Transformer Oil and Winding Temperature Rise Test

Temperature rise test of Transformer is included in type test of transformer. In this test we

check whether the temperature rising limit of transformer winding and oil as per specification or

not.

Temperature Rise Test for Top Oil of Transformer:

First the LV winding of the transformer is short circuited.

Then one thermometer is placed in a pocket in transformer top cover. Other two thermometers

are placed at the inlet and outlet of the cooler bank respectively.

The voltage of such value is applied to the HV winding that power input is equal to no load

losses plus load losses corrected to a reference temperature of 75oC.

The total losses are measured by three wattmeters method. During the test, hourly readings of

top oil temperature are taken from the thermometer already placed in the pocket of top cover.

Hourly readings of the thermometers placed at inlet and outlet of the cooler bank are also noted

to calculate the mean temperature of the oil.

Ambient temperature is measured by means of thermometer placed around the transformer at

three or four points situated at a distance of 1 to 2 meter from and half-way up the cooling surface

of the transformer.

•Temperature rise test for top oil of transformer should be continued until the top oil temperature

has reached an approximate steady value that means testing would be continued until the

temperature increment of the top oil becomes less than 3oC in one hour. This steady value of top

oil is determined as final temperature rise of transformer insulating oil.

Temperature rise limits of transformer when it is oil immersed, given in the table below

NB: These temperature rises limits mentioned in the above table are the temperature rise above

the temperature of cooling medium. That means these are the difference between winding or oil

temperature and temperature of cooling air or water.

Winding Temperature Rise Test on Transformer

After completion of temperature rise test for top oil of transformer the current is reduced to its

rated value for transformer and is maintained for one hour.

After one hour the supply is switch off and short circuit and supply connection to the HV side

and short circuit connection to the LV side are opened.

But, the fans and pumps are kept running (if any).

Then resistance of the windings are measured quickly. But there is always a minimum 3 to 4

minutes time gap between first measurement of resistance and the instant of switching off the

transformer, which can not be avoided.

Then the resistances are measured at the same 3 to 4 minutes time intervals over a period of 15

minutes.

Graph of hot resistance versus time is plotted, from which winding resistance (R2) at the

instant of shut down can be extrapolated.

From this value, θ2, the winding temperature at the instant of shut down can be determined by the

formula given below-

Where, R1 is the cold resistance of the winding at temperature t1.

For determining winding temperature rise we have to apply the above discussed indirect method.

That means hot winding resistance is measured and determined first and then from that value we

have to calculate the winding temperature rise, by applying resistance temperature relation

formula. This is because unlike oil the winding of transformer is not accessible for external

temperature measurement.

History of Megger

The device is being used since 1889, popularity raised during 1920s, since long back devise is

same in its uses and purpose of testing, few real improvement appeared in recent years with its

design and quality of tester. Now high quality option are available which are easy to use and quite

safe.

What is Megger?

Insulation resistance quality of an electrical system degrades with time, environment condition

i.e. temperature, humidity, moisture and dust particles.

It also get impacted negatively due to the presence of electrical and mechanical stress, so it’s

become very necessary to check the IR (Insulation resistance) of equipment at a constant regular

interval to avoid any measure fatal or electrical shock.

Uses of Megger

The device enable us to measure electrical leakage in wire, results are very reliable as we shall

be passing electric current through device while we are testing.

The equipment basically use for verifying the electrical insulation level of any device such as

motor, cable, generator winding, etc.

This is a very poplar test being carried out since very long back. Not necessary it shows us

exact area of electrical puncture but shows the amount of leakage current and level of moisture

within electrical equipment/winding/system.

Important parts:-

Analog display:- Analog display provided on front face of tester for IR value recording.

Hand Crank:- Hand crank used to rotate helps to achieve desired RPM required generate

voltage which runs through electrical system.

Wire Leads:- Used same as in electronic tester i.e. For connecting tester with electrical system.

Advantages of Hand Operated Megger

Still keeps important in such high-tech world as it’s an oldest method for IR value

determination.

