How to Protect Transformers? | Devices | Electrical Engineering

For power transformers, the protection is to be provided usually against dangerous overloads and excessive temperature rise. Dangerous overloads may be due to external faults or the internal ones. External faults, however, are cleared by the relay system outside the transformer within the shortest possible time in order to avoid any danger to the transformer due to these faults. Hence the protection for internal faults is to be provided in such transformers.

Differential protection is the most important type of protection used for protection against internal phase-to-phase and phase-to-earth faults. The other protection systems employed for protection of transformers against internal faults are Buchholz protection, core-balance leakage protection, combined leakage and overload protection, restricted earth-fault protection.

Buchholz Protection of Transformers:

On the occurrence of internal fault in an oil-immersed power transformer gas is usually generated, slowly for an incipient fault (such as sparking, small arcing, loose connections in conducting path etc.,) and violently for heavy faults. Most short circuits caused either by impulse breakdown between adjacent turns at the end turns of the winding or as a poor initial point contact which will immediately heat to arcing temperature.

The heat generated by the large local currents causes the transformer oil to decompose and generate gases, which can be used in detection of winding faults.

The relays based on this principle are:

(i) Pressure relays and pressure relief devices which act on the measurement of the total accumulated pressure,

(ii) Rate of pressure rise relay, which acts on the measurement of the rate of formation of the gas and

(iii) Gas accumulator relay, most commonly known as Buchholz relay, actuated by the gas formed.

Buchholz protection employing Buchholz relay is the simplest form of protection and is most commonly used on all oil-immersed transformers provided with conservator.

Core-Balance Leakage Protection of Transformers:

An earth fault usually involves a partial breakdown of winding insulation to ground. The resulting leakage current is quite small as compared to short-circuit current. The earth fault may continue for a long time and cause considerable damage before it ultimately develops into a short circuit and removed from the system. Under such circumstances it is advisable to provide earth-fault protection in order to ensure that the earth fault or leak is removed in the early stages. An earth-fault relay used for it is essentially an overcurrent relay of low setting and operates as soon as earth fault or leak develops. One method of protection against earth faults in transformers is the core-balance leakage protection.

This system consists of three primary conductors surrounded by the magnetic circuit of a current transformer. This has a single secondary winding which is connected to the relay operating coil. Under normal conditions i.e., when there is no earth fault the instantaneous sum of the currents in the three phases is always zero, and there is no resultant flux in the core of the CT no matter how much the load is out of balance.

Thus no current flows through the relay operating coil and trip circuit remains open. When an earth fault occurs, the sum of the three currents is no longer zero and a current is induced in the secondary of the CT causing the trip relay to operate and isolate the transformer from the bus-bars.

Combined Leakage and Overload Protection of Transformers:

The core-balance protection described above suffers from the disadvantage that if the fault occurs between phases the relay does not operate. This shortcoming is overcome by employing three separate CTs. In this system of protection two overload relays and one earth leakage relay are connected. The overload relays used are high current setting ones and are arranged to operate against phase-to-phase faults while the earth fault relay has low current setting and operates under earth or leakage faults only.

The two overload relays are sufficient to protect all the three phases while the earth-fault or leakage-fault relay is energized by the resultant currents from all the three CTs in case of leakage fault. The trip contacts of the overload relays and earth-fault or leakage relay are connected in parallel, as illustrated in the figure. So the circuit breaker will trip in the event of energisation of either overload relay or leakage relay. Thus the protection against faults and short circuits either to earth or between phases is achieved.

Biased Differential Protection of Transformers:

In order to avoid undesirable operation on heavy external faults due to CT’s errors and ratio change as a result of tap changing use of biased or percentage differential relay is made, restraining winding being energized by the through current. Figure 9.4 shows the arrangement of percentage differential relaying for power transformers.

The power transformer is star connected on one side and delta connected on the other. The CTs on the star-connected side are delta-connected and those on delta-connected side are star-connected. The neutrals of CT star and power transformer star connections are grounded. The restraining coils are connected across the secondary windings of CTs. The operating coils are connected between the tapping points on the restraining coils and the star point of the CT secondary windings.

The operating coils normally carry no current as they are balanced on both sides of the CTs. On the occurrence of internal fault in the power transformer windings, the balance is disturbed and the operating coils of the differential relays carry current corresponding to the difference of the current between the two sides of the power transformers and operate the relays to trip the main circuit breakers on both sides of the power transformer.

Harmonic Restraint Relay of Transformers:

The operation of the relays because of magnetising inrush current can be avoided by using kick fuses across the relay coils or using relays with inverse and definite minimum time (IDMT) characteristics. However, for EHV transformers, the relay current and time ratings necessary to ensure stability on the magnetising inrush current caused by switching-in the transformer are not adequate for providing high speed protection.

