Soft Load & Open/Closed Transition

[jimgineer]

Can someone please explain to me what drives the desicion between open transition and closed transition (and where this language is used: I've only heard it used to describe ATS operation), and what relationship this has to 'soft loading' etc. The long winded engineery answer that explains all the ins and outs is appreciated... Or a link to a past thread where this has been covered.

Thanks very much

 

[dpc]

Are you talking about motor starters or transfer switches?

 

[jimgineer]

Transfer Switches, I think.

 

[jimgineer]

This is actually in reference to switchgear and its operation - closing normally open breakers upon a return to utility power after an outage.

Sorry for the vagueness-

 

[davidbeach]

Open transition = break before make.

Closed transition = make before break.

If you are looking at closed transition switching using breakers that tie a utility circuit to any source it is not normally tie to your utility will be very interested in what you are doing and will have the final say in the matter. Closed transition transfer switches where it all one motion and there is no intermediate both closed position may be acceptable without additional protection, but for a breaker scheme expect to protect the utility circuit against fault contribution from your source.

 

[waross]

The word transfer may be substituted for the word transition in this context.
I will refer to line power and generator power for clarity.
Open transition.
The line power breaker opens and then the generator power breaker closes. Above about 100 amps, a definite time delay should be used in the open position. The delay should be enough to allow the residual voltage of any spinning motors to decay.
Fast open transition. Basically, a normally closed contact on the generator power breaker closes the line power breaker. This is dangerous in the larger sizes. The line power may be connected to spinning motors before the residual voltage has decayed. If the line power is out of phase with the generator power, (and the probability is high that it will be) the motor will be subject to mechanical shock and possible damage. The system may experience high transient currents.
Very fast, synchronized transition. A synchronizing circuit delays the transition until the phase relationships between the line power and the generator power are correct. Then a very fast mechanism operates to complete an in phase transfer before the motors are able to drop out of sync.
Closed transition. The line power is synchronized with the generator power feed. Then the line power breaker is closed. Now the generator power breaker is opened. As David has indicated, this raises serious issues with the supply utility and with fault current levels. The utility must give permission and a protection engineer must review the protection scheme. A system that is safe with either generator power or line power may be very unsafe and fail catastrophically if a fault occurs during the closed period of the transfer.

Bill
--------------------
"Why not the best?"
Jimmy Carter