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Design and installation of grounding grid for substation
With the rapid development of the electric power industry, the requirements for grounding systems in power networks have become increasingly stringent. The grounding system within a substation plays a critical role in ensuring the safe and stable operation of the facility, directly affecting both equipment integrity and personnel safety. However, due to incomplete design, inaccurate construction, or improper testing, numerous accidents caused by faulty grounding grids have occurred in recent years. These incidents have not only damaged primary equipment but also led to secondary control cable failures, which can spread faults into the control room, causing larger-scale outages. Therefore, a well-designed and properly maintained grounding grid is essential for the reliable operation of the entire power system.
1. Ground Network Design
Currently, many substation grounding designs lack comprehensive details. Most layout diagrams only show general grounding grid configurations with minimal specifics, such as the main lines, without clearly indicating the grounding connections for special equipment or high-density areas. Important zones like the control room, high-voltage room, and wall bushings often lack individual grounding plans. Additionally, the design department rarely provides detailed calculations or explanations for key parameters like soil resistivity or grounding resistance. Many engineers are unsure where to obtain soil resistivity data, how to measure it accurately, or whether the measurements reflect the actual soil stratification. When calculating short-circuit currents, some designers struggle to choose appropriate shunt coefficients for fault points and lightning protection lines, leading to overly optimistic grounding resistance values that may not meet real-world conditions. A thorough and accurate design is crucial for ensuring the long-term performance and safety of the grounding grid.
1.1 Calculation of Ground Short-Circuit Current
According to the power industry standard DL/T 621-1997, the ground short-circuit current is calculated using two formulas: I = (Imax - In)(1 - Kel) and I = In(1 - Ke2), with the higher value being used. Here, I represents the current flowing through the grounding grid during a fault. Imax refers to the maximum short-circuit current when the system is grounded, and this formula applies primarily to effectively grounded systems. This value can be obtained from the operating or relay protection departments, or calculated based on system conditions. Typically, the maximum short-circuit current is determined using the most severe operating mode, such as single-phase grounding.
In is the short-circuit current that flows into the transformer’s neutral point during a ground fault. If the transformer’s neutral is not grounded, In = 0, simplifying the equation to I = Imax(1 - Kel). If there is one neutral point, In = 30% of Imax; with two neutral points, In = 50% of Imax. These values should be calculated and verified by the relay protection department.
Kel is the shunt coefficient for all lightning protection lines connected to the substation’s grounding network. Based on expert analysis, this coefficient depends on the number of outgoing lines. For example, if there is one line, Kel = 0.15; for two lines, Kel = 0.28; for three lines, Kel = 0.38; for four lines, Kel = 0.47; and for five or more lines, Kel ranges between 0.5 and 0.58, depending on the shunting effect across the corridor.
Ke2 is the shunt coefficient for external grounding of the lightning protection line, typically set at 0.18. This is suitable for cases where the transformer’s neutral point is externally grounded.
When considering future growth, it's important to account for a 10-year development plan and multiply the calculated values by a factor of 1.2–1.5. In areas with difficult soil dispersion, a flow coefficient of 1.25 should be applied. Based on these values, it becomes clear that when two neutral points are grounded, the ground fault current could potentially exceed the internal fault current.
1.2 Soil Resistivity Ï
Soil resistivity (Ï) is a fundamental parameter in grounding system design. When selecting a substation location, the soil conditions must be carefully evaluated. If the surface soil has a high resistivity, it may be impossible to achieve the required grounding resistance (R ≤ 2000/I). Proper soil stratification and resistivity measurement are essential to ensure the grounding grid meets safety standards.
1.3 Grounding Resistance Requirements
According to DL/T 621-1997, the grounding resistance should satisfy R ≤ 2000/I, meaning that the product of the grounding current and resistance should not exceed 2000V. With the increasing use of microcomputer-based protection systems, achieving this requirement has become more challenging. Some countries have even set stricter limits, requiring IR < 650V. To address this, measures such as laying copper grounding bars are often used to reduce resistance.
2. Grounding Grid Construction and Installation
Poor technical skills among some construction teams, combined with limited supervision, can lead to substandard grounding grid installations. Issues such as poor welding, disconnected grounding lines, or improperly placed main network checkpoints can compromise the effectiveness of the grounding system. Some contractors treat the final layout as an as-built drawing without documenting any changes made during construction, which can result in inconsistencies and potential hazards.
To prevent such issues, strict inspections and tests must be conducted by qualified personnel during intermediate and final acceptance stages. Any deviations from the design should be corrected promptly to ensure the quality and reliability of the grounding system.
During installation, attention should be given to the following:
- Galvanized flat steel on the main line should be installed vertically to minimize corrosion.
- The control room’s grounding should form a closed loop. When the main line passes through the control room, grounding wires should extend to the upper floor, and building reinforcement should be connected to the main grounding line.
- Wall bushings should be grounded outdoors, with each group’s grounding wire connected to the main line to enhance safety.
- Primary equipment grounding wires must not be connected to the grounding flat steel in cable trenches or suspended through them.
- After installing the grounding grid, clean original soil should be used for backfilling, avoiding dirty soil or gravel, which can affect performance.
3. Grounding Grid Impedance Testing
Once the grounding grid is constructed, its resistance must be accurately tested to verify the design and provide precise parameters for the operating team. Due to factors like soil composition, physical state, and seasonal variations, the grounding resistance can change over time. During testing, the grounding rod should be placed away from the substation to avoid interference from buried metal objects or layered soil, which can distort readings. Tests should be conducted under similar humidity and temperature conditions to ensure consistency.
The quality of the grounding grid is vital for the safe and reliable operation of the substation. It requires careful attention from power authorities throughout the design, construction, and testing phases. By ensuring reasonable design, high-quality construction, and accurate testing, the grounding system can perform optimally and contribute to the stability of the entire power network.