• The Transactions of The Korean Institute of Electrical Engineers
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• v.65 no.1
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• pp.23-30
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• 2016

Wake effects cause wind turbine generators (WTGs) within a wind power plant (WPP) to produce different levels of active power and subsequent reactive power capabilities. Further, the impedance between a WTG and the point of interconnection (POI)-which depends on the distance between them-impacts the WPP's reactive power injection capability at the POI. This paper proposes a voltage control scheme for a WPP based on the available reactive current of the doubly-fed induction generators (DFIGs) and its impacts on the POI to improve the reactive power injection capability of the WPP. In this paper, a design strategy for modifying the gain of DFIG controller is suggested and the comprehensive properties of these control gains are investigated. In the proposed scheme, the WPP controller, which operates in a voltage control mode, sends the command signal to the DFIGs based on the voltage difference at the POI. The DFIG controllers, which operate in a voltage control mode, employ a proportional controller with a limiter. The gain of the proportional controller is adjusted depending on the available reactive current of the DFIG and the series impedance between the DFIG and the POI. The performance of the proposed scheme is validated for various disturbances such as a reactive load connection and grid fault using an EMTP-RV simulator. Simulation results demonstrate that the proposed scheme promptly recovers the POI voltage by injecting more reactive power after a disturbance than the conventional scheme.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.65 no.9
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• pp.1479-1485
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• 2016

This paper presents DC voltage recovery time improvement method in DC link of High Voltage Direct Current (HVDC) with offshore wind farm. The wind farm should be satisfied Low Voltage Ride Through(LVRT) control strategy when grid faults occur. The LVRT control strategy indicates actions which have to be executed according to the voltage dip ratio and the fault duration. However, The LVRT control strategy makes between wind farm and power system through DC Link voltage when grid fault occurs. The de-loading scheme is one of the method to control the DC voltage. But de-loading scheme need to long DC voltage recovery time. Thus, this paper proposes an improved de-loading scheme and we analysis DC voltage and active power reference through a simulation.

• Journal of Power Electronics
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• v.11 no.6
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• pp.931-937
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• 2011

This paper presents a method for the performance improvement of a shunt active power filter (SAPF) using the indirect current control (ICC) scheme. Compared to the conventional direct current control (DCC) scheme, the ICC gives better performance with a lower number of sensors. A simplified and efficient control algorithm using a low cost Intel 80C196KC microcontroller is implemented using only two current sensors for the source current and one voltage sensor for the DC-link voltage of the SAPF circuit. The objective is to eliminate harmonics and to compensate the reactive power produced by non-linear loads such as an uncontrolled rectifier feeding an inductive load. The APF is realized using a three phase voltage source inverter (VSI) with a dc bus capacitor. Experimental results are presented to prove the better performance of the ICC method over the DCC one.

• 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.

• Proceedings of the KIEE Conference
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• 1991.07a
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• pp.583-586
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• 1991

This paper presents a comprehensive study on the stability of several control schemes for the induction motor driven by current source inverters. A stator voltage-controlled current source inverter drive system without a speed sensor is investigated in order to find appropriate control schemes, which are primarily based on direct or, alternatively, indirect frequency control scheme. The overall control systems with either voltage control loop or current and voltage control loops provided in addition to each frequency control scheme are analyzed by utilizing the root locus method and simulated by computer to illustrate the validity of this analysis.

• Proceedings of the KIPE Conference
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• 2017.07a
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• pp.74-75
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• 2017

This paper proposes a control scheme of the voltage ripple suppression for the permanent magnet exciter generator. The output voltage of the permanent magnet excitation generator is affected by the field current, load current and the engine speed. The engine speed can be controlled by the governor. But, the actual frequency is changed at the starting and a sudden load variation. As a result, output voltage overshoot and undershoot can decrease the power quality in the grid system. The proposed control scheme uses a frequency factor to control the field current of the generator for the voltage ripple reduction. Because of the linkage flux is proportional to the frequency, the instantaneous frequency can consider the linkage flux. The proposed control method shows the improved control performance for the permanent magnet excitation generator through simulation.

• The Transactions of the Korean Institute of Power Electronics
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• v.22 no.4
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• pp.285-296
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• 2017

This paper proposes an improved modulation scheme for the medium voltage modular multi-level converter (MMC), which operates in the nearest level control and applies in the medium voltage direct current (MVDC) system. In the proposed modulation scheme, the offset (neutral-to-zero output) voltage is adjusted, with the phase voltage magnitude, thereby maintaining a constant value with N+1 level in the controllable modulation index (MI) range. In order to confirm the proposed scheme's validity, computer simulations for the 22.9 kV - 25 MVA MMC were performed with PSCAD/EMTDC, as well as hardware experiments for the 380 V - 10 kVA MMC. The proposed modulation scheme offers to build a constant pole voltage regardless of the MI value, and to build a phase voltage with improved total harmonic distortion (THD).

• Journal of Power Electronics
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• v.16 no.4
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• pp.1551-1564
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• 2016

The high penetration level of inverter-based distributed generation (DG) power plants is challenging the low-voltage ride-through requirements, especially under unbalanced voltage sags. Recently, a flexible injection of both positive- (PS) and negative-sequence (NS) reactive currents has been suggested for the next generation of grid codes. This can enhance the ancillary services for voltage support at the point of common coupling (PCC). In light of this, considering distant grid faults that occur in a mainly inductive grid, this paper proposes a complete voltage support control scheme for the interface inverters of medium or high-rated DG power plants. The first contribution is the development of a reactive current reference generator combining PS and NS, with a feature to increase the PS voltage and simultaneously decrease the NS voltage, to mitigate voltage imbalance. The second contribution is the design of a voltage support control loop with two flexible PCC voltage set points, which can ensure continuous operation within the limits required in grid codes. In addition, a current saturation strategy is also considered for deep voltage sags to avoid overcurrent protection. Finally, simulation and experimental results are presented to validate the effectiveness of the proposed control scheme.

• The Transactions of the Korean Institute of Power Electronics
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• v.8 no.1
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• pp.40-47
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• 2003

A low-cost current-sensing scheme for the motor drives with MOSFET is described. Many motor drives usually employ the common current sensors to measure current for the purpose of control or protection. These current sensors, however, significantly burden the power circuit with the size and cost. The proposed current-sensing scheme utilizes information concerning MOSFET's On-voltage and On-resistance. An analogue circuit detecting On-voltage can overcome the above disadvantages because the circuit is small and is made at a low cost, and the fuzzy inference for On-resistance is also simply designed based on MOSFET's characteristics. The validity of this scheme will be experimentally verified by adopting the current control of a battery car.

• 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.