Magnetic Balance Test on Transformers T ransformers:Magnetic Balance Test is performed on transformer to c heck whether the core and winding of the t ransformer is in balance position or not. Magnetic Balance is a test of Critical use as it helps us to enhance the efficiency and life of transformers The Magnetic Balance test is conducted on Transformers to identify inter turn faults and magnetic imbalance. The magnetic balance test is usually done on the star side of a transformer. A two phase supply 440V is applied across two phases, say, 1U and 1V. The phase W is kept open. The voltage is then measured between U-V and U-W. The sum of these two voltages should give the applied voltage. That is, 1U1W + 1V1W will be equal to 1U1V. For instance, if the voltage applied is 440V between 1U1V, then the voltages obtained can be 1U1V = 1U1W + 1V1W 440V = 260V + 180V The voltages obtained in the secondary will also be proportional to the voltages above. This indicates that the transformer is magnetically balanced. If there is any inter-turn short circuit that may result in the sum of the two voltages not being equal to the applied voltage. The Magnetic balance test is only an indicative test for the transformer. Its results are not absolute. It needs to be used in conjunction with other tests.
Tan Delta testing - Principle and Method Tan Delta is a a diagnostic test conducted on the insulation of cables and windings. It is used to measure the deterioration in the cable. It also gives an idea of the aging process in the cable and enables us to predict the remaining life of the cable. It is alternatively known as the loss angle test or the dissipation factor test. Principle The Tan Delta test works on the principle that any insulation in its pure state acts as a capacitor. The test involves applying a very low frequency AC voltage. The voltage is generally double the rated voltage of the cable or winding. A low frequency causes a higher value of capacitive reactance which leads to lesser power requirement during the test. Besides, the currents will be limited easier measurement.
enabling
In a pure capacitor, the current is ahead of the voltage by 90 degrees. The insulation, in
a pure condition, will behave similarly. However, if the insulation has deteriorated due to the entry of dirt and moisture. The current which flows through the insulation will also have a resistive component. This will cause the angle of the current to be less than 90 degrees. This difference in the angle is known as the loss angle. The tangent of the angle which is Ir/Ic (opposite/adjacent) gives us an indication of the condition of the insulation. A higher value for the loss angle indicates a high degree of contamination of the insulation. Method of Testing The cable or winding whose insulation is to be tested is first disconnected and isolated. The test voltage is applied from the Very Low Frequency power source and the Tan delta controller takes the measurements. The test voltage is increased in steps upto the rated voltage of the cable. The readings are plotted in a graph against the applied voltage and the trend is studied. A healthy insulation would produce a straight line. The test should be continued only if the graph is a straight line. A rising trend would indicate weak insulation which may fail if the test voltage is increased beyond the rated voltage of the cable.
Interpretation
of the test data
There are not standard formulae or benchmarks to ascertain the success of a tan delta test. The health of the insulation which is measured is obtained by observing the nature of the trend which is plotted. A steady, straight trend would indicate a healthy insulation, while a rising trend would indicate an insulation that has been contaminated with water and other impurities. Insulation
Resistance Measurement
Insulation Resistance Measurement is an important check in the maintenance of electrical equipment such as motors, transformers. It is estimated that nearly 80% of all maintenance activities in the industry is related to checking the insulation of machines. It is therefore vital that the engineer has a fair idea of the principle behind the measurement of Insulation Resistance and the methods used. Insulation resistance is measured using a meggar. In the normal operation of machinery, the insulation is subjected to moisture, oil, dust, electrostatic stress due to machine operation and a host of other elements. Hence, insulation ages and deteriorates. It is vital that the health of the insulation be monitored continually to avoid sudden, catastrophic failure of machines. Principle of Insulation Resistance Measurement The method used to measure insulation resistance is based on Ohm¶s law. A high voltage is applied across the resistance; the current that flows through the insulation is measured. The ratio of voltage and current gives the resistance. The value of the insulation resistance is usually in the order of mega ohms Instruments
used in Measurement
The instrument used to measure Insulation Resistance is known as the Megger. It is similar in principle to the ohmmeter except for the fact that a higher voltage is used. The typical meggars have a test voltage of 500V, 2500V or 5000V. The Meggar has a high internal resistance hence, there it is safe to use despite the high voltage generated. The meggar has 3 terminals. Line, Earth and Guard. The test voltage appears on the ³Line´ Terminal. This terminal is connected to the winding whose insulation needs to be checked. The ³Earth´ Terminal is connected to the ground. The ³Guard´ Terminal is connected to the surface of the insulation to measure the surface currents which tends to flow along the surface of the insulation. Method of Insulation Resistance Measurement The winding to be tested should first be isolated. The other windings of the machine which are not being tested should be connected to the ground. The voltage is applied to the winding and the reading is taken after about 60 seconds. The reading is noted. After the test is over, the winding needs to be ³discharged´. This is because the insulation acts as a dielectric forming a capacitor between the winding and the earth. This can store charge and can deliver a shock if not discharged. Discharging can be done by connecting to the ground.
