• The Transactions of The Korean Institute of Electrical Engineers
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• v.57 no.12
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• pp.2167-2172
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• 2008

A large scale embedded generator or a micro-grid is usually interconnected to the grid through the dedicated line. Immediately after the removal of the grid supply, it must be disconnected and remain disconnected until the normal grid supplies are restored. This paper proposes an islanding detection algorithm based on complex power at the intertie. Complex power at the intertie delivered to the grid has a value depending on power flow during normal operation. When loss of mains occurs, complex power at the intertie moves to a value depending on the parameters of the dedicated line. The performance of the proposed algorithm was investigated under various operating and fault conditions. The proposed algorithm can detect an islanding operation even if no power is exchanged prior to loss of mains and discriminate the fault condition from islanding.

• Progress in Superconductivity and Cryogenics
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• v.22 no.1
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• pp.7-11
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• 2020

As demand for electricity in urban areas increases, it is necessary to improve electric power stability by interconnecting neighboring substations and high temperature superconductor (HTS) power cables are considered as a promising option due to its large power capacity. However, the interconnection of substations reduces grid impedance and expected fault current is over 45 kA, which exceeds the capacity of a circuit breaker in Korean grid. To reduce the fault current below 45 kA, a HTS power cable having a fault current limiting (FCL) function is considered by as a feasible solution for the interconnection of substations. In this study, a FCL HTS power cable of 600 MVA/154 kV, transmission level class, is considered to reduce the fault current from 63 kA to less than 45 kA by generating an impedance over 1 Ωwhen the fault current is induced. For the thermal design of FCL HTS power cable, a parametric study is conducted to meet a required temperature limit and impedance by modifying the cable core from usual HTS power cables which are designed to bypass the fault current through cable former. The analysis results give a minimum cable length and an area of stainless steel former to suppress the temperature of cable below a design limit.

• Progress in Superconductivity and Cryogenics
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• v.4 no.2
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• pp.38-41
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• 2002

We have conducted Preliminary investigation to propose a suitable site for superconducting fault current limiter (SFCL) installation in the KEPCOs 154kV grid This investigation limited the application SFCL to the bus-tie position of the grid in the Seoul area. First, we calculated maximum Potential fault current for all substations. Then, among substations where the fault current exceeds the CB capacity, rye selected two substations where buses are being operated untied. For the selected two. S S/S and M S/S. fault currents at the M S/S were estimated to be 22.5㎄ and 24.3㎄ for two buses untied respectively, but 44.2㎄ if buses were tied. Simulation using a hypothetical SFCL of 5 Ohm impedance showed that it controlled the fault current up to 20. 1㎄ for bus-tie Position, 28.4㎄ and 29.9㎄ for both buses. respectively, each of which are under the capacity of the currently installed 31.5㎄ GIS. For both substations a SFCL with 5 Ohm impedance successfully controls the fault current under the CB capacity, and 10 Ohm SFCL may be recommendable to maximize the SFCL effect.

• Journal of Power Electronics
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• v.8 no.1
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• pp.10-22
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• 2008

The wind-driven doubly fed induction generator (DFIG) is currently under pressure to be more grid-compatible. The main concern is the fault ride-through (FRT) requirement to keep the generator connected to the grid during faults. In response to this, the paper introduces a novel model and new control scheme for the DFIG. The model provides a means of direct stator power control and considers the stator transients. On the basis of the derived model, a robust linear quadratic (LQ) controller is synthesized. The control law has proportional and integral actions and takes account of one sample delay in the input owing to the microprocessor's execution time. Further, the influence of the grid voltage imperfection is mitigated using frequency shaped cost functional method. Compensation of the rotor current pulsations is proposed to improve the FRT capability as well as the generator performance under grid voltage unbalance. As a consequence, the control system can achieve i) fast direct power control without instability risk, ii) alleviation of the problems associated with the DFIG operation under unbalanced grid voltage, and iii) high probability of successful grid FRT. The effectiveness of the proposed solution is confirmed through simulation studies on 2MW DFIG.

