Distance And Differential Protection Distance And Differential Protection

UNIT-2 Distance and Differential Protection

Distance Protection


Introduction



Distance protection is a widely used protective scheme for the protection of high and extra high voltage (EHV) transmission and subtransmission.



Over current relays have been found unsuitable for the protection of transmission lines because of their inherent drawbacks of variable reach and variable operating time due to changes in source impedance and fault type.



REACH:

a distance relay operates when the impedance (or

component of impedance) as seen by the relay is less than a preset value. This preset impedance or corresponding distance is called the reach of the relay. (max. length of line up to which relay can protect).

Distance… 

Distance relays have been developed to overcome the problems associated with the use of over current relays for the protection of transmission lines.



Distance relays are double actuating quantity relays with one coil energized by voltage and the other coil energized by current. As shown in the figure.



The torque produced is such that when V/I reduces below a set value, the relay operates.



During a fault on a transmission line the fault current increases and the voltage at fault point reduces.



The ratio V/I is measured at the location of CT's and VT's.

Distance… 

The voltage at VT location depends on the distance between the VT and the fault. If fault is nearer, measured voltage is lesser. If fault is further, measured voltage is more.



Hence assuming constant fault resistance each value of V/I measured from relay location corresponds to distance between the relying point and the fault along the line.



Hence such protection is called Impedance Protection or Distance Protection.

Distance…

Distance… 

Modern distance relays provide high speed fault clearance. They are used for the protection of transmission and sub transmission lines at 220kV, 132kV, 66kV and 33kV. Sometimes even at 11kV.



For 132kV and 220kV systems, the recent trend is to use carrier current protection where relaying unit used are distance relays. In case of failure of carrier signal, they act as back-up protection.



A distance protection scheme is a non-unit system of protection. A single scheme provides both primary and back-up protection.

Universal Torque Equation 

Most of the protection relays consist of some arrangement of electromagnets. These electromagnets have either current windings or voltage windings and in some cases both the windings.



Current through the windings produce magnetic fluxes and the torque is produced by interaction between the fluxes of both the windings.

Torque developed by current winding = K1 I2 Torque developed by voltage winding = K2 V2

Universal… 

hence , the net torque developed by the interaction of two fluxes is = K3 V I cos (θ- ĩ) where θ = angle between V and I ĩ = maximum torque angle

The universal relay torque equation is given as T = K1 I2 + K2 V2 + K3 V I cos (θ- ĩ) + K4 where K1, K2, K3 = tap settings or constants of I and V K4 = mechanical restraint due to spring or gravity

Classification of Distance Relays 1)

Impedance relays

2)

Reactance relays

3)

MHO relays

4)

Angle impedance relays

5)

Quadrilateral relays

6)

Elliptical and other conic section relays

The principle of R-X Diagram 

R-X diagrams are useful in plotting characteristics of Distance Relays.

Impedance Relay



An impedance relay is a voltage restrained over current relay.



The relay measures impedance up to the point of fault and gives tripping command if this impedance is less than the relay setting Z.



Relay setting Z is known as replica impedance and it is proportional to the set impedance i.e. impedance up to the reach of the relay.



The relay monitors continuously the line current I through CT and the bus voltage V through PT and operates when the V/I ratio falls below the set value.

Impedance…


Construction

Impedance relay (Induction cup type)

Impedance… Let IF = line current when fault occurs at point X VF = supply voltage when fault occurs at point X iF = current supplied to current coil when fault occurs vF = voltage supplied to voltage coil when fault occurs V = normal supply voltage I = normal line current ZL = V/I = impedance of healthy section ZF = VF/IF = impedance when fault occurs

Impedance… 

The relay is connected at point A. The fault occurs at point X. The voltage coil receives voltage vF and current coil receives current iF when fault occurs.



The setting of the relay is selected, such that it protects the transmission line up to point B.



Thus for any fault between A-B similar to that shown at point X the impedance under fault condition will be less than the predetermined value of impedance ZL and relay will operate.

Impedance…




Operating characteristics:



To realize the characteristics of an impedance relay, current is compared with voltage at the relay location.



The current produces a positive torque

(operating torque) and the

voltage produces a negative torque (restraining torque). Therefore it is called as voltage restrained over current relay.

Impedance…



The equation for the operating torque of an electromagnetic relay can be written as:

T = K1 I 2 − K 2V 2 + K 4

since (K3 = 0)

Where K1 and K2 are constants, Neglecting the effect of the spring used, which is very small, the torque equation can be written as

T = K1 I 2 − K 2V 2 

This means the operating torque is produced by the current coil and the restraining torque by the voltage coil, which means that an impedance relay is a voltage restrained over-current relay.

Impedance…



For the operation of the relay, the following condition should be satisfied 2

K1 I > K 2V V2 I2 V

2

or

K 2V 2 < K 1I 2

K1 < K2
where K is constant

I Z
The relay operates if the measured impedance Z is less than the given constant.

Impedance…

operating characteristics of an impedance relay

Impedance…

operating characteristics of an impedance relay on the R-X diagram

Impedance…



Any value of Zf less than the radius of the circle produces positive torque. Any value of Zf more than the radius, of circle produces a negative torque and relay does not operate.

Impedance…

Operating time characteristics 

The curve I represents actual characteristics. Curve II is simplified representation of the same (right angle instead of curve).



The relay unit used for distance protection are double actuating quantity instantaneous relays.



The electromagnetic relays of balanced beam type or induction cup type are preferred.

Impedance…




Disadvantages



It is non-directional. It responds to the faults on both sides of CT,VT location. Hence it cannot discriminate between internal and external faults.



It is affected by arc resistance of line fault and results in under-reach.



It is sensitive to power swings as a large area is covered by the circle on each side on R-X plane.

Impedance…


Directional Impedance Relay



Directional features senses the direction in which the fault power flows with respect to the location of CT and VT. Directional impedance relay operates for following conditions:



Impedance between fault point and relay location is less than the relay setting Z.



The fault power flows in a particular direction from relay location. The direction power flow is sensed by measuring phase angle between voltage and current.



With Directional Characteristic added to the plain impedance characteristic, the results in a characteristic with a sector of a circle.

Impedance…

operating characteristics of directional impedance relay on the R-X diagram

Impedance… 

Consider a locus of fault point on transmission line (locus OY).



