Ping-Heng Wu

Zero-Sequence Voltage Injection for DC Capacitor Voltage Balancing Control of the Star-Connected Cascaded H-Bridge PWM Converter Under Unbalanced Grid

This paper presents a dc capacitor voltage balancing convtrol method for the star-connected cascaded H-bridge PWM converter in the static synchronous compensator (STATCOM) applications. The proposed control utilizes the zero-sequence voltage injection to accomplish the dc capacitor voltage balancing, and the this operation of zero-sequence voltage does not affect the original reactive power control. The proposed control method also works for the low-voltage ride-through operation. The control algorithm is verified with a 220-V 1kVA STATCOM based on star-connected cascaded PWM converter, and the test results verify that all the dc capacitor voltages are still regulated at the commanded value even as the grid voltage sag occurs.

A flexible DC voltage balancing control based on the power flow management for star-connected cascaded H-bridge converter

This paper presents the flexible dc capacitor voltages balancing control method for star-connected cascaded H-bridge converters. In this paper, the power flows are analyzed by investigating all the converters control freedoms. Based on the analyses, users can accomplish the dc capacitor voltage balancing with both negative-sequence current and zero-sequence voltage. These two kinds of control options are further integrated together with the defined weighting factor K. The low-voltage ride-through operation of a proposed control method is also discussed in this paper. The control method is verified with a 220-V 1-kVA static synchronous compensators based on the star-connected cascaded H-bridge converters, and the experimental test results show that all the dc voltages are well regulated at the commanded value with different weighting factor K.

Managed DC voltage utilization technique for the renewable energy source based on the star-connected cascaded H-bridges converter

The modular multilevel cascaded converter with single-star bridge-cell (MMCC-SSBC) is very suitable for next generation high-scale renewable energy resource system for the low switching loss, low output current distortion, and high reliability. The voltage balancing control is an important issue for MMCC-SSBC to maintain system operation which can be accomplished by negative sequence current and/or zero sequence voltage injection. The pure negative sequence current injection results in unbalanced output current, and thus increasing the stress of power electronic devices. On the other hand, pure zero sequence voltage injection increases the DC voltage utilization, and thus increasing the risk of over-modulation. This paper provides a flexible voltage balancing by regulating the effectiveness between these two injections to manage the DC voltage utilization and reduce the imbalance of output currents. The laboratory test results verify the proposed method, which are tested under unbalanced output power between each phase of MMCC-SSBC and asymmetrical voltage faults.

Delta-connected cascaded H-bridge converter application in unbalanced load compensation

The delta-connected cascaded H-bridge converter has been applied to compensate flicker and unbalance load to meet power quality requirement. Based on the detailed power analysis, this paper presents a dc capacitor voltage-balancing technique for the converter. This paper also proposes a current limit control technique to avoid overstressing the power semiconductor devices of the converter. The control techniques in this paper are verified in a 220-V, 1-kVA laboratory test bench.

A fault tolerant operation technique for STATCOMs based on star-connected cascaded H-bridges multilevel converter

As more and more renewable and distributed energy sources are connected to the grid, the static synchronous compensator (STATCOM) plays an important role in power system to increase the power factor and regulate the load voltage. In general cases, these STATCOM systems are required to install at the medium voltage level (1kV to 50kV) and regulated for the high reliability. Thus, the modular multilevel cascaded converters are applicable to these requirements. This paper provides a zero sequence voltage injection, so called neutral point shift, for fault tolerant operation as the module of bridge cell is broken. The limit of zero sequence voltage injection and the proposed phase/clustered voltage balancing control are also introduced in this paper. The control algorithm is applied with a 7-level star-connected cascaded converter and test results verify the proposed fault tolerant operation and phase/clustered voltage balancing control.

A voltage balancing control based on average power flow management for the delta-connected cascaded H-bridges converter

The electric arc furnaces (EAFs) are the major disturbances in the medium-voltage level of power system for the low frequency voltage variations, which lead to incandescent lamp flickering. In order to alleviate these voltage variations, the modular multilevel cascaded converter with the topology of single-delta bridge-cell (MMCC-SDBC) is used to the medium-voltage level flicker compensator application. The voltage balancing control of MMCC-SDBC is a fundamental issue to maintain the system operation. This paper presents a voltage balancing control by zero sequence current injection based on the average power flow management. Besides, the proposed method has ability to operate at unbalanced grid voltages since both the positive and negative sequence voltages are considered. The laboratory test results show the verifications of proposed method, which are tested at a down-scaled 220V and 1kVA converter.

A peak current limit control technique in low-voltage ride through operation of the star-connected cascaded H-bridges converter

As smart grid techniques grows quickly and complexly, the Static synchronous Compensator (STATCOM) has become one of the most important role to manage the power flow and regulate the grid voltage. As grid voltage faults occur, the STATCOM becomes more crucial to increase the system stability. This paper present the STATCOM application by the modular multilevel cascaded converter (MMCC) based on single start bridge cell (SSBC) for the medium-voltage level to VAR compensation. In low-voltage ride-through (LVRT) operation, this paper proposes the peak current limit control (PCLC) to meet the grid requirement and utilize the converters current capacity. Moreover, this paper also presents a average power balancing control by zero-sequence voltage injection to balance converters voltages and without affects the converters output current.

The Delta-Connected Cascaded H-Bridge Converter Application in Distributed Energy Resources and Fault Ride Through Capability Analysis

This paper presents a flexible dc voltage balancing control technique of the modular multilevel cascaded converter with single-delta bridge-cells (MMCC-SDBC) in the distributed energy resources (DERs) system for maintaining the stable operation under different unbalanced conditions. The unbalanced conditions, for example, the different output powers of the DERs in bridge cells, and the unbalanced grid faults are considered in the proposed method. In addition, the comparison of the fault ride through capability between MMCC with single-star bridge-cells and MMCC-SDBC is addressed. The laboratory test results are given to verify the proposed method and the analysis.

The delta-connected cascaded H-bridge converter application in distributed energy resources and fault ride through capability analysis

There are two unbalanced conditions for cascaded converter in Renewable Energy Source (RES) system. One is from the different output power of RES in bridge cells, and the other is grid voltage unbalanced fault. This paper presents a flexible voltage control technique of Modular Multilevel Cascaded Converter with single-delta bridge cells (MMCC-SDBC) in the RES system for maintaining the stable operation under the unbalanced conditions. Moreover, the comparison of the fault ride through capability between MMCC with Single Star Bridge Cells (MMCC-SSBC) and MMCC-SDBC is addressed in this paper. The laboratory test results are given to verify the proposed method and the analysis.

A Transformer Inrush Reduction Technique for Low-Voltage Ride-Through Operation of Renewable Converters

The low-voltage ride-through (LVRT) capability has become one of the most important issues since more renewable and distributed energy resource are installed in the grid. The LVRT capability means the converter should remain grid-connected and inject required current to support gird voltage. On the other hand, the sudden reduction of grid voltage may result in magnetic flux deviation in the step-up transformer as grid voltage sag occurs. The deviation of magnetic decays very slowly due to the high efficiency transformer, and it would lead to inrush current as grid voltage restores to normal level. This paper proposes a method to mitigate the magnetic flux deviation and limit the peak current without excelling the current capability of semiconductor devices by using a close-loop current control during LVRT operation. The proposed method can manage the converters output peak current and reduce the risk of inrush current as grid voltage restores.