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What is Circuit Breaker - Operating Principle and Arcing Phenomenon

What is a Circuit Breaker?

As the name indicates, circuit breaker means the device which breaks (Open) the circuit under the abnormal condition and protects the system from hazards. 

The function of a circuit breaker is to isolate the faulty point of the power system in case of abnormal conditions such as faults. 

Another important device which is protective relay detects abnormal conditions and sends a tripping signal to the circuit breaker after receiving tripping command from the relay, the circuit breaker isolates the faulty part from the power system

A circuit breaker essentially consists of fixed and moving contacts, called electrodes.  These contacts are placed in the closed chamber containing a fluid containing medium (either liquid or gas) which quenches the arc formed between the contacts.  

Under normal operating conditions, these contacts remain closed and will not open automatically until and unless the system becomes faulty. 

The contacts can be opened manually or by remote control whenever desired. 

When a fault occurs on any part of the system, the trip coils of the breaker get energized and the moving contacts are pulled apart by some mechanism, thus opening the circuit.

Operating principle of Circuit Breaker

When the contacts of a circuit breaker are separated under fault conditions, an arc is struck between them. The current is thus able to continue until the discharge ceases. 

The production of arc not only delays the current interruption process but it also generates enormous heat which may cause damage to the system or to the breaker itself. 

Therefore, the main problem in a circuit breaker is to extinguish the arc within the shortest possible time so that heat generated by it may not reach a dangerous value. 

Arc Phenomenon in Circuit Breaker

When a short-circuit occurs, a heavy current flows through the contacts of the circuit breaker before they are opened by the protective system. 

At the instant when the contacts begin to separate the contact area decreases rapidly and large fault current causes increased current density and hence rise in temperature. 

The heat produced in the medium between contacts (usually the medium is oil or air) is sufficient to ionize the air or vaporize and ionize the oil. The ionized air or vapor acts as a conductor and an arc is struck between the contacts. 

The potential difference between the contacts is quite small and is just sufficient to maintain the arc. The arc provides a low resistance path and consequently, the current in the circuit remains uninterrupted so long as the arc persists. 

During the arcing period, the current flowing between the contacts depends upon the arc resistance. 

The greater the arc resistance, the smaller the current that flows between the contacts. The arc resistance depends upon the following factors:
  1. The degree of ionization - the arc resistance increases with the decrease in the number of ionized particles between the contacts.
  2. Length of the arc - the arc resistance increases with the length of the arc i.e. separation of contacts.
  3. Cross section of arc - the arc resistance increase with the decrease in the area of cross section of the arc.

Types of Circuit Breaker

The basic types of circuit breakers according to the medium of arc interruption are

What is an HVDC System?

HVDC stands for High Voltage Direct Current. An HVDC electric power transmission system uses direct current for the bulk transmission of electrical power, in contrast with the more common alternating current systems. 

For long-distance distribution, HVDC systems are less expensive and suffer lower electrical losses. For shorter distances, the higher cost of DC conversion equipment compared to an AC system may be warranted where other benefits of direct current links are useful.

When HVDC Started?

The transmission and distribution of electrical energy started with direct current. In 1882, a 50-km-long 2-kV DC transmission line was built between Miesbach and Munich in Germany. At that time, conversion between reasonable consumer voltages and higher DC transmission voltages could only be realized by means of rotating DC machines.

The HVDC (high voltage DC) transmission made the modest beginning in 1954 when a 100 kV, 20 MW DC link was establish between Swedish mainland and the island of Gotland. 

Until 1970, the converter stations utilized mercury arc valves for rectification; the successful use of thyristors for power control in industrial devices encouraged its adoption in HVDC converters by development of high power semiconductor devices. 

The latest technology in HVDC system is to use LTT’s (Light triggered thyristors) or GTO’s (Gate turn off) or IGBT’s (Insulated gate bipolar transistor).Control is obtained by using VSC (Voltage source converter) using PWM (Pulse width modulation) technique.

