Integrating Wind Energy into the Grid - An Overview



1. The supply and distribution of electricity

Power grids consist of hierarchies. On the European continent, the highest level is the UCTE grid. Germany's transmission grids feed into that grid at the extra high-voltage level (220 kV and upwards). The four operators of transmission grids in Germany (EnBW Transportnetze AG, E.ON Netz GmbH, RWE Transportnetze Strom GmbH, and Vattenfall Europe Transmission GmbH) regulate loads within Germany over large areas between power plants and energy consumers (conglomerations). Below the extra high-voltage grids, there are distribution grids (high-voltage of 110 kV and medium voltage of 10-30 kV) that spread the loads across substations within the region. These substations then route the electricity to individual consumers in low-voltage grids (230 V / 380 V).



Figure 1: The four transmission grid operators in Germany


There are specific rules for the operation of the individual grids (see Figure 2).



Figure 2: Hierarchy of the sets of rules



Both electricity generation and consumption fluctuate over the course of a day or year; in addition, there are random fluctuations. Total power generation has to be tailored to power consumption, and any differences must be compensated for by feeding power to or getting power from adjacent grids.



Figure 3: Baseload, intermediate load, and peak load in power grids


Translations:     Grundlast = Baseload, Mittellast = intermediate laod, Spitzenlast = peak load



Fluctuations in power generation are not by any means restricted to wind power, though wind power is admittedly especially intermittent because power generation depends on the wind. Thus, the larger the share of wind power in a particular grid, the more standby power will have to be available in that grid. Coal plants, gas turbines, pumped storage bands, and other renewable sources of energy such as biomass plants can provide this standby power for hours, minutes, and seconds.



Figure 4: Typical load curve for power consumption and generation in conventional power plants


Translations: Wasserkraft = hydropower, Kernenergie = nuclear power, Braunkohle = lignite, Erdgas = natural gas, Steinkohle Spitzenstrom = surge current, Pumpstrom = pumping electricity



2. The history of wind energy in the grid

In the first few years of commercial wind power, wind energy only made up a few percent of total grid capacity. Generally, wind turbines were connected at the medium-voltage level. There was thus no need for special sets of rules to regulate operation. Like other distributed power generators, wind turbines were merely expected to be neutral players -- in other words, not to consume or produce much reactive power (ideally,  = 1) and to switch off as quickly as possible if there are any problems in the grid (short circuit, drop-in voltage, or deviation in frequency) in accordance with VDEW guidelines. The limit values and specifications for grid operators also set limits for undesired effects on the grid, such as flicker, harmonic distortion, etc.

But the share of wind power in individual grids grew, and wind power currently covers demand for electricity completely in some regions when the wind is blowing strongly.



Figure 5: Typical curve for power consumption and the generation of wind power



As the size of wind farms increased, connections to high-voltage and extra high-voltage grids also became more common. The offshore wind farms planned will also be connected at these levels. In addition, sparsely populated areas generally do not have densely meshed grids, though a lot of wind power may be produced in such areas.

Here, manufacturers of wind turbines and grid operators face technical challenges. For the past few years, they have been working with research and measurement institutes to develop new control systems for wind turbines and to come up with better sets of rules for connections to the grid.



3. Rules for connections to the grid

The goal of these new rules is to move beyond treating wind turbines as passive generators that are ideally "neutral" as defined above and merely latch on to the "actual" grid structure; now, they are to be used as active elements in transmission and distribution grids. Their technical properties should allow them, for instance, to react appropriately to changes in voltage and frequency or to overloads in certain lines in order to increase the quality of power supply instead of being a burden.

In Germany, E.ON and Vattenfall have come up with grid connection guidelines for their zones, and the VDN has also created a set of rules as an independent association. In addition, a number of other countries in Europe and elsewhere are currently developing similar rules for their own situations.

These rules for grid connections mainly concern power limitations for individual wind turbines or entire wind farms, such as increasing or reducing output at preset increments in certain situations, or specifications of the output factors / reactive power that wind farms must provide. In this way, standby power can be provided over periods of minutes or seconds.

Special attention still has to be paid to actions taken during malfunctions. Wind turbines and wind farms used to have to be switched off immediately ("nonselective shut down") when, for example, a short-circuit had led to a drop in voltage, but this drastic action also caused adjacent wind farms to fail, thus reducing overall power generation. In the worst-case scenario, such measures could set off a chain reaction, reducing voltage further down the grid so that more remote wind turbines would also have to be switched off, possibly leading to a major power outage.

Therefore, all modern sets of rules for grid connections require wind turbines to be kept on the grid during certain types of voltage dips to keep the voltage level up. At the same time, the various sets of rules for grid connections specify in detail -- sometimes even contradicting each other -- which kind of voltage dips (duration, depth, etc.) wind turbines must react to. Figure 6 shows Vattenfall's guidelines as an example; here, the blue line is the voltage limit relative to the duration of the disturbance. Below that line, wind turbines must remain connected to the grid.



Figure 6: Vattenfall's voltage curves for its grid connection guidelines.
Below the blue line, wind turbines cannot be disconnected from the grid
if there are malfunctions in the grid.


Translation: Netzspannung = power supply voltage



Measurements can be provided to demonstrate that certain wind turbines fulfill the technical requirements for grid connection. In addition to international guidelines on measurements to assess effects on the grid (flicker, harmonic distortion, etc.) in IEC 61400-21, for instance, there are now also guidelines specially developed to assess the technical requirements for grid connections, such as Germany's "Technical Guidelines for Wind Turbines -- Part 4: Determining Grid Connection Values" (Technische Richtlinie für Windenergieanlagen – Teil 4: Bestimmung der elektrischen Eigenschaften: Kraftwerksverhalten). These measurements investigate, for example, the potential of power limitations using target values or the provision of reactive power (capacitive and inductive).

A wind turbine's response to errors can also be measured. To this end, the turbine's electrical drive system can be installed on a test bench, or voltage dips can be created artificially in the grid to test wind turbines under real-world conditions using special equipment that does not disturb other participants in the grid. A number of measurement and research institutes, including WINDTEST Grevenbroich GmbH, perform such intricate measurements.

Manufacturers of wind turbines can use these measurements to have their products certified by such authorities as the FGH. These certificates confirm that a wind turbine complies with the technical requirements of a given set of rules for grid connections.
Such sets of rules, guidelines, and measurements can be used to predict the response of entire wind farms before they have even been installed. This information then serves as the basis for negotiations of grid connection agreements between operators of wind farms and grid operators.



4. The future of wind power with respect to grid integration

Germany's Energy Agency (dena) produced a well-known "Grid Study" in 2005 that dealt with the expansion of wind power in Germany. The study included scenarios up to 2020 and investigated the results of grid integration on the development of other power plants, the cost of electricity, and power security. The main finding in the study was that wind power can be expanded considerably. Installed capacity is expected to amount to 36 GW by 2015, which would mean that wind power would make up around 14% of total power generation capacity.

Nonetheless, only around 850 km -- some 5% of the current transmission lines -- would have to be added to reach that figure. Furthermore, no additional power plants would have to be built to provide standby power as the current power plants would suffice. This expansion of wind energy up to 2015 would probably increase the price of electricity by around 0.5 €cents per kilowatt-hour.



References, for further information:

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Text and Figures:
Dr. rer. nat. Markus Koschinsky, Eric Effern
WINDTEST Grevenbroich GmbH, Germany
http://www.windtest-nrw.de