Friday, June 6, 2014

Transformer Faults causes

Major causes of Faults in Transformers:-
1. Most common kind of fault being the turning to primary problems due to weakening of insulating material.
2. Power transformers in which there are On-load and off load tap changers this may possibly also difficulties.
3. All large transformers are oil submerged sort there is a possibility of oil leakage. Which may additionally results in problems that are significant.
4. Transformers experience large inrush currents if they happen to be un loaded that are abundant in harmonic content during switching.
5. A transformer might develop inter turn problems giving rise to local hot spots within the rotating.
6. Transformers may suffer from over fluxing due to under frequency operation at rated voltage. Over fluxing might also be triggered when the transformer is put through over-voltage at the rated frequency.
7. In case of sustained overload states, the transformer should not be permitted to function for long duration.

PROTECTION OF POWER
(A) DIFFERENTIAL PROTECTION
 This plan is useful for the protection of transformers against short-circuits that were internal. It supplies the best overall defense for internal errors. Nevertheless in the event of underground high-impedance grounding it cannot supply ground fault protection.
The differential present in transformers affects and should be considered while employing differential protection. These variables can result in a differential current also underbalanced power in & out conditions: 1.Magnetizing inrush current- The ordinary magnetizing current brought is 2-5% of the rated present. However during Magnetizing inrush the current may be as large as 8-30times the rated current for commonly 10 cycles, dependant on the transformer resistance.
2. Over-excitation-
This is usually of issue - transformer units. Transformers are commonly designed to function simply below the flux saturation level. Any further boost from the max voltage that is allowable degree (or Voltage/Frequency proportion), might cause saturation of the core, in turn leading to large boost in the excitation present drawn by the transformer.
3. CT Saturation
CT vividness can be led to by Outside problem voltages. This may cause relay operating current to flow due to distortion of the saturated CT current.
That secondary voltage levels and different main, is the major & secondary CT different types and percentages 5. Transformer Differential exchange To take into account the variants that were above less sensitive Percent Differential Relays with percentage characteristics in the range of 1-5 are used to transformers. Moreover, numeric relays restraints and in modern micro-processor can be implemented.
4.  Transformer Relay Connections:
Percent Differential Relay Connections Harmonic Restraint: The percent differential structure seems to mal-operate due to magnetizing inrush. The inrush current waveform is full of harmonics whereas the fault current that is interior consists of only the part that is essential. Therefore to solve the difficulty of inrush current, which will be neither an unusual state or a fault, additional discipline is produced which comes to picture simply during inrush condition and is ineffective during internal faults.
(B) LIMITED earth-fault Safety:
 A percent differential relay includes a particular minimal worth of pick up for faults that are inner. The relay not detects faults with current below this value. Turning-to- core faults, which are single stage to earth type, involving resistance that is high, fall in this group. Thus for such sort of faults GROUND FAULT PROTECTION that was CONFINED is used. This type of protection's reach must be limited to the winding of the transformer ; otherwise it might work for any ground fault, everywhere in the system, beyond the transformer the title of this structure.

EARTH FAULT PROTECTION FOR THE DELTA aspect OF DELTA STAR TRANSFORMER:
(C) Overcurrent PROTECTION:
Over-current protection is employed for the purpose of providing backup protection for transformers that were big. (above 5MVA).Two phase problem and one earth fault relay is sufficient to provide OC protection to star delta transformer.

(D) Safety AGAINST OVERFLUXING:
The magnetic flux increases when voltage increases. This causes increased iron reduction and magnetizing current. The core and core products gets heated and the insulation is changed. Safety is required where over-fluxing due to sustained over-voltage can occur. The flux density also raises and hence has over-fluxing's same result. The expression for flux Where phi flux, f = regularity, E = used voltage and K phi K E/f gives transformer, is a continuous.
To restrain flux, the percentage E/ f is controlled. The ratio must be detected, when it exceeds a threshold value. Digital circuits with appropriate relays are offered to quantify this ratio. Over-fluxing will not require tripping that is high velocity thus instantaneous functioning is unwanted when brief disturbances occur. But the transformer needs to be isolated in one or two minutes at the most if over-fluxing lasts.

