Guide to MV Transformers for Tesla Megapack ( North American)
As the demand for sustainable energy solutions grows, 
the Tesla Megapack has emerged as a leading choice for large-scale energy storage projects. Key to the seamless integration of the Megapack is the Medium Voltage (MV) transformer, which plays a critical role in ensuring efficient energy transfer and distribution. This guide aims to provide comprehensive insights into selecting and deploying MV transformers for Tesla Megapack installations in the North American market.
Numerous types of MV transformers are compatible with the Tesla Megapack. Today, our discussion will delve into these options in detail, with a specific focus on the North American context. This article provides detailed information on transformer specifications, datasheets, and specific requirements aligned with the Tesla Megapack's standards.
In the North American market, the oil-immersed pad-mounted transformer stands out as the preferred choice for seamlessly integrating with the Tesla Megapack. Its widespread adoption can be attributed to its ease of installation, exceptional efficiency, and design features meticulously crafted to align with the connection requirements of the Tesla Megapack.
Pad Mounted Transformer Manufacturer
Daelim Transformer stands as a distinguished pad-mounted transformer manufacturer, dedicated to delivering excellence in every aspect of our products and services. Our pad mounted transformers boast a comprehensive range of UL and cUL certifications, guaranteeing strict adherence to rigorous safety standards.
Leverage our extensive experience in executing successful Battery Energy Storage System (BESS) projects throughout North America. Place your trust in Daelim Transformer to furnish bespoke BESS transformer solutions precisely tailored to meet your project requirements with unparalleled precision and reliability. Contact us today to discover how we can empower your sustainable energy initiatives.
Contact Daelim TransformerTesla Megapack And MV Transformer Design
For Tesla Megapack, design-specific transformer configurations are required depending on the system's duration. Here are the transformer requirements for both 4-hour and 2-hour Tesla Megapack systems
For Tesla Megapack - 4 Hour System
- The transformer must have two or four low-voltage outputs.
- These outputs are situated on both sides of the transformer.
- Each output is connected to the Tesla Megapack unit accordingly.
Tesla Megapack - 2 Hour System
- The transformer should feature one low-voltage output.
- This output is located on one side of the transformer.
- The output is connected to the Tesla Megapack unit.
These specifications ensure that the transformer aligns precisely with the energy storage requirements and setup of the respective Tesla Megapack system duration.
MV Transformer Design Standards For Tesla Megapack
The MV Transformer design shall be outdoor, oil-immersed 3 phase pad mounted type. Transformer characteristics should comply with the latest revisions of ANSI, NEMA, and particularly the relevant IEEE Standards. The transformer design must also hold UL certification.
Standards
- C57.12.00 - IEEE Standard General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers
- C57.12.28 - IEEE Standard for Pad-Mounted Equipment--Enclosure Integrity
- C57.12.29-2014 - IEEE Standard for Pad-Mounted Equipment--Enclosure Integrity for Coastal Environments
- C57.12.34 - IEEE Standard Requirements for Pad-Mounted, Compartmental-Type, SelfCooled, Three-Phase Distribution Transformers, 10 MVA and Smaller; High-Voltage, 34.5 kV Nominal System Voltage and Below; Low-Voltage, 15 kV Nominal System Voltage and Below
- C57.12.90 - IEEE Standard Test Code for Liquid-Immersed Distribution, Power, and Regulating Transformers
- C57.154 - IEEE Standard for the Design, Testing, and Application of Liquid-Immersed Distribution, Power, and Regulation Transformers Using High-temperature Insulation Systems and Operating at Elevated Temperatures
Learn more about IEEE C57 Standard
The percent impedance voltage, as measured on the rated voltage connection, shall be per transformer datasheet for target impedances, and the tolerance on the impedance shall be +/- 7.5% of the nominal value.
The guaranteed efficiency shall be per transformer datasheet with a +/- 0.1% allowable deviation in efficiency on individual units but the average for the lot order of transformers shall match the value specified on the datasheet.
The medium voltage bushings shall be 900A deadbreak 40kA Short-time current rating as specified on the transformer datasheet (Section 3). Bushings shall be epoxy/polymer type.
Supplier to notify Tesla if porcelain bushings are offered. Bushings shall be externally removable and be supplied with a removable stud. The transformer shall be provided with six (6) medium voltage loop feed bushings. Medium voltage bushings shall comply with IEEE 386, 2016.
