High-voltage transformers are among the most important elements in an electric-power system. Each one of them is affected by various external factors: overvoltage, partial discharge (PD), overheating, vibrations, etc., which are created by a strong electric field, thermal effect, humidity, impurities, factory defects, dissolved water and gas in oil-type-transformer insulation. These and other factors, caused by the environment, reduce the life of a device. Thus, the evaluation of the device condition is one of the most important factors for a system-safety evaluation, which ensures a reliable and economical electrical-network operation. This work reviews different contact and non-contact methods, used to evaluate the conditions of transformers by measuring the level of PD. The selected method, i.e., the non-contact measurement of electromagnetic-wave radiation was used to evaluate the voltage-transformer status. The experiment was performed at a 110 kV substation. The authors discuss the efficiency of the selected method to evaluate the voltage-transformer insulation condition.