# Control and sizing of modular multilevel converter-based STATCOM with hybrid energy storage system for large-scale integration of wind farms with the grid

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## Abstract

A configuration of energy storage system with STATCOM features (E-STATCOM) using modular multilevel converter (MMC) is presented in this paper. It helps to integrate large wind farms into the grid complying grid codes. The E-STATCOM has the capability to provide active and reactive power supports according to the requirements. The proposed topology can handle higher power at improved efficiency compared to the conventional converter-based configurations. The energy storage system (ESS) of E-STATCOM is formed with battery and ultracapacitor to meet the demand of both high-power-density and high-energy-density loads. Also, the combination can improve the transient performance and lifespan of the battery. In this paper, the integration of hybrid energy storage system (H-ESS) with the MMC to form an E-STATCOM and associated control-related issues are discussed. An algorithm to balance the state of charge among the distributed ESS is proposed. A procedure to determine size of the H-ESS is also discussed. The complete system is simulated in PSCAD, and the effectiveness of E-STATCOM is checked for the integration of a large wind farm with the grid.

## Keywords

STATCOM Energy storage system Battery Ultracapacitor Wind farm Modular multilevel converter DC–DC converter## List of symbols

- \(\alpha _i\)
Multiplication factor of an \(i\mathrm{th}\) converter at the DC-link of MMC

- \(C_\mathrm{mod}\)
Capacitance of ultracapacitor module

- \(I^\mathrm{ref}_L\)
Reference inductor current for a DC–DC converter

- \(I_\mathrm{DC}\)
DC-link current of MMC

- \(I_{d,\mathrm{max}}\)
Maximum limit of the

*d*-axis current- \(I_{d,\mathrm{min}}\)
Minimum limit of the

*d*-axis current- \(I_{q,\mathrm{max}}\)
Maximum limit of the

*q*-axis current- \(I_{q,\mathrm{min}}\)
Minimum limit of the

*q*-axis current*M*DC–DC converters connected at the DC-link of MMC

- \(P_\mathrm{bat}\)
Active power provided by the battery energy storage system

- \(P_\mathrm{req.}\)
Power rating of each DC–DC converter at the DC-link

- \(P_\mathrm{sm}\)
Active power output of a submodule in MMC

- \(P_\mathrm{uc}\)
Active power provided by the ultracapacitor energy storage system

- SoC
State of charge

*t*Time of SoC algorithm operation

- \(V_\mathrm{o}\)
Output voltage of a DC–DC converter at the DC-link

- \(V_\mathrm{bat}\)
Terminal voltage of each battery module at the submodule of MMC

- \(V_\mathrm{uc\_max}\)
Maximum voltage of each UC module

- \(V_\mathrm{uc\_min}\)
Minimum voltage of each UC module

- \(V_\mathrm{uc}\)
Voltage of each UC module

- \(\eta \)
Efficiency of the DC–DC converter used to integrate the ultracapacitor modules

- \(C_\mathrm{uc}\)
Capacitance of each UC unit

- \(E_\mathrm{dis}\)
Amount of energy that can be discharged from UC–ESS module

- \(I_\mathrm{rated}\)
Nominal current of an ultracapacitor unit

## Notes

### Acknowledgements

The authors would like to thank the Department of Science and Technology (DST), New Delhi, India, for financial support under the Grant SERI 2016 with Project Code DST/TMD/SERI/S103.

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