Typical appliances and loads in residences, commercial use and industries are run on alternating current (AC) power due to the efficiencies of usage, generation and distribution of AC power. As such, the use of power conversion devices is widely accepted for commercial or grid power from independent direct current (DC) sources, such as batteries and solar cells used to generate electrical power AND/or store it for later use.
Many known power conversion devices utilize a DC source connected to a commutating or power conversion device bridge, which creates a square, stepped or sinusoidal voltage wave by using several switching devices. The DC power source is insulated from the AC output using layers of components such as switches and transformers. The generated voltage is filtered and modified to match the grid requirements by the way of number of feedback mechanisms such as pulse width modulation techniques and filters.
FIG. 11 shows one example of a known power conversion device in the form of a power conversion bridge circuit 1100. Bridge circuit 1100 allows DC power to be converted into AC power via manipulation of switches S1, S2, S3 and S4 to produce a desired AC output waveform. Table 1 illustrates how the switching states of switches S1-S4 affect an output voltage Vx of bridge circuit 1100.
TABLE 1Switch S1Switch S2Switch S3Switch S4VXCommentONOFFOFFON+VDCPositive waveOFFONONOFF−VDCNegative waveONOFFONOFF0Net zeroOFFONOFFON0Net zero
With bridge circuit 1100, the use of appropriate switching sequence and timing allows the creation of a desired waveform, and also allows the control of harmonic content in the output waveform. Further, ripple and harmonics in the output signal may be filtered by using resonant loads, tank circuits, or external filters with band pass characteristics to limit the presence of noise in output.
In some distributed DC networks, plural DC sources may be interconnected in a series-parallel combination. The series combination of DC sources gives higher DC voltages and may be used to bring operational voltages to desired levels. On the other hand, the overall power is increased by combining multiple legs of sources in parallel.
However, one issue with a DC network of series-parallel combination is that any individual source mismatch may lead to significant loss of power in the network. Specifically, any defect in one source throttles the output from all series-connected sources. Likewise, parallel legs having sources with lower voltages may sink the power from other legs instead of sourcing, and thus may reduce the overall output. This may happen in case of solar cells arrayed together, where the output from a cell will be degraded or reduced due to shading of the cells. Even a single cell which is shaded will reduce its own output, as well as the output of the module in which the cell is located. Likewise, a shaded module will reduce the output from a string of series-connected modules. In many solar installations, multiple strings of series-connected modules are paralleled together to achieve desired output power levels. Thus, the impact of a loss of power from a cell can be severe and result in significant power loss for an entire solar cell array when connected in this manner. Further, in some environments, even during normal operation, there may be a statistical variation between the power output from individual DC sources as high as ten percent.