IEC 61000-4-18-2019 pdf free.Electromagnetic compatibility (EMC)- Part 4-18:Testing and measurement techniques – Damped oscillatory wave immunity test.
4.2slow damped oscillatory wave phenomenon
This phenomenon is representative of the switching of disconnectors in HVIMV open-airsubstations, and is particularly related to the switching of HV busbars.
In substations,the opening and closing operations of HV disconnectors give rise to sharpfront-wave transients, with rise times of the order of some tens of nanoseconds.
The voltage front-wave includes reflections due to the mismatch of the characteristicimpedance of HV circuits involved. In this respect, the resulting transient voltage and currentin HV busbars are characterized by a fundamental oscillation frequency that depends on thelength of the circuit and on the propagation time.
The oscillation frequency ranges from about 100 kHz to a few megahertz for open-air sub-stations,depending on the influence of the parameters mentioned above and the length of thebusbars, which can vary from some tens of metres to hundreds of metres (400 m can occur).
In this respect,the oscillation frequency of 1 MHz can be considered representative of mostsituations, but 100 kHz has been considered appropriate for large HV substations.
The repetition frequency is variable between a few hertz and a few kilohertz depending on thedistance between the switching contacts. For contacts in close proximity,the repetitionfrequency is at its maximum, while for contact distances close to allowing re-ignitions betweenthe contacts,the repetition rate is at its minimum and is equivalent to twice the powerfrequency with respect to each phase (100/s per phase for 50 Hz grids and 120/s per phasefor 60 Hz grids).
IEC 61000-4-18 The repetition rates of 40/s and 400/s represent a compromise,taking into account thedifferent durations of the phenomena,different frequencies considered and the energy towhich the circuits under test are subjected.
Repetitive oscillatory transients can be generated by switching transients and the injection ofimpulsive currents in power systems (networks and electrical equipment).
4.3Fast damped oscillatory wave phenomenon4.3.1General
The fast damped oscillatory wave immunity test should cover phenomena present in twospecific environments:
– substations of the power network (produced by switchgear and controlgear);- all installations exposed to high-altitude electromagnetic pulse (HEMP).4.3.2Disturbances produced by switchgear and controlgear
During opening or closing disconnector operations,between both contacts of the operateddevice,a large number of restrikes take place due to the slow speed of the contacts.Therefore,disconnector switch operations generate very fast transients, which propagate astravelling waves in the busbars ‘of the substation. The electrical length of conductors andbusbars will determine the oscillation frequencies of the transient overvoltages.
For air insulated substations (AIS) these transients will radiate an electromagnetic field in thesubstation environment. Measurements have been performed in air insulated substationsusing instruments with a large frequency bandwidth [1]2.These measurements have shownthat transient phenomena with frequencies higher than 1 MHz can also take place in thesesubstations.
For gas insulated substations(GIS), these transients propagate inside the metallic enclosure,which contains the SF6 gas. Due to the skin effect,high frequency transients are confinedinside the enclosure and cause no problems. Transient current is transferred to the externalsurface of the enclosure tube at any enclosure discontinuity.As a consequence, the enclosurepotential rises and the current flowing on the enclosure surface radiates an electromagneticfield in the substation environment. The transient ground potential rise is a direct source oftransient common mode currents in the secondary circuits.The radiated electromagnetic fieldalso induces common mode currents in the secondary circuits.IEC 61000-4-18 pdf download.