School of Electrical, Electronics and Computer Engineering
School of Electrical, Electronics and Computer Engineering
School of Electrical, Electronics and Computer Engineering
SUBMITTED BY
NAME: SATHEESH KUMAR NATARAJAN
STUDENT NO: 099120124
DATE: 11TH JAN’ 2010
It is beneficial to perform periodic tests on transformer oil to determine whether it is
capable of fulfilling its role as an insulator and heat transfer medium. Some of the most
Common tests for transformer oil are: Dissolved Gas Analysis, Screen Tests, Water
Content, Metals in Oil, and polychlorinated biphenyl (PCB) content.
Most of the internal faults in a transformer can be detected through oil analysis Under
electrical and thermal stresses, a variety of gases, combustible and non-combustible,
are generated in transformers as a result of dielectric fluid degradation. These
generated gases dissolve in the dielectric fluid . DGA is an effective and reliable tool to
detect incipient faults in oil immersed transformers
Dissolved Gas Analysis (DGA) is widely accepted as the most reliable tool for the
earliest detection of thermal (overheating) and incipient faults (arcing and/or corona) in
transformers and load tap changers. DGA testing is recommended initially at the start of
the maintenance program, and annually thereafter, or at any time unusual conditions
occur. These conditions include:
The kinds and amounts of gases produced are indicative of their cause (corona, thermal
effects, sparking, or electric arcing). The results are reported in Parts-Per-Million (PPM)
by volume, the following are the 7 combustible gases that are considered:
Hydrogen (H2) Corona or low energy electrical discharges
Methane (CH4) Corona or overheating /partial discharge
Acetylene (C2H2) low energy sparking
Ethylene (C2H4) Severe localized overheating or high energy arcing
Ethane (C2H6) Thermal fault in oil / Electrical discharge
Carbon Monoxide (CO) Cellulose overheating/Degradation of cellulose
Carbon Dioxide (CO2) Cellulose overheating/Degradation of cellulose
Carbon Dioxide (CO2) is considered, and the ratio of CO2 to CO (Cellulose Insulation)
is evaluated to understand the condition of the cellulose components (woods and
papers). Nitrogen (N2) and Oxygen (O2) are also considered.
According to the Dissolved gas Analysis of the gas collected from Buchholz relay of
transformer 2 at site E, transformer 2 experiences severe problem which has drew the
engineers to carry the analysis on all ten units in five substations.
By analyzing the result of the dissolved gas analysis conducted on the ten units reveals
that the other transformers are also experiencing the problem even though the problem
is not as severe as the one experienced by transformer 2 at substation E.
The figures representing the percentage of different gases present in each and every
transformers present in the five substations has been interleaved in table-1 exceeds the
accepted level of percentage of gases which may be allowed in a transformer.
The corona type low energy discharge is the inception of partial discharge which is
experienced by each transformer in five substations which is represented by the
hydrogen (H2) gas in the presence of methane (CH4) whose PPM are well above the
accepted value. The presence of acetylene (C 2H2) in the transformer-2 indicate that the
Partial discharge has developed into low energy sparking with higher temperatures. The
suspected identification for the presence of acetylene (C 2H2) is arcing in in-tank tap-
changer. This problem is confined to transformer-2, since the value representing the
amount of acetylene (C2H2) present in all other transformers are well within the
allowable range . When the sparking escalates to arcing then the presence of ethylene
(C2H4) the transformers in all five substations experiences thermal fault in oil which is a
principal indicator for the presence of ethylene (C 2H4) Since the value representing the
amount of ethylene (C2H4) present in each transformer exceeds the allowable range,
all the transformer are subjected to problems associated with the presence of that
particular gas element .The major problem undergone by each transformer is the
overheating of cellulose which is represented by the amount of CO2 and CO gases and
also thermal fault in oil discovered in transformer test analysis.
The major problems experienced by all the transformers is the overheating of cellulose ,
followed by thermal fault in oil, over heating of oil and transformer-2 at substation E in
particular suffer severe partial discharge and arcing additionally.
Partial Discharge:
A partial discharge (PD) is the dissipation of energy caused by the buildup of
Localized electric field intensity. In high voltage devices such as transformers, this
buildup of charge and its release can be symptomatic of problems associated with
aging, such as floating components and insulation breakdown.
Corona is considered to be partial discharge and occurs at areas of high electrical
stress such as sharp points along an electrical path. Partial discharge is commonly
explained as being intermittent unsustained arcs which are shot off of the conducting
material like a stream of electrons. If these arcs contact solid insulating material, they
can cause serious damage .
When partial discharge is initiated, high frequency transient current pulses will appear
and persist for nano-seconds to a micro-second, then disappear and reappear
repeatedly. Pd currents are difficult to measure because of their small magnitude and
short duration.
Arcing:
Insulation Overheating:
Cellulose is normally used to insulate transformer windings. If a transformer becomes
overloaded for any reason, the windings will generate excessive heat and deteriorate
the cellulose. A DGA test can identify an overloaded transformer when high carbon
monoxide, high carbon dioxide, and in extreme cases elevated methane and ethylene
levels are detected. If a transformer is operated in an overloaded manner for a long
period of time, the deteriorating condition of the cellulose will shorten its life. By
detecting insulation breakdown at an early stage.
OIL Overheating:
Overheating of the oil liquid is a different problem. A DGA test will indicate high thermal
gases (methane, ethane, ethylene) as a result of overheating of the oil these gases are
formed from the breakdown of the oil caused by heat Heating maybe caused by poor
contacts on a tap changer. or loose connections on a bushing or a grounding strap, or
circulating currents in the core due to an unintended core ground.
SERVICE
All the transformers are left to service,With effective result from the DGA test we can
come to conclusion about the problems that occurred or can occur in near future , by
using the specific method to pin point every problems which are known to us through
DGA test and taking effective measures to rectify each problems.
In general actions that can be taken once a problem is detected would depend on the
severity of the problem. If conditions are not severe the transformer should simply be
monitored closely. If the condition becomes worse and thermal combustibles elevate,
the transformer will need to be taken out of service. If the combustibles are stable and
remain present, then the transformer should be inspected at the next outage or a
scheduling of downtime should result.
If the measures are not taken effectively it may get worse because Each and every
problems experienced by the transformers mentioned in table -1 has their own demerits
with each one leading to another if the corrective work are not been executed
effectively.
Arcing
Arcing can be generated in many areas of a transformer. Insulation breakdown in the
windings, from coil to coil or coil to ground. will result in arcing. A portion of the
insulation may become deteriorated to the point that it can no longer sustain any
difference in electrical potential. If a winding shorts from turn to turn, or phase to phase,
or phase to ground, arcing will occur and the transformer could fail. If windings are
involved in an arcing condition, the transformer usually will be de-tanked and repaired.
A loose connection may also cause arcing but of greater significance would be arcing
due to insulation breakdown.
Insulation/oil Overheating
The result of DGA associated with all other transformer suffer partial discharge even
though it is not as severe as the transformer-2 at substation E still effective
measurements are needed to rectify the fault. Most of the transformer suffer thermal
fault in oil which indicated the high temperature in the oil.
Transformer-1 at substation D is the least suffer compared with all other transformers
with only one major problem which is overheating of cellulose which turned out to be a
common problem among all the transformers.