WO2010000350A1

WO2010000350A1 - Estimation and control of wear in wind turbine slip ring brushes

Info

Publication number: WO2010000350A1
Application number: PCT/EP2009/003449
Authority: WO
Inventors: Heng DENG; Tie Ling Zhang; Yin BO
Original assignee: Vestas Wind Systems A/S
Priority date: 2008-07-01
Filing date: 2009-05-13
Publication date: 2010-01-07
Also published as: GB2461533A; GB0812038D0

Abstract

The condition of brushes in a wind turbine slip ring assembly is monitored and controlled by deriving a wear index from measurements of rotor rotational speed and current over time. The index value may be used to schedule maintenance and may be used to control ambient conditions around slip ring brushes such as pressure, temperature and humidity to prolong brush life.

Description

Estimation and Control of Wear in Wind Turbine Slip Ring Brushes

FIELD OF THE INVENTION

This invention relates generally to wind turbines and more specifically to the estimation and control of wear of slip ring brushes in a wind turbine induction generator.

BCKGROUND OF THE INVENTION

Wind turbines generally use a three phase asynchronous induction generator to convert mechanical power from rotation of the wind turbine blades into electrical power. The generator comprises a stator and an electrical rotor arranged in a housing, and a slip ring assembly which is mounted on the rotor shaft and connected to the rotor windings to transfer current between the electrical rotor and a frequency converter. The slip ring assembly comprises contact rings mounted on the electricalrotor, which rotate over static slip ring brushes mounted in a slip ring housing to provide a rotating electrical contact.

Slip ring brushes are usually carbon based, for example, a graphite or carbon silver alloy. Wear of the slip ring brushes very frequently necessitates unscheduled maintenance that is more expensive than scheduled maintenance and so undesirable. Although referred to as brushes, slip ring brushes are solid blocks of material.

Wind turbines are unmanned as it is unsafe to enter the turbine when it is in operation. Wind turbines are intended to run all day every day. Thus, there are no personnel to check the day to day running of wind turbines and wind turbine generators are required to operate with higher reliability than other induction generators. Moreover, wind turbines are often located in remote inaccessible locations where maintenance is expensive due to the cost of transporting personnel to the wind turbines and the time taken. If wind turbines are located in off-shore wind farms, maintenance difficulties are exacerbated whilst operating in hostile environments. Wind turbines typically have a scheduled maintenance programme and any additional maintenance is highly undesirable as it is estimated to double the cost of scheduled maintenance.

Wearing slip ring brushes have been found to be one of the main causes of wind turbine failures. If the brushes are not replaced once they have worn past a certain point, the turbine must be shut down. When maintenance is performed, the remaining length of the brushes must be measured manually to check that there is sufficient brush remaining until the next scheduled periodic maintenance.

We have therefore appreciated that there is a need to be able to monitor the condition of slip ring brushes used in wind turbines and to prolong the life of slip ring brushes to avoid the need for unscheduled maintenance and to lengthen the time between instances of scheduled maintenance.

SUMMARY OF THE INVENTION

The present invention aims to address these requirements. One aspect of the invention provides a method of estimating the condition of slip ring brushes by deriving a wear index as a function of electrical rotor current and electrical rotor rotational speed over time. Another aspect of the invention resides in the control of variables within the induction generator housing in response to the derived wear index to increase the lifetime of the brushes.

More specifically, there is provided a method for estimating the condition of a slip ring brush in a wind turbine generator having an electrical rotor, comprising determining the rotor current, determining the rotor rotational speed, deriving a measure of wear speed from the determined rotor current and rotational speed, and deriving a wear index from repeated derived measurements of wear speed over time.

The invention also provides a monitoring system for a wind turbine comprising a generator having an electrical rotor and a slip ring brush assembly, the system comprising a means for determining electrical rotor rotational speed, a sensor for sensing electrical rotor current, and means for deriving a measure of wear speed from the sensed rotor current and rotational speed and for deriving a wear index from repeated measurements of wear speed over time.

Embodiments of the invention have the advantage that an estimate of brush condition may be derived from the principal factors which affect wear rates. Preferably, an indication is given that maintenance is required when the wear index reaches a predetermined value. This has the advantage that maintenance of the wind turbine may be scheduled to avoid the high costs of unscheduled maintenance.

Preferably, the wear index is obtained from a value of wear speed integrated over time. Wear speed is obtained from the product of rotor current and rotational speed. This has the advantage that the changes in wear speed over time can be monitored and deviations from an expected deterioration in brush condition detected before a catastrophic failure.

The invention also provides a method for controlling a wind turbine generator having an electrical rotor, and a slip ring brush assembly in a slip ring housing, comprising deriving a slip ring brush wear index from measurements of rotor current and rotor rotational speed over time and, in response to the derived wear index, controlling the ambient conditions around the slip ring brush in the slip ring housing.

