is to conduct dc current for normal superconducting coil operation and the thyristor stage is to provide fast arcless dc current interruption for quench protection (QP). Compared with the all solid-state switches and vacuum interrupters of equal capabilities the proposed switch is much cheaper than the dc breakers and has lower power dissipation in current conducting state "Kuchinski in Proc. Power Ele., 899-903, 1995;" "Benfatto IEEE Power Del., Vol. 3, No. 4, pp. 1372-1376, Oct. 1998." Also, it is much faster than the vacuum interrupters. The system is designed to break 40-kA dc current within 200 ms. Also, KSTAR device, a configuration of the energy dump system and coil power supplies are presented     

Performance investigation of a current-controlled voltage-regulatedPWM rectifier in rotating and stationary frames

Active front-end rectifiers with reduced input current harmonics and high input power factor will be required in the near future for utility interfaced applications. In order to meet the new and more stringent regulations with force-commutated switches, the voltage source inverter approach is superior to the conventional current source approach, in terms of number of components and control options. However, the straightforward power angle control of the rectifier is characterized by a slow response and potential stability problems. This paper proposes a current-controlled PWM rectifier as an alternative. It provides near sinusoidal input currents with unity power factor and a low output voltage ripple. Moreover, it produces a well-defined input current harmonic spectrum, exhibits fast transient response to load voltage variations, and is capable of regenerative operation. PWM pattern generation is based on a carrier technique and the current controller is implemented in the: (a) stationary (abc) frame; and (b) rotating (dqo) frame. The design and the performance of the two controller options are investigated and compared     

Single-stage three-phase buck-boost type AC-DC converter with highpower factor

A new control process for single-stage three-phase buck-boost type AC-DC power converters with high power factor, sinusoidal input currents and adjustable output voltage is proposed. This converter allows variable power factor operation, but this work focus on achieving unity power factor. The proposed control method includes a fast and robust input current controller based on a vectorial sliding mode approach. The active nonlinear control strategy applied to this power converter, allows high quality input currents. Given the comparatively slow dynamics of the DC output voltage, a proportional integral (PI) controller is adopted to regulate the converter output voltage. The voltage controller modulates the amplitudes of the current references, which are sinusoidal and synchronous with the input source voltages. Experimental results from a laboratory prototype show the high power factor and the low harmonic distortion characteristics of the circuit     

Global cost optimization of 1-10 MWh toroidal SMES

The toroidal superconducting magnetic energy storage (SMES) configuration has a much reduced stray field over a solenoidal SMES. This allows the entire SMES facility to be placed in a high density environment where both personnel and equipment can be located closer to the magnet. In the past, this concept has been resisted due to the increased magnet cost for a large toroid over a solenoid storing an equivalent amount of energy. Preliminary studies have indicated that when the entire system cost is considered, the penalty may not be so excessive in systems storing a few MWh as is the case for applications like transmission stability and spinning reserve. The author is developing a cost model for the toroidal SMES system which will assist in selecting the optimum architecture, shape, and aspect ratio for toroidal coil modules which can be used for systems storing from 1 to 20 MWh. The global optimization model includes detailed magnetic, structural, and thermal considerations and provides cost analysis for all aspects of construction. Construction and material costs are based on cost estimating relationships and firm estimates derived from construction of a number of large industrial magnets and other equivalent systems at both Lockheed Martin (formerly General Dynamics) and Bechtel