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Reactive power 2
- Thread starter mab2008
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Hi folks,
I think all this confusion is because we, as engineers, have the tendency to model everything and, sometimes, forget to come back to the basics.
Also, we always keep trying to visualize mathematical entities as being physical entities.
In reality, there exist the electric and the magnetic filed, which can be associated to voltage (V) and current (A), respectively.
The other electrical engineering "quantities" (S (VA), P (W) and Q (var)) are derived mathematically from these two physical entities (V and A). These "quantities" are defined with base on alternating current (AC) systems with linear components and sinusoidal wave.
The best way to "see" if the vars is leadding or lagging is looking the angle between the voltage and current.
rmw, regarding to synchronous compensator (SC), it can be "visualize" as a voltage source generating a voltage that can be (a) higher, (b) lower or (c) equal to the voltage at the terminal on which it is connected.
In the case (a), the SC (over-excited) will "delivery" current (90 degree leading from the voltage) to the system as current flows from high to low potential (voltage). In this case the SC works as a capacitor.
Conversely, in the case (b), the SC (under-excited) will be "absorbed" the current (90 degree lagging from the voltage), as its voltage is lower than that on the terminal. In this case the SC works as an inductor.
In the case (c), no current will flow. The SC will be is "sleeping".
In all cases, some energy is consumed to supply the losses (friction, heating, etc), as davidbeach rimind us.
This same "picture" can be used for a generator, but in this case there will be a prime mover connected to the generator shaft.
Best regards,
Herivelto Bronzeado
I think all this confusion is because we, as engineers, have the tendency to model everything and, sometimes, forget to come back to the basics.
Also, we always keep trying to visualize mathematical entities as being physical entities.
In reality, there exist the electric and the magnetic filed, which can be associated to voltage (V) and current (A), respectively.
The other electrical engineering "quantities" (S (VA), P (W) and Q (var)) are derived mathematically from these two physical entities (V and A). These "quantities" are defined with base on alternating current (AC) systems with linear components and sinusoidal wave.
The best way to "see" if the vars is leadding or lagging is looking the angle between the voltage and current.
rmw, regarding to synchronous compensator (SC), it can be "visualize" as a voltage source generating a voltage that can be (a) higher, (b) lower or (c) equal to the voltage at the terminal on which it is connected.
In the case (a), the SC (over-excited) will "delivery" current (90 degree leading from the voltage) to the system as current flows from high to low potential (voltage). In this case the SC works as a capacitor.
Conversely, in the case (b), the SC (under-excited) will be "absorbed" the current (90 degree lagging from the voltage), as its voltage is lower than that on the terminal. In this case the SC works as an inductor.
In the case (c), no current will flow. The SC will be is "sleeping".
In all cases, some energy is consumed to supply the losses (friction, heating, etc), as davidbeach rimind us.
This same "picture" can be used for a generator, but in this case there will be a prime mover connected to the generator shaft.
Best regards,
Herivelto Bronzeado
Y'all,
In school way,way back, I got questioned for a correct figure in one of my answers! The professor didn't understand how I got the answer when my formulas are in a mess! We came to an understanding (i.e. me learning) that I got my conventions in reverse!
Having said that, we all can come to an agreement if our conventions in the flow of power, current, etc are the same.
I was taught any quantity entering the system will be considered "positive" and quantities leaving the system as " negative"; outgoing vars-negative, incoming vars-positive, etc.
In school way,way back, I got questioned for a correct figure in one of my answers! The professor didn't understand how I got the answer when my formulas are in a mess! We came to an understanding (i.e. me learning) that I got my conventions in reverse!
Having said that, we all can come to an agreement if our conventions in the flow of power, current, etc are the same.
I was taught any quantity entering the system will be considered "positive" and quantities leaving the system as " negative"; outgoing vars-negative, incoming vars-positive, etc.
insinyur02
Mechanical
please explain:
- why increasing voltage from generator when power factor at lagging pf = 0.84, reduces PF to 0.82 and increases MVAR and kV from generator.
1. if the generator is leading which absorbs VAR? what to do to have PF to 1, and vice versa.
2. is this correct? power station charge companies based on their real power, not apparent power (total).
- why users with low PF get penalty? is it because VAR (phantom power) is difficult to quantify?
- why increasing voltage from generator when power factor at lagging pf = 0.84, reduces PF to 0.82 and increases MVAR and kV from generator.
1. if the generator is leading which absorbs VAR? what to do to have PF to 1, and vice versa.
2. is this correct? power station charge companies based on their real power, not apparent power (total).
- why users with low PF get penalty? is it because VAR (phantom power) is difficult to quantify?
insinyur02,
PF is the cosine of the angle between the current and the voltage. when you turn the exciter knob to increase the excitation, the angle between the voltage and the supplied current increases. A 0.84 lagging PF means an angle of 32.86 degrees; A 0.82 lagging PF means an angle of 34.91 degrees! See how the angle increases.
On Q 1., All you have to do is increase the exciter voltage until your generator reads 0.9 or 0.85 (depending on the generator temp and what the grid requires you to operate at).
On Q 2., MWHr is energy that power companies charge most users. But users who cannot maintain higher than the specified lowest power factor limit (usually 0.85 lagging) are penalized because the system voltage will go down if loads are too inductive. These MVAr-Hrs (amount of reactive VA times the hours) are sometimes metered on customers known to have PF difficulty and not hard to quantify. Please review your power triangle math.
PF is the cosine of the angle between the current and the voltage. when you turn the exciter knob to increase the excitation, the angle between the voltage and the supplied current increases. A 0.84 lagging PF means an angle of 32.86 degrees; A 0.82 lagging PF means an angle of 34.91 degrees! See how the angle increases.
On Q 1., All you have to do is increase the exciter voltage until your generator reads 0.9 or 0.85 (depending on the generator temp and what the grid requires you to operate at).
On Q 2., MWHr is energy that power companies charge most users. But users who cannot maintain higher than the specified lowest power factor limit (usually 0.85 lagging) are penalized because the system voltage will go down if loads are too inductive. These MVAr-Hrs (amount of reactive VA times the hours) are sometimes metered on customers known to have PF difficulty and not hard to quantify. Please review your power triangle math.
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