THE UNIVERSITY OF LAHORE,ISLAMABAD CAMPUS COURSE: POWER SYSTEM PROTECTION Lab Work Sheet 1 Student Name:________________ Reg No: _____________________ Date: _______________________ Marks obtained: ____________________ Checked date: _______________ Instructions: Complete all activities in lab and get checked by the lab engineer. Home task should be completed before next lab and must checked by the lab engineer. At the end of all activities and home task students advised to write a summary in their own words. Late submission will lead to deduction of marks. Date must be properly mentioned. LAB EAXPERIMENT NO: 1 INTRODUCTION TO POWER PROTECTION SYSTEM Objective: To learn about electrical power protection system components. Relay: A relay is an electrically operated switch. It is a mechanical device. Relays are used where it is necessary to control a circuit by a low-power signal.or where several circuit must be controlled by one signal. Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults; in modern electric power systems these functions are performed by digital instruments still called "protective relays" Transformer: A transformer is an electrical device that transfers energy between two or more circuits through electromagnetic induction. Transformer types: There are various types of transformer. Two of them are explained below; 1. current transformer (CT) 2. potential transformers (PT) Current Transformer (CT): A current transformer (CT) is used for measurement of alternating electric currents. Current transformers, together with potential transformers (PT), are known as instrument transformers. A current transformer isolates the measuring instruments from what may be very high voltage in the monitored circuit. Current transformers are commonly used in metering and protective relays in the electrical power industry. Potential transformers (PT): Potential transformers (PT) are a parallel connected type of instrument transformer, used for metering and protection in high-voltage circuits or phasor phase shift isolation. They are designed to present negligible load to the supply being measured and to have an accurate voltage ratio to enable accurate metering. A potential transformer may have several secondary windings on the same core as a primary winding, for use in different metering or protection circuits. The primary may be connected phase to ground or phase to phase. The secondary is usually grounded on one terminal. Circuit breaker: A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and interrupt current flow. Unlike a fuse, which operates once and then must be replaced, a circuit breaker can be reset manually or automatically to resume normal operation. Circuit breakers are made in varying sizes, from small devices that protect an individual household appliance up to large switchgear designed to protect high voltage circuits feeding an entire city. Types of circuit breakers: Low-voltage circuit breakers Low-voltage (less than 1,000 VAC) types are common in domestic, commercial and industrial application Sulfur hexafluoride (SF6) circuit breakers A sulfur hexafluoride circuit breaker uses contacts surrounded by sulfur hexafluoride gas to quench the arc. They are most often used for transmission-level voltages and may be incorporated into compact gas-insulated switchgear. Vacuum circuit breaker A vacuum circuit breaker is such kind of circuit breaker where the arc quenching takesplace in vacuum. The technology is suitable for mainly medium voltage application. Oil circuit breaker Mineral oil has better insulating property than air. In oil circuit breaker the fixed contact and moving contact are immerged inside the insulating oil. Whenever there is a separation of current carrying contacts in the oil, the arc in circuit breaker is initialized at the moment of separation of contacts, and due to this arc the oil is vaporized and decomposed in mostly hydrogen gas and ultimately creates a hydrogen bubble around the arc. This highly compressed gas bubble around the arc prevents re-striking of the arc after current reaches zero crossing of the cycle. The oil circuit breaker is the one of the oldest type of circuit breaker Isolator: Isolator is used to ensure that an electrical circuit is completely de - energised for service or maintenance. Such switches are often found in electrical distribution and industrial applications, where machinery must have its source of driving power removed for adjustment or repair. Highvoltage isolation switches are used in electrical substations to allow isolation of apparatus such as circuit breakers, transformers, and transmission lines, for maintenance. The isolator or disconnector is usually not intended for normal control of the circuit but only for safety isolation. Isolators may be used in high voltage devices like transformers. Difference between isolator and circuit breaker • Isolator is an off-load device while, circuit breaker is an on-load device. • Isolator is a switch operated manually, which separate the circuit from the power main and discharges the trapped charges in the circuit. • Circuit breakers operate automatically, triggered by electromechanical mechanism inside and are a safety feature for abnormal loads and voltages in the circuit. Fuse: Fuse is a type of low resistance resistor that acts as a sacrificial device to provide over current protection, of either the load or source circuit. Its essential component is a metal wire or strip that melts when too much current flows through it, interrupting the circuit that it connects. Short circuits, overloading, mismatched loads, or device failure are the prime reasons for excessive current. Fuses are an alternative to circuit breakers.