This article will show you how to test a transformer. By applying simple Ohm’s law i.e. Type tests of the transformer are to verify whether the transformer is designed as per the customer’s expectations and design specifications. The transformer tank must be grounded during the test. On the other hand to measure the iron losses open circuit test is conducted. During the test, it is important not to remove current or voltage leads while current is still flowing through the transformer. By measuring a transformer's winding resistance from one HV transformer bushing to another, transformer resistance testing can reveal a great deal of information about the transformer. Wattage Rating Output transformers are rated in wattage. Unlike DC system, there are no fixed positive and negative poles in AC system, and hence, transformers cannot have fixed positive and negative terminals. If a de-mag operation is not performed, excess residual flux in the transformer core may cause a large inrush currents on the primary side that could trip the protective relays. A relatively safe way to test transformers is to hook up a small AC voltage (1-5VAC) and measure the voltages across the other windings. (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, Calculation of winding temperature at the end of temperature rise. We use helical windings low voltage, high capacity transformers, where the current is higher, at the same time windings turns are lesser. Voltmeter leads shall be independent of the current leads to protect it from high voltages which may occur during switching on and off the current circuit. Direct loading method. The table below demonstrates how test data for a simple 13,200-208/120V three-phase transformer with three primary de-energized tap changer positions can be recorded. Next measurement… When finished - Demagnetize. Hi friends, in this article, I am going to describe about polarity of transformer windings, if you are interested, keep reading. Before conducting a transformer winding resistance test, it is important to observe all safety warnings and take proper precautions. Large values may cause inaccuracy by heating the winding and thereby changing its temperature and resistance. By using the test procedure outlined above we can test our spare output transformers to see if we have one that will meet our approximate 30:1 turns ratio requirement. Transformer oil testing; Transformer winding resistance measurement; Magnetic balance test; Routine tests of transformer include all the type tests. Instead, replace it. During measurement, the current has to be at the value of IN or close to this value as far as possible. To minimize observation errors, polarity of the core magnetization shall be kept constant during all resistance readings. The tertiary winding reduces the impedance of the circuit so that the fault current easily passes to the ground. Remove the neutral ground and isolate the CT from any associated burden. Photo: TestGuy, Transformer Winding Resistance Test Connections - 3-phase Delta Winding. Since the transformer’s primary winding is designed for a higher voltage (220 V, 230 V) than the secondary (12 V, 24 V, etc. ), it is wound with a large number of windings. In addition to the obvious faulted transformer winding (i.e., an open winding or … Photo: TestGuy. Due to the low resistance of the transformer winding, the currents that circulate can end up being quite large and dangerous. Photo: TestGuy, 3-Phase Transformer Dual Winding Resistance Test Connections. For star connected three-phase winding, the resistance per phase would be half of the measured resistance between two line terminals of the transformer. Since the IR value of transformer insulating oil may vary with temperature. Inductance. Full disclaimer here. Ideally these windings would have zero resistance, but in reality, transformers experience power loss due to load resistance, which can easily be tested with an ohmmeter. Large values may cause inaccuracy by heating the winding and thereby changing its resistance. Keep in mind that each transformer configuration is different and your specific setup may not apply to what is shown below, consult the user manual that came with your test kit for more information. Many power and distribution transformers are equipped with tap changers to increase or decrease the turn's ratio depending on the supply voltage. Visually Verify Transformer. When conducting an insulation resistance test on transformer windings, a 2.5 or a 5 kV megger can be used. When using modern test sets with a de-mag function, it's recommended that both current and voltage leads be attached to a high side winding for the demagnetization process. For star connected winding, the resistance shall be measured between the line and neutral terminal. This is an internal problem with the windings and typically means the transformer will not function. This test checks the resistance of the non-safety insulation between the screens and windings of the transformer. Opens where there is supposed to be resistance or resistance where there is supposed to be an open are dead giveaways. A winding resistance test of a transformer is carried out as a type test, routine test and also as a field test. The ohmmeter will display either a blank screen or the word ‘open’. At the end of the test, the test voltage shall be rapidly reduced up to 1/3 the full voltage before disconnection.The test shall be performed on all the windings. If the transformer housing is swollen or shows burn marks, please do not check it further. The test shall be carried out by any one of the following methods.Direct loading method. The oil temperature at the time of testing is taken as the average of top oil and bottom oil temperatures of transformer. The single-phase applied voltage wave shape shall be approximately sinusoidal. So, the transformer has to go through this type of test during the manufacturing itself. In this method of measurement of winding resistance, the test current is injected to the winding and corresponding voltage drop across the winding is measured. The standard Megger testing equipment is used, the ‘line’ terminal of which is connected to the winding or core bolt under test. Steps to Perform Polarity Test; Additive Polarity: In additive polarity, the same terminals of the primary and the secondary windings of the transformer are connected Subtractive Polarity: In subtractive polarity, different terminals of the primary and secondary side of the transformer is connected. However the good news is when either the primary or secondary winding developed a short circuit, it will usually produced a bad smell and the transformer gets very hot on its core or … Interpretation of winding resistance results is usually based on a comparison of each resistance value with each adjacent winding at the same tap. An approximation is developed to calculate the temperature of winding at that condition, as follows: The average temperature of insulating oil should be taken 3 to 8 hours after de-energizing transformer and when the difference between top and bottom oil temperatures becomes less than 5oC. A step-down transformer is one that reduces the input voltage. A test set used for a power transformer would be much different than one designed for small instrument transformers. Note: The measurement of winding resistance of a transformer shall be carried out at each tap. In such cases, it's acceptable to measure the resistance of each winding at only the designated tap position. (Optional) Perform a power-factor or dissipation-factor tip-up test on windings rated greater than 