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Megapack MV Transformer
- Transformers shall comply with NEC Article 450.
- Transformers shall be of dead-front, pad-mount design.
- Transformer neutral shall not be bonded from primary to secondary. On 4-hour projects, a single transformer neutral bushing shall be brought out in one (1) of the two (2) LV termination cabinets, to prevent unintentional looped connections.
- Transformer primary terminations shall be configured for loop-feed.
- Transformers shall have a grounded isolation shield between the high- and low- voltage windings.
- Current Transformers (CTs) shall be mounted internally to the tank under oil to monitor primary current flow through the transformer. MV CTs installed inside the tank shall be 150:1A 5P10 3VA or better accuracy class. Supplier has the option to provide a 5A secondary current rating, pending Tesla approval. If 5A secondary is provided, supplier to revise Slot Z – Current
- Inputs on the SEL-751 relay from “1” to “5” in digit 15 of the part number. ITI tape wrapped BCTs or equivalent. CT ratio to ensure that full load current is within nominal output of transformer.
- Transformer equipped with a load break switch to disconnect transformer from the MV energized circuit while maintaining the rest of the circuit energized. Operating handle shall have provisions to lock the switch on both the open and closed positions.
- If required on the datasheet the transformer shall be designed to HIGH seismic qualification level per IEEE 693. Frequency modifying devices shall not be allowed without prior approval.
- The certified report shall include calculations and summary of the results of the seismic analysis.
MV Transformer Low voltage cabinet (4hr system)
- Transformer LV bushings shall be located in two
separate LV cabinets in opposing transformer segments. 
Low voltage cabinet shall be provided with tin-plated spade-type bushings. The spacing of the connection holes shall be a standard 1.75” on center 2-hole NEMA spacing, per ANSI C57.12.34 figure 19.- Each compartment shall allow for adequate space to install half the parallel set of LV conductors as specified in transformer datasheet.
- The quantity of connection holes shall be adequate to install half the parallel set of LV conductors as specified in transformer datasheet
- Ground bar shall be adequately sized to terminate equipment grounding conductors from Megapacks.
- LV cabinet shall be supplied with front bolt-on covers or removable doors.
- Front covers or doors shall be provided with a 2” minimum size Infra-Red (IR) window, outdoor rated for the same environment as the transformer. The window shall be located to allow thermography scans of the transformer electrical connections without the need to opening the cabinet doors or covers. The IR window shall be equipped with a protective cover and security access key.
- All keys and locks provided should be keyed alike.
- Install two (2) 1-5/8” Unistrut rails for mounting a field network enclosure on the side of one (1) LV cabinet. The exact dimensions for the rails shall be determined during detailed design.
- LV cabinet shall have sill panel around the base of the LV cabinet to facilitate cable installation. The dimensions for the openings shall be determined during detailed design. Bottom sill shall provide unobstructed access to perform the cable installation with cables intended to be laid in place.
MV Transformer Low voltage cabinet (2hr system)
- Transformer LV bushings shall be located in (1) one LV
cabinet perpendicular to the MV cabinet segment. - Low voltage cabinet shall be provided with tin-plated spade-type bushings. The spacing of the connection holes shall be a standard 1.75” on center 2-hole NEMA spacing, per ANSI C57.12.34 figure 19.
- Each compartment shall allow for adequate space to install all of the parallel sets of LV conductors as specified in transformer datasheet.
- The quantity of connection holes shall be adequate to install all of the parallel sets of LV conductors as specified in transformer datasheet.
- Ground bar shall be adequately sized to terminate equipment grounding conductors from Megapacks.
- LV cabinet shall be supplied with front bolt-on covers or removable doors.
- Front covers or doors shall be provided with a 2” minimum size Infra-Red (IR) window, outdoor rated for the same environment as the transformer. The window shall be located to allow thermography scans of the transformer electrical connections without the need to opening the cabinet doors or covers. The IR window shall be equipped with a protective cover and security access key.
- All keys and locks provided should be keyed alike.
- Install two (2) 1-5/8” Unistrut rails for mounting a field network enclosure on the side of the LV cabinet. The exact dimensions for the rails shall be determined during detailed design.
