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Curremt Equalizing for a Multi-Modular Current Source Converter Li Jun, Dehong Xu
K.W.E. Cheng
College of Electrical Engineering Zhejiang University Hangzhou, 310027, P.R. China
Department of Electrical Engineering The Hong Kong Polytechnic University Hong Kong S A R
Abstract - Phase-shifted S P W M technique is adopted to control directly-paralleled multi-modular current source converter for Super-conducting Magnetic Energy Storage System (SMES) to
cancellation originally expected can not be reached pronouncedly [71. To eliminate the DC current un-balance and AC current reach high power and low harmonics. The phase-shifting of the distortion result from camer phase shifting without adjusting carriers results a serious current unbalance between each converter modules when the high power switch devices work on the amplitude and phase angle of the modulation waves of low switch rate. This paper analyses the problems of the classic each module individually, this paper introduces the carriercurrent balancing methods and introduces the carrier-swapping swapping method into phase-shifted SPWM technique for a method into phase-shifted S P W M technique, to eliminate such multi-modular current source converter. Experiment results current unbalance in a multi-modular current source converter are given to prove the effectiveness of current equalization without adjusting the amplitude and phase angle of the provided by the camer-swapping method. modulation waveforms of each module respectively. Experimental results are given to prove the effectiveness of current 11. PHASE SHIFTED SPWM CONTROLWITHOUT CARRIERequaIization provided by the carrier-swapping method. a
SWAPPING
I. INTRODUCTION
SMES is a perfect component for Flexible AC Transmission System (FACTS) because of its high efficiency and fast response [l],[2]. A bi-direction power converter known as Power Condition System (PCS) is needed by SMES as a tie between the super-conductive coil and the utility system. The PCS can be either a current source converter or a voltage source converter connected to a two-quadrant boost chopper. The super-conductive coil is inherently a current source, the current source converter have more advantages to be the PCS for SMES than the voltage source converter [3],[4],[5]. High power converter is needed for a high power SMES. Current source converter can be paralleled directly to reach high power. Proper PWM control strategy should be applied to such multi-moduIar current source converter to reduce the harmonic while the switch frequency o f the high power device is restricted to a low rate. Phased-shifted SPWM control is a good way to reduce the harmonic and attain linearity characteristics far a multi-modular current source converter [SI. Care should be taken to balance the current of each module in the multi-modular converter. When the camer frequency is lower (as 3 or 6 times of the modulation signal), the average DC current o f each module differs significantly because of the phase shifting of the carriers. A current equalizing control loop is applied to each module to control the amplitude and phase angle of the modulation signal for current balancing [5],[6]. Thus, the modulation signals for each paralleled converter differ from each other seriously, harmonics self-
0-7803-8975-1/05/$20.WQ2M15 IEEE.
A . Main Circuir A main circuit of eight-modules current source converter is shown in Fig, 1. Each module is a six-valves current source converter. AC side of each module is connected together directly. DC side of converter module i s paralleled through DC inductors.
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Figurc I. Main circuit of an eight-modules current source converter for
SMES
B. Phase-shified SPWM Control Phase-shifted SPWM control is applied to this multimodular current source converter to reduce harmonics while the switch frequency is low. SPWM control for current source converter need tri-logic signal [XI. The signals are produced as follows: X , = sign [v,. ( w f 1 - v , ( w , t + +,)I X b=sign[v,(wt)-v,(w,t+#,)l
x, = srgn [ v , ( w t ) - V < ( W , l + +,)I sign (XI = [I, z J yo = +cx. - X . 5 ) yb
=
f(xb
-
xc)
U, = +(X
