philosophy on transformer protection

Philosophy of Transformer Protection

INTRODUCTION

The Power Transformer is one of the most important links in a power transmission and distribution system. A Transformer fault will cause a large interruption in power supplies and the impact is more serious than a transmission line outage and also cause damage to power system stability.

Philosophy of Transformer Protection (Contd...)

• It consists essentially of a magnetic core built of insulated silicon steel laminations upon which distinct sets of coils are wound.

• The alternating current applied to one of the windings establishes an alternating magnetic flux in the core, which induces an e.m.f in the windings.

• The formula connecting induced voltage, flux and number of turns is as follows:

• E= 4.44 f Øm N E - RMS value of the induced emf in

the winding. f - frequency of supply in Hz

Øm - Total magnetic flux through the core (max.value) in Webers.

N - No. of turns in the winding.



Transformer protection falls under two major categories :

• Protection of the system against the effects of faults arising inside the

transformer.• Protection of the transformer against

the effect of faults occurring on any part of the system (external).


FAULTS INTERNAL TO THE TRANSFORMER

Earth faultsPhase to Phase faults Inter turn faultsCore faultsTank faults


EARTH FAULTS: In this case, the fault current is controlled mainly

by the leakage reactance of the winding and generally the currents are of high magnitude.

PHASE – TO – PHASE FAULTS: Faults between phases within a transformer are

relatively rare, if such a fault does occur it will give rise to a substantial current compared to the earth fault current.


• INTER – TURN FAULTS:• A high voltage transformer connected to an overhead

transmission system is very likely to be subjected to step fronted impulse voltages. Hence the risk of partial winding flash over is high. It is opined that 70% to 80% of all transformer failures arise from faults between turns or inter turn faults.

• A short circuit of a few turns of the winding will give raise to heavy faults current in the short-circuited loop, but the impact on terminal currents will be very small because of the high ratio of transformation between the whole windings and the short circuited turns.


CORE FAULTS:• If any portion of the core insulation becomes

defective, it will cause sufficient eddy currents to flow, causing serious over heating, which may reach a magnitude sufficient to damage the winding.

• The additional core-loss, although causing severe local heating, will not produce a noticeable change in input current and could not be detected by normal electrical protections. However, it is very much essential to detect this condition before a major fault has been created.


Core faults (contd.)• Fortunately, in an oil immersed transformer, the

local heating will cause breakdown of some of the oil with an accompanying evolution of gas, which will escape to the conservator.


TANK FAULTS:

• Loss of oil through tank leak, failure of welded joints etc. may lead to a dangerous condition.


EXTERNAL SYSTEM CONDITIONS:

Over-load System faults (phase to phase/ phase to earth) Over voltage Reduced system frequency.


OVERLOAD

• Over load causes increased copper loss and a consequent temperature rise.

• Overloads can be allowed for limited period depending on the initial temperature and the cooling conditions.

• System short circuits produce a relatively intense rate of heating of the feeding transformers, the copper loss increasing in proportion to the square of the per unit fault current.


OVERLOAD (contd.)

• Large fault currents produce severe mechanical stresses in transformers, the maximum stress occurs during the first cycle of asymmetric faults current and so cannot be arrested by automatic tripping of the circuit.

• Hence the control of such stresses is to be taken care of at the time of design itself.

System faults(Ph-Ph and Ph-E)

• Transformer needs to be protected from feeding external phase to phase and phase to ground faults on connected 400kV or 220kV system.

• These faults include uncleared faults in 220kV system and 400kV system. Backup directional OC protection is used.

• The protection to be coordinated properly to operate with a time delay to avoid unwanted tripping of ICT for external faults.



OVER VOLTAGE Over Voltage conditions are of two types:

1. Transient Surge Voltage Transient over voltages arise from switching and

lightning disturbances and are liable to cause inter- turn faults.

These over voltages are usually limited by shunting the high voltage terminals to earth either with a plain rod gap or by lightning arrestors (surge diverters).


2. Power Frequency Over-voltage

• Power frequency over voltage causes both an increase in stress on the insulation and a proportionate increase in the working flux.

• The latter causes an increase in the iron loss and a disproportionate increase in magnetizing current.

• Under conditions of over excitation of core, the core bolts which normally carry little flux may be subjected to a large component of flux diverted from the highly saturated region of core along side.


Power frequency over voltage (contd.)• Under this condition the bolts may get heated to a

temperature, which destroys their own insulation and will damage the coil insulation if the condition continues. Reduction in frequency has an effect on flux density, which is similar to that of over voltage.

• In other words, the transformer can operate with some degree of over voltage with a corresponding increase in frequency. But operation must not be continued with a high voltage input at a low frequency.


