Electric Automation Forum
Forum » General Discussion » What is the role of VARS in system voltage improvement?
Topics: What is the role of VARS in system voltage improvement? on General Discussion
#1
Start by
ziaullah jan
09-01-2014 03:47 AM

What is the role of VARS in system voltage improvement?

When the system voltage or terminal voltage of a generator is low why we increase excitation which in result increase VARS? what is the role of VARS in system voltage improvement?
09-01-2014 05:49 AM
Top #2
Jim Phipps, P.E.
09-01-2014 05:49 AM
When the field excitation of a synchronous generator is increased, the terminal voltage of the machine also increases because there is more magnetic flux from the field on the rotor cutting across the stator windings which produces more terminal voltage. In AC power systems, reactive power flows from regions of higher voltage magnitude to lower voltage magnitude. Thus, when the terminal voltage is increase above the remote system voltage, reactive power will flow from the generator to the system. When the terminal voltage is decreased below the remote system voltage, reactive power will flow from the system to the generator.
09-01-2014 08:30 AM
Top #3
Jim Phipps, P.E.
09-01-2014 08:30 AM
Reactive power (VARs) are needed to control the system voltage. When power is transmitted through a power system element like a transmission line or power transformer, the flow of current through the inductance of the element absorbs reactive power and produces a voltage drop. When the sum of voltage drops becomes too large, additional reactive power must be added to the system to boost the voltage back up again. Reactive power is generated (voltage rise) by shunt capacitor banks and increasing the field excitation on synchronous machines (motors and generators). Reactive power is absorbed (voltage drop) by power system inductive reactances, shunt reactors and reducing the field excitation on synchronous machines.
09-01-2014 11:11 AM
Top #4
Alan Maltz
09-01-2014 11:11 AM
Ziaullah,

To simplify Jim's comments, we increase the excitation to produce VARs that overcome the voltage drop caused by the reactive elements in a grid just like we increase the fuel to produce Watts that overcome the decelerating torque (drop in speed) caused by the loads.

Alan
09-01-2014 01:40 PM
Top #5
virgilio rodriguez
09-01-2014 01:40 PM
Ziaullah,

When there is a deficit of reactive power (kVAr) supply in the system, the voltage will drop.
When there is an excess of reactive power (kVAr) supply in the system, the voltage will rise.
So the system voltage is an indicator if there is an excess or deficit of reactive power supply.
09-01-2014 04:07 PM
Top #6
Kevin Kabamalan
09-01-2014 04:07 PM
however, increased excitation is heatloss.
09-01-2014 06:24 PM
Top #7
Cornwell Takawira
09-01-2014 06:24 PM
Well said Jim, thanks.
09-01-2014 08:48 PM
Top #8
ziaullah jan
09-01-2014 08:48 PM
Thanks to all contributors and specially to Allan & Jim by giving such a fruitful comments.
09-01-2014 09:08 PM
Top #9
Anand
09-01-2014 09:08 PM
In case of Alternators, there are basically are two different types of Voltages. One of it is called "INDUCED EMF" and the other "TERMINAL VOLTAGE".
TERMINAL VOLTAGE = [INDUCED EMF] - [VOLTAGE DROP IN THE GENERATOR ARMATURE (GENERALLY THE STATOR WINDING)]
If an Alternator is operating Isolated, Increase or Decrease of EXCITATION FIELD CURRENT has direct corresponding effect of Increase or Decrease of Terminal Voltage.
If however, the same Alternator is operated in parallel to a big Grid System, such a big Grid System is termed as INFINITE BUS. Theoretically an INFINITE BUS behaves as an OCEAN and just as Ocean Level doesn't changes depending on how much water is coming to the Ocean from a river that falls into it and the River Level too at mouth of the Estuary remains the same as that of the Ocean, the Generator Terminal Voltage remains (theoretically) constant irrespective of change in DC FIELD EXCITATION Current, even though the INDUCED EMF does change.
What happens in such a case that if DC FIELD EXCITATION Current is increased, the correspondingly Increased INDUCED EMF required the Generator to come to Lagging Power Factor so that the Voltage Drop (Armature Reaction Drop) in Armature (Stator) is SUBTRACTIVE so that INDUCED EMF - (POSITIVE ARMATURE REACTION DROP) = TERMINAL VOLTAGE.
If on the other hand the DC FIELD EXCITATION Current is decreased, the correspondingly Decreased INDUCED EMF required the Generator to come to Leading Power Factor so that the Voltage Drop (Armature Reaction Drop) in Armature (Stator) is subtractive so that ADDITIVE so that INDUCED EMF - (NEGATIVE ARMATURE REACTION DROP) = TERMINAL VOLTAGE.TERMINAL VOLTAGE. [PLEASE NOTE THAT (A) + (-B) = (A - B) and
(A) + (+B) = (A + B). Subtractive Voltage Drop is like (-B) and Additive Voltage Drop is lie (+B).
09-01-2014 11:16 PM
Top #10
Francis Luces
09-01-2014 11:16 PM
Increasing the excitation of a generator increases the system voltage at which the generator is connected. This is illustrated when a vector diagram of a classical machine is drawn. Since power system is a non-linear consumer of reactive power, it will be helpful to stabilize system voltage by adjusting the excitation of a generator. At high system demand, the voltage of the system drops while at light loading, overvoltages in the system will be experienced due to ferranti effect of transmission lines. To correct this deficiencies, power dispatch engineers will often resort to call the attention of plant operators to adjust MVAr/excitation of generating units, but it should be noted that MVAr of generators is limited since their are following reactive power capability curves depending on the fuel types of their prime movers.
Reply to Thread