• Journal of Power Electronics
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• v.17 no.3
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• pp.733-743
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• 2017

This paper develops a coordinated control strategy of the doubly fed induction generator (DFIG) system based on repetitive control (RC) under generalized harmonic grid voltage conditions. The proposed RC strategy in the rotor side converter (RSC) is capable of ensuring smooth DFIG electromagnetic torque that will enable the possible safe functioning of the mechanical components, such as gear box and bearing. Moreover, the proposed RC strategy in the grid side converter (GSC) aims to achieve sinusoidal overall currents of the DFIG system injected into the network to guarantee satisfactory power quality. The dc-link voltage fluctuation under the proposed control target is theoretically analyzed. Influence of limited converter capacity on the controllable area has also been studied. A laboratory test platform has been constructed, and the experimental results validate the availability of the proposed RC strategy for the DFIG system under generalized harmonic grid voltage conditions.

• Proceedings of the KIPE Conference
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• 2012.07a
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• pp.154-155
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• 2012

The dynamic voltage restorer (DVR) is an effective protection device for wind turbine generator based on doubly-fed induction generator (DFIG) operated under the unbalanced voltage dip conditions. The compensating voltages of DVR depend on the voltage dips and on the influence of the zero sequence components. If the $Y_0/{\Delta}$ step-up transformers are used, there are no zero sequence components on the DFIG side. However, if the $Y_0/Y_0$ step-up transformers are used, the zero sequence components will appear during faults. The zero sequence components result in the high insulation costs and the asymmetric of the terminal voltages. This paper proposes a method for controlling zero sequence components in DVR to protect DFIG under unbalanced voltage dips. Simulation results are presented to verify the effectiveness of the proposed control method.

• Proceedings of the KIPE Conference
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• 2007.11a
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• pp.103-107
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• 2007

In doubly fed induction generators (DFIGs), control of rotor currents allows for adjustable speed operation, active, and reactive power control. This paper presents a DFIG control strategy that enhances the active and reactive power control with controllers that can compensate for the errors caused by current measurement path in the DFIG system. The errors can be divided into two categories: offset and scaling errors. These can induce the speed, active, and reactive power pulsations, which are one and two times the fundamental slip frequency in the DFIG. And these undesirable ripples can do the DFIG harm. In this paper, a new compensation algorithm is proposed. Therefore, the proposed algorithm has several advantages: to implement is easy; it require less computation time; it is robust with regard to the variation of the induction generator parameters. In this paper, a new algorithm is introduced by using the integral of phase currents to measure the current ripples of rotor-side converterin the DFIG system. The experiment results are shown the effectiveness of the proposed method.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.57 no.12
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• pp.2173-2178
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• 2008

A doubly-fed induction generator(DFIG) system has been widely used in the modem wind turbines due to variable-speed operation, high efficiency and small converter size. It is well known that an inter-turn fault of a generator is very difficult to be detected. The DFIG system uses a wound rotor induction machine so that the magnetizing current of the generator can be fed from both the stator and the rotor. This paper proposes a protection algorithm for a DFIG using the d-q equivalent circuit in the time domain. In the case of a DFIG, the voltages and currents of the rotor side as well as the voltages and currents of the stator are available. The proposed algorithm estimates the instantaneous(i.e., converted into the stationary frame) induced voltages from the rotor and the stator sides. If the difference between the two estimated induced voltages exceeds the threshold, the proposed algorithm detects the inter-turn fault. The algorithm can detect a inter-turn fault of a winding. The performance of the proposed algorithm is validated using a PSCAD/EMTDC simulator under inter-turn fault conditions and normal operating conditions such as an external fault and the change of the wind speed.

• Journal of Electrical Engineering and Technology
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• v.10 no.2
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• pp.504-510
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• 2015

Because wind generators (WGs) in a wind power plant (WPP) produce different active powers due to wake effects, the reactive power capability of each WG is different. This paper proposes a hierarchical voltage control scheme for a WPP that uses a WPP controller and WG controller. In the proposed scheme, the WPP controller determines a voltage error signal by using a PI controller and sends it to a doubly-fed induction generator (DFIG). Based on the reactive current-voltage ($I_Q-V$) characteristic of a DFIG, the DFIG injects an appropriate reactive power corresponding to the voltage error signal. To enhance the voltage recovery capability, the gains of the $I_Q-V$ characteristic of a DFIG are modified depending on its reactive current capability so that a DFIG with greater reactive current capability may inject more reactive power. The proposed scheme enables the WPP to recover the voltage at the point of common coupling (PCC) to the nominal value within a short time after a disturbance by using the adaptive $I_Q-V$ characteristics of a DFIG. The performance of the proposed scheme was investigated for a 100 MW WPP consisting of 20 units of 5 MW DFIGs for small and larger disturbances. The results show the proposed scheme successfully recovers the PCC voltage within a short time after a disturbance.

