Maintenance and Testing

of
Substation Equipment

PREPARED BY : S.K.MISHRA

1
 Presentation Outline
• Concepts of Maintenance and Testing of
Substation Equipment
• Failure Modes in Electrical Equipment
• Maintenance &Testing of substation equipment
– Transformers /CT/CVT
– Breakers/Isolators
– Hardware
– Protection System
– Earthing/Safety
2
 Substation equipments
• Switching :
– Circuit breakers
– Isolators
• Protection &Measurement
– CT
– CVT
– Relaying
• Transformers and Reactors
• Hard wares : Conductors, connectors, terminals, cables etc
• Earth and earthing devices
• Communication equipment
3
 Objective
• Minimize accidents
• Reduce unplanned shutdowns
• Reduce down time
• Lengthens mean time between failure(MTBF)
• Reduce mean time to repair (MTTR)

4
 Purpose of Maintenance& Testing
• Recognize Factors causing deterioration of
Equipment Conditions
• Establish root cause
• Provide means for correcting them

5
 CONCEPT OF MAINTENANCE
Run to failure Preventive Predictive Proactive

Small Items Subject to Random

Non critical wear Failure
Unlikely to fail Known failure Not subject to RCFA
Redundancy pattern wear FMEA
Maint. Design
Induced
Failure

6
 CONCEPT OF TESTING
ACCEPTANCE Routine
Special testing NDT
TEST Testing

FAT Site testing RLA

Type Test Periodic RCFA
Design test Functional IRT
Com. Overhauling Vibration
acceptance Maintenance Wear Debris
test test etc
Technologies Over the Age
NDT

Vibration Wear
IRT Acoustic
Debris

Oil
Corona Partial Motor
Analysis
Discharge Current
Specific
Air Born signature
test
Ultrasonic

8
Failure mechanism in electrical system.

• Root cause of all electrical failures are

– Moisture(The equipment must be dry)
– Tightness (there should not be any loose part)
– Temperature ( the equipment must be cool)

9
Failure Modes(Insulation System)
Mechanical Stress Switching
Temperature Hot Temp coefficient is negative
Spots Weak spot > More
current>more hot> more
current
Environment Moisture , chemical, dirt, oil
Electrical stress Corona , surge, partial
discharge
Thermal Aging Arrhenius Law ½ for every 10
Degree c more temp

10
 Failure Modes(Hardwares)
Mechanical Stress Switching
Temperature Hot Temp coefficient is negative
Spots Weak spot > More
current>more hot> more
current
Environment Moisture , chemical, dirt, oil
Electrical stress Corona , surge, partial
discharge
Thermal Aging Arrhenius Law ½ for every 10
Degree c more temp

11
 Failure Modes
Equipment
Failure

Damage Deterioration
• Overhead Insulation system
Primary, service, Stress Grading system
secondary lnes
• Poles
• Conductors
• Insulators and
associated
hardwares
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Conductor Corrosion

Steel
Wire 2. Rusting ,
Loss of
coating
Zinc
3. Wet Coating
+drying 1. Sacrificial
migration migration Zn
Aluminum
to Al

13
Conductor Corrosion

14
Insulator Failure

Drift from Cooling

Tower Falls On
switch yard

15
 Failures
Failure:
•Grid Failure due to tripping of 220 KV
Switch yard
•Failure of ICT-1R , tripping of 220 KV Bus,
Partial Grid failure
•Flash Over across Insulators in 220 KV and
400 KV switchyard.
•Flashover and tracking most severe during
winter and foggy weather condition.
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 Failures
• Plumes from the cooling tower condenses and fall on the
switch yard as drizzle during winter season. The effect is more
severe on 220 kV switchyard and Generator Transformer and
Station Transformer bay.
• Drizzle intensifies during winter &foggy weather condition.
•  
• Drift from Cooling Tower Falls On switch yard
• Drift
•
The salt and pollutants carried by drift of cooling tower cause
wetting of the insulators. The effective creepage distance of
insulators reduces.
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ISOLATOR FINGER
EQUALISING RING
String Insulator Flashover
 220- KV UNIT # CT -Y- PH TOP DOME WAS HOT.CT OIL HAS ON
TESTING WAS FOUND VIOLATING ALL DGA NORM.REPLACED
Thermal image Visual image
*>150.0°C
9CT -R- PH

