• 대한전기학회:학술대회논문집
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• 대한전기학회 2001년도 하계학술대회 논문집 A
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• pp.233-236
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• 2001

Parallel operation scheme to several transformers is adopted because of the load increase, economic problem, or load shedding. For the transformer's parallel operation, loads proportional to each transformer's capacity must be allotted, and circulation currents must be limited as much as without causing any problem in a real operation. But, both transformers in parallel operation can be tripped when either faults at lower voltage side of a transformer or faults in a bus occurs. Therefore, parallel operation scheme to distribution transformers in Korea is not adopted in a normal state but only when loaded or load-shedded. These are due to the insufficiency of the construction in communication network and AVR scheme. Besides that, those are because bus bar protection scheme to lower voltage side of a transformer is not applied. In spite of enormous initial investment costs, advanced countries take so much account of power system reliability and stable supply that they adopt the parallel operation scheme in a normal state. One of the problems in parallel operation is the overheat of transformers due to the excessive circulation currents. This paper presents the scheme that controls voltages between both transformers using circulation currents that occurs in parallel operation.

• 한국철도학회:학술대회논문집
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• 한국철도학회 2003년도 춘계학술대회 논문집
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• pp.444-449
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• 2003

This paper presents a vector control method for the parallel-connected motor drive system. In this paper new estimation scheme of rotor flux position is presented to reduce sensitivity due to load difference between the motors. To confirm the validity of the proposed control method, we compare a simulation result of the proposed control method with that of the conventional indirect vector control method. The simulation results show that the proposed control method is effective the step change in load torque.

• Journal of Electrical Engineering and Technology
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• 제10권3호
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• pp.1048-1057
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• 2015

This paper proposes a sensorless speed control algorithm for parallel-connected dual Surface-mounted Permanent Magnet Synchronous Motors (SPMSMs) fed by a single inverter. For stable parallel operation of synchronous motors with a single inverter, each motor has to be constantly kept in the synchronization state regardless of load torque. If the master motor with the larger load is controlled, the synchronous state will be maintained. Therefore, detection of the master motor is essential. Conventionally, the master motor is determined by comparing the rotor position error from the relation between the back-EMF for torque angle and the flux position; consequently, the position sensor is deemed essential for finding the rotor position. The parallel sensorless speed control method proposed in this paper uses no position sensor, instead it compares the flux-axis current from the connection between the back-EMF for torque angle and current in unbalanced load conditions. The results of simulation and experiment conducted verify the efficacy of the proposed method.

• Journal of KIEE
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• 제10권1호
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• pp.35-42
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• 2000

An equivalent load sharing control based on the frequency and voltage droop concept for parallel operation of two three-phase Uninterruptible Power Supply (UPS) systems with no control interconnection lines is presented in this paper. First of all, due to the use of active power and reactive power as control variables, the characteristics of output powers according to amplitude and phase differences between output voltages of two UPS systems are analyzed. Secondly, simulation results under different line impedance demonstrate the feasibility of the wireless parallel operation control. Finally, experiments are presented to verify the theoretical discussion with two three-phase 20kVA UPS systems employed TMS320C32, a kind of real time digital signal processor (DSP).

• 전기학회논문지
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• 제62권3호
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• pp.424-430
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• 2013

This paper presents a new design works for the braking chopper system which is included in the propulsion system of KTX high speed train. Due to the current fed type synchronous motors used in the propulsion system, some different behaviors are shown comparing to the voltage type other chopper systems. Specially this chopper system acts either braking controlling or regenerative power controlling system with a parallel resistive load in the propulsion system. In this paper, an improved simple high power IGBT brake chopper system has proposed which is able to be replaced with an existing complicated GTO chopper system. The analytical approaches to the parallel load type current chopper system and the propper snubber circuits calculation were explained in this paper to control new chopper system. In addition, the thermal resistance of the cooling system for power dissipation of IGBT modules was calculated also. Finally several PC simulations have been done to clarify its availability.

