• The Transactions of the Korean Institute of Power Electronics
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• v.19 no.4
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• pp.332-341
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• 2014

A cascaded H-bridge multi-level STATCOM(STATic synchronous COMpensator), which is composed of many cell inverters with independent dc-sources, generates inevitably dc-side voltage unbalance among phases when it compensates unbalanced load. It comes from the difference of flowing active power in each phase when this compensator makes negative-sequence current to eliminate the unbalance of source-side current. However, this unbalance can be controlled by injecting zero-sequence current which is decoupled with grid currents, so the compensator can work well during this balancing process. Both a feedback control algorithm, which produces zero-sequence current proportional to dc-side voltage unbalance within each phase, and a feedforward control algorithm, which makes zero-sequence current directly from the compensator's negative-sequence current, were proposed. The dc-side voltage of each phase can be controlled stably by these proposed algorithms in both steady-state and transient, so the compensator can have fast response to satisfy control performance under rapid changing load. These balancing controllers were implemented and verified via simulation and experiment.

• Proceedings of the KIPE Conference
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• 2016.11a
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• pp.143-144
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• 2016

The circulating current control within the phase legs is one of the main control objectives in a modular multilevel converter (MMC) under different operating conditions. This paper proposes a control strategy of circulating currents in the MMC under unbalanced voltage by using a proportional-resonant (PR) controller. Under the unbalanced voltage, the circulating currents in the MMC consists of three components such as positive-sequence, negative-sequence, and zero-sequence circulating currents. With the PR controller, all components of the circulating current will be directly controlled in the stationary reference frame without decomposing into positive- and negative-sequence components. Thus, the ripples in the circulating currents and the DC current are suppressed under the unbalanced voltage. The effectiveness of the proposed method is verified by simulation results based on PSCAD/EMTDC simulation program.

• Journal of Electrical Engineering and Technology
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• v.13 no.3
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• pp.1241-1250
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• 2018

Parallel three-phase voltage source inverters in a direct connection configuration are widely used to increase system power ratings. A zero-sequence circulating current can be generated according to the switching method; however, the zero-sequence circulating current not only distorts current, but also reduces the system reliability and efficiency. In this paper, a model predictive control scheme is proposed for parallel inverters to drive an interior permanent magnet synchronous motor with zero-sequence circulating current suppression. The voltage vector of the parallel inverters is derived to predict and control the torque and stator flux components. In addition, the zero-sequence circulating current is suppressed by designing the cost function without an additional current sensor and high-impedance inductor. Simulation and experimental results are presented to verify the proposed control scheme.

• Journal of Electrical Engineering and Technology
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• v.13 no.2
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• pp.550-558
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• 2018

This paper presents the current limit strategy of voltage controller of delta-connected H-bridge static synchronous compensator (STATCOM) under an unbalanced voltage fault event. When phase to ground fault happens, the feasibility to heighten the magnitude of sagging phase voltage is considered by using symmetric transformation method in delta-structure STATCOM. And the efficiency to cover the maximum physical current limit of switching device is considered by using vector analysis method that calculate the zero sequence current for balancing the cluster energy in delta connected H-bridge STATCOM. The result is simple and obvious. Only positive sequence current has to be used to support the unbalanced voltage sag. Although the relationship between combination of the negative sequence voltage with current and zero sequence current is nonlinear, the more negative sequence current is supplying, the larger zero sequence current is required. From the full-model STATCOM system simulation, zero sequence current demand is identified according to a ratio of positive and negative sequence compensating current. When only positive sequence current support voltage sag, the least zero sequence current is needed.

• The Transactions of the Korean Institute of Electrical Engineers B
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• v.55 no.1
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• pp.54-60
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• 2006

A power limiting algorithm is proposed for stable operation of grid-connected inverter in case of grid voltage unbalance considering the operation limit of inverter. During the voltage unbalance the control performance of Inverter. is degraded and the output power contains 120Hz ripple due to the negative sequence of voltage. In this paper, conventional dual sequence current controller is implemented to solve these problems using separated control of positive and negative sequence. Especially the maximum power limit which guarantees the maximum rated current of the inverter is automatically calculated as the instant grid voltage changes. As soon as the voltage recovers the proposed algorithm can return to the normal power control mode accomplishing low voltage ride through. Proposed algorithm is verifed using PSCAD/EMTDC simulations and tested experimentally at 4.4kW wind turbine simulator set-up.

