The objective of power system protection is to isolate a faulty section of electrical power system from rest of the live system so that the rest portion can function satisfactorily without any severer damage due to fault current.
Actually circuit breaker isolates the faulty system from rest of the healthy system and this circuit breakers automatically open during fault condition due to its trip signal comes from protection relay. The main philosophy about protection is that no protection of power system can prevent the flow of fault current through the system, it only can prevent the continuation of flowing of fault current by quickly disconnect the short circuit path from the system. For satisfying this quick disconnection the protection relays should have following functional requirements.
Protection system in power system
basic concept of Protection system in power system and coordination of protection relays.
The secondary of current transformer is connected to the current coil of relay. And secondary of voltage transformer is connected to the voltage coil of the relay. Whenever any fault occurs in the feeder circuit, proportionate secondary current of the CT will flow through the current coil of the relay due to which mmf of that coil is increased. This increased mmf is sufficient to mechanically close the normally open contact of the relay. This relay contact actually closes and completes the DC trip coil circuit and hence the trip coil is energized.
The mmf of the trip coil initiates the mechanical movement of the tripping mechanism of the circuit breaker and ultimately the circuit breaker is tripped to isolate the fault.
The functional requirements of protection relay
Reliability
The most important requisite of protective relay is reliability.
They remain inoperative for a long time before a fault occurs; but if a fault occurs, the relays must respond instantly and correctly.
Selectivity
The relay must be operated in only those conditions for which relays are commissioned in the electrical power system. There may be some typical condition during fault for which some relays should not be operated or operated after some definite time delay hence protection relay must be sufficiently capable to select appropriate condition for which it would be operated.
Sensitivity
The relaying equipment must be sufficiently sensitive so that it can be operated reliably when level of fault condition just crosses the predefined limit.
Speed
The protective relays must operate at the required speed. There must be a correct coordination provided in various power system protection relays in such a way that for fault at one portion of the system should not disturb other healthy portion. Fault current may flow through a part of healthy portion since they are electrically connected but relays
associated with that healthy portion should not be operated
faster than the relays of faulty portion otherwise undesired interruption of healthy system may occur. Again if relay associated with faulty portion is not operated in proper time due to any defect in it or other reason, then only the next relay associated with the healthy portion of the system must be operated to isolate the fault. Hence it 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.
Important Elements for power system protection
Switch gear
Consists of mainly Bulk oil Circuit breaker, Minimum oil Circuit breaker, SF6 Circuit breaker, Air Blast Circuit breaker and Vacuum Circuit breaker etc. Different operating mechanisms such as solenoid, spring, pneumatic, hydraulic etc. are employed in Circuit Breaker. Circuit Breaker is the main part of protection system in power system it automatically isolate the faulty portion of the system by opening its contacts.
Protective gear
Consists of mainly power system protection relays like current relays, voltage relays, impedance relays, power relays, frequency relays, etc. based on operating parameter, definite time relays, inverse time relays, stepped relays etc. as per operating characteristic, logic wise such as differential relays, over fluxing relays etc. During fault the protection relay gives trip signal to the associated circuit breaker for opening its contacts.
Station Battery
All the circuit breakers of electrical power system are DC (Direct Current) operated. Because DC power can be stored in battery and if situation comes when total failure of incoming power occurs, still the circuit breakers can be operated for restoring the situation by the power of storage battery. Hence the battery is another essential item of the power system. Some time it is referred as the heart of the electrical substation. A Substation battery or simply a Station battery containing a number of cells accumulate energy during the period of availability of A.C supply and discharge at the time when relays operate so that relevant circuit breaker is tripped.
What is relay ? Definition of protective relay
A relay is automatic device which senses an abnormal condition of electrical circuit and closes its contacts. These contacts in turns close and complete the circuit breaker trip coil circuit hence make the circuit breaker tripped for disconnecting the faulty portion of the electrical circuit from rest of the healthy circuit.
Pickup level of actuating signal: The value of actuating quantity (voltage or current) which is on threshold above which the relay initiates to be operated.
If the value of actuating quantity is increased, the electro magnetic effect of the relay coil is increased and above a certain level of actuating quantity the moving mechanism of the relay just starts to move.
Reset level: The value of current or voltage below which a relay opens its contacts and comes in original position.
Operating Time of Relay ‘ Just after exceeding pickup level of actuating quantity the moving mechanism (for example rotating disc) of relay starts moving and it ultimately close the relay contacts at the end of its journey. The time which elapses between the instant when actuating quantity exceeds the pickup value to the instant when the relay contacts close.
Reset time of Relay ‘ The time which elapses between the instant when the actuating quantity becomes less than the reset value to the instant when the relay contacts returns to its normal position.
Reach of relay ‘ A distance relay operates whenever the distance seen by the relay is less than the pre-specified impedance. The actuating impedance in the relay is the function of distance in a distance protection relay. This impedance or corresponding distance is called reach of the relay.
