Selected 5 Interview Questions for Electrical Engineers asked for PSUs

Q1. For a 100 kW generator, only 50 kW of load is connected. Will the generator generate only 50 kW or 100 kW?

A1. Under steady state conditions, the (mechanical input - rotational losses) will be equal to the electrical power output. Hence, the generator will generate only 50 kW, even though its full capacity is 100kW.

Q2. What happens if we connect more than 100 kW of load?

A2.  If we connect more than 100kW of load, there will be flow of more current in the lines than their capacity and if present, the fuses shall blow off. If that doesn't happen then the wires will get overheated.

Q3. What is the difference between "earth" and "neutral"?

A3. The "neutral" refers to the star point in a star connection. The star connection of the 3 phase windings of an alternator, transformer or a 3 phase motor has one end of each phase-winding connected together, forming the neutral point or the star point. Neutral is not present in a delta connection.

"earth" refers to the zero potential level ( the ground) to which an electrical equipment is connected for personnel safety. The neutral of star connection can be earth to protect the 3 phase machine for large short circuit currents.

Q4. At what condition, a synchronous motor acts as a synchronous condenser?

A4.  When a synchronous motor is run unloaded, its primary function becomes improving the pf pf the system, by generating or absorbing reactive power. As we know that increasing the If changes its pf from leading to lagging(inverted-V curve). Thus a synchronous condenser is a synchronous motor designed with large field current. There is no shaft extension. Larger If requires greater amount of copper and large area of cross section of field poles as compared to ordinary synchronous motor of same rating.

Q5. How is a 100 MVA transformer cooled?

A5.  80 MVA -> ONAN (Oil-Natural Air-Natural)

       120 MVA -> ONAF (Oil-Natural Air-Forced)

       200 MVA -> OFAF (Oil-Forced Air-Forced)

       250 MVA -> OPAF (Oil-Pumped Air-Forced)

Hence a 100 MVA transformer has to be ONAF(Oil-Natural Air-Forced) cooled.  

"Soon I will be ready with the next five. If you want explanation on any of the above, or want to get your own question answered,or want to give a better answer to these questions, post them in the  comments section below."

Why the stator of the turbo-generators normally has a star winding, instead of a delta winding?

One of the main reasons for the larger generators producing voltages not more than around 22kV is the insulation restrictions. Higher voltages between the windings require more insulation in the stator windings which turn out not to be feasible for larger output voltages.

In case of a star-connected winding, the line voltage is √3 times the phase voltage. And the winding insulation depends on the phase voltage levels. For a delta-connected winding, line voltage is equal to phase voltage. Thus, for the same line voltage generated, amount of insulation required in a star winding will be lesser(due to lesser phase voltage) than that required in a delta winding.

VL = √3Vp               For star connection

   VL = Vp                   For Delta connection

• Insulation between the winding depends on the voltage levels
• Heating of the winding depends on the current levels.

NOTE: If you don’t find the answers given by me complete or justified, or want to add your views to them; you are free to give your own explanations through the comment section. You can also post your queries on the post.And we shall discuss them here.