INTERNSHIP REPORT ON THE INSTRUMENTATION OF LIMESTONE STA

INTERNSHIP REPORT ON THE INSTRUMENTATION OF LIMESTONE STA

Construction and Internals of Power Transformer:

HV (high voltage) and LV (low voltage) sides:

HV side has windings around the core through the bushings. And the LV side having less number of windings it comes out through the bushes. High quality insulated electric copper conductors wrapped to special insulated table to be used for windings.

(a) (b)

(c) (d)

Drain Valve:

A transformer has drain valve at the bottom to replenish oil and drain out to flush tank and check for internal corrosions.

Conservator Tank:

Then comes entire Transformer tank that is filled with oil. It has two purposes.

a) To cool down and reduce extra heat.

b) Act as an insulator.

Cooling is of ON (oil natural) and AN (air natural) type.

The hot oil by transformer actions goes up and cool oil comes down. Then hot oil is entered the air cooling fans and get cooled and then goes down. Transformer that is cooling the windings of a transformer gets cooled by air by natural action by the process of ON and AN.

High Power transformer has a rubber diaphragm inside the conservator tank so that air does not come in and direct contact with oil. This rubber diaphragm goes up or down depending upon the temperature of the oil. Air is allowed to come inside the conservator tank or expel outside the conservative the depending upon the movement of the rubber diaphragm.

Transformer Core:

Transformer core is made up of steel laminations. To achieve high level of precision and to control vibrations and sound level the proper cutting of plate is need to be done and special care is taken out when stacking the laminations.

Bushings:

The high voltage conductors that pass through the grounding wall of a transformer are known as bushings of a transformer.

The insulator and the external connections must have the strength to withstand the short circuit.

Bushings are low cost compared to the high voltage assets that protect. Bushing failures can have substantial consequences to the transformer, human safety and environment.

Serious consequences could be occur when porcelain bushes fails. Shards of porcelain could be launched and cause potential risks to the human safety and surrounding objects.

The transformer oil is contaminated once it comes in contact with the atmosphere The transformer oil can also be ignited and cause problems for the human safety

and environment

These serious consequences can be avoided by use of new CRS bushings.It uses a solid insulation material there is a substantial decrease in human risks.

Breather:

Breather is an accessary of an oil filled transformer

It is attached to the oil conservator tank Serves as the breathing point of the unit, When the insulation oil of the transformer gets heated up, oil expands and goes

to the conservator tank Oil pushes the air out of the conservator tank through the breather Breather is filled with silica gel which adsorbs moisture.

When the insulation oil of the transformer cool down, oil retracts. During retraction process, fresh air is sucked from the atmosphere through

breather.

The basic purpose of silica is to absorb moisture. As it absorbs moisture its color changes from i. deep blue to light blue ii. Light blue to pink color. Pink color indicates that silica gel has been saturated and it has to be replaced. In atmosphere there is no pure dry air because it is mixture of moisture and dust

particles. If the dust particles are allowed the oil of the transformer gets contaminated. Therefore oil cup is used to collect dust particles when the air is sucked by the

breather.

Buchholz’s Relay:

It is a mechanical device By heating of the arc isulating material is destroyed and gas is released. So there

was need to collect the gas bubbles from the transformer to a place where quality and quantity of the gas can be estimated.

So for this purpose buchollz named scientist gave the idea and this relay is known by his name as buchollz’s relay.

This relay is fitted in the pipe connecting the transformer tank with the

conservator tank. It consists of two main floats.

When both the floats sink ,each of them short circuits two contacts by closing a circuit so the fault could be detected.

It is also be capable to detect the low magnitude fault such as inter turn

fault,incipient winding fault and core fault etc. Upper bucket is manually lower and cause the mercury switch to operate alarm. Lower bucket is manually lower and cause the mercury switch to operate trip.

In mechanical interlock there are two positions:

a) T/L (Test Lock):

To test the function of the relay move the with skrew driver in anticlockwise direction till the slot on the test key point towards the T/L position.

Both the floats will operate and mercury switch of upper and lower float will show continuity.

During transportation or storage of the relay,the test key should remain in T/L position.

During topping up of transformer oil the test key should remain in T/L position.

b) S (Service):

During commissioning of the relay the key should remain in service position.

