Research On Cascaded H-Bridge Converter For Energy Storage And Its Control Techniques

The energy storage system(ESS) can not only be used as power source but also the load, so it plays a vital role of bidirectional adjustment in increasing the ability of grid to accommodate the electric power generated by renewable energy sources. With the penetrance of large scale renewable energy sources increasing with each passing day, the capacity and voltage level of ESS also become higher and higher as the effective method to smoothing the fluctuation of renewable generation output. The cascade H-bridge converter as the power condition system(PCS) for energy storage has the following advantages. The converter can achieve high ac output voltage by using low rated voltage switching devices. The battery energy management system is designed easily for its distributed configuration. The system is expanded easily for the modular construction based on H-bridge. So the cascade H-bridge converter has greater potential in the field of high voltage and large capacity ESS. The paper researches on the cascade H-bridge converter as the power condition system for energy storage, including the questions as follows:(1) The battery or supercapacitor has the ability of short-term high power discharge. The PCS power level is designed several times as the rated power of ESS in order to using the ability. When the ESS working under its rated power, the PCS will exist a large surplus capacity which leading to the low utilization. This problem is seldom concerned at present. In order to improve the problem, the paper proposes a unified control strategy which combines the ESS function of smoothing the fluctuation of renewable generation output and the ESS function of filtering. The strategy firstly achieves the smoothing function, and then achieves the active power filtering with the surplus capacity of PCS.(2) The distributed configuration of cascade H-bridge converter for energy storage essily leads to the imbalance of the DC link voltage or capacity of energy storage units. Based on the power model of cascade H-bridge converter, the paper studies the phase to phase DC link voltage balance problem in terms of energy which shows the relationships among the phase to phase DC link voltage differences, injecting power for each phase and the equalization time. Then the paper proposes the quick control strategies of DC link voltage balancing by injecting the maximum zero-sequence voltage or the maximum negative-sequence current, and deduces the solutions of the maximum zero-sequence voltage and the maximum negative-sequence current.(3) The DC link current of cascaded H-bridge converter for energy storage system contains a lots of ripple current components. The ripple current will bring bad effects to the energy storage units. The paper firstly analyzes the DC, low-frequency and high-frequency components of H-bridge DC link current by theoretical derivation and simulation analysis. Based on the H-bridge equivalent circuit model of ripple current, the paper researches on the propagation characteristics of suppressing DC ripple current by three passive filtering methods. Then the paper proposes two circuit topologies of the cascade H-bridge converter for energy storage with buck/boost DC active filter and the cascade H-bridge converter for energy storage with DC active filter based on Coupling transformer. Finally the paper elaborates the control strategies for suppressing the DC ripple current by using DC active filter.(4) The cascaded H-bridge multilevel converter is used as inverter for energy storage usually. However it is found that it also can be used as the DC/DC converter for energy storage. The paper firstly proposes the AC-DC converter circuit topology for energy storage based on cascaded H-bridge struture. Then the paper researches on the operating principle and the key control strategy of the cascaded H-bridge circuit being used as the DC/DC converter.(5) The experimental prototype of five-level cascaded H-bridge PCS is developed, including the main circuit hardware, the control circuit hardware and the control software. Based on the prototype, the effectiveness of part of the related control schemes for cascaded H-bridge PCS is validated initially. Then the experimental platform of hybrid energy storage system is built to be coordinated with grid-connected PV generation. The experimental results are provided to show the effect of smoothing PV output fluctuation by using ESS.