Zunfang Chu

An Improved Pulse Width Modulation Method for Chopper-Cell-Based Modular Multilevel Converters

In this paper, an improved pulse width modulation (PWM) method for chopper-cell (or half-bridge)based modular multilevel converters (MMCs) is proposed. This method can generate an output voltage with maximally 2N+1 (where N is the number of submodules in the upper or lower arm of MMC) levels, which is as great as that of the carrier-phase-shifted PWM (CPSPWM) method. However, no phase-shifted carrier is needed. Compared with the existing submodule unified pulse width modulated (SUPWM) method, the level number of the output voltage is almost doubled and the height of the step in the staircase voltage is reduced by 50%. Meanwhile, the equivalent switching frequency in the output voltage is twice that of the conventional SUPWM method. All these features lead to much reduced harmonic content in the output voltage. What is more, the voltages of the submodule capacitors can be well balanced without any close-loop voltage balancing controllers which are mandatory in the CPSPWM schemes. Simulation and experimental results on a MMC-based inverter show validity of the proposed method.

A novel inner current suppressing method for modular multilevel converters

Ideally, the inner (the upper or lower arm) current of a modular multilevel converter (MMC) is ideally assumed to be the sum of a dc component and an ac component of the fundamental frequency. However, as ac current flows through the submodule (SM) capacitors, the capacitor voltages fluctuate with time. Consequently, the inner current is usually distorted and the peak/RMS value of it is increased compared with the theoretical value. The increased currents will increase power losses and may threaten the safe operation of the power devices and capacitors. This paper proposes a closed-loop method for suppression of the inner current in an MMC. This method is very simple and is implemented in a stationary frame, and no harmonic extraction algorithm is needed. Hence, it can be applied to single-phase or three-phase MMCs. Besides, this method does not influence the balancing of the SM capacitor voltages. Simulation and experimental results show that the proposed method can suppress the peak and RMS values of the inner currents dramatically.Ideally, the inner (the upper or lower arm) current of a modular multilevel converter (MMC) is assumed to be the sum of a dc component and an ac component of the fundamental frequency. However, this current is usually distorted and the peak/RMS value of it is increased compared with the theoretical result. This is because ac current flows through the submodule (SM) capacitors and the capacitor voltages fluctuate with time. The increased currents will increase power losses and may threaten the safe operation of the power devices and capacitors. This paper proposes a novel close-loop method for suppression of the inner current in MMC. This method is very simple and is implemented in stationary frame, and no harmonic extraction algorithm is needed. Hence, it can be applied to single-phase or three-phase MMC. What is more important, this method does not influence the balancing of the SM capacitor voltages. Simulation and experimental results show that the proposed method can suppress the peak and RMS values of the inner currents dramatically. Meanwhile, the harmonic contents in the output current can also be suppressed satisfactorily even when the SM capacitor voltage ripple factor is as large as about ±19%. Therefore, the proposed method can also be adopted to reduce the SM capacitance requirement.

Power Module Capacitor Voltage Balancing Method for a ±350-kV/1000-MW Modular Multilevel Converter

This letter focuses on the power module (PM) capacitor voltage balancing of a

Design of Power Electronic Transformer Based on Modular Multilevel Converter

\pm 350

A three-phase 10 kVAC-750 VDC power electronic transformer for smart distribution grid

-kV/1000-MW modular multilevel converter (MMC). For this MMC, the current flowing through the IGBTs will exceed the nominal current of the adopted commercial high-voltage (3.3 kV or higher) IGBT products, i.e., 1500 A. As for the PM voltage balancing of this MMC, the switching frequency must be kept low to reduce the losses and the accurate calculation of the PM switching frequency is of great importance to the design of the cooling system. This letter presented a low-switching frequency PM balancing method by introducing the balancing adjusting number (BAN). The PM switching frequency calculation method is also presented with different BANs. Computer simulation and the real-time control hardware in the loop test results based on RT-LAB show the effectiveness of the proposed PM balancing method and the accuracy of the switching frequency calculation method under the rated power.

Prototype of smart energy router for distribution DC grid

Power electronic transformer acts as a key component of the future automation in smart grid, which is the replacement of conventional distribution transformers by a power transformer. In this paper , the optimum design of a power electronic transformer(PET) based on topology of Modular multilevel converter (M2C). M2C is an emerging and highly attractive topology intended for high- or medium-voltage power electronic devices. In the design process, the modulation scheme of M2C is analyzed. A control method for balancing the M2C DC buses has been introduced. The PET includes a dual-active-bridge DC/DC converter. The DC-DC converter realizes magnetic isolation of PET and reduces the transformer size. Meanwhile, both primary-side and secondary-side switches can achieve the zero-voltage-switching(ZVS) .The PET performs power transformers functions, at the same time, which has advantages such as power factor correction, elimination of voltage sag and swell, and reduction of voltage flicker in load side.

Power module voltage balancing method for a ±350 kV/ 1000 MW modular multilevel converter

In this paper, a power electronic transformer (PET) topology for distribution grid is presented based on modular multilevel converter (MMC). The presented topology is of three-phase type and have two dc voltage terminals, i.e. the high-voltage dc terminal and the low-voltage dc terminal. So, it can be connected directly to high-voltage direct current (HVDC) systems and renewable energy systems simultaneously. This feature makes it possible to perform as an “energy router” in smart grids. What is more important, the proposed PET can substantially reduce (by 44.4% for the studied case) the number of medium frequency transformers (MFTs), compared with the existing solutions. Computer simulation and experimental results on a 10 kV prototype show validity of this PET.

Startup strategy of VSC-HVDC system based on modular multilevel converter

Smart energy router (SER), also called as power electronics transformer (PET) or solid state transformer (SST), will serve as a critical component in the next-generation electric power system. Taking advantage of modular multilevel converter (MMC) and input series output parallel (ISOP) topologies, new type of smart energy router has been developed. Presented architecture using two stages with AC/DC conversion in the medium-voltage (MV) side, and ISOP type DC/DC conversion in the low-voltage (LV) side, thus allows interaction with DC grid and local renewable sources, etc. Compared to existing topologies, proposed SER uses less power switches and high-frequency transformers. The details related to the 2-MVA SER prototype, including the electrical design of the circuits and control system is presented in this paper. Simulation and experimental results on this prototype with distribution DC grid show validity of this SER.

A novel circulating current suppressing method of modular multilevel converter

In this paper, a power module capacitor voltage balancing method is presented for a ±350 kV/ 1000 MW modular multilevel converter (MMC) oriented for a back-to-back grid interconnecting project. In this method, a Balancing Adjusting Number (BAN) is proposed to adjust the voltage balancing effect and the switching frequency of the power modules. An accurate switching frequency calculation method is also presented. Verification results show the effectiveness of the proposed voltage balancing method and the correctness of the switching frequency calculation method.

An improved voltage balancing method of modular multilevel converters

Based on modular multilevel converter (MMC) for high voltage direct current (HVDC) system, the startup strategy is researched. The equivalent charging circuits and physical models for uncontrolled and controlled charging process of MMC-HVDC system are derived and the charging current stresses are analyzed. In traditional methods, series resistors in the arm or in the dc lines are used to limit the surge current when the MMC station unlocked at controlled charge step from dc side. In this paper, new startup method without extra source is proposed. With this method, charging current of MMC from dc side at controlled stage is minimized. Experimental results testify the effectiveness of this strategy.