Chul-Sang Hwang

Complementary Power Control of the Bipolar-type Low Voltage DC Distribution System

In this paper, a new power control strategy for the bipolar-type low voltage direct current (LVDC) distribution system is being proposed. The dc distribution system is considered as an innovative system according to the increase of dc loads and dc output type distribution energy resources (DERs) such as photovoltaic (PV) systems and energy storage systems (ESS). Since the dc distribution system has many advantages such as feasible connection of DERs, reduction of conversion losses between dc output sources and loads, no reactive power issues, it is very suitable solution for new type buildings and residences interfaced with DERs and ESSs. In the bipolar-type, if it has each grid-interfaced converter, both sides (upper, lower-side) can be operated individually or collectively. A complementary power control strategy using two ESSs in both sides for effective and reliable operation is proposed in this paper. Detailed power control methods of the host controller and local controllers are described. To verify the performances of the proposed control strategy, simulation analysis using PSCAD/EMTDC is being performed where the results show that the proposed strategy provides efficient operations and can be applied to the bipolar-type dc distribution system.

Voltage transient analysis of a PMSG wind power system using controller-hardware-in-the loops

This paper presents voltage transient analysis of a proposed 3MW permanent magnet synchronous generator (PMSG) wind power generation system (WPGS) using a real time digital simulator (RTDS). The WPGS that is modeled with realtime digital simulator (RTDS) and RSCAD simulators are characterized for voltage transient analysis. Furthermore, a newly actual controller-hardware-in-the-loop simulation (CHILS) is employed to analyze voltage transient to the system. For validation, two typical voltage sag conditions are considered as three-phase-line to-ground and single-line-to-ground faults. Experimental results validate that the modeled system operates safely under transient grid conditions by pitch control as well as circuit breaker operation.

A Novel Frequency Tracker for Islanded-Mode Operation in Microgrid

This paper proposes a method for frequency control of islanded microgrid with battery energy storage system. For frequency control of islanded microgrid, battery energy storage system uses a phase locked loop algorithm with positive sequence components for a fast frequency estimation. Microgrid is a power system with small inertia because it has small capacity generators and inverter systems for renewable energy. So, Islanded microgrid`s frequency varies fast and large as small generation and load changes. To reduce frequency variation of islanded microgrid, it needs a device with fast frequency response. For fast frequency response, a fast frequency tracking is important. To show the validation of proposed fast frequency tracking algorithm, battery energy storage system with proposed algorithm is tested in microgrid pilot plant.

Analysis of voltage faults in the grid-connected inverter of a wind power generation system using real-time digital simulator

This paper presents the analysis of voltage faults of the grid-connected inverter of a 3MW wind power generation system (WPGS). The analysis using a real time digital simulator (RTDS) is performed and evaluated for voltage faults to enable to withstand voltage dips during transient disturbance. For validation, typical voltage sag conditions and faults are deployed to the system. Simulation and experimental results validate that the analytical model is introduced and implemented under transient grid conditions.

Simulation Analysis of a Renewable Energy Based Microgrid using RTDS

Due to enhanced demands on quality, security and reliability of the electric power energy system, a microgrid has become a subject of special interest. In this paper, output characteristics of energy storage system (ESS) with an electric double layer capacitor (EDLC) and battery energy storage system (BESS) of a renewable energy based microgrid were analyzed under grid-connected and islanded operation modes. The microgrid which consists of photovoltaic and wind power turbine generators, diesel generator, ESS with an EDLC, BESS and loads was modeled using real time digital simulator. The results present the effective control patterns of the microgrid system.

Characteristic analysis of three-phase four-leg inverter based load unbalance compensator for stand-alone microgrid

This paper analyses a three-phase four-leg voltage sourced inverter (VSI) based load unbalance compensator (LUC) including its control algorithm, which is a component of a microgrid. The purpose of proposed three-phase four-leg VSI based LUC is to improve power quality of the stand-alone microgrid. Power quality of the microgrid which was installed in Mara-island, Korea is analysed using a real operational data. In this work, the microgrid in Mara-island which includes a photovoltaic power generation system, a diesel generator, a battery energy storage system, and a power management system is modelled in PSCAD/EMTDC, and proposed three-phase four-leg VSI based LUC is also modelled and applies to the microgrid. Power flow and stability of the modelled microgrid with the LUC is analysed under variable irradiance and unbalance loads. The results show that the proposed LUC helps to improve stability of the stand-alone microgrid. The proposed three-phase four-leg VSI based LUC and its control algorithm can be effectively utilized to the stand alone microgrid which has large unbalance loads.

