CLASSIFICATION OF POWER SUPPLY

1.      Class IV           - Normal PS 415V 3 ph – 2500 KVA

2.      Class III – Diesel Generator set, 415V, 3 ph, 1010KVA – Self exited.

3.      Class II   - UPS – 230V, 50 KVA

4.      Class I    - Battery pS – 110VDC, 550AH & 425 AH-ROP S/stn, 300 AH – PREFRE S/stn.

Class I – Control ckt VCB, Exit lighting

Class II – Control room all area monitors, SCADA, trx scanners.

Class III – E2 fans & lighting

Class IV – All are normal supply.

FRLS: Flame Retardant Low Smoke Cable: Used in High block

TRANSFORMER

A Transformer is a static piece of apparatus which transforms electrical energy from one circuit to another circuit without changing the frequency either stepping up or stepping down the voltage.

Principle: The principle of operation of a transformer is based upon the theory of mutual induction between two electrical circuits linked by a common magnetic flux.

The rating of a transformer is the maximum power which can be drawn from it without the temperature rise in the winding exceeding the safe limits for she particular class of insulation employed.

The rated capacity of a transformer is expressed in kVA not in kW. Generally the rating of a transformer is determined by its temperature rise. The temperature rise is caused by the losses in the machine. Copper loss depends upon the value of the load current and iron loss on voltage. Therefore, total loss of a transformer depends on volt ampere (VA) and is independent of load power factor. A certain amount of current will produce the same I²R loss at any value of power factor. This loss limits the output of the machine. The output in kilowatts is proportional to the power factor. For a given kW load, if the power factor decreases, the load current increases proportionately causing more losses and temperature rise in the machine. For the above reason transformers are generally rated in kVA not in kW.

Power factor of transformer: The power factor of a transformer is very low and lagging on no load. But the power factor on load is nearly equal to the power factor of the load which it is carrying.

Normal phase difference between the voltage: The no load current in a transformer normally lags behind the voltage by about 75⁰.

Material used for the cores of a transformer: Laminations of specially alloyed silicon steel (silicon content 4 to5%) are used due to its high electrical resistance, high permeability, non-ageing characteristics and minimum iron loss.

The iron core is used in a transformer to provide continuous easy magnetic path of low reluctance. The magnetic leakage is reduced to a minimum by sectionalizing & interleaving the primary and secondary windings. Iron cores are made laminated to reduce the eddy current loss. The laminations are insulated from each other by insulating varnish or thin paper to break the path of eddy currents thus to reduce the eddy current loss.

Grain oriented laminations: The grain oriented laminations are such cold rolled laminations specially annealed to orient crystals i.e. the grains in a uniform direction in the direction of rolling which produce very high permeability and low hysteresis loss. These laminations are punched and assembled in such a way that the grains of different lamina lie in the same direction as the magnetic field and not across it. A special care is taken during tightening of iron cores to make it free from vibration and humming sound. The max. flux density in transformer core is 1.6 to 1.8 Weber per metre².

The phase relationship between the primary and secondary voltages of a transformer is 180⁰ out of phase.

CRGO Steel: It means Cold Rolled Grain Oriented silicon steel manufactured by coiled rolling to give easy direction to all its crystals.

To magnetize itself less magnetizing force is required. Accordingly requirement of magnetizing current is reduced. Therefore it is widely used for making transformer cores.

Special attention is given to assemble a transformer core in such a manner that the crystal direction is parallel to the flux path, otherwise the core will offer high reluctance thereby lowering magnetic permeability.

Permolloy: It consists of iron and nickel alloyed with copper and molybdenum having initial permeability of 2500 and maximum permeability up to 100000.

It is heat treated up to 1100⁰C and curie temperature varies between 420-580⁰C depending on the composition. It is used in the manufacture of sensitive relays.