• The Transactions of the Korean Institute of Power Electronics
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• v.14 no.4
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• pp.293-298
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• 2009

This paper presents a variable gain current control algorithm for the stabilized grid connection between the grid and a doubly fed induction(DFIG) as a wind power generator. The performance of a RSC current controller depends highly on accurate machine parameters, and especially requires a fast and robust response regardless of the disturbances such as voltage sag. However, parameter variations of a DFIG occur at the point of grid connection, which affects the current controller gains based on DFIG parameters after a DFIG is connected to the grid. Thus, performance degrades when actual machine parameters depart from values used in the control system. In the proposed algorithm, current controller gains of the rotor side converter(RSC) are changed after a DFIG is connected to the grid. The simulation results and experimental results for a 750kW are shown to illustrate the feasibility of the proposed algorithm.

• The Transactions of the Korean Institute of Power Electronics
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• v.16 no.1
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• pp.11-19
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• 2011

This paper describe development of a hardware simulator for the DFIG wind power system, which was designed considering wind characteristic, blade characteristic, and blade inertia compensation. The simulator consists of three major parts, such as wind turbine model using induction motor, doubly-fed induction generator, converter-inverter set. and control system. The turbine simulator generates torque and speed signals for a specific wind turbine with respect to the given wind speed which is detected by Anemometer. This torque and speed signals are scaled down to fit the input of 3.5kW DFIG. The MSC operates to track the maximum power point, and the GSC controls the active and reactive power supplied to the grid. The operational feasibility was verified through computer simulations with PSCAD/EMTDC. And the implementation feasibility was confirmed through experimental works with a hardware set-up.

• New & Renewable Energy
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• v.5 no.3
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• pp.4-12
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• 2009

The effect of change in DFIG (doubly-fed wind power generator) rotating speed and active power on the grid was analyzed to understand the characteristics of wind power using the wind power simulator connected to the grid at Gochang Power Quality Test Center. Electric power quality improvement devices (DVR, STATCOM, SSTS) and electric power quality disturbance application devices for 22.9 kV grid are equipped at Gochang Power Quality Test Center. Induction motor and VVVF inverter were used to emulate the blade of a wind power generator, and a simulator for Cage wound induction generator and DFIG was developed. The trial line were assumed to be 20 km and 40 km in length, and variable wind speed pattern was set using wind speed data from Ducjeokdo to verify the power characteristics of the wind power generator according to rotating speed.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.60 no.12
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• pp.2183-2189
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• 2011

Since main portion of the required electric power in Jeju Island is provided from the mainland through two HVDC lines, Jeju HVDC has a most significant impact on Jeju power system. Average wind speed of Jeju Island is the highest among several candidates in South Korea. So, Jeju Island has been a suitable site for the construction of wind farms where several wind farms are now operating and several others to be sited. Since the large-scale wind generation could have adverse impacts on the stable operation of Jeju power system, wind power is also important for the stability of Jeju power system. Therefore, accurate modeling of Jeju HVDC and wind farms is required for stability analysis of Jeju power system. In this paper, PSS/E-based dynamic modeling of Jeju HVDC and DFIG wind farms is proposed. Model-writing technique of PSS/E is used to develop USRAUX model and USRMDL model for controlling the frequency of HVDC and imposing an operation limit of wind power, respectively. Dynamic characteristics of Jeju HVDC and DFIG wind farms are analyzed through the dynamic simulations. The simulation results show the effectiveness of the developed models for Jeju power system.

• The Transactions of the Korean Institute of Electrical Engineers B
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• v.55 no.6
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• pp.325-336
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• 2006

This paper deals with the dynamic analysis of variable speed wind power systems with doubly-fed induction generators (DFIG). First, the mathematical modeling of wind farm which consists of turbine rotor, DFIG, rotor side and grid side converter and control systems is presented. In particular, the equation for dynamic modeling of the DFIG and the AC/DC/AC converter is expressed as dq reference frame. And then, on the basis of mathematical modeling for each component of wind farm, dynamic simulation algorithms for speed and pitch angle control of wind turbine and generated active and reactive power control of the DFIG and the AC/DC/AC converter are established. Finally, Using the MATLAB/SIMULINK, this paper presents dynamic simulation model for 6MW wind power generation systems with the DFIG considering distribution systems and performs the dynamic analysis of wind power systems in steady state. Moreover, this paper also presents the dynamic performance for the case when the voltage sag in grid source and phase fault in bus are occurred.