Substation Structural Design 3

[WARose]

My arm has been twisted into doing a electrical substation for a client. Never done one. Have done a few transformer bases.....but that's been a while. After reviewing what I can (including old threads here), I have a few philosophical questions. I will put the question itself in bold....so to separate the wheat from the chaff. I would appreciate responses from experienced substation structural design engineers.....although some of this may have to be answered by a electrical guy/gal.

1. I am (highly) confused as to what codes are to be used for this. ASCE has a MOP: ASCE 113. And it has some good info. But it's load factors are different than current IBC codes. IEEE has the code for seismic design: 693-2018. IBC doesn't reference either of those documents in the version I use. So what code(s) do I use for this? Is it a matter of: I'd take a run at it with ASCE/IBC.....and if those other codes give something more conservative.....then run with them?

2. Is it common to have a lot of equipment up on pedestals....but everything just one one mat.....or individual footings?

3. What kind of vibration issues can I expect with this? I remember transformers have some unbalanced forces (especially because of the associated pumps and so on). Anything else? Ballpark, what kind of frequencies & unbalanced forces are we talking here?

4. At least with some of this equipment, I've seen containment for possible leaks. Is common practice to contain the whole area?

5. What kind of fire protection is needed for some of this stuff? This subject is one of the few times this kind of equipment is specifically mentioned in the IBC. Apparently some transformers need fire walls near other stuff....what else?

I may have a few more questions as I think of them......thanks in advance.

 

[AskTooMuch]

I'll try to answer a few based on few substations we put in our plant.

2. Usually individual foundations for transformers, etc.. substation building on drilled piers, poles on drilled piers or steel piles.

4. I have not seen the whole area is contained. Whole area maybe raised due to expected flood elevation.

5. I've only see one with firewall, majority don't have any.

 

[canwesteng]

2. Depends on your site.

3. Haven't heard of vibrations issues with transformers, even on powerplant scale/electroplating plant scale. There will be a pump for anything substantial as they will be oil cooled, but the pump itself is quite small. The vibrations from the actual transformer itself are more of a noise concern, if it is a substantial transformer it will be very loud.

4. Containment generally only for oil filled equipment (transformers mainly, perhaps capacitors/switchgear are oil cooled if large enough). Really depends on the scale, if you have piles of large equipment that needs to be oil cooled you might see the whole area as containment.

5. You'll have to dig into NFPA 70 to determine this, there is no one size fits all answer. Will depend on how close transformers are to other stuff iirc.

 

[OldDawgNewTricks]

I have designed several substation buildings that contain transformers, swtitchgear, etc. So my answers pertain to buildings, not exterior substations. Please disregard if this doesn't apply to your situation.

1. I use IBC for the building design. I haven't looked, but maybe ASCE 113 & IEEE 693 would apply to the exterior, non-building structures?

2. Inside the buildings, all the equipment is on housekeeping pads.

3. No special vibration design for the equipment I have encountered.

4. Containment is usually required for oil-filled transformers. We usually put concrete curbs with ramps at the doors and use the entire building floor as containment.

5. IDK. I leave that up to the Electrical and Fire Protection engineer. Sometimes they need a 3-hour vault depending upon what else is around.

 

[miningman]

what are the voltages / power ratings involved?? Answers may affect the necessary grounding requirements.

 

[WARose]

Thanks for the replies so far.

[blue](OldDawgNewTricks)[/blue]

I have designed several substation buildings that contain transformers, swtitchgear, etc. So my answers pertain to buildings, not exterior substations. Please disregard if this doesn't apply to your situation.

Thanks but I'm not talking about one of those prefabbed deals.

[blue](miningman)[/blue]

what are the voltages / power ratings involved?? Answers may affect the necessary grounding requirements.

Don't know that yet. Would grounding significantly impact any of the issues I raise in the OP? I don't remember grounding being that big of a issue on previous transformers I've done.

 

[DayRooster]

I have designed substations and transmission lines but it was a while ago…so I might be a bit rusty.

1. If it’s a dead-end then the loads are based on IEEE load factors because line tension is critical. If it’s a transformer equipment containment foundation then it’s ASCE load factors. If it’s a control house then it’s ASCE (maybe IBC requirements depending upon the buildings occupancy). If I recall ASCE 113 does a good job of explaining the various structures…otherwise this document is also a good design manual:

https://www.rd.usda.gov/files/UEP_Bulletin_1724E-300.pdf

Trying to recall where this is called out but for the life of me I can’t recall. Can’t recall if it’s buried within the electrical codes or if the utility dictates it…

2. Typically drilled piers with the exception of the transformer which would be a mat (with potential fire walls). Shallow foundations are an alternative but not the preference from my past experience.

