How to accurately test a three-phase transformer?

Three Phase Transformer December 23, 2020

Testing of transformers is a crucial process that must be conducted throughout its life span. The timely testing of transformers not only improves the working but also increases the life span of a transformer. But, more than conducting tests technicians with the right knowledge must also practice testing techniques. The testing of transformers is generally conducted to ensure that all electrical, mechanical, and thermal standards are met.

The most important question that arises here is, how can you test a transformer accurately? Is there any accurate testing procedure followed for every transformer category? This blog highlights commonly conducted tests on a three-phase transformer.

  1. Voltage Ratio Test: This one is a simple and routine test of the transformer. For conducting this test, you need to supply the three-phase 415V to HV (High Voltage) windings, keeping LV (Low Voltage) open. Measure the induced voltage at both terminals to find out the actual voltage ratio of the transformer. Repeat the same with all the tap positions separately.
  2. Winding Resistance Rest: This test helps in determining I2R losses and checking winding temperatures. It ensures the healthiness of the equipment i.e., checking loosened ends, broken conductor strands, high voltage leads, bushings, etc. Make all the connections required for the test as per your transformer’s configurations. Pass the known DC current through the winding under test and record the voltage drops across each terminal. Compare the resistance value with each adjacent winding at the same tap. If all the readings are within the 1% limit of each other, the specimen has passed the test. Also, ensure checking each tap while conducting this test.
  3. Magnetic Balance Test: If you want to study imbalance, if any, in the magnetic circuit, conduct the magnetic balance test. Firstly, apply 3 phase 415V to the primary winding and remove any one phase (for delta fashion winding), and any two (for star fashion winding). Record voltages in three phases using a multimeter in the primary and secondary winding. Repeat for the other two phases. If the results conclude that flux in one produced by one limb, is the sum of flux produced by the other two limbs, your core is in a healthy condition.
  4. Magnetizing Current Test: It locates the defects in magnetic core structure, shifting of winding, the problem in tap changers, etc. At first, take the taps changer to its lowest position and open all HV & LV terminals. Then apply 3 phase 415V on the line terminals and measure the supply voltage and current in each phase. Now repeat the test while keeping the tap changer at its normal and highest positions.
  5. Insulation Resistance Test: an essential type test, ensuring the soundness of the overall insulation system of the power transformer manufacturer. To begin with, disconnect all the line and neutral terminals of the transformer. Connect Megger leads to LV & HV bushing studs to determine IR value in between LV & HV windings. Next, connect Megger leads to HV bushing studs and transformer tank earth point to measure IR value in between HV winding and earth point. Repeat the same with LV bushing studs to record insulating resistance IR value between LV winding and earth. Normally, you can see two similar higher readings on the two outer phase limbs on the transformer core and one lower reading on the center limb phase.
  6. Short Circuit Test: Firstly, isolate the power transformer from service. Remove HV/LV jumps and disconnect neutral from the earth/ground. Short the LV phases and connect short-circuited ones to neutral. Energize the HV side by the supply of the LV side. Now, measure the current in neutral, high lines, and high voltage currents. The wattmeter will indicate the total cu loss of the transformer.

These tests will determine what part of your setup is faulty and if it needs repair or an immediate replacement. The reason for performing tests using set procedures is because random testing can be as hazardous as using a faulty transformer. Moreover, testing the transformer timely & accurately has its own benefits. It increases safety and eliminates the risk of hazard, boost equipment efficiency, & reduce expenses & losses.

Get your transformers tested with the assistance of your transformer manufacturer, and continue enjoying the benefits of a healthy transformer.

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