When the installation of the transformer and its accessory equipment has been completed, the installation inspection has been performed, all major mechanical and electrical problems have been found and repaired, and all construction around transformer is completed, then installation testing can begin. All the mechanical or electrical work on a transformer should be completed to prevent a change in the transformer that could affect test results. It is essential that all the testing be done at this point. The only time the base guideline test data can be obtained is while it is new and has no operating hours. All tests should be performed (without any being omitted) because some of the tests overlap each other, providing backup results of the same test, which can confirm, deny, or raise a question as to test results accuracy.
Prior to the acceptance of the installation, you will be required to perform various tests and document the test data and results on the Form P-025.
Each test may determine more than one thing or detect more than one problem. All test are necessary for that reason. Normally a problem should be verified or the results verified by more than one test.
Refer to ANSI/IEEE Standard C57.125, 1990 for additional information on documentation and analysis of transformers. Also refer to C57 Appendix A for further information on the tests discussed in this section. A copy of this Appendix is contained later in this module. The installation testing should be performed just prior to energizing the transformer. There should never be a delay of more than one month between testing and energizing the transformer (less if possible). This prevents possible damage or change in the condition of the transformer.
Table 3 indicates which field test are appropriate for the following conditions: • New Transformer Installation
• Shop Rework of Transformer
• Periodic Maintenance of Transformer • Field Rework at Plant Side
• Troubleshooting Transformer Problem • Factory Testing of New Transformer
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Notice the notes section which provides codes for varying conditions that may exist. This table is not all inclusive but provides a general guideline for field testing.
The installation electrical tests are necessary to determine that the operation of the transformer is correct, and to ensure the proper setup of the auxiliary and protective equipment and controls prior to energizing the unit. These tests also provide a benchmark for subsequent operational tests that may be performed in the future. There are eleven installation tests that should be performed on power transformers (see Form P-000 and Form P-025):
• Turns -Ratio Tests • Polarity Test
• Winding Resistance
• Winding-Insulation Resistance • Polarization-Index Test
• Core-Ground Inspection and Test • Tank-Ground Inspection and Test • Tap-Setting Inspection
• Power Factor
• Applied Voltage Test (Hi-Pot)
• Oil Test
– Field Test – Eight Lab Test • Gas Analysis
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Turns-Ratio Tests
The purpose of this test is to test each phase winding turn-to-turn insulation system in a transformer. A transformer turns ratio (TTR) test primarily detects inner winding short circuits or tap changer alignment problems. The test performs polarity checks, and verifies the no-load voltage ratio of all transformer tap positions. If exciting current is also measured, the test may possibly detect high resistance due to loose connections or grounded conductors. This test is also used to investigate the possibility of problems following a transformer exposure to severe duty faults. Before starting the test ensure that the TTR tester is calibrated or has been calibrated within the past 6 months. If it has not, perform a quick calibration test according to TTR manufacturer's requirements. A TTR test should be performed on all new transformers after installation. This test is an easy way to verify that the internal connections are correct and that there are no winding to winding shorts. This test also verifies that a transformer meets its nameplate ratings.
A TTR test should be made at the following two points:
• At all connection positions of the no-tap changer for deenergized operation with the no-load tap changer on the rated voltage position (or center tap)
• At all load tap changer positions with the tap changer for the deenergized operation on the rated voltage position (or neutral)
The TTR test is performed using instruments designed for this purpose. The most widely used instrument for this test is convenient for field use because it requires no outside power. It has a hand-crank power supply providing a very low-voltage power source, such as 8-10 volts and 50 to 60 Hz. There are also motor-driven sets. Using this type of equipment, the test may be performed even if the oil is removed from the transformer. Two windings, primary and secondary, on one phase are connected to the instrument and the internal bridge elements are varied to produce a null indication on the detector (no reading.) Usually, the exciting current is also being measured. Compare the measured ratios with the ratios on the nameplate. They must compare to within 0.5% , but should be even closer to actual values. It is imperative that ratio measurements be made on all taps to confirm the proper alignment and operation. Before the results are analyzed, there should be a great amount of data available that was accomplished with the instrument being used.
When performing this test due to a maintenance function (transformer has been energized), be sure to follow proper safety guidelines prior to making the turn ratio measurement. A check is made to verify that the transformer is de-energized, that power supply devices are properly "locked-out." Safety-ground connections are removed from the winding circuits only as needed to perform the test and then replaced when finished.
The basic method of performing this test is accomplished by using a TTR test set. A typical test set is one manufactured by the James G. Biddle Company (catalog No. 55003, Figure 86). The TTR test set contains a small AC power source of about 8 volts which is applied to the low voltage winding of the transformer to be tested and also to the low voltage winding of the parallel reference transformer, located in the TTR set (Figure 87). The ratio of the reference transformer is adjusted until a detector comparing the high voltage winding of each transformer (the transformer under test and the reference transformer) is nulled. The turns ratio of the transformer under test can then be read directly from the dials on the TTR test set.
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Figure 87. Schematic Diagram for Transformer Turns-Ratio (TTR) Test Set
When testing a transformer, the turns ratio is measured for all combinations of winding pairs. For a two winding transformer there is only one combination LV to HV but for a three winding transformer there are three combinations (LV to HV, HV to TV and LV to TV; LV = low-voltage winding, HV = high-voltage winding, TV = tertiary-voltage winding). See Figure 88 for polyphase transformer connections.
For transformers with a no-load tap changer, the turn ratio is measured between the high- voltage winding on each tap position and the rated low-voltage winding.
In accordance with ANSI standards, the measured turn ratios must be within 0.5% of nameplate values. Ratios exceeding this acceptance value should be investigated to identify potential problems.