140.0
120.0 Max
93.5
100.0 9CT -Y-PH
80.0
60.0
40.0
20.0 Max
149.4
0.0

*<0.0°C
 GT-2 220 KV BUSHING HOT SPOT.THE BUSHING WERE FOUND
DEFECTIVE AND WAS REPLACED

*>100.0°C GT2YPH BUSHING

100.0
90.0
Max
80.0
235.0 GT2 BPH BUSHING
70.0
60.0
50.0
Max
40.0
156.0
30.0
20.0

*<14.3°C
MONITORING OF OIL LEVELOF
CONCERVATOR
 Key Activities
• Electrical preventive maintenance &testing.
• Electrical repairs.
• Analysis of failures or root cause analysis.
• Trending of maintenance and testing data

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Systematic approach to maintenance

Failure cause

Establish
Proximate cause to failure Presence of
systemic causes

Compare to past Compare to similar Compare to similar

failure equipment conditions

26
Systematic approach to maintenance

If problem is generic

Systematic
Other utility information approach to
root cause
analysis

If Manufacturer
If plant specific If location specific
specific

27
Systematic approach to maintenance

If problem is Plant specific

Systematic
approach to root
cause analysis
Attributes

To unique system Environmental

Operation factors
design Factors

28
Maintenance and Testing
of
Transformer

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Testing of Oil filled Transformer
 Objective of Testing
• Control transformer heat
• inspect and maintain auxiliary device
• test and maintain insulating system
• maintain bushing
• maintain oil
• maintain protective devices
 Transformer testing
• IR and PI value
• Moisture content
• Ratio test
• resistance test
• magnetization current
• impedance voltage
• tan delta and capacitance of transformer
• tan delta and capacitance of bushing
• earth resistance
• oil testing
• recovery voltage test
 Power factor testing
• Capacitance between H&L can
be represented by -CHG, CHL, CLG
• IN an Ideal capacitor the current and
voltage are in qudrature I.e I r = 0.
Dielectric loss angle is 0.
• Power factor is cos =sin =Ir/I.
• Dissipation factor = Tan =Ir/Ic.
• For very low loss angle PF=DF
 Significance of PF and DF
• Change in capacitance indicates:
– presence of moisture layer.
– Short circuit.
– Open circuit in the capacitance network.
• Change in Tan delta indicates:
– chemical deterioration due to time and temperature. Localized
overheating.
– Contamination by moisture, carbon deposits, bad oil, dirt and other
chemical
– several leakage through cracks and surface.
– Ionization
 Interpretation
• Between measurement of same unit after
successive interval of time .
• Between measurements on duplicate units
or similar parts under same environment
condition.
• Testing at different test voltage .
– If loss factor increases sharply above certain
voltage it indicates ionization in voids.
 Typical values
• Equipment loss factor AT 20 C
• oil filled transformers
– new (high voltage ) 0.25 to 1 %
– 15 year old )do 0.75 to 1.5 %
– low voltage 1.5 to 5 %
– bushings> 110 kv 0.3 to 3 %
– OIL FILLED< 110 KV 1.5 TO 4%
– SOLID OR DRY 3 TO 10%
 INFLUENCES on Testing
• TEMPERATURE
– Loss increases with temperature .
Temperature correction must be applied to 20
c before making any interpretation .
• Surface leakage :
– specimen must be clean and dry before
application of voltage.
 Influences on Testing
• Humidity :
– if relative humidity is <50 %. A thin layer of surface film of moisture
forms and dissipates rapidly due very low volume absorption of
porcelain.
– Best time of testing : late morning through afternoon , dry and
sunny condition
• Electrostatic Interference :
– due to capacitive coupling between energized equipment and
specimen. It is very difficult to evaluate nature of influence.
– Typical symptom : negative loss factor
 Case study -1
Thermal imaging of Bushing
 Case study : GT Bushing .
• Tan delta GRAPH
1.4
1.2
1
0.8 R
Y
0.6
B
0.4
0.2
0
TAN DELTA
 CAPACITANCE TESTING
CAPACITANCE GRAPH
490
500
450
389 391
400
350
300 R
250 Y
200 B
150
100
50
0
CAPACITANCE
 ANALYSIS
• Hot spot on the bushing was appearing
repeatedly and was spreading.
• As both capacitance and tan delta were found
high there was definitely moisture
contamination.
• Finding : The bushing was sent to BHEL Bhopal
for repairing. It was found that the inner
insulation was completely damaged.
 Precaution
• .Bushing test is carried out
by Ust Mode through Tan Tan delta
delta tap cap
• Tan delta cap is the most
vulnerable point in this
testing. There are
instances of failure of
bushing after this tap was
left open after testing .
 Transformer Failure
There was Paint in the mating surface
giving high resistance to the earth
connection

