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1996-2009 Operation Technology, Inc. Workshop Notes: Reliability Assessment
Reliability Assessment
Slide 2 1996-2009 Operation Technology, Inc. Workshop Notes: Reliability Assessment
Power System Reliability
Analysis
Reliability
The probability of a system performing its function adequately for the period of time and operation conditions intended
Adequacy
Sufficient facilities within the system to satisfy customer demand
Security
Ability of the system to respond favorably to disturbances arising within that system
Slide 3 1996-2009 Operation Technology, Inc. Workshop Notes: Reliability Assessment
Power System Reliability
Analysis
Used in system planning and operation
Reliability Assessment for:
Generation station and generation capacity
Composite generation and transmission system
Distribution system
Substation and switching stations
Protection system
Slide 4 1996-2009 Operation Technology, Inc. Workshop Notes: Reliability Assessment
Power System Reliability
Analysis
Various Indices to Measure Reliability
Customer Interruption Frequency
Customer Interruption Duration
Customer Curtailment Power/Energy Not Served
Reliability Worth Study
Minimize Total Cost: Reliability Cost and Consumer Interruption Cost
Slide 5 1996-2009 Operation Technology, Inc. Workshop Notes: Reliability Assessment
Distribution System Reliability
Analysis
Concerned with availability and quality of power
supply at each customers service entrance
Adequacy Analysis
Statistics show that failures in distribution systems
contribute as much as 90% towards the
unavailability of supply to a load as compared with
other parts of electric power systems
Slide 6 1996-2009 Operation Technology, Inc. Workshop Notes: Reliability Assessment
Reliability Analysis Using ETAP
Assesses distribution system reliability level for radial and looped systems with a very efficient
algorithm
Considers single and double contingencies
Assesses reliability level for system and each load point based on component failure model and
system configuration
Performs sensitivity analysis to identify the optimal location to make greatest improvement on system
reliability at minimum cost
Slide 7 1996-2009 Operation Technology, Inc. Workshop Notes: Reliability Assessment
Single & Double Contingencies
Consider a system with two or more parallel branches.
In case of Single Contingency Analysis failure of only one branch at a time is considered.
In case of Double Contingency Analysis simultaneous failure of two branches at a time is also considered in addition to failure of one branch at a time.
Slide 8 1996-2009 Operation Technology, Inc. Workshop Notes: Reliability Assessment
Single & Double Contingency
Example
Single Contingency:
Failure of T5 or T6 at a time is considered.
Double Contingency
Simultaneous failure of T5 & T6 and failure of T5 or T6 at a time is considered.
Slide 9 1996-2009 Operation Technology, Inc. Workshop Notes: Reliability Assessment
Component Model
A - Active Failure Rate (No of Failures/Year)
Causes the operation of the protection devices aroundthe failed component, i.e. a short-circuit fault
Failed component itself (and those components that aredirectly connected to this failed component) restores to
service after repair or replacement
P- Passive Failure Rate (No of Failures/Year)
Does not cause the operation of protection around thefailed component, i.e. an open circuit fault
Failed component itself restores to service after repairor replacement
Slide 10 1996-2009 Operation Technology, Inc. Workshop Notes: Reliability Assessment
Component Model
Mean Time To Repair in hours (MTTR)
Time required to repair a component outage
and/or restore the system to its normal operating
state
Mean Repair Rate (No of repairs per year) ( )
= 8760/MTTR
Mean Time To Failure (years) (MTTF):
MTTF = 1.0/( A+ P)
Slide 11 1996-2009 Operation Technology, Inc. Workshop Notes: Reliability Assessment
Component Model
Mean Time Between Failure (Year) (MTBF)
MTBF = MTTF + MTTR/8760
Forced Outage Rate (Unavailability) (FOR)
FOR = MTTR/(MTBF*8760)
Switching Time
Time in hours for isolating a fault occurred at the component
Assume that CB/Fuse trip a fault instantaneously
Time for replacing a failed element by a spare one,in hours rP
Slide 12 1996-2009 Operation Technology, Inc. Workshop Notes: Reliability Assessment
Single-Component Concepts
Two-State Model
A two-state up/down representation is used for the operation/repair cycle of a component (such
as lines, cables, transformers, breakers, fuses,
switches, loads and busbars)
DOWN
= ( A+ P)Up
Down
MTTF
..
..
