We get the percentage impedance. as required). So we have to choose a transformer suitable for parallel operation whose phase sequence and phase divergence are same. This value of magnetic flux depends on the current of I2. Repeat this test between other phases as example (A & a , A & b , A & c , A & n , B & a , B & b , B & c , B & n , C & a , C & b , C & c , C & n ). Polarization index = 10 minutes value/1 minute value. These tests are carried out at the worksite of the transformer. Measure insulation resistance between low voltage windings and the main body of the transformer. Oil condition of OLTC to be examined in every year. These tests are part of the routine tests. Let us first discuss about the action to be taken on power transformer in monthly basis. So primary to secondary and primary to earth of a current transformer are measured with 2.5 or 5 KV megger. Measure insulation resistance between low voltage windings and high voltage windings. For confirming the specifications and performances of an electrical power transformer it has to go through a number of testing procedures. Thirdly, Electrical special transformer tests. This involves a transformer at no load condition and on load condition. A schedule of such maintenance of current transformer is preferred below for ready reference. In case of an autotransformer, a magnetic balance test of transformer should be repeated for LV winding also. Now disconnect the transformer neutral from ground. Moreover very frequent maintenance of bus PT or CVT may not also be possible as far taking shutdown of such PT or CVT total bus section would be out of protection and metering. What is a water cut meter and where is it used? The angle between Im and Ia is 90 (as Im lagging). The voltage tap changer is set to position 1. Power Transformer Maintenance And Acceptance Testing (on photo: Austin Energy’s Substation Maintenance Power Transformer hot oil processing and vacuum filling; credit: austinenergy.blogspot.com) It has no moving parts, requires minimal maintenance, and is capable of withstanding overloads, surges, faults, and physical abuse that may damage or destroy other items in … If a current transformer does not perform properly, there may be a huge disturbance in the system due to malfunctioning of protection relays. Check the continuity of all internal connections. The current drawn in this case (I0) should not exceed 1% of the transformer’s rated current as the transformer is opened at primary. For that, trip contacts and alarm contacts of the said devices are shorted by a small piece of wire, and observe whether the concerned relays in remote panel are properly working or not. The following is helpful; If the extinction voltage is less than the transformer’s operating voltage, there is a real danger to the transformer, which means that the partial discharge will never stop if it happens to the transformer. Most testing routines will include most of these tests. If it is a three-phase transformer, each phase is tested individually. • NETA defines the standards by which electrical equipment is deemed safe and reliable. How to do the maintenance of a transformer. This test ensures that each circuit is properly wired properly and that all connections are tight. If any failure happens in the transformer then it will affect the whole system. In conclusion: Therefore, we know that there are many types of transformer testing done at the factory or the transformer’s worksite, as mentioned above, and all revolve around the same tests. Insulation resistance test of transformer. Thermo vision scanning of primary terminals and top dome of a live CT should be performed at least once in a year. If there is a higher current than the other two phases, then the transformer is stopped and checked for the reasons. In a step-down transformer designed to reduce the voltage, the number of turns in the secondary coil must be lower than in the first, while in step-up transformer, the secondary coil must have more turns than the first coil. Simultaneous readings of voltage are taken to the low-voltage and high-voltage windings area after the voltage is applied to one winding. Then measure the line voltage on the low voltage side as well as the phase voltage. A varying magnetic flux will be created in the transformer’s core by the varying current in the primary winding and thus a varying magnetic field through the secondary winding and the varying magnetic field will induce a voltage in the secondary winding and this effect is called induction. If it is higher than 5%, the transformer test fails. Resistive value of earth connection and rizer must be measured annually with clamp on earth resistance meter. It is recommended that tank and core should always be grounded when this test is performed and that each winding is short-circuited at the bushing terminals. Determine the voltage amplitude at which partial discharges of a specified low magnitude commence with increasing and crease with decreasing voltage. Here also if no failure of insulation, the test is successful. Verify guarantees, design calculations, and manufacturing quality. Similarly, we perform various tests on the transformer to ensure the transformer’s required technical specifications and analyze its performance. To determine the magnitude of the specified discharge quantity at a specified voltage. All electrical equipment needs to be maintained safely from some of the failures. Verify guaranteed values to prove the quality of the core steel and the quality of core staking. The calibration of OTI and WTI must be carried once in two years. Transformer testing is one of the most important lectures. The ratio is calculated under no-load conditions, using a tool known as a turns ratio tester. In this case (the case of partial discharge), we call the voltage value. This scanning can be done with help of infra-red Thermo-vision Camera. • NETA: • The InterNational Electrical Testing Association (NETA) is an accredited standards developer for the American National Standards Institute (ANSI). This current causes a magnetic flux in the iron core. We can also neglect voltage drop that means the measured power, in this case, represent iron loss (P0) only. 8 Common Testing Procedures for Electrical Transformers, Here Are Some Tips on How to Install an Electrical Transformer, AE can also be used to estimate corrosion in reinforced concrete, The NEC Requires GFCI Protection for Receptacles Located Outdoors, A Multimeter Is Not Just for Electricians, How the IRS Determines Independent Contractor Status, How Nonprofits Benefit From Measuring Outcomes, 7 Types of Electrical Conduit—EMT, PVC, RMC, and More. Some variations in testing results in natural, depending on the moisture, cleanliness and the temperature of the insulation, but to pass, the insulation must demonstrate a higher resistance than prescribed international standards for the type of transformer. Absorption coefficient = 1 minute value/15 secs. If any failure happens in the transformer then it will affect the whole system. As such it is desirable to perform various preventative maintenance activities to ensure the transformer maintains a high level of performance and a long functional life. This test is crucial, and it is considered the backbone of transformer testing. This article is related to Transformer Maintenance Tests, and what assessment is needed in transformer. (Supervisory Control and Data Acquisition), Programmable Logic Controllers (PLCs): Basics, Types & Applications, Diode: Definition, Symbol, and Types of Diodes, Thermistor: Definition, Uses & How They Work, Half Wave Rectifier Circuit Diagram & Working Principle, Lenz’s Law of Electromagnetic Induction: Definition & Formula. Thermal induced failures in transformers are caused by the paper insulation of the windings to lose its physical strength to the point where it can no longer withstand the mechanical movement that occurs inside a transformer. For testing the transformer, de-energize the transformer to begin inspection and maintenance. In this test, it does tests like dielectric and short circuit test, measurement of zero sequence impedance of three-phase transformers, measurement of acoustic noise level, measurement of harmonics of no-load current, and it also measures the power taken by the fans and oil pumps. The test is successful if no collapse of the test occurs. Insulation breakdown will also cause transformer failure. In a 3 phase transformer, it is essential to carry out a vector group test of transformer. Determine the resistance and reactance of windings (R1, R2, X1, X2) of the transformer at primary and secondary sides. Select the tapping of the off-circuit tap changer. Take the voltmeter readings from the primary winding. The voltage induced in different phases of a transformer in respect to neutral terminals given in the table below. We can calculate the duration of the test from this relation: Repeat the test at each tap changer point of the transformer. Therefore, we must correct the measured value during the test according to the following equations.

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