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The purpose and significance of transformer no load test and load test

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The transformer loss is an important performance parameters, on one hand, it indicates the efficiency of the transformer in operation, and on the other hand, it also indicates whether the performance of design can meet the requirements. The transformer no load loss and no load current measurement, load loss and short-circuit impedance measurements are routine testing for transformer.

For the transformer no load test, that is applied the rated voltage to any loop coils, when other coils are open, to measure the transformer no load loss and no-load current. The no-load current and rated current are expressed as percentage.

1, The load test is to measure the no load loss and current loss at rated voltage, in the process of testing, open circuit in high voltage side, pressurization in the low voltage side, the test voltage is the rated voltage in low voltage side, when in low test voltage, the test current is a few percent of rated current.

2,The power capacity options of the transformer load test: to ensure the power waveform distortion does not exceed 5% of regulator capacity, the no load capacity of test object should be under 50; when use the voltage regulator to pressure, the regulator should be less load capacity 50% of capacity; when using generators tests, load capacity should be less than 25% of generator capacity.

In no load test, the test voltage adopt the rated voltage of low pressure side, which focus on measuring no load loss. The main loss is iron loss. The iron loss is irrelevant to the size of load, that is the no load loss is equal to load iron loss, while it’s in the rated voltage situation. If the voltage deviate from the nominal value, and the transformer core saturation magnetic flux density at the magnetization curve section, that the no load loss and no-load current will change dramatically, therefore, the no load test should be carried out at rated voltage.

   Note: When measuring the on load and load loss, as the power factor is rather low, that may equal to cosφ ≤ 0.1. So it’s required to adapt low power factor wattmeter.

3, It can be found the following defects through the transformer no load test: poor insulation between the silicon steels, core interelectrode, partial short-circuit chip burning chip to chip, partial damaged insulation in bolts, banding strip, plate, and iron yoke, then forming a short circuit, the magnetic circuit silicon steel loosening, dislocation, gap become large, the core multiple earthing, coils short circuit in interturn and interlayer, coil has turns, ranging from short circuit or parallel branch turns interlayer ampere-turns imbalance, misuse the high consuming and low-quality silicon steel or in wrong design and calculation.