phase sequence relays

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mivey

Senior Member
in fact the gist of my problem here is that one of our friends said above (perhaps u mivey) there are some relays which can find phases themselves and break the supply if it reverses so it is not problem that even if the supply is RTS and not RST (reverse of the conventional sequence) at the moment of connection and motor runs and after a while if the sequence returns back to its conventional sequence so the relay will break the supply since at the moment it defined the sequence of the supply it takes the sequence at that moment as reference so that it will break if it reverses,
generally i see relays which have defined supply phase names as RST and it means if the conventional supply is RST so u cant run a motor in reverse direction true?
but if the relay is programmable so u can preset the sequence as RTS and it will run reverse and there is no problem from then on true?
anyway i wont ask any question about this thank u for all ur help tries...
If I understand what you are asking, you want to know how we separate out the negative sequence currents/voltages? It is a simple mathmatical computation. See this link for a brief overview:
http://www.gedigitalenergy.com/smartgrid/Dec07/7-symmetrical.pdf
 
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mivey

Senior Member
as u said only names change, the next peaks will be at the same time with other scheme, in reality there is no need to draw a picture, if they have 120 degree in both cases it means they have the same values at the same times exept that only phase names changing,
Yes .
 

electrics

Senior Member
no what i am asking is this,
if i am not wrong anyone above said to me that some relays can define the RST phases themselves, is it possible?
i see relays which have terminals as R S T so it means these (and perhaps all ) relays will be connected to the utility and it accepts these phases either true or wrong as RST and whenever the sequence turns out reverse it breaks true??

and if it is the way described above (second one) so it is possible that u can connect the S to r , R to s and T to t So it is a wrong connection but the relay will be able to feel it ?? if so how will it make this?? i am asking of a REFERENCE .. or is this so easy that i cant see because of simplicity??
 

electrics

Senior Member
excuse me i think i cant express myself fine lets give a try again:)
if a relay has RST terminals and u connect RTS to these terminals how the relay will know that it is not RST but RTS ??
it has no reference since for example maximum values are reached at the same time..
so it has to have something electrodynamical since the direction of a rotor will produce a a ClockWise or AntiClockWise direction so that it can dedect that the connection is true or not
if it is not up to electrodynamics so what can make a relay detect that connection is true or not?
if it accepts the first connection sequence as RST so it is easy that the relay dedects any reversal since any one phase's value will lag or delay with respect to the reference phase (perhaps R Phase)
if a relay can dedect the phase sequence at the moment it is connected to the supply (as i said some guy said so to me in this subject) so is it possible that this relay can dedect the phase names separately? (ı guess this is impossible)
 

rcwilson

Senior Member
Location
Redmond, WA
The relays have three terminals for voltage input, say they are labeled 1, 2, 3. A circuit inside the relay energizes the contact output if the voltages on the terminals reach their positive peak in the 1,2,3 sequence. If the phase sequence is 1-3-2 the relay does not actuate.

The relay can't tell if it is phased red-yellow-blue, RST, ACB or Tom, Dick and Harry. It is just looking at its terminals and saying yes = 1,2,3 or no.

Here's an example of how I use them. An overhead crane has up-down, east-west, north-south controls with reversing AC motors. A phase sequence relay monitors the incoming power because the motors will run backwards if the phase sequence is wrong. When the crane is first wired up and the relay does not pick up, we swap two phases, the relay picks up and we get control power. When I push the UP button, the hoist goes up. It doesn't matter if the substation or the panelboard has RST or RTS, ACB, or CBA phasing. It only matters what is feeding the motor.

If an electrician upstream ever swaps the phases for some reason, the crane will stop working. (I investigated a fatality where the crane was used on temp power during construction and swapped to permanent power with different phase rotation. When the operator pushed the down button, the hoist went up, past the upper travel limit switches, breaking the hoist cables.) The relay is not looking for a specific phase just for the "correct" phase sequence and motor rotation direction for the system it is wired to.
 

mivey

Senior Member
no what i am asking is this,
if i am not wrong anyone above said to me that some relays can define the RST phases themselves, is it possible?
i see relays which have terminals as R S T so it means these (and perhaps all ) relays will be connected to the utility and it accepts these phases either true or wrong as RST and whenever the sequence turns out reverse it breaks true??

and if it is the way described above (second one) so it is possible that u can connect the S to r , R to s and T to t So it is a wrong connection but the relay will be able to feel it ?? if so how will it make this?? i am asking of a REFERENCE .. or is this so easy that i cant see because of simplicity??
You bring the wires to the relay in a certain order. If the relay expects terminal R to lead terminal S to lead terminal T and you hook up the wires differently, then it will think you have a reverse sequence.

I'm sure it is possible for a relay to be programmed to accept the reverse connection as "normal" or you might could just use the opposing open or closed contact, depending on what you were doing. I just looked at a couple and they expect you to bring in the wires in the positive sequence. By positive, I mean the normal connection that gives you the desired motor rotation. It expects the wire at terminal R to lead S to lead T. It doesn't matter how the wires are labeled.

If we are coordinating with another relay, we need to be consistent in which wire we label R.


