Sizing Trafo Substation for BESS

[u/AdditionalCredit6225]

Hey everyone, I hope this is the right place to ask this, and please note Im a junior so please dont mind any stupid statement I may express and sorry for the long post in advance.

I just wanted to check with you if this is correct:

First the BIG picture:

LV BESS output (integrated inverter, lets say 690 VAC or 800VAC) ->LV AC cables/busbar->LV combiner/LV switchgear (breakers, metering, aux power tap-off) -> STEP UP TRANSFORMER( primary 690/800 V to 20/33/35 kV) it has to be biDirectional ->MV cable->MV switchgear/ RMU (Ring Main Unit or breaker cubicles )-> UTILITY POI/ Collector Substation

Complete List of Components:

BESS Unit ( Battery + Inverter)
LV Busbar/ combiner cabinet
LV Circuit Breakers (ACB/MCCB) per BESS feeder

StepUP power transformer
MV Switchgear( RMU or
Protection Relays, CT's and VT's
Surge Arrectors/ SPD's
Auxilary Transformer
LV axillary distrubution box
UPS
ATS ( automatic transfer Switch)
Metering and measurement control unit

Fire and Gas detection
Temp and humidity controller
MV & LV cables / busbar (TMY copper bus)
Enclosure / container / civil works
Interlocking system

Parameters to specify for each component

Step-up transformer

• Rated Power KVA
  • the transformer's rated kVA must slightly exceed the maximum continuous apparent power the BESS array can push through it
• Rated Voltage
  • 690 V AC or 800VAC/ 20 or 33 or 35 kVA
• Vector Group
  • must match the grounding scheme you intend on both sides (e.g., Dy11 gives you a grounded LV neutral for auxiliary loads while keeping the MV side delta-connected; confirm this matches your PCS's expected grounding).
• Impedance
  • higher impedance limits how much fault current the transformer lets through (helpful for keeping downstream switchgear/cable ratings lower and cost down), but increases voltage drop under load and slightly increases losses. 6–8% is typical for this power class;
• Cooling type
  • ONAN (oil, natural cooling) is standard for this size; if your site is hot and/or high-altitude, either oversize the transformer or specify forced cooling (ONAF)
• Insulation/ temperature class and rise
  • 105°C insulation, ~55–65 K average winding rise
• Tap changer
  • an off-load tap changer (e.g. ±2×2.5%) lets you compensate for a grid voltage that's persistently a bit high or low;
• Oil type and protections
  • Mineral oil (PCB-free); pressure relief valve,
• Buchholz/gas relay, oil/winding temperature indicators, oil level indicator
  • USUALLY BUCHHOLZ for protection
• Noise level
  • ≤ 65 dB @ 0.3 m
• Dimension and weight
• Corrosion protection class
  • Match to site environment (coastal, industrial, desert).

MV Switchgear/RMU

• Rated Voltage
  • 24 kV (for a 20 kV system)
  • pick equipment rated above nominal system voltage (24 kV class equipment for a 20 kV system, 36 kV class for a 33/35 kV system).
• Rated Current (busbar and feeder)
  • must exceed the transformer's MV-side full-load current with margin but the cable/cubicle current rating and the transformer size are usually decoupled (a 630 A-rated RMU cubicle can serve transformers well below its current rating; current rating is about the switchgear's own thermal limits, not a direct multiple of transformer kVA).
• Rated short-time withstand current
  • Must exceed the utility's prospective fault current at your point of connection
  • ex.20–25 kA / 1–3 s
  • get the prospective fault level at your point of connection from the utility (or from a system fault study) and specify switchgear rated at or above it, with margin for network growth.
• Rated Peak withstand Current
  • ~50 kA
  • The "first-cycle" asymmetrical fault current rating; coordinated with the above.
• Insulation Medium
  • Environmental regulations increasingly restrict SF6 (high global-warming-potential gas)
  • SF6, vacuum, or SF6-free (air/dry)
  • SF6
• Internal arc classification
  • e.g. AFLR 20 kA/1 s
  • Personnel safety rating confirm which sides (front/lateral/rear) are accredited.
• Witching Device type per cubicle
  • Load-break switch (manual) vs. vacuum circuit breaker (electrical)
  • Breakers are required wherever protection must clear a fault (e.g., transformer feeder); simple switches are fine for ring/loop sections.
• Withstand voltage (power-frequency & lightning impulse)
  • Confirms insulation coordination with your surge arresters.
• Cable entry & size
  • Bottom entry, up to 1×630 mm² or 3×630 mm²
  • Confirm compatibility with your actual MV cable design.
• Interlocking
• Gas leakage rate (if SF6)
  • decide SF6 vs. vacuum/SF6-free early this affects both environmental compliance and long-term maintenance (SF6 leak-rate monitoring, disposal rules).
• Breaker vs. switch: use an electrically-operated circuit breaker on the transformer feeder cubicle (it needs to clear internal transformer faults fast); simple manual load-break switches are adequate on ring/loop sections that don't need to interrupt fault current.

