5.2.1. Verification of SOC Equalization

The experimental waveforms of rectifier mode operation in steady state are shown in Figure 17. During the operation, the references of *Pac* 800 W and an external 600 W DC load is connected to the DC-link throughout. In Figure 17a, the five-level modulation waveform of line voltage is presented with quite balanced capacitor voltages. Sinusoidal grid currents with low harmonic components are shown in Figure 17b. Circulating current of phase A is shown in Figure 17c with fundamental frequency component to equalize the SOC difference between upper and lower arms. Battery current with low harmonic components is also shown in Figure 17c to verify the effectiveness of the proposed capacitor voltage filter scheme. As the batteries operate in discharge mode, the SOC equalization process of the 12 battery modules are shown in Figure 17d. Due to the large capacity of the battery unit, the SOC equalization process is long, hence, the SOCs are sampled every ten minutes and presented by MATLAB.

**Figure 17.** Experimental waveforms of steady-state operation of the MMC-BESS. (**a**) Modulated voltage waveform. (**b**) Grid currents and capacitor voltage. (**c**) Circulating current and battery current. (**d**) SOC equalization process.

5.2.2. Verification of Proposed Battery Side-Based Capacitor Voltage Control

The experimental waveforms of dynamic performance with and without the proposed control strategy at battery side are shown in Figure 18. The reference of AC power was 800 W throughout, and DC load was changed from 800 W to 500 W. Seen from Figure 18a, without the proposed control strategy at battery side, the battery current responded slowly, causing voltage fluctuation on capacitor voltage. Compared with Figure 18a, the battery current responded faster and capacitor voltage was stable during the power flow transfer process in Figure 18b. This comparison was in accordance with the simulation results in Figure 15.

**Figure 18.** Experimental waveforms of dynamic performance of the MMC-BESS. (**a**) Without the proposed control strategy. (**b**) With the proposed control strategy.

### 5.2.3. Verification of Dynamics of Rectifier Mode Operation

The dynamic performance of rectifier mode operation is shown in Figure 19. Utilizing the proposed control strategy based on battery side capacitor voltage control, the DC-link voltage was controlled to 120 V throughout, AC power was regulated through the reference value directly according to the indications in Figure 19. The external 600 W DC load was connected to the DC-link during the operation, which is illustrated in the general view of the waveform. Seen from the experimental results, the AC power and DC-link voltage had no direct interactions due to the existence of additional power flow provided by the BESS, which is different from conventional MMCs. The dynamics of AC power and DC-link voltage were both able to satisfy the requirements of the system as a rectifier or a pure PCS.

**Figure 19.** Experimental waveforms' dynamic performances of rectifier mode operation with proposed control strategy.
