In the world of power transmission, medium-voltage (MV) cables (typically 6kV to 35kV) play a crucial role. If you look closely, you'll find that MV cables have a very fixed design in terms of the number of cores: either single-core or three-core. You almost never see four-core or five-core MV cables, which is a significant difference from common low-voltage cables.

Behind this lies a set of rigorous electrical engineering principles. Why can't MV cables have more cores like low-voltage cables? And if we need a three-phase MV power supply with a grounding wire, what are the feasible alternatives?

1. The Mystery of MV Cable "Single-Core" or "Three-Core": Electrical Stress and Eddy Currents

The design of MV cables is constrained by two core electrical principles: the electrical stress on the insulation layer and the eddy currents generated by electromagnetic induction.

l Electrical Stress

·  The insulation layer of an MV cable is much thicker than that of a low-voltage cable because the higher the voltage, the greater the electric field strength the insulation must withstand.

·  The design of single-core or three-core cables ensures a uniform distribution of the electric field within the insulation layer, avoiding localized concentrations. In a three-core cable, the three phase conductors are twisted together, creating a symmetrical electromagnetic field that helps balance electrical stress and ensure the safety of the insulation.

·  Integrating a fourth conductor, such as a grounding wire (typically a neutral wire), into the MV cable would break the electrical symmetry of the three-phase cable. The presence of the grounding wire would cause the electric field strength in its vicinity to increase sharply, creating an electric field concentration point. This would greatly reduce the life of the insulation and could even lead to partial discharge and insulation breakdown.

l Eddy Current

·    As three-phase current flows through the cable, it generates an alternating magnetic field. In a single-core cable, the three cables are laid in parallel, and their respective magnetic fields cancel each other out, reducing electromagnetic induction on surrounding metal structures.

· In a three-core cable, the three phase conductors are tightly twisted together, and their magnetic fields form a symmetrical vector sum, minimizing the external magnetic field effect of the cable. This effectively prevents the generation of eddy currents in the cable's metal armor, pipes, or trays, thereby reducing heat and energy loss.

·  If a fourth conductor (grounding wire) is added to the three-core structure, this electromagnetic symmetry is destroyed. The extra conductor (even without current) will distort the original magnetic field, causing eddy currents to be generated in the metal armor around the cable. This will not only cause the cable to heat up and increase losses, but it also poses a threat to the long-term safe operation of the cable.

Conclusion: Based on the strict consideration of insulation safety and electromagnetic symmetry, the design standard for MV cables only allows for single-core or three-core structures

2. Alternatives for Three-Phase MV + Grounding

Since a grounding wire cannot be integrated into an MV cable, how do we connect a three-phase MV power supply with a grounding wire in actual projects? There are three common and reliable alternatives.

l Alternative 1: Using three single-core cables + one single-core grounding wire

n Structure: This is the most common solution. Three independent single-core cables are used as the three phase conductors (A, B, C), and a separate single-core cable is laid as the grounding wire (PE)

n Advantages:

² High Flexibility: You can choose a grounding wire with a different cross-sectional area based on actual needs. For example, in ungrounded neutral systems, the grounding wire may only need a very small cross-section.

² Electrical Safety: This independent laying method completely avoids issues of electrical asymmetry and eddy currents.

² Easy Maintenance: If one of the phase or grounding wires fails, it can be replaced individually without having to replace the entire cable.

n Application Scenarios: Most MV power transmission projects, especially when cables need to be laid over long distances or in separate zones.

l Alternative 2: Using a three-core cable + a separately laid single-core grounding wire

n Structure: A complete three-core cable is used for the three phase conductors, and a separate single-core cable is laid in parallel in the cable trench, tray, or conduit as the grounding wire.

n Advantages:

² Installation Efficiency: Laying one three-core cable is faster than laying three single-core cables, saving construction time.

² Compact Structure: Especially in confined spaces, the overall outer diameter of a three-core cable is usually smaller than that of three single-core cables bundled together, making efficient use of space.

n Application Scenarios: Confined environments such as compact substations, cable wells, and tunnels where space is a premium.

l Alternative 3: Using the Metal Armor as a Grounding Wire (PE)

n Structure: This solution is mainly used for single-core or three-core MV cables with metal armor (such as copper tape or aluminum tape). In this case, the metal armor layer of the cable can be used directly as a shielding and grounding protection layer.

n Advantages:

² Cost Savings: No need to purchase and lay a separate grounding wire.

² Simplified Construction: Eliminates the extra step of laying a separate grounding wire, simplifying the construction process.

² Powerful Grounding Protection: The armor layer provides very reliable shielding and grounding protection for the cable, effectively preventing leakage and electromagnetic interference.

n Application Scenarios: Primarily used in systems with directly grounded neutrals. In such systems, the cable armor is usually connected to the system ground, serving as a path for fault currents. This solution is common in projects like urban underground utility tunnels and large industrial park

Conclusion

MV cables are limited to single-core or three-core designs based on the rigorous consideration of electrical stress balance and electromagnetic symmetry. The need for a grounding wire is met through proven and reliable alternatives such as laying a separate single-core cable or utilizing the cable's own metal armor.

Correctly understanding these design principles and alternatives not only helps us make better cable selections but also ensures the long-term stability and safety of the power system.