A Superconducting Magnetic Energy Storage-Emulator/Battery Supported Dynamic Voltage Restorer
Abstract
This study examines the use of superconducting magnetic and battery hybrid energy storage to compensate grid voltage fluctuations. The superconducting magnetic energy storage system (SMES) has been emulated by a high current inductor to investigate a system employing both SMES and battery energy storage experimentally. The design of the laboratory prototype is described in detail, which consists of a series-connected three phase voltage source inverter used to regulate AC voltage, and two bidirectional DC/DC converters used to control energy storage system charge and discharge. ‘DC bus level signaling’ and ‘voltage droop control’ have been used to automatically control power from the magnetic energy storage system during short-duration, high power voltage sags, while the battery is used to provide power during longer-term, low power under-voltages.
EXISTING SYSTEM:
Methods to mitigate long-term voltage disturbance, such as load disconnection or modification of loads for greater low-voltage ride-through capability may be impractical. Alternatively, supply voltage can be stabilised by tap changing transformers, uninterruptable power supplies (UPS), shuntconnected compensators, or dynamic voltage restorer (DVR) systems. Tap changing transformers have been shown to suffer from a slow response time and can only output discrete voltage levels. UPS systems provide the complete voltage waveform during a power failure and may prove costly and unnecessary in the event of partial voltage sags. A DVR is a series-connected device capable of voltage compensation with fast response time by injecting a voltage in series with the supply. DVR systems can be self-supporting by using power from the grid to mitigate disturbances. Alternatively, DVR systems can use energy storage to provide power during compensation such as capacitors for short-term storage or batteries [for longer-term storage. Nielsen and Blaabjerg have shown that capacitor-supported DVR systems can suffer from relatively poor performance for severe and long duration sags. A recent study has shown that an ultra-capacitor based DVR can be used to mitigate short-term voltage sags lasting less than one minute. Wang and Venkataramanan [have shown that flywheels are a viable short-term energy storage technology for use with voltage restorer systems both experimentally and by simulation. Kim et al. have described a 3 MJ/750 kVA SMES-based DVR system and shown experimental results confirming that SMES is suitable for the compensation of short-term voltage sags. Shi et al. have used a system-level simulation to also show that SMES energy storage is capable of compensating voltage sags lasting 100ms.
PROPOSED SYSTEM:
Short-term voltage compensation alone may not be sufficient to protect a sensitive load as both long-term and short-term voltage stability has been shown to present a problem for many consumers. For this reason, this study considers the use of SMES/battery hybrid energy storage to compensate long and short-term voltage fluctuations. Woong et al. have also considered a SMES/battery hybrid and shown it is viable for smoothing of renewable energy generator output power and can result in reduced energy storage system capacity and prolonged battery life. Li et al. have shown that a SMES/battery energy storage system can improve battery lifetime in electric buses. Deng et.al. have presented a SMES/battery hybrid system for reducing peak grid power in an electric vehicle charging station. Nie et al. have also presented a SMES/battery hybrid system and shown its feasibility in dealing with long term and short term charge/discharge events in a wave energy conversion system. This study extends previous simulation-based SMES/battery hybrid system studies by considering the hardware implementation of a SMES/battery energy storage system. The design is shown to be capable of interfacing SMES and battery energy storage systems and controlling their power sharing to support a three phase load, during both long-term and short-term voltage sags. This has benefits in terms of improved long-term voltage support capability and reduced costs compared with a purely SMES-based system. Additional benefits include reduced battery power rating requirement and an improvement in expected battery life compared with a battery-only system.
CONCLUSIONS
The performance a novel hybrid DVR system topology has been assessed experimentally and shown to effectively provide voltage compensation for short-term sags and long-term under-voltages. A prototype system has been developed which demonstrates an effective method of interfacing SMES and battery energy storage systems to support a three phase load. The system has been shown to autonomously prioritise the use of the short-term energy storage system to support the load during deep, short-term voltage sags and a battery for lower depth, long-term under-voltages. This can have benefits in terms of improved voltage support capability and reduced costs compared with a SMES-based system. Additional benefits include reduced battery power rating requirement and an expected improvement in battery life compared with a battery-only system due to reduced battery power cycling and peak discharge power. IX.
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