A generalized approach for design of contingency versatile DC voltage droop control in multi-terminal HVDC networks

The non-deterministic nature of power fluctuations in renewable energy sources impose challenges to the design of DC voltage-droop controller in Multi-Terminal High-Voltage DC (MTDC) systems. Fixed droop control does not consider converters’ capacity and system operational constraints. Consequently, an adaptive droop controller is counseled for appropriate power demand distribution. The previous adaptive droop control studies based on the converters’ Available-Headroom (AH) have lacked the demonstration of the droop gain design during consecutive power disturbances. In this paper, the design of the adaptive DC voltage droop control is investigated with several approaches, based on the permitted converters’ global and/or local AH and Loading Factor (LF). Modified adaptive droop control approaches are presented along with a droop gain perturbation technique to achieve the power-sharing based on the converters’ AH and LF. In addition, the impact of Multi-Updated (MU), Single-Updated (SU), and Irregular-Updated (IU) droop gains is investigated. The main objective of the adaptive droop control design is to minimize the power-sharing burden on converters during power variations/consecutive disturbances while maintaining the constraints of the DC grid (i.e., voltage and power rating). The presented approaches are evaluated through case studies with a 4-terminal and 5-terminal radial MTDC networks.Qatar Foundation; Qatar National Research FundScopu

Smooth Regulation of DC Voltage in VSC-MTDC Systems Based on Optimal Adaptive Droop Control

DC voltage stability and power balance are the key conditions for stable operation of DC transmission systems. The droop control does not depend on inter-station communication and has the advantage of multi-station cooperative unbalanced power dissipation, which has been widely used in the voltage source converter based multi-terminal direct current (VSC-MTDC) system. However, its control will cause DC voltage deviation, and there are a series of problems such as unstable system control and overload operation of the VSC-station due to improper setting of droop coefficient. In this paper, the infeasibility of DC voltage error-free correction under droop control mode is theoretically analyzed, and a VSC-MTDC cooperative optimization droop control strategy with DC voltage "quasi-error-free" adjustment ability is proposed. The strategy adjusts the droop coefficient in real-time by monitoring the DC voltage deviation and the power margin of the VSC-station to solve the problem of voltage deviation caused by unbalanced power consumption. At the same time, an additional DC voltage stabilizer is designed to automatically adjust the power reference value and restore the DC voltage. Finally, a five-terminal VSC-MTDC system is built under PSCAD/EMTDC to verify the feasibility of the proposed strategy

Adjustable inertial response from the converter with adaptive droop control in DC grids

In a DC grid, the inherent inertial support from the DC capacitors is too small to resist step changes or random fluctuations from the intermittent power resources, which results in lower DC voltage quality. In this paper, an adaptive droop control (ADC) strategy is proposed to achieve an increased inertia from the droop controlled converter. The adaptable droop coefficient according to the DC voltage variation enables fast swing of the droop curve, so that the converter can provide inertial power for the DC grid like synchronous generators in AC grids. The design of the ADC including the calculation and limitation of the adaptable droop coefficient is analyzed in detail. The small-signal analysis of the DC grid with ADC is provided to identify its stability issue. Experimental tests on a controller hardware-in-the-loop (HIL) platform of a low-voltage (LV) DC grid are carried out to validate the proposed method. In this LV DC grid, the proposed ADC is implemented on the energy storage system (ESS) which provides inertial support to improve the DC voltage quality under different power fluctuations, and smooths the power transmitted to AC grid