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CHAPTER 8

FAILURE MODES AND EFFECTS ANALYSIS OF WT

8.1 INTRODUCTION

Wind energy stands out to be one of the most promising new and

renewable sources of generating electrical power for any country. Wind

electric generator converts kinetic energy available in wind to electrical

energy by using rotor, gear box and generator. The wind energy is widely

used because of its environmental friendliness and many countries have good

wind potential to harness energy.

Failure Modes and Effects Analysis (FMEA) is a step-by-step

approach for identifying all potential problems in a design, a manufacturing or

assembly process, or a product or service. “Failure Modes” means the ways,

or modes, in which something might fail. Failures are any errors or defects,

especially one that affects the end user, and can be potential or actual.

“Effects Analysis” refers to studying the consequences of those failures. Its

most visible result is the documentation of collective knowledge of cross

functional teams. FMEA can be adopted during design stage to prevent

failures afterwards it used for control the failures during operation. The

different types of FMEA are System FMEA, Design FMEA, Process FMEA,

Service FMEA and Software FMEA. FMEA helps us to identify the different

prospective failures and used to develop the requirements that minimize the

likelihood of those failures.

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FMEA technique is designed for the following reasons.

1. To identify potential failure modes for a product or process

2. To assess the risk associated with those failure modes

3. To endow with numerical rank for issues in terms of

importance

4. To prioritize according to their serious consequences

5. To identify and carry out corrective actions for the most

serious issues.

8.2 REQUIREMENTS OF FMEA

In general, FMEA requires the identification of the following basic

information:

1. Item(s)

2. Function(s)

3. Failure(s)

4. Effect(s) of Failure

5. Cause(s) of Failure

6. Current Control(s)

7. Recommended Action(s)

8.3 NECESSITY OF FMEA

1. When a process, product or service is being designed or

redesigned

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2. When an existing process, product or service is being applied

in a new way

3. Before developing control plans for a new or modified

process

4. When enhancement goals are planned for an existing process,

product or service

5. When analyzing failures of an existing process, product or

service

6. Round the clock life of the process, product or service

8.4 FMEA PROCEDURE

1. Assemble the cross functional team with diverse knowledge

about the process, product or service and define the scope

2. Establish the ground rules

3. Collect data and review all relevant information

4. Identify the parts, components, system or processes to be

analyzed

5. Identify the potential failures, effects and causes

6. Determine the severity rating, or S

7. Determine the occurrence rating, or O, used to estimate the

probability of failure

8. Determine the detection rating, or D, used to estimate the

controls systems used to detect the cause or its failure mode

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9. Calculate the Risk Priority Number, or RPN, which equals

S × O × D. and also calculate the Criticality by multiplying

Severity by Occurrence, S × O

10. Evaluate the risk associated with the issues identified by the

analysis

11. Prioritize and assign corrective actions

12. Perform corrective actions and re-evaluate risk

13. Distribute, review and update the analysis as appropriate

8.5 GROUND RULES AND ASSUMPTIONS OF FMEA

Before detailed analysis takes place, ground rules and assumptions

are usually defined and it must include,

1. Standardized mission profile with specific fixed duration

mission phases

2. Sources for failure rate and failure mode data

3. Fault detection coverage that system built-in test will realize

4. Whether the analysis will be functional or piece part

5. Criteria to be considered (mission abort, safety, maintenance,

etc.)

6. System for uniquely identifying parts or functions

7. Severity category definitions

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8.6 WT HIERARCHY FOR FMEA

The WTS can be classified into two main categories according to

pitch control, constant and variable pitch. Similarly, according to speed, it is

classified into constant and variable speed turbines. The WT consists of rotor

system, gear system, brake system, generator system, hydraulics system,

electrical system etc as shown in Figure 1.3

A system FMEA is made up of various systems. A FMEA sub

system is a subset of a system FMEA. A component FMEA is a sub set of sub

system. In this research, the WT sub assemblies are considered for evaluation.

The main parts of the rotor considered for the analysis are hub, main bearing,

blade and rotor control system.

Figure 8.1 WT Hierarchy for FMEA

The failure of the WTS is defined through three stages as shown in

Figure 8.1. The WT is placed in the first stage (stage I); whereas, WT sub

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assemblies and parts are in second (stage II) and third (stage III) levels

respectively.

