reactive power in transmission underground cables 3

[prismpower]

we are designing 4 X 33kV line of about 30km long. according to the Etap simulation, reactive power is generated in the line on light loads. it is considerable amount of mvars. how can i reduce this amount of reactive power?

cheers

 

[davidbeach]

Shunt reactors are about your only option. You could also move the individual conductors further apart from each other.

 

[Mbrooke]

Around here the practice is to switch on shunt reactors at the busses where the cables connect to under light load conditions.

 

[7anoter4]

I agree with you Mbrooke.
Let's say there is about 4 parallel sets of 3 single-core 33 kV 400 mm^2 copper XLPE insulated cables in underground.
According to IEC 60502-2 Table B.2 – Current ratings for single-core cables with XLPE insulation Rated voltage 3,6/6 kV to 18/30 kV * Copper conductor 572 A
and Table B.19 – Correction factors for groups of three-phase circuits for 12 single core cables at 400 mm distance [factor 0.64] then 572*.64=366 A.
Conductor diameter 25 mm and shield diameter 43 mm.
The capacitance between conductor and shield of one cable is:
2.3/18/ln(43/25)=0.2356 microF/km
4 parallel cables and 30 km 4*0.2356*30=28.27 mF
Capacitive reactance:
Xc=10^6/2/pi()/50/28.27=112.6 ohm
Since the capacitance is shunt connected then:
Qc=33^2/112.6=9.67 MVAR.
The inductive reactance will be:
XL=2*pi*f*(0.2*LN(2*S/dcond)+K)/10^3
XL=2*pi()*50*(0.2*LN(2*400/25)+0.0514)/10^3=0.234 ohm/km and
30*0.234=7.02 ohm per 30 km.
Since the inductance is series connected then the reactive power depends on current flowing through. For full load will be:
QL100%=30*4*3*366^2*0.234/10^6=11.28 MVAR.
It seems at full load the reactive power required by cables is slight inductive.
If the current will decrease up to 20% then the inductive reactive power will be only:
QL20%=30*4*3*(366*.2)^2*0.234/10^6=0.45 MVAR
In order to compensate 9.22 MVAr you need a shunt inductance of:
XLshunt=33^2/9.22=118.1 ohm and a rated current of 33/sqrt(3)/118.1=161 A
If you will increase the distance between cables to 1000 mm[1m] you'll get XL=0.2915 ohm and QL20%=0.56 MVAR only.

 

[bacon4life]

A couple of other idea that may be far less practical than shunt reactors:
[ul][li]During light loads, turn 1, 2 or 3 of the conductor sets off.[/li]
[li]Decrease the capacitance:

for overhead, increase phase spacing​

for underground, increase the insulation thickness to increase the spacing between the phase conductor and the tap shield​

for underground, change insulation types because the relative permittivity of the insulation impacts cable capacitance​

for underground, increase the cable size and reduce the number of parallel conductors​

[/li][/ul]

 

[JJHorak]

On David's comment of moving cables farther apart:
I d not think that will decrease phase to ground or phase to phase capacitance of sheilded cables, but I suppose it will increase inductance a tad.

On bacon4life comments:
Nice set of options. Increasing voltage rating of cable will force increased insulation.

J. Horak, P-R Engineering, Colorado

 

[prismpower]

great answers! thanks

 

[anksyete]

It depends on cable size also. If it is 240m2, you can consider 60kvar/km

 

[HamburgerHelper]

JJHorak,

This calculator supports what you say about shielded cables. The inductance changes but the charging current looks to be independent of the spacing because of the shield. I suppose the shield isn't thick enough to contain the flux. Moving the cables apart would help the cable burn up VARs.

http://nepsi.com/resources/calculators/calculation-of-cable-data.htm

 

[bacon4life]

Moving the cables apart would increase reactive power usage at heavy loads, but the OP was about light loads where the cable inductance has little impact on reactive power usage. Increasing the reactance would increase the total range of reactive power consumption, and make it more likely to need multiple steps of shut reactance or additional voltage control devices.