A New Start-up Method for a Load Commutated Inverter for Large Synchronous Generator of Gas-Turbine

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.

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

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

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

Fig. 4. Configuration of an LCI system

Fig. 5. Principle of forced commutation mode

Fig. 6. Principle of natural commutation mode

Fig. 7. Control structure of an LCI system

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

Fig. 9. Block diagram of initial angle detection

Fig. 10. Block diagram of PLL

Fig. 11. Block diagram of the current controller

Fig. 12. Principle of forced commutation

Fig. 13. Modeling of the load commutated inverter

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

Fig. 15. Start-up within forced commutation mode

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

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

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

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

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

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

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)

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)

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

Table 2. Relationship between rotor position and thyristor pair

Table 3. Parameters of the synchronous machine