Synchronous motor and Under voltage protection

[RRaghunath]

Motor undervoltage protection is generally definite time based. This is widely employed for induction motors.

In case of Synchronous motors, the vendor provides a curve indicating the voltage versus stability time.

What is the practice followed, is Inverse characteristic for undervoltage protection of Synchronous motors common! look forward to your views / experiences.

Thanks in anticipation.

 

[slavag]

Hi Raghun.
My expirence is 4 synch motors 6.9kV ( 10.5 and 12.5 MW).
Setting of 27 function was 0.8Un t=3sec.( with this setting motors in operation about 25 years)
I don't here about Inverse base 27 for synch motors.
I know is not help you, but ..
From my point of view, you don't need inverse time based 27.
I don't see any advanages.
Best Regards.
Slava.

 

[oldfieldguy]

Raghun--

In immediate memory, I dealt with three synchronous motors, 13.8 kV, 14,000 horsepower. Original protection was 27/47 electromechanical relay (GE ICR) set at 0.9 PU. It did have an inverse time characteristic.

old field guy

 

[RRaghunath]

Thanks to both of you, slavag and oldfieldguy for sharing your experience.

The numerical relay for motor protection P241 has inverse characteristic for 27 but with fixed characteristic (Relay Operating time = TMS/(1-Multiple). How is it in GE ICR (if you can recollect, oldfieldguy).
The 7UM62 of Siemens also has similar characteristic but 7SJ which is also used for motor protection has only fixed characteristic.

 

[slavag]

Hi Raghun.
I think it's C-type curve. it's fixed curve with
k-setting. We used for these motors generator protection,include 87,81,27,59 and 40 (of course other standard motor protection also). Simple motor protection from my point of view is not enough.
We aslo add to BB 6.9 kV ( 3 motors are connected to same bus) additional 27 relay as back-up with t=3.5 or 4 sec.
Best Regards.
Slava

 

[oldfieldguy]

Raghun-

The ICR has a left and a right contact on an induction disk frame. The user gets to choose two of three parameters in setting up the realy. We used the left contact, set at 0.8 p/u for tripping on an undervoltage conditiona, and the right contact closes at 0.9 p/u for a permissive to start. Setting the relay thusly gives an inherent amount of delay proportional to the undervoltage, but the user cannot change the delay without changing the undervoltage closing point of the right contact.

Here's the online manual:

http://www.geindustrial.com/publibrary/checkout/Installation and Instruction|icr/geh1783|PDF

It was compromise, for sure, but these relays were very common on motor protection circuits here. They are slowly disappearing as users upgrade to modern digital protection devices.

old field guy

 

[slavag]

Hi Raghun.
What you decide ? Used inverse or DT UV function?
(I will meeting with some customer on the next weak about protection of 4 x syn motors 20MW each one).
Regards.
Slava

 

[RRaghunath]

Our standard specification is to have Inverse characteristic for under voltage protection, matching with the stability characteristic of the synchronous motor.

Recently, while doing the protection setting calculations, I observed the characteristic available in the motor protection relay is no way near to the motor characteristic and further the characteristic is not selectable, unlike the overcurrent characteristics where we have SI/VI/EI etc.
Thus, I was wondering whether the inverse characteristic is giving any additional benefit at all.
Just to elaborate I have compared the relays of Siemens as well as AREVA (type P241 and type 7UM62 respectively) and find there is no difference with regard to the undervoltage inverse characteristic.

 

[slavag]

Hi Raghun.
Something intresting:
I now provide review of old protection of this 4 synch motors. Supplier dosen't used any undervoltage functionality. this pump station in operation from 1952-1953. But protection from 1993-1994.
Regards
Slava

 

[RRaghunath]

Thanks.
I wonder what is the practice in UK or USA.