AT & TD. Protection Relay
- Thread starter qrenzo
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rcwilson
Senior Member
- Location
- Redmond, WA
AT= Amp Tap, TD = Time dial. AT picks the left-right position of the trip curve, TD sets the up-down.
On old electromechanical relays, the pickup point of the curve was set by placing a screw in the proper tap setting. The relay had a tapped current transformer on the input with taps at (for example) 0.1, 0.25, 0.5, 0.75, 1.0, 2.5, 3.0, 4.0 and 5.0 for a standard 5 amp input. Place the tap screw in the 2.0 amp tap and apply 2.0 amps to the relay input and the relay disk would just start to turn against its coil spring. (Think of a watthour meter rotating disk held back by a coil spring). With tap current applied, the disk slowly rotates until the trip contact mounted on the disk's rotating shaft touches the stationary trip contact. The AT or tap point is the current at which the relay starts to react or the disk "picks up" and just starts to move. (Most relays would never trip with current less than 110% - 120% of the tap value).
That value is where you start to draw the overcurrent relay curve at infinite time at the top of the graph. It determines the horizontal position of the trip curve on the time vs current graph.
AT is also known as Tap, PU for pickup or Long Time pick up setting. Check the relay manual to see if it is expressed in actual amps or per unit amps.
TD or time dial determines the relative vertical position of the curve. A large time dial number means more time delay at the same constant current.
On mechanical relays a round dial with numbers on it at the top of the disk's shaft set the "home" or starting position of the disk. The stationary contact was located about 11:00 on the relay. At TD=0 the disk is rotated clockwise to 11:00 closing the trip all the time. At TD= 10, the home position is back so the moving contact starts at about 2:00 position. It takes a longer time for the fault current to spin the disk 270 degrees and close the trip contact.
Modern relays mimic these characteristics using digital calculations or analog circuits. But the terms still stick with us.
On old electromechanical relays, the pickup point of the curve was set by placing a screw in the proper tap setting. The relay had a tapped current transformer on the input with taps at (for example) 0.1, 0.25, 0.5, 0.75, 1.0, 2.5, 3.0, 4.0 and 5.0 for a standard 5 amp input. Place the tap screw in the 2.0 amp tap and apply 2.0 amps to the relay input and the relay disk would just start to turn against its coil spring. (Think of a watthour meter rotating disk held back by a coil spring). With tap current applied, the disk slowly rotates until the trip contact mounted on the disk's rotating shaft touches the stationary trip contact. The AT or tap point is the current at which the relay starts to react or the disk "picks up" and just starts to move. (Most relays would never trip with current less than 110% - 120% of the tap value).
That value is where you start to draw the overcurrent relay curve at infinite time at the top of the graph. It determines the horizontal position of the trip curve on the time vs current graph.
AT is also known as Tap, PU for pickup or Long Time pick up setting. Check the relay manual to see if it is expressed in actual amps or per unit amps.
TD or time dial determines the relative vertical position of the curve. A large time dial number means more time delay at the same constant current.
On mechanical relays a round dial with numbers on it at the top of the disk's shaft set the "home" or starting position of the disk. The stationary contact was located about 11:00 on the relay. At TD=0 the disk is rotated clockwise to 11:00 closing the trip all the time. At TD= 10, the home position is back so the moving contact starts at about 2:00 position. It takes a longer time for the fault current to spin the disk 270 degrees and close the trip contact.
Modern relays mimic these characteristics using digital calculations or analog circuits. But the terms still stick with us.
Thank you for the respond. What I still don't understand is:
1. Why is the transformer inrush current plotted as an X in the first picture and not as a curve?
2. What is the meaning of ADJ in "51-ADJ" ? Does it mean ADJusted? If so, then what's the point in plotting the original "51" curve next to the "51-ADJ" curve ?? Because as i see it, in the graphic the TD & AT for both are the same (TD = 2, AT = 5)
1. Why is the transformer inrush current plotted as an X in the first picture and not as a curve?
2. What is the meaning of ADJ in "51-ADJ" ? Does it mean ADJusted? If so, then what's the point in plotting the original "51" curve next to the "51-ADJ" curve ?? Because as i see it, in the graphic the TD & AT for both are the same (TD = 2, AT = 5)
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another question:
3. Why is F1 located lower than F2 in the abscissa of the graphic in the first picture, when in the Single Line Diagram of the picture it says that F1 = 16,566 A and F2 = 15,538 A ??
4. The relays are placed closer to the Transformer than the CB both in the primary and secondary side of the transformer. What are the advantages and disadvantages of this formation? compared to, for example when the CBs are closer to the transformer than the relays?
3. Why is F1 located lower than F2 in the abscissa of the graphic in the first picture, when in the Single Line Diagram of the picture it says that F1 = 16,566 A and F2 = 15,538 A ??
4. The relays are placed closer to the Transformer than the CB both in the primary and secondary side of the transformer. What are the advantages and disadvantages of this formation? compared to, for example when the CBs are closer to the transformer than the relays?
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JoeStillman
Senior Member
- Location
- West Chester, PA
3. F1 is on the primary side of the transformer. I believe the plot shows the transformed current.
4. The relays protect what's downstream of their CT's. Placing the CT's upstream of the breaker protects more of the gear. On the primary side, the relay is not intended to protect the incoming cable so its sfer to place it downstream of the breaker, where the CT primary can be de-energized locally.
1. Why is the transformer inrush current plotted as an X in the first picture and not as a curve?
2. What is the meaning of ADJ in "51-ADJ" ? Does it mean ADJusted? If so, then what's the point in plotting the original "51" curve next to the "51-ADJ" curve ?? Because as i see it, in the graphic the TD & AT for both are the same (TD = 2, AT = 5)
2. What is the meaning of ADJ in "51-ADJ" ? Does it mean ADJusted? If so, then what's the point in plotting the original "51" curve next to the "51-ADJ" curve ?? Because as i see it, in the graphic the TD & AT for both are the same (TD = 2, AT = 5)
SG-1
Senior Member
- Location
- Ware Shoals, South Carolina
The disc speed on an induction disc electro-mechanical is controlled by the applied current. In the case of a Westinghouse CO type relay the disc speed could be farther adjusted ( calibrated ) by changing the magnetic restraint force. This was done by adjusting the amount of magnetic force between a fairly powerful horseshoe magnet & the induction disc.
I believe the picture is showing that the relay is capable of being set anywhere between the two curves with the Time Dial set on 2 & the TAP set to 5 Amps or that amout of variation is possible. You would have to read the instruction manual for the relay associated with that picture for more information.
I believe the picture is showing that the relay is capable of being set anywhere between the two curves with the Time Dial set on 2 & the TAP set to 5 Amps or that amout of variation is possible. You would have to read the instruction manual for the relay associated with that picture for more information.
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