Elucidation of DC Magnetic Deviation in Converter Transformers Used for a Self-Commutated BTB System during Single-Line-to-Ground Faults

Recently, many scientists and engineers of power systems and power electronics have been involved in multilevel converters intended for achieving large-capacity power conversion without transformers. Two of the representatives are: i) the diode-clamped multilevel converter (DCMC), and ii) the capacitor-clamped multilevel converter (CCMC). A three-level DCMC, or a neutral-pointclamped (NPC) converter has been put into practical use. If the level of voltage is more than three in a DCMC, inherent voltage imbalance occurs in the series-connected dc capacitors, thus resulting in requiring an external balancing circuit (such as a buck-boost chopper) for each dc-link capacitor. Furthermore, a significant increase in the clamping diodes required renders assembling and building of each leg more complex and difficult. The similar problems exist in a CCMC in terms of clamping capacitors.To solve the aforementioned problems, attention is paid to a modular multilevel converter (MMC) for high-or medium-voltage power conversion without line-frequency transformers. Fig. 1 shows a half-bridge circuit based on an MMC. The MMC consists of cascade connection of multiple bidirectional PWM chopper-cells in Fig. 1(b), thus requiring voltage-balancing control of each choppercell. However, no paper or article has been presented or published on the voltage-balancing control with theoretical and experimental verifications.This paper deals with a modular multilevel converter with focus on its PWM control method and operating performance. Combining averaging control with balancing control enables the converter to achieve voltage balancing without any external balancing circuit. The viability of the MMC as well as the effectiveness of the PWM control method is confirmed by computer simulation using the "PSCAD/EMTDC" software package, and experiment using a 250-W laboratory system. Fig. 2 shows experimental waveforms of Fig. 1 obtained by the laboratory system. The voltage command of each dc capacitor was set as v * C = 70 V, while the frequency of the load voltage v 0 was set as f = 50 Hz respectively. Experimental results show that each dc voltage was regulated at 70 V without any steady-state error. Moreover, this paper proposes the dual MMC for low-voltage large-current power conversion. (a) half-bridge circuit (b) chopper-cell Fig. 1. Circuit configuration of a modular multilevel converter. Fig. 2. Experimental waveforms of Fig. 1 ( f = 50 Hz, v * C = 70 V, E = 140 V).-13 -

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Elucidation of DC Magnetic Deviation in Converter Transformers Used for a Self-Commutated BTB System during Single-Line-to-Ground Faults

Cited by 2 publications

References 22 publications

A novel transformer model using magnetic circuit

A novel transformer model using magnetic circuit

PWM Control and Experiment of Modular Multilevel Converters

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