Hak-Jun Lee

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.

A Current Controller Design for Current Source Inverter-Fed AC Machine Drive System

A current source inverter (CSI) requires a capacitor filter for the commutation of switching device as well as for attenuating switching harmonics. Hence, the CSI-fed ac machine has a second-order system in the continuous time domain. This paper presents a design methodology for the closed-loop current controller of the CSI-fed ac machine drive system. A multiloop current controller design using a pole/zero cancellation method is employed with a transfer function matrix. To decouple the cross-coupling terms which cause mutual interferences between the d- and q-axes in the synchronous reference frame, two types of controller are proposed and implemented using different decoupling method. Additionally, active damping methods are incorporated to enhance the stability of the system. A stability analysis in discrete-time domain is investigated to verify the feasibility of the proposed closed-loop current controller. To evaluate the effectiveness of the proposed current controller, computer simulations and experimental tests were performed and the results are discussed.

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.

System configuration and control strategy using FPGA for Hybrid vehicle power Control Unit

In this paper, it is presented a configuration to miniaturize HPCU (Hybrid vehicle Power Control Unit) and a control strategy using FPGA logic. To miniaturize the HPCU, the film capacitor is used as a DC-link capacitor instead of the electrolytic capacitor. Additionally, for the minimization of control hardware, a FPGA based controller is used to control multi-module of power converter. To verify the feasibility of the control strategy and FPGA based control hardware, the laboratory based load test is performed. With FPGA based hardware, the sampling frequency of the digital current control loop can be increased by 30% compared to DSP software based control system.

FPGA-based motion controller with a high bandwidth current regulator

In this paper, a motion controller based on a Field Programmable Gate Array (FPGA) with a high performance current regulator is presented. This system contains a processor and a high bandwidth current regulator as a peripheral, and all of these are implemented on a single-chip FPGA. The execution time of the current regulator is less than 200 ns at 100 MHz clock frequency. Thanks to this fast execution, the inevitable digital delay, which is 1.5 times the sampling period in a PWM inverter has been reduced to a half of the sampling period. As the result, the control bandwidth of the current regulator has been extended up to about 3 kHz at 10 kHz PWM switching frequency. By exploiting the performance of the current regulator, the speed regulation bandwidth also enhanced up to 700 Hz. The experimental results are presented to demonstrate the validity of the proposed system and the performance of the regulator.

A current controller design for current source inverter-fed PMSM drive system

A current source inverter (CSI) requires a capacitor filter for the commutation of switching device as well as attenuating switching harmonics. This paper presents the current controller design of CSI-fed PMSM (Permanent Magnet Synchronous Machine) drive system in order to attenuate resonance due to the LC filter. Basically, the CSI-fed drive system is modeled as a 2nd order system. Hence a multi-loop current controller design using a pole/zero cancellation method is proposed with a transfer function matrix. To compensate the cross-coupling term, two types of controller are implemented using different decoupling method. Additionally, active damping methods are proposed to enhance the stability of the system. To evaluate the effectiveness of the proposed current controller, computer simulation and experiment have been performed and the results are discussed.

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 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.

Layout of IGBT-based Current Source Converter for low stray inductance

In this paper, a layout of Insulated Gate Bipolar Transistor(IGBT)-based Current Source Converter (CSC) to reduce stray inductance is proposed. Due to the rapid evolution of IGBT, it has become a competitive switching device in comparison with other high power switching devices even in MW power conversion system. The higher switching frequency of IGBT might reduce the size of other passive component of CSC significantly, and the demerits of CSC can be diminished remarkably, while the merits of CSC can be retained. However, since the switching speed of IGBT is high, the overshoot of the voltage across IGBT due to the stray inductance increases. And, the switching loss and voltage stress would be prohibitive. To decrease the stray inductance, a layout of power circuit exploiting magnetic coupling between bus bar connecting switching devices is devised in this paper. The stray inductances of two different layouts of IGBT-based CSC are compared to verify the effectiveness of the proposed strategy, experimentally.

System configuration and control strategy for compound type hybrid excavator with ultra capacitor

This paper presents a power converter configuration for the compound type hybrid excavator with an ultra capacitor, which is directly connected in parallel to dc-link capacitor without DC/DC converter. With this structure, the efficiency of the system has been improved compared to the conventional compound type hybrid excavator where an ultra capacitor was connected via DC/DC converter. And, a power control strategy for the suggested configuration is described. To suppress the oscillation of the voltage of the ultra capacitor, an observer for the internal voltage of the ultra capacitor is devised. To evaluate the effectiveness of the proposed configuration and power control strategy, extensive computer simulations have been performed and the results are discussed. The feasibility of the simulation results is experimentally verified based on mid-sized excavator.