• 전기학회논문지
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• 제66권10호
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• pp.1481-1487
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• 2017

This paper proposes a sensorless speed control algorithm for parallel-connected dual Surface-mounted Permanent Magnet Synchronous Motors fed by a single inverter. For stable parallel operation of synchronous motors with a single inverter, each motor has to be constantly kept in the synchronization state regardless of load torque. If the master motor with the larger load is controlled, the synchronous state will be maintained. Therefore, detection of the master motor is essential. Conventionally, the master motor is determined by comparing the rotor position error from the relation between the back-EMF for torque angle and the flux position. consequently, the position sensor is deemed essential for finding the rotor position. In this paper, we proposed a method that decides the magnitude of the load from the power angle of two motors due to the load variation and selects the motor to control through the sign function for the sensorless speed control without the position sensor. The results of simulation and experiment conducted verify the efficacy of the proposed method.

• 전력전자학회:학술대회논문집
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• 전력전자학회 2003년도 춘계전력전자학술대회 논문집(2)
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• pp.987-993
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• 2003

The module UPS can flexibly implement expansion of power system capacities. Furthermore, it can be used to build up the parallel redundant system to improve the reliability of power system operation. To realize the module UPS, load sharing without interconnection among parallel connecting modules as well as a small scale and lightweight topology is necessary. In this paper, the three-arm converter/inverter is compared with the general full-bridge and half-bridge topology from a practical point of view and chosen as the module UPS topology. The switching control approaches based on a pulse width modulation of the converter and inverter of the system are presented independently The frequency and voltage droop method is applied to parallel operation control to achieve load sharing. Two prototype 3kVA modules are designed and implemented to confirm the effectiveness of the proposed approaches. Experimental results show that the three-arm UPS system has a high power factor, a low distortion of output voltage and input current, and good load sharing characteristic.

• 대한전기학회논문지
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• 제35권7호
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• pp.272-277
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• 1986

The system that follows to the resonance frequency of high frequency MOSFET inverter and varies according to the changes of load characteristics is proposed. Also we suggested a method how to select the resonant load type between series and parallel circuit for a given inverter type. It leads to the conclusion that in the case of high impedance loads, parallel resonant circuits are preferable, on the other hand, for low impedance loads, series resonant circuits are more preferable. For frequency tracking, a PLL circuit is used as main control element to detect the phase difference of current and voltage of load. The realized apparatus composed of control circuit and voltage type full-bridged MOSFET elements as main parts of inverter. A stable frequency follow-up characteristics are obtained for 1.2MHz, 1.5KW high frequency output and power is always supplied to the load with unity power factor.

• 전력전자학회:학술대회논문집
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• 전력전자학회 2001년도 전력전자학술대회 논문집
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• pp.501-505
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• 2001

The parallel operation system of UPS is used to increase reliability of power source at critical load. But parallel UPS system has a few defects, impedance is different from each other and circulating current occurs between UPSs, due to line impedance and parameter variation, though controlled by the same synchronization signal. According to such characteristic of parallel UPS, balanced load-sharing control is the most important technique in parallel UPS operation. In this paper, a novel power deviation compensation algorithm is proposed. it is composed of voltage controller to compensate power deviation that be calculated by using active and reactive current deviation between inverters on synchronous d-q reference frame.

• 전력전자학회:학술대회논문집
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• 전력전자학회 2002년도 추계학술대회 논문집
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• pp.158-162
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• 2002

The parallel operation system of multiple UPS(Uninterruptible Power Supply) is used to increase power capacity of the system or to secure higher reliability at critical loads. In the parallel operation of the two UPSs, the load sharing control to maintain the current balance between them is a matter of consequence. In this paper, a highly precise load sharing controller is proposed and implemented for the parallel operation system of two UPSs. After that, a good performance of the proposed method is verified by simulation in the parallel operation system with two UPSs.