Current in LC Circuit Connected to the Source of EMF

In parallel connection current is minimum.The circuit at resonance is called current magnifier.Initially both behave as a SC path.

At PF correction bases does not lie resonance ?

What is SC?

What is the basis of assuming that source impedance does not have an impedance?

PF correction capacitors may have inductrance in series for several reasons. This series impedance is in the circuit of the applied emf. The applied emf is cancelled by the voltage drop across the PF capacitor and inductor in series, satisfying Kirchoff's voltage law.

If you can draw the circuit diagram of what you are explaining, we can have better understanding of the same.

My qwestion may be idiotic.

Source impedance (for instance of an outlet 3 phases 400V) is very low. Load connected to it is inductive (an inductive motor without load for example), with a low PF. Parallel with the load capacitors bank is connected improving the PF. Why the reactive currents prefer to flow from the L to C and vice versa ? Why they do not flow the other direction - back to the sourse - through the source, that is, by the path of less impedance ?

Source only supplies the "Effective Current" of the parallel combination of the "Inductor" and the "Capacitor"

BY VIRTUE OF KIRCHOFF'S CURRENT LAW THIS IS THE ONLY SOLUTION.

Your impression that the current is not flowing through the source is not at all correct. Both the inductor and capacitor are passive elements and cannot supply the current. The effective current always comes from the source.

If you disconnect the load (R,L,C), the source is open-circuited and there is no current..only the open-circuit voltage. The circuit needs to be completed externally to make a current flow...

Or do you mean something else?

Ignoring the load parameters (ind motor), let's say the impedance of the C is 5 Ohms, the impedance of the source (transformer) - 0,005 Ohms.

In the absence of the C a back emf from the coils of the lightly loaded motor will flow back into the source in the form of reactive current, let's say of 5 A, 50 times per second in Europe, 60 times in the USA.

Why, after connecting the C, should the reactive current flow the path of more impedance of 5 Ohms (charging the C), instead of the path of less impedance of 0.005 Ohms (heating the lines from the source and the coils of the source)?

You failed to notice that capacitive reactance is negative compared to inductive reactance which is positive. You are only comparing the magnitude and ignoring the direction.

Yes, 50 times per second in one direction, 50 times in the other.

There lies the answer. A 5 MVAR capacitor compensates a 5 MVAR inductor with the source having 0 MVAR or 0 Amperes.

the capacitive impedance is lower [being negative] compared to source impedance. The source therefore has lesser current.

Do the reactive currents of L and C flow through the source, towards each other and mutually compensating each other in the case of PF correction ?

My big misconception may lie in believing reactive current of L flows through C charging C, and the reactive current of C flows through L creating flux in L.

Individual reactive currents of L and C will not flow into the source. Only the effective reactive current flows into the source. Hence measuring power factor at the source shows improvement, when C is used in parallel with L at the load side.

Thus if we have 5 MVAR reactor and 5 MVAR capacitor, the source does not see any reactive power component. And source is not the least impedance path, as the impedance needs to be considered not only by magnitude but also by its sign.

Thank you Mr Ramaswami for your answers.

But, then, would I be correct if I put it so:

with parallel L and C we select such their values that frequency of the source becomes = the inner frequency of the LC curcuit, and there takes place a potential resonance, Ul = -Uc, Z of the circuit becoming unlimitedly great, thus the impedance of the source becoming unlimitedly great, reactive currents of the L and C flow through each other.

Sorry that my terminology is not always correct as my native language is not English.

The potential across the L and C will be same at any given instant [they are in parallel connection as in power factor correction and Induction motor example discussed]

i.e Ul = Uc , The inductive component of the current is partly supplied by the source and partly supplied by the capacitor. When compensation is such that net power is lagging.

Ul = Uc and inductive power is wholly met by capacitive power at unity power factor

Ul = Uc and Capacitive power is partly met by inductive power and the absorption of the balance capacitive power from the source, for leading power factor.

Thus you see, that source may contribute, may not contribute or may even absorb reactive power depending on the situation.

In all the cases there is actually only one source , with neither C nor L active Q generators.

The source impedance always remains same - never infinity nor 0 or anything else. This is what we use for example in Transient stability calculations for generator model. The X"d, X'd, Xd , and similary for Q -axis of the sources remain constant. They do not vary irrespective of combination of L and C chosen in the example considered.

We are not (approaching) a parallel resonance in PF correction ? (we actually do not want PF =1)

I did not understand your comment.

Parallel resonance is a condition, where in the driving point impedance at a node reaches maximum for a harmonic frequency.

Our discussions so far have centered around fundamental frequency only.

the point is source impedance at a given frequency is constant.