Components of HVDC Transmission System

To assist the designers of transmission systems, the components that comprise the HVDC system, and the options available in these components, are presented.

The main elements of an HVDC system are:
  1. Converter unit
  2. Converter transformer
  3. AC filters & Capacitor banks
  4. DC filters
  5. Reactive Power source
  6. Smoothing Reactor
  7. DC Switchgear

HVDC Converter Station

A HVDC converter station uses thyristor valves to perform the conversion from AC to DC and vice versa. The valves are normally arranged as a 12- pulse converter. The valves are connected to the AC system by means of converter transformers. The valves are normally placed in a building and the converter transformers are located just outside.

The power transmitted over the HVDC transmission is controlled by means of a control system. It adjusts the triggering instants of the thyristor valves to obtain the desired combination of voltage and current in the DC system.

Several other apparatus are needed in a converter station, such as circuit breakers, current and voltage transducers, surge arresters, etc.

HVDC Converter Transformer

The converter transformers adapt the AC voltage level to the DC voltage level and they contribute to the commutation reactance. Usually they are of the single phase three winding type, but depending on the transportation requirements and the rated power, they can be arranged in other ways. 

Almost all of HVDC power converters with thyristor valves are assembled in a converter bridge of twelve-pulse configuration. The construction of the standard 12- pulse converter transformer system (CTS), it can be obtained with either of the following arrangements.

The converter transformers are the heaviest equipment in a HVDC converter station. Single units can often have a total weight of 200 - 550 tons.

The converter transformer serves several functions:

  • Supply of AC voltages in two separate circuits with a relative phase shift of 30 electrical degrees for reduction of low order harmonics, especially the 5th and 7th harmonics.
  • Act as a galvanic barrier between the AC and DC systems to prevent the DC potential to enter the AC system.
  • Reactive impedance in the AC supply to reduce short circuit currents and to control the rate of rise in valve current during commutation.
  • Voltage transformation between the AC supply and the HVDC system.
The HVDC converter transformer can be built as three-phase units or as single-phase units depending on voltage and power rating. When built as three phase transformers there is generally one unit with the valve winding arranged for star connection and the other unit for delta connection. In single-phase design the two valve windings are generally built on the same transformer unit.

Design wise the internal voltage distribution from the DC-voltages on the transformer valve terminals will need a different insulation build-up as compared to the insulation system in a conventional transformer.

Converter Valve

The quadruple valve structure is suspended from the ceiling of the valve hall via porcelain insulators. At the top and bottom of the structure, a metallic framework ensures the mechanical stability of the valves. Between the frameworks, the different levels in the valve are mechanically fixed by means of threaded epoxy rods.

Thyristor valves are the heart of the HVDC conversion process. Modern valves have an excellent performance record and very small losses.The thyristor valves do the actual conversion from AC to DC or vice versa. The basic circuit used is the Graetz bridge consisting of six valve functions, but in order to eliminate the largest harmonics, two such bridges are connected in series forming a 12-pulse converter.

The valves are normally located in a valve building and arranged as three structures (quadruple valves) suspended from the ceiling of the valve hall, but other arrangements do exist.

AC Filters

Conventional HVDC converters always have a demand for reactive power. At normal operation, a converter consumes reactive power in an amount that corresponds to approximately 50 % of the transmitted active power. The least costly way to generate reactive power is in shunt connected capacitor banks. Some of these capacitor banks can then be combined with reactors and resistors to form filters providing low impedance paths for the harmonics in order to limit them from entering into the AC network.

Active DC filters

The principle of the active DC filter is to inject a current generated by a power amplifier into
the DC circuit canceling the DC side harmonics coming from the HVDC converter. The amplifier is controlled by a high speed digital signal process controller.

For overhead line HVDC transmissions the DC filter takes care of telephone interference. The smoothing reactor, which is installed also for other reasons, is an important element in the filtering of DC side harmonics. For overhead line transmissions, it is normally necessary to install additional filter circuits between the pole bus (outside the smoothing reactor) and the neutral bus. 

