Switchgear & Protection - 1 | GATE Notes & Videos for Electrical Engineering - Electrical Engineering (EE) PDF Download

Need For The Protection Scheme

As per the technology has improved & Generally all of the Machineries are being operating with the help of External Energy Resource & at most of the places the external energy source is Being Electrical one, Hence Because as per our knowledge the ratings of Equipment in the system is so large along with Electrical apparatus operates at various voltage levels and may be enclosed or placed in open. Under abnormal operating conditions protection is necessary for Safety of electrical Equipment's & Safety of human personnel.

so we require a protective systems for the safety purposes of human being & Electrical Equipment's, The main reason for designing a protected Power System is to isolate the faults & the components affected as a result of the fault.

The main function over here is to cater to the issue of tripping of circuit breakers. These devices also oversee that the automated operation is maintained. Another related task for the protective devices is for monitoring the system and this is about collecting the data. The most important parameter in this regard is the operating quality. The cost of protection is directly dependent on the kind of device in use.

Causes of Power System Failure
It is very important to understand the reasons behind these, if you were to understand the root causes that govern the reliability of a power system. This section deals with all those important reasons on which the reliability of a power system depends. Following are the main causes for power system failure

• Cables laid beneath/Underground cables
• Transformer breakdowns
• Lightning
• Tree Contact
• Insulation Failure
• Flash Over
• Physical Damage
• Birds & Squirrels
• Snakes & Insects
• Bears and cattle

Among the above given reasons for Power System Failure the former five to six such as Cables laid Beneath/Underground Cables/Transformer Breakdowns/Lightning/Tree Contact/Insulation Failure etc., reason are the main cause of the occurrence of Fault.

2012 Blackout in India

The July 2012 India blackout was the largest power outage in history, occurring as two separate events on 30 and 31 July 2012. The outage affected over 620 million people, about 9% of the world population, or half of India's population, spread across 22 states in Northern, Eastern, and Northeast India. An estimated 32 GW of generating capacity was taken offline in the outage.

History of Electrical Infrastructure in India

1. The Indian electrical infrastructure was generally considered unreliable. The Northern grid had previously collapsed in 2001.
2. 27% of power generated was lost in transmission or stolen, while peak supply fell short of demand by an average of 9%.Hence the nation suffered from frequent power outages that could last as long as 10 hours.
3. About 25% of the population, about 300 million people, had no electricity at all.
4. The power generating stations are hooked onto an interconnected network of transmission lines and substations. These generating stations supply electricity through these transmission lines.
5. The stability of the grids depends on a delicate equilibrium of demand supply chain. The amount of load is directly proportional to the amount of power generated.

Different Types Of Fault Occur in Power System

Configuration of Fault Types

Probability of Occurrence of Fault in Power System
Single Line to Ground Fault (85%)
Line to Line Fault  (8%)
Double Line to Ground Fault (5%) 
Three-Phase Fault  (2%)

Protection Scheme in Power System
Protection systems must not interfere with or limit the normal operation of the system but must continuously monitor the system to detect electrical failure or abnormal electrical conditions. Further important aspects in the design of the power system are:

• Incorporation of features aimed at preventing failures, and
• Provisions for mitigating the effects of failure when it occurs.

The type of electrical failure that causes greatest concern is the short-circuit, or ‘fault’ as it is usually called, but there are other abnormal operating conditions peculiar to certain elements of the system that also require attention. Some of the features of design and operation aimed at preventing electrical failure are listed below

• Provision of adequate insulation.
• Coordination of insulation strength with the capabilities of lightning surge arresters
• Use of overhead ground wires and low tower-footing resistance.
• Design for mechanical strength to reduce exposure, and to minimise the likelihood of failure caused by animals, birds, insects, dirt, sleet, bush fires, etc.
• Proper operation and maintenance practice.

