Designing and testing high-efficiency wind generator

Medium speed permanent magnet generators (MS PMGs) deliver more than 98% efficiency - the highest of any commercial wind generator design. Efficiency is also high at partial loads in low wind conditions, enabling the optimum annual production of kWh. This article spotlights the design and testing effort behind ABB’s multi-MW medium-speed and medium-voltage generator-converter package.

Since MS PMGs were launched in 2011, they have attracted a lot of attention in the markets. Major benefits of the medium-speed design include high reliability for large offshore powers, compact size and lower weight, which enables low-turbine top head mass and easier logistics. MS PMGs can be used with the full converter concept to provide total control and an advanced grid compliance strategy.

MS generators are available for multi-MW powers up to 7 MW and more, and can be implemented in three different ways. In the fully integrated design, the gearbox and generator share the same frame, bearings and shaft. The semi-integrated design has the generator and gearbox integrated via a supporting flange, while in the modular design the generator is a separate unit which is mounted independently of the gearbox.

Multi-megawatt wind turbines aim for a low nacelle head weight and a minimum cabling effort between the generator, converters and transformer. As a result, medium voltage (MV - most commonly 3.3 kV) is often the optimum choice of system voltage, because it uses significantly lower currents than low-voltage solutions, and therefore reduces losses in the power conversion components (electrical drivetrain) and long cable runs.

Specify - design - test

When the customer, a leading wind turbine manufacturer, commissioned ABB to supply an MS PMG generator-converter package, the first step was to determine the basic design specifications for the generator. The nominal speed was set at 400 rpm in conjunction with the gearbox supplier and customer. The nominal electrical power was set at 7.35 MW with an efficiency of >98%. Additional boundary conditions for the design were water cooling with an inlet water temperature of 50˚C, and weight and dimension limits.

Once the basic specifications had been settled, the design work - including a large number of simulations, optimisations, calculations, and analyses - began. After it had been established that an 18-pole design was optimal, the best pole shape and pole shoe design were determined through a major analysis effort that included hundreds of finite element iterations. Attention was also paid to the switching frequency, in order to avoid excitation frequencies that could cause resonances, not only in rotating parts but also in the frame and cooler components. This work paved the way for real testing of prototypes, as this is the only way to effectively evaluate how the generator and converter interact.

The choice of converter was ABB’s PCS 6000 - a medium-voltage full power IGCT (integrated gate commutated thyristor)-based converter. Its low-loss, high-reliability design allows operation with a moderate switching frequency while providing good harmonic performance in the output voltage. The low-loss design and reduced component count also mean that the cost of energy is reduced over the whole lifetime of the turbine. A further advantage is that the converter’s modularity allows a tailored mechanical arrangement of the components, depending on whether it is placed in the tower, nacelle or a separate container outside the turbine. The converter also incorporates a number of features to ensure grid code compliance.

Back-to-back integration tests

The integration tests were performed using a back-to-back set-up at ABB’s generator plant in Helsinki, Finland. Two generators were mechanically coupled and both were connected to the grid through a frequency converter. One converter drove its generator as a motor, which drove the other generator. This generated power back to the grid through its own converter. As a result, only the losses of the whole system had to be covered from the grid. The set-up allowed the generator to be run at nominal active power.

The tests confirmed that almost all of the design specifications had been met or exceeded. The generator temperature rise was class B, as calculated, and the final temperatures of the converter components remained below the relevant limits. The three-phase short circuit test after temperature rise showed that the design target of protecting the generator magnets against demagnetisation had been realised. The generator vibration and emitted noise levels were well below the relevant IEC criteria. The generator’s efficiency at the nominal point was 98.17%, and it exceeded expectations at other loading points.

Optimising the switching frequency

A vibration acceleration sensor was fixed at the axial middle of the stator yoke in order to measure the yoke vibration response to switching frequency. The switching frequency and/or its side bands can excite the stator natural mode, and this might lead to resonance. The switching frequency over the whole speed range in different modulation modes was tested. The best results were achieved with a fixed switching frequency over the whole speed range, asynchronous mode, equal to or higher than 720 Hz. Asynchronous mode provided favourable results because neither the switching frequency nor its side bands coincided with dangerous resonance points at any rotation speed.

The back-to-back tests of the electrical drivetrain therefore showed that the generator-converter package met the customer’s specifications and IEC requirements. This solution’s combination of medium speed and medium voltage now provides many significant benefits, not only for the customer, the turbine manufacturer, but also for the wind farm operator and end customer. MS PMGs provide efficiency with compact size and low weight. MV systems enable low current solutions that minimise system and cable losses, make generator design easier and allow the use of extremely robust MV converters with high availability.