Mumetal: It consists of iron and nickel alloyed with copper and chromium having initial permeability of 20000 and maximum permeability up to 110000. It is heat treated up to 1100⁰C and curie temperature is 430⁰C. It is used for manufacturing instrument transformers and miniature transformers.

Turn ratio of a transformer: The ration of number of turns in the primary to the number of turns in the secondary winding is called the turn ratio or the ratio of transformation of the transformer which is indicated by a constant ’K’. Then N₁/N₂ = E₁/E₂= K

Voltage ratio of a transformer: Voltage ratio is the ratio of the voltage between the line terminals of one winding to that between the line terminals of another winding at no load.

Counter e.m.f. in primary winding of a transformer: When an alternating voltage is applied to the primary winding it causes a flow of alternating current which creates alternating flux. This alternating flux is linked with both the primary and secondary windings and hence induces self induced emf on the primary winding and a mutually induced emf on the secondary winding. Since the primary self induced emf is in the opposite direction to the applied voltage, it is known as counter emf or back emf of the transformer.

e.m.f. equation: E_rms = 4.44φ_m.f.T. volts

The different types of coils used in the transformer winding and their uses:

(a) Spiral type coil: Spiral type coil: This consists of layers wound in continuous length from top of bottom of the coils, the conductors of which consist of a number of square or rectangular strips in parallel. These coils are only used for low voltage winding carrying a heavy current of more than 100 amps.

(b) Cross over type coil: This type of coil consists of a number of layers having a number of turns per layer, the conductor being round wire or a strip insulated with a paper covering. Cross over coils are wound on formers. These coils are used for high voltage windings of distribution transformers and current up to about 20Amps.

(c) Helical type coil: This type of coil is wound in the form of a helix and consists of a number of rectangular strips wound in parallel radically so that each separate turn occupies the total radial depth of the winding. These coils area used for lower voltage winding of larger transformer at voltages 11kV to 33 kV. Multilayer helix can be used for higher voltage winding up to 500 kV.

(d) Continuous disc type coil: This coil consists of a number of discs wound continuously from a single wire or a number of strips in parallel. In each disc a number of turns are wound radially over one another and the conductor passes from disc to disc uninterruptedly. These coils are used for both higher voltage winding from 33kV to 500kV and lower voltage winding from 11kV to 66kV above the rating of 1MVA.

Ideal transformer: The transformer having an overall efficiency of 100% is called ideal transformer. For an ideal transformer, Output = Input.

Losses in Transformer: (i) Core losses or iron losses or constant losses consisting of hysteresis and eddy current losses in the core and (ii) Copper losses in the primary and secondary windings.

Since hysteresis is a kind of loss and affect the efficiency of transformer, so it is always objectionable.. The hysteresis loss depends on (i) the quality and constituents of iron core, (ii) the frequency and (iii) the flux density. Hysteresis loss is more than eddy current loss in a transformer. The main purpose of using silicon steel in the laminations of transformer core is to reduce the hysteresis loss. The core flux is always constant on every load i.e no load to full load. No load current produces core flux and supplies iron losses and copper loss on no load.

Load current of a transformer: When a load is connected to the secondary side of a transformer the transformer is said to be on load and the current circulating through the secondary winding via load is called the load current. 

Primary current on load: When the load current(I₂) flows through the secondary winding a counterbalancing current (I₁) is induced on the primary winding varying inversely to their respective turns. The total primary current on load (I₁) is equal to the vector sum of the primary counter balancing, current (I₁) and the no load current (I₀) which will be approximately equal to (I₁) as I₀ is practically very small.

If the load current increases with the increase in load current corresponding transformer circuit modeling the behavior of a real transformer which represents the primary circuit as well as secondary circuit in one circuit with suitable parameters for easy calculation with the addition of a  shunt circuit to account for no load current.

Approximate equivalent circuit of the transformer: The approximate equivalent circuit to the transformer means a slight modification of the equivalent circuit by transferring the shunt circuit to the primary terminal with close approximation effecting very little change in the result.