Mounting of Bucholz relay:

Arrow on the relay should be pointing towards conservator tank. The relay should be mounted within angle of 1 to 9 degrees. The pipe at the apparatus side should be straight for a length of 5 times the

nominal diameter of the pipe bore. The pipe at the conservator side should be straight for a length of 3 times the

nominal diameter of the pipe bore. Sharp bends in the pipe work should be avoided

Transformer Nameplate :

Type TSUN 6338LPK No 20600246

Rated Power 2000 kVATemp rise oil/winding amb. 52°C

No. of phases 3Ins. Class A

Rated frequency 50HzType of Cooling ONAN

Total Losses 27580WVector group Dyn 11

Rated Voltage HV/LV (No Load) 6300/415 VRated Current HV/LV 183.3/2782.4 AImpedance Voltage 8.12 %

Duty Cont.Winding Temp. Rise 53°C

Duration of short Circuit 3 sec

HIGH VOlTAGE AND LOW VOLTAGE SIDES:

HIGH VOLTAGE LOW VOLTAGE

ConnectionVolts

TerminalTap Changer Connections Position Connection Volts Terminal

A

B

DELTA

6615A5 A4 B5 B4 C5 C4 1

STAR 415

0abc

6457.5A4 A6 B4 B6 C4 C6 2

6300A6 A3 B6 B3 C6 C3 3

6142.5A3 A7 B3 B7 C3 C7 4

5985A7 A2 B7 B2 C7 C2 5

KVA:

The transformation capability of the transformer. It is always rated in KVA.

Volts at NO LOAD:

It has HV and LV voltages. HV side is known as primary side and LV side is known as secondary side.

It tells that how much the primary side and secondary side will vary depending upon the load factor.

The HV side does not vary unless and until we adjust switching tapes..

It has LV voltages of 6300 rated voltages. While the standard voltages at LV side are 415 volts.

Current (Amperes):

The current ratings at LV sides are 183.3,and 2782.4 at HV sides At LV side as we step down the voltages up to 6300 volts our current increases up to

2782.4 A at LV side.

Phase:

Connections:

HV side is delta connected having voltage and currents at positions

Type of Cooling:

It is of two types.a ) ON(oil natural) Cooling.b ) AN (air natural) Cooling.

A

B

o Frequency:Rated frequency is 50Hz.

o Weight:

Oil weight 1050kgCore and oil assembly weight 17000kgsLiftable weight 2355kg

Switching positions are at 360° round shaped circle starting from 1 to so on.When we select position 1 the voltages are 145206V and current is 39.8A respectively.The main purpose of positioning is to vary the HV side voltages depending on the load.But these varying voltages are in the limit and we can vary to a small limit at each postion means not in the limit of 1000V.This variations of voltages at each position is set by the manufacturer.The LV side voltages remains irrespective of the selected positions.

(a) (b)

To achieve voltages of 145206V and current of 39.8A we have to adust switching position 1.For this purpose we have to join 5 and 6 join taps as shown in above figure.Join taps tells us that we have to engage HV side windings to get the desired position.The number of windings actively involve in transformation process varies.

Transformer Testing

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We perform transformer testing in order to

locate the parameters of a transformer. In order to access their conditions put up the proper maintenance check out proper working operation save from any electrical hazards have environmental and human safety avoid from greater risk

BDV TESTING OF A POWER TRANSFORMER:

The oil works as insulating material and serves as coolant (exhaust heat being produced in transformer).

Transformer oil deteriorates because of aging and moisture ingress, requiring periodic testing.

In BDV testing, oil sample is subjected to a steadily increasing alternating voltage until breakdown occurs in a BDV test kit.

The breakdown voltage is the voltage reached at the time of the first spark appears between the electrodes.

The test is carried out six times on the same cell filling and the electric strength of the oil is the arithmetic means of the six results obtained.

Testing kit has two electrodes having gap of 2.5 mm.