A high speed biased differential relay incorporating a harmonic restraint feature is immune to the magnetising inrush current. The magnetising inrush currents have a high component of even and odd harmonics (about 63% of second harmonics and 26.8% of third harmonics) while harmonic component of short-circuit currents is negligible. The use of these facts is made for restraining the relay from operation during initial current inrush.

The harmonic restraint differential relay is sensitive to fault currents but is immune to the magnetising currents. The operating coil of the relay carries only the fundamental component of current only while the restraining coil carries the sum of the fundamental and harmonic components.

Basic circuit of a harmonic restraint differential relay is illustrated in Fig. 9.8. The restraining coil is energized by a direct current proportional to bias winding current as well as the direct current due to harmonics. Harmonic restraint is had from the tuned circuit (X_C – X_L) that allows only the fundamental component of current to enter the operating circuit.

The dc and higher harmonics (mostly second harmonics) are diverted into the rectifier bridge feeding the restraining coil. The relay is adjusted so that it will not operate when the harmonic current exceeds 15% of the fundamental current. Both the dc and higher harmonics are of large magnitude during magnetising inrush.

The relay may fail to operate due to harmonic restraint feature if an internal fault has considerable harmonics that may be present in the fault current itself due to an arc, or due to saturation of CT. Also, if a fault exists at the instant of energization of transformer harmonics present in the magnetising current may prevent the operation of the relay. This problem can be overcome by providing instantaneous overcurrent relay in the differential circuit which is set above the maximum inrush current but will operate in less than one cycle on internal faults. Thus fast tripping is ensured for all internal faults.

The other method used is harmonic blocking. In this method the harmonic component of magnetising inrush current is used for blocking a separate relay, called the blocking relay, whose contacts are in series with the contacts of the differential relay. The blocking relay contains a 100 Hz blocking filter in operating coil and 50 Hz blocking filter in the restraining coil. During inrush currents the second harmonic component is predominant and the blocking relay is blocked. The blocking relay contacts remain open. During short circuits, fundamental component is predominant, so blocking relay operates and relay contact circuit is closed.

Self-Balance Protection System of Transformers:

The self-balance protection system for the protection of alternators can also be employed to power transformers without any modification except that the same type of equipment has to be employed for both primary and secondary sides. The protective transformers (CTs) can be located in the oil of the transformer tank. This system of protection of power transformers is not much used because it cannot provide protection to transformer terminals and the connected cables up to switchgear.

Differential Magnetic Balance Protection System of Transformers:

This system is necessarily a combination of circulating current and self-balance protection systems. The main advantages of this protection system are increased stability and sensitivity and its application to power transformers provided with load tap-changers. Figure 9.9 illustrates a scheme representing a power transformer having primary (low-voltage) side connected in delta and secondary (high-voltage) side connected in star.

The current transformers used are bus type and are connected for circulating current protection. Their current ratings are so adjusted that they provide equal secondary currents. The relay operating coil instead of being connected to the equipotential point of the pilot wires is fed from another winding on hv side CT (CT₂). Since the core flux of Iv side CT (CT₁) is more because of its more ampere-turns, the turns of hv side CT are increased so as to make core flux of CT₂ zero under all load conditions resulting in no current flowing in the relay operating coil under normal operating conditions. The CT₂ is made with high permeability alloy core so as to reduce magnetising current and provide an accurate balance.

For any through fault CT₂ continues to have no flux and so the relay operating coil remains inoperative. On occurrence of fault in protection zone, say at F, excessive current flows through CT₁ causing flow of current in the relay operating coil. Thus relay is energized and the circuit breaker gets tripped.

Self Stabilising Magnetic Balance Protection System of Transformers:

For the protection of power transformer having tappings it is necessary that the protective CT connected on hv side (i.e., CT₂) must also be capable of changing its current ratio whenever power transformer tappings are changed i.e., CT₂ windings need some modification.

It is explained as- in Fig. 9.10 (a) plain magnetic balance protection system is illustrated, the relay connections are not shown for sake of simplicity. CTs used are bus-bar type. In Fig. 9.10 (b) self-stabilizing circuit for magnetic balance protection system is shown. In this circuit the magnetic core of CT₂ is divided into two halves P₁ and P₂ and the secondary winding is so wound that the flux developed by the two halves P₁ and P₂ is equal and opposing each other. Thus in normal operating conditions no emf is induced in the secondary winding and the relay operating coil remains inoperative.

 

When the transformer is operating under normal operating conditions and carrying full-load currents, the flux developed by the two halves is equal and relay winding coil is un-energized. Now when the tappings of the main transformer are changed, mmfs of the two halves are changed causing the flux developed by them to be different. So an emf, proportional to the difference of the two fluxes, will be induced in the relay coil, as shown in Fig. 9.11.

