Energy storage system with dual-active-bridge converter
An energy storage device includes: a number of cells; and a dual-active-bridge converter connected to the cells, wherein the cells are floating relative to the system and are galvanically isolated therefrom. The energy storage device can be included in an energy storage system that includes: a grid tie unit comprising at least one DC/AC converter; and multiple pods connected to the grid tie unit, each pod including: a number of cells; and a power electronics unit, wherein the cells are floating relative to the system and are galvanically isolated therefrom.
1. An energy storage device comprising:
a grid tie unit comprising at least one DC/AC converter, the grid tie unit coupling the energy storage device to an external AC system;
a DC bus coupled to the grid tie unit;
a plurality of pods connected to the DC bus, each pod of the plurality of pods including:
a plurality of battery cells; and
a power electronics unit including a dual-active-bridge converter connected to the plurality of battery cells, wherein the dual-active-bridge converter includes a first bridge coupled to the plurality of battery cells and a first side of a transformer, and a second bridge coupled to a second side of the transformer and the DC bus, the power electronics unit configured to:
selectively operate the dual-active-bridge converter in a direct mode and an indirect mode, wherein in the direct mode a positive-voltage of the first bridge overlaps a positive-voltage of the second bridge during a switching cycle of the dual-active-bridge converter;
sample a current of the transformer at two or more sample points in the switching cycle of the dual-active-bridge converter to generate current samples;
based on the current samples, estimate an average output current of the dual-active-bridge converter;
determine that a power requirement of the pod is below a level that ensures soft switching of the dual-active-bridge converter and, in response, place the dual-active-bridge converter in a burst mode of operation, wherein the burst mode of operation includes introducing a wait state where no current is rectified by the dual-active-bridge converter; and
align a switching edge of a pulse of the switching cycle with an end of the wait state.
2. The energy storage device of claim 1 , wherein the dual-active-bridge converter of the power electronics unit samples the current.
3. The energy storage device of claim 1 , wherein the two or more sample points are chosen based on a midpoint of an on-time of a bridge leg of the first bridge.
4. The energy storage device of claim 1 , wherein sampling the current of the transformer at two or more sample points in the switching cycle of the dual-active-bridge converter includes sampling of a first current during the direct mode of operation and sampling a second current during the indirect mode of operation.
5. The energy storage device of claim 1 , wherein the power electronics unit is further configured to vary a switching behavior of the dual-active-bridge converter every half switching cycle to obtain a current control bandwidth on an order of a switching frequency of the dual-active-bridge converter.
6. The energy storage device of claim 1 , wherein a duration of the burst mode of operation is dependent on a ratio of power requirement to minimum power.
7. The energy storage device of claim 1 , wherein during the wait state sufficient current is maintained in an inductance of the dual-active-bridge converter to ensure that the switching edge is a soft switching edge.
8. The energy storage device of claim 1 , wherein the plurality of pods are arranged as a plurality of packs, each of the plurality of packs including:
a cabinet including one or more of the plurality of pods; and
a thermal control system configured to monitor and regulate a temperature of the one or more of the plurality of pods.