Transformer Protection

BySusobhan PatraSusobhan Patra

5/8/2014

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).

POWER TRANSFORMERSThe term power transformer is used to refer to those transformers used in the generator and the distribution circuits, and these are usually rated at 500 KVA and above. Power systems typically consist of a large number of generation locations, distribution points, and interconnections within the system or with nearby systems, such as a neighboring utility. The complexity of the system leads to a variety of transmission and distribution voltages. Power transformers must be used at each of these points where there is a transition between voltage levels.

CAUSE OF FAULTS IN POWER TRANSFORMER

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.

2. Power transformers are generally provided with on-load tap changing (OLTC) gear. This is another major area of occurrence of fault.

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.at the time of switching.

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.

Tap changer

PROTECTION OF POWER TRANSFORMERS(A) DIFFERENTIAL PROTECTIONThis scheme is employed for the protection of transformers against internal

short circuits. It provides the best overall protection for internal faults. The following factors affect the differential current in transformers and

should be considered while applying differential protection.

1. Magnetizing inrush current The normal magnetizing current drawn is 25% of the rated current. However during Magnetizing inrush the current can be as high as 830 times the rated current for typically 10 cycles,

2. Overexcitation This is normally of concern in generatortransformer 2. Overexcitation This is normally of concern in generatortransformer units. Transformers are typically designed to operate just below the flux saturation level. Any further increase from the max permissible voltage level could lead to saturation of the core, in turn leading to substantial increase in the excitation current drawn by the transformer.

3. CT Saturation External fault currents can lead to CT saturation. This can cause relay operating current to flow due to distortion of the saturated CT current.

4. Different primary and secondary voltage levels, that is the primary & secondary CTs are of different types and ratios

Transformer Differential RelayTo account for the above variables less sensitive Differential Relays with percentage characteristics in the range of 15 to 60% are applied to transformers. Additionally, in modern microprocessor and numeric relays harmonic restraints can be applied.Transformer Differential Relay Connections:

Harmonic Restraint:The differential scheme tends to maloperate due to magnetizing inrush.The inrush current waveform is rich inharmonics whereas the internal faultharmonics whereas the internal faultcurrent consists of only thefundamental component. So to solvethe problem of inrush current, whichis neither an abnormal condition nora fault, additional restraint isdeveloped which comes to pictureonly during inrush condition and isineffective during internal faults.

(B) RESTRICTED EARTH FAULT PROTECTION :A differential relay has a certain minimum value of pick up for internal faults. Faultswith current below this value are not detected by the relay.Winding-to-core faults, which are single phase to ground type, involving highresistance, fall in this category.Therefore for such type of faults RESTRICTED EARTH FAULT PROTECTION isused. The reach of such a protection must be restricted to the winding of thetransformer; otherwise it may operate for any ground fault, anywhere in the system,beyond the transformer, hence the name of this scheme.

(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.

HV Bushing & connection

LV Connection to bus

(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 in a transformer is given by = K E/fWhere = flux, f = frequency, E = applied voltage and K is a constant.

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 does not overheat the winding. The maximum allowed temperature is about 95C. Thus the protection against overload depends on the winding temperature.In thermal image technique, a temperature sensing device like silicon resistor is placed 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 H.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 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.BUCHHOLZ RELAY : 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. In between the transformer tank and the conservator the Buchholz relay is placed and it is a slow acting device, the minimum operating time is 0.1 s and the average time is 0.2 s. When an incipient fault such as a winding-to-core fault or an inter-turn fault occurs onto-core fault or an inter-turn fault occurs onthe transformer winding, there is severeheating of the oil. This causes gases to beliberated from the oil. There is a build-up ofoil pressure causing oil to rush into theconservator. A vane is placed in the path ofsurge of oil between the transformer and theconservator. A set of contacts, operated bythis vane, is used as trip contacts of theBuchholz relay This output of Buchholzrelay may be used to trip the transformer.

BUCHHOLZ RELAY :

GAS ANALYSIS :

The trapped gases in the conservator can give valuable clue to the type of damagethat takes place inside the transformer. This is because the insulation between thewinding turns, the insulation between the stampings of the core and the oil, allliberate specific gases when they get heated up due to a fault. The presence ofthese gases can be used as a signature of a particular type of damage that mayhave taken place inside the transformer.

PRESSURE RELIEF VALVE :

An oil pressure relief valve is fitted at the top of the transformer tank. It is a spring controlled valve placed at 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. They often occur because of deterioration of insulation in the transformer. This produces arcing which in turn overheats the insulating oil and causes the tanks to rupture; further arcing then will start a fire. Fires are also initiated by lightning and occasionally by dirty insulators on the outside of the tanks.In spite of protection by these measures and expert maintenance, the risk of fire remains quite high, and a fire protection system is always recommended and often required. In addition, suppression systems are frequently installed.