Meeting grid code requirements for wind farms in South Africa

 

The growing global demand for energy, coupled with the increasing popularity of the “green” lifestyle, has fueled the search for clean, renewable energy resources in recent years. Wind energy, in particular, is being harnessed throughout the world and is quickly becoming the utility industry’s most popular renewable energy source.

Taking advantage of this naturally occurring potential energy, by converting it into consumable electrical energy, is a logical idea; however, the nature of wind makes it challenging to connect this type of energy to the transmission grid.  The best wind resources are often located in remote, sparsely populated areas, which means that any power generated by wind farms in these areas must be transported long distances to higher demand locations. Taking advantage of this naturally occurring potential energy, by converting it into consumable electrical energy, is a logical idea; however, the nature of wind makes it challenging to connect this type of energy to the transmission grid.  The best wind resources are often located in remote, sparsely populated areas, which means that any power generated by wind farms in these areas must be transported long distances to higher demand locations.

Furthermore, the power from wind farms can only be generated when the wind is blowing, which can be predicted to a certain degree, but not entirely controlled. Both of these problems with wind power can result in transmission system thermal and stability limitations.

In addition, while wind turbine technology has improved substantially over the past 30 years, turbines can still cause flicker and harmonic issues that can be problematic for other nearby equipment as well as violate harmonic/flicker standards and guidelines. Turbines also have varying voltage-withstand capabilities. A severe voltage drop or rise has the potential to trip the turbine off-line, reducing the availability and reliability of the wind farm.  In order to overcome the challenges associated with wind generation, utilities all over the world have come up with interconnection requirements and grid codes that allow the wind farms to appear more like conventional generation to the rest of the grid. Nearly every major country or region has specific new generation interconnection requirements or guidelines, including South Africa.

South African situation

South Africa has embraced the abundance of natural wind and solar resources. As of June 2017, 55 renewable projects in South Africa are fully operational which have added 2942 MW of generation to the transmission grid. This includes 1471 MW of wind, 1344 MW of solar PV, 200 MW of concentrated solar and 14,3 MW of hydroelectric generation [1]. The Department of Energy has committed to 13 225 MW of renewable generation by the year 2025 [1].

Grid code requirements 

The growing penetration of renewable generation leads to growing concern over the impact of these generation sources on the transmission grid. The public electric supply utility, Eskom, provides sustainable electricity solutions to grow the economy and improve the quality of life to the people in South Africa and the region [2].

In order to ensure the safety and reliability of the transmission system, Eskom has well-defined requirements which all wind farms in South Africa must meet before they can be connected to the rest of the South African transmission system.

The latest requirements are described in Eskom’s Grid Connection Code for Renewable Power Plants (RPPs) Connected to the Electricity Transmission System (TS) or the Distribution System (DS) in South Africa, Version 2.9 (July 2016).

These grid code requirements ensure that the wind farm itself has enough capability to maintain availability and reliability to the grid.

The grid code requirements include steady state requirements such as achieving a full capacitive to inductive power factor range for the full range of generation as well as voltage regulation requirements. Wind farms must be capable of being able to achieve various control objectives – voltage, power factor, or constant reactive power.

In voltage regulation, the wind farm must operate on a specified droop profile, and for all control objectives, the wind farm must meet the new set points within a speed of response requirement of 30 seconds [3].

On the transient side of the grid code, there are two main requirements. The first is that low voltage and high voltage ride through (LVRT/HVRT) characteristics are defined for the wind farm.

For these defined 3-phase faults and surges of different durations, a wind farm must be able to ride through the event without tripping off-line. In addition to riding through the events, the wind farm must support the grid during these events by supplying capacitive reactive current to the grid during low voltage events and absorbing inductive reactive current from the grid during high voltage events.

The current targets are defined within the grid code. The transient current characteristic must be met for any high voltage event above 1,1 pu and for any low voltage event below 0,9 pu at the point of common coupling.

The reactive current target must be met within 60 ms of the fault. This speed of response emulates a traditional synchronous generator, and commands the need for dynamic reactive current support to achieve this requirement [3].

Similar to all other grid code requirements, the South African requirements are imposed at the point of connection of the wind farm. From the grid’s perspective, the full wind farm will then look similar to conventional synchronous generation.

A wind farm will often takes advantage of a variety of reactive resources to fully meet the grid code requirements. When the turbines themselves cannot fully meet all of the grid code requirements this may require the installation of some ancillary equipment.

For example, the full dynamic reactive capability of a wind farm can come from various resources including: the reactive capability of the turbines themselves, the continuous and overload capability of a static synchronous compensator (STATCOM) flexible AC transmission system (FACTS) device, and from switched capacitor or reactor banks.

Meeting the grid code requirements

The grid code requirements have been written in such a way that a wind farm can emulate traditional generation with dynamic reactive capability. The nature of these requirements demands that any ancillary equipment that is supplied to meet the requirements must also be dynamic in nature.

Traditional mechanically switched capacitor and reactor banks can provide reactive compensation, but typically do not have the speed that is mandated by the grid code. They also have other drawbacks which include the potential to cause harmonic resonances.

A STATCOM contains power electronic components which can continuously supply both capacitive and inductive current up to its rating and is used to control system voltage or power factor.

STATCOM devices, manufactured by AMSC and called D-var, are available in 4 Mvar (12 Mvar for 2 sec.) building blocks.

This is a current injection device with the injected current being independent of the voltage as is the case with a static var compensator (SVC). The response time of a STATCOM is sub-cycle, and typically also offers a short term overload capability.

This overload capability is tremendously beneficial in reducing the cost of an overall system by providing a smaller continuously rated device designed with the overload capability required to solve short-term transient events.

The design is modular and can be expanded by combining multiple 4 Mvar building blocks. The flexible control software allows the D-var STATCOM to be employed for a variety of applications.

It also allows for other reactive resources to be integrated seamlessly into the overall reactive compensation system.

These other reactive resources could include the existing reactive capability of the turbines themselves and mechanically switched capacitor and reactor banks. The device’s master controller (MCE) utilises patented soft-switching capability, completely mitigating the step voltage change when switching a shunt bank.

These features allow one dynamic reactive compensation system to be comprised of various resources that expands the wind farm’s overall reactive capability at a minimal cost.

One master controller with creative and flexible controls behind the hardware allows for the use of different reactive resources to meet the various grid codes.

AMSC and Reactive Power have worked together to supply hybrid reactive compensation systems, comprising  STATCOMs and switched shunts to various wind farms throughout South Africa, allowing them to fully meet all South African voltage, power factor, and LVRT/HVRT related grid code requirements.

References

[1] SAWEA: “Wind Stats and Facts”, www.sawea.org.za/resource-library/wind-map.html, June 2017.
[2] Eskom: www.eskom.co.za/Pages/Landing.aspx.
[3] Eskom: “Grid Connection Code for RPPs in South Africa – Version 2.9”, July 2016.

Contact Kevin Talbot, Reactive Power, Tel 011 708-0044, kevin@reactivepower.com

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