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Note for Switch Gear and Protection - SGP By mahaveer singh

  • Switch Gear and Protection - SGP
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Er.Mahaveer singh Contact no:8387057001,8005829571,Mahaveers024gmail.com Btech 6sem/3rdyear electrical ,MITS JADAN PALI Fundamentals of Power System Protection Objectives :• • • • • • ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ We will provide an overview of electrical energy systems. Make a case for protection systems. Describe necessity of apparatus and system protection. Define a relay element. Discuss evolution of relays from electromechanical to numerical relay. Describe functioning of a circuit breaker. Why do we need Protection? Electrical power system operates at various voltage levels from 415 V to 400 kV or even more. All such equipment undergo abnormalities in their life time due to various reasons. A worn out bearing may cause overloading of a motor. A tree falling or touching an overhead line may cause a fault. A lightning strike (classified as an act of God!) can cause insulation failure. Under frequency or over frequency of a generator may result in mechanical damage to it's turbine requiring tripping of an alternator.{49.5-50.5} Types of Protection :-Protection systems can be classified into apparatus protection and system protection. • Transmission Line Protection and feeder protection • Transformer Protection • Generator Protection

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• Motor Protection • Busbar Protection What is a Relay? • A relay is a logical element which processes the inputs (mostly voltages and currents) from the system/apparatus and issues a trip decision if a fault within the relay's jurisdiction is detected. • A relay R1 is used to protect the transmission line under fault F1. An identical system is connected at the other end of the transmission line relay R3 to open circuit from the other ends as well.

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• To monitor the health of the apparatus, relay senses current through a current transformer (CT), voltage through a voltage transformer (VT). VT is also known as Potential Transformer (PT). • The relay element analyzes these inputs and decides whether (a) there is a abnormality or a fault and (b) if yes, whether it is within jurisdiction of the relay. • A relay sends a tripping signal to circuit breaker(CB) which opens the circuit. • A real life analogy of the jurisdiction of the relay can be thought by considering transmission lines as highways on which traffic (current/power) flows. Classification or the types of relays depend on the function for which they are used. Some of the categories include protective, reclosing, regulating, auxiliary and monitoring relays. Protective relays continuously monitor these parameters: voltage, current, and power; and if these parameters violate from set limits they generate alarm or isolate that particular circuit. These types of relays are used to protect equipments like motors, generators, and transformers and so on. Reclosing relays are used to connect various components and devices within the system network, such as synchronizing process, and to restore the various devices soon after any electrical fault vanishes, and then to connect transformers and feeders to line network. Regulating relays are the switches that contacts such that voltage boosts up as in the case of tap changing transformers. Different Types of Relays Depending on the operating principle and structural features relays are of different types such as electromagnetic relays, thermal relays, power varied relays, multi-dimensional relays, and so on, with varied ratings, sizes and applications.

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1. Electromagnetic Relays These relays are constructed with electrical, mechanical and magnetic components, and have operating coil and mechanical contacts. Therefore, when the coil gets activated by a supply system, these mechanical contacts gets opened or closed. The type of supply can be AC or DC. DC vs AC Relays Both AC and DC relays work on the same principle as electromagnetic induction, but the construction is somewhat differentiated and also depends on the application for which these relays are selected. DC relays are employed with a freewheeling diode to de-energize the coil, and the AC relays uses laminated cores to prevent eddy current losses. Attraction Electromagnetic Relays Type These relays can work with both AC and DC supply and attract a metal bar or a piece of metal when power is supplied to the coil. This can be a plunger being drawn towards the solenoid or an armature being attracted towards the poles of an electromagnet as shown in the figure. These relays don’t have any time delays so these are used for instantaneous operation. Induction Type Relays These are used as protective relays in AC systems alone and are usable with DC systems. The actuating force for contacts movement is developed by a moving conductor that may be a disc or a cup, through the interaction of electromagnetic fluxes due to fault currents.

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