Motor Differential Relay Tripping

Sounds to me as though it is doing its job! You most likely have an impending fault in the making. That is exactly what a differential relay is meant to protect against!

The first thing on a strange differential relay operation is meggering.

accordint to the our relay co-ordination 87 m set value is 5% only

As I understand that relay is sensing 11-12% differential current at 80% of the speed and when motor is started the figure might have come down much less than the reported differential current.

First understand why is differential current registered. Till 80% of the speed, motor draws almost the full starting current. At this current, if CT characteristic are not matching or if there is difference in CT secondary burdens, the secondary currents are likely to vary even if there is not fault with motor winding.

Now the next step is how to get rid of this problem. My suggestion is that since you are using numerical relay, there are 2 settings are available for differential current sensing. You may select 15% setting till the start time of the motor and after that the relay will automatically switchover to differential relay setting of 10%. 5 to 10%setting is acceptable value. Making it very sensitive may not yield any result. You may have instantaneous trip on differential current after it switchover from start setting to running setting.

You may also try using higher size of cable for Motor end CT for balancing CT secondary burdens and see the impact on value of Differential current on starting.

The cables lengths of 130mts: check the routing of the cables and try to have them far from any possible pick-ups.

High voltage cables can induce substancial currents on these cables and cause the imbalances...

CT cable and power cable runs through different cable tray but they are very close

Hi

From your test at 80% load the diff current measured by the relay wais 11-12%. This value is not abnormal. You can select a safe setting between 15-20%.

Due to ageing of CTs Motor etc. the differential current might be higher than the previous value.

raju

circuit insulation may be proven by meggar before raising the setting.5% setting advice is absolutely correct if the system is ok.

Please confirm whether the ct secondaries are connected in Star or delta.If it is in delta the delta formation to be checked properly.As you have told that stability test is OK , it shall be alright. Secondly whether earthing at both ct is advised by relay manual.

In relay it is not mentioned the CT star point earthing at both the end but we did earthing at both the ends with is the cause in doing earthing at at both the CT's and how it will affect in differential protection

You Have not answered my other question , Whether the CT secondaries are connected in Star or Delta

The CT secondaries are connected in Star

Hello, everyone!

jcg154

if you have testing equipment with regulated current source I suggest simple test which can tell you where is the problem motor or CT-s and their connection...My idea is that you simulate working condition in one phase with regulated current where one wire should be on the star connection of the motor and another wire before CT-s on entry of motor and then regulate current from 0 to 100A which should be enough for trip condition of your differential relay if CT-s or their secondary connection are bad. If impedance of one motor phase is big and you can not provide enough current you should bridge motor phase with another wire .If CTs polarity and its sec.connection are OK differential current should be almost 0 and you can see current value on the screen

Same test you should do on another 2 phase of the motor.

Good hunting!

The same method we carried out as mentioned- above between the two CT's by bridging the motor as before. We Applied 0-100A regulating current, the different current sensed is (Rph-0.01, Yph-0.012, Bph-0.01)

Unbalance appears a bit high at such a low current. If checked with a good dmm unbalance should be about .001-.006A.If ct ratios are matching.Polarity appear to be matching

CT stability test method between panel CT & motor CT, you can get the drawings from the following link. Please tell us you have found any wrong in this method or any different method to found out the fault

http://cid-2dd1473da1a13751.skydrive.live.com/self.aspx/ZCL-PL-NCR/YER-ZUARI-87M%20testing.pdf

Dear all

Little bit corrections

CT stability test method Dwg (between panel CT & motor CT), you can get from the following link. If any mistakes in that method please tell to us or if you have any other different kind of method to find out the faults

http://cid-2dd1473da1a13751.skydrive.live.com/self.aspx/ZCL-PL-NCR/YER-ZUARI-87M%20testing.pdf