While this charge and discharge of the windings is initiated, an observer can also watch for buildup and collapse of the CT secondary current on a low-scale
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8. TESTING POWER TRANSFORMERS High-voltage transformers are some of the most important (and expensive) pieces of equipment required for operating a power syst em. The purchase, preparation, assembly, operation and maintenance of transformers represent a large expense to the power system. 8.1 OVERVIEW When transformers are received from the factor y or reallocated from another location it is necessary to verify that each transformer is dry, no damage has occurred during shipping, internal connections have not been loosened, the transformer™s ratio, polarity, and impedance agree with its nameplate, its ma jor insulation structure is intact, wiring insulation has not been bridged, and the transformer is ready for service. Physical size, voltage class, and kVA rating are the major factors that dictate the amount of preparation required to put transformers in service. Size and kVA rating also dictate the kind and number of auxiliary devices a transformer will require. All of these factors affect the amount of testing necessary to certify that a transformer is ready to be energized and placed in service. There are a multitude of checks and tests perf ormed as a transformer is being assembled at a substation. The test engineer may not di rectly perform all of the following tests and inspections but must be sure they are satisfa ctorily completed, so that the final decision over transformer bank readiness for energization can be made. Some tests and procedures may be performe d by specialists during the assembly phase. Special tests, other than those listed, ma y also be required. Many require special equipment and expertise that construction electr icians do not have and are not expected to provide. Some tests are performed by an a ssembly crew, while other tests are done by the person(s) making the final elec trical tests on the transformers. BPA has hundreds of power transformers in stalled throughout the system, and few of them are identical. The following information is not intended to describe, or include, the details for performing the entire array of tests needed to prepare transformers for service, only the tests that may be performed by fiel d personnel. Even though details have been limited, descriptions should allow field personne l to perform, or assist in performing, the basic tests they may be asked to do. Pro cedures and tests are described somewhat generically, but apply to most transformers in one way or another. Also, the following test descriptions provide an anchor point from which to ask for help when needed. The following items are discussed or described: Nameplate Data Power Meggering Auxiliary Components and Wire Checks Lightning Arrestors Hand Meggering Temperature Devices
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CT Tests Winding Temperature and Thermal Image Bushing Power Factoring Remote Temperature Indication Transformer Power Factoring Auxiliary Power Voltage Ratio Automatic Transfer Switch Polarity Cooling System Transformer-Turns Ratio Bushing Potential Device Tap Changers Auxiliary-Equipment Protection and Alarms Short-Circuit Impedance Overall Loading Zero Sequence Trip Checks Winding Resistance Before proceding with transformer measuremen ts the test engineer will become familiar with the safety rules of Section 2. THESE RULES MUST BE FOLLOWED FOR ALL TEST PROCEDURES. Following is an approximate sequence for transformer testing: 1. Inspect transformer and parts fo r shipping damage and moisture. 2. Check nameplate and prints for pr oper voltages and external phasing connection to the line or bus. 3. Check calibration of all thermal gauges and hot-spot heater, bridge RTDs and associated alarm contacts. Contact settings should be similar to the following: One stage runs all the time (forced cooling) 2nd stage at 80°C 3rd stage at 90°C Hot-spot alarm 100°C (trip at 110°C when applicable) Top-oil alarm 80°C at 55°C rise and 75°C at 65°C rise OA = no fans or pumps FA =fans running FOA = fans and pumps running 4. Check and Megger all wiring point to point: Fans, pumps, alarms, heaters, tap changers, and all other devices on the transformer and interconnecting cables 5. All banks above 150 MVA should be vacuum dried. Do not apply test voltages to the winding during the vacuum drying process. Make certain the terminals are shorted and grounded during oil circulation because of the large amount of static charge that can build up on the winding. 6. After the tank has been filled with oil, confirm that an oil sample was sent to the Chemical Lab and that its results are en tered in the bank test reports. Note the oil level and temperature at completion of filling. 7. Power operate to verify proper rotation of pumps and fans and correct operation of the under load (UL) tap changer, when provided. Also, check heater, alarms and all other devices for proper operation. 12. Following are the winding tests to be performed:
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Ratio and Polarity (Voltage Method or TTR). The preference is that all large power Transformers (>1 MVA) be tested with TTR test set. Impedance DC winding resistance Megger and Power Factor windings, bushing and arrestors. Note: Wait until 24 hours after completion of oil filli ng for Power Factor testing. 13. Load CT circuits overall and flash for polarity. 14. Before energization, trip-check bank protection schemes and make sure the gas- collection relay is free of gas. 15. When energizing a bank or picking up lo ad, monitor bank currents and voltages, including UL tap-changer operation. 16. Check proper phasing and voltage of the bank to the system before load is picked up. When possible, larg e transformers (>1 MVA) should remain energized for eight hours before carrying load. 17. Make in-service checks on meters and relays. 18. Release to Operations and report energization information to the TNE office. 19. Turn in revised prints and test reports , which should include the following: All test data Moisture and oil data Problems incurred In-service data Time energized and release to operation Any unusual problem that information will aid in future equipment testing 8.2 NAMEPLATE DATA a nd TERMINAL MARKINGS Collecting nameplate data is not testing, but it must be done for all equipment. This data is recorded by the person(s) performing the equipment tests. The act of recording the nameplate data also helps test personnel familiarize themselves with the unit to be tested. For a transformer, much of the needed information can be obtained from the main nameplate. If there is an under load tap cha nger, it too will have a nameplate. CTs have name plates and may have them on the bus hing pockets where they are mounted with additional information on a nameplate placed inside the coole r-control cabinet door (typical on large transformers). Bushi ngs, fuses, fan and pump motors, lightning arrestors, and disconnect switches will also have individual nameplates. An attempt should be made to fill in all pertinent spaces on the data sheet. A miscellaneous information space is provided on data sheet s for information that pertains to the transformer but does not have a specified plac e to record it. Recording miscellaneous information not identified specifically by a te st data sheet may be important as well.
