3.8 MW 11 kV Slip Ring Motor

[maypot]

Hi,

I have an existing 1.0 MW 11 k V slip ring motor with a MPR 2000 relay protection relay. We are upgrading the motor to a 3.8 MW 11 kV slip ring one. The MPR basically provides the functions viz thermal overload, locked rotor current, phase lost sequence ,earth fault , under over volt etcc.
Does a 3.8 MW deserve in addition to the core motor protections also the differential protection. What are the current standards < if any > for a motor of this size ?

Thks.

Bob

 

[jraef]

There is a school of thought among large industrial users here in the US to add differential protection on motors over 2500HP (3.3MW). It is a potential nuisance trip issue in my opinion, unless you have a good IEEE 87 relay which is masked during start-up, when there are often differential issues because of transformer saturation. I would recommend a GE Multilin 469 which has Fn 87 built-in, as well as RTD inputs. You could also keep your existing relay and add a stand alone 87 relay, but just be aware of nuisance trips as being an issue.

Also, check out this previous thread for more options.
thread237-92596

&quot;Venditori de oleum-vipera non vigere excordis populi&quot;

 

[maypot]

Thks jraef , this forum is really invaluable.

 

[jraef]

There is a very good article by Richard Nailen about differential protection for AC motors in the latest (April 2005) edition of Electrical Apparatus magazine, but unfortunately they do not publish on line. Here is a link to their site where you can buy a copy of the magazine.

http://www.barks.com/eacurr.html

&quot;Venditori de oleum-vipera non vigere excordis populi&quot;

 

[aolalde]

maypot:

For differential protection the motor must be constructed with accessible input and output terminals per phase. This means the motor will have 6 leads from T1 to T6. The easier connection is with three CT’s sensing the currents of T1&T4, T2&t5 and T3&T6.
Since the current in is equal and with 180° offset to the output, the regular condition is zero current.
If for whatever reason the in and out currents are different a large current will be detected by the CT, tripping the current relay.
Apparently this type of protection will prevent major destruction of the core lamination in the event of a winding failure.

 

[mc5w]

However, obtaining wound primary high voltage CTs for your application with 2 primary windings may be difficult.

To use 600 volt rated window CTs in your application on an ungrounded or resistor grounded application you need short shielded cables with the shields grounded at one and the same end. There are some other precautions that are needed. There is an instrument transformer manual over at General Electric that tells you how to do this with 600 volt CTs.

For a solidly grounded system you could use short lengths of unshielded 8,000 volt cable.