How does reactive power effect system voltage
How does reactive power effect system voltage
(OP)
I understand the general concepts that a decrease in reactive power will cause system voltage to decrease and that an increase in reactive power will cause an increase in system voltage but for some reason I am not able to see this analytically. For instance I know that when capacitors are added to a system or that when synchronous generators are overexcited there is an increase in reactive power (MVARS) which in turn can increase the system voltage. Conversely I know that when there is not enough reactive power in the system to supply inductive loads such as motors, transformers, etc... then the system voltage will drop.
Is there a way that someone can explain this to me analytically? Perhaps I am just missing something simple. Am I able to somehow see it by drawing vectors that include voltage vectors with leading or lagging current vectors?
Thanks
Is there a way that someone can explain this to me analytically? Perhaps I am just missing something simple. Am I able to somehow see it by drawing vectors that include voltage vectors with leading or lagging current vectors?
Thanks






RE: How does reactive power effect system voltage
regardless of the phase angle of a current, it causes I2R losses.
Depending on the type of reactive current or power (capacitive or inductive) and the dirrection of the reactive current, the effect may be a voltage drop or a voltage rise. There are instances when the flow of reactive power may be the opposite direction to the flow of real power.
Vectors for reactive currents are 90 degrees out of phase with the applied voltage. However the phase angle of the load current has an effect on the total voltage drop.
The addition of capacitors may cause a reactive voltage drop.
The addition of capacitors may cause a cancellation of part or all of an inductive voltage drop with a net voltage rise.
The addition of capacitors may cause increased excitation of the supply generator with an attending rise in voltage.
Bill
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"Why not the best?"
Jimmy Carter
RE: How does reactive power effect system voltage
RE: How does reactive power effect system voltage
The voltage rise can be approximated by:
% volt rise = (kvar·XL)/(10·kV²)
Where: XL = the inductive reactance of the line (ohms)
kvar = three-phase capacitor size
kV = line-to-line voltage
This voltage rise is above the voltage as reduced by load voltage drop. That is, if there is 5% voltage drop with no capacitors, and 6% voltage rise caused by the capacitors, then the net voltage rise will be 1%.
RE: How does reactive power effect system voltage
RE: How does reactive power effect system voltage
When a generator is connected to a relatively large grid:
The throttle setting determines the output of real power.
The voltage setting determines the amount and direction of reactive power produced.
The grid determines the voltage.
In one instance the addition of capacitors or the production of leading reactive power may cancel reactive voltage drops and improve the voltage.
In another instance and under light load conditions, the addition of capacitors may and the induction of the supply transformer may act as a series LC circuit and cause voltages high enough to damage equipment.
Many years ago power factor was measured and penalties assessed based on the ratio of a months reactive power consumption to real power consumption. It was common to correct plant power factors by connecting an unswitched bank of capacitors 24/7.
During the night when the major load was just a part of the normal lighting the voltage may rise high enough to cause rapid failure of the lights.
Note: These are two distinct and separate effects.
I have seen an instance where diesel generators were run unloaded as synchronous condensers to cancel the reactive voltage drop of the transmission line feeding a city so as to improve the supply voltage regulation for the city.
Bill
--------------------
"Why not the best?"
Jimmy Carter