Chapter 7 System Protection

Two Types of Protection 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.

Objective of System Protection 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

Protective Relaying Relay devices monitor the power system’s voltages and currents through CTs and PTs. Programmed to initiate Trip 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

Electromechanical Composed of coils 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

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.

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 Instantaneous Trip Setting – Trip as fast as possible; no time delay

Inverse Time Current Curves

Coordination: Transformer Protection Fuse Only

Coordination: Transformer Protection CB & Fuse

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) and Time OverCurrent (51) Measurements taken from CTs mounted directly on CB bushing Senses fault types of L-G, L-L, L-L-L

Protection

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.

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

Transmission Protection Much different 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 the T-Lines Zone or Distance Relaying Zones overlap to provide Redundancy CTs & PTs used extensively to “feed” quantities to relays

Transmission Protection Directional Relaying detects which bus the fault is located Over and UnderVoltage Relays Turn on and Turn 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

Generator Protection Differential Relays for Winding 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

Generator Synchronization Synchronization Relay 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) 2 Voltage (Magnitudes match both sides of CB) 3 Phase Angle (Sine waves match both sides of CB) 4 Rotation (ABC)

Read Chapter 7!