Protective Relays | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE) PDF Download

Protective Relays

BASIC RELAY TERMINOLOGY

Relay 

• A relay is an automatic device by means of which an electrical circuit is indirectly controlled (opened or closed) and is governed by a change in the same or another electrical circuit

Operating Force or Torque 

• A force or torque which tends to close the contacts of the relay.

. Restraining Force or Torque 

• A force or torque which opposes the operating force or torque

Pick-up (level) 

• The threshold value of the actuating quantity (current, voltage etc.) above which the relay operates.

Reset or Drop-Out (level) 

• The maximum value of the actuating quantity below which contacts are opened is called the reset or drop-out value.

Operating Time 

• It is the time which elapses from the instant at which the actuating quantity exceeds the relays pick-up value to the instant at which the relay closes its contacts.

Reset time 

• It is the time which elapses from the moment the actuating quantity falls below its reset value to the instant when the relay comes back to its normal (initial) position.

Setting 

• The value of the actuating quantity at which the relay is set to operate.

Back-up Relay 

• A back-up relay operates after a slight delay, if the main relay fails to operate.

Back-up Protection 

• The back-up protection is designed to clear the fault if the primary protection fails. It acts as a second line of defence.

Primary Protection 

• If a fault occurs, it is the duty of the primary protective scheme to clear the fault. It acts as a first line of defence.

Undervoltage Relay 

• A relay which operates when the system voltages falls below a certain preset value.

Directional or Reverse Power Relay 

• A directional relay is able to detect whether the point of fault lies in the forward or reverse direction with respect to the relay location.  It is able to sense the direction of power flow, i.e., whether the power is flowing in the normal direction or the reverse direction.

Time-Lag Relay 

• A time-lag relay operates after a certain preset time lag. 
• These relays are used in protection schemes as a means of time discrimination.

Moving Iron Relay 

• This is dc polarized, moving iron type relay. There is an electromagnet, permanent magnet and a moving armature in its construction.

Thermal Relay 

• This relay utilizes the electrothermal effect of the actuating current for its operation.

Conduction Relay 

• This is MHO relay whose diameter (passing through the origin) lies on the R-axis.

Offset Mho Relay 

• In an offset mho relay, the mho characteristic is shifted on the R-X diagram to include the origin.

Angle Impedance Relay 

• The characteristic of this relay on the R-X diagram is a straight line passing at an angle and cutting both the axes. It is kind of a distance relay and is also called on Ohm relay.

Earth Fault Relay 

• A relay used for the protection of an element of a power system against earth faults is known as an earth fault relay.

 Phase Fault Relay 

• A relay used for the protection of an element of a power system against phase faults is called a phase fault relay

Negative Sequence Relay 

• A relay for which the actuating quantity is the negative sequence current 
• This type of a relay is used to protect electrical machines against overheating due to unbalanced currents.

Zero Sequence Relay 

• A relay for which the actuating quantity is the zero sequence current. This type of a relay is used for earth fault protection.

Starting Relay or Fault Detector 

• This is a relay which detects abnormal conditions and initiates the operation of other elements of the protective scheme.

Overreach 

• Sometimes a relay may operate even when a fault point is beyond its present reach.

Burden 

• The power consumed by the relay circuitry at the rated current is known as its burden.

Restricted Earth Fault Protection 

• It refer es to the differ ential prote ction of transformers or alternators against ground faults. 
• It is called restricted because its zone of protection is restricted only to the winding of the alternator or transformer. 
• It does not respond to faults beyond its zone of protection.

 Residual Current 

• It is the algebraic sum of all currents in a multi phase system. It is denoted by Ires. 
• In a 3-phase system Ires = I_a + I_b + I_c.

PROTECTIVE RELAYS

• A protective relay is an automatic device which detects an abnormal conditions in an electrical circuit and causes a circuit breaker to isolate the faulty element of the system.

Functional Characteristics A protective relay is required to satisfy four basic functional characteristics.

Reliability 

• A protective relay must operate reliably when a fault occurs. The reliability of a protective relay should be very high, a typical value being 95%.

Sensitivity

• A protective relay should be sensitive enough to operate when the magnitude of the actuating quantity exceeds its pick-up values.

