In a 10kV power grid, when a single-phase grounding fault occurs, the voltage of the non fault phase will rise to the line voltage (10kV). Why not choose 10kV for medium voltage cable U ₀, but 8.7kV? This is mainly based on the following technical basis:
1. Definition of MV cable rated voltage
In standards, the rated voltage of cables is usually expressed as U ₀/U (Um):
U ₀: Single phase to ground rated voltage (phase voltage)
U： Two phase rated voltage (line voltage)
Um： maximum working voltage
For medium voltage cables of 8.7/15kV:
U ₀=8.7kV (single phase to ground rated voltage)
U=15kV (rated voltage between two phases)
Um=17.5kV (maximum operating voltage)
2.How is the rated phase voltage of 8.7kV determined?
The rated phase voltage U ₀ of the cable is not directly selected based on the maximum instantaneous voltage under fault conditions (such as 10kV), but is comprehensively considered based on the normal operating voltage of the system and fault conditions:
(1) Based on the normal operating status of the system
Under normal operating conditions:
The phase voltage of the 10kV system U ₀=10kV/√ 3 ≈ 5.77kV.
If the medium voltage cable uses 8.7kV as U ₀, it means that the insulation design margin of the medium voltage cable is much higher than the normal operating voltage.
(2) Consider voltage rise during single-phase grounding fault
When a single-phase grounding fault occurs, the phase voltage of the non faulty phase will rise to the line voltage level (10kV).
However, the insulation design of medium voltage cables is not entirely based on short-term fault voltage selection, but should take into account normal operating conditions, fault tolerance time, and long-term stability.
The U ₀ of 8.7kV is sufficient to ensure the reliable operation of medium voltage cables during faults, while considering both economy and safety margins.
(3) Compliant safety margin
International standards such as IEC 60502-2 and GB/T 12706.2 stipulate that:
A medium voltage cable with a rated voltage of 8.7/15kV, with U ₀=8.7kV, can withstand a voltage of 8.7kV for a long time.
In the case of short-term faults (such as single-phase ground faults), medium voltage cables can withstand short-term overvoltages higher than 8.7 kV, but there is no need to directly set U ₀ to 10 kV.
3.Why not directly choose 10kV as the U ₀?
If the cable is directly selected as U ₀=10kV, although it may seem to improve the fault tolerance, it is not suitable in practical engineering, mainly due to the following reasons:
(1) The insulation margin is sufficient
The insulation thickness and dielectric strength of the cable with a rated voltage of 8.7/15kV have been designed to withstand the voltage rise during single-phase grounding faults without affecting safe operation.
If U ₀ is increased to 10kV, it means that the insulation layer needs to be further thickened, increasing manufacturing costs, and such a high margin is not required in actual engineering.
(2) Economic considerations
Thickening the insulation layer not only increases manufacturing costs, but also increases the outer diameter and weight of the cable, affecting the difficulty of cable laying and increasing construction and maintenance costs.
At present, the 8.7kV design is reliable enough and there is no need to add additional costs.
(3) Unified engineering standards
International standards (IEC, GB) generally adopt 8.7/15kV as the recommended cable grade for 10kV power grids. Using 10kV U ₀ cables will result in incompatibility with existing standards and affect engineering applications.
In summary, selecting cables with 8.7/15kV specifications in a 10kV power grid is the best choice that has been verified through long-term practice.