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an investigation of dual stator winding induction machines

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<strong>an</strong>d additional power supplies. Directly connected parallel three-phase converters from<br />

on source is the way to overcome those problem, however since the two three-phase<br />

PWM converters share the same voltage sources, circulating currents c<strong>an</strong> exist in all <strong>of</strong><br />

the phases. To avoid the circulating current, inter-phase reactors are used as zero-<br />

sequence current damper, however the reactors c<strong>an</strong> only provide high imped<strong>an</strong>ce at high<br />

frequency level <strong>an</strong>d c<strong>an</strong> not dampen the low-frequency circulating current. In [9.4], it is<br />

found that the zero-sequence excitation <strong>of</strong> parallel connected rectifiers comes from the<br />

discontinuous point <strong>of</strong> the Space Vector Modulation (SVM) method. Since the<br />

conventional control method with only q- <strong>an</strong>d d-axis components c<strong>an</strong>not eliminate the<br />

disturb<strong>an</strong>ce, a SVM without using zero vectors is used to reduce the pure zero-sequence<br />

current. By using the proposed modulation method, a circulating current at switching<br />

frequency on each phase c<strong>an</strong> still be observed, which c<strong>an</strong> be considered as small<br />

disturb<strong>an</strong>ces. Another synchronized current control method has been proposed in [9.7], in<br />

which two parallel connected three-phase converters are controlled as a three-phase six-<br />

leg converter. The switching frequency <strong>an</strong>d harmonic current content were reduced in<br />

steady state while the quick response has been observed at tr<strong>an</strong>sient state. The circulating<br />

current <strong>an</strong>d zero sequence current c<strong>an</strong> be regulated to be zero, however, the control <strong>of</strong> the<br />

system will be very complicated if more converters are paralleled. In [9.2], a zero-<br />

sequence model is developed based on <strong>an</strong> average model <strong>of</strong> parallel connected<br />

converters. Then a control variable associated with space vector modulation was<br />

introduced for the control <strong>of</strong> zero sequence current. A high perform<strong>an</strong>ce zero sequence<br />

current control was proposed to suppress the circulating current. In [9.8], a method<br />

combining the space-vector modulation scheme with a variable-structure control was<br />

22

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