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In-Service Testing and Diagnosis of
Gapless Metal Oxide Surge Arresters
According to IEC60099-5
Title 22
Overview of presentation
• Motivation for condition monitoring of metal oxide surge arresters (MOSA)
• “The Surge arrester life”
• Service experience
• Examples of arrester failures
• Characteristic properties of MOSA (ZnO)-arrester
• Aging and causes of failure
• Consequences of failure – transformer failures
Title 33
Overview of presentation contd.
• Surge arrester condition assessment
• IEC 60099-5 about “Diagnostic indicators of metal oxide surge arresters in service”
• Monitoring equipment and field application for third harmonic analysis with compensation
• Testing strategy and risk assessment
• Case studies
Title 44
Background and Motivation
• The Metal Oxide Surge Arrester (MOSA) is a cheap and passive component, but protecting crucial apparatus.
• Overlooked despite severe consequences if it fails.
• MOSAs can age and fail due for a number of different reasons.
• May offer inadequate overvoltage protection, especially if the rated voltage is selected too low.
• Diagnostic indicator: Resistive leakage current increases with time increasing risk for failure.
Title 55
Power System Overview –Typical Location of Surge Arresters
Typical location of
surge arresters:
In substations
At the end of
transmission lines
At cable ends
At transformers,
generators, capacitors
etc
Location depending on
voltage level,
equipment and local
conditions
Title 66
The Surge Arrester Life
The normal destiny of the surge arrester is to be:
specified, purchased, installed- and forgotten…
Most common maintenance practice:No testing of surge arresters –
Only replacement after breakdown.“surge arresters are inexpensive no big deal to replace!!!”
Is this really an acceptable practice?
Title 77
The Surge Arrester Life
Why care about surge arresters?
1. The arrester is your “bodyguard” for protecting important apparatus against the “overvoltage terrorists”
2. You cannot see if an arrester is bad, but you can measure it.
The big question is:
Are the arresters fit for fight?
Title 88
• Failure rate depending on arrester quality, dimensioning and local conditions
• Typical failure effects on arresters: Explosion and external damages –
visual detection Puncturing and causing earth fault -
indicated by earth fault relay, can be difficulty to locate
Aged arrester with reduced protection level –cannot be found without checking the arrester
Surge Arrester Service Experiences
Title 99
Failure of 400kV Surge Arrester
Title 1010
Failed Arrester hanging with Bus Pipe
Title 1111
Shattered Pieces of Surge Arrester Stacks
Title 1212
Damaged Surge Monitor and shattered Pieces of Arrester stack
Title 1313
Another failed Surge Arrester
Title 1414
More…Failure of Surge Arrester
Title 1515
Surge Arrester Properties
Main objectives: Protect important apparatus against dangerous overvoltages
• Low resistance during surges so that overvoltages are limited
• High resistance during normal operation, to avoid negative effects on the power system
• Sufficient energy absorption capability for stable operation
Title 1616
Equivalent Circuit Diagrams
MO discharge resistor
MO ArresterSiC Arrester
SiC discharge resistor
Series spark gap
and RC control
Title 1717
Voltage Current CharacteristicsMOSA (ZnO) and SiC Arresters
Title 1818
• Thermal instability and arrester failure can occur at
operating voltage in case the temperature of the blocks is too high.
MOSA must be correctly selected with respect to:
o continuous operating voltageo different kinds of overvoltageso ambient temperatureo pollutiono ageing
Thermal Instability
Title 1919
1. A lightning strike causes a discharge in a MOSA
2. The lightning causes an earth fault in the network
3. Single line to earth fault causes voltage increases on the two healthy phases
4. The earth is disconnected by a circuit breaker
5. Disconnection of the fault can cause increased TOV due to load dropping
6. Circuit breaker reclosing cause additional arrester energy due to switching overvoltages
Total Power Dissipation Accumulated ofSequence of Incidents in the Network
Title 2020
Rated voltage must be chosen high enough based on:
- Normal operation conditions- Ambient temperature- Continuous voltage- Surface contamination- Ageing- Accumulated energy from previous discharges
Critical factors to avoid failure
Title 2121
The choice of MOSA is always a compromise
Increased nominal/rated voltage: Possibility that the MOSA will withstand the
stress increases Reduced protection margin
Arresters with higher energy class: reduced risk for arrester failure Price increases
The choice of MOSA is a compromise between protectionlevel, voltage withstand and energy absorption
Title 2222
Design of Porchelain-MOSA –eks. Cooper Power Systems
Title 2323
Design of Polymeric-MOSA – (ABB)
Title 2424
Ageing of MOSA
Normal operating voltage causes ageing
Pollution and overvoltage surges can cause ageing from overloading of all or some of the blocks
Moisture entry through sealing gaskets, may lead to shorting of ZnO discs and overstressing of healthy ZnO blocks.
