Auto Bus Transfer & Momentary Paralleling

Do your own homework

http://cr4.globalspec.com/thread/108787

http://cr4.globalspec.com/thread/108735

http://cr4.globalspec.com/thread/108707

http://cr4.globalspec.com/thread/108692

http://cr4.globalspec.com/thread/108634

"...while momentary paralleling of the two buses , what are the points to be carefully taken care..."?

1. Never, ever allow that to occur,

2. Connecting two different sources together without proper synchronization equipment,

3. Even "momentarily", is a formula for instant disaster...

Let someone else design the logic.

1. You will need a broom, a dustpan, and an air filter with a blower (or sucker).

2. After this momentary event, go around sweeping up pieces of the copper bus, MCC cabinet and copper dust that settled to the floor, and any vaporized personnel in the area when the cabinet exploded.

3. Turn the blower (sucker) on, and filter all the carbon and other smoke out of the air that you let out of all the equipment by breaking the electrical seals, due to faulty phase in parallel. The extra solid parts of the smoke, well you may want to do your laundry.

4. Send any surviving personnel straight to the audiologist to assess the amount of hearing loss from the big bang.

There is about 10¹³¹ J/cm³ in the universal ether, give or take 20-30 orders of magnitude. You let out only 10-124 fraction of this, so don't worry, there is still a great deal left in that one cm³.

Crestfallen!

My queries were not understood by you & an unexpected denouement came out.

1. Momentary paralleling is done with a synchronizing relay & the desired Incomer being tripped or isolated after a preset time. In this case only one incomer is taking care the total load after the bus coupler is made ON. This combination will vary from case to case basis in 2 IC+ 1BC orientation. Where is the problem in this operation as most industry sectors are adopting this in their process plant where uninterrupted power supply is must. In this case the fault current & other design parameters are well taken care of. What is your views ?

2. In Auto bus transfer scheme the bus coupler needs to be made ON in case any unwanted fault trip happened in any one of the Incomers.In this automatic operation the main worry is about the motor feeders what I have raised in my query. This operation also adopted by many industries for their unmanned substations. Have you come across any such operation where motor feeders are there in the buses. If yes what precautions or protection you are practicing.

I need an technical answer from an expert.

Regards

Hire one, then.

I am no expert in that field, whatsoever, so good luck, it sounds far above my pay grade.

I have never seen nor even heard of such an arrangement as that. How large are these motors you refer to?

Momentary paralleling is adopted when one of the incomers is planned to be shutdown for maintenance purpose or otherwise. The switchboard fault level calculation considers both the incomers simultaneously 'on' to take care of eventuality of the desired incomer failing to open. When it comes to safety, electrical interlocking is not considered good enough.

In Auto bus transfer, there is fast bus transfer scheme in which the supply interruption is typically for about 20ms and no motors are tripped in this scheme. This scheme is widely used in coal based power stations.

The second type of auto bus transfer is Slow bus transfer scheme, wherein, the transfer happens after all the motors are tripped and the bus voltage falls to below 40% rated (typical). This is more common in industrial substations.

Hope the above is helpful.

Contact Beckwith Electric for information on their Motor Bus Tranfer equipment. Sounds like they have a solution for what you are asking: http://www.beckwithelectric.com/products/mbt.html

Yes , Mr. Raghun the answer sounds meaningful & certainly it will help me.

But I astonished looking at the answers #3 & #4 as if the question is new.

Now Mr Raghun kindly say something on the motor back emf while the power of one side bus goes off for a while & how the sudden torque impact on motor shaft wud be taken into consideration. As the motor will act as a generator during the power off condition. What protection/precaution should be considered here?

What information need to be collected from the motor vendor while designing such logic to make it full proof?

Apparently, there is no such item as any mechanical or electric device that is fool proof, if that is what you meant to say.

As to your other comment: If perfect synchrony of phases is not maintained during paralleling, you may find the pieces lying nearby. Gook luck.

The motor, large or small, is an induction motor is it not. Thus there should be some back emf generated in the stator coils from circulating current in the rotor, but I do not suspect this to be a large number, based on design of motors to have tolerable hysteresis. The physical interactions that accelerate the rotor disappear upon loss of the imposed rotating magnetic field. Induction motors being driven are not usually considered efficient generators of electricity.

I suppose you are asking with regard to fast bus transfer scheme. As I mentioned, the power interruption is limited to about 20ms and within such a short time, the system voltage and the motor back emf are not expected to drift much in terms of phase angle. In any case, standard motors are designed to withstand 150% of rated voltage briefly.

OK, then, maybe there is no real problem?

Thanks Mr. Raghun!

You are talking about 20ms i.e fast bus transfer relay operating time like Basler or Artech solonic make. But before that there are normal u/v relay operating time plus breaker tripping time(1 no breaker) plus breaker closing time (bus coupler) which will come around 200ms( approx) with all operating times added together.

Hope 200ms will not hamper the operation so far the motor function is concerned.

Need your feedback once again.

Hope has no place in an engineering study, especially one where you will be liable for the damage that will occur. 10-12 cycles is a lifetime when it comes to this type of analysis. I have done forensic studies on the damage that is the result of "hope".

Be honest and tell your client to budget more so that you can subcontract this to a professional who has done this before.

U/V based transfer is not called fast bus transfer.

In generation plants, when the generator protection acts it opens the auxiliary bus incomer as well. Such a case, the generator protection output that trips the aux bus incomer is used also to initiate the fast bus transfer.

The scheme checks that there is no protection operation in the incomer and gives a close command to the standby incomer which closes after verifying that the main incomer CB is already Off by then.

Thus, the close command to standby incomer is almost simultaneous as the opening command to the main incomer. The difference between the closing and opening times of the CB (typically 20ms) is the only interruption time.

Another pre-requisite in this scheme is that there should be a device monitoring the two supplies (main and standby) and maintain a permissive contact in the transfer scheme as the supplies are in synch.

There are comprehensive microprocessor based devices available in the market which can perform fast bus transfer and in case of failure of fast bus transfer, can do U/V based bus transfer or slow transfer. These devices also allow manual transfer from the front facia or from the control desk. You can google to find more.

Auto Changeover Scheme explained well here.

https://electengmaterials.wordpress.com/2019/01/17/auto-changeover-scheme/

Thanks & Regards

Source: https://electengmaterials.wordpress.com

Thank you Mr.Raghun, You are right.

Bus transfer schemes explained here with the help of single line diagram.

https://electengmaterials.wordpress.com/2018/11/15/single-line-diagram-of-power-plant/

Thanks & Regards

Edit: The above link is not working. Here is the new link.

https://electengmaterials.com/single-line-diagram-of-power-plant/

Source: https://electengmaterials.com

Thanks & Regards

Edit: The above link is not working. Here is the new link.

https://electengmaterials.com/auto-changeover-scheme/

Source: https://electengmaterials.com