Why During Start up AVR control is in manual and stays upto 85% speed then excitation breaker opens and again around 95% speed the excitation breaker(41DC) closes and control changes to Auto?
Why can't the excitation control is in Auto right from the start up??
If suppose during start up the control is in manual why the breaker opens at 85% speed and closing again at 95%speed??
When the excitation during start up is in manual control how it is under control(because we never adjusted the voltage during start up manually)??
Is the generator used as a motor during the start-up of the GE 9FA turbine at your site?
> Is the generator used as a motor during the start-up of the GE 9FA turbine at your site?
Yes we use generator as a starting motor with the help LCI.
This is a very basic description of how the generator field power loop works in the units with a static starting device (SSD) or static frequency converter (SFC) or load commutated inverter (LCI) at our site.
The field circuit breaker (FCB) feeds a three winding transformer. One of the secondary windings feeds the main excitation (brushless). It takes control when the generator is going to be synchronized to the grid and during operation. The AVR controls this excitation. The other feeds the starter excitation (via brushes). This one is in control during the unit run up (and cranking/washing). It is controlled by the SFC control system.
Both of them are separated and have separated controls. That's why you don't "see" the AVR doing anything during startups. So, during run up, you don't need the AVR, the SFC is in control.
When you give the unit the start order, the generator rotor field will be established injecting current through the FCB, the starter excitation winding, a thyristor bridge and the starter excitation brushes.
The SFC supplies then the generator stator with variable frequency and reduced voltage. The generator works as a synchronous motor speeding up/down to the different speeds (purge speed, firing speedů) following the set points (in my case CSFSTP) the speedtronic sends to the SFC control, until the set has reached a sufficient speed for the gas turbine to operate autonomously (in your case 85%, in my case L14HC=84%).
When this speed has been reached, the gas turbine can speed up by its own means, so the starter excitation brushes can be withdrawn, the FCB opened and the SFC stopped.
Just before the unit is launched to the grid (L14HS=95%) the FCB closes again and the AVR takes control to regulate voltage/current into the rotor to match the unit's terminal voltage to the grid voltage.
Hope this helps.
Thank you very much for your valuable time in clarifying
my doubt. its really s very good explanation but still I have some questions to ask you.
Do you mean FCB means Dc breaker or Ac breaker. considering FCB as DC breaker I can not see any transformer between FCB and generator and considering as AC I observe its feeding only and directly to the rotor sowhere will be the source for starter excitation
does the starter excitation system means field flashing
If it is other than field flashing breaker kindly differentiate between them.
What I described applies to de GTs with SFCs we have at our plant, yours could be different. (we don't have an excitation DC breaker on our units). In the GCP circuit diagram you should be able to see how's the generator field power loop at your site. I can imagine various ways of getting the alternator to work as a motor, but the basic concept is just one: create a constant field in the rotor then create a rotating field in the stator so the rotor turns.
Here we have two independent channels that feed excitation current to the rotor windings:
1 The starter excitation (used to create the rotoric field when the SFC is working). From the 400Vac bus, through the FCB (Field Circuit Breaker/41E/excitation breaker), one of the windings of the three winding excitation transformer and a dedicated thyristor bridge/controlled rectifier (via brushes). The SFC control has a 4-20mA output to control the excitation at this point. This could be the reason why you don't see your AVR "doing anything" during run ups.
2.0. The generator excitation. From 400Vac bus, through the FCB again, the other winding of the three winding excitation transformer and a dedicated thyristor bridge/controlled rectifier (brushless). This is the excitation used to create the rotoric field to synchronize the unit to the grid and during operation connected to the grid. The AVR controls the triggering points of the thyristors at this stage.
2.1. Excitation booster. From 125Vdc through a static switch (downstream the thyristor bridge of the generator excitation). It's used when there's an important drop of the generators terminal voltage. I'm not really sure if this is what you call "field flashing".
Hope this answers your questions.