A mobile substation is a completely self-contained trailer mounted unit comprised of transformer, cooling equipment, high and low voltage circuit protection, metering, relaying, AC and DC auxiliary power supplies and surge protection. Mobile substations are good option, especially in rural area or isolated areas where electricity is difficult to access. The mobile substation or portable substation is fully equipped with different power system equipment, and must be able to be shipped from one place to another with fully assembled mode.

Mobile Substation Manufacturing 

Once need is established, there are a number of different areas to be investigated before a specification is written and a proposed listing of equipment is developed. For our blog page please click link

Selection of Mobile substation equipment:

Deciding substation equipment and their ratings is important task of substation design. Selection of voltage level plays an important role in designing substation and its equipments. Major parameter considered in selection of voltage level depends on amount of power to be transferred. Depending upon requirement, mobile substation is placed for temporary service, which has to match the system voltage level. Main switchyard equipment are

1. Conductor

2. Insulator

3. Lightning arrester

4. Bushings

5. Current transformers

6. Voltage transformer

7. Disconnector / Earthing switches

8. Circuit breaker

9. Central control cubide

10. Drive mechanism

11. Power transformer

12. Vacuum circuit breaker

13. Battery

14. Air conditioner

MVA Rating of Mobile Substations

For facilitating road transportation mobile transformer attached to mobile substation are generally manufactured within limit of 60 MVA rating. Generally single incoming and single outgoing feeder is preferred. Current rating of incoming or outgoing feeders should be less than 2000A otherwise the transformer cooling system and associated equipment rating may matter the mobility of substation. Hybrid  Insulation is usually used for most mobile substations. Multiple voltages requirement (HV and LV) will add size and weight to the unit, thus reducing MVA rating of the transformer and mobile substation. LTC requirement will add weight and size to the mobile unit, thus reducing MVA rating of the transformer and mobile substation. Bottom mounted bushings to reduce overall height. Compact construction by means of an optimized tank design. ODAF cooling for the transformer. HV BIL reduction have great impact on mobile transformer weight and size reduction. LV BIL’s usually are maintained at full level up to 25 kV, 150 kV BIL. Impedances for mobile transformer is much higher than a conventional unit. Typically they are 2 times the ANSI standard values. Losses are not evaluated for mobile transformer. The losses are much higher than conventional unit. Mobile transformer noise levels are typically NEMA standard. Lower noise levels should be avoided if possible because the cooler size and weight becomes excessive. The LTC is typically on the LV side for LV regulation. For multiple LV voltages, it is advisable to go with 2:1 ratio’s LV’s to avoid the requirement of a series transformer for the LTC. Because this will add weight and size to the transformer.

Mobile Substation SMS Series

Power Transformer:

75°C or 95°C rise unit. This provides maximum capacity while keeping the weight to a minimum. Upgraded thermal insulation systems permit the higher temperature rise without sacrificing the life expectancy of the transformer (NOMEX Design). ODAF cooling consists of transformer oil coolers and circulating pumps. The cooling is automatically started by energizing the transformer. Without the coolers in operation the unit cannot carry load. The oil preservation system is sealed tank type with an optional inert gas system.

Conservator designs can be considered for 550kVBIL or higher when road height allows. The transformer tank will withstand 15 PSI and full vacuum. Polymer bushings can be substituted for weight savings. Where there is multiple high voltage, externally operated de-energized switches are furnished to make series-multiple connections. In a similar manner a low voltage series-multiple or delta-wye switch can be furnished with the operating mechanism external to the tank to eliminate having to open the transformer.

Several switch options are available to increase the functionality of the transformer:

1. LTC

2. DTC

3. Series/Parallel

4. Delta/ Wye

Transformer cooling:

The heat exchanger system consists of one or two Brand X coolers. These are mobile transformer type oil coolers complete with copper liner tubes and aluminum extruded fins, pumps and fans. The system incorporates two pipe systems for the flow of the oil. The heat exchanger is designed to provide the maximum cooling efficiency. The cooling system can consists of two pumps and multiple fans . When one pump shuts down, the cooling system is capable of providing adequate cooling for continuous operation at 70% rated load indefinitely with no adverse or abnormal affect on the transformer. Pumps and fans shall not be controlled by winding temperature and must be energized during normal operation. All piping between the transformer and the heat exchanger will be seamless metal and incorporate stainless steel flex joints to allow for vibration and expansion without any adverse effects. Pipe fittings and valves are suitable for high pressure applications.

High Voltage section:

This section typically includes a combination of the following:

  • Circuit Breaker and Disconnect
  • Circuit Switcher
  • Power Fuses
  • Surge Arresters

Circuit Breaker and Disconnect Switch can be on a separate trailer (typical for 115kV and higher) .Circuit Switcher can be on a separate trailer, typical when the weight and dimensions of the main unit exceed the specification or D.O.T. limits.

For voltages up to and including 38kV, MSU typically equipped with either a fused disconnect switch or vacuum circuit breaker with disconnect switch .For voltages up to 69kV, the choice is typically a fused disconnect switch, circuit switcher, SF6 circuit breaker.

Low Voltage Section:

This section generally includes:

  •  Surge Arresters
  • Potential Transformers
  • Current Transformer
  • Circuit Breaker and Disconnect
  • Recloser
  • Station service transformer
  • Cable Reels
  • Circuit Breaker is normally vacuum circuit breaker.
  • Disconnect switches can be gang operated type or Hooksticks.


This section generally includes :

The controls, protection and metering is mounted in an outdoor cabinet. There are many protective relay’s available. Transformer differential protection, over-current relays and sudden gas pressure relays are common. As the substation becomes more sophisticated so does the relaying.

  • Main Control Panel (NEMA 4)
  • Battery and Charger (Lead Acid or Ni Cd)
  • Inverters

Advantages of Mobile Substations:

1) Mobility in service

2) Quick access to service

3) Less maintenance cost

4) Increase in reliability

5) Maximum KVA with minimum height

Applications of Mobile substations:

1) Mobile substations are employed to provide emergency power supply during in unexpected situations.

2) It is also used to supply power when ordinary conventional substation is under maintenance or fault.

3) It is employed for providing temporary power supply to industrial plants in remote locations.

4) Mobile substation are used to provide electricity in remote and difficult area where conventional power cannot be provided.