2750 KVA Bess Transformer-34.5/0.4 KV

Engineered for BESS applications, delivering reliable, high-performance power conversion under demanding grid and cycling conditions

01 General

1.1 Project Description

This unit comes out of a project delivered in early 2025, when a U.S.-based power infrastructure contractor placed an order for a 2750 kVA three-phase pad-mounted transformer for a BESS site in Hinkley, California. The requirement was straightforward on paper-step down medium voltage to low voltage for the energy storage and conversion system-but the operating conditions were less forgiving, both electrically and environmentally.

The bess transformer itself is designed in accordance with IEEE Std C57.12.34-2022. It’s rated at 2750 kVA, with a 34.5 kV primary and 0.4 kV secondary, Dyn1 vector group. Cooling is KNAN, and FR3 natural ester is used as the insulating fluid. Nothing unusual in isolation-but taken together, it’s a configuration that fits well with inverter-based BESS systems, where stability, thermal behavior, and safety margins all start to matter a bit more than in conventional distribution setups. With an impedance of 7.5%, the transformer provides stable three-phase four-wire output, making it well suited for inverter-based BESS systems and associated auxiliary loads.

The installation site presents:

Extreme ambient temperatures from -13.2°C to 45.7°C (ASHRAE 20-year return)
Elevated fault current levels on the medium-voltage system (~28 kA)
Continuous cycling operation typical of grid-scale BESS systems

1.2 Technical Specification

2750 kVA pad mounted bess transformer specification and data sheet

Delivered to	America
Year	2025
Type	Three phase pad mount transformer
Standard	IEEE Std C57.12.34-2022
Rated Power	2750KVA
Frequency	60HZ
Feed	Loop
Front	Dead
Phase	three
Cooling Type	KNAN
Liquid insulant	FR3 Oil
Primary Voltage	34.5 kV
Secondary Voltage	0.4 kV
Vector Group	Dyn1
Winding Material	Aluminum
Impedance	7.5%
Efficiency and Losses Standard	99.53%
Tap Changer	NLTC
Tapping Range	±2*2.5%
No Load Loss	2.6 kW
On Load Loss	17.3 kW
Accessories	surge arrester

1.3 Drawings

2750 kVA pad mounted bess transformer drawing and nameplate

2750 kVA pad mounted bess transformer nameplate

02 Engineered for BESS Performance

This pad mounted bess transformer is specifically engineered for battery energy storage system (BESS) applications, with key design features focused on system compatibility, safety, and reliable performance under dynamic operating conditions.

2.1 BESS-Oriented Electrical Configuration

The unit adopts a 34.5 kV delta primary and 400/230 V grounded wye secondary (Dyn1 equivalent), providing a stable three-phase four-wire output required for inverter-based systems. The two-winding aluminum design, combined with 7.5% impedance, supports effective protection coordination under high fault levels (~28 kA) while maintaining overall system stability.

2.2 Thermal and Insulation Design

Designed for continuous charge/discharge operation, the transformer utilizes ONAN liquid cooling with a 65°C temperature rise limit, ensuring reliable thermal performance under repetitive load variations. The aluminum winding structure is optimized for both heat dissipation and mechanical strength under short-circuit conditions. The core and winding assembly is designed to minimize losses and withstand thermal stress under high cycling BESS operation

Transformer Core Stacking for 2750 kVA BESS Pad Mounted Transformer

2750 kVA pad mounted bess transformer active part

2.3 Protection and System Integration

To meet the realities of a BESS installation, the protection scheme wasn’t treated as an add-on but as part of the core design. The bess transformer is fitted with an internal loadbreak switch, bayonet expulsion fuses, and current-limiting fuses, working together as a coordinated protection system. Surge arresters rated at 24.4 kV MCOV are included as well-mainly to handle overvoltage events from switching or grid disturbances, which are not uncommon in these systems.

Grounding follows a solidly grounded configuration, with bonding at the main distribution board. It’s a fairly standard approach, but important here-it keeps the system behavior predictable and ensures compatibility with typical BESS grounding schemes, especially on the downstream side.

2750 kVA BESS Pad Mounted Transformer bayonet expulsion fuses

2750 kVA BESS Pad Mounted Transformer loadbreak switch

2.4 Electrical Interface and Maintainability

The transformer is configured with 600 A deadfront loop-feed bushings on the high-voltage side and side-mounted NEMA 8-hole spade terminals on the low-voltage side, enabling safe and efficient connection in underground distribution systems and high-current BESS applications.

Cable interfaces are designed to accommodate 500 kcmil aluminum MV conductors with concentric neutral and grounding, and parallel 1000 kcmil LV cables, ensuring reliable current carrying capacity.