A problem in Electrical Power Station

[khaled]

I was working in electrical power station controlled by GE Fanuc PLC, this station of three generators, it can be synchronised manually or automatically according to the load demand, for the manual case there is a three position selector switch, this switch is used to select the number of generators to be in the service, P1 for 1 generator, P2 for two generators & P3 for three generators. Now the problem is that if I want to change from position P2 to P3 one of the generators will be diconnected electrically from the Bus for amoment then it will be synchronised againe in the same moment the theird generator will start and will be synchronised with the other two generators. this problem occurs also in case of changing from P3 to P2, i.e. two generators will be disconnected one time for amoment then one of them will connected again. Now the quistion is it a fault in the selector switch or it is a mistak in the program of the PLC? when I followed the PLC program I found that there is no latching signal while changing from P2 to P3, or From P3 to P2, i.e. to change from P2 to P3, P3 signal should come before P2 signal going off.
Thank you.

 

[BobB]

The fault is in the design of the system. You could relace the switch with "sliding" contacts or use a keep relay in the PLC (latch).
beerchug

 

[Steve Bailey]

Has the system always behaved the way it currently does? Was it designed to operate the way it does? Did someone in your facility accept the system from the builder in its current form? Has someone replaced the original selector switch recently?

These are questions that you ought to find answers for before you assume that there is a fault in the system. You may not like the way it works, but before making changes, you should find out if there is a valid reason for it operating the way it does.

 

[BobB]

My apologies for my tearing in at a million miles an hour. You are quite right Steve, I should not have replied in the way I did. Unfortunately the brain goes much quicker than the fingers and I have a very bad habit of half saying things and expecting people to understand what I mean.
This appears to be a very simple power station setup, one that I have done many times before. I believe that the switch only selects how many sets to place on line. If this is so, it is very likely that the designer/programmer has probably made the same mistake that I made ONCE many years ago. When the switch is in between positions, a break is seen by the PLC programme as "set 2 and/or 3 not required". In other words, the switch inputs are not latched in the PLC and the contacts are not of the sliding type. This will probably then cause the circuit breaker(s) to open momentarily.
If this is the case, the easiest way to overcome the problem would be to use a sliding switch that only breaks the desired contact in the correct position and not between points.
You would have to be very careful and check that this is the intent of the switch and any possible negative effects that replacing it would have on the performance of the programme. It is also possible that a sliding switch may muck up the programme, that is expecting single discrete inputs. If this is the case, a latching circuit would probably be neded in the programme.
Have you checked the original design criteria and switch configuration to see what was intended, what was installed and how it has been implemented in the PLC?
Come back with some more answers before I put my foot in it again.

 

[Tom Jenkins]

There are lots of ways to skin any given cat. You can use make before break contacts on the switch, or time delays on the PLC inputs (de-bouncing in effect) and so on.