Method for on-line assessment and indication of transformer conditions

There is described a method of monitoring the fault gases in the headspace of a transformer and providing an indication of transformer conditions.

BRIEF DESCRIPTION OF THE INVENTION
 This invention relates in general to a method of on-line detection of fault
 gases in a transformer head space to provide an assessment and indication
 of the transformer operating conditions.
 BACKGROUND OF THE INVENTION
 Transformers generally consist of copper and/or aluminum conductors,
 insulated with paper or varnish, wound into a variety of winding
 configurations, separated by pressboard spacers to allow oil flow for
 cooling, and a variety of pressboard barriers for insulation between
 windings and ground. A silicon steel laminated core links the windings.
 The assembly is contained in a heavy steel tank with porcelain bushings to
 connect to the windings, external cooling heat exchangers, and various
 accessories. The transformer is filled with insulating oil to provide
 insulation and to carry away heat from the windings. Provisions are made
 for expansion and contraction of the oil with temperature changes due to
 loading and ambient. In addition, air and moisture are excluded from the
 system in order to maintain dielectric integrity and avoid excessive aging
 of the materials.
 Starting in the early 1960s, the practice of monitoring selected gases
 dissolved in the oil to diagnose operating problems inside oil-filled
 transformers became a normal industry practice. Various types of
 dielectric or excessive heating problems break down the insulating oil and
 solid materials into characteristic gases that dissolve in the oil and
 collect in the head space. It became industry practice for users to take
 periodic oil samples from transformers for testing in a laboratory to
 identify developing operating problems. Considerable effort over the years
 has gone into trying to categorize certain gases and ratios of these gases
 to interpret the oil samples and diagnose problems. Because of high
 variability between different samples, different laboratories, different
 lab tester, it has been generally considered an "art" subject to different
 diagnoses from different laboratories. An on-line transformer gas monitor
 has been the "holy grail" for power transformer users.
 It is generally known that an arc under insulating oil generates acetylene
 gas (C.sub.2 H.sub.2) along with a much larger quantity of hydrogen
 (H.sub.2). It is probable that hydrogen bubbles are produced that rise to
 the top of the transformer faster than the gas can be absorbed into the
 oil. It is also probable that acetylene rides up with the hydrogen bubbles
 and therefore will be present throughout the transformer head space rather
 quickly. Gases diffuse much faster throughout gas than through oil, and
 the fault gases are uniformly distributed in the headspace, whereby a
 sample is representative of the headspace mixture. Also, transformer
 winding cooling ducts and insulation barriers tend to provide a vertical
 path for cooling oil flow and for "rising" bubbles in "rising" oil flow to
 the top of the transformer. This contrasts with a highly restricted and
 possible totally blocked horizontal oil path in large power transformers
 with directed oil flow. For fault gases to get from the point of
 generation deep in the windings to a location on the tank wall where an
 oil sample is typically taken, or an on-line analyzer may be located, the
 gases most likely transfer from rising oil to the gas head space first and
 then back into the oil at the interface. Interfacial transfer takes place
 in accordance with the relative saturation Ostwald coefficients for the
 individual gases. Thermosyphon action alone, no pumping, will slowly
 circulate oil near the head space, it takes quite some time to reach some
 degree of equilibrium with the head space, throughout the transformer. It
 may take 24 hours or more for the fault gases caused by an arc deep in the
 windings to reach the tank wall where an oil sample is taken or an on-line
 gas analyzer may be located. Pumped oil circulation will reduce this time,
 however even then significant time will elapse before any of the fault
 gases generated by the initial arc can be sensed at the tank wall. During
 this amount of time, a catastrophic failure of the transformer can
 develop. Close monitoring of the head space gases for rapidly increasing
 levels of acetylene and hydrogen would provide an indication of arcing and
 could be used to generate a dangerous condition and/or a trip signal.
 Other failure modes in aged transformers that happen due to cumulative
 effects of aging, shrinkage, loose clamping, and winding deformation are
 associated with through-faults (short circuits). The tendency is for
 individual conductors to bend and twist mechanically during
 through-faults. Failure may occur several days, weeks or months after the
 "final" through-fault. Failure starts with shorted turn or strands,
 resulting in very high localized current, not sensed by differential
 relaying, and there may not be any partial discharge activity involved.
 There is burning of conductor and strand insulation and generation of CO
 and ethylene. This may even include acetylene without arcing. Close
 monitoring of the head space gases after a through-fault could be used to
 generate an indication of failure and to generate a trip signal. Here
 again, bubbles and debris may reach some critical stress region and cause
 a major failure. This can be a high energy arc that causes tank rupture.