MTTF
MTTR MTTR
UP
Slide 13 1996-2009 Operation Technology, Inc. Workshop Notes: Reliability Assessment
Model for Components in
Series/Parallel
Two Components in Series
sys
rr
sys
rrrrsysr
2211)
22)(
11(
2211
1, r1
Component 1 Component 2
21sys
2, r2
Slide 14 1996-2009 Operation Technology, Inc. Workshop Notes: Reliability Assessment
Model for Components in
Series/Parallel
Two Components in Parallel
1, r1
Component 1
Component 2
2, r2
)21
(21
22111
)21
(21 rr
rr
rrsys
21
21rr
rr
sysr
Slide 15 1996-2009 Operation Technology, Inc. Workshop Notes: Reliability Assessment
System Modeling
Fault Current Interruption
Only overcurrent protection devices (CB and fuse) can interrupt fault current
Fault current interruption is assumed to be instantaneous
Assumed to have no effect on components with multiple source connection and isolated from
fault by CB/fuses
Slide 16 1996-2009 Operation Technology, Inc. Workshop Notes: Reliability Assessment
System Modeling
Fault Isolation
All switching devices can isolate faults. CBs and fuses isolate fault instantaneously
Switches isolate fault at switching time of the faulted component
Switching time for a load is equal to that of the closest component
Slide 17 1996-2009 Operation Technology, Inc. Workshop Notes: Reliability Assessment
System Modeling
Normally Open Tie: Open tie PD can be closed
(switching time) to provide back up power
Two terminal buses should be energized
Can have several PDs connected in series and with one or more open
Slide 18 1996-2009 Operation Technology, Inc. Workshop Notes: Reliability Assessment
Library for Reliability Analysis
Component Reliability
Data for each type of component - transformer, bus, line, etc.
Active Failure Rate
Passive Failure Rate
Repair Time
Switching Time
Replacement Time
Typical data from IEEE Standard
Slide 19 1996-2009 Operation Technology, Inc. Workshop Notes: Reliability Assessment
Library for Reliability Analysis
Sector Customer Interruption Cost
Standard Industrial Classification (SIC) is used to divide customers into seven categories of
large user, industrial, commercial, agriculture,
residential, government & institutions and office
& buildings.
Sector Customer Damage Functions (SCDF) are interruption costs for several discrete outage
durations.
Slide 20 1996-2009 Operation Technology, Inc. Workshop Notes: Reliability Assessment
Library for Reliability Analysis
A log-log interpolation of the cost data is used where the interruption duration lies between two
separate times.
If fault duration is outside the range, a linear extrapolation with the same slope as that
between the two largest durations are used to
calculate the interruption cost.
Slide 21 1996-2009 Operation Technology, Inc. Workshop Notes: Reliability Assessment
Distribution System Reliability
Indices
Nejjei ,
Average Failure Rate at Load Point i, i(f/yr)
e,j - The average failure rate of element j (or element
combination j, such as double contingency).
Ne - The total number of the elements whose faults will
interrupt load point i.
Annual Outage Duration at Load Point i, Ui(hr/yr)
Nejijrjei
U,
rij --Failure duration at load point i due to a failed element j.
Slide 22 1996-2009 Operation Technology, Inc. Workshop Notes: Reliability Assessment
Distribution System Reliability
Indices Average Outage Duration at Load Point, ri(hr)
iiU
ir /
Expected Energy Not Supplied Index at Load Point, EENSi (MWhr/yr)
iUiP
iEENS Pi - the average load of load point i.
Expected Interruption Cost Index at Load Point, ECOSTi(k$/yr)
Nejjeij
rfiP
iECOST
,)(
The EENS and ECOST for a bus are calculated based on loads that
are directly connected to that bus due to the outage of that bus.
Where f(rij) is the SCDF.
