A simple question about settings of protection relays.

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Kaaud

Kaaud

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First I want to thank you guys for the expert advice I've been getting from this forums.

My questions about this short circuit study and settings are:

1-What are the I/U (A/V) highlited by red?? whats is 0.01xCTsec and what is the punit they are talking about??

2-Is it right to have same undervoltage settings for all upstream and downstream relays with no descrimination?? Thx in advance.

3-How can we have a CT 1200/1 and the primary pickup value is 2160 A (second row) ??

 
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alright already!!!

alright already!!!

Kaaud said:
First I want to thank you guys for the expert advice I've been getting from this forums.

My questions about this short circuit study and settings are:

1-What are the I/U (A/V) highlited by red?? whats is 0.01xCTsec and what is the punit they are talking about??


2-Is it right to have same undervoltage settings for all upstream and downstream relays with no descrimination?? Thx in advance.

3-How can we have a CT 1200/1 and the primary pickup value is 2160 A (second row) ??

Click to expand...


Response:

1) I/U is, as the title lists, the range of the available settings in CT increments. Line 2 shows an adjustability of 0.01 to 15 times the CT rating. Under the Settings header is the actual selection within the given range, in this case 1.8.
Your ? #3 follows from this: 1.8 * 1200/1 = 2160A

2) The 27 undervoltage settings are all discretionary. The system likely never dips to 70%, and if it does, everything alarms or shuts down. You may find it best to coordinate the U/V settings from source to load, but 70% looks like on/off to me.

3) answered in 1

John M
 
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Kaaud said:
1-What are the I/U (A/V) highlited by red?? whats is 0.01xCTsec and what is the punit they are talking about??

2-Is it right to have same undervoltage settings for all upstream and downstream relays with no descrimination?? Thx in advance.

3-How can we have a CT 1200/1 and the primary pickup value is 2160 A (second row) ??
Click to expand...

mayanees covered it pretty well. Maybe a few extra pieces will help.

1. I/U is european for I = current, U = voltage
A/V = North American(?) A = current, V = voltage

2. U/V settings are generally operations oriented. When the power goes off-line, the system needs to open any feeders that will make it harder to start up. Generally all others are left closed. U/V settings don't coordinate very well. When the voltage goes below 70% it is generally on the way tp zero within a few doxem milliseconds. One usually picks a 27 setting that is below any normal inrush dips. Past that point, any further drop is the power going out.

Critical loads that require power as soon as possible generally have the 27 shut off. You don't want them to trip.

So, having all relays set to the same value would tell me the company doing the coordination maybe did not understand the operational needs - and maybe the relays are set that way because that is what you need and they did understand.

Interesting note: The delay on the 27 is 2sec. The CBs don't trip for a short (less than 2sec)voltage dip - even if the voltage dips to zero
You will have to understand the operations and the coordination to make effective decisions.

3. As mayanees said, the range of settings for the 50 (instantaneous OC) is .01 to 15. Why they added "xCTsec", I don't know. Of course the relay only sees the CT secondary current - you already knew that. All the values are given in feeder amps. I don't know what they expected to tell you with that phrase.

As for the value being above the CT rating, instantaneous OC is always (unless there is something really weird going on) above the CT. As you knew, the 50 relay is responding to a short circuit. The current is generally above In, the CTs are above their rating and are going into saturation. The output is not linear. You can get the curves for the CT that will show the inaccuracy. However, for a 50 that really doesn't matter. The important part of a 50 setting is to be inside of the equipment damage curve.

cf
 
Couple of suggestions.

Get the manufacturer's book on the relays (looks like there are two different types).

Get copies of the IEEE Red (IEEE 141, Electric Power Distribution) and Buff (IEEE 242 Protection and Coordination) books.

These three will a lot on understanding the coordination scheme.

You have a medium sophisticated system. From what we can see, I'm guessing 6600V distribution, 10MW utility connection, 1 - 2MW gen, 1 - 1.5MW gen

Dig out the curves and lay it out - I'd even do it by hand if that was all I had available. It will be interesting

I have no idea what a "punit" is (are?)

cf
 
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punit is per unit value. if I remember correctly, you base your calculations on a single reference, eg 10 mva. if you have a xfmr rated at 15mva then divide 15/10 to get 1.5 pu to get the equivalent 10 mva value.
 
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