 Through-fault failures are probably the most difficult to detect because
 deformed winding conductors can be extremely close together and appear
 normal. Voltage between turns ranges from a few volts to hundreds of
 volts. The slightest oil space will provide adequate insulation. However,
 the slightest movement turns the situation into a serious problem. Close
 monitoring of the head space gases for a sudden spike in CO, ethylene, and
 possibly acetylene can be sensed as a dangerous and/or trip condition.
 Other gases are generated very slowly by low energy partial discharges or
 pyrolysis of insulating materials that might dissolve into the oil rather
 than form bubbles. These are typically incipient problems that would be
 considered cause for caution and merit more frequent and closer
 monitoring. In general, a "change" in the long-term trend of gassing is
 cause for alarm. "Unchanging" low levels of gases, other than acetylene,
 are normally considered as a "normal" operating condition.
 Pat. No. 5,659,126 describes a method for monitoring dissolved gases in the
 electrical insulating oil supply of an electrical transformer in which a
 blanket of gas containing a fault gas is present in the headspace above
 the insulating oil supply contained in the transformer. The method
 includes transferring a sample of the gas from the headspace to a gas
 chromatograph instrument, and measuring by gas chromatograph techniques
 the gas concentration level of the fault gases contained in the gas
 sample. The output from the gas chromatograph is processed by a computing
 device which calculates the related gas concentration level of the fault
 gases present in the oil supply. The computing device is informed of a
 partition function based on Henry's Law and converts the measured fault
 gas concentration level in the headspace to a measurement of the
 corresponding concentration level of the same fault gas in the oil supply.
 The resulting data are used for producing a reading of the fault gas
 concentration in the transformer oil supply to provide an indication of a
 specific transformer fault. The problem with measurement of dissolved or
 headspace gases is that there are many factors, including temperature,
 pressure, existing degree of saturation, and partitioning coefficients of
 individual gases that affect the concentration of the dissolved gases
 present in the oil and the gases in the head space. It is very difficult
 to sense an unchanging gas condition because of the dynamic changes in gas
 distribution through out the transformer.
 It would be very valuable to have a way to identify normal and cautious
 operating conditions without having to take into account all of the
 variables. Certain gases are slowly and consistently generated over the
 years in all operating transformers. CO.sub.2 is an example. The
 concentration may vary in the head space as temperature and pressure vary
 even though the total volume of CO.sub.2 in the transformer is essentially
 constant on a weekly/monthly basis. Considerable attention has been given
 over the years to interpreting transformer faults based upon the ratios of
 the various fault gases. This relates to the different generation rates of
 the individual gases at specific local oil temperatures. Generally, more
 than one gas is produced by any fault condition. However, these efforts
 were typically based upon oil samples tested in a laboratory, not on-line
 at the transformer. Also, it is typically assumed that equilibrium
 conditions exist between the gas producing problem, all of the oil in the
 transformer and between that oil and the head space gases. In reality, it
 is very doubtful that equilibrium ever exists in an operating transformer
 because of variations in load, ambient temperature pumped oil flow and
 thermosyphon oil flow. It is next to impossible to interpret gases this
 way reliably on a short time basis. CO.sub.2, or an introduced known
 volume of a tracer gas, can be used as a base reference. By comparing
 fault and oxygen gases to nitrogen, being essentially 100%, and the
 varying CO.sub.2 concentration related to temperature, pressure, oil
 circulation and its saturation characteristic, the varying concentrations
 of other gases can be compared in a way that reveals whether total gas
 content in the transformer is changing, plus how fast it is changing, as
 an indicator of a gas generation condition inside the transformer. The
 significance of such "true" changes, once identified, is well established
 by experience in the industry. This would provide a means for reliable
 determination of normal (green), cautious (yellow) or dangerous (red)
 operating conditions.
 OBJECTS AND SUMMARY OF THE INVENTION
 It is a general object of the present invention to provide a method for
 on-line assessment and indication of transformer conditions.
 It is another object of the present invention to provide on-line assessment
 of and indication of transformer conditions which is substantially
 independent of external factors such as temperature, pressure, saturation
 and partitioning coefficients of the individual gases in the headspace.
 It is another object of the present invention to provide a method of
 assessing transformer conditions which can adequately remove all variables
 from an assessment to provide reliable assessment on a real-time basis.
 In one aspect, the present invention relates to a method of preventing
 catastrophic failure of transformers due to arcing which comprises
 detecting rapidly increasing levels of hydrogen together with traces of
 acetylene in the headspace of a transformer and providing an alarm and/or
 trip signal.
