r/BESS

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