Dear All,

we were having problems with our Mvar recently, it's sudden drop from 90 to -135 and after a few minutes back to 90. Anyone experience this before? we suspecting problems with ex2000 excitation system

regards

 

Dear anonymous,

A positive VAr reading indicates reactive current is being "produced" by the generator because the exciter regulator is supplying more energy to the generator rotor than is required to make the generator terminal voltage equal to the voltage of the grid the generator is operating in synchronism with.

Producing reactive current in a synchronous AC machine (generator) requires higher than "normal" current flows through the generator rotor windings. Higher current flows result in more heat being generated in the generator rotor, and there are limits to the amount of heat that can be removed from the generator rotor which means there are limits to the amount of reactive current that can be produced by a generator. Most generators are capable of supplying reactive current to the grid they are connected to without harming the generator.

A negative VAr reading indicates reactive current is being "consumed" by the generator because the exciter regulator is *not* supplying sufficient energy to the generator rotor to make the generator terminal voltage equal to the voltage of the grid the generator is operating in synchronism with.

One of the potentially serious side-effects of negative VArs is that the generator rotor can be driven (by the prime mover) to "slip a pole" and jump by 180 degrees or 360 degrees which can be catastrophic to the generator, the load coupling, and/or the prime mover--severely catastrophic.

There are usually protective relays which monitor reactive current and even generator rotor current to prevent such a catastrophic event. Most generators are not capable of being operated as reactive loads on the system they are connected to, not for very long anyway.

Now, you say you are producing 90 MVArs (MEGA VArs) and then the indication jumps to -135 MVArs (MEGA VArs)--which is a change of 225 MVArs, which means there is a huge change in the amount of current that is flowing out of the exciter to the generator rotor. That change, if it can be believed, indicates that the exciter is going from producing "excessive" current to producing almost zero current. I would presume that if you looked at the reactive capability curve for the generator you would see that the -135 MVAr reading is pretty close to, if not more than, the under-excited limit for the generator for the load being experienced.

Which would lead one to believe there is some problem with either the EX2000 exciter regulator or the VAr meter/transducer/calculation. Which would mean there is likely some alarm(s) being annunciated by the EX2000; what are the EX2000 alarms being annunciated prior to and during one of these events?

Also, it would lead one to believe that either the under-excited reactive current limit has been changed/forced or some protective relay(s) have been "bypassed".

OR, there is some problem with the way that the reactive current (VAr indication) is being obtained/displayed.

If the reactive current is changing by as much as you have indicated, and it's a real condition, then something is very seriously wrong with the equipment at your site--or it's being operated in a manner which is unsafe for the equipment and any personnel in the area of the equipment.

In any case, get someone to site who can assist with troubleshooting the issue(s).

 

Assumed that your EX2k is wired to the HMI, did you see any diagnostic alarms from the EX2k? The diagnostic alarms are usually in form of error codes which you could cross reference from the manual.
Do you have the UEL (under excitation limiter) installed and active for your EX2k? If yes what is the set points for alarm and trip? You could check this with your generator under excitation curves.
Do you have PSS function in your EX2k? If yes, was it in service and active during the incident?

If there are no diagnostic alarms present during the incident and the PSS was active, your generator might just be responding to the grid. Check with your grid operator to confirm whether there was any grid disturbance during the period.