load flow and motor starting studies

[kookypedia]

Guys,
I used to study load flow with normal voltage level for utility and define several scenarios in distribution level such as loosing an MCC etc.
I was looking at another study that has only 3 scenarios with 95%, 100% and 105% voltage levels for utility and nothing in distribution level!
Are we really responsible for this much fluctuation in utility voltage? I thought we consider only 2% fluctuation (and relays should trip for more than 2%) and we are responsible for other scenarios that can happen in distribution level.
Do you know any IEEE or PIP standard that clarifies this?

 

[magoo2]

ANSI C84.1 is probably your best bet.

It defines the more popular voltage levels for transmission, distribution and services.

Typically, you can expect a +/-5% maximum variation from nominal voltages.

 

[kookypedia]

Thanks a lot, it helped a lot!

 

[bacon4life]

I hope you are not serious about tripping for a 2% voltage fluctuation. On a daily basis the transmission system swings more than that. Just closing in a single section of transmission capacitors can change the voltage by 2% to 3%. In the western USA,

https://www.wecc.biz/Reliability/TPL-001-WECC-CRT-3.pdf

provides default transmission voltages from 90% to 110% for some contingencies. Voltage deviation is limited to 8% for single contingencies such as a transformer or a transmission line. Allowable voltage deviation is not specified for larger contingencies like bus faults. Utilities are also allowed to adopt less strict criteria than these.

 

[Variac200]

To trip on a voltage fluctuation of 2% would make the system less reliable. For LV/MV networks, some people go with a range of +/- 10%.

 

[kookypedia]

But, ANSI C84.1 says service voltage for 13.8 kv should be between 97.5%-105%.
Doesn't it mean I should trip for more than 105% and less than 97.5%?

http://www.powerqualityworld.com/2011/04/ansi-c84-1-voltage-ratings-60-hertz.html

 

[bacon4life]

Although the voltage levels specified in the ANSI standard appear to be clear, C84.1 doesn't specify the time period for the measurement. One interpretation might be the 3 phase average voltage over a 1 hour period. Anther interpretation might be the lowest phase voltage for a single cycle. Utility distributions LTCs/voltage regulators usually have time delay of at least 30 seconds before adjusting voltage in order to avoid responding to transient voltages.

You should only trip protective relays in order to protect equipment from harm, not just because of a number in a standard. You might want to take a look at the ITIC curve. Tripping on anything inside the curve may expose your customers to unnecessary outages.

http://www.powerqualityworld.com/2011/04/itic-power-acceptability-curve.html

 

[kookypedia]

Bacon4life,
Thanks for information on ITIC curve.
What about load flow studies?
My ETAP shows reasonable results when I have 97.5% in utility (voltage in all feeders are greater than 90%).
I would be bellow 90% in many gears if I use 95% in the utility!

 

[7anoter4]

For load flow study, in my opinion, IEEE Std 399-1997 ch.6.4 Load flow solution methods it could be recommended. Also motor starting study is here: Chapter 9 Motor-starting studies.
IEEE Std 399-1997 IEEE Recommended Practice for Industrial and Commercial Power Systems Analysis

 

[kookypedia]

Thanks Zanoter4,
I looked at Brown and Red books and they exactly talk about standard voltage ranges etc for different studies!