Small-signal sequence impedance models have been developed for different types of wind turbines and used in industry to study wind farm and high-voltage direct current (HVDC) system harmonics and resonances. Compared to other small-signal methods, a major advantage of system modelling and analysis based on sequence impedance is its scalability: an equivalent impedance model can be easily built for any complex system by aggregating the impedance of individual turbines and the network. A recent development in the small-signal sequence impedance theory is the modelling of frequency coupling to improve the accuracy of system analysis and explain a common characteristic of harmonics created by system resonance. In light of this new development, this study presents and compares different aggregation methods to build farm-level impedance models in the presence of coupling in individual turbine responses. The objective is to identify practical methods that can meet the needs of different system analyses and are easily to use. Experiences of China State Grid and TenneT with the application of sequence impedance models in renewable energy and HVDC system resonance and harmonic analysis are also discussed, along with an overview of their ongoing efforts to develop new grid codes and system analysis tools based on sequence impedances.
In many cases, the increasing load and the growing proportion of injected power from distributed energy generation results in the violation of voltage constraints in low voltage distribution grid. As an alternative to additional cables or substations, the application of distribution transformers equipped with on-load tap-changers (OLTC) is under development. This paper presents their benefits, and a method to control them with a voltage range as input parameter is proposed. The computational model for load flow calculation with OLTC, the simulation environment and the applied stochastic, synthetic profiles are described. The simulation results prove that the proposed method is suitable to keep the voltage within the constraints. It also features the advantage of less switching operations compared to a fixed voltage control method. This aspect leads to less wear and for this reason to a decrease of maintenance effort.
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