There are many devices that can be found in the substation.

One of the categorizations for substation is between primary and secondary substation equipment.

Which device types belong to each category?

  • \$\begingroup\$ I'm not an expert, but my working definition would be that "primary" equipment would be anything through which power flows on its way to the customer, the failure of which would mean the immediate disruption of delivery. "Secondary" equipment would be the systems that provide status monitoring, control and maintenance functions. Failures in the latter might affect the ability to respond to events, but would not immediately disrupt power delivery. \$\endgroup\$ – Dave Tweed Mar 23 '17 at 14:53
  • \$\begingroup\$ Quite possibly this could refer to metering equipment, using a transformer rated meter. There are CTs and PTs that reduce high currents/voltages to lower levels, so they're easier (and safer) to measure. The primary side of these transformers and the equipment connected is "primary equipment", and the secondary side is "secondary equipment". \$\endgroup\$ – Steve Mar 23 '17 at 15:45
  • \$\begingroup\$ Also possibly of some use: cfroundtable.org/meetings/060206/eaton.pdf \$\endgroup\$ – Steve Mar 23 '17 at 15:46

In North America, primary refers to over 1000V while secondary is under 1000V. So a primary substation would involve transformation, say from 66kV to 13.8kV, while a secondary substation would involve transformation, say from 13.8kV to 480V.

IEC standards (common in Europe) define "primary" and "secondary" differently. For example, 11kV switchgear is available in primary and secondary styles. I am not familiar with the differences.

  • \$\begingroup\$ I do not think that the question is about considering the primary/secondary side of the transformer but it rather about the main equipment (primary) and the supportive equipment (secondary). \$\endgroup\$ – Hazem Aug 27 '17 at 6:37

These paragraphs from "Substation Automation Systems Design and Implementation" book by Evelio Padilla, may give you the answer.

Physically, all electric substations are comprised of a group of high voltage apparatus whose individual sizes depend on substation operation voltages, which is called the primary equipment, as well as by a lot of low voltage smaller components, which as a whole are called the secondary system.

The group of high voltage apparatus comprises changing‐state equipment or switchgear (circuit breakers, disconnectors and earthing‐switches,used to maintain or to interrupt the energy flux from/to transmission lines or load feeders connected to the substation, instrument transformers (voltage transformers and current transformers) that reflect voltages and currents present at high voltage terminals of primary equipment, and also in most cases, power transformers to change voltage levels according to substation purpose


The primary equipment of a transmission substation operates at standardized rated voltages, such as 72.5, 123, 145, 245, 420, 525 and 765 kV, whilst in most distribution substations, primary equipment works at lower standardized voltages, such as 7.2, 12, 24 or 36 kV. Such equipment is manufactured in such a way that its insulation in service must reliably withstand predefined over‐ voltage levels; otherwise they can become severely damaged due to external flashovers or by internal insulation breakdown.


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