3. No vibration issues but there are issues with air flow clearances around the transformer along with containment and potentially fire protection (NFPA 850). Containment designs can get pretty wild depending upon fire requirements. Ever seen a rock sandwich?

4. Yes you will need containment to satisfy EPA. Also might need containment per NFPA if it applies too.

5. Correct, if NFPA 850 applies (which it always did for me) then you need fire protection requirements for the transformer. This could mean huge containment plus giant fire walls. All depends on the transformer size and location to other structures.

 

[WARose]

Thanks DayRooser. That's good info.

 

[DayRooster]

No problem. BTW, that was the quick response from memory. If no one else responds with more information then let me know and I can always go digging through old files…

 

[dik]

Missed this...

I've worked on several... switch yards and transformer yards. some additional considerations. Transformer containment pits for coolant and difficulty in removing rainwater. I've done half a dozen where we've used a proprietary product called Sorbweb that allows water to pass, but gels when transformer oil contaminates it. Containment pit is lined with an EPDM liner and has a Sorbweb 'window'.

Trenches and ducts for cables.

Physical spacing of equipment.

Electrical grounding grid for the area. Sparkies are big on grounding. This is a series of interconnected grounding cables in a grid pattern buried beneath 6" or so of clean stone.

Grounding of all elevated steel structures. Equipment supports, structures, etc. All parts have to be interconnected with grounding cables.

Use of HDG with HDG stainless grounding lugs and NEMA connectors and Belleville washers. Access or contact with all equipment must be grounded so there is limited potential if you touch 'something' while standing on 'something'.

Grounding of all concrete structures and cadwelds to connect cable to reinforcing.

Design of buried enclosures for uplift, drainage and grounding.

Construction of blast containment walls to protect equipment and buildings from transformer explosions.

Construction of 'A' type frames for 'dead end' structures.

The enclosures are generally enclosed with chainlink fencing which has to be grounded. Also access to the facility has to be part of the grounding grid and this has to extend, generally a metre outside the chainlink fence.

I'll likely think of a couple of other items...




Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

Headings from my project notes related to this:

DESIGN LOAD INFORMATION (REV 180519)
WIND LOADING (REV 190524)
ICE LOADING (REV 171124)
DUST/DEBRIS LOADING (ADDED 161222)
DUCT VAULT LOADING (REV 160505)
DUCT VAULT SERVICE LOADS (ADDED 160505)
DUCT BANK SERVICE LOADS (ADDED 160505)
A-FRAME DESIGN (ADDED 160505)
ELECTRICAL AND MECHANICAL EQUIPMENT LOADING MATRIX NOTES (ADDED 131120)
ELECT EQUIP (REV 170521)
FILL MATERIALS (REV 170521)
1-1/2" CLEAN CRUSHED STONE (FOR SORBWEB CONTAINMENT BASIN) (REV 120103)
2" CLEAN GRAVEL (FOR ADSORPTION TRENCHES) (REV 170521)
FIRE QUENCHING STONE (ALBARRIE CONTAINMENT SYSTEM) (ADDED 151217)
INSULATING STONE (NON-CONDUCTIVE GRANITE OR LIMESTONE) (REV 160119)
SAND (CONDUCTOR BEDDING SAND) (REV 160119)
TRENCHING AND CONDUIT INSTALLATION (REV 161212)
FACILITIES SITEWORK (GENERAL) (ADDED 151224)
FACILITIES SITEWORK (PAD AREA) (ADDED 151224)
FACILITIES SITEWORK (DUCT BANK) (ADDED 151224)
FACILITIES SITEWORK (P/C DUCT VAULT) (REV 160505)
GROUNDING LUGS AND GROUNDING TABS (STAINLESS STEEL) (REV 170825)
UTILITY POLES (ADDED 131120)
WIRES AND CABLES (REV 170521)
SEGMENTAL PRECAST CABLE TRENCHES (ADDED 140718)
CABLE TRAY (REV 170213)


Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

There could be a dozen different types of electrical equipment in a building or on a platform, A frame, or whatever. I found it easier to deal with this by creating a 'load matrix' identifying the equipment with loads, dimensons, etc. I had a couple of spreadsheets to design platforms, A Frames, etc. I'll see if I can still find them... work done a decade or so back. For one transformer yard I did, there were 6 large transformers (bigger than most houses) and the coolant was equivalent to 2-1/2 tanker cars of transformer oil, for each.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Lomarandil]

My experience matches DayRooster's reply

----
just call me Lo.

 

[HTURKAK]

I have similar experience with DayRooster (Structural) post.. You did not mention power rating and the location..when located in a petrochemical /industrial plan , blast proof design may be required.

I will suggest you to get and look ;

The Electric Power Engineering Handbook ( by Leonard L. Grigsby )

You can find a lot of info. for Oil containment, animal security, grounding, fire protection, seismic considerations...

