Low voltage DC distribution system comprising of a small wind generator and a battery energy storage system

Abstract

Current trends in electrical power consumption indicate that there is an increase in the number of electrical appliances that consume direct current (DC) power such as computers, lighting appliances, TVs and other electronic-based apparatus. There is also a widespread adoption of distributed generators near the load centres, which are mostly DC (photovoltaic systems, battery storage systems and fuel cells), to improve the voltage profile and reduce power losses. Distribution of electric power in DC form therefore, serves to eliminate the several conversion stages common in AC distribution systems. Consequently, there is greater savings in the cost of system implementation and higher reliability due to a decreased number of converters. The growth of DC microgrids has further encouraged the harnessing of other none DC renewable energy sources such as wind which is abundant and a clean source. However, interfacing of wind energy source to a DC distribution demands a high voltage boost active rectifier and proper feedback control to handle its intermittent nature which has the potential to introduce voltage flicker in the distribution system.

This research focuses on the design of a small wind energy conversion (SWEC) system ideal for interfacing low voltage wind generator to a DC distribution system. The proposed SWECS comprises of a three-phase diode rectifier cascaded with an interleaved tapped-coupled inductor boost DC-DC converter. This converter topology provides very high boost voltage rectification, low input- and output-side current ripple, good input disturbance rejection and output voltage regulation. A good combination of the coupled-inductor turns ratio and duty cycle of the converter is chosen to provide the necessary boost ratio while keeping the device blocking voltages within acceptable limits. Consequently, the proposed SWECS interface have a small structure, simple PWM control circuitry and high efficiency.

To further address the intermittent nature of wind energy resource, a low voltage battery energy storage system (BESS) is incorporated in the distribution system. Low voltage BESS offers the best solution in energy storage due to their low cost, mature technology and few cell failures. However, they require a high voltage gain and bidirectional power flow converter for proper operation and efficient integration to a distribution system. In this regard, the research focuses on the design of an efficient, high boost and low cost converter suitable for interfacing this low voltage BESS to the distribution system. The proposed interface comprises of a bidirectional tapped-coupled inductor boost DC-DC converter. This converter adopts the concept of converter interleaving to enable current ripple cancellation at the converter input and output, so that the size of converter components are significantly reduced. Further reduction in inductor size is demonstrated through application of coupled inductors, in which two-phase inductors share the same core, to provide the same effective inductance as two larger non-coupled phase inductors. A novel passive lossless snubber circuit is employed to clamp the voltage spikes across the active switches without altering the normal operation of the converter.