A battery that reduces a factory’s grid demand during discharge can increase that demand during recharge. The existing transformer must be reviewed in both states. Comparing the PCS kW rating with spare transformer nameplate capacity leaves out reactive power, temperature, voltage conditions and the actual timing of site loads.
For a project behind an existing MV/LV transformer, the first purchasing decision should be whether the installation and connection agreement permit the proposed operating envelope. The storage rating follows that study.
The 100 kW / 215 kWh sizing guide explains the equipment-level power and energy limits. Transformer headroom is a separate system constraint that can prevent the cabinet from operating at its nominal power.
Establish the existing limit before adding battery duty
Collect the transformer data sheet, nameplate, temperature class, impedance, cooling arrangement, protection settings and maintenance condition. Obtain time-series loading at the connection and identify anticipated production changes. A spare-capacity estimate based on one quiet shift is not a design input for a factory that runs seasonal peaks.
The Schneider Electric Electrical Installation Guide on transformer rating treats apparent-power demand, anticipated extensions and installation temperature as selection considerations. Those factors remain relevant when storage changes the load profile of an existing installation.
Confirm the load data’s measurement interval and whether it reports kW, kVA or both. A billing demand value may not capture short load steps, and a kW-only series cannot fully describe current loading when reactive demand changes. Record the source of each limit rather than treating every number as interchangeable capacity.
| Input | Engineering unit or record | Decision affected |
|---|---|---|
| Existing load profile | Timestamped kW, kvar, kVA and current as available | Recharge headroom and operating limits |
| Transformer rating | kVA with specified cooling and temperature conditions | Permitted continuous duty |
| Site environment | Ambient profile, ventilation and enclosure conditions | Thermal assessment and any derating |
| Network data | Voltage, fault level, impedance and protection study | Connection, switching and fault coordination |
| Operating agreement | Import/export permissions and approved control requirements | Lawful operating envelope |
Separate active power from current loading
For an undistorted balanced system, active and reactive power combine into apparent power. Real installations may also have harmonic and unbalance effects, so the engineer should use the applicable measurement and study method rather than relying on a simplified triangle alone.
The Electrical Installation Guide definition of power factor explains the relationship between active power and apparent power, which forms the basis of equipment rating. A battery reducing active import does not necessarily remove the site’s reactive demand.
A PCS may provide reactive support if its equipment, control configuration and connection approval allow it. That service can use converter current capability that would otherwise be available for active-power dispatch. Request the operating capability curve and the priority between active and reactive commands.
Do not assume a marketing statement about power-factor support makes the existing transformer adequate. The study must include other compensation equipment, harmonics, operating voltage and protection coordination. It should also explain how the system responds when the PCS is unavailable.
Review a sequence of operating states
Begin with the facility’s normal import. Add maximum permitted battery recharge during the coincident site load. Then review discharge, PV production, export where permitted, and credible equipment failures. Each state should have a declared network direction and a corresponding control limit.
The dispatch model used in the peak-shaving sizing method should include recharge constraints. Otherwise it may promise a reduction at the main peak while creating an unacceptable load later in the day.
| State | Relevant interaction | Required conclusion |
|---|---|---|
| High factory load plus battery recharge | Imports combine at the transformer | Recharge is limited to approved headroom |
| Battery discharge during production | Active import falls while reactive demand may remain | Current and voltage remain within reviewed limits |
| Low load with PV and battery discharge | Power may reverse at the connection | Export permission and protection are addressed |
| Loss of the control meter | Headroom calculation becomes unreliable | Documented fallback prevents unreviewed duty |
| PCS out of service | Expected peak reduction or reactive support disappears | Site operation has a credible contingency |
| Planned production expansion | Historical load no longer represents the facility | Operating limits are reviewed before expansion |
Distinguish reverse power flow from rewiring a transformer
Power exported through the normal transformer arrangement is not the same question as connecting a general-purpose transformer’s windings in a reversed voltage-conversion role. The equipment manufacturer and system designer must review the actual application. Generic statements that transformers are reversible do not establish suitability for a proposed BESS connection.
The DOE distributed-energy interconnection checklist is a useful process reference for discussions with the utility. Local grid requirements, protection approval and the installation’s electrical study govern the project, rather than a foreign checklist.
The protection review should consider changed fault contributions, direction-sensitive functions, isolation, switching sequences and the settings used in each mode. These are design responsibilities for competent engineers. A storage vendor’s export-limit function is not a replacement for the required protection study.
Carry the study limits into the quotation and SAT
When evaluating the HMX commercial storage range, send the approved connection envelope with the enquiry. Request charge-power limits, the relevant meter interface, active/reactive priority and fallback behaviour as controlled project requirements.
At site acceptance, demonstrate the permitted states with the installed metering and settings. Do not attempt an unapproved maximum-export test merely to reach a cabinet rating. Record how recharge is restricted when factory load rises and how the owner can identify a control or meter problem.
The useful handover result is an operating envelope the owner can enforce and review when site production changes. Transformer headroom should be a live constraint in the dispatch configuration, with a documented engineering basis. Sources reviewed on 10 October 2026.