DETECTION AND ANALYZATION OF BEARING CURRENT ON THE MAIN SHAFT OF WIND TURBINE

PHD-STUDENT: JIAN ZHAO

ELECTRIC POWER ENGINEERING

Reference group Chalmers group

Olle Bankeström, SKF, Göteborg

Lars Jacobsson Rabbalshedes Kraft Ola Carlson, Main supervisor

Roger Magnusson, Skellefteå Kraft Yujing Liu, Examiner

Christofer Åslund, Göteborg Energi Xiangdong Xu, Supervisor

Rahul Kanchan, ABB CRC, Västerås

Sponsor: Energimyndigheten and reference group

Test result from SKF

Test result from SKF

Grease color changed & roller race damaged

AimThe main aim of the project is to understand the origin of bearing currents

in wind turbines and thus the possibility to counteract the same.

The objectives of the project are as follows:

• To develop knowledge that describes how bearing currents occur.

• To develop methods for measuring bearing currents.

• To develop methods that prevent the bearing currents.

Common mode voltage in pitching system

5

hub

main bearing

pitching system

power cable

Grid

Pitching converter

Pitching motor

Neutral

PWM feed converter with 3 bridges has 8

combination of switch state, S0 and S7 state

have no loop in winding， the voltage direct

drop on the neutral point

Common mode voltage

6

Converter

Frame

Frame

Stator

CB1 CB2

CRF

CSWF

ShaftRotor

CSWR

Stator

windings

CB2CB1 CRF

CSWR

3CSWFVCM,S

Stator

windings

Frame

RotorShaft

CM voltage

VAG

VBG

VCG

VCM

Laboratory test of CM voltage

7

Grid

InductionMotor

Converter

V

VCM

A

Induction machine

Load machineGround

Neutral point

• Converter feed induction machine

• Neutral point floating

• CM voltage peak up to 240V

0

200

400

-200

5 ms

DC/V

2.5 ms

-400

peak value 240V

On board test in Big

Glenn

8

Electrical and mechanical System in Big Glenn wind turbine

9

Grid

Vcm

Generator

Frequency converter

Generator

Main bearing

Converter

Transformer

Power cable for

pitching system

Pitching system

Blade

Test System

10

Picoscope

Rogowski Coil

Test coil setup

11

Generator side bearing current

12

Observations from Big Glenn

• There was a noise from the bearings

• There were high temperatures of the bearings

• There were high current trough the bearings

The wind turbine is taken down,

the bearing problems was one reason

Measurements from

Gårdsten, 2 MW

14

Simplified turbine geometry

Main bearing

Sensor ASensor B

Generator

&

converter

Rotor

Gearbox

2

-2

8

0

0.8

A

-0.8

3.2

-3.2

-8

A

8

3.2

-3.2

-8

A2

-2

0

0.8

A

-0.8

Bearing Current during pitching

No pitch operation current(B－A) ≈ 0.4 A

Main frequency at 5k Hz

Pitching operation current(B－A) ≈ 3 A

• Conclusion

Electrical pitching system generate CM voltage

CM voltage damage the main bearing

• Future workBuild more advanced online realtime test system

Analysis and eliminate CM voltage