Capacitor and Inductor Basics 1

[djs6588]

I am a recently hired planning engineer and unfortunately the college I had graduated from did not offer many power courses, we took a little of everything and did not pick a set path. I feel I have a good understanding of the basics but it really bothers me when I do not completely understand what is going on and one thing I am struggling to wrap my head around is cap banks and exactly how they work. I have some ideas which seem right to me but I am looking for some reassurance that I am correct or why I am wrong.

1) If inductors make the current lag and you add caps to make the voltage lag this is just bringing the p.f. closer to unity. But whether it is at unity or not the inductors still need kvar to build up the magnetic field. The caps are now supplying the energy needed for the inductors magnetic field?
2) If the caps are now supplying the energy to build up the magnetic field how is this happening? In the first quarter of the sine wave, when the energy is being stored in the inductor, woundn't the capacitor be charging and not be able to supply the energy needed? If someone could help me understand what happens in a cap and inductor in the first half of the sine wave I would appreciate it.
3) If there is a cap 1 span away from an inductive load, and I am correct in thinking the caps supply the kvars to the inductor, then would the current in this 1 span of wire be higher than the span before it since there is additional current flowing between the cap and inductor?
4) How does a cap bank raise the voltage? My thinking was since there is less current being supplied by the generator then there are less line losses and that raises the voltage.

I appreciate any help. Thanks.

 

[wareagle]

Check here
http://www.google.com/search?sourceid=navclient&aq=hts&oq=&ie=UTF-8&rlz=1T4ADFA_enUS493US493&q=Capacitor+and+Inductor+Basics#hl=en&rlz=1T4ADFA_enUS493US493&q=circuit+analysis+capacitors+and+inductors&revid=77625114&sa=X&ei=VR0oUZrJBYH89QSBtYGQBQ&ved=0CIIBENUCKAA&bav=on.2,or.r_gc.r_pw.r_qf.&bvm=bv.42768644,d.eWU&fp=fd0b0c069f530852&biw=1512&bih=678"] "capacitor basics" [/url]
This is a complex subject to try and explain here.

 

[cvance]

“If inductors make the current lag and you add caps to make the voltage lag this is just bringing the p.f. closer to unity.” – in simple terms, yes this would be true.

“But whether it is at unity or not the inductors still need kvar to build up the magnetic field. The caps are now supplying the energy needed for the inductors magnetic field?” – Inductive loads are considered var consumers, where the imaginary component of their impedance is a positive value. Capacitors are considered var suppliers, where the imaginary component of their impedance is a negative value. Mathematically inductive loads would result in positive vars and capacitive loads would result in negative vars. Therefore, adding capacitors to a distribution line will provide var support (adding a negative var value to a larger positive var value equals a net smaller number) offsetting the reactive inductive component of the load profile.
This reduces the reactive component of power, which reduces the hypotenuse of your power triangle (MVA), without changing the real power component of the subject load (watts).

“If the caps are now supplying the energy to build up the magnetic field how is this happening?”
Read above statement.

“In the first quarter of the sine wave, when the energy is being stored in the inductor, wouldn’t the capacitor be charging and not be able to supply the energy needed” – this type of thinking is incorrect. The time required to get to system stability (saturation of your inductor or full charge to your capacitor), which is considered steady state (the condition for all normal system circuit analysis) is a function of your circuit’s natural impedance (also called surge impedance). Forget this concept until you have a solid understanding of power fundamentals, as this is a topic barely touched upon in fundamental power courses and discussed more in depth in a transients in power systems course.

“If there is a cap 1 span away from an inductive load, and I am correct in thinking the caps supply the kvars to the inductor,” - if your capacitor var output is the same as your inductive load var input then yes. Typically you would not size your capacitor bank to be equal or greater than your var load, so realistically, your capacitor will be supplying some vars but not all vars.

“then would the current in this 1 span of wire be higher than the span before it since there is additional current flowing between the cap and inductor?” – No. There is not an increase in current flow to your load based on the capacitor. Your load current is the same regardless of the addition of capacitors or not. Adding the capacitor will reduce the overall vars seen upstream from the point of the capacitor application, resulting in a reduction in current flow.

“How does a cap bank raise the voltage? My thinking was since there is less current being supplied by the generator then there are less line losses and that raises the voltage.” Yes.

I recommend getting a hold of a text in basic power fundamentals. This would help you to understand some concepts to be able to ask more in depth questions.

Also try to get a copy of
IEEE std 1036 – 2010 IEEE Guide for Application of Shunt Power Capacitors

If you have any more questions or want to get more specific on a single focus, post a reply