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Unit substation fundamentals

What is a unit substation?

A unit substation steps down utility distribution voltages to ideal in-plant distribution voltages that match operating conditions. They can be applied inside or outdoors and consist of a close-coupled assembly made up of three main parts: 

  • Primary electrical distribution equipment
  • A three-phase power transformer 
  • Secondary electrical distribution equipment
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Front-view outline of a unit substation configuration
There are two types of unit substations: primary unit substations and secondary unit substations. The two configurations are defined by equipment voltage, rating of the transformer and the type of equipment incorporated within the assembly. The transformer is the major component of the assembly and understanding what type of transformer technology to specify is the most important design consideration. Once the type of unit substation is determined, it is much easier to select the appropriate transformer. 

Primary unit substation

Primary unit substations are ideal for stepping down utility distribution voltages to in-plant distribution voltages. These customizable assemblies can be applied for primary voltage classes up to 69 kV and secondary voltage class starting at 34.5 kV and below.

All configurations are standardized upon applicable ANSI & NEMA® Standards to provide complete electrical and mechanical control over coordination of the three close-coupled sections. Primary unit substations also offer exceptional design flexibility, allowing nearly any switchgear type as secondaries

For primary unit substations, the following electrical ratings are most typical:

  • Primary voltage: 6.9-69 kV
  • Transformer kVA: 500-20,000 kVA
  • Secondary voltage: 2.4 kV – 34.5 kV

Secondary unit substation

Customizable secondary unit substations are used to step down utility distribution voltage to in-plant distribution voltages when primary voltage classes are below 38 kV and secondary voltage classes range from 208 V to 600 V.

These configurations are also standardized upon applicable ANSI & NEMA® Standards to provide complete electrical and mechanical control over coordination of the three close-coupled sections. A secondary unit substation often offers dedicated pull-apart terminal blocks and pre-made harnesses to eliminate the headaches associated with wiring control and communications devices inside a substation.

For secondary unit substations, the following electrical ratings are most typical:

  • Primary voltage: 2.4-38 kV
  • Transformer kVA: 300-3750 kVA
  • Secondary voltage: 208, 240, 480 and 600 V

 

Advantages of unit substations

Unit substations provide many advantages compared to traditional approaches of stepping down utility distribution voltages. For example, the close-coupled design allows the transformer and electrical equipment to be located in close proximity to the load concentration, which minimizes the lengths of cabling and buses to reduce power losses and exposure to faults. This close-coupled design can also improve voltage regulation and service continuity.

Additionally, the unit substation design is more compact than traditional approaches by nature to help maximize footprint for revenue-generating equipment while reducing on-site labor and delivery costs. This straightforward design with simplified accessories also helps reduce maintenance costs.

Further, standardization of assembly configurations can be accomplished easily with all accessories and features meeting applicable NEMA and IEEE guidelines in order to quickly meet increasing power needs. And, by utilizing highly researched and thoroughly tested unit substation designs, you can achieve high short-circuit strength to reliably meet the demands of modern systems.

Transformer selection

To meet specific application needs, unit substations can incorporate liquid-filled and/or dry-type transformers. Specialty transformer designs are also available enabling you to: