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@pillay said:
@doc70 said:
I have a 13800V to 600V 2MVA step-down transformer.I need to megger test this unit, I have a megger good for 5KV and also a hi-pot DC tester (Megger brand).
I am unclear of how to make the physical connections between the transformer and the testers.
make sure the transformer is not connected to any thing like the potential transformer,than carry out
[1] usual insulation test between the frame and the connections terminations of the ht side.
[2] test between frame and the lv side terminations.
[3] test between ht terminations and LV terminations.
[4] test between phases at HT side
[5] test between phases at LV side.
[6]
Dear Doc70 ;
Refer to Mr. Pillay said, I want to mentioned that the Insulation test Steps are mentioned by Mr. Pillay are 100% correct, but there’s 2 points :
– If the primary voltage is ” 13.8kV “, the ” Insulation voltage ” should be around ” 17.5kV ” ( depending on the manufactere information ), so, what are the obtaining results with ” 5kV ” ?
– If the secondary voltage is ” 600V “, the ” Insulation voltage ” should be around ” 800V ” ( depending on the manufactere information ), so, you can’t do the insulation test for the secondary with ” 5kV “.
Dear ;
About the mentioned way to locate the Earth Fault’s place, I have the following points :
1 – You don’t mention to the Earthing System, as with ” IT ” we use normally ” IMD equipments ” that locate the earth fault’s place.
2 – If we used the correct CB equipped with Earth Fault Protection that are chosen depending on the ” Earth Fault Protection Discrimination Study “, we can know directly at least the panel where this fault is occured, and by your way, we can locate the relevant outging.
3 – You said that the 1st earth fault should be cleared before the 2nd happen, that is 100% correct only in ” IT Earthing System “, but with other system ” TT, TN-S, TN-C “, each fault should be cleared as there’s no relation between 2 earth faults
Dear Mr. Anilsabaji ;
First, as you said the VSD can withstand up to “ 65kA ”, but I want to add that this value is depending on the VSD’s Type and the manufactures.
About your example “ 8 Alternators ” I have the following comments :
1 – As we know all that many CB manufactures produce a special rang of CB called “ Limiter CB ” that can limit the Short Circuit Currents, and also they added in the catalogs some special tables containing the “ Upstream & Downstream Values ” of the short circuit currents for each “ Limiter CB ” as a guide for the users, so, we can use them where we need to limit the short circuit currents even to a value smaller than “ 10kA ”.
2 – As I understood that each Alternator is “ 2000kVA, 690V, 60Hz, X”d=20% ”, and by a simple calculation we know that exactly at the connection points “ Isc3max = 9.23kA ”, and by using the necessary cables ( only 15meters ) it drops to “ ≈ 9kA ”, that means by splitting these 8 alternators into 2 groups of “ 4 ”, the Short Circuit Current value at BB will be “ ≈ 36kA ”, so, we don’t have any problem for the big VSD, and for the small the problem can be solved by using a Limiter CB.
3 – Also we know all the there are some weakness points of Fuses, like : 1 of 3 fuses blown that are using to protect an Outgoing, where to know it we should use a special fuses equipped with “ Striker ” with also a special base of fuse holder equipped with “ Auxiliary contact ” by which we will have a signal once happen.
4 – Using the Fuses to protect circuits oblige to have Fuses as spare parts from all installed kinds and ratings to be able to replace any blown one.
Dear ;
I have some commitments that are :
– Where there’s harmonics in a network, the phases’ currents will be increased accordingly, and the Neutral current will be composed of :
– The Vectorial Sum of the 3 phases’ currents.
– The Sum of the 3rd harmonics that means “ 3, 9, 15, 21, … ”
So, the Neutral current will be equal or bigger than the phase current depending on the values of the 3rd harmonics, but when we haven’t the Neutral current will be only the Vectorial sum of 3 phases’ currents that’s small.– When we have harmonics in a network, and depending on the harmonics’ values and types, we will decide the sizes of the Neutral lines, but not using always “ Double-size ” of these lines.
– When we do the study of any project, as we have all necessary information about the loads, we should know if there are harmonics or not, if yes, what the kind of them to find the best solution at the beginning. Either for Phases or Neutral sizes cables.
– As recommendations, You said about the Neutral lines “ …. or separate neutrals for each phase ”, please explain how you can separate the neutral if you have a machine generating harmonics that should be supplied by “ 3 P + N ” ???? We can use your solution only for Lighting Circuits supplied by “ 3 P + N ” but not for other Single Phase Circuits. Normally, we supply single each circuit by Phase + Neutral, and accordingly the IEC… standard it’s not permitted to use the same neutral for many Single Phase Circuits.
It's very good what ” Paintamelec ” does, but please let me know if the Technical specifications insist to have ” Full Type Tested panels ” and as you aren't producer of CB's what do you do in this Case ?
It's very good what ” Paintamelec ” does, but please let me know if the Technical specifications insist to have ” Full Type Tested panels ” and as you aren't producer of CB's what do you do in this Case ?
michaelt said:
can anyone advise what would be best set up to use with a delta delta 690/480v transformer being used to supply two 15kw vsd with emc filter to ground . insulation metering versus imedance relay and ground monitoring , i take it ground monitoring equipment for a delta/star set up will not work with the former it system unless with impediance relay, or am i missing something ,this is for offshore situation . and a retro fit .
Dear
For ” IT ” system :
– If the transformer's secondary is with Neutral : the Neutral point can be either connected to the Earth point by ” Impedance ” or not connected ( completely opened ).
– If the transformer's secondary is without Neutral : one phase ( any one ) can
be either connected to the Earth point by ” Impedance ” or not
connected ( completely opened ).– Some of ” Insulation Monitoring ” are internally equipped by ” Impedance “, so, we don't need it. Noting that we can know from the schematic diagram of intallation if we need an impedance or not.
Tegan Smith said:
I'd have to say that despite reading this article I'm still undecided
Dear ;
Please let us know what's the kind of application where you want to use the ” IT “.
Tapan said:
Hello,
I wanted to inquire if there was a code requirement to have two separate Automatic Transfer Switches i.e. one for life safety systems such as elevator load, fire pump etc. and another for utility loads. Is it general design principle to have two separate ATS for life safety and general utility loads?
Thank you.
Regards,
Tapan.5
Dear ;
First, there's no code, but depending on some points we do our choices, that are :
1 – The total power's value of the life loads, and the power's value of the general loads that should be also supplied from the generator, certainly after integrating the ” ku – utilisation factor ” and ” ks – simultaneity or diversity factor “.
2 – The place of the life loads and the place of the general loads.
3 – If all these loads ” Life & General ” are supplying from the same panel or not.
4 – Depending of the supplying ways of these loads.
Jitendra A. Khetani said:
Folllowings are advantage & disadvantage of MCCB & HRC FUse:
1) MCCB to be replaced once it trips on short circuit. Hence shut down for replacement of MCCB.
2) In PCC, Fault current is more, You must use Control fuse instead of MCB as fuse is rated for 80KA or 120KA while MCB is rated for 10KA or 15KA.
3) Wherever you use MCCB, MCCB with Earth fault protection release must be taken, otherwise MCCB will not trip on Ground fault and your Main breaker will trip. But if had use fuse, Fuse will blow off in case of Ground fault.
4)Fuse replace ment will take time.
5) Blowing of fuse of one phase crate single or 2 phase supply & burning of motor.
Jitendra A. Khetani
Dear ;
Excuse-me, but there's some things in your comments should be clarified that are :
1 – About tripping of MCCB on short circuit : If the values of ” Breaking Capacities on Service – Ics “ of installed MCCB or even MCB in any LV network have been correctly chosen depending on the values of ” Isc3max “, no need to replace them after tripping on short circuit.
2 – About the Breaking Capacities of ” MCB ” : certainly that most of used MCB's have ” 6, 10, or 15kA ” but many manufacturers like ” Schneider Electric, ABB, Legrand, hager, and others ” have MCB's with ” 25, 35, and even 50kA ” but the tests' sequences are depending on ” IEC 60947-2 “.
3 – About the tripping of MCCB on Ground Fault ( or Earth Fault ) :
A – The MCCB without earth fault protection can trip on Earth Fault if the value of the magnetic protection built in the MCCB is
smaller than the value of the earth fault current.
B – If the outgoing MCCB installed on the top of the earth fault circuit is not trip but the main MCCB tripped that means the magnetic
protection value of the outgoing MCCB is bigger than the Earth Fault Current, so, the chosen of this protection is not correct and
even the magnetic protection value of the main MCCB is smaller than the Earth Fault Current !!!!!
C – Normally, depending on the used Earthing System, we should calculate the earth fault current value, then decide if the MCCB
should be without or with an Earth Fault Protection, and even this decision will be done depending on the used Earthing System.
For exp. in ” TN-C ” we can't use the Earth Fault Protection, so, in this case, we have only the Magnetic Protection that
should trip on earth fault.
crosswind50000 said:
100 % agree with ghali . but should have to pay attention if the motor is located in haz. the motor shall not overheat and surface temperature going beyond the temperature class in case the overload relay is not well set.
Dear ;
Please explain more clear what is the relation between the surface's temperature and the Overload relay ? Noting that the working of Overload relay is depending on the circulated current value and not of the heat temperature value.
FredG said:
Hello,
Thanks for your explanation. It's a very good synthesis. Therefore, I have 3 questions.
Firstly, you mentionned you're trying to use fuses only with small loads because the protection of phase missing is not assured by the standard fuses. But Schneider sales engineer mentionned me we can use an electronic equipment (fuse monitor) to detect the phase failure. Is it expensive? Did you already used?
Secondly, you mentionned also problem to have fuses above 630A. Do fuse manufacturers range limit their range at 630A? Which manufacturers can I choose?
And Finally, why do you prefer to use fuses to protect capacitors? I have some applications where I have to protect capacitors. So, I'm very interesting to have your position.
Another question, I've also some customers who are using Drives. I was thinking to use fuse UR for these appplications. What is your position on this topic?Best regards,
FredG
Dear ;
1 – About ” Fuse Monitor ” from Schneider : till now I don't use it, because it's expensive more than ” Fuse with Striker + Fuse Holder with Auxiliary Contact “.
2 – ” NH fuses” ” more than ” 630A ” : as most of manufactures limit their rang ” up to 630A ” but some of them like ” Ferraz Shawmut ” produces NH Fuses ” up to 1250 A “.
3 – About the protection for ” 3 Ph-LV Capacitors ” : I use the Fuses to protect these Capacitors for the following reason :
A – No need of ” missing phase protection ” for Capacitors.
B – These Capacitors are composed from ” 3 Single Phase Capacitors “, and there's no a big problem if one of them will be disconnected
due to a fuse blown.
C – About the external body :
1 – If it's made from ” Insulation Material “, the possibility to have and Earth Fault is too limited.
2 – If it's made from ” Conductive Material “, and as the installation's places of Capacitors are either near the Main Distribution
Board or near the Sub-Main Distribution Boards where the value of the Earth Fault is enough to blow the fuse due an Earth Fault.
4 – About the protection of the Drives : as the values of the fault currents in the Drives are limited, and most of Drives have inside ( by programming ) the phase missing protection, we can use the UR fuses.
Dear ;
The standard said that each touchable metallic pieces should be earthed, so, if the rear cover, even screwed, can be touchable it should be earthed.
Dear Mr. Laurent ;
I tried to login to this library by using the same password sent by Electrical Engineering Community without success, please let me know whta should i do.
thanks in advance.
Dear ;
Certainly, we have the same mainly kind of protections ” Overload & Short Circuit ” when using the standard fuses, and as the fuses haven't any mechanism, the protection against these faults are 100% assured. Personally, and for general protection, I use the fuses for some applications but not all time.
I prefer to use the fuses in the following applications :
1 – Fuses type ” Cylindrical ” for all control circuits as the breaking capacity for this kind is too high ” 100-120 kA “, so, we can use them at any levels of LV network whatever the values of the maximum short circuit currents, and also for some small loads ” 3 phases ” when there no motors, because the protection of phase missing ( fuse blown due to fault between phase & neutral ) is not assured by the standard fuses. Noting that with another kind of fuses ” Fuse with Striker ” using with a special fuse holder ” equipped with auxiliary contacts ” we can assure the protection for this fault, but the cost will be high.
2 – Fuses type ” NH without Striker ” to protect the Capacitors.
In general applicationsI ” Mains & Outgoings ” protections, I don't prefer to use the fuses for the following reasons :
1 – As the Tripping curves can't be modified, we can't find the necessary rating for all places in the network, especially when the values of the minimum short circuit currents or the earth fault currents are small.
2 – We can't realize in all cases the ” Discrimination or Selectivity ” either for Overload or/and Short Circuits Faults.
3 – We can't realize in all cases the ” Cascading or Buck-up “.
4 – If some loads are ” Motors ” and we use the standard fuses ( without Striker ) to protect these outgoings, we should absolutely use one of the following solutions to assure the protection against phase missing ( one fuse blown ) :
A – The Electronic Equipment ” Phase Failer “.
B – Using the ” Fuses with Striker ” with the special ” Fuse Holders ” or ” Switch-Fuse Disconnectors ” ( depending on the ratings )
equipped with auxiliary contacts.
C – Using the ” Electronic Thermal Relays ” with contactors.
5 – As the Earth Fault Current in ” TT ” earting system is small, the fuses can't assure the protection against Earth Faults, so, we should use also an ” E.L.P. “.
6 – The fuses' ratings that we can find too easy are ” up to 630 A “, where only some specialized manufactures produce ” up to 1250 A “.
7 – When we use the fuses, we should have the necessary stock of all fuses used in the project ( types & ratings ), to be able to replace any blown one.