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
‘ Definite time Relays
2. Inverse time Relays with definite minimum time(IDMT)
3. Instantaneous Relays
4. IDMT with inst.
5. Stepped Characteristic
6. Programmed Switches
7. Voltage restraint over current relay
Based on of logic the protection relay can be categorized as
‘ Differential
2. Unbalance
3. Neutral Displacement
4. Directional
5. Restricted Earth Fault
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.
Standard lead numbers used in control circuit of protection of power system
Certain lead numbers are standardized as follows and should be compulsorily adopted with ferrules at terminations of leads.
Main DC Positive supply ‘ J1
Main DC Negative supply ‘ J2
DC Positive bus inside panel ‘ K1
DC Nagetive bus inside panel ‘ K2
Remote Close – K15R
Remote Trip – K5R
Local Close – K15L
Local Trip – K5L
Metering CT secondaries ‘ D11, D31, D51, D71 etc.
Protection CT secondaries ‘ C11, C31, C51, C71 etc.
Special Protection CT secondaries ‘ A11, A31, A51, A71 etc.
PT scondaries ‘ E11, E31, E51, E71 etc.
Electromagnetic relay working
Practically all the relaying device are based on either one or more of the following types of electromagnetic relays
a) Magnitude measurement
b) Comparison
c) Ratio measurement
types of electromagnetic relays.
i. Attracted Armature type relay
ii. Induction Disc type relay
iii. Induction Cup type relay
iv. Balanced Beam type relay
v. Moving coil type relay
vi. Polarized Moving Iron type relay
Attraction Armature Type Relay
Attraction Armature Type Relay is the most simple in construction as well as its working principle. These types of electromagnetic relays can be utilized as either magnitude relay or ratio relay. These relays are employed as auxiliary relay, control relay, over current, under current, over voltage, under voltage and impedance measuring relays.
Hinged armature and plunger type constructions are most commonly used for these types of electromagnetic relays. Among these two constructional design, hinged armature type is more commonly used.
We know that force exerted on an armature is directly proportional to the square of the magnetic flux in the air gap. If we ignore the effect of saturation, the equation for the force experienced by the armature can be expressed as,
F = (KI2 ‘ K’)
Where F is the net force, K’ is constant, I is rms current of armature coil, and K’ is the restraining force.
The threshold condition for relay operation would therefore be reached when
KI2 = K’
If we observe the above equation carefully, it would be realized that the relay operation is dependent on the constants K’ and K for a particular value of the coil current.
From the above explanation and equation it can be summarized that, the operation of relay is influenced by
i. Ampere ‘ turns developed by the relay operating coil
ii. The size of air gap between the relay core and the armature.
iii. Restraining force on the armature.
Sepam protection relays:
A complete range for many different needs
Protection relays constantly monitor the power network and trip the circuit breakers
to isolate the faulty portion under fault conditions: overload, short-circuit, insulation
fault.
The Sepam range of protection relays is designed for all protection applications in
medium-voltage public and industrial distribution networks.
A multi-functional range of digital relays:
Each Sepam series offers all the functions required for the intended application:
‘ effective protection of life and property
‘ accurate measurements and detailed diagnosis
‘ integral equipment control
‘ local or remote indications and operation.
A Sepam solution for every application
‘ For each electro technical application, Sepam offers the relay suited to the protection
‘ needs of your network:
‘ substations (incomer or feeder type)
‘ transformers
‘ motors
‘ generators.
‘ capacitors.
‘ bus bars.
Integration in a remote-control system
To manage MV substations, Sepam protection relays can be connected to Easergy
T200I remote-control and monitoring interfaces (RTU) for operation with PowerLogic
power-monitoring units and Flair fault detectors.
This flexible solution includes a number of functions:
‘ protection of incoming and outgoing circuits
‘ detection of fault currents
‘ switch management
‘ database containing event logs and measurements
‘ backed-up power supply
‘ communication via SCADA
‘ local and remote access via a web server.
External tripping
A Sepam series 10 A can receive, via a logic input, a tripping order from an external protection device.
Trip-circuit supervision
This function continuously monitors the trip circuit to make sure that it has not been interrupted. The system shown opposite runs a low current through the trip circuit.
Sepam checks that the current is effectively present.
Remote circuit-breaker control
The circuit breaker can be remotely controlled via the communication system. A Sepam logic input is used to select the operating mode (local or remote).
Record of last fault
Displays the characteristics of the last fault. Sepam indicates the fault source, the value of the three phase currents and the earth current at the time of tripping.
The information is stored in memory until the next fault.
CT:
Function
PE50032 The specifically designed CSH120 and CSH200 core balance CTs are used for direct residual current measurement. The only difference between them is the diameter.
Due to their low voltage insulation, they may be used only on cables with earthed shielding.