smart grid monitoring of transformers by dga by michel duval
TRANSCRIPT
1
Smart Grid Monitoring of Transformers
by DGA
Michel Duval
CIGRE Thailand, Bangkok 2013
2
Electric Power
3
Power Transformers
4
Catastrophic Failures
55
Failures in Service
- The failure rate of power transformers in service (internal
failures needing repairs) typically is 0.3% per year.
- For a population of 2000 transformers, this means 6
transformers will fail in the next year.
- Less than 1 will fail catastrophically.
- 1994 will not fail.
- 200 (i.e., 10% of the population at or above IEEE/IEC
condition 1) will form abnormal amounts of gases because
of faults.
66
The Monitoring Dilemma
- Nobody knows which 6 of the 2000 transformers will fail
next year.
- To identify them, all the transformers need to be monitored,
including the 1800 operating normally, just for the purpose of
detecting the 6 that will fail and need repairs and the less
than 1 that may eventually fail catastrophically.
- In economic terms, the cost of monitoring is justified as long
as it does not exceed the cost of not detecting the 6 failures
and the catastrophic one (typically, >20M$).
77
Smart Grid Monitoring
- A smart grid is a modernized electrical grid that uses
information and communications technology in an
automated fashion to improve the efficiency, reliability and
sustainability of the production and distribution of electricity.
- It implies a re-engineering of the electricity services industry.
- It requires monitoring tools for evaluating on a real-time
basis the condition of electrical equipment, so as to optimize
asset utilization, system reliability and load capabilities
88
Monitoring Tools
-General tools for monitoring oil temperature, oil pressure,
partial discharges, etc, in transformers are available.
-However, for the early detection of faults and failures and
evaluating the condition of transformers, the main
monitoring tool is dissolved gas analysis (DGA).
-More than 1 million DGA analyses are performed by ~600
laboratories and ~ 40,000 on-line gas monitors each year
worldwide.
99
Basic Types of Faults
Detectable by DGA
-PD: partial discharges of the corona-type in voids in paper
insulation, as a result of poor drying or impregnating with oil.
-D1: low-energy discharges, such as partial discharges of
the sparking-type in oil or paper, tracking on paper, small
arcing, arc-breaking activity in LTCs.
-D2: high-energy discharges, e.g., flashovers, high-energy
arcing, short-circuits with power follow through, with
Buccholz alarms and tripping.
1010
Basic Types of Faults
Detectable by DGA
-T1: thermal faults of low temperature T < 300ºC, because
of overloading, insufficient cooling, design problems.
-T2: thermal faults of 300 <T< 700ºC, because of defective
contacts, welds, with carbonization of paper.
-T3: thermal faults of high temperature T >700ºC, because
of high circulating currents in core and coil, short circuits in
laminations, often in oil only.
1111
Additional Sub-Types of Faults
Detectable by DGA
-S: stray gassing of oil (in oil only) at T <200ºC, because of
the chemical instability of oils produced since ~ 2000.
-O: overheating of oil or paper at T <250ºC, therefore
without carbonization of paper.
-R: catalytic reactions of water with e.g., galvanized steel.
-these low-temperature faults, including corona PDs, are of
little concern in transformer.
1212
Additional Sub-Types of Faults
Detectable by DGA
-T3/T2 in oil only: at T >700/ 300ºC, of minor concern as
long as they do not evolve into faults D1, D2 or C.
-C: carbonization of paper at T >300ºC, potentially more
dangerous (loss of insulating properties of paper).
1313
DGA Diagnosis Methods
-Key Gas, Rogers and IEC methods. Limitations are high %
of wrong diagnosis (50%) or undiagnosed cases (30%),
respectively.
-Duval Triangle 1, allowing to detect the 6 basic types of
faults (PD, D1, D2, T3, T2, T1 + DT).
-Duval Triangles 4 and 5, allowing to detect the 5 sub-types
of faults (S, O, R, T3/T2 in oil, C), and to distinguish
between faults of lesser concern in oil and more serious
faults in paper.
1414
Duval Triangles 1, 4 and 5
Triangle 1
Triangle 4 Triangle 5
151515
Use of Triangles 4 and 5
-Triangles 4 and 5 should never be used in case of faults
identified with Triangle 1 as faults D1 or D2.
-Triangle 4 should be used only for faults identified first
with Triangle 1 as low temperature faults PD, T1 or T2, or
when there is a high level of H2.
-Triangle 5 should be used only for faults identified first
as high temperature thermal faults T2 or T3.