Capacitors or filter circuits may also have to be installed between the neutral bus and ground. The filter types used on the DC side are essentially the same as those used on the AC side, i.e. series resonance filters and high pass filters.

HVDC smoothing reactor

A DC smoothing reactor is a high inductance coil connected in a series with main DC pole
circuit between the converter Bridge and DC line pole. 

HVDC smoothing reactors can be of air-core design as well as and oilinsulated units. A DC reactor is normally connected in series with the converter.
The main objectives of the reactor are:
  • To smooth the ripple current in DC
  • To reduce the risk of commutation failures by limiting the rate of rise of the DC line current at transient disturbances in the AC or DC systems.
  • Prevention of resonance in the DC circuit.
  • Reducing harmonic currents including limitation of telephone interference
Main two types of smoothing reactors are-
  1. Air-insulated smoothing reactor
  2. Oil-insulated (filled) smoothing reactor

Typical layout of HVDC transmission system

Figure shows the typical layout of the HVDC substation which is having AC switchyard and DC switchyard and converter building in between. And this converter building contains converter transformer, converter bridge etc. The AC filters are provided near the AC switchyard for the compensation.

4 Major Types of Electrical Substations

The assembly of apparatus used to change some characteristic (e.g. voltage, a.c. to d.c., frequency, p.f etc.) of electric supply is called a sub-station. Sub-stations are an important part of the power system. The continuity of supply depends to a considerable extent upon the successful operation of sub-stations.

There are four major types of electric substations. 
  1. switchyard at a generating station
  2. customer substation
  3. system station
  4. distribution station

Generating Station Switchyards

The first type is the switchyard at a generating station. These facilities connect the generators to the utility grid and also provide off-site power to the plant. 

  • Generator switchyards tend to be large installations that are typically engineered and constructed by the power plant designers and are subject to planning, finance, and construction efforts different from those of routine substation projects. 
  • Because of their special nature, the creation of power plant switchyards will not be discussed here, but the expansion and modifications of these facilities generally follow the same processes as system stations.

Customer Substations

The second type of substation, typically known as the customer substation, functions as the main source of electric power supply for one particular business customer. 
  • The technical requirements and the business case for this type of facility depend highly on the customer’s requirements, more so than on utility needs.

System Stations

The third type of substation involves the transfer of bulk power across the network and is referred to as a system station. Some of these stations provide only switching facilities (no power transformers) whereas others perform voltage conversion as well. 
  • These large stations typically serve as the endpoints for transmission lines originating from generating switchyards and provide the electrical power for circuits that feed transformer stations. 
  • They are integral to the long-term reliability and integrity of the electric system and enable large blocks of energy to be moved from the generators to the load centers. 
  • These system stations are strategic facilities and usually very expensive to construct and maintain.

Distribution Substation

The fourth type of substation is the distribution station. These are the most common facilities in power electric systems and provide the distribution circuits that directly supply most electric customers.
  • They are typically located close to the load centers, meaning that they are usually located in or near the neighborhoods that they supply, and are the stations most likely to be encountered by the customers.
  • Depending on the type of equipment used, the substations could be
    • Outdoor type with air-insulated equipment
    • Indoor type with air-insulated equipment
    • Outdoor type with gas-insulated equipment
    • Indoor type with gas-insulated equipment
    • Mixed technology substations
    • Mobile substations

Starting of Synchronous Motor

Synchronous motors are not self starting. Some additional devices should be used to start the motor. Basically there are two methods for starting synchronous motors:
  1. Induction motor starting (Damper winding)
  2. Auxiliary motor starting.

Damper Winding Starting

Most of the synchronous motors have salient pole structure. A winding consisting of heavy copper bars is installed in slots in the pole faces. These bars are all shorted together at both ends of the rotor.

When 3-phase supply is fed to the stator, a rotating magnetic field is produced in the air gap which induces currents in the bars which further produces flux. The interaction of the fluxes produces torque in the direction of field rotation. In other words, the motor is started as an induction motor, the bars in the pole face slots forming a sort of squirrel cage rotor.

