SIERRA SERIES – up to 36KV Prefabricated Compact Substation

SIERRA SERIESSIERRA II series, type tested prefabricated compact substation, suitable for secondary distribution.

No costs for on site assembly and minimal civil engineering. The small profile makes SIERRA II easy to install discretely in sensitive locations. It guarantees an high level of personal safety.


SIERRA II is totally enclosed metal cladding, self-contained and weatherproof, ready to be placed upon a concrete base pad.

  • The unit is typically divided into three rooms for the medium voltage switchgear, low voltage switchboard and distribution transformer.
  • The transformer is located in the middle of the substation while the MV and LV compartments are located at both ends of the substation adjacent to the corresponding bushings of the transformer.
  • The substation may be operated from the outside (Non Walk-in Accessibility Type B).
  • The electrical equipment is cooled by natural ventilation through openings in the substation.
  • The unit is mainly made of corrosion resistant 2mm AlZn-coated steel sheet. The aluminium and the zinc alloy coating provide protection against corrosion due to harsh environment conditions. In addition, 3mm galvanized steel sheets are used to make the support in the base frame (oil collection pit) stronger.
  • The oil collection pit, which can take 100% of the transformer’s oil volume, is integrated into the substation base frame.
  • Enclosure, with closed door is IP43. Doors can be fitted with gaskets in order to have degree of protection IP54.
  • Last finish coat is a powder paint of polymer resin base.

MV Compartment

This compartment can be equipped with wide varieties of MV switchgear. For voltages up to 24kV, the basic configuration is a SF6 insulated RMU which consist of two load break switches and one switch-fuse combination for transformer protection, all contained in a single sealed tank. Same configuration is also available for voltages up to 36kV.

Transformer Compartment

This compartment is specially designed to provide the transformer with adequate airflow cooling through well sized openings. The compartment is provided with a pressure relief system integrated in the ceiling/roof. As an option, a floor mesh can be provided to prevent entry of small animals and other foreign objects, assuring in the meanwhile continuous airflow and circulation. Oil-immersed or dry type transformers can be accommodated on this compartment.

Low Voltage Section

This compartment contains the Low Voltage Distribution Board (LVDB), which is fed from the secondary side of the transformer. The LVDB can be designed in different arrangements and configurations in order to suit any specific application. Inside the panel can be installed Air circuit breakers (ACB’s), Moulded case circuit breakers (MCCB’s), LV Fuse switches Instrumentation and metering


Natural ventilation is provided through ventilation for air intake at the bottom of the walls. The air is extracted through ventilation openings at the top of the walls, and natural air circulation ensures sufficient cooling to the distribution transformer.

As standard SIERRA II is classified and type-tested in accordance with temperature class K20. Temperature class K10 can be achieved by increasing the ventilation area for the transformer room.

Ground System

The substation forms a complete metallic bolted structure. Each compartment is provided with grounding busbars. Bonding and interconnection of the grounding buses are made of 70mm2 bare stranded copper conductors.

The ring main unit (RMU) and the low voltage panel (LVDB) are provided with grounding bus that can be connected to the grounding system at site.

Standards Reference

The SIERRA II range is designed and type-tested in accordance with EN 61330, which covers the following test programs:

  • Test to verify the insulation level of prefabricated substation
  • Test to prove the temperature rise of the main components contained in a prefabricated substation
  • Test to prove the capability of the earthing circuits to be subjected to rated peak and rated short-time withstand current
  • Functional test to prove satisfactory operation of the assembly
  • Test to verify the degree of protection
  • Test to verify the withstand of the enclosure of the prefabricated substation against mechanical stress
  • Additional type test in accordance with EN 61330 and PHELA Recommendation n.7 05/2004
  • Test to assess the effects of arcing due to internal fault.

Main components are type-tested in accordance with their individual standards:

Medium-voltage switchgear

EN 60265: High-voltage switches
EN 60298: Metal-enclosed high-voltage switchgear
EN 60694: High-voltage switchgear

Low-voltage switchboard

EN 60439: Low-voltage switchgear and control gear

Distribution transformer

EN 60076: Power transformers

Compliance to other applicable Standards and Council Directives is indicated on the Declaration of Conformity draft in accordance to EN 45014 “General criteria for suppliers declaration of conformity”.

Moreover WE WALTER has adopted a quality management system for engineering, construction and testing processes compliant with the requirements of EN ISO 9001:2000.

Transformer Rated PowerkVA630-12501600-2500630-1250
Primary Rated VoltagekVA12-2412-2424-36
Internal arc withstand current 1"kA161616
Overall dimensionsL (mm)300030003000
W (mm)210021002100
H (mm)234026402640