A UPFC with reduced DC bus capacitance for voltage regulation and phase balancing in LV distribution networks with high PV penetrations

The penetration level of photovoltaic (PV) power in the low voltage (LV) distribution networks is rapidly increasing. The European Photovoltaic Industry Association reported the installed capacity of PV systems globally reached 177GW at the end of 2014. The annual rate of installations, 38.7GW in 2014, continues to increase. A large part of this is installed as residential systems connected to LV networks. The Australian Clean Energy Regulator has published a report showing the total installed capacity of PV in Australia exceeded 4.5GW in 2015. The impacts of high PV penetrations in LV residential distribution networks include voltage rise, voltage unbalance, and reverse power flow which may limit the level of photovoltaic penetration within the LV distribution networks. A four-leg compensator based on a unified power flow controller (UPFC) concept is proposed for simultaneous voltage and current compensation in LV distribution systems. As a voltage compensator, this compensation device is shown to be capable of regulating the positive, negative and zero sequence voltage in LV distribution networks under high PV penetrations. At the same time, as a current compensator, the device is capable of power factor correction, zero sequence or neutral current compensation, harmonic current compensation and a degree of negative sequence current compensation. Instantaneous reactive power theory shows that DC-bus capacitor power will fluctuate at twice mains frequency during any unbalanced operation of the regulator. Real and instantaneous power balance of the UPFC can be maintained by allowing the input shunt converter to draw a small positive and negative sequence current respectively. Instantaneous power balance with negative sequence current makes it possible to reduce the DC bus capacitance which allows long life ceramic or polypropylene capacitors to replace electrolytic capacitors. The operation of the proposed UPFC based four-leg compensator has been demonstrated by extensive simulation studies. These confirm that the device can perform the full range of series and parallel compensation duties for the compensation of voltage and current respectively. It has also been demonstrated that the DC bus voltage can be controlled using relatively small DC bus capacitors. The simulation work is further confirmed by experimental work with a small scale laboratory model. The laboratory system used commercial inverter modules rated at 600V and 15A per phase to construct two inverters with four phase legs. These were configured as a parallel connected inverter and a series inverter with series voltage injection transformers. The four-leg inverters shared a common DC bus and were equipped with voltage and current transformers and controlled using a Texas Instruments Delfino processor.