Why we adopt setting greater than 10% of the CT secondary current for a differential relay?. Is this the problem of CT saturation ?. If so how it happend?. Please help me
Basically for transformers/ motors where CT,s used for differential protection may get saturated even if the fault is outside the zone of there protection.Let us suppose two CT,s one on the neutral end and other on the HV end of a motor.If a fault occurs near the HV end CT but outside their zone in which they operate the CT closer can get saturated due to high voltage and so a difference current will operate the relay so normally 30% setting is used for transformers and motors as CT s are not 100 percent perfectly matched and a error current is always present which can operate the relay.CT ratio mismatch ,Knee point voltage mismatch,Difference in accuracy class of CT,s can also be a source of mismatch and false operaion of differential relay.
saturated even if the fault is outside the zone of there protection.Let us suppose two CT,s one on the neutral end and other on the HV end of a motor.If a fault occurs near the HV end CT but outside their zone in which they operate the CT closer can get saturated due to high voltage and so a difference current will operate the relay so normally 30% setting is used for transformers and motors as CT s are not 100 percent perfectly matched and a error current is always present which can operate the relay.CT ratio mismatch ,Knee point voltage mismatch,Difference in accuracy class of CT,s can also be a source of mismatch and false operaion of differential relay.
Naveed,
is that italic text above to say that CT shall be set 30% of the CT secondary ?
or to set maximum loading for transformer & motor ?
It means that differential protection(relay) to be set to operate at 30% of In (nominal current).Obviously the CT,s secondaries would also be of the Nominal current e.g. differential relay MBCH(GEC ALSTOM) has 1A nominal current.CTs ratio will be 11500A/1A....I hope you got my point and I understood your question as well.
Is it possible to set the relay 10% of the CT nominal current ,If the CT Characteristic are almost same for neutral end and HV end of motor and the class of CTs - Class X.
Do you know the Generator protection - SHAFT CURRENT PROTECTION ?. How it can measure?. What is the scheme?.
Is it possible to set the relay 10% of the CT nominal current ,If the CT Characteristic are almost same for neutral end and HV end of motor and the class of CTs - Class X.
Do you know the Generator protection - SHAFT CURRENT PROTECTION ?. How it can measure?. What is the scheme?.
sorry for late reply.Actually if you are setting for a motor you can set 10% and give it a try.If there is error current of more then 10% then your motor will trip immediately when start command is given on differential protection.So then you will have to increase the setting.Secondly regarding your question for shaft current it is measured by means of brushes connected to the shaft of the generator and then the brushes are connected to earth.This scheme is valid only if Shaft is grouned on one side otherwise circulating currents will flow and damage the shaft.For current measurement use a clamp type multimeter and measure the current by placing around the ground wire connecting the shaft to the ground.
CT Saturation
Re: CT Saturation