Angle ROY = α depends upon the phase angle between V and I with given setting of directional element, the operating torque is positive within the semi-circle with radius Z and on right hand side of the inclined line of directional characteristic DD‘



For faults on one side of the relay location, angle ‘α’ lies between angle DOD'. Hence relay operates for two conditions: (i) Locus OXY should have angle α with angle DOD' given by Directional Feature. (ii) Impedance measured by relay should be less than the Setting Z.

Impedance…


Torque equation of Directional Impedance Relay



The directional relay responds to the phase angle between V and I at relay location. Suppose torque of directional unit is given by,

T = K1VI cos(φ − θ ) where T = Torque K1 = Constant V = Voltage supplied to relay coil I = Current supplied to relay coil Φ = Phase angle between V and I θ = Angle of maximum torque

Impedance… when the relay is on verge of operation T =0 hence cos (Φ - θ ) =0 i.e. (Φ - θ ) = ± 900 Hence for positive torque, Φ should be within θ ± 90°.  This directional characteristic when presented on R-X diagram is a straight line (DOD) for which Φ is within θ ± 90°.

Impedance… 

However, impedance characteristic puts another conditions, i.e. V/I < Z represented by a circle on R-X diagram. Hence the net characteristic of directional impedance relay is a semi-circle above a straight line passing through zero. The radius of circle corresponds to measured impedance.

Impedance…

Impedance… 

The impedance unit may be given a current bias, i.e. the voltage coils is supplied by additional voltage proportional to line current (say DI), Basic Torque equation gets modified to

T = K1 I 2 − K 2(V + DI ) 2 (V + DI ) is the voltage supplied to voltage coil of impedance relay. 

The characteristic when plotted on R-X diagram is a circle with radius V/I and with centre shifted from origin. The circle may be completely 'offset' from the origin so much so that origin is left out of the circle.

Reactance Relay 

In this relay the operating torque is obtained by current and the restraining torque due to a current-voltage directional element. This means, a reactance relay is an over-current relay with directional restraint.



The directional element is so designed that its maximum torque angle is 90o, i.e. ĩ = 90o in the universal torque equation T = K1 I2 - K3 V I cos (θ- ĩ) = K1 I2 - K3 V I cos (θ- 90) = K1 I2 - K3 V I sin θ

Reactance… for the operation of the relay, K1 I2 > K3 V I sin θ

K1 sin θ < 2 K3 I

VI

K1 Z sin θ < K3 K1 X < K3

Reactance…

Operating characteristic of reactance relays

Reactance…


Construction

Reactance relay (Induction cup type)

Reactance… 

The structure used for the reactance relay can be of induction cup type.



It is a

four-pole

structure. This has operating, polarizing and

restraining coils. 

The operating torque is produced by the interaction of fluxes due to the windings carrying current coils, i.e. interaction of fluxes of poles 1, 2 and 3 and the restraining torque is developed due to the interaction of fluxes due to poles 1, 3 and 4.



The operating torque will be proportional to I2 and restraining torque proportional to VI cos (θ-90). The desired maximum torque angle is obtained with the help of R-C circuits as shown in figure.

Reactance…


Disadvantage



This relay as can be seen from the characteristic, is a non-directional relay. This will not be able to discriminate when used on transmission lines, whether the fault has taken place in the section where the relay is located or it has taken place in the adjoining section.



If directional unit is added then it will not trip for high power factor load.

Mho or Admittance Relay 

In the impedance relay a separate unit is required to make it directional, while the same unit can not be used to make a reactance relay with directional feature.



The mho relay is made inherently directional by adding a voltage winding called polarized winding.



This relay works on the measurement of admittance Y∟θ . This relay is also called angle impedance relay.

Mho…


Construction

Mho relay (Induction cup type)

Mho… 

This relay also uses an induction cup structure. It also has an operating coil, polarizing coil and restraining coil.



In this relay operating torque is obtained by V and I element while the restraining torque is obtained by a voltage element. Thus an admittance relay is a voltage restrained directional relay.



The operating torque is produced by the interaction of the fluxes due to the windings carried by the poles 1, 2 and 3. While the restraining torque is produced by the interaction of fluxes due to the windings carried by the pole 1, 3 and 4.

Mho… 

Thus the restraining torque is proportional to the square of the voltage (V2) while the operating torque is proportional to the product of voltage and current (VI). The torque angle is adjusted using series tuning circuit.

Mho…


Torque Equation From the universal torque equation T = K3 V I cos (θ- ĩ) - K2 V2

For the relay to operate K3 V I cos (θ- ĩ ) > K2 V2

V 2 K3 < cos(θ − τ ) VI K2

K3 Z< cos(θ − τ ) K2 

This is the equation of a circle having diameter ‘K3/K2’ passing through origin. And this constant ‘K3/K2’ is the ohmic setting of the relay.

Mho…
Operating Characteristics

Operating characteristic of Mho relays

Mho… 

As seen from the torque equation, the characteristics of this relay is a circle passing through origin with diameter as ‘K3/K2’.



It is called MHO relay because its characteristic is a straight line when plotted on an admittance diagram (G-B axes).



The relay operates when the impedance seen by the relay falls within this circle. This shows that this relay is inherently directional without any additional directional unit required.



The angle ‘α’ can be adjusted to be 45o, 60o, 75o and so on. This angle is maximum torque angle.

Mho… 

The setting of 45 o is used for high voltage (33 or 11 kV) distribution lines, the setting 60o is used for 66 or 132 kV lines while the setting of 75o is used for 275 and 400kV lines.



Characteristic is directional and will operate for faults in one direction only.



Relative reach of the relay goes on changing for various ratios of R / x.

Distance Protection or 3-zone Protection 

Whenever over-current relaying is found slow or is not selective distance protection should be used. Since the fault currents depend upon the generating capacity and system configuration, the distance relays are preferred to the over current relays.



Consider the figure. which consist of two line sections AB and CD; it is desired to provide distance protection scheme.



The protection scheme is divided in three zones. Say for relay at A, the three zones are Z1a, Z2a and Z3a.



Z1a corresponds to approximately 80% -90%length of the line AB and is a high speed zone. No intentional time lag is provided for this zone.

Distance…



This unit is not to set to protect the entire line to avoid undesired tripping due to over reach. If the relay operates for a fault beyond the protected line, this phenomena is called over reach.

Distance… 

Second zone Z2a for relay at A covers remaining 20% length of the line AB and 20% of the adjoining line. In case of a fault in this section relay at A will operate when the time elapsed corresponds to the ordinate Z2a.



The main idea of second zone is to provide protection for the remaining 20% section of the line AB.