In rotor system the blades, hub, main bearing and nose cone were

considered and in gear box system the low speed shaft, high speed shaft, input

shaft, input pinion, bearings, gears, cooling system, sensors and temperature

control were considered In brake system the important parts are brake pad,

spring, brake shoe and mechanical control. In generator system the important

components are the contactors, shaft, bearing, flexible coupling, generator

cooling system, top terminal box and generator control at control panel.

The key yaw system elements taken into account of analysis are

yaw motor, yaw gear, yaw planetary, yaw bearing, yaw brake, drives,

controllers and wind vane. The important rotor hydraulic system assembly

constituents are accumulator, hydraulic cylinder, hydraulic pump, oil

reservoir, pump and hydraulic transmission system. The hydraulic brake

system with hydraulic pump, oil reservoir and hydraulic brake control, brake

solenoid, accumulator and hydraulic cylinder were considered.

The Table 8.1 shows the severity, occurrence and detection rating

scale for WT in high uncertain wind area of Muppandal site.

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Tabl

e 8.

1 Se

veri

ty, O

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e an

d D

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ale

for

WT

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Det

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Ver

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igh

Ver

y H

igh

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ecu

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will

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Hig

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that

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that

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curr

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ill d

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failu

re m

ode.

137

Tab

le 8

.1 (C

ontin

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Scal

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Seve

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Det

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Low

Low

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138

8.7 FMEA

A failure mode is defined as the behavior in which a system, sub

assembly, part etc. could potentially fail to meet the design intent. FMEA is

an important reliability tool to explore the ways or modes in which each

system element can fail and assesses the consequences of each of these

failures. Failures are prioritized according to how serious their consequences

are, how frequently they occur and how easily they can be detected.

The purpose of the FMEA of wind turbines is to take actions to

eliminate or reduce failures, starting with the highest-priority ones. Risk

Priority Number (RPN) is used to evaluate the risk associated with the

potential problems and it helps to identify the critical failure modes associated

with the design or process. The RPN can be calculated by using

Equation (8.1).

RPN = Severity (S) X Occurrence (O) X Detection (D) (8.1)

The severity, occurrence and detection are rated for 10 point scales.

Therefore, RPN varies from 1 to1000, that is 1 represents absolute best and

1000 represents the worst.

8.7.1 Severity

The Severity (S) is used to estimate the most serious effect of

failures. This is the severity rating, or S. Severity is rated on a scale from 1 to

10 in this research, based on the seriousness required for the WT components,

where 1 means ‘No Effect’ and 10 means ‘Failure’ affecting the system

operation and safety without any warning.

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8.7.2 Occurrence

The Occurrence (O) is used to estimate the likelihood that the cause,

if it occurs, will produce the failure mode and its particular effect and it is also

called as likelihood. This rating estimates the probability of failure occurring

for that reason during the lifetime of the system. Occurrence is usually rated

on a scale from 1 to 10, where 1 is extremely unlikely and 10 is inevitable. A

failure cause is used to identify the design weakness of the system that may

result in a failure.

8.7.3 Detection

The Detection (D) is termed as the effectiveness. It is used to

estimate the effectiveness of the controls to prevent or detect the cause or

failure mode. The assumption is that the cause has occurred. This rating

estimates how well the controls can detect either the cause or its failure mode

after they have occurred, but, before the customer is affected. Detection is

usually rated on a scale from 1 to 10, where 1 means the control is absolutely

certain to detect the problem and 10 means the control is certain not to detect

the problem values. The inspection is carried out by visual, auditive and

olfactive. The maintenance method followed in wind farms are time based

preventive maintenance.

8.8 FMEA FOR WT

In this research, the twenty numbers of 250 kW WT and its seven

major sub assemblies such as rotor, gear box, brake system, generator, yaw

system, rotor hydraulic control system and brake hydraulic system were

considered. The sub assemblies considered for FMEA analysis include parts

and its control system.

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8.8.1 Failure Modes Considered For WT

The twenty failure modes considered for 250 kW WT and its sub

assemblies.

