This paper introduces a method to improve the primary frequency contribution of grid connected variable speed wind turbine generators (WTGs). Using their energy reserve margins, deloaded WTGs are controlled to provide relief to the grid during depressed frequency conditions. The frequency support from individual WTGs is regulated based on the available reserve, which depends on the prevailing wind velocities. By continuously adjusting the droop of the WTG in response to wind velocities, its primary frequency response is significantly improved in terms of reduced stresses on WTGs during low wind speeds. The impact of variable droop operation on two aspects of WTG operation is investigated-primary frequency contribution and smoothening power fluctuations caused due to changes in wind speed. Also highlighted is the usefulness of this control when adopted by wind farms.

Primary frequency regulation by deloaded wind turbines using variable droop

Active power reserves are needed for the proper operation of an electrical system. These reserves are continuously regulated in order to match the generation and consumption in the system and thus, to maintain a constant electrical frequency. They are usually provided by synchronized conventional generating units such as hydraulic or thermal power plants. With the progressive displacement of these generating plants by non-synchronized renewable-based power plants (e.g. wind and solar) the net level of synchronous power reserves in the system becomes reduced. Therefore, wind power plants are required, according to some European Grid Codes, to also provide power reserves like conventional generating units do. This paper focuses not only on the review of the requirements set by Grid Codes, but also on control methods of wind turbines for their participation in primary frequency control and synthetic inertia.

Participation of wind power plants in system frequency control: Review of grid code requirements and control methods

Review of contribution to frequency control through variable speed wind turbine

Traditionally, inertia in power systems has been determined by considering all the rotating masses directly connected to the grid. During the last decade, the integration of renewable energy sources, mainly photovoltaic installations and wind power plants, has led to a significant dynamic characteristic change in power systems. This change is mainly due to the fact that most renewables have power electronics at the grid interface. The overall impact on stability and reliability analysis of power systems is very significant. The power systems become more dynamic and require a new set of strategies modifying traditional generation control algorithms. Indeed, renewable generation units are decoupled from the grid by electronic converters, decreasing the overall inertia of the grid. ‘Hidden inertia’, ‘synthetic inertia’ or ‘virtual inertia’ are terms currently used to represent artificial inertia created by converter control of the renewable sources. Alternative spinning reserves are then needed in the new power system with high penetration renewables, where the lack of rotating masses directly connected to the grid must be emulated to maintain an acceptable power system reliability. This paper reviews the inertia concept in terms of values and their evolution in the last decades, as well as the damping factor values. A comparison of the rotational grid inertia for traditional and current averaged generation mix scenarios is also carried out. In addition, an extensive discussion on wind and photovoltaic power plants and their contributions to inertia in terms of frequency control strategies is included in the paper.

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Power systems with high renewable energy sources: A review of inertia and frequency control strategies over time

The increasing penetration of wind power may influence the frequency stability of power systems. Therefore, new control schemes are necessary for wind turbines and power systems to support the frequency control. Currently, most of the published control methods can be classified into 3 levels, i.e., wind turbine level, wind farm level and power system level. The wind turbine level control enables wind turbines, particularly the variable speed wind generators, to provide dynamic response and power reserves for the primary frequency control by implementing the inertial, droop or deloading controller. The wind farm level control distributes the central control command from the system to the local wind turbines and energy storage units for the desired generation. The power system level control coordinates wind farms with conventional power plants for the secondary control to recover the frequency to the reference value faster than for the no coordination control case. This paper presents a review on the latest studies in relation to the 3-level frequency control of power systems with wind power penetration.

Review on frequency control of power systems with wind power penetration

This paper is devoted to the active power control of DFIG wind turbines. An adaptation of the usual power set point control has been proposed in order to improve the transient behavior for high wind speeds. Several strategies for the active power control are studied. In the high wind speed range, the pitch control seems the most relevant to release a power margin while, in the low wind speed range, the increase of the rotation speed is more convenient. The power margin set point variation rate has to be limited in order to avoid torsion oscillations of the shaft. Nevertheless, the reaction speed of the wind turbine lies in the time frame of seconds, so that the participation to the primary frequency control is feasible.

N. Bragard

Papers

Active Power Control Strategies of DFIG Wind Turbines