Transfer of STS at Phase Angles Difference

[Pgee630]

Hi,

Would like to seek some advices on STS. The design consultant designs with the source 1 and source 2 input to a single phase STS in different phases (e.g. S1 on L1 and S2 on L2, S1 and S2 originate from different power transformer). Assume this is incorrect / bad practice as normally S1 and S2 should be on same phase to ensure STS transfer reliably. Please advise and comment. Thanks

 

[waross]

Do you have a diagram?
What is the capacity of the STS?
Are there any large motor loads?

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Palletjack]

Isn’t that what it’s designed to do?
It’s an open transition switch so for example I could have a single phase STS on my house and have two or three phases of a three phase line feeding the switch..
Say I put A phase on S1, B phase on S2, and C phase on source S3.

For MV I would expect it to be on different transformers. For this I would want the displacement and phase angles pretty close between sources.

 

[waross]

S1, Source 1?

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Pgee630]

@waross

It's a single phase 30A 220V STS supplying critical load IT load, that's why a reliable transfer is a must.

 

[waross]

How fast does the switch transfer?

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Pgee630]

@waross

Transfer time is just few milliseconds with synchronous inputs.

 

[Palletjack]

What switch?
It’s hard to give a definitive answer with so little information.
Some static transfer switches are designed to transfer regardless of angular difference, others have settings to allow transfer within 45 degrees.

Generally speaking, it’s a customer issue.
When your switching between a cycle (20ms) you have to determine if your load will handle a separate source that’s more than 45 degrees out.

 

[waross]

L1 and L2 on different power transformers tells us nothing.
The power transformers may be on different phases.
The phase angle difference may be zero, it may be 15 Degree, it may be 180 degrees or it may be something else.
How long does it take the STS to recognize an outage?
When the power fails, this scheme may restore power within 1/2 cycle rather than one full cycle. We don't know. We don't have the information that was available to the designer.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Pgee630]

The two sources ultimately coming from the same utility feeder, so suppose won't have major difference between their phase angles if they are in the same phase, e.g. L1, right?
However, if the inputs are from L1 and L2, then the difference might not be expected to be minor right? Given we don't know which brand of STS will the end user install (can transfer in any angle differences or have a limitation to a certain extent), does it make sense to have the inputs from the same phase? The question is: if there is no any limitation/constraints to assign the input sources coming from the same phase, is it logical still to make them to be from different phases so to make a asynchronous transfer, rather than a better synchronous transfer? Unless we are very sure all the STS products in the market can perfectly transfer in all angle difference, right?

 

[waross]

Two single phase systems derived from three phase systems may have a wide choice of phase angles.
Is this a hot transfer or a transfer on failure of one source?

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Pgee630]

It's transfer on failure