Assistant engineer interview questions – 7

• Where is the buckles relay used ?
• In power tranformer and voltage above 750KVA

Mock Interview questions

• How to differentiate the 220 kv and 110 kv tower with out any specification ?
• Calculate the number disk in the insulator .Each disk can handle the 11kv
Number of insulator calculation for example 220 KV line .

Number of insulator =120/(11*root2)+1

so the Number of insulator :10

Here root2 for the insulation depends on the peak voltage
+1 for the more protection

Lighting surge voltage,switching surge voltage

lighting surge voltage:

when the electrical equipment like fan ,tv etc are on off . Over voltage transient surge can occur in the AC power lines .Near by any lightening strike on the transmission line .Nearby lightning strikes can also generate transient surges in AC .

switching surge voltage:

when the sudden interruption in any circuit surges will produce .Switching surge is sudden hike in voltage level because of switching.When switching is done suddenly, there is increase in voltage for some seconds.You can see when heater is switched off, voltage surge occurs and there is change in brightness of bulb for second.

Application of zig-zag connection in the transformer
Harmonic voltages suppression
Provides low impedance to zero sequence currents
Less cost compared to Scott transformer
It provides perfect isolation between ground and component
full form of EHV,HVDC ?

EHV: Extra High Voltage transmission line .It will carries 345kv – 500kv
HVDC;High Voltage Direct current transmission line

What is IDMT relay

Inverse definite minimum time rely .IDMT relay is operating inverse to the current carrying by it.its minimum time relay.

what is the need of Busbar protection

Minimising damage at the fault location
Maintaining power system stability
localising isolation to avoid wide spread disruption

Performance required for the protection relays ?
fault clearance performance
Reliability
cost