In case of an arcing fault in section AB which adds to the impedance of the line as seen by the relay at A, the adjustment is such that the relay at A will see that impedance in second zone and will operate. The operating time of second zone is normally about 0.2 to 0.5 second.

Distance… 

The third zone unit at A provides back up protection for faults in the line CD, i.e., if there is a fault in the line CD and if for some reason the relay at C fails to operate then relay at A will provide back up protection. The delay time for the third zone is usually 0.4 to 1.00 sec.



Incase the feeder is being fed from both the ends and say the fault takes place in the second zone of line AB (20% of the line AB), the relay at B will operate instantaneously (because it lies in the first zone of BA) where as the fault lies in the second zone of the relay at A.



This is undesirable from stability point of view and it is desirable to avoid this delay. This is made possible when the relay at B gives an inter trip signal to the relay at A in order to trip the breaker qickly rather than waiting for zone-2 tripping.

Applications of Distance Relays


For ground fault protection



Since the resistance of the ground is a variable quantity, a ground fault relay should be independent of the resistance. Consequently, reactance relays are normally preferred for ground fault relaying.




For phase fault protection



For short transmission lines reactance type relay is used because more of the line can be protected at high speed. This is due to the fact that a reactance relay is practically independent of the arc resistance which may be large compared with the line impedance.

Applications of Distance… 

The mho type relay is most suited for long lines where especially there are more chances of severe synchronizing power surges on the system.



The mho relay occupies the least space on an R-X diagram for a given line section and is, therefore least affected by abnormal system conditions except the line faults.



Since mho relay is most affected by arc resistance, it is used for long lines. This relay is more reliable than the other two because the relay has one set of contacts.

Applications of Distance… 

The impedance relay is less affected from synchronizing power surges as compared to reactance relay and also this relay is less affected from arc resistance as compared with the mho relay. The impedance relay is, therefore, used for protecting medium length transmission lines.

“S M L R-

R I M”

directional restrained over current relay (DROC)

I - voltage restrained over current relay (VROC) M - voltage restrained directional relay (VRDR)

Differential Protection
Introduction 

Differential protection is a method of protection in which an internal fault is identified by comparing the electrical conditions at the terminal of the electrical equipment to be protected.



"A differential relay responds to vector difference between two or more similar electrical quantities". From this definition the following 'aspects are known: 1. The differential relay has at least two actuating quantities say I1, I2, 2.The two or more actuating quantities should be similar i.e. current/current. 3. The relay responds to the vector difference between the two i.e. to I1 – I2, which includes magnitude and/or phase angle difference.

Differential… 

Differential protection is generally unit protection. The protected zone is exactly determined by location of CT's or VTs. The vector difference is achieved by suitable connections of current transformer or voltage transformer secondary's.

Differential…


Applications of Differential Protection

I.

Protection of Generator, Protection of Generator-Transformer Unit.

II.

Protection of Transformer.

III. Protection of Feeder (Transmission Line) by Pilot wire differential protection. IV. Protection of transmission Line by Phase Comparison Carrier Current Protection. V.

Protection of large motors.

VI. Bus-zone protection.

Differential…


Types of Differential Relays



A differential relay is a suitably connected over current relay which operates when the phasor difference of currents at the two ends of a protected element exceeds a predetermined value.

I.

Simple (basic) differential relay

II.

Percentage (biased) differential relay

III. Balanced (opposed) voltage differential relay

Differential…


Types of Differential Protection

I.

Simple (basic) differential Protection

II.

Percentage (biased) differential Protection

III. Balanced (opposed) voltage differential Protection

Differential…


Simple ( basic) Differential Relay or Merz-Price Protection Scheme



A simple differential relay is also called basic differential relay.



A simple differential relay is an over current relay having operating coil only which carries the phasor difference of currents at the two ends of a protected element.



It operates when the phasor of secondary currents of the CTs at the two ends of the protected element are connected together by a pilot wire circuit.



The operating coil of the over current relay is connected at the middle of the pilot wires.

Differential…


Simple ( basic) Differential Relay or Merz-Price Protection Scheme

Merz-Price … 

Behavior

of

Simple

Differential

Protection

during

Normal

Condition 

Figure illustrates the principle of simple differential protection employing a simple differential relay..



The CTs are of such a ratio that their secondary currents are equal under normal conditions or for external (through) faults.



Under normal conditions the secondary currents I1s and I2s of CT1 and CT2 respectively are equal to the secondary load current I’L.

Merz-Price… 

The secondary currents, under normal

conditions simply circulate

through the secondary windings of the two CTs and the pilot leads connecting them, and there is no current through the spill or difference circuit , where the instantaneous over current (OC) relay is connected. 

Hence, the OC relay does not operate to trip the circuit breakers (CBs). Since the currents circulate in the CT

secondary's this differential

protection scheme is called “ circulating current differential protection scheme or “ Merz-Price protection scheme. 

The boundaries of the protected zone is determined by the locations of the CTs.

Merz-Price… 

Behavior of Simple Differential Protection during External Fault

Percentage … 

I1 = current entering the protected zone = IF I2 = current leaving the protected zone = IF hence, I1 = I2 = IF and through fault current = (I1+I2)/2 = IF I1s= I2s = IF/n where n = CT ratio secondary value of through fault current = (I1s+I2s)/2 = I‘F

Merz-Price… 

Behavior of Simple Differential Protection during Internal Fault



If the differential currents (I1s-I2s) is higher than the pick-up value of the over current relay, the relay will operate and both the circuit breakers will be tripped out isolating the protected equipment from the system.

Merz-Price…


Disadvantages of Simple Differential Protection Scheme



The current transformer are connected through

cables called pilot

cables. The impedance of such pilot cable generally cause a slight difference between

the currents at the ends of the section to be

protected. A sensitive can operate to a very small difference in the two currents , though there no fault existing. 

The relay is likely to operate inaccurately with heavy through current flows. This is because the assumed identical current transformers may not have identical secondary currents due to the constructional errors and pilot cable impedances.

Merz-Price… 

Under severe through fault conditions, the current transformer may saturate

and cause unequal secondary currents. The difference

between the currents may approach the pick-up value to cause the inaccurate operation of the relay. 

Under heavy current flows, pilot cable capacitances inaccurate operation of the relay.

may cause




Percentage or Biased Differential Relay



The disadvantage of moloperation of the simple differential relay due to CT errors during heavy external (through) faults is overcome by using percentage differential relay which is also called biased differential relay.



Percentage differential relay provides high sensitivity to light internal faults with high security (high restraint) for external faults and makes differential protection scheme more reliable.



