Protection on HV Side of Delta Star Transformer

Protection on HV Side of Delta Star Transformer

We are using 11 /6.9 Kv 3.15 MVA transformer for 6.6 KV motors. The transformer is Delta /Star On the 11 KV side a 7SJ62 relay of siemens make is provided. The relay has 4 current inputs. Three CT's on the 11 kV side have been wired to 3 sets of inputs and the the 4th input has been wired to 6.9 KV side neutral side CT. The three CT,s on the 11 KV side offer Over current and earth fault protection to the transformer on the primary side. The earth fault current is calculated internally by the relay using the three phase CT currents. The earth fault protection has been set for 10% and instantaneous protection. The problem is that on the 6.6 KV side, the earth fault protection is IDMT based. Even if an earth fault occurs on the star side, this should not be reflected in the primary side which is delta connected, but it is frequently observed that for an earth fault on secondary side or starting of multiple motors simultaneously also is resulting in unbalance currents on the primary side sufficient enough to cause tripping of the transformer on earth fault protection. It may b e worth mentioning that there is no stabilising resistance had been used in the CT secondary circuit. Kindly suggest how to overcome these problems Re: Protection on HV side of Delta Star transformer 10/05/2012 11:00 PM

Can you give the setting details of relays at both HV side and LV side. How do you infer the HV side relay has tripped due to earth fault on the LV side.Is there any recordings on the relay, if i f you have recordings please furnish.We have t o analyse before offering any suggestion. Is this incidence of tripping recent one or from commissioning time. How many heavy duty motors are connected and its details also required. Re: Protection on HV side of Delta Star transformer 10/07/2012

Thanks for the prompt reply and sorry for the delay in providing the data. The Primary current of the transformer is 165 Amps and the secondary current is 263 Amps. The load on the transformer is 2 Nos of 315 KW motors running on Variable Frequency Drives and 2 Nos of 508 KW motors on Direct on line starting. Few more loads are to be added to this board. The starting current of the 508 KW motors are 320 Amps for 3 Seconds (Pump Application). The 508 KW motors are provided with a Micom P242 Motor Management relay with an earth fault setting of 10% and a time delay of 500 milliseconds. The transformer secondary had been provided with a Micom P142 Feeder Management Relay and the Over current and earth fault settings for this relay are all IDMT settings.

Earthfault setting is 10% and TMS is 0.2. The CT ratio on Secondary is 300/5 The transformer Primary had been pr ovided with instantaneous earth fault protection at 10% without any time delay. The CT ratio on the transformer primary is 180/5 The over current settings on both sides of the transformer are in order to take care of the 508 KW motor starting with the other 3 motors in operation. Also only one motor is started at a time and generally the time between starting of one motor and another motor is more than 10 minutes, hence starting of all motors at a time is ruled out. In the present case one 508 KW motor burnt during starting. As the motor protection relay was provided with 500 milliseconds for earth fault protection. This relay did not operate immediately, the feeder management relay P142 was provided with IDMT feature and required time to operate as the magnitude of the earth fault was limited to 200 Amps because of the Neutral grounding resistor. During that period none of the motors were running on the transformer The unbalance caused on the transformer secondary due to the burning of the motor reflected on the primary side as minor unbalance and caused the transformer primary to trip on earth fault protection. The disturbance recording obtained from the relay 7SJ62 also indicates that the unbalance was not not too high to cause the trip (the unbalance was derived from the three phase current transformers readings as measured by the relay). But the transformer tripped on HV earth fault. The relay was subsequently taken out of service for testing and the tests performed on the relay indicated that the relay performance during test was perfectly normal. The settings of the relay have been reviewed by Siemens personnel and have found every thing to be perfectly as per requirement. Inspite of all the above the tripping had occurred. Hope that clarifies the queries. Expecting a prompt reply Reply Re: Protection on HV side of Delta Star transformer 10/07/2012

Curves drawn based on the present settings.You will notice that settings are not properly coordinated.Red curve: LV side relay.Black curve :HV side curve For any fault current the relay on HV side is bound to trip first ,before LV side as per the curves.

Curve1 based on present setting

Curves based on proposed settings The present settings are : 6.6 kV side: Ie=10% tms0.2 ,NI,IDMT 11 kV side: Ie=10% delay=0,DT Proposed setting 6.6kV side: Ie=10% DT 0.025 sec 11 kV side Ie=0.12% DT 0.15 sec

Try the proposed settings,I hope this will solve the problem of HT side tripping before LV side tripping We have given some suggestions for your posting to sort out the issues. We would like to know whether you have tried out and what is the out come. If you have sorted out the issue what was the problem ? This feed back will immensely help us. Re: Protection on HV side of Delta Star transformer 10/06/2012

As has been rightly observed by you, the primary side earthfault protection in the relay is offered by measuring/monitoring the zero sequence components in the HV side phase currents. When multiple motors are a single large capacity motors are started, there is bound to be zero sequence components in the phase currents, due to the magnetising inrush currents in the motor(s). Not surprised that the earth fault relay on the primary side (which is set to instantaneous) is tripping. You may introduce a time delay of about 100 milli-secons to avoid tripping or add a stabilising resistor in the HV side CT secondary wires. Re: Protection on HV side of Delta Star transformer 10/06/2012

Please before I can offer any solution, I want you to send me the AC PROTECTION DIAGRAMS used for the TRANSFORMER PROTECTION. Has the Transformer been provided with DIFFERENTIAL Protection? Meanwhile, I have the following suggestions to give regarding the temporary solution pending the time I will get your DRAWINGS:(i) Increase the INSTANTANEOUS setting of the O/C & E/F relays on the PRIMARY side of the Transformer( SET AT X4). (ii)The setting of the IDMT Relay on the 6.6kv side of the transformer should be increased to 20% (iii) The motors should NOT all be STARTED SIMULTANEOUSLY. The Motors should be started one after the other. This will solve this unwanted tripping of the transformer. Please forward my requests to my mail box