proposed hybrid inverter system
a. topology and properties
a complete power circuit diagram of the proposed system is illustrated in fig. 3. it is composed of a three-phase thyristor controlled rectifier, a load commutated inverter followed by a dc-link inductor, and a three-phase voltage source inverter. the voltage source inverter is connected with the load commutated inverter in parallel through a small lc filter. basically, the proposed system has a combined inverter topology of a load commutated inverter and a voltage source inverter. notice that although this configuration is similar with the topology of an active power filter or a tandem inverter, its purpose and operation are quite different from them.
the load commutated inverter operates in the square-wave mode with converter-grade thyristors. consequently thyristors in the load commutated inverter turn on and off only once per cycle of the output current and their switching loss isnegligible.
the main function of the voltage source inverter is injecting sinusoidal phase voltages to the induction motor in order to regulate the motor speed as well as provide a safe commutation angle for the load commutated inverter. the induction motor speed is regulated by transiently adjusting the output voltage amplitude and frequency of the voltage source inverter. in addition, the phase angle of the output voltage is controlled by shifting the firing angle of the load commutated inverter suitably to obtain a safe load commutation angle. therefore, the leading power factor for the load commutated inverter operation is entirely obtained by the voltage source inverter over the whole speed range of the induction motor. based on the leading power factor for the load commutated inverter provided by the voltage source inverter, the proposed system can run an induction motor without the dc-commutation circuit as well as output ac capacitors of the conventional load commutated inverter based induction motor. therefore, the proposed system can successfully solve all problems caused by the output capacitors and the forced dc-commutation circuit. in addition, the proposed scheme can generate sinusoidal motor voltages and currents, leading to a reduction in the low-order harmonics injected into the motor. this allows elimination of the torque pulsation and harmonic losses due to output currents of the conventional load commutated inverter, which produce low-order harmonics. small lc filter is required to smooth out the pulsewidth-modulated voltages generated by the voltage source inverter.
figure 4 shows a per-phase equivalent circuit of the proposed system. the proposed system has a parallel connection of two inverters, the load commutated inverter represented by the current source ithy and the voltage source inverter by the voltage source vpwm. the voltage source inverter impresses a sinusoidal motor phase voltage vo to the motor. moreover, it provides leading power factor for safe commutation of the load commutated inverter. a motor phase current io is determined by the sinusoidal motor phase voltage vo controlled by the voltage source inverter. concurrently the load commutated inverter also supply a current ithy to the motor. therefore, the motor phase current io is the sum of the lci output current, ithy and the vsi output current, ivsi. from the operating point of view, the fast voltage source inverter operates as a master inverter and the slow load commutated inverter as a slave. as a result, the proposed system can show a fast system transient response compared with the conventional load commutated inverter based induction motor since the proposed system has time response close to the sampling period.
figure 5 shows a current vector diagram of the proposed system. the phase angle φrepresents the leading power factor angle for safe commutation of the load commutated inverter. this angle can be controlled by adjusting the phase angle between the motor phase voltage and the gating instant of the load commutated inverter. therefore, this strategy ensures safe commutation of the load commutated inverter over all operating speed of the induction motor. the phase angleθrepresents the power factor angle of the induction motor.
in terms of power rating supplied to the motor, the load commutated inverter supplies the real power to the motor load, while the voltage source inverter provides the real power corresponding to phase shift between the lci output current and the motor phase current, as well as the reactive power. the load commutated inverter is not comparable to the voltage source inverter from cost point of view. therefore, the voltage source inverter power rating should be kept minimum to make the proposed system a cost-effective solution. because the voltage source inverter should supply its output current equal to the difference between the motor phase current, io and the lci output current, ithy, the vsi output current, ivsi is proportional to the phase angle between ithy and io, corresponding toφ+θ. since the voltage source inverter power is desired to be as small as possible compared with the load commutated inverter power, the phase angleφ+θ should be maintained to be a minimum value. this condition can be obtained by adjusting the leading angleφto the minimum value satisfying the safe commutation, and controlling the induction motor with good power factor. since a high power induction motor shows a better power factor characteristics than a small rating motor, it is expected that the power factor angleθis small in high power motor applications. it makes the proposed system more competitive and useful for high power applications.
(to be continued )
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