Adaptive Droop Control for Effective Power Sharing in Multi-Terminal DC (MTDC) Grids

Abstract

Abstract-Following a converter outage in an MTDC grid, it is critical that the healthy converter stations share the power mismatch/burden in a desirable way. A fixed value of powervoltage droop in the DC link voltage control loops can ensure proper distribution according to the converter ratings. Here a scheme for adapting the droop coefficients to share the burden according to the available headroom of each converter station is proposed. Advantage of this adaptive (variable) droop scheme for autonomous power sharing is established through transient simulations on an MTDC grid with four bipolar converters and DC cable network with metallic return. Results for both rectifier and inverter outages under two different scenarios are presented. Post contingency steady-state operating points obtained from transient simulation are shown to be consistent with those derived analytically. Impact of varying droop coefficients on the stability of the MTDC grid is established. An averaged model in Matlab/SIMULINK which has been validated against detailed switched model in EMTDC/PSCAD is used for the stability and modal analysis.

Review of VSC HVDC connection for offshore wind power integration

“…The converter stations with the higher values of droop will participate more in the DC voltage control and therefore their power share will be smaller [12,64,65]. In a fixed droop scheme, the droop factor at each terminal is calculated according to the corresponding converter ratings.…”

“…the difference between the rated capacity and the present loading. For dealing with this issue, [65] suggests a modification of the initial DC voltage droop controller. In the proposed control scheme, the droop factor is a function of the available headroom of each converter.…”

“…This approach with the variable droop factor would also be useful in the future, when the multi-terminal HVDC grids enter the power market. It will allow the owners of the converter stations to share the power mismatch appropriately, and they could use spare capacity also as ancillary service [65].…”

“…In AC systems, active power-frequency droop is used in the governor control loop, for avoiding conflicts in reference values of frequency. In a similar way, in the DC grids, the active power-DC voltage droop can be used to modify the reference values of the DC voltage, for avoiding conflicting DC voltage set values at the terminals [65]. According to [64], proper calculation of the initial reference values (before their modification according to droop factor) is needed for a more precise control scheme.…”

Control of multi-terminal HVDC networks towards wind power integration: A review

“…Besides, large droop gains can provoke oscillations in the DC voltage [29][30][31]. To improve performance under various loading conditions, references [12,32,33] have proposed an adaptive scheme that modified the droop gains depending on the system state.…”