15
16
Mixtures of Faults
-mixtures of faults sometimes occur rather than « pure »
faults and may be more difficult to identify with certainty.
-for instance, mixtures of faults D1 and T3 may appear as
faults D2 in terms of gas formation.
17
New Faults vs. Old Faults:
-when a new fault appears, as evidenced by a change in
gas pattern, a more precise identification of the new fault
may be obtained by subtracting the gas concentrations
corresponding to the old fault from those corresponding to
the new one (incremented values).
-this, however, introduces additional uncertainty on the
subtracted value.
1818
Interpretation of CO and CO2
-Until recently, CO and CO2 were considered as good
indicators of paper involvement in faults. Recent
investigations at CIGRE, however, have shown that this is
not always the case.
-High concentrations of CO (>1000 ppm) and/or low
CO2/CO ratios (<3), WITHOUT the formation of significant
amounts of hydrocarbon gases, are NOT an indication of a
fault in paper, particularly in closed transformer, but are
rather due to oil oxidation under conditions of limited supply
of O2.
191919
CO2
and CO from Closed Transformers
Ref: I.Hoehlein, CIGRE TF15 (2010)
56 MVA, 220kV
Manufactured 2006
Rubber Bag
19
2020
Interpretation of CO and CO2
-High concentrations of CO (>1000 ppm) and low CO2/CO
ratios (<3), TOGETHER WITH the formation of significant
amounts of hydrocarbon gases, may be an indication of a
fault in paper (to be confirmed with Triangles 4 or 5 and
furans).
-High concentrations of CO2 (>10,000 ppm), high CO2/CO
ratios (>20) and high values of furans (>5 ppm) are an
indication of the slow degradation of paper at relatively low
temperatures (<140°C), down to low degrees of
polymerization (DP) of paper.
2121
Interpretation of CO and CO2
-Low concentrations of CO and CO2, below condition 1 of
IEC or IEEE (750 and 7500 ppm, respectively), correspond
to normal gassing in transformers without faults.
-Intermediate concentrations of CO, CO2 and CO2/CO ratios
may indicate a slow degradation of paper and intermediate
DPs of paper, of no concern at all for the normal operation
of the transformer.
2222
Interpretation of CO and CO2
-Zero or very low rates of change of CO and CO2 do not
necessarily mean the absence of a fault in paper. Localized
faults in paper often do not produce detectable amounts of
CO and CO2 against the usually high background of these
gases in service.
-However, they do produce significant amounts of the other
hydrocarbon gases, allowing the detection of faults in paper
with Triangles 4 or 5.
2323
Example of a Localized Fault in Paper
242424
24
CIGRE Risk of Failure vs. CO2
-The risk of failure is very low at high CO2 values, which are
strongly correlated with paper degradation and low DPs of
paper, suggesting that the risk of failure at low DPs of paper is
also very low, not very high as generally mentioned.
-Indeed, large numbers of transformers have been observed at
CIGRE to operate quite normally with DPs of paper < 200.
-And no cases have been reported so far of transformers with
DPs < 200 that failed because of the mechanical weakness of
paper, even when subjected to external short-circuits.
24
252525
25
Transformers at Risk of Failure
-So, in a large majority of cases, low DPs of paper do not
mean the « end-of-life » of transformers as generally
assumed.
-The main concern with low DPs of paper is the shrinkage of
paper and loosening of windings, not the mechanical (tensile)
strength of paper. This can be mitigated by reclamping
transformers.
-Transformers most at risk of failure are gassing transformers
that cannot be fixed.
25
26
-Type of fault (electrical, thermal)
-Location of fault (paper, oil)
-Gas concentrations, gassing rates (conditions 1 to 4)
Factors Influencing the Interpretation
of DGA Results
27
Typical / Condition 1 Values
-Typical /condition 1 values of IEC/ IEEE correspond to a
given percentile (90%) of the population of DGA results
-They mean that 90% of DGA results for dissolved gases
are below these 90% Typical values
-They are used to concentrate maintenance efforts on the
10% of the population with the highest gas levels and
therefore most at risk
28
Typical / Condition 1 Values
-Below typical/ condition 1 values, gas formation is
considered not to be a concern for the equipment.
-Below these values, it is recommended to use “normal”
sampling frequency (monthly, semi-annual, etc.,..) and not
to attempt a diagnosis.
-Above these values, it is recommended to use „increased‟
sampling frequency (e.g., monthly or weekly) and a DGA
diagnosis may be attempted.
