Transformer Protection and Testing

The type of protection available for a given transformer depends on the size, complexity, and revenue derived there from it. For these reasons, different protection schemes are employed for power and distribution transformers.

The type of protection used should minimize the time of disconnection for faults within the transformer and reduce the risk of catastrophic failure. Any extended operation of the transformer under abnormal conditions such as faults or overloads compromises the transformer's life. This implies that the speed of isolation upon a fault is essential.

Causes of Transformer Failure

Transformers throughout the power system, experience different levels of through-fault currents in terms of magnitude, duration, and frequency. 

Through-faults in transformers can produce physical forces that cause insulation wear and friction-induced displacement in the winding. These effects are cumulative and should be considered over the life of the transformer.

Classification of Failures in Transformers

Failures in transformers can be classified into:

1. Winding failures due to short-circuit (turn-to-turn faults; phase to phase faults; phase to ground & open circuited winding faults)

2. Core faults (core insulation failure; shorted laminations; corroded laminations)

3. Terminal failures (open leads; loose connections; short circuits

4. Abnormal operating conditions (over fluxing, over-voltage & overloading

External faults

On-load tap changer failures (mechanical, electrical & short-circuits)

The distribution transformer steps down the distribution feeder voltage to the utilization voltage (11kv/415v). For best efficiency, distribution transformers should be operated between 50% and 75% of the total load rating.

Devices used for Distribution Transformer Protection -

1. Fuses

2. Lightning Arresters

3. Transformer Neutral Earthing

Fuses

Fuses are used as the primary protection for distribution transformers because of their simplicity & low cost.

They are short-time devices and can be applied to protect distribution transformers of up to 33kV

Generally, a fuse is designed to snap or rupture when its current rating is exceeded. Fuses rupture for a variety of reasons.

The knowledge of the magnitudes of the transformer's primary and secondary current will help in the correct selection of the primary and secondary fuse ratings. Hence, when properly selected, fuses offer adequate protection for the distribution transformers against overload and short circuit currents.

Examples -

What are the correct J&P or RMU and HRC cartridge fuse ratings suitable for a 300kVA 11/0.415kV transformer assuming a 4-way feeder pillar is used with 4 overhead cables?

It is noteworthy that continuous overloading is one of the main causes of premature aging and breakdown of distribution transformer winding insulation.

2. Lightning and Transient Overvoltage

Distribution transformers are subjected to transient over-voltages resulting from the networks to which they are connected. These voltages are either the result of direct or induced lightning strikes on the HV or LV network or of surges generated during switching by switchgear operations.

These resultant voltages are usually higher than the rated breakdown voltage of the insulation and are usually responsible for the premature aging of the transformer winding insulation.

Protection against Transient Overvoltage resulting from Lightning

There are two major means by which overvoltage can be protected -

1. Spark Gap Protection

2. Lightning Arresters

1. Spark Gaps

They are the simplest and least expensive protection scheme for overvoltage. It costs two conducting electrodes separated by a gap usually filled with a gas such as air, designed to allow an electric spark to pass between the conductors.

B. Lightning Arresters

Although very expensive. Lightning arresters provide protection with greater performance. It protects the transformer from high voltage surges caused by lightning strikes.

Location of Lightning Arresters

The lightning arrester should be located as close as possible to the apparatus or equipment to be protected, particularly if an overhead line terminates in a transformer. During wave propagation phenomena, the transformer represents a point of almost total reflection and the stress that it is subjected to can reach approximately twice the maximum voltage of the incident wave.

3. Transformer Neutral Grounding

Transformer neutral or star point is usually grounded in order to protect the transformer against the dangerous effects of over-voltages and heavy short circuit current by holding the neutral potential close to ground potential.

Neutral earthing ensures the rapid disconnection of faulty apparatus from the system without undue delay and enables the use of protective relaying (earth fault relays) for fault clearance.

The ground resistance must be low (typically 2 ohms or less for distribution substations). In practice, distribution transformer neutral grounding is effected in the feeder pillar by providing a link between the earth bar and neutral bar.

Neutral grounding can be solid, through resistance or reactance, or both.

4. Buchholz Protection

The Buchholz relay is a mechanical safety device sensing the accumulation of gas in large oil-filled transformers. It will cause an alarm on the slow accumulation of gas or initiate the operation of the transformer breaker to isolate the transformer if the gas is produced rapidly in the transformer oil.

The relay is installed in the pipe between the transformer main tank and the conservator. It responds to internal arcing faults and slow decomposition of insulating materials. It can also detect low oil levels in the transformer due to leakage.

It has two elements, a float switch, and a combined hinged flap and float switch. Gases generated due to internal failure in the transformer tank cause streams of bubbles that move upwards and towards the conservator tank but are trapped in the Buchholz chamber.

