Protection System in Power System
This portion of our website covers almost everything related to protection system in power systemincluding standard lead and device numbers, mode of connections at terminal strips, color codes in multi-core cables, Dos and Don’ts in execution. It also covers principles of various power system protection relays and schemes including special power system protection schemes like differential relays, restricted earth fault protection, directional relays and distance relays etc. The details of transformer protection, generator protection, transmission line protection & protection of capacitor banks are also given. It covers almost everything about protection of power system.
The switchgear testing, instrument transformers like current transformer testing voltage or potential transformer testing and associated protection relay are explained in detail. The close and trip, indication and alarm circuits different of Circuit breakers are also included and explain.
Objective of power system protection
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
Let’s have a discussion on basic concept of Protection system in power system and coordination ofprotection relays.
In the picture the basic connection of protection relay has been shown. It is quite simple. 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.
Electrical Protection Relays or Protective Relays
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.
Power System Protection
Electrical Protection Relay
Code of practice Electrical Protection
Three Phase Vector Diagram
Electrical Fault Calculation
Electromagnetic Relay
Electrical Protection Relay
Code of practice Electrical Protection
Three Phase Vector Diagram
Electrical Fault Calculation
Electromagnetic Relay
Now let’s have a discussion on some terms related to protective relay
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.
Power system protection relays can be categorized into different Types of relays.
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
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
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
1. 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.
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.
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
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
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.
(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.
(a) Buchholz
(b) OSR
(c) PRV
(d) Water level Controls etc.
3. Pressure Switches.
4. Mechanical Interlocks.
5. Pole discrepancy Relay.
List Different protective relays are used for different power system equipment protection
Now let’s have a look on which different protective relays are used in different power system equipment protection schemes
Relays for Transmission & Distribution Lines protection
| SL | Lines to be protected | Relays to be used |
|---|---|---|
| 1 | 400 KV Transmission Line | Main-I: Non switched or Numerical Distance Scheme Main-II: Non switched or Numerical Distance Scheme |
| 2 | 220 KV Transmission Line | Main-I : Non switched distance scheme (Fed from Bus PTs) Main-II: Switched distance scheme (Fed from line CVTs) With a changeover facility from bus PT to line CVT and vice-versa. |
| 3 | 132 KV Transmission Line | Main Protection : Switched distance scheme (fed from bus PT). Backup Protection: 3 Nos. directional IDMT O/L Relays and 1 No. Directional IDMT E/L relay. |
| 4 | 33 KV lines | Non-directional IDMT 3 O/L and 1 E/L relays. |
| 5 | 11 KV lines | Non-directional IDMT 2 O/L and 1 E/L relays. |
Relays for Transformer protection
| SL | Voltage Ratio and Capacity of Transformer | Relays on HV Side | Relays on LV Side | Common Relays |
|---|---|---|---|---|
| 1 | 11/132 KV Generator Transformer | 3 nos Non-Directional O/L Relay 1 no Non-Directional E/L Relay and/or standby E/F + REF Relay | - - | Differential Relay or Overall differential Relay Overflux Relay Buchholz Relay OLTC Buchholz Relay PRV Relay OT Trip Relay WT Trip Relay |
| 2 | 13.8/220 KV 15.75/220 KV 18/400 KV 21/400 KV Generator Transformer | 3 nos Non-Directional O/L Relay 1 no Non-Directional E/L Relay and/or standby E/F + REF Relay | - - | Differential Relay or Overall differential Relay Overflux Relay Buchholz Relay OLTC Buchholz Relay PRV Relay OT Trip Relay WT Trip Relay |
| 3 | 220 /6.6KV Station Transformer | 3 nos Non-Directional O/L Relay 1 no Non-Directional E/L Relay and/or standby E/F + REF Relay | 3 nos Non-Directional O/L Relay | Differential Relay Overflux Relay Buchholz Relay OLTC Buchholz Relay PRV Relay OT Trip Relay WT Trip Relay |
| 4 | Gen-volt/6.6KV UAT | 3 nos Non-Directional O/L Relay | 3 nos Non-Directional O/L Relay | Differential Relay Overflux Relay Buchholz Relay OLTC Buchholz Relay PRV Relay OT Trip Relay WT Trip Relay |
| 5 | 132/33/11KV upto 8 MVA | 3 nos O/L Relay 1 no E/L Relay | 2 nos O/L Relays 1 no E/L Relay | Buchholz Relay OLTC Buchholz Relay PRV Relay OT Trip Relay WT Trip Relay |
| 6 | 132/33/11KV above 8 MVA & below 31.5 MVA | 3 nos O/L Relay 1 no Directional E/L Relay | 3 nos O/L Relay 1 no E/L Relay | Differential Relay Buchholz Relay OLTC Buchholz Relay PRV Relay OT Trip Relay WT Trip Relay |
| 7 | 132/33KV, 31.5 MVA & above | 3 nos O/L Relay 1 no Directional E/L Relay | 3 nos O/L Relay 1 no E/L Relay | Differential Relay Overflux Relay Buchholz Relay OLTC Buchholz Relay PRV Relay OT Trip Relay WT Trip Relay |
| 8 | 220/33 KV, 31.5MVA & 50MVA 220/132KV, 100 MVA | 3 nos O/L Relay 1 no Directional E/L Relay | 3 nos O/L Relay 1 no Directional E/L Relay | Differential Relay Overflux Relay Buchholz Relay OLTC Buchholz Relay PRV Relay OT Trip Relay WT Trip Relay |
| 9 | 400/220KV 315MVA | 3 nos Directional O/L Relay (with dir.highset) 1 no Directional E/L relay. Restricted E/F relay 3 nos Directional O/L Relay for action | 3 nos Directional O/L Relay (with dir.highset) 1 no Directional E/L relay. Restricted E/F relay | Differential Relay Overflux Relay Buchholz Relay OLTC Buchholz Relay PRV Relay OT Trip Relay WT Trip Relay Over Load (Alarm) Relay |
Points to be remembered in respect of protection of transformers
i. No Buchholz relay for transformers below 500 KVA capacity
ii. Transformers up to 1500 KVA shall have only Horn gap protection
iii. Transformers above 1500 KVA and upto 8000 KVA of 33/11KV ratio shall have one group control breaker on HV side and individual LV breakers if there is more than one transformer.
iv. Transformers above 8000 KVA shall have individual HV and LV circuit breakers.
v. The relays indicate above shall be provided on HV and LV
vi. LAs to be provided on HV & LV for transformers of all capacities and voltage class.
vii. OLTC out of step protection is to be provided where Master follower scheme is in operation
viii. Fans failure and pumps failure alarms to be connected.
ix. Alarms for O.T., W.T., Buchholz (Main tank & OLTC) should be connected.
ii. Transformers up to 1500 KVA shall have only Horn gap protection
iii. Transformers above 1500 KVA and upto 8000 KVA of 33/11KV ratio shall have one group control breaker on HV side and individual LV breakers if there is more than one transformer.
iv. Transformers above 8000 KVA shall have individual HV and LV circuit breakers.
v. The relays indicate above shall be provided on HV and LV
vi. LAs to be provided on HV & LV for transformers of all capacities and voltage class.
vii. OLTC out of step protection is to be provided where Master follower scheme is in operation
viii. Fans failure and pumps failure alarms to be connected.
ix. Alarms for O.T., W.T., Buchholz (Main tank & OLTC) should be connected.