- LV cabinet shall have sill panel around the base of the LV cabinet to facilitate cable installation. The dimensions for the openings shall be determined during detailed design.
- Bottom sill shall provide unobstructed access to perform the cable installation with cables intended to be laid in place.
Accessory Cabinets
A liquid level gauge, pressure gauge, and dial type thermometer each with dry type alarm contacts must connect to a separate NEMA 4 or NEMA 4X rated enclosure outside of the LV or MV compartments. The location of this enclosure must be such that, when open, the gauges can be read from grade level without the use of a ladder. Pressure gauges shall usePSI for units of measurement. Temperature gauges shall use degree Celsius for units of measurement & be installed in a dry, threaded well to allow removal of gauge for testing or replacement without spilling oil.
The transformer shall have an externally mounted SEL-751 relay (P/N: SEL-751001A0X1A0XA11C10 inside a NEMA 4 or NEMA 4X box. The relay shall be mounted on a swing-out sub panel. Transformer manufacturer shall include digital input connections to the relay from all transformer gauges (level, pressure, and temperature). Each additional relay connection points shall be wired to terminal blocks inside the enclosure to allow for future use without the need to connect to the relay directly.
All control wiring shall be stranded, tinned copper SIS wire, minimum #14 AWG, insulated for 600V AC, 90°C.
¾” metallic conduit chase shall be factory installed between the MV cabinet and SEL relay enclosure.
No more than two wires shall be terminated on any one terminal block or connection. Wire splicing is not permitted.
Terminal blocks shall be phenolic or polyamide block rated 600V and not less than 20A.
Identify each terminal on each block with a permanent designation.
Refer to Tesla provided FNE + RIB panel drawings.
Oil sampling port shall be external to any cabinet (MV or LV) and provided with a dedicated lockable cover. Preferred sampling location is near the LV Cabinet.
Learn more about Pad Mounted Transformer Accessories
Megapack 4800 kVA Transformer DATA SHEET–4HR Projects
Data Sheet
| DESCRIPTION | UNIT | DATA |
| Tesla Part Number | P/N | 1943161-0X-B |
| Enclosure rating | ANSI | Outdoor |
| Operating Frequency | Hz | 60 |
| Capacity | kVA | 4800 |
| Cooling Class | - | KNAN |
| Impedance @ nominal Voltage on Base Rating | Z% | 7.5% ±7.5% |
| Efficiency @ 100% of Maximum MVA 75℃ Rating | % | >=99.0% |
| Total losses at Max. MVA cannot exceed | % | 1.00% |
| Windings | 2-3 | 2 |
| Rated Voltage of MV Terminals (Nominal) | kV | 34.5 |
| Rated Voltage of LV Terminals (Nominal) | V | 480/277 |
| Basic Impulse Level MV Terminals (BIL) | kV | 150 |
| Basic Impulse Level LV Terminals (BIL) | kV | 30 |
| MV Winding Config | Delta/wye | Delta |
| LV Winding Config | Delta/wye | Wye |
| Loop or Radial System Design | - | Loop Feed |
| Livefront or Deadfront | - | Deadfront |
| Connection LV Terminals | (Cable or Bus Duct) | Cable, Compression lugs |
| Oil/Fluid | Type | Less Flammable Fluid |
| Type of Material Used in Coils | - | Al or Cu |
Standard Features and Accessories
- Nameplate
- Dial-type thermometer
- Liquid level gauge
- Pressure-vacuum gauge
- 1 in drain valve with sample valve
- Pressure relief valve
- 1 in upper fill/filter press connection
- Color: RAL 9016 Traffic White
- Medium Voltage Switch: Pad Mount type, pad-lockable
- SEL-751
- Internal CT’s
- NC alarm contacts on temperature, liquid level, and pressure/vacuum gauges and pressure relief device
- Seismic anchoring as per IEEE 693
- LV busbar on segments 2 and 4 (sides of the transformer)
- The structural support base for ground screws or similar support
- Structural base maximum anchoring/support points: 4
- IR window MV and LV Cabinets
Megapack 4800 kVA Transformer DATA SHEET–2HR Projects
Data Sheet
| DESCRIPTION | UNIT | DATA |
| Tesla Part Number | P/N | 1943164-0X-B |
| Enclosure rating | ANSI | Outdoor |
| Operating Frequency | Hz | 60 |
| Capacity | kVA | 4800 |
| Cooling Class | - | KNAN |
| Impedance @ nominal Voltage on Base Rating | Z% | 7.5% ±7.5% |
