Phase sequence correction 1

[AusLee]

Hello,

I am having requirements for protection against chnage in phase sequence from the public network, therefore i have phase sequence relays installed in the ATS and in the case of a wrong phase seq on the netowrk, it will work on the generator.

The problem is that people think that the generator is running because there is no public network, so they do not pay attention whereas what should be done is go and see why it started and if it was because of a phase sequence fault then they should contact the local authority as soon as possible since running on gnenerator is much more expensive than running on public network. This is not practical and i do not think they will bother every time. I am thinking of linking this alarm to the set of conditions of a problem in the generator (high coolant temp, low oil pressure, ...) with the only exception that in this case this alarm will be on while the genset is on. This is the cheapest solution.

I am thinking of a more expensive one, a phase sequence corrector. I have my network design for RST sequesnce and i connect the generator accordignly, and i am thinking of the possibilities that can come from the netwrok, they are 6: RST, RTS, SRT, STR, TRS and TSR. So i need 6 phase sequence relays and 6 contactors, with no interlock because logically only one relay can fire at a time. Now i can relax, come what come from the network. Of course it is expensive but i would like to ask if it is viable and if you can point me to something similar?

Thanks.

 

[jtkirb]

how often do they roll the phases going into your building? I assume you mean rotation when you speak of sequence?

You only need to care if the seq is ABC or ACB. It's either right or wrong.

You may need to step back and re-think your problem...or give more information on what the real problem is.

JTK

 

[davidbeach]

As jtkirb says there are only two sequences, ABC and ACB, all others are phase shifts and are of no concern. You could just wire an alarm to an additional contact on your phase sequence relay.

What kind of utility would ever cause something like this to be necessary? How would they be regularly reversing phase rotation?

 

[jtkirb]

I get this question quite a bit- how do we line up phase A of our generator to phase A of the utility? Like DB said- only two ways- right or wrong.

Use the seq relay on the utility voltage to prevent the utility breaker from closing if not correct and send an alarm. That's all you have to do.

JTK

 

[AusLee]

Ok ,thanks. I use the pahse sequence on the generator and it is clockwise. For the network, i do not know how frequently they change it but with only 2 contactors as you said i will have this in all my future projects. Let me ask this: i can do it on the LV side with 2 contacotrs to insure that i have Always correct phase sequence from the network, but do you advise i do it on the MV side? I think it will be more cumbersome then. Taking Schneider Electric's line of products i would have:

one IM cubicle (MV IN)
one IM cubicle (MV Out)
then i will have one NSM cell which works as an MV ATS: either this or that side. The inputs of this cell will not be connected to the busbar of the IMs, rather i will have two sets of cables coming out of the IM busbas, one ABC and one ACB, and one phase sequence relay will tell which side of the NSM shall activate, and from then i connect the other metering and transformer protection cells directly to the output busbar of the NSM. Cost of 1 NSM module compared to the cost of 2 LV contactors (kilo amps)+cubicle can be advantageous. what do you think?

 

[rbulsara]

Auslee:

I will be littel blunt than others..

I find it hard to believe any utility co changeing phase sequence ever....

As for generators, you check it when you install it first time and thereafter you need not worry unless you think the generator is going to rotate in reverse.

And what exactly are you trying to do with your last post? Make a sequence changer?? You are out of your mind. You do not need to do that.

Please leave this to some professional and experience engineers and hire a help.

 

[AusLee]

rbulsara - i will be more blunt.

The phase sequence is imposed by the client who is a government administration and have had problems with the elctrical authority related to phase sequence. So it does not matter what you believe, what matters is what they knwo and what are paying and asking for to be installed.

As for generators, like i said, there is not problem and i hard wire it correctly in accordance with the project upon installation.

About my last post. Yes i am trying a sequnce changer. Why would Not i need that? You are the one who is out of his mind until you prove that i am creating a short circuit condition.

Kindly do not reply unless you have constructive and informed feed back related to the question.

 

[rbulsara]

Whatever you say, Auslee. Good Luck.

 

[davidbeach]

AusLee, rbulsara is correct. You are either dealing with a non-problem or you are somewhere that the utility is not capable of a basic standard of care.

If you really need to do something, and I'm not convinced that any real utility would create the problem you describe, it could be done fairly easily. You need to take service at one location, you mention MV, so that would work. Your first piece of gear needs one incoming breaker and two outgoing breakers, both outgoing breakers rated for synchronizing duty. One of the outgoing breakers will be wired to the load ABC and the other wired to the load ACB. VTs on the supply side of the incoming breaker would supply reference voltage to a voltage relay that will trip the incoming breaker on any disturbance of the utility. When the voltage relay sees good utility voltage, of either phase rotation, it would start a timer, with a minimum time of at least 30 seconds. After the timer expired, the relay would allow one of the two outgoing breakers to close (it could even do the closing) while blocking the close on the other breaker. The need for synchronizing duty breakers is because the open outgoing breaker will be seeing out of phase voltages across its open contacts.

 

[rcw retired EE]

AusLee - I have inspected, designed or installed about 300 generator installations and many industrial electrical facilities around the world. Most of them had some sort of phase sequence protection that would trip the main breaker or prevent closing if the utility phase sequence was incorrect. None of them had the ability to change the phase sequence using contactors or breakers.

