DC Cascaded Energy Storage System Based on DC Collector With Gradient Descent Method

With the continuous development of distributed energy, the energy storage system (ESS) is indispensable in improving power quality. Aiming at the application of large-capacity storage battery access to medium voltage dc power grid, a dc cascaded ESS based on the dc collector is proposed, and the characteristic, topology, and control are presented in detail. In this scheme, the low-voltage storage batteries are accessed to medium voltage dc bus directly with dc collectors, which not only has higher efficiency, but also improves power density. Especially, the diversity of state of charge is only reflected in the duty cycle of submodules (SMs), rather than the voltage deviation. Further, a variable angle carrier phase-shifted modulation with gradient descent method is discussed to eliminate the dominant current harmonics. By calculating the harmonic amplitude and solving the Jacobian matrix, the gradient that reduces harmonic current is obtained, and the optimal combination of the carrier phases is deduced by iterative calculation. Meanwhile, the method can be extended in the dc collector with any number of SMs to eliminate the dominant harmonics. Finally, the experimental prototype is built and the benefit of the proposed solution is verified.