A novel three-phase utility interface for photovoltaic, wind-electric and fuel cell systems.

摘要

Photovoltaic, wind-electric and fuel cell based renewable energy sources are beginning to supplement the conventional fossil and nuclear based sources for generating electricity. Fuel cells and solar cells produce dc voltage and current. In most wind electric systems, the wind turbine drives an ac generator which produces three-phase voltages at a variable frequency depending on the wind speed. This variable frequency ac voltage is rectified into a dc voltage. Thus, to interface the renewable energy sources with the utility, a power conditioner is required to convert the dc voltage to the line frequency ac voltage.; The objective of the work presented in this thesis is to develop an interface with the three phase utility, which is reliable, economical and has low line current distortion. The power flow from the dc source is controlled by two step down dc-dc converters feeding a thyristor inverter, connected to the three phase utility through a wye-delta isolation transformer. The step down dc-dc converters, while controlling the power flow, also modulate the dc link currents by the third and the sixth harmonic components in order to shape the currents on the utility side to have a THD of less than 5%. A novel control concept allows the power factor of operation to be controlled around unity, from slightly lagging to slightly leading, even in the presence of the thyristor inverter. It is also shown that soft switching concepts can be incorporated into the interface, thereby reducing the Electro-Magnetic Interference at high power levels.; Results from an experimental prototype validate the operation of the interface. This interface has lower high frequency switch kVA rating as compared to other common three-phase utility interface topologies and is a significant step towards making the utility interactive large photovoltaic, wind-electric and fuel cell systems economically feasible.