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1941-1943 - new type mobile transformer power unit capable of supplying electricity for a city of 70,000 population


CD 1966096 E&MP34.030

Electric Power Plants

December 11, 1941 - 1943

Not a war machine of destruction, as it appears, but first of a new type mobile transformer power unit capable of supplying electricity for a city of 70,000 population.

This is one of six such units being made by General Electric at its Pittsfield Works for the American Gas and Electric Company.

Each will be mounted on a specially designed railroad car so that they can be readily transported to any locality where invading bombers or other accidents might interrupt or cause an emergency power shortage.

What appears to be guns protruding from the "tank" are huge porcelain insulators through which connections are made with the electrical power systems.

additional related text found


Fig. 1.

The G-E factory-built portable substation - valuable for seasonal and planned temporary service and as a fast-stepping pinch-hitter in emergencies.

CENTRAL-STATION operators speak enthusiastically of the portable substation units employed on their systems. The ready portability of these units not only permits their quick installation whenever and wherever needed, but also contributes to decreased operating expense by making a single unit sufficient for many types of temporary and stand-by service along far-flung points of the distribution system. The most common of these are:

  • 1. Emergency use - to continue the service of a regular substation in time of trouble.

  • 2. Stand-by service - to handle seasonal and other types of temporary overloads.

  • 3. To continue service during inspection or repair work.

  • 4. To supply power for construction jobs and numerous other temporary power needs.

Installation is merely a matter of parking and grounding the unit, elevating the disconnecting switch structure and running conductors to the high-voltage and low-voltage lines.

Fig. 2.

A power company built this portable substation, using a G-E single-circuit unit substation arranged for connection to overhead circuits.

GENERAL ELECTRIC offers fully equipped portable substations, each assembled on a trailer, forming a compact, portable unit that is ready at a momentís notice to take the road to the job.

These units are designed along the conservative, dependable lines characteristic of the standard G-E factory-built substation units - so many of which are in successful operation - with modification based on a detailed study of portable-substation requirements, such as flexibility of application and compliance with state-highway regulations.

Fig. 1 shows a design in which the equipment is especially arranged for maximum portability. Starting with the incoming high-voltage lines (at the right), it includes:
  • 1. A novel, high-voltage, air-break disconnecting switch on a mounting which can be lowered for transportation and conveniently raised to a safe overhead position for operation.

  • 2. High-voltage lightning arresters for protection against overvoltage.

  • 3. A three-phase power transformer with load-ratio-control equipment, if desired, for voltage regulation.

  • 4. Low-voltage circuit breaker.

  • 5. Low-voltage metering and protective equipment.

  • 6. Low-voltage lightning arresters.

Experience has indicated the desirability of two general types of portable substations to provide the desired balance between size, weight, and output - one using a self-cooled transformer, and the other a forced-cooled transformer.


A self-cooled portable-substation unit is especially suited for the smaller sizes and, as it requires no attention, it is ideal for duty that may include extended periods of continuous service. All the desirable features of a nonportable substation unit, such as voltage-regulating equipment, can be included, as in Fig. 1.

In some cases, the requirements are filled by a standard substation unit, modified but slightly and trailer-mounted, as in Fig. 2. Such equipments have been furnished for use as spare units for other substations on the same system. If desired, the unit can be easily removed from the trailer and interchanged with another similar unit.

The core and coils are placed upright. The drop section in the center of the trailer provides a low center of gravity while maintaining sufficient road clearance.

A gross weight of 35,000 pounds is the legal limit of most states. This self-cooled unit, with all the features of a standard substation unit, will meet this limit in ratings of 1000 kva at 33,000 volts, and 1500 kva at 13,200 volts, with a 25-per-cent increase in rating available if fans are added for air-pressure cooling.


With an extremely compact, high-capacity forced-oil-cooled transformer, it is possible to provide minimum weight and size for a given rating. This unit, therefore, is ideal for rapid emergency service. Because these units may frequently be called upon to travel long distances, at fairly high speeds, over secondary roads, and across bridges which limit the gross weight to ten tons or less, they are specially designed for portability.

The transformer core-and-coil assembly is placed on its side to meet limiting dimensions. Uniform cooling is obtained by forcing the oil through ducts between the coils.


Fig. 3

Another portable substation for which General Electric supplied equipment. equipment includes a totally enclosed oil pump and motor, and a cooler with fans. For cases which require the highest capacity consistent with road limitations, designs are available which dispense with voltage-regulating and other accessory equipment.

Because the core-and-coil assembly is placed on its side, a low center of gravity can usually be obtained with a straight-frame trailer. This compact forced-cooled unit will meet a weight of about 20,000 pounds, with a rating of 1500 kva at 33,000 volts, and 2000 kva at 13,200 volts.


The major equipment of both the self-cooled and the force-cooled types closely corresponds, except for arrangement, to that used with the G-E single-circuit substation unit.* Load-ratio-control equipment, when desired, is mounted in a separate oil-filled compartment. The low-voltage circuit breaker is also placed in a separate compartment, as are the breaker control and the low-voltage metering equipment. These compartments, together with valves and operating handles, can be locked to prevent operation by unauthorized persons.

When space limitations permit, the standard oil conservator can be furnished. Otherwise, the standard automatic gas-seal equipment is recommended. Where close supervision of the unit is expected, the gas-oil seal or the hermetic gas seal can be furnished. The hermetic gas seal is desirable in cases where extremely close weight limitations apply.

A relief diaphragm is provided for protection against excessive internal pressure that might be generated by a sustained short circuit. A hinged door is reclosed by a spring, when the diaphragm breaks, to prevent entrance of moisture.

Standard pellet-type or Thyrite-type lightning arresters, adapted to the voltage and the nature of the service, are mounted adjacent to the high-voltage as well as the low-voltage bushings. Metal screening is regularly provided to guard against accidental contact with live parts.

The portable substation is arranged for symmetrical weight distribution on the trailer in order to provide stability when on the road. The complete substation is assembled as a unit and is so secured to the trailer body that strains and deflections in the latter are not transmitted to the substation structure.

The trailer is designed to meet all state-highway and I.C.C. regulations. It can be turned in a very small circle. The large steering circle on the front axle maintains stability when turning, and, together with a special reach rod used for connection to the service truck, prevents weaving on the road. Low-pressure dual tires and two-line vacuum air brakes are regularly furnished. There are mudguards over all wheels.

The transformer, accessory equipment, and trailer of these portable substations comprise a single co-ordinated unit, which is completely assembled at the factory. This means that the manufacturer is able to design and arrange all component equipment in such a way as to provide the maximum practical degree of safety and dependability of operation in units of minimum practical weight and dimensions.

Portable-substation units of the two general types described above are suited to a range of applications sufficiently wide to assure that, in all cases, a unit closely adapted to the needs of the job is available. The ratings mentioned above are typical and others are available to meet unusual requirements. We shall be glad to prepare, on request, detailed specifications for any specific application.

Completely described in Bulletin GEA-2840

Here are a few of the pioneer portable substations for which General Electric supplied equipment. The experience gained in the manufacture of early units such as these has contributed greatly to the development of General Electric portable substations, which meet the requirements of the broad range of present-day applications. 

Original Caption by Science Service
©General Electric

National Museum of American History


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