This aspect of the invention also provides a controller system for a wind turbine generator having an electrical rotor, and a slip ring brush assembly in a slip ring housing, the system comprising means for determining rotor rotational speed, a sensor for sensing rotor current, means for deriving a slip ring brush wear index from the determinedrotor rotational speed and current over time, and a controller for controlling ambient conditions within the slip ring housing.

Embodiments of this aspect of the invention have the advantage that by controlling ambient conditions in response to the wear index, parameters that affect the wear rate of the brushes can be adjusted to reduce the wear rate, prolong the life of the brushes and so lengthen the time required until scheduled maintenance is necessary.

Preferably, the speed of a fan controlling airflow in the slip ring housing is adjusted. This has the advantage of controlling slip ring and slip ring brush temperature which may reduce the wear rate.

Preferably, the humidity of air in the slip ring housing is adjusted. This may be done by control of humidity in the wind turbine nacelle in which the generator and slip ring assembly is mounted. The slip ring assembly fan draws in air from the nacelle so that changes in nacelle humidity affect the humidity of air around the brushes in the slip ring housing.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings, in which:

Figure 1 is a schematic view of a wind turbine induction generator and slip ring assembly;

Figure 2 is an exploded view of the slip ring assembly;

Figure 3 is a perspective view of a slip ring brush gear assembly; Figure 4 is a perspective view of a double slip ring brush holder and brushes used in the assembly of Figure 2;

Figure 5 is a perspective view of the slip ring assembly;

Figure 6 is a schematic view showing the position of slip ring brushes relative to slip rings; and

Figure 7 is a flow chart illustrating the control of ambient variables in response to a derived wear index.

Figure 1 shows a schematic view of a wind turbine. Turbine blades 10 are driven by incident wind and, via a drive train 12, cause rotation of an electrical rotor 14 of a three phase induction generator 16. A stator 18 has a three phase winding which is connected directly to the transmission grid and the three phase rotor winding is connected via a slip ring and brush assembly 20 to the rotor side C _rotor of a converter 22, the grid side of which C gri_d is coupled via coupling inductors L to the grid. C rotor and C g_ri_d are AC/DC converters with DC voltage being provided by a capacitor 24 arranged between the converters. A control system 26 generates a pitch angle command to set the pitch angle of the turbine blades and provides voltage controls V_r and V₉ to C _rotor and C _grid respectively to control wind turbine power, DC voltage and reactive power at the grid terminals. Modern large scale wind turbines also have sophisticated control systems which can sense and control various parameters within the turbine assembly and in the connection between the turbine and the grid.

Figure 2 shows the slip ring and brush assembly. A brush frame 30, shown in more detail in Figure 3, has three pairs of slip ring brushes and a ground contact brush and is arranged around slip ring assembly 32 which is shown in more detail in Figure 5. A capacitor frame 34 is attached to the brush frame and 3-brush ground contact 36 is received at one end of the slip ring assembly 32. The brush frame 30, slip ring assembly 32 and contact 36 are received in a slip ring housing 38 which is mounted on the main generator housing over the rotor shaft. The shaft extends through an aperture 40 in the housing and the slip ring assembly is fixed to the shaft to rotate with the rotor. A filter housing 42 is attached to the end of the slip ring housing remote from the generator housing to seal the slip ring assembly. The filter housing contains a fan and a filter. The fan draws air into the slip ring assembly from the wind turbine nacelle, within which the generator and slip ring assembly is mounted. The air flow generated reduces the temperature of the slip ring brushes, which reduces the rate of which they wear. It also removes abrasive carbon wear particles produced by the brushes so limiting the damage caused by those particles.

Figures 3 and 4 show the slip ring brush gear assembly and a double brush holder in more detail. The assembly of Figure 3 comprises three pairs of double brush holders 41 of the type shown in Figure 4 each having a pair of brushes 43 and mounted on a frame 30 together with a ground contact brush 44. As can be seen from Figure 4, the brushes are spring loaded so that they are biased towards the slip ring ensuring that contact is maintained as they wear. One slip ring is provided for each of the three phases with a fourth providing a ground contact. Thus, it can be seen that there are four graphite brushes per phase and a further brush for contact with the ground slip ring.

The slip ring assembly is shown in Figure 5 and comprises the three slip rings 50 and a driving ring 51. The rings are mounted on a bush 54 and are separated by ribbed isolating rings 52. The slip ring assembly has a central aperture 56 which receives an end of the rotor.

Figure 6 is a schematic view showing four brushes in contact with their slip ring, with the direction of rotation being indicated by the arrow. For each pair of brushes, the leading brush 58a, b will wear more quickly than trailing brush 60a, b and it has been shown that the wear rate of the leading brush is about twice that of the trailing brush. One maintenance option is to reverse the double brush holder, which is symmetrical, so that the leading brush becomes the trailing brush and vice versa.

