Chia-Tse Lee

A New Droop Control Method for the Autonomous Operation of Distributed Energy Resource Interface Converters

Microgrid is widely accepted as an effective mean of integrating various distributed energy resources (DERs) through their interface converters to provide electric power of high quality and reliability. These distributed resources interface converters (DICs) are required to operate in an autonomous fashion without any communication for enhanced reliability. Conventionally, the real power-frequency droop control and the reactive power-voltage droop are adopted as the de-centralized control strategies in these DICs for the autonomous power sharing operations. However, the reactive power sharing of Q-V droop control often deteriorates if there are mismatched transmission line impedance characteristics. In this paper, a Q-V dot droop control method with V dot restoration mechanism is proposed to improve the sharing of reactive power. Its operation principle and control method are explained and analyzed. In addition, simulation and experimental results are presented to validate the effectiveness of the proposed method.

A Low-Voltage Ride-Through Method With Transformer Flux Compensation Capability of Renewable Power Grid-Side Converters

With the growing penetration of renewable energy resources, the grid operators place higher emphasis on their grid integration requirements. The low-voltage ride-through (LVRT) capability is one of the most important issues for maintaining the grid stability. This paper proposes a LVRT technique for effective utilization of the ampere capacity of the grid-connected converter to meet the LVRT requirement, and to reduce the potential post-sag inrush current of the transformer within the grid-connected converter system. This technique precisely manages the peak value of converters phase current while performing the transformer flux compensation and the current injection for full utilization of the ampere capacity without the risk of triggering the overcurrent protection. The operation principles of the proposed method is explained, and laboratory test results are presented for validation.

A reactive current injection technique for renewable energy converters in low voltage ride-through operations

Distributed energy resources have assumed an important role in meeting the energy demand of todays world, thanks to their dramatic growth in recent years. Their potential in reducing the green house gas emission also leads to great attentions and expectations. As DERs are integrated into the utility grid, they must follow the grid connection requirement for the stability of the power system. This paper focus on the reactive current injection during the low voltage ride through of the grid interface converter. A positive sequence and negative sequence reactive current injection technique is proposed to meet the low voltage ride through requirement, its the operation principle and the control design of the proposed method are explained in details, and its effectiveness is validated by laboratory test results.

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.

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.

Average power balancing control of a STATCOM based on the cascaded H-bridge PWM converter with star configuration

This paper presents the average power balancing (APB) control method for the modular multilevel cascaded converter based on single-star bridge cells in the static synchronous compensator applications. The proposed control method accomplishes both the reactive power compensation and the dc bus voltage balancing control even under grid voltage sags. The low-voltage ride-through capability of the proposed method will become more and more important as more distributed generation resources are integrated into the grid. Laboratory test results are provided to validate the proposed APB control method.

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.