HVDC meshed multi-terminal networks for offshore wind farms: Dynamic model, load flow and equilibrium

Abstract

This paper presents sufficient conditions for the existence of an equilibrium point of multi-terminal HVDC (MT-HVDC) network for offshore applications. Newton's method is used both as a computational tool and as the basis for proving the existence and uniqueness of the equilibrium. A simple but generalized dynamic model of the MT-HVDC grid for offshore wind farm applications is presented which is non-linear due to the constant power loads with droop regulation. The classic Kantorovitch theorem is used to define the requirements for the existence of the equilibrium and the super-convergence of Newton's method starting from voltages close to 1pu. Finding this equilibrium is equivalent to the power flow in power systems applications. Computational results corroborate the requirements for equilibrium as well as the convergence of the algorithm in a realistic MT-HVDC grid. Index TermsOffshore wind energy, Multiterminal HVDC systems, HVDC, transient stability, equilibrium point, dynamic systems.

Dynamics and Stability of Meshed Multiterminal HVDC Networks

“…1) Currents control strategy: The control of the MMC is realized by the application of PI controllers to the currents at the grid side, the application of the circulating current suppression control and the zero sequence circulating current with PI too. Without loss of generality a first order current system modeled as (12) has a general controller, which is described by (13) and (14).…”

“…The stability study of the equilibrium point is also presented with the evaluation of a Lyapunov function obtained by the Krasovskii's method [11], [12]. The paper represents a significant extension of the contributions included in [13], [14], including a grid representation based on graph theory, which includes the frequency dependence of transmission line/cable models. Additionally, it presents a more realistic model of the Modular Multilevel Converter together with conditions for its simplifications.…”