Mechanical

1. Misalignment of shaft

2. Fracture failure

3. Bed bolt shear

4. Friction

5. Wear out

6. Over heat

7. Brittle seals

8. Body crack

9. Material

10. Lightening

11. Flutter

12. Impact load

Electrical

1. Voltage fluctuation

2. Sensor fault

3. Control panel fault

4. Irregular power supply and output

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5. Cable fault

6. Control system fault

8.8.2 Failure Mechanism of WT

1. Prolonged excessive vibration of individual components

2. Manufacturing defect in gear box casing

3. Fatigue and dynamic load

4. Excessive wear

5. Contamination

6. High voltage

7. Prolonged high temperature

8.8.3 Failure Effect of WT

1. Loss of power generation

2. Failure of WT components

3. Shutdown

4. Generator burn out

5. Control panel failure

6. Oil pitting

7. Tip open when running

8. Yaw motor rapture

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8.9 RANKING OF EFFECTS OF WT BY INITIAL SEVERITY

The average severity of twenty numbers of 250 kW WT is from 5 to

8 as shown in Figure 8.2.

Figure 8.2 Ranking of Effects of WT by Initial Severity

The maximum severity of 8 is allotted to rotor. The severity for

rotor is high because if there is any mild crack or damage caused in the blade

or tiny fault of the main bearing and rotor shaft, which will reduce the

generation significantly. Next to rotor, the generator and gear box have a

severity of 7. The present design of the gear box of the WT has a short life

due to improper material, improper lubrication system and inefficient cooling

system. The electrical fault in the generator is somewhat predictable than

mechanical fault in the rotor. But the generators are the second key element

for power generation. The severity allotted for yaw system, brake system and

rotor hydraulic system is 6 based on their individual component seriousness to

the system. The low severity is given for hydraulic control of the brake

system as 5, because, once hydraulic control fails, the mechanical and

electrical actuated brake will take care.

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8.10 RANKING OF CAUSES OF WT BY INITIAL OCCURRENCE

The occurrence of individual component of the WT is assigned

according to the failure rates as specified range in the Table 8.1. The high

frequency of failures of yaw system, hydraulic system of rotor and generator

had brought the occurrence rate to 6 as shown in Figure 8.3. The reason for

the variation of occurrence from 4 to 6 is that the single failure mode

probability of failure occurs in between 0.0001 and 0.0004. In the constant

pitch and constant speed WT, the average failure rates of the yaw, hydraulic

control of rotor and generator lie between 0.00038 failures per hour,

0.000354and 0.000313. The failure rate per hour of gear box system is in

between 0.0000103 and 0.000467. The average failure rate for 20 wind

turbines per hour of gear box system is 0.00286. Therefore, the occurrence

allocated as 5. The failure rate per hour of hydraulic control of brake system

is in between 0.000122 and 0.000398 and its average failure rate is 0.000259.

The lowest failure rate per hour in the rotor is obtained as 0.0001833 and

brake is 0.000191. The occurrence value is assigned as 4.

Figure 8.3 Ranking of causes of WT by Initial Occurrence

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8.11 RANKING OF CAUSES OF WT BY INITIAL DETECTION

The detection is rated by the scales as rated on the Table 8.1. The

detection refers to the likelihood of detecting a root cause before a failure can

occur. It enforces requirement of any measurement device. The brake system

of the WT has poor level of design control of detection as indicated in Figure

8.4.since it is difficult to identify the wear of brake pad. Therefore, the highest

detection value assigned for brake system as 9. In the same way, the fault

detection is unfeasible for the yaw system and all hydraulic rotor and

hydraulic system of brake controls. The detection value assigned to them is 7.

The brake system, yaw system and hydraulic control of brake and rotor

system require urgent action to detect the failure mode before it occurs. The

gear system has a detection device such as temperature sensor, vibration

sensor and gear oil level indicator. The rotor and generator have a good

detection device.

Figure 8.4 Ranking of causes of WT by Initial Detection

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8.12 CAUSES RANKED BY INITIAL RPN

The causes ranked by initial RPN is shown in Figure 8.5 and

Appendix 2. It shows the relative risk of the WT components which have high

priority. RPN is calculated by multiplying the Severity by the Occurrence by

the Detection of the risk. In this study, the highest value of RPN obtained is

252 for yaw system and the lowest is 160 for rotor. It is noted that the most of

the failures are contributed by yaw, hydraulic control of rotor, brake and gear.