The schematic diagram of the percentage (biased) differential relay is shown in figure.



This relay has two coils. One coil is known as restraining coil or bias coil which restrains (inhibits) the operation of the relay.

Percentage … 

The another coil is the operating coil which produces the operating torque for the relay.



When the operating torque exceeds the restraining torque, the relay operates. The operating coil is connected to the mid-point of the restraining coil as shown in figure.



Nr and No are the total number of turns of the restraining coil and the operating coil respectively.



Since the restraining coil is tapped at the centre, it forms two sections with equal number of turns, Nr/2.

Percentage … 

The restraining coil is connected in the circulating current path in such a way that current I1s flows through one section of Nr/2 turns and I2s flows through the another section of Nr/2, so that the complete restraining coil of Nr turns receives the through fault current of (I1s+I2s)/2.



The operating coil, having No number of turns is connected in the difference (spill) path, so that it receives the differential (spill) current, (I1s-I2s).

Percentage … 

The operating condition of the percentage differential relay can be derived as follows:



The relay operates if the operating torque produced by the operating coil is more than the restraining torque produced by the restraining coil.



As the torque is proportional to the ampere-turns (AT), the relay will operate

when the ampere-turns of the operating coil (AT)o, will be

greater than ampere-turns of the restraining coil, (AT)r. Ampere-turns of the left-hand section of the restraining coil = (Nr/2) I1s. Ampere-turns of the right-hand section of the restraining coil = (Nr/2) I2s Total ampere-turns of the restraining coil, (AT)r = (Nr/2) (I1s+I2s)

Percentage … Thus it can be assumed that the entire Nr turns of the restraining coil



carries a current (I1s+I2s)/2. The current (I1s+I2s)/2 which is the average of the secondary currents of



the two CTs (CT1 and CT2) is known as the ‘through current’ or restraining current, Ir. Hence Ir = (I1s+I2s)/2 the ampere turns of the operating coil, (AT)o = No (I1s-I2s) Neglecting spring restraint, the relay will operate when, (AT)o > (AT)r or

No (I1s-I2s) > Nr (I1s+I2s)/2

or

(I1s-I2s) > (Nr /No) (I1s+I2s)/2 Id = K Ir

where, Id = (I1s-I2s) is the differential current through the operating coil. Hence it is also called the differential operating current. Ir = (I1s+I2s)/2 is the restraining current or through current, and

K = Nr/No = slope or bias

K ( slope or bias) is generally expressed as a percentage value. The relay will be on the verge of operation when: (I1s-I2s) = (Nr/No) (I1s+I2s)/2 or

Id = K Ir (K or slope = 10%, 20%, 30% and 40% ) the slope (K) of the relay determines the trip zone.

Percentage … 

Thus, at the threshold of operation of the relay, the ratio of the differential operating current (Id) to the restraining current (Ir) is a fixed percentage; and for operation of the relay the differential operating current must be greater than this fixed percentage of the restraining (through fault) current. Hence this relay is called ‘percentage differential’ relay. The percentage differential relay is also known as ‘biased differential relay’ .



Under normal and external fault conditions, the restraining torque produced by the restraining coil is greater than the operating torque produced by the operating coil, hence the relay is inoperative. When the internal fault occurs, the operating torque becomes more than the restraining torque and the relay operates.

Percentage … 

The percentage differential relay does not have a fixed pick-up value. The relay automatically adapts its pick-up value to the

restraining

(through) current. 

As the restraining current goes on increasing, the restraining torque also increases and the relay is prevented from maloperation.

Percentage …


Characteristics



It can be seen that

except at low currents the characteristics is a

straight line. Thus the ratio of the differential operating current to the average restraining current

is a fixed percentage. Hence the relay

name is percentage differential relay.

Percentage …


Stability Characteristics



It can be seen that

except at low currents the characteristics is a

straight line. Thus the ratio of the differential operating current to the average restraining current

is a fixed percentage. Hence the relay

name is percentage differential relay.




Stability Characteristics



The point of intersection of the relay and internal fault characteristics gives the minimum internal

Percentage … fault

current required for the correct

operation of the relay (IF min). 

The point of intersection of the relay and external fault characteristics gives the maximum external fault current ( IF max) is known as “through fault stability limit”.



Stability ratio (s) = IF max /IF min



The immunity of the percentage differential relay to moloperation on external (through) fault can be increased by increasing the slope of the characteristics. Hence it is universally used to protect generators and transformers.




Settings of Percentage Differential Relay

i.

Basic setting or sensitivity setting



% basic setting is the ratio of Minimum current in operating coil only to cause operation (bias is zero) to the rated current of the operating coil .



10% - 20 % for generators and 20% for transformers

ii.

Bias setting (k)



It is the ratio of the number of turns in the restraining coil (Nr) to the turns in the operating coil (No). Or

( I 1s − I 2 s ) N × 100 r % Bias (k) = × 100 = ( I 1s + I 2 s ) / 2 No 

10% for generators and 20%-40% for transformers.




Balanced (Opposed) Voltage Differential Protection



In this case, the secondary's of the CTs (CT1 and CT2) are connected in such a way that under normal operating conditions and during external faults, the secondary currents of CTs on two sides oppose each other and their voltages are balanced. Hence no current flows in pilot wires and relays.



During internal fault , however, a differential current proportional to (I1I2) in case of single end fed system and proportional to (I1+I2) in case of double-end-fed system flows through the relay coils.



If the differential current flowing through the relay coils is higher than the pick-up value, the relays operate to isolate the protected equipment from the system.



Balanced … 

Since no current flows through the secondary's of CTs under normal operating conditions, the whole of the primary ampere-turns are used in exciting the CTs.



This creates large flux causing saturation of CTs and inducing high over voltages which can damage the insulation of CT secondary's.



For this reason, the CTs used in such protective scheme are air-core type so that the core does not get saturated and overvoltage's are not induced during zero secondary current under normal operating mode.

Feeder Protection 

There are several methods of protection of transmission lines. The first group of non-unit type of protection which includes

1)

Over Current (non-directional ) a. Time Graded over current protection b. Current Graded over current protection

2)

Distance protection (directional) a. Pilot wire differential protection (i) circulating current differential relaying (ii) Transley scheme b. Carrier Current Protection



Such non-unit type protections do not have pilots. The discrimination is obtained by coordinating the relays settings. Fuses are used in distribution systems, where relays and circuit breakers are not necessary and fuses are preferable due to their low cost, current limiting features etc.