No external source required to operate.

Cheaper available in market.

Disadvantages of Hand Operated Megger

At least 2 person required to operate i.e. one for rotation of crank other to connect megger with

electrical system to be tested.

Accuracy is not up to the level as it’s varies with rotation of crank.

Require very stable placement for operation which is a little hard to find at working sites.

Unstable placement of tester may impact the result of tester.

Provides an analog display result.

Require very high care and safety during use of the same.

Construction of Megger

Circuit Construction features :-

•Deflecting and Control coil : Connected parallel to the generator, mounted at right angle to

each other and maintain polarities in such a way to produced torque in opposite direction.

•Permanent Magnets : Produce magnetic field to deflect pointer with North-South pole magnet.

•Pointer : One end of the pointer connected with coil another end deflects on scale from infinity

to zero.

•Scale : A scale is provided in front-top of the megger from range ‘zero’ to ‘infinity’, enable us to

read the value.

•D.C generator or Battery connection : Testing voltage is produced by hand operated DC

generator for manual operated Megger. Battery / electronic voltage charger is provided for

automatic type Megger for same purpose.

•Pressure coil resistance and Current coil resistance : Protect instrument from any damage

because of low external electrical resistance under test.

Working Principle of Megger

Voltage for testing produced by hand operated megger by rotation of crank in case of hand

operated type, a battery is used for electronic tester.

500 Volt DC is sufficient for performing test on equipment range up to 440 Volts.

1000 V to 5000 V is used for testing for high voltage electrical systems.

Deflecting coil or current coil connected in series and allows flowing the electric current taken

by the circuit being tested.

The control coil also known as pressure coil is connected across the circuit.

Current limiting resistor (CCR and PCR) connected in series with control and deflecting coil to

protect damage in case of very low resistance in external circuit.

In hand operated megger electromagnetic induction effect is used to produce the test voltage

i.e. armature arranges to move in permanent magnetic field or vice versa.

Where as in electronic type megger battery are used to produce the testing voltage.

As the voltage increases in external circuit the deflection of pointer increases and deflection of

pointer decreases with a increases of current.

Hence, resultant torque is directly proportional to voltage and inversely proportional to current.

When electrical circuit being tested is open, torque due to voltage coil will be maximum and

pointer shows ‘infinity’ means no shorting throughout the circuit and has maximum resistance

within the circuit under test.

If there is short circuit pointer shows ‘zero’, which means ‘NO’ resistance within circuit being

tested.

Work philosophy based on ohm-meter or ratio-meter. The deflection torque is produced with

megger tester due to the magnetic field produced by voltage and current, similarly like ‘Ohm's

Law’.

Torque of the megger varies in ration with V/I, (Ohm's Law :- V = IR or R = V/I). Electrical

resistance to be measured is connected across the generator and in series with deflecting coil.

Produced torque shall be in opposite direction if current supplied to the coil.

High resistance = No current :- No current shall flow through deflecting coil, if resistance is

very high i.e. infinity position of pointer.

Small resistance = High current :- If circuit measures small resistance allows a high electric

current to pass through deflecting coil, i.e. produced torque make the pointer to set at ‘ZERO’.

Intermediate resistance = varied current :- If measured resistance is intermediate, produced

torque align or set the pointer between the range of ‘ZERO to INIFINITY’.

Voltage and Turn Ratio Test of Transformer

Transformer Ratio Test:

The performance of a transformer largely depends upon perfection of specific turns

or voltage ratio of transformer. So transformer ration test is an essential type test of

transformer. The voltage should be applied only in the high voltage winding in order to avoid

unsafe voltage.

Procedure of Transformer Ratio Test

First, the tap changer of transformer is kept in the lowest position and LV terminals are kept

open.

Then apply 3-phase 415 V supply on HV terminals. Measure the voltages applied on each

phase (Phase-phase) on HV and induced voltages at LV terminals simultaneously.

After measuring the voltages at HV and LV terminals, the tap changer of transformer should be

raised by one position and repeat test.

Repeat the same for each of the tap position separately.

