Wind turbines have become major players in today’s electricity-generation market, with year-over-year market growth, internationally, hitting 44 percent in 2014, according to the Global Wind Energy Council. However, this rapid growth doesn’t come without complications. In this post, I’ll cover how wind’s intermittent nature can affect grid operations, and describe how one solution – hybrid VAr compensation – can aid efforts to incorporate more of this carbon-free energy into overall electricity supplies.
Growth, with no signs of slowing down
Though it was once only a marginal contributor to international electricity supplies, wind has become an equal partner with traditional coal and natural gas thermal-generation technologies in many regions of the world. By the end of 2013, for example, wind turbines generated 34 percent of Denmark’s electricity and 21 percent of Spain’s supplies, according to the World Wind Energy Association (you can see more interesting statistics from the WWEA’s 2013 World Wind Energy Report here). And the historic commitments announced at the conclusion of December’s United Nations Conference on Climate Change in Paris will likely create even more demand for wind energy.
Mitigating wind’s intermittency impacts
But wind turbines, though they emit no greenhouse gases, can create complications for grid operators, especially as they become significant contributors to total electricity supplies. Obviously, turbine production can vary with the wind, itself, from minute to minute, and such intermittency can raise havoc with transmission and distribution systems designed for steady, reliable operation. Additionally, turbine drive type, either fixed- or variable-speed, also can play a role in how they impact the grid to which they are connected as shown in Figure 1.
For these reasons, grid operators often impose requirements on wind facilities at the point of common coupling, including:
- Voltage range limits, which can be fixed at a national level or by local standards, and can be defined as a function of a facility’s operating time. Typical values are between 80 percent and 120 percent of nominal value.
- Frequency range limits, with typical values ranging from 47Hz to 53Hz.
- Reactive power limits (which can vary with drive type), which typically represents 30 percent of installed power capacity.
- Harmonics limits, typically governed by international standards, such as the Institute of Electrical and Electronics Engineers Standard 519.
- System response times and response to voltage dips, typically defined by national grid codes.
Power quality problems can be a two-way street
Importantly, wind-turbine operation, itself, can be negatively affected by power-quality problems in the grid to which it is connected. These can include voltage dips that can shut down wind-farm operations, and harmonics that can damage turbine equipment. Figure 2 illustrates the two-way power-quality relationship shared between wind turbines and their connected grids.
Creating a more harmonious relationship
So, what options do wind developers have for ensuring power-quality issues don’t create problems for either the connected grid or their own operations? In my view, hybrid VAr compensation offers the best solution for power-quality improvement and voltage stabilization. It combines cost-effective static solutions including capacitors and reactor banks, along with dynamic electronic devices, to provide grid-code compliance at a competitive price.
As shown in Figure 3 below, this solution incorporates:
- Automatic capacitor banks, which inject capacitive energy into the network.
- A shunt reactor that supplies inductive energy in no-load situations or when cable length is a priority.
- An electronic static compensator that can inject a reactive, capacitive or inductive current continually, and in less than one cycle, to compensate for rapid fluctuations in reactive power consumption.