Induction motor action will bring the motor to nearly synchronous speed. At synchronous speed there is no relative motion between the poles of the air gap field and the pole face bars. No current is induced in the bars at synchronous speed, and no torque would be produced by them. However, the maximum speed developed on induction motor action is very close to synchronous speed and the rotor falls into step when the dc field current is switched on.

The field winding terminals are usually shorted through a resistor during starting until such time that the field is excited. This has two advantages. 
  • First it protects the slip ring insulation from the high ac voltage induced in the field during starting. 
  • Second the current circulating in the field winding would provide a small additional accelerating torque.

Auxiliary Motor Starting

The auxiliary motor may be a dc shunt motor or an induction motor having the same number of poles as the synchronous motor or two poles less as compared to synchronous motor. The job of the auxiliary motor is to bring the synchronous motor to synchronous speed or near synchronous speed. 

The auxiliary motor is mechanically coupled to the synchronous motor. No load is put on the synchronous motor during starting. Therefore, the auxiliary motor has to overcome only the inertia of the synchronous motor and may have its rating much smaller than that of the synchronous motor being started.

When the speed is near synchronous speed, the 3-phase supply is switched on to the armature and DC supply to the field circuit of synchronous motor. The synchronous motor pulls into step, its speed rises to synchronous speed and it continues to run at this speed.

If the induction motor has two poles less than synchronous motor, the induction motor speed is higher than the synchronous speed of the synchronous motor. Once the higher speed is attained, the supply to induction motor is switched off and 3-phase supply to synchronous motor is switched on. 

When the speed of the synchronous motor almost equals the synchronous speed, it's field winding is energized. The synchronous motor pulls into step and starts running at synchronous speed. However, in modern days this method of starting is hardly used. It is mainly the damper winding which is utilised for starting the synchronous motor.

Need of a Starter (3 & 4 Point) in DC Motors

We use 3 and 4 point starters in DC motors. What happens if a DC motor is started without a starter? Let's check it.

Consider a dc motor at standstill. When a dc motor is at rest the speed of the motor is zero, therefore the back emf is zero.

We know that Back Emf, Eb= V - IaRa
Therefore, Ia = (V - Eb)/Ra

Armature resistance Ra in a DC motor is very small. Now at rest Eb=0 and Ra is negligible. Hence from the above equation, Ia will be very high.

Therefore when the motor is directly connected to the supply lines a heavy current will flow through the armature conductors. This heavy current is very dangerous.

In order to limit this heavy current, we use the starter in a DC motor. 

The example given below will give you more idea about this.

How Starter Works?

To understand how the starter works, consider a 400V, 20kW, dc motor with a total resistance of 0.5 Ohms

If this motor is directly connected to the supply, it will draw a current of 400/0.5= 800 Amps.

The full load current of the same motor is 64 Amps. The starting current is therefore 12.5 times that of full load current. 

This gives rise to heating and mechanical forces of 150 times full load values.

When running back emf oppose the applied voltage and thus a small current flows.

Problems of heavy inrush current at the starting time
✈ Heavy sparking at the commutator and even flashovers.
✈ Damage to the armature windings.
✈ Damage to rotating parts of the motor.
✈ Large dip in supply voltage.

Hence for the protection of the motor from high current during starting period (say 5 to 10 seconds), it is necessary to connect a high resistance in series with the armature of the motor at the starting period. 

The resistance should gradually cut in steps as the motor gains speed and develop back emf.

Hence, the starter is essential for DC motors.

Current Transformer (CT) - Construction and Working Principle

The large alternating currents which can not be sensed or passed through normal ammeter, and current coils of wattmeters, energy meters can easily be measured by use of current transformers along with normal low range instruments.

A transformer is a device which consists of two windings called primary and secondary. It transfers energy from one side to another with suitable change in the level of current or voltage.
Related: Working Principle of Transformer
A current transformer (CT) is a type of transformer that is used to measure AC current. It produces an alternating current (AC) in its secondary which is proportional to the AC current in its primary. Current transformers, along with voltage or potential transformers are Instrument transformer.