| Efficiency @ 100% of Maximum MVA 75℃ Rating | % | >=99.0% |
| Total losses at Max. MVA cannot exceed | % | 1.00% |
| Windings | 2-3 | 2 |
| Rated Voltage of MV Terminals (Nominal) | kV | 34.5 |
| Rated Voltage of LV Terminals (Nominal) | V | 480/277 |
| Basic Impulse Level MV Terminals (BIL) | kV | 150 |
| Basic Impulse Level LV Terminals (BIL) | kV | 30 |
| MV Winding Config | Delta/wye | Delta |
| LV Winding Config | Delta/wye | Wye |
| Loop or Radial System Design | - | Loop Feed |
| Livefront or Deadfront | - | Deadfront |
| Connection LV Terminals | (Cable or Bus Duct) | Cable,Compression lugs |
| Oil/Fluid | Type | Less Flammable Fluid |
| Type of Material Used in Coils | - | Al or Cu |
Standard Features and Accessories
- Nameplate
- Dial-type thermometer
- Liquid level gauge
- Pressure-vacuum gauge
- 1 in drain valve with sample valve
- Pressure relief valve
- 1 in upper fill/filter press connection
- Color : RAL 9016 Traffic White
- Medium Voltage Switch: Pad Mount type, pad lockable
- SEL-751
- Internal CT’s
- NC alarm contacts on temperature, liquid level, and pressure/vacuum gauges and pressure relief device
- Seismic anchoring as per IEEE 693
- LV busbar on segments 2 (1 side of the transformer)
- The structural support base for ground screws or similar support
- Structural base maximum anchoring/support points: 4
- IR window MV and LV Cabinets
4400 kVA Pad mounted transformer for Megapack 2 XL
Specifications
- Power Rating: 4400kVA
- Primary Voltage: 34.5kV Delta
- Secondary Voltage: 480V Grounded Wye
- Phase: 3-Phase (4 Wire)
- Frequency: 60 HZ
- Temperature Rise: 65 °C @ 46°C Ambient
- Cooling Class: KNAN
- %IZ: 6.50% +/- 7.5% @4400kVA
- BIL HV: 150kV
- BIL LV: 30kV
Transformer design
- To accommodate two Tesla Megapack 2XL’s on each side of the transformer, for a total of four Megapack 2XL’s.
- To meet the latest revisions of the following battery storage manufacturer Tesla transformer requirements:
- Tesla Megapack – Above Grade Transformers. Transformer to be designed to accommodate the 4-hour Megapack configuration.
- Tesla Megapack 2XL Design and Installation Manual
- Appropriately for the conditions of use including solar duty, ambient temperatures, and harmonic loading (k-factor of 2 or higher)
- To meet all applicable UL and IEEE standards including C57.12.00, C57.12.28, C57.12.34, C57.12.70, C57.12.80, C57.12.90, and C57.18.10
- To be pad mount, outdoor, UL nameplated
- With natural ester insulating fluid
- To be sealed-tank construction
- As dead-front loop feed HV configuration
- For step-down, bi-directional operation
- With continuous windings without splices
- With an electrostatic shield between the HV and LV windings and an externally accessible ground shield bushing
- With a NEMA 3R termination cabinet
- With a five position tap changer with +/- 2@ 2.5% HV taps
- With Bay-O-Net or internal expulsion and coordinated back-up current-limiting primary fuse protection
- One-piece 600A HV bushings in HV connection cabinet
- With a HV two position load break oil-immersed rotary switch
- LV Spade terminals to accommodate horizontal lug spacing of 2.0”
- With a 14-hole grounding bus plate and will accept NEMA two-hole ground lugs in the LV compartment
- HV Compartment to have a standard NEMA two-hole ground pad
- With liquid thermometer, liquid-level and pressure/vacuum gauges with Form C alarm contacts prewired to terminal blocks in externally accessible enclosure
- With a Schrader-type valve
- With a cover-mounted automatic pressure relief valve as required to prevent tank rupture
- Oil drain, fill, and oil sample and pressure relief valves
- With a nitrogen blanket
- With seismic anchoring provisions for the site indicated above and for 3/4" hardware
- With RAL-7035 color finish in accordance with C57.12.18-2014 and spare touch-up paint
- Rated for continuous operation
Reading on: The Impact of Impedance on Transformer Design
Transformer Factory Acceptance Test
Factory Acceptance Test shall include all tests required by IEEE C57.12.90 specifically the as listed below designated as “Routine” and “Design”.