Break Down Voltage Test Values:-

What should be Break Down Voltage values for the following Transformers?1. 22/0.4kV, 1000kVA2. 0.4/125 KV, 160.9 KVA3. 132/6.3 KV, 26 MVA

Above three transformers having different voltages. Take Higher side Voltages1. 22,000 V2. 125, 000 V3. 132, 000 V

According to IS 6792:1992, BDV value should be30 kV Minimum: Transformers up to 72.5 kV40 kV Minimum: Transformers from 72.5 kV up to 170 kV50 kV Minimum: Transformers from 172 kV up to 420 kV

Example-1: 22/0.4kV, 1000kVA

The BDV value should be greater than 30 KV (Voltage is 22000 V)

Example-2: 0.4/125 KV, 160.9 KVA

The BDV value should be greater than 40 KV (Voltage is 125,000 V, greater than 72,500 V)

Example-3: 132/6.3 KV, 26 MVA

The BDV value should be greater than 40 KV (Voltage is 132,000 V, greater than 72,500 V

(oil testing through drain valve) (oil testing through buchholz relay)

Collection of Oil Samples:

Six samples of 37.5MVA 33KV / 11KV Transformer are taken. Time gap between each test is 5 minute. Electrode/arc gap in the testing kit is 2.5 mm.

Results are as follows

a. 85 kV

b. 89 kV

c. 75 kV

d. 90 kV

e. 88 kV

f. 65 kV

Such types of variation are often seen and cause confusion whether the unit is ready for operation or not.

One common reason for variation in result is insufficient time gap between oil samples.

Instead of 5 minutes, time gap between each sample should be around 15-20 minutes.

The techniques used for oil sampling follow the guidelines of international recognized standards that are known as:

ASTM (American Society for Testing Materials) IEC (International Electro technical Commissioning) AS (Australian Standards) EPA (Environmental Protection Agency)

Bottle Sampling:

The recommended identified point is bottom drain valve Lye absorbing pieces under drain valve to avoid ground to be contaminated by

the oil spills. Place a container under the samling valve to capture oil

The main valve of the sampling point should be closed Now carefully remove the bolts and nuts from the sampling port Clean the sampling port with clean cotton Now install the flinch adopter to the sampling port Push the tubing of the flinch into the sample bottle and start to lose the valve Gradually turn the bottle to allow the oil to swill and warm the bottle. It will help us

to prevent condensation Don’t shake the bottle as it will introduce moisture and air bubbles into the

sample Tight the transformer drain valve and empty the tubing of flinch adopter

Glass Syringe and stopcock sampling:

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First identify the sampling valve which is bottom drain valve Lye absorbing pads under drain valve to avoid ground to be contaminated by the

oil spills Place a container to capture oil that is going to spill The main valve of the transformer should be closed Lose nuts and bolts to remove the sealing kit Clean the sampling port with the neat piece of cotton Install the flinch adopter at the sampling port

Ensure that the stopcock is attached to the syringe and attached the flinch tubing to the stopcock

Set the stopcock at position 1 and push on the plunger

Note: Pulling on the plunger may results in formation of bubbles Set stopcock at position 3 and press the plunger to swill the oil into the container Then open the stopcock and fill the syringe up to 50mm,close stopcock and

remove the tubing, pack the syringe immediately into box, unblock the adopter, install the sealing kit of sampling port

Remember that the oil remaining oil into the tubing and adopter should be flush through to the waste container

Sampling using a buchholz sampling device:

Buchholz relay is introduced in conservator tank designed transformers.It is introduced to provide protective functions and accumulate gases released from the transformer.

(Buchholz relay b/w T/F and tank) (Bubbles travelling towards the oil tank)

When due to some reasons gases are not dissolved in the insulating liquid they evolve in the form of bubbles.

These bubbles find their way to the conservator through the pipe. So these bubbles are removed by the Bucholz relay before they reach the

conservator tank and contaminate the oil. The Bucholz samples are taken from the gas sampling device. In case if device is not available samples are taken directly from the buchholz Buchholz can be sampled either for oil or gas If Bucholz contains humiliated gas then it need to be sampled and gas can be

released using variety of samples We can use either glass syringes or gas sampling bags

First task is to accumulate gas through the buchholz into the sampling device

TO take samples through the sampling device open the bottom port of the sampling device and allow the oil to drain,Notice that the oil level in the device decreases

Now the sampling device is full of gas close the bottom port Once the gas is collected in the device retrieve the gas by attaching the sampling

tubing to the top port of the sampling device and connect the sampling container either the glass syringe or sampling bag

Now open the top port valve of the sampling device to release Notice that the oil level will rise again as the gas is pushed out from the device Note that all the gas is released by opening the top valve port till the oil start to

flow out through the tubing

DG(Dissolved Gas) Analysis of a power transformer:

DGA analysis is not for determining the quality of oil in the transformer.

It is carried out to determine any abnormalities in the internal winding or

paper insulating of transformer.

