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Topics: Differential Protection of 6600V Induction motor on Power Supply
Start by
Baljinder Bansal
08-17-2014 04:09 AM

Differential Protection of 6600V Induction motor

In differential Protection (like in Transformers) we have CT's installed at Primary side and secondary side of the t/f. How is the Differential Protection of the 6600V Induction motor is achieved as the CT's are installed in the panel of Induction motor that too on the feeder of the motor?

Some body told me that Differential CT's are also installed on the Incomer of the switchboard on which Motor panel is installed but he couldn't explain his Answer?
08-17-2014 06:48 AM
Top #2
08-17-2014 06:48 AM
How have dreamt that the 6600 V Induction Motor concerned is protected by Differential Protection? I am asking this question because it is very unusual to provide Differential Protection for such motors.
Theoretically speaking, there can be two scenarios as follows.
Scenario-1: Stator Winding of the motor concerned is Star Connected and the Star Point has been takn out through a Bushing and is available for installation of a CT through which the Star Point can either be grounded. In such a Scenario three CTs, each of proper ratio so as to correspond to at least 110% of Full Load Current should be provided for phases at outgoing terminals of the Switchgear and the CT of same Ratio provided at Motor Neutral for measurement of Neutral Current to Ground (which would normally be Zero). With such 4 CTs a RESTRICTED EARTH FAULT Setup can be created so as to provide highly sensitive protection to motor stator windings.
Scenario-2: All 6 Terminals of the Motor Stator Windings are available outside through respective Bushing. In such Scenario conventional Differential Protection Setup can be made.

However, in real life providing any of the two types of protection as described above would be highly unconventional and uneconomical - based on Cost/Benefit Ratio.
08-17-2014 09:36 AM
Top #3
John Levine, P.E.
08-17-2014 09:36 AM
To provide the maximum protection to large motors (typically over 2500 HP) you want 3 kinds of current protection. 1) You want a set of CT's to provide phase over current protection. 2) You want a CT to go around all 3 conductors (zero sequence CT) to provide phase to ground protection,. 3) You want either 1 set or 2 sets of CT's to provide differential protection. This will protect against phase to phase low levels of current. In order to do differential protection, all six leads must be brought out of the motor so you can look at the current going into and out of each phase. You can download the GE Multilin 469 manual and see the recommended wiring to accomplish this. See figures 3-17 to 3-19. In real life this is provided and is very inexpensive insurance to prevent major damage to large motors.
08-17-2014 11:46 AM
Top #4
08-17-2014 11:46 AM
CTs have to be introduced on the line side(U1 V1 W1) and and on neutral side (U2 V2 W2) of the motor and then star is formed for motor differential protection.
08-17-2014 02:07 PM
Top #5
Ing. Ole Knudsen
08-17-2014 02:07 PM
We have just installed 17 off such motors, including Diff. protection.
Motors: 11kV, 2.7MW each.
1: Protection Relay is ABB 630 REM
2: Core balance protection, through large CT at bottom of feeder switchgear cabinet. It is always a battle to get the electricians to connect the cable armour to earth outside this summation CT, and to lead any potential screen connections back out, to have them earthed outside this CT.
3: Standard Over Current protection, through individual CTs for each phase. These CTs are installed in the main switchgear panels.
4: Differential protection, through additional CTs installed in each motors Star Point junction box.

This installation is just about to be commissioned, over the next couple of months.

The main question will be to configure the protection relays correctly, since the different burden on each set of CTs per phase will be quite different, due to the difference in cable lengths. The star point CTs are up to about 400m from the switchgear, and furthermore, they are of different manufacture, but I have been reassured that these modern relays can be set up to compensate for all these differences.

If these motors were to be delta connected, for 6.6kV, then the differential protection would have to include a separate set of CTs in the main junction box. There is no reason why these CTs could not serve as the O/C CTs.
08-17-2014 04:39 PM
Top #6
subrata deb
08-17-2014 04:39 PM
The question is why some time we have to provide separate differential protection for motor
inspite of having so many protections? Probably , the answer is where the starting current
of the motor is too high & very close to the system short circuit current, we have to set the
instantaneous relay above the starting current in order to allow the motor to start.But in that
process the relay will fail to detect any short circuit current which is less than the relay setting current.But the differential protection will operate whenever there is a fault in the
region between the two sets of CT ( line side & neutral side)
08-17-2014 07:01 PM
Top #7
Ing. Ole Knudsen
08-17-2014 07:01 PM
Maybe it is not quite so simple.
It is quite easy for the modern relay to have a number of settings with some time-dependent correlation with a scale of overcurrent limits.
This style will allow the starting inrush current profile, but once the motor is up to speed, it can not easily distinguish between some partial fault, while the motor is only part loaded.
This is where a differential protection relay can come in handy, because it can sense if any partial current into one end of a winding does not come out of the other end of that same winding. It could have a small proportion going to frame, or to another winding due to some insulation breakdown in the winding. In order to minimise damages to the stator iron, and maybe even limit the number of coils that need to be replaced after a fault, it is of benefit to detect this type of fault as early as possible.
Your mention of protection problems during start-up is not easily solved through the differential protection, because it is difficult to keep the diff. prot. CTs from saturating during the increased starting current, and especially during the first few cycles while the whole magnetic system undergoes its first un-symmetric stabilisation.
08-17-2014 09:25 PM
Top #8
shashi bhargava
08-17-2014 09:25 PM
Mr Baljinder Bansal,
I think the differential protection is necessary in all large motors. It does justify the cost / benefit ratio. The answers above are perfect and you can take a cue from this.
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