Core losses can be determined by short circuit test. In this test the low voltage secondary of the transformer is short circuited through an ammeter. At first a small voltage is applied to the high voltage primary side and the voltage is gradually raised in steps till full load current flows through the secondary. Since the test voltage is very small in comparison to the normal voltage the core losses are negligible and the wattmeter connected on the primary side records the full load copper losses.

Since no load current is very small and no load copper loss is very small in comparison with the iron losses so the copper losses are neglected in open circuit test.

Copper losses can be determined by short circuit test. In this test the low voltage secondary of the transformer is short circuited through an ammeter. At first a small voltage is applied to the high voltage primary side and the voltage is gradually raised in steps till full load current flows through the secondary. Since the test voltage is very small in comparison to the normal voltage the core losses are negligible and the wattmeter connected on the primary side records the full load copper losses.

Copper loss varies inversely with the power factor.

Impedance voltage: The impedance voltage of a transformer is the voltage required to circulate rated current through a winding of the transformer when another winding is short circuited with the respective windings connected as for rated voltage operation. It is usually expressed in percent of the rated voltage of the winding in which the voltage is measured.

Efficiency of the transformer: The ordinary or commercial efficiency of a transformer is defined as the ratio of the output in watts to the input in watts. Since the efficiency is based on the power output in watts and not on volt-amperes, the power factor plays an important role in determining the efficiency.

The condition for the maximum efficiency of a transformer is (Iron losses = Copper losses).

The efficiency of a power transformer depends on the power factor of the load and the percentage of the load.

Since the transformer is a static apparatus without any rotating parts there is no frictional loss. The only losses are iron losses and copper losses in the primary & secondary. Therefore, the efficiency of a transformer becomes so high about 95% to 98%.

All day efficiency of a transformer: It is defined as the ratio of energy output in 24 hours to energy input in 24 hours.

Regulation of a transformer: The regulation of a transformer is the difference between the no load and full load secondary voltages expressed in terms of the no load voltage with constant primary voltage.                 

                                              No load voltage – Full load voltage

Percentage regulation = -----------------------------------------------  x 100%

                                                               No load voltage

                                                  E – V                                  IR cosφ  + IX sinφ

                                        =   ---------------   x 100%    =   -------------------------  x 100%

                                                     E                                                     E

Where R = Equivalent resistance in terms of secondary.

             X = Equivalent reactance in terms of secondary.

              E = Induced e.m.f.  of secondary

              φ = Phase difference between voltage and current

              I  = Full load current.

Effect of power factor and load on the regulation of a transformer: For lagging power factor of load current the regulation will be positive i.e. the secondary voltage will be reduced with the increase in the load current.

For leading power factor of load current the regulation will be negative i.e. the secondary voltage will be negative i.e. the secondary voltage will be increased with the increase in the load current.

Percentage impedance of the transformer: The impedance voltage is the vector sum of resistance voltage and reactance voltage. The percentage impedance is the value of impedance voltage expressed as a percentage of rated voltage. Thus % impedance = % Impedance voltage.

          I₁Z’₁

=    ---------  x 100

            V₁

Where I₁ = Rated primary current

             Z₁  = Impedance with reference to primary.

             V₁  = Rated primary voltage.

Tap changing: Tap changing is a process of changing the ratio of transformation by increasing or decreasing the number of active turns in one winding with respect to another winding for maintaining a constant voltage on a power system.

There are two types of tap changing arrangements:

1. OFF load tap changing: Off load tap changing gears can be operated only when all the loads on the transformer are cut off. The voltage ratio of a transformer can be varied by about + 10% by means of OFF load tap changing gears.

2. ON load tap changing gears can be operated even when the load is on. The voltage ratio of a transformer can be varied by about + 16% by means of ON load tap changing gears. The tapings are generally provided on the high voltage side since the current loading is comparatively small on the high voltage side.