The factors that affect wear of the slip ring brushes fall generally into two categories: mechanical friction and erosion. Both are affected by many operational factors. Friction produces carbon dust which itself can accelerate wear, while electrical erosion results from vaporisation of carbon with little physical residue. The physical residue is problematic as it is conductive and can give rise to the risk of flashovers and short circuits when it is between parts at different voltages. The filter assembly and suction system described with respect to Figure 2 alleviates this problem.

It is well understood that temperature and humidity both affect mechanical friction. Current loading and the mechanical characteristics of the motor are also relevant as is the contact pressure between the slip ring and the brush.

Erosion is affected by the size of the rotor current, the contact pressure and a number of other factors. Sparking, which leads to erosion can be caused by an improper film build up on the conductive surface or threading of the surface caused by abrasive wear particles.

Other factors that affect electrical erosion include the brush neutral setting, interpole strength, low brush spring pressure, poor brush seating, high mica and commutator eccentricity. Sparking increases with current loading and motor speed.

A regression model may be developed to model the wear speed of the brush. This model takes into account three factors:

1) Atmospheric conditions, such as pressure p, temperature, T and humidity, h; 2) Commutator condition including film evaluation, fe and undercut quality q;

3) Brush assembly design including brush grade, brush length, holder design and spring pressure Sp.

The quality of film is an important factor in determining the coefficient of friction between the brush and the slip ring and is strongly affected by the temperature of the brush surface. The film is a microscopic layer of metal carbon composite formed when electric current is passed between a metallic slip ring and the brush in the presence of water vapour.

Taking factors 1 to 3 into account, the speed of wear of a brush (Wspeed) may be represented as:

Wspeed = f(p,T,h,fe,q;Sp) (1)

for a given generator design and brush assembly design. A value for Wspeed may be derived from a theoretical analysis, experimentation and simulation.

We have appreciated that, for a given brush and generator design, all the factors that affect wear speed are dependent on the rotor current l_r(t) and the rotor rotational speed ω_r(t). It is thus possible to express the value Wspeed as

Wspeed oc l_r(t) x ω_r(t) (2)

The cumulative wear length of a brush over time may be expressed as a wear index

t Wear Index = J Wspeed dt or J l_r(t) W_r(t) dt (3)

Thus, the wear index is derived from repeated derived measurements of wear speed over time, where wear speed is derived from sensed rotor current and rotational speed and is preferably the product of those sensed values.

A threshold value of the wear index may be established for a given brush design and the wear index is then calculated from measurements of the rotor current and rotational speed in operation. When the calculated wear index is greater than the predetermined threshold, maintenance may be scheduled to replace the worn out brushes. The existence of the wear index is extremely beneficial as it enables the state of the brushes to be monitored and maintenance to be scheduled, so avoiding the unnecessary costs of unscheduled maintenance and avoids the loss of production caused by turbine failure and the reduction in operating efficiency that is caused by badly worn, but still functional brushes. The wear index may be displayed graphically to provide an easily understood representation of the change in wear index over time.

The two components of the wear index, the rotor current and the rotational speed are parameters that are routinely measured in wind turbines. Rotor current sensors measure the rotor current and an encoder measures the rotor position from which rotational speed can be calculated. Commercial wind turbines use SCADA (Supervisory Control and Data Acquisition) control systems which sense and communicate a wide range of wind turbine parameters including the rotor current and rotor speed. These parameters are measured and communicated by the applicant's existing SCADA periodically. The derivation of the wear index may be performed by the wind turbine controller, for example by a local controller embedded onto the converter controller board or turbine control system. Alternatively it may be signalled to a remote location such as a wind farm central control station. The derived wear index is compared against an acceptable value of the index and, if the derived value is unacceptable, the wind turbine controller, or remote controller, signals that maintenance is necessary.

It will be appreciated that the wear index is an estimate of the condition of all brushes in the slip ring housing. It follows, therefore, that all brushes should be replaced at the same time so that the index has wearing to all brushes.

