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PROTECTION OF POWER
TRANSFORMER
1
TRANSFORMER
Introduction
A transformer is a static electrical device used in
electric power systems to transfer power be...
2
power applications. Transformers are essential for the
transmission,distributionandutilization of electrical
energy.
Ind...
3
and voltage induction from primary to secondary circuits
in indicated directions.
POWER TRANSFORMERS
The term power tran...
4
Fig.An example of a power transformer used in electric power system
CAUSE OF FAULTS IN POWER TRANSFORMER
Transformers ar...
5
3. All large transformers are oil immersed type.There is a
possibility of oil leakage.
4. Transformers experience large ...
6
The following factors affect the differential current in
transformers and should be consideredwhile applying
differentia...
7
4. Different primary and secondary voltage levels, that is
the primary & secondary CT’s areof different types and
ratios...
8
Fig. Percentage Differential Relay Connections
Harmonic Restraint:
The percentage differential scheme tends to maloperat...
9
A percentage differential relay has a certain minimum
value of pick up for internal faults.Faults with current
below thi...
10
EARTH FAULT PROTECTION FOR THE DELTA
SIDE OF DELTA STAR TRANSFORMER :
(C) OVER CURRENT PROTECTION :
Over current protec...
11
(D) PROTECTION AGAINST OVERFLUXING :
The magnetic flux increases when voltage increases. This
results in increased iron...
12
To control flux, the ratio E/ f is controlled. When the ratio
exceeds a threshold value, it has to be detected. Electro...
13
voltage of the bridge.Also the voltage across the silistor is
applied to a static control circuit which controls coolin...
14
Working : When an incipient fault such as a winding-to-
core fault or an inter-turn fault occurs on the transformer
win...
15
The Buchholz relay also has another set of contacts
operated by a float. Thesecontacts stay open when the
transformer t...
16
PRESSURE RELIEF VALVE : An oil pressure
relief valve is fitted at the top of the transformer tank. It is
a spring contr...
17
Protection of a power transformer against fire
(H) PROTECTION AGAINST LIGHTNING:
Lightning overvoltage surges originate...
18
impulse and long duration continuing current impulse
which affects the transformer insulation system.
Protection agains...
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Protection of power transformer

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A brief description of protective schemes used in the protection of power transformers used in power distribution.

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Protection of power transformer

  1. 1. PROTECTION OF POWER TRANSFORMER
  2. 2. 1 TRANSFORMER Introduction A transformer is a static electrical device used in electric power systems to transfer power between circuits through the use of electromagnetic induction. Transformers are devices that transfer energy from one circuit to another by means of a common magneticfield. When an alternating current flows in a conductor,a magnetic field exists around the conductor.If another conductor is placed in the field created by the first conductor such that the flux lines link the second conductor,then a voltage is induced into the second conductor.The use of a magnetic field from one coil to induce a voltage into a second coil is the principle on which transformer theory and application is based. Transformers range in size from thumbnail-sized used in microphones to units weighing hundreds of tons interconnecting the power grid.A wide range of transformer designs are used in electronic and electric
  3. 3. 2 power applications. Transformers are essential for the transmission,distributionandutilization of electrical energy. Induction Law The transformer is based on two principles: 1. An electric current can produce a magnetic field. 2. A changing magnetic field within a coil of wire induces a voltage across the ends of the coil (electromagnetic induction). Referring to the figure here, current passing through the primary coil creates a magnetic field. The primary and secondary coils are wrapped around a core of very high magnetic permeability, so that most of the magnetic flux passes through both the primary and secondary coils. Any secondary winding connected load causes current
  4. 4. 3 and voltage induction from primary to secondary circuits in indicated directions. POWER TRANSFORMERS The term power transformer is used to refer to those transformers used in the generatorand the distribution circuits, and these are usually rated at 500 KVA and above. Power systems typicallyconsist of a large number of generation locations, distribution points, and interconnections within thesystem or with nearby systems, such as a neighboring utility. The complexity of the system leads to avariety of transmission and distribution voltages. Power transformers must be used at each of these pointswhere there is a transition between voltage levels.
  5. 5. 4 Fig.An example of a power transformer used in electric power system CAUSE OF FAULTS IN POWER TRANSFORMER Transformers are prone to variety of faults : 1. The most common type of fault being the winding to core faults because of weakening of insulation.Phase faults inside the transformers are rare.However,such faults may occur on terminals,which fall within the transformer protection zone. 2. Power transformers are generally provided with on-line tap changing (OLTC) gear.This is another major area of occurrence of fault.
