The generator inside a doubly-fed turbine

Power plant owners are looking for a reliable system which can guarantee continuous operation and maximum production of energy with the lowest lifetime cost. The doubly-fed (DF) concept offers a cost-effective way to achieve variable speed operation, a good price per kWh, and to satisfy basic grid code requirements.

The DF generator is a wound rotor asynchronous machine where the rotor windings are connected to a small converter via slip rings and brushes. The generator feeds power both from the directly connected stator (approximately 2/3 of Pn) and also from the rotor (approx. 1/3 of Pn).

The converter enables control of the generator speed, power and power factor, thus giving a wider operational speed range and the ability to feed reactive power to support the grid. A partial converter, rated at 1/3 Pn, can be used because a +/- 30 per cent speed range is sufficient for wind turbine applications. Losses are also limited to one third those of a full power converter.

Complying with grid codes
The increasing proportion of wind power in the grid system means that wind power plants must be built to support the existing network, for example by feeding reactive power to the system. Plants must also be able to operate during a short fault period in the grid – ie, they need ride-through capabilities. This means that the turbine must be able to feed power so that the safety devices provided can operate correctly and protect the grid.

These requirements are specified in the different national grid codes in each country. Conventional fixed speed turbines need additional equipment to comply with grid codes. The DF system, with its converter control, can meet basic grid code requirements and it also offers variable speed operation for an improved energy yield.

Engineering challenges
In the DF concept, the generator has to be fitted with a reliable slip ring unit to feed power from the rotor via the converter. This involves high currents and high temperatures, so efficient cooling and the correct brush materials are a must.

Doubly-fed wind power generator
The need to produce reactive power (p.f. 0.90 cap.) results in high rotor temperatures and can also affect the bearing temperatures. Efficient yet low noise cooling of the whole generator is therefore essential. In addition, sudden gusts of wind can cause unexpected overspeeds that subject the rotor to mechanical stresses.

The use of a converter imposes great demands in terms of the rotor winding insulation and bearing currents.
The generator has to be engineered to cope with the high voltage peaks from the different types of converter used, and to ensure that it is a good technical match with the converter. Designing an optimized package requires detailed knowledge of both the converter and generator, and of the ambient conditions and altitude where the turbine will be located. If this knowledge is not available, unexpected problems may occur

Rotor and slip rings designed for reliability
The rotor and slip ring unit play a key role in ensuring that the generator will provide over 20 years’ reliable operation. It has to withstand sudden high overspeeds, as well as the over-temperatures that can occur during reactive power production.

The rotor in a DF generator is more challenging than its counterpart in an induction machine. The DF rotor design is based on windings, like the stator, rather than the copper bars used in a standard squirrel cage machine. This makes the manufacture of DF rotors much more complex. In addition, the rotor winding insulation has to withstand voltage peaks as high as 2.5 kV from the different converters used in the industry.
Proven ABB slip ring design
One of the most important details in the rotor is the winding-end support: it must be strong enough to secure a long lifetime, even with sudden high overspeeds, but at the same time it must allow for efficient cooling when maximum reactive power is produced.
The slip ring unit has to withstand harsh environmental conditions and high currents (up to 1000 A or even more, depending on the power level). There are many important factors involved in designing a reliable unit, such as selecting the optimum materials for the slip rings and brushes, ensuring that dust is not allowed to accumulate, and enabling easy serviceability. Efficient cooling is crucial for the reliable functioning of the unit. The slip ring unit is also the most critical component from the end user’s point of view due to the changing operating conditions and the need for regular servicing.

Why the doubly-fed system is so popular
The structure of the doubly-fed generator

Fixed speed turbines do not fully utilize the power of the wind. Variable speed operation enables DF turbines to capture the energy contained in gusts and therefore achieve a higher yield. Only a small, partial converter - 1/3 Pn - is required, and the converter also enables reactive power production, thus satisfying basic grid codes. Lower losses in the partial converter also mean high total system efficiency at nominal speed. Therefore, the DF concept is, both technically and economically, an effective and flexible solution that satisfies basic end-user requirements.

Patented ABB rotor

DF generators from ABB
In 2010 ABB introduced a new family of slip ring generators featuring an enhanced rotor design. The generators will fit most 1.5 – 2.0 MW doubly-fed turbines that are in use around the world. The concept is based on a standard design with either air or water cooling and a modular component structure. Customer-specific modifications are possible for different interface connections.

The patented rotor design enables higher overspeed and voltage peak withstand with efficient cooling. In 2011 the platform’s power range has been extended to cover powers up to 5 MW and more.

Highlights of ABB´s DF generator platform
Patented rotor
New winding-end support ring - for sudden uncontrolled overspeeds
Increased insulation level (2,5 kV) - to suit most converter designs
Extremely low Total Harmonic Distorsion (THD) - especially at the 5th and 7th multiplies
Improved cooling - for reliable continuous reactive power production
Reliable slip ring unit
Joint development - half a century in house design experience
Superior cooling air flow - for reliable reactive power production
Specialized manufacturing know-how - from the leading slip ring manufacturers
Proven bearing insulation
ABB has many years of experience in proven bearing end shield insulation construction

A leading supplier of wind power technology
ABB is the largest global supplier of electrical solutions for turbines and wind parks, and it is the market and technology leader in generators, converters, motors, circuit breakers & contactors, transformers and HVDC. ABB has supplied a number of turn-key wind park grid connections and High Voltage DC transmission systems for distant offshore wind parks.

Over the last 30 years ABB has supplied 30 000 generators to leading wind turbine customers all over the world - corresponding to a total of 30 GW of power.

ABB has been building reliable megawatt class doubly-fed generators since 1997, based on more than 120 years of experience in electrical machines and over half a century of know-how in manufacturing heavy-duty slip ring motors.

Leading turbine manufacturers rely on ABB´s proven technology for all drivetrain types, from direct drive to medium and high speed, and for reliable, optimized generator - converter package.

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