Restricted Earth Fault Protection of Transformer

An external fault in the star side will result in current flowing in the linecurrent transformer of the

affected phase and at the same time a balancing current flows in the neutral current transformer,

hence the resultant electric current in the relay is therefore zero. So this REF relay will not be

actuated for external earth fault. But during internal fault the neutral current transformer only carries

the unbalance fault current and operation of Restricted Earth FaultRelay takes place. This scheme

of restricted earth fault protection is very sensitive for internal earth fault of electrical power

transformer. The protection scheme is comparatively cheaper than differential protection scheme

Restricted earth fault protection is provided in electrical power transformer for sensing internal

earth fault of the transformer. In this scheme the CT secondary of each phase of electrical power

transformer are connected together as shown in the figure. Then common terminals are connected

to the secondary of a Neutral Current Transformer or NCT. The CT or Current

Transformer connected to the neutral of power transformer is called Neutral Current Transformer or

Neutral CT or simply NCT. Whenever there is an unbalancing in between three phases of the power

transformer, a resultant unbalance current flow through the close path connected to the common

terminals of the CT secondaries. An unbalance current will also flow through the neutral of power

transformer and hence there will be a secondary current in Neutral CT because of this unbalance

neutral current. In Restricted Earth Fault scheme the common terminals of phase CTs are

connected to the secondary of Neutral CT in such a manner that secondary unbalance current of

phase CTs and the secondary current of Neutral CT will oppose each other. If these both electric

currents are equal in amplitude there will not be any resultant current circulate through the said close

path. The Restricted Earth Fault Relay is connected in this close path. Hence the relay will not

response even there is an unbalancing in phase current of the power transformer.

Basic Operation

Restricted Earth Fault Operation Under normal conditions and by application of Kirchhoff’s laws the sum of currents in both current transformers (CTs) equals zero. If there is an earth fault between the CTs then some current will bypass the CT's and the sum of currents will not be zero. By measuring this current imbalance faults between the CTs can be easily identified and quickly cleared.

Fault detection is confined to the zone between the two CTs hence the name 'Restricted Earth Fault'.

REF protection is fast and can isolate winding faults extremely quickly, thereby limiting damage and

consequent repair costs. If CTs are located on the transformer terminals only the winding is protected.

However, quite often the secondary CT is placed in the distribution switchboard, thereby extending the

protection zone to include the main cable.

Without REF, faults in the transformer star secondary winding need to be detected on the primary of the

transformer by the reflected current. As the winding fault position moves towards the neutral, the

magnitude of the current seen on the primary rapidly decreases and could potentially not be detected

(limiting the amount of winding which can be protected). As the magnitude of the currents remain

relatively large on the secondary (particularly if solidly earthed), nearly the entire winding can be

protected using REF.

ANSI Designation

Often 87N or 64REF are used to denote restricted earth fault protection. The intent is that both mean the same thing.

Which is the correct or better to use may be open to interpretation.

To me, 87N is more appropriate as REF is a differential type protection (ANSI 87 – differential protection relay). I

think 64 is strictly more relevant to detecting earth/ground faults due to breakdown of insulation.

This was a question asked below (in the discussion). If anyone has any views, please join that thread.

It should be remembered that the protection as illustrated covers only the secondary of the transformer.

Sometimes REF protection is added to the primary as well (although if primary protection is required I

would prefer to consider full differential protection).

As it is essential that the current in the CTs be balanced during normal conditions (and through faults),

historically REF has been implemented using High Impedance Relays. CT's have also been specified as

matched pairs and the impedance of leads/wires and interconnecting cables has had a large influence on

the functioning of the relay. Measurement errors associated with these issues have been responsible for

nuisance tripping and the system can be difficult to commission. This may be the reason some people

avoid the use of REF. Recent advances in numerical relay technology have all but eliminated these

issues, making the implementation of REF relatively easy, ensuring no nuisance tripping and simplifying

commissioning.

In the diagram 87N is the ANSI Device Number for restricted earth fault.

See Also - See more at: http://myelectrical.com/notes/entryid/150/restricted-earth-fault-

protection#sthash.z6zN8BoC.dpuf

What is Buchholz Relay ?