  6. 6. 5 3. All large transformers are oil immersed type.There is a possibility of oil leakage. 4. Transformers experience large inrush currents that are rich in harmonic content at the time of switching if they happen to be unloaded. 5. A transformer may develop inter turn faults giving rise to local hot spots within the winding. 6. Transformers may suffer from over fluxing due to under frequency operation at rated voltage.Over fluxing may also be caused when the transformer is subjected to over voltage at the rated frequency. 7. In case of sustained overload conditions,the transformer should not be allowed to operate for long duration. PROTECTION OF POWER TRANSFORMERS (A) PERCENTAGE DIFFERENTIAL PROTECTION This scheme is employed for the protection of transformers against internal short circuits. It provides the best overall protection for internal faults.However in case of ungroundedor high impedance grounding it cannot provide ground fault protection.
  7. 7. 6 The following factors affect the differential current in transformers and should be consideredwhile applying differential protection. These factors can result in a differential current even underbalanced power in & out conditions: 1.Magnetizing inrush current– The normal magnetizing current drawn is 2–5% of therated current. However during Magnetizing inrush the current can be as high as 8–30times the rated current for typically 10 cycles, depending upon the transformer andsystem resistance. 2. Overexcitation–This is normally of concern in generator–transformer units.Transformers are typically designed to operate just below the flux saturation level. Anyfurther increase from the max permissible voltage level (or Voltage/Frequency ratio), could lead to saturation of the core, in turn leading to substantial increase in theexcitation current drawn by the transformer. 3. CT Saturation – External fault currents can lead to CT saturation. This can cause relayoperating current to flow due to distortion of the saturated CT current.
  8. 8. 7 4. Different primary and secondary voltage levels, that is the primary & secondary CT’s areof different types and ratios 5. Phase displacement in Delta-Wye transformers. Transformer Differential Relay To account for the above variables less sensitive Percentage Differential Relays withpercentage characteristics in the range of 15 to 60% are applied to transformers. Additionally, inmodern microprocessor and numeric relays harmonic restraints can be applied. Transformer Differential RelayConnections:
  9. 9. 8 Fig. Percentage Differential Relay Connections Harmonic Restraint: The percentage differential scheme tends to maloperate due to magnetizing inrush.The inrush current waveform is rich in harmonics whereas the internal fault current consists of only the fundamental component. So to solve the problem of inrush current, which is neither an abnormal condition nor a fault, additional restraint is developed which comes to picture only during inrush condition and is ineffective during internal faults. (B) RESTRICTED EARTH FAULT PROTECTION :
  10. 10. 9 A percentage differential relay has a certain minimum value of pick up for internal faults.Faults with current below this value are not detected by the relay. Winding-to-core faults, which are single phase to ground type, involving high resistance,fall in this category. Therefore for such type of faults RESTRICTED EARTH FAULT PROTECTION is used.The reach of such a protection must be restricted to the winding of the transformer; otherwise it may operate for any ground fault, anywhere in the system, beyond the transformer, hence the name of this scheme.
  11. 11. 10 EARTH FAULT PROTECTION FOR THE DELTA SIDE OF DELTA STAR TRANSFORMER : (C) OVER CURRENT PROTECTION : Over current protection is used for the purpose of providing back up protection for large transformers. (above 5MVA).Two phase fault and one ground fault relay is sufficient to provide OC protection to star delta transformer.
  12. 12. 11 (D) PROTECTION AGAINST OVERFLUXING : The magnetic flux increases when voltage increases. This results in increased iron loss and magnetizing current. The core and core bolts gets heated and the lamination insulation is affected. Protection against overfluxing is required where overfluxing due to sustained overvoltage can occur.The reduction in frequency also increases the flux density and thus has the same effect of overfluxing. The expression for flux ina transformer is given by Φ = K E/f Where Φ = flux, f = frequency, E = applied voltage and K is a constant.
