synchronization with one generation

[odlanor]

A 13.8kV auxiliary services system consists of 5 substations (SE1, SE2, SE3, SE4, SE5) interconnected in a closed loop configuration. Each substation has loads and only SE1 has generation.
IIn normal condition the ring is closed (1-2-3-4-5-1). Assuming that the connection 4-5 was opened at 5 terminal. So, the ring had been cut in two ramifications: SE1-SE2-SE3-SE4 and SE1-SE5.
question:
How to synchronize the terminal 5 with 4 to close the ring again?
Note that there is only one source of generation and therefore the parties to be synchronized in the same spin speed, is never found.

 

[DTR2011]

Is there a utility source as well? What kind of loads? If there are motor loads (critical), consider a fast bus transfer. Is this a power plant?

Beckwith Electric has some good papers about this subject and synchronizing as well.

http://www.beckwithelectric.com/support/tech-papers.html

 

[odlanor]

the problem is:
qow to synchronize 2 3-phase sequence voltage vectors out of phase , both running at same frequency?

 

[Marmite]

If you only have one source then your voltages can't go out of synch by opening a ring switch unless you carry out work on the cable and mess up the phasing. Please post a single line diagram. Is there another source on the network?
Regards
Marmite

 

[mgtrp]

The voltages can be at different phase angles depending on the loads etc, especially if there are long distances between substations. If the difference is too pronounced, then you need to bring them closer together by adjusting the voltage in some way (changing transformer taps, etc).

A simple synch-check relay on the close circuits is all that is really needed to confirm that the angle differences aren't too large before closing. If the substations are close, so there is no risk of the phase angle difference being large, you might even be able to just close the circuit breaker.

 

[odlanor]

mgtrp,
yes, you can close CB but you keep two ramifications circuit eletrically separated although is physically closed.
distribution Currents after close will be the same as before. From operational viewpoint there will be doubts.

 

[mgtrp]

I'm not quite sure that I'm following you, sorry.

 

[davidbeach]

Unless your ring stretches over many, many km, you won't every get to the point that you have too large a phase angle across your open point. Closing at 15 degrees is no big deal. In your case I wouldn't worry about 30 degrees (not that you could ever get there in any normal situation) unless there were extremely large synchronous motors involved. If your local generation is in fact the only source to the ring under any condition, I wouldn't even waste the time, effort, or expense of synch check relays.

 

[odlanor]

davidbeach,
this is a 13.8kV close ring auxiliary system of about 1.3km , to supply a hydropower plant of 25 generator(25x80MVA).
I do not think is necessary check synchronism.
My viewpoint is that you only synchronize when voltage and phase angle are inside limit of synchronization AND there a little difference of frequency in order that phase sequence voltage are overlap and generators can be atracted or engaged.
With one generator we have a same frequency and even though limits are satisfied and you close CB , systems will not engaged for a short time after synchronization. This could be represent a worry for operation team.

 

[odlanor]

davidbeach,
this is a 13.8kV close ring auxiliary system of about 1.3km , to supply a hydropower plant of 25 generator(25x80MVA).
I do not think is necessary check synchronism.
My viewpoint is that you only synchronize when voltage and phase angle are inside limit of synchronization AND there a little difference of frequency in order that generators can be atracted or engaged.
With one generator we have a same frequency and even though limits are satisfied and you close CB , systems will not engaged for a while after synchronization. This could be represent a worry for operation team.

 

[isquaredr]

SE5 & SE4 are tied through SE1, what's the issue here or am I missing something? Probably a diagram would help if there are added complications.

 

[odlanor]

I guess they are not tied yet. There are two sequence phase voltages running at same frequency but with offset of angle.
I understand to synchronize two generaror , voltage phasors must be overlapped. Although ring is phisically closed , there are no reorganization of distributing current bettween ramifications. This is very teoric interpretation of eletromagnetism that I am trying to understand.

 

[davidbeach]

It will rebalance itself when the breaker closes. Basic Ohms law. There are no issues in closing the breaker.

 

[odlanor]

davidbeach,
I am not sure about your affirmative. There are no issues in closing the breaker. The problem is out phase keeping same frequency. I need to think to reply later.

About synchronization:
This system is grounding with high resistance , with phase-to-earth current no more the 10A.
All fault to ground will alarm. There are 1 hour to look for the fault and recomposition of system.
Do you think is necessary synchronism to close CB , in condition of this fault?

 

[davidbeach]

OK, I don't get the concern. If the SE1-SE4 path is longer and has more load than the SE1-SE5 path, the phase angle at SE4 will lag that at SE5, so what? When you close between SE4 and SE5 you will be back to EXACTLY what you had before it opened between SE4 and SE5. Angles and current flows will shift and rebalance. Two different operating states but there is nothing that would keep one from transitioning to the other if the conditions (such as closing the SE4-SE5 tie) change.

 

[odlanor]

assume before close:
- SE1-SE4-5(open terminal at SE5) path, the phase angle at 5 will lag 30 that his source SE1.
- SE1-SE5 path the phase angle at SE5 will lag 10 that his source SE1.
assume you close CB with 30-10 = 20 between SE5 and 5 points of synchronization BEFORE closing.
This difference (20) will keep AFTER close because frequence is the same at both side.

SO you will not have rebalance of currents!

 

[davidbeach]

Nope. How would that angle (can't imagine the angles get that big, but what ever) be maintained unless that was the angle that would have been in place be for the loop was opened? Once SE4-SE5 is closed, if there is less impedance between the generation at SE1 and the load at SE4 via SE5 than there is via SE2-SE3 then current will flow to the load at SE4 via SE5 and the angles will change. On the other hand, if there is less impedance to the load at SE5 via SE2-SE3-SE4, than directly, the current will flow around the loop that way. The angles, what ever they are, are only valid for one condition, and when you close the tie you change the conditions and the angles change. Again, simple Ohms law.

 

[waross]

You said it better than I would have David.

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

 

[odlanor]

davidbeach,
You are completely right. Now I am convincent.

Lets go to 2nd problem
This system is grounding with high resistance , with phase-to-earth current no more the 10A.
All fault to ground will alarm. There are 1 hour to look for the fault and recomposition of system.

Do you think is necessary synchronism to close CB , in condition of this fault
OR
do you have any specific reference literature for me to do some avaliation?