How to Select a Power Transformer for a Substation

How to select a power transformer for a substation

The transformer you put into a substation sets the reliability, efficiency and cost of everything downstream. This guide walks through the five steps of the selection, then covers the details that most often get decided late and cost money to change.

Step 1: determine the required power rating

Start from the substation load demand and calculate the required kVA or MVA. Consider:

Current maximum demand
Future load growth, typically projected 10 to 20 years ahead
Diversity factor
Diversity of connected loads

The rating decision is really a question about the planning horizon. A unit sized exactly to today’s demand will be overloaded within a few years in a growing area; a unit sized for a demand forecast that never arrives leaves capital and losses on the table. Utilities usually settle this by fixing the planning horizon in the specification — commonly 10 or 20 years — and sizing to the forecast peak at the end of it. Our capacity calculation guide sets out the arithmetic.

Learn more about substation transformer specifications.

Step 2: determine voltage levels

Set the primary and secondary levels from:

Transmission voltage (for example 110 kV, 220 kV, 500 kV)
Distribution voltage (for example 10 kV, 35 kV)
Tolerance requirements, typically ±5%

Voltage levels follow the network, not preference. What matters is the tap range: the transformer needs enough tapping to hold the secondary voltage within tolerance across the range of primary voltage the network actually delivers, including the light-load voltage rise that appears on long lines.

Step 3: choose the transformer type

Based on your application:

Step-down transformer, for voltage reduction in distribution substations
Step-up transformer, for voltage increase in collector substations
Auto-transformer, for regulation between similar voltage levels

Number of units

One large unit is cheaper per MVA than two smaller ones, but a single unit is a single point of failure. Substations that cannot accept an outage, or that expect growth in steps, normally install two units at partial rating each, so that one can carry the critical load while the other is out of service.

Step 4: select cooling method

Choose between:

ONAN, oil natural air natural, for small to medium loads
ONAF, oil natural air forced, for medium to large loads
OFAF, oil forced air forced, for large ratings

Read our guide on cooling methods. Multi-stage cooling is common at substation level because it gives an emergency rating with auxiliaries running without paying for forced cooling at normal load.

Step 5: consider other factors

Impedance voltage
No-load and load losses
Efficiency requirements
Sound level limits
Environmental conditions (altitude, temperature, humidity)
Installation constraints

Impedance and the rest of the design

Impedance voltage sets two things at once: voltage drop under load and short-circuit current through the switchgear. Low impedance gives better regulation but higher fault levels, which may force higher-rated switchgear throughout the substation. Impedance is therefore a system decision, and it should come from a short-circuit study rather than from a default value.

Losses and cost of ownership

Substation transformers run continuously, so no-load loss is charged 24 hours a day. Many utilities now assess offers on capitalised loss cost rather than equipment price. If your specification does not do this, the cheapest compliant bid will simply have the worst losses — a decision that is repeated every year for the life of the unit.

Sound level

Substations near housing have to meet noise limits, and those limits are measured at the boundary, not at the transformer. Site the unit and choose the cooling class with that in mind.

Selection checklist

Fix the planning horizon and size to the forecast peak.
Confirm primary and secondary voltage, tap range and tolerance.
Decide unit count and redundancy arrangement.
Fix the connection type: step-down, step-up or auto-transformer.
Set cooling class from rating and load profile.
Specify impedance from the system study.
Specify losses and capitalise them in the evaluation.
Check sound level against site limits.
Confirm ambient and altitude against the design assumptions.
Agree the test schedule, tap changer type and protection scope.

Common specification mistakes

Copying the previous project’s specification without rechecking the load forecast
Leaving impedance to the supplier, so bidders offer different values and the prices are not comparable
Buying on equipment price only, which rewards the highest losses
Forgetting tap range on a long or weak network
Allowing no space for cooling air, maintenance access, or the future second unit

Talk to our engineering team

Hebei Youge Electrical Equipment Co., Ltd. designs and builds power transformers to
IEC 60076 and IEEE C57, with routine tests on every unit and a test report supplied with
the equipment. Send us your rating, voltage class and site conditions and we will come back
with a technical proposal.

How to Select a Power Transformer for a Substation: Complete Guide