In-Young Suh

A master and slave control strategy for parallel operation of three-phase UPS systems with different ratings

Parallel operation of UPS system has been used to increase power capacity of the system or to secure reliable supply of power to critical loads. During parallel operation, load sharing control to maintain the current balance is critical for reliable operation, since load sharing is very sensitive to differences in components of each module such as amplitude /phase difference, line impedance, and output LC filters. To solve these problems various control algorithms have been researched. However, these methods cannot be applied to UPS systems with different ratings. For this case, master and slave control algorithms for parallel operation is adequate. If the ratings of UPS systems are different, the value of passive LC filters will be different, and it will affect current sharing. This paper presents the general problems associated with parallel operation of UPS systems, and control strategy for parallel operation with different ratings. The validity of the proposed control strategy is investigated through simulation and experiment with two UPS systems.

Instantaneous control of average power for grid tie inverter using single phase D-Q rotating frame with all pass filter

This paper proposes a new instantaneous power control method using the rotating frame with an all pass filter for a single-phase grid tie inverter. The recently proposed single-phase rotating frame is reanalyzed based on the phasor notation that has been widely used to understand linear electrical circuits and then the relationship between them is defined. A grid tie inverter circuit model with a LCL filter is derived in a single-phase rotating frame. A novel average power control method with an all pass filter suitable for digital implementation is introduced. Various results from simulations are shown to verify the proposed scheme and actual experiments taken from a 3 kW grid tie inverter will be carried out in a near future.

A new control scheme of unified power quality compensator-Q with minimum power injection

Voltage sags are one of the most frequently occurring power quality problems challenging the power systems today. The unified power quality conditioner (UPQC) is one of the major custom power solutions that are capable of mitigating the effect of supply voltage sags at the load or point of common coupling (PCC). A UPQC-Q employs a control method in which the series compensator injects a voltage that leads the supply current by 90/spl deg/ so that the series compensator at steady state consumes no active power. However, the UPQC-Q has the disadvantage that its series compensator needs to be overrated. Thus it cannot offers effective compensation. This paper proposes a new control scheme for the UPQC-Q that offers minimum power injection. The proposed minimum power injection method takes into consideration the limits on the rated voltage capacity of the series compensator and its control scheme. The validity of the proposed control scheme is investigated through simulation and experimental results.

A study on DVR control for unbalanced voltage compensation

This paper presents a new control scheme for a DVR (dynamic voltage restorer) system consisting of series voltage source PWM converters. To control the negative sequence components of the source, it is necessary to detect the negative sequence components. Generally, a filtering process is used which has some undesirable effects. This paper suggests a new method for separating positive and negative sequences components. This control system is designed using differential controllers and digital filters. The positive and negative sequences are extracted and controlled individually. The performance of the presented controller and scheme are confirmed through simulation and actual experiment with a 2.5 kVA prototype DVR system.

Current Mode PWM Controller for a 42V/14V bidirectional DC/DC Converter

The conventional way to implement a bidirectional converter with boost/buck has been to use two general purpose PWM ICs with a single supply voltage In this case, when one direction mode is in operation, the other is disabled and the output of the error amplifier of the disabled IC may be saturated to a maximum value or zero. Therefore, during mode transition, a circuit which can disable the switching operation for a certain time interval is required making it impossible to get a seamless transition. In this paper, the limitations of the conventional 42V/14V bi-directional DC/DC converter implemented with general current mode PWM ICs with a single supply voltage are reviewed and a new current mode PWM controller circuit with an dual voltage system is proposed. The validity of the proposed circuit is investigated through simulation, and experiments.

Development of 20kW energy storage system for Smart Green Town

The main purpose of this paper is to introduce the development and structure of 20kW Energy Storage System (ESS). The basis of the capacity selection of 20kW ESS, which will be installed in ‘Se-hwa sanatorium’, is introduced. The Smart Green Town is defined and its structure is also introduced in this paper. The possibility of success with this system will be verified as an ESS through the simulated results of the grid-connected mode and the stand-alone mode.

Development of the power conditioning system for high power fuel cell system

This paper presents the design, development and performance of a power conditioning system (PCS) for application to a 250 kW Molten Carbonate Fuel Cell (MCFC) generation system. A DSP controller was used to control the dc-dc and dc-ac converter operation for grid connection and power injection to the grid. The controller must also supervise the total PCS operation while communicating with the fuel cell system controller. A control method for parallel operation of dc-dc converters was proposed and verified. A 250 kW prototype was successfully built and tested. Experimental performances are compared to minimum target requirements of the PCS for MCFC.

A Study on the Effects of New Facts Installations on Existing Distance Protection Relays

Abstract The UPFC, which is one of the many FACTS devices that are in use in the modem power system, is able to independently control the active and reactive power in the transmission systems by injecting a controlled voltage in series. The series injected voltage by the UPFC can be perceived as a change in transmission line impedance by existing distance protection relays and affect their correct operation. This paper first uses EMTDC models of the UPFC and surrounding power system of the Kangjin substation located in Jeonla-namdo province in Korea to investigate the impedance variation according to different UPFC operating conditions under steady and transient states. Then the operation of the UPFC protection controls, and impedance variations seen by the distance protection relay were reviewed under fault conditions to investigate the possibility of relay misoperation, and the need for relay protection setting modifications.