Optimized High Power Converters for Renewable Energy Systems

Posted on 29 June 2019

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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 realized. This upsurge has strongly reignited the demand for renewable energy system components throughout the supply chain, including wind power and PV converters.

By Alan Tims, Solutions Business Development Manager Asia, Semikron Solution Center

 

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

The Semikron Solution Centers saw this effect first hand in 2010 by experiencing phenomenal growth in order intake of more than 300% for standard renewable energy converters, and strong interest for new project developments.

The Semikron Solution Centers group is unique within the power electronics sector as it is the only global group of companies who specialize in the design and manufacturing of power electronics assemblies and platforms. Semikron has been integrating semiconductors, drivers and other key components into power electronics assemblies for more than 50 years within its various subsidiaries. Semikron combined these subsidiaries in 2003 to form the Semikron Solution Centers group, which now comprises eight Solution Centers, one from each continent, whose key strategy is to manufacture and support power electronics platforms and solutions direct to the local market.

Clear power topology trends are emerging for the converters, where 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 behavior of generating equipment during grid disturbances.

Power ratings have consolidated, for landbased systems the typical power rating for new developments is between 2 MW and 3 MW, mainly using water-cooled converters, bottoming out at 1.5 MW with a mixture of double-fed induction generator (DFIG) and SG applications, air and water-cooled. Offshore applications have leveled off at 5 MW to 6 MW, with some pilot applications surpassing this level. Whilst 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 this strong market resurgence has placed mounting pressure on the converter suppliers to offer readily available standardized 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 platform: 450 kW to more than 2.5 MW

A new inverter platform is now available which enables the system integrator to readily source an easy to use versatile inverter to meet these market demands. The Semistack for renewable energy is a high power three phase inverter platform that uses the SKiiP intelligent power module (IPM) with integrated heat sink, power module, driver and protective sensors/functions, and is optimized for renewable energy applications. The modular topology, the wide selection of SKiiP IPM power ratings, the choice of air or water-cooling, and the ability to connect the inverters in parallel allows for a wide range of user applications from around 450 kW to more than 2.5 MW full power systems such as those used in SG wind power applications.

The base inverter configuration comprises three half bridge phase cells mechanically arranged in a vertical configuration. Each phase cell contains one SKiiP IPM 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 realize a complete 4 quadrant converter or for paralleling the stacks to obtain higher power ratings.

The Semistack for renewable energy comprises of three modular halfbridge (GB) phase cells

Air and water-cooled versions are available; both versions are configured using the same overall mechanical topology. The user can choose between a simple air-cooled solution typically used in solar applications and lower power DFIG wind power applications, or a higher power water-cooled solution typically used in larger SG wind applications.

The versatility of the Semistack for renewable energy is demonstrated in the realization of the following applications. A typical 1.5 MW DFIG system1 can easily be organized by combining two air-cooled Semistack inverters into a four quadrant configuration. The complete converter fits into a 600mm wide cabinet and measures only 1200mm in height enabling the complete assembly to fit into a standard 2000mm cabinet with room for other equipment.

The maximum power density is realized by the water-cooled chassis

Similarly two water-cooled inverters with four bay SKiiPs can be combined in the same manner to configure a full power four quadrant SG application up to 1.5 MW2.

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

Higher power ratings are achieved using the water-cooled versions which are available in two chassis sizes. The larger chassis 4/3 accommodates either the four bay or three bay SKiiP IPMs. It is possible to combine four bay and three bay stacks together in a four quadrant configuration. This allows for optimization of SKiiP modules where the requirements for the generator side converter are often more severe than the grid side. The 4/3 chassis accommodates full power applications up to 1.5 MW2 or 2.5 MW3 with two inverters in parallel. With a power density of more than 10 kVA/litre the Semistack for renewable energy has at least a 20% advantage over its closest rivals. The smaller 3/2 chassis uses the three bay and two bay SKiiPs for lower power requirements that are typically used in DFIG systems where the converter is rated to approx. 30% of the system power, and for high power solar systems.

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

Only two cooling circuit water couplings are necessary for each inverter as the separate cooling plates are internally connected in parallel to avoid thermal stacking. Two quick connect fluid connectors for inlet and outlet are located at the bottom of each inverter to easily facilitate the fluid connection to the external cooling system.

The Semistack for renewable energy can easily be coupled together to form four quadrant and higher power configurations such as the 2.5MW 4Q SG configuration 3

Functional design

Owing to the functional design of the Semistack 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 Semistack 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.

At the heart of the Semistack are the SKiiP3 intelligent power modules. The SKiiP is the highest power IPM on the market and is the most widely used high power module in the wind power market today. More than 57GW of global wind power generation is driven by Semikron devices today. Hence the Semistack for renewable energy is a natural choice for wind power applications.

Fully qualified

By definition a Semikron Solution Center platform is a standard product or family of standard products that are clearly defined and qualified. The Semistack for renewable energy, like other Solution Centers platforms undergoes a rigorous qualification process. During this process the Semistack is subjected to a range of arduous type tests in accordance with a combination of international and in-house standards, including electrical, thermal, thermal cycling, environmental and shock & vibration.

Additionally each SKiiP IPM is fully tested during its own production process. Every fully assembled Semistack undergoes a full suite of tests during production including isolation, operational load tests and short circuit tests. Additionally an optional burn-in is available 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.

The heart of the Semistack for renewable energy, SKiiP intelligent power module, the most powerful IPM on the market

Benefits for the renewable market

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

Future proofing is assured with the Semistack for renewable energy as it is compatible with the next generation SKiiP IPM. The Semistack is ready to take advantage of the new 4th generation SKiiP technology with its increased power density and new digital driver when it becomes available.

 

 

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