Byung Moon Han

Real-Time Hardware Simulator for Grid-Tied PMSG Wind Power System

This paper describes a real-time hardware simulator for a grid-tied Permanent Magnet Synchronous Generator (PMSG) wind power system, which consists of an anemometer, a data logger, a motor-generator set with vector drive, and a back-to-back power converter with a digital signal processor (DSP) controller. The anemometer measures real wind speed, and the data is sent to the data logger to calculate the turbine torque. The calculated torque is sent to the vector drive for the induction motor after it is scaled down to the rated simulator power. The motor generates the mechanical power for the PMSG, and the generated electrical power is connected to the grid through a back-to-back converter. The generator-side converter in a back-to-back converter operates in current control mode to track the maximum power point at the given wind speed. The grid-side converter operates to control the direct current link voltage and to correct the power factor. The developed simulator can be used to analyze various mechanical and electrical characteristics of a grid-tied PMSG wind power system. It can also be utilized to educate students or engineers on the operation of grid-tied PMSG wind power system.

Novel islanding detection method for distributed generation

This paper describes the development of a novel islanding detection method for inverter-based distributed generation, which uses the signal cross-correlation scheme between the injected reactive current and the power frequency deviation. The existing method injects the 5% reactive current to the rated current for detecting the frequency deviation, which brings about reduction of power quality. On the contrary, the proposed method injects the 1% reactive current to the rated current which brings about negligible degradation of power quality. The proposed method detects the islanding state by calculating the cross-correlation index between the injected reactive current and the frequency deviation. The operational feasibility was verified through computer simulations with PSCAD/EMTDC software and experimental works with a 3kVA hardware prototype. The proposed method can detect the islanding state effectively without degrading the power quality at the point of common connection point.

Study on resonant fly-back converter for DC distribution system

This paper describes a resonant fly-back converter with thyristor switches, which can be used for the DC distribution with a PWM voltage-source inverter. The proposed converter operates to change high-voltage DC into low-voltage DC with isolation and large converting ratio. The converter has a main thyristor switch and an LC resonant circuit with an auxiliary thyristor switch for commutation in the primary side of the gap transformer. The operation of the proposed converter was verified through computer simulations with EMTP and experiments with a hardware scaled-model. The proposed converter could be implemented with commercially available components and proven technologies for supplying electricity to the rural or remote customer.

New combined power-conditioning system for superconducting magnetic energy storage

This paper proposes a new combined power-conditioning system for large-scale superconducting magnetic energy storage; it consists of a DC chopper and a PWM (pulse-width modulation) voltage-source converter. The proposed system can independently control the active and reactive power of the utility network by regulating the chopper duty cycle and the converter firing angle. The operational concept was verified through mathematical analyses using an equivalent circuit. The dynamic interaction was analyzed using a simulation model with EMTP (Electromagnetic Transients Program). The analysis results show that the new system is feasible for development with commercially available components and technologies.

Power conditioning system for fuel cell with 2-stage DC-DC converter

This paper proposes a grid-tied power conditioning system for the fuel cell, which consists of an LLC resonant DC-DC converter and a 3-phase inverter. The LLC resonant converter boosts the fuel cell voltage of 26–48V up to 400V, using the hard-switching boost converter and the high-frequency unregulated LLC resonant converter. The operation of proposed power conditioning system was verified through simulations with PSCAD/EMTDC software. The feasibility of hardware implementation was verified through experimental works with a laboratory prototype, which was built with 1.2kW PEM fuel-cell stack, 1kW high gain step-up converter, and 2kW PWM inverter. The proposed system can be utilized to commercialize a real interconnection system for the fuel-cell power generation.

Distance relaying algorithm using a DFT-based modified phasor estimation method

In this paper, we propose a distance relaying algorithm using a discrete Fourier transform (DFT)-based modified phasor estimation method to eliminate the adverse influence of exponentially decaying DC offsets. Most distance relays are based on estimating phasors of the voltage and current signals. A DFT is generally used to calculate the phasor of the fundamental frequency component in digital protective relays. However, the output of the DFT contains an error due to exponentially decaying DC offsets. For this reason, distance relays have a tendency to over-reach or under-reach in the presence of DC offset components in a fault current. Therefore, the decaying DC components should be taken into consideration when calculating the phasor of the fundamental frequency component of a relaying signal. The error due to DC offsets in a DFT is calculated and eliminated using the outputs of an even-sample-set DFT and an odd-sample-set DFT, so that the phasor of the fundamental component can be accurately estimated. The performance of the proposed algorithm is evaluated for a-phase to ground faults on a 345 kV, 50 km, simple overhead transmission line. The Electromagnetic Transient Program (EMTP) is used to generate fault signals. The evaluation results indicate that adopting the proposed algorithm in distance relays can effectively suppress the adverse influence of DC offsets.

DC Microgrid Operational Method for Enhanced Service Reliability Using DC Bus Signaling

This paper proposes a DC microgrid operational strategy and control method for improved service reliability. The objective is to supply power to as many non-critical loads as possible, while providing an uninterrupted power supply to critical loads. The DC bus signaling method, in which DC voltage is an information carrier, is employed to implement the operational strategy in a decentralized manner. During grid-connected operation, a grid-tied converter balances the power of the microgrid by controlling the DC voltage. All loads are connected to the microgrid, and operate normally. During islanded operation, distributed generators (DGs), a backup generator, or an energy storage system balances the power. However, some non-critical loads may be disconnected from the microgrid to ensure the uninterrupted power supply to critical loads. For enhanced service reliability, disconnected loads can be automatically reconnected if certain conditions are satisfied. Control rules are proposed for all devices, and detailed microgrid operational modes and transition conditions are then discussed. Additionally, methods to determine control parameter settings are proposed. PSCAD/EMTDC simulation results demonstrate the performance and effectiveness of the proposed operational strategy and control method.

Switching-level simulation model of MMC-based back-to-back converter for HVDC application

Switching-level simulation model of MMC-based BTB (back-to-back) converter for HVDC (high voltage DC transmission) application is very important to analyze its operation and transient. However, switching-level simulation model for the actual MMC-based BTB converter is very difficult because it consists of more than 150 sub-modules for each arm. In this paper a switching-level simulation model for the 11-level MMC-based BTB converter was developed with PSCAD/EMTDC software. It has 12 sub-modules for the positive arm and another 12 sub-modules for the negative arm. The developed simulation model includes the DC-voltage balance algorithm, the circulating-current reduction algorithm, the harmonic reduction algorithm, and the redundancy operation algorithm. So, the developed simulation model can be utilized to design the MMC-based BTB converter and to develop its protection scheme.

Operation analysis of novel UPFC based on H-bridge modules

This paper proposes a novel UPFC based on H-bridge modules, isolated through single-phase multiwinding transformers. The dynamic performance of proposed system was analyzed by simulation with EMTDC, assuming that the UPFC is connected with the 138-kV transmission line of one-machine-infinite-bus power system. The proposed system can be directly connected to the transmission line without series injection transformers. It has flexibility in expanding the operation voltage by increasing the number of H-bridge modules.

Reactive-power compensator using thyristor PWM current source inverter

This paper describes a static synchronous compensator with thyristor switches, which operates as a current-source PWM inverter with assistance of commutation circuit. The system operation was verified in detail through computer simulations with EMTP, and experiments with a scaled hardware model. The proposed system has two advantages of low switching loss and easy expansion of operation voltage through series connection of thyristors.