I would like to seek guidance on the development of the Lightning Earthing Impedance Measurement Project.
The brief overview of the project is as follows,
This overview outlines the concept of measuring the earthing impedance with the variable frequency AC source to predict the behaviour of power transmission earthing system under the influence of transient fault / switching transient / lightning transient conditions.
In practice, the impedance of earthing systems is measured at power frequency to gauge the effectiveness of earthing system. The power system frequencies are 50/60 Hz. However, the frequencies under the various abnormal conditions could be several tens of kHz due to the transients generated in the system during fault conditions / switching transients / lightning transients. The earthing system is expected to efficiently absorb the high currents produced under these abnormal conditions. This calls for the characterization of the earthing system at various frequencies ranging from power frequency to several tens of kHz.
Following are the steps of such a measurement
A) Low distortion sine wave generator produces variable frequencies ranging from 10Hz to 100kHz @ typically 10 volts peak to peak.
B) A power amplifier of 100kHz bandwidth amplifies the signal produced by the sine wave generator to a level of typically 1kW to constitute a variable frequency AC source.
C) Variable frequency AC source injects the current into the earthing system at the various frequencies of interest (typically ranging from 10Hz to 100kHz).
D) The rise of earth potential at the point of injection of current in the earthing system is measured with the differential voltage probe (100kHz bandwidth).
E) Injected current magnitude is monitored with the wide bandwidth (100kHz) current sensor.
F) The acquired waveform of current and voltage are processed through the lock-in amplifiers / software filtering to minimize the effects of various noise such as power frequency noise, power system harmonics, traction noise, broadband noise, communication signal noise etc..
G) The RMS value of voltage and current are obtained from the filtered waveforms.
H) The ratio of RMS voltage to RMS current is obtained at various injected frequencies to show the value of earthing impedance of the system under test.
I) The magnitude of earthing impedance in ohms is plotted against the frequency in Hz in log scale to exhibit the behaviour of earthing systems under the influence of various frequency conditions ranging from 10Hz to 100kHz.
I look forward to useful suggestions from the forum members with regards to the development of the above mentioned Lightning Earthing Impedance Measurement System.