MiniSub – Dry Type

This dry type design that Power Systems has developed a unique design dry-type (air insulated) padmount transformers as defined by the CSA specification C227.4, and SF6 switching technology using the Arc-Whipper© three position switch.

The concept of the dry units is “ Distributed Energy” Spreading out the KVA over the building by using the primary incoming voltage to bring power to smaller substations located where the loads are.


  • Compact size only slightly larger than a standard transformer alone
  • Includes up to three primary switches and protection
  • Maintenance free SF6 switching technology
  • Voltage ranges up to 15kV
  • Selection of low voltage breaker and metering options
  • Increased Safety and Reliability
  • All assembled and tested as one unit

Sample Design Comparison



Power Systems Technology uses the most reliable, technologically superior power transformer design in the market today.

This is as a result of continuing development in all phases of the design and manufacturing processes in meeting the requirements of a broad range of customers and the most onerous applications.

Additionally, these transformers have demonstrated improved withstand ability to short circuits and will provide a life expectancy considerably in excess of current industry standards.

Core Construction

Cores are manufactured from high quality non-ageing, cold rolled, grain oriented silicon steel laminations. Cores are precision cut to close tolerances using modern equipment, to eliminate burrs and minimize losses. They feature step lap, miter cut, stepped core construction

Coil Construction

Coils are either layer (barrel) or disc wound to suit the voltage class. Typically, disc wound coils are utilized above 8.7 kV class. Available in aluminum or copper, windings are designed with wire or foil conductors for optimum performance for the application.

Disc wound coils feature comb construction to electrically balance voltage stresses over the full length of the coil. All internal connections are brazed or TIG or MIG welded.


Typically dry type power transformers are manufactured with a class H, 220°C insulation system. High temperature resistant materials are used including Nomex Aramid papers, silicone coated fiberglass, Nomex sleeving, supersil duct sticks and pressure sensitive glass tape. All materials meet or exceed UL standards for use on dry type power transformers.

Core & Coil Assembly

As a completed assembly, the coils are held rigidly in place between insulators clamped to the upper and lower core frames under high compression.

SF6 Switchgear

Visible break conforms to widely accepted operating safety practice, giving clear visual confirmation of contact position of ALL THREE PHASES through the viewing windows in the tank. The large open gap allows field voltage testing without cable removal.

Manual, Remote or Automatic Switching

Arc Whipper® is equipped for manual operation as standard and includes: External handles for operation by hand; quick make/break spring operators; bushing oriented line diagram; positive position indicators; padlock provisions; and operating instructions. Remote operation can also be provided by motor operators with pushbutton or SCADA control. Automatic transfer switches use fast acting motor operators with voltage/current sensing and full function control. Units are factory wired/adjusted

Construction Features
Arc Whipper® SF6 loadbreak switches are ruggedly constructed, incorporate proven design and are rated/tested in accordance with ANSI standards C37.71-1984 and C37.72-1987 for long life with minimal maintenance. Switch contacts are equipped with very high speed auxiliary arcing contacts to quickly interrupt in ¼ to ½ cycle, leaving main contact surfaces undamaged by arcing. The arcing contact allows main contacts to be designed for high continuous, fault close and momentary current performance. Switches are factory filled with non-toxic, non-flammable, high dielectric SF6 gas insulation. SF6 quality and stability are maintained by use of totally sealed tank construction, an internal purifying absorbent and the extremely fast Arc Whipper® interrupter. A color coded, field replaceable pressure gauge and self-sealing fill valve allow monitoring and addition of SF6 as necessary.

Switch tanks are ¼” thick steel with a highly corrosion/abrasion resistant 5 mil epoxy finish. External fittings and fasteners are marine grade stainless steel or bronze. For increased corrosion resistance, stainless steel tanks can be
provided. Current carrying parts are high conductivity electrolytic grade copper with plating, locking fasteners, and torque wrench assembly to assure permanent low resistance connections. Moving contacts are self-aligning, self cleaning (wiping type) and designed to increase contact pressure with increasing currents. Contact assemblies are supported by high strength, molded glass polyester insulation with flex connectors at critical locations to prevent stressing and misaligning due to high current or mechanical forces.