 In another aspect, the present invention relates to a method of assessing
 transformer conditions which comprises establishing a known volume of a
 reference gas or tracer gas in the operating transformer and comparing the
 fault and oxygen gases to the varying concentration of the reference or
 tracer gas related to pressure, temperature, etc. to provide an indication
 of whether the gas composition in the transformer is changing, and the
 rate of change.

DETAILED DESCRIPTION OF THE INVENTION
 The present invention provides a system and method for on-line assessment
 of and indication of transformer conditions. Faults in a transformer, such
 as arcing, corona discharge, insulation failure, etc., result in gases
 which can be used to identify faults and provide an indication of the
 transformer condition. Fault gases typically present in gas samples
 include ethane (C.sub.2 H.sub.6), ethylene (C.sub.2 H.sub.4), acetylene
 (C.sub.2 H.sub.2), methane (CH.sub.4), carbon dioxide (CO.sub.2), carbon
 monoxide (CO) and hydrogen (H.sub.2). Dry nitrogen is generally added to
 the headspace.
 Referring to FIG. 1, gases in the headspace 11 of transformer 12 are
 periodically sampled and applied to a gas analyzer 14. The sampling is
 controlled by the programmable computer 16 which controls the sample valve
 17. The computer also controls operation of the gas analyzer 14. The gas
 analyzer is preferably a gas chromatograph. The output signal from the gas
 analyzer is applied to a processor 18 which operates on the signal and
 provides output signal which are a measurement of the concentration of
 each fault gas in the gas sample. The measurement is preferably in parts
 per million (ppm) of the fault gas per volume of the gas in the gas
 sample. The individual fault gas concentration measurements are applied to
 the programmable computer.
 As described above, an arc in the transformer under the insulating oil
 generates acetylene gas along with a large quantity of hydrogen. The gas
 bubbles rise to the top and enter the headspace. By monitoring these gases
 in the headspace a warning signal can be generated and the transformer
 taken out of service, thereby avoiding catastrophic damage to the
 transformer and associated power system. To this end, the programmable
 computer is programmed to detect sudden increases in the hydrogen and
 acetylene concentrations and to provide an output danger (red) signal
 which can be used to automatically disconnect the transformer.
 Also as previously described, failures of aged transformers can also be
 caused by through faults (short circuits). Such faults generate fault
 gases which rise to the headspace. Generally, a change in the long term
 trend of gassing is a cause for alarm. However, the concentration of the
 fault gases can change as a result of changes in temperature, pressure,
 etc. Any system which is intended to provide warning or caution signals
 because of changes in long term trends must take this into account. In
 accordance with one aspect of the present invention, this is achieved by
 using a reference gas such as the CO.sub.2 in the headspace or by
 introducing a known volume of a tracer gas into the head space. The volume
 of CO.sub.2 in the headspace can be the reference gas since its volume is
 essentially constant on a weekly/monthly basis. The fault gas
 concentration is compared by the computer to the changing reference gas
 concentration due to temperature, pressure, etc., thereby eliminating the
 effects of temperature, pressure, etc., on the fault gas concentration and
 give a true indication of any changes. This would then provide a reliable
 indication of transformer conditions.
 To this end, the computer is programmed to receive the measure of gas
 concentration including reference gas concentration, carbon dioxide in the
 present example, and perform the comparison. If the fault gases show
 changes above those due to temperature, pressure, etc., the computer
 provides an output caution (yellow) signal. If there are no changes, the
 output will show a normal (green) condition.
 The computer 16 may be programmed so as to analyze the gases for hydrogen
 and acetylene more often than for other gases which change more slowly.
 Thus, danger or red conditions which occur are monitored so as to detect
 arcing and provide a timely alert.
 Substantial benefits can be obtained by loading power transformers beyond
 current practices that are based on nameplate ratings and thermal
 algorithms that are general in nature and usually conservative. Generation
 of ethylene, ethane and/or CO generally results from excessive heating due
 to leakage flux, a defective connection or other abnormal condition
 directly related to transformer loading. Reliable detection of a true
 increase in any or all of these gases can be related to daily loading
 events and be used as a precursor of true short-term and continuous
 loading limits and initiate the cautious condition. Dynamic experience
 gained from a population of on-line monitors could evolve criteria for
 loading families of transformers beyond nameplate rating in addition to
 establishing specific limits for individual units.
 Thus, there has been provided a reliable fast-response method for on-line
 assessment and indication of transformer conditions. The method can
 generate signals indicating dangerous conditions requiring removal of the
 transformer from service, or cautious conditions for assessing load
 conditions and providing criteria for long term operating and loading of
 the transformer.