Hello. Welcome to this module on overcurrent protection relays. Let us start with the topic on introduction to the overcurrent protection. At the end of this topic, you will be able to state the need of overcurrent protection, discuss the protective devices used for overcurrent protection, list the types of overcurrent, illustrate the protection coordination, show the applications of overcurrent relay. Let's start with the definition of overcurrent. When a short circuit or overload situation occurs in an electrical circuit, then the normal load current will be exceeded. It is known as overcurrent. Now, let's see why do we need overcurrent protection. In the electrical circuits, when voltage and current levels exceed from the safe state to the abnormal state, it might damage the electrical equipment. Occurrence of the damage depends upon the value of current and duration of fault. To avoid this, we need overcurrent production. Let's understand what's an overcurrent relay and its functions. A relay which senses and sends tripping signals when the current exceeds a predetermined set value is called an overcurrent relay. Its functions are as follows. One, detect abnormal fault current and or load current. Two, isolate faulty part of the system. Three, fast operation to minimize damage. With an overcurrent relay, we will achieve reliability and efficient operation of the power system. The factors that influence the amount of fault current are voltage and impedance of source, impedance of a transformer, contribution of the motor, the distance between the source and the fault. Now, we will see factor to reduce magnitude of fault current. We can lower the source impedance. This can be reduced by making improvements in the utility system, adding generators, and improving the transmission network. Also, to limit the maximum fault current, distribution transformers' impedance can be used. In the initial few cycles, any motor such as induction, synchronous motor must be treated as a source for computing fault current. As we travel away from the source, the fault current usually decreases. An exception being a system with large motor loads at the remote ends. Next, we will see about types of overcurrent. They are overload, short-circuit, ground fault. Let's see each type in detail now. Let's start with overload overcurrent. It happens when an electrical circuit is needed to carry load current in excess of the rated-load capacity whether by original design or by modification. Overload load situations can develop in a building's electrical power distribution system at the service, feeder, or branch circuit level. Even when a motor is mechanically overloaded, an overcurrent will occur. Next. Short-circuit overcurrent. This is a high-magnitude fault overcurrent that effectively provides a low resistance in parallel with the associated load's impedance. Next. Ground fault overcurrent. Your short-circuit situation affecting just one of the circuit conductors and the grounded metal raceway or an electrical distribution or usage equipment enclosure is known as ground fault overcurrent. Faults in power system can arise due to short circuits, ground contact, isolated neutral systems, high-impedance grounded systems, open phases. Next, let's see the statistics for the electrical fault occurrence. For single-phase-to-ground, it is 70-80 percent. For phase-to-phase-to-ground, it is 10-17 percent. For phase-to-phase, it is 8-10 percent. For a three-phase, it is 2-3 percent. Now, let's see the protective devices. Protective devices that are used for overcurrent protection are: fuse, recloser, sectionalizer, circuit breaker, and relay. Let's discuss each protective device in brief. Fuse. It is an electrical protective device consisting of a metallic wire strip, that melts to open an electric circuit if the current exceeds a safe level. The melting is because of the excessive heat generated by the square of current and resistance of the strip. Recloser. It is high voltage circuit breaker with inbuilt current, voltage sensor, and protective relay. It isolates electric power when the trouble occurs and restores power automatically once fault is removed. Sectionalizer. It isolates a faulty section of the line from the rest of the distribution system when a permanent fault occurs, and automatically observing pre-selected number of current interrupting operations by recloser. Circuit breaker. It's a device that interrupts aberrant or fault current after protective relays detect a fault in electrical system. It's a mechanical device that interrupts the flow of high amplitude or fault current, which also acts as a switch. Protective relay. It is designed to send trip signals to circuit breaker when the detection of fault occurs in the power system. The circuit breaker isolates the fault from the healthy portion. Next, we move to primary requirements of overcurrent protection. For permissible overcurrent, high starting currents, and current surges, overcurrent protection with time added is the fundamental requirement. High current set instantaneous relay is used if delay is not permissible. The protection should be coordinated with neighboring overcurrent protection so that, relay nearest to fault operates first. This minimizes the system unwarranted disruption due to the fault. Overcurrent relay are applied on various protection devices. Let's see each one by one. It protects motor stator windings against short circuits and overloads. For high rating of motors, over-current relays are used. Transformer protection. It is used mostly at power-transformer locations for external fault backup protection. On some sub-transmission lines, distance relay is not economically viable. When distance relays are utilized for phase faults, most transmission lines have primary ground-fault protection. On most lines with pilot relaying as main protection, overcurrent relay can be used as ground backup protection. Let us summarize all that you have learned. The need for overcurrent protection, types of overcurrent, protective devices used for overcurrent protection, protection coordination, and applications of overcurrent relay.
