Interaction between the voltage‐droop and the frequency‐droop control for multi‐terminal HVDC systems

Abstract

MTDC systems need to provide a high level of reliability as well as efficient support to the interconnected AC grids. This can be achieved by equipping each converter with a dual controller combining both the voltage-droop and the frequency-droop control techniques. In this article, the theory behind the coupling between the two droops is investigated and an electromagnetic transient study of a 5-terminal HVDC grid is discussed to validate the theoretical results. The use of the dual controller allows the whole system to be more flexible. However, the frequency droop coefficient must be corrected to comply with the TSO's requirements.

Impact of the DC cable models on the SVD analysis of a Multi-Terminal HVDC system

“…The third DC cable model is basically PI section but with several parallel branches [10], as depicted in Figure 1(c), where the parallel branches have been calculated through a fitting algorithm to fit the frequency response of the frequency-dependent elements of a wide-band cable [11]. Finally, the fourth DC cable model is an improved PI section where the coupling between the core and the screen of the cable is represented by a mutual inductance between the two [6], [12], as pictured in Figure 1(d). This coupling generates a current that flows through the resistance of the screen, and since this resistance is significant, the transient state of the system is better damped overall than when the screen is totally neglected (as with a classical PI section).…”

“…As illustrated in Figure 2, the two GSCs are equipped with voltage-droop controllers (see [3]- [6]) such that they participate in the DC voltage regulation by adapting their power withdrawal from the DC grid according to the equation…”

“…The impact of the voltage-droop parameter on the MTDC system can easily be assessed from a static point of view, as in [6] and [15], where a power reference modification ∆P * g of one of the converters generates a computable DC voltage deviation. However, from a dynamic point of view, it is necessary to investigate how the voltage-droop parameter impacts the system when the latter is excited by oscillatory inputs like the output power of the WFC, or poorly-damped power oscillations generated by the AC grids.…”

“…The master-slave command strategy of a DC link is not suitable for MTDC systems since the single AC bus connected to the master converter would have to provide all the balancing power and that the loss of this "master" converter would mean the loss of the whole system. To remedy this problem, the voltage droop control technique described in [3]- [6], where several VSCs participate in the DC voltage regulation at the same time, is probably the most adequate control strategy to safely and reliably operate any MTDC system. However, in those references, the voltage-droop parameter is either selected from a static point of view, or simply arbitrarily chosen equal to 0.05 p.u./p.u.…”

Small-signal modelling for in-depth modal analysis of an MTDC system

“…On this last topic, the master-slave control technique used to operate the existing HVDC links is not suitable for meshed HVDC grids [3]. The voltage-droop control technique, described in [4]- [6], seems to be the best solution to safely and efficiently controlling MTDC systems.…”