Design Of Grounding Resistance Measurement Device For Grounding Grid Based On Correlation Analysis Method

As grounding resistance is a major technical indicator that affects the performance of the grounding grid,a regular measurement of grounding resistance plays an important role in ensuring not only the normal distribution of current for grounding conductors but also the lightning protection performance of substations,overhead lines and grounding devices,so as to maintain the safe operation of electrical systems.Among the existing methods of measuring grounding resistance,the fall-of-potential method requires a large amount of data to draw a potential distribution curve,and the level of efficiency is low.The hetero-frequency measurement method is disadvantaged by such problems as sine wave distortion and the mutual inductance coupling of lead wires.Likewise,numerical calculation methods are constrained by such drawbacks as the difficulty in dividing elements and establishing boundary conditions.Without the excavation of grounding grid,it is always a challenge facing technicians to design a measuring device capable to measure the grounding resistance of the grounding grid accurately and consistently.Herein,a method used to measure the grounding resistance of the grounding grid is proposed by applying the m-sequence pseudo-random signal and the relevant identification technology.Besides,a grounding resistance measuring device is developed for the grounding grid.In order to select the excitation signal with better measurement effect,the grounding resistance simulation model was established.Then,both simulation and comparison were conducted for the measurement results of the sinusoidal signal and the pseudo-random signal.It was verified that the grounding resistance as measured by the pseudo-random signal with a larger amplitude was comparable to the standard level.Based on the proposed principle of cross-correlation method system identification,the designed measuring device took STM32 controller and LabVIEW host computer as the core.The STM32 controller generated a 5th-order m-sequence pseudo-random signal,and collected current and voltage signals in real time through control of the 24-bit analog-to-digital converter ADS1256.The LabVIEW host computer was used to complete the polarity conversion of the signal,the correlation operation,and the fast Fourier transform of the current and voltage signals.It can not only display the grounding resistance accurately and stably,but also fulfill the function of human-computer interaction.The accuracy of the grounding grid grounding resistance measurement device was verified through correlation analysis.Firstly,the resistors were tested at different levels of resistance in the laboratory,and the measurement error was found to fall within the permissible range.Secondly,the grounding resistance of the lightning protection devices in the buildings on the campus was measured.The measurement results comply with the requirements set out in the specifications of the national standard building lightning protection design.Finally,an on-site measurement was conducted for the grounding resistance of the grounding grid of Changchun Yangpu 220 k V substation.The measurement results meet the requirements for grounding systems of 110 k V and above.Moreover,laboratory and field measurement results were obtained to demonstrate the practicability of the designed grounding resistance measurement device.To conclude,it was verified that the method of calculating grounding resistance with pseudo-random signal is advantaged by high precision and the robustness to external interference.