This invention relates to an apparatus for preventing thermal damage to an electrical power transformer.
Electrical power transformers have been used for many years and have been changed little in design over those years. They are normally cooled by circulating oil through the windings and the conductors are insulated from each other with cellulose, that is, paper. If the cellulose insulation gets too hot for too long a time, it can become structurally weak and allow the conductors to touch each other causing a catastrophic short-circuit.
xe2x80x9cCurrentxe2x80x9d in a power transformer is defined as the electrical current in one of the main windings.
xe2x80x9cHot Spot Temperaturexe2x80x9d in a transformer is defined as the temperature of the cellulose insulation within the transformer, at its hottest location. This temperature is usually a calculated value, since the location and parameters for a particular transformer design are not usually known accurately.
An alternative is a measured hot spot temperature: the maximum of several fiber-optic-sensed temperatures at carefully selected points within the transformer. This is an expensive and delicate method; hence the widespread use of the usual calculated method.
xe2x80x9cThermal Damagexe2x80x9d in a power transformer is defined as structurally-weakened cellulose insulation within the transformer.
The structural damage is cumulative, that is, if the strength is reduced by a certain amount during an over-temperature condition, then the insulation does not recover its strength; and subsequent over-temperature conditions cause additional damage.
xe2x80x9cLoss of Lifexe2x80x9d is the accumulated damage to insulation at the hottest spot location. It is defined by a mathematical function taken from a Guide of the Institute of Electrical and Electronics Engineers: C57.91-1995 xe2x80x9cIEEE Guide for Loading Mineral Oil Immersed Transformers.xe2x80x9d It depends on two factors: the degree of excessive hot spot temperature and the time over which the condition persists.
xe2x80x9cTotal loss of lifexe2x80x9d of the insulation in a power transformer is defined as the condition for which the cellulose insulation has reached a proportionxe2x80x94such as one-halfxe2x80x94of its original structural strength.
Up till now, thermal damage has been assumed to occur either when a current limit has been exceeded or when a hot spot temperature limit has been exceeded.
It is one object of the present invention therefore to provide an improved apparatus for preventing thermal damage to an electrical power transformer.
According to one aspect of the invention there is provided an apparatus for preventing thermal damage to an electrical transformer comprising:
a processor
an input current transducer,
an input ambient temperature transducer,
means for an operator to enter a setting of loss of life limit beyond which thermal damage would occur,
an output trip contact arranged to be activated when the setting is exceeded,
an output alarm contact arranged to be closed by the processor,
the processor being arranged to repeatedly calculate a time based on a prediction algorithm that uses as input the present value of current and the present value of ambient temperature and to close the output alarm contact when the prediction algorithm predicts a trip condition in a time less than a given value, such as thirty minutes.
Preferably the processor is arranged to begin the accumulated loss of life calculation only when the hot spot temperature exceeds its normal rated value.
Preferably the processor is arranged to reset its calculated accumulated loss of life to zero whenever the hot spot temperature falls below its normal rated value.
Preferably there is provided a second output alarm contact arranged to be closed when the prediction algorithm predicts a trip condition in a time less than a smaller value, such as fifteen minutes.
Preferably the set time period, for example one half hour, is selected so as to provide the operator with a sufficient period of time to take effective remedial action.
Preferably the calculation is carried out on the assumption that the present current and ambient temperature will remain unchanged during the set time period.
Preferably there is provided a time display and wherein the processor is arranged to indicate on the time display a remaining portion of time from the present time to the time at which the setting would be exceeded.
Preferably the processor is arranged to cancel the alarm condition in the event a calculated time period of any subsequent calculation is greater than the predetermined set time period.
Preferably there is also provided an output contact that closes at a set maximum value of hot spot temperature regardless of whether or not the accumulated loss of life limit has been reached.
Preferably the hot spot temperature trip is included in the prediction algorithm.
Preferably if either the accumulated loss of life setting is exceeded or the hot spot temperature setting is exceeded the prediction algorithm generates an alarm contact closure.
The invention as defined above therefore provides an electronic hardware-software combination that predicts when the insulation in an electrical power transformer will suffer undesirable thermal damage, up to say thirty minutes before such thermal damage would occur. An alarm output contact closes, and a xe2x80x9ctime-to-tripxe2x80x9d countdown begins, from thirty minutes down to zero, at one minute intervals. If conditions change, that is remedial action is taken to alleviate the condition, then the prediction system continuously re-predicts.