RTDS based Transient Analysis of PMSG Type wind Power Generation System

The operation of permanent magnet synchronous generator (PMSG) type wind power generation system (WPGS) can be affected by the utility condition. Consequently, transient condition of utility should be analyzed for the safe and reliable operation of WPGS. This paper presents transient analysis results of a PMSG type WPGS using real time digital simulator (RTDS). A fault condition was applied to the transient analysis of PMSG type WPGS as the transient grid condition. The simulation results were analyzed to show the operational characteristic of PMSG type WPGS under the transient phenomenon of utility.

Design of conduction cooling system for a high current HTS DC reactor

A DC reactor using a high temperature superconducting (HTS) magnet reduces the reactors size, weight, flux leakage, and electrical losses. An HTS magnet needs cryogenic cooling to achieve and maintain its superconducting state. There are two methods for doing this: one is pool boiling and the other is conduction cooling. The conduction cooling method is more effective than the pool boiling method in terms of smaller size and lighter weight. This paper discusses a design of conduction cooling system for a high current, high temperature superconducting DC reactor. Dimensions of the conduction cooling system parts including HTS magnets, bobbin structures, current leads, support bars, and thermal exchangers were calculated and drawn using a 3D CAD program. A finite element method model was built for determining the optimal design parameters and analyzing the thermo-mechanical characteristics. The operating current and inductance of the reactor magnet were 1,500 A, 400 mH, respectively. The thermal load of the HTS DC reactor was analyzed for determining the cooling capacity of the cryo-cooler. The study results can be effectively utilized for the design and fabrication of a commercial HTS DC reactor.

Digital Signal Processor-Based Power Management System Implementation for a Stand-Alone Microgrid on a Small Island in Korea

This chapter presents a power management system (PMS) for the control of a stand-alone microgrid that was installed in Mara Island’s microgrid system in Korea. Most stand-alone microgrids can control or confine the start, stop, or output of each distributed generator using the energy management system, but these functions cannot guarantee stability against disturbances that occur transiently and unexpectedly due to their reliance on communications. Therefore, Mara Island’s microgrid system developed and applied a PMS that does not rely on communications to secure the stability of the transient system. The PMS controls controllable active and reactive power, particularly that of energy storage systems, for the controllable active power of a diesel engine. First, we performed a controller hardware-in-the-loop simulation to verify the PMS performance. The microgrid system was modeled in a real-time digital simulator, connecting the PMS designed by a digital signal processor. Second, the PMS was installed in Mara Island’s microgrid system in Korea, which includes a photovoltaic power generation system, a diesel engine, a battery energy storage system, and the PMS. These systems are connected to a 380-V, one-feeder distribution subsystem. The loads of the microgrid usually vary from 40 to 120 kW. The results show that the proposed PMS helps to improve the stability of the stand-alone microgrid. The stability and utilization of the system can be increased by utilizing the PMS in accordance with the purpose of the microgrid system.

Development of an effective control scheme for a DC microgrid with energy storage system

Power control is important in the operation of dc microgrid, particularly for the power sharing between different types of power sources including PV, wind power generation systems. In this paper, an effective control scheme for dc microgrid with energy storage system (ESS) is proposed. The power sharing scheme using two ESSs in a dc microgrid for stable operation is proposed. Detailed effective control scheme of voltage and power control is described. To verify the performances of the proposed effective control scheme, the simulation analysis using PSCAD/EMTDC is conducted. The results demonstrate the effectiveness of the control scheme that can improve power balancing and sharing in connection with ESSs for stable operation of dc microgrid under various generation and load conditions.