Module 9 - Protection Flashcards
What is protection?
Set of events in a power system from fault detection to fault clearance
What do Protective Devices do?
DETECT, LOCATE and INITIATE the removal of faulted components from power network in minimum desirable time.
Causes of faults?
- Mechanical deterioration of equipment
- Overvoltage, overcurrent and overheating
- External damage from animals, birds, collision etc.
- Natural events like storms, thunder etc.
Main features of a protection system
Selectivity:
Detect and isolate the faulted item ONLY
Stability:
Leave all the healthy circuits alone to ensure continuation of supply
Sensitivity:
Detect even the smallest fault or system abnormality and operate correctly before fault causes irreparable damage.
Speed:
Operated as fast as possible/timely manner
Dependability:
MUST TRIP when needed
Security:
MUST NOT TRIP when not needed.
Principles of Protection
-Monitoring voltage (voltage transformers) and current (current transformers)
-Detect variance in parameters to discriminate between abnormal and normal conditions (relays do this)
-Do the protection (circuit breakers do this)
What is a burden?
- The load on VTs and CTs
- impedance connected to transformer secondary winding, but may also be VA delivered to load.
How do relays work?
- Disc can rotate and is restrained by spiral spring
- current is induced in disc, interacts with magnetic field and produces torque
-If overcurrent occurs (Input current greater than pick-up current) then the disc will rotate to close relay contacts - larger the input current, larger the torque and faster closing
- after current removed or reduced to below pick-up, spring resets contacts.
What are isolators?
- type of disconnecting mechanical switch to ensure circuit is completely isolated from supply mains
- isolators are extremely slow compared to circuit breakers
What are circuit breakers?
- Protective electrical device that works as switch to stop slow of current
- have means of extinguishing the arc at moment of contact separation and operate very fast
- isolators do not have these arc extinguishing features
What are sectionalizers?
- Used in distribution lines in conjunction with auto-reclosers
- when there’s a fault in downstream line of sectionalizer, line may be tripped due to an upstream auto re-closer
what is an auto-recloser
- with fault detected by sectionalizer, auto-recloser will trip
- re closer will attempt to automatically reenergize line in case fault was temporary
- if fault still exists, auto re-closer will trip again
How are voltage transformers operated?
- High impedance load
- almost open circuit
How are current transformers operated?
- Very low impedance load
- almost short circuit
Why do instrument transformers deliver low power?
- ensures voltage regulation of VT
- limits voltage across burden which increases the magnetizing current in the CT
- ensures amp-turns balance is maintained
Measuring vs Protection instrument transformers
- Measuring instrument transformers are accurate but cant handle high fault currents
- Protection instrument transformers can handle high fault currents but at the cost of accuracy
How to choose a CT?
- burden
-short circuit currents that might cause saturation - ratio and angle errors that would create relay inputs that do not represent primary conditions
What are electromechanical relays?
- attracted armature
- moving coil
- induction
- thermal
- motor operated
- mechanical
What are static relays?
- analogue electronics, not coils and magnets
- no moving parts
What are digital relays
- Use microprocessors and microcontrollers
What are numerical relays
specialised digital signals processor (DSP) as computational hardware together with software.
Types of overcurrent relays
- Instantaneous overcurrent relays
- Definitive minimum time-daly overcurrent relays
- inverse definitive minimum time (IDMT)
What are the five main components of a power protection scheme?
- Relays
- Circuit breakers
- current and voltage transformers
- fuses
- batteries
what is the function of a RELAY
sense faults by sensing current and voltage and initiate tripping to disconnect faulted equipment or sections
What are the purposes of CURRENT AND VOLTAGE TRANSFORMERS
step down the actual high currents and voltages that exist in power systems to levels which can be used by relays
