ABSTRACT
Temperature monitoring system for power transformers submerged in oil applied preferentially in transformers submerged in oil and it presents several facilities in the sense of becoming temperature monitoring more reliable and safe, with highlight for the redundant measurement of temperature of the top of the oil; self-calibration of the temperature measurement entrances by means of internal references and specific electronic circuits; timing between successive activation of the groups of forced cooling when the deactivation of monitoring system on screen; internal watches with auxiliary alimentation without battery; safety reinforced in the alarms signaling for elevated temperature and finally automatic adjustment of the alarm values of the temperature differential of the commuter of derivations in load.
CHAPTER ONE
1.0 INTRODUCTION
The work focuses on “TEMPERATURE MONITORING SYSTEM FOR POWER TRANSFORMERS WITH AUTOMATIC CIRCUIT BREAKER”; notedly of a system that offers a series of facility that guarantee larger reliability and safety of operation related to the monitoring of temperature of this kind of equipment; with highlight for the redundant measurement of temperature of the top of the oil, having two temperature sensors; self-calibration of the temperature measurement entrances by means of sensors that send signals for specific electronic circuits (automatic circuit breaker); timing between successive activation of the forced cooling groups when of the deactivation of the monitoring system occured; internal watches with auxiliary alimentation by “super capacitors”; alarms signalling of elevated temperature accomplished by two microcontrollers that change information to each other through photocouplers and finally automatic adjustment of the alarm values of the temperature differential of the Commuter of Derivations in Load (CDC).
1.1 BACKGROUND OF THE INVENTION
The transformers and reactors of power are equipments broadly used in systems of generation, transmission and distribution of electric power of average, high and extra-high tension.
These equipments generally use as insulating and of heat transfer ways, some kind of oil, which can be mineral, petroleum derivative, vegetable, silicone-based and so many another.
Such equipments are mounted in tanks and are provided of a nucleus, which constitute in magnetic plates, on which are mounted several coilings, such as primary, secondary and tertiary. The set nucleus more coilings is denominated active part, and works completely immersed in insulating oil, which should have its temperature supervised or monitored constantly, together with the temperature of the coilings.
Some transformers also have several intermediary derivations in their coilings, usually denominated “taps”, which allow the selection of the wished transformation relation between primary and secondary of the transformer, enabling thus the regulation of the exit tension of the transformer. The selection of tap to be used is made by a denominated equipment “Commuter of Derivations in Load” or “Commuter Under Load”, herein referred just as “commuter”, which is able to make tap change with the energized transformer and without interrupting the chain supply for the load. Also the commuter operates immersed in insulating oil, however in a compartment separated of the oil of the transformer in order to avoid the contamination of that, since in the tap change operations the contacts of the commuter interrupt electric arcs that deteriorate gradually the oil in which they are immersed.
1.2 OBJECTIVE OF THE PROJECT
To allow its secure operation, the power transformers are equipped with devices for thermal supervision, with the objective of controlling automatically the system of forced ventilation, alert the operator in case of high temperatures and to make the emergency disconnection of the transformer in case of extreme temperatures through a circuit breaker.
1.3 SCOPE OF THE PROJECT
These devices of thermal supervision usually make the measurement of the temperature of the top of the oil, obtained through a sensor installed in the cover of the transformer, region where usually is the hottest oil, and also the temperature of the coils, obtained from indirect form due to the difficulty in install sensors in this region, because of the elevated electric potential of the coiling related to mass.
The sensor used for temperature measurement of the top of the oil generally is a sensor of variable resistance RTD type (for example, Pt100 ohms to 0° C.) installed in the cover of the transformer. This sensor is connected for circuits in a measurement entrance in the temperature monitoring system that measure the electric resistance of the sensor, reading that is further converted in the value of corresponding temperature. Monitoring systems of existing temperature have means to make the calibration of this measurement, such as through variable resistors (trimpots) or by software, using in any of the cases an external pattern as reference for the calibration
The temperature of the coiling is obtained by calculation, basing on temperature of the oil summed to an elevation or gradient of temperature that is calculated with base in the load chain of the transformer.
From the value obtained for the temperature of the coiling, the temperature monitoring system makes the automatic command of the forced cooling system of the transformer, which is formed by heat radiators or dressing rooms equipped with fans for forced circulation of air and/or bombs for forced circulation of the insulating oil. The fans and/or oil bombs are generally divided into two groups, so that a group enters operation automatically when the temperature reaches determined landing, and other group is activated when the temperature increases more, reaching a superior landing.
When the forced cooling is commanded in the automatic way above described, the refrigeration groups are activated in a sequential way like increases the temperature. This fact introduces as a consequence the unlike use of the equipments of each group, once that those activated to smaller temperatures will operate for more time that the activated to largest temperatures. Or still in a worse condition, in which some cooling groups never will act, depending on conditions of ambient temperature and shipment of the transformer. To avoid this occurrence, generally it is included in the activation circuit of the refrigeration groups a manual selector key, which allows inverting the activation order of the groups. To be effective, the position of this key should be inverted manually by the operator from time to time
1.4 SIGNIFICANCE OF THE PROJECT
The temperature monitoring system has an exit contact for the automatic activation of each refrigeration group, contacts that are never closed simultaneously, but always with a compulsory interval in order to avoid that all the motors be energized at the same time, what it would cause a elevated departure chain.