Sparking Between Core

support and Tank

Earth

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 Tanks
• Earthing at single point
• Leakages
• Insulation between core support and Tank
• Magnetization of Turret Bolts

45
Thermal Imaging OF transformer
 Analysis
• Thermal imaging of transformer
require in-depth analysis of the
transformer design.
• It can be employed to detect gross
defect inside Tap-changer. However in
the tank area there is huge variation in
thermal pattern.
• Detection of oil level in conservator , oil
coolers chocking are very successful.
 Over-fluxing of GT core
• Overfluxing of GT
can cause severe
damage to the core
.
• The setting of the
protection relay
and Flux
characteristics are
to be closely
coordinated
 D.G.A(dissolved gas analysis)

• This is the most effective and sure way of

ascertaining fault inside the transformers.
Analysis of fault gases and trending have
been proved successful in detecting many
incipient faults inside transformers.
 Case study ICT
The
overheating
was on the
compensating
winding
connection
.No testing
except
DGAcan
detect such
kind of fault
Compensating coil connection
Presence of stray wire
 Transformer Components
• Insulating Oil :
– Storage
– Issues of Mixing of oils
– Oil leakages and
– Oil Filtration
– Oil filing /sampling
• Bushing
• Protection : Bucholz, PRV, Elect Protection

53
 Insulating Oil
• Prolonged Operation >75 c causes oxidation
and sludging, aging of oil impregnated paper
and pressboards
• New oil > 30 KV, Processed >55 kv and < 10
ppm

54
 Recommended Tan Delta test values for
Transformers
• Oil filled transformers 0.5 % to 1 % at 20 c.
there should be any change in capacitance
value as change in capacitance indicate
winding or core displacement.9change less
than 10 %)
• Dry type power transformer PF TiP UP bench
mark should be established

55
 Recommended values
• Transformer oil : new 0.05 %, used oil less
then 0.5 % , value more than 1 % at 20 c is bad
and need reconditioning.
• Bushings :less than 0.5 % for 400 kv and less
than 0.7 % for 220 kv, there should not be
abrupt change
• Motors :PF Tip -Up 0.5 to 1 %

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 Maintenance &Testing
of insulating oil
• Dielectric break-down voltage
• Acidity test
• Interfacial tension
• color test
• power factor test
• specific gravity
• water content test or moisture test
• furfural analysis
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 Maintenance &Testing
of insulating oil
• TCGA (total combustible gas analysis
• DGA (dissolved gas analysis)
• hydrocarbon bonds of insulating oil under
extreme stress and temperature conditions
breaks giving rise to different hudrocarbon
gases like H2, CO2, CH4 etc.

58
Dissolved gas analysis (DGA)Interpretation

Operating Interpretations
condition
Nitrogen +5 % or Normal operation
less oxygen
N2, co, co2 Winding insulation
overheating
N2, CH4, Oil overheated,
C2H6,H2,C2H4 minor fault

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Dissolved gas analysis Interpretation

N2,H2, Small quantity Corona

of C2H4,C2H6 discharge
N2,H2,C2H4,C2H6, Corona involving
CO2, CO paper insulation
N2,high H2,CH2, High energy
CH4, C2H6 arcing
N2,high H2,CH2, High energy
CH4, C2H6 and arcing involving
CO,CO2 winding
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 FUNDAMENTAL INSULATION RESISTANCE

• Capacitance charging current (function of time

constant of the test specimen0
• dielectric absorption current (irreversible
charging current small magnitude)
• surface leakage
• partial discharge current
• volumetric leakage current

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 DC Tests
• IR (insulation resistance)
• PI (polarization index)
• High potential test
• easy to perform, equipment inexpensive
• off line test require outage.
• Non very sensitive to main problems like loose
windings, thermal degradation, stress erosion