Abnormalities in the internal winding or paper insulation can be detected

by complete overhaul of transformer if DG Analysis is not carried out.

Due to faults that are happening inside the transformer heat will be

formed and due to this energy the long organic oil chain is getting broken.

As a result so many gases is formed such as CO2, C2H4, C2H2, C2H6,

C3H6, C3H8, CH4, CO, H2, O2, N2 etc.

The principal or key gases associated with each type of issue are shown below:

Hydrogen (H2): generated by partial discharges

and arcing

Methane (CH4): generated by relatively low elevated

temperatures (150° C).

– Acetylene (C2H2): generated by arcing.

– Ethane (C2H6): generated by high temperatures

(<300° C)

– Ethylene (C2H4): generated by high temperatures

(>300° C)

– Carbon Monoxide (CO): generated by oxidation of

cellulose insulation

– Carbon Dioxide (CO2): generated by oxidation of

cellulose insulation

In DG analysis concentration of different gases like nitrogen, oxygen,

carbon monoxide, carbon dioxide, hydrogen, methane, ethane, ethylene

and acetylene etc. is determined.

Example:

26.25 MVA, 220/11.5 KV is being tripped by Bucholz relay. DG Analysis

is being carried out having following results

Gas Concentration in ppm

H2 28

O2 173831

N2 593952

CH4 8

C2H4 4

C2H6 1

C2H2 2

C3H6+C3H8 1

CO2 615

CO 124

What could be interpreted from these results?

Permissible Limits:-

Please see the chart

It gives the permissible Limits of Gas levels in relation to age of the

transformer

In your Transformer none of the above limit is exceeded.

If anything exceeded, then we can go for analysis of fault.

Therefore your Transformer is Healthy.

Please confirm whether the transformer is recently commissioned. In that

case, there is every possibility that the trapped air in the winding and

other areas During Hot Oil circulation get released due normal vibration

and collected at Buchollz relay.

Dissolved key gas concentration limits in ppm

Status Normal Modest Concern Major Concern

Imminent Risk

Transformer Results as mentioned above

H2 100 101-700 701-1800 >1800 28CH4 50 121-400 401-1000 >1000 8

C2 H 2 5 36-250 51-80 >80 2C2 H 4 50 51-100 101-200 >200 4C2 H6 50 66-100 101-150 >150 1CO 200 351-570 571-1400 >1400 124CO2 5000 2500-4000 4001-10000 >10000 615

Remarks:

Normal:

In this condition it is stated that below this level indicates that the transformer is

operating in a satisfactory manner.

If any individual gas concentration exceeds the specified level, it is needed to

investigate further

Modest Concern:

Within this range indicates greater than normal combustible gas concentrations.

Any individual combustible gas exceeding specified levels should be

investigated.

Check either any trend is present or not.

Major Concern:

Within this range indicates a high level of decomposition.

Any single combustible gas exceeding these levels should be investigated

immediately.

Take immediate action to establish a trend, as faults are probably present.

Imminent Risk:

Within this range indicates excessive decomposition of Beta Fluid and cellulose.

Continued operation could result in failure of the transformer.

Our Results:

The results shown here illustrates that the transformer is healthy and is in proper

working condition.

None of the limit is exceeded, so there is no need to go for the analysis of the

fault.

The following examples are extreme but serve to illustrate how the dissolved gas-in-oil test can be used

to highlight active problems.

EXAMPLE 14 The utility suspected that the magnetic shunt pads attached to the inside of this transformer tank had

come loose and were either touching the core (layered steel structure around which the copper winding is

installed) or were coming very close. The result was intermittent unintentional core grounding and stray

flux causing localized overheating of the oil.

EXAMPLE 25 It was suspected that a single line to ground through-fault caused the failure of this

transformer. The fault was of enough energy to destroy one of the windings. Acetylene was a

predominant gas possibly indicating that arcing may have occurred. The acetyleneto-ethylene ratio

indicated the problem could be either high temperature overheating of the oil, arcing or both. The problem

was likely in the winding because it involved cellulosic materials, as witnessed by the comparatively high

carbon monoxide and carbon dioxide concentrations.

EXAMPLE 35 A technician noticed that this transformer was enveloped in a cloud of steam during a

rainstorm. This prompted an investigation where it was found that the temperature indicator pegged. It

was estimated that the temperature in the unit had been greater than 200°C. It was discovered also that

the unit became highly overloaded during some switching functions due to current imbalances in the three

phases. The condition probably existed on and off for two years. An internal investigation found the

cellulosic paper used as an insulant was brittle and crumbly. Again, the key gas indicators for this

condition were carbon monoxide and carbon dioxide.