Degree of ageing depends on the nature/ quality of the granular layer.
Increase in resistive leakage current may bring the arrester to thermal instability and complete arrester breakdown.
Title 2525
Incorrect arrester specification corresponding toactual system voltage and overvoltage stress
Overloading due to: Temporary overvoltages (cracking, puncturing). Switching overvoltages (cracking, puncturing,
flashover). Lightning overvoltages (change of
characteristic/ageing, flashover, puncturing).
External pollution or moisture penetration .
Consequence of aging: Increase in the continuous resistive leakage current .
This is a good indicator of the arrester condition.
Metal Oxide Surge Arresters - Causes for Failure
Title 2626
Reduced overvoltage protection –- Increased risk of equipment failure and
outages for instance breakdown intransformer, bushings, switchgear
Possible break-down of porcelain housings:- Risk of personal injury- Risk of damage to other equipment
Consequences of Arrester Failures
Title 2727
The US insurance company HSB reports, as reasons for transformer failures:
Electrical disturbances: 29%Lightning: 16%
→ Has the arrester done its job?
Reasons for Transformer Failures
Title 2828
YES -Transformers Do Fail
Title 2929
YES -Transformers Do Fail
Title 3030
SiC - Arresters with spark gaps:No reliable in-service method available off-line tests: Spark-over test and grading current measurement Dielectric loss (the Doble test)
Metal oxide surge arresters without gaps:In service tests are possibleOn line tests: Continuous leakage current during normal service. Available in-service methods discussed in Amendment
1 to IEC 60099-5: ‘’Diagnostic indicators for metal oxide surge arresters in service’’.
Possibilities for Surge Arrester Condition Assessment
Title 3131
Condition check performed on regular basis will:
• Increase the safety for the operational and maintenancestaff.
• Give early warning signals utilize life time and takeaged arresters out of service before they fail.
• Prevent costly arrester failures and service interruptions.
• Prevent damages to other equipment, e.g. transformer bushings.
Why Condition Assessment of Surge Arresters?
Title 3232
IEC 60099-5 Part 5:Selection and Application Recommendation
Title 3333
Methods for Monitoring of degradation of MOSA
Visual inspection– Locating external abnormalities on the arrester and gives
practically no information about the internal of the arresterSurge counters– Frequently installed on MOSA, but has no practical use for
diagnosis of condition of the arrestersTemperature measurements – Thermo Vision– Frequently used method. Detects the increased block
temperature on the housing surface of the arrester.Leakage current measurements– Most used diagnostic method. For in-service testing, the
method with indirect determination of the resistive leakage current with compensation for harmonics in the voltage (THRC) is providing the best available information quality with respect to diagnostic efficiency.
Title 3434
Conventional Surge Counters
Title 3535
”Modern” Surge Counter –ABB EXCOUNT II
Mod.
1
Mod.
2
SURGE COUNTING:
- Number
- Time stamp
- Current amplitude classif.
•
•
•
•
•
•
CONDITION MONITORING:
- Total leakage current
- Resistive leakage current
(Method B2 - IEC 60099-5)
• •
•
Title 3636
”Monitoring Spark Gaps”, from TriDelta
Title 3737
IEC 60099-5:Leakage Current Measurements
It
Ic:
0.2-3 mAIr:
10-600A
U
Title 3838
Ic
= 100 Ic
= 100
Ir = 30I
r = 10
It = 100,5 I
t = 104,5
Measurement of the total leakage current
example:
IEC 60099-5:Leakage Current Measurements
It
Ic:
0.2-3 mA
Ir:
10-600AU
Title 3939
The total leakage current increases with only 4% when the resistive part is triple.
This small change in It is difficult to read on the mA – meter.
