Journal of Electrical Engineering and Technology

The Korean Institute of Electrical Engineers (대한전기학회)
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A New Start-up Method for a Load Commutated Inverter for Large Synchronous Generator of Gas-Turbine

  • An, Hyunsung (Dept. of Electrical and Electronic Engineering, Chungnam National University) ;
  • Cha, Hanju (Dept. of Electrical and Electronic Engineering, Chungnam National University)
  • Received : 2017.02.08
  • Accepted : 2017.08.24
  • Published : 2018.01.01
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Abstract

This paper proposes a new start-up method for a load commutated inverter (LCI) in a large synchronous gas-turbine generator. The initial rotor position for start-up torque is detected by the proposed initial angle detector, which consists of an integrator and a phase-locked loop. The initial rotor position is accurately detected within 150ms, and the angle difference between the real position and the detected position is less than 1%. The LCI system operates in two modes (forced commutation mode and natural commutation mode) according to operating speed range. The proposed controllers include a forced commutation controller for the low-speed range, a PI speed controller and a PI current controller, where the forced commutation controller is connected to the current controller in parallel. The current controller is modeled by Matlab/Simulink, where a six-pulse delay of the thyristor and a processing delay are considered by using a zero-order hold. The performance of the proposed start-up method is evaluated in Matlab/Psim at standstill and at low speed. To verify the feasibility of the method, a 5kVA LCI system prototype is implemented, and the proposed initial angle detector and the system performance are confirmed by experimental results from standstill to 900rpm.

Keywords

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Fig. 1. Configuration of a gas-turbine power station

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Fig. 2. Conventional method for a start-up system

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Fig. 3. Operating characteristic curve of a gas-turbinesystem

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Fig. 4. Configuration of an LCI system

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Fig. 5. Principle of forced commutation mode

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Fig. 6. Principle of natural commutation mode

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Fig. 7. Control structure of an LCI system

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Fig. 8. Simplified representation of a silent-pole synchro-nous machine

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Fig. 9. Block diagram of initial angle detection

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Fig. 10. Block diagram of PLL

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Fig. 11. Block diagram of the current controller

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Fig. 12. Principle of forced commutation

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Fig. 13. Modeling of the load commutated inverter

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Fig. 14. Initial rotor position detection: (a) Initial angle :50¡Æ(b) Initial angle : 220¡Æ

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Fig. 15. Start-up within forced commutation mode

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Fig. 16. Overall start-up of the LCI system

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Fig. 17. Prototype of the 5kVA LCI system and MG-set

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Fig. 18. Initial angle detection (qinit = 115¡Æ): (a) Measuredtheta (2rad/div), (b) Detected theta (2rad/div)

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Fig. 19. Initial angle detection (qinit = 230¡Æ): (a) Measuredtheta (2rad/div), (b) Detected theta (2rad/div)

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Fig. 20. Selection of thyristor pair: (a) Theta (q, 2rad/div),(b) B phase input current, (c) C phase input current

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Fig. 21. Response of current control: (a) Step response ofDC current, (b) DC link current from forcedcommutation

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Fig. 22. Initial start-up in forced commutation mode: (a)DC link voltage (Vdc, 250V/div), (b) DC linkcurrent (Idc, 2A/div), (c) machine voltage (Eac,25V/div), (d) Theta (q, 2rad/div)

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Fig. 23. Three-phase current in forced commutation mode:(a) Machine voltage (Eac, 25V/div), (b) A-phasemachine input current (Ia, 2A/div), (c) B-phasemachine input current (Ib, 2A/div), (d) C-phasemachine input current (Ic, 2A/div)

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Fig. 24. Overall start-up of the LCI system: (a) DC linkvoltage (Vdc, 250V/div), (b) A-phase machineinput current (Ia, 5A/div), (c) machine voltage (Eac,250V/div), (d) rotor speed (wr, 200rpm/div)

Table 1. Relationship between rotor position and polarity of back-emf

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Table 2. Relationship between rotor position and thyristor pair

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Table 3. Parameters of the synchronous machine

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