Oil testing is an important part of a utility’s electric apparatus condition assessment. As these cases

illustrate, dissolved gas-in-oil analysis is the most important diagnostic test for detecting a wide range of

problems

The principal or key gases associated with each type of issue

are shown below:

– Hydrogen (H2): generated by partial discharges

and arcing

– Methane (CH4): generated by relatively low elevated

temperatures (150° C).

– Acetylene (C2H2): generated by arcing.

– Ethane (C2H6): generated by high temperatures

(<300° C)

– Ethylene (C2H4): generated by high temperatures

(>300° C)

– Carbon Monoxide (CO): generated by oxidation of

cellulose insulation

– Carbon Dioxide (CO2): generated by oxidation of

cellulose insulation

Status

Hydrogen H

2

Methane

CH4

Acetylene

C2 H 2

Ethylene

C2 H 4

Ethane

C2 H6

Carbon monoxide

CO

Carbon dioxide

CO2

Normal 100 120 35 50 65 350 2500

Modest concern

101-700 121-400 36-250 51-100 66-100 351-570 2500-4000

Major Concern

701-1800 401-1000

51-80 101-200 101-150

571-1400 4001-10000

Imminent Risk

>1800 >1000 >80 >200 >150 >1400 >10000

__________________

The insulating liquid is in contact with the internal components.

Gases formed by normal and abnormal events within the transformer are

dissolved in the oil.

By analyzing the volume, types, proportions, and rate of production of dissolved gases, much diagnostic information can be gathered.

Therefore we perform the DG analysis to estimate the volume of dissolved gases and to check the quality of oil.

Fault Gases:

Since these gases can reveal the faults of a transformer, they are known as "fault gases".

Gases are produced by oxidation, vaporization, insulation decomposition, oil breakdown and electrolytic action.

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Evaluation of Possible Faults by the Key Gas Method:

The four general fault types produce a unique gas that indicates the fault type.

“Key Gas Method” is used as an indication of which fault type to examine in greater detail.

Fault Type: Thermal decomposition of Cellulose

Principal Gas: Carbon Monoxide

Characteristics: Decomposition products of cellulose include CO and CO2. If the

cellulose is saturated with Beta Fluid, the decomposition products will include

hydrocarbon oxides (as above)

Fault Type: Corona –partial discharge:

Principal Gas: Hydrogen

Characteristics: Corona discharges produce hydrogen and methane. If the corona

occurs in cellulose, the gas profile will also include CO and CO2

Fault Type: Arcing

Principal Gas: acetylene

Characteristics: Arcing always generates large amounts of acetylene and hydrogen.

Carbon oxides may be present if the fault involves cellulose. Carbon may be present in

the oil.

Dissolved Gas Sampling:

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Sampling Equipments

Sampling Flask:

A piece of firm is there at the top to protect the flasks By using flasks to collect samples of nitrogen or oxygen biological activities can

be killed by mercury present in the flask Lid of the flask has the rings that are fixed between the glass and the plunger to

covered properly

Sampling Rack:

Used to hold the sampling flasks It has a clip and tubing and at the back it has a wrench to hold the gas cylinders

C02 Cylinder for sample collection:

Normally 5 ponds cylinder having 5 ponds C02 is used Use small elastic belts to hold the cylinder The regulator is used to control the flow

Tubing for sample collection:

Two thin nylon tubes are used One for C02 and the other for water

Part 2:

How to do Sampling:

Remove the black vinyl cap and attach tygon tubing Start the CO2 gas to flow Insert the tubing from the cylinder into the flask Flush the area between the o-rings with CO2 la

Inspection and Electrical maintenance of a power Transformer

The present maintenance trend is to reduce cost, which in some cases means lengthening the intervals of time to do maintenance or eliminating the maintenance completely.

The utility, or company, realizes some savings on manpower and material by lengthening the maintenances cycle, but by doing this, the risk factor is increased.

A few thousand dollars for a maintenance program could save your utility or company a half-million dollar transformer.