Features that mitigate the immediate effects of an electrical failure

1. Design to limit the magnitude of short-circuit current
   • By avoiding too large concentrations of generating capacity
   • By using current-limiting impedance
2. Design to withstand mechanical stresses and heating owing to short-circuit currents.
3. Time-delay under-voltage devices on circuit breakers to prevent dropping loads during momentary voltage dips.
4. Ground-fault neutralizers.

Features for promptly disconnecting the faulty element

• Protective relaying
• Circuit breakers with sufficient interrupting capacity
• Fuses

Features that mitigate the loss of the faulty element

• Alternate circuits.
• Reserve generator and transformer capacity.
• Automatic Reclosing.

Features that operate throughout the period from the inception of the fault until after its removal, to maintain voltage and stability

• Automatic voltage regulation
• Stability characteristics of generators

Thus, protective relaying is one of several features of system design concerned with minimizing damage to equipment and interruptions to service when electrical failures occur. 

Zone of Protection in Power system

Primary & Back Up Protection

Primary protection (Main protection) is the essential protection provided for protecting an equivalent/machine or a part of the power system. As a precautionary measure, an addition protection is generally provided and is called ‘Backup Protection’. If any fault occurs in the protected area, the primary protection act first. If primary protection fails to act, the back-up protection comes into action and removes the faulty part from the healthy system. 

Advantages of Back-up Protection
Main protection can fail due to failure of one of the components in the protective system such as relay, auxiliary relay CT, PT, trip circuit, circuit-breaker, etc. If the primary protection fails, there must be an additional protection, otherwise the fault may remain uncleared, resulting in a disaster. When main protection is made inoperative for the purpose of maintenance, testing, etc. the Back-up protection acts like main protection. 

• As a measure of economy, Back-up protection is given against short-circuit protection and generally not for other abnormal conditions. The extent to which back-up protection is provided, depends upon economic and technical considerations.
• The cost of back-up protection is justified on the basis of probability of failure of individual component in protection system, cost of the protected equipment, importance of protected equipment, location of protected equipment, etc. 

Secondary Protection
Back-up protection is very important for stable and reliable power system, As we know that, it is not possible to design a 100% secure and efficient system because there are possibilities of failure in the connected CTs, PTs, circuit breaker etc. in the system. If it happens, then it will destroy our whole switching system. If the primary protection operation falls into trouble, then secondary protection disconnects the faulty part from the system. Moreover, when we disconnect primary protection for testing or maintenance purpose, then secondary or back-up protection will act as primary protection. In the above fig, relay “X” (1 Sec time setting) provides backup protection for each of the four connected lines to the main bus.

Types of Secondary or backup protection

• Relay Backup Protection
• Breaker Backup Protection
• Remote Backup

1. Backup Relay Protection
The equipment installed in power system network is very costly. The main equipment of the power system network is Power transformer which costs a lot. Reliability of electrical protection on such equipment should be high enough.

Not only the Transformer, the extra high voltage lines of the system demand extra reliability of protection. The backup relaying schemes provide this extra reliability to the system. 

Backup relays are extra relaying schemes attached to the equipment or part of the network with their own relaying system.

The main function of backup relay, to operate in any failure of tripping of Circuit Breaker due to main relays. The relay attached to the system may fail due to

• Mechanical defect of moving parts of the main relay,
• Failure of DC supply to the relay,
• Failure of tripping pulse to the breaker from relay,
• Failure of Current or Voltage to the relay from CT or PT circuits etc.

In this typical situation there should be another line of protection called back up relaying. Hence, back up relaying essentially have everything separate from main relaying scheme. This is because backup relay must not fail to operate in the event of failure of main relays.

There are some situations when we have to disconnect main relays from the system for preventive maintenance or trouble shootings. In those cases due to presence of back up relays, we do not have to interrupt the equipment or circuit. During this time back up protection scheme takes care of the protection of the system.

2. Breaker Backup Protection
Breaker failure relays are required to give a rapid trip when the primary circuit breaker does not break properly at example:  a short circuit in the network. The faulty section of the network could in this way be tripped separately.

