Abstract
Multi terminal VSC-HVDC systems are a promising solution to the problem of connecting offshore wind farms to AC grids. Optimal power sharing and appropriate control of DC-link voltages are essential and must be maintained during the operation of VSC-MTDC systems, particularly in post-contingency conditions. The traditional droop control methods cannot satisfy these requirements, and accordingly, this paper proposes a novel centralized control strategy based on a look-up table to ensure optimal power sharing and minimum DC voltage deviation immediately during post-contingency conditions by considering converter limits. It also reduces destructive effects (e.g., frequency deviation) on onshore AC grids and guarantees the stable operation of the entire MTDC system. The proposed look-up table is an array of data that relates operating conditions to optimal droop coefficients and is determined according to N-1 contingency analysis and a linearized system model. Stability constraints and contingencies such as wind power changes, converter outage, and DC line disconnection are considered in its formation procedure. Simulations performed on a 4-terminal VSC-MTDC system in the MATLAB-Simulink environment validate the effectiveness and superiority of the proposed control strategy.
Highlights
Renewable energy sources (RESs), and offshore wind farms (OWFs), have started to play a principal role in power systems [1,2,3,4]
The development of multi-terminal HVDC (MTDC) systems has been facilitated by advances in power semiconductor switches, voltage source converters (VSCs), current source converters (CSCs), and DC breakers [6]
DC voltage control and proper power sharing in postcontingency conditions are some of the main challenges of MTDC systems [12], while the master–slave, voltage margin, and voltage droop methods are the three basic control schemes [13]
Summary
Renewable energy sources (RESs), and offshore wind farms (OWFs), have started to play a principal role in power systems [1,2,3,4]. Reference [40] develops a control strategy consisting of a linear quadratic controller and adaptive droop control It improves the stability in post-contingency conditions and prevents the DC voltages of VSC stations from reaching their limits. Reference [41] uses a secondary control layer based on a consensus algorithm between adjacent converters to adjust adaptive droop coefficients considering the tradeoff between voltage regulation and power sharing. The optimal droop coefficients satisfy this purpose, while minimizing the DC voltage variation and maintaining all converters (with or without droop control mode) within their limits They are determined based on the initial loading of converters, the stability constraint, and converter limitations by linearizing the system around the steady-state operating point in various contingencies. If any power constraint is activated by the problem (7) solving process, the droop coefficient of its related converter is permitted to change
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