Inspection: It First de-energize the transformer

Look for physical damages and check proper insulations Loose connections and defective wires The most important the signs of corrosions Verify whether the cores or properly grounded or not Load tap changer should be on the proper positions since at wrong positions it

may results in excessive load currents ,under voltage and over voltage conditions Transformer itself should be properly grounded

To carry out inspection in more details following tests should be conducted.

1. *Insulation Resistance:

Tap Change of Power Transformer

Regulating the voltage of a transformer is a requirement that often arises in a power system

The voltage control is performed by changing the turns ratio. This is done by provision of taps in the winding.

In an application it may be needed

1. To supply a desired voltage to the load.

2. To counter the voltage drops due to loads.

3. To counter the input supply voltage changes on load.

Fig 42: (Tap Changing and Buck Boost Arrangement)

Note: Taps are provided on the HV winding

The volts per turn available in large transformers is quite high and hence a change of even one turn on the LV side represents a large percentage change in the voltage.

Also the LV currents are normally too large to take out the tapping from the windings. LV winding being the inner winding in a core type transformer adds to the difficulty of taking out of the taps.

Hence irrespective of the end use for which tapping is putt, taps are provided on the HV winding.

Buck-Boost Arrangement:

Provision of taps to control voltage is called tap changing. In the case of power systems, voltage levels are sometimes changed by injecting

a suitable voltage in series with the line. This may be called buck-boost arrangement. In addition to the magnitude, phase

of the injected voltage may be varied in power systems.

Tap Changing is affected when:

a) The transformer is on no load or No load Tap Changer (NLTC)

In this arrangement the tap positions are changed when the transformer is taken out of the circuit and reconnected.

b) The load is still connected to the transformer or On Load Tap Changer (OLTC):

In this arrangement taps are changed without the interruption of the load current As there is no permission of switching off the transformer during tap changing. The

tapping arrangement is placed in separate diverter tank attached to electrical power transformer main tank.

Inside this tank, the tap selectors are generally arranged in a circular form. The diverter switches have contacts operating in rapid sequence with usually four separate make and break units.

Schemes of On-Load tap changing:

Reactor method:

This method employs an auxiliary reactor to assist tap changing. The reactor has a center tapped winding on a magnetic core.

The two ends of the reactor are connected to the two bus bars to which tapping switches of odd/even numbered taps are connected.

(Reactor Method of Tap Changer with table of switching)

When only one tap is connected to the reactor the shorting switch S is closed minimizing the drop in the reactor.

The reactor can also be worked with both ends connected to two successive taps. In that case the switch ’S’ must be kept open.

The reactor limits the circulating current between the taps in such a situation. Thus a four step tapped winding is used for getting seven step voltage on the secondary (see the table of switching).

Tap Switches Closed

1 1,s

2 1,2

3 2,s

4 2,3

5 3,s

6 3,4

7 4,s

8 4,5

9 5,s

The advantages of this type of tap changer are:

1. Load need not be switched off.

2. More steps than taps are obtained.

3. Switches need not interrupt load current as an alternate path is always provided.

Major Objection:

The major objection in this scheme is that the reactor in the circuit is always generating extra loss.

Parallel winding, transformer method:

In order to maintain the continuity of supply the primary winding is split into two parallel circuits each circuit having the taps as shown in Fig. 44.

Two circuit breakers A and B are used in the two circuits. Initially tap 1a and 1b are closed and the transformer is energized with full primary voltage.

To change the tap the circuit breaker A is opened momentarily and tap is moved from 1a to 2a. Then circuit breaker A is closed.

When the circuit A is opened whole of the primary current of the transformer flows through the circuit B.

A small difference in the number of turns between the two circuit exists. This produces a circulating current between them.

Next, circuit breaker B is opened momentarily , the tap is changed from 1b to 2b and the breaker is closed.

In this position the two circuits are similar and there is no circulating current.

Fig 44: (Parallel Primary Winding Tap Changer)

Note: The circulating current is controlled by careful selection of the leakage reactance. Generally, parallel circuits are needed in primary and secondary to carry the large current in a big transformer. Provision of taps switches and circuit breakers are to be additionally provided to achieve tap changing in these machines.

Series booster method:

In this case a separate transformer is used to buck/boost the voltage of the main transformer. The main transformer need not be having a tapped arrangement. This arrangement can be added to an existing system also.

Fig. 42 shows the booster arrangement for a single phase supply. The reverser switch reverses the polarity of the injected voltage and hence a boost is converted into a buck and vice versa.