Bushing: Bushing is a type of porcelain or ebonite post insulator put on the top or side of the transformer tank though which connection are made to the external circuits.

Types of bushing:

 (a) Porcelain insulator bushing used up to 33 kV

(b) Oil filled bushings consisting of a hollow porcelain cylinder of special shape with a hollow tube shaped conductor through its centre used for the voltage above 33 kV

(c) Capacitor type bushing made of thick layers of bakelized paper alternating with thin graded layers of tin foil being covered by a porcelain rain shed and filled up with bitumen in the annular space between the rain shed and the bushing used in outdoor substation for the voltage above 33kV.

Auto transformer: An auto transformer is a single winding transformer provided with a number of taps in which the single winding performs the functions of both the primary and the secondary.

Basic difference between distribution transformer and auto transformer: The basic difference is that the distribution transformer has two separate windings and there is no electrical connection between the primary and the secondary while an auto transformer has a single winding in which the primary and secondary are electrically connected.

Advantage of Auto transformer: (i) Being single winding transformer it requires less copper and becomes lighter in weight. (ii) The cost is less than a two winding transformer, (iii) As it is provided with large number of taps desired voltages can be obtained by adjusting suitable taps.

The Auto transformers is used in high voltage line a fault in the primary may affect the secondary side in which case the secondary load equipments may be damaged. So it is not used for high voltages.

Auto transformers are used in case of low voltage and low transformation ration e.g. starting equipments of there phase induction motor, control equipments of single phase and three phase electr5ic locomotives, voltage control of power and lighting circuits, regulating transformers, boosters to raise the voltages in a.c. circuits etc.

Variable transformer: A variable transformer (named as Variac, Dimmer-stat etc) is nothing but an auto transformer having continuously variable tap so that any output voltage desired could be obtained.

Difference forms of connections used in primary and secondary windings in three phase transformers:

Star/Star connection is best suited for small high voltage transformers with balanced load.

Star/Delta connection is applied to the transformers at sending end of the transmission line where the voltage is to be stepped up.

Delta/Delta connection may be used in large low voltage transformers where the insulation problem is not so urgent.

Delta/Star connection is generally applied in step-down distribution transformers to provide a 3-phase, 4 wire supply for three phase loads as well as single phase loads which may be balanced or unbalance.

Symbolic vector groups for identification of 3-phase transformer:

(i) Group -1 (Yyo, Dd0, Dzo, Zd0)

(ii) Group-2 (Yy6, Dd6, Dz6, Zd6)

(iii)Group-3 (Dy1, Yd1, Yz1, Zy1)

(iv) Group -4 (Dy11, Yd11, Yz11, Zy11)

Transformer vector group Dy11 and impedance voltage 5%: A transformer with vector group symbol (Dy11) means a transformer having primary winding delta connected and secondary winding star connected and belonging to group-4 with + 30⁰ or -330⁰ phase displacement between primary and secondary winding as referred to the standard counterclockwise vector rotation.

Impedance voltage 5% means when 5% of normal voltage is applied across one winding it produces normal full load current to flow through the secondary winding when its terminals are short circuited.

Parallel operation: To connect two or more transformers in parallel is called parallel operation of transformers.

The necessity: (i) To share the increasing demand of the load, (ii) to continue the supply in case of any fault and periodical maintenance, (iii) To get maximum operational efficiency.

Condition for parallel operation:

(i)   The voltage transformation must be same

(ii) The polarity must be same

(iii) The percentage impedance should be same

(iv) The phase sequence must be same

(v) The vector diagram and the phase displacement must be same.

If the voltage ratios of the transformers running in parallel are not identical, the secondary voltage will be different for which a circulating current will flow through the two secondary’s if the transformers area connected in parallel. The circulating current will act as extra load without any useful work being done and cause heating of transformers even on no load.