Current Transformer Symbol / Circuit Diagram 

A current transformer (CT) basically has a primary coil of one or more turns of heavy cross-sectional area. In some, the bar carrying high current may act as a primary. This is connected in series with the line carrying high current.
The secondary of the current transformer is made up of a large number of turns of fine wire having small cross-sectional area. This is usually rated for 5A. This is connected to the coil of normal range ammeter.

Related: Why Current Transformer (CT) Seconday Should not be Open ?

Working Principle of CT

These transformers are basically step up transformers i.e. stepping up a voltage from primary to secondary. Thus the current reduces from primary to secondary. 

So from current point of view, these are step down transformer, stepping down the currant value considerably from primary to secondary. 

N1 = Number of Primary Turns
N2 = Number of Secondary Turns
I1 = Primary Current
I2 = Secondary Current

For a transformer,
I1∕I = N2/N1

As N2 is very high compared to N1, the ratio I1 to I2 is also very high for current transformers. Such a current ratio is indicated for representing the range of current transformer. 

For example, consider a 500:5 range then it indicates that C.T. steps down the current from primary to secondary by a ratio 500 to 5. 

I1∕I = 500/5

Knowing this current ratio and the meter reading on the secondary, the actual high line current flowing through the primary can be obtained. 

Construction of Current Transformer

There are three types of constructions used for the current transformers which are,
  1. Wound Type CT
  2. Torroidal (Window) Type CT
  3. Bar Type CT

Wound Type Current Transformer – The transformers primary winding is physically connected in series with the conductor that carries the measured current flowing in the circuit. The magnitude of the secondary current is dependent on the turn’s ratio of the transformer. 

Torroidal (Window) Type Current Transformer – These do not contain a primary winding. Instead, the line that carries the current flowing in the network is threaded through a window or hole in the torroidal transformer. Some current transformers have a “split core” which allows it to be opened, installed, and closed, without disconnecting the circuit to which they are attached. 

Bar-type Current Transformer – This type of current transformer uses the actual cable or bus-bar of the main circuit as the primary winding, which is equivalent to a single turn. They are fully insulated from the high operating voltage of the system and are usually bolted to the current carrying device. 

1. Wound Type Current Transformer

In wound type construction the primary is wound for more than one full turn on the core.
The construction is shown below.
In a low voltage wound type current transformer, the secondary winding is wound on a bakelite former. The heavy primary winding is directly wound on the top of the secondary winding with a suitable insulation in between the two. Otherwise the primary is wound completely separately and then taped with suitable insulating material and assembled with the secondary on the core.

The current transformers can be ring type or window type. Some commonly used shapes for the stampings of window type current transfonncrs are shown in the figure below.

The core material for wound type is nickel-iron alloy or an oriented electrical steel. Before installing the secondary winding on core it is insulated with the help of end collars and circumferential wraps of pressboards. Such pressboards provide additional insulation and protection to the winding from damage due to the sharp corners.

2. Bar Type Current Transformer

In this type of current transformer, the primary winding is nothing but a bar of suitable size. The construction is shown in the Figure.

The insulation on the bar type primary is bakelized paper tube or a resin directly moulded on the bar. Such bar type primary is the integral part of the current transformer. The core and the secondary winding are same in bar type transformer.

The stampings used for the laminations in current transformers must have high cross-sectional area than the ordinary transformers. Due to this, the reluctance of the interleaved comers remains as low as possible. Hence the corresponding magnetizing current is also small. The windings are placed very close to each other so as to reduce the leakage reactance. To avoid the corona effect, in bar type transformer, the external diameter of the tube is kept large.

The windings are so designed that without damage, they can withstand short circuit forces which may be caused due to short circuit in the circuit in which the current transfomter is inserted.

For small line voltages, the tape and varnish are used for insulation. For line voltages above 7 kV the oil immersed or compound filled current transformers are used.

