Jae- Jung

Control Strategy for Improved Dynamic Performance of Variable-Speed Drives With Modular Multilevel Converter

This paper presents a control scheme for the modular multilevel converter (MMC) to drive a variable-speed ac machine, especially focusing on improving dynamic performance. Theoretically, the energy balance in the MMC cell capacitors is prone to be unstable at start-up and low-frequency operations. In addition, the MMC topology essentially requires advanced control strategies to balance energy and suppress the voltage pulsation of each cell capacitor. This paper proposes a control strategy for the robust dynamic response of MMC even at zero output frequency employing leg offset voltage injection. The leg offset voltage for balancing the arm energy is produced by direct calculation without the circulating current control loop controller. Thanks to the highly dynamic leg offset voltage from direct calculation and not conventional circulating current controller, the dynamic performance of an MMC at low speeds has conspicuously improved. The ac machine has been driven from standstill to rated speed without excessive cell capacitor voltage ripples utilizing this proposed strategy. The simulation and experimental results verify that stable operation is guaranteed down to <;2% of the rated speed under 40% step load torque disturbance.

Control of the Modular Multilevel Converter for variable-speed drives

This paper presents a control strategy of the entire frequency range operation for Modular Multilevel Converter (MMC), especially focusing on variable speed drive of an AC machine. The structure of MMC essentially requires energy balancing control so as to mitigate the voltage pulsation of each cell capacitor in converter arms. In the proposed control strategy, two operation modes are employed. One is a low frequency operation mode for start-up and low speed operation of the AC machine, and the other is a normal frequency operation mode from medium to rated speed of the AC machine. To reduce the pulsation, this paper proposes the energy balancing control strategies at each operation mode. Theoretically, the energy balancing control of the capacitors is prone to be unstable at low frequency operation. In order to prevent the instability, a special control strategy is introduced. The strategy exploits a common mode voltage and a circulating current with high frequency component in low frequency operation mode. With the proposed control scheme, the speed control range of the AC machine driven by MMC can be down to zero speed without instability of voltage of the cell capacitors. Experimental results for the energy balancing control are shown to demonstrate the effectiveness of the proposed control strategy.

A comprehensive cell capacitor energy control strategy of a modular multilevel converter (MMC) without a stiff DC bus voltage source

Cell capacitor energy control of a Modular Multilevel Converter (MMC) is conventionally done by controlling leg current and modulation strategy. In most of literatures, leg current transient is analyzed under an assumption that the DC bus is a stiff DC voltage source. In a real MMC-based HVDC transmission system, however, theres no such virtual stiff DC voltage source and the conventional regulation method can lead to poor dynamics of cell capacitor energy control and even make system unstable. In this paper, the MMC model is revised for circulating current transient analysis. Based on the revised model, a new comprehensive cell capacitor energy control strategy is proposed by updating leg capacitor energy reference on-line and injecting positive and negative sequence circulating currents. Validity of the proposed method is verified by a 7-level scaled version prototype experimental setup.

Principle, control and comparison of modular multilevel converters (MMCs) with DC short circuit fault ride-through capability

Lack of DC short circuit fault blocking and ride through capability is one of main issues in applications of Modular Multilevel Converter (MMC) to actual HVDC transmission system. Recently, several topologies have been proposed to provide DC short circuit fault blocking capability and/or DC fault ride through capability. In this paper, the operation principles, functionalities, and characteristics of several topologies are compared. And, its revealed that the conventional leg capacitor energy balancing strategy by regulating DC component of leg current fails under DC short circuit fault. A corresponding new leg capacitor energy balancing method by common mode voltage injection is proposed. Validity of the proposed method is verified by computer simulation.

A novel control strategy of a modular multilevel converter (MMC) based VSC-HVDC transmission system

In the conventional control strategy of the VSC-HVDC system based on the MMC, direct modulation was employed and the terminal behavior of the MMC was similar to that of the two-level converter. The DC bus voltage of the power dispatcher side was regulated indirectly by controlling voltage regulator side DC bus voltage, and the transmission line current was determined passively by the power flow. Fluctuation of the transmission line voltage would occur during rapid power flow variation due to the inherent capacitor-inductor coupling in the DC transmission line. In this paper, a new concept of the control strategy is proposed. At first, by the proposed control strategy AC grid current control, DC bus current control, and arm capacitor voltage balancing control of the MMC are fully decoupled. Secondly, the DC bus of the MMC operates as a controlled voltage source and the power flow is controlled by regulating the transmission current directly and actively, and the transmission line voltage fluctuation is fully suppressed during power flow variation. Validity of the proposed control strategy is verified by both full scale simulation and down scale experiment.