Construction of Buchholz Relay

Buchholz relay in transformer is an oil container housed the connecting pipe from main tank to

conservator tank. It has mainly two elements. The upper element consists of a float. The float is

attached to a hinge in such a way that it can move up and down depending upon the oil level in

the Buchholz relayContainer. One mercury switch is fixed on the float. The alignment of mercury

switch hence depends upon the position of the float.

The lower element consists of a baffle plate and mercury switch. This plate is fitted on a hinge just in

front of the inlet (main tank side) of Buchholz relay in transformer in such a way that when oil

enters in the relay from that inlet in high pressure the alignment of the baffle plate along with the

mercury switch attached to it, will change.

In addition to these main elements aBuchholz relay has gas release pockets on top. The electrical

leads from both mercury switches are taken out through a molded terminal block.

Buchholz Relay Principle

The Buchholz relay working principleof is very simple. Buchholz relay function is based on very

simple mechanical phenomenon. It is mechanically actuated. Whenever there will be a minor internal

fault in the transformer such as an insulation faults between turns, break down ofcore of transformer,

core heating, the transformer insulating oil will be decomposed in different hydrocarbon gases,

CO2 and CO. The gases produced due to decomposition oftransformer insulating oil will accumulate

in the upper part the Buchholz container which causes fall of oil level in it.

Fall of oil level means lowering the position of float and thereby tilting the mercury switch. The

contacts of this mercury switch are closed and an alarm circuit energized. Sometime due to oil

leakage on the main tank air bubbles may be accumulated in the upper part the Buchholz container

which may also cause fall of oil level in it and alarm circuit will be energized. By collecting the

accumulated gases from the gas release pockets on the top of the relay and by analyzing them one

can predict the type of fault in the transformer.

More severe types of faults, such as short circuit between phases or to earth and faults in the tap

changing equipment, are accompanied by a surge of oil which strikes the baffle plate and causes the

mercury switch of the lower element to close. This switch energized the trip circuit of the circuit

breakers associated with the transformer and immediately isolate the faulty transformer from the rest

of the electrical power system by inter tripping the circuit breakers associated with both LV and HV

sides of the transformer. This is how Buchholz relay functions.

Buchholz Relay Operation – Certain Precaution

The Buchholz relay operation may be actuated without any fault in the transformer. For instance,

when oil is added to a transformer, air may get in together with oil, accumulated under the relay

cover and thus cause a false Buchholz relay operation.

That is why mechanical lock is provided in

that relay so that one can lock the movement of mercury switches when oil is topping up in the

transformer. This mechanical locking also helps to prevent unnecessary movement of breakable

glass bulb of mercury switches during transportation of the Buchholz relays.

The lower float may also falsely operate if the oil velocity in the connection pipe through, not due to

internal fault, is sufficient to trip over the float. This can occurs in the event of external short circuit

when over currents flowing through the winding cause overheated the copper and the oil and cause

the oil to expand.

What is a Buchholz relay?

Buchholz relay is a type of oil and gas actuated protection relay universally used on all oil

immersed transformers having rating more than 500 kVA. Buchholz relay is not provided in

relays having rating below 500 kVA from the point of view of economic considerations.

Why Buchholz relay is used in transformers?

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.

Buchholz relay will sense such faults and closes the alarm circuit.

Working principle

Buchholz relay relies on the fact that an electrical fault inside the transformer tank is

accompanied by the generation of gas and if the fault is high enough it will be accompanied by a

surge of oil from the tank to the conservator

Whenever a fault occurs inside the transformer, the oil in the transformer tank gets overheated

and gases are generated. The generation of the gases depends mainly on the intensity of fault

produced. The heat generated during the fault will be high enough to decompose the transformer

oil and the gases produced can be used to detect the winding faults. This is the basic principle

behind the working of the Buchholz relay.

Construction

Buchholz relay can be used in the transformers having the conservators only. It is placed in the pipe

connecting the conservator and the transformer tank. It consists of an oil filled chamber. Two hinged

floats, one at the top of the chamber and the other at the bottom of the chamber which accompanies

a mercury switch each is present in the oil filled chamber. The mercury switch on the upper float is

connected to an external alarm circuit and the mercury switch on the lower is connected to an

external trip circuit.

Operation

Operation of the Buchholz relay is very simple. Whenever any minor fault occurs inside the

transformer heat is produced by the fault currents. The transformer oil gets decomposed and gas

bubbles are produced. These gas bubbles moves towards the conservator through the pipe line.

