High-power converters for renewable energy systems

The renewable energy market is rebounding strongly from the global financial crisis. Projects that were previously delayed or mothballed have been restarted and many new projects are being realised. This upsurge has strongly reignited the demand for renewable energy system components throughout the supply chain, including wind power and PV converters.

The Global Wind Energy Council (GWEC) forecasts an annual average growth rate of 20.9% for total installed capacity over the next 4 years. This will more than double the total installed capacity of wind power to more than 400 GW by 2014. The growth outlook is especially poignant for Asia, where China was the world’s largest wind power market in 2009 and today is now only second behind the USA in their accumulated base of wind generation capacity.

Similarly for photovoltaic, strong growth in the cumulative installed PV capacity of 40% was forecasted in 2010, with Germany remaining the largest market, while new markets in Southern Europe, USA and Asia are feted to see good growth - particularly in Asia if the Chinese government ratifies its domestic PV growth stimulus plan, which is predicted to set a target of 30 GW installed base by 2020.

Converter topology trends

Clear power topology trends are emerging for the converters.

• In the wind power sector new developments are focused on permanent magnet synchronous machines (SG) which use full power converters. This configuration offers a higher overall efficiency, gives a wider range of useable wind speed and enables the converters to accommodate the new regulations concerning behaviour of generating equipment during grid disturbances.
• For land-based systems power ratings have been consolidated, with typical power ratings for new developments of between 2 and 3 MW. These converters can be either water or air cooling for the permanent magnet synchronous machines and the double-fed induction generators. 
• Offshore applications have levelled off at 5 to 6 MW, with some pilot applications surpassing this level. While there is a definite trend towards medium voltage (MV) for offshore systems, many new developments continue to use low-voltage (LV) converters connected in parallel.
• In solar applications, power technology trends for grid connected systems typically range up to 500 kW per individual inverter with some new developments underway at 1 MW plus.

The general maturing of the wind power and solar market coupled with a strong market resurgence has placed mounting pressure on the converter suppliers to offer readily available standardised converters that are pre-qualified; hence low technical risk and flexible enough to be used in a range of power ratings. Time to market has become the key driver and converters for renewable energy applications have become commodity items.

Flexible platforms

New inverter platforms are available with 450 kW to more than 2.5 MW capability. These high-power, three-phase inverter platforms use intelligent power modules, integrated heatsinks, power modules, protective sensors and driver sensors to provide optimised and renewable energy solutions.

Modular topology, wide selection of power ratings, the choice of air or water cooling and the ability to connect the inverters in parallel provides solutions for a wide range of applications, which provides great versatility. It enables system integrators to readily source and easily solve their specific application demands.

The base inverter configuration comprises three half-bridge phase cells mechanically arranged in a vertical configuration. Each phase cell contains one intelligent power module and cooling plate or heatsink, DC bus with long-life polypropylene capacitors, AC connections and snubbers. The individual phase cells are connected together by a low-inductance DC coupling and housed within a rigid mechanical frame. The inverter stacks can be connected together by means of a DC busbar to realise a complete 4-quadrant converter or for paralleling the stacks to obtain higher power ratings.

An inverter for renewable energy comprises three modular half bridge (GB) phase cells.

The versatility of an inverter for renewable energy is demonstrated in the realisation of the following applications. A typical 1.5 MW DFIG system can easily be organised by combining two air-cooled inverters into a four-quadrant configuration. A complete converter fits into a 600 mm wide cabinet and measures only 1200 mm in height, enabling both of the inverters to fit into a standard 2000 mm cabinet with room for other equipment.

Maximum power density is realised by the water-cooled chassis.

Higher power ratings are achieved using the water-cooled versions. For example, two water-cooled inverters with four intelligent power module bays can be combined in the same manner as the air-cooled versions to configure a full-power, four-quadrant permanent magnet synchronous machine application.

A compact 4Q 1.5 MW air-cooled DFIG configuration allows room at the base of the cabinet.

Functional design

Owing to the functional design for renewable energy, the assemblies can be easily connected together at the DC link by means of a low-inductance bus coupling connected between stacks. This makes it easy to configure four-quadrant applications and to parallel the inverters for higher power. Moreover, a brake chopper can be added by using another inverter assembly. This flexibility enables the systems integrator to easily build a range of different power ratings using the same components. A simple AC busbar is available as standard to easily facilitate cable connections at the front of the inverter assembly, or an optional AC busbar kit can be used to orientate the AC connections to the bottom of each inverter assembly.

Two chassis designs are available in water-cooled versions: the chassis 4/3 and the chassis 3/2.

Fully qualified

Inverter platforms undergo a rigorous qualification process. During this process the inverter is subjected to a range of arduous type tests in accordance with a combination of international and vendor’s in-house standards. This includes electrical, thermal, thermal cycling, environmental, and shock and vibration testing. Additionally, each intelligent power module and final assembly undergoes a full suite of tests during production including isolation, operational load tests and short circuit tests. Typically, vendors offer a burn-in service for customers who require an elevated level of severity. This ensures maximum robustness and reliability during the long service lifetimes necessary for high-value, grid-connected power-generation equipment.

An intelligent power module is at the heart of an inverter.

Benefits for the renewable market

A converter system supplier can benefit from the ability to outsource a standard product for renewable energy solutions. The system supplier now has the freedom to choose from a range of qualified models. This eliminates the need for costly investment in design and manufacturing resources, and minimises technical risk. The flexibility within the product family and functional design has added another dimension to the supply chain thus enabling the systems suppliers to meet the short time to market for commodity demands that are prevalent in the market.

An important responsibility is placed on inverter manufacturers in the need to future-proof their products by ensuring compatibility with the next generation of inverters. This will allow users to take full advantage of the next generation of inverter technology with its increased power density and new digital drivers when it becomes available.