Calculating load and no load losses of cables
Calculating load and no load losses of cables
(OP)
How do you calculate the load and no load losses of a cable in kW please? I'm used to dealing with these for transformers but it doesn't seem to be as straight forward for cables. Thanks!






RE: Calculating load and no load losses of cables
Rafiq Bulsara
http://www.srengineersct.com
RE: Calculating load and no load losses of cables
RE: Calculating load and no load losses of cables
Cable manufacturer can provide the information on dielectric losses.
David Castor
www.cvoes.com
RE: Calculating load and no load losses of cables
What is the conductor configuration? eg Single conductor, three conductor bundle, free air, in conduit.
You have dielectric losses, (resistive and capacitive) capacitive charging currents, and possibly corona losses.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Calculating load and no load losses of cables
RE: Calculating load and no load losses of cables
I'm trying to work out out how to adjust the fiscal metering as the commercial boundary is at the 'wrong' end of the cable.
Another possibly stupid question, I've got values quoted for resistane and inductance, are they likely to be per phase or a 3 phase value?
RE: Calculating load and no load losses of cables
But at 400V, no load losses will be negligible. Most of the very small no-load current will be due to cable capacity to ground and will be quadrature current. This will not affect kW metering. A second order effect will be the I2R loss caused by the charging current. My gut feeling is that it may take several years for the value of the no-load losses to equal the cost of your time to calculate them.
But please continue and let us know what you find out (in dollars). I think that this is the type of problem that should be solved ONCE. It will give you a feel for a real world application that you often don't get in class and goes to developing judgment or feel for your work.
Your resistance values will be per foot or per 100 Feet. (Or the SI equivalent). The values will be applied to each cable and the current in the cable. The total losses will be the sum of the individual cable losses.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Calculating load and no load losses of cables
Dielectric losses are proportional to frequency, system voltage, conductor screen radius, relative permittivity, conductor screen stress and dielectric loss angle. For cables upto about 50kV the dielectric losses are small in comparison to the conductor losses. However, the dielectric losses increase with voltage and are very significant at higher voltages.
Regards
Marmite
RE: Calculating load and no load losses of cables
David Castor
www.cvoes.com
RE: Calculating load and no load losses of cables
Is the dielectric loss what I am calling the no load loss? I've seen it quote in kW/km (I'm UK based) but wasn't sure if it was the no load loss or the load loss at cable rating.
I need to correct the kVAr metering too hence the interest in the no load reactive loss. I've seen the capacitance quoted, can I calculate the susceptance from that?
Although it probably isn't really worth calculating for just this one site I'll undoubtedly have to do it again sometime. Definately worth a spreadsheet! This is actually a rare chance for me to do classwork (it's been too long since I was at university), I'm normally dealing with faults and commissioning new systems.
RE: Calculating load and no load losses of cables
A value of kW/km is probably dielectric loss. Load loss is dependent on the load current. Dielectric shunt losses are not.
David Castor
www.cvoes.com
RE: Calculating load and no load losses of cables
RE: Calculating load and no load losses of cables
RE: Calculating load and no load losses of cables
That's a nice link, jghrist. Never heard of the company.
David Castor
www.cvoes.com
RE: Calculating load and no load losses of cables
RE: Calculating load and no load losses of cables
----------------------------------
If we learn from our mistakes I'm getting a great education!
RE: Calculating load and no load losses of cables
RE: Calculating load and no load losses of cables
Regards
Marmite
RE: Calculating load and no load losses of cables
RE: Calculating load and no load losses of cables
Another source (Rating of Electric Power Cables, George J. Anders, IEEE Press, 1997) uses the formula
C=e·10^-9/[18·ln(D/d)] where C is in F/m.
This formula gives results the same as the Prysmian formula when converted to the same units.
Using and example from Anders, 400 kV PPL cable with D=94.6 mm, d=58.6 mm, f=60 Hz, e=2.8, tan(delta)= 0.001 gives
C=0.325 uF/km with Anders and Prysmian
C=32.5 mF/km with Brugg
W=6.53 kW/km with Anders
W=6.50 kW/km with Prysmian
W=654 kW/km with Brugg
RE: Calculating load and no load losses of cables
Regards
Marmite
RE: Calculating load and no load losses of cables
I've saved all the links so I can bring myself up to speed on cables, we're about to split my group at work in half and I think our cable guy will end up in the other group so it's no bad thing if I branch out a bit.
RE: Calculating load and no load losses of cables
For three phase cable/conductor, is the right simple equation to use to calculate losses:
P = i^2R
OR
P = 3 * i^2R
Thanks!