introduction
load commutated inverter (lci)-based synchronous motor drives have been traditionally used in very high power applications such as pumps, compressors and fans drives. the merits of the load commutated inverter based system are resulted from the fact that since it employs converter grade thyristors and utilizes natural commutation of the thyristors. it provides simplicity, robustness, cost effectiveness, and very low switching losses.. moreover, because it has the currentsource inverter (csi) topology, it has inherent advantages of csi:
1) short-circuit protection, the output current being limited by the regulated dc-link current,
2) high converter reliability, due to the unidirectional nature of the switches and the inherent short-circuit protection,
3) instantaneous and continuous regenerative capabilities .. due to all these features, a number of researches have been conducted in the last two decades, to control the load commutated inverter based induction motor drive and improve its performance, especially in medium to high power applications..
however, the conventional load commutated inverter-based induction motor drives have shown some serious difficulties. since the system has thyristor-based topologies, it should guarantee safe commutation for thyristors, requiring that the load commutated inverter is faced with a leading power factor in all the operating regions. the leading power factor required for natural commutation is generated by additional capacitors connected in parallel with the induction motor, since the motor cannot provide the leading power factor through excitation control employed for the synchronous motor. as the power rating of the induction motor is increased, a larger capacitance is required to create higher leading var requirement taken by the capacitor, which could become unreasonably high. output capacitors also set up resonance phenomena by the interaction with the motor inductance, seriously restricting the drives' performance and inherent instability in the high frequency region .. large output capacitors may cause an undesirable self-excitation under certain conditions, a problem which becomes aggravated at higher speeds.. this approach to generate the leading power factor through the output capacitor, although very widely used, has fundamental problems resulted from the approach itself. at the startup and during the low speed operation, the leading vars generated by the output capacitors decrease, resulting in the lagging power factor, thus load commutation is not possible. therefore a complex and costly forced dc-commutation circuit is required for the load commutation at lower speed region .. moreover, the squarewave motor current waveforms in low speed region, rich in low order harmonics, can produce voltage spikes in stator leakage inductance of the motor, potentially hazardous for the winding insulation..
in this paper, a novel hybrid solution for the load commutated inverter based induction motor drive, using a parallel assembly of a load commutated inverter and a voltage source inverter (vsi), is proposed. the operation of the proposed circuit is investigated and described. it is shown that all problems caused in the conventional load commutated inverter-induction motor system can be overcome by the proposed solution. this hybrid solution has the following features.
1) the leading power factor for load commutation of the load commutated inverter is fully provided by the voltage source inverter in all operating regions.
2) the safe commutation for the load commutated inverter is achieved by active control of the leading power factor angle through the voltage source inverter in all applications.
3) all problems caused by the output capacitor in the conventional load commutated inverter based induction motor drives, such as fundamental and harmonic resonance, and inherent instability in the high frequency region, can be solved since the vsi emulates output capacitor.
4) by avoiding the use of complex and costly forced dccommutation circuit, the potential risk of commutation failure regarding the dc-commutation circuit and the torque pulsation of the motor can be eliminated.
5) the motor currents and voltages under all running conditions are pure sinusoidal, containing little harmonic components.
6) the proposed system shows fast dynamic response by the vsi operation.
standard load commu- tated inverter based induction motor drive
a typical schematic diagram of a load commutated inverter based induction motor drive is shown in fig. 1. it consists of a three-phase controlled rectifier at input side and a current source inverter at the output side with a large dc link inductor. the amplitude of the currents supplied to the motor is controlled by the phase-controlled line converter through a dc link inductance. the dc link inductor reduces the current harmonics and ensures that the input of load inverter and hence, to the motor appears as a current source. the dc current magnitude as well as motor current magnitude can be controlled by adjusting the firing angle of the line converter. the load inverter can control only the fundamental frequency of motor currents by selecting the gating instances of thyristors. for successful commutation of thyristors in the load inverter, the output current of the load inverter, ithy must lead the corresponding motor phase voltage, voutput. since motor phase currents in induction motors always lag the corresponding motor phase voltages by the induction motor characteristics, a leading power factor is obtained by the output capacitor and the dc-commutation circuit. the output ac capacitors are required to provide a phase shift of the motor phase current, resulting in a leading power factor. a vector diagram of fig. 2 explicitly explains how the output capacitor provides a phase shift of the current, resulting in a leading power factor angle φ. this leading power factor allows thyristors in the load inverter to commutate at speeds above critical frequency of induction motors.
the output capacitor also smoothes out the output current waveform coming from the inverter to nearly sinusoidal in the high frequency operation by providing a low impedance path for current harmonics. however, at the startup and during the low speed region, these output capacitors cannot make enough leading angle because the capacitor currents are too small due to high impedance of the capacitors. thus, additional forced dc-commutation circuits are required so as to facilitate the commutation from one phase to another phase, by effectively bypassing the flow of dc link current around the load. with the operation of this circuit, the induction motor can start up and bring the operation to above the critical speed, which will ensure load commutation by output capacitors. however, this conventional load commutated inverter based induction motor system with the output capacitor and the dc commutation circuit has shown some drawbacks.
1) since output ac capacitors should fully compensate the effect of inductance in the induction motor in order to provide a phase shift, the required capacitor size must be increased proportional to the power rating of an induction motor.
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