Capacitors inject negative vars? 1

[countrytoad]

Inductance is lagging, corresponds to positive Q or vars or reactive power.
Capacitance is leading, corresponds to negative Q.

Inductive machines or components absorb (negative) vars, yes?
Capacitance injects, generates, or produces (negative) vars, yes?

Why is the convention that capacitance injects negative vars, not that capacitance absorbs positive vars and inductance injects positive vars?

It may be that I just don't understand this correctly...


Is my convention wrong? This post states otherwise:

http://www.eng-tips.com/viewthread.cfm?qid=139432:

"By convention, reactive power, like real power, is positive when it is “supplied” and negative when it is “consumed.”"

 

[ScottyUK]

Per the established conventions: a capacitor acts as source of vars, while an inductor acts as a sink of vars.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[countrytoad]

But a capacitor will create a leading current, therefore negative vars (a negative power triangle). So a capacitor acts as a source of negative vars? Or positive vars?

Much Thanks!

 

[dpc]

Vars in a power system are generally assumed to be lagging (inductive) vars. Under this convention, capacitors produce vars.

The analogy would be positive watts and negative watts. Do motors produce negative watts? I suppose, but the convention is that they consume watts produced by generation sources.

As has been mentioned many times in other threads - if the flow of watts and vars are in the same direction, the power factor is lagging. If the watts and vars are in opposite directions, the power factor is leading.

 

[electricpete]

Good comments already.

Some more comments may or may not help. Here are mine:

There are of course (at least) two equivalent ways to look at the world:
1 - assume vars are a positive quantity, and describe the situation under consideration by the direction in which the vars flow.
2 - establish a reference direction for reactive power flow, and decide whether the situation under consideration represents positive or negative var flow with respect to the reference direction.

Inductance is lagging, corresponds to positive Q or vars or reactive power.
Capacitance is leading, corresponds to negative Q.

Your statement about positive Q for inductance and negative Q for reactive follows the 2nd approach, where you have assumed the reference direction of var flow is into the device (load convention for the device).

Inductive machines or components absorb (negative) vars, yes?Capacitance injects, generates, or produces (negative) vars, yes?

No. [positive] Vars flow from power system into the inductive machine. So we say induction machines absorb vars (with the understanding we're talking about positive vars... approach 1). Likewise positive vars flow from cap into system. This terminology of contrasting "absorb" and "inject" is more aligned with approach 1 (assuming positive vars) since your adjust your verb (to "absorb" or "inject") to suit the situation (both cases vars assumed positive).


=====================================
(2B)+(2B)' ?