 It is not necessary that the ratings of two transformers to be the same for parallel operation but they should share the load in proportion to their ratings.

Incorrect polarity results a dead short circuit and due to unequal percentage impedance the load sharing of two transformers will not be in proportion to their kVA ratings.

Effect of percentage impedance on load sharing: Since the percentage impedance determines the voltage drop between no load to full load, with equal percentage impedances the transformers running in parallel will share the load in proportion to their capacities. The transformer with a lower percentage impedance will have a lesser voltage drop and therefore it will take greater share of the load. On the contrary the transformer with a higher percentage impedance will have a higher voltage drop and it will not take its due share of the load.

In parallel operations of two transformers, both will share equally load if their impedances are equal. If the ratings of the two transformers are equal but percentage impedances are different the loads will be shared inversely as the impedances of the transformers expresssed in percent of normal voltage. In case of different percentage impedances for different ratings of transformers the percentage impedances have to be converted to the same basic kVA. Then the loads will be shared inversely as the converted impedance of the transformers. 

In parallel operation of transformers of different capacities. The rated output of the smallest transformer in the group should not be less the 33x 1/3 percent of the rated output of the largest transformer in the group.

Protection requires to transformer:  A transformer requires protection for the following hazards:

(i)   Overloading and short circuit fault.

(ii) Internal fault: (a) Loose contact,

(b) Short circuit between adjacent turns &

(c) Earth fault in the winding or leads.

(iii) External hazards:

(a)        Lightning surges &

(b)        System overvoltage.

Protective equipments required for a large transformer: (i) Over current relay, (ii) Differential relay, (iii) Earth leakage relay & (iv) lightning arrestor.

Instrument transformer: Ana instrument transformer is a transformer designed to represent the condition of current or voltage and of phase position in the primary circuit with acceptable accuracy in the secondary circuit. It changes the voltage or current in a power circuit to values which render them convenient for measurement.

The instrument transformers differ from the power transformers mainly by their volt ampere rating. The instrument transformers have very low rating of only a few tens of VA e.g. 10VA, 50 VA, 100 VA etc. Whereas the power transformers have high kVA rating e.g. 25kVA, 200kVA, 500kVA, 1000kVA even up to 1000 MVA.

Two types of instruments transformers: 1. Current transformer, 2. Potential transformers.

Current transformer: (i) Air-cooled current transformers and (ii) Oil immersed current transformers.

Types of current transformers according to the magnetic circuit: (i) Ring type, (ii) Core type, (iii) Shell type.

Error in CT’s: There are two types of errors, e.g. ratio error and phase difference error. The ratio error is the amount by which the secondary current differs from exact proportionality to the primary current and is expressed as a percentage of the rated secondary current.

The phase difference error is the angle by which the secondary current differs in phase from the primary current and is expressed in minutes of arc.

Categories of current transformers: There are two categories of CTs, e.g.

(i) Measuring current transformers are used with ammeters, watt meters, kVA meters, kWh meters, power factor meters etc.

(ii) Protective current transformers are used with over current relays, earth fault relays, differential protection, impedance protection etc. Current transformer is used in a Tong tester.

Only one turn of very thick conductor is used in the primary of a current transformer: In the primary of a current transformer only one turn of very thick conductor having negligible resistance is used not only for reducing the induced voltage but also for reducing the voltage drop to nearly zero. As large current flows through the primary and small current as per current ratio flows through the secondary, if one turn is selected for primary the secondary turns will be smaller.

Material used for the core of CT : In order to reduce the ampere turns required the core of a CT must have high permeability, small iron loss, a low flux density and low phase difference error. Two suitable materials are (i) Silicon steel having maximum permeability of 4500 at flux densities in the neighborhood of 5000 lines/cm² with phase difference error of 1.5 to 2⁰, (ii) Mumetal, a nickel iron alloy containing a small amount of copper having maximum permeability of about 80000 at flux densities of about 3500 lines/cm² with phase difference error of below 0.5⁰.