Stability 

• This is the ability of the protective system to remain inoperative under all load conditions, and also in case of external faults.
• The relay should remain stable when a heavy current due to an external fault is flowing through it.

Speed 

• A protective relay should neither be too slow which may result in damage to the equipment, nor should it be too fast which may result in undesired operation during transient faults.

ELECTROMAGNETIC RELAYS

Attracted Armature Relay

• The coil is energised by an operating quantity proportional to the system current or voltage. 
• The operating quantity produces a magnetic flux which in turn produces an electromagnetic force.
  The electromagnetic force is proportional to the square of the flux in the air gap or the square of the current. 
• This type of relay is used for the protection of small machines, equipment etc.
• It is also used for auxiliary relays, such as indicating flags, slave relays, alarm relays etc. 
• The actuating quantity of the relay may be either ac or dc. 
• In dc relays, the electromagnetic force of attraction is constant. 
• In the case of ac relays, sinusoidal current flows through the coil and hence the force of attraction.
  
• The total force is a double frequency pulsating force.
• This may cause the armature to vibrate at double the frequency. Consequently, the relay produces a humming sound and becmes noisy. 
• This difficulty can be overcome by making the pole of the electromagnet of shaded construction. 
• The reset to pick-up ratio for attracted armature type relays is 0.5 to 0.9. For this type of a relay, the ratio for ac relays is higher as compared to dc relays.

Advantages 

• It has low VA burden. 
• It is an instantaneous relay. 
• It has very high operating speed.

Induction Disc Relay

• The rotating disc is made of aluminium. 
• One half of each pole of the electromagnet is surrounded by a copper band known as the shading ring. 
• The shaded portion of the pole produces a flux which is displaced in space and time with respect to the flux produced by the unshaded portion of the pole.
•  The two alternating fluxed displaced in space and time cut the disc and produce eddy currents in it. 
• Torques are produced by the interaction of each flux with the eddy current produced by the other flux. The resultant torque causes the disc to rotate. 
• It is used for overcurrent protection. 
• Disc type units give an inverse time current characteristic and a re slow compared to the induction cup and attracted armature type relays. 
• Used for slow-speed relays. 
• The torque is proportional to the square of the actuating current if single actuating quantity is used. 
• A permanent magnet of high coercive force is employed to produce eddy current braking to the disc. The magnets should remain stable with age so that its accuracy will not be affected. 
• The braking torque is proportional to the speed of the disc. When the operating current exceeds pick-up value, driving torque is produced and the disc accelerates to a speed where the braking torque balances the dirving torque. 
• The disc inertia should be as small as possible, so that it should stop rotating as soon as the fault current disappears. 
• At a current below pick-up value, the disc remains stationary by the tension of the control spring acting against the nromal direction of disc rotation.
   

Induction of Cup Relay

• It possesses high sensitivity, high speed and produces a steady non-vibrating torque. 
• Its operating time is to the order of 0.01 second. Thus with its high torque/inertia ratio, it is quite suitable for higher speeds of operating. 
• It is less sensitive to dc transients.
• Induction cup type relays were widely used for distance and directional relays. 
• The theory given below is true for both disc type and cup type induction relays. 
• Figure 2.7 shows how force is produced in a rotor which is cut by f₁ and f₂. These fluxes are alternatig quantities and can be expressed as follows

Where q is the phase difference between f₁ and f₂. The flux f₂ leads f₁ by q. 

Voltages induced in the rotor

• The induced eddy currents in the rotor are in phase with their voltages.

•  The forces produced

F = Kf₁ f₂ sinq. In this expression, f₁ and f₂ are rms values. 

• If the same current produces f₁ and f₂ the force produced is given by F = KI₂ sin q 
• A stationary iron core is placed inside the rotating cup to decrease the air gap without increasing. 
• The spindle of the cup carries an arm which closes contacts. 
• A spring is employed to provide a resetting torque. 
• When two actuating quantities are applied, one may produce an operating troque while the other may produce restraining torque.

Protective Relays

Moving Coil Relay This type of a relay has a permanent magnet and a moving coil. It is also called a permaent magnet d.c. moving coil relay. The actuating current flows in the moving coil.

