Compact Medium-Voltage Substation: Parts, IAC, Testing

Short answer

A compact (prefabricated) substation combines, in a type-tested enclosure, the medium-voltage switchgear, the transformer, the low-voltage panel and their connections. IEC 62271-202 covers it up to 52 kV on the medium-voltage side and requires type tests, among them the internal arc test, because it is installed in places accessible to the public.

What it is and where it is found

A compact substation, also called a prefabricated substation, compact transformer station or kiosk, is the point where the medium-voltage network is converted to low voltage for a building, a neighborhood, an industrial plant or a solar plant. IEC 62271-202 defines it as a type-tested assembly that contains, in one enclosure, transformers, medium- and low-voltage switchgear, connections and auxiliary equipment. It covers cable-connected substations with a rated voltage above 1 kV and up to 52 kV on the medium-voltage side, for outdoor use and in places accessible to the public. The 2022 edition changed the title to “prefabricated alternating current substations” and extended the scope to those that contain only medium-voltage switchgear.

On the single-line diagram, the incoming medium-voltage cable enters the switching cubicle; from there it goes to the distribution transformer, and the secondary feeds the low-voltage panel, which distributes to the users.

Main parts

Enclosure: of concrete, metal or composite materials, with ventilation and, where applicable, a pit for the transformer oil.
Medium-voltage switchgear: normally a ring main unit (RMU) with load-break switches for the ring incomers and outgoers and, for the transformer, a switch-fuse combination or a breaker with relay.
Transformer, oil-immersed or dry-type.
Medium-voltage (cables or busbars) and low-voltage connections.
Low-voltage panel with its main breaker and outgoing feeders; see low-voltage panel and MCC.
Auxiliaries: lighting, metering, sometimes a current transformer (CT) for measurement and telecontrol equipment.

How it works

The assembly is designed and tested as a unit, because the enclosure changes the behavior of each component:

Thermal: the transformer dissipates its losses inside a closed space; the inside air heats up and the transformer runs hotter than in open air. The enclosure class expresses that extra heating according to the type test, and from it the permissible load factor is calculated; IEC 62271-202 covers it for oil-immersed and dry-type transformers.
Internal arc: if an arc occurs in the cubicle or in the connections, the hot gases and pressure must escape where there are no people. The internal arc test verifies the IAC classification, with accessibility A (authorized personnel), B (public) or AB.
Dielectric and short circuit: the connections and the enclosure are tested for short-time and peak withstand currents and for the rated insulation levels.

The operating sequence is imposed by the RMU interlocks: the earthing switch cannot be closed with the load-break switch closed, nor can the cable compartment be opened without the earth closed.

Variants

With interior access (walk-in): the operator enters a corridor to operate.
Without interior access (non-walk-in): operated from outside with the doors open; it is more compact.
Surface, semi-buried or underground: IEC 62271-202 accepts all three.
Pad-mounted: the North American model, in which the transformer has its medium- and low-voltage compartments attached; it is specified according to IEEE C57.12.34.
Industry and solar plants: transformer stations in a container or on a structure (skid), with switchgear, transformer and panel, specified with the same criteria. At a solar plant the assembly works the other way round from distribution: it receives the energy from the inverters at low voltage and steps it up to the collector network; its parts and standards are in transformer station of a solar plant.

The compact substation is for medium-to-low voltage only. At higher levels, the equivalent is medium-voltage switchgear in a building and the GIS substation.

Rated characteristics and nameplate

IEC 62271-202 defines, in addition to the common characteristics of switchgear:

Rated voltage and insulation levels on the medium-voltage side.
Rated current and temperature rise.
Permissible short-time and peak withstand currents.
Rated power of the substation, which the 2022 edition defines with three parameters.
Enclosure class.
Internal arc classification (IAC): accessibility, arc current and duration.

The assembly nameplate identifies the manufacturer, type and serial number, the standard and these characteristics. Each component (RMU, transformer, panel) also carries its own nameplate.

Integration with protection and control

Transformer protection is usually by current-limiting fuses in the switch-fuse combination, coordinated with the transformer; see medium-voltage fuse. For larger ratings a breaker with a 50/51 and 50N/51N overcurrent relay is used, which can be self-powered from its CTs. The criteria for choosing between the two solutions are in transformer protection with fuse or breaker, and the functions and power supply of the relay in transformer cubicle relay. The transformer contributes its temperature trips and alarms, which in hermetically sealed units can be grouped by an integral DGPT2 relay, and surge arresters protect the cable incomer.

When the substation is remotely controlled, a controller or RTU reports the position of the load-break switches, the alarms and the fault passage indicators. In IEC 61850, load-break switches are modeled as XSWI, the breaker as XCBR and the control as CSWI.

Commissioning and maintenance tests

Inspection: assembly, ventilation, pit, grounding and interlocks.
RMU: contact resistance, operations, interlocks and gas pressure; see RMU testing.
Cables: VLF or applied voltage test and partial discharge.
Transformer: ratio, winding resistance, insulation and, for oil-immersed units, dielectric strength.
Protection: fuses of the correct rating, or relay injection and actual breaker trip.
Low-voltage panel: insulation, trip units and phase sequence.
Maintenance: thermography, cleaning, ventilation and moisture checks, and partial discharge.

Common failures and what to check first

Poorly made medium-voltage cable terminations.
Condensation from poor ventilation, with surface discharges.
Transformer overloaded by not considering the enclosure class.
Fuses of the wrong rating or type, or a blown fuse whose striker did not open the load-break switch, leaving the transformer on two phases.
Damaged or bypassed interlocks.
Loose low-voltage connections.

After a trip, check first the fuses or the relay record and the condition of the cable terminations. The switching order for energizing and de-energizing the assembly is in energizing a compact substation.

Testing and commissioning with Tiza Group

For the complete assembly we perform the tests and commissioning in substations, industry and solar plants, in Colombia or elsewhere on request: write to us.

Fuse or breaker to protect an MV transformer

References

IEC 62271-202:2022, Prefabricated substations for alternating current.
IEC 62271-200, Metal-enclosed switchgear and controlgear for 1 kV to 52 kV.
IEC 62271-103 and IEC 62271-105, Load-break switches and switch-fuse combinations.
IEC 60076-1 and IEC 60076-11, Power transformers and dry-type transformers.
IEC 61439-2, Low-voltage switchgear and controlgear assemblies: power assemblies.
IEEE C57.12.34, Three-phase pad-mounted distribution transformers.

Frequently asked questions

What does the enclosure class of a compact substation mean?

It indicates how much extra heating the transformer undergoes inside the enclosure compared with operating in open air, according to the IEC 62271-202 type test. The higher the class, the more the transformer load has to be reduced so as not to exceed its temperature limits.

What is the difference between a compact substation and a pad-mounted transformer?

The IEC 62271-202 compact substation is an enclosure that contains switchgear, transformer and panel as a type-tested assembly. The pad-mounted unit, common in North America, is a distribution transformer with medium- and low-voltage compartments attached to its tank, specified according to IEEE C57.12.34.

What does an IAC AB classification mean on a compact substation?

That the internal arc test demonstrated protection for the operator, authorized personnel (A), and for the general public (B), on the sides tested, with the arc current and duration stated next to the classification.