The above transformer ratio test can also be performed by portable transformer turns ratio

(TTR) meter.

They have an in built power supply, with the voltages commonly used being very low, such as

8-10 V and 50 Hz. The HV and LV windings of one phase of a transformer are connected to the

instrument, and the internal bridge elements are varied to produce a null indication on the

detector.

Let's have a discussion on transformer turns ratio (TTR) meter method of turn ratio test of

transformer.

A phase voltage is applied to the one of the windings by means of a bridge circuit and the ratio

of induced voltage is measured at the bridge. The accuracy of the measuring instrument is < 0.1

%.

The significance of polarization index test.Let I be the total initial current during polarisation index test or PI test.IC is the capacitive current.IR is resistive or conductive current.IS is surface leakage current.IP is polarization current of the insulator.

Tan Delta Test |Loss Angle Test | Dissipation Factor Test

Principle of Tan Delta Test

A pure insulator when is connected across line and earth, it behaves as a capacitor.

In an ideal insulator, as the insulating material which acts as dielectric too, is 100 % pure,

the electric current passing through the insulator, only have capacitive component.

There is no resistive component of the current, flowing from line to earth through insulator as

in ideal insulating material, there is zero percent impurity.

In pure capacitor, the capacitive electric current leads the applied voltage by 90o.

In practice, the insulator cannot be made 100% pure.

Also due to the ageing of insulator the impurities like dirt and moisture enter into it. These

impurities provide the conductive path to the current.

Consequently, leakage electric current flowing from line earth through insulator has also

resistive component.

Hence, it is needless to say that, for good insulator, this resistive component of leakage electric

current is quite low.

In other way the healthiness of an electrical insulator can be determined by ratio of resistive

component to capacitive component.

For good insulator this ratio would be quite low. This ratio is commonly known as tanδ or tan

delta. Sometimes it is also referred as dissipation factor.

In the vector diagram above, the system voltage is drawn along x-axis. Conductive electric

current i.e. resistive component of leakage current, IR will also be along x-axis.

As the capacitive component of leakage electric current IC leads system voltage by 90o, it will

be drawn along y-axis.

Now, total leakage electric current IL(Ic + IR) makes an angle δ (say) with y-axis.

Now, from the diagram above, it is cleared, the ratio, IR to IC is nothing but tanδ or tan delta.

NB: This δ angle is known as loss angle.

Method of Tan Delta Testing

The cable, winding, current transformer, potential transformer, transformer bushing, on

which tan delta test or dissipation factor test to be conducted, is first isolated from the system.

A very low frequency test voltage is applied across the equipment whose insulation to be tested.

First the normal voltage is applied. If the value of tan delta appears good enough, the applied

voltage is raised to 1.5 to 2 times of normal voltage, of the equipment.

The tan delta controller unit takes measurement of tan delta values.

A loss angle analyser is connected with tan delta measuring unit to compare the tan delta values

at normal voltage and higher voltages, and analyse the results.

During test it is essential to apply test voltage at very low frequency.

Therefore, required apparent power for tan delta test would become high enough which is not

practical.

So to keep the power requirement for this dissipation factor test, very low frequency test

voltage is required.

The frequency range for tan delta test is generally from 0.1 to 0.01 Hz depending upon size and

nature of insulation.

There is another reason for which it is essential to keep the input frequency of the test as low as

possible.

As we know,

How to Predict the Result of Tan Delta Testing

There are two ways to predict the condition of an insulation system during tan delta or

dissipation factor test.

First, one is, comparing the results of previous tests to determine, the deterioration of the

condition of insulation due ageing effect.

The second one is, determining the condition of insulation from the value of tanδ, directly.

No requirement of comparing previous results of tan delta test.

If the insulation is perfect, the loss factor will be approximately same for all range of test

voltages.

But if the insulation is not sufficient, the value of tan delta increases in the higher range of test

voltage.

Insulation Dielectric Test of Transformer

The dielectric test of transformer is generally performed in two different steps, likewise,

separate source voltage withstand test and induced voltage withstand test of transformer, which

we have discussed one by one below.

