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14362648-Chapter 7 System Protection.ppt
- 1. Chapter 7 SystemProtection
- 2. Two Types ofProtection • System Protection deals with the electric grid using • Protective Relays, fault currents, grounding, circuit breakers, fuses, etc. • Personal Protection deals with safety • Rubber gloves and blankets, grounding jumpers, tagging, etc • This chapter is about SYSTEM PROTECTION.
- 3. Objective of SystemProtection • Remove faulted equipment from the grid before it damages other equipment. • Protection of equipment is key; it is not intended to protect people • Protects equipment from power faults and/or lightning • Effective equipment grounding enhances system protection • Relays operate faster, lightning is directed to earth
- 4. Protective Relaying • Relaydevices monitor the power system’s voltages and currents through CTs and PTs. • Programmed to initiateTrip or Close signals to CBs when settings are exceeded • Also send alarms to System Operators • All Battery Powered (DC) so they are functional if main AC power is out • The Stabilizing force against unwanted Destabilizing forces (faults) • Protective Relays are • Electromechanical • Solid State
- 5. Electromechanical • Composed ofcoils of wire, magnets, spinning disks and electrical contacts. Very Mechanical in nature. • Advantages • Self Powered, simple-single function design (ie Overcurrent Relay, Underfrequency Relay) • Absolutely Secure – Can’t be “hacked” • Disadvantages • One relay per phase, difficult to set up, adjust and requires more frequent maintenance and testing
- 6. Solid State (electronic/microprocessor) •Electronic with No moving parts. Modern • Advantages • Multi-function (TOC, UnderFreq, UnderVolt, differential, etc.) • Small space requirements • Some self-testing • Remote access • Fault location and advanced information storage • Disadvantages • External Power Required • Software can be complex • Many “eggs” or functions in one “basket” or device.
- 7. Inverse Time –Current Concept • The time to trip a circuit breaker shortens as the fault current increases • High Faults close to a substation, CB can open in less than 2 cycles • Minimum Pickup (or trip) Setting – Never trips below this setting • InstantaneousTrip Setting –Trip as fast as possible; no time delay
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- 11. Distribution Protection Schemes •Feeders are normally fed Radially out of stations • Power flows in one direction…downline • Typical Protection: • Underfrequency Relays • Overcurrent Protection with Reclosing Relays • A,B,C and G • Instantaneous (50) andTime OverCurrent (51) • Measurements taken from CTs mounted directly on CB bushing • Senses fault types of L-G, L-L, L-L-L
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- 13. Distribution Protection Schemes •Temporary Faults (Lightning) – Open Instantaneous then Close before downline fuses blow. • Permanent Faults (Tree, Car) – O/C, O/C, O/C, Lockout OR downline fuse blows before lockout.
- 14. Relay Coordination • Goal:Remove faulted element with the least customers out • Most downstream clearing device from the fault clears the fault first • Upstream devices act as backup clearing devices
- 15. Transmission Protection • Muchdifferent than distribution protection because lines are normally loop fed. • Multiple Generation Sources, Buses, Lines, Substations and Circuit Breakers • Power Flow can occur in either direction on theT-Lines • Zone or Distance Relaying • Zones overlap to provide Redundancy • CTs & PTs used extensively to “feed” quantities to relays
- 16. Transmission Protection • DirectionalRelaying detects which bus the fault is located • Over and UnderVoltage Relays • Turn on andTurn off Station Capacitor Banks • Trip breakers for abnormal conditions • Differential Relay Protection for buses, transformers, generators • Power in must equal Power out • If not, there is a problem, so we trip
- 17. Generator Protection • DifferentialRelays forWinding Short Ckt Protection • Field Ground Protection • Motoring • Lack of mechanical energy from prime mover • Power flows INTO the Generator • Underfrequency Relay for Loss of Excitation • Could lose synchronism • Frequency Relays • Volts per Hz relays
- 18. Generator Synchronization • SynchronizationRelay purpose is to connect 2 lines together OR place a spinning generator online. • Permissive Relays “look” for 4 conditions: • 1 Frequency (60 Hz both sides of CB) • 2Voltage (Magnitudes match both sides of CB) • 3 Phase Angle (Sine waves match both sides of CB) • 4 Rotation (ABC)
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