Transformers : Testing

 

Transformer forms the heart of any electrical system. This is an equipment that helps electrical engineers tremendously to deal with the transmission loss and the heavy drop of voltage at the customer end.

A transformer is essentially an equipment that steps up or steps down alternating (current)  voltages without changing the frequency. Any transformer can be used as a step up or a step down transformer depending upon the requirement. The higher voltage side carries less current and the lower voltage side carries higher current ie, N1/N2 = VI/V2 = I2/I1 whereas N1 and N2 are number of turns on primary and secondary sides, V1 and V2 are voltages on Primary and secondary and I1 and I2 are current in primary and secondary. The major parts of a transformer are two windings – primary and secondary and a core that carries the flux. The power supply on one of the windings induces a magnetic field and due to this, a voltage is induced in the other winding. This is the simple statement of faraday's law of induction. To carry the magnetic field that causes induction in the other field, a highly permeable magnetic material is used, so that there is no loss of flux between the windings. Any ferrous material can do this job, but to reduce loss of flux and the resultant loss in energy. In addition, the losses in the core material causes excessive heating of core, which is not desirable. So, it is important to have the best quality core in the transformer that can carry maximum flux without incurring too much loss. In the modern days, there are a number of types of core materials that really minimise the core to very great extend. The most commonly used and easy to handle among the core material is Cold Rolled Grain Oriented (CRGO) steel.

This write up doesn't intend to dwell at length with the design or constructional aspects of a transformer. Instead, I shall describe in brief, the different tests carried out on transformers.

Tests on a transformer can be carried out in three categories:

1) type tests or special tests

2) Routine tests

3) Factory acceptance tests

Type tests :

Most common type tests that are carried out on a transformer are

a) Lightning Impulse test

b) Dynamic short circuit test

c) Temperature rise test

In addition to this, there a number of other special tests like Tan delta measurement, Zero phase sequence measurement, Harmonics measurement, Sweep Frequency Response Analysis. These tests are categorised as optional and are carried out on transformers based on the customer requirements

Routine tests and Factory acceptance tests:

Normally these tests are same. Routine tests are carried out by the manufacturer's quality control department on 100% quantity and acceptance tests are carried out in the presence of the buyer's representative. Buyer witnesses the routine tests on randomly selected samples according to customer's discretion or as per an agree Quality assurance Plan (QAP)

Tests on transformers are carried out as mentioned in the Indian standard specification IS 2026 or International Electro-technical commission (IEC - 76).

Routine tests that are to be carried out on a transformer are

1) Visual examination

2) Dimensional verification

3) Turns ratio measurement

4) Vector group verification

5) magnetic balance check

6) Insulation value measurement

7) Power frequency High voltage test

8) Induced Overvoltage test

9) No Load Loss and No load current measurement

10) Load Loss and Impedance voltage measurement

11) Winding resistance measurement

12) Tests on accessories(if any) like Bocholt relay, Winding temperature  and oil temperature indicators, Magnetic Oil gauge, Pressure release valves

While type tests are meant for determine the soundness of the design, the routine tests are meant for determining the quality of each product.

Impulse test:

This is a test that simulates a lightning that may strike a transformer. This determines the capability of the insulation to withstand sudden surge in voltages to a very dangerous level. A lightning impulse that is similar to that of an actual impulse is generated using a lightning generator. This impulse, usually of a shape of 2/50microsecond is applied on each limb of the transformer with all other windings are shorted and connected to tank and earth. Another feedback, that is the leakage current flowing due to the impulse also fed to the oscilloscope. A transformer is considered withstood this test if the waveform that are observed on the oscilloscope after the application of impulses a few times on each limbs as stipulated in the specification, are similar. In case of a failure or an internal flashover, the wave form will have a distorted shape

Dynamic short circuit test :