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Terminal marking of power transformers is determined by ANSI standards. Two- winding transformers have termin als designated by H and X (e.g. H1, H2, X1, X2,), where H is the higher voltage-rated winding and X is the lower voltage winding. As viewed from the high-voltage side, H 1 bushing terminal will be locat ed on the right. Three-or- more-winding transformers will have winding de signation H, X, Y and Z, where H is the high-voltage winding (or, the highest kVA-rated winding in case windings have the same voltage rating) and X, Y, and Z are for decreasing winding voltage ratings. 8.3 AUXILIARY COMPONENTS AND WIRE CHECKING The size, type, and location of a transformer dictate the am ount of external equipment associated with it. A transformer may be outfitt ed with devices that are not to be used at the time of installation. Even if not expect ed to be placed in se rvice, all auxiliary equipment should be checked for proper operatio n to assure it is not defective and could be utilized in the future if needed. This is especially true for a new transformer, in order to verify that what has been receiv ed is fully functional. All wiring on the transformer should be check ed and verified prior to energization. Check control panels, terminal cabinets, and cabl es routed to the transformer. Torque all screw, nut, and bolt terminals for tightness, including the wires on CTs where they originate at the connection boxes on the high-voltage bushings. If there is an UL tap changer, its wiring must also be checked. Wire checking a transformer™s auxiliary equi pment is useful for several reasons. A thorough check might prevent damage or destruct ion of a unit that is difficult, expensive, or impossible to replace. This process also provides personnel an opportunity to become familiar with the equipment. A thorough wiring check forces personnel to look at the equipment in detail, serves as a cross check for drawings, and verifies that documentation and prints actually represent the physical eq uipment. It helps a ssure that wires and components are properly sized, se cure, and ready for service. 8.4 HAND MEGGERING (DC Hi-Potent ial Insulation Testing) Most hand-crank Meggers have output voltages from 250 to 500 volts DC. All wiring on transformers should be Meggered at 250 or 500 VDC. Meggering transformer wiring is emphasized because of the numerous small terminal boxes mounted on large power transformers. Conduit connecting them together can have moisture accumulation or water leaks. In addition, when wiring is pulled through the metal conduit on a transformer, occasionally th e insulation is scraped down to the bare wire. Also note that any box mounted on a vertic al surface should have a small drain hole drilled at the bottom in case water leaks in fr om a loose conduit join t. Larger boxes or cabinets usually have resistive heaters and air-vent holes covered by screens to prevent
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moisture accumulation. Terminal boxes mount ed on horizontal surfaces must have good weather seals for their covers . Any gasket with questionable ability to provide a watertight seal should be replaced. Early completion of wire checking and low- voltage-component meggering is advisable, especially when large transformers are to be tested. Completion of these tasks up front is important because it allows application of power to alarm and control circuits without worry of causing damage. Having auxiliary power available helps facilitate operational checks, especially when UL tap changers need to be operated to perform various tests. Changing tap positions manually by hand cranking the mechanism is a slow and tiresome process. 8.5 CT TESTS Transformer bushing CTs should be tested usi ng the Current Ratio test method before the transformer has been completely assembled. CTs should be tested before they are mounted on the transformer. In some cases, CTs may have to be tested by connecting test leads to both ends of an installed bushing. This can be difficult! If the CTs are already mounted in the transf ormer, large (high-capacity) current-testing leads can be pulled through the CT centers before bushings have been inserted. Occasionally it is not possible to perform a Cu rrent Ratio test. CT tap ratios can be verified by applying a voltage across the full CT winding Œ a Tap Voltage Ratio test — then measuring the voltage drop across each in dividual tap. This is a simple test to perform, and voltage ratios will be directly proportional to the CT turns ratio between taps. This Tap Voltage Ratio test, however, should not be chosen as a substitute for a Current Ratio test. The voltage method should be rega rded as the last alternative. Testing the equipment at rated current offers more assura nce that it will perform as expected when placed in service. The Current Ratio met hod reflects this philosophy; the Tap Voltage Ratio method does not. The Tap Voltage me thod cannot establish true orientation (polarity) of the installed CT, or test the primary to seconda ry current ratio, and leaves some points unverified. In addition to Tap Voltage Ratio, a secondary Tap Current Ratio test can be performed. For this test, rated or less current is injected through a tap input and the output current of the full CT winding is measured by transformer action. It is equivalent to the procedure used for performing a Short-Circuit Impe dance test on an autotransformer. CT POLARITY It is still necessary to verify CT polarity. One method used to establish CT polarity in power transformers is commonly referred to as “Flashing the CTs.” This test can be performed by applying 6-to-12 volts DC to th e transformer bushings, using a hot stick to make and break the test circuit. An autom obile battery is often most convenient because
