Electrochemical energy storage (EES) unit is different from traditional high-voltage equipment in two main characteristics: active and modular structure

such as a direct connected to 35kV EES unit

which has about 25-50 submodules in a single phase. There are new features in the overvoltage propagation and distribution within EES units

the overvoltage protection scheme

the insulation coordination

and the insulation withstand tests of EES unit. This article takes the EES unit with 29 submodule cascaded direct connecting to AC 35kV as an example to calculate and analyze the overvoltage distribution inside the EES unit under lightning impulse

standard operating impulse

steep wave and long wave operating impulse

including the distribution of overvoltage to ground in the main circuit and the longitudinal overvoltage distribution along the submodule chain. According to the characteristics of withstand overvoltage

the insulation of EES units is divided into pole to ground insulation and longitudinal insulation (the insulation between adjacent components) in the paper. The results show that lightning impulse mainly acts on the pole to ground insulation. To limit the lightning overvoltage amplitude of the EES unit's pole to ground insulation

at least 2 sets of lightning arresters are required

one set installed on the busbar and the other set installed on the EES unit side of the grid connected reactor (see Figure 3). By properly configuring lightning arresters

the maximum amplitudes of lightning overvoltage between each submodule and ground are basically the same

but the frequency of occurrence varies

with higher frequency on the side closer to the power grid. The overvoltage of the pole to the ground caused by operating impulse is lower than that caused by lightning impulse

but it will generate overvoltage on the longitudinal insulation of the EES unit. The overvoltage between the two poles of the submodule can reach about 1.6 times the peak of the steady-state voltage and generate roughly 5kA overcurrent. Finally

the article proposes a theoretical basis and method for conducting insulation withstand tests on EES unit using submodules instead of the entire EES unit and provides a principle for determining the insulation coordination coefficient of EES unit based on Gaussian distribution. The overvoltage protection scheme

insulation coordination coefficients

and insulation withstand test methods proposed in this article can serve as important references for EES unit insulation design and insulation safety demonstration.