# 7 Testing of Transformers

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1 7 Testing of Transformers The structure of the circuit equivalent of a practical transformer is developed earlier. The performance parameters of interest can be obtained by solving that circuit for any load conditions. The equivalent circuit parameters are available to the designer of the transformers from the various expressions that he uses for designing the transformers. But for a user these are not available most of the times. Also when a transformer is rewound with different primary and secondary windings the equivalent circuit also changes. In order to get the equivalent circuit parameters test methods are heavily depended upon. From the analysis of the equivalent circuit one can determine the electrical parameters. But if the temperature rise of the transformer is required, then test method is the most dependable one. There are several tests that can be done on the transformer; however a few common ones are discussed here. 7.1 Winding resistance test This is nothing but the resistance measurement of the windings by applying a small d.c voltage to the winding and measuring the current through the same. The ratio gives the winding resistance, more commonly feasible with high voltage windings. For low voltage windings a resistance-bridge method can be used. From the d.c resistance one can get the a.c. resistance by applying skin effect corrections. 44

2 3 A2 S s ~ 1 a A2 a2 + A1 a1 A1 a1 (a)a.c.test (b)d.c.test Figure 18: Polarity Test 7.2 Polarity Test This is needed for identifying the primary and secondary phasor polarities. It is a must for poly phase connections. Both a.c. and d.c methods can be used for detecting the polarities of the induced emfs. The dot method discussed earlier is used to indicate the polarities. The transformer is connected to a low voltage a.c. source with the connections made as shown in the fig. 18(a). A supply voltage s is applied to the primary and the readings of the voltmeters 1, 2 and 3 are noted. 1 : 2 gives the turns ratio. If 3 reads 1 2 then assumed dot locations are correct (for the connection shown). The beginning and end of the primary and secondary may then be marked by A 1 A 2 and a 1 a 2 respectively. If the voltage rises from A 1 to A 2 in the primary, at any instant it does so from a 1 to a 2 in the secondary. If more secondary terminals are present due to taps taken from the windings they can be labeled as a 3, a 4, a 5, a 6. It is the voltage rising from smaller number towards larger ones in each winding. The same thing holds good if more secondaries are present. 45

3 Fig. 18(b) shows the d.c. method of testing the polarity. When the switch S is closed if the secondary voltage shows a positive reading, with a moving coil meter, the assumed polarity is correct. If the meter kicks back the assumed polarity is wrong. 7.3 Open Circuit Test W A Io Im jxm Ic Rc (a)physical Arrangement (b)equivalent Circuit Figure 19: No Load Test As the name suggests, the secondary is kept open circuited and nominal value of the input voltage is applied to the primary winding and the input current and power are measured. In Fig. 19(a), A, W are the voltmeter, ammeter and wattmeter respectively. Let these meters read 1, I 0 and W 0 respectively.fig. 19(b) shows the equivalent circuit of the transformer under this test. The no load current at rated voltage is less than 1 percent of nominal current and hence the loss and drop that take place in primary impedance r 1 +jx l1 due to the no load current I 0 is negligible. The active component I c of the no load current I 0 46

4 represents the core losses and reactive current I m is the current needed for the magnetization. Thus the watt meter reading W 0 = 1 I c = P core (38) I c = W 0 (39) 1 I m = I0 2 I2 c or (40) R c = 1 I c andx m = 1 I m (41) 1 Io Figure 20: Open Circuit Characteristics The parameters measured already are in terms of the primary. Sometimes the primary voltage required may be in kilo-olts and it may not be feasible to apply nominal voltage to primary from the point of safety to personnel and equipment. If the secondary voltage is low, one can perform the test with L side energized keeping the H side open circuited. In this case the parameters that are obtained are in terms of L. These have to be referred to H side if we need the equivalent circuit referred to H side. 47

5 Sometimes the nominal value of high voltage itself may not be known, or in doubt, especially in a rewound transformer. In such cases an open circuit characteristics is first obtained, which is a graph showing the applied voltage as a function of the no load current. This is a non linear curve as shown in Fig. 20. This graph is obtained by noting the current drawn by transformer at different applied voltage, keeping the secondary open circuited. The usual operating point selected for operation lies at some standard voltage around the knee point of the characteristic. After this value is chosen as the nominal value the parameters are calculated as mentioned above. 7.4 Short Circuit Test The purpose of this test is to determine the series branch parameters of the equivalent circuit of Fig. 21(b). As the name suggests, in this test primary applied voltage, the current and power input are measured keeping the secondary terminals short circuited. Let these values be sc, I sc and W sc respectively. The supply voltage required to circulate rated current through the transformer is usually very small and is of the order of a few percent of the nominal voltage. The excitation current which is only 1 percent or less even at rated voltage becomes negligibly small during this test and hence is neglected. The shunt branch is thus assumed to be absent. Also I 1 = I 2 as I 0 0. Therefore W sc is the sum of the copper losses in primary and secondary put together. The reactive power consumed is that absorbed by the leakage reactance of the two windings. W sc = I 2 sc (r 1 + r 2 ) (42) Z sc = sc I sc (43) (x l1 + x l2 ) = Z 2 sc (r 1 + r 2 )2 (44) 48

6 A W sc (a)physical Arrangement Isc r1 jxl1 r 2 jx l2 sc (b)equivalent Circuit Figure 21: Short Circuit Test 49

9 In the equivalent loss method a short circuit test is done on the transformer. The short circuit current is so chosen that the resulting loss taking place inside the transformer is equivalent to the sum of the iron losses, full load copper losses and assumed stray load losses. By this method even though one can pump in equivalent loss inside the transformer, the actual distribution of this loss vastly differs from that taking place in reality. Therefore this test comes close to a load test but does not replace one. A W 1 2Io Io Io 1 I 2 I2 I2 I 2 A W 2 s Figure 22: Back to Back Test - Phantom Loading In Phantom loading method two identical transformers are needed. The windings are connected back to back as shown in Fig. 22. Suitable voltage is injected into the loop formed by the two secondaries such that full load current passes through them. An equiv- 52

10 alent current then passes through the primary also. The voltage source 1 supplies the magnetizing current and core losses for the two transformers. The second source supplies the load component of the current and losses due to the same. There is no power wasted in a load ( as a matter of fact there is no real load at all) and hence the name Phantom or virtual loading. The power absorbed by the second transformer which acts as a load is pushed back in to the mains. The two sources put together meet the core and copper losses of the two transformers. The transformers work with full flux drawing full load currents and hence are closest to the actual loading condition with a physical load. 53

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[ Assignment View ] [ Eðlisfræði 2, vor 2007 26. DC Circuits Assignment is due at 2:00am on Wednesday, February 21, 2007 Credit for problems submitted late will decrease to 0% after the deadline has passed.

### The Two-Wattmeter Method

The Two-Wattmeter Method In a three phase, wye or delta three wire system, under balanced or unbalanced conditions, with any power factor, the two-wattmeter method is a practical and commonly used method

### APJ ABDUL KALAM TECHNOLOGICAL UNIVERSITY. EE100 Basics of Electrical

APJ ABDUL KALAM TECHNOLOGICAL UNIVERSITY EE100 Basics of Electrical SAMPLE QUESTION PAPER Maximum Marks 100 Part- A (10 questions) Attempt all the questions 10*4=40 1. We have two identical 10 V electromotive