Need some engineering guidance

[spurlockda]

Hi folks,

thanks for reading my query. I am working on an expeditionary airfield and we have a genny supplying 120/208 240/416 Volts 4 Wire, 3 Phase and I am powering a tower module with the associated electronics as well as an OpsCen/Command and Control module. A lot of the equipment (most actually except the radars) are running 120 VAC single phase. As you can imagine, clean power is critical to the electronics. Plus we have a second 30kW, 120/208 240/416 Volts 4 wire, 3 Phase backup genny that I would like to run with an auto bus tie so that in case of an emergency or unscheduled shutdown of the pri genny, the backup will kick in automatically with very little to no interruption of service.

i've talked to some folks who are not the real experts like you all and they are telling me that the best way to get single phase is to convert it to DC and then to single phase AC. They are saying that will automatically filter the power?????

Any guidance on where I can find the correct solution or build a solution would be greatly appreciated.

Thanks again,

 

[petersonra]

Sounds like good advice to me.

 

[Sahib]

They are saying that will automatically filter the power?????

Yes. Superior performance in that respect. However, it is a better idea to include SPD and EMI/RFI filter on input and output for protection, particularly in bypass mode.

 

[jeremy.zinkofsky]

Hi folks,

thanks for reading my query. I am working on an expeditionary airfield and we have a genny supplying 120/208 240/416 Volts 4 Wire, 3 Phase and I am powering a tower module with the associated electronics as well as an OpsCen/Command and Control module. A lot of the equipment (most actually except the radars) are running 120 VAC single phase. As you can imagine, clean power is critical to the electronics. Plus we have a second 30kW, 120/208 240/416 Volts 4 wire, 3 Phase backup genny that I would like to run with an auto bus tie so that in case of an emergency or unscheduled shutdown of the pri genny, the backup will kick in automatically with very little to no interruption of service.

i've talked to some folks who are not the real experts like you all and they are telling me that the best way to get single phase is to convert it to DC and then to single phase AC. They are saying that will automatically filter the power?????

Any guidance on where I can find the correct solution or build a solution would be greatly appreciated.

Thanks again,

What they mean by "automatically filter the power" is that when an inverter converts AC to DC and back to AC, the output is a pure sine wave. So you will not see any voltage drop across the inverter. Additionally, if normal power cuts out for a brief time (i.e. brown-outs) the DC bus of the inverter will provide power to the load until standby kicks on or normal power is restored. So the load always sees a constant and clean source of power.

The most reasonable way to accomplish this is to specify a UPS. Your battery capacity will be low as you will only need to provide power until the generator transfers and starts up. Install a UPS ahead of all electrical distribution but load side of the transfer switch. You want the normal and standby sources to pass through the UPS on its way to the load.

 

[Jraef]

If you need battery backup in the event of a generator shutdown, by all means buy a UPS, and for the cleanest power, get one that is "double conversion", meaning it is always conditioning the power. Cheaper UPS units will be "standby" where they only turn in when the line power begins to go out of spec.

If you don't need battery backup, then a UPS adds battery maintenance and servicing requirements that may be undesirable in a remote location. In that case, you can get a simpler "power conditioner".

In either case use the 208/120 3 phase output of your generator, condition it at the 3 phase side, then on the conditioned side you will have 3 separate 120V L to N clean power circuits available. Try to balance your 120V loads as best you can.

 

[jeremy.zinkofsky]

If you need battery backup in the event of a generator shutdown, by all means buy a UPS, and for the cleanest power, get one that is "double conversion", meaning it is always conditioning the power. Cheaper UPS units will be "standby" where they only turn in when the line power begins to go out of spec.

If you don't need battery backup, then a UPS adds battery maintenance and servicing requirements that may be undesirable in a remote location. In that case, you can get a simpler "power conditioner".

In either case use the 208/120 3 phase output of your generator, condition it at the 3 phase side, then on the conditioned side you will have 3 separate 120V L to N clean power circuits available. Try to balance your 120V loads as best you can.

He may find it difficult to procure a power conditioner that is rated for 30KW. If he can, it will most likely be very expensive. Also, a power conditioner will not provide any "ride-through" power if the normal power cuts off for a second or less and comes back on. That little amount of time without power may be detrimental to the sensitive electronics he is feeding.

 

[Ingenieur]

This sounds like a mil installation
cost may not be an issue
get 1 large ups then feed a panel for the 1 ph loads
or get some pou ups's for the critical loads
size it to carry you thru genset xfer

 

[publicgood]

We should preface this UPS topic that depending on the selection, there may be generator-to-ups sizing criteria that needs to be considered. Also in-rush depending on the loads. Not to mention DC link options.

