Reactive Power Management of Power Networks with Wind Generation

Efficient energy management in Europe and the larger application of energy derived from renewable sources, together with energy saving concerns, constitute the basis for the compulsory pack of measures that aim to reduce greenhouse gas emissions and fulfill the Kyoto protocol approved by The United Nations Framework Convention on Climate Change. These measures intend to reduce the emission of greenhouse gases by the year 2012 and in following years [1].

In recent years technological advances in power electronics have facilitated the development of electronic equipments that offer the ability to handle large amounts of power; consequently, the use and application of this technology into electrical power systems have increased significantly. These electronic devices, called Flexible AC Transmission System (FACTS), are based on electronic power converters and they provide the ability to make quick adjustments and to control the electrical system. FACTS devices can be connected in series, in parallel, or in a combination of both. The benefits they offer to the electrical grid are widely referenced in scientific literature. These benefits include improvement of the stability of the grid, control of the flow of active and reactive power on the grid, loss minimization, and increased grid efficiency.

The behavior of power systems with large wind energy farms requires a detailed examination of the available wind turbine technologies to evaluate the power capabilities of each technology and their operating regions. Wind turbines can be classified between fixed-speed (small speed changes due to generator slip) and variable-speed.

Every optimization problem employed to study electric power systems consists of an objective function and a group of constraints to be observed by this function that concurrently define the problem itself. The main constraints associated with the problem of reactive power planning are related to load flow equations. These problems are widely known as optimum load flow. Current operating conditions of reactive systems imply the necessity of redefining the reactive power planning problem, though bearing in mind its performance under normal conditions and faults or disturbances. The employment of these constraints by the planning of reactive power leads to a second optimization model known as optimum load flow with security constraints. Finally, more recent studies have raised the possibility of including voltage stability into the objective function, as well as into the problem constraints of reactive power planning, to maximize the voltage stability margin. Thus, a third optimization problem is derived, namely optimum load flow with security and voltage stability constraints. This optimization method aims to guarantee the existence of voltage stability margins in the system under faults and disturbances.

Beyond any doubt, we may consider century 21st as the one devoted to renewable energy. According to the International Energy Agency (IEA) [1], Renewable Energy Sources shall provide about 35 % of the European Union’s (EU) electricity by 2020, and within this context, wind energy is set to contribute the most – nearly 35 % – of all the power coming from renewable sources. This evolution is based on sustainability scenarios, like the BLUE one [2] related to the reduction of greenhouse emissions. However, the appropriate integration of such renewable energy into power networks still presents major challenges to Power Systems Operators (PSO) and planners.

Reactive Power Management is a critical issue when dealing with the planning and operation of power networks with high wind energy penetration. Reactive Power Management entails the requested operation and planning actions to be implemented in order to improve the voltage profile and the voltage stability in power networks [1]. An efficient reactive power planning could be obtained by choosing an optimum location of var sources during the planning stage, whereas efficient reactive power dispatch could be achieved by scheduling an optimum regulation of the voltage set point at the generators connection point and at the VAR settings during the reactive power dispatch [2] and [3].