The other group of protection of line is unit-type of protection such as pilot wire differential protection, carrier current protection based on phase comparison method; etc.

Type of Protection 1. Over current Protection a. Time graded or current graded b. Directional or non-directional

c. Earth-fault Protection

Remarks Applied as main protection for distribution lines and back-up for main lines, where main protection is of distance or other faster type. Inverse definite minimum time relays preferred for time graded systems. Instantaneous relays for current graded systems. Separate earth-fault protection is necessary in addition to phase fault protection. Type of earthing and magnitude of earth fault current should be considered.

2. Distance protection a. Pilot wire differential protection

For important lines of relatively shorter length (a few tens of km).

b. Carrier Current Protection

Where length of transmission line is long and simultaneous opening of circuit-breakers at both ends is necessary.




Pilot

Wire Protection using Circulating Current Differential

Relaying



The differential circulating current protection principle can be readily applied to feeder protection. Two CT's are connected in each protected line, one at each end. Under healthy/external fault conditions the secondary currents are equal and circulate in pilot wires.



The relay is connected between equipotential points of pilot wires. For external faults and normal condition the differential current I1-I2 of two CT's is zero and relay does not operate.



During internal faults this balance is distributed and differential current flows through the relay operating coils.


Pilot Wire Protection Using Transley Scheme 

This scheme is a balanced voltage scheme with the addition of a directional feature. Figure shows the schematic arrangement of the scheme.



An induction disc type relay is used at each end of the protected line section.



The secondary windings of the relays are interconnected in opposition as a balance voltage system by pilot wires.



The upper magnet of the relay carries a summation winding to receive the output of current transformers.



Under normal conditions and in case of circulates through the pilot wires electromagnets of the relay.

external faults, no current

and hence through the lower

Transley… 

In these conditions no operating torque is produced. In case of internal faults, current

flows

through

the pilot wire and the lower

electromagnets of the relay. 

In this condition the relay torque is produced from the interaction of the two fluxes, one of which is produced directly from the local CT secondary current flowing through the upper magnet of the relay.



The second flux is produced by the current flowing through the lower magnet. The current flowing through the lower magnet is relatively small. Therefore, this scheme is suitable for fairly long pilots having loop resistance up to 1000 ohm.

Transley…

Figure. Transley Scheme




Limitations of Pilot Wire Protection of Line.



Pilot wire protection needs additional expenditure of Pilot wires, the Pilot wires need supervision to check. Open circuits and short circuits on Pilot wires lead to relay failure.



The Pilot wires are put at the same time along with power conductors. In cable systems, Pilot cables are put in the same trench of power cable.



Voltages are induced in pilot wires due to the field of power conductors: This voltage should be limited to 5-15 volts.



Overhead Pilot wires are exposed to lightening and high voltage surges. They must be protected by means of lightning arresters. Similarly they should not come in contact with power circuit.



According to the rules the voltage across Pilot is limit to about 200V and current to 200mA.



For short lines of less than 16 km the Pilot wires give most economical form of high speed relaying. For lines up to 16 km Pilot wire protection is popular, It used even for lines up to 50 km .in rare cases.



Beyond the length of 16 km. carrier current Pilot relaying is more economical and preferable.


Carrier Current Protection 

This is most widely used scheme for the protection of EHV and UHV power lines.



In this scheme a carrier channel at high frequency is employed. The carrier signal is directly coupled to the same high voltage line that is to be protected.



The frequency range of the carrier signal is 59kHz to 700kHz. Below this range, the size and the cost of coupling equipment becomes high whereas above this range, signal attenuation and transmission loss is considerable. The power level is about 10-20 W.



In this scheme the conductor of the power line to be protected are used for the transmission of carrier signals. So the pilot is termed as a power line carrier.



The main disadvantage of conventional time-stepped distance protection is that the circuit breakers at both ends of the line do not trip simultaneously when a fault occurs at one of the end zones of the protected line section. This may cause instability in the system.



Where high voltage auto-reclosing is employed, non-simultaneous opening of the circuit breakers at both ends of the faulted section does not provide sufficient time for the de-ionization of gases.



In a carrier current scheme, the carrier signal can be used either to prevent or initiate the tripping of a protective relay. When a carrier signal is used to prevent the operation of the relay, the scheme is known as carrier–blocking scheme. When the carrier signal is employed to initiate tripping, the scheme is called a carrier inter tripping or transfer tripping or permissive tripping scheme.

1.

Coupling capacitor



The carrier equipment is connected to the transmission line through 'Coupling Capacitor' which is of such a capacitance that it offers low reactance (1/wc) to carrier frequency but high reactance power frequency.



Thus coupling capacitors allows carrier frequency signals to enter the carrier equipment but does not allow 50 Hz power frequency currents to enter the carder equipment.



To reduce impedance further a low inductance is connected in series with coupling capacitors to form a resonance at carrier frequency.

2.

Line Trap Unit



Line trap unit is inserted between bus bar and connection of coupling capacitor to the line.



It is a parallel tuned comprising L and C. It has a low impedance (less than 0.1 ohm) to 50 Hz and high impedance to carrier frequencies.



This unit prevents the high frequency signals from entering the neighboring line, and the carrier currents flow only in the protected line.

3.

Protection and Earthing of Coupling Equipment



Over voltages on power lines are caused by lightning, switching, faults, etc. produce stress on coupling equipment and line trap‘ unit.



Non-linear resistors in series with a protective gap is connected across the line trap unit and inductor of the coupling unit. The gap is adjusted to spark at a set value of overvoltage.

4.

Electronic Equipment There are generally identical units at each end: (i) Transmitter unit. (ii) Receiver unit (iii) Relay unit.

(i)

Transmitter unit.



Frequencies between 50 to 500 kHz are employed in different frequency bands. Each band has certain band width (say 150-300kHz, 90-115kHz).

(ii) Receiver unit. 

The high frequency signals arriving from remote end are received by Receiver. The receivers, the signal sand feeds to carrier receiving relay unit



Receiving unit comprise. - An attenuator, which reduces the signals to a safer value. - Band pass filter, which restricts the acceptance of unwanted signals (signals from adjacent sections, spurious signals.) - Matching transformer or matching element to match the impedances of line and receiving unit.




Applications of Carrier Current Protection Scheme



Pilot channel such are carrier current over the power line provides simultaneous tripping of circuit-breakers at both the ends of the line in one to three cycles. Thereby high speed fault clearing is obtained, which improves the stability of the power system.

1.

Fast, simultaneous operating of circuit-breakers at both ends.

2.

