Power quality - Cracking the wind farm grid code
Over recent years there has been a continuous increase in installed wind power generation 
capacity throughout Europe. This has caused the transmission system operators (TSOs) to review their grid connection rules - otherwise known as grid codes - to limit the impact of wind farms on network power quality and stability. Peter Jones of ABB UK investigates
New rules demand power plants of any kind should support the electricity grid, not just in normal operation but also in case of voltage dips. Some of the key considerations are steady state and dynamic reactive power capability, continuously acting voltage control and fault ride-through behaviour.
The result of these new considerations is that some commonly used turbine designs may have limitations in meeting the grid code requirements of some countries, especially for steady state and dynamic reactive power. For wind farms where these types of turbine are installed the solution is to install appropriate "add-on" reactive power equipment to achieve the necessary grid code compliance for operation and power production.
Reactive power compensation
Reactive power control provided by generators or capacitor banks alone may be too slow for the sudden load changes found in wind farms. ABB offers two appropriate reactive power compensation solutions, the SVC (Static Var Compensator) and the Statcom (STATic COMpensator).
The first approach, the SVC, is based on conventional capacitor banks, together with parallel thyristor controlled inductive branches, which consume the excess of reactive power generated by the capacitor bank. This type of equipment can be directly connected to the intermediate voltage bus, which interconnects the wind farms (up to 69 kV). When needed, it is also possible to connect the SVC to the high-voltage network via a dedicated transformer.
The second, more advanced, approach to compensation for reactive power is the use of a Voltage Source Converter (VSC) incorporated as a variable source of reactive power. Compared to other solutions a voltage source converter is able to provide continuous control, very dynamic behavior due to fast response times and with single phase control also compensation of unbalanced loads. The ultimate aim is to stabilise the grid voltage and minimize any transient disturbances.
ABB's Statcom converters are based on power converter System (PCS) platforms providing the following control features:
- Power factor correction (cos phi control)
- Voltage control
- Active harmonics cancellation
- Flicker mitigation
- Unsymmetrical load balancing
The Statcom features the same state-of-the-art power electronic voltage source converter (VSC) technology used in ABB's PCS 6000 range of products, such as the ACS 6000 range of medium voltage drives. It is a purely static device, with no switched passive elements, that provides outstanding performance for both steady state and dynamic operation, with the added advantage of a small installation footprint.
ACS and PCS 6000 converter units are based on three-level IGCT (integrated gate commutated thyristor) phase modules. The IGCT is the state-of-the-art semiconductor element for this power range. Large numbers of these converter units have been sold worldwide and they have a proven track record of performance and reliability.
A particular advantage of the ABB Statcom in wind farm applications is its fast dynamic voltage control and its behaviour during both balanced and unbalanced grid faults (fault ride-through), which enable it to help meet stringent grid code requirements.
Reactive power requirements
An analysis of the requirements of the UK's grid code suggests the required reactive power is approximately one third of a wind farm's nominal active power. Typical wind farm nominal power ranges from 30 MW up to 100 MW for on-shore installations. Therefore, the required reactive power compensation is in the region of 10 MVar to 35 MVar. For large wind farms, typically several hundred MW, ABB recommends the more traditional SVC. While, as a rule of thumb, the Statcom is appropriate for small to medium sized wind farms.
Typically, the very compact Statcom power electronic modules are placed inside a cabinet. This also houses other equipment such as the DC link, cooling system and controls. Only a few additional external components are needed, such as the Statcom transformer, grid filter and heat exchanger.
STATCOM functionality
The STATCOM is equipped with a set of functions in order to help wind farms to fulfill the grid code requirements. These include:
- Steady state reactive power supply or absorption. This function can be fulfilled by following a reactive power set-point, a set-point for a power factor at the connection point of the wind farm or by operating according to a linear reactive power versus voltage characteristic (Q/U characteristic).
- The implementation of the latter case also fulfills the voltage control requirement generally called for in the grid codes. The grid companies often require a certain flexibility to change the basic behaviour of the voltage control scheme. A reduced set of changeable parameters has to be available, especially the target voltage and the slope of the linear characteristic.
- Smoothly follow a set-point ramp. This is stepless, in contrast to solutions based on switched passive component
- Meeting the dynamic requirements of the grid codes, e.g. a step in the set-point is followed within less than 1 second without notable overshoots or oscillations.
- During voltage dips (balanced or unbalanced), the Statcom injects reactive current in the order of value of the nominal Statcom current and therefore helps to support the grid voltage.
Typical application
A typical application of a Statcom is a wind farm in Scotland that has used it to overcome its initial difficulties in complying fully with the grid code requirements in terms of: steady state reactive power supply; voltage control and dynamic reactive power supply. In addition, it also has helped to meet harmonic requirements. Since the wind farm is connected via two 33kV cable connections to the nearest 132kV/33kV substation, it is split into two parts that can also be connected via a coupling switch. However, both wind farm strings are required to run autonomously. Therefore, two 12.5 Mvar units were required.
Both converter units were placed inside the wind farm substation building, this allowed a cost efficient installation in a well protected environment. Both transformers were located outdoors next to the converter units. Although the output voltage waveform of the Statcom is close to sinusoidal, a small harmonic filter was still needed to maintain harmonic distortion within acceptable limits. The filter is located on a steel structure close to the transformer. The heat exchangers for the converter closed loop water cooling system are also located outdoors.
Tests carried out with the units in continuous operation at nominal power have shown that they now enable the wind farm to comply with all the grid code requirements for steady and dynamic reactive power capability and voltage control. The behaviour of the wind farm during grid faults has also been modelled, and the results show that the units can be expected to support the wind farm during balanced faults - they inject reactive current in such a way that they help to maintain the voltage. And even during heavy unbalanced faults the Statcoms support the voltage.
ABB's latest development to help UK wind farm operators achieve grid code compliance is a containerized Statcom that provides a mobile, easily transportable, source of reactive power. It is designed primarily to act as a temporary solution while the need for a long term, permanent reactive power installation is assessed. The mobile unit is housed in a standard shipping container that incorporates the VSC, multi-voltage step-up transformer (to provide flexibility of use between the UK and Ireland) and associated control equipment. All the equipment is factory built and commissioned to provide the fastest possible deployment when it is delivered to site, and only basic foundation work with a small plinth is required.
A single modular unit can provide up to 10 MVar of reactive power, making it ideal for use with wind farms up to around 30 MW. However, the modular approach enables a number of units to be piggy-backed together to create larger solutions.
Cracking the grid code requirements present a definite challenge for wind farm developers in certain countries. Mainly because the steady state and dynamic reactive power injection/absorption requirements are difficult to fulfill with some wind farm designs. Therefore "add-on" equipment is often needed to comply with the grid codes. Statcoms present a cost-effective and efficient method of providing reactive power compensation for small to medium sized wind farms.
Share this article with others
Post to del.icio.us
Printed from http://www.electricalreview.co.uk/features/117949/Power_quality_-_Cracking_the_wind_farm_grid_code.html
Comment on this article
Skip to comments
We encourage users to analyse, comment on and even challenge Electrical Review's articles, including the one above - 'Power quality - Cracking the wind farm grid code'
User reviews and comments that include profanity or personal attacks or other inappropriate comments or material will be removed from the site.
Additionally, entries that are unsigned or contain "signatures" by someone other than the actual author will be removed. We will take steps to block users who violate any of our posting standards, terms of use or privacy policies or any other policies governing this site.