 

[Jraef]

He may find it difficult to procure a power conditioner that is rated for 30KW. If he can, it will most likely be very expensive. Also, a power conditioner will not provide any "ride-through" power if the normal power cuts off for a second or less and comes back on. That little amount of time without power may be detrimental to the sensitive electronics he is feeding.

Sola makes 3 phase power conditioners up to 75kVA. He didn't say he needed to "hold up" at all, he just said he needed "clean power". That's a power conditioner, not a UPS. That's why I said, IF he needs battery backup in the event of a generator shutdown, THEN buy a UPS. But if it's not necessary, it adds a maintenance burden that may be unwanted in an "expeditionary airfield".

In around 1998 I did a large project with the Air Force for self-contained mobile radar sites. They did indeed need battery backup because in the heat of battle, even a 1 second loss of radar as a transfer switch came on was enough to lose track of a target. So we used 2 x 25kVA on-line double conversion UPS units; one for the electronics, one for the radar motors and magnetrons. 2 years later the Air Force was buying new UPS units like crazy, because nobody was maintaining them and they didn't have the resources in the field to do so, nor to even bother with replacing the batteries. It was cleaner and simpler for them to just swap out the entire UPS units. But hey, it was just tax dollars at work...

 

[spurlockda]

Thanks Jraef

Thanks Jraef

Sola makes 3 phase power conditioners up to 75kVA. He didn't say he needed to "hold up" at all, he just said he needed "clean power". That's a power conditioner, not a UPS. That's why I said, IF he needs battery backup in the event of a generator shutdown, THEN buy a UPS. But if it's not necessary, it adds a maintenance burden that may be unwanted in an "expeditionary airfield".

In around 1998 I did a large project with the Air Force for self-contained mobile radar sites. They did indeed need battery backup because in the heat of battle, even a 1 second loss of radar as a transfer switch came on was enough to lose track of a target. So we used 2 x 25kVA on-line double conversion UPS units; one for the electronics, one for the radar motors and magnetrons. 2 years later the Air Force was buying new UPS units like crazy, because nobody was maintaining them and they didn't have the resources in the field to do so, nor to even bother with replacing the batteries. It was cleaner and simpler for them to just swap out the entire UPS units. But hey, it was just tax dollars at work...

Jraef,
You are spot on. My needs are for special circumstances that require complete aviation operations in the middle of nowhere. Includes air search radar, weather radar, sat comms, and etc. While I appreciate all of the other replies, I must have mistakenly written it to seem focused on the UPS function and filtering.

My requirement documents for the contract state that I must be able to down convert some of the three phase power to single phase and provide "clean" power to the devices. An additional requirement include the automatic redundant buss tie to a back up genset. There are other Key Performance Parameters but those are in the stretch goals so I can defer to them later. UPS with batteries are one of those.

The other posters confirmed that converting to DC then back to AC is the key to a pure sine wave thus satisfying the "clean" requirement.

Short of buying an AC-DC-AC converter, is it a relatively simple matter to build one? The reason I ask about building one is so that I can customize it to make it modular and easily transportable.

Thanks again everyone. As a hopefully successful Veteran Contractor (who is currently operating on a shoestring budget), I hope that I can parlay this contract into a success so I can pay for better equipment for my business.

Dave

 

[Jraef]

...

Short of buying an AC-DC-AC converter, is it a relatively simple matter to build one? The reason I ask about building one is so that I can customize it to make it modular and easily transportable.

Thanks again everyone. As a hopefully successful Veteran Contractor (who is currently operating on a shoestring budget), I hope that I can parlay this contract into a success so I can pay for better equipment for my business.

Dave

The AC to DC to AC part of that is relatively easy to make, the clean sine wave output is not. That would be one advantage of buying a good quality double conversion UPS, the mfr of the UPS will have already designed that output PWM algorithm and filtering system to get you that clean sine wave output. Cheap UPS units will be square wave or "modified" square wave / step wave outputs because that is cheap and easier to make, however the output sine wave is anything but "clean" with regard to many sensitive electronics.

Given this situation, I think you will find it less expensive and easier to just buy a standard double conversion UPS with as small of a battery backup as possible, then if your "stretch goals" are required, bump up the battery capacity. You MAY even find a UPS supplier willing to sell you just the power unit, no batteries and charger system and make that optional altogether.