The derived wear index may be used also to improve and extend the brush life. As the rotor speed and rotor current readings are taken very frequently, for example, every few seconds, the change in wear index can be monitored over time. Once the measured index reaches a predetermined value, various system parameters which affect wear rates, for example, the pressure, humidity and temperature of air can be controlled. As mentioned previously, the slip ring housing has a forced air flow through it. The speed of the fan in the filter housing may be adjusted to reduce the temperature of the brushes in response to an increase in the wear index. It is common practice to include a temperature sensor in the slip ring housing to sense the temperature of the slip ring and measurements from this sensor may be used to estimate brush temperature. Thus, the derived index may be compared to a reference value and if the comparison is unacceptable, the ambient conditions around the slip ring brushes within the slip ring housing are controlled, preferably, by changing the temperature and/or the humidity of air flowing through the housing. Increasing the airflow will also affect the air pressure. The ambient conditions may be controlled by the wind turbine controller at the wind turbine or other control system which is remote from the wind turbine. The slip ring assembly is attached to the generator which is located in the wind turbine nacelle. The humidity of air in the nacelle can be sensed and the humidity of air passing through the slip ring assembly can be varied by changing the suction fan speed to vary the speed of airflow. Thus, humidity in the slip ring housing is not measured directly but the humidity of air which is drawn into the slip ring housing from the nacelle is measured and can be controlled in response to changes in the wear index of the brushes. This in turn controls the humidity of air surrounding the brushes and so can reduce wear rates by maintaining humidity below or at desired levels. This process is illustrated by the flow chart of Figure 7 which shows the measurement of I _rotor and ω _rotor and the derivation of the wear index. If this index is acceptable, the measurement is merely repeated periodically. If it is not acceptable, slip ring temperature and nacelle air humidity are measured and, if appropriate, adjusted. In the case of slip ring temperature, adjustment is by varying the slip ring fan speed.

Thus, the embodiment described provides a wear index which may be derived from existing measured wind turbine parameters, particularly rotor speed and rotor current. The index may be monitored so that maintenance schedules may be adjusted in accordance with slip ring brush condition thereby reducing the risk of turbine failure and reducing the need for expensive unscheduled maintenance. Moreover, the change in wear index can be monitored over time and this information is used to alter parameters such as airflow through the slip ring housing to reduce temperature and humidity and so prolong brush life. This in turn enables maintenance schedules to be extended and so maintenance costs reduced.

Many modifications to the embodiments described are possible and will occur to those skilled in the art without departing from the scope of the invention which is defined by the following claims.

Claims

1. A method for estimating the condition of a slip ring brush in a wind turbine generator having an electrical rotor, comprising determining the rotor current, determining the rotor rotational speed, deriving a measure of wear speed from the determined rotor current and rotational speed, and deriving a wear index from repeated derived measurements of wear speed over time.

2. A method according to claim 1 , comprising indicating that maintenance of the slip ring brush is required when the wear index reaches a predetermined value.

3. A method according to claim 1 or 2, wherein the wear speed is the product of the rotor current and rotational speed and the wear index is the wear speed integrated over time.

4. A method according to claim 1 , 2 or 3, comprising comparing the derived wear index with a reference value of the wear index and, if the comparison is unfavourable, controlling ambient conditions within a housing containing the slip ring brush.

5. A method for controlling a wind turbine generator having an electrical rotor, and a slip ring brush assembly in a slip ring housing, comprising deriving a slip ring brush wear index from measurements of rotor current and rotor rotational speed over time and, in response to the derived wear index, controlling the ambient conditions around the slip ring brush in the slip ring housing.

6. A method according to claim 4 or 5, wherein the controlling of ambient conditions comprises varying the speed of a fan controlling airflow through the slip ring housing.

7. A method according to claim 4, 5 or 6, wherein the controlling of ambient conditions comprises varying the humidity of air in the wind turbine nacelle within which the generator and slip ring housing are mounted.

8. A monitoring system for a wind turbine comprising a generator having an electrical rotor and a slip ring brush assembly, the system comprising a means for determiming rotor rotational speed, a sensor for sensing rotor current and means for deriving a measure of wear speed from the sensed rotor current and rotational speed and for deriving a wear index from repeated measurements of wear speed over time.

9. A monitoring system according to claim 8, comprising an indicator for signalling that the slip ring brushes require maintenance when the wear index reaches a predetermined value.

10. A monitoring system according to any of claims 8 or 9, comprising a controller for varying ambient conditions in the slip ring brush assembly if the derived wear index compares unfavourably with a predetermined value.

11. A control system for a wind turbine generator having an electrical rotor, and a slip ring brush assembly in a slip ring housing, the system comprising means for determining rotor rotational speed, a sensor for sensing rotor current, means for deriving a slip ring brush wear index from determined rotor rotational speed and current over time, and a controller for controlling ambient conditions within the slip ring housing in response to the derived wear index.

12. The invention of claim 10 or 11 , wherein the controller comprises means for varying the speed of a fan controlling airflow through the slip ring housing.

13. The invention of claim 10, 11 or 12, wherein the controller comprises means for varying the humidity of air in the wind turbine nacelle within which the generator and slip ring housing are mounted.

14. A wind turbine having a monitoring system according to any of claims 8 to 10.

15. A wind turbine having a controller according to any of claims 11 to 13.

16. A wind farm comprising a plurality of wnd turbines according to claim 14 or 15.