Measuring Transformer Losses

What is Transformer Loss?

Utility transformers convert high transmission voltages to lower levels for residential and commercial use, but contribute almost 4% to total energy loss due to load and no-load losses. Load losses occur when a transformer is active and include resistive and stray losses. No-load losses happen when a transformer is energized but not loaded.

Figure 1. Transformer loss types
 

Why is Measuring Transformer Loss Important?

In general, measuring transformer loss helps ensure safer operation and optimizes delivery of electricity. For utility operators, minimizing losses is crucial as they only generate revenue from power delivered to end users. When commissioning a new transformer, operators specify no-load losses and load losses, and if these guaranteed loss values are exceeded, the manufacturer faces significant penalties.

During loss testing (especially no-load testing), low power factor signifies high core losses and requires specialty instruments to more accurately measure power.
 

What is Needed to Measure Transformer Loss?

Loss measurements include actual losses from a transformer's core and virtual losses caused by uncertainties in the measurement system. Standards IEC 60076 and IEEE C57.12.00 outline methods for measuring transformer losses, include consideration of measurement errors, and cover both power loss and power efficiency testing.

Power Loss: P_loss = P_out – P_in (Watts)
Power Efficiency: h = P_out / P_in (Percent)

To perform loss measurements, engineers typically use a three-phase-transformer power loss measuring system and an integrated shunt reactor loss measuring system (not covered in this handbook). A typical transformer power loss measuring system consists of three high-voltage reference capacitors (including corresponding active high-voltage dividers), three zero-flux current transformers (including corresponding current channels), and one three-phase high precision power analyzer (i.e., Wattmeter).
 

Figure 2. Transformer power loss measuring system example with a precision power analyzer
 

A precision power analyzer provides accurate measurements across power factors ranging from 0.0001 to 1, which makes it ideal for transformer testing through the minimization of measurement errors. Precise measurement of input (P_in) and output power (P_out) allows engineers to better evaluate and enhance transformer design efficiency.
 

Learn more about the importance of measuring transformer loss by reading the Yokogawa Test&Measurement case study Royal Smit Transformers Gains High Value From Accurate Loss Validation.