feeding 40km circuits on 33kV 3

[prismpower]

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.

 

[Mbrooke]

What ratings are those cables/insulators/gear built by (IEC or IEEE)? Does the load have its own tap changers? You can run equipment at exactly its max rating, continuously, ie 34.5kv class equipment at 38kv. Both caps and running the source "hot" can do, but you will have to play it right. For example, the supply substation will have to produce about 33.1kv under light load to keep voltages at the right values at the source, and step up in supply vltage as load rises to maintain the correct values. Otherwise 36kv will produce high voltages during light loads at the source. Also be aware that if you loose load while running hot (ie feeder or circuit trip at the load) the voltage will go up on what ever is remaining.


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.

 

[prismpower]

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

 

[bacon4life]

The whole distance is cable? How large of load is it? In addition to capacitors for heavy load periods, you may need reactors to keep the voltage down during light load periods.

 

[RRaghunath]

If you operate with 36kV at source end, you will have problem during light load conditions, i.e. the voltage at receiving end will be >36kV. This is because of Ferranti effect. The voltage rise could be substantial as the capacitance of cable is much larger compared to the Overhead line of same length.
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

 

[Mbrooke]

Bacon for life is right, you might also need reactors for light loads if the whole thing is cable.


You can run the system at 36kv provided it does not exceed 36kv at any point or part during normal operation.

 

[jghrist]

If you have not already purchased and installed the cable, then certainly use 20.8/36 kV cable. What is the minimum required utilization voltage? If it is less than 36 kV, you may be able to use load drop compensation to keep the maximum voltage on the cable less than 36 kV while maintaining the minimum utilization voltage.

 

[prismpower]

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

 

[submonkey]

Hi prismpower,

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

 

[prismpower]

i was thinking the same; to use transformers that can accept lets say 27kV or 26kV with onload tap changer. my question is; what is the relation between voltage drop and losses? or there is no relation at all?

 

[waross]

Your voltage drop will be the directed sum of two components.
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

 

[submonkey]

Hi,

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

 

[Mbrooke]

And also monitor voltage stability. If the voltage dips at the source station during peak load and the load tap changer starts boosting at the load substation, you could end up with voltage collapse or run away voltage depressions on the load being served.