Uses / Advantages of Current Transformer

Current transformers are used extensively for measuring current and monitoring the operation of the power grid.

Along with voltage leads, revenue-grade CTs drive the electrical utility's watt-hour meter on virtually every building with three-phase service and single-phase services greater than 200 amperes. 

High-voltage current transformers are mounted on porcelain or polymer insulators to isolate them from ground. 

Current transformers can be mounted on the low voltage or high voltage leads of a power transformer.

Often, multiple CTs are installed as a "stack" for various uses. For example, protection devices and revenue metering may use separate CTs to provide isolation between metering and protection circuits, and allows current transformers with different characteristics (accuracy, overload performance) to be used for the devices.

Classification or Types of Protective Relays

The IEEE defines protective relays as: “relays whose function is to detect defective lines or apparatus or other power system conditions of an abnormal or dangerous nature and to initiate appropriate control circuit action ”. 

Relays detect and locate faults by measuring electrical quantities in the power system which are different during normal and intolerable conditions. The most important role of protective relays is to first protect individuals, and second to protect equipment

All the relays consist of one or more elements which get energized and actuated by the electrical quantities of the circuit. Most of the relays used are electromechanical type which work on the principles of electromagnetic attraction and electromagnetic induction.

There are different types of protective relays available now. They can be broadly classified in to the following types. Each of them will be explained in detail in coming articles.

  1. Electromagnetic attraction type Relays
    1. Solenoid Type
    2. Attracted Armature
    3. Balanced Beam Type
  2. Induction Type Relays
    1. Induction Disc Type
    2. Induction Cup Type
  3. Direction Type Relays
    1. Reverse Current Type
    2. Reverse Power Type
  4. Relays based on Timing
    1. Instantaneous Type
    2. Definite Time Lag Type
    3. Inverse Time Lag type
  5. Distance type Relays
    1. Impedance Type
    2. Reactance Type
    3. Admittance Type
  6. Differential Type Relays
    1. Current Differential Type
    2. Voltage Differential Type
  7. Other Types of Relays
    1. Under voltage, current, power relay
    2. Over voltage, current, power relay
    3. Thermal Relay
    4. Rectifier Relay
    5. Permanent Magnet Moving Coil Relay
    6. Static Relay
    7. Gas Operated Relay
Protection relays can also be classified in accordance with their construction, actuating
signal, application and function.

Classification based on  Construction
Depending upon the principle of construction, the following four broad categories are found.

  • Electromechanical Relay
  • Solid State Relay
  • Microprocessor Relay
  • Numerical Relay
Classification based on Actuating Signals

The actuating signal may be any of the following signals including a numbers of different combinations of these signals depending upon whether the designed relay require a single or multiple inputs for its realization.
• Voltage
• Power
• Frequency
• Temperature
• Pressure
• Speed
• Others

Classification based on Function
The functions for which the protection system is designed classify the relays in the following few categories.
• Directional Over current Relay
• Distance Relay
• Over voltage Relay
• Differential Relay
• Reverse Power Relay
• Others

It is important to notice that the same set of input actuating signals may be utilized to design to relays having different function or application. 
For example, the voltage and current input relays can be designed both as a Distance and/ or a Reverse Power relay.

Electromagnetic Attraction Type Relays

The electromagnetic attraction type relays operate on the principle of attraction of an armature by the magnetic force produced by undesirable current or movement of plunger in a solenoid. These relays can be actuated by AC or DC quantities. The various types of these relays are. 

  1. Solenoid Type
  2. Attracted Armature Type
  3. Balanced Beam Type

1. Solenoid Type 

In this relay, the plunger or iron core moves into a solenoid and the operation of the relay depends on the movement of the plunger. 

2. Attracted Armature Type : 

This relay operate on the current setting. When currant in the circuit exceeds beyond the limit, the armature gets attracted by the magnetic force produced by the undesirable current. The current rating of the circuit in which relay is connected plays an important role in the operation of the relay. 

