This paper presents the results of testing carried out to compare a widely used Phasor Measurement Unit with Power Quality measurement module with the relevant standard for power quality measurement instruments. The purpose of this work is to determine the reliability of the Wide Area Measurement System and phasor measurement units installed in Estonian transmission system for power quality measurements. The accuracy of the phasor measurement units is assessed based on standards IEC 61000-4-30 (class A specifications) and IEC 62586-2 standards for power quality instruments. If required, the tests carried out are adapted based on the PMU capabilities. In addition, the impact of current input on the measurement error in the allowed range of the device has been carried out. Results show, that the PMU passes all selected tests with Class A measurement error.