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Understanding reverse power relay

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ubern

Military
Jun 29, 2008
4
I am trying to understand the reverse power relays to our steam driven generators. This is a discription of the device from the book:
"Potential signal flows through the top coil over a rotatable disc and transformer action induces a circulating current in the disc. Current signal flows through the bottom coil under the disc and produces magnetic flux that passes through the portion of the disc containing the circulating current. This causes motor action that rotate the disc" I can tell you from the drawing that the potential signal is taken across 2 phases of the potential transformer while the current signal is inline with a single phase of the current transformer. "A lag loop or shorted coil on the magnetic pole of the botom coil brings the lower pole flux in phase with thte upper pole flux at unity power factor reverse power flow conditions thereby creating maximum rotational forces. As the disc rotates, contacts mate and energize the reverse power relay"
Here is what i dont understand:
-If one of the two generators looses its prime mover then the other generator will maintain voltage, therefore the potential signal should not change.
-A difference in phases between voltage and current would be the same throughout the circuit so it cant be used to tell if real power is flowing into or out of a machine.
-Its an AC circuit so i know you cant say "current is flowing into or out of" the machine as it is always doing both equally.
- the only difference i can think of is that now there will be less current flow through the current transformer. But if this was the mechanism that activated the reverse power relay then the relay would be energized when the machine was unloaded.

What am i missing?...how is this circuit able to tell which direction real power is flowing?

Also, how does the lag loop bring pole fluxes in phase at unity power factor?
 
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It is all about power factor.

As you know, unity power factor means that both voltage and current are in phase, if you are consuming power at PF=1. Or opposite (180 degree difference between voltage and current) if you are generating power at PF=1. "Generating at PF=1" could also be written "Consuming at PF=-1".

Now, it depends how your relay is connected. Sometimes a lag ring is needed on one of the poles to retard the flux resulting from that pole. In this case, the connection and the lag ring have been chosen so that you have zero phase difference at PF=1. No phase difference means no rotating flux - and the disk does stand still.

If you generate power, there is a phase difference that makes the disk try to rotate in the direction where the reverse power contact is not actuated. And, if the power direction is reversed, the phase difference changes sign so that flux direction changes direction and makes the disk rotate in the other direction so that the reverse contact is actuated.

Modern reverse power relays do not need coils, lag rings or disks to do this work. They just measure phase difference between voltage and current. As long as the difference is "positive", the relay is OK. It is only when it changes sign that a "reverse" contact is activated.

The lag ring works like a shorted circuit which delays the build-up of the flux (a current to oppose the flux is circulating - Lenz' Law) so that the flux is delayed = phase lag.

Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
It sounds like you are making this difficult.
Consiter two sine waves, one voltage, and one current. When the voltage is at a positive peak, where is the current?
Is the current above or below the zero line? If it's above the zero line then the power flow is out, if it's below the power flow is in.
If the current peaks before the voltage then the var flow is capacitive, if the current peaks after the voltage then the var flow is inductive.

 
No, I am not making it difficult. I tried to answer the OP's question about how a reverse polarity relay works. Not just what power direction is.

Your explanation is OK and simple to understand. But are you sure that someone that says "Its an AC circuit so i know you cant say "current is flowing into or out of the machine as it is always doing both equally" is prepared to accept that current is above or below zero and take it from there? I am not so sure.

I am not so sure that my explanation will be accepted either. But I did not try to "make it difficult".

Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
Ok so let me see if i understand this correctly: Lets say when the generator is generating then voltage is leading current by a PF of .98. in this situation current positive peak and voltage positive peak are preaty close to each other because when voltage is positive then current is trying to go out of the machine. But when the prime mover goes away then when voltage is at peak positive then current is close to peak negative because when voltage is positive, current is trying to go into the machine.

Does this mean that when a machine changes from a generator to a motor, that the phase angle between voltage and current changes by 180 degrees?
 
Yes! Glad you got it.

Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
Actually in my previous post I wasen't refering to you Skogsgurra.

Often times the writers of these manuals know the material so well that they forget who is going to be reading them. It's common to get confused, and it's better to just step back and take another look from a different perspective.
 
Sorry Cranky. Never occured to me that the question as such was "difficult". Things got sorted out quite well. I think.

Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
I think ubern was just to deep into how the relay works, and not understanding how to apply it. I've done this before, and strangley also with a directional power relay.
 
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