These gas bubbles get collected in the relay chamber and displaces oil equivalent to the volume of

gas collected. The displacements of oil tilts the hinged float at the top of the chamber thereby the

mercury switch closes the contacts of the alarm circuit.

The amount of gas collected can be viewed through the window provided on the walls of the

chamber. The samples of gas are taken and analyzed. The amount of gas indicates the severity of

and its color indicates the nature of fault occurred. In case of minor faults the float at the bottom of

the chamber remains unaffected because the gases produced will not be sufficient to operate it.

During the occurrence of severe faults such as phase to earth faults and faults in tap changing gear,

the amount of volume of gas evolves will be large and the float at the bottom of the chamber is tilted

and the trip circuit is closed. This trip circuit will operate the circuit breaker and isolates the

transformer.

Advantages of Buchholz relay

Buchholz relay indicates inter turn faults and faults due to heating of core and helps in the

avoidance of severe faults.

Nature and severity of fault can be determined without dismantling the transformer by testing

the air samples.

Limitation of Buchholz relay

It can sense the faults occurring below the oil level only. The relay is slow and has a minimum

operating range of 0.1second and an average operating range of 0.2 seconds.

The Buchholz relay has two oil-filledchambers

with floats and relays arranged vertically one over the other. If high eddy currents, local

overheating, or partial discharges occur within the tank, bubbles of resultant gas rise to the top of

the tank. These rise through the pipe between the tank and the conservator. As gas bubbles

migrate along the pipe, they enter the Buchholz relay and rise into the top chamber.

As gas builds up inside the chamber, it displaces the oil, decreasing the level. The top float

descends with oil level until it passes a magnetic switch which activates an alarm. The bottom float

and relay cannot be activated by additional gas buildup. The float is located slightly below the top

of the pipe so that once the top chamber is filled, additional gas goes into the pipe and on up to

the conservator. Typically, inspection windows are provided so that the amount of gas and relay

operation may be viewed during testing.

If the oil level falls low enough (conservator empty), switch contacts in the bottom chamber are

activated by the bottom float.

These contacts are typically connected to cause the transformer to trip. This relay also serves a

third function, similar to the sudden pressure relay.

Buchholz relay inside

A magnetically held paddle attached to the bottom float is positioned in the oil-flow stream

between the conservator and transformer tank. Normal flows resulting from temperature changes

are small and bypass below the paddle. If a fault occurs in the transformer, a pressure wave

(surge) is created in the oil. This surge travels through the pipe and displaces the paddle. The

paddle activates the same magnetic switch as the bottom float mentioned above, tripping the

transformer.

The flow rate at which the paddle activates the relay is normally adjustable. See your specific

transformer instruction manual for details.

Protecting Oil Type Transformer with Buchholz Relay

Introduction

Buchholz relay is a gas-actuated relay installed in oil immersed transformers for protection

against all kinds of faults. Named afteri ts inventor mr. Max Buchholz (1875–1956) in 1921, relay

is used to produce an alarm in case of incipient (i.e.slow-developing) faults in the transformer and

to disconnect the transformer from the supply in the event of severe internal faults. It is usually

installed in the pipe connecting the conservator to the main tank.

It is a universal practice to use Buchholz relays on all such oil immersed transformers having

ratings in excess of 750 kVA. The Buchholz relay is a protective rely for equipment immersed in oil

for insulating and cooling purpose.

It is intended mainly for transformers or choke coils having a conservator vessel.

The relay responds to the accumulation of gas or air inside the apparatus when the oil levelis too low or the flow of oil unusually strong. The relay neither gives a warning signal or disconnects the endangered equipment.

The Buchholz relay operates even on very slight faults which are just in process of developing, so

that greater damage may be prevented.

Top

Protection Range

The relay is particularly effective in case of:

1. Falling oil level owing to leaks

2. Short circuited core laminations

3. Short – circuits between phases

4. Broken-down core bolt insulation

5. Earth faults

6. Bad contacts

7. Puncture of bushing insulators inside tank

8. Overheating of some part of the windings

In the event of a fault, oil or insulations decomposes by heat, producing gas or developing an

impulse oil flow. To detect these phenomena, a Buchholz relay is installed.

