Considering the alternatives

4 January, 2010

The United Kingdom is Europe's windiest region. In January 2009, U.K. wind installations totalled 3,240 megawatts (MW) with the capacity to power 1.3 million homes and overt the emission of 3,682,563 tons of carbon dioxide annually, according to the British Wind Energy Association (BWEA).

The U.K. is also presently the world's leading market for offshore wind energy. According to the BWEA, approximately 2,300 MW of offshore wind power is expected to be online globally by the end of 2009, with the region accounting for about half of that total. The BWEA projects the U.K. will have more than 5,000 MW of offshore wind capacity online by year end 2015. Fuelling continued growth of the market, the U.K. government recently set an ambitious target of generating 15% of all of Great Britain and Northern Ireland's energy from renewables by 2020. The BWEA expects wind power to account for the lion's share of these renewables, or a total of approximately 33 gigawatts (GW) of capacity. Industry research firm Emerging Energy Research predicts the U.K.'s offshore wind market will grow to nearly 10 GW by 2020.

The rapid expansion of wind power markets in the U.K. and around the world has created a critical need for power conversion systems equipped to meet the unique requirements of the industry. Responding to these specific needs, American Superconductor (AMSC) has developed the PowerModule PM3000W; a fully programmable, flexible and modular power converter designed to address the challenges related to wind power conversion. This modular power converter platform can be utilised across multiple wind energy applications - streamlining manufacturing processes and reducing time to market, and therefore, lowering the overall cost of wind energy converter systems.

Optimising wind turbine output

To harness the best wind resources both on-and offshore, larger-scale wind farms often must be located in highly remote areas with severe weather and temperature conditions, and weak power grids. While the intermittent nature of wind power alone presents unique challenges for power conversion, these added factors make the need for tailored solutions a vital component to the success of wind installations.

Offering flexibility and cost efficiency, the PM3000W is compact, highly scalable and designed for rapid integration into a wide range of wind turbines with power ratings from 750 kilowatts (kW) for the basic PM3000W building block - up to 6 MW. This solution has additionally been engineered to withstand the severe climatic changes and poor power grid conditions encountered at remote wind farms.

Today, electric utilities are requiring that wind installations be equipped with capabilities such as power factor correction, voltage control, volt-ampere-reactive (VAR) support and fault ride through. Installed at power grid substations, AMSC's D-VAR solution provides these capabilities, allowing wind farms to meet grid interconnection standards now being enforced.

Canada, Australia, New Zealand, Spain and the U.K. are among the first to adopt such standards to protect the grid from the transient voltage stemming from wind farms that can damage power grid infrastructure and result in blackouts. Others are expected to soon follow suit. AMSC's D-VAR solution is now deployed at more than 50 wind farms around the world for this purpose.

In developing the module, AMSC drew upon its broad capabilities, including:

• The company's experience connecting entire wind farms to power grids using its D-VAR solution
• Its extensive power electronics hardware and software experience
• The engineering capabilities of its wholly-owned AMSC Windtec subsidiary which provides licenses and customised designs for wind turbines ranging in power from 750 kW to 10 MW
• The knowledge gained in the deployment of thousands of its PowerModule PM1000 power converters in wind turbines

The PM1000 architecture includes printed circuit board (PCB) design with state-of-the-art Insulated Gate Bipolar Transistors (IGBTs), control algorithms and air or liquid cooling. As with the PM3000W, several PM1000 converters can be configured in parallel to enable higher power-rated converters and systems, making it ideal for use in a broad range of industries and applications.

Well equipped versatility

The PM3000W power electronic converter incorporates algorithms and external communication protocols to enable universal generator connectivity. It is one of the first configurable power converter building blocks developed specifically to bridge the needs of both wind turbine generators and wind farm grid interconnection by providing low voltage ride through (LVRT) and ground fault protection directly inside the wind turbine. It can also be used with most, if not all, wind turbine generators - the most common types being synchronous/ asynchronous, induction/permanent magnet, double-fed/full conversion, 50 Hz/60 Hz.

Most wind turbines today operate in variable-speed mode, and the control system regulates the rotor speed to obtain peak efficiency in fluctuating winds. Operating in variable-speed mode, however, requires the use of power converters. In addition, today's state-of-the-art controllers integrate signals from dozens of sensors to control rotor speed, blade pitch angle, generator torque, and power conversion voltage and phase; and in conjunction with the power converter are integral for providing fault ride through. The controller is also responsible for critical safety functions such as shutting down the turbine when extreme conditions are encountered, for example hurricane-force wind speeds and grid power outages.

Lower switching losses have been achieved with the PM3000W due to faster, cleaner turn-on/turn-off characteristics of IGBT components - with higher output efficiency gained from lower losses. The converter utilises thermally and mechanically enhanced IGBT modules and features wind power specific controls, interfaces and application modules, and mono-frame construction with slide mounts - offering versatility. IGBTs employed in the PM3000W, for instance, are rated at over the standard 125°C.

The device is a 4-quadrant converter; the topology features two dual-DSP embedded controllers, two 3-phase power processors and a dynamic brake. The two embedded controllers allow implementation of two separate control functions with independent software for each power processor (one for line side and one for generator side), giving the customer the ability to program certain PLC functions locally in the converter. The power converter is liquid cooled in a double conversion architecture with a common DC bus. A dynamic brake is included and provides DC bus protection against over-voltage. This compact design yields a power density of up to 130 W/in3 (7.9 W/cm3). Multiple PM3000W power converters can additionally be configured in parallel to create higher power-rated converter systems up to 6 MW.

Add-ons and extras

AMSC also produces customised wind turbine electrical systems and core electrical components, which include the PM3000W converter, pitch and yaw controls, and advanced SCADA systems to monitor the turbine's control. These electrical systems and components provide AC to DC to AC power conversion (double conversion) and enable reliable, high-performance wind turbine operation by controlling power flows, regulating voltage, monitoring system performance and controlling the pitch of wind turbine blades to maximise efficiency.