Transfer Switch Phase Differential

[ohmhead]

Well Mivey what you are saying is the type of winding between the two are the 30 deg secondary problem are you saying use a different secondary on each meaning wye on one and a delta on the other or y on one y on other to eliminate the phase shift ?

 

[gar]

100221-1016 EST

ohmhead:

Run the experiment I described in post #18 and you will see a phase shift between the two voltages. Try to relate this to how three phase transformers can be connected. Also see if you can find some source material on drawing vector diagrams.

.

 

[charlie b]

The emergency source is a. . . .

The 13kv terminates in a rotary puffer switch, which then feeds two transformers.

Are you using the word "emergency" in a loose, "conversational English" sort of way? You would never be telling us that an Article 700 "Emergency" power source comes from the same utility feed as the normal source? :-?

 

[A-1Sparky]

Yeah, sorry Charlie. I guess I should have specified "alternate" source. The "alternate" source, while operating under normal utility-powered conditions, originates form the 480Y277v gear, which is fed from the backup generator upon loss of utility power. I guess that I expected to see identical sine waves produced from the same utility source. Time to do some transformer studying! Thanks for all the replies, guys.

 

[A-1Sparky]

I've been doing some reading up on in-phase monitoring for tranfer switches, and it seems as though such a system is inferior to a time-delay neutral system. It seems like changing over to a time-delay neutral system would solve my problem. Is there something I'm missing?

 

[mivey]

Well Mivey what you are saying is the type of winding between the two are the 30 deg secondary problem are you saying use a different secondary on each meaning wye on one and a delta on the other or y on one y on other to eliminate the phase shift ?

On a wye-wye bank (and a delta-delta bank), the primary and secondary voltages share the same transformer so there is no phase shift. On the delta-wye bank, the line-line voltage on the delta side goes through one transformer but the line-line voltage on the wye side goes through two transformers. Thus, the delta and wye voltages have a relative phase shift from the primary to secondary.

I'm saying one alternative is to get rid of the delta-wye bank and replace it with a wye-wye bank so you have all wye-wye banks and no phase shift.

The other option is to add a delta-wye bank in the circuit that currently has only wye-wye banks. This will get you a 30 degree phase shift in this circuit to match the 30 degree shift in the other circuit. Just be sure the delta-wye is connected the same as the other bank because you can either get a 30 degree shift or a 210 degree shift (30 plus 180 degrees, or a 30 degree shift with the polarity reversed).

 

[mivey]

I've been doing some reading up on in-phase monitoring for tranfer switches, and it seems as though such a system is inferior to a time-delay neutral system. It seems like changing over to a time-delay neutral system would solve my problem. Is there something I'm missing?

I think so. You can't tie two different voltages together without modifying the voltages to make them match.

 

[A-1Sparky]

Mivey, the voltages would be the same. The 480v coming from the generator is stepped down via a 480v delta primary to 208Y120v secondary transformer before it reaches the transfer switch.

 

[mivey]

Mivey, the voltages would be the same. The 480v coming from the generator is stepped down via a 480v delta primary to 208Y120v secondary transformer before it reaches the transfer switch.

If you have two voltages that have the same peak value, it does not mean they are the same unles they peak at the same time. A voltage that has a phase shift is a different voltage from one that is not shifted. As I see it, you have two scenarios:

#1: two 30 degree shifts trying to tie to one 30 degree shift:

--------13kV,0 deg---------
|.........................|
{delta....................{delta
}wye......................}wye
|480Y/277,30 deg..........|208Y/120,30 deg
|.........................|
|.........................|
{delta....................|
}wye......................|
|208Y/120,60 deg..........|
|.........................|
|--------[cabinet]--------|

#2: one zero degree sift plus one 30 degree shift trying to tie to one zero degree shift

--------13kV,0 deg---------
|.........................|
{wye......................{wye
}wye......................}wye
|480Y/277,0 deg...........|208Y/120,0 deg
|.........................|
|.........................|
{delta....................|
}wye......................|
|208Y/120,30 deg..........|
|.........................|
|--------[cabinet]--------|

 

[jim dungar]

Actually there are several different scenarios that can cause your observed out of phase problem. They are almost always a result of misidentification of phases in the facility.

One possible scenario is where the leads are swapped the same way primary and secondary (Swap H1 and H3, and swap X1 and X3), resulting in a 60? phase shift with normal phase rotation (that is what flips everybody out).

This is easily identifiable by measuring all of the different voltage combinations.

X1 - X1' = Vln
X2 - X2' = Vln
X3 - X3' = Vln
X1 - X2' = 2Vln
X1 - X3' = Vln
X2 - X1' = Vln
X2 - X3' = 2Vln
X3 - X1' = 2Vln
X3 - X2' = Vln

 

[mivey]

Actually there are several different scenarios that can cause your observed out of phase problem. They are almost always a result of misidentification of phases in the facility.