Slide 23 1996-2009 Operation Technology, Inc. Workshop Notes: Reliability Assessment
Distribution System Reliability
Indices Interrupted Energy Assessment Rate Index at Load Point,
IEARi ($/kWhr)
System Average Interruption Frequency Index, SAIFI
(f/customer.yr)
System Average Interruption Duration Index, SAIDI
(hr/customer.yr)
iEENS
iECOST
iIEAR
iN
iNiSAIFI
servedcustomer ofnumber Total onsinterrupticustomer ofnumber Total
Where N is the number of customers at load point i
iN
iNi
USAIDI
servedcustomer ofnumber Totaldurationson interrupticustomer of Sum
Slide 24 1996-2009 Operation Technology, Inc. Workshop Notes: Reliability Assessment
Distribution System Reliability
Indices Customer Average Interruption Duration Index,
CAIDI(hr/customer interruption)
Average Service Availability Index, ASAI(pu)
iiN
iNi
UCAIDI
onsinterrupticustomerofnumberTotalsdurationoninterrupticustomerofSum
8760
8760
demanded hoursCustomer service available ofhoursCustomer
iN
iUi
Ni
N
ASAI
Where 8760 is the number of hours in a calendar year
Slide 25 1996-2009 Operation Technology, Inc. Workshop Notes: Reliability Assessment
Distribution System Reliability
Indices
Average Service Unavailability Index, ASUI(pu)
System Expected Energy Not Supplied Index, EENS (MWhr/yr)
ASAIASUI 1
EENS = Total energy not supplied by the system = EENSi
System Expected Interruption Cost Index, ECOST(k$/yr)
ECOST = ECOSTi
Slide 26 1996-2009 Operation Technology, Inc. Workshop Notes: Reliability Assessment
Distribution System Reliability
Indices
Average Energy Not Supplied Index, AENS
(MWhr/customer.yr)
System Interrupted Energy Assessment Rate Index,
IEAR($/kWhr)
iN
iEENS
AENSservedcustomer ofnumber Total
system by the suppliednot energy Total
EENSECOSTIEAR
RA Example 1 Using ETAP
1996-2009 Operation Technology, Inc. Workshop Notes: Reliability Assessment Slide 27
RA Example 1 Using ETAP
Data:
Active failure rate for breakers: a = 0.003 failure/year
Passive failure rate for breakers: p = 0.002
failure/year
Failure rate for Bus, Utility: = 0.001 failure/year
MTTR for breakers: 30 hours
MTTR for buses, utility: 2 hours
1996-2009 Operation Technology, Inc. Workshop Notes: Reliability Assessment Slide 28
RA Example 1 Using ETAP
Reliability Indices at LP1:
Failure rate for the main bus:
CB1 fails actively OR passively CB2 and CB3 fail actively Utility fails Main bus itself fails
yearfailure
BusMainUtilityCBaCBaCBpaLP
/013.0
001.0001.0003.0003.0005.0
3211
The main bus would be de-energized if:
1996-2009 Operation Technology, Inc. Workshop Notes: Reliability Assessment Slide 29
RA Example 1 Using ETAP
Annual unavailability for the main bus:
yearhour
MTTR
MTTRMTTR
MTTRMTTRU
BusMainBusMain
UtilityUtilityCBaCB
CBaCBCBpaCBLP
/334.0
001.02001.02003.030003.030005.030
33
22111
Time to replace the main bus:
hoursU
rLP 692.25013.0
334.01
1996-2009 Operation Technology, Inc. Workshop Notes: Reliability Assessment Slide 30
RA Example 2 Using ETAP
Results for the Single Contingency case
1996-2009 Operation Technology, Inc. Workshop Notes: Reliability Assessment Slide 31
RA Example 2 Using ETAP
Results for the Double Contingency case
1996-2009 Operation Technology, Inc. Workshop Notes: Reliability Assessment Slide 32
RA Example 2 Using ETAP
Calculations for the Double Contingency case:
For simplicity for hand calculations:
Failure rates for the breakers connecting the transformers to the buses are taken to be zero
Failure rates of the two transformers are taken to be 1; MTTR = 200 hr.
Therefore the failure rate at Bus 2 due to double contingency:
yearfailuresrr
rr
double /0436681.0
8760
200120011
8760
)200200(11
87601
8760
)(
2211
2121
1996-2009 Operation Technology, Inc. Workshop Notes: Reliability Assessment Slide 33
RA Example 2 Using ETAP
Calculations for the Double Contingency case:
Failure rate for the single contingency case:
Therefore total failure rate at Bus 2 :
yearfailures
gledoubleBus
/0546681.0011.00436681.0
sin2
yearfailures
A
CBBusMainBus
P
CB
A
CBUgle
/011.0
62111sin
1996-2009 Operation Technology, Inc. Workshop Notes: Reliability Assessment Slide 34
RA Example 3 Using ETAP
1996-2009 Operation Technology, Inc. Workshop Notes: Reliability Assessment Slide 35