Breaker failure relays are referred to as back-up protection devices which are applied as stoppage protection relays for the most relays. As case in point Breaker failure relay is employed as a backup device for differential relay of an influence Power Transformer and additionally for distance relay of a conductor.

Working Principle of Breaker Back-up Protection
There are three conditions must be satisfied For operation of breaker failure relay which are

1. Tripping signal from any main protection relay of the faulted equipment is exist.
2. Circuit breaker of the faulted equipment is still connect (n.o. auxiliary contact from circuit breaker is used).
3. There is current passing through this equipment even if it is value is very small.

Although one of the second and third condition is sufficient for healthy operation of breaker failure relay, it is preferable to use both for more security.

Essential Qualities of Protection

1. Speed

• When electrical faults or short circuits occur, the damage produced is largely dependent upon the time the fault persists. Therefore, it is desirable that electrical faults be interrupted as quickly as possible.
• High-speed fault detecting relays can now operate in as little time as 10 milliseconds and output relaying in 2 milliseconds. The use of protection zones minimizes the requirement for time-delayed relaying.

2. Selectivity

• Selectivity is the ability of a protection relay to accurately locate the fault and as well as classify it.
• A relay should also be able to suggest whether or not the fault is in its jurisdiction.
• This jurisdiction of a protection relay is known as its protection zone.

3. Sensitivity

• The protection must be able to distinguish between healthy and fault conditions, i.e., to detect, operate and initiate tripping before a fault reaches a dangerous condition.
• On the other hand, the protection must not be too sensitive and operate unnecessarily.
• The ability of relaying to fulfil the sensitivity requirement is improved through the use of protection zones.

4. Reliability

• The protective system must function whenever it is called upon to operate, since the consequences of non-operation can be very severe. This is accomplished by duplicate A and B protections and duplicate power supplies.
• This is the quality of the relay that determines its ability of not failing ever.
• This quality can be achieved by redundancy. Redundancy in protection depends on the criticality of the equipment to which the protection relay is connected.

% Reliability = (No. of correct trippings X 100) / (No. of desired trippings + No. of incorrect trips)

5. Dependability

• A Protection Relay is said to be dependable if it trips only when there is a fault current.
• It can be measured in terms of the ‘certainty’ of its tripping only when it has to trip.
• Dependability of a relay can be improved by improving its sensitivity.

% Dependability = (No. of correct trippings X 100) / (Total No. of desired trippings)

Classification of Relay

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.

Functions of protective Relay:

• To sound an alarm or to close the trip circuit of a circuit breaker so as to disconnect Faulty Section.
• To disconnect the abnormally operating part so as to prevent subsequent faults. For e.g. Overload protection of a machine not only protects the machine but also prevents Insulation failure.
• To isolate or disconnect faulted circuits or equipment quickly from the remainder of the system so the system can continue to function and to minimize the damage to the faulty part. For example – If machine is disconnected, immediately after a winding fault, only a few coils may need replacement. But if the fault is sustained, the entire winding may get damaged and machine may be beyond repairs.
• To localize the effect of fault by disconnecting the faulty part from healthy part, causing least disturbance to the healthy system.
• To disconnect the faulty part quickly so as to improve system stability, service continuity and system performance.
• Transient stability can be improved by means of improved protective relaying.
• To minimize hazards to personnel.

Terminologies of 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 electromagnetic 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.

