به دلیل عملکرد رله ارت فالت می باید تست و بازدیدهای لازم از ترانس بعمل آید. در زیر برخی توصیه های معمول در این زمینه آمده است.
1- Insulation resistance-dielectric absorption testlng.
The insulation resistance test is probably the best known and most often used electrical test for insulation. It is used primarily to detect low resistance paths to ground or between windings that result from carbonization, deterioration, or the presence of moisture or dirt. It will not indicate the actual quality of the insulsion, but when conducted under controlled condltions, with the data compiled for a number of service intervals, trending data can be developed, and definite conclusions can be drawn as to the insulation’s rate of deterioration.
(a) High and medium voltage insulation systems are usually designed to withstand large potentials and large quantities of electricity. Because of this, special equipment must be used to perform resistance tests. Ohm’s Law applies for all systems, and no matter how high the applied voltage, or how “resistance” the insulating material, there will be a measurable leakage current, and there will be a resultant resistance value.
Because of these conditions, leakage currents are usually stated in micro-amps (one millionth of an amp) and resistance values in megohms (one million Ohms).
Most hand-held meters are not capable of reading these extremes accurately, and special equipment is used. Even if a unit can read these extremes accurately, it must also be able to supply the necessary quantities of electricity to charge the massive conductors and contacts found in a transformer.
(b) An insulation resistance test is usually performed with a megger, an instrument that is not only capable of reading high resistance values, but is also able to produce the necessary currents and voltages to obtain the readings. Megger test potentials are usually applied at 500, 1,000, 2,500, and 5,000 volts DC. These potentials are obtained by using a motor driven or hand-crank operated magneto. The hand crank units are both lightweight and portable, and because they require no batteries or external source, they are also extremely dependable. Motor-driven units, on the other hand, are capable of achieving higher and more constant test voltages, but are practically useless withoutbatteries or a external source. Both units are available in models capable of producing accurate readings for resistance levels as high as 100,000 megohms.
(c)The following conditions should be observed when performing an insulation resistance test: Make sure that both the tank and core iron are solidly grounded. Disconnect any systems that may be connected to the transformer winding, including high and low voltage and neutral connections, lightning arrestors, fan systems, meters, and potential transformers.
Potential transformers are often located on the line sides of breakels or disconnects; when the disconnecter is opened, there will still be a path available to ground. Short circuit all high and low voltage windings together at the bushings connections; jumpers should be installed to ground, and no winding should be left floating. The ground connection on grounded windings must be removed. If the ground cannot be conveniently removed, the test cannot be performed on that winding. Such a winding must be treated as part of the grounded circuit.
(d) Using a megohmmeter with a minimum scale of 20,000 megohms, measure the insulation resistance across the connections as shown in figure 7-l.
(e) The terminal markings are referenced as follows:
The L terminal is the line or “Hot” terminal of the instrument, where the test potential is generated. The E terminal is the “Earth” or ground connection. The G terminal is the “Guard” terminal, it is used to isolate a certain portion of the circuit from the test.
(f) These test connections are considered the bare minimum for a maintenance testing cycle, and should be applied only to a transformer that has already been in service. They will not detect shorts between the individual windings on the high or low side.
(g) The test voltages should be as close as possible to the voltage rating of the component to which it is being applied.
(h) All final insulation resistance values should be corrected to 20 °C to compensate for varying conditions at the time of the test, and to allow for comparison of readings taken at different test intervals. The winding temperature, and not the atmospheric temperature, should be used for insulation resistance tests. It is important to note that when a transformer is de-energized, there is a proportional change between the actual temperature of the windings and the exterior tank or oil temperature indicated by the temperature gauges.
Average readings should be taken for various points on the transformer tank, and then the insulation resistancereadings corrected to 20 °C.
(i) There are many schools of thought as to what is considered an acceptable insulation resistance value. A widely accepted rule of thumb for insulation resistance values is “the kV rating of the item under test plus one megohm.” This should be considered as a bare minimum value, and any values equal to five times this amount should be investigated. If the investigation reveals nothing, then the humidity and condition of the item under test should be considered. A 10 megobm resistance value for a piece of 5 kV equipment should not be accepted without investigation, but if the humidity is high, and the insulation is dirty, that value may be acceptable.
