METHOD AND APPARATUS FOR MEASURING PARTIAL DISCHARGE CHARGE VALUE IN FREQUENCY DOMAIN
The charge value of a partial discharge pulse in an electrical cable is provided by analyzing the frequency spectrum of the propagated pulse in a spectrum analyzer 21 to generate sets of spectrum signals, each including a magnitude and a frequency of a component of the pulse. A laptop computer has a processor, receives the spectrum signals, and has one or more application computer programs operable to perform the calculations of Equations 2-6, finds the bandwidth and energy of the spectrum signals from which the computer programs derive unknown time domain parameters including the maximum value of the pulse and the time interval of the pulse at half its maximum value. With those parameters, the program reconstructs a time domain signal of the pulse and integrates that signal to provide the charge value of the partial discharge.
1 . A method for measuring the charge of a partial discharge (PD) pulse comprising the steps of:
receiving a PD voltage pulse;
analyzing the PD voltage pulse to generate frequency spectrum signals wherein each frequency spectrum signal includes a magnitude and frequency;
processing said frequency spectrum signals to generate signals representative of
(a) the maximum magnitude of the PD voltage pulse; and
(b) the time interval of the PD voltage pulse at half the maximum (FWHM) value of the PD voltage pulse;
generating a time domain signal representative of the PD voltage pulse from the derived full width at half the maximum value of the pulse and the derived maximum value of the PD pulse;
for predetermined impedance, integrating the PD voltage pulse over time to generate a signal representative of magnitude of the charge in the PD pulse.
2 . The method of claim 1 wherein said processing step comprises the steps of:
measuring bandwidth of the frequency spectrum signals;
from said bandwidth, deriving the signal representative of full width at half the maximum value of the PD pulse (FWHM);
deriving a magnitude of energy of said frequency spectrum signals;
deriving the maximum magnitude of the PD voltage pulse from the magnitude of energy of said frequency spectrum signals.
3 . The method of claim 1 , further comprising the step of converting the PD pulse to a digital format and storing the digital format for latter frequency analysis.
4 . A method for measuring the charge of a partial discharge (PD) pulse in an electrical cable while said cable online and part of an electrical energy distribution system, comprising the steps of:
sensing a PD voltage pulse in said electrical cable while said electrical cable is online;
analyzing the PD voltage pulse to generate frequency spectrum signals wherein each frequency spectrum signal includes a magnitude and frequency;
measuring bandwidth of the frequency spectrum signals;
deriving a signal representative of full width at half the maximum value of the PD pulse (FWHM) from the bandwidth measurement;
deriving a magnitude of energy of said frequency spectrum signals;
deriving a maximum magnitude of the PD voltage pulse from the magnitude of energy of said frequency spectrum signals;
generating a time domain signal representative of the PD voltage pulse from the derived full width at half the maximum value of the pulse (FWHM) and the derived peak value of the PD pulse;
for predetermined impedance, integrating the PD voltage pulse over time to generate a signal representative of magnitude of the charge in the PD pulse.
5 . The method of claim 4 wherein the PD pulse has a Gaussian form.
6 . The method of claim 4 wherein the PD voltage pulse is analyzed with a spectrum analyzer to provide the magnitude and signals in the frequency spectrum.
7 . An apparatus for method for measuring the charge of a partial discharge (PD) pulse in an electrical cable online as part of an electrical energy distribution system, comprising:
a sensor for detecting a PD pulse in said online electrical cable;
a spectrum analyzer coupled to the sensor for receiving the PD voltage pulse and for generating frequency spectrum signals wherein each frequency spectrum signal includes a magnitude and frequency;
a processor having a central processing unit, one or more memories, and programs stored in the one or more memories for operating the processor and for:
calculating bandwidth from the frequency spectrum signals;
calculating full width at half the maximum value of the PD pulse (FWHM) from the bandwidth measurement;
calculating a magnitude of energy of said frequency spectrum signals;
calculating a maximum magnitude of the PD voltage pulse from the magnitude of energy of said frequency spectrum signals;
generating a time domain signal representative of the PD voltage pulse from the derived full width at half the maximum value of the pulse (FWHM) and the derived peak value of the PD pulse;
for a predetermined impedance, integrating the PD voltage pulse over time to generate a signal representative of magnitude of the charge in the PD pulse.
8 . The apparatus of claim 7 , further comprising an analog-to-digital converter for converting the output of the sensor into a digital format and a memory for storing the digital format of the sensor signal for later processing.
9 . The apparatus of claim 7 wherein the spectrum analyzer is an analog swept frequency analyzer which generates digital output signals of the magnitude and frequency of components of the sensed PD pulse.
10 . The apparatus of claim 7 wherein the spectrum analyzer is a digital frequency analyzer which generates digital output signals of the magnitude and frequency of components of the sensed PD pulse.