Loading of 500 KV transmission lines

[jan63]

I am giving data about the 500 KV transmission lines of our power station.Going by the current carrying capacity we can further load the lines.But is the conductor the only consideration.We are adding 960 MW to our station.Will present lines carry the additional load
Conductor capacity No.of Conductors Present Max load

Line 1 896 A per conductor 4 1356A

Line 2 896 A per conductor 4 1076 A

Line 3 896 A per conductor 3 1012 A

Line 4 901 A per conductor 4 1092 A

Line 1&3 has a CT of 1600/1 while lines 2&4 have CT of 3000/1

 

[Inrushman]

Hi Jan63,

System stability shall be checked against the new capcaity / load.

The economic operation of the existing line ( Kelvin rule ) may lead to a unoptimized conductor for the new current rating, although the conductor can carry ( if the current rating is beyond the capcaity e.g. 4*896 = 3584 A ),

in optimal case, the new current capacity shall be 1356+(960/(1.732*500K*.8)) = 2742 A, without any operational marings, which is less than the available line-1 capacity ( i don't know the connection scheme, however other lines shall be checked similarly), CT rations & knee point shall be checked again for adequacy.

 

[WhiteyWhitey]

Jan63.

Assuming all of the lines are Overhead.

The lines you are connected to would have been built to a MVA rating.

From this MVA rating the Current in the conductors can be derived.

A higher current in the lines will cause the lines to heat up and expand. This means they sag lower to the ground and also could put more tension on the towers (Someone with better experience should confirm that i think)

If you are planning to exceed the MVA rating of the transmission lines you will need to do a study of the towers, the conductors and their clearance to ground to ensure that the additional sag on the lines will not cause any safety issues along the route.

The best suggestion as always (and always the frustrating one) is if you are not familiar with the ratings procedure get some assistance from a qualified engineer / consultant.

Cheers,
Andrew

 

[davidbeach]

I'm sure that lines are built to an Amp rating, not an MVA rating. As long as the max voltage isn't exceeded the line won't care about MVA as such but Amps are the limiting factor. Throw in a voltage and you can calc an MVA but any change in voltage requires a new MVA while the current rating remains the same.

Networked lines are almost never loaded even close to maximum load during N-0 conditions. If they are, there has to be a RAS or SPS to respond to contingences that would leave the line overloaded. For the OP, how many lines out of service would be credable? Remove that many line leaving the lowest capacity lines and don't overload them. May also be stability issues to consider.

 

[RRaghunath]

Thermal rating of OHLs is only relevant when line lengths are small. The 500kV OHLs have substantial inductive reactance which causes voltage drops as the current magnitude increases. The voltage drop is partly compensated by the line capacitance. This is called Ferranti effect.

Thus, the loading of lines is limited by the available voltage at the receiving end.
Surge impedance loading is the load in MVA at which the sending and receiving end voltages are same, i.e. the inductive reactance and capacitive reactance cancel out each other.
There are methods to increase the loading capacity of the OHLs beyond their surge impedance loading level, use of series capacitors in the OHL circuit for example.

 

[jan63]

Thamk you for the response.The matter discussed with some one here told me that normally 500 KV lines are safe to be loaded upto 1100 MW.Our lines are not long about 70 km each but then the do not end at far end and there is an in&out arrangement.I think it will require the study of the system as a whole.

 

[scottf]

As far as the CTs are concerned, if you exceed their continuous rating is a function of the ratio and rating factor, not just the ratio alone.

 

[waross]

One limit on loading which you should check is the maximum voltage drop now and the maximum voltage drop your On Load Tap Changers can accommodate. Loading past the capacity of the tap changers to correct the voltage may be a deal breaker.
This is related to Raghun's post.
In support of WhiteyWhitey's post;
This is not trivial. There was an incident years ago near Vancouver Canada. I am not sure of the voltage but it may have been 500kV, there are 500kV lines feeding Vancouver.
It was a very hot summer day and the system was carrying a heavy load due in part to air conditioning loads. The combination of high ambient temperatures and high current cause a span to come so close to the top of a small hill that there was a flashover to a small tree. The fire department put the fire out and were investigating the site when the line flashed again and two fire fighters died.
Re tower loading, that is worse in the winter. More sag means less tension in the conductors and the towers must support the weight of the conductors.
In the winter you have less sag causing more conductor tension and ice loading causing more tension and more tower loading.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[slavag]

Hi.
Im agree with David. Current is a limit, not MVA.
Of course a voltage drop too, but it's another issue, as Bill pointed.

Becouse this, we add a thermal image fuction, for signal only, to line protective relays.

Best Regards.
Slava

 

[stevenal]

I seem to be missing something here. Ampacity is 896, load is 1356 A, and you wish to add load?

 

[ScottyUK]

Stevenal,

There are four conductors in the example you quote, the ampacity is for one of them.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[stevenal]

Four conductors as in three phase plus neutral? Or four bundled conductors per phase?

 

[davidbeach]

At 500kV I'd go with bundling.

 

[jan63]

these are four conductors per phase

 

[QBplanner]

I don't know your position is but normally system planners or real time operating engineers determine what the maximum 500kV line transfer with equipment and P&C engineers.

It is a complicate system issue to determine the 500kV level line loading depending on the network topology including configuration (Loop, parallel, redial ), length(80-100km thermal, 80-320km voltage regulation/stability beyond 320km it is a steady state stability issue), conductor types(3 4, or 6 bundled the more the lower impedance the higher charging), voltage support (The stronger the better) and every other tiny spots like equipment rating and protection settings.

 

[bacon4life]

You should also double check the ratings for the circuit breakers, the air break disconnect switches and the substation bus, especially in circuits 1 & 3.

 

[m3ntosan]

Check everything that may give limiting restrictions:
Protections, busbar, bushings, circuit breakers, disconnectors, CTs, line traps, etc.

Like Inrushman said, you will affect the angle stability, generators will operate with an increased angle and losing an OHL will be much harder to cope with.