Separate Source Voltage Withstand Test of Transformer

This dielectric test is intended to check the the ability of main insulation to earth and between

winding.

Procedure

All three line terminals of the winding to be tested are connected together.

Other winding terminals which are not under test and also tank of the transformer should be

connected to earth.

Then a single-phase power frequency voltage of shape approximately sinusoidal is applied for

60 seconds to the terminals of the winding under test.

The test shall be performed on all the windings one by one.

The test is successful if no break down in the dielectric of the insulation occurs during test.

In this transformer testing, the peak value of voltage is measured, that is why

the capacitor voltage divider with digital peak voltmeter is employed as shown in the diagram

above.

The peal value multiplied by 0.707 (1/√2) is the test voltage.

The values of test voltage for different fully insulated winding are furnished below in the table.

Induced Voltage Test of Transformer

The induced voltage test of transformer is intended to check the inter turn and line end

insulation as well as main insulation to earth and between windings-Keep the primary winding of

transformer open circuited.

Apply three phase voltage to the secondary winding. The applied voltage should be twice of

rated voltage of secondary winding in magnitude and frequency.

The duration of the test shall be 60 second.

The test shall start with a voltage lower than 1/3 the full test voltage, and it shall be quickly

increased up to desired value.

The test is successful if no break down occurs at full test voltage during test.

Vector Group Test of Power Transformer

Vector Group Test of Transformer

The vector group of transformer is an essential property for successful parallel operation of

transformers.

Hence every electrical power transformer must undergo through vector group test of

transformer at factory site for ensuring the customer specified vector group of transformer.

The phase sequence or the order in which the phases reach their maximum positive voltages,

must be identical for two paralleled transformers.

Otherwise, during the cycle, each pair of phases will be short circuited.

The several secondary connections are available in respect of various primary three phase

connection in a the three phase transformer.

So for same primary applied three phase voltage there may be different three phase secondary

voltages with various magnitudes and phases for different internal connection of the transformer.

Let's have a discussion in detail by example for better understanding.

We know that, the primary and secondary coils on any one limb have induced emfs that are in

time-phase.

Let's consider two transformers of same number primary turns and the primary windings are

connected in star.

The secondary number of turns per phase in both transformers are also same.

But the first transformer has star connected secondary and other transformer has delta

connected secondary.

If same voltages are applied in primary of both transformers, the secondary induced emf in

each phase will be in same time-phase with that of respective primary phase, as because the the

primary and secondary coils of same phase are wound on the same limb in the core of

transformer.

The following table gives the connections for which from the view point of phase sequence

and angular divergences, transformer can be operated parallel.

According to their vector relation, all three phase transformers are divided into different vector

group of transformer.

All electrical power transformers of a particular vector group can easily be operated in parallel

if they fulfill other condition for parallel operation of transformers.

Impulse Test of

Transformer

Lighting is a common phenomenon in transmission lines because of their tall height.

This lightning stroke on the line conductor causes impulse voltage.

The terminal equipment of transmission line such as power transformer then experiences this

lightning impulse voltages.

Again during all kind of online switching operation in the system, there will be switching

impulses occur in the network.

The magnitude of the switching impulses may be about 3.5 times the system voltage. Insulation

is one of the most important constituents of a transformer.

Any weakness in the insulation may cause failure of transformer.

To ensure the effectiveness of the insulation system of a transformer, it must confirms the

dielectric test.

But the power frequency withstand test alone can not be adequate to demonstrate the dielectric

strength of a transformer.

That is why impulse test of transformer performed on it. Both lightning impulse test and

switching impulse test are included in this category of testing.

Lightning Impulse

The lightning impulse is a pure natural phenomenon.

So it is very difficult to predict the actual wave shape of an lightning disturbance.

From the data compiled about natural lightning, it may be concluded that the system

disturbance due to natural lightning stroke, can be represented by three basic wave shapes.

1. Full wave

2. Chopped wave and

3. Front of wave