If under this tap changed condition the load on the transformer is increased, mmf of the two halves will increase but the difference of fluxes developed will decrease. Thus with the increase in load on power transformer, the difference in fluxes developed by the two halves of core of CT₂ decreases, as shown in Fig. 9.11. Now if the relay is so designed that its minimum operating voltage is much more than the induced voltage under any desirable load condition but with no fault, as illustrated by OD in Fig. 9.11, then stability is ensured. In practice OD is made twice the minimum ordinate.

Restricted Earth-Fault Protection of Transformers:

Earth fault relays connected in residual circuit of line CTs provide protection against earth faults on the delta or unearthed star-connected windings of power transformers. The connections of restricted earth-fault protection for star-connected and delta-connected windings are shown in Fig. 9.12. A CT is fitted in each connection to the protected and the secondaries of CTs are connected in parallel to a relay.

Ideally, the output of the CTs is proportional to the sum of zero sequence currents in the line and the neutral earth connection if the latter is within the protected zone. For external faults zero sequence currents are either absent or sum to zero in the line and neutral earth connection. For internal faults, the sum of zero sequence currents is equals twice the total fault current.

In Fig. 9.13 the star-connected side is protected by restricted earth-fault protection.

When there is an earth fault outside the protective zone, say at F₁, it causes the currents I, and I₁ in CT secondaries as illustrated in Fig. 9.13. So the resultant current in earth fault relay is negligible. For an earth fault within the protected zone, say at F₂, only current I₂ exists, being negligible. Thus current I₁ flows through the earth-fault relay. Thus restricted earth-fault relay does not operate for earth fault beyond the protective zone of the transformer.

For an earth fault near the neutral point of the transformer the voltage available for driving earth fault current is small. For the relay to sense such fault, it has to be too sensitive and would, therefore, operate for spurious signals, external faults and switching surges. Hence the relay is set as per practice, so as to operate for earth fault current of the order of 15% of rated winding current. Such setting protects restricted portion of the winding, hence the name restricted earth-fault protection.

Stabilizing resistor is connected in series with the relay to avoid magnetising inrush current and also saturation of CT core.

Frame Leakage Protection of Transformers:

The transformer is mounted insulated from the ground, as illustrated in Fig. 9.14. The transformer tank is connected to earth through a CT to which an instantaneous earth fault relay is connected. In the case of an earth fault in the transformer (breakdown of insulation in any winding of the transformer), there is a flow of current to the earth over this connection causing the relay to operate. Such an arrangement is usually provided where banked transformers are provided with a single overall differential protection and it is difficult to find as to which transformer is faulty.

Generator-Transformer Unit Protection:

In hv transmission the bus-bars are operated at higher voltages than that of generation; it is common practice to connect the generators directly to step-up transformers. In this protection scheme no circuit breaker is interposed in between the generator and transformer. The main advantage of such protection is that it simplifies the protection, mainly the differential protection for both generator and transformer can be combined together by employing CTs on the neutral side of the generator and on the hv side of the power transformer, as illustrated in Fig. 9.15.

Because of the occurrence of magnetising inrush current transients the relay settings in this protection scheme must be considerably higher than those for protecting a generator only. The zone of differential protection includes the stator windings of the generator, the step-up transformer and the intervening connections.

It is necessary to take care of the phase shift within the power transformer and the connections of CTs. If a unit transformer is tapped off at the generator terminals, this also has to be taken care of by suitable connections of the CTs for protection. CTs located on the neutral side of the generator are star-connected while the CTs on the secondary (hv) side of the main transformer are delta-connected so as to cancel the 30° displacement between line currents introduced by delta-connected primary of the main transformer.

The unbalance caused between CT pairs due to load of unit transformer is avoided by providing another set of star-connected CTs in the primary leads of the latter. In healthy condition, the sum of secondary currents of these CTs and the secondary currents of the generator star-point CTs is equal to the currents in the pilot wires from the secondaries of the delta-connected CTs on the secondary side of the main transformer. On occurrence of fault differential relays are energized. The hv winding of the main transformer is protected against earth faults by the restricted earth fault protection scheme.

From the schematic diagram of generator-transformer unit protection shown in Fig. 9.15, it is obvious that the stator winding of the generator and the LV or primary windings of the main transformer and unit transformer comprise a separate circuit having no electrical connection with the hv circuits. So an earth fault at any point of this separate circuit will cause a flow of current through the earth connection and through a PT connected in series with it. An alarm relay connected across the secondary winding of PT will get energized and give the necessary signal.