• Damping is provided by an aluminium former. The operating time is about 2 cycles. 
• The operating torque is produced flowing to the interating torque is produced flowing to the interaction between the field of the permanent magnet and that of the coil. The operating torque is proportional to the current carried by the coil. 
• The torque exerted by the spring is proportional to deflection. The relay has on inverse operating time current characteristic. 
• It responds to only dc actuating quantities. 
• Moving coil relays are most sensitive type electromagnetic relays. 

The universal relay torque equation 

T = K₁I₂ + K₂V₂ + K₃VI cos (q – t) + K

• By assigning plus or minus signs to some of the terms and letting others be zero and sometimes adding some terms having a combination of voltage and current, the operating characteristics of all types of relays can be obtained.

DISTANCE RELAYS

A relay which measures impedance or a component of the impedance at the relay location is known as distance relay 

• It is used for the protection of a transmission line. 
• These are mainly classified as :
  • Impedance relays
  • Reactance relays
  • Mho relays.

Impedance Relay 

• A relay which measures impedance at the relay location is called an impedance relay. 

Torque Equation T = K₁I² – K₂V²

• The effect of spring torque is neglected in above equation.

Operating Principle 

• For the operation of the relay the operating torque should be greater than the restraining torque.

K₁I² – K₂V²

Here V and I are the voltage and current quantities fed to the realy

or Z < constant (design impedance)

Remember: 

• The impedance relay will operate only if the impedance seen by the relay is less than a prespecified value. 
• It is relay is a voltage restrained over-current relay. 
• The operating characteristic of an impedance relay on V-I diagram.

• The initial bend in the characteristic is due to the presence of spring torque. 
• Since the operation of the relay is independent of the phase relation between V and I, the operating characteristic is a circle and hence it is a nondirectional relay.

• The impedance relays normally used are high speed relays.

Reactance Relay 

• A relay which measures reactance at the relay location is called a reactance relay. 
• In this relay the operating torque is obtained by current and the restraining torque due to a currentvoltage directional element.

Remember:  A reactance relay is an over-current relay with directional restrained.

Operating Principle 

• The directional element is so designed that its maximum torque angle is 90°, i.e., t = 90°.

For the operation of the relay 
 

Remember: 

• For the operation of the relay the reactance seen by the relay should be smaller than the reactance for which the relay has been designed.

Important Points: 

• The resistance component of the impedance has no effect on the operation of the relay. 
• It responds only to the reactance component of the impedance. 
• Is a non-directional relay.

MHO Relay It measures a particular component of the

 
factor angle and q is the design angle to shift MHO characteristics on the R-X diagram. Its characteristic on the R-X diagram is a circle passing through the origin.

• It is also known as an admittance or angle admittance relay. 
• In this relay the operating torque is obtained by the V-I element and restraining torque due to the voltage element.

Remember: 

• A mho relay is a voltage restrained directional relay.

Operating Principle 

T = K₃ VI cos (q – t) – K₂V² 

For the relay to operate 

K₃VI cos (q – t) > K₂V²

• The relay operates when the impedance seen by the relay falls within this circle.
• The relay is inherently directional so that it needs only one pair of contacts which makes it fast tripping for fault clearance and reduces the VA burden on the current transformers.

DIFFERENTIAL RELAYS

A relay which operates in response to the difference of two actuating quantities. 

• The differnetial relay is one that operates when the vector difference of two or more similar electrical quantities exceeds a pre-determined value. 
• A differential relay, should have :
  • Two or more similar electrical quantities, and
  • These quantities s houl d have phase displacement (normally approx 180°), for the operation of the relay.

Remember: 

• The most common application of differential relay is the current differential type.

Current Balance Relay  

• The dotted line represents the equipment to be protected.  The voltage induced in the secondary of the the CTs will circulate a current through the combined impedance of the pilot wires and the CTs.  In case the operating coil is not connected between the equipotential points there will be difference current through the operating coil of the relay and this may result in maloperation of the relay.  When the operating coil of the relay is not connected between the equipotential points, even though the current through each CT is same, the burden on the two CTs is unequal. This causes the heavily loaded CTs to saturate during through fault, thereby causing dissimilarity in the characteristics of the two CTs which results in maloperation of the relay.
• For an internal fault, consider fig. (a) when the circuit is fed from one end and fig. (b) when the circuit is fed from both the ends. In both the cases, a current will flow through the operating coil of the relay and it will operate. This form of protection is known as Merz-Price protection.

