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High-Speed PMGs Open up Full Converter Technology

As their mechanics are broadly interchangeable with the widely used doubly-fed induction generator, high-speed PMGs are an promising approach for turbine makers looking to gain the benefits of a switch to full converter technology. By following this path, converters decouple generators from the grid, enabling total control and full grid compliance.

 

As an industrial sector, wind power is attracting new companies on an almost daily basis. As a result, even though the markets are enjoying substantial growth, competition is getting fiercer.

To establish their place in the market, wind turbine manufacturers are therefore seeking competitive designs and are also considering different drivetrain concepts. Their research will supply differing conclusions, depending on their specific target market. Some firms choose a doubly-fed (DF) induction generator concept, while others go to a full converter solution using permanent magnet generators (PMGs), all making the right decisions according to their own strategies.

An ABB high-speed permanent magnet generator

Newcomers to the market are more likely to pioneer new approaches while experienced players are better able to test new drivetrain designs while continuing to sell their current models.

Turbine original equipment manufacturers (OEMs) need to ensure their products are of high quality and offer the highest production of energy with the lowest lifetime costs to be accepted by the plant operators and investors. Lifetime cost, price/kWh, low O&M cost, weight, supply-chain availability — there are many criteria to consider and there are also many uncertainties involved.

Profitable Power Production

The growth of wind power depends greatly on the strength of the grid in different countries. This is why turbines with high power quality and the ability to stabilise the grid by feeding in reactive power are favoured and have become more so in light of increasingly demanding grid code requirements.

Today, power plant owners and grid operators also demand the maximum continuous energy production, and the maximum load factor with the minimum of maintenance downtime, as well as low operational costs. For these reasons, full converter (FC) applications have become increasingly popular.

In the FC concept all generated power goes through the converter, enabling the turbine to operate across the full speed range— theoretically from 0% to 100% of its speed operation — while maintaining grid synchronous output. The converter's DC-link totally decouples the generator and the mechanical drivetrain (of the rotor, mainshaft and gearbox) from the grid. The grid independent voltage also means that changes in the grid do not affect the generator dynamics. This decoupling also enables the maximum drivetrain damping, leading to easier turbine design, resulting in low weight.

Thus the full converter concept multiplies all the benefits of the doubly-fed system and much more. It offers a full speed range, full grid compliance with the most advanced grid fault support and ride-through function, full control of the generator and the grid and total grid de-coupling of mechanical parts. Such generator topologies, which allow a 100% speed variation, can also obtain greater aerodynamic efficiency.

The permanent magnet generator — with the highest efficiency in all speeds and especially at low wind speeds — is the best choice in terms of the full converter concept to maximise electricity production while minimising the total lifetime cost. The use of efficient and robust permanent magnet generators will maximise the overall economics of a wind turbine installation and with low maintenance requirements it is ideal for offshore applications too.

Permanent Magnet Generator Technology

A permanent magnet generator (PMG) is a synchronous machine in which the rotor windings have been replaced by permanent magnets. As a result it needs no separate excitation and the excitation losses of the rotor — which normally account for about 20%—30% of total generator losses — are eliminated. This enables high power density and leads to a smaller size and the highest efficiency at all speeds, offering the maximum annual production of kWh with the lowest lifetime cost.

ABB's high-speed permanent magnet generator rotor

Great experience is required to design the permanent magnet generator in order to make it a reliable product for serial production. The most critical factor for the proper operation of a PM machine is the magnetic circuit design itself — and the magnet selection from different NdFeB magnet materials available — as well as the correct positioning and fastening of the magnets for different speed range solutions. Experience in stabilising and ageing of the magnets and corrosion protection is also very important for reliable operation and a long operational lifetime.

Special design knowledge is needed to prevent high temperatures in the rotor, which in a fault situation like a short circuit could cause weakening of the magnetic properties. A 'cold' rotor also minimises problems with bearing temperature sensors tripping as a result of heat conduction from the rotor along the shaft to the bearing.