3. Balanced Beam Type : 

In this relay, the armature is fastened to a balanced beam. For normal current, the beam remains horizontal but when current exceeds, the armature gets attracted and beam gets tilted causing the required operation 

Induction Type Relays

These relays work on the principle of an electromagnetic induction. The use of these relays is limited to AC quantities. The various types of these relays are, 

  1. Induction Disc Type
  2. Induction Cup Type

1. Induction Disc Type

In this relay. a metal disc is allowed to rotate between the two electromagnets. The electromagnets are energised by alternating currents. The two types of constructions used for this type are shaded pole type and watthour meter type. 

2. Induction Cup Type : 

In this relay, electromagnets act as a stator and energised by relay coils. The rotor is metallic cylindrical cup type. 

Directional Type Relays

These relays work on the direction of current or power flow in the circuit. The various types of these relays are 

  1. Reverse Current Type
  2. Reverse Power Type

1. Reverse Current Type : 

The relay is actuated when the direction of the current is reversed ur the phase of the current becomes more than the predetermined calve 

2. Reverse Power Type : 

The relay is actuated when the phase displacement between applied voltage and current attains a specified value. 

Relays Based on Timing

In relays the time between instant of relay operation and instant at which tripping of contacts takes place can be controlled. The time is called operation time. Based om this, the time relays are classified as, 
  1. Instantaneous Type
  2. Definite Time Lag Type
  3. Inverse Time Lag type

1. Instantaneous Type : 

In this type no time is lust between operation of relay and tripping of contacts. No intentional time delay is provided. 

2 Definite Time Lag Type : 

In this type intentionally a definite time lag is provided between operation of relay and tripping of contact. 

3 Inverse Time Lag type : 

In this type, the operating time Is approximately Inversely proportional to the magnitude of the actuating quantity. 

Distance Type Relays 

These relays work on the principle of measurement of voltage to current ratio. In this type, there are two coils. One coil is energized by current while other by voltage. The torque produced is  proportional to the ratio of the two quantities. When the ratio reduces below a set value, the relay operates. 

The various types of these relays are, 
  1. Impedance Type
  2. Reactance Type
  3. Admittance Type

1. Impedance Type : 

In this type, the ratio of voltage to current is nothing but an impedance which is proportional to the distance ol the relay form the fault point. 

2. Reactance Type : 

The operating time is proportional to the reactance which is proportional to the distance of the relay from the fault point. 

3. Admittance Type : 

This is also called mho type. In this type, the operating time is proportional to the admittance. 

Differential Type Relays 

A differential relay operates when the vector difference of two or more electrical quantities in the circuit in which relay is connected. exceeds a set value. These are classified as, 
  1. Current Differential Type
  2. Voltage Differential Type

1. Current Differential Type : 

In this type, the relay compares the current entering a section of the system and the current leaving the section. Under fault condition, these currents are different.

2. Voltage Differential Type :

In this type, two transformers are used. The secondaries of the transformers are connected in series with the relay in such a way that the induced e.m.fs are in opposition under normal conditions. Under fault condition, primaries carry different currents due to which induced e.m.f.s no longer remain in opposition and the relay operates. 

Other Types of Relays 

Various other types of relays which are used in practice are,

1. Under voltage, current, power relay : 
This relay operates when the voltage, current or power in a circuit falls below a set value

2. Over voltage, current, power relay : 
This relay actuates when the voltage, current or power In a circuit rises above a set value.

3. Thermal Relay : 
This relay actuates due to the heat produced by the current in the relay coil.

4. Rectifier Relay : 
In this relay, the quantities to be sensed are rectified and then given to the moving coil unit of the relay.

5. Permanent Magnet Moving Coil Relay : 
In this relay, the coil carrying current is free to rotate in the magnetic field of a permanent magnet. This is used for DC only.

6. Static Relay : 
This relay uses some electronic method for sensing the actuating quantity. It uses a stationary circuit.

7. Gas Operated Relay : 
The gas pressure is adjusted according to the variations in the actuating quantity. This gas pressure is used to actuate the relay. Buchholz relay is an example of such type of relay.