Ic
= 100 Ic
= 100
Ir = 30I
r = 10
It = 100,5 I
t = 104,5
Measurement of the total leakage current
example:
IEC 60099-5:Leakage Current Measurements
Title 4040
IEC 60099-5:Leakage Current Measurement
IEC 60099-5, clause 6.1.6.1.2:
“At given values of voltage and temperature, the resistive component of the leakage current is a sensitive indicator of changes in the voltage-current characteristic of non-linear metal-oxide resistors.
The resistive current can, therefore, be used as a tool for diagnostic indication of changes in the condition of metal-oxide arresters in service.”
Title 4141
Equivalent Circuit of MOSA
Ic in the same size as It.
Ir is nonlinear and depends on voltage level and temperature.
U sinusoidal (fundamental component only): I1c, I1r, I3r
Harmonics in the operating voltage U: I1c, I1r, I3r, I3c
I3r (and Ir) is generated by the arrester itself and can be used as a diagnostic indicator.
It
Ic:
0.2-3 mA
Ir:
10-600AU
Title 4242
The resistive current component:
• is typically 5-20% of the total leakage current under normal operating conditions.
• is a sensitive indicator of changes in the voltage-current characteristic.
• depends on the voltage and temperature.
Typical Voltage - Current Characteristics
Title 4343
IEC 60099-5 says: Error range for third harmonic leakage current without compensation for different phase angles of system voltage third harmonics:
Includes various voltage-current characteristics of nolinear metal-oxideresistors.
1% third harmonic in voltage may give ±100% measurement
error.
(Norway: 0,1– 0,9% harm.)
Method B1: 3rd harmonic analysis of leakage current:
IEC 60099-5:Leakage Current Measurements
120°
270°
Title 4444
3rd harmonic analysis chosen is used as a basis to obtain feasibility/reliability measurements in three-phase applications on-site.
Presences of harmonics in the operating voltage generate harmonic capacitive leakage currents that is indirectly measured and compensated for.
The key for compensation is application of field probe for indirect measurement of the 3rd harmonic capacitive leakage current generated by the operating voltage.
The total and “true” resistive leakage current Ir is calculated from I3r
and arrester data (incl. correction for temperature and voltage).
Method B2: Harmonic analysis of leakage current
using third harmonic with compensation:
IEC 60099-5:Leakage Current Measurements
Title 4545
IEC 60099-5:Leakage Current Measurements
• Weakness with Method B1: The presence of harmonics in the system voltage have been a great problem since these harmonics may interfere with the harmonics generated by the nonlinear resistance of the arrester.
• Favorable effect by Method B2: It introduces a field probe that allows a compensation for the harmonic currents generated by the harmonics in the voltage. This implies that the method shows low sensitivity to harmonics in the voltage.
Method B2: Measurement of resistive leakage current using 3rd harmonic analysis with compensation for harmonics in the system voltage.
Title 4646
Method B2 is ranked to be the best field method for evaluation of ageing and deterioration of MOSA.
Properties of on-site leakage current measurements:
A HV-DC test is effective but off line and complex
IEC 60099-5: Summary
Title 4747
Measurement of total leakage current.
► Poor sensitivity. Insufficient method.
Direct measurement of resistive leakage current.►Attractive, but not usable on site.
Method B1: 3rd harmonic analysis of the leakage
current.►High sensitivity to harmonics in the voltage.
Method B2: 3rd order harmonic analysis of the leakage current with compensation.►Ranked by IEC 60099-5 as most reliable on site.
Available diagnostic methods:
IEC 60099-5: Summary of Performance
Title 48www.doble.no 48
Deployment of LCM 500 accessories
1. Gapless MOSA
2. Insulated base
3. Grounding wire
4. Clip-on CT500– it(t)
5. Counter
6. Field probe – ip(t)
7. Arrester pedestal
8. Telescopic rod
9. LCM 500 unit
The Field Probe shouldNEVER exceed this limit
1
5
2
7
3
96
8
4
Title 49
TO
GE
TH
ER
WE
PO
WE
R T
HE
WO
RL
D
www.doble.com
Performance of testing
First of all – connect
the instrument to
earth
FP should be placed
as close as possible
to the base of the
arresters
CCT should be
placed above any
counter/a-meters
Title 5050
Requirements: Separate earth lead & insulated base for each arrester.
