Wind Turbine DFIG Slip Ring Fault: Case Study

Over 60% of all wind turbines are currently using the doubly fed induction generators (DFIGs). At the time of their introduction they offered higher efficiency (in sync with grid with variable wind turbine speed). And also better power factor control (import/export of reactive power).

In addition, reduced converter cost (only 25-30% of power passes through the convertor) compared to the previous generation of generators. This is possible by connecting the rotor windings to the grid via a multi-phase slip ring unit and a voltage converter. The slip ring unit consists of a set of spring-loaded brushes. They ride on slip rings mounted on the rotor, for each phase.

A number of generator faults are detectable by accelerometers in the load zone of the generator bearings, including faults with the slip ring assembly. This case study shows how early detection of slip ring defects could result in a fast, inexpensive repair. If, on the other hand the slip ring fault is not corrected at an early stage of development, this could lead to a catastrophic failure where the generator is destroyed.

MACHINE/INDUSTRY/PROCESS

A multi-MW wind turbine in a wind park was operating at full production. At that time, Brüel & Kjær Vibro condition monitoring system temporarily disconnected for approximately two months. Shortly after, when the condition monitoring system went back online, the customer notified the Brüel & Kjær Vibro Surveillance and Diagnostics Service Centre. And, requested the initial data analysis.

OBSERVATION/DIAGNOSIS

Then, Brüel & Kjær Diagnostics Group analysed the data using a detailed time waveform and frequency spectrum, as shown in Figures 1-2. Increased vibration indicated a potential problem with generator slip ring unit.

Figure 1: The time waveform showing amplitude modulation of a little less than 2.5 Hz.

OBSERVATION/DIAGNOSIS (Cont.)

Figure 2: Increased vibration amplitude for several harmonics as measured by the accelerometer located on the non-driven end generator bearing.

They issued an alarm report by phone conversation with the customer explaining the problem, in the following step. And, below is an extract from the recommendation section of the report:

• In addition, generator inspection within 1-2 weeks is a recommendation.

• Another recommendation is to perform an up-tower generator test run with particular focus on the slip ring unit. Check the height of the slip ring brushes to be within the acceptable limits. Then, inspect the condition of the cooling groves, insulation ring and the current clamps.

RESULTS/FEEDBACK

A site inspection was done one day after the report was issued.

Figure 3: Slip ring shown with worn out cooling groves (left), and worn out brushes (right).

RESULTS/FEEDBACK (Cont.)

Aftermath, the slip ring unit and brushes were replaced. In this way, the wind turbine returned to production with lower vibrations measured on the generator bearings.

Figure 4: Vibration amplitudes returned to normal (6x harmonic shown), as measurement by the accelerometer on the non-driven end generator bearing.

BENEFITS/COST SAVINGS ESTIMATION

There are clear benefits in detecting a slip ring fault early. The cost and downtime in replacing the slip ring unit is small in relation to the maintenance and downtime with a catastrophic failure of the generator.

• Slip ring unit replacement: Approximately, 4000 € plus a few hours downtime (500-1000 €) • Generator replacement: With approximation, 100 000 € (includes crane) plus downtime (four weeks at 2000 € per day), with a total of 156 000 €

Hence, this gives a savings of 151 000 €, which does not include labor.