What
should be the value of the Insulation Resistance?
The Insulation Resistance thus measured is usually in the order of mega ohms. A general rule of the thumb is that the minimum value should be greater than 1 mega ohm for every 1kV rating of the machine. Thus, for a machine rated for 11kV, the minimum acceptable value would be 11 mega ohms. Temperature has a direct impact on the value of the Insulation Resistance. The Insulation Resistance decreases with increase in temperature. Thus the values should be normalized for a standard temperature. That is, a value measured at 20 deg. C cannot be compared with a value measured at 30 deg. C. The value at 30 deg. C needs to be corrected. A general rule of thumb is that the insulation resistance decreases by a factor of 2 for every 10 degree rise in temperature. Hence, the value taken at 30 deg. C needs to be multiplied by a 2 to get a value corrected to 20 degrees. How do we ensure a good value of IR? The Insulation Resistance of a machine depends chiefly on the dryness of the windings. The entry of moisture into the windings lowers the Insulation Resistance. The ingress of moisture can be prevented by ensuring that the windings are kept dry. Special heaters known as anti-condensation heaters are provided in machines to keep them dry. It must be ensured that these heaters are kept on. How do we improve the Insulation Resistance value? If machines are found with low Insulation Resistance values below the permissible limits, heating the windings by connecting lamps around them is an effective method of driving moisture from the windings. If no improvement is seen even after heating, other reasons such as insulation wear or deterioration can be suspected. Other parameters related to the health of the Insulation are the Polarization Index(PI), tan delta, hipot test, step test, etc
Polarization Index Polarization Index is an indicator which gives an idea of the cleanliness of the windings. It is a ratio of the Insulation Resistance Measured for 10 minutes to the insulation resistance value measured after 1 minute. Since it is a ratio; it does not have any units.
The Polarization Index should be above 2.0 to be permissible. Machines having PI below 2.0 cannot be operated.
The Polarization Index test works on the principle that impurities in a winding act as charge carriers and are responsible for the leakage current which flows when the insulation is tested. These impurities can be polarized over a period of time. By measuring the rate of polarization, we can determine the amount of impurities in the winding and the cleanliness of the winding. The Polarization Index does not have any significant relation with temperature upto 50 deg. C. However, the Polarization Index test should not be conducted at a temperature beyond 50 deg. C
TEMPERATURE-RISE TEST PURPOSE OF THE MEASUREMENT The purpose is to check that the temperature rises of the active part of the transformer: oil, windings, core, do not exceed the limits agreed on or by the standards. The test system is different whether we have to carry out the tempeature rise test on one transformer liquid filled (oil or silicone liquid) or dry-type. In fact in the first case we use the short-circuit method while in the second case we use the back-to-back method. PERFORMANCE OF TEST ON LIQUID FILLED TRANSFORMERS told above, the test is performed by using the short-circuit method. The temperature rise of windings is determined by the resistance method. The supply and measuring facilities as well as the measuring circuit are the same as in the load losses measurement and in the resistance measurement. In addition thermometers are used for the measurement of the temperature of the oil, cooling on different points of the tank and the ambient temperature. The test is performed as follows:. As
Cold resistance measurement The resistance and the corresponding oil temperature are measured. Resistances are measured between line terminals e.g., A-B. The winding temperature is the same as oil temperature. Determination of oil temperature rise The power to be supplied to the transformer is the sum of the no-load losses and the load losses on the nominal tapping on which the temperature-rise test is to be generally performed. With this power the transformer is warmed up to thermal equilibrium. The supply values and the temperatures of different points are recorded at suitable time intervals. The oil temperature rise above the cooling medium temperature can be calculated from the steady state conditions. Determination of windings temperature rise Without interupting the supply the current is reduced to rated current for min. 1 h. The supply and the temperatures are recorded as above. When the current has been cut off the hotresistance measurement is performed. The test connection is changed for carrying out the resistance measurement and after the inductive effects have disappeared the resistance -timecurves are measured for a suitable period of time (zero time is the instant of switching off the supply). The resistance is measured between the same line teminals as in the cold resistance measurement. The resistance of the windings at shut-down are obtained by extrapolating the resistance-timecurves to the instant of switching off. The temperature rises of the windings above the oil temperature are calculated on the basis of the "hot" and "cold" resistance values and the oil temperature. The temperature rises of the windings above the cooling medium temperature are found by adding the temperature to the before mentioned winding temperature rises. The ambient temperature is measured by means of three sensors Pt 100 of a digital thermometer, which are placed at different points around the transformer at a distance defined by the standards approximately half-way up the transformer. sensors Pt 100 of the some thermometer are used for: - Top oil temeprature on an oil filled thermometer pocket on the tank cover. Other
- Temperatures of oil coming in and going out of the cooling radiators.
