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. Energy storage system hybridisation is shown to be advantageous by reducing battery peak power demand compared with a battery-only system, and by improving long term voltage support capability compared with a SMES-only system. Consequently, the SMES/battery hybrid DVR can support both short term high-power voltage sags and long term under voltages with significantly reduced superconducting material cost compared with a SMES-based system.
- Dynamic Voltage Restorer (DVR)
- Energy Storage Integration
- Superconducting Magnetic Energy Storage
Figure 1. Hybrid energy storage DVR system configuration.
EXPECTED SIMULATION RESULTS:
Figure 2. Simulated PLL Algorithm results: (a) Simulated voltage sag with phase jump (b) Phase jump angle (c) Blue trace: supply phase angle. Red trace: PLL output: ‘Pre-sag compensation’ with controller gains: kp = 0.5, ki = 5, (d) Blue trace: supply phase angle. Red trace: PLL output: ‘In phase compensation’ with controller gains kp = 200, ki = 50.
Figure 3. Hybrid System Experimental results: 0.1s Three phase sag to 35% of nominal voltage. (a) Supply voltages (b) Load voltages (c) DC Link Voltage (d) Battery Current (e) SMES-inductor current.
Figure 4. Battery System Experimental results: 0.1s Three phase sag to 35% of nominal voltage. (a) Supply voltages (b) Load voltages (c) DC Link Voltage (d) Battery Current.
Figure 5. Hybrid System Experimental results: Long-term three phase under voltage (a) RMS supply phase-voltage. (b) RMS load phase-voltage (c) DC Bus Voltage (d) Battery Current (e) SMES-inductor current.
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.
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