Types of Relays
Types of protection relays are mainly based on their characteristic, logic, on actuating parameter and operation mechanism.
Based on operation mechanism protection relay can be categorized as electro magnetic relay, static relay and mechanical relay. Actually relay is nothing but a combination of one or more open or closed contacts. These all or some specific contacts the relay change their state when actuating parameters are applied to the relay. That means open contacts become closed and closed contacts become open. In electromagnetic relay these closing and opening of relay contacts are done by electromagnetic action of a solenoid.
In mechanical relay these closing and opening of relay contacts are done by mechanical displacement of different gear level system.
In static relay it is mainly done by semiconductor switches like thyristor.
In digital relay on and off state can be referred as 1 and 0 state.
Based on Characteristic the protection relay can be categorized as-
  1. Definite time relays

Definite Time Over Current Relay
This relay is created by applying intentional time delay after crossing pick up value of the current. A definite time over current relay can be adjusted to issue a trip output at definite amount of time after it picks up. Thus, it has a time setting adjustment and pick up adjustment.

  1. Inverse time relays with definite minimum time(IDMT)

Ideal inverse time characteristics can not be achieved, in an over current relay. As the current in the system increases, the secondary current of the current transformer is increased proportionally. The secondary current is fed to the relay current coil. But when the CT becomes saturated, there would not be further proportional increase of CT secondary current with increased system current.
From this phenomenon it is clear that from trick value to certain range of faulty level, an inverse time relay shows exact inverse characteristic. But after this level of fault, the CT becomes saturated and relay current does not increase further with increasing faulty level of the system. As the relay current is not increased further, there would not be any further reduction in time of operation in the relay. This time is referred as minimum time of operation.
Hence, the characteristic is inverse in the initial part, which tends to a definite minimum operating time as the electric current becomes very high. That is why the relay is referred as inverse definite minimum time over current relay or simply IDMT relay.

  1. Instantaneous relays.

Instantaneous over current relay, which operates very fast with no intentional time delay and the operating time of these relay can be as low as 0.01sec . These relay operates only when the impedance between the relay and the source zs is very small compared to the impedance to the impedance of the protected section zl .

7. Voltage restraint over current relay.
The voltage restrained over current protection function(51V) is used to protect generators. The operation set point is adjusted according to the voltage to take into account cases of faults close to the generator which cause voltage dips and short circuit current. This protection function is three-pole. It picks up if one ,two or three phase currents reach the voltage-adjusted operation set point.
Based on of logic the protection relay can be categorized as-
  1. Differential.

Principle of Differential Protection scheme is one simple conceptual technique. The differential relay actually compares between primary current and secondary current of power transformer, if any unbalance found in between primary and secondary currents the relay will actuate and inter trip both the primary and secondary circuit breaker of the transformer.
2. Unbalance.
3. Neutral displacement.
4. Directional.
The over-current protection can be given directional feature by adding directional element in the protection system. Directional over-current protection responds to over-currents for a particular direction flow. If power flow is in the opposite direction, the directional over-current protection remains un-operative.
Directional over-current protection comprises over-current relay and power directional relay- in a single relay casing. The power directional relay does not measure the power but is arranged to respond to the direction of power flow.
Directional operation of relay is used where the selectivity can be achieved by directional relaying. The directional relay recognizes the direction in which fault occurs, relative to the location of the relay. It is set such that it actuates for faults occurring in one direction only. It does not act for faults occurring in the other direction. Consider a feeder AC (Fig. 9) passing through sub-section B. The circuit breaker CB3 is provided with a directional
[IMG]file:///C:\Users\arptst\AppData\Local\Temp\msohtmlclip1\01\clip_image002.emz[/IMG](Fig. 9) Principle of directional protection
Relay `R' which will trip the breaker CB3 if fault power flow in direction C alone. Therefore for faults in feeder AB, the circuit breaker CB3 does not trip unnecessarily. However for faults in feeder BC the circuit-breaker CB3 trips
Because it's protective relaying is set with a directional feature to act in
direction AC

Another interesting example of directional protection is that of reverse power protection of generator (Fig. 10). If the prime mover fails, the generator continues to run as a motor and takes power from bus-bars.
[IMG]file:///C:\Users\arptst\AppData\Local\Temp\msohtmlclip1\01\clip_image003.emz[/IMG](Fig. 10) Reverse powers protection against motoring action of a generator
Directional power protection operates in accordance with the direction of power flow.
Reverse power protection operates when the power direction is reversed in relation to the normal working direction. Reverse power relay is different in construction than directional over-current relay.
In directional over-current relay, the directional element does not measure the magnitude of power. It senses only direction of power flow. However, in Reverse Power Relays, the directional element measures mag­nitude and direction of power flow.

