1. Chapter 7 System Protection

2. 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.

3. 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

4. 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

5. 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

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

8. Inverse Time Current Curves

9. Coordination: Transformer Protection Fuse Only

10. Coordination: Transformer Protection CB & Fuse

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

12. Protection

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
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

16. 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

17. 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

18. 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)

19. Read Chapter 7!