| Tests requested by Tesla. Reference: IEEE C57.12.00-2015, Table 17 | No | Design |
| Resistance measurements of all windings on the rated voltage tap | Yes | Yes |
| Ratio tests on the rated voltage connection as listed on the nameplate | Yes | No |
| Polarity and phase relation tests on the rated voltage connection | Yes | No |
| No-load losses and excitation current at 100% of rated voltage and at rated power frequency on the rated voltage tap connections | Yes | No |
| No-load losses and excitation current at 110% of rated voltage and at rated power frequency on the rated voltage tap connections | No | Yes |
| Impedance voltage and load loss at rated current and rated frequency on the rated voltage connection, and at the tap extremes of the first unit of a new design | Yes | No |
| Operation tests of all devices | Yes | No |
| Control (auxiliary) and cooling consumption losses, if applicable. | Yes | No |
| Temperature rise at minimum and maximum ratings of the first unit of a new design | No | Yes |
| Dissolved gas in oil analysis | Yes | No |
| Audible sound level | No | Yes |
| Winding insulation resistance | Yes | No |
| Lightning impulse | Yes | Yes |
| Low frequency - Voltage Applied | Yes | No |
| Low frequency - Voltage Induced | Yes | No |
| Partial discharge test | No | Yes |
| Lifting and moving devices | No | Yes |
| Oil Leaks | Yes | No |
| Current Transformer Factory Acceptance Testing | Yes | No |
| Turns ratio & phase angle accuracy test. | Yes | No |
| Winding resistance test | Yes | No |
| Polarity test | Yes | No |
| Excitation test | Yes | No |
| Insulation resistance test | Yes | No |
| Relay acceptance test | Yes | No |
| Power up relay | Yes | No |
| Check wiring from CT to each relay input to ensure phase & polarity is correct | Yes | No |
| Check wiring from each alarm contact to the relay through the SEL HMI program | Yes | No |
(*) Note: The routine column refers that all units shall be tested. The design column refers that only 1 unit shall be tested.
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Medium Voltage cabinet
Medium voltage enclosures shall be in ANSI Outdoor or greater enclosure.
Conform to ANSI C57.12.28, with doors that are capable of being removed from the hinges during transformer service.
Be provided with LOTO capabilities for disconnect switch.
All keys and locks provided should be keyed alike.
Doors shall be provided with a 2” minimum size Infra-Red (IR) window, outdoor rated for the same environment as the transformer. The window shall be located to allow thermography scans of the transformer electrical connections without the need to opening the cabinet doors or covers. The IR window shall be equipped with a protective cover and security access key.
Deadbreak bushings to be mounted at a minimum of 4 feet from transformer base.
Equipment Marking and Labeling
Signage shall be provided on all electrical equipment in accordance with Applicable Law, including NEC and NFPA 704 requirements.
Signage shall be weather-proof, corrosion-proof, UV-stabilized, and fade-resistant and
shall be capable to last the duration of the minimum design life.
Signs shall be attached using non-corrosive materials suitable to meet the Minimum Design Life.
Transformer shall have an engraved permanent identification label visible and readable from a distance of 48 inches that provides the unique identification number as indicated on the electrical drawings and in the LOTO procedure.
All equipment identifications shall be unique, and match provided design documents.
Equipment identifications shall be consistent across all references.
Permanent labels shall be suitable for wet locations and rated for UV resistance if exposed.
Structural Support Base
The transformer shall be provided with a steel structural support to allow direct transformer installation on top of ground screws, helical piles, driven piles or similar type of foundations.
Drawings shall indicate locations for anchor bolts.
Conclusion
The pad-mounted transformer serves as the ideal companion for the Tesla Megapack in BESS projects. If you find yourself uncertain about selecting the right transformer for your Tesla Megapack BESS project, drop Daelim Transformer a line! We are committed to helping you acquire the transformer necessary to energize your BESS project effectively. Trust in Daelim Transformer to provide the expertise and support you need to make your project a success.
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