Classification of Relay on the basis of technology

• Electromagnetic Relay 
• Static Relay 
• Micro-Processor Based Relay

Classification of Relay on The Basis of Their Function

• Over-Current Relay
• Under-Voltage Relay
• Impedance relay
• Under Frequency relay
• Directional relay

Other Category of Type of relay

1. Based on Characteristic
   • Definite time Relays.
   • Inverse definite minimum time Relays (IDMT)
   • Instantaneous Relays
   • IDMT with Instantaneous.
   • Stepped Characteristic
   • Programmed Switches
   • Voltage restraint over current relay
2. Based on Logic
   • Differential
   • Unbalance
   • Neutral Displacement
   • Directional
   • Restricted Earth Fault
   • Over Fluxing
   • Distance Schemes
   • Bus bar Protection
   • Reverse Power Relays
   • Loss of excitation
   • Negative Phase Sequence Relays etc.
3. Based on Actuating parameter
   • Current Relays
   • Voltage Relays
   • Frequency Relays
   • Power Relays etc.
4. Based on Operation Mechanism
   • Electro Magnetic Relay
   • Static Relay ⇒ (i)Analog (ii)Relay Digital Relay (iii) Numerical /Microprocessor Relay
   • Mechanical relay: These relay are subdivided into following Category of relay 
5. Thermal
   • OT Trip (Oil Temperature Trip)
   • WT Trip (Winding Temperature Trip)
   • Bearing Temp Trip etc.
6. Float Type
   • Buchholz
   • OSR
   • PRV
   • Water level Controls etc.     

• Pressure Switches.
• Mechanical Interlocks.
• Pole discrepancy Relay

Classification of Protective Scheme

• Over-Current Protection
• Distance Protection
• Differential Protection
• Carrier-Current Protection

1. Over Current Relay:
A relay that operates or picks up when it’s current exceeds a predetermined value (setting value) is called Over Current Relay.
Over-current protection protects electrical power systems against excessive currents which are caused by short circuits, ground faults, etc. Overcurrent relays can be used to protect practically any power system elements, i.e. transmission lines, transformers, generators, or motors.
Overcurrent includes short-circuit protection. Short circuits can be Phase faults, Earth faults, Winding faults, Differential and distance protection.

Over-current protection is useful for the following:

• Detect abnormal conditions
• Isolate the faulty part of the system
• Speed Fast operation to minimize damage and danger
• Discrimination Isolate only the faulty section
• Dependability / reliability
• Security / stability
• Cost of protection / against the cost of potential hazards

Type of Over Current Relay:

• Instantaneous Over Current (Define Current) Relay
• Define Time Over Current Relay
• Inverse Time Over Current Relay (IDMT Relay): Moderately Inverse, Very Inverse Time, Extremely Inverse
• Directional over Current Relay.

Application of over current relay: Motor protection, Transformer protection, Line protection, Distribution protection

2. Distance Protection Relay

• Distance protection is widely used in the transmission network, it is also called impedance protection because relay operates with respect to fault impedance of the transmission line (Z=V/I).
• It calculates the apparent impedance of a line with the help of voltage & current input connected to the relay. If measured impedance falls below set impedance trip command is issued to clear the fault.
• Measuring element is Current Transformer and Voltage Transformer. Usually, relay having 4 zones of the transmission line. Zone1, Zone2, Zone3 will be forward zones (Towards the Line) and zone 4 will be Reverse zone (Towards the Source).

Distance Relay are further classified as

• Impedance Relay 
• Reactance Relay
• Mho Relay

3. Differential Protection Relay

• It is used for transformer and generator protection. It simultaneously compares the phaser difference & magnitude of the current entering & leaving the protected zone. Differential protection is unit protection, relay works on the principle of Kirchoff's Current Law.
• Measuring element is Current Transformer. The differential current measured between the incoming current and Outgoing current must be negligible current during stable and through fault condition.
• In case of in-zone fault or unstable condition (due to CT saturate) relay will sense the differential current and issue the trip signal.

4. Carrier-current protection

• For long overhead lines, the power line itself may be used as the interconnecting channel between the terminal equipment.
• Carrier-current protection is the most widely used scheme for the protection of Extra High Voltage (EHV) and Ultra High Voltage (UHV) power lines.
• The carrier signal is directly coupled to the power line itself which is to be protected.
• Carrier-current protection is faster and superior to distance protection schemes and is more reliable when used for long transmission lines, although the terminal equipment are more expensive and complicated.