Underexcitation limiters functionality when generatovoltage regulator is in Manual mode of operation

[Soloten]

I am trying to understand if Underexcitation limiter (UEL) is functional if the voltage regulator is set to manual mode when connected to a large system. By functional, I mean if the UEL will enforce the excitation limit when in manual mode. My understanding is that if the Automatic Voltage Regulator (AVR) is functional, then the UEL is a component of the AVR that enforces excitation limits so that the field isn't underexcited below a set level in an attempt to achieve a voltage set point. Now when one changes to manual mode with fixed field current, I first thought how the VAR contribution/absorption of the generator will change, but what if the system conditions change suddenly wherein the generation is supposed to absorb VARs - i.e. become underexcited. At this point, what happens, given the regulator is in manual mode?

I understand this may not be a very practical scenario but I'm trying to understand the UEL as well as the response of the system if it were in manual and a system voltage increase scenario occurred..

 

[redlinej]

I believe this will depend on the AVR manufacturer. Most manufacturer will provide modes of operation and explain them in details.

I will take a look at ABB UNITROl 1020 manual and see what its says.

 

[RRaghunath]

In Manual mode, the excitation current that is set manually is maintained, to my understanding. So, any changes in the system VAR status could destabilise the machine as it is not practical for the operator to respond that quickly. Limiters are non-functional too.
That's the reason, redundant AVR channels are being provided these days instead of one Auto and one Manual channel.

 

[Hoxton]

Traditionally the manual mode was for commisioning and fault finding. One could put into manual mode if the auto section failed. There were the dangers as stated above.

Nowadays we have reliable electronics and the need for manual mode is lessened. the is also the danger of unwary operators entering manual mode without being fully aware of the dangers.

 

[redlinej]

See attached a screen shot, of the different modes of operation, of the ABB unitrol AVR. In manual mode there is no limiter active.