# Meggering of 3-phase Synchonous Generator



## speedtronic (Jun 17, 2008)

For phase to phase Meggering of 3-phase Synchonous Generator, we have to open its flexible links (between phases) & its star point. We then connect Megger's positive lead to phase (which we want to Megger) & we short remaining two phases with ground?(& connect megger's negative lead to it) Why?? What if we donot short other two phases with ground?


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## nick (Feb 14, 2008)

well let me think mutual inductance depends on what voltage is if its high voltage you can get a voltage back on the windings that are left open and these can induce a voltage at a high potential into the stator coil you are measuring .kind of like the ct current transformer you must short these out when working on these . as they can produce a high voltage output .it bites you . but let me think about your question , what voltage and what test high pot test or a standard megger test ? when we high pot test we ground phases to be safe .we even ground these to ground after test to discharge voltage that builds up on conductors after time. next high voltage insulation on conductors windings the air gap between the coiled conductors is small but a charge can build up between these and hold a dc test voltage which will discharge if grounded . it acts like a capacitor at high voltage and at high voltage it is a capacitor just the copper wires insulation coating meaning the magnet wire insulation . this voltage can be a multipule of say 15 times what your test voltage is . did this help best to ya take care speedtronics


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## speedtronic (Jun 17, 2008)

*Meggering of 3-phase Equipment*

Dear Nick, thanx very much for your explanation. I was actually asking about standard meggering. Now please tell me that for phase to ground meggering of a 3-phase power T/F. Do we need to perform same practice? I mean like shorting of other two phases to ground?
Moreover, kindly tell me something about high pot. test. I know that it is used to measure maximum voltage withstanding capability of an equipment. BY injecting high voltage, what do we measure at output? & what should be its range?
Also kindly tell me about stator winding inter-turn faults? what they actually are? As I know, In case of inter-turn winding fault the current on both side of the winding is same so differential protection can not be used? then what kind of protection is used for it?


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## nick (Feb 14, 2008)

no you dont have to short out the transformer windings to ground with a standard megger at 500 /1000v test voltage just isolate thes apart . i thought you were high potting that synchronous generator which that would be high voltage . just megg it open , a high pot test uses hv at 50kv to 150 kv dc/ac test voltage for mostly cables buss bars windings hv stuff .the test is a slow step ,meaning voltage is ramped up from zero to selected test voltage in 500v or 1000v/5000v increment steps each step is 1 minute long . the last step of the test will be the maxinum voltage selected by the manufacture of that cable / buss bar / winding ,this can be 5 times the rated voltage of that insulation . example if its 16000 v cable then you go to 5 times that rated voltage on the last step usally not always . you must slowly apply voltage up or you will blow a hole in the cable or insulation in a winding or insulated buss bar . a microamp meter / voltage kv meter on tester monitors the volts and microamps during testing , this is when you can tell if your cable is good or bad. amps increase too fast = problem .amps float up or down= problem amps never go up with voltage increasing = problem a slow discharging current bleeding down slowly =problem ect ect ect ,example air is a conductor at 75kv it will conduct ,if your test cable termination is exposed to air it will give a bad test it bleeds off into the air to ground iniozation happens , we wrap our open ends of terminations with sandwich plastic wrapping and a rubber bands its rated for 12000 volts one wrap . you learn to understand how that microamp meter works to locate a problem. example a up amp reading then a down amp reading its usally a bad load break or lose connection or crimp terminal bad like a high resistance bad connection , can be wet damp or dust . to calculate your megg ohm test reading using a high pot you use ohms law ,[ ohm = E / I = the test megg] E= kv volts I= microamps reading but you record all steps during each minute meaning each step on spread sheet for a record of that cable buss bar or winding . this can be a distructive test if the wrong person does it wrong but its the real deal thing and if it passes the tested insulation its good . the range for hv generator / hv motors is what the factory specs , and the hv cable or the hv buss bar . next a[ internal fault in a winding] is hard to locate with this test meaning a short of two turns a high pot test will make it worst it will blow it apart !!! ,you use a inductometer or a microhm meter for this . theres also a ac high freq test when i was in the service , it uses ac high frequency to test a winding any winding , it will give you henrys and will pass or fail a winding by comparing each a b c at one time .if its got a bad turn just a few windings shorted on one phase it tells us the inbalance of inductance on that phase to the others and which one is bad , just add most high voltage gen or motors or winding in transformers today have[ thermal sensors ]inbedded in the cores near the windings and indicate when a hot spot or short is going bad but were talking high voltage . safety always ground any cable or winding or buss for 4 hours after test it will build up a charge the next day you come back to work guess what you get shocked real good .after test GROUND IT !!!as [differentail relays] if secondary shorts internally the primary picks this up the relays should work fine as they function to sense current unbalance so if [pri a phase] is higher due to high current on [ secondary a phase] it would or should trip with the correct relaying . pri ct to secondary ct the coupling of windings does not mater i will think this one out ? in my opinon the best way to test without distructive problems is a high freq test not the old high pot test as voltage would be at the outer skin of the copper conductor less voltage on dielectric and a better safe test due to this and less time to perform the test ? comments does this make sense take care best to ya Speedtronic


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## nick (Feb 14, 2008)

*directional elements*

well i found what your looking for , to give you the correct answer and not have any mis givings on this they make a turn to turn fault detection relay current diff protection type uses a two stage slope adapts to internal or external fault of transformer with half high or low side connections , also they do alot more freq phase voltage lots microcontoller based stuff if you like to see go to http://www.selinc.com/sel-487e.htm best to ya take care :thumbsup:


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