This test simulates a sudden in-rush of current through the windings, that may be a few thousand times more than the rated current of the transformer. A well built transformer is expected to withstand this sudden inrush for a fraction of a second. Current in-rush of this order may happen in a transformer in case of an internal short-circuit. Requirement of current and the duration are determined by the buyer depending on the fault conditions where the transformer is used. The test value is determined using the impedance value. A voltage that is 10 times the value of impedance voltage that has been measured during the load loss measurement, is applied on the winding under test with the other winding short circuited. A high current in the  range of kilo amperes flows through the windings under this condition. The current and voltage are fed to the oscilloscope and wave shape is observed. The transformer is considered withstood the test, if routine tests conducted after this tests show the same results as it was tested before the short circuit test

Temperature rise test :

This test is mainly for checking the efficiency of the cooling system of the transformer. On load, a transformer produces heat on account of losses in the core and coils. The cooling system in a transformer is expected to dissipate this heat effectively so that there will not be excessive heat and resultant damages to the insulation. Depending on the insulation class, there are limitations on the temperature rise in the transformer. For example, for a Class F insulated transformer, the maximum temperature rise that can occur shall be 45Degree Celsius over and above the ambient temperature. This implies that a maximum of 45+Ambient temperature only shall be measured even after the transformer is put on full load for several hours together. Temperature rise requirement is the choice of the buyer, depending on the working condition of the electrical system

In a temperature rise test, even after several hours of full lad on the transformer, there shall not be rise in temperature above the specified limit.

This test is carried out by applying a total measured losses on the transformer. For this, first, one of the windings is short circuited and voltage is gradually applied on the other winding till the watt meters read the sum of Load losses extrapolated to 75 Degree Celsius and the No Load losses. This load will be maintained till a steady state of temperature is attained. This means, once a steady state is achieved, heat generated and heat dissipated will be equal. At this point, temperature of the cooling medium can be noted. In case of an oil cooled transformer, this will be the maximum top oil temperature attained by the transformer.

Immediately after attaining temperature stability, the load is switched off and the resistance of the windings are measured. From the readings, the temperature of the windings can be calculated. The measured values shall be with in the limits stipulated by the buyer

Routine tests :

1) Visual examination : This is to ensure the correctness of construction, workmanship, quality of welding on the transformer tank, surface painting and so on. In addition to this, correctness of construction and provision of accessories can also be checked with the help General arrangement drawings and Bill of materials ( GAD and BOM). It is important to have all accessories like temperature indicators, Bucholtz relay(in case of oil cooled transformers), tap changers in case of tapings, shut off and filtering valves in case of oil cooled transformers as mentioned in the BOM and GAD

2) Dimensional verification : This shall be checked using  GAD.

3) Turns Ratio test : This is one of the most important tests on transformer. With the help of a Turns ratio measuring kit, the exact turns ratio of a transformer can be accurately measured. The working principle of a Ratio Measurement kit is to apply a small voltage in the rage of a 120 Volts on the winding with higher number of turns and measuring the induced voltage on the other winding. Voltage is applied on the winding with higher number of turns in order to avoid high voltage being fed to the instrument. The fed voltage and the induced voltages are measured in the Ratio Meter and by adjusting the knobs in the instrument, a null deflection can be achieved in the galvanometer. These knobs are nothing but adjustable potentiometers that are graduated to read the turns ratio directly once the null deflection is achieved

Vector Group determination : Three phase transformers are grouped in different vector groups based on customer requirement. Vector group of a three phase transformer must be known, if transformers are meant for parallel operation, as , transformers of same vector group only are used for parallel operation. This verification is done by, first plotting the diagram of a vector group to which the transformer has been constructed. Then a three phase LT supply is applied on one of the windings and voltages are measured as per the plotted diagram.

For example if the transformer is of vector group DYn11, this means, the primary winding is of delta connection and the secondary is star connected with it neutral brought out. The figure 11 stand for the clock position of 11o clock , that means, the phase angle difference between the HV and LV windings are 30degree on plus side

Magnetic Balance Test :

This test is meant to determine the distribution of flux in the core of a three phase transformer. To check this, first, an LT supply to the range of 400V is applied on one phase of  the winding with a higher number of turns. This induces voltages on other phases. The induced voltages are measured on both primary and secondary windings.

If the voltage is applied on the U-phase of the transformer, the sum of the voltages measured across V and W phases shall be equal to the voltage on U phase. Similarly the induced voltage on secondary U phase will be equal to the sum of the voltages measured on V and W phases.

