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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 of a substation plays a critical role in ensuring the safe and stable operation of the facility, as well as protecting both personnel and equipment from potential hazards. However, due to incomplete design, inaccurate construction, or improper testing, numerous accidents caused by faulty grounding grids have occurred in recent years. These incidents not only damaged primary equipment but also led to secondary control cable failures, causing widespread disruptions in the control room. This highlights the crucial role that a properly designed and maintained grounding grid plays in maintaining the reliability and safety of the entire power system.
1. Grounding Grid Design
Currently, many substation grounding designs are basic and lack detailed specifications. The layout diagrams often only show the main grid lines without marking specific grounding points for special equipment. Areas with high equipment density are frequently overlooked, and there is no separate grounding plan for critical zones such as the control room, high-voltage room, and wall bushings. Additionally, the design department usually does not provide clear calculation methods for the grounding grid, nor do they explain how key parameters like soil resistivity and short-circuit current are determined. Many designers are unclear about where to obtain soil resistivity data, how it should be measured, or whether it accurately reflects soil layering. When calculating the grounding short-circuit current, the selection of shunt coefficients for fault points and lightning arresters is often arbitrary, leading to an overly low resistance value in the design. A comprehensive and accurate grounding grid design is essential for ensuring the long-term stability and safety of the power system. Key design parameters include the ground short-circuit current, soil resistivity, and grounding resistance, which directly affect the performance of the grounding system.
1.1 Calculation of Ground Short-Circuit Current
According to the power industry standard DL/T 621-1997, the formula for calculating the ground short-circuit current is I = (Imax - In)(1 - Kel) and I = In(1 - Ke2), taking the maximum value. Here, I represents the current flowing through the grounding grid during a short circuit. Imax refers to the maximum ground short-circuit current when the system is grounded. This formula applies specifically to effectively grounded systems and can be obtained from the operating or relay protection departments, or calculated manually. Typically, the maximum operating mode is used when a single-phase ground fault occurs. In represents the short-circuit current flowing into the transformer's neutral point during a ground fault. If the transformer’s neutral point is ungrounded, In equals zero, simplifying the equation to I = Imax(1 - Kel). For transformers with one neutral point, In is approximately 30% of Imax, while for two neutral points, it is around 50%. 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 grounding grid. According to expert recommendations, Kel depends on the number of outgoing lines. For one line, it is 0.15; for two lines, 0.28; three lines, 0.38; four lines, 0.47; and five or more lines, between 0.5 and 0.58, depending on the shunting effect across the corridor. Ke2 is the shunt coefficient for external grounding of the transformer’s neutral point, typically 0.18. When considering future expansion, the calculated values should be multiplied by a development factor of 1.2–1.5, and in areas with poor soil dispersion, a flow coefficient of 1.25 may be applied. Based on these calculations, it becomes evident that when two neutral points are grounded, the ground short-circuit current could exceed the internal value.
1.2 Soil Resistivity Ï
Soil resistivity (Ï) is a fundamental parameter in determining the effectiveness of the grounding grid. When selecting a substation location, the soil conditions must be carefully evaluated. If the surface soil has a high resistivity, it may not meet the required grounding resistance (R ≤ 2000/I). Therefore, additional measures such as using copper grounding bars may be necessary to reduce resistance. In some countries, stricter limits like IR < 650V are enforced due to the increased sensitivity of microcomputer-based protection systems.
1.3 Grounding Resistance Requirements
According to DL/T 621-1997, the grounding resistance of the system must satisfy R ≤ 2000/I, meaning that the product of the grounding current and resistance should not exceed 2000 volts. However, with modern protection systems, achieving this limit is challenging, especially in areas with high soil resistivity. As a result, alternative solutions such as installing copper grounding strips are often employed to ensure compliance.
2. Grounding Grid Construction and Installation
Poor technical skills among some construction teams and limited supervision can lead to issues such as poor welding, disconnected grounding lines, or failure to mark changes during installation. Some contractors treat the final layout as an as-built drawing without documenting any modifications made during construction. To prevent such problems, strict inspections and tests must be conducted by qualified personnel. Intermediate and final acceptance checks should be carried out, and any defects found should prompt immediate rework to maintain quality standards.
During construction, attention should be given to several key aspects:
- The galvanized flat steel on the main grounding line should be installed vertically to minimize corrosion.
- The control room’s grounding should form a ring network. When the main line passes through the control room, grounding wires should be drawn from both sides to the upper floor, and building reinforcement should be connected to the main grounding line for better performance.
- Wall bushing grounding should be placed outdoors, with each group’s grounding wire connected to the main line to enhance safety for personnel and indoor equipment.
- Primary equipment grounding wires must not be connected to the flat steel in the cable trench or suspended through it.
- When backfilling after laying the grounding grid, clean original soil should be used beneath the grid, avoiding the use of contaminated soil or gravel.
3. Grounding Grid Impedance Testing
After the grounding grid is constructed, an accurate measurement of its resistance is essential to verify the design and provide reliable parameters for the operating team. Due to factors such as soil composition, physical state, and seasonal variations, the resistance of the grounding grid can change over time. During testing, the grounding rod should be placed far from the substation to avoid interference from complex soil conditions or buried metal objects. To ensure consistency, the test should be conducted under similar humidity and moisture conditions as those assumed during the design phase.
In conclusion, the quality of the grounding grid is a critical factor in ensuring the safe and reliable operation of a substation. It requires careful attention from power authorities throughout the design, construction, and testing phases. Efforts should be made to achieve a reasonable design, precise construction, and accurate testing to guarantee the long-term performance and safety of the system.