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TTR Testing Indications
The indicator windows, above the four decade switches, give the actual turns ratio of the transformer. This is only true when the TTR test set is in a balanced condition. Because of this, you must first balance the TTR test set before reading the dials. The test set is balanced when the following conditions occur:
• The test set crank is being turned. • The voltmeter reads 8 volts.
• Ammeter (excitation current meter) reading decreases toward zero. • Null detector has no deflection.
Figure 89 shows the TTR test set indications that are present when the test set is balanced. These readings show a turns ratio of 115.415:1. Note that for the accuracy of this instrument, the instrument manufacturer suggests that the instrument should be read to the nearest tenth, resulting in a reading of 115.42:1.
Expected Test Results
The voltage ratings, or ratios, which are indicated on transformer nameplates are based on the number of turns of wire on the primary and secondary. So, the turns ratio between the primary and secondary is equal to the voltage ratio between the primary and secondary. This ratio is only true for phase voltages. The following equation shows this relationship. This equation should be compared to each position of the transformer NLTC position:
Vp
Ps = Calculated Turns Ratio
where: Vp is Primary Phase Voltage Vs is Secondary Phase Voltage
The expected turns ratio value is equal to the calculated value + 0.5%. If the value that is measured is not in this range, there is a problem with the transformer or the test set.
Interpretation of Data
There are two different starting points to consider in interpretation of TTR test data. These are balanced and unbalanced readings.
Balanced Readings - A balanced reading that is + 0.5% of the calculated turns ratio is the
normal indication. In this case, the transformer is fine. A balanced reading that is more than + 0.5% of the calculated turns ratio shows a problem. The problem is either a wrong connection or a short circuit.
If the reading is out of specification, check to make sure the TTR tester has the right connections. If the connections are right, the problem may be shorted turns, which will require an internal inspection or a detanking of the transformer to find the problem.
Unbalanced Readings - If a balanced reading cannot be obtained, several problems are
possible. These are:
• Wrong connection or test method • Open circuit in windings
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First verify that the proper test method is being used for the transformer under test (turns ratio or inverse turns ratio). Next, make sure the connections are right. If these are all right, a short- or open-circuit exists in the windings. The type of problem can be determined by the other indications on the TTR test set.
Open-circuit indications are as follows: • Normal exciting current • Normal generator voltage
• No deflections of the null detector Short-circuit indications are as follows:
• High exciting current • Low generator voltage
Again, an internal inspection or a detanking of the transformer might be required to find the problem.
Review of Sample Data
Figure 90 shows the TTR test set readings from a transformer with the following nameplate voltage ratings.
• Winding Configuration : Delta-Wye • Primary Voltage : 13,800 Volts • Secondary Voltage : 480y/277 Volts
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To find out if these readings are good or bad, you must first calculate the turns ratio, using nameplate data as follows:
Turns Ratio = VP VS =
13,800 Volts
277 Volts = 49.819:1 Next, find out what the allowable range of measured readings is as follows:
(49.819) (0.5%) = 0.2491
Allowable Range = 49.819 + 0.2491 = 50.0681 = 49.819 – 0.2491 = 49.5699
Now, compare the measured value to the calculated value. The measured value is 49.705. These readings do not show a problem.
Figure 91 shows the TTR readings from the same transformer taken one year later. Look closely at the indications given in the figure. Notice that there is normal excitation current, normal generator voltage, and the null detector is in the center position. At a first glance, these readings may appear normal, until the values in the windows are read closely.
Since these are all at zero, there should be some deflection of the null detector. And when there is no deflection, this transformer, most likely, has an open winding circuit.
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Winding-to-Winding Polarity Test
Polarity
Standard American practice is to mark winding terminals as X1, X2, etc., in the low voltage winding; H1, H2, etc., in the high-voltage winding starting from the ends of the winding which go around the core in the same direction, as shown in Figures 92 and 93. By definition terminals H1 and X1 have the same polarity.
Polarity is of no importance in a singly connected distribution transformer, but it is of importance if transformers are to be paralleled or banked. Polarity can be determined by the connection of Figure 92 or Figure 93, but it can be most conveniently checked at the same time as the TTR ratio by the connection where wrong polarity is immediately apparent.
Three-Phase Polarity and Phase Sequence
Polarity, taken phase by phase, is no different in a three-phase transformer from what it is in a single-phase transformer. However, because of the various ways in which the windings can be connected, polarity alone does not describe the relation between the primary and secondary systems of a three-phase voltage. Figure 94 shows a number of the possible connections of three-phase windings, together with rules for determining phase sequence. The markings of leads H1, H2, and H3, shown in Figure 94, indicate that the induced voltages of these leads to neutral pass through their positive peak values in this order in time. An angular displacement of 30 electrical degrees exists in Group 2 of Figure 94.
If the terminals of all phase windings are available, checking polarity phase by phase is simple and straightforward. If one winding is connected in Y with the neutral not available, three-phase voltage must be applied and measurements of voltage made according to Figure 94.
Voltmeter Flicks-Method Polarity Test
Test Method - The polarity of a transformer is a designation of the relative instantaneous
directions of currents in its leads. Primary and secondary leads are said to have the same polarity when at a given instant the current enters the primary lead in question and leaves the secondary lead in question in the same direction as though the two leads formed a continuous circuit.
Terminals which lead out from the transformer in the same direction around the core, H1 and X1, have the same polarity, and the instantaneous induced voltage at these terminals has the same sign. If a voltage is applied to H1 - H2, and connections made as shown by the dotted lines, the polarity is correct if V < H1H2.
Figure 92. LV/HV Winding Markings/ Polarity Voltage Reading Method Polarity Test