Auto-reclosing

simultaneous

reclosing

signal

is

sent

thereby

simultaneous (1 to 3 cycles) reclosing of circuit-breaker is obtained. 3.

Fast clearing prevents shocks to systems.

4. Tripping due to synchronizing power surges does not occur, yet during internal fault clearing is obtained. 5. For simultaneous faults, carries current protection provides easy discrimination. 6. Fast (2 cycle) and auto-reclosing circuit-breakers such as air blast circuit-breakers require faster relaying. Hence, the carrier current relaying is best suited for fast relaying in conjunction with modern fast circuit-breakers

7. Other uses of carrier equipment. The carrier current equipment is used for several other applications besides protection. These are : (a) Station to station communication. In power station, receiving stations and sub-stations telephones are provided. These are connected to carrier current equipment and conversion can be carried out by means of "Current Carrier Communication". (b) Control. Remote control of power station equipment by carrier signals. (c) Telemetering

Bus zone/Busbar Protection


Differential Current Protection

•

Figure shows a scheme of differential current protection of bus zone.

•

The operating principle is based on Kirchhoff’s law. The algebraic sum of all the currents entering and leaving the bus zone must be zero, unless there is a fault therein.

•

The relay is connected to trip all the circuit breakers. In case of a bus fault

the algebraic sum of currents will not be zero and relay will

operate. •

The main drawback of this scheme is that there may be false operation in case of an external fault . (saturation of CTs)

Figure. Differential current protection of bus-zone




High Impedance Relay Scheme

•

Figure shows a scheme of differential protection using high impedance relay.

•

A sensitive dc polarized relay is used in series with a tuning circuit which makes the relay responsive only to the fundamental component of the differential current of the CTs.

•

The tuning circuit makes the relay insensitive to dc and harmonics there by making it more stable on heavy external faults.

•

To prevent

excessive voltages on internal faults

a non-linear

resistance(thyrite) and a high set over current relay connected in series with the non-linear resistance are employed. The high set relay provides fast operation on heavy faults.

Bus zone/Bus bar Protection

Figure. Differential protection using high impedance relay

Rotating Machines Protection


Generator Protection 

Generator differential protection



Stator earth fault protection



Negative phase sequence protection



Against unbalanced loading



Inter-turn fault protection



Reverse power protection



Rotor earth fault protection



Temperature sensor in slots



Over current relay in stator and rotor circuit



Surge arrestor for surge over voltages



R-C surge suppressors

Rotating Machines Protection 

Rotating machines include synchronous generators, synchronous motors, synchronous condensers and induction motors.


Protection of Generators (i) Stator protection (a) Stator windings by percentage differential protection (b) Restricted earth-fault protection by differential system (c) Protection against stator inter-turn faults

Generator… (ii) Rotor protection (a) Field ground-fault protection (b) Rotor temperature alarm (c) Protection against rotor over heating because of un balanced threephase stator currents (or) Negative sequence protection (d) loss of excitation protection

(iii) Miscellaneous a.

over voltage protection

b.

Over speed protection

c.

Protection against motoring

d.

Protection against vibration and distortion of rotor

e.

Bearing over heating protection

f.

Protection against auxiliary failure

g.

Protection against voltage regulator failure

h.

Protection against pole slipping

i.

Field suppression

j.

Back-up protection

stator… 1. Stator Protection (a) Percentage Differential Protection

stator…

Stator…


Principle of Operation



The differential protection is that which responds to the vector difference between two or more similar electrical quantities.



In generator protection, the current transformers are provided at each end of the generator armature windings.



When there is no fault in the windings and for through faults, the currents in the pilot wires fed from CT connections are equal. The differential current I1-I2 is zero.

Stator… 

When fault occurs inside the protected winding, the balance is disturbed and the differential current I1-I2 flows through the operating coil of relays causing relay operation.



Thereby the generator circuit-breaker is tripped. The field is disconnected and discharged through a suitable impedance.

Stator…


Construction



The percentage differential relay has an operating coil and a restraining coil, one for each phase.



The restraining coil is connected centrally in pilot wires. The operating coil is connected between mid-point of restrains coil neutral pilot wire.



Differential relay provides fast protection to the stator winding against phase to phase faults and phase to ground faults.



If neutral is not grounded or is grounded through impedance, additional sensitive ground fault relaying should be provided.

Stator…


Advantages



high speed operation, about 15 ms. with static protection



low setting



full stability on external faults.



The biasing of the differential relay eliminates the problems associated with CT's.

Stator…


Applications



Differential protection which protects only generator is arranged to trip main circuit breaker and to suppress the field.



Differential protection does not respond to through faults and overloads.



Differential protection gives a complete protection to generator windings against phase to phase faults.



Differential protection is recommended for generators above 2 MVA rating.

Stator… (b) Restricted earth fault protection by differential scheme

stator… 

When neutral is solidly grounded, it is possible to protect complete alternator of transformer winding against phase to ground fault.



However, neutral is earthed through resistance to limit earth-fault currents. With resistance earthing, it is not possible to protect complete winding from earth-fault and the % of winding protected depends on the value of neutral earthing resistor and the relay setting.



While selecting the value of resistor and earth fault relay setting, the following aspects should be kept in mind :

i.

The current rating of resistor, resistance value, relay setting, etc. should be selected carefully.

ii.

Setting should be such that the protection does not operate for earthfaults on EHV side.

stator… 

Earth faults are not likely to occur near the neutral point due to less voltage w.r.t. earth. It is a usual practice to protect about 80 to 85% of generator winding against earth-faults. The remaining 20 to 15% winding from neutral side left un-protected by the differential protection.



In additional to differential protection, a separate earth-fault protection is provided to take care of the complete winding against earth faults.

stator…


Operation



During earth fault If in the alternator winding, the current, If flows through a part of the generator winding and neutral to ground circuit.



The corresponding secondary current Is flows through the operating coil and restricted earth-fault coil of the differential protection. The setting of the restricted earth fault relay can be selected independent of the setting of the over current rely.



If the earth-fault If occurs at point ‘f’ of alternator winding Vaf is available to drive earth-fault current ‘If‘ through the neutral to ground connection.



If point is nearer to terminal a (nearer to the neutral point) the forcing voltage Vaf will be relatively less. Hence earth fault current reduce.

If will

Stator… 

It is not practicable to keep the relay setting too sensitive to sense the earth-fault currents of small magnitudes. Because, if too sensitive, the relay may respond during through faults of other faults due to inaccuracies of CT's, saturation of CT's etc.