  13. 13. 12 To control flux, the ratio E/ f is controlled. When the ratio exceeds a threshold value, it has to be detected. Electronic circuits with suitable relays are available to measure this ratio. Overfluxing does not require high speed tripping and hence instantaneous operation is undesirable when momentary disturbances occur. But the transformer should be isolated in one or two minutes at the most if overfluxing persists. (E) PROTECTION AGAINST OVERHEATING : The rating of a transformer depends on the temperature rise above an assumed maximum ambient temperature. Sustained overload is not allowed if the ambient temperature is equal to the assumed ambient temperature. The maximum safe overloading is that which doesnot overheat the winding.The maximum allowed temperature is about 95°C. Thus the protection against overload depends on the winding temperature which is usually measured by thermal image technique. In thermal image technique,a temperature sensing device like silicon resistor is placed in the transformer oil near the top of the transformer tank. A CT is employed on the L.V. side to supply current to a small heater. Both the temperature sensing device and the heater are placed in a small pocket. The silistor is used as an arm of a resistance bridge supplied from the stabilized dc source. An indicating instrument is energized from the out of balance
  14. 14. 13 voltage of the bridge.Also the voltage across the silistor is applied to a static control circuit which controls cooling pumps and fans,gives warning of overheating and ultimately trips the transformer circuit breakers. (F) PROTECTION AGAINST INCIPIENT FAULTS: INCIPIENT FAULTS: Faults which are not serious at the beginning but which slowly develops into serious faults are known as incipient faults. BUCHHOLZRELAY : It is a gas actuated relay. When a fault develops slowly,it produces heat, thereby decomposing solid or liquid insulating material in the transformer. The decomposition of the insulating material produces inflammable gases. The Buchholz relay gives an alarm when a specified amount of gas is formed. The analysis of the gas collected in the relay chamber indicates the type of the incipient fault. There is a chamber to accommodate Buchholz relay,in between the transformer tank and the conservator.The Buchholz relay is a slow acting device,the minimum operating time is 0.1 s and the average time is 0.2 s. Too sensitive settings of the mercury contacts is undesirable because they are subjected to false operation on shock and vibration caused by conditions like mechanical shock to the pipe,tap changer operation and heavy external faults.
  15. 15. 14 Working : When an incipient fault such as a winding-to- core fault or an inter-turn fault occurs on the transformer winding, there is severe heating of the oil. This causes gasesto be liberated from the oil around 350°C. There is a build-up of oil pressure causing oilto rush into the conservator. A vane is placed in the path of surge of oil between thetransformer and the conservator. Aset of contacts, operated by this vane, is used as tripcontacts of the Buchholz relay This output of Buchholz relay may be used to trip thetransformer.
  16. 16. 15 The Buchholz relay also has another set of contacts operated by a float. Thesecontacts stay open when the transformer tank is filled with oil. However, in case ofleakage of oil or decomposition of oil, the float sinks causing the contacts to close. Lossof oil will no doubt cause the transformer temperature to rise but does not warrant immediate tripping. Hence, normally these contacts are wired to an alarm which alertsthe operator. GAS ANALYSIS : The trapped gases in the conservator can give valuable clue to the type of damage thattakes place inside the transformer. This is because the insulation between the windingturns, the insulation between the stampings of the core and the oil, all liberate specificgases when they get heated up due to a fault. The presence of these gases can be usedas a signature of a particular type of damage that may have taken place inside thetransformer.
  17. 17. 16 PRESSURE RELIEF VALVE : An oil pressure relief valve is fitted at the top of the transformer tank. It is a spring controlled valve located at the end of an oil relief pipe protruding from the top of the tank. Whenever a surge in the oil is developed, it bursts , thereby allowing the oil to discharge rapidly. It operates when the pressure exceeds 10 psi but closes automatically when the pressure falls below the critical level.This avoids the explosive rupture of the tank and the risk of fire. (G) PROTECTION AGAINST FIRE : Power transformers are subject to fires from many sources. Theyoften occur because of deterioration of insulation in the transformer.This produces arcing which in turn overheats the insulating oil andcauses the tanks to rupture; further arcing then will start a fire. Firesare also initiated by lightning and occasionally by dirty insulators onthe outside of the tanks. Proper maintenance can reduce these risks. Careful protection againstfaults by shielding, grounding, lightning arresters, interrupting devicesand relays can also decrease the opportunity for a destructive fire. In spite of protection by these measures and expert maintenance, therisk of fire remains quite high, and a fire protection system is alwaysrecommended and often required. In addition, suppression systemsare frequently installed.
  18. 18. 17 Protection of a power transformer against fire (H) PROTECTION AGAINST LIGHTNING: Lightning overvoltage surges originate from atmospheric discharges and they can reach their peak within a few microseconds and subsequently decay very rapidly. The surge voltage can reach up to 10 times the rated transformer voltage and they pose the greatest threat to transformers on the distribution networks. The charge from the surge produces both short duration high current
  19. 19. 18 impulse and long duration continuing current impulse which affects the transformer insulation system. Protection against such overvoltage surges can be achieved by using Lightning Arresters.The distance between the lightning (surge) arrester and the equipment to be protected should be as short and straight as possible. Therefore, integration of the surge arrester in the equipment to be protected gives almost an ideal protection. Fig. Lightning Arrester used to protect the powertransformer against lightning surges. ----------------------------------------------------------------------

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