29
90% Typical (Condition 1) Values
for Concentrations at IEC (2007), in ppm
(vs. source)
30
90% Typical (Condition 1) Values
for Concentrations at IEEE (2013), in ppm
(vs. kV, MVA, age, %O2)
313131
90% Typical (condition 1) Values for
Gassing Rates, in ppm/month
31
32
Pre-failure (Condition 4) Values
-CIGRE has evaluated the probability of having a failure-
related event (PFS) in service vs. gas concentration and
gassing rate.
-Based on these PFS curves, pre-failure (condition 4)
values have thus been established, as well as intermediate
conditions 2 and 3.
333333
33
Risk of Failure vs. Gases Formed
(PFS = Probabilityof Failure in Service)
33
343434
CIGRE/IEC Sampling Intervals vs.
Concentrations in Service, in ppm
34
353535
CIGRE/ IEC Sampling intervals vs.
Gassing Rates in Service, in ppm/month
35
36
Actions Recommended by IEC at
Conditions 1-4
-Condition 1: increase oil sampling frequency for DGA
-Conditions 2-3: consider complementary tests
(infrared scans, acoustic tests, PD tests, effect of load).
-Conditions 3-4: consider transformer inspection.
-Condition 4: consider transformer repair or replacement.
37
Transformer Parameters Influencing
Conditions 1-4
-CIGRE (2006)/ IEEE (2013):
-Operating conditions (load, climate)
-Age (new, old)
-Type (power, core, shell, instrument, reactor).
-MVA, voltage
-Open or closed
-CIGRE WG47 (2013):
-Fault type
3838
Occurrence of Faults in Service at CIGRE
3939
Effect of Type of Thermal Fault
on Condition 1 Values at CIGRE
(ppm)
(ppm)
4040
Effect of Type of Electrical Fault
on Condition 1 Values at CIGRE:
(ppm)
(after deletingC2H2 < 2 ppm)
(ppm)
4141
Effect of Type of Fault
on Condition 4 values at CIGRE:
(in ppm, using previous adjustment factors)
4242
Comparison with Cases of High Gas Levels
without Failure at CIGRE:
(in ppm)
4343
Monitoring with DGA
-Monitoring off-line (by manual or laboratory DGA) is mostly
used but cannot detect faults occurring between two oil
samplings (e.g., every year, month or week).
-On-line multi-gas or hydrogen monitors can detect abnormal
and/or quick-developing faults occurring within days or hours
between oil samplings.
4444
Abnormal and Quick-Developing Faults
-Abnormal gassing (above condition 1 for concentrations
and gassing rates) will occur in 200 of the 2000
transformers.
-Quick-developing faults (above condition 4 for gassing
rates) will typically occur in 20 to 40 of them.
4545
Detection of Quick-Developing Faults with a Multi-
Gas Monitor in a 3-Phase GSU Transformer
Day 2 – 16:00
Day 2 – 20:00
Day 3 – 00:00
Day 3 – 04:00
Day 2 – 12:00
Day 3 – 12:00
Day 3 – 16:00
Day 3 – 08:00
Day 23 – 04:00 to Day 24 – 08:00 Followed by transformer failure
C2H2 = 800 ppm/day!
4646
700 MVA Transformer
C2H2 = 45 ppm/day!
4747
336 MVA Transformer(Placed in Service -1969)
C2H4 = 300 ppm/day!
4848
1100 MVA Transformer
C2H4 = 300 ppm/day!
4949
-Gassing rates were all significantly above condition 4
values in the previous 4 examples.
-The corresponding transformers were removed from
service 1 to 3 days after looking at monitor readings, before
potential catastrophic failure.
-However, it might have been better to remove them from
service earlier.
-Without an on-line monitor, these transformers would
possibly have suffered catastrophic failures.
5050
On-Line Monitoring with Multi-Gas Monitors
- Multi-gas monitors will detect all types of faults, even in
their early stages at condition 1, and without false alarms.
However, they are more expensive than hydrogen only
monitors.
- The recommendation of CIGRE (TB # 409, 2010) is
therefore to use multi-gas monitors in critical transformers
(GSU, nuclear, transmission) and in abnormally gassing
transformers.
5151
Fault Detection with Hydrogen Monitors
Note: for faults T3 in paper (C), curve for H2 is a bit higher.Ref: Duval, TSUG 2013.
5252
Fault Detection with Hydrogen Monitors
-Hydrogen monitors are most sensitive to stray gassing of
oil S (occurring in ~ 25% of cases), and to corona partial
discharges PD (occurring in only 0.3% of cases).
