S." "Nomura

National Institute for Fusion Science (NIFS-Repository)

Thyristor converters are very promising as a largecurrent power conditioning system for superconductingmagnets. However, the traditional thyristor convertersoperate with a lagging power factor on the ac side re-quiring large reactive power compensation eﬀorts. Theobjective of this work is to discuss the feasibility of theseries compensation of thyristor converters using vari-able series capacitors1). The series capacitors can controlthe magnet voltage through a unity power factor thyris-tor converter with a resulting leading power factor seenfrom the grid. Therefore, combined with the traditionalthyristor converter, the combined converter system en-ables the magnet voltage control with a unity power fac-tor. Additionally, by the eﬀect of the leading and lag-ging current combinations, the combined converter sys-tem may also reduce the harmonic currents.Fig. 1 shows the simulation circuit of the combinedthyristor converter system in order to evaluate the re-duction of harmonic currents. The combined thyristorconverter system is composed of the 12-pulse convertersystem. Fig. 2 shows the simulation waveforms whenthe ﬁring angle of the pure thyristor bank and the lead-ing angle of the series compensated thyristor bank areselected to 7.5 deg., respectively. In this case, the ac cur-rent of each branch has 30 deg. of the phase diﬀerence.Therefore the total current waveform through the gridbecomes the similar current waveform of the 24-pulsepure thyristor converter even when the actual thyristorconverter system consists of the 12-pulse conﬁguration2).This result veriﬁes that the combined thyristor convertersystem can be expected to reduce the harmonic currentsFig. 1: Simulation circuit of the 12-pulse thyristor bankbased on the combined thyristor converter system.Fig. 2: Simulation waveforms of the 12-pulse combinedthyristor converter system. The current of each branchcorresponds to the current indicated in Fig. 1.by selecting an optimal combination of the ﬁring angleof the pure thyristor bank and the leading angle of theseries compensated thyristor bank. Instead of the unitypower factor thyristor converter in the series compensa-tion, the leading power control capability and the har-monic current reduction of the 12-pulse diode rectiﬁer isalso experimentally evaluated by using gate-commutedseries capacitors3).1) S. Nomura, J. A. Wiik, “Feasibility study on currentsource conversion for superconducting magnets us-ing series compensated thyristor converters,” IEEETrans. Appl. Supercond., vol. 22, no. 3, 5400804(2012).2) D. Saito, S. Nomura, “Power factor improvement andharmonic current reduction of thyristor converters us-ing series capacitors,” 2012 IEE-Japan Industry Ap-plications Society Conference, Y30 (in Japanese).3) I. Matsumoto, S. Nomura, “Leading power factor con-trol and AC current waveform improvement of mul-tiplex diode rectiﬁers using gate-commuted series ca-pacitors,” 2012 IEE-Japan Industry Applications So-ciety Conference, Y31 (in Japanese).296§9. Feasibility Study on Series Compensated Thyristor Converters for Superconducting MagnetsNomura, S. (Meiji Univ.)

Feasibility Study on Series Compensated Thyristor Converters for Superconducting Magnets

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