 

[dauwerda]

It looks like you have some good responses already but I'll add my two cents just to share more (similar) experiences.

2. So what code(s) do I use for this?

ASCE 113 is what I see used 95% of the time (or is at least the starting document). Many utilities have their own internal standards that they follow for loading criteria, most are similar to ASCE 113, some are a mashup of ASCE 113 and current ASCE 7 (ASCE 113 is still based on ASCE 7-05). Following the IBC/ASCE 7 will definitely be more conservative for wind loading (due to factors in load combinations), but you would probably still need to lean on ASCE 113 for applying short circuit loads.

ASCE 113 basically uses a watered down ELF approach from ASCE 7 for seismic, I typically follow the current version of ASCE 7 for seismic loads.
The only time seismic may get a little messy is if it is a high seismic region that you must follow the IEEE 693 (this is usually driven by the customer/utility). IEEE 693 is generally used for the seismic certification of the electrical equipment (through FEM and shake table testing), however if the equipment is being supported on structures, this will obviously impact the response of the equipment, so the structure design also needs to be considered. The equipment supplier may supply the structure to match what they tested with, or the structure will need to be designed to match the performance the equipment was tested on/for.

2. Is it common to have a lot of equipment up on pedestals....but everything just one one mat.....or individual footings?

Everything is typically on individual foundations witch equipment and bus being supported by steel structures. Transformers and breakers would be on mat foundations. Bus supports and equipment stands are usually on individual drilled piers (unless soils are not conducive to this). I'm not sure if frost is an issue, but my experience has been that breakers are typically connected using flexible connectors and the foundation is usually allowed to float i.e. not extend below frost. Large transformers are typically on mat foundations that do extend down to frost or are supported on piles with void form - this is to ensure there is no differential movement. Where as small pad-mount transformers are supported on small (sometimes even precast or fiberglass) concrete pads.

What kind of vibration issues can I expect with this?

None that are structural. The utility I used to work at used to put rubber pads (old conveyor belt) between the concrete foundation and the transformer base to help reduce noise.

Is common practice to contain the whole area?

Personally, I have only seen this done once as a retrofit to an existing substation - It's not an ideal solution, if there is also a fire, it can spread across the entire substation with the oil rather than being contained to one spot. Secondary oil containment is driven by the EPA and is part of the Spill Prevention, Control, and Countermeasure (SPCC) Plan

https://www.epa.gov/sites/production/files/documents/spccbluebroch.pdf.

The real requirement is that you can't discharge oil (spills) into navigable waterways and if you have a certain amount of oil (over 1320 gallons) you must have an SPCC plan. This plan usually relies on secondary oil containment - which may range from soil/clay berms around the equipment to concrete walls and floors. I have even worked on some sites with limited room that basically had a concrete catch basin around the transformer and an underground vault that the oil was routed to through pipes (the vault had an "oil-minder" pump installed to drain rain water that collected).

What kind of fire protection is needed for some of this stuff?

The simplest (and cheapest) solution is to provide enough space between equipment to not need any other fire protection (fire walls between transformers is probably one of the most common). This of course is driven by site constraints. If my memory is correct, 50ft was the magic number, but you should certainly verify this with the NFPA.

 

[WARose]

Thanks dauwerda. More good info.

Follow up question (for anyone interested): where are the "dead end" loads for power lines specified? ASCE 113? Or somewhere else?

Thanks for all replies so far.

 

[dauwerda]

The primary load on the deadend structure is due to wire tension. This will be determined by whoever is doing the line design and can vary quite a bit (whether the line coming in is at full, high tension or slack spans in from a deadend T-Line pole outside the substation). I have designed deadend structures that support over 30 kips of tension at each phase location and I have designed them that support only 1 kip at each phase location. ASCE 74 is the go-to for line design, but ultimately you will need to get this loading information from whoever is doing the line design.

Also note, most deadends (whether they are H-Frames or A-frames) are tapered tube steel and are typically designed by the manufacturer, you would just provide a drawing showing geometry, load requirements and any attachment details.

 

[WARose]

Also note, most deadends (whether they are H-Frames or A-frames) are tapered tube steel and are typically designed by the manufacturer, you would just provide a drawing showing geometry, load requirements and any attachment details.

Are the foundations (for them) left to manufacturers as well? Thanks (again) for your replies.

 

[dauwerda]

The manufacturer will typically design (and supply) the anchors, but the foundation design itself would be done by others (you). Typically, the manufacture would prepare (and stamp) a calculation package based on the information supplied, this would include the anchor details and foundation reactions for the structure for you to use to do the foundation design.

 

[WARose]

Question for anyone interested: I've heard some talk about protection from explosion.....when is that required? When NFPA 850 says so?

Does anyone make some prefabbed firewalls that can handle this as well?

EDIT: BTW, are transformers the main items to be worried about with explosion/fire?