GSU tx OLCTC

The OLTC changes the voltage regardless of which side it is installed on. The reason that they are usually installed on the high voltage side is because that is the low current side and the switching mechanism doesn't have to be as robust. And don't forget you can also adjust the output voltage/VARs via the excitation system.

The voltage signal is feedback from LV Primary to AVR OLTC, if there was a voltage drop on the grid, generator AVR would adjust the field current in order to match with the voltage grid through the feedback from synchronizer, the tx avr sees this condition as voltage drop, it tends to reduce its winding in order to increase the primary winding (LV side of tx), but the primary side of tx voltage is fed from the generator, no matter how many steps were reduced, the voltage on the primary side is fixed by the generator, my assumption this may cause the secondary side voltage to drop (HT side of tx) as primary voltage is fixed by the generator and also due to secondary winding reduction by OLTC, if this were to happen, the HT side (33kv) would no longer syncronized with grid because the amount of voltage reduction on HT side(33kv) is greater than the voltage reduction by the grid/generator. I'm perplexed how is possible for OLTC step up TX to work if the generator is paralleled with the grid?, what I can see now NLTP is the best option.

RAMCONSULT gives a good description of what could be happening.

My thought is that typically the generator will have two modes of operation..

1) AVR Voltage control when it needs to synchronise and connect to the Grid, say with the OLTC preset to middle tap.

2) Power factor control when Grid connected, typically unity where the generator just tries to run at a steady MW and nearly constant amps without reacting directly to volts. N.B. often the OLTC does the voltage or VAr/power factor control at Grid voltage, or just keeping the generator working at its rated voltage despite Grid voltage changes [just consider that rated MVA at 95% volts requires 5% overcurrent from the generator while 105% voltage requires rotor overcurrent].

I am not clear if the question is about a theoretical system with AVR and OLTC or an actual plant. The term AVR [Automatic Voltage Regulator] is commonly used for the "excitation" control system with the general understanding that it is not just a fixed "zero regulation" voltage system but can operate with a current feed back which causes a voltage fall with increasing lagging current (droop) or/and a slower loop which behaves like a human operator who adjusts the voltage knob (often the operator sets voltage set-point via an up/down motor drive or electronic register which can be switched to auto control by a raise/lower controller).

If this is a new fault on a proven plant, is it possible that some (Grid breaker?) auxiliary selection to the AVR/Excitation control [or OLTC] is not operating as it should when connection is made to the Grid??

It is possible an AVR "voltage droop" is not working or is set too small. This would not matter or be noticed until connected to Grid. Once Grid connected, however, the AVR could have excessive gain leading to instability.

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