PowerTech converters for stable grids in wind farms

PowerTech Green Line 1000 is proving its worth in China’s largest test wind farm

The Chinese wind power market has expanded rapidly in recent years, and extensive work is being done to develop grid feed-in regulations for wind turbines and their feasibility in the field. As renewable energy production continues to grow, the technological requirements for renewable energies are also rising. Renewable energies also have to face the same tasks as classic power plant units: demand-based energy generation, security of supply and grid support. The Chinese authorities have already taken inadequate wind farms out of service as a result. PowerTech reactive power converters offer a cost-effective, easy-to-integrate retrofit solution even for this market.

While coal dominated power generation in China for decades, the Chinese government is now increasingly focusing on wind energy as a clean and economical alternative. For the first time in 2010, China became the country with the highest installation rate of new wind turbines – in total, over 14,000 MW of capacity was installed. The country is currently the international leader with over (40,000 MW) 211 GW (as of 2018) of wind turbines installed.

Test centre in Zhangbei

One way to solve the current challenges in the Chinese wind power industry is the wind turbine test centre opened in January 2010. The National Wind Power Integration Research and Test Center (NWIC), located in Zhangbei province, offers a specialised environment for research and development, test sequences, training courses and professional education. The largest test park in the country is organised and controlled by the China Electric Power Research Institute (CEPRI), a subsidiary of the State Grid Corporation of China (SGCC). The institute was founded in 1951 and is headquartered in northern Beijing. Its main task is research and development on all aspects of energy supply in China.

The Zhangbei test park is designed for extensive prototype testing of wind turbines as well as for the typical grid connection tests (grid codes). All the necessary requirements are set up at the central feed-in point. This includes a low-voltage ride through (LVRT) system, grid simulators with a capacity of 6 MW and an energy storage system with an output of 2.5 MW.

Research into grid compatibility

In cooperation with one of the top 10 wind turbine manufacturers, extensive tests are being conducted at the Zhangbei test park to establish whether the turbines can feed reactive power into the grid as specified by the grid codes. Further tests are also underway to check the frequency control requirements to support grid stability as well as grid-stabilising properties of the wind turbines if certain faults arise in the power system.

As a long-standing supplier and technology partner, PowerTech Converter is contributing its Green Line 1000 – a reactive power converter that meets the demand of grid operators for providing a reactive current during a temporary voltage drop at the wind turbine. More than 130 MW of wind power capacity has already been fitted with PowerTech’s Green Line 1000.

LVRT requirements fully satisfied

The Green Line 1000 is connected on the 690 V side parallel to the wind generator of the respective system. It consists of a pulsed IGBT converter with an intermediate voltage circuit. When nominal conditions exist on the grid side, the inverter operates in stand-by mode. In the event of a line-side voltage drop, it switches into active mode and supplies an adjustable reactive current of up to 2500 A.

Following the grid feed-in regulations tested in Zhangbei, the Green Line 1000 starts feeding reactive current into the grid when the grid voltage drops below 90 percent of the nominal voltage. The reactive current volume increases in linear proportion to the extent of the line voltage dip, and up to an adjustable value. This means that all common LVRT requirements are fully met: Balanced grid dips are controlled up to 15 percent of the line voltage – unbalanced dips even up to 0 percent of the line voltage. It is able to ride through several consecutive line voltage dips. The DCU control unit developed by PowerTech independently and intelligently detects and handles each of these incidents.

Well over 100 grid voltage dips were carried out at the Zhangbei test park. The Green Line 1000 worked perfectly in ever case. Given the harsh climatic conditions in Zhangbei, the fact that no thermal or electrical problems occurred is a great triumph for PowerTech and proof for the wind turbine manufacturer that their own turbines are already able to comply with the defined grid quality standards in line with international grid feed-in regulations.

PowerTech reactive power converter ‘Green Line 1000’ provides improved grid integration

The accelerated expansion of renewable energies necessitates more adaptable grids and grid-supporting functions provided by power plants. Moving forward, new wind turbines will have to meet these requirements. PowerTech reactive power converters make it possible to implement them quickly and cost-effectively. They enable wind turbines to maintain grid support in the event of voltage dips (FRT – Fault Ride Through). Under practical conditions, the Green Line 1000 has already proven itself in systems from various manufacturers with a total output of 130 MW.