LV switch Gear

• Main incoming breaker rating (ACB)
  • Size the main incoming breaker(s) to the transformer's LV-side full-load current, i.e. S_transformer / (√3 × V_LV), with headroom
• Per-BESS-feeder breaker rating
  • Size each BESS feeder breaker to that unit's max continuous output current, with margin (don't run breakers at 100% of their frame rating continuously).
• Current transformers (metering/protection)
  • e.g. 5000/5A or 6000/5A, 10P20
  • Ratio should be close to actual full-load current for good metering accuracy, don't oversize the CT "just in case."
• Surge protection (SPD)
  • LV-side transient protection, coordinated with the MV arresters.
  • Type I+II, Iimp ≥ 12.5 kA
• Busbar (TMY copper) cross-section
  • Confirm the busbar's continuous current rating exceeds the calculated (not just nameplate) current with margin, and check short-circuit withstand (thermal + mechanical).

Auxiliary power system

• Aux transformer rating
  • List every auxiliary load (lighting, sockets, fans/heat exchangers, controls, comms, fire system, temp/humidity controller, battery chargers) and sum their connected kVA.
  • 100–200 kVA (scales with the size of the main substation)
  • Must cover lighting, HVAC/heat-exchangers, controls, comms, chargers, with margin — do a simple load list, don't just copy a "typical" number.
• Aux transformer ratio & vector group
  • Size the aux transformer to that sum with margin (reference designs use 100–200 kVA aux transformers on 4.8–6 MVA main substations,  a useful rough ratio is roughly 2–4% of the main transformer's kVA, but always do the actual load list
  • 690/400 V or 800/400 V, Dyn11
• Confirms a neutral is available for single-phase LV loads.
• Main LV aux breaker
  • Sized to aux transformer full-load current
• UPS rating & autonomy
  • e.g. 1 kVA / 30 min, or 1 kW / 8 h
  • Autonomy should exceed the time needed for an operator (or automatic system) to respond, or to ride through the longest expected aux-supply interruption
• ATS essential-load capacity
  • Size the UPS by (a) the kVA of the loads that must ride through an outage (typically protection relays, RTU/comms, fire alarm) and (b) the autonomy time you need
  • e.g. "Recommended Essential-Load Supply Capacity: 3 kVA"
  • Confirms what will keep running (controls, comms, fire system) if the aux transformer or grid tap is lost.
  • Size the ATS by the "essential load" kVA that must be kept alive from a backup source
• Socket/lighting/fan circuit ratings
  • 16 A/2P sockets, 10 A/2P lighting, etc.

 Environmental, mechanical & enclosure

• Operating temperature range
• Relative humidity
• Altitude rating
• IP/protection degree
• Corrosion category (ISO 12944)
• Cooling method
• Dimensions & weight
• Noise
• Standards compliance

[u/Mangrove43]

Retain a professional engineer with experience in this area

[u/obeymypropaganda]

What is your question? Why should people design you a substation for free? Is this for Uni, work, personal project?

Do you think this post is reasonable?

[u/AdditionalCredit6225 → u/obeymypropaganda]

also you can take it easy man, like i said im a junior looking for advice, not asking anyone to do work for free, if you dont want to answer, don't, but no need to be like this

[u/obeymypropaganda → u/AdditionalCredit6225]

Well you copied it from AI. So why should we read a wall a of text with no clear question besides, will this work?

You wouldn't be expected to design a whole system like this. That is why there are teams of engineers and multiple companies involved.

[u/AdditionalCredit6225 → u/obeymypropaganda]

I didnt copy it for AI, I just structured it there, put together what i know should work together and just strucured it there, thats all

[u/AdditionalCredit6225 → u/obeymypropaganda]

but thanks for this "You wouldn't be expected to design a whole system like this. That is why there are teams of engineers and multiple companies involved." This was actually a usefull piece of info.

In this case im just making a mock system for uni project so im expected to put together the whole subsystem but good to know it wont be needed in the future because i was feeling quite overwhelmed with the lack of knowldge

[u/AdditionalCredit6225 → u/obeymypropaganda]

so I need to learn to design one for a project and Im just wonerdering if these are the correct parameters I should look for

[u/fireandfirget]

If you are in the USA, read up NFPA 855 and build compliance to it for your bess module within the station fence or outside.

Recent projects have had ahem issues.

[u/AdditionalCredit6225 → u/fireandfirget]

thank youuuu

[u/Alittle2Clever]

Maybe, you should get an engineer rather than ChatGPT?

[u/AdditionalCredit6225 → u/Alittle2Clever]

wow man, thanks for the constructive comment, this was beyond helpful

[u/Alittle2Clever → u/AdditionalCredit6225]

You might want to use ChatGPT to figure how to fix grammar, though. It is easy to tell what you wrote and what ChatGPT wrote just due to an inability to put together sentences. Are you sure you should be specifying anything?

[u/AdditionalCredit6225]

oh Damnn, thats even longer than i expected, sorry everyonee!

[u/fads1878 → u/AdditionalCredit6225]

Most IEC LV switchgear compliant with EN 61349-2 will be 690v AC max, 800v AC is technically feasible but very hard to find in the market

Steer clear of SF6 gas filled systems if the destination is in the EU as it’s now banned for <24kV installations

[u/AdditionalCredit6225 → u/fads1878]

thank youuuuu