A140 Do’s and Don’ts for Power Transformers (Part-5): Selecting and sizing a power transformer

Introduction

Selecting and sizing a power transformer is a critical first step in designing any reliable electrical distribution or transmission system. It involves matching the transformer’s kVA or MVA rating, percentage impedance (Z%), insulation class, cooling class, oil and winding temperature rise and voltage capabilities to the specific demands of the load, while accounting for system short‑circuit levels, future growth, and ambient conditions. A well‑sized transformer not only ensures optimal efficiency and voltage regulation under normal and peak loads but also minimizes losses, extends equipment life, and reduces total lifecycle costs. Conversely, under or over‑sizing can lead to overheating, excessive inrush currents, poor power quality, and unnecessary capital expenditure. In this process, Engineers should balance technical requirements, load profiles, harmonics, temperature rise limits, and fault duties with the applicable standards (IEEE, IEC, and National Codes) and practical considerations such as site constraints, safety margins, and long‑term operational flexibility.

The following critical mistakes made during the selection and sizing of power transformers should be avoided:

• DO-NOT Oversize Transformer

Significantly oversized transformers operate inefficiently at low loads. They have higher no-load (excitation) losses, increased capital cost, larger footprint, and potentially higher installation costs. They also draw higher inrush currents. Size based on realistic load profiles, factoring in diversity, future plausible growth and acceptable overload capability according to international standards, IEC 60076-7:2018 - Loading guide for mineral-oil-immersed power transformers or IEEE C57.91-2011 - IEEE Guide for Loading Mineral-Oil-Immersed Transformers and Step-Voltage Regulators and National Codes.

• DO-NOT Base your Choice Solely on Initial Cost

Cutting corners on purchase price often means compromising on efficiency, thermal performance or longevity. These are the factors that drive total lifecycle cost far more than the quoted price. 

• DO-NOT Neglect Temperature Rise and Ambient Conditions

Always check the oil and winding temperature‐rise limits in relation to the worst‑case ambient. IEC 60076-1:2011 – General, specifies requirements for operation under normal service conditions; however, it allows the purchaser to stipulate a higher minimum temperature for the cooling medium. 

• DO-NOT Assume Nameplate MVA equals Usable Capacity

Continuous loading must respect the transformer’s rated MVA at its oil and winding temperature‐rise limits, not the short‑term overload capacity. Confusing short‑term emergency ratings with sustained operation leads to overheating. 

• DO-NOT Underestimate the Impact of Impedance (%Z)

Low‐impedance units may cause excessive inrush currents and impact coordination, while overly high impedance can lead to unacceptable voltage drops. The impedance should be matched to system short‑circuit levels and coordination requirements. 

• DO-NOT Skip Harmonic and Non‐linear Load Analysis

Transformers feeding VFDs, rectifiers, or other non‐linear loads need de‑rated kVA or K‐factor design to handle increased eddy‐current and dielectric losses. Ignoring harmonics invites premature aging and failures.

• DO-NOT Overlook Environmental and Mounting Constraints

 Selecting a unit without confirming clearances, foundation capacity, seismic requirements, or ingress protection ratings can derail installation and lead to costly site modifications.

• DO-NOT Fail to Verify Standards and Vendor Documentation

Skipping a thorough review of National Code, IEC and IEEE standards compliance, factory routine test reports, and Guaranteed Performance data risks accepting units that underperform or won’t integrate properly with protection scheme  

• DO-NOT Overlook Special Duties

Duties such as frequent on/off cycling or parallel operation require tailored designs. 

• DON'T Assume Standard Cooling is Always Sufficient:

High ambient temperatures, restricted airflow, or unusual duty cycles can cause standard cooling (ONAN) to be inadequate, leading to overheating. Assess the installation environment and load profile. Specify forced air (ONAF) cooling to handle peak loads or high ambient temperature without exceeding temperature rise limits. 

• DO-NOT Skip Factory Acceptance Tests (FAT)

They catch manufacturing defects early. Review and compare FAT results with the factory routine test results.

Conclusion

Selection and sizing transformers requires careful analysis of the power system’s requirements, load profiles, environmental conditions, and lifecycle costs. Rushing through or overlooking this process or relying solely on rules of thumb can lead to inefficient, unreliable, or even unsafe installations. To avoid these pitfalls, always consult the relevant standards (IEEE, IEC, and applicable National Codes) and involve experienced power system engineers.