Minimum Short Circuit Ratio Requirement for MMC-HVDC Systems Based on Small-Signal Stability Analysis

Abstract

This paper determines the minimum short circuit ratio (SCR) requirement for a modular multilevel converter based high-voltage direct current (MMC-HVDC) transmission systems. Firstly, a simplified model of MMC is introduced; the MMC is represented by its AC and DC side equivalent circuit. Next, by linearizing the MMC subsystem and the DC network subsystem, the deduction of the small-signal models of MMC subsystem, the small-signal model of the DC network and MMC-HVDC are carried out successively. Thirdly, the procedure for determining the minimum SCR requirement of MMC-HVDC is described. Finally, case studies are performed on a two-terminal MMC-HVDC system under four typical control schemes. The results show that the restraint factors for the rectifier MMC is predominantly the voltage safety limit constraint, and the restraint factors for the inverter MMC are mainly the phase locked loop (PLL) or the outer reactive power controller. It is suggested that the minimum SCR requirement for the sending and the receiving systems should be 2.0 and 1.5 in the planning stage.

Evaluation of the use of short-circuit ratio as a system strength indicator in converter-dominated power systems

“…The study in [5] investigates the minimum required short-circuit ratio for stable AC-DC systems using the SCR to obtain a system representation by means of an equivalent AC system impedance. A similar approach is found in [6], [7] where the system equivalent representation is also calculated in a straightforward manner by using the information of the SCR.…”

Robust multivariable control design for HVDC systems considering AC grid impedance uncertainties

“…Commonly, the AC grid operator provides the maximum and minimum values of the short circuit ratio (SCR) at the point of common coupling (PCC), and the upper and lower bounds for the grid equivalent impedance (in the form of X and R) [4]. The upper bound of grid impedance which corresponds to the minimum SCR poses a major challenge to the control system of the VSCs, especially in the case of a very weak AC grid whose SCR can be lower than 2 [5,6]. Thus, the equivalent AC grid model has to capture this characteristic to ensure that a control design based on such grid model is valid for a wide range of impedance variations.…”