The power rating of this transformer need be a small fraction of the main transformer as it is required to handle only the power associated with the injected voltage.

Note: One precaution to be taken with this arrangement is that the winding must not be open circuited. If it gets open circuited the core (B in fig) gets highly saturated.

In spite of the small ratings and low voltages and flexibility, this method of voltage

control costs more mainly due to the additional floor space it needs. The methods of

Moving Coil Voltage Regulators:

Voltage regulation discussed so far basically use the principle of tap changing and hence the voltage change takes place in steps.

Fig 45: (Moving Coil Voltage Regulator)

Applications like a.c. and d.c. motor speed control, illumination control by dimmers, electro-chemistry and voltage stabilizers need continuous control of voltage. This can be obtained with the help of moving coil voltage regulators.

Fig. 45 shows the physical arrangement of one such transformer. a, b are the two primary windings wound on a long core, wound in the opposite sense. Thus the flux produced by each winding takes a path through the air to link the winding.

These fluxes link their secondaries a2 and b2.A short circuited moving coil s is wound on the same limb and is capable of being held at any desired position.This moving coil alters the inductances of the two primaries.

The sharing of the total applied voltage thus becomes different and also the induced emf in the secondaries a2 and b2. The total secondary voltage in the present case varies from 10 percent to 20 percent of the input in a continuous manner.

The turns ratios of a1:a2 and b1:b2 are 4.86 and 10.6 respectively.5/4.86+95/10.6= 10% when s is in the top position. In the bottom position it becomes 95/4.86+5/10.6= 20%. By selecting proper ratios for the secondaries a2 and b2 one can get the desired voltage variation.

Sliding contact regulators: These have two winding or auto transformer like construction.

The winding from which the output is taken is bared and a sliding contact taps the voltage.

Fig 45: (Sliding Contact Regulator)

The minimum step size of voltage change obtainable is the voltage across a single turn.

The conductor is chosen on the basis of the maximum load current on output side. In smaller ratings this is highly cost effective.

Two winding arrangements are also possible. The two winding arrangement provides electrical isolation also.

Electrical Maintenance:

It involves fault diagnosis, routine servicing and repair of electrical components By this smooth running of production can be governed

Electriacal maintenance involves: Preventive maintenance Operational maintenance Corrective maintenance

(a) (b)

(c) (d)

Preventive maintenance:

It depends upon the RCM analysis It is performed to avoid failures ,unnecessary production loss and safety

violations

Cast Resin Transformers (CRT):

CRT are dry type Transformers and maintenance free Insulation failure due to overheating of winding is the cause for failure of

Transformer winding. To monitor rise of temperature in CRT temperature sensors are located at various points inside winding and are connected to the tripping controls of the breakers.

Carry out routine cleaning of dust specially when the transformers are installed outdoor. Dust formation along with moisture causes insulation failure and flash over. If air bubbles remain in the castings it will cause insulation failure.

We can replace the oil filled transformers in most of the substations to avoid the transformer failure for at least ten years.

Operational maintenance:

It is the care and minor maintenance of equipment It consists of inspecting ,cleaning ,servicing ,preserving ,lubricating and adjusting It also includes the minor parts replacement

Corrective maintenance:

It involves the regular task to maintain system in optimum operating condition In this maintenance equipment is maintained after breakdown It is the most expensive type

Its major functions includes: Eliminating breakdown Eliminating deviation Eliminating unnecessary repairs

Precautions to be taken before starting Maintenance:

The transformer should be disconnected from supply Windings and Tank should be solidly earthed Flame should not kept near the transformer at the time of maintenance work The manufacturer’s recommendations must be followed for maintenance

Maintenance Schedule of Power Transformer as per Standard Specifications:

ISI 1886-1967 Recommended Maintenance

Maintenance Schedule for Transformers of Capacities 1000kVA and above

S.No. Inspection Frequency

Item to be inspected

Inspection Notes Actions required if abnormal Conditions

1 Hourly Ambient Temperature

……………………… ……………………

2 Hourly Ambient Temperature

Check that temperature rise is reasonable

Shutdown the transformer and investigate either persistently higher than normal

3 Hourly Oil Temperature

4 Hourly Load ampere Check against the rated figures

……………………

5 Hourly Voltage Check against the rated figures

……………………

6 Daily Oil level in transformer

Check against transformeroil level

If low,top up for with dry oil examine transformer for leaks

7 Daily Oil level in bushings

…………………… ………………….