The trapped gases displaced the oil in the Buchholz chamber consequently lowering the upper float.

The operation of the switches connected to an external alarm and trip circuit is then initiated for incipient faults and serious faults respectively

Buchholz protection is an important protective scheme for power transformers as it detects fault within the transformer particularly in the incipient stages to avoid major breakdown and sudden failure of power supply.

Electrical Protection

Electrical protection was issued to protect the power transformer from unbalanced short circuit current and overvoltages.

When electrical protection operates, isolation of the power transformer is instantaneous because of the magnitude of current and voltage involved.

Differential Relay Protection

The differential protection is an arrangement that covers the transformer and provides the best overall protection for both phase and ground faults

Testing of Power Transformers

Insulation Resistance Test

This involves the use of a portable 500volts - 5000 volts insulation resistance tester. For a transformer with a voltage rating of 11/0.415kV, the HV winding is tested by applying 5000 volts at the HV terminal with respect to the ground. The LV side is tested with 1000 volts.

All values obtained are recorded in meg-ohms. Any value below 100 meg-ohms is regarded as bad and is usually seen as a sign of deterioration of the insulation of the windings or ingress of moisture in the oil/windings.

The results are recorded as follows:

HV - E = 300Mohms

LV - E = 250 Mohms

HV - LV = 900Mohms

Insulation Test

A 60kV or 80kV DC insulation tester is used for this purpose. For a transformer with a voltage rating of 11/0.415kV, 25kV DC is applied on any of the primaries (HV) winding terminals for 1 minute, and 2kV is applied on the secondary LV winding terminal for 30 seconds both with respect to the ground.

The leakage current is noted if possible. Any considerable drop in the voltage applied is indicative of a fault in the windings of the transformer. The continuity of the windings in the transformer is also checked

The results are recorded as follows:

3. Ratio Test

The ratio test is used to check the transformation ratio in the windings of the transformer. It helps detect any abnormality in the windings.

There are two types of transformer ratio tests -

a. Voltage Ratio Test

b. Turns ratio test

a. Voltage Ratio Test

This is used to check the ratio of voltage transformation in the windings of transformers. This test is usually carried out on distribution transformer using a single phase source of ac power supply e.g Generator

The procedure of the Transformer Ratio Test

1. Firstly keep the transformer tap changer in the lowest position and LV terminals to be kept open

2. Apply 3-phase 415V supply on HV terminals. Measure the voltages applied on each phase (Phase to phase) on HV and induced voltages at LV terminals simultaneously

3. After measuring the voltages at HV and LV terminals, the tap changer of the transformer should be raised by one position, and repeat test

4. Repeat the same for each of the tap positions separately

Turns Ratio Test

The ratio meter is used to carry out this test and it is one of the final tests to be conducted before energizing a new power transformer

Procedures for Turns Ratio Test

1. Isolate the equipment and ground all the incoming and outgoing cables from the transformer bushing terminals connections

2. Connect the H-designated three-phase test lead with the military-style connector at one end to the mating connector on the test set marked with an H.

3. Connect X designated three-phase test of lead military-style connector at one end to the mating connector on the test set marked with an X.

4. Connect the H1, H2, and H3 designated test lead to the corresponding H1, H2, and H3 transformer terminal/bushing. Connect the H0 test lead if the H0 terminal/bushing is present

5. Connect the X1, X2, and X3 designated test leads to the corresponding X1, X2, and X3 transformer terminals/bushings. Connect the X0 test lead if the X0 terminal/bushing is present

6. Perform turns ratio measurements for all tap positions

7. Confirm that the measured ratio is within 0.5% of the calculated ratios

Excitation Test

In this test, a single phase a.c supply voltage 230V is applied to the secondary terminals of the transformer with the primary terminals open-circuited.

The voltage is applied to all the phases one after the other with the phase voltage at the secondary terminals measured and noted.

Note that the transformer is operated in reverse (in step-up mode) and a dangerously high voltage will be available at the primary terminals therefore no measurement can be conducted there.

An excitation test is very powerful and can indicate that a transformer is faulty even when the insulation resistance test, insulation test, and ratio test results are okay.

The result of a test on a 500kVA 11kV/415V dy11 transformer is shown below:

Transformer Earth Resistance Test

Transformer or substation earth resistance test is usually carried out prior to the commissioning of transformer in service

This is achieved by measuring the resistance to the flow of ground current using an earth resistance tester

Typical earth resistance acceptable for distribution substations is 2ohms

For power transformer substations, this value has to be improved to less than 1 ohm in order to prevent damage to equipment by providing a low impedance path between a fault and the source of ground fault current. It also helps to facilitate the operation of protective devices and minimize the build-up of static charges