As the nickel-iron alloy reduces the magnetizing current and core loss to about 10% of that of the corresponding silicon steel values, Mumetal is best suited for the core of CT where a high degree of accuracy is desired.

Effect of saturation on the performance of CT: On reaching saturation the transformer ratio is affected resulting in an increase of exciting current due to reduction in magnetizing inductance and introducing a phase error in the CT deviating primary and secondary currents from their desired phase opposition i.e. 180^{0 .}

The increase in exciting current causes the reduced secondary output and hence reduced speed of over current relays. The saturation of CT disturbs the balance in differential relays thus affecting the stability of protection.

Therefore the effect of saturation makes the operation of the CT unsatisfactory.

Basic difference between current transformers and potential transformers: (i) A CT is connected in series with the line whereas a PT is connected across the supply lines. (ii) In a CT the number of turns is inversely proportional to the current but in a PT the number of turns is directly proportional to the voltage. (iii) In a CT the primary has one or more turns of heavy conductor and the secondary has large number of turns of thin wires whereas in a PT the primary has a large number of turns of thin wires and the secondary has a few number of turns of comparatively thick wires.

Types of potential transformer and application:  There  are   two types of potential transformers:

(i) Electromagnetic type potential transformer, in which primary and secondary windings are used on magnetic core like usual transformers.

(ii) Capacitor type potential transformer, in which a series capacitor group is used with an auxiliary voltage transformer. The primary voltage is applied to the series capacitor group and the voltage across one of the capacitor is taken to the primary of the auxiliary voltage transformer. Then the secondary of the auxiliary voltage transformer is finally taken to the metering and/or control panel.

Potential transformers are used with voltmeter, kWh meter, power factor meter, frequency meter, protective relays for the purpose o measurement and protection. Electromagnetic type potential transformers are used up to 66kV lines while capacitor type potential transformers are used for the voltages 66kV and above.

Effect of open circuiting the secondary of a CT: If the secondary of a CT is open circuited the whole current in the primary becomes magnetizing current & a dangerously high voltage will appear across the secondary which may cause insulation breakdown, damage to the magnetic property of its iron core, overheating and also death to life. For these reasons open circuiting of a CT should never be permitted.

Current limit of CT is 50 amps and voltage limit of PT is 750 volts.

Standard secondary rating of PT is 110 volts and that of CT is 5 amps.

Basic difference between a power transformer and a potential transformer according to the operating condition: Though the primary windings in both power transformer and potential transformer are energized continuously at a substantially constant voltage, the secondary of a power transformer is connected to a a load which may vary between zero and the maximum which he transformer is capable of carrying whereas the load or burden connected to the secondary of a potential transformer does not vary as a rule. The permissible voltage drop in potential transformer between zero and maximum burden is much smaller than the corresponding limit in a power transformer.

Earthing arrangement for:

(a) 132 kV or 220 kV generator transformers: The metallic frame is earthed by two separate and distinct connections with earth. The neutral point is directly earthed by not less than two separate and distinct connection with earth each having its own electrode thus permitting the grading of insulation in the transformer from the terminal end to the neutral point.

(b) 11kV or 33kV substation transformers: The frame is earthed by two separate earth electrodes as above but for neutral earthing resistance earthing not less than two numbers may be used to limit the fault current which is expected to be too high in this case.

(c) 11lB/0.433 kV distribution transformers: The frame is earthed by two separate earth electrodes and the neutral of the secondary star winding is earthed by not less than two nos separate and distinct connections directly to earth each having its own electrode.

(d) Transformer with delta winding: The frame is earthed by two separate earth electrodes. An earthing transformer may be used to get the star neutral point which may be earthed by two separate connections to the earth either directly or through a resistance if desired.

(e) Instrument transformers: Cases or frames of instrument transformers, the secondary windings of current transformers and one point of the secondary winding of potential transformer shall be earthed which may be done by connection to the earth bus.