The RPN of yaw system and rotor hydraulic control are high and this is due to

the high severity, absence of detective mechanisms and increased frequency

of failures over a period of time. The brake system and gear system have a

RPN of 216 and 210 respectively. Although the rotor has high severity, the

RPN is less (168) due to low detection and occurrence. But, rotor as a whole

system of hydraulic and mechanical system, the risks are extremely large. The

generator has high severity and occurrence, but it has low RPN (168) because

of diminutive detection value.

Figure 8.5 Ranking of causes of WT by Initial RPN

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8.13 MODIFICATION AND ANALYSIS OF WT COMPONENTS

The modification and redesign is implemented on trail basis in a

specific location number 10, where the failures of individual components are

high. The failure of yaw system during high wind causes reduction in power

generation and imparts high thrust on the blade which causes major failure.

To reduce the yaw failure an active redundant soft yaw system is fixed. The

soft yaw system provides smooth start and stop without noise and yaw brakes

are totally eliminated. Initially, the numbers of failures were 6 to 10 per year

over a span of fifteen years. After implementation, the failures come down by

20 to 33 % when compared with conventional yaw system. The redundant

yaw system not only affords smoothness to the system but also eliminates the

risks of the rotor system.

The gear oil test facility is incorporated in the site to test kinematic

viscosity, density, flash point and metal content. The gear oil analysis is

performed on site laboratories so that the information about the future damage

and the correct time for oil change are easily revealed. The vibration meter is

fixed on various parts of WT components like shaft, gear, yaw system,

generator and various positions of the tower. Based on the outcome of the

vibration meter, the bolts are adequately torque to make the structure rigid for

increasing the span of MTTF. The pressure gauges have been set in rotor and

brake hydraulic system to gauge the system pressure to ensure smooth

operations of the turbine. To measure the rate and extend of wear of the brake

pads, infrared position sensor was installed at appropriate place. This helps

for planning predictive maintenance instead of breakdown maintenance.

8.14 RANKING OF CAUSES BY INITIAL AND REVISED RPN

The modification of WT components are shown in Appendix 3 and

Figure 8.6. After modification, the RPN varies from 96 to 150 indicating

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reasonable improvement in reducing the relative risks considerably. The

reduction of occurrence by diminishing the failure rate and the reduction of

detection by augmenting new measurement systems can trim down the RPN

to some extent. In two other important components, the rotor and generator,

with a reduction of failure rate and high detection rate, the net RPN is

relatively low though they have high severity indices.

Figure 8.6 Ranking of causes of WT by Initial and Revised RPN

The gear system is ranked third in the RPN causes. It is proved that

the gear system is highly critical and it requires best control system. The

hydraulic control of brake has the least RPN of 175.

8.15 CAUSES RANKED BY RPN % REDUCTION

The improvements in the components RPN in term of percentage

are represented in the graph shown in Figure 8.7. It can be observed that it is

maximum in yaw system as 52% and lowest of 40 %in the rotor. The highest

reduction of RPN in yaw is due to the provision of active standby redundant

yaw system. The installation of the infrared position sensor in brake system

resulted in 50% reduction in RPN. Simple periodic maintenance and

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predictive maintenance of shaft, sleeve ring, bearing and cooling system of

generator brings the RPN reduction. The new design, pressure regulation

system brought the improvements in hydraulic controls of brake and rotor.

Figure 8.7 Causes Ranked by RPN % Reduction

8.16 CRITICALITY ANALYSIS USING SEVERITY AND

OCCURRENCE OF WT

The Criticality analysis is a procedure by which each potential

failure mode is ranked according to the combined influence of severity and

probability of occurrence. The criticality value is obtained by the product of

severity and occurrence. The criticality of failure is independent of detection

because detection is failure prevention through design.

