Why Cable Sheaths Carry Current
In single-core high-voltage cables, the metallic sheath or screen that surrounds the main conductor serves dual purposes: it provides a defined ground reference for the electric field, ensuring radial electrostatic stress distribution within the cable insulation, and it carries fault current during ground fault conditions, protecting the insulation from thermal damage. Under normal operation, however, the alternating current in the main conductor induces a voltage in the metallic sheath through electromagnetic induction — essentially, the cable behaves as a 1:1 transformer with the conductor as primary and the sheath as secondary.
The induced sheath voltage is proportional to the conductor current, cable length, and the mutual inductance between conductor and sheath, which depends on cable geometry and spacing. For a typical 132kV single-core cable carrying 500A, the induced sheath voltage can reach 50-100V per kilometer of cable length. If both ends of the sheath are solidly grounded, this induced voltage drives a circulating current through the sheath-ground loop. The magnitude of this current can approach 50-80% of the conductor current in long cable circuits — representing significant I²R losses, sheath heating, and reduced cable ampacity.
Sheath Bonding Methods
To manage induced sheath voltages and currents, several bonding configurations are employed in high-voltage cable installations:
Single-Point Bonding
In single-point bonding, the sheaths of all three phases are connected together and grounded at one end of the cable route only. At the ungrounded end, sheath voltages appear during normal operation. Surge voltage limiters (SVLs) — typically nonlinear metal oxide varistors — are installed at the ungrounded end to limit transient overvoltages during switching surges or lightning events. Single-point bonding eliminates circulating currents entirely during normal operation, maximizing cable ampacity, but the sheath voltage at the open end can reach hazardous levels during faults, requiring careful SVL coordination.
Cross-Bonding
Cross-bonding is the most widely used method for long high-voltage cable circuits. The cable route is divided into three equal minor sections. At each joint between minor sections, the sheaths are transposed: phase A sheath connects to phase B in the next section, phase B to phase C, and phase C to phase A. After three minor sections — one complete transposition cycle — the vector sum of induced voltages is theoretically zero, and the sheaths can be solidly grounded at both ends of the major section without significant circulating current. In practice, minor section lengths cannot be perfectly equal, and residual circulating currents of a few percent of conductor current are typical.
Both-Ends Bonding
In both-ends bonding, sheaths are solidly grounded at both cable ends. This is the simplest configuration but results in the highest circulating currents and is generally limited to short cable runs — typically under 100 meters — where induced voltages are low enough that circulating current losses are acceptable.
Why Monitor Sheath Circulating Current?
Changes in sheath circulating current over time are a sensitive indicator of developing problems in high-voltage cable systems:
- Sheath bonding failure: Corrosion or mechanical damage to sheath bonding connections can cause open circuits, resulting in dangerous induced voltages appearing on accessible sheath connections.
- SVL degradation: In single-point bonded systems, degradation of surge voltage limiters changes the impedance of the sheath grounding path, altering circulating currents.
- Water tree growth: Water ingress into cable insulation creates conductive paths that modify the electric field distribution and can manifest as changes in sheath current characteristics.
- Joint degradation: Partial discharge or overheating at cable joints can alter local capacitance and resistance, producing measurable changes in sheath current.
The SCYC-HLJC2304 Integrated Monitoring Solution
The SCYC-HLJC2304 high-voltage cable sheath circulating current and partial discharge online monitoring system integrates sheath current measurement with PD detection in a unified platform. Current sensors — either split-core CTs for retroactive installation or solid-core types for new builds — clamp around each sheath grounding conductor to measure circulating currents on all three phases simultaneously.
The monitoring device maintains a continuous record of sheath currents, computing phase imbalance and trending changes over time. Configurable alarm thresholds alert operators to abnormal conditions, while the integration with PD monitoring provides complementary diagnostic information: a simultaneous increase in sheath current and PD activity strongly suggests a developing cable fault requiring investigation.
The system’s IP68-rated sensors are suitable for installation in cable trenches and manholes where occasional flooding may occur. Wireless communication from the monitoring device to the substation data concentrator eliminates the need for additional signal cables along the cable route.
Related Products from Qingdao Britop
- SCYC-HLJC2304 Cable Sheath Circulating Current and PD Monitoring System
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