Customized slip rings by simulation

2014-10-20 - The slip ring unit (SRU) is one of the key components in doubly-fed (DF) generators. Environmental simulation enables it to be customized for each site.

Turbine OEMs install their standard wind turbines in many different environments, ranging from coastal areas to dry, high altitude locations. Environmental conditions affect the operation and service life of the generator SRU. Significant benefits can be achieved for end users if the operation conditions are simulated and the results used to customize the SRU for the specific site where the generator is installed.

The demanding SRU

In DF generators reactive and active power is fed to the grid not only from the stator but also from the rotor, which involves high currents and high temperatures. Because the rotor is rotating, reliable slip rings are needed to get the power out from inside the generator.
The SRU comprises a hub with phase slip rings and current rods, a brush rocker with brush holders, phase brushes, and rotor cable connecting studs. Improving the operation of the SRU in different site conditions requires considerable experience. Designing a reliable SRU involves many important factors, including material selection between the slip ring and the brushes, dust removal, efficient cooling and easy servicing. The changing operation condition cycles and need for regular servicing also make this one of the most critical components for the end user.

Modular SRU for easy customization

Many other factors - such as different grid codes and load conditions - affect the size and optimum construction of the SRU. Turbine manufacturers also use the software and hardware configuration of the converter to deliver more reactive power in order to comply with grid support requirements. Different OEM specs involving reactive power, speed range and overspeed affect the performance of the SRU.
Selecting the correct material combinations and setup for the SRU components can therefore be a challenging task.

New innovations in SRU design

To help turbine OEMs meet the often unknown environmental conditions at installation sites, ABB has worked with leading SRU manufacturer to create an innovative modular slip ring design which enables load optimization and effective cooling based on exact load conditions.
A development initiative was launched with the aim of achieving a flexible standardized design which would enable perfect optimization for each individual case. The focus was on reliable, trouble-free operation with long service intervals and low maintenance needs in order to maximize availability and kWh production.
The design was based on a new modular construction which can be easily optimized for each load case. This also contributed to efficient cooling and dust removal.
Efficient cooling and easy maintenance
At the same time an advanced brush set up was also developed which enables easy brush installation and maintenance. To standardize the construction for different load cases, the same brush size is used and the number of brushes needed for each phase is optimized, resulting in a cost effective solution.A widely known brush grade, proven in most environmental conditions, was selected as the standard option.

The unit is straightforward and robust, and it features efficient cooling, easy maintenance and a rigid structure for high overspeed endurance.
All the main components, including the brushes, are made by one of the biggest global turn-key slip ring manufacturers. High quality, partial discharge resistant insulation materials are used so that the SRU can withstand the voltage stresses encountered in converter duty. Every SRU will undergo a 5 kV HV test. A partial discharge test is also performed in order to verify the high quality of the insulation.

Tailor made testing in simulated site conditions

A new slip ring testing laboratory was built to enable R&D to support customers by performing comprehensive testing with the exact installation site conditions. The results are used to customize the setup of the new SRU components according to the load and environmental conditions.
The new SRU testing laboratory follows the conditions of IEC standard 60068-1 7th edition on environmental testing procedures. Wind turbine load conditions can be emulated based on IEC 61400. The requirements of these two standards can be combined to create tailored test programs, which can also incorporate the customer’s other requirements and site conditions.
“We work with turbine OEMs throughout the generator development process to find the best solutions for each case. And now with our new R&D lab we can customize the SRU for the exact site conditions and help wind park operators to maximize their production time,” says Raimo Sakki, product manager for wind power generators at ABB.

Different environmental and load parameters can be simulated based on ABB’s broad experience in generators, research studies, environmental conditions and customer specifications. Using separate converters in both the rotating and feeding system sides it is possible to test all major turbine operation points while the climate chamber controls the humidity and ambient temperature.
The tests are performed with a real SRU housing and fans, in a configuration similar to a complete generator in wind turbine nacelle conditions. In addition to overspeed and high voltage tests, the comprehensive test program also includes verification of partial discharge, brush wear and air flow measurement for easy dust removal.

Benefits for turbine OEMs and end users

The test results are used to select the correct combination of SRU components by optimizing parameters such as the slip ring size and number of brushes, as well as the brush and ring material. Load optimization ensures safe operation temperatures and effective cooling.
For turbine OEMs and end users the main benefit of using pretested components is that it eliminates the need for design changes and avoids unnecessary turbine shut downs. Using site specific optimization also minimizes maintenance and enables long service intervals. This results in continuous operation and maximized kWh production.

Responding to market demand, ABB is expanding its well-proven DF generator family, investing in a new SRU test laboratory to offer advanced reliability with durable slip rings that are customized for the exact site conditions.

Over the last 30 years ABB has supplied more than 35,000 wind power generators for all drivetrain types from low speed, direct drive and medium speed hybrid to high speed PMGs. It has been manufacturing doubly-fed, semi-variable speed generators since 1997.

ABB has more than 50 years of experience in these asynchronous, wound rotor slip ring motors and generators, and it is also a leading manufacturer of optimized generator-converter packages.
The doubly-fed (DF) generator is a wound rotor slip ring 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 (approx. 2/3 of Pn) and the rotor (approx. 1/3 of Pn).

The converter enables control of the generator speed, power and power factor, thus giving a wider speed range to produce power and the ability to feed reactive power to support the grid. A partial converter sized to 1/3 Pn can be used, because a +/-30% speed range is sufficient for wind applications.

With this system it is possible to supply reactive power and thus satisfy basic grid codes. It also offers a high total system efficiency.

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    Low brush wear under installation site conditions
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