Reliability week 17 and 18

Define reliability

The probaiility that a [sub assembly/component/system] will meet its required function under stated conditions for a specified period of time.

Define a repairable system

A system that can be fixed/restored to working order after a failure through repair/maintenance like part replacements or adjustments (include major components like gearboxes, generators, pitch systems, assemblies of sub assemblies)

Define a non repairable system

one that cannot be repaired once it fails and must be replaced entirely after a single failure (often at component level, like batteries, bearings, power electronics etc.)

What is the failure rate?

THe rate at which failures occur within a specific period (per unit time). Measures as the number of failures per turbine per operating period, (usually a yea) or failures per MWh

What does failure rate indicate?

How often critical components within a wind turbine are likely to fail and need repair or replacementWh

What is failure intensity?

Related to the severity and impact of the failures, such as downtime and costs assocaited with failures, uncluding repair costs and lost revenue. describes the reliability or failure rate over time

What is wind turbine?

the total amount of time that a wind turbine does not operate due to a failure

What is the failure intesnity visualised as?

A bathtub curve

What is the bathtub curve?

A graphical representation of reliability showing failure rate over time. Failure rate is in the middle of the graph when failure intensity is constant

What is the equation for the failure rate ?

$\lambda = \frac{1}{MTBF}$ MTBF is mean time between failures

What is the equation for the repair rate?

$\mu=\frac{1}{MTTR}$ MTTR is mean time to repair

What does the operability time graph look like?

MTBF is between start and the end of the first failure when its operable again

MTTF is from start to the beginning of first failure

LD is between start of failure to the MTTR start

MTTR is from somewhere between the start and end of the failure where LD ends , to the end of the failure

What is LD

That is logistial delays

What is MTBF

Mean time before failure

What is MTTF

Mean time to failure

What is LD

Logistical delays

What is MTTR

Mean time to repair

What is the availability?

The fraction of the time ina year that the wind turbine can generator electricity

What is the usual availability of an onshore WT?

95-97%

What is the usual lifetime of an onshore WT?

20 - 30 years

What is the operational availability (technical)

$$A_{op}=\frac{MTTF}{MTBF}=\frac{MTBF-MTTR-LD}{MTBF}=1-\frac{\lambda}{\mu}-LD.\lambda$$

What is the manufacturers availability? (commercial)

$$A_m = \frac{MTBF-MTTR}{MTBF}=1-\frac{\frac{1}{\mu}}{\frac{1}{\lambda}}=1-\frac{\lambda}{\mu}$$

Which is larger, the operational availability or the manufactueres availabilty?

The manufacturers because the logistical delays arent considered in the manufacturers

What are examples of logistical delays?

• response time, if the turbine is offshore the maintenance team will take time to assess and respond to the failure

• Spare parts availability - if a part failed, obtaining the replacement may take time, could be from specialised manufacturers

• Weather conditions - adverse weather can delay repair actions as repair crews and equipment need to reach the wind turbine

What is the capacity factor?

This is the annual energy production as a percentage of the theoretical maximum for the turbine/farm $C = 100\times \frac{AEP}{8760\times P}$ AEP is annual energy production

What is the cost of energy CoE

The current price of a unit of electricity at a specific point in time

What is the LCOE?

The levelized cost of energy LCOE represents the average cost of producing a unit of electricity over the entire lifespan of a power generation project.

What is the difference between the CoE and the LCOE?

The CoE is the current price, while the LCOE considers the total cost spread across a power plant’s entire lifespan

What are the factors for the CoE?

1. Wind regime

2. Energy capture efficiency of the wind turbine generators

3. availability of the wind turbine

4. lifetime of the wind turbine

5. capital costs (CapEx)

6. financing cost

7. operation and maintenance cost (OpEx

How do you calculate LCOE?

ICC - initial capital cost

FCR - fixed charge rate

AOE annual operating expenses

AEP annual net energy production

What is the LLC?

The land lease cost

What is the O and M?

The levelised oepration and maintenance cost

What is the LRC

The levelised replacement/renovation cost

What is teh equation for the OpEx?

The LLC + O&M +LRC

Land lease cost + operation and maintenance cost + levelised replacement cost

What is the trade off when trying to reduce the LCOE?

To reduce the LCOE, installing the WT in deep waters increases their power capacity, but increases their failure rate

What % of costs are O and M for offshore and onshore?

23% of their total investment costs for offshore

5% of costs for onshore

What will cause cost reductions for WT?