CCT = Clip-on Current Transformer
CCT
Short circuit of insulated base will lead to circulating currents in the fundament and the earth lead.
CCT
Electromagnetic field can introduce current in this loop.
Leakage Current Measurements
Title 5151
Risk Assessment
Based on the level and development of resistive leakagecurrent Ir over time:
1. Trend analysis over time
In general look for increasing trend
Baseline reading when the arrester is new. If Ir
increases by 300-400%, this confirms severe ageing
2. Compare to maximum recommended values from arrester manufacturers
3. Compare Ir for arresters of the same make and type:
The three phases in a line or bay
All arresters in the grid
4. Combination of step 1-3
Title 5252
Steps in the final evaluation:
1. It and Ir are unrealistically high: Circulating currents? Check the insulated base and arrester grounding.
2. Ir higher than expected: Temporary heating? Consider to re-test in approx. 1 day to confirm measured value.
3. Confirmed high reading of Ir: Monitor continuously or proceed with step 4.
4. Contact arrester manufacturer and consider replacement.
Risk Assessment
Title 5353
Testing Strategy
1. Classify all your MOSAs (name of substation, bay/line and phase, nameplate data (manufacturer, type designation, year/date of commissioning etc.), historical data/failure rates, importance etc.).
2. Establish threshold levels/maximum recommended levels for the resistive leakage current for each arrester type.
3. Define action limits (good condition, satisfactory, re-test/monitor continuously, replace).
4. Define measurement regularity (normal, frequent, monitor continuously, after special fault situations).
5. Define verification actions after replacement (laboratory test, dissection/inspection).
6. Evaluate measurements, action limits, regularity of measurements and verification tests to possibly improve the testing strategy.
Title 54
1. Measurements at a 420kV GIS
2. Measurements at a Petro-Chemical factory
3. Measurements at an Oil Refinery
4. Power Utility
5. 110kV Transmission line
6. City substation
7. 420kV Substation
8. Coastal site
9. 110kV substation
Case studies
54
Title 5555
Measurements at 420 kV GIS Substation (1/3)
Case 1: 24 arresters, type A, B and C - 420 kV
The utility wanted to assess the arrester conditions because of surge arrester failures in the past.
Max. recommended leakage current values:
Type A = 167μA (167 μA = 100%)
Type B = 100μA (100 μA = 100%)
Type C = 675 μA (675 μA = 100%)
Title 5656
0
20
40
60
80
100
120
1 2 3 4 5 6 7 8 9 10 11 12
Arrester number
Resis
tive l
eakag
e c
urr
en
t in
perc
en
t
of
max.
reco
mm
en
ded
(100%
)
Bay 1
Bay 2
Bay 3
Bay 4
420 kV MOSA at transmission utility
0
20
40
60
80
100
120
1 2 3 4 5 6
Arrester number
Resis
tive l
eakag
e c
urr
en
t in
perc
en
t o
f m
ax.
reco
mm
en
ded
Bay 7
Bay 8
Type A: 100% ~ 165 A Type B: 100% ~ 165 uA
0
500
1000
1500
2000
2500
3000
1 2 3 4 5 6
Arrester number
Resis
tive l
eakag
e c
urr
en
t (u
A)
Bay 5
Bay 6
Type C: 100% ~ 675 uA
Measurements at 420 kV GIS Substation (2/3)
Title 5757
Measurements at 420 kV GIS Substation (3/3)
Measurements showed:
1 arrester of type C with app. 375% of max. recommended value
1 arrester of type A with app. 90% of max. Recommended value
The rest of the arresters had values from 70% and lower.
Conclusion: The two arresters showing the highest values were replaced to reduce the risk of outages. New measurement is recommended in one year.