The Buchholz relay is installed at the middle of the connection pipe between the transformer oil

tank and the conservator, so that, upon a fault development inside a oil transformer, an alarm is

set off or the transformer is disconnected from the circuit.

Construction and technical characteristics

Buchholz relay contruction

Position Description

1-2 Release terminals

3-4 Alarm terminals

5 Earth terminal

6 Breather cock

7 Valve of pneumatic test

8 Mechanical test of the alarm/release circuit

9 Cable gland

10 Drain plug

Casing Non porous weatherproof compact casting of light aluminium alloy painted.

Cover Non porous weatherproof compact casting of light aluminium alloy painted. On the cover

are located : the terminal box, the valve of pneumatic test, the breather cock, the button

for mechanical test of alarm and trip circuits.

Inspection windows Special tempered glass with graduated scale in cm3.

Contacts – They can be mercury switches or magnetic actuated switches (reed contacts).

On request it’ s possible to supply change-over switches.

Switches characteristics Rated voltage: 24 ÷ 250V AC or DC

Rated current: 0,5 A (10000 tests)

Breaking capacity: 2 A AC (cos = 0,4 ÷ 25% - 50Hz) 2A DC (T = L / R = 40 msec).

Mechanical protection degree IP 54

Insulation 2000V 50Hz between terminals and earth for a 60 seconds time.

Working temperature Oil temperature range: - 25 / +115°C.

Vibration test (in normal operative conditions) Oscillation amplitude: 2mm

Time diagram:

0Hz ÷ 100Hz 30 sec.

100Hz (200 vibrations/sec.) 60 sec.

100Hz ÷ 0Hz. 30 sec.

Contacts capacity to withstand vibrations Mercury sw: 150 horizontal vibrations/sec. (75Hz) first signals of closing contacts 120 vertical

vibrations/sec. (75Hz) first signals of closing contacts.

Magnetic sw: 200 horizontal vibrations/sec. (100Hz) no one signal of closing contacts 200 vertical

vibrations/sec. (100Hz) no one signal of closing contacts. Top

Function of Buchholz relay

In the following the operation of a Buchholz relay is explained using the example of a double-

float Buchholz relay. The relay is built in the connecting pipe between the transformer tank and

the conservator. During normal operation it is filled completely with insulating liquid.

Due to buoyancy the floats are at their top position. If a fault occurs inside the transformer, the

Buchholz relay responds as follows:

Gas accumulation

Gas accumulation in Buchholz relay

Fault: Free gas is available in the insulating liquid.

Response: The gas in the liquid moves upwards, accumulates in the Buchholz relay and

displaces the insulating liquid level. The moving float actuates a switch contact (magnet contact

tube). An alarm signal is tripped. The lower float is not affected as from a certain gas volume the

gas flows through a piping to the conservator.

Insulating liquid loss

Insulating liquid loss in Buchholz relay

Fault: Insulating liquid loss due to leakage.

Response: As the liquid level falls the top float moves downward. An alarm is tripped. If the liquid

loss continues, conservator and piping as well as the Buchholz relay will be emptied. As the liquid

level falls, the lower float moves downward. The moving float actuates a switch contact so that the

transformer is disconnected.

Insulating liquid flow

Fault: A spontaneous incident generates a pressure wave moving in the direction of the

conservator.

Response: The liquid flow reaches a damper arranged in the liquid flow. If the flow rate exceeds

the operating threshold of the damper, the latter moves in flow direction. Due to this movement a

switch contact is actuated so that the transformer is disconnected.

Insulating liquid flow - Buchholz relay

The upper and lower switching system form a functional unit in the single-float Buchholz relay. In

case of a fault, the single-float Buchholz relay normally isolates the transformer immediately from

the mains system.

Top

Wiring Diagrams

Standard Wiring Diagram

Buchholz relay - Standard wiring diagram

Mounting Sketch

Buchholz relay - Mounting sketch

Advantage

1. It is the simplest form of transformer protection.

2.It detects the incipient faults at a stage much earlier than is possible with other forms of

protection.

Disadvantage

1. It can only be used with oil immersed transformers equipped with conservator tanks.

2. The device can detect only faults below oil level in the transformer. Therefore, separate

protection is needed for connecting cables.

