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Seeking Answers to Power and System Engineering Related Questions

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Junior Member level 2
May 2, 2015
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Guys I have an oral exam on Tuesday and there are some questions which could come up in the interview and i think its too challenging for me anyways.. Could you please help me answering some of these (briefly) couz i personally got no idea

Hi guys. I have few questions which I couldnt find anywhere or in the books and Its kind of like a marked coursework and I want to have a bit of idea if anyone knows something for the following questions.. I will appreciate it so much :)
U can just answer the ones you could.. i really need this please

1)What is the optimal operating condition of the transformer in terms of efficiency, voltage regulation and load current?

2)What is the purpose of reactive compensation in a power system?

3)How does connecting a capacitor to an inductively loaded line improve its voltage regulation?

4) Why is it generally desirable to maintain a near-unity overall system power factor from the perspective of a generator?

5) Why is it important for a transmission system to be able to operate even with lost lines?

6) How could ypu permanently compensate the inductive reactance of the line to minimise its reactive power absorption?


1) efficiecy: this is when P_out/P_in is max.
This should be at nominal load, but in detail it depends on transformer parameters

1) voltage regulation. This is when input to output impedance is minimal.
This again depend on transformer parameters. Prim and Sec series inductivity, prim and Sec resistance, coupling of prim to sec.

1) load current.
In my eyes this is problematic to answer. No current = lowest power dissipation, but also no output power. Therefore 0% efficiency.
I can't definitely say what the optimal current is...

2) reactive current causes extra power dissipation in transmission lines and transformers. Therefore you compensate reactive power to improve overall efficiency.

3) somehow it is answered in 2).
But if you compensate an inductive load with a capacitor you might create a resonant. This has to be avoided. This is at cos(phi) = 1.
Therefore one tries to compensate for cos(phi) of 0.95 or max 0.98 or so...

4) see 2)

5) don't know what this means

6).. with a capacitor..(if here the fixed inductive reactance of the line itself is meant)

Hope this helps



1. a) The optimal operating condition of a transformer in terms of efficiency is that its copper losses should be equal to core losses. The copper losses are the variable losses which depends on primary and secondary current while the core losses are constant for given frequency and dimensions of transformer
b) In order to achieve zero voltage regulation, the necessary condition is that the load power factor should be leading such that the p.f. = cos inverse (Xe2/Ze2)..............where xe2 and ze2 are the transformer reactance and impedance referred to secondary side.

Answer for 2, 3 and 4. The purpose of reactive power compensation in a system is to improve the power factor. If the power factor of a system is unity then, the system will be drawing only active power which is the power responsible for doing work, and if the power system is less than unity, it will be drawing reactive power, which is not utilized in doing real work but in changing the magnetic fields and results in heating of the devices and ultimately reduced efficiency.
Connecting a capacitor to an inductively loaded line provides the reactive power to the line which initially it was drawing from the generator or supply bus. Hence it draws less current from the supply and results in lower voltage drop which results in decreased voltage regulation.

6. Shunt inductors, Series capacitors , synchronous motors(with no shaft load and varying excitation) and static var compensators are some of the remedies used to reduce reactive power consumption of the line.

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