Help requested understanding the settings on a 27 UNDERVOLTAGE relay on the main bus

[bdn2004]

I'm looking at the existing system with three undervoltage relays connected to the three 12.47kV feeders into the Plant
... there's a 12470/240-120V CPT on each one.

The test data shows:

Bus #1: PU at 109V and drops out at 105V.
109V/120V = 90% voltage, on the primary side this would be 11,223V

Bus #2: Another 12kV bus has the same relay and it says the Instantaneous function is set to DO = 55.91V and PU = 55.25V, and the time delay function is disabled. 55.91V/120V = 47% voltage. On the primary side 12,470V * .47 = 5860.9V

Bus #3: Another bus has this function done by a 47 relay. With settings PU V = 100 and DO V = 80V and an operating time of .0254 seconds
100/120 = .83, 80/120 = 0.67, => 10,350V and 8354V

Why is there such differences in all these settings ? Also, what is the difference between the PU value and the DO V ?

 

[bdn2004]

And as aside to this...I talked to the Utility engineer who services this Plant. He told me that because of the capacitor banks installed on the system ... the voltage will never dip below 1%.

Are the above settings typical ?

 

[bdn2004]

https://www.eng-tips.com/viewthread.cfm?qid=6520

https://www.eng-tips.com/viewthread.cfm?qid=6516

These are related posts.

 

[bdn2004]

Wouldn't if you get a fault on any feeder the voltage is going to zero - so an instantaneous only setting doesn't seem to make much sense. This is on the main breaker and it would trip off the whole system.

I've seen in a couple of other forums the 80% value for 2 seconds is a safe setting.

 

[davidbeach]

"Never dip below 1%" is a pretty bold claim while being highly ambiguous. More likely meaning than never dipping below 1% would not dipping by more than 1%, on other words never dipping below 99%. In either case it will get below that number. Any outage is below 1% and many system events will cause the voltage to dip below 99%, including many events far, far, away. Typical voltage bandwidth is +/- 5%.

What types of 27 relays? Are you looking at NC or NO contacts?

Electromechanical 27 relays have NC contacts; connecting them to a voltage source with enough voltage causes the contacts to open. They will then close if voltage gets too low. In a modern relay, the 27 function will be connected to a NO output and the relay will close the output when the voltage falls below the set point.

I'm not sure how you could have a PU above the DO value, but somebody may have labeled an EM relay differently than I might. The PU value should be less than the DO value. That way as the voltage falls it get to the PU value after it's already below the DO value. When the voltage then increases it rises above the PU value and then gets to the DO value and the contact opens. If there was only one value you would have the relay chattering if the voltage was right at the setting but not perfectly stable. That's how relays get destroyed.

 

[bdn2004]

The question I had asked the Utility engineer was what is the min/max voltage on their system. I wanted to put it in the power software and run the analysis with both values. He said in the rural areas the voltage does fluctuate by percentages - but not at this Plant because of the capacitor banks installed.

This is a 12.47kV new system and switchgear, not exactly like the old system. The old system has electromechanical 27 relays.

The new system was not specified with these 27 electromechanical relays. And it’s not shown on the drawings.

The multi-function electronic relay that controls the main breaker has this function - and the Plant is asking that the new system operate just like the old system. I’m assuming there is a NO contact to be programmed on the relay that closes, then opens the breaker when the monitored voltage input and time delay values are met.

I’m just getting the conversation started trying to understand what to set this to. I was trying to get some guidance from the existing ones.

 

[crshears]

He said in the rural areas the voltage does fluctuate by percentages - but not at this Plant because of the capacitor banks installed.

I've never heard that capacitor banks peg the local / site voltage at a given value, instead they raise it by some amount that varies directly as the prevailing system voltage.

The only way that voltage tolerances could be held that tight is by use of a static VAR compensator, something which is far more complex and sophisticated than simple "capacitor banks".

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[bdn2004]

Hmmm I’ll check into that. Considering my source I don’t usually have any reason to doubt. He’s a very intelligent person at a very high level with the Utility.

The capacitor banks are the NESCO 3-stage harmonic filtering type. They are the size of a yellow school bus. I’ll call them and ask them and post what they say. I’ve called them before.