On control of cascaded H-bridge converters for STATCOM applications
Doktorsavhandling, 2017

Cascaded H-bridge (CHB) converters are today considered the industrial standard for STATCOM applications, mainly due to their small footprint, high achievable voltage levels, modularity and reduced losses. However, there are still areas of research that need to be investigated in order to improve the performance and the operational range of this converter topology for grid-applications. The aim of this thesis is to explore control and modulation schemes for the CHB-STATCOM, highlighting the advantages but also the challenges and possible pitfalls that this kind of topology presents for this specific application. The first part of the thesis is dedicated to the two main modulation techniques for the CHBSTATCOM: the Phase-Shifted PulseWidthModulation (PS-PWM) and the Level-Shifted PWM (LS-PWM) with cells sorting. In particular, the focus is on the impact of the adopted modulation technique on the active power distribution on the individual cells of the converter. When using PS-PWM, it is shown that non-ideal cancellation of the switching harmonics leads to a nonuniform active power distribution among the cells and thereby to the need for an additional control loop for individual DC-link voltage balancing. Theoretical analysis proves that a proper selection of the frequency modulation ratio leads to a more even power distribution over time, which in turns alleviates the role of the individual balancing control. Both PS-PWM and cells sorting schemes fail in cell voltage balancing when the converter is not exchanging reactive power with the grid (converter in zero-current mode). To overcome this problem, two methods for individual DC-link voltage balancing at zero-current mode are proposed and verified. Then, the thesis focuses on the operation of the CHB-STATCOM under unbalanced conditions. Two different strategies for DC-link capacitor voltage balancing, based on zero-sequence voltage/ current and negative-sequence current control, are investigated and compared. The comparison shows that none of the investigated control strategies allow for the full utilization of the converter capacity. Aiming at enhancing the converter utilization, two alternative control strategies, based on the proper combination of the zero-sequence and negative-sequence current control, are proposed and investigated. Finally, the operation of the CHB-STATCOM when controlling the voltage at the connection point in case of unbalanced system conditions is considered. It is shown that even if the device is intended for compensation of the positive-sequence voltage only, control of the negativesequence voltage might be necessary in order to avoid undesired overvoltages. Three structures for the negative-sequence voltage control are investigated and compared.

cascaded H-bridge converters


Modular Multilevel Converters (MMC)


EA-salen, Hörsalsvägen 11, Chalmers
Opponent: Prof. Vassilios Agelidis Department of Electrical Engineering, Technical University of Denmark, Denmark.


Ehsan Behrouzian

Chalmers, Energi och miljö, Elkraftteknik

Impact of frequency modulation ratio on capacitor cells balancing in phase-shifted PWM based chain-link STATCOM

2014 IEEE Energy Conversion Congress and Exposition, ECCE 2014,; (2014)p. 1931-1938

Paper i proceeding

An overview of multilevel converter topologies for grid connected applications

15th European Conference on Power Electronics and Applications (EPE). 2-6 Sept. 2013, Lille, France.,; (2013)p. 1-10 (Article no. 6634634)

Paper i proceeding

Investigation of negative sequence injection capability in H-bridge multilevel STATCOM

2014 16th European Conference on Power Electronics and Applications, EPE-ECCE Europe 2014,; (2014)p. Art. no. 6910883-

Paper i proceeding

Individual capacitor voltage balancing in H-bridge cascaded multilevel STATCOM at zero current operating mode

17th European Conference on Power Electronics and Applications, EPE-ECCE Europe 2015, Geneva, Switzerland, 8-10 September,; (2015)

Paper i proceeding

Investigation of Negative-Sequence Injection Capability of Cascaded H-Bridge Converters in Star and Delta Configuration

IEEE Transactions on Power Electronics,; Vol. 32(2016)p. 1675-1683

Artikel i vetenskaplig tidskrift

Impact of Switching Harmonics on Capacitor Cells Balancing in Phase-Shifted PWM Based Cascaded H-Bridge STATCOM

IEEE Transactions on Power Electronics,; Vol. 32(2016)p. 815-824

Artikel i vetenskaplig tidskrift

Interconnected transmission systems are complex and require careful planning, design and operation. The continuous growth of the electrical power system, as well as the increasing electric power demand, has put a lot of emphasis on system operation and control. It is under this scenario that the use of High Voltage Direct Current (HVDC) and Flexible AC Transmission Systems (FACTS) controllers represents important opportunities and challenges for optimum utilization of existing facilities and to prevent outages .

One of the most well-known FACTS device is STATic COMpensator (STATCOM). The main hardware part of a STATCOM is voltage source converter (VSC). The use of high-performance and cost-effective high power VSCs is a prerequisite for the realization of a STATCOM. Up to some years ago, the implementation of VSCs for high-power applications was difficult, which limited the use of STATCOM for low power applications. However, in the last decades the new generation of converters, called multilevel converters, has paved the way for the use of STATCOM for high-power applications. Among the multilevel VSCs family, the modular configurations such as Cascaded H-Bridge (CHB) converters are the most interesting solutions for high-power grid-connected converter.

Although CHB converter are today the accepted solution for the implementation of STATCOM, challenges still exist both from a control and from a design point of view. The existing control methods are not utilizing the converter in the most appropriate way. This is for this reason that this thesis focuses on the control of CHB converters for STATCOM appplication and proposes a comprehensive control system to utilize the converter in the most optimize way.


Elektroteknik och elektronik



Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4319


Chalmers tekniska högskola

EA-salen, Hörsalsvägen 11, Chalmers

Opponent: Prof. Vassilios Agelidis Department of Electrical Engineering, Technical University of Denmark, Denmark.