A comprehensive AC side single line to ground fault ride through strategy of a modular multilevel converter for HVDC system

The AC side Single Line to Ground (SLG) fault is one of the most frequent faults in power systems. And, in an HVDC system based on modular multilevel converter it calls for the fault ride through strategy to transmit maximum possible electricity during the fault to secure power system stability. It presents different characteristics of SLG faults at the voltage regulator side and the power dispatcher side. In this paper a comprehensive fault ride through strategy for AC side SLG fault occurred at both converters of the HVDC system is proposed. The proposed method presents fast dynamics and promises maximum possible electricity transmission during the faults. Voltage fluctuation and current overshoot in transmission line during SLG fault can be fully suppressed by the proposed method. Moreover, the proposed control strategy is free of inter-station communication and secures the reliability of HVDC transmission system. Validity of the proposed method is verified by simulation of a ±200kV, 400MW point-to-point HVDC system (216 sub-modules per arm).

A cell capacitor energy balancing control of Modular Multilevel Converter considering the unbalanced AC grid conditions

This paper presents a control scheme for the regulation of cell capacitor energy balancing of a Modular Multilevel Converter (MMC) for HVDC transmission systems, considering the unbalanced AC grid conditions. It is essential that the MMC balancing control should be valid not only for the balanced normal operations but also for the asymmetrical grid fault conditions. This paper proposes the control scheme that has the ability of seamless mode change between balanced and unbalanced grid condition. Applying the proposed method, the capacitor energy balancing operation is successfully realized with improved dynamic responses. Finally, the simulation results verify the validity of the proposed method.

A cell capacitor energy balancing control of MMC-HVDC under the AC grid faults

This paper presents the techniques for regulation of system balancing in modular multilevel converter (MMC) based HVDC system under the AC grid double line fault condition. Due to the special structure of MMC, the balancing of the cell capacitor voltage is crucial for any operating circumstance of the system. Also, the responses against the grid faults of HVDC system based on MMC are different to those based on the conventional voltage sourced converters. Therefore, even under the nonpermanent fault in AC grid, the balancing of the cell capacitor energy of MMC has to be guaranteed continuously. Among AC grid fault modes, the double line fault causes one of the most severe transients in MMC. In this paper, a balancing control method of MMC based on an offset voltage injection strategy in the output reference voltages is proposed. By the virtue of the proposed method, the HVDC system based on MMC can have the enhanced fault ride-through capability against the double line fault in AC grid. The validity of the proposed method is verified by simulation results of a ±200kV, 400MVA point-to-point HVDC system.

A switching frequency reduction and a mitigation of voltage fluctuation of modular multilevel converter for HVDC

A switching loss of Modular Multilevel Converter(MMC) might increase drastically in HVDC system because the number of sub-module(SM) is proportional to the DC-link voltage. And, a special strategy for reducing switching frequency has been significant research issue in terms of overall operating efficiency of MMC for HVDC system. The voltage fluctuation of capacitor in SM, however, increases as the switching frequency decreases, and the capacitor with large capacitance which is the main portion of equipment cost for SM is required to mitigate the voltage fluctuation. In this paper, the switching frequency reduction strategy is proposed using the sorting method with a virtual capacitor voltage of individual SMs. In addition, this paper presents the 2nd order harmonic circulating current injection to suppress the voltage fluctuation. By numerical loss analysis, it is identified that the 2nd order harmonic current injection does not incur severe additional loss. Thanks to the harmonic current injection, the capacitance of SM capacitor could be reduced by 33% at the cost of only 0.05 % efficiency degradation in the given simulation condition. To evaluate the effectiveness of the proposed strategies, the computer simulation with 400 kV, 400MA, 221-level MMC has been performed and the results are discussed. Additionally, validity of the proposed strategies has been verified by 7-level down scaled prototype experimental setup.

A new topology of multilevel VSC converter for hybrid HVDC transmission system

In this paper, the existing Modular Multilevel Converter (MMC) topologies for Line Commutated Converter (LCC)-Voltage Source Converter(VSC) connected as a hybrid High Voltage DC (HVDC) transmission system are reviewed and a new topology of multilevel converter for a hybrid HVDC is introduced. Among the existing MMC topologies for the hybrid HVDC, an MMC structure consisted of Half-Bridge SubModule(HBSM)s and Full-Bridge SubModule(FBSM)s has characteristics such as reduced system cost, low operation loss, but still keeping capability to cope with DC short circuit fault. However, it is very difficult for the conventional hybrid MMC structure, where each arm of MMC is consisted of mixed HBSM and FBSM, to balance the submodule capacitor voltages under sliding of DC bus voltage. For solving the defect of the conventional structure of MMC, an asymmetric MMC, where in a leg an arm is consisted of HBSM and other arm of FBSM, is devised. The proposed asymmetric MMC can regulate the DC bus voltage freely without uncontrollable submodule capacitor voltages. The problems of the conventional structure of MMC and the validity of asymmetric MMC are verified by both computer simulation and experiment results.