To increase the safety of the operation, the exit contacts of the temperatures monitoring system for activation of the refrigeration groups generally are of an Usually Closed type (NF), of a way that in failure case in the temperatures monitoring system, or in case of auxiliary alimentation lack for that system, groups of forced refrigeration will be activated, reducing the risk of over heating of the transformer while monitoring system is out of operation.
Equally important in the transformers is the monitoring of the oil temperature of the compartment of the commuter under load. As in normal conditions of operation the commuter is not an important source of heat, the oil temperature of the commuter will tend to be alike or lightly smaller than the oil temperature of the transformer, accompanying the variations of that. For this reason, the thermal supervision of the commuter is made through the calculation of the temperature difference between commuter and transformer. In normal conditions this differential will be next to zero, and in case of a thermal defect in the commuter this differential will increase. It is fitting then a limit value for this differential, which if overtaken will activate an alarm indicating possible defects in the commuter.
The temperature monitoring systems frequently have an internal watch, which supplies referenced date and schedule information for several ends, as, for instance, periodic records in memory of the temperature measurements and of the occurrence in events (alarms disconnections, activations of the forced cooling, and others). During the normal operation of the temperature monitoring system, the necessary energy for the operation of the watch is provided by own auxiliary tension with which is fed monitoring system. However, when of the referred alimentation lack, it is necessary to keep the watch in operation, in order to avoid the same has to be hit in the return of the energy. This task is generally guaranteed by use of an internal battery in the temperature monitoring system.
1.5 LIMITATION OF THE PROJECT
The systems nowadays used to the monitoring of temperatures in transformers have some inconvenient, which are:
- They use only a sensor for measurement of the temperature of the top of the oil, what it obliges, in defect case in this sensor, to the retreat of the service transformer, because the sensor is installed in the cover of the transformer, region with risk of electric discharge due to the proximity with the high tension conductors. This disconnection of the transformer just for repair in a temperature sensor can carry elevated costs for the owner of the transformer, be for revenue loss during the disconnection of the transformer, be for stop of the production in industries that depend on the electric power, be for penalties imposed by government regulators agencies.
- The measurement circuits of the temperature sensors of the oil need to be calibrated periodically to make the resistance reading with the required precision, what it consumes time of the maintenance people and obliges the periodic disconnections of the transformer for calibration, carrying the costs described in the previous item.
- In case of a defect in the temperature sensor of the oil, as a bad operation in the sensor that causes increase in your resistance, that increase will be interpreted as if it was a temperature increase, provoking incorrect measurement of the temperature of the top of the oil, what it can carry for false alarms or even untimely and unnecessary disconnection of the transformer, causing interruption in the electric power and great prejudices supply.
- In case of failure in the temperatures monitoring system, or in case of auxiliary alimentation lack for that system, the exit contacts for activation of groups of forced refrigeration will be closed simultaneously, since they are of NF kind, causing the simultaneous departure of all the refrigeration groups, what it carries a departure elevated chain of the motors that can provoke the disconnection of protection circuit breakers of this circuit. To avoid that, it is compulsory the use of relays of external timing in the temperatures monitoring system.
- In substations non-assisted, where there is not a present operator, or still if the operator forget to change the position of the selector key that inverts the activation order of the groups of forced refrigeration, these groups will be used of unlike form, what it can carry several inconvenient, such as the excessive and precocious waste of some fans and bombs while others remain without use. It can occur also problems associated to the inactivity for long periods of the equipment that stay without operating, just as the drying of the lubricant grease.
- The battery used internally to the temperature monitoring system to keep the hit of the watch in case of auxiliary tension lack must to be obligatorily substituted at the end of its useful life or next to it. That aggregates costs as work hand used in the maintenance substitution and costs, mostly in facilities in remote locations and without the presence of an operator, considering although this operation will be able to demand the service retreat of the transformer, since the monitoring of temperature will be inoperative.
- The temperature monitoring systems are equipped with several exit contacts, used mostly for the ignaling of current alarms of high temperatures; activation of the groups of forced cooling and for disconnection of the transformer for very high temperatures. The maid solution nowadays in existing systems for activation of these exit contacts generally consists in the use of an exit pin of the microprocessor or microcontroller associated to each exit contact, taking over this pin a certain logical level (generally 1) for the condition of closed contact and the logical level opposite (generally 0) for the contact condition of open. This solution introduces some deficiencies with regard to the reliability of the temperature monitoring system against improper activations of these contacts; especially the contacts for disconnection of the transformer, which if activated can improperly cause interruption in the electric power supply, with financial and social prejudices. In the existing, eventual systems external interferences (tension surges, intense electromagnetic field, extreme temperatures) or even internal defects to the microprocessor (or microcontroller) can lock it and change improperly the logical level of its exits, causing activation of the contacts associates to them.
1.5 PURPOSE OF THE PROJECT
The purpose of this work is to design a means of monitoring the oil temperature of a transformers continuously and protects them from overheating. Since transformers are vital elements of the electric power transmission and distribution infrastructure, they need to be monitored to prevent any potential faults. Failures in a transformer can easily costs several million dollars to either repair or replace, and will also cause a loss of service to customers and revenue until the symptom is found and repaired.
In transformers the need for monitoring the oil temperature is very much necessary. Turning the transformers OFF whenever the temperature is greater than its maximum operating temperature protects the transformer from damage and also the life-time of the transformers can also be increased. Since the oil is present inside the transformer there is a necessity to develop a system which can sense the temperature of the oil inside the transformer and automatically switches ON the Circuit breaker if temperature exceeds the limit.
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