Byongjun Lee

Reactive reserve-based contingency constrained optimal power flow (RCCOPF) for enhancement of voltage stability margins

This paper presents a new concept of reactive reserve-based contingency constrained optimal power flow (RCCOPF) for voltage stability enhancement. This concept is based on the fact that increase in reactive reserves is effective for enhancement of voltage stability margins of post-contingent states. In this paper, the proposed algorithm is applied to voltage stability margin of interface flow. Interface flow limit, in the open access environment, can be a main drawback. RCCOPF for enhancement of interface flow margin is composed of two modules, modified continuation power flow (MCPF) and optimal power flow (OPF). These modules are recursively performed until satisfying the required margin of interface flow in the given voltage stability criteria.

Frequency-Shift Acceleration Control for Anti-Islanding of a Distributed-Generation Inverter

This paper proposes frequency-shift acceleration control for anti-islanding of an inverter-based distributed generator. The proposed control is based on frequency positive feedback in the synchronous dq frame. Overall, the scheme of the proposed control is described. The design methodology of the frequency-shift acceleration gain is presented based on small-signal stability and step input response. The proposed control is intended for zero nondetection zone, noncompromised power quality, and easy implementation without additional devices required. Simulation and experimental results verify that the proposed anti-islanding scheme and the gain design approach are effective and useful in real applications.

Development of voltage stability constrained optimal power flow (VSCOPF)

This paper presents an application of optimization techniques incorporating voltage stability constraints. L-index that indicates voltage collapse proximity is adapted into the OPF formulation. This paper shows the impact of incorporation of voltage security constraint into optimal power flow formulation in which the active power dispatch problem is associated with guaranteeing adequate voltage security levels in power systems. It provides the optimal solution both minimizing generation cost and guaranteeing voltage security level, Numerical examples using simple 3-bus system and IEEE 30-bus system are also presented to illustrate the capabilities of the proposed formulation.

Development of Enhanced Electric Arc Furnace Models for Transient Analysis

Electric arc furnaces (EAFs) are a main cause of voltage flicker due to the interaction of the high demand currents of the loads with the supply system impedance. In order to adequately understand and analyze the effects on the power system from these loads, obtaining an accurate representation of the characteristics of the loads is crucial. In this paper, a mixed chaotic EAF model to represent the low-frequency and high-frequency variations of the arc current, respectively, and a chain-shaped chaotic EAF model to characterize the current variation have been proposed. The concept of chaotic parameters, such as chaotic resistance, inductance or admittance, also has been proposed for the characterization of arc furnace operation and the highly nonlinear physical processes. In addition, a DLL–Dynamic Link Library–module, which is a FORTRAN interface with TACS–Transient Analysis of Control Systems–is developed to implement the chaotic load models in the EMTP–Electromagnetic Transients Program. The simulation results are presented in comparison with the actual data to illustrate the validity of the models.

Justification of Effective Reactive Power Reserves With Respect to a Particular Bus Using Linear Sensitivity

A constant reactive power (Q) reserve (CQR) is measured by the difference between the maximum reactive power generation and its current. However, all reactive power sources must not affect the entire power system when severe contingencies occur. Thus, generators must be evaluated as to whether or not they will have an impact on the maximum permissible loading of buses. This definition considers some generators to still have reactive power reserves when the system faces voltage collapse. This remaining reactive power reserve, although available, cannot be used in an arbitrary manner. This paper proposes the concept of an effective reactive power reserve and it is calculated on the basis of a sensitivity analysis. The computed reactive power reserve is analyzed using static and dynamic simulations in bulk power systems.

Hybrid Control System for Managing Voltage and Reactive Power in the JEJU Power System

This paper proposes a hybrid voltage controller based on a hierarchical control structure for implementation in the Jeju power system. The hybrid voltage controller utilizes the coordination of various reactive power devices such as generators, switched shunt devices and LTC to regulate the pilot voltage of an area or zone. The reactive power source can be classified into two groups based on action characteristics, namely continuous and discrete. The controller, which regulates the pilot bus voltage, reflects these characteristics in the coordination of the two types of reactive power source. However, the continuous type source like generators is a more important source than the discrete type for an emergency state such as a voltage collapse, thereby requiring a more reactive power reserve of the continuous type to be utilized in the coordination in order to regulate the pilot bus voltage. Results show that the hybrid controller, when compared to conventional methods, has a considerable improvement in performance when adopted to control the pilot bus voltage of the Jeju island system.

Fault Response of a DFIG-based Offshore Wind Power Plant Taking into Account the Wake Effect

In order to meet the low voltage ride-through requirement in a grid code, a wind power plant (WPP) has to stay connected to a grid, supporting the voltage recovery for a grid fault. To do this, a plant-level controller as well as a wind generator (WG) controller is essential. The dynamic response of a WPP should be analyzed in order to design a plant-level controller. The dynamic response of a WPP for a grid fault is the collective response of all WGs, which depends on the wind speed approaching the WG. Thus, the dynamic response of a WPP should be analyzed by taking the wake effect into consideration, because different wind speeds at WGs will result in different responses of the WPP. This paper analyzes the response of a doubly fed induction generator (DFIG)-based offshore WPP with a grid fault taking into account the wake effect. To obtain the approaching wind speed of a WG in a WPP, we considered the cumulative impact of multiple shadowing and the effect of the wind direction. The voltage, reactive power, and active power at the point of common coupling of a 100 MW DFIG-based offshore WPP were analyzed during and after a grid fault under various wind and fault conditions using an EMTP-RV simulator. The results clearly demonstrate that not considering the wake effect leads to significantly different results, particularly for the reactive power and active power, which could potentially lead to incorrect conclusions and / or control schemes for a WPP.

Hybrid PSO-Complex Algorithm Based Parameter Identification for a Composite Load Model

This paper proposes a hybrid searching algorithm based on parameter identification for power system load models. Hybrid searching was performed by the combination of particle swarm optimization (PSO) and a complex method, which enhances the convergence of solutions closer to minima and takes advantage of global searching with PSO. In this paper, the load model of interest is composed of a ZIP model and a third-order model for induction motors for stability analysis, and parameter sets are obtained that best-fit the output measurement data using the hybrid search. The origin of the hybrid method is to further apply the complex method as a local search for finding better solutions using the selected particles from the performed PSO procedure.

Hierarchical voltage control of a wind power plant using the adaptive IQ-V characteristic of a doubly-fed induction generator

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 (IQ-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 IQ-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 IQ-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.

Study of the Effectiveness of a Korean Smart Transmission Grid Based on Synchro-Phasor Data of K-WAMS

Recent technological achievement in areas of distributed computing, networking, high speed communications and digital control, as well as the availability of accurate GPS time source, are rapidly becoming the enabling factors for the development of a new generation of real time power grid monitoring tools. KDN (Korea electric power Date Network Co.) R&D department with KERI, Korea University and LSIS has embarked on long term research and development work in the field of wide area systems specifically applied to the power transmission grid in Korea. Primary focus will be on wide area measurement and monitoring, analysis, assessment technique and tools aimed at preventing the propagation of power grid instabilities. Of special interest are voltage stability and small signal stability. This project aligns with KEPCO (the Korea Electric Power Company). In this paper, case studies performed with the use of the developed system are presented.