Power systems loaded with non-linear loads often carry harmonic currents leading to harmonic voltage distortions or result in additional stresses on other components connected to the power system. The harmonics already existing in the power system are also referred to as "ambient harmonics". To provide a basis for HVDC or SVC design, or Power Quality characteristics, measurements shall be carried out investigating the existing ambient harmonic levels at a determined network point.
Measurement of ambient harmonics on high voltage power lines are performed using a standard voltage transformer mounted on a truck, which has been characterized up to harmonic component #25, together with dedicated software, accomplishing IEC 61000-4-7.
Harmonics contain continuous monitoring can be performed in high voltage systems for extended periods of a week or more.