Bigger and more efficient turbines, lower capital and operating costs and other tech and economic advancements

How does reliability change with size?

Which components cause the highest failure rate?

Pitch systems 18.7%, 1.18 (failures per unit time or MWh)

protection units 12.1%, 1.17 (failures per unit time or MWh)

Overall lubrication system 11.2%, 1.08 (failures per unit time or MWh)

yaw system 9.1%, 0.88 (failures per unit time or MWh)

Which components of WT have the higher portion of the total dowbtime?

Yaw systems (698h, 23.9%)

pitch system (503h, 17.2%)

power generation system (306h, 10.5%)

electrical grid (265h, 9.1%)

failure rates of generators and converters DFIG vs PMG

In DFIG more failures per year for generator,

In PMG more failures per year in converter

Failure rate by severity for DFIG, PMG, PRC, FRC

What is a FOWF

Floating Ofshore Wind Farm

What is a SCSG type FOWF?

Squirrel cage synchronous generator

this is a wound field synchronous generator or an electrically excited sycnhronous machine where the rotor field is created by the current

requries a field excitation system andused with a gearbox or a medium speed drive train

can control reactive power via excitation

Advantes of a SCSG

No rare earth magnets

good voltage and reactive power contol

mature technology

Challenges of SCSG

More components from excitation system and slip rings/burshes

more maintenance if offshore

gearbox often present which is a major failure risk

What is a PMSP type FOWF?

A permanent magnet synchronous generator, rotor magnetic field comes from permanent magnets. Used in direct drive systems (no gearbox) and requires a fully rated power converter

Advantages of a PMSG?

No gearbox - good for reliability

no excitation system - fewer components

higher efficiency

(best for offshore wind)

Challenges of PMSG

Large and heavey geneartor

depends heavily on power electronics reliability because needs fully rate converter

uses rare earth materials

what are the key components which fail in WTs?

Blade, gearbox, pitch system, yaw system

Causes of failure by fracture and crackings?

• Design defects

• poor quality

• change in process

  • lack of production eceprience

What is the simple payback period analysis?

THis determines the relative economic benefits compres revenue with consts to determine the time in years required to recoup an initial investment $SP=\frac{ICC}{E_a*P_e}$ #

E_a is annual electricity production and Pe is the price for electricity

Benefits of an induction machine with no converter at a fixed speed

• simple robust design

• no brushes of excitation system

• low failure rate of the machine itself

What are the risks of an induction machine with no converter at a fixed speed?

• Requires a gearbox

• no control over speed (higher mech stress)

  • poor grid support capabilit

What are the benefits of an induction machine with a fully rated converter?

• enables variable speed → reduced mechanical loads

• removes need for stric grid synchronisation

  • better control → smoother operation

Risks of an inuction machine with a fully rated converter?

• Fully rated convverter has a high failure rate

• more electrical complexity

• still may require a gearbox

What is the reliability trade off of an induction machine with a fully rated converter?

Decreased mechnaical stress by increased electrical failure risk

benefits of a DFIG?

partially rated converter - 30%

• lower converter losses

• lower cost

widely used so mature tech

What are the risks of the DFIG?

Slip rings and brushes

• wear and require maintenance

• major offshore reliability issues

gearbox required

sensitive to grid faultsWh

What is the realiability trade off for DFIG?

Lower converter losses and more mature tech, but multiple vulnerable conponents, gearbox, slip rings, converter

Benfits of a wound rotor synchronous machine WRSM

Controlled excitation → good voltage / reactive power contorl

no permanent magnets which improves costsWha

What are the risks of a wound rotor synchornous machine?

Excitation system - slip rings and burshes

more components mean more failure points

often used with a gearboxWh

What is the reliability trade off of a wound rotor synchronous machine?

Controllable excitation and no magnets but added electrical complexity reduces offshore reliability

Benefits of a permanent magnet synchronous machine?

• No gearbox

• no excitation system

• fewer moving parts

  • high efficiency

Risks of a permanent magnet synchrnous machine

• fully rated converter required

• large heavy generator

• thermal and structural stresses

  • rare earth material dependency

what is the main failure point for a permanent magnet synchronous generator?

The fully rated power converter

Why is the gearbox a big reliability problem?

Low failure rate but very high repair time, offshore means this is extremely costly

What is the power electronics reliability trade off?

• high failure rate

• easier to replace

• modular