Earthing transformer: An earthing transformer is a transformer which is intended primarily for the purpose of providing a neutral point for grounding purposes. It may be a two winding unit with a delta connected primary and a star connected secondary or a single winding three phase auto-transformer with windings in interconnected star or “Zigzag”.

When the necessity arises for earthing the neutral of one part or another of an interconnected transmission or distribution network at a place where no natural neutral point is available, an earthing transformer is used for that purpose. The neutral point of the earthing transformer is connected to earth directly or through a current limiting impedance while the terminals are connected to the three phase lines.

 Arrangements for proper ventilation of transformer installed indoor in an enclosed room: For proper ventilation the transformer should be kept well away from the wall to make free movement of air round all the four sides. To ensure proper air circulation for the efficient cooling of the transformer a minimum are of 1 sq/ meter for inlet per 1000kVA should be provided as near the floor as possible and a minimum area of 2 sq. meters for outlet per 1000kVA should be provided in the opposite side of air inlet as high as the building allows to enable the heated air to escape readily and be replaced by cool air.

Name Plate: Indoor type Transformer

Make	Vijay Electrical Ltd (Hydrabad)
Type	Dry type Resin Cast
Capacity	2500 KVA, 2.5 MVA
No load Voltage	HV: 33000 volts, 33 KV LV : 433 volts
Current:	HV: 43.738 Amps LV : 3333.43 Amps
Type of cooling	AN  ( Air Natural)
Frequency	50Hz
Imp. Voltage	8.76%
Core, Coil Assy	9830 Kg
Enclosure & fitting	2320 Kg
Total Weight	12150 Kg
Off ckt Tap changing links	+ in steps of 2.5 variation in HV
Phase	3
Scanner	total 8 channels. Alarm – 130oC, Trip 140oC
Class of insulation	H (180o)
Vector gr.(Connection symbol)	Dyn11
Yr of manufacturing	2004
Insulation Level	HV-Kv : L1170/AC-70 LV55354-Kv: AC-3
Serial  No
Guaranteed max. temperature rise in winding	over an ambient of 45oC-95oC
Neutral CT	4000/1

Dry type Resin cast: Advantage

1.      Improves earth tracking resistance

2.      High di-electric strength

3.      Moisture repellent

4.      Flame retardant

5.      High insulation resistance

6.      Easy for maintenance.

PROTECTION (AUXILIARY RELAY TYPE VAA-ALSTOM)

1.      Over current

2.      Earth Fault

3.      Restricted Earth fault

4.      High winding temperature protection. ALARM- 100⁰C, TRIP – 110⁰C

5.      Door interlocking

6.      Neutral displacement relay.

TYPES OF TRANSFORMER

1.      CORE type, 2. BERRY type, 3. SHELL type, 4. DISTRIBUTION, 5. Power, 6. AUTO-TRANSFORMER, 7. SINGLE & THREE PHASE.

In Transformer/Alternator, star point is earthed because to keep unbalance load & line voltage constant. So, electrical equipment is safe.

                                     PT         PV             SI

Transformer ratio:    ----  =  -------- =    --------

                                     ST         SV             PS

PT: Primary winding  - high turns wire small/high voltage   33Kv

       Secondary winding – low turns wire thick/low voltage        110 V

CT: Primary winding – low turns wire max thick / high current        100 Amps

       Secondary winding high turns wire thin/ low current           1 Amps

VECTOR GROUP Dyn11

Primary is Delta connected and Secondary is Star connected. As per Watch minute pointer indicates Primary Voltage & Hour pointer indicates Secondary voltage. When there is 11 o’clock in the watch it means there is 30⁰ angle between two pointers.  i.e Secondary voltage leads by 30⁰ to primary voltage. When primary voltage leads to Secondary voltage, it is in anti-clockwise direction of the watch.