2.5 kV. While physically and electrically isolating the circuitry fro… The transformer turns ratio test is used to make sure that the ratio between the windings of the primary and secondary coils follow the proper specifications. We are a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for us to earn fees by linking to Amazon.com and affiliated sites. Transformer winding resistance measurements are obtained by passing a known DC current through the winding under test and measuring the voltage drop … Photo: TestGuy, Transformer Winding Resistance Test Connections - 3-phase Wye Winding. The first type of test is called output transformer testing, and the second type is called a transformer continuity test. Prior to testing, short the primary winding of the CT under test by connecting H1 and H2, then short the secondary winding of the CT under test by connecting X1 and X2-X5. Unless one of the wires has continuity to the transformer core, the polarity doesn't matter. Transformers are very versatile devices. Polarity Test is performed to determine the correct polarity of the transformer. In addition to the obvious faulted transformer winding (i.e., an open winding or shorted turn), subtler problems can be detected. Insulation resistance. All other steps to be taken during transformer winding resistance measurement in these methods are similar to that of current voltage method of measurement of winding resistance of transformer, except the measuring technique of resistance. If you're very careful while testing, you can reverse the transformer winding connections and connect the secondary windings to BP1 & BP2 and the primary windings to optional BP3 and BP4. This test is also known as full load test, copper loss test and winding loss test. The megger test or insulation test is carried out to ensure the healthiness of the overall insulation system of an electrical power transformer. BP3 & BP4 can be connected to the secondary of a step-up transformer, or the primary of a step-down transformer. Consider this transformer circuit: If the transformer is measured without oil, the winding temperature is normally assumed the same temperature as the surround air. The megger will help to detect areas of deteriorating insulation where the windings may soon short. Even though the voltages induced in the secondaries of the transformers are AC, the same circulating currents flow in each of the secondary windings. Test equipment connections should be made in the following order: For single-phase and simple Delta-Wye configurations, the following connections can be used. How do I test my welding transformer? You can do a quick test for each winding for an open while the transformer is still connected in a circuit. Back-to-back method. IR tests are performed to test the integrity of the winding insulation but at very high voltages. Make LE's efforts sustainable. A shorted primary or secondary winding is still checked well under resistance tests. But it has a smaller current flow, so it uses a thinner wire of a longer length. Windings are the conductors wrapped in various forms like helical, disc, cylindrical, crossover which generates MMF that is carried by the core to other windings for having the different levels of voltages. When measuring winding resistance, the reading should be observed and recorded once the resistance value has stabilized. In bridge method of measurement of winding resistance, the errors are minimized. This test is also known as no load test, iron loss test or core loss test. Generally, transformer windings are immersed in insulation liquid and covered with paper insulation, hence it is impossible to measure the actual winding temperature in a de-energizing transformer at the time of transformer winding resistance measurement. If the transformer has a winding temperature gauge, use these readings, otherwise the winding temperature is assumed the same as the oil temperature. Make sure that the bushing porcelains are cleaned since dirt deteriorates the insulation resistance. The main principle of the bridge method is based on comparing an unknown resistance with a known resistance. It is used for testing the high rating transformer. The resistance measured by Wheatstone bridge. This test ensures that the transformer will provide the proper step-up or step down in voltage. The basic concept of energy transfer between mutual inductors is useful enough between a single primary and single secondary coil, but transformers dont have to be made with just two sets of windings. By applying simple Ohm’s law i.e. There are different types of windings used for different kinds of applications and arrangements. This test verifies the polarity of the windings, along with checking for the accuracy of the turns of the winding. Because changing ratio involves a mechanical movement from one position to another, each tap should be checked during a winding resistance test. If all readings are within one percent of each other, the specimen is considered to have passed the test. "On-load" tap changers and voltage regulators may be operated with the test set left on while changing from tap to tap, this not only saves time but can also verify the make before break function of the tap changer. Measuring the resistance of transformer windings assures that each circuit is wired properly and that all connections are tight. Short Circuit Test. Regardless of the configuration, the resistance measurements are normally made phase-to-phase and the readings are compared with each other to determine if they are acceptable. As in delta connection the resistance of individual winding can not be measured separately, the resistance per winding shall be calculated as per the following formula: The resistance is measured at ambient temperature and then converted to resistance at 75oC for all practical purposes of comparison with specified design values, previous results and diagnostics. Photo: Testguy. In the first step, consider the winding 1 and 2 are short-circuited. Insulation resistance. The windings consist of the current-carrying conductors wound around the sections of the core, and these must be properly insulated, supported and cooled to withstand operational and test conditions.. The oscillator's AC signal tests the integrity of the primary and secondary windings; broken wires or shorts inside the transformer will produce no output. Current Transformer Winding Resistance Test Connections. Explanation With Connection Diagram Rated current is applied to the transformer windings at … Modern test equipment for this purposes utilizes a Kelvin bridge to achieve results; you might think of a winding resistance test set as a very large low-resistance ohmmeter (DLRO). This test calculates the angular displacement and relative phase sequence of the transformers and can be conducted at the same time as ratio and polarity tests. The voltages of the phase of primary and secondary windings in each transformer can be recorded and comparisons made to get the phase relation between them. The test set uses a 16 VAC, 60hz signal to determine the AC load resistance in ohms. The output of the transformer varies from 160 – 1000 kVA from 0.23-15 kV. During routine maintenance, it may not always be feasible to test each tap due to time constraints or other factors. The measured resistance should be corrected to a common temperature such as 75°C or 85°C using the following formula: Once all testing is complete, perform a demagnetization operation on the transformer. Testing the transformer in your welder requires you to understand how the transformer has been put together.