What impact does an ungrounded 10KV high-voltage single-core cross-linked polyethylene power cable have on its operation?

It is absolutely not allowed for both ends to be grounded through overvoltage protectors; the metal shielding layer (shield and armor) of high-voltage single-core cables must have one point that is directly grounded. If it is not directly grounded, a capacitive voltage division will occur between the metal conductor, the metal shield, and the ground, resulting in a very high voltage on the metal shield!

There should be one point of grounding, and only one. Grounding at two points creates induced circulating currents, which can cause the cable to heat up over time, accelerating its aging and even leading to fires. If one point is grounded, an induced potential may appear at the ungrounded end; I have measured an induced potential of 70 V. It is recommended to lay single-phase cables in a triangular configuration to reduce induced voltage.

The last edit to this post was made by jjli618 on 2010-7-26 at 12:57. Electrical safety regulations stipulate that the non-live metal enclosures of electrical equipment must be grounded; therefore, the aluminum coating or metal shielding layer of cables must also be grounded. Cables with voltage levels of 35 kV and below typically use a two-terminal grounding method. This is because most of such cables are three-core cables; under normal operation, the sum of the currents flowing through the three conductors is zero, and there is essentially no magnetic flux outside the aluminum coating or metal shielding layer. As a result, there is basically no induced voltage at either end of the aluminum coating or metal shielding layer, and therefore no induced current flows through it when both ends are grounded. However, when the voltage exceeds 35 kV, single-core cables are mostly used. The relationship between the core of a single-core cable and its metal shielding can be regarded as that of the primary winding of a transformer. When current flows through the core of a single-core cable, magnetic field lines intersect the aluminum coating or metal shielding layer, resulting in an induced voltage at its ends. The magnitude of the induced voltage is proportional to the length of the cable circuit and the current flowing through the conductors. When the cable is very long, the induced voltages on the sheath can add up to levels that pose a risk to human safety. In the event of a short-circuit fault, operational overvoltage, or lightning strike, very high induced voltages can be generated on the shielding, which may even cause the insulation of the sheath to break down. At this point, if the two ends of the aluminum coating or metal shielding layer are still connected to ground, large circulating currents will occur in these layers; such currents can range from 50% to 95% of the current flowing through the core. This results in losses that cause the aluminum coating or metal shielding layer to heat up. Not only is this a waste of a large amount of electrical energy, but it also reduces the cable’s current-carrying capacity and accelerates the aging of the cable’s insulation. Therefore, single-core cables should not have their ends grounded. However, when one end of the aluminum coating or metal shielding layer is not grounded, the following problem arises: when lightning current or overvoltage waves flow along the core, a very high surge voltage appears at the ungrounded end of the cable’s aluminum coating or metal shielding layer ； When a short circuit occurs in the system and short-circuit current flows through the conductor core, a relatively high power-frequency induced voltage will also appear at the ungrounded end of the cable’s aluminum coating or metal shielding layer. If the insulation of the cable’s outer sheath cannot withstand such overvoltage and gets damaged, it will lead to multiple points of grounding, thereby creating circulating currents. Therefore, when using one-end interconnected grounding, measures must be taken to limit overvoltage on the shielding layer. During installation, special connection and grounding methods should be employed at appropriate positions on the aluminum-clad or metal shielded layer, in accordance with the specific conditions of the circuit and on the principle of economic rationality; simultaneously, shielding layer protectors should be installed to prevent the insulation of the cable shielding layer from being damaged. Therefore, when installing high-voltage cable lines, in accordance with the requirements of GB50217-1994 \"Code for Design of Cables in Electric Power Engineering\", when a single-core cable line has its metal sheath grounded at only one point, the induced voltage at any point on the metal sheath should not exceed 50–100 V (and should not be greater than 50 V if no safety measures are taken to prevent arbitrary contact with the metal sheath) ； If effective measures are taken, it shall not exceed 100V), and it shall be insulated from ground. If the voltage exceeds this specified value, segmented insulation with metal sheaths or cross-connected wiring after insulation should be employed. To reduce the induced voltage on adjacent auxiliary cables and communication cables caused by single-core cable lines, cross-interconnection wiring should be used as much as possible. For cables of short length, single-point grounding can be used. To protect the insulation of the cable sheath, a sheath protector should be installed at the ungrounded end. It can be seen that there are the following grounding methods for high-voltage cable lines: 1. One end of the shielding layer is grounded directly, while the other end is grounded through the shielding layer protection——this method can be adopted ； 2. The midpoint of the shielding layer is directly grounded, while the ends are shielded through the protective grounding of the shielding layer – this is the common approach ； 3. Shield cross-interconnection ---- Common method ； 4. Cable reversal and interconnection of metal sheaths – the most effective grounding method ； 5. Grounding both ends of the shield——not commonly used, only applicable to very short cables and cable circuits with light loads.

For the explanations provided above, answers to all of them can be found in relevant standards or books. The key issue is that protective covers are used at both ends of the cable, and it is grounded through these covers; in effect, neither end of the cable is properly grounded. The construction team followed the design plans provided by the design institute exactly. The problem lies in the fact that there were no previous examples of such designs, nor were any design regulations stating that single-core power cables must be grounded. This is where the error in this design lies. I wonder if any expert in cables could explain the specific hazards of not grounding or what consequences it might lead to.