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UBC Theses and Dissertations

Control of single phase parallel inverter systems Vorell, Kourosh


We have proposed two new control methods which allow two or more single phase PWM inverter modules to operate in parallel. Our main goals are to achieve balanced load sharing among all inverter modules and to eliminate any circulating currents in the system. The first proposed control technique is the current imbalance method in which each inverter module is provided with a current imbalance signal. The current imbalance signal is calculated for each module as the difference between the expected module current an the actual module current. Inverter modules use the current imbalance signal to deliver the expected current to the load; thus, the system can achieve balanced load sharing. Computer simulations of this control method provides satisfactory results for typical load values. However, at low loads, inverters do not share the load current and the system suffers from circulating currents. The prototype testing of the current imbalance method indicates more problems with this technique. Due to low noise immunity and sensitivity of the system, the load current is not fully shared among modules even for typical loads. The second proposed control technique is the single voltage control method which is similar to a conventional current mode control, with the exception of having two or more current loops (inverter modules). This technique uses a single voltage control block which provides a reference current for all the inverter modules. The effect of adding or removing inverter modules on the stability of the system has been studied. The system can be designed to tolerate a few faulty modules. Both the computer simulation and prototype testing of this technique give satisfactory results: good load sharing, no circulating currents, and stable operation over a large range of loads. Therefore, we recommend using the single voltage control method in most applications.

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