By Amanda Freick, VP Strategy

The Perfect Storm for Transformer Design 

Utilities are facing a convergence of pressures that make transformer efficiency and performance more important than ever. 

• Aging transformer fleets: Many units in North America are 30+ years old and sized for a different era of predictable loads and minimal electronics. 
• Distributed energy resources (DERs) and EV growth: Rooftop solar, batteries and EV chargers are being added rapidly, often behind the meter. These resources change not only power magnitude, but also direction, timing, and harmonic content. 
• Mismatch between design and reality: Transformers are still commonly sized by peak kVA estimates. Yet real-world performance depends on load type, power factor, harmonic signature and asymmetry. 

The result is that field performance often deviates from design expectations, leading to efficiency loss, reliability concerns and costly overbuilds. 

Technical Drivers of Transformer Performance 

Both design and operating conditions shape transformer efficiency. Four scientific properties determine how well a transformer performs in the field: 

• Winding resistance → Copper losses 
  These I²R losses grow with load current and drive efficiency degradation under high or unbalanced loads. 

• Leakage reactance → Voltage regulation 
  Flux that does not couple windings causes reactance. Too much reactance leads to voltage sag during spikes such as EV charging. 

• Core material → Hysteresis losses 
  Grain-oriented silicon steel vs. amorphous alloys makes a big difference, particularly at light load, where amorphous cores reduce heating. 

• Magnetizing inductance → No-load losses 
  Governs standby losses, which dominate when transformers are lightly loaded. 

These factors create the familiar efficiency curve. Transformers reach peak efficiency where copper and core losses balance. At lower loads, core losses dominate. At higher loads, copper losses spike, heat rises and lifespan shortens. 

Why Load Type Matters 

It isn’t just how much load you put on a transformer—it’s what kind of load. 

• Resistive loads (heaters, incandescent lights): near-unity power factor, clean waveforms, high efficiency. 

• Inductive loads (motors, HVAC, pumps): create lagging reactive power, causing higher losses. 

• Capacitive loads (lightly loaded lines, PF correction banks): can amplify harmonics when unmanaged. 

• Non-linear loads (LEDs, VFDs, solar inverters, EV chargers): inject harmonics that distort waveforms, increase winding heating and degrade voltage regulation. 

In today’s systems, transformers rarely see ideal resistive loading; they see a cocktail of inductive and non-linear sources that stress the unit differently than design models assume. 

Case Studies: What Field Data Reveals 

Lodge Analysis – Harmonics in Action 

A transformer serving a large lodge was rated at 500 kVA due to a 250hp fire pump being fed from the same transformer. The result was chronic underloading, as the transformer sizing was based on the pump’s inrush current. 

• The lodge’s base load included VFDs for HVAC and pools, AV systems and advanced lighting controls. 

• The most severe harmonics occurred when the 250 HP fire pump was off. 

• Persistent 5th harmonic current pushed voltage THD into the 6–9% range, far above IEEE 519 limits. 

• Nearby homes, not the lodge, reported power quality issues such as flicker. 

Because feeder SCADA missed this localized problem, the utility initially had no explanation. Transformer-level monitoring identified the issue and traced it directly to the oversized, underloaded transformer. 

Bus Depot Electrification – $4M (AUD) Savings 

In another project, Endeavour Energy in Australia needed to electrify a major bus depot. Initial models called for a $6.5 million (AUD) upgrade to transformers and switchgear. 

Monitoring revealed that: 

• Local transformers had ample headroom 

• Load from the depot could be managed with smart EV charging schedules 

• The required investment dropped to $2.5 million (AUD)

That’s a $4 million savings on a single site. With 56 similar depots in Sydney alone, the potential avoided cost across the system exceeds $100 million. 

Four Field Lessons for Designers 

Data from monitoring thousands of transfomers across the world highlights four consistent patterns: 

• Oversizing isn’t always safe: Too much headroom increases impedance, which amplifies voltage distortion and lowers efficiency. 
• DER impacts are local: Reverse power flow and harmonics affect transformers directly, not necessarily the whole feeder. 
• Transformer sizing is often mismatched: It’s not uncommon to find transformers sized for worst case scenarios which leads to consistent underloading. 
• Planning assumptions miss edge behaviors: Real-world imbalance, harmonics, and load shapes diverge from models. 

Moving Forward 

For utilities, the implications are clear: 

• Update sizing criteria to include harmonics and DER impacts 
• Implement monitoring strategies for loading and PQ verification 
• Review specifications now for DOE 2029 compliance 
• Consider harmonic-tolerant transformer designs where non-linear loads are prevalent 

The takeaway: Transformer performance cannot be reduced to kVA alone. Modern distribution design requires integrating efficiency, power quality and DER impacts into every decision. Utilities that act now will unlock capacity, save money and deliver cleaner, more reliable power to their customers.