MV/HV Switchgear Doubts

1. Usually DC source is taken from UPS or DC Battery Charger. This supplly is seperated from 3 phase AC Source that come from transformer to Switchgear cubicle.

With DC Source we can test closing/tripping coil of contactor although main 3 phase supply is not energized yet.

beside that, usually we have push button station near motor. this is supplied with DC source as a common. this purpose is to keep voltage drop still small.

2.mechanical latch purpose is to prevent if electrical latch have a problem (error reading/writing, delay working, lost connection). and to save operator from prosedure mistake.

3. Of course in HV/MV , we need vaccum contactor. because vaccum is need to prevent the appear of the arching/spark . in the past , we use oil contactor.

Here for single application you have raised three question but all these depend on design selection of your MV/HV motor contactor with latch.

While preparing designe philosophy it is decided that which type of control supply you want use & selection must be like that your MV/HV motor can get trip/off during Total Power failure.

Here your team have demanded 110v DC control supply so you have got Mechanical latch MV/HV motor contactor,where two circuits are there one is for closing & 2nd one is for tripping & only pulse contact is required to close or trip/off of MV/HV Motor contactor rest of jobs of holding of these contactor can be done by mechanical latch and 110v DC would be available during Total power failure also because it comes from your DC-UPS.

From supplier you can also get Vacuum contactor without mechanical latch & with control supply 230V AC,Here OEM(Original Equipment Manufacturer) supply Separate PT of 11 or 6.6 or 3.3 kV/230V AC with each Vacuum switchgear trolley of your MOTOR(depends on 11 or 6.6 or 3.3kv motor rating).This PT gets supply supply of 11 or 6.6 or 3.3kv(Base on your voltage rating of MV or HV board) as input directly from Busbar during racked-in/Energised condition of MV/HV contactor.For test position of Vacuum contactor separate AC supply would be there which comes from your normal DB. Here Vacuum contactor coil gets pick-up continuously during ON condition of breaker(it works like LT Motor contactor),All interlock or stop command would be given in series,breaking of any interlock or command Vacuum contactor coil gets drop & during Total Power failure also Busbar(from where control supply taken for MV motor) will get dead because of that Motor Vacuum contactor gets drop.

Whatever I know upto 800kw Motor Vacuum contactor has been used as cost cutting measure, beyond 800kw Motor there is some technical limitation is there for quencing of Arc in Vacuum contactor that is why above 800 or 850kw motor Vacuum Breaker is used.For Vacuum contactor you dont need to provide Short Circuit Protection for motor because of HT fuses & also your Power cable size is also reduced drastically for same kw Motor MV/HV motors if you use Vacuum contactor instead of Vacuum breaker.

I hope you get all your answer.

thanks

SJ

"Whatever I know upto 800kw Motor Vacuum contactor has been used as cost cutting measure, beyond 800kw Motor there is some technical limitation is there for quencing of Arc in Vacuum contactor that is why above 800 or 850kw motor Vacuum Breaker is used.For Vacuum contactor you dont need to provide Short Circuit Protection for motor because of HT fuses & also your Power cable size is also reduced drastically for same kw Motor MV/HV motors if you use Vacuum contactor instead of Vacuum breaker"

Dear SJ, is it not reverse ? or we should say :

For Vacuum breaker you dont need to provide Short Circuit Protection for motor because of HT fuses & also your Power cable size is also reduced drastically for same kw Motor MV/HV motors if you use Vacuum breaker instead of Vacuum contactor"

thank you...

I have worked in the middle east for almost 16 years. I was a distribution engineer. The same questions as yours did come to my mind long time ago. I have asked my fellow engineers and this is why:

1. The control circuits is DC supplied in case of power failure you can still operate the switch.

2. Mechanical latch is secondary that you can charge the spring manually.

3. We have vacuum, oil and SF6 breaker. Oil preferable because it is abundant and easy to maintain. SF6 advantage are light weight and firesafe. Vacuum switch the difficulty is maintenance and handling.

Thanks all of you.....Now I get the the Advantage of using Vacuum Contactor with HRC Fuse instead Conventional C.B. Thanks once more all of you.