Hsin-Chih Chen

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.

A flexible low-voltage ride-through operation for the distributed generation converters

With more and more distributed energy resources (DERs) are installed in the utility grid, the utility requires the DER generation system to remain grid-connected and injects reactive and active power to support grid voltage during voltage sags. In this paper, a positive- and negative-sequence current injection method is proposed to meet the low-voltage ride through (LVRT) requirement. The proposed method predefined a current constraint to avoid the overcurrent during the LVRT operation and adjust the positive-sequence reactive current to reduce the DC-bus voltage ripple. Comparisons of the proposed method and other LVRT techniques are also presented.

Robust current control and commutation tuning for an IPMSM drive

The operating performance improvement of an interior permanent magnet synchronous motor (IPMSM) drive via robust current control and intelligent commutation tuning is studied in this paper. First, the nominal motor parameters are estimated and a robust current control scheme is designed to possess close and robust winding current tracking performance. It is known that the torque generating capability of an IPMSM is affected by the changes of commutation instant and field excitation. The proof is given to show that the latter can be equivalently achieved by tuning the commutation instant, and the effects of these two variations on the IPMSM drive performances under speed open-loop and closed-loop conditions are observed analytically and experimentally. Then accordingly, an intelligent tuning approach is developed to automatically determine the advance of commutation instant. The minimum current command is achieved to obtain better torque generating capability equivalently. A DSP-based IPMSM drive is established and the effectiveness of the proposed control approaches is demonstrated experimentally.

A Low-Voltage Ride-Through Technique for Grid-Connected Converters With Reduced Power Transistors Stress

With more and more distributed energy resources being installed in the utility grid, grid operators start imposing the low-voltage ride-through requirement on such systems to remain grid-connected and inject reactive and/or active current to support grid voltage during fault conditions. This paper proposes a positive and negative sequence current injection method to meet such requirement. In the meantime, the proposed method can reduce the second harmonic ripples in the dc link of the power converter without violating the peak current constraint of the power transistors. The long-term reliability of the converter system can thus be improved.

Average power control of DC bus voltages of cascaded H-bridge multilevel converters

This paper presents an average power balancing control technique for the modular multilevel cascaded converter (MMCC) based on single-star bridge cells (SSBC) in the static synchronous compensator (STATCOM) applications. Detailed power flow analyses of the MMCC-SSBC converter, from the bottoem level of individual H-bridge modules, then the middle level of per-phase circuit, to the top level of the three-phase circuit, are performed. By utilizing the outcome of such power flow analyses, the proposed method can precisely control all the DC bus voltages of the MMCC-SSBC while performing the reactive power compensation, even under extreme conditions like voltage sags, thus enhance the low voltage ride through functionalities of the STATCOM, which is very critical for renewable energy deployment. The proposed control algorithm is verified by a MMCC-SSBC test bench in the laboratory under normal and fault grid conditions.

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 DC Bus Voltage Balancing Technique for the Cascaded H-Bridge STATCOM With Improved Reliability Under Grid Faults

As more distributed renewable energy sources are installed in the utility grid, the static synchronous compensators are applied to manage the power factor and the grid voltage in the medium-voltage level of the power system. The cascaded multilevel converter with single-star topology is well applied to this application. The dc-capacitor voltage balancing control is a fundamental issue, and it can be accomplished by negative-sequence current and/or zero sequence voltage injection. Unfortunately, the zero sequence voltage injection increases the risk of over-modulation and the negative-sequence current injection results in high peak current. This paper provides the voltage balancing strategy to manage the peak current or the modulation index. Besides, all the asymmetrical grid voltages are considered in the proposed method to satisfy the grid fault operation. Laboratory test results verify that the proposed method limits the peak current or manages the modulation index during the grid fault operations.