Issues on Distribution Network Grounding Resistance Threshold and Calculation

Summary of Issues Related to Distribution Network Grounding Resistance Threshold and Calculation

In the operation of the distribution network, the insufficient ability to identify the grounding resistance is a key issue affecting fault judgment. To reasonably set the threshold, multiple factors need to be comprehensively considered.

I. Difficulties and Directions in Balancing Thresholds

The operating conditions of the grounding resistance are extremely complex. The grounding media may include tree branches, the ground, damaged insulators, damaged arresters, wet sand, dry turf, dry grassland, wet turf, reinforced concrete, asphalt pavement, etc. The grounding forms are also diverse, including metal grounding, lightning discharge grounding, tree branch grounding, resistance grounding (subdivided into low-resistance and high-resistance, and there is also extremely high-resistance grounding, and there is no authoritative division standard for high-resistance and low-resistance).

There are also arc grounding forms such as insulation failure grounding, disconnection grounding, short-gap discharge arcs, long-gap discharge arcs, and intermittent arcs. To balance the threshold between sensitivity and reliability, it is necessary to combine the actual operation data of the distribution network, the proportion of fault types, conduct a large number of simulation simulations and field tests, analyze the grounding resistance characteristics under different operating conditions and forms, build a threshold calculation model covering multiple influencing factors, and dynamically adjust the threshold.

II. Key Value of Grounding Resistance Calculation

For the problem of high-resistance grounding, calculating the value of the grounding resistance is of great significance for fault judgment. Due to the high difficulty in identifying high-resistance grounding faults, accurately calculating the resistance value can provide a core basis for judging the nature of the fault and locating the fault point, assist operation and maintenance personnel in quickly handling the fault, and avoid the expansion of the fault.

III. Optimization of Grounding Fault Confirmation Process

After a grounding fault occurs, the three-phase current sampling value variation can be extracted, combined with data such as voltage and zero-sequence components, and algorithms (such as wavelet transform, Fourier analysis, etc.) can be used to process the signal, accurately identify the fault characteristics, lay a foundation for subsequent resistance calculation and threshold judgment, and improve the accuracy and timeliness of grounding fault detection.

Confirm the grounding fault: After a grounding fault occurs, take the variation of the three - phase current sampling values：

N is the number of sampling points in a power frequency cycle.

Suppose there is a fault in Phase A. The calculation is the difference between the sampling value of the fault - phase current and the average value of the variation of the sampling values of the two non - fault - phase currents.

Let the capacitance to ground of each phase of the line be c. The three-phase currents flowing through the line terminal are i_A, i_B, and i_C respectively; the capacitance currents of each phase to ground are i_CA, i_CB, and i_CC respectively; the line load currents of each phase are i_LA, i_LB, and i_LC respectively.

In an actual power grid, the three-phase line load currents remain unchanged before and after a fault occurs, that is,iLA＝i′LA，iLB＝i′LB，iLC＝i′LC.

Then, the variation of each phase current of the faulty line before and after the fault can be calculated as:

Confirmation of the ground fault current value: the difference between the variation of the fault-phase current sampling value and the average of the variations of the sampling values of the two non-fault phases in the faulty line：

Then, the grounding fault resistance value can be calculated as: