What Is The Protection Of Transformers
There are different kinds of transformers such as two winding or three winding electrical power transformers, auto transformer, regulating transformers, earthing transformers, rectifier transformers etc. Different transformers demand different schemes of transformer protection depending upon their importance, winding connections, earthing methods and mode of operation etc.
It is common practice to provide Buchholz relay protection to all 0.5 MVA and above transformers. While for all small size distribution transformers, only high voltage fuses are used as main protective device. For all larger rated and important distribution transformers, over current protection along with restricted earth fault protection is applied.
Differential protection should be provided in the transformers rated above 5 MVA.
Depending upon the normal service condition, nature of transformer faults, degree of sustained over load, scheme of tap changing, and many other factors, the suitable transformer protection schemes are chosen.
All equipment installed in a power electrical system have standardized ratings for short-time withstand current and short duration power frequency voltage. The role of the protections is to ensure that these withstand limits can never be exceeded, therefore clearing the faults as fast as possible.
In addition to this first requirement a system of protection must be selective. Selectivity means that any fault must be cleared by the device of current interruption (circuit breaker or fuses) being the nearest to the fault, even if the fault is detected by other protections associated with other interruption devices.
As an example for a short circuit occurring on the secondary side of a power transformer, only the circuit breaker installed on the secondary must trip. The circuit breaker installed on the primary side must remain closed. For a transformer protected with MV fuses, the fuses must not blow.
They are typically two main devices able to interrupt fault currents, circuit breakers and fuses :
- The circuit breakers must be associated with a protection relay having three main functions:
- Measurement of the currents
- Detection of the faults
- Emission of a tripping order to the breaker
- The fuses blow under certain fault conditions
Transformer Protection :
Stresses generated by the supply
Two types of over voltages may stress and even destroy a transformer:
- The lightning over voltages due to lightning stroke falling on or near an overhead line supplying the installation where the transformer is installed
- The switching over voltages generated by the opening of a circuit breaker or a load break switch for instance.
Depending of the application, protection against these two types of voltage surges may be necessary and are often ensured by means of ZnO surge arrestors preferably connected on the MV bushing of the transformer.
Stresses due to the load
A transformer overload is always due to an increase of the apparent power demand (kVA) of the installation. This increase of the demand can be the consequence of either a progressive adjunction of loads or an extension of the installation itself. The effect of any overload is an increase of the temperature of oil and windings of the transformer with a reduction of its life time.
The protection of a transformer against the overloads is performed by a dedicated protection usually called thermal overload relay. This type of protection simulates the temperature of the transformer’s windings. The simulation is based on the measure of the current and on the thermal time constant of the transformer.
Some relays are able to take into account the effect of harmonics of the current due to non-linear loads such as rectifiers, computers, variable speed drives etc. This type of relay is also able to evaluate the remaining time before the emission of the tripping order and the time delay before re-energizing the transformer.
In addition, oil-filled transformers are equipped with thermostats controlling the temperature of the oil.Dry-type transformers use heat sensors embedded in the hottest part of the windings insulation.
Each of these devices (thermal relay, thermostat, heat sensors) generally provides two levels of detection:
- A low level used to generate an alarm to advise the maintenance staff,
- A high level to de-energize the transformer.
Internal faults in oil filled transformers
In oil filled transformers, internal faults may be classified as follow:
- Faults generating production of gases, mainly:
- Micro arcs resulting from incipient faults in the winding insulation
- Slow degradation of insulation materials
- Inter turns short circuit
- Faults generating internal over pressures with simultaneously high level of line over currents:
- Phase to earth short circuit
- Phase to Phase short circuit.
Some Most Usable Protection Of Transformers :
Buchholz Relay :
Buchholz relay is used for the protection of transformers from the faults occurring inside the transformer. Short circuit faults such as inter turn faults, incipient winding faults, and core faults may occur due to the impulse breakdown of the insulating oil or simply the transformer oil.
Differential Protection Relay :
The relay whose operation depends on the phase difference of two or more electrical quantities is known as the differential protection relay. It works on the principle of comparison between the phase angle and the magnitude of the same electrical quantities.
PRD Protection :
Pressure Relief Device is a device which is used for avoiding high oil pressure builds up inside the transformer during fault conditions. It is fitted on the top of the main tank. The PRD allows rapid release of excessive pressure that may be generated in the event of a serious fault.
WTI And OTI Protections :
WTI means winding temperature Indicator and OTI means Oil Temperature Indicator which indicates the winding temperature & oil temperature of the transformer and operates the alarm, trip, and cooler control contacts.This instrument operates on the principle of thermal imaging and it is not an actual measurement.
Restricted Earth Fault Protection:
The sensitivity of REF protection is more than that of Differential Protection. Normally the setting of differential protection is kept 20% which can not be reduced further. While the setting for Restricted Earth Fault is 5%. Thus if a fault occurs near the neutral point in star connected winding then differential protection wont sense it but REF will sense and trip the machine.
Over current Earth Fault Protection :
The combined over current and earth-fault relay is a secondary relay which is connected to the current transformers of the object to be protected. The over current unit and the earth-fault unit continuously measure the phase currents and the neutral current of the object.