8 Daily Leakage of water into cooler

…………………… Replace if crack or broken

9 Daily Relief If Broken Example for short

diaphragm circuit etc.10 Daily Dehydrating

BreatherCheck that air passage is free, Check color of active agent, examine for cracks and dust deposits

If silica gel is pink ,check by spare charge, The old charge may be reactivated for use again

11 Quarterly Bushing Check for dielectric strength and water content

Clean or replace

12 Quarterly Oil in Transformer

Quality of Oil Take suitable action to restore Quality of Oil

13 Quarterly Cooler for bearing motors and operating mechanisms

Lubricate bearings check gear box, Examine contacts check manual controls and interlocks

Replace burnt or worn contacts or other parts

14 Half Yearly Oil Cooler Test for pressure ……………………

15 Half Yearly Oil in Transformer

Check for acidity and sludge

Filter or replace

16 Yearly Oil filled bushing

Test Oil Filter or replace

17 Yearly Gasket joints …………………… Tighten the bolts evenly to avoid uneven pressure

18 Yearly Cable Boxes Check for sealing arrangement for filling holes. Examine compounds for cracks

Replace Gaskets if leaking

19 Yearly SurgeDiverters and Gaps

Examine for cracks and dust deposits

Clean or replace

20 Yearly Relays, Alarms and their circuits

Examine Relay and Alarm Contacts ,Their operations ,Fuses ,Check Relay accuracy etc.

Clean the components and replace fuses and contacts if necessary. Change the settings if necessary

21 Yearly Earth Resistance

…………………… Take suitable actions if resistance is high

22 a) 5 Yearly a )1000 to 3000 kVA

Overall inspection including lifting of core and coils

Wash by hosing down with clean dry oil

b) 7-10 Yearly

b ) above 3000kVA

Overall inspection including lifting of core and coils

Wash by hosing down with clean dry oil

ISI 1886-1967 Recommended Maintenance

Maintenance Schedule for Transformers of Capacities less than 1000kVA

S.No. Inspection Frequency

Item to be inspected

Inspection Notes Actions required if abnormal Conditions

1 Hourly Load Current Check against rated figures

……………………

2 Hourly Voltage Check against rated figures

……………………

3 Daily Dehydrating Breather

Check that air passages are clear, Check color of active agent

If Silica Gel is pink ,Change by spare charge.The old charge may be reactivated for use again

4 Monthly Oil level in Transformer

Check the transformer oil level

If low top up with dry oil. Examine transformer for leaks

5 Quarterly Bushings Examine for cracks and deposits

Clean or replace

6 Half Yearly Non-Conservator tank

Check for moisture under cover

Improve ventilation ,Check oil

7 Yearly Oil in transformer Check for dielectric strength and water

Take suitable actions to improve

content, Check for acidity and sludge

quality of oil

8 Yearly Earth Resistance …………………… Take suitable actions if Earth Resistance is high

9 Yearly Relays ,Alarms and their circuits

Examine Relay and alarm contacts, their operation fuse etc. Check relay accuracy etc.

Clean the components and replace contacts and fuses if necessary. Change the setting if necessary

10 2 Yearly Non Conservator Transformers

Internal inspection above core

Filter oil regardless of conditions

11 5 Yearly ……………………

Overall inspection including lifting coils

Wash with clean dry oil

NOTE:

One of the reasons for failure of distribution transformers is the supply of sub-standard transformers, which is a result of improper vendor rating and lack of routine test prior to dispatch by the manufacturer & issue for use. It is therefore proposed as follows:

Vendors should be approved only after confirming the adequacy of the manufacturing & testing facilities at the works.

Procurement should be on the guaranteed technical particulars and not necessarily L-I rate.

Routine tests should be observed at the works and the transformer should be tested prior to issue for utilization.

Maintenance Tests:

Two important tests that could prevent field failure are:

1. Using an infrared scan on a transformer could locate “hot spots”. By using this test we could detect system overloads, loose defective

components and damaged switchgears An infrared camera is used to detect the hotspots Hotspots are easily eliminated before before they cause failures IR Thermography technique is used in these cameras

IR Thermography Inspection helps:

Increase equipment life Reduce downtime Electrical thermography inspections save money Prevent catastrophic failure Lower repair costs Lower risk Prevent lost production Non-destructive Infrared testing

2. Dissolved gas analysis test of the oil by a lab