-Such faults will commonly produce thousands of ppm of H2
without being a concern for the transformers. If the limit in
hydrogen monitors is set at average condition 1 values for
H2 (100 ppm or 7 ppm/month), this may result into many
false alarms.
5353
Fault Detection with Hydrogen Monitors
-Faults D1/D2 at dangerous condition 4 of CIGRE will
produce 0.5 ppm/day of C2H2 together with only 1 or 2
ppm/day of H2.
-If the limit for H2 in the monitor is set at average condition 1
(100 ppm), the monitor will detect these faults only in their
late stages (condition 3 or 4), when dangerous levels of 25
to 50 ppm of C2H2 have already formed.
-If it is set at 5 ppm over a period of 3 days, this may result
into many false alarms.
5454
Fault Detection with Hydrogen Monitors
- In case of thermal faults T3/T2/T1/O the main gas formed is
C2H4, CH4 or C2H6, together with 3 to 10 times less of H2. If
the limit for H2 is set at 100 ppm, the monitor will detect
these faults only in their late stages (condition 3 or 4).
- Decreasing the limit for H2 in the monitor (e.g., to 50 or 20
ppm) will increase the number of false alarms due to faults
S or corona PD of lesser concern.
- The recommendation of CIGRE (in TB # 409, 2010) is
therefore to use hydrogen monitors in non-critical
transmission and distribution transformers, and in
transformers with no previous gassing history.
55
Examples of On-Line Gas Monitors
56
Multi-Gas Monitors
Monitors of the chromatographic type:
-after gas extraction, will separate individual gases on a GC
column, then measure them with GC detectors.
-TM8, TM3 (Serveron)
-Calisto 9 (Morgan Schaffer)
57
Monitors of the Chromatographic-Type:
-use the same standardized, NIST-traceable technique
as laboratories.
-provide automatic recalibration at fixed intervals as
laboratories do.
-require some maintenance (change of carrier gas,
calibration gas mixture, GC columns every 3 to 5 years).
58
Monitors of the Infrared-Type:
-after gas extraction, will measure directly individual gases with
an infrared detector, and H2 with a solid state sensor
-Transfix 8, Transport-X 7 (GE-Kelman) use a photo-acoustic
(PAS) detector.
-LumaSense 9 uses a non-dispersive IR detector.
59
Monitors of the infrared type:
-do not require change of carrier gas and gas mixture.
-cannot measure H2 , O2 by infrared, requiring the use of
relatively inaccurate solid state sensors for that purpose.
-some may need recalibration because of contaminations
in the ambient air (SF6, oil vapours, solvents) and lamp
fade with time; some cannot be recalibrated in the field.
-require change of infrared lamp ~ every 5 years.
-contain several moving parts.
60
Hydrogen Monitors
-Hydran (GE): measures 100% of the H2 + 18% of the CO
present in oil with a PTFE membrane and fuel cell detector.
-Calisto 2 (Morgan Shaffer): measures H2 only with a PTFE
membrane, GC and TCD detector.
-TM1, Qualitrol, Weidmann: measure H2 with an inorganic
membrane and an H2Scan Pd solid state sensor.
-TM1 (Serveron): improved version of H2Scan.
6161
Other Applications of DGA
-DGA can also be used to detect faults in LTCs, using for
example Duval Triangle 2 for compartment types, and Triangles
2a to 2e for in-tank types.
-it can also be used for oils other than mineral oils, such as
natural esters (FR3, BioTemp), synthetic esters (Midel) and
silicone oils, using for example Duval Triangles 3.
6262
DGA in LTCs at IEC/ IEEE:
Duval Triangle 2 for compartment types Duval Triangles 2 for in-tank typesN1 (MR types M, D)N3 (MR types VR, VV)N4 (MR types R, V)N5 (MR types G, UZD)
636363
Duval Triangles 3 for Non-Mineral oils
Mineral oil
FR363
646464
64
DGA in wind farm transformers at CIGRE
64
-Because they are usually Padmount transformers not
designed for that purpose, many tend to form lots of gases,
as a result of:
-Corona PDs, because of poor oil impregnation.
-Stray gassing of oil, because of abnormal overheating.
656565
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Stray gassing of oil at CIGRE
-With mineral oil, H2 at T<120C and CH4, C2H6 at
T>200C.
-With vegetable oils (e.g.,FR3), H2 at T<70C and C2H6
at higher temperatures (Triangles 6 and 7).
-With silicone oils, H2 at T>200C.
65