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work vehicles are usually available at the job si te, but a lantern battery will work as well. The transformer winding resistance is usually enough to limit the cu rrent flow from a 12- volt car battery, but adding series (current-limiting) resistance (a load box) to the test circuit is advisable in any test ci rcuit with an automotive battery. Be aware that the DC test circuit will genera te a voltage kick when disconnected. Take precautions to prevent electric shock. If perf orming this test directly on CTs, always include a current-limiting resistance (a load box) in the flash lead connections. Lantern batteries have high internal resistance and don™t need an extra series resistor. Arc flash on a power transformer can be lim ited if the transformer windings are short circuited on the side opposite those being flashed through. WARNING! A transformer winding that is carrying DC current will generate a large voltage across the winding when disconnected. To prevent electric shock, use a means of insulation from the connection when breaking the test connection. A hot-stick tool is recommended. WARNING! When using lead-acid car batteries never make or break circuit connections on a battery terminal. Lead-acid batteries produce hydrogen gas when charging and have been known to explode if ignited by an electric spark when connecting directly to both battery terminals. To make a test circuit for flashing a CT, conn ect battery positive (the positive terminal of a car battery) to the polarity end, or high-voltage terminal, of the transformer. Add series resistance, such as a resistive load box, into th e test circuit to limit short-circuit current. Current-limiting the short-circ uit DC test current can reduce core magnetization. Connect battery negative (car chassis or frame ground) to the test lead used with a hot- stick tool. The hot-stick lead is used to touch the nonpolarity end of the bushing or station ground if a grounded transformer is being tested. Current must be allowed to flow through the bushings and CTs long enough to build up a charge in the transformer windings. A hot stick is required to make and break this charging path because an extremely large arc can be generated as the magnetic field collapses. An analog voltmeter (such as a Simpson VOM) is connect ed across the CT secondary terminals with the meter polarity side referenced to CT polar ity (X1). While this charge and discharge of the windings is initiated, an observer can also watch for buildup and collapse of the CT secondary current on a low-scale ammeter plugge d into an appropriate set of relays or ammeters. The test meters deflect upscale on charge and downscale on discharge, if the CT polarity is correct. Begin the test by momentarily touching the bushing cap (or ground connection) with battery negative for one or two seconds. Increase the DC application time as needed to get enough meter deflection to assure results. If the transformer requires some time to build a char ge in its windings, there may not be very much positive deflection on contact, whereas there may be a much greater negative deflection on break.
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transformer field assembly is correct, and that the transformer is ready for service. In addition, these test data reports become a valuable tool when compared with later diagnostic tests used to assess transformer condition. Single-phase test procedures can be used to measure the ratio and impedance of two-winding transformers, three-winding transformers, autotransformers, and three-phase transformers. Moreover, in the case of three-phase transformers (with a Wye connection) and grounding banks, zero-sequence impedance measurements are made with the single-phase pr ocedure. Comparisons between measurements are useful when single-phase tests are made on three id entical transformers or on each phase of a three-phase transformer, as it is unlikely th at each single-phase unit or each phase of a three-phase transformer would have sustained the same damage. Safety Before proceeding with any measurements in a high-voltage substation, the test engineer must be thoroughly familiar with the job. Make sure the transformer bank being tested is de-energized, out of service, and isolated from the power system before climbing on it or connecting it to any test leads. Follow all safety rules and be aware of any energized equi pment in the working area. Never uncoil test leads by throwing them in energized yards. Ground test equipment and test circuits to avoid stray voltages from ener gized lines, lightning or close-in faults. Take care to check the polarity of the test voltage. The grounded leg of the 115-VAC source shall be connected to ground for safety. WARNING! Extreme caution must be observed when te st-energizing high-ratio transformers (10 to 1, for example), because high voltages will be present at the transformer terminals. Care must be taken not to energiz e bus or equipment that electricians or other personnel could be working on and that test equipment does not contact energized equipment. If equipment terminals are accessible or if the bus is connected to the transformer terminals, conceivably transferring test pot entials to other locations, fence off the exposed areas with guards as required by safety procedures, warn working personnel of test-energized potentials, and if necessary prov ide a Safety Watcher. If possible, ratio test transformers before terminal connections to buses have been made. 8.8.1 SINGLE-PHASE POLARITY The polarity designation of each transforme r winding is determined by the relative direction of instantaneous current or voltage as seen at the transformer terminals. For example, primary and secondary leads are said to have the same polarity when, at a given instant, current enters the primary lead in question, the instantaneous induced voltage in the secondary is increasing when the impre ssed voltage on the primary is increasing, or conversely, if they are both decreasing at the same instant.
