Renewable energy power systems, such as wind energy power systems and solar energy power systems, often include a power converter with a regulated DC bus. For example, wind power systems, such as wind driven doubly-fed induction generator (DFIG) systems or full power conversion systems, can include a power converter with an AC-DC-AC topology. Solar power systems can include a power converter that has a DC-DC-AC topology or simply a DC-AC topology.
In a typical configuration, an energy storage system can be coupled to the DC bus of a power converter in a renewable energy power system. The energy storage system can be used, for instance, to apply power to the DC bus of the power converter during transient conditions or to store energy generated by a generator. For example, a switching power supply, such as a DC to DC converter, can be provided to transfer energy back and forth between the DC bus of the power converter and the energy storage device. A bi-directional DC to DC switching power supply can allow for power flow from the energy storage device to the DC bus during transient conditions but also to allow power flow from the DC bus to the energy storage device, for instance, to charge the energy storage device.
However, in such a configuration, large cabinet filters are often needed to filter out heavy ripple, including common mode noise and differential mode noise, before energy is provided to one or more energy storage devices in the energy storage system. These filters can be very expensive, with the cost increasing as the size of the filter increases. Further, in such a configuration, the one or more energy storage devices in the energy storage system, such as one or more batteries, are not electrically isolated from the power converter in the renewable energy system, and thus are susceptible to damage from fault currents. Moreover, integrating an energy storage system into the DC bus of a power converter typically requires a purpose-built power converter, and such an energy storage system is not typically suitable to be retrofitted to an existing renewable energy power system.