Modular Multilevel Voltage Source Converter Research Articles

The Alternate Arm Converter “Extended-Overlap” Mode: AC Faults

This article presents ac fault ride-through strategies for the “extended-overlap” operating mode of the alternate arm converter (AAC), which is a type of modular multilevel voltage source converter that has been proposed for HVdc transmission applications. The AAC offers several benefits over the half-bridge modular multilevel converter, such as requiring fewer submodules with a smaller capacitance and providing dc fault ride-through capability. Novel symmetrical and asymmetrical ac fault ride-through strategies are described and these strategies are experimentally validated by using a small scale prototype.

The Series Bridge Converter (SBC): AC Faults

The series bridge converter (SBC) is a modular multilevel voltage source converter proposed for HVdc transmission applications. This topology is attractive for offshore and inner-city installations due to reduced volume and footprint, when compared with the modular multilevel converter. However, the required reduction in converter ac-side voltages during ac faults is particularly difficult to achieve. In this letter, a novel SBC boost operation mode and control strategy for ac fault ride-through is described and then validated in a steady state by simulation.

Protection of transmission lines in multi-terminal HVDC grids using travelling waves morphological gradient

This paper presents a new method for the protection of transmission lines in voltage source converter based multi-terminal HVDC grids. The fault generated travelling waves at the faulted line ends are filtered by series inductors. The filtered voltage waveform processed by morphological gradient accompanied by fault voltage drop is employed for fast single-end protection of HVDC lines. The proposed method is stable against external DC and AC faults, as well as faults at opposite pole and the breaker opening transients from the adjacent lines. Moreover, it does not require communications between the line ends, it is capable to detect and discriminate relatively high resistance faults, and uses relatively low sampling rate. The effectiveness of the proposed protection method is validated by simulation study on a four-terminal HVDC grid including modular multilevel voltage source converters.

The Alternate Arm Converter (AAC)—“Short-Overlap” Mode Operation—Analysis and Design Parameter Selection

This paper presents converter operation principles and theoretical analyses for “short-overlap” mode operation of the alternate arm converter (AAC), which is a type of modular multilevel voltage source converter that has been proposed for HVdc transmission applications. Fourier series expressions for the ideal arm current and reference voltage are derived, for the first time, in order to develop an expression for the submodule capacitance required to give a selected peak–peak voltage ripple of the summed submodule capacitor voltages in an arm. The dc converter current contains nonnegligible low-order even harmonics; this is verified by deriving, for the first time, a Fourier series expression for this current. As the dc converter current needs to be filtered to form a smooth dc grid current, a novel dc filter arrangement is proposed, which uses the characteristics of a simplified dc cable model, as well as the capacitance of the dc link and additional dc-link damping resistance, in order to form a passive low-pass filter. Results obtained from a simulation model, which is based on an industrial HVdc demonstrator, are used in order to verify the presented converter operation principles and theoretical analyses.

High-Frequency Operation of a DC/AC/DC System for HVDC Applications

Voltage ratings for HVdc point-to-point connections are not standardized and tend to depend on the latest available cable technology. DC/DC conversion at HV is required for interconnection of such HVdc schemes as well as to interface dc wind farms. Modular multilevel voltage source converters (VSCs), such as the modular multilevel converter (MMC) or the alternate arm converter (AAC), have been shown to incur significantly lower switching losses than previous two- or three-level VSCs. This paper presents a dc/ac/dc system using a transformer coupling two modular multilevel VSCs. In such a system, the capacitors occupy a large fraction of the volume of the cells but a significant reduction in volume can be achieved by raising the ac frequency. Using high frequency can also bring benefits to other passive components such as the transformer but also results in higher switching losses due to the higher number of waveform steps per second. This leads to a tradeoff between volume and losses which has been explored in this study and verified by simulation results with a transistor level model of 30-MW case study. The outcome of the study shows that a frequency of 350 Hz provides a significant improvement in volume but also a penalty in losses compared to 50 Hz.

Modeling and Simulation of MMC-HVDC Energy System

In order to study the steady-state performance of modular multilevel voltage source converter (MMC-HVDC), a 21-level MMC-HVDC system is modeled by PSCAD/EMTDC software. This system uses the DC voltage and active power controls, which are designed by the fundamental operation principles and mathematical models of MMC-HVDC. By analyzing the steady-state performance of MMC-HVDC, the corresponding simulation waveforms verify the correctness and validity of the simulation model.

Superconducting Magnet Based VSC Suitable for Interface of Renewable Power Sources

A novel Direct Current Control (DCC) modular multilevel Voltage-Source Converter (VSC) based on High- Superconducting Magnet Energy Storage (SMES) has been proposed. The design of the conduction-cooled magnet and the current carrying test results are introduced. Compared with the exiting types of converter, the Superconducting Magnet based VSC can be more effective in coping with the unbalanced power between the AC and DC sides, which makes it more suitable for the usage of the interface of the renewable power with the main grid. The concerned parameters of the converter have been deduced. The topology and the control strategy have been verified by a digital model simulated in Matlab Simulink environment.