Percentage impedance: 8.76

Ø  If 33 Kv supply is give to HT side and if  3333.33 amps full load current is flowing then secondary voltage will be 400v in place of 433v ie 8.76% less.

Ø  If secondary if short circuited , we are giving 8.76 v in place of 33kv, then from secondary 3333.33 amp current will flow.

Transformer: The transformer is of resin cast type with primary winding connected in delta and secondary winding connected in star. HV & LV winding are separately cast under vacuum with unfilled epoxy resin system and with fiber glass reinforcement core is constructed out of cold rolled grain oriented silicon steel and the exposed cut surfaces are given epoxy varnish coating. Core clamping is done with mild steel channels. Tie rods and clamping rods with FRP sheet insulation between core channels and lamination. HV & LV coils are assembled concentrically with FRP and supporting blocks at top and bottom winding. Temperature is monitored with the help of PT-100 sensors connected to a Digital winding temperature scanner. WTS is provided with two set points for Alarm and trip.

Periodic Inspection & Maintenance:

i) Ensure that the transformer is kept free of dust and other obstructing material on the enclosure.

ii) Check protection circuit periodically.

iii) Check the winding temperature scanner and limit switches of doors.

Iv)  Ensure that the transformer is not overloaded.

Ø  In case of repair, it should be de-energized and the enclosure case should be grounded. This ground should be connected to all transformer coil terminals these connections must be removed before the transformer is placed in service.

Ø  Dry type resin cast transformer, require very little maintenance from time to time. However, inspection should be made at regular intervals and corrective measure shall be taken when necessary to ensure the most satisfactory service from this equipment evidence of  rusting.  Corrosion of enclosure and deterioration of the insulation and point should be checked and corrective measures shall be taken whenever necessary.

Ø  Winding should be inspected for dirt especially accumulation on insulating surface or where it would tend to restrict air flow. For loose connections for the condition of tap changing links or terminal boards and for the general condition of the transformer. Accumulations of dirt on the coils, insulators, lead connections of ventilated. Dry type transformers should be removed to permit free circulation of air particular attention should be given to the cleaning of top and bottom ends of coils assemblies and to the cleaning of ventilating ducts.

Ø  The winding may be cleaned with vacuum cleaner, air blower or with compressed air, care should be taken to maintain adequate ventilation during cleaning, lead supports, tap changing links and terminal boards, bushings and other major insulating surfaces should be brushed or wiped with a dr4y link free cloth.

Connection of CT for transformer: The CTs are delta connected on the star side and star connected on the delta side in order to compensate for the phase difference in the primary and secondary currents of power transformer.

The ratio of the CT on the HV side of a 3 phase Star-Delta, 33/11 kV transformer if the protecting CT on the LV side has a ration of 300:5. If 300 Amps is the line current on the LV side the CT secondary current is 5 Amps.                                         11

The line current on HV(Star side = 300 x ------ = 100 Amps

                                                                          33

The CTs on the star side are delta connected and the current required on the relay side of the CT is 5 amps. Therefore, the current in the CT

Secondary (phase current) is 5/√3. Therefore the CT ratio on the HT side will be 100: 5/√3.

Bus bar protection schemes: There are mainly two classes of bus bar protection schemes:

(a) Differential protection, the most sensitive and reliable method of protection.

(b) Fault bus protection also known as frame leakage protection gives the earth fault protection.

Protection schemes for feeder line: The feeder line protection schemes are as follows:

(a) Time graded over current and earth fault protection.

(b) Instantaneous over current protection.

(c) Differential pilot wire protection with wire pilot system or carrier pilot system.

(d) Carrier current protection with phase comparison relaying or directional comparison relaying mainly used for transmission lines of 132 kV and above.

(e) Distance protection employing plain impedance relay, reactance relay or MHO relay.

(f) Reverse power protection generally employed in interconnected system of generating units or stations, ring and parallel feeders.