A Dual VSG-Based M3C Control Scheme for Frequency Regulation Support of a Remote AC Grid Via Low-Frequency AC Transmission System

Abstract

This paper proposes a low-frequency ac (LFAC) transmission system capable of providing frequency regulation support for a remote ac grid aiming at enhancing its frequency stability. The LFAC transmission system using modular multilevel matrix converters (M3Cs) for a direct ac-ac conversion is receiving noticeable attention as an alternative solution for a long-distance transmission system. One of its potential applications is the system connection between a large ac grid and a remote ac grid, whereas the latter usually suffers from a fluctuant frequency due to lack of inertia. The core of this work is addressing a novel control scheme of the remote ac grid-side M3C, which makes both interactions with the LFAC system and the remote ac grid behave like a synchronous generator using a dual virtual synchronous generator (VSG) control scheme. This control scheme can enhance the frequency regulation in the remote ac grid, which the existing control schemes cannot provide. Average dc capacitor voltage versus active power (V ave -P) droop control is proposed to coordinate the power flow between the VSG controls applied in both sides of the M3C. To demonstrate the problem of the existing control scheme, as well as the benefits offered by the proposed control scheme, transient performance including load variation and fault events in the remote ac grid is studied and examined in the EMTDC/PSCAD software environment.INDEX TERMS Frequency control, low-frequency ac transmission, modular multilevel matrix converter, power control, power grids, power transmission, remote grid, virtual synchronous generator.

Frequency selection method for urban low frequency multi‐terminal interconnected systems

“…Under the control of the M3C device at both ends, the transmitted power on the line can be adjusted, which makes adjacent partitions mutual backup. The objectives of the M3C external loop controller at both ends of each transmission line are as follows [30, 33]: Considering the strong robustness of the power frequency main network, the active power external loop of power frequency side usually operates at the constant average voltage of sub‐module capacitor control (CAVSCC) to maintain the M3C power balance. The low‐frequency side of sending partition (SP) operates at the constant outlet voltage control (COVC) to provide the slack bus of interconnection system; the low‐frequency side of receiving partition (RP) operates at the constant active/reactive power control (CAPC/CRPC), regarded as load bus. The reactive power external loop of both power frequency sides operates at the CRPC. The reference value depends on the actual operation modes. …”

Structure optimization based on phase‐locked loop and controller parameters optimization of Y‐connected modular multi‐level converter for fractional frequency offshore wind power system under weak grid

“…Based on the state equations of the above control strategies, circuit Equations (7) and PLL Equations (8), this paper selects the following parameters as state variables:…”

“…FFTS can greatly reduce the line impedance by reducing the frequency, thereby solving the problem of the limited transmission capacity in the long-distance HVAC system [5][6][7]. Compared with HVDC, the system-level control and protection strategies of FFTS are simpler and easier to implement, similar to the traditional HVAC system [8,9]; its feasibility has been verified in [10,11].…”

Dual‐passivity‐based control strategy of modular multilevel matrix converter

“…Until now, the research of M3C is still in the stage of theoretical research, and there is no mature application at home and abroad. At present, the research on the control of M3C mainly includes the input‐side, 12‐15 the cluster voltage and circulating currents, 10,16‐20 and the output‐side 15,20‐22 . There are many literatures on the cluster voltage and circulating currents control.…”

“…However, there are few studies on the control of the M3C input‐side and output‐side. At present, proportional integral (PI) control is the main control method 12‐15 . For example, in Reference 23, a double αβ 0 transformation is proposed to separate the input‐side current, the output‐side current and the cluster current from the nine clusters, that is, to realize the decoupling control.…”