• Journal of Electrical Engineering and Technology
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• v.12 no.2
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• pp.751-760
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• 2017

This paper proposes a point of common coupling (PCC) voltage compensation algorithm using a current limitation strategy for use in distributed generation (DG). The proposed strategy maintains the PCC voltage by prioritizing currents when an output current reference is larger than the current capacity of the power condition system (PCS) of the DG. With this strategy, the DG outputs the active current, reactive current, and the negative sequence current. The DG uses the reactive current for maintaining the PCC voltage within a normal range; the negative sequence current is used for reducing the PCC voltage unbalance. The proposed method was verified using PSIM simulation and experimental results.

• The Transactions of the Korean Institute of Power Electronics
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• v.27 no.4
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• pp.283-290
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• 2022

This study proposes a voltage control scheme in a synchronous reference frame to improve the dynamic characteristics of double-conversion UPSs. UPSs need to control positive and negative sequence voltage, so that positive and negative sequence extractors are generally used to obtain each sequence of the voltage and current. Voltage and current controllers for each sequence are implemented. However, the extractor causes considerable delay, and the delay restricts the control performance, especially for the current controller. To improve the dynamics of the current controller, the proposed scheme adopts a unified current controller without separating positive and negative sequences. By using discrete-time current controller, the control bandwidth can be extended significantly so that negative sequence current can be controlled. To enhance the performance, an additional feed-forward technique for output voltage regulation is proposed. The validity of the proposed controller is verified by experiments.

• The Transactions of the Korean Institute of Power Electronics
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• v.19 no.5
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• pp.399-406
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• 2014

This study investigates the structure and operation of the ITER VS converter and proposes a sequence control method for six series-connected VS converters to reduce reactive power. The operation and the proposed sequence control method are verified through RTDS simulation. The ITER VS converter must supply voltage/current to the superconducting magnets for plasma current vertical stabilization, and the four-quadrant operation must proceed without a zero-current discontinuous section. The operation mode of the VS converter is separated into a 12- and 6-pulse circulating current and transition modes according to the size of the load current. The output voltage of the unit VS converter is limited because of the rated voltage; however, the superconducting coil must increase the operating output voltage. Thus, the VS converter must be connected in a 6-series to provide the required operating output voltage. The output voltage of the VS converters is controlled continuously; however, reactive power is limited within a minimized value of the grid. In this study, the unit converter is compared with converters connected in a 6-series to determine a suitable sequence control method. The output voltage is the same in all cases, but the maximum reactive power is reduced from 100% to 73%. This sequence control method is verified through RTDS simulation.

• Journal of Power Electronics
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• v.19 no.1
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• pp.234-243
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• 2019

Studying the control strategy of a microgrid under the load unbalanced state helps to improve the stability of the system. The magnitude of the power fluctuation, which occurs between the power supply and the load, is generated in a microgrid under the load unbalanced state is called negative sequence reactive power $Q^-$. Traditional power distribution methods such as P-f, Q-E droop control can only distribute power with positive sequence current information. However, they have no effect on $Q^-$ with negative sequence current information. In this paper, a stationary-frame control method for power sharing and voltage unbalance compensation in islanded microgrids is proposed. This method is based on the proper output impedance control of distributed generation unit (DG unit) interface converters. The control system of a DG unit mainly consists of an active-power-frequency and reactive-power-voltage droop controller, an output impedance controller, and voltage and current controllers. The proposed method allows for the sharing of imbalance current among the DG unit and it can compensate voltage unbalance at the same time. The design approach of the control system is discussed in detail. Simulation and experimental results are presented. These results demonstrate that the proposed method is effective in the compensation of voltage unbalance and the power distribution.

• Proceedings of the KIPE Conference
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• 2001.10a
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• pp.586-590
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• 2001

In this paper, a novel control scheme compensating source voltage unbalance and harmonic currents for hybrid active power filters is proposed, where no low/high-pass filters are used in compensation voltage composition. The phase angle and compensation voltages for source harmonic current and unbalanced voltage components are derived from the positive sequence component of the unbalanced voltage set, which is simply obtained by using digital all-pass filters. Since a balanced set of the source voltage obtained by scaling the positive sequence components is used as reference values for source current and load voltage, it is possible to eliminate the necessity of low/high-pass filters in the reference generation. Therefore the control algorithm is much simpler and gives more stable performance than the conventional method. In addition, the source harmonic current is eliminated by compensating for the harmonic voltage of the load side added to feedback control of the fundamental component.