One possible scenario is where the leads are swapped the same way primary and secondary (Swap H1 and H3, and swap X1 and X3), resulting in a 60? phase shift with normal phase rotation (that is what flips everybody out).

This is easily identifiable by measuring all of the different voltage combinations.

X1 - X1' = Vln
X2 - X2' = Vln
X3 - X3' = Vln
X1 - X2' = 2Vln
X1 - X3' = Vln
X2 - X1' = Vln
X2 - X3' = 2Vln
X3 - X1' = 2Vln
X3 - X2' = Vln

I think what you are describing is a polarity change. The zero deg angle (call it phase "A") becomes 180 deg, etc. The closest phase from the other circuit to "A" is either at 120 deg or 240 deg, both being 60 degrees away. The OP had 20-25 degrees difference.

 

[big john]

...The differential is significant enough that the in-phase monitor in the transfer switch is not allowing a full transfer upon loss of utility voltage.

Elaborate on this problem. If both of your sources are from the same utility feed, obviously "loss of utility" isn't exactly correct, because that would kill all the power to your ATS, right?

If this ATS has a synch-check, I'm assuming it's a closed-transition ATS?
Are you trying to exercise it when both sources are energized, and it refuses to transfer?
Or does the ATS not transfer when you lose one of your sources?

What is actually going on?

-John

 

[jim dungar]

I think what you are describing is a polarity change. The zero deg angle (call it phase "A") becomes 180 deg, etc. The closest phase from the other circuit to "A" is either at 120 deg or 240 deg, both being 60 degrees away. The OP had 20-25 degrees difference.

Actually, I was giving a reason to measure all of the voltages. Simply comparing Va-Va' is not sufficient.

There are several different things that can cause a 'shift'. Unless the OP was using an O-scope, the actual angle is unknown. In the situation I listed as an example, there was a polarity change on the primary side causing what appeared to be a phase rotation problem on the secondary, which was fixed by swapping the X leads. This created the 60? error. But a standard delta-wye transformer has a 30? shift, so it is possible the difference is between them is 30?.

The key is to be looking for 2Vln, which indicates a reversed phase sequence, or as you called it a 'polarity change'.

 

[ohmhead]

Well thanks Gar & Mivey & Jim it just started to make some sense and then came the generator .

And we were getting close to the end of the tunnel .

 

[SG-1]

I've been doing some reading up on in-phase monitoring for tranfer switches, and it seems as though such a system is inferior to a time-delay neutral system. It seems like changing over to a time-delay neutral system would solve my problem. Is there something I'm missing?

Open transition with a time-delay neutral would work if you can tolerate lose of loads both on the transfer and the re-transfer back to the normal source.

 

[mivey]

Actually, I was giving a reason to measure all of the voltages. Simply comparing Va-Va' is not sufficient.

I don't think he did.

There are several different things that can cause a 'shift'. Unless the OP was using an O-scope, the actual angle is unknown.

He was.

I used a Fluke oscilloscope.

In the situation I listed as an example, there was a polarity change on the primary side causing what appeared to be a phase rotation problem on the secondary, which was fixed by swapping the X leads. This created the 60? error. But a standard delta-wye transformer has a 30? shift, so it is possible the difference is between them is 30?.

The key is to be looking for 2Vln, which indicates a reversed phase sequence, or as you called it a 'polarity change'.

I agree that could be this case in another situation, but I'm just not buying it here. I think that would be too obvious on a scope.

Add: the voltage differences he was getting were consistent with the phase difference he observed on the scope.

 

[mivey]

Open transition with a time-delay neutral would work if you can tolerate lose of loads both on the transfer and the re-transfer back to the normal source.

True enough. I think ohmhead mentioned switching with the motors off line.

I'm not sure what kind of delay he would need to let the system settle down before transfer.

 

[jim dungar]

I don't think he did.

I took measurements between A1 and A2, B1 and B2, and C1 and C2.

I am not trying to say I know what is causing the problem, only that we don't have enough information to narrow down the possibilities.

 

[gar]

100221-2045 EST

jim:

The combination of posts 1 and 3 provides the information that an oscilloscope was used, and an angle derived from the scope.

.

 

[steve066]

I've been doing some reading up on in-phase monitoring for tranfer switches, and it seems as though such a system is inferior to a time-delay neutral system. It seems like changing over to a time-delay neutral system would solve my problem. Is there something I'm missing?

Depends on what your loads are. As Mivey mentioned, if you have any large motors, you don't want to hit them with a large phase difference, unless you shut them off during a live-source to live-source transfer. But since you know you will always have exactly 60 degrees of phase shift between the two sources, you could probably just turn off the in-phase monitoring. Switching at 60 degrees of phase difference just happens to be the maximum phase difference you can switch at, and the current surge won't exceed the motors rated starting current.

I'm not really sure what you mean by a time delay neutral. I've heard of solid neutral, a switched neutral, and overlapping neutrals.

Steve