PERCENT VOLTAGE DROP ON TRANSMISSION LINE 4

[Tesla1971]

I was recently asked and performed a percent voltage drop calculation for a transmission line.

I have never done this calculation before so prior to submitting my work to the customer, I wanted to see if my answer was acceptable.

The only problem is I can't find any literature that lists what would be deemed acceptable results...5%, 10%, etc.

Does anyone know the parameters for acceptable percent voltage drop on a transmission line?

Thanks

 

[davidbeach]

It depends. What is being done for voltage regulation? How much variation can that voltage regulation system smooth out?

 

[Tesla1971]

I don't know anything about how the voltage is to be regulated.

This is a bid project and the customer is simply proposing several combinations of Transmission Line Sizes and MW on the line.

And from that, I was calculating the Voltage Drop.

My answer came out to be 10%, which seems high to me, but on the next combination, i got around 6%.

So I don't know if 10% is high and should be discarded as a choice or if that's typical.

There aren't any sites, manuals, etc. i have been able to find that say point blank: the acceptable voltage drop along a transmission line should be between 1%-7%, or whatever it turns out to be.

My only example would be the NEC where it calls out appropriate voltage drops in cables carrying power to equipment. I believe its somewhere between 3%-5%.

Is a number cited in this manner for transmission lines?

 

[waross]

A transmission line often feeds a power transformer with an on-load tap changer. The voltage drop is generally within the range of the tap changer.
The NEC requirement is for unregulated utilization circuits and has no application to transmission lines.


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

 

[cranky108]

So we can conclude you know the proposed loading, and source end voltage level. 10% seems high for voltage drop, are you including the rise for the capactance?

If this is a long line the capactance should provide some support for the voltage, and reduce the voltage loss. But at 10% the line should be very long, and the wire very small. I would expect the cost of larger wire to be small to the cost of the rest of the line.

Is this a composit core conductor by chance?

 

[TurbineGen]

What is your load? An LTC? What is the nature of the load? Is it highly cyclical, or relatively constant? If the load is an LTC with a moderately cyclical load, so long as you are within the tap changer's range, you should be ok so long as the customer is ok with the losses. This however is also a question of economics? How much does your customer value the kWs lost in the transmission line?

------------------------------------------------------------------------
If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[Tesla1971]

Let me say first off thank you to all that have responded. Your input has been very helpful.

It took a couple of your threads but it suddenly clicked for me as to why the voltage regulation at the end is important.

You can't lose more voltage along the line than your LTC can bring back up. So if your LTC has 5% voltage regulation, your voltage drop needs to be less than 5%.

 

[bacon4life]

Also be sure to check with your utility to see what their voltage range they provide. We typically run towards the top of our bandwidth, so we can swing +3-7% under normal operation and up to -8% after some contingencies.

If this is truly transmission, I would have expected a load flow study with absolute limits on the voltage rather than an allowable voltage drop for just your section of line.

The LTC also compensates for voltage drop in the transformer, so you may need more than a 5% LTC for a line drop of 5%.

 

[prc]

Tesla, I believe it depends mainly on the power factor of the load on the lines. Approximately voltage regulation is IR Cos phi + IX sin phi where phi is the power factor angle.If load is unity Pf,second term disappear.R & X are the resistive and reactive component of equipments (trf &lines)You can get approximate R & X figures from IEC 60909-2 Electrical equipment data for Short circuit current calculations.