Patent Application
20070273338
Each output phase of bridge 60 is summed with an output phase of bridge 50. For this discussion, three phase utility grid 90 can be considered as three, zero-impedance voltage sources 91, 92 and 93 with a common, grounded neutral at terminal 80. As such, the currents through inductors 61 and 51 will algebraically add and the resultant, composite current waveform will be sourced into the utility grid 90 at terminal 81. The same will hold true for the remaining two phases. Capacitors 71, 72 and 73 are used to provide a second filter pole at high frequencies where the impedance of a non-ideal utility grid is non-zero.
Power can only be delivered into utility grid 90 from bridge 60 if the voltage across photovoltaic array 10 is higher than the instantaneous utility grid voltage for a given phase 91, 92 or 93. On hot days, photovoltaic array 10 will not have a maximum power point voltage high enough to enable bridge 60 to create the plus and minus current peaks into the utility grid. Under these conditions, the current sinewave will distorted or “flat-topped”. This is when bridge 50 is brought into play to provide the missing peaks. Boost circuit 30 is a non-isolated boost circuit. When power semiconductor switch 35 is closed, inductors 31 and 32 are charged. When switch 35 is opened, the energy stored in inductors 31 and 32 is transferred to capacitors 41 and 42. Switch 35 is operated at high frequencies as part of a closed loop voltage regulation circuit, resident on control board 100. The DC voltage at the input of bridge 50 across capacitors 41 and 42 is regulated to a voltage just high enough to allow power to be sourced into utility grid 90 by bridge 50 during the peak excursions of the utility voltages 91, 92 and 93.
In renewable energy applications, it is crucial to convert power from a renewable energy source, such as a photovoltaic array, at very high conversion efficiencies. Also, the National Electric Code prohibits a photovoltaic array with high enough voltage to source undistorted power directly into a 480/277 Vac utility grid without the use of a lossey, 60 Hz transformer or autotransformer. The invention enables a direct, code-compliant 480/277 Vac grid connection with high power conversion efficiencies. The high efficiency is achieved by processing the bulk of the throughput power once with low voltage bridge 60 and a much smaller amount of power with boost circuit 30 and high voltage bridge 50.
One additional advantage afforded by this novel power converter topology is that a significant high frequency ripple current cancellation can be had even when high voltage bridge 50 is regulating zero current per phase. High frequency pulse modulation is used to create the sinusoidal currents for each phase in both high voltage bridge 50 and low voltage bridge 60. In the invention, high frequency pulse modulation for low voltage bridge 60 is out of phase with that of high voltage bridge 50 to provide substantial high frequency ripple current cancellation at the current summation points of the two bridges. Because of the ripple current cancellation, the size and cost of filter inductors 51-53 and 61-63 can be reduced significantly.
Also, because high voltage bridge 50 works at higher voltages, switches 54-59 must be rated for higher voltages than switches 64-69. The higher voltage switches 54-59 are less efficient and more costly and would not be well suited for use in bridge 60 where high conversion efficiency is critical. The expense of higher voltage switches 54-59 is leveraged in this invention by using bridge 50 to pull down the open circuit photovoltaic array voltage to the maximum power point voltage when the power converter is started up with an energized photovoltaic array. The open circuit voltage is typically pulled down in less than a second to a safe level where bridge 60 switches begin to operate and supplant bridge 50 currents.