Each of the concepts of low-, medium- and high-speed permanent magnet generators is unique and leads to a different construction and morphology. Experience is therefore also needed not only in the electrical design, but also in the mechanical architecture, for instance in using new materials for reliable fixing of magnets. Working with extremely powerful magnets is also challenging in terms of series production of the machines.

The selection between low, medium and high speed is individual and any one of the configurations can be the 'right' one for a certain company when applied in a particular market.

In the low-speed, gearless, direct-drive configuration, the generator is normally integrated into the turbine construction, which leads to a large project with experienced partners needed for solving the mechanical design challenges. Long development times are normal and the construction is larger and heavier when compared with the use of geared solutions with higher speeds. In addition, there are also not so many large-scale suppliers for these big generators. Nonetheless, particularly in the lower MW range, the low speed system offers a good choice in the right market.

A medium-speed configuration with an integrated single or two-step low-speed gearbox represents a more compact design where the generator is typically integrated into the gearbox. But, here again, experienced partners are needed in solving the mechanical design challenges and development time is long. This concept offers reasonable sizing, even at over 5 MW capacity, and low-speed gearing is widely accepted as being more reliable.

The high-speed configuration using a traditional three stage gearbox is mechanically similar to doubly-fed systems, having the same gear design and speed range, again enabling a small generator.

This provides the most straightforward route for new companies and experienced turbine manufacturers of existing doubly-fed system to broaden their offering to gain the benefits of the full converter concept. It also offers the smallest size and weight, making it an ideal generator for series production in multi-MW capacity machines. However, the manufacturer must have the necessary experience of rotor construction to ensure that the magnets remain in place in this high-speed application, especially with sudden overspeeds.

The Easiest Way to Full Converter Concept?

In their search for competitive designs and in their drivetrain concept studies many companies find it easy and safe to use the upgrade strategy, building on a design that they already know.

An illustration of the full converter concept

However, it must also be considered that advancing design in this way might not meet future market demand. End users are actively looking at designs that promise them the highest production of energy with the lowest life time costs and in this context the full converter concept using high-speed permanent magnet generators offers an easier and faster way to get into the markets.

For instance, developing a new high-speed full converter turbine is far more straightforward than the totally new constructions required for the development of low- and medium-speed designs.

The high-speed PMG is to a certain extent mechanically interchangeable with the doubly-fed generator (with the same speed range and able to use the existing gear design). As a result, companies building turbines today using the doubly-fed concept find it easier to upgrade to full converter technology using PMGs, following their own reliable turbine construction and saving money and time. 'Only' the partial converter has to be changed to a larger full-power converter to be fitted somewhere in the existing nacelle.

When looking at the overall costs, a physically small generator is relatively easy and fast to install, as well as economical to transport. And, if something goes wrong, it is also much easier to disassemble and take down. Furthermore, when the time comes that still larger turbines are required, there is even the possibility of doubling the power output by using, for example, a 'double output gearbox' and two of the same small generators used in the earlier model. This proven generator platform, needing only slight modifications, therefore makes the whole process faster.

For the wind park developer, the smaller size also means that high-speed PMGs are an easier component to stock and protect from the weather at the site (compared to the larger low-speed gearless units). And for the generator supplier, the small size means easier serial production without the requirement for special manufacturing facilities that are required to build larger-sized units. Thus PMGs offer the potential for a rapid production start-up in new markets and fast deliveries to the turbine OEMs coupled with their inherent advantages of efficiency — particularly at low speeds — improved O&M regime and enhancement of electricity production and consequently a shorter return on investment period.

Indeed, recognising this, some of the world's leading wind turbine manufacturers have already begun selecting high-speed permanent magnet generators as their choice as the future solution for a full converter technology.

Timo Heinonen is a manager at ABB Wind Power Generators

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