DC Cascaded Energy Storage System Based on DC Collector With Gradient Descent Method
Authors: Feng An; Biao Zhao; Bin Cui; Yushuo Chen; Lu Qu; Zhanqing Yu; Rong Zeng
Abstract:
With the rapid development of distributed energy such as wind and photovoltaic power, large-capacity battery energy storage systems (ESS) have become crucial for stabilizing power grid fluctuations and improving power quality. However, traditional schemes for medium-voltage DC grid access face issues like high cost, series voltage deviation due to inconsistent state of charge (SOC), and fault surge currents from concentrated capacitors. To address these problems, this paper proposes a DC cascaded ESS based on a DC collector, along with a harmonic suppression strategy.
The DC collector is a modular power electronic device with multiple low-voltage ports (for batteries) and a centralized medium-voltage port (for grid connection). It eliminates concentrated capacitors, enabling online fault module isolation without large surge currents, and features flexible control and fault redundancy. Notably, SOC imbalance is only reflected in submodule (SM) duty cycles, avoiding DC voltage deviation that risks power device damage. Its SMs are classified into non-isolated (half-bridge, full-bridge, etc.) and isolated (single-stage, double-stage) types, each optimized for cost, power loss, and fault handling.
For harmonic suppression, a variable carrier phase-shifted modulation with gradient descent method (VCPWM-GDM) is proposed. By taking harmonic amplitude as the objective function and carrier phases as control variables, it solves the Jacobian matrix to obtain the harmonic reduction gradient, then iteratively deduces optimal carrier phases. This method overcomes limitations of traditional modulation, suppressing both fundamental and higher harmonics and adapting to any number of SMs.
Systematic parameter design (SM number, switching frequency, filters) is provided. A 30kW prototype with four SMs verifies the scheme: smooth unlocking, excellent dynamic performance during power step-changes, flexible online SM cut-in/out, and significant harmonic suppression (total harmonic distortion reduced from 15.8% to 4.54%). With moderate computational burden (68μs calculation time), the scheme achieves efficient, reliable, and low-cost access of large-capacity batteries to medium-voltage DC grids, promising broad applications in renewable energy integration.