MAGNETISING INRUSH:

• The phenomenon of magnetizing inrush is a transient condition, which occurs primarily when a transformer is energized.

• It is not a fault condition and therefore does not necessitate the operation of protection, which on the contrary must remain stable during the inrush transient, which is a major factor that is to be taken care of in the design of transformer protection.


Magnetic inrush (contd.)

Harmonic content of inrush waveform• The waveform of transformer magnetizing

current contains a proportion of harmonics, which increases as the peak flux density is raised to the saturating condition.

• As long as the waveform is symmetrical about the horizontal axis, only odd harmonics will be present.


Magnetic inrush (contd.)• The energizing conditions, which result in an

offset inrush current, produce a waveform, which is not symmetrical about horizontal axis.

• Such wave typically contains both even and odd harmonics.

• Typical inrush currents contain substantial amounts of second and third harmonics.


OVER HEATING PROTECTION

• The rating of a transformer is based on the temperature rise above assumed maximum ambient temperature.

• At a lower ambient temperature some degree of overload can be safely applied.

• Short period overloads are also permissible to an extent dependent on the previous loading conditions.


Over heating Protection (contd.)• Protection against overload is based on winding

temperature.

• The winding temperature is measured by thermal imaging.

• The winding temperature indicates the winding hot-spot temperature of oil-immersed transformers.

• The sensing bulb is placed in oil filled pocket located in the hottest oil of the transformer (on the top of transformer oil tank). The thermal image device consists of a heater and shunt network fed from a current transformer in one phase of transformer LV side.


Over heating protection (contd.)• The current proportional to the transformer load-

current from the CT is passed through the heater, which simulates corresponding winding gradient (winding to top oil temperature differential).

• The measuring portion of WTI reacts to both the top oil temperature sensed by the bulb and the winding gradient simulated by the heater.

• Thus the WTI directly displays the temperature in the hottest part of the winding.

Precautions during O&M:

OPERATING THE TRANSFORMER WITHIN THE SPECIFIED LIMITS OF TEMPERATURE AND VOLTAGE

PROPER CHECKING AND MAINTENANCE OF OLTC

PROVIDING SUITABLE PROTECTIVE RELAYS AND MONITORING DEVICES

QUALITY OF OIL OIL LEVEL MONITORED TO AVOID BREAKDOWN

OF INSULATION SILICAGEL BREATHER TO AVOID ENTRY OF

MOISTURE

Design of protection:• To design the protective scheme it is necessary to

have an idea/ intimate knowledge of faults. A fault can be detected by particular type of protection equipment and some of the protection equipments are more sensitive than the other.

• Most protective schemes applied to the transformers are based on the current balance principle of magnitude comparison of current flowing into and out of the transformer.


Protective relays:• Protective relays limit the damage in case of fault and

monitors to prevent the fault. Therefore fast and reliable protective relays should be used.

NORMAL PROTECTIONS FOR TRANSFORMER

• BUCHHOLZ RELAY(OLTC & Main Tank)• OVER LOAD PROTECTION RELAYS• OVER CURRENT PROTECTION• GROUND FAULT PROTECTION• DIFFERENTIAL PROTECTION• PRESSURE RELIEF DEVICE PROTECTION• OIL MONITORING(DGA)• OVER FLUXING PROTECTION


Differential Protection:The following points are to be consider when applying

the Differential Protection:The CT secondary connections must be arranged so

that any phase shift due to vector group of the main transformer is compensated for.

The CT ratios must chosen to suit the main transformer ratio so that the differential currents will be zero during normal operation and where the main transformer ratio is variable by means of tap changing.

Effect of magnetizing inrush current.


• The Protective zone of a Differential relay includes faults in Transformer, faults on Buses or cables between CT and transformer and then rapidly initiate disconnection of the supply to the main transformer. Then damages as well as non-selective tripping of other protective relays are prevented.

The Transformer Differential relay must be able to cope with the following conditions:


The Transformer Differential relay must be able to cope with the following conditions:

• When energizing the transformer after the fault, it is possible to obtain a large inrush current in the exciting winding. The magnitude and duration of the inrush current depends on

o Instant of switching ino Design of transformero Type of transformer connectiono Method of neutral groundingo The fault MVA rating of transformero When transformers connected in parallel.


• Generally, the magnitude of the inrush current can be 5-10 times the rated current when switching in to high voltage side and 10-20 times the rated current when switching in to low voltage side.

To prevent the relay to operate when energizing or to delay the operation, an instantaneous relay must have a magnetizing inrush restraint and thereby utilizing certain characteristic difference between inrush current and fault current.


philosophy on transformer protection

Documents

main transformer

transformer

protective

magnetic inrush

inrush current

rated current

fault current

main transformer