A change in phase sequence is so uncommon that if it should occur it is a relatively simple change to swap two of the incoming cables. Most phase sequence changes occur due to maintenance or expansion work done on the power system. The change is detected when power is returned and tested. It is a simple matter to shut down and reconnect correctly. In all of the plants I worked in over the last 30 years I recall only one phase sequence change and it happened inside the plant and not on the utility supply. (I’m not counting the initial motor connections that have to be changed about 55% of the time.)

Most owners and clients do not want to spend the extra money for automatic phase sequence changers. The added cost and complexity is not worth the possible small savings in future down time. The phase sequence relay provides equipment protection so there is no danger of damage by not having automatic phase swapping. I would be more concerned about a failure in the phase swapping mechanisim and the related control complexity.

 

[AusLee]

Thank you.

davidbeach - thank you for confirming that it can be done and for the specs needed for every item. Except for the syncro issue i would like to think that i got it right. I will send this to our design offices and have them elaborate it in a complete submittal. Thanks again.

rcwilson - i'm not at 300 yet, still in my 20s and all you say is true to me. A simple flip of the cables can do it but as i said in my initial post that i have to wire it to an alarm and in many projects this is far or uanttended to be heard. This is why i am looking for a way to do it automatically.

I had an MV ATS designed around and NSM (Merlin Gerin) switch. When the client say that the cell can handle only 1000 maneuvers he immediately asked it to be replaced by 2 DM circuit breakers. The added complexity delayed the project by 1 month at the manufacturer's, so They ask for complexity.

Money is not a problem this is why i have not left Africa yet. Money is the last thing you worry about, if you are of the kind who seeks worry but per se one it is not.The homopolar component I0 is "limited" by the utility to 1000Amps; i Never have a normal cut-out in energy, Always an undervolatge for some 5 seconds, sometimes more then out: you know and i know what kind of maneuvers cause that but so that others will forgive me my threpasses i shall do the same.

 

[jraef]

AusLee,
I'll back you up on the reversal problem. I do a lot of international work and I can attest to the fact that phase sequence reversals are quite common, especially in developing nations. It most often happens during storm outages or even political uprsings when people sabotage equipment and workers are under extreme pressure to reconnect. While it is usually caught immediately, I have heard tales of it not being corrected for days or weeks. If that is unacceptable, as it sound like it is for you, then your solution has merit, and davidbeach's suggestions are appropriate.

I was also involved in one remote lumber mill in Washington State who had their Public Utility lines reversed after a storm and the plant ran in reverse for 5 minutes before anyone realized what was happening, causing massive equipment damage. That was 15 years ago but to this day they still insist that every motor controller has a phase reversal protection relay on them.

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[RalphChristie]

Although we use a lot of phase reversal schemes on some of our portable mini-substations, (shift a lot due to the activities in the mine) I must admit that I have never actually seen one on a utility supply or on a generator. In a way I want to agree with rcwilson - why not just start the generator when there is no power, and use a phase sequence relay to check rotation on the utility side. If the phase rotation is wrong the utility breaker must open and someone has to change direction on the utility side of the breaker. (without the generator starting) If it would happen, a phase change could quickly be done and immediate action could be taken.

However, if the client still insists on a such a scheme, it is easily done like David suggested.

One other thing - although your answer will probably be no - is there any chance (at any time) for paralleling the utility and the generator?

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[AusLee]

Ralph - i agree with you as i agreed with rcwilson that it is easily rectified. But for my peace of mind and that of the client i want to have it automated. I have come 3 solutions: one using an ATS cubicle, David's suggestion which is more expensive but allows a larger number of maneuvers by contrustion of the breaker and one solution using 2 incoming instead of one: this is the cheapest and i will discuss it with the authority next time we meet.

For the paralleling of generator and network, it will be a phase-to-phase short circuit if the sequence is wrong, so one more reason for no i do not have it in this case.

Thanks again for your help.

 

[waross]

As I understand it, you are using circuit breakers for the ats.
One breaker for the Utility connection and one breaker for the Generator connection.
Consider two breakers for the utility connection, reverse connected. Use one or two phase monitors to select which breaker to close for the proper phase rotation.
I have seen simmilar circuits on a small scale on refrigerated shipping containers. As you may know, a refrigeration compressor has a plate under the oil pump rotor that can turn a few degrees in either direction. This changes the suction and discharge ports on the oil pump so that lube oil is delivered regardles of the direction of rotation of the oil pump, ONLY WHEN THE COMPRESSOR IS NEW. In an old machine the plate cannot be depended on to still be free to rotate. Common practice is that the first connection of a hermetic compressor is unimportant, but thereafter it must continue to run in the same direction. Reversal of an older compressor often results in lubrication failure.
There was a common circuit on three phase hermetic compressors on shipping containers. These units are plugged in at random and phase rotation cannot be predicted. There were two main contactors instead of one. There was a phase monitor and the proper contactor was energised to always assure correct rotation of the motor regardless of incoming phase rotation.
Seems like what you need and basically all you need is one more main breaker.

 

[AusLee]

Hello again,

You are right in what you say, MV or LV it's the same, although i think on the MV side it can be cheaper and less cumbersome: the MV space has always room for one more unit, but the MDB rooms are always calculated just fit

Anyway, i have made an AutoCAD drawing on:

http://lndang.tripod.com/

and that's more than enough for me - Thanks again.