Grounding Resistance Testing of Substation Ground Grids
In recent years, many substations have experienced expansion and equipment damage accidents caused by lightning strikes, most of which are closely related to unqualified grounding resistance of the ground grid.
The ground grid serves as both working grounding and protective grounding. When the grounding resistance is too high, the following hazards may occur:
During ground faults:
The neutral point voltage offset increases, causing excessive voltage between the sound phase and the neutral point. This may exceed insulation limits and damage equipment.
During lightning strikes or surge events:
The large current generates a high residual voltage, threatening nearby equipment through backflashover. The effective lightning protection level of conductors and equipment is reduced, leading to potential damage.
For personnel safety:
Excessive grounding resistance endangers the safety of operators and maintenance personnel working in the substation.
Over time, due to the corrosive effect of soil on the grounding device, corrosion occurs, increasing the grounding resistance and affecting safe operation. Therefore, regular monitoring and accurate measurement of grounding resistance are essential.
However, testing during transformer operation is often affected by:
Current interference from the live ground grid,
Mutual interference between test leads.
Since the grounding resistance of large ground grids is typically below 0.5 Ω, even small interference can cause large errors. Inaccurate testing may lead to misjudged faults or unnecessary reconstruction, resulting in additional cost and risk.
Based on practical research and field experience, the following summarizes the principles, methods, and precautions of grounding resistance testing for substation ground grids.
The grounding impedance of a grounding device is determined by measuring the potential difference and current through it.
To minimize measurement error, the current electrode (C) should be placed as far as possible from the grounding device under test (G).
Typical layout distances:
| Wiring Method | Distance Between Current Pole (C) and Ground Grid (dcG) | Distance Between Potential Pole (P) and Ground Grid Edge | Voltage Lead Length |
|---|---|---|---|
| Parallel wiring | 4–5 × diagonal length (D) of ground grid | Variable | 0.618 × current lead length |
| Triangle wiring | ≥ 2 × D | Equal to current lead | Equal to current lead |
In this method, the grounding device and test electrodes are arranged as shown below:
Symbols:
G — grounding device under test
C — current pole
P — potential electrode
D — large diagonal of grounding device
dcG — distance between C and edge of G
x — distance between P and edge of G
I — test current
U — measured potential difference
Testing steps:
Inject current I between G and C.
Move the potential electrode P outward from G every 50–200 m.
Record potential difference U at each point.
Plot U–x curve; the flat (zero-gradient) section represents the potential plateau.
The grounding impedance is calculated as:
[
Z = \frac{U_m}{I}
]
If the flat section is unclear (due to underground interference or coupling), increase the distance of the current loop or adopt another verification method.
This is the most common approach, with two variations:
The current line and potential line are arranged in the same direction.
dcG satisfies circuit layout (typically 4–5 × D).
dPG = (0.5 ~ 0.6) dcG.
Move potential electrode P three times, each time by 5 % of dcG.
If results vary by ≤ 5 %, take the average as the final value.
Note:
For large grounding devices, this method is less suitable.
If unavoidable, keep current and potential lines as far apart as possible to reduce mutual inductive coupling.
The current and potential lines form an angle θ (typically 30°–45°).
dcG ≈ 4–5 D, dPG ≈ dcG.
The measured value can be corrected using:
[
Z = Z'' / \cos(\theta)
]
where Z'' is the measured impedance.
If soil resistivity is uniform, an isosceles triangle wiring (dcG = dpG = 2 D, θ ≈ 30°) is recommended for improved accuracy.
When using a grounding resistance tester, the wiring principle is similar to the three-pole method.
Connection points:
E — connected to the tested ground grid.
P1 — connected to the tested ground grid (shorted with E under normal condition).
P2 — voltage probe; length = 0.618 × current line length.
C — current line; length = 4–5 × diagonal length (D).
For ground resistance ≤ 0.5 Ω, it is recommended not to short E and P1.
This minimizes lead/contact resistance influence and improves measurement accuracy.
Environmental conditions:
Ground resistance is highly affected by soil moisture.
Testing should be conducted:
During dry seasons and unfrozen soil conditions.
Avoid immediately after rain, snow, or lightning.
Data validity:
Accurate measurement provides a reliable basis for maintenance and corrective planning.
Safety significance:
Maintaining qualified grounding resistance effectively prevents:
Dangerous step and touch voltages,
Equipment insulation failures,
Personnel electric shock incidents.
Thus, regular testing ensures safe and stable operation of substation equipment and provides a secure working environment for staff.
The JYD Grounding Down Wire Continuity Tester—also known as the Large Grid Earthing Resistance Tester—uses advanced power-supply technology.
It is a highly automated portable instrument designed to measure on-resistance values between the grounding down-conductors of various substation equipment.
Key features:
Output current up to 10 A
High precision and repeatability
Suitable for large ground grid resistance verification and continuity testing
The grounding resistance of a substation ground grid is a crucial parameter for ensuring:
Equipment protection
Lightning and fault current safety
Personnel safety
Through systematic testing using the potential drop, three-pole, or tester methods, and by following proper test arrangements and environmental precautions, substations can effectively monitor grounding condition, detect corrosion or degradation early, and maintain reliable operation.
Kingrun Transformer Instrument Co.,Ltd.
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