(j) The final criterion for evaluating insulation resistance values should be the amount of change from the manufacturer’s factory test values, or from the last test interval. The manufacturer should be contacted if any values are significantly lower than the factory values.
(k) To obtain useful data that is indicative of the dielectric capabilities of the transformer’s insulation, it is recommended that a polarization index or dielectric absorption ratio be computed for all resistance readings.
The polarization index is determined by holding the applied voltage of the megohmmeter constant, and taking resistance readings at the end of l- and 10-minute intervals. The apparent increase in the resistance is due to the dielectric charging of the insulation. The polarization index is computed by dividing the lminute value into 10-minute value.
(l) The dielectric absorption ratio is computed in the same way, except that 60.second intervals are used. These values should, theoretically, be independent of temperature or other outside factors.
(m) The polarization index and dielectric absorption ratios are also subject to different methods of interpretation. In any case, they should alw.ays be greater than one, and any downward trend in their value over a number of test intervals indicates deterioration that should be investigated.
2- Winding resistance measurements.
If a measurement of the winding resistance shows no appreciable change from the factory test values, then it can be assumed that there are no loose connections. Maintenance testing should include only the appliedtap position. Three-phase wye windings should be measured phase to neutral, and delta windings should be separated to read individual windings, if possible. If the windings cannot be separated, three separate readings should be taken, with each winding measured in parallel with the other two, and the results evaluated as a function of the parallel and series connectionsinvolved. In this instance, the comparison of the three readings (the difference should be no greater than 1 percent) will indicate whether or not there are any problems.
(a) The winding resistance can be measured with a low resistance ohmmeter, or with a Kelvin bridge. Be sure to make good contact with the winding leads, and to wait 3 minutes after initial contact before taking a reading. This delay is necessary due to the induction created by the transformer windings.
Because the windings will store energy, it is important to shut off the test set and allow the energy to dissipate before removing the test leads.
(b) If the factory test values are available, or if the transformer cannot be disconnected, the resistance values for each winding should be compared to those of the adjacent windings. A difference of one percent indicates a potential problem.
3- Contact resistance. Loose connections can result in overheating and possible equipment, failure.
All high and low voltage and ground connections should be inspected, and if any abnormal conditions are noted, the contact resistance should be measured to ensure that solid contact is being made. This testing works especially well in conjunction with infrared scanning. If a connection shows hot on the IR scan, and its contact resistance cannot be lowered by tightening, it should be replaced.
4- DC high potential testing. The DC high potential test is applied at above the rated voltage, and can cause damage to the transformer if special precautions are not taken. When a leakage current passes through the insulation system of an oil-tilled transformer, different amounts of the total voltage are dropped in the solid (paper) and liquid (oil) parts of the insulation. These voltage drops are caused by the resistance of each insulating component, and heat is created. Under normal AC operation, only a small amount (l/4) is dropped across the solid insulation. The remaining 3/4 is dropped in the oil, where the heat can be easily dissipated, and little harm is done.
(a) When a DC potential is applied, nearly 3/4 of the voltage is dropped across the solid insulation. This changing stress is further complicated when higher than operating level voltages are applied. DC Overpotential testing is of little value as a maintenance test, and is usually conducted for acceptance purposes, or after repair of transformers. In any event, high potential testing should not be conducted unless a satisfactory result is obtained for the insulation resistance. It ishighly recommended that the manufacturer be contacted before performing this test, and that only manufacturer’s procedures be followed in conducting this test.
(b) DC Step Voltage Testing is often performed on transformers at less than the rated voltage of the winding under test. Voltages are applied in equal increments at timed intervals (usually 1 minute) and the rate of change of the leakage currents is monitored. When the applied potential is plotted against the leakage currents (on Log-Log paper) the rate of change should yield a reasonably linear slope. Leakage current jumps of more than 100-150 percent times the previous value usually indicate a problem, and the test should be discontinued so that the circuit can be investigated. Like all of the other tests, this test is especially useful when repeated tests over extended time intervals are considered, and trending data is generated.