Title 5858
Measurement at a Petro-Chemical factory (1/1)
Case 2: 6 arresters, 145kV, installed 1984
Factory owner anxious due to: very high production loss if outages
old arresters, condition unknown
Measurements showed: 2 units with app. 130%(Ir max rec.= 130µA=100%)
3 units with app. 95%
1 unit with app. 70%
Conclusion: All 6 arresters were replaced to reduce
outage risk
Title 5959
Measurement at a Oil Refinery (1/1)
Case 3: 6 arresters, 300kV, installed 1984
Refinery owner anxious due to: old arresters, condition unknown
very high production loss if outages
Measurements gave: 2 units with app. 60% (Ir max rec.= 130µA=100%)
2 units with app. 50%
2 units with app. 35%
Conclusion: All arresters OKNew measurements recommended in one year
Title 6060
Utility performing routine tests annually (1/2)
Case 4: Annually routine testing with LCM for all arresters in the grid
Condition monitoring to avoid: Sudden failures
blasting of arresters
outages
Philosophy: Use a simple test to detect “bad” arresters in service- no outage
necessary!
Have set max resistive leakage current to 500μA
Verification of LCM measurements in laboratory (capacitance, tanδ,
IR and dissection)
Cooperate with arrester manufacturer to improve arrester design
based on measurement experience
Title 6161
Utility performing routine tests annually (2/2)
70 surge arresters have been removed from service, based on resistive leakage current measurements with LCM
Conclusion: Utility statement:” Using LCM with third harmonic resistive
current measurement technique is very effective in detecting defective/ aged surge arresters”
Removed arresters showed: 90% damaged due to moisture ingress
10 % severely aged
The problem increased during the rainy season
The sealing gaskets were improved and replaced by o-rings in cooperation with the arrester manufacturer
Title 6262
Measurements at 110 kV Transm.line (1/2)
Case 5: Leakage current measurements as part of the condition based maintenance for 110 kV system
Utility is using LCM II for the following purposes: Identify and remove “bad” arresters
Replacing standard leakage current meters which are both ineffective
and easily damaged by extreme weather and pollution
Measuring philosophy:
As a preventive approach, arresters are measured before monsoon with the LCM II
Removed arresters are tested in laboratory for verification of high leakage currents (IR testing, waveform analysis etc.)
Title 6363
Measurements at 110 kV Transm.line (2/2)
Conclusions:
3 arresters have been removed based on leakage current measurements so far
All three showed high leakage currents of respectively 293μA, 570μA
and 7070 μA!!!
Remaining arresters of same make showed low and normal values
(<80μA)
Standard analog meters in the field were not showing readings in the
alarm region
Title 6464
Measurements at a City Substation (1/1)
Case 6: 18 arresters, 300 kV, majority installed in 1980
The utility was concerned due to arrester failures (arrester explosions) in the past
Measurements showed: 3 units with close to 300% (Ir max rec.=130μA=100%)
4 units in the area app. 70-100%
The rest showed low/normal values <50%
Conclusion/our recommendations: Replace 3 arresters
Monitor 4 arresters closely
Measure the rest again in app. 1-2 years
Title 6565
Measurements at 420 kV System (1/1)
Case 7: 6 arresters, 420 kV, commissioned in 1988
Max recommended resistive leakage current for all 6
arresters are Ir = 165μA (= 100%)
Measurements showed:All arresters had Ir values between 37-55%
Conclusion: All arresters are considered to be in good condition. New measurements are recommended in one year
Title 6666
Measurements at Coastal Site (1/1)
Case 8: 6 arresters, 145 kV, commissioned in 2002
Max recommended leakage current for all 6 arresters are
Ir = 130μA (= 100%)
Measurements gave:All arresters had Ir values between 35-46%
Conclusion: All arresters are considered to be in good condition. New measurements are recommended in two years
Title 6767
Measurements at 110 kV Substation (1/2)
Case 9: 18 arresters, 110 kV – measured in 2007
Max recommended leakage current not known baseline
established by averaging measurements for all 18arresters
Measurements gave: Two arresters had significantly higher readings (230% and 400%
respectively
Conclusion: The two arresters were taken out of service for laboratory testing. The test showed ingress of moisture that caused internal heating and increase of resistive leakage currents
Title 6868
Test of 110kV MOSAs, early 2007
Measurements at 110 kV Substation (2/2)
Title 6969
Surge arresters protect valuable assets from overvoltages generated by lightning strikes or switching operations.
MOSA will deteriorate over time due to electrical and thermal stress.
Leakage Current MeasurementMethod B2 using third harmonic with compensation according to IEC 60099-5 has been used successfully world wide for surge arrester monitoring.
This method is easy and efficient for field application for any make of metal oxide surge arresters.
Summary
Title 7070
QUESTIONS?