No Load and No Load Current Measurement :

This test measures the total losses a transformer causes due to its core. Rated voltage of the transformer is applied on one of the transformers with the other winding open circuited. Measurement of losses is measured using wattmeter. Wattmeter can be connected either in Two watt meter configuration or three watt meter configuration. The current flowing through the circuit during this test shall also be noted. The buyer may specify the limit of No Load loss and No load current

Load Loss and Impedance voltage measurement :

Load loss is the total power consumed by the windings. In another way, this is the total power loss on account of resistance of the windings. One of the windings is short-circuited and a voltage is gradually applied on the other winding till rated current is flowing through the windings. If rated current is flowing through one winding, obviously  rated current will be flowing through the other winding too. The circuit for measuring the Load Loss is similar to tha used to measure No Load Loss. The voltage that is required to drive the rated current with one winding short circuited is called impedance voltage. This voltage gives indication about the short circuit capacity of a transformer during a fault occurs. Generally impedance of a transformer is mentioned in percentage. The measured impedance voltage is calculated as a percentage of the rated voltage. For example if the impedance voltage is measured as 3300V for a winding with rated voltage of 33000Volts, the percentage impedance will be 10%.

Winding Resistance measurement :

This is measured using resistance bridges. Thermostat common resistance bridge used is Kelvin's double bridge. The resistance values are directly measured using this equipment. resistance values are used to calculate the theoretical total losses caused due to the resistance of the windings. Total losses measured using wattmeter will be a combination of I(square)x R and the stray losses.

High Voltage test at power frequency :

This test is done to ensure the soundness of insulation. A high voltage at the rated frequency of the system, is applied on one of the windings with the other winding shorted and connected to the tank and earth. The value of the high voltage that needs to be applied is selected from IS/IEC based on the rated voltage of the winding on which the test is carried out. This test is done on each windings for a duration of one minute. The transformer is considered withstood the test if the applied voltage does not collapse to zero or there are no flashovers inside or outside the transformer.

Insulation Resistance Value measurement :

Insulation resistance is an indication of dryness of the windings. Windings that are dried properly is expected to have insulation resistance in the range of Mega Ohms. A DC voltage of 500 to 5000V depending upon the rated voltage of the winding is generated in a small hand held equipment and is applied to one winding with the other terminal of the equipment connected to the tank and earth point. The equipment is commonly called Merger. The value of insulation resistance depends on the current that passes from the winding to the earth point (leakage current) - higher the current lower is the insulation resistance value and vice versa

Induced Over Voltage test :

The purpose of this test is to ascertain the strength of insulation between two adjacent turns. During the test, a voltage double the value that may normally appear between two turns during normal working condition appears between the turns. To achieve the double voltage across each turn, a voltage double the rated voltage of one winding is applied on it. Since a voltage above the rated voltage is applied  across the winding, the core may get saturated as the core is designed to carry flux when rated voltage is applied on windings.As the frequency is inversely proportional to flux, in order to avoid oversaturation of core, when the voltage is doubled, frequency also can be doubled so that the equation, Flux= Voltage/frequency remains unchanged. Frequency can further be increased if the generator is designed to generate a voltage in that frequency. Duration of the test may be determined as per IS/IEC. Generally for a Double voltage and Double frequency test, the test duration is one minute. A transformer is considered withstood the test if the applied voltage does not fall to zero during the test. If the voltage falls, this indicates that there is certain interterm short circuit in the transformer and due to this circulating current is flowing. This circulating current trips the test circuit

Efficiency of the transformer :

The rating of a transformer is done in VA ( Volt Ampere). The reason is that, a transformer does not influence the power factor of a circuit. It is the power factor of the equipment that are connected to the transformer that determines the total power factor of the circuit

If the rating of the transformer is mentioned as 100KVA, this indicates that, the input to this transformer is 100KVA. So the output of this transformer will be value which is after the deduction of Load Losses and No-load Losses. So, efficiency of a transformer can be calculated as (Input - Losses) x 100/ Input . This is indicated in %.