Negative Sequence Relays TUESDAY, JULY 12, 2011

Generator Protection Part 12

1. Negative Sequence Relays

The negative relays are also called phase unbalance relays because these relays provide

protection against negative sequence component of unbalanced currents existing due to unbalanced

loads or phase-phase faults. The unbalanced currents are dangerous from generators and motors

point of view as these currents can cause overheating. Negative sequence relays are generally used

to give protection to generators and motors against unbalanced currents.

A negative sequence relay has a filter circuit which is operative only for negative sequence

components. Low order of over current also can cause dangerous situations hence a negative

sequence relay has low current settings. The earth relay provides protection for phase to earth fault

but not for phase to phase fault. A negative sequence relay provides protection against phase to

phase faults which are responsible to produce negative sequence components.

The Fig. 1 shows the schematic arrangement of negative phase sequence relay.

Fig. 1 Negative phase sequence relay

Basically it consists of a resistance bridge network. The magnitudes of the impedances of all the

branches of the network are equal. The impedances Z1 and Z3are purely resistive while the

impedances Z2 and Z4 are the combinations of resistance and reactance. The currents in the

branches Z2 and Z4 lag by 60o from the currents in the branches Z1 and Z3. The vertical branch B-D

consists of inverse time characteristics relay. The relay has negligible impedance.

Fig. 2

The current IR gets divided into two equal parts I1 and I2. And I2 lags I1 by 60o. The phasor

diagram is shown in the Fig. 2.

Ī1 + Ī2= Īrs

Let I1 = I2 = I

The perpendicular is drawn from point A on the diagonal meeting it at point B, as shown in the

Fig. 2. This bisects the diagonal.

... OB = IR /2

Now in triangle OAB,

cos 30 = OB/OA

... √3/2 = (IR/2)/I

... I = IR/√3 = I1 = I2 ............(1)

Now I1 leads IR by 30o while I2 lags IR by 30

o.

Similarly the current IB gets divided into two equal parts I3 and I4. The current I3lags I4 by 60o.

From equation (1) we can write,

IB /√3 = I3 = I4 ...............(2)

The current I4 leads by IB while current I3 lags IB by 30o.

The current entering the relay at the junction point B in the Fig. 1 is the vector sum of , and .

Irelay = Ī1 + Ī3 + ĪY

= IY + (IR/√3) (leads IR by 30o) + IB/√3(lags IB by 30

o)

The vector sum is shown in the Fig. 3 when the load is balanced and no negative sequence

currents exist.

Fig. 3

It can be seen from the Fig. 3 that,

Ī1 + Ī3 = -ĪY

... Ī1 + Ī3 + ĪY = 0

Thus the current entering the relay at point B is zero. Similarly the resultant current at junction D

is also zero. Thus the relay is inoperative for a balanced system.

Now consider that there is unbalanced load on generator or motor due to which negative

sequence currents exist. The phase sequence of C.T. secondary currents is as shown in the Fig.

4(a). The vector diagram of I1, I3 and IY is shown in the Fig. 4(b) under this condition.

The component I1 and I3 are equal and opposite to each other at the junction point B. Hence

I1 and I3 cancel each other. Now the relay coil carries the current IYand when this current is more

than a predetermined value, the relay trips closing the contacts of trip circuit which opens the circuit

breaker.

Fig. 4 Negative sequence current

Zero Sequence Currents : The zero sequence components of secondary currents are shown in

the Fig. 5(a). We know that,

Fig. 5 Zero sequence currents

ĪR = Ī1 + Ī2

ĪB = Ī3 + Ī4

These sums are shown in the Fig. 5(b) and (c). It can be seen from the Fig. 5(d) that,

Ī1 + Ī3 = ĪY in phase with IY

The total current through relay is Ī1 + Ī3 +ĪY. Thus under zero sequence currents the total current

of twice the zero sequence current flows through the relay. Hence the relay operates to open the

circuit breaker.

To make the relay sensitive to only negative sequence currents by making it inoperative under

the influence of zero sequence currents is possible by connecting the current transformers in delta

as shown in the Fig. 6. Under delta connection of current transformers, no zero sequence current

can flow in the network.

Fig. 6 Delta connection of C.T.s

1.1 Induction Type Negative Sequence Relay

Another commonly used negative sequence relay is induction type. Its construction is similar to that

of induction type over current relay. The schematic diagram of this type of relay is shown in the Fig.

7.