Merz-Price Protection It is modifed by biasing the relay. This is commonly known as percentage differential protection.

• The relay consists of an operating coil and a restraining coil. The operating coil is connected to the mid-point of the restraining coil.

Operating Principle

• The differential current through the operating coil is (i₁ – i₂) is and the equivalent current in the restraining coil is (i₁ + i₂)/2.
• The torque developed by the operating coil is proportional to the ampere-turns.
• Torque developed by the operating coil To µ (i₁ – i₂)n_o
  where n_o is the number of turns in the operating coil.
• The torque due to restraining coil

Where n_r is the number of turns in the restraining coil.

• At balance

or

Voltage Balance Relay 

• Here the CTs at two ends are connected in opposition as shown in fig.

• The relative polarity of the CTs is such that there is no current through the relays under balanced 
• The requirement of CT is that they should induce voltages in the secondary linearly with respect to the current. 
• Since the magnitude of the fault current is very large, in order that the voltage should be a linear function of such large currents the CTs should be air-cored

OVER CURRENT RELAY
A relay which operates when the actuating current exceeds a certain preset value (its pick-up value).

Instantaneous Relay  

• An instantaneous relay has no intentional time delay in its operation and it is about 0.1 sec. 
• The instantaneous relay is more effective where the impedance Z_s between the source and the relay is small compared with the impedance Zl of the section to be protected.

Inverse Time Current Relay 

• A relay in which the operating time is inversely proportional to the magnitude of the operating current. 
• Its characteristics can be obtained with induction type of relays by using a suitable core which does not saturate for a large value of fault current.

Definite Time Current Relay  

• A relay in which the operating time is independent of the magnitude of the actuating current. 
• Its characteristics is shown by characteristic 1 in above figure.

Inverse Definite Minimum Time (IDMT) Relay 

• A relay which gives an inverse time characteristic at lower values of the operating current and definite time characteristic at high values of the operating current. 
• These relays are widely used for the protection of distribution lines. 
• Its characteristics can be achieved by using a core of the electromagnet which gets saturated for currents slightly greater than the pick up current. 
• Its characteristics is shown by characteristic 2 in above figure. 
• Characteristics

Very Inverse Relay 

• A relay in which the saturation of the core occurs at a later stage. 
• Its characteristics is shown by characteristic 3 in above figure. 
• Characteristics
  

Extremely Inverse Relay 

• A relay in which the saturation occurs at a still later stage. 
• Its characteristics is shown by characteristic 4 in above figure. 
• Characteristics
  

Current Setting 

• The actual rms current flowing in the relay expressed as a multiple of the setting current (pickup current) is known as the plug setting multiplier (PSM).
  
  
• Suppose, the rating of a relay is 5 A and it is set at 200% i.e. at 10 A. If the current flowing through the relay is 100 A, then the plug setting multiplier will be 10. The PSM = 4 means 40 A of current is flowing, PSM = 6 means 60 A of current is flowing and so on.

Time Setting 

• The term time multiplier setting (TMS) is used for these steps of time settings. The values of TMS are 0.1, 0.2, ..., 0.9, 1. 
• Suppose that at a particular value of the current or plug setting multiplier (PSM), the operating time is 4 s with TMS = 1. The operating time for the same current with TMS = 0.5 will be 4 × 0.5 = 2 s. The operating time with TMS = 0.2 will be 4 × 0.2 = 0.8 s.

Buchholz Relay

Whenever a fault takes place in a transformer the oil of the tank gets overheated and gases are formed.

• The generation of the gases may be slow or violent dependong upon whether the fault is a minor or heavy short circuit. 
• The generation of gases is used as a means of fault detection. 
• Commonly used for protection in all transformers provided with conservators. 
• For a minor or incipient fault, the slow generation of gas gives rise to gas bubbles which try to go the sonservator but are trapped in the upper portion of the relay chamber, therey a fall in oil level takes place. This disturbs the equilibrium of the gas float. The float tilts and the alarm circuit is closed through the mercury switch and the indication is given. 
• For a heavy fault, large volumes of gases are generated which causes violent displacement of the oil and impinge upon the baffle plates of the lower float and thus the balance of the lower float is disturbed. The lower float is tilted and the contacts are closed which are arranged to trip the transformer.