  1. Restricted earth fault.

An external fault in the star side will result in current flowing in the line current 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 Fault Relay 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

6. Over fluxing.
7. Distance schemes.
8. Bus bar protection.
9. Reverse power relays.
10.Loss of excitation.
11.Negative phase sequence relays etc.
Based on actuating parameter the protection relay can be categorized as-
1.Current relays.
2. Voltage relays.
3. Frequency relays.
4. Power relays etc.
Based on application the protection relay can be categorized as-
1.Primary relay.
2. Backup relay.
Primary relay or primary protection relay is the first line of power system protection whereas backup relay is operated only when primary relay fails to be operated during fault. Hence backup relay is slower in action than primary relay. Any relay may fail to be operated due to any of the following reasons,
1) The protective relay itself is defective.
2) DC Trip voltage supply to the relay is unavailable.
3) Trip lead from relay panel to circuit breaker is disconnected.
4) Trip coil in the circuit breaker is disconnected or defective.
5) Current or voltage signals from CT or PT respectively is unavailable.
As because backup relay operates only when primary relay fails, backup protection relay should not have anything common with primary protection relay.
Some examples of Mechanical Relay are-
1. Thermal
(a) OT trip (Oil Temperature Trip)
(b) WT trip (Winding Temperature Trip)
(C) Bearing temp trip etc.
2. Float type
(a) Buchholz
(b) OSR
(c) PRV
(d) Water level Controls etc.
3. Pressure Switches.
4. Mechanical Interlocks.
5. Pole discrepancy Relay.

SPDT relay is an electromagnetic switch consisting of a coil (terminals 85 & 86), 1 common terminal (30), 1 normally closed terminal (87a) and a normally open terminal (87).
Fig1. Poles of a SPDT Relay Switch
When the coil of the relay is at rest (not energized), the common terminal (30) and the normally closed terminal (87a) have continuity.
Fig2. Un-energized Relay Position
When the coil is energized, the common terminal (30) and the normally open terminal (87) have continuity.
Fig3. Energized Relay Position
Basically, the coil is an electromagnet that causes the arm that is always connected to the common (30) to pivot when energized thereby breaking contact with the normally closed terminal (87a) and making it with the normally open terminal (87).
Below is an example of how the connections are made to a load in a circuit using a SPDT relay switch?
Fig4. Connection of an SPDT Relay to Load
When the relay coil is energized, contact is established between the common (COM) and normally-open (NO) terminal thereby completing the supply connections to the load. Hence, the load is switched on when the relay is energized. Also check this article to understand the working of relays with the help of pictures, here - How Relay works

Different Types of Relay
1. Single Pole Single Throw (SPST)- these types of relay comprise of 4 terminals. Two terminals are used as coil points and other two can be used to connect or disconnect the circuit (A and B).
2. Single Pole Double Throw (SPDT)- these types of relay comprise of 5 terminals two for coil one for common terminal(C) and rest two can be connected to the common terminal.
3. Double Pole Single Throw (DPST)- these types of relay comprise of 6 terminal two for coil and other four for connecting and disconnecting two device. In other words it contains two SPST relay in one package.
4.DoublePole Double Throw(SPDT)- these types of relay comprise of 8 terminal two for coil and another two as common point and rest for connecting and disconnecting devices. In another words in this two SPDT relay are connected in one package.
Comparison of Different Type Relay:
Characteristic Electro Mechanical Relay Static Relay Digital Relay Numerical Relay
Technology Standard 1st generation Relays. 2nd generation Relays. Present generation Relays. Present generation Relays.
Operating Principle They use principle of electromagnetic principle. In this relays transistors and ICs r been used They use Microprocessor. Within built software with predefined values They use Microprocessor. Within built software with predefined values
Measuring elements/ Hardware Induction disc,Electromagnets, Induction cup, Balance Beam R, L, C, Transistors, Analogue ICs comparators Microprocessors, Digital ICs, Digital Signal Processors Microprocessor s,Digital ICs, DigitalSignal processors
Measuring method Electrical Qtysconverted intomechanical force, torque Level detects,comparison withreference value in analogue Comparator A/D conversion,Numericalalgorithmtechniques A/D conversion,Numericalalgorithmtechniques
Surrounding Environment Depend upon gravitation and the value changes to the surrounding magnetic fields also. There value may vary with respect to temperature also.
Relay Size Bulky Small Small Compact
Speed of Response Slow Fast Fast Very Fast
Timing function Mechanical clock works, dashpot Static timers Counter Counter
Time of Accuracy Temp .Dependant Temp. Dependant Stable Stable
Reliability High Low High High
Vibration Proof No Yes Yes Yes
Characteristics Limited Wide Wide Wide
Requirement of Draw Out Required Required Not Required Not Required
CT Burden High Low Low Low
CT Burden 8 to 10 VA 1 VA <0.5 VA <0.5 VA
Reset Time Very High Less Less Less
Auxiliary supply Required Required Required Required
Range of settings Limited Wide Wide Wide
Isolation Voltage Low High High High
Function Single Function Single Function Multi Function Single Function
Maintenance Frequent Frequent Low Very Low
Resistance 100 mille ohms 10 Ohms 10 Ohms 10 Ohms
Output Capacitance < 1 Pico Farad > 20 Pico Farads > 20 Pico Farads > 20 Pico Farads
Deterioration due to Operation Yes No No No
Relay Programming No Partially Programmable Programmable
SCADA Compatibility No No Possible Yes
Operational value indication Not Possible Possible Possible Possible
Visual indication Flags, targets LEDs LEDs, LCD LEDs, LCD
Self monitoring No Yes Yes Yes
Parameter setting Plug setting, dial setting Thumb wheel,dual in line switches Keypad for numeric values,through computer Keypad for numeric values,through computer
Fault Disturbance Recording Not possible Not possible possible possible