Element Theory
Equipment
Connections
Something Old
Something New
100
What is the equation for I2?
I2 = (Ia + a^2*Ib + a*Ic)/3
100
Where are the CTs on this breaker?
On the bushings
100
Do relays with voltage connections always need PTs?
No, occasionally system voltage is below the 300 V rating and PTs are not required.
100
What elements should be considered for breaker failure timing?
Relay output operation time, breaker operation time, retrip time (if applicable), security factor.
100
Which relays have time domain protection?
T400L, T401L, 411L, 751 (technically?)
200
In a generic sense, how is fault location calculated in SEL relays?
Ohms law
Vf/If = Zf
Zf/Zl = "distance" to fault
200
Why would someone choose a fuse saving scheme?
If the fuses are spread out and/or difficult to replace, it is worth it to minimize fuse replacements.
200
Name some possible IN connection options
Physical residual connection, flux summing CT, transformer neutral, cap bank neutral, NGR
200
How does fast bus tripping work?
Upstream/bus relay will have faster than normal OC elements, downstream relay will send blocking signal if OC is detected on feeder.
200
How is arc-flash better than typical 50 elements?
Faster response time, more sensitive, more secure with flash supervision.
300
How do you calculate secondary ohms from primary ohms?
Zpri* (CTR/PTR) = Zsec
300
What is the rating of a C400, 2000:5 CT tapped down to 800:5
C160 Rating
300
What is the difference between open delta and broken delta?
Open delta provides three phase-phase voltages, broken delta provides one 3V0 measurement.
300
What are a few benefits of HiZ grounding?
Limiting fault current, minimizing damage to system, allows system to continue to provide power during faults.
300
List all event types you can think of (sample rates don't matter)
Filtered
Raw
Differential
COMTRADE
High Impedance
Compressed
400
What are some differences between DCB and POTT?
DCB is a blocking scheme, and will allow tripping if comms fail (more dependable)
POTT is a permissive scheme, and will block the POTT scheme if comms fail (more secure)
400
What are some benefits of cap banks?
Power factor control, compensation for inductive loads, power quality
400
If you pass the primary through a CT twice, how does that affect the secondary current?
Secondary current will double.
400
How would you test a forward directional 50G element?
Check the line angle, current should lag voltage by that angle, make sure I2 and I0 magnitude supervisions are satisfied, check Z2F threshold and make sure the impedance is lower (higher magnitude) than threshold.
400
Why is anti-islanding protection important for microgrids?
Often, the microgrid source cannot support the microgrid load by itself, and will result in unacceptable power quality and potential damage.
500
What would be the compensation for a DABY transformer with wye-wye CTs?
W1 = 0
W2 = 1
500
Name every piece of equipment you recognize in this picture:
Recoser, pole, cross arm, conductor, insulator
500
Can CTs be paralleled? If yes, how and why would it be done?
Yes, if CT inputs aren't available or in Hi-Z bus diff.
500
How does an electromechanical 51 relay work?
Current passes through the inputs, magnetizing the main coil/main core. This magnetization induces an electromagnetic force on the disc, which then moves (resisted by the time dial spring and the permanent magnet) until closing the trip contact.
500
What protection elements should be used for generator protection?
50/51, 27/59, 81, 46, 47, 40, 25, 64,78,87