In a power system consisting of generators, transformers, transmission, and distribution circuits, it is inevitable that sooner or later some failure will occur in the system. When a failure occurs on any part of the system, it must be quickly detected and disconnected from the system, because if the fault is not cleared quickly, it may cause unnecessary interruption of service to the customers and prevents the effects of fault from spreading into the system.
The detection of a fault and disconnection of a faulty section is achieved by using fuses or relays in conjunction with circuit breakers.
A fuse performs both detection and interruption functions automatically, but its use is limited for the protection of low voltage circuits only.
For high voltage circuits (above 3.3 kV), relays and circuit breakers are employed for protection of power system for faults. The relays detect the fault and supply information to the circuit breaker which performs the function of circuit interruption.
Protective Relays:
A protective relay is a device that detects the fault and initiates the operation of the circuit breaker to isolate the defective element from the rest of the system.
A typical relay circuit is shown in figure below.
Fundamental Requirements of Protective Relays:
The main function of protective relays is to cause the prompt removal from service of any element of the power system when it starts to operate in an abnormal manner or interfere with the effective operation of the rest of the system.
In order that protective relay system may perform function satisfactorily, it should have the following qualities:
(i) Selectivity: It is the ability of the protective system to select correctly that part of the system in trouble and disconnect the faulty part without disturbing the rest of the system.
(ii) Speed: The relay system should disconnect the faulty section as fast as possible for the following reasons:
- Electrical apparatus may be damaged if they are made to carry the fault currents for a long time.
- A failure on the system leads to a great reduction in the system voltage. If the faulty section is not disconnected quickly, then the low voltage created by the fault may shut down consumers’ motors and the generators on the system may become unstable.
- The high-speed relay system decreases the possibility of development of one type of fault into the other more severe type.
(iii) Sensitivity: It is the ability of the relay system to operate with low value of actuating quantity. Sensitivity of a relay is a function of the volt-amperes input to the coil of the relay necessary to cause its operation. The smaller the volt-ampere input required to cause relay operation, the more sensitive is the relay.
Thus, a 1 VA relay is more sensitive than a 3 VA relay.
(iv) Reliability: It is the ability of the relay system to operate under the predetermined conditions.
(v) Simplicity: The relaying system should be simple so that it can be easily maintained. Reliability is closely related to simplicity. The simpler the protection scheme, the greater will be its reliability.
(vi) Economy: The protective relaying used must be economical. As a rule, the protection devices should not cost more than 5% of total cost.
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. Over current relays can be used to protect practically any power system elements, i.e. transmission lines, transformers, generators, or motors.
Over current 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 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 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.
Universal Relay Torque Equation
The universal relay torque equation can be given as
where I = RMS value of current in current coil
V = RMS value of voltage fed to the voltage coil
ϕ = Electrical angle between V and I
T = The maximum torque angle K1, K2
and K3 = Relay constant
K = Mechanical restraining torque
Differential Protection:
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 a unit protection, relay works on the principle of Kirchhoff'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.
Applications of Differential protection: Transformer, Generator and Cable Protection.
INTRODUCTION TO CIRCUIT BREAKERS
A circuit breaker can make or break a circuit either manually or automatically under all conditions, i.e., no-load, full-load and short-circuit conditions.
In other words, a circuit breaker is a piece of equipment which can
(i) make or break a circuit either manually or by remote control under normal conditions.
(ii) break a circuit automatically under fault conditions.
(iii) make a circuit either manually or by remote control under fault conditions
Operating principle: A circuit breaker essentially consists of fixed and moving contacts, called electrodes. Under normal operating conditions, these contacts remain closed and will not open automatically until and unless the system becomes faulty.
Of course, 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 circuit breaker get energised and the moving contacts are pulled apart by some mechanism, thus opening the circuit.
When the contacts of a circuit breaker are separated under fault conditions, air 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 beat which may cause damage to the system or to the circuit 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.
Principles of Arc Extinction:
The factors responsible for the maintenance of arc between the contacts are as follows:
(i) potential difference between the contacts
(ii) ionized particles between contacts
Arc Voltage: It is the voltage that appears across the contacts of the circuit breaker during the arcing period.
Restriking voltage: It is the transient voltage that appears across the contacts at or near current zero during arcing period.
Recovery voltage: A transient recovery voltage for high-voltage circuit breakers is the voltage that appears across the terminals after current interruption. It is a critical parameter for fault interruption by a high-voltage circuit breaker, its characteristics can lead either to a successful current interruption or to a failure.
CLASSIFICATION OF CIRCUIT BREAKERS
Circuit breakers can be classified as:
(i) Oil circuit breakers which employ some insulating oil (e.g. transformer oil) for arc extinction.
(ii) Air-blast circuit breakers in which high pressure air-blast is used for extinguishing the arc.
(iii) Sulphur hexafluoride circuit breakers in which Sulphur hexafluoride (SF6) gas is used for arc extinction.
(ii) Vacuum circuit breakers in which vacuum is used for arc extinction.
CURRENT CHOPPING
It is the phenomenon of current before the natural current zero is reached. Current chopping mainly occurs in air-blast circuit breakers because they retain the same extinguishing power irrespective of the magnitude of the current to be interrupted. When breaking low currents (e.g., transformer magnetizing current) with such breakers, the powerful de-ionizing effect of air-blast causes the current to fall abruptly to zero well before the natural current zero is reached. This phenomenon is known as current chopping and results in the production of high voltage transient across the contacts of the circuit breaker.
CIRCUIT BREAKER RATINGS
Under fault conditions, a circuit breaker is required to perform the following three functions as follows:
1. It must be capable of opening the faulty circuit and breaking the fault current.
2. It must be capable of being closed on to a fault.
3. It must be capable of carrying fault current for a short time while another circuit breaker (in series) is clearing the fault.
As per the above functions performed by a circuit breaker, circuit breakers have three ratings:
Breaking capacity: It is current (rms) that a circuit breaker is capable of breaking at given recovery voltage, power factor, and rate of rise of Restriking voltage
Making capacity: The peak value of current (including d.c. component) during the first cycle of current wave after the closure of circuit breaker is known as making capacity.
Short-time rating: It is the period for wit the circuit breaker is able to carry fault current while remaining closed.
The short-time rating of a circuit breaker depends upon its ability to withstand
(a) the electromagnetic force effects
(b) the temperature rise.
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