Transformer testing needs to be carried out to ensure that the distribution transformers are built with
• adequate electrical strength to withstand over voltage (due to switching surges) impinging on the winding without causing flashover; and
• adequate mechanical strength to bear the mechanical stresses developed on the winding during short circuits.
The following tests must ideally be conducted on the units before their acceptance:
• testing of windings − insulation and mechanical strength;
• testing of insulating transformer oil; and
• other tests.
Testing of Windings − Insulation and Mechanical Strength
• adequate electrical strength to withstand over voltage (due to switching surges) impinging on the winding without causing flashover; and
• adequate mechanical strength to bear the mechanical stresses developed on the winding during short circuits.
The following tests must ideally be conducted on the units before their acceptance:
• testing of windings − insulation and mechanical strength;
• testing of insulating transformer oil; and
• other tests.
Testing of Windings − Insulation and Mechanical Strength
This involves four types of tests, which we have described briefly in Table.
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| Testing of Windings |
We give below the findings of the Central Power Research Institute (CPRI) about the short circuit test.
Box: Findings of Central Power Research Institute on Short Circuit Test
Box: Findings of Central Power Research Institute on Short Circuit Test
Testing of Insulating Transformer Oil
At present, transformer oil is subjected to Breakdown Voltage (BDV) test to ensure its electrical strength. Other tests to confirm important transformer characteristics such as acidity, resistivity, etc., are not carried out before accepting bulk supply. Thus, there is every possibility that manufacturers use inferior quality of oil. This can lead to poor insulation resistance between High
Voltage to Earth, Low Voltage to Earth and High and Low Voltages and reduced cooling rate. Moreover, it can give rise to abnormal temperature increases even before loading the transformer to its rated capacity.Thus, it is important to check whether the oil used is a new one or a reconditioned one or a reclaimed one before the transformer is installed.Both water and water saturated oils are heavier than clean and dry oil, and sink to the bottom of the container.
The following tests are usually conducted on the transformer oil:
• inspection of samples;
• acidity test;
• analysis of dissolved gases; and
• electric strength test.
We now briefly describe these tests.
•Inspection of Samples: Colour and odour of the oil provide useful information on the quality of oil and its fitness for use. In Table we list the factors that should be watched out for during inspection.
At present, transformer oil is subjected to Breakdown Voltage (BDV) test to ensure its electrical strength. Other tests to confirm important transformer characteristics such as acidity, resistivity, etc., are not carried out before accepting bulk supply. Thus, there is every possibility that manufacturers use inferior quality of oil. This can lead to poor insulation resistance between High
Voltage to Earth, Low Voltage to Earth and High and Low Voltages and reduced cooling rate. Moreover, it can give rise to abnormal temperature increases even before loading the transformer to its rated capacity.Thus, it is important to check whether the oil used is a new one or a reconditioned one or a reclaimed one before the transformer is installed.Both water and water saturated oils are heavier than clean and dry oil, and sink to the bottom of the container.
The following tests are usually conducted on the transformer oil:
• inspection of samples;
• acidity test;
• analysis of dissolved gases; and
• electric strength test.
We now briefly describe these tests.
•Inspection of Samples: Colour and odour of the oil provide useful information on the quality of oil and its fitness for use. In Table we list the factors that should be watched out for during inspection.
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| Inspection of Oil Samples |
•Acidity Test: Transformer oil deteriorates gradually while in service due to oxidation. The acidity in the oil causes rusting of ironing work inside the tank above the oil level and the attached varnish on the windings. The recommended limits for acidity test and the action required are given in Table.
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| Action Required for Various Acidity Levels of Transformer Oil |
•Analysis of Dissolved Gases: The permissible concentrations of dissolved gases in the oil of a healthy transformer are given in Table .
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| Permissible Concentrations of Dissolved Gases in Transformer Oil |
•Electric Strength Test (Breakdown Voltage Test): This is the most commonly known test applicable to mineral insulating oils, and it was
originally developed to test the breakdown voltage of the oil. We describe the method of its measurement in Box .
Box : Method of Measurement of Breakdown Voltage
An oil test cell is used in which an alternating voltage is applied between two metal spheres 12.5 mm in diameter with a gap of 2.5 mm between them. The voltage is increased until breakdown occurs. The flashover must be quickly stopped to allow six successive measurements of the rupture voltage on the same sample. The value of the electric strength of the sample tested is the average of the six measurements.
originally developed to test the breakdown voltage of the oil. We describe the method of its measurement in Box .
Box : Method of Measurement of Breakdown Voltage
An oil test cell is used in which an alternating voltage is applied between two metal spheres 12.5 mm in diameter with a gap of 2.5 mm between them. The voltage is increased until breakdown occurs. The flashover must be quickly stopped to allow six successive measurements of the rupture voltage on the same sample. The value of the electric strength of the sample tested is the average of the six measurements.
Other Tests
We now briefly describe some other tests, which are usually conducted on transformers. These include temperature rise test, drying out of transformer, and open circuit and Sumpner’s test.
• Temperature rise test is performed to measure the temperature rise of the main and conservator tanks. The transformer passes the test if the rise is within the specified limits given by the manufacturer.
• Drying out of transformer is necessary if
− tests indicate the presence of moisture in transformer oil;
− the oil does not withstand the dielectric strength test; and
− the insulation resistance readings are not satisfactory.
Box : Method of Drying Out the Transformer
Normally HOT OIL CIRCULATION method should be used for drying out the distribution transformer. In special circumstances, where this method does not give satisfactory results, SHORT CIRCUIT WITH HOT OIL CIRCULATION should be used. In this method, both core and winding inside the tank are simultaneously dried out/streamlined with filter. The moisture dries out from the windings into the oil and is removed from the oil by evaporation and filtering.
•Open circuit and Sumpner’s tests: The open circuit test is carried out on a transformer for calculating no load parameters and core losses.Sumpner’s back to back test is done for testing performance on full load for two similar transformers.
We now briefly describe some other tests, which are usually conducted on transformers. These include temperature rise test, drying out of transformer, and open circuit and Sumpner’s test.
• Temperature rise test is performed to measure the temperature rise of the main and conservator tanks. The transformer passes the test if the rise is within the specified limits given by the manufacturer.
• Drying out of transformer is necessary if
− tests indicate the presence of moisture in transformer oil;
− the oil does not withstand the dielectric strength test; and
− the insulation resistance readings are not satisfactory.
Box : Method of Drying Out the Transformer
Normally HOT OIL CIRCULATION method should be used for drying out the distribution transformer. In special circumstances, where this method does not give satisfactory results, SHORT CIRCUIT WITH HOT OIL CIRCULATION should be used. In this method, both core and winding inside the tank are simultaneously dried out/streamlined with filter. The moisture dries out from the windings into the oil and is removed from the oil by evaporation and filtering.
•Open circuit and Sumpner’s tests: The open circuit test is carried out on a transformer for calculating no load parameters and core losses.Sumpner’s back to back test is done for testing performance on full load for two similar transformers.
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