What is the impact of multiple PCS units in parallel on EMC testing in an energy storage system?

When multiple PCS units are operating in parallel, EMC testing results will differ from single-unit testing. The main impacts include:

1. Noise superposition: Noise from each unit may be in phase or out of phase, resulting in overall conducted and radiated noise levels that may be higher or lower than those of a single unit. Typically, the most unfavorable combination of operating conditions needs to be tested.

2. Circulating currents and resonances: Circulating currents between parallel units may generate new differential-mode and common-mode noise frequency components. The parallel system and the grid impedance may form new resonance points, leading to noise amplification at specific frequencies.

3. Increased grounding system complexity: With multiple units sharing a common ground, the grounding network impedance and ground loops may become more complex, affecting common-mode noise discharge and immunity.

4. Test representativeness: Standards typically require testing the parallel system as a whole. This necessitates considering not only individual unit EMC during design but also the interactions during parallel operation. For example, using interleaved modulation to disperse harmonics, optimizing parallel busbar design to suppress circulating currents, and ensuring a good common ground.