Advanced Electronics (concepts And Installs) Flashcards
The level gauges outside sub 5 attached to the two transformers, the ones showing the winding temp alarms and such, how do they function?
They work on 125-130 volts dc. At the transformer itself, the box attached to the side of the transformer is what contains the terminal blocks, with load wires at top (not standard; we usually have load on right and bottom side, BUT, at a peckerhead, whatever works better takes precedence).
The positive dc wire is labelled. It is then jumpered to a few other terminal block contacts on the same side (one for each alarm/input to the flex io), to provide line voltage or incoming supply power to other points along it. The power then goes into the device (in this case, a level gauge switch etc) which looks like a gas meter and is located at a higher height against the wall of transformer, and once the contact closes at a certain setting (ie certain temp setting), power completes its circuit, ending at the flex io (which acts as a neutral bus bar basically), which is located in the top compartment of the cells located inside the sub (at the tie bar cell specifically).
This flex io then communicates with the main program, and, at sub 1, should the CT (which is in a box atop the transformer) or a level guage or alarm reads a problem setting at the transformer in sub 5, the cell feeding that transformer in sub 1 should trip, via the communications system that connects the flex io’s together.
If reading an open circuit voltage, and the flex io/plc input is low, then everything is good. What can happen if you accidentally have your ohmeter setting on the voltmeter?
The ohmeter provides a lower impedance internally in the meter compared to the voltmeter setting. In this case, the ohmeter provides an alternative path for the circuit.
If you are measuring across an open (whether at a contact or terminal block), and you’re in the ohmeter setting, power goes through the voltmeter, to the flex io, completing the circuit and simulating a closed circuit. This is why ohms should always be measured on a dead circuit.
When you close a tie breaker with a transformer on each side, why doesn’t it short?
Because the transformers are in parallel and off the same feed. Same point electrically.
BUT this only applies if the phases are paralleled too. Phase A on transformer A must attach to the same point on the bus bar as phase A on transformer B. Otherwise, line to line short on the bus bar.
Without a smart system for closing a tie to connect two transformers, what can happen?
Multiple things.
1) one possible thing is that, if one transformer is open and one is closed, and the open one is then closed, the inrush of the second transformer, coupled in with amps already going through system to previously closed breaker may trip the breaker feeding both transformers.
