What problems do online monitoring of partial discharges in cables and monitoring of sheath current address, respectively?

Online monitoring of partial discharges in cables focuses on insulation defects, while sheath loop current monitoring focuses on abnormalities in the grounding current of the metal sheath. The former is used to detect partial discharge risks in areas such as cable joints and the cable body; the latter determines whether there is abnormal heating or operational hazards in the sheath grounding system by monitoring the current at the sheath grounding point.

When selecting equipment for online monitoring of partial discharges in cables and sheath current, considerations should be based on on-site equipment, control logic, communication methods, and maintenance procedures. Power equipment accessories and online monitoring devices typically do not operate in isolation; rather, they work in conjunction with transformers, switchgear, cables, backend platforms, alarm circuits, and operations and maintenance procedures. Clearly defining the application scenario is more beneficial for procurement and subsequent maintenance than simply listing model numbers.

1. Core Features and Target Audience

2. When selecting a model, first consider the on-site requirements

Partial discharge monitoring is suitable for assessing the condition of insulation. Once insulation defects occur in cable joints, terminations, or the cable body, partial discharges may appear before the fault escalates. Sheath circulating current monitoring is suitable for assessing the condition of the metal sheath grounding system; abnormal circulating currents may lead to overheating, increased losses, or risks to the grounding system. The two monitoring methods focus on different aspects and cannot be used as substitutes for one another.

3. Deployment and Operations Considerations

Cable monitoring must be integrated with the line registry. The registry should include the line name, voltage level, length, joint locations, terminal locations, grounding method, and historical defects. This way, when an alarm is triggered, operations and maintenance personnel can quickly correlate the data with the on-site location, thereby improving the efficiency of re-inspection and maintenance.

When requesting a quote or discussing technical matters, we recommend having the following information ready: equipment model numbers, on-site photos, installation locations, wiring configurations, backend integration requirements, and details about the existing system. For retrofit projects, you should also provide the nameplates of the original equipment, photos of the terminals, and a description of the control circuits; for new construction projects, you should plan the power supply, communications, panel space, measurement point names, and alarm levels simultaneously during the design phase.

4. Frequently Asked Questions

1. Can partial discharge and sheath current be used interchangeably?

No, one focuses on insulation discharge, and the other focuses on sheath grounding current.

2. For which cables is sheath circulation suitable?

The documentation mentions that it can be used for monitoring grounding points in the sheaths of high-voltage cables rated at 10 kV and above.

3. Why is it necessary to maintain a route log?

This makes it easy to quickly pinpoint the source of an alarm to a specific circuit, connection, or grounding point.

Addendum: How to Interpret Detection Signals

Partial discharge alarms and abnormal sheath circulating currents in cables generally cannot be evaluated in isolation. Partial discharge signals indicate that the insulation condition may be deteriorating, while abnormal sheath circulating currents suggest potential issues with the grounding system or the sheath circuit. If both types of signals show abnormalities simultaneously, the level of concern should be elevated; if only a single signal is abnormal, the assessment should be made in conjunction with line load, joint locations, historical defects, and on-site re-measurements. For critical cable lines, it is recommended to compile partial discharge trends, sheath current trends, joint logs, and maintenance records into a single operations and maintenance file.

During on-site implementation, the names of the measurement points must match those on the line drawings. For example, a specific cable terminal, an intermediate joint, or a sheath grounding point should all be clearly numbered in the system. This way, when an alarm is triggered, maintenance personnel can directly locate the cable manhole, terminal cabinet, or grounding point, thereby reducing troubleshooting time.

7. Summary

The value of online monitoring of partial discharges in cables and sheath current lies not merely in completing a single installation, but in making it easier to detect, record, and address equipment conditions. Clearly defining the target equipment, interfaces, control logic, and O&M procedures during the selection process will ensure more stable operation in the long run. If there are any questions regarding model numbers, dimensions, interfaces, or technical details, refer to the project technical agreement and the manufacturer’s latest documentation.

5. When deploying on-site, first identify the monitoring targets.

Although online monitoring of partial discharges in cables and online monitoring of sheath circulating currents both fall under the category of cable condition monitoring, they address different failure mechanisms. The former focuses on detecting discharge signals that may occur within the insulation system or at accessories, and is suitable for determining whether there are signs of insulation degradation in the cable body, intermediate joints, terminations, and other locations; the latter focuses on monitoring changes in the metal sheath current and is suitable for identifying anomalies caused by grounding methods, cross-connections, sheath insulation, and changes in operational load. If these two methods are simply understood as “cable monitoring” at the project site, it can easily lead to unclear placement of measurement points and difficulty in interpreting alarms, which may also affect subsequent maintenance decisions.

During the procurement and solution discussion phases, it is recommended to first clarify the voltage rating of the cable line, the installation method, the number of joints, the terminal locations, the distribution of grounding boxes, and whether an integrated online monitoring platform is already in place. For partial discharge monitoring, attention should be paid to sensor placement, signal acquisition, interference suppression, and trend analysis; for sheath current monitoring, attention should be paid to the monitoring circuit, grounding box locations, data upload, and anomaly threshold management. If the line’s operating environment is complex, these two methods can be used as complementary measures but should not be used as substitutes for one another.

6. How Operations Personnel Use Monitoring Results

The value of a monitoring device lies not merely in triggering a single alarm, but in helping operations and maintenance personnel form a continuous assessment of the system’s status. Partial discharge data must be evaluated in conjunction with operational load, environmental interference, historical trends, and on-site re-measurements; sheath current data must be analyzed in conjunction with load current, grounding methods, and line structure. In cases where trends continue to rise, significant discrepancies exist among similar measurement points, or alarms recur frequently, on-site verification should be arranged. For occasional data fluctuations, external factors such as construction, thunderstorms, and switch operations should first be ruled out to prevent the monitoring system from becoming a mere alarm device.