Add: Bob said it clearly, I think.
 

electrics

Senior Member
"contact output if the voltages on the terminals reach their positive peak in the 1,2,3 sequence. If the phase sequence is 1-3-2 the relay does not actuate.
"
here is the problem, the relay has no reference , how can it decide that they reach positive peaks in order? if it has a reference say that the first one of the terminals is R phase so it is easy to determine the sequence but as u can see from the 3 phase sine wave scheme there is no any difference between RST and RTS , they reach the same peak value at the same time intervals...
 

mivey

Senior Member
"contact output if the voltages on the terminals reach their positive peak in the 1,2,3 sequence. If the phase sequence is 1-3-2 the relay does not actuate.
"
here is the problem, the relay has no reference , how can it decide that they reach positive peaks in order? if it has a reference say that the first one of the terminals is R phase so it is easy to determine the sequence but as u can see from the 3 phase sine wave scheme there is no any difference between RST and RTS , they reach the same peak value at the same time intervals...
They do not reach the peaks in the same order or at the same time.

Let the relay have terminals Lassie, Flipper, and Arnold. The relay expects whatever voltage is on terminal Lassie to reach a peak value before terminals Flipper and Arnold. It expects terminal Flipper to reach a peak before terminal Arnold. It knows that terminal Lassie is reaching a positive peak every 1/60 of a second.

If during the 1/60 of a second after terminal Lassie reaches a peak, terminal Flipper reaches a positive peak before terminal Arnold, then the sequence is normal. If during the 1/60 of a second after terminal Lassie reaches a peak, terminal Arnold reaches a positive peak before terminal Flipper, then the sequence is abnormal
 

electrics

Senior Member
ooh excuse me, i figured it in a way that the phase sequences is parading before a check point:) u are totally right, i was obsessed with the idea that relay has a check point (just an imagination )and and from there checks the maximums
EXCUSE ME ; it is very simple indeed and i am ashamed not to understand it, i made u exhausted my dear friend thank u one thousand times...................
 

rcwilson

Senior Member
Location
Redmond, WA
There are many ways of doing this. Think about your voltage vectors. Here is one way.

Internally the relay has a phase shifting RC or RL or transformer circuit designed to produce an output when the sequence is correct. Assume RST voltage sequence. Phase shift the R voltage 120 degrees lagging so it matches up with S. Connect a relay coil to actuate on the sum of the shifted R and the original S voltage. They are in phase so the voltage should be about twice the fundamental. If the phase sequence is RTS, the R voltage is shifted the opposite direction and the sum of the two voltages is less than the relay coil setting.

There are many ways of implementing it. Start drawing your vector diagrams and play with inductor, capacitor, resistor and phase shifting transformer circuits and you can come up with a way of doing it also.
 

electrics

Senior Member
okay lad, pls tell me how a RC or RL circuit lags a voltage 120 degree, i know how current is lagged with RC and RL :) But i am really ignorant of phase shifting, can u show me a pic or make a visual explanation?
 

mivey

Senior Member
ooh excuse me, i figured it in a way that the phase sequences is parading before a check point:) u are totally right, i was obsessed with the idea that relay has a check point (just an imagination )and and from there checks the maximums
EXCUSE ME ; it is very simple indeed and i am ashamed not to understand it, i made u exhausted my dear friend thank u one thousand times...................
But now that I have made a drawing, you are going to have to suffer the consequences and look at it :grin::

Lassie-Flipper-Arnold.jpg
 

mivey

Senior Member
okay lad, pls tell me how a RC or RL circuit lags a voltage 120 degree, i know how current is lagged with RC and RL :) But i am really ignorant of phase shifting, can u show me a pic or make a visual explanation?
If you understand how a current can have a phase shift, then just run that phase shifted current through a resistor and guess what the voltage across that resistor will be?
 

mivey

Senior Member
I found a good example that illustrates the phase shift across the capacitor. One scope channel measures the voltage across the capacitor and the other channel measures the voltage across a small resistor that has the capacitor current running through it. The resistor voltage is out of phase with the capacitor voltage because the capacitor current & voltage are out of phase.

http://www.physics.ucla.edu/demoweb...cuits/the_90d_phase_shift_of_a_capacitor.html
 

mivey

Senior Member
okey mivey i know phase shifting is possible till 90 degree but what about 120 degree?
Then make sure the resistor is not small as compared to the capacitor. The impedance is a combination of the resistor and capacitor. It is quite easy to get a 60 degree shift. Stage two of those and you have 120 degrees. You could do a phase shift digitally or through an op-amp circuit. I suspect most have a digital phase-shifter now.
 

rcwilson

Senior Member
Location
Redmond, WA
okey mivey i know phase shifting is possible till 90 degree but what about 120 degree?

Note that I said you could make a circuit of R -C-L components and transformer connections. You can wind a transformer to get jalmost about any phase shift you want.

For phase sequnece detection, a shift of less than 90 is all that's needed.
 

electrics

Senior Member
I examined the link u gave me but still couldnt get how can it be achieved, it seems to be explaining how a phase shift less than 90 degree can be achieved. you say "Stage two of those and you have 120 degrees" what does stage two mean?
if u dont want to give answer i feel respect for u still since i always ask what i know:)
in fact i dont know how phase can be shifted with RLC circuits..
 
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