feeding 40km circuits on 33kV
feeding 40km circuits on 33kV
(OP)
hello,
we have a 36kV power transformer with on load tap changer, 36kV switch-gear. we have load on 40kms from the substation. shall we procure higher voltage cables rating (20.8/36 max 42kV instead of 18/30 max 36kV) to allow the tap-changer compensate the voltage drop? or we should use capacitors? are the 36kV designed to operate contentiously on 36kV while they are usually used for 33kV?
what about the losses with relation to the voltage drop?
sorry for the many questions. but we should take proper decision in this regards.
cheers.
we have a 36kV power transformer with on load tap changer, 36kV switch-gear. we have load on 40kms from the substation. shall we procure higher voltage cables rating (20.8/36 max 42kV instead of 18/30 max 36kV) to allow the tap-changer compensate the voltage drop? or we should use capacitors? are the 36kV designed to operate contentiously on 36kV while they are usually used for 33kV?
what about the losses with relation to the voltage drop?
sorry for the many questions. but we should take proper decision in this regards.
cheers.






RE: feeding 40km circuits on 33kV
Caps: this is doable once you understand the electrical (reactive) properties of the 33kv line and its load. This to will have to be played right, as the caps will need to be switched on/off in response to current/pf/voltage. It is typical to deploy multiple switched caps on the line spread evenly appart, probably about 3 to 5 on your line- 2 might do- at least that is how I've seen it done.
Now, I know this may not be possible, but considering the distance involved I would look into upgrading to 46, 66 or 69kv. 25 miles is steep for a 33kv line, though not unheard of. At those distances higher voltages are typical , at least in this small part of the globe.
RE: feeding 40km circuits on 33kV
thanks Mbrooke,
we cant step the system up due to non-technical constraints. and we have on load tap changer that can compensate the voltage accordingly. what i want to confirm is that we can run continuously at 36kV without any issues.
cheers
RE: feeding 40km circuits on 33kV
RE: feeding 40km circuits on 33kV
Hence, I think it is not a good idea to operate the source at 36kV on permanent basis. Providing capacitor bank at the receiving end at 33kV bus or at lower voltage level in order to compensate the reactive power requirement of the loads connected is beneficial. As we know from theory, it is flow of lagging VARs that is primarily responsible for the voltage drop.
Further, you may have to control the transformer tap at source to lower below 33kV at source bus, in order to maintain voltage at receiving end from rising to unacceptable high level, during low load conditions.
Rompicherla Raghunath
RE: feeding 40km circuits on 33kV
You can run the system at 36kv provided it does not exceed 36kv at any point or part during normal operation.
RE: feeding 40km circuits on 33kV
RE: feeding 40km circuits on 33kV
thanks all for the valuable replies,
the required voltage at the receiving end is 33kV, hence we will not exceed 36kV at the transmitting side. can 18/30 max 36kV cables sustain 36kV continuously?
yes we have issue with reactive power and we should work on it. but our main concern is the voltage drop.
cheers
RE: feeding 40km circuits on 33kV
I believe that 36kV cables can sustain 36kV continuously, based
on the definitions for "highest voltage for equipment" in the
IEC standards. You may want to check the applicable standards
if you are not in IEC land, and/or confirm with your cable
manufacturer.
A potential issue with intentionally operating at 36kV is the
lack of any safety margin. The 36kV rating exists because it
allows some margin for systems designed to operate at 33kV.
A system that normally runs at 36kV will, on occasion, exceed
36kV due to transients, non-ideal behaviour of equipment, load
rejection, etc.
Can you use an on-load tap changer at the load end to accept
a voltage lower than 33kV?
Thanks,
Alan
RE: feeding 40km circuits on 33kV
RE: feeding 40km circuits on 33kV
There will be a resistive voltage drop and a reactive voltage drop acting at right angles to the resistive drop.
The relationship between losses and voltage drop will depend on the X:R (X-Reactive:R-Resistive) ratio and Pythagorus' theroem.
The losses will be the I2R losses as calculated by the line current and the effective AC resistance of the line.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: feeding 40km circuits on 33kV
I agree with waross regarding the loss relationship.
To make a sensible decision, I think you should model
the system using a load flow program (or excel if
you're desperate).
It seems that you have option (1): run with 36kV at the
sending end and, say, 33kV at the receiving end or
option (2): run with 33kV at the sending end and a bit
less than 30kV at the receiving end (higher currents,
slightly more drop).
The difference is an on-average change in the running
voltage by 3 to 5 kV, which will make the currents a
bit higher.
It might be easier from an equipment supply perspective
to buy higher current equipment than it would be to use
a non-standard voltage.
You should check a few other things as well, like the
angle difference across the line when loads are starting,
and the minimum fault current at the remote end.
Thanks,
Alan
RE: feeding 40km circuits on 33kV