# Sensorless Current Pulsation Compensation in a Hybrid Energy Storage
## 4 Conclusions  
The solution presented in the paper enhanced the capabilities of a hybrid energy storage application by reducing the pulsation of the current in the DC link and, as a consequence, preventing undesired heating of the electrochemical storage. The addition of a capacitor bank with an active DC/DC converter in parallel with the electrochemical energy storage creates a protection circuit for unbalanced operation of the inverter. The unbalanced state of operation of the inverter is undesirable for the electrochemical energy storage but desired by the grid operator as a means to reduce grid voltage asymmetry, created by renewable energy sources and asymmetrical loads. The presented solution is based on a dual-active bridge topology, which possesses the desired dynamics for this application as it does not introduce any delays in the current control loop. The dual active bridge is controlled by a proportional-resonant controller, and it aims to generate sinusoidal current with a phase opposite that of the DC link pulsation. By doing so, it significantly reduces pulsation during energy storage. The reference signal for the compensator is calculated instead of measured, removing the need for an additional current sensor in the DC link of the converter. The calculation is done by the four-wire inverter controller and is based on the measured output currents of the converter. The reference is passed to the compensator by a communication protocol (in this case, the SPI protocol was used). The compensator presented in the paper managed to reduce the pulsation of the energy storage current by more than 80% -- reducing it from 8 to less than 1.5 A. It has to be pointed out that dynamic changes in the pulsation are compensated instantly, and the start of the compensator takes less than 5 pulsation cycles. The dynamics of the dual-active bridge converter driven by the sinewave confirmed claims of zero delay in phase to the input current control loop. It must be pointed out that the execution rate of the PR controller affects delays in the control loop, and at the same time it cannot be too high, as that would require controller coefficients to become small values (in the \(10^{-9}\) range), which would make the control loop inaccurate. This creates an issue if the compensator is required to suppress the higher harmonics, which can also appear in the DC link during unbalanced operation of the four-wire inverter, and would require a multi-rate control loop with separate execution rates for each of the PR controllers. Figure 12: Step change of the pulsation in the DC link current. Further research in this application should include compensation of higher harmonics based on a multi-rate parallel PR controller connection. Additional research could also investigate a dual active bridge with two outputs on the secondary side: one connected to the energy storage and operating with DC current, and the other working as a pulsation compensation circuit that would operate with AC current. The solution could be based on a transformer with two secondary windings, which would reduce the number of H-bridges in the application by one and thus increase power density. The proposed compensator could also be used in applications with three-phase, four-wire inverters with unidirectional power sources, like solar panels or fuel cells, since inverters fed by those sources have strictly limited asymmetrical operation capabilities due to current pulsation in the DC link. Adding the compensator to the DC links of those applications would be a cost-effective way of expanding the range of asymmetric operation of those inverters. Conceptualization, K.F. ; methodology, D.Z. ; software, K.F. ; validation, D.Z. and K.F. ; formal analysis, D.Z. ; investigation, D.Z. and K.F. ; resources, D.Z. ; data curation, D.Z. and K.F. ; writing\(-\)original draft preparation, K.F. ; writing\(-\)review and editing, D.Z. and K.F. ; visualization, K.F. ; supervision, D.Z. ; funding acquisition, D.Z.