Soon-Ryul Nam

A modeling method of a high impedance fault in a distribution system using two series time-varying resistances in EMTP

More reliable algorithms for detecting a high impedance fault (HIF) require the voltage and current data at a relaying point instead of a faulted branch when HIF occurs. Thus, an accurate modeling method of HIF is essential for the development of a reliable detecting algorithm. The data should contain the complex characteristics of HIF such as buildup and shoulder as well as nonlinearity and asymmetry. This paper presents a modeling method of the above-mentioned HIF characteristics. Among the experiment data on a 22.9 (kV) distribution system conducted by Korea Electric Power Corporation (KEPCO), experiment data showing all the above characteristics is chosen in this paper. Two series time-varying resistances (TVRs) controlled by transient analysis of control systems (TACS) in EMTP are employed for modeling. One TVR is used for nonlinearity and asymmetry from the voltage-current characteristic for one cycle in the steady state after HIF, and then the other TVR for buildup and shoulder from the waveforms in the transient state after HIF. The comparison of the modeling results with the experiment data shows close correspondence. With the developed HIF model, the voltage and current at the relaying point are obtained with various load condition and fault condition such as fault distance and inception angle.

New modified fourier algorithm to eliminate the effect of the DC offset on phasor estimation using DFT

This paper proposes a new modified Fourier algorithm to eliminate the adverse influence of a decaying dc offset when Discrete Fourier transform (DFT) is used to estimate the phasor of the fundamental frequency component. The proposed algorithm decomposes the DFT process into even sample set DFT and odd sample set DFT. The effect of the dc offset is eliminated by using the recursive relationship between the outputs of even sample set DFT and odd sample set DFT. The performance of the algorithm is evaluated by using computer- simulated signals and EMTP-generated signals. The evaluation results indicate that the proposed algorithm can estimate the accurate phasor of the fundamental frequency component regardless of not only the primary decaying dc offset but also the secondary decaying dc offset caused by CT circuit itself including its burden.

Adaptive protection schemes of Distributed Generation at distribution network for automatic reclosing and voltage sags

In this paper, the adaptive protection schemes of utility interconnected distributed generation (DG) units for distribution automatic reclosing and voltage sag are proposed. Based on the general transient responses of DG units with synchronous generator during a fault by the reclosing types, i.e. radial reclosing and passive reclosing, classifying algorithms of reclosing types with varying fault resistance are proposed. Also, mathematical formulation of adaptive protection coordination schemes for reclosing types and voltage sags are introduced. The proposed algorithms are evaluated by the case study using PSCAD/EMTDC simulation tool.

Improved operating scheme using an IEC61850-based distance relay for transformer backup protection

Problems occur when zones overlap in the conventional step distance protection scheme. If the same zones from different relays overlap, discriminating between the real fault location and the virtual one is difficult, and malfunction can occur. In this case, the zone length or time delay is usually modified to separate the overlapped zones. However, these changes affect the coordination of the power protection system. A clear example of this involves transformer backup protection. Generally, when Zone 2 of a transformer backup protection relay overlaps Zone 2 of an adjacent line backup protection, the method of modifying this zone may not be obvious. To overcome this problem, this paper describes an improved operating scheme using an IEC61850-based distance relay for transformer backup protection.

Modeling of DFIG wind turbines considering fault-ride-through grid code

In modern power systems, wind energy is one of the most important among the various renewable sources. Recent grid codes require wind farms to remain dynamically stable during a voltage dip and to supply active and reactive power into systems. Fault ride through (FRT) refers to the capability of generation plant to remain connected and to offer network support throughout a serious voltage disturbance on the power system. Doubly fed induction generator (DFIG) technology is the dominant technology in the growing global market for wind power generation, due to the combination of variable-speed operation and a cost-effective partially rated power converter. However, DFIG is sensitive to dips in supply voltage and risks damage to its power converter without specific protection to ride-through grid faults. The induction generator very quickly loses internal magnetization in proportion to the reduced voltage. The demagnetization produces large currents on both stator and rotor circuits. Since this rotor current is typically far greater than the ratings of the converters power electronic devices, specific protection measures should be provided to avoid damage to the rotor-side converter devices and dc-link capacitors. Generally, this protection is achieved by rotor crowbar applications that temporarily connect the rotor circuits through a resistor, diverting current from the rotor-side converter and rapidly de-energizing the rotor. In this paper, a DFIG wind turbine with a rotor crowbar is modeled in order to investigate its dynamic responses to ride-through grid faults. PSCAD/EMTDC is used to simulate various fault conditions. In the simulations, the behaviors of the model are presented and analyzed such as the response of the pitch controller, the voltage of the DC-link, voltages and currents related to the stator and rotor. To assess the validity of the modeling, the simulation results are compared to a DFIG wind turbine without a rotor crowbar.

Dynamic modeling of DFIG wind farms and HVDC for the power system in Jeju Island

In Jeju Island, which is the largest island in South Korea, main portion of the required electric power is provided from the mainland through two HVDC lines. Since Jeju HVDC is operated on frequency control mode and it covers entire power variations in Jeju Island, it has a most significant impact on the power system in Jeju Island. The large-scale wind generation also could have significant impacts on the power system in Jeju Island. This is because the total capacity of wind power is considerable large compared to the load in Jeju Island and it can threaten the stable operation of the power system. In this paper, dynamic modeling of wind farms and Jeju HVDC is developed using PSS/E. Wind farms are composed of doubly fed induction generators (DFIGs). Model writing technique is used to develop user-defined models for controlling HVDC frequency and imposing maximum penetration limit. The simulation results indicate the effectiveness of the dynamic modeling for stability studies of the power system in Jeju Island.