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
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.
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
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:
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
Features that mitigate the immediate effects of an electrical failure
Features for promptly disconnecting the faulty element
Features that mitigate the loss of the faulty element
Features that operate throughout the period from the inception of the fault until after its removal, to maintain voltage and stability
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.
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.
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
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
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
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.
1. Speed
2. Selectivity
3. Sensitivity
4. Reliability
% Reliability = (No. of correct trippings X 100) / (No. of desired trippings + No. of incorrect trips)
5. Dependability
% Dependability = (No. of correct trippings X 100) / (Total No. of desired trippings)
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:
Terminologies of Protective Relay
Classification of Relay on the basis of technology
Classification of Relay on The Basis of Their Function
Other Category of Type of relay
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:
Type of Over Current Relay:
Application of over current relay: Motor protection, Transformer protection, Line protection, Distribution protection
2. Distance Protection Relay
Distance Relay are further classified as
3. Differential Protection Relay
4. Carrier-current protection
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