The entire e.n.o. 82 plant system, fitted with the Green Line 1000, received its Type Certificate on the wind turbine level from Germanischer Lloyd. The simple integration into existing plants as well as a first-class price-performance ratio tipped the scales for e.n.o. energy in favour of the Green Line 1000. Stefan Bockholt, Head of Development at e.n.o. energy, was impressed after joint tests: ‘It confirmed the superb technical performance data – a convincing demonstration.’ Reactive power characteristics can be individually configured, which makes it possible to preset the respectively applicable grid code. For the e.n.o. 82 wind turbine, the Green Line 1000 was placed parallel to the original converter. Unwanted resonances and beat frequencies could be avoided thanks to the variably adjustable pulse frequency in the PowerTech device. As a result, this wind turbine will be able to produce and feed energy reliably and, most importantly, in line with the grid in the future.

e.n.o. –Built on Quality for a Safe Future

The e.n.o. energy GmbH evolved from a project planning office established in 1999 and now operates as a project developer, wind turbine manufacturer and provider of turnkey wind farms. The company generates its turnover in two key areas: The first business area comprises the development, project planning and realisation of wind farms in Europe, with Germany currently the main target market. The second area involves the production and sale of proprietary wind turbines. In addition, e.n.o.o. energy provides maintenance, servicing and overhaul of wind turbines

Wind farm in Abla. Two power converters per windmill

The small community of Abla lies at the foot of the Spanish Sierra Nevada at an altitude of about 1,000 m in a rough, treeless, windy area. Suzlon is building a wind farm here which will have a total of 49 type S88 wind turbines. We have been supplying Suzlon with our power converters for generator control of this successful wind turbine for many years. Used in combination with the Green Line 1000, the plant in Abla also meets the requirements of the German Renewable Energy Act (EEG).

The energy requirements of a large city

Each wind turbine in Abla has a capacity of 2.1 MW, so that the entire wind farm has a capacity of more than 100 MW. This means that the wind turbine can generate sufficient energy to meet the needs of a large city. Suzlon has chosen the PowerTech Green Line 1000 for this wind farm. They were specially developed at the request of Suzlon for this type of wind turbine. The power converter keeps the plant connected to the grid even during brief voltage dips.

Sturdy, durable, affordable

Knorr-Bremse PowerTech wind power converters are used by Suzlon worldwide in large quantities as the main component of this type of wind generator. The simple, robust and cost-effective solution is perfect for wind turbines with high output. An asynchronous generator with slip-ring rotor and electronically controlled additional rotor resistance is used for switching.

• Nominal capacity: 2.5 MW

• Slip range: 0–16.7%

• Maximum rotor current: 500 A

• Dimensions: 800 x 600 x 1,780 mm (W x D x H).

• Cooling: Forced convection (min. 1,050 m/h)

• Ambient temperature: -20 to +50 °C

The PowerTech converter for wind turbines can increase the slip range of an asynchronous generator with slip-ring rotor to 16.7 percent of the nominal speed via additional resistances in the rotor circuit. Via a diode rectifier, an intermittently switched IGBT short-circuits the rotor circuit in a certain duty cycle, briefly bridging the additional resistance. Depending on the duty cycle, this will obtain a mean value for the rotor resistance (chopper principle). During gusty weather, the rotor resistance is increased, the duty cycle of the IGBTs is reduced, and the additional energy in the flywheel mass of the drive train is stored in the form of an increase in rotor speed. The increased output can be transferred to the grid in a controlled manner so that the drive train is loaded with an almost constant torque. The pitch control returns the system to the specified operating point.

System complies with Renewable Energies Act

In recent years, energy supply companies, especially in Europe, have been calling for wind farms to be perfectly integrated into the power supply system. To achieve this, the wind turbines must remain connected to the grid for a certain period of time during grid shortages and be able to maintain a reactive current. This is why we have developed the compact Green Line 1000 power converter, which perfectly masters these characteristics. The Green Line 1000 can generate up to 2,500 A as a reactive current during a mains fault and thereby complies with the requirements of the Renewable Energies Act.

Successful Collaboration

In total, almost 600 Green Line converters are active in Suzlon wind turbines. They are installed in a wide variety of geographical locations – from the USA, to Spain, Portugal, Africa and Northern China. Our converters master the wide range of climatic conditions prevailing there with ease.