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Figure 47: Single-Phase Transformer Polarities A. Subtractive B. Additive Transformer polarity relates to how winding leads are brought out to bushing terminals. These connections are determined by transfor mer design, winding directions and internal- lead clearance requirements. Transformer polarity is either subtractive or additive. If the instantaneous polarity (as defined above) of adjacent terminal s is the same, transformer polarity is subtractive (See Fig. 47A). If diagonally opposite terminals of a transformer have the same instantaneous polarit y, transformer polarity is additive (See Fig. 47B). Transformer winding polarity locations are important when identical windings on a transformer are to be paralleled, when paralleling transformers with identical ratios and voltage ratings, when determining three-pha se connections of transformers, and to establish the correct connections for three-phase transformers that operate in parallel with the power system. Polarity between transfor mer windings may be determined either by comparison with a transformer of known polar ity, DC flashing or the AC method. Only the latter two methods are used by TNE. 8.8.2 TRANSFORMER POLARITY BY DC FLASHING METHOD The DC Flashing test uses 1.5-V to 6-V dry-cell batteries and a DC voltmeter with 1.5-V to 10-V scales. Meter deflection directions will be more discernible if the meter™s mechanical zero position can be adjusted in an upscale direction to allow deflecting in both directions. As shown in Fig. 48, the test is conducted by connecting the voltmeter to the transformer low-voltage terminals and c onnecting the battery intermittently to the high-voltage winding. Figure 48: Polarity Test by DC Flashing
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In Fig. 48, transformer polarity is subtractive if the mete r shows an upscale kick when test connection SW is closed and a downscal e kick when test connection SW is opened. In this case bushings ﬁA a nd Bﬂ would be labeled ﬁX1 and X2,ﬂ respectively as polarity terminals are adjacent. Transformer polarity would be additive if the meter shows a downscale kick when test connection SW is cl osed and upscale kick when test connection SW is opened. In this case bushings ﬁA and Bﬂ would be labeled ﬁX 2 and X1,ﬂ respectively as polarity termin als are diagonally opposite. Note: The inductive kick when the battery circ uit is opened will be much larger than when the battery circuit is closed. Adequate meter deflection will require a dr y-cell battery that is in good condition, low- resistance connections to bushing terminals, and positive make-and-break connections to terminal H 1. The usual procedure is for the person operating the battery connections to say, ﬁMakeﬂ when connecting the positive battery terminal to the H 1 bushing and ﬁBreakﬂ when opening the connection. Then, observation of meter deflect ions determines transformer-winding polarity. Al so, verify meter terminal markings by measuring a dry cell prior to testing. WARNING! Do not become in series with the test lead s by holding the battery clip lead in one hand while also hand contacting the H1 terminal. 8.8.3 POLARITY TEST BY THE AC METHOD If the AC method is used to determine windi ng polarity, a voltage may be applied to the high-voltage winding (H1 to H2) and the two adjacent bushi ngs of the high- and low-voltage windings (H2 to B) are jumpered together (Refer to Fig. 49, below).
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Figure 49: AC Method for Testing Transformer Polarities For the voltmeter, when V POL = VH – VX the transformer functi ons with subtractive polarity and terminals A and B would be labeled ﬁX 1 to X2,ﬂ respectively. Conversely, when VPOL = VH + VX the transformer has additive polarity and terminals B and A would be labeled ﬁX1 to X2,ﬂ respectively as polarity terminals are diagonally opposite each other. The AC Polarity test method can be conve niently made during th e Transformer Ratio test. The methodology used to check ratio include the Voltage method and the Transformer Turns Ratio (TTR) test set. The test is used to check nameplate voltage values for the range of taps on the transformer. 8.8.4 RATIO AND POLARITY VOLTM ETER TEST OF TWO-WINDING TRANSFORMERS The circuit for making ratio and polarity test s of a two-winding transformer is shown in Fig. 50.
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