• Journal of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.23 no.10
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• pp.14-20
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• 2009

It is very important for power stability to suppress the excessive fault current happened frequently in the real power grid The superconducting fault current limiter (SFCL) is one of the most effective ways to reduce the fault current among the facilities developed so far. In this paper, we have investigated the operating characteristics of the power grid with the SFCL according to three types such as the single, double and triple line-to-ground faults. In addition, we analyzed the consumption power of the superconducting units based on the working data of the SFCL. We confirmed that the fault current could be limited lower than its peak value to 85 percentage in initial fault condition and to 85 percentage after one cycle in the matrix-type SFCL. The consumption powers of the superconducting units were almost equal by reduction of the difference of the critical current between superconducting units element.

• Journal of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.22 no.8
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• pp.18-25
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• 2008

The paper presents a modeling of the grid-connected wind turbine generation system on MATLAB/Simulink and aims to perform simulations for analysis of the system's characteristics. It performs a pitch regulation for control of the wind generator's output with respect to wind speed variation, and presents a relationship between interconnecting transformer's connections and fault current by reviewing the variations of fault current according to transformer connections in a grid-connected wind power generation system. It also investigates the effect of grounding methods of the interconnecting transformer's neutral point on fault current variations. The simulation results show the differences of fault currents, voltages and generator's characteristics for a line-to-ground fault according to interconnecting transformer's four different connections, and the differences of fault currents of the system according to grounding methods of the transformer neutral point. Therefore, the case studies demonstrate the effectiveness of the proposed simulation model on Simulink.

• Proceedings of the KIEE Conference
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• 2011.07a
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• pp.91-92
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• 2011

Beat is a phenomenon, where the magnitudes and frequencies of the voltage and the current fluctuate. This paper describes a quantitative analysis of the beat phenomenon of a wind farm using the envelope of a current during intertie/grid fault conditions. In this paper, the ratio of the crest to the trough of the envelope curve and the time interval between adjacent troughs are defined and used to evaluate the beat phenomenon quantitatively. Beat phenomena under various fault and wind conditions are analyzed. The proposed quantitative analysis seems simple but effective in the more understanding of beat phenomenon of a wind farm, and thus can be used as a basis for operation and/or protection of an intertie.

• Proceedings of the KIEE Conference
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• 2011.07a
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• pp.706-707
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• 2011

The insertion of the crowbar system in the doubly fed induction generator rotor circuit for a short period of time during grid disturbance enables a more efficient way of limiting transient rotor current and hence protecting the rotor side converter (RSC) and the DC - link capacitor. When crowbar is activated at fault occurrence and clearance time, RSC is blocked while DC -link capacitor and the grid side converter (GSC) can be controlled to provide reactive power support at the PCC and improve the voltage which helps to comply with grid codes. In this paper, control strategies for crowbar system to limit the rotor current during fault is presented with RSC and GSC controllers are modified to control PCC voltage during disturbance to enhance DFIG wind farm to comply with some strict grid codes. Model simulated on MATLAB/Simulink verify the study through simulation results presented.

• Proceedings of the KIEE Conference
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• 2009.07a
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• pp.476_477
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• 2009

This paper compares the several cases with High Impedance Fault(HIF) conditions and analyzes the impact of the grid-connected photovoltaic system at the HIF conditions. Simulations are conducted by using Electro-Magnetic Transient Program (EMTP) and using the results, the analysis is presented.

• Journal of Electrical Engineering and Technology
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• v.1 no.2
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• pp.137-144
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• 2006

This paper proposes novel protection schemes for grid-connected distributed generation (DG) units using the fault location algorithm. The grid-connected DG would be influenced by abnormal distribution line conditions. Identification of the fault location for the distribution lines at the relaying point of DG helps solve the problems of the protection relays for DG. The proposed scheme first identifies fault locations using currents and voltages measured at DG and source impedance of distribution networks. Then the actual faulted feeder is identified, applying time-current characteristic curves (TCC) of overcurrent relay (OCR). The method considering the fault location and TCC of OCR might improve the performance of the conventional relays for DG. Test results show that the method prevents the superfluous operations of protection devices by discriminating the faulted feeder, whether it is a distribution line where DG is integrated or out of the line emanated from the substation to which the DGs are connected.