It is essential to protect high capacity transformers against external and internal electrical faults.
External Faults in Power Transformer
External Short Circuit of Power Transformer
The short – circuit may occur in two or three phases of electrical power system. The level of fault current is always high enough. It depends upon the voltage which has been short-circuited and upon the impedance of the circuit up to the fault point. The copper loss of the fault feeding transformer is abruptly increased. This increasing copper loss causes internal heating in the transformer. Large fault current also produces severe mechanical stresses in the transformer. The maximum mechanical stresses occur during first cycle of symmetrical fault current.
High Voltage Disturbance in Power Transformer
High voltage disturbance in power transformer are of two kinds,
- Transient Surge Voltage
- Power Frequency Over Voltage
Transient Surge Voltage
High voltage and high frequency surge may arise in the power system due to any of the following causes,
- Arcing ground if neutral point is isolated.
- Switching operation of different electrical equipment.
- Atmospheric Lightening Impulse.
Whatever may be the causes of surge voltage, it is after all a traveling wave having high and steep wave form and also having high frequency. This wave travels in the electrical power system network, upon reaching in the power transformer, it causes breakdown of the insulation between turns adjacent to line terminal, which may create short circuit between turns.
Power Frequency Over Voltage
There may be always a chance of system over voltage due to sudden disconnection of large load. Although the amplitude of this voltage is higher than its normal level but frequency is same as it was in normal condition. Over voltage in the system causes an increase in stress on the insulation of transformer. As we know that, voltage , increased voltage causes proportionate increase in the working flux.
This therefore causes, increased in iron loss and proportionately large increase in magnetizing current. The increase flux is diverted from the transformer core to other steel structural parts of the transformer. Core bolts which normally carry little flux, may be subjected to a large component of flux diverted from saturated region of the core alongside. Under such condition, the bolt may be rapidly heated up and destroys their own insulation as well as winding insulation.
Under Frequency Effect in Power Transformer
As, voltage as the number of turns in the winding is fixed.
From, this equation it is clear that if frequency reduces in a system, the flux in the core increases, the effect are more or less similar to that of the over voltage.
Internal Faults in Power Transformer
The principle faults which occurs inside a power transformer are categorized as,
- Insulation breakdown between winding and earth
- Insulation breakdown in between different phases
- Insulation breakdown in between adjacent turns i.e. inter – turn fault
- Transformer core fault
Internal Earth Faults in Power Transformer
Internal Earth Faults in a Star Connected Winding with Neutral Point Earthed through an Impedance
In this case the fault current is dependent on the value of earthing impedance and is also proportional to the distance of the fault point from neutral point as the voltage at the point depends upon, the number of winding turns come across neutral and fault point. If the distance between fault point and neutral point is more, the number of turns under this distance is also more, hence voltage across the neutral point and fault point is high which causes higher fault current. So, in few words it can be said that, the value of fault current depends on the value of earthing impedance as well as the distance between the faulty point and neutral point. The fault current also depends up on leakage reactance of the portion of the winding across the fault point and neutral. But compared to the earthing impedance,it is very low and it is obviously ignored as it comes in series with comparatively much higher earthing impedance.
Internal Earth Faults in a Star Connected Winding with Neutral Point Solidly Earthed
In this case, earthing impedance is ideally zero. The fault current is dependent up on leakage reactance of the portion of winding comes across faulty point and neutral point of transformer. The fault current is also dependent on the distance between neutral point and fault point in the transformer. As said in previous case the voltage across these two points depends upon the number of winding turn comes across faulty point and neutral point. So in star connected winding with neutral point solidly earthed, the fault current depends upon two main factors, first the leakage reactance of the winding comes across faulty point and neutral point and secondly the distance between faulty point and neutral point. But the leakage reactance of the winding varies in complex manner with position of the fault in the winding. It is seen that the reactance decreases very rapidly for fault point approaching the neutral and hence the fault current is highest for the fault near the neutral end. So at this point, the voltage available for fault current is low and at the same time the reactance opposes the fault current is also low, hence the value of fault current is high enough. Again at fault point away from the neutral point, the voltage available for fault current is high but at the same time reactance offered by the winding portion between fault point and neutral point is high. It can be noticed that the fault current stays a very high level throughout the winding. In other word, the fault current maintain a very high magnitude irrelevant to the position of the fault on winding.
Internal Phase to Phase Faults in Power Transformer
Phase to phase fault in the transformer are rare. If such a fault does occur, it will give rise to substantial current to operate instantaneous over current relay on the primary side as well as the differential relay.
Inter Turns Fault in Power Transformer
Power transformer connected with electrical extra high voltage transmission system, is very likely to be subjected to high magnitude, steep fronted and high frequency impulse voltage due to lightening surge on the transmission line. The voltage stresses between winding turns become so large, it can not sustain the stress and causing insulation failure between inter – turns in some points. Also LV winding is stressed because of the transferred surge voltage. Very large number of power transformer failure arises from fault between turns. Inter turn fault may also be occurred due to mechanical forces between turns originated by external short circuit.
Core Fault in Power Transformer
In any portion of the core lamination is damaged, or lamination of the core is bridged by any conducting material that causes sufficient eddy current to flow, hence, this part of the core becomes over heated. Sometimes, insulation of bolts (Used for tightening the core lamination together) fails which also permits sufficient eddy current to flow through the bolt and causing over heating. This insulation failure in lamination and core bolts causes severe local heating. Although these local heating, causes additional core loss but can not create any noticeable change in input and output current in the transformer, hence these faults cannot be detected by normal electrical protection scheme. This is desirable to detect the local over heating condition of the transformer core before any major fault occurs. Excessive over heating leads to breakdown of transformer insulating oil with evolution of gases. These gases are accumulated in Buchholz relay and actuating Buchholz Alarm.