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Current behavior

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 DAR &PI
• Dielectric absorption ratio: IR 60 SEC/IR 15 SEC
• PI(Polarisation Index) VALUE: IR 600 SEC/IR 60
SEC

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 DC High potential test
• This is a go -no -go test generally no
recommended as routine test.
• Acceptance test is 80% factory test
• routine test is 65 % of factory test.
• This is a destructive testing.
• Step voltage test is preferred for cable

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 Evaluation of test data
• Minimum value thumb rule 1 M ohm per
rated Kv = 1 M ohm
• manufacturer information
• comparison with acceptance or commission
value
• previous test value
• comparison with values of similar units

66
 Factors affecting the tests
• Contamination like dirt, oil, moisture,acids,
salts
• contamination at terminal points.
• Higher temperature or extremely low
temperature.

67
 Test method
• Grounded specimen test method.(GST)
• Ungrounded specimen test method.(UST)
• grounded specimen test method with guard
(GST-G)
• power factor Tip-up test

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Control Panels

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 Why the Problem Persists
IP 1st 2nd 3rd 4th
Character Character Character Character
Internation Against Against Additional Additional
al ingress of ingress of requireme requireme
Protection
solid water nt nt
foreign
objects
1: 50mm dia 1: vertically A: Back of H
2:>2.5 mm dia Dripping Hand M
4:1 mm dia 2: dripping 15 B: Finger S
5; Dust angle C:Tool W
Protected 3: Spraying D: Wire
6: Dust 4: Splashing
protected 5: Jetting
Dust Tight 7: Immersion
 Termination
• Control Cable termination:
– Stranded Copper Conductor
– Bare portion of solid conductor shall be tinned
and terminated directly without using cable lugs.
– All cable entry points shall be vermin and dust
proof. Unused opening shall be effectively closed
Switchyard Components

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 SF6 Breaker
• SF6: self healing gas : Gas absorbs the free electron
generated by arc which cause the gas to ionize.
• Maintenance : Leakage of Sf6 Gas
• Contamination :
– Conducting particle
– Moisture ,40 ppm
– Oil contamination
– Gaseous contamination
– Arc decomposition product

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 Breakers
• Timing analysis
• Contact resistance
• Dynamic resistance
• Travel analysis
• Insulation resistance
• Auxiliary contact travel measurement
• Closing coil and tripping coil healthiness

74
 SF6 Breaker Flashover

Any number of
testing can not
substitute
cleaning
 Conductors
• Damage
– Visual inspection
– IR Thermography
– Radio interference test
– Corona detection

76
 Surge Arresters
• LCM
• Insulation resistance
• Leakage current monitoring
• Cleanliness

77
 Case study of LA
• LA are integral Part of
Generator transformers and
due to close proximity to GT
are very important for
availability of GT.
• In many cases there are
damage to Bushings due to
explosion on LA.
• There are very little
information on testing and
predictive maintenance of
LA
 400 KV MALERKOTLA LINE CVT -Y- PH MIDDLE STACK.ON
TESTING FOUND VERY HIGH TAN DELTA .REPLACED

*>50.0°C
MALERKOTLA CVT -Y- PH
50.0

45.0
Max
40.6

40.0

35.0 Max
54.8

30.0

*<30.0°C
 Capacitance& tan delta Graph
MEGGER VALUE CAPACITANCE
 Tan delta graph
• Tan delta value of
one of the stack
was very high.
• Tan delta value of
different
manufacturer are
different . Hence
bench marking of
each type is
required
 Findings
• Based on Tan delta
and capacitance
value LA was taken
out .
• Internal inspection
shows severe
water
contamination and
arcing
 Findings

Water droplets
Rusted parts and tracking marks
 Current Transformers and CVT
• C-Tan delta C1 & C2
• Oil DGA
• Secondary burden test
• Ratio and VI

85
 Insulators
• Puncture detection
• Cleanliness
• IR Thermography
• Radio Interference
• Acaustic

86
 Earthing
• All metal support to be earthed to prevent
danger to public and live stock
• Reduce risk of back flash due to travelling wave
• Typical : Tower on concrete : 20 ohm
– Additional earthing if soil is rocky
– Running electrode /earth wire
– All earth connection must be bonded ( can carry
upto 50 A current)

87
 Thank you
santanu23@yahoo.co.in

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