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Loss of Reactive Power on a Power Transmission Line

05/07/2010 1:57 PM

Overhead Line's information : 420 kV, 2x954 MCM, 45 km.

Substation A ............... Substation B

240 MW ....................... - 240 MW

16 MVAr ....................... - 7 MVAr

we read same values from MW meter (of course as opposite - and +) from each substation.

but we can't read same values for reactive power.

where is the 9 MVAr ? what is the reason ?

thanks ahead for your answers

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#1

Re: Loss of Reactive Power on a Power Transmission Line

05/07/2010 4:52 PM

The short answer is that probably the circuit is not what you think it is. Remember, reactive power is not real power. It comes from the lag in current due to transmission line effects, motor and supply power factors and a lot of other lesser effects from the variety and phase of other sources on the grid at that time.

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#2

Re: Loss of Reactive Power on a Power Transmission Line

05/08/2010 12:35 AM

The 9 MVAR are simply consumed by inductive impedance of the 45 km of the line, while the 7 MVAR are the real substation B MVAR demand

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#4
In reply to #2

Re: Loss of Reactive Power on a Power Transmission Line

05/08/2010 3:55 AM

Long transmission lines are usually capacitive. For short, it may be either.

For this case it might well be a capacitive reactance. In that case the pf of the supplier should have been better than the pf of the receiver.

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#3

Re: Loss of Reactive Power on a Power Transmission Line

05/08/2010 3:06 AM

My wild guess is there is some 9 MVAR capacitor connected on substatiob B, which is not accounted for by substation A. So kindly see schematic of each station and where and how the meters are connected.

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#7
In reply to #3

Re: Loss of Reactive Power on a Power Transmission Line

05/08/2010 3:51 PM

Unlikely.

If a capacitor is connected at B, it will supply reactive power downstream to reduce B's demand for it, therefore, if 16MVAr is coming down to B because B requires it and a capacitor is connected to B, then the 16MVAr will be reduced and less will be coming from A where the meter should record less than 16MVAr.

Most likely, the 9MVAr is consumed on the transmission line. The amount is within the range expected.

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#5

Re: Loss of Reactive Power on a Power Transmission Line

05/08/2010 4:25 AM

hi

this is ravi

Friend that 9MVAR is consumed by the transmission line. because the line has impedance. the capacitive reactance is less than the inductive for short distances. even if it is lont transmisson line, in no load and lightly loaded condition the capacitive reactance is higher than the inductive.

there are many equipments which need reactive power like induction motor, transformers, transmission lines etc. they need some reactive power for their operation.

for every 1M of power to transfer in line there is a need of 1MVAR based on the line capacity.

because of this reactive power consumption the receiving end voltage will dropped down i.e. Vs>Vr. this drop is due to reactive power consumption only

if sending end and receiving end voltage are equal then that line is called loss less line and there is no reactive power loss in the line, but practically its not possible.

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#6
In reply to #5

Re: Loss of Reactive Power on a Power Transmission Line

05/08/2010 10:19 AM

in no load and lightly loaded condition the capacitive reactance is higher than the inductive.

What load has got to do with the reactances? In no/light load conditions, the receiving voltage exceeds the sending voltage (Ferranti Effect)

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#8
In reply to #5

Re: Loss of Reactive Power on a Power Transmission Line

05/09/2010 3:53 AM

I just want to understand your statement "for every 1M of power to transfer in line there is a need of 1 MVAR based on the line capacity."

At PF= 1 (ideal case), 1 MW = 1 MVA. But are you saying to drive 1 MW power into the transmisson line you need 1 MVAR reactive componenet and hence total MVA is = sqrt2 or 1.414 MW ?? May be you have practical field knowledge! Am I riight in understanding that power factor is so bad ??!! What is the distance of transmission line which results in such poor PF? What corrective steps are taken in such cases?

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#9

Re: Loss of Reactive Power on a Power Transmission Line

05/11/2010 11:44 AM

As others have indicated, there is a capacitor installed between the substations. The capacitor is the 45 kM transmission line. The phase conductors constitute the plates of the capacitor, while the air between them is the insulating layer. At 45 kM, the plate area is roughly 1100 m2, which is why the capacitive reactance is so large.

Although not stated, I assume that the MVAR readings at both stations show a lagging power factor. The leading MVAR's generated by the capacitance of the transmission line supply a portion of the reactive power demand of the Substation B load.

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#10

Re: Loss of Reactive Power on a Power Transmission Line

05/11/2010 11:28 PM

Substation B has negative reading for both MW and MVAR , while Substation A readings are positive , this means that Sub B is supplying power and sub A is consuming it

Sub A MVAR demand is 16 MVAR, Sub B supply only 7, the remaining 9 MVAr are produced by the capacitive reactance of the line

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