Hence a practice is to protect about 85% of the generator winding against phase to earth fault and to leave the 15% portion 'unprotected by the differential protection against earth-faults.



A separate earth-fault protection covers the entire winding against earth-faults.

Stator…

Stator… 

R=V/I where R = impedance in ohms between neutral and ground V = line to neutral voltage I= full load current of largest machine or transformer



% of winding un protected =

R × I o ×100 V

where R =ohmic value of impedance IO = minimum operating current in primary of CT V = line to neutral voltage

Stator… (c) Protection against Stator Interturn Faults 

The inter-turn faults are detected by measuring the residual voltage of generator terminals.



This voltage appears across the tertiary winding which is connected to operating winding of a three element directional relay.



The quadratic winding is operated from secondary side of the voltage transformer.



During normal condition, the residual voltage is zero, i.e., VRES = VRN + VYN + VBN = 0



This balance is disturbed during inter-turn fault on any of the single windings. And the residual voltage is fed to the relay coil.

Stator…

rotor… 2. Rotor Protection (a) Field Ground-fault Protection 

As the field circuit is operated ungrounded, a single ground fault does not affect the operation of the generator or cause any damage. However, a single rotor fault to earth increases the stress to the ground in the field when stator transients induce an extra voltage in the field winding.



In case a second ground fault occurs, a part of the field winding is bypassed, thereby

increasing the current through

portion of the field winding.

the remaining

rotor… 

This causes an unbalance in the air gap fluxes, thereby creating an unbalance in the magnetic forces on opposite sides of the rotor.



The unbalancing in magnetic forces makes the rotor shaft eccentric. This also causes vibrations.



A high resistance is connected across the rotor circuit. The centre point of this is connected to earth through a sensitive relay. The relay detects the earth faults for most of the rotor circuit (Fig.) except the centre point of rotor.

rotor… 

Other methods of rotor earth fault protection include d.c. injection method and a.c. injection method, (Fig.). A single earth fault in the rotor circuit completes the circuit comprising voltage Source S, sensitive relay earth fault. Thereby the earth fault is sensed by the voltage relay. D.C. injection method is simple and has no problems of leakage currents.

rotor…

rotor… (b) Rotor Temperature Alarm 

This protection is employed only to large sets and indicates the level of temperature and not the actual hot spot temperature. It is not practicable to embed thermocouples in rotor winding since the slip ring connections would be complicated. Resistance measurement is adopted.



The rotor voltage and current are compared by a moving coil relay. The voltage coil of the relay is connected across the slip ring brushes. The current coil is connected across the shunt in the field circuit.



Double actuating quantity moving coil relay is used, the restraining coil being circuit coil and the operating coil is the voltage coil (Fig.). Resistance increases with temperature.

rotor…

The relay measures the ratio V/I == R (which gives a measure of rotor temperature).

rotor… (c) Protection against Rotor Overheating because of Unbalanced Three-phase Stator Currents (or) Negative Sequence Protection 

The negative sequence component of unbalanced stator currents cause double frequency current to be induced in the rotor iron. If this component become high severe overheating of the rotor may be caused.



The unbalanced condition may arise due to the following reasons: (i). When a fault occurs in the stator winding (ii). An unbalanced external fault which is not cleared quickly (iii). Open-circuiting of a phase (iv). Failure of one contact of the circuit breaker

rotor… 

The time for which the rotor can be allowed to withstand such a condition is related by the expression. I22t = K where I2 = negative sequence component of the current t = time K = constant ; 7 for turbo generator with direct cooling 60 for a salient pole hydro generator



Figure shows a protective scheme using a negative filter and relay.

rotor… 

The over current relay used in the negative phase sequence protection has a long operating time with a facility of range setting to permit its characteristic to be matched to I22t characteristics of the machine.



A typical time range of the relay is 0.2 to 2000s.



It has shaded pole construction with a Mu-metal shunt. The negative sequence filter

gives an output proportional to I2. it actuates an alarm

as well as the time current relay which has a very inverse characteristic.

rotor…

(d) Loss of field Protection 

A 'loss of field' or 'field failure' can be caused by opening of field switch or field circuit-breaker. The behavior of the generator depends upon whether the generator connected singly to a load or whether the generator is connected in parallel with other units or the system.



If it is a single unit supplying a local load, the loss of field causes loss of terminal voltage and subsequently loss of synchronism depending upon the load conditions.

rotor… 

If the generator is connected in parallel with other units it can draw the magnetizing currents from the bus-bars and continue to runs as induction generator.



The power-output of the generator is reduced while running as induction generator. The slip frequency e.m.f. is induced in the rotor.



The stator currents may increase above normal current rating of generator during the run as induction generator. High currents may cause voltage drop and overheating of generator bus-bars; stator winding, etc.



Fig. illustrating the loss of field protection by means of an under-current relay connected across a shunt in series with the field winding.

Protection of Transformers


Types of Faults Encountered in Transformer (i) over heating (ii) winding faults (a) phase to phase faults (b) earth faults (c) inter turn fault (iii) open circuits (iv) through faults (v) over fluxing

Protection of Transformers…


(i) over heating



Over loads, short circuits and failure of cooling system are causes of over heating.



Over heating depends on type of transformer and class of insulation .



Generally thermal over load relays and temperature relays sounding the alarm are used to provide protection against over heating. Similarly temperature indicators are also provided.

Protection of Transformers…


(ii) winding faults



Over heating and mechanical shocks cause to deteriorate the winding insulation, which leads to short circuit between phases or between the phase and ground and also short circuit between adjacent turns of same phase.



The differential protection scheme is generally used for such type faults of above 5 MVA rating of transformers. For below 5 MVA rating over current protection is used.




(iii) Open circuit faults



The open circuit in one of the three phases is open and it is dangerous as it causes the undesirable heating of the transformer.

Protection of Transformers…


(iv) Through faults



Through

faults are the external faults which occurs out side the

protected zone. 

Through faults are not detected by the differential protection.



The over current relays with under voltage blocking, zero sequence protection and negative sequence protection are used to give protection against through faults.

Protection of Transformers…


(v) Over fluxing



The flux density in the transformer core is proportional to the ratio of the voltage to frequency i.e., V/f.



The V/f relay called volts/hertz relay is provided to give the protection against over fluxing operation.




Other faults



Tap changer faults



High voltage surges due to lightening and switching, incipient faults also occur in transformer.



The Buchholz relay is used for oil immersed transformers to give the protection against incipient faults .