RESULT The temperature rises are calculated as follows:
Oil temperature rise The temperature rise of top oil
is:
(10.1)
PN = rated losses Pk75°C + P0 Pm = power supplied during the test X = exponent according to the standard = 0.8 ( ONAN distribution trafo) = final temperature rise of oil by temperature rise oil-time curve The average temperature rise
of the oil is:
(10.2)
= oil temperature going into the radiators = oil temperature coming from the radiators = average ambient temperature
Windings temperature rise
The average oil temperature
before the hot-resistance measurement is:
(10.3)
= top oil temperature the average winding temperature
is:
(10.4)
R 1 = cold resistance R 2 = hot resistance km = 235 for copper km = 225 for aluminium = the average oil temperature during cold resistance measurement The average temperature gradient g of the winding above the oil temperature is: (10.5)
IN = winding rated current Im = test current y = exponent according to the standard = 1.6 ( ONAN) The average winding temperature rise (10.6)
above the ambient temperature is:
Result The report indicates: - cold resistance values and the corresponding oil temperature - oil and cooling system temperatures and the corresponding losses - hot resistances at shut-down - temperature rises calculated from the measuring results.
TEMPERATURE RISE TEST FOR DRY TYPE TRANSFORMERS ACCORDING TO IEC 726 PURPOSE OF THE MEASUREMENT The purpose is to check that the temperature rise of the active part of the transformer windings and core do not exceed the agreed limits those stated by the standards. PERFORMANCE TESTS ON DRY TYPE TRANSFORMERS The temperature rise test on this kind of transformers is performed by using the simulated load method. This method is used either for an enclosed and non-enclosed dry-type unit air natural cooled or forced cooled. Temperature tests are made by utilising the rises obtained from two tests, one with no-load loss only, and one with load losses only, i.e., short-circuit run. The no-load test, at rated voltage (fig. 4.1), is continued until steady-state conditions are obtained; indivi dual winding temperature rises, resistance after shutdown.
Fig. 4.1
is determined via measurement of winding
Circuit to temperature rise test (no-load losses at nominal voltage)
G1 supply regulator, T1 transformer to be tested, T3 current transformers, P1 - P2 - P3 digital wattmeter. The short-circuit runs with rated current (fig. 3.1) flowing in one winding and the other winding short-circuited is stated immediately following the no-load run, and continued until steady-state conditions are obtained; individual winding temperature rises, measurement of winding resistance after shutdown.
Fig. 3.1
is determined via
Circuit to temperature rise test (short-circuit at nominal current)
G1 supply regulator, T1 transformer to be tested, T3 current transformers, P1 - P2 - P3 digital wattmeter. The resistances of the windings at shut-down are obtained by extrapolating the resistance-timecurves to the instant of swithing off. The calculation method of the windings temperature is:
= average ambient temperature
R 2 = hot resistance R 1 = cold resistance
km = 235 for copper km = 225 for aluminium
The total winding rise, , of each winding, with rated current in the winding and normal excitation of the core, is calculated by the following formula:
Winding temperature-rise correction for reduced current When the input test current ( It) is below the rated value of current (In), but not less than 90% (In), of the temperature rises, of the windings, shall be measured by the resistance method when steady-state conditions have been reached, and corrected to rated load conditions, by the formula:
The value of q shall be taken as: AN transformers: 1.6 AF transformers: 1.8 Determination
of constant temperature conditions
The ultimate temperature rise is reached when the temperature rise becomes constant; this is considered to have been achieved when the temperature rise does not vary by more than 2% of the permissible temperature rise per hour or 2 K per hour, whichever is smaller. During the test, the ambient temperature is measured by means of three pt100 probes. Other pt100 probes are used for checking the temperature in centre of top yoke, on the LV windings up and bottom. RESULTS
The report indicates: - cold resistance value and the corresponding ambient temperature - core and windings temperatures and their corresponding losses - temperature rises calculated from the measuring results.
Performance of test on dry-type transformers The temperature rise test on this kind of transformers is performed by using the back-to-back method when two similar transformers are available. The two transformers one of which is the unit under test, are connected in parallel as shown on the following fig. 10.1. One side of the transformers (generally the H.V. side) is excited at the rated voltage of the transformer under test. By means of an injected voltage by a special regulator, the rated current is made to flow in the transformer under test.
Fig. 10.1
The circuit for temperature rise test on dry-type transformer
G supply regulator, T1 transformer under test, T2 similar transformer, T3 T5 current transformers, T4 voltage transformer, P 1 wattmeter, P2 - P4 ammeters, P3 voltmeters (r.s.m. value). During the test the ambient temperature is measured by means of three sensors Pt 100 of a digital themometer which are placed at different points around the transformer under test. Other sensors of the same thermometer are used fo checking the temperature in centre of top yoke and as close as practicable to the innermost low-voltage winding conducors at the top of the winding. After the transformer has got to steady state conditions the hot measurement of
windings resistances is carried out. The resistances of the windings at shut-down are obtained by extrapolating the resistance-time-curves to the instant of switching off. The calculation method of the windings temperature is: (10.7)
R 1 = cold resistance R 2 = hot resistance km = 235 for copper km = 225 for aluminium = the average oil temperature during cold resistance measurement