The criticality index is the quantitative measure of criticality of the

failure mode that combines the probability of the failure mode’s occurrence

with its severity ranking. For each severity categorization, the criticality

index is calculated for each of the corresponding failure. The result is a rank

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ordering of failure modes within each severity classifications. The index is

defined as

k k p k pC t (8.2)

Where

Ck = Critical index of failure mode ‘k’

kp = Fraction of the component p’s failure having failure

mode k given component ‘p’ has failed

k = Conditional probability that failure modes ‘k’ will

result in the identified failure effect

p = Failure Rate of component ‘p’

t = Duration of time in analysis

The generator is considered as a highly critical component and it has

a criticality value of 42 as shown in Figure 8.8.

Figure 8.8 Criticality Ranking of WT

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8.16.2 Initial Occurrence and Severity matrix

Subsequently, yaw is placed in the second level of criticality,

because of the high frequency of failure and moderate severity. After that, the

gear system and rotor system have high criticality ranks due to the high

severity and very moderate occurrence. The brake has low criticality because

of low severity. The criticality is trivial in hydraulic controls of brake and

rotor.

Figure 8.9 Initial Occurrence and Severity matrix

8.15.1 Revised Occurrence and Severity matrix

The Figure 8.8 compares the failure modes by way of a Criticality

ranking. The occurrence / severity matrix shown in Figure 8.9 and 8.10

exposes that all the seven components of wind turbines are having very high

priority causes because, they are above the high priority line. It implies that

the immediate rectification action is required to solve the crisis. The yaw

(coordinates 6, 6) and the generator (co ordinates 7, 6) are fall on very high

priority causes. Both rotor and brake hydraulic systems require action slightly

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lower than the other priority causes. After implementation, the occurrence is

reduced with less investment and priority is brought nearer to the high priority

line as shown in Figure 8.10. But, the graph exposes that it requires some

serious design modifications.

Figure 8.10 Revised Occurrence and Severity matrix

Figure 8.11 Yearly generation at redesign implemented location

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After implementation, the generation obtained in the year 2012 is

5,99,874kW.hr, which is very high as shown in Figure 8.11, when compared

with the past generations over 15 years.

The FMEA study carried out successfully to twenty numbers of

constant speed and constant pitch 250 kW wind turbines over a span of 15

years by using Xfmea software. This study has a prospective of improving the

reliability and availability of wind turbines by modifying and redesigning the

components, redundancy system, effective monitoring mechanism and proper

preventive maintenance. From the FMEA, it is evident that immediate actions

are required in generator, yaw system, rotor system and gear system. The

generator is constructed overhanging and this leads to repeated misalignment

of shaft due to vibrations. The high uncertainty wind causes voltage

fluctuations and the inadequate performance of sleeve ring, carbon brush and

bearing diminish the reliability and availability of the generator. The failures

in the generator are early detected and restricted with help of the continuous

monitoring of vibration meter. It is inevitable that the yaw system requires

redundant system to increase its performance. The improvement in lubrication

system and efficient cooling system makes 47% improvement in RPN of the

gear box.

The rotor is overstressed due to the frequent failure of yaw

mechanism. It is found that the smooth and efficient yaw mechanism with

active redundancy reduces 52 % of RPN. The crack on the blade edge and

tip, improper blade material and fatigue load brought down the rotor

efficiency. It is necessary to set up design control for detecting the brake

failure. The frequent inspection of the brake pad with a help of infrared

position sensor, reduces failure to 50%. It is suggested that the yaw system

and brake system should be inspected in weekly schedule and preventive

maintenance be carried out effectively. The FMEA has the potential to

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improve the reliability of WTSs at high uncertain wind environment, where

reliability plays an important role. Furthermore, it is believed that in time, it

will play a key function in the monitoring and maintenance to make wind

power generation a more efficient.

8.17 SUMMARY

The step-by-step Failure modes and effects analysis is carried out in

this chapter for identifying all potential problems of WT and its sub assembly.

The RPN is computed for existing WT based on severity, occurrence and

detection. After modification, the revised RPN is obtained. The reduction of

occurrence by diminishing the failure rate and the reduction of detection by

augmenting new measurement systems can trim down the RPN to some

extent. The revised RPN indicates 40 to 52 percent improvement in reducing

the relative risks considerably. The criticality index derived from FMEA is

calculated for each consequent failure.