Facilitating the integration of wind turbines into power networks while maintaining frequency stability

Frequency stability in large power networks has been traditionally maintained by the inertia of synchronous generators. However wind turbines that are coupled to the power system through a power electronics (PE) interface do not provide the inertia that is required to maintain frequency stability. This will have a material effect on the ability of the power system to manage incidents arising from the loss of a generator or sudden increase in demand. To facilitate the increased integration of wind turbines an inertial component needs to be synthetically supplied by the wind turbines to maintain the stability of the frequency. In this paper the impact of PE interfaced generation has been assessed on a simplified representation of the UK power system appropriate for determining the frequency stability. The test network used in simulations was validated for a significant number of recorded system incidents. The studies carried out established the sensitivity of frequency stability to changes in the power system’s generation mix (increase of wind generation) and system conditions including frequency support provided by loads. It then assessed the impact of employing a synthetic inertia component associated with wind turbines and the contribution it makes to frequency stability in the network. The synthetic inertia controller was then designed to give different inertial outputs. Sensitivity of inertial response to controller parameters is clearly established. Finally it is demonstrated that by employing suitably tuned synthetic inertia controllers more wind turbines can be integrated into the system while maintaining required frequency stability of the power system. INTRODUCTION Frequency Control is managed by many elements on a power system: one of which is the inherent inertia of large machines rotating shafts that are electrically coupled to the power system. However in facilitating government targets for a reduction in CO2 suppliers of electricity are changing the face of the electricity generating quite dramatically and are turning to wind turbines to provide energy to the system. The UK targets in practical terms require 37% of electricity to be produced from renewable sources by 2020 [1]. This changing generation background provides many challenges to the power system industry. One of these areas is frequency stability of the network following a large disturbance on the network. The majority of the wind turbines being connected are of the variable speed type where much of the mechanical mass of the wind turbine and in some instance all of it is situated behind the PEs and therefore not electrically coupled to the power system, as shown in figure 1. a) Doubly Fed Induction Generator b) Full Converter Wind Turbines Figure 1 Schematics of PE interface connected Wind Turbines The practical impact of this is that the inertial contribution of the wind power turbine will not be provided to the power system [2-5]. The effect of this increase in generation that has a much reduced, or even no inertia at all, will be that the system inertia (the cumulative effect of all the synchronous machines inertia) will also be reduced and the effect (bigger drop in frequency for smaller inertia) can be clearly seen in figure 2. 1320MW Loss, Comp H 4 v H 2 49 49.2 49.4 49.6 49.8 50 50.2 0 5 10 15 20 25 30 Time (s) F re q u e n c y ( H z ) H = 4 H=2 Figure 2 Effect of reducing the system inertia With this impact in mind the manufacturers have been quick to show that the wind turbines can provide a synthetic inertia contribution [6, 7]. The inertial contribution is realised by not operating the wind turbine at the optimal Grid Gearbox Rotor Convert Doubly Fed Induction generato