Arc Whipper® SF6 loadbreak switches are designed, tested and rated per applicable sections of IEEE, ANSI, NEMA, CSA and other industry standards including:

ANSI C37.71-1984 Standard for Subsurface Load Interrupting Switches.
ANSI C37.72-1987 Standard for Padmounted Load Interrupting Switches.
ANSI/IEEE 386 Standard for Separable Connectors and Bushings.
IEC 265 International Standards for Load Interrupting Switches.
ASTM D-2472 Specification for Commercial Type Electrical Grade SF6.
ANSI C57.12.28 Standard for Padmount Enclosures.
Note 35 kV information available on request

Application Information
Arc Whipper® SF6 loadbreak switches are completely submersible, fully deadfront, and corrosion resistant for application in the harshest conditions including areas subject to flooding, severe contamination, rodents and snakes, blowing snow/dust, sea coasts, high altitude, and even Division 2 hazardous locations. Non-toxic, non-flammable SF6 insulation permits installation indoors or outdoors in high traffic areas. Compact size and reduced weight provide solutions where overall space is important such as in high-rise buildings, front lot lines and in small vaults.

Switch Operators and Options
Arc Whipper® SF6 load break switches are equipped with external handles for manual operation as standard. The handle can be padlocked in all switch positions and has an anti-reversing feature for additional safety. Interlocks can be provided as may be required by local practice and for increased safety. In addition, remote/SCADA and fully automatic operators are available for increased levels of control and system automation. These motor operators attach to the switch handle and may be easily disconnected for manual operation.

Manual Operation
The handles are removable and can be attached to the switch operating shaft through an arc of 360 degrees in 45 degree increments for maximum mechanical advantage during operation or may be removed and stored on the padlockable hanger bracket when not in use. Rotating the handle toward the desired position causes quick make/break, spring operated contact movement independent of the operating handle movement. Latches prevent contact blow off and provide positive position indication. Additional features include: padlock provisions in all positions, bushing oriented line diagram and instructions. Three position switches are equipped with anti-pass through provisions to prevent inadvertent operation to the third position.

Key Interlocks
Are available as required to prevent paralleling feeders or to limit access to energized equipment or to otherwise
coordinate proper system operation. Keylocks are mounted on the specified switched ways with locking provided for the required position.

Automatic Load Transfer Switches
The MiniSub is ideal for maintaining power to critical loads such as hospitals, airports, shopping centers, and industrial facilities. Upon loss of preferred primary feeder voltage, the load is automatically transferred to an alternate source. Optional automatic retransfer, overcurrent detection and fault isolation provide additional system flexibility. Units include fast acting 120 VAC motor operators attached to the switch operating handles. Voltage sensing is done through a set of elbow mounted detectors and relay combination which supply data to the full function automatic control with self-contained 120 VAC/24VDC control power source. All items are factory assembled, wired, adjusted, and tested for simplified installation and operation.

Visible Break, Make and Ground
Each switched way is equipped with a viewing window for clear, visual confirmation and assurance of contact position of ALL PHASES. Conforms to local code and operation practices which require visible break and (where equipped) visible grounding during system operation, maintenance, repair, or testing.

Anti-reverse Operating Handles

Eliminate the possibility of interrupting currents beyond the capability of the switch by preventing immediate operation after closing on a fault. A latch on the operating handle must be reset prior to subsequent operation allowing upstream circuit protection time to clear the fault.

Primary Cable Terminations
ANSI/IEEE standard 386 defines the specific interfaces dimensions that 200 and 600 amp, elbows, inserts and equipment bushings must conform to insure interchangeability. The 200 amp load break, and 600 amp non load break connectors provide a convenient and safe way to connect all primary cables. With the use of a Raychem PILC kit, lead cables can be connected using the same style. While the cables and terminations are barriered from access, the elbow test point offers an additional security and safety, as well as a voltage reference for automatic or SCADA type operations This termination provides a deadfront waterproof connection.

Low Voltage

There is a wide variety of accessories available in the secondary compartment. This is specific to each project and designer’s needs.

This can include:

  • Digital metering for customers or utilities
  • Separate metering access door
  • Main Breakers
  • Ground fault protection
  • Feeder Breakers

The Connection

This unique connection between the SF6 switch and the transformer allows the switch(s) to be plugged in for a water tight connection. This feature allows the switch to be removed in the field for service, if required. At transformer can be replaced with this method in a few hours. This connection maintains the submersible criteria of the MiniSub unit.