Energy storage system for renewable energy source

1. A renewable energy power system, comprising:

a power converter having a DC bus;

an energy storage system coupled to the DC bus of the power converter, the energy storage system comprising an energy storage device and a switching power supply coupled between the energy storage device and the DC bus of the power converter; and

a control system configured to operate the energy storage system in a first mode where power flows in a first direction from the DC bus to the energy storage device and in a second mode where power flows in a second direction from the energy storage device to the DC bus;

wherein the switching power supply comprises a bi-directional resonant DC to DC power converter, the bi-directional resonant DC to DC power converter configured to accommodate bi-directional power flow between the energy storage device and the DC bus;

wherein the bi-directional resonant DC to DC power converter comprises a resonant circuit, a first switching element and a second switching element, and wherein the first switching element and the second switching element are coupled in series, the first switching element and the second switching element are each coupled to a node; and

wherein the resonant circuit is coupled to the node between the first switching element and the second switching element.

2. The renewable energy system of claim 1 , wherein the bi-directional resonant DC to DC power converter comprises a filtering circuit coupled to the resonant circuit.

3. The renewable energy system of claim 2 , wherein the first switching element is a first insulated gate bipolar transistor having a gate, a collector, and an emitter, the second switching element is a second insulated gate bipolar transistor having a gate, collector, and an emitter, wherein the collector of the first switching element is coupled to a positive terminal of the DC bus, the emitter of the first switching element is coupled to the collector of the second insulated gate bipolar transistor, and the emitter of the second switching element is coupled to a negative terminal of the DC bus.

4. The renewable energy system of claim 3 , wherein the first switching element is coupled in parallel with a first diode and the second switching element is coupled in parallel with a second diode.

5. The renewable energy system of claim 2 , wherein the resonant circuit comprises a first LC circuit having a first inductor and a first capacitor, the first inductor coupled to the node between the first switching element and the second switching element, the first capacitor coupled between the first inductor and a negative terminal of the DC bus.

6. The renewable energy system of claim 5 , wherein the filtering circuit comprises a second LC circuit having a second inductor and a second capacitor, wherein the second inductor is coupled to a node between the first inductor and the first capacitor and the second capacitor is coupled between the second inductor and the negative terminal of the DC bus.

7. The renewable energy system of claim 2 , wherein the control system is configured to coordinate the switching of the first switching element or the second switching element with zero current crossings of a current flowing in the respective switching element.

8. The renewable energy system of claim 1 , wherein the resonant DC to DC power converter is a buck converter, boost converter, zero current switching (ZCS) converter, zero voltage switching (ZVS) converter, or a buck-boost converter.

9. The renewable energy system of claim 1 , wherein the DC bus is coupled between an AC to DC converter and a DC to AC converter in a wind power system.

10. The renewable energy system of claim 1 , wherein the DC bus is coupled between a DC to DC converter and an AC to DC converter in a solar power system.

11. A method of operating an energy storage system, comprising:

selecting operation of the energy storage system in a first mode;

providing power from a DC bus to an energy storage device via a bi-directional resonant DC to DC converter coupled between the DC bus and the energy storage device; and

controlling the bi-directional resonant DC to DC converter such that power flows in a first direction from the DC bus to the energy storage device;

wherein the bi-directional resonant DC to DC power converter is configured to accommodate bi-directional power flow between the energy storage device and the DC bus

wherein the bi-directional resonant DC to DC power converter comprises a resonant circuit, a first switching element and a second switching element and wherein the first switching element and second switching element are coupled in series, the first switching element and second switching element are each coupled to a node; and

wherein the resonant circuit is coupled to the node between the first switching element and the second switching element.

12. The method of claim 11 , wherein the method further comprises selecting operation of the energy storage system in a second mode;

providing power from energy storage device to the DC bus via the bi-directional resonant DC to DC converter coupled between the DC bus and the energy storage device; and

controlling the bi-directional resonant DC to DC converter such that power flows in a second direction from the energy storage device to the DC bus.

13. The method of claim 12 , wherein the bi-directional resonant DC to DC converter comprises a first switching element and a second switching element coupled in series with one another, a resonant circuit coupled to a node between the first switching element and the second switching element, and a filtering circuit coupled to the resonant circuit.

14. The method of claim 13 , wherein controlling the bi-directional resonant DC to DC converter such that power flows in a first direction from the DC bus to the energy storage device comprises coordinating the switching of the first switching element with zero current crossings of a current flowing in the first switching element.

15. The method of claim 13 , wherein controlling the bi-directional resonant DC to DC converter such that power flows in a second direction from the energy storage system to the DC bus comprises coordinating the switching of the second switching element with zero current crossings of a current flowing in the second switching element.