Enhancing the Performance of Pre-Charging Strategy of a Modular Multilevel Static Synchronous Compensator
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Enhancing the Performance of Pre-Charging Strategy of a Modular Multilevel Static Synchronous Compensator Samuel N. Duarte1
· Pedro M. Almeida1 · Pedro G. Barbosa1
Received: 30 March 2020 / Revised: 26 May 2020 / Accepted: 9 September 2020 © Brazilian Society for Automatics–SBA 2020
Abstract This paper presents an improvement in an energizing strategy for a three-phase four-wire modular multilevel converter connected to an electric network as a static synchronous compensator. The new procedure comprises three stages. During the first stage, two thyristors control the current drained from the grid in order to charge the capacitors of the submodules of the positive and negative arms of two phases of the converter. After that, the energy stored in these capacitors is shared with the others submodule capacitors by firing the semiconductor switches of the converter in a strategic way. In addition of limiting the current drained from the grid during all the energizing procedure, the proposed strategy does not produce overvoltage at the converter dc terminals. The results of digital simulations are presented to validate the mathematical analysis and to demonstrate the performance of the proposed energizing strategy. Keywords Pre-charging algorithm · Inrush current · Modular multilevel converter · STATCOM
1 Introduction The modular multilevel converter (MMC) is an emerging power electronic converter which the dc bus voltage is shared between several series connected submodules (SM). This feature makes the MMC an interesting topology of static converter for high-power high-voltage applications such as high-voltage dc transmission systems (HVDC) and flexible ac transmission systems (FACTS) (Lesnicar and Marquardt 2003; Abildgaard and Molinas 2012). In addition to its scalability, the capacity to synthesize low distorted voltages, while the switching losses are kept lower than other multilevel converter topologies, is motivating the use of MMC to drive medium-voltage motors (Du et al. 2018), to integrate large wind and solar power plants into the grid (Sharifabadi et al. 2016), to operate as active power filters (Ghetti et al. 2012), and static power compensator or conditioning (Orcajo et al. 2020; Duarte et al. 2019). Figure 1 shows the schematic diagram of a three-phase four-wire MMC connected to an electric network as a static synchronous compensator (STATCOM–MMC). The fourth
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Samuel N. Duarte [email protected] Federal University of Juiz de Fora, Electric Engineering Graduate Program, Juiz de Fora, MG 36.036-900, Brazil
leg of the MMC can be controlled as a single-phase converter to compensate for zero-sequence unbalances at the point of common coupling (PCC) in a manner similar to that shown in Duarte et al. (2019). In this figure, the MMC legs are composed by two arms, one positive and the other negative, each one with m SM and an inductor connected in series. These submodules are simple dc–ac static power conversion units comprising semiconductor switches and, at least, one capacitor. The main co
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