Protection of Transformers…


The following information is necessary while selecting the protection scheme for a power transformer.

1. Particulars of transformer (a) kVA (b) Voltage ratio (c) Connections of windings (d) Percentage reactance (e) Neutral point earthing, value of resistance (f) Value of system earthing resistance (g) Whether indoor or outdoor, dry or oil filled (h) With or without conservator.

Protection of Transformers… 2. Length and cross-section of connecting leads between CT's and relay panel. 3. Fault level at power transformer terminals. 4. Network diagram showing position of transformer, load characteristics.


Types Protection of Transformers

1)

Percentage differential protection (a) Star-delta

(b) Star-Star

2)

Protection against Magnetizing Inrush current

3)

Buchholz Relay

1) Percentage differential Protection of Transformers 

Percentage differential protection is used for the protection of large power transformers having rating of 5 MVA and above.



This scheme is employed for the protection of transformers against internal short circuits.



The following table gives the way of connecting C.T secondary's for the various types of power transformer connection. Power Transformer connections

C.T connections

primary

secondary

primary

secondary

Star

delta

delta

star

delta

delta

star

star

star

star

delta

delta

delta

star

star

delta

Protection of Transformers… 

For the sake of understanding, the connection of the C.T secondary's in delta for star side transformer and the connection of C.T secondary’s in star for delta side of power transformer is shown in the fig. a and b

(a) Merz prize or Percentage Differential Protection for stardelta Transformer 

The primary of the power transformer star connected

while the

secondary is delta connected. Hence to compensate the phase difference the C.T secondary’s on the primary side must be connected in delta while C.T secondary’s on delta side must be connected in star. 

The restraining coils are connected across the C.T secondary winding while the operating coils are connected between the tapping points on the restraining coils and star point of C.T secondary’s.



It is important to note that this scheme gives protection against short circuit faults between the turns i.e. inter turn faults also.

Fig. Merz prize Protection for star-delta Transformer

(b) Merz prize Protection for star-star Transformer 

Both primary and secondary of the power transformer are connected in star and hence C.T. Secondary's on both the sides are connected in delta to compensate for the phase displacement.



The star points of both the windings of the power transformer are grounded. The restraining coils are connected in the C.T secondary's.



The operating coils are connected between the tapings on the restraining coil and the ground.

Fig. Merz prize Protection for star-star Transformer

2. Protection against Magnetizing Inrush Current 

When an unloaded transformer is switched on, it draws a large initial magnetizing current which may be several times the rated current of the transformer. This initial magnetizing current is called the magnetizing inrush current.



As the inrush current flows only in the primary winding, the differential protection will

see this inrush current

as an internal fault. The

harmonic contents in the inrush current are is different than those in usual fault current. 

As the second harmonic is more in the inrush current than in the fault current, this feature can be utilized to distinguish between a fault and magnetizing inrush current.

Magnetizing Inrush… 

Figure shows a high speed biased differential scheme incorporating a harmonic restraint feature.



The

relay of this scheme is made insensitive to magnetic inrush

current. The operating principle is to filter out the harmonic from the differential current, rectify them and add them to the percentage restraint. 

The tuned circuit Xc XL allows only current of fundamental frequency to flow through the operating coil.



The dc and harmonics, mostly second harmonics in case of magnetic inrush current, are diverted into the restraining coil. The relay is adjusted so as not to operate when the second harmonic (restraining) exceeds 15% of the fundamental current (operating).

Magnetizing Inrush…

Magnetizing Inrush… 

The minimum operating time is about 2 cycles.



The dc offset and harmonics are

also present in the fault current,

particularly if CT saturates. The harmonic restraint relay will fail to operate on the occurrence of

an internal fault which contains

considerable harmonics due to an arc or saturation of the CT. 

To overcome this difficulty, an instantaneous overcorrect relay (the high set unit) is also incorporated in the harmonic restraint scheme.



This relay is set above the maximum inrush current. It will operate on heavy internal faults in less than one cycle.

3) 

Buchholz Relay Buchholz relay is a gas operated relay used for the protection of oil immersed transformers against all type of internal faults.



It uses the principle that due to the faults, oil in the tank decomposes, generating the gases. The 70% component of such gases is hydrogen which is light and hence rises upwards towards conservator through the pipe.



Buchholz relay is connected in the pipe as shown in the figure. due to the gas collected in the upper portion of the Buchholz relay, the relay operate and gives an alarm.

Buchholz Relay…

Figure. basic arrangement of Buchholz relay

Buchholz Relay…

Figure. construction of Buchholz relay

Buchholz Relay…


Operation



Internal faults like, insulation fault, core heating, bad switch contact, faulty joints etc.



When the fault occurs the decomposition of oil in the main tank starts due to which the gases are generated. The hydrogen tries to rise up towards conservator but in its path it gets accumulated in the upper part of the Buchholz relay. Through passage of the gas is prevented by the flap valve.



When gas gets accumulated in the upper part of housing , the oil level inside the housing falls. Due to which the hallow float tilts and close the contacts of the mercury switch attached to it. This completes the alarm circuit to sound an alarm.

Buchholz Relay…


Due to this operator knows that there is some incipient fault in the transformer.




The testing results give the indication, what type of fault is started developing in the transformer. Hence transformer can be disconnected before fault grows into a serious one.




The alarm circuit does not immediately disconnects the transformer but gives only indication to the operator. This is because some times bubbles in the oil circulating system may operate the alarm circuit through actually there is no fault.




The connecting pipe between the tank and conservator should be as tight as possible and should slope upwards conservator at small angle from the horizontal . This angle should be between 10 to 11o

Buchholz Relay…


Advantages

1.

Normally a protective relay does not indicate the appearance of the fault. It operates when fault occurs. But Buchholz relay

give an

indication of the fault at very early stage, by anticipating the fault and operating the alarm circuit. 2.

It is the simplest protection of the transformers.

Buchholz Relay…


Limitations

1.

Can be used only for oil immersed transformers having conservator tanks.

2.

Only faults below oil level are detected.

3.

Setting of the mercury switches can not be kept too sensitive otherwise the relay can operate due to bubbles, vibration, earth quakes and mechanical shocks etc.

Buchholz Relay…


Applications

1.

Local over heating

2.

Entrance of air bubbles in oil

3.

Core bolt insulation failure

4.

Short circuited laminations

5.

Loss of oil and reduction in oil level due to leakage

6.

Bad and loose electrical contacts

7.

Short circuit between phases

8.

Winding short circuit

9.

Bushing puncture

10. Winding earth fault