Coordinated Start-Up Control and Inter-Converter Oscillations Damping for MMC-HVDC Grid

Abstract

Modular multilevel converter (MMC)-based high-voltage direct current (HVDC) grid is a promising technology for multiple offshore wind farms (OWFs) to form interconnected offshore grids. However, in the MMC-HVdc grid, the converter terminals connecting the OWFs could only be passively pre-charged by the ones connecting active ac networks via the dc lines during the start-up process. Under this condition, the start-up process of HVdc grid of the passive-side converters becomes significantly complicated. This paper, considering an HVdc grid integrating multiple passive-side converters, investigates its start-up process with three main contributions: 1) a modified nearest level modulation (NLM) method is proposed for the passive-side converters to mitigate the inrush currents after they are deblocked; 2) based on the dc-side equivalent models, the inter-converter resonances and oscillations between the passive-side converters are analyzed after they are deblocked, and; 3) an oscillation damping controller is proposed to provide the electrical damping of inter-converter oscillations between the passive-side converters. The simulation results in PSCAD/EMTDC from a four-terminal MMC-HVDC grid with two terminals connecting the OWFs verify the effectiveness of the proposed start-up control schemes.INDEX TERMS HVDC grid, modular multilevel converter (MMC), start-up control, inrush currents, inter-converter oscillations. NOMENCLATURE T iTerminal4) SM Sub-module HB Half-bridge CB Circuit breaker VSC Voltage source converter MMC Modular multilevel converter HVDC High-voltage direct current PTP Point-to-point NLM Nearest level modulation SCR Short circuit ratio PCC Point of common coupling MTDC Multi-terminal HVDC The associate editor coordinating the review of this manuscript and approving it for publication was Ahmad Elkhateb.

MMC Based MTDC Grids: A Detailed Review on Issues and Challenges for Operation, Control and Protection Schemes

“…In [113] a coordinated control strategy is proposed for damping oscillations of a VSC based MTDC system connected with wind power plants. An oscillation damping controller to provide electrical damping for inter converter oscillations in an MMC based HVDC system is proposed by [114]. A detailed analysis for low frequency oscillations for an off-shore wind farm connected to MMC based MTDCC grid have been provided by [115].…”

Modeling and Harmonic Stability of MMC-HVDC With Passive Circulating Current Filters

“…Typically, MMCs with voltage control schemes are used in renewable energy integration applications, such as wind farms in the North Sea [4] in Europe that are connected to the onshore grids using MMC-HVDC systems, while MMCs with phase current controls are typically utilized in large-scale grid connections, such as most of the onshore two-terminal HVDC systems. There are also other advanced control strategies used for different MMC application purposes [28][29][30]. For simplicity, an MMC with an AC voltage control strategy without an inner current control loop is adopted in this paper as an example to conduct the following discussion.…”

Duty-Cycle Predictive Control of Quasi-Z-Source Modular Cascaded Converter Based Photovoltaic Power System

“…Therefore, accommodating high voltage (HV) dc transmission can lower power loss [1]. Conventional solutions commonly use a central inverter with a bulky step-up transformer or the cascaded multilevel inverter, in order to invert the dc voltage from the PV panels into a HV ac bus [2]- [4]. The high ac voltage is then converted to a high dc voltage for the longdistance transmission.…”