Fig. 7 Induction type negative sequence relay

The central limb of upper magnet carries the primary which has a centre tap. Due to this, the

primary winding has three terminal 1, 2 and 3. The section 1-2 is energized from the secondary of an

auxiliary transformer to R-phase. The section 2-3 is directly energized from the Y-phase current.

The auxiliary transformer is a special device having an air gap in its magnetic circuit. With the

help of this, the phase angle between its primary and secondary can be easily adjusted. In practice it

is adjusted such that output current lags by 120orather than usual 180

ofrom the input.

So, Ix = Input current of auxiliary transformer

IR1 = Output current of auxiliary transformer

and IR1 lags IR by 120o

Hence the relay primary carries the current which is phase difference of IR1 and IR .

Positive Sequence Current : The C.T. secondary currents are shown in the Fig. 8(a). The Fig.

8(b) shows the position of vector IR1 lagging IR by120o. The Fig. 8(c) shows the vector sum of IR1 and

- IY.

The phase difference of IR1 and IY is the vector sum of IR1 and - IY. It can seen from the Fig. 8(c)

that the resultant is zero. Thus the relay primary current is zero and relay is inoperative for positive

sequence currents.

Fig. 8 Positive sequence currents

Negative Sequence Currents : The C.T. secondary currents are shown in the Fig.. 9(a). The

Fig. 9(b) shows the position of IR1 lagging IR by 120o. The Fig. 9(c) shows the vector difference of

IR1 and IY which is the relay current.

Under negative sequence currents, the vector difference of IR1 and IY results into a current I as

shown in the Fig. 9(c). This current flows through the primary coil of the relay.

Fig. 9 Negative sequence currents

Under the influence of current I, the relay operates. The disc rotates to close the trip contacts

and opens the circuit breaker.

This relay is inoperative for zero sequence currents. But the relay can be made operative for the

flow of zero sequence currents also by providing an additional winding on the central limb of the

upper magnet of the relay. This winding is connected in the residual circuit of three line C.T. This

relay is called induction type negative and zero sequence relay.

The schematic arrangement of induction type negative and zero sequence relay is shown in the

Fig.10.

Fig. 10 Induction type negative and zero sequence relay

Translay Scheme

The translay relay is another type of differential relay. The arrangement is similar to overcurrent

relay but the secondary winding is not closed on itself. Additionally copper ring or copper shading

bands are provided on the central limb as shown in the Fig. 2.

Fig. 2 Translay relay

These type of relays are used in the feeder protection and the scheme is called Translay

scheme. In this scheme, two such relays are employed at the two ends of feeder as shown in the

Fig. 3.

Fig. 3 Translay scheme of feeder protection

The secondaries of the two relays are connected to each other using pilot wires. The connection

is such that the voltages induced in the two secondaries oppose each other. The copper coils are

used to compensate the effect of pilot wire capacitance currents and unbalance between two

currents transformers.

Under normal operating conditions, the current at the two ends of the feeder is same. The

primaries of the two relays carry the same currents inducing the same voltage in the secondaries. As

these two voltages are in opposition, no current flows through the two secondaries circuits and no

torque is exerted on the discs of both the relays.

When the fault occurs, the currents at the two ends of the feeder are different. Hence unequal

voltages are induced in the secondaries. Hence the circulating current flows in the secondary circuit

causing torque to be exerted on the disc of each relay. But as the secondaries are in opposition,

hence torque in one relay operates so as to close the trip circuit while in other relay the torque just

holds the movement in unoperated position. The care is taken that at least one relay operates under

the fault condition.

Role of copper ring : Mainly relays may operate because of unbalance in the current

transformers. The copper rings are so adjusted that the torque due to current induced in the copper

ring due to primary winding of relay is restraining and do not allow the disc to rotate. It is adjusted

just to neutralise the effect of unbalance between current transformers. The copper rings also

neutralise the effect of pilot capacitive currents. Though the feeder current is same at two ends, the

pilot capacitive currents may allow in the pilots. This current leads the secondary voltage by 90o. The

copper rings are adjusted such that no torque is exerted on the disc, due to such capacitive pilot

currents, by adjusting the angle between the induced current in the disc and secondary current to be

90o.

The advantages of this scheme are,

1. Only two pilot wires are required.

2. The cost is very low.

3. The current transformers with normal design can be employed.

4. The capacitive effects of pilot wire currents do not affect the operation of the relays.