Zero-sequence circulating current suppression strategy for parallel power conversion systems in lithium battery formation and capacity grading power supply systems

Parallel operation of multiple power conversion system (PCS) units has emerged as the dominant strategy for scaling the capacity of battery formation and grading power supplies. Nevertheless, such parallel PCS configurations, sharing a common DC bus, exhibit zero-sequence circulating current (ZSCC), which induces excessive current ripple amplitude in the output waveform and critically degrades battery cycle life. Conventional ZSCC mitigation approaches inherently entail a trade-off between current harmonic distortion and DC-link midpoint voltage balancing, thereby elevating controller complexity and impairing dynamic response under time-varying load conditions. To address this limitation, this work uncovers the fundamental coupling mechanism between ZSCC generation and midpoint potential regulation inherent in traditional space-vector modulation schemes. Accordingly, we propose a Large and Zero Vector Modulation (LZVM) strategy that restricts voltage vector synthesis exclusively to large vectors and three redundant zero vectors, thereby eliminating medium vectors, which perturb the DC-link midpoint potential, and small vectors, which exacerbate common-mode voltage. By minimizing the average common-mode voltage over a single switching period, LZVM enables fully decoupled ZSCC suppression and midpoint potential balancing. A parallel experimental system comprising six 215 kW three-level PCS units was established. The proposed LZVM modulation strategy was evaluated against the traditional LMZVM modulation strategy across several dimensions: common-mode voltage suppression, midpoint potential balance, ZSCC suppression, and THD of grid-connected currents. Experimental results confirm that the proposed LZVM demonstrates significant advantages in ZSCC suppression while minimally affecting the harmonic characteristics of output currents.