We have Siemens RG3 T4s excitation system installed at our power plant with steam turbine. It was operating at 72MW and 75MVArs and suddenly it goes upto 93 Mvars, and generator protection relay gives a command to control system to close the main valve and trip the machine as well as GCB. The terminal voltages of generator goes to 11.57kV while rated terminal voltages are 10.5kV and V/f setting was 2. 42 for 1 second which was met and protection relay operated.
Now I have some questions:
1 Will the Over excitation limiter did not come into service? How can I Check this in RG3 controller? Anybody have a guide?
2. AVR was on volt control mode. Is this the reason which may increase the Mvars to maintain terminal voltages? Because at that time grid voltages were less around 125kV in 132kV system
Regarding the questions you are asking some questions coming through my mind:
In order to try to facilitate troubleshooting of the site issue I would ask you:
1- You should have a look on the commissioning report for the Generator control protection panel.
2- I am not familiar with siemens AVR but philosophy is the same for all OEMs.
Try to connect to IED protection relay and AVR FOR a check up of the fault alarms trips that occurs and try to get some trends file.
At this state I cannot say its because of this or that.. cause Two things are differents overvoltage and overexcitation are differents things, and V/H LIMITER or other loss of synchronism can occurs in such situation ( grid voltage/ frequency fluctuations).
As you described some value are exceeding the setpoint protection and for sure you will need to work closely with grid representatives.
During my experience on GE and Alstom equipments worldwide I faced kind of situation with Plant supplying power to a cement factory.
So we do not know how your plant is operating with grid or island mode proces in case of.
Try to find out by events trends recorders why the plant operated at Mvars value swinging as you described this is on e of the key for get the problem solved.
I do not know if you got loss of synchronism protection enabled in your IEd protection relays ...
Try to troublesshooting the problem by having access to the tripping diagramm ...
I would try to give you some support as much as i can.