(E) PROTECTION AGAINST OVERHEATING:
A transformer's evaluation depends on the temperature rise above an assumed maximum ambient temperature. Sustained overload isn't allowed if the normal temperatures is equivalent to the ambient heat that was presumed. The maximum secure overloading is that which does not overheat the winding. The maximum allowed temperature is about 95degC. Consequently the winding heat that's typically measured by thermal picture method is depended on by the safety against overload. In thermal image approach, a temperature detection apparatus like plastic resistor is placed in the transformer oil near the top. A CT is applied on the L.V. aspect to furnish current to a small heater. The temperature sensing the heater and device are put in a pocket that was small. The silistor is employed as a provider of a resistance bridge provided from the dc source that was stabilized. A device that was indicating is revived in the out of balance current of the bridge. Additionally the voltage through the silistor is used to some static control circuit which controls fans and cooling pumps, offers forewarning of overheating and ultimately trips the transformer circuit breakers.
(F) Safety AGAINST INCIPIENT FAULTS:
INCIPIENT FAULTS:
Faults that are not significant at the beginning but which slowly develops into severe mistakes are known as incipient faults.
BUCHHOLZRELAY:
This is a gasoline actuated relay. When a fault grows slowly, it creates warmth, therefore decomposing insulating material that is fluid or sound in the transformer. The disintegration of the insulating materials creates inflammable gases. The Buchholz relay gives an alarm when a given amount of gasoline is created. The evaluation of the gasoline collected in the exchange chamber indicates the fault's sort. A step is to adapt Buchholz relay, in between the transformer tank as well as the conservator. The Buchholz relay is a slow-acting device, the minimum operating period is 0.1 s and the average time is 0.2 s. Overly sensitive configurations of the mercury associates is unwanted since they're afflicted by false process on shock and shaking due to states like physical shock to tap changer procedure, the pipe and serious external problems.
Operating: When an incipient problem for example a -to- primary fault or an inter -change problem occurs on the transformer winding, there's intense heating of the oil. This causes gases to be separated from your oil. A build-up is of oil pressure creating oil to rush into the curator. A vane is put into surge of oil between the transformer and the conservator's path. A set of contacts, operated by this vane, is employed as trip contacts of the relay this output of Buchholz relay may be utilized to trip the transformer.
The Buchholz relay also has yet another set of contacts controlled by a drift. These contacts remain open when the transformer tank is filled with oil. However, in circumstance of leakage of disintegration or oil of petroleum, the drift sinks causing the contacts to close. Loss of oil will no doubt cause the transformer temperature to climb but will not warrant falling that is immediate. Consequently, generally these contacts are wired to alarm.
GAS EVALUATION:
The trapped gases in the curator can give precious clue to the sort of injury that takes location. This really is due to the fact that the insulation between the oil, all, the insulating material between the stampings of the core and the winding turns free specific gases when they get heated-up as a result of a fault. The presence of these gases can be used as a personal of a special type of harm that may have taken place inside the transformer.

PRESSURE reduction device:
An oil-pressure relief valve is fitted on top of the transformer tank. It's a spring restricted device located by the end of an oil relief pipe protruding in the highest part of the tank. Whenever a surge in the acrylic is developed, it bursts , thereby permitting the oil to discharge quickly. It works although the pressure exceeds 10 psi but closes automatically when the pressure drops below the crucial level. This avoids the tank's explosive rupture and the potential of hearth.
(G) Safety AGAINST FIRE :
 Power transformers are susceptible to fires from several sources. They often happen due to worsening of insulating material. This produces arcing which in turn over heats the insulating oil and causes the tanks to split; additional arcing then may start a fire. Fires are also originated by lightning and occasionally by grimy insulators on the outside the tanks. These dangers can be reduced by proper maintenance. Cautious protection against faults by shielding, grounding, lightning arresters, stifling devices and relays can also decrease the opportunity for a destructive fire. In spite of defense by these actions and maintenance that is skilled, the risk remains not fairly low, and there is a fire protection method often needed and always recommended. In addition, suppression systems are frequently installed.

Safety against fire (hydrogen) Safety of an electric transformer AGAINST LIGHTNING:

Lightning overvoltage spikes result from atmospheric discharges plus quite rapidly and later disintegrate their peak can be reached by them within several microseconds. The spike current can reach up to 10 times the rated transformer current and they present the greatest threat to transformers to the supply networks. Both short duration current that is high is produced by the cost in the surge impulse and long-duration continuing present impulse which influences the transformer insulation system. Protection against such over voltage spikes can be accomplished through the use of Lightning Arresters. The distance between the lightning (surge) arrester and the gear to be protected must be as brief and straight as possible. Hence, almost a safety that is ideal is given by consolidation of the surge arrester in the gear to be shielded.