Substation reliability depends not only on how quickly a fault can be repaired, but also on how early developing problems can be identified. Power transformers, gas-insulated switchgear (GIS), circuit breakers, HV cables, busbars and protection systems operate under continuous electrical, thermal, mechanical and environmental stress.
For maintenance teams, the practical question is therefore not whether these assets need attention, but how maintenance should be planned. A purely reactive strategy waits for a failure or obvious performance problem. A predictive or condition-based strategy uses inspections, testing, online monitoring and equipment data to identify changes before they develop into a major outage or emergency repair.
Modern monitoring technologies make this shift increasingly practical. Partial discharge (PD) monitoring can support condition assessment of GIS, power transformers and HV cable systems, while digital sensing and protection technologies can provide synchronised data from remote parts of a network. Together, these tools help operators move from time-based intervention alone toward maintenance decisions based on actual asset condition and risk.
Reactive maintenance is often described as a “fix it when it breaks” approach. Equipment remains in service until a fault, failure or noticeable performance issue occurs, after which the maintenance team investigates the problem and carries out repair or replacement.
This approach can be reasonable for low-criticality equipment where a failure has limited operational consequences. It becomes much less attractive when the asset is expensive, difficult to access or essential to system reliability.
A sudden failure of critical substation equipment can result in:
The main limitation is that some electrical defects provide little obvious warning to operators unless the equipment is inspected, tested or continuously monitored.
Predictive maintenance takes a condition-led approach. Instead of waiting for equipment to fail, maintenance teams track the health and performance of important assets and look for signs of deterioration, abnormal behaviour or changing operating conditions.
Depending on the asset, this can include visual inspection, electrical testing, temperature and mechanical monitoring, protection-system data, waveform analysis and partial discharge detection. The objective is not to perform more maintenance. It is to direct maintenance effort toward the assets that actually need attention and to plan intervention at a more appropriate time.
Substations contain assets that are costly, interconnected and operationally critical. A defect in one major component can affect protection, isolation and power transfer beyond the equipment where the problem started. Transformer failures can require lengthy repair or replacement, while faults in GIS or cable systems can be difficult to diagnose and locate without suitable monitoring data.
Predictive maintenance gives operators more information about asset condition, making it easier to prioritise inspection, testing, spares planning and outage work. This is particularly valuable when equipment is ageing, access is difficult or maintenance resources must be focused on the highest-risk assets.
Not every asset requires the same level of monitoring. The maintenance strategy should reflect the equipment’s criticality, age, operating environment, duty and potential consequence of failure.
Transformer monitoring can include loading, temperature, insulation condition and abnormal electrical activity. Where partial discharge monitoring is appropriate, UHF, HF and acoustic methods can be used for detection, diagnosis and location of PD activity.
GIS is compact and highly reliable, but internal defects can be difficult to identify by external inspection alone. UHF and complementary PD detection methods can provide useful condition information, while circuit-breaker operating performance and protection behaviour should also be monitored and tested.
Cable systems are exposed to electrical, thermal and environmental stress, and faults may occur in the cable itself, joints or terminations. Condition assessment, partial discharge monitoring and accurate fault or PD location can reduce the time needed to investigate long or inaccessible cable routes.
Cable systems are exposed to electrical, thermal and environmental stress, and faults may occur in the cable itself, joints or terminations. Condition assessment, partial discharge monitoring and accurate fault or PD location can reduce the time needed to investigate long or inaccessible cable routes.
A properly designed and maintained earthing system remains essential for electrical safety and fault performance. Routine inspection and testing should therefore remain part of a complete substation maintenance programme even when advanced condition monitoring is deployed.
Partial discharge monitoring is especially relevant to high-voltage insulation systems because it provides another source of condition information between scheduled maintenance intervals. The supplied ITL/SDMT product literature includes both permanent online monitoring systems and portable instruments for GIS, transformers and HV cables.
For GIS, the PD700 is described as a high-sensitivity, multi-channel 24/7 UHF monitoring system with an advanced timing system for real-time location of PD sources. Its PD Expert analysis system is designed to recognise and classify PD patterns, reject noise and generate alarms. For site testing, the PD71 provides HF and UHF analysis, while the PD71X combines HF, UHF and acoustic channels and can use pulse-arrival timing to improve PD location. The PD74i handheld detector integrates UHF, acoustic, HF, TEV and acoustic-imaging methods in one portable platform.
For power transformers, the PD700T is a 24/7 UHF monitoring system intended for transformers fitted with UHF sensors. The supplied flyer recommends a minimum of four UHF sensors for accurate location and describes a time-of-flight algorithm that calculates PD propagation paths, supporting location and spatial separation of different PD sources. Portable options include the PD71T for UHF location and analysis and the PD71X for mixed HF, UHF and acoustic sensing. The PD74i can also be used for transformer testing.
For HV cables, the PD700C monitors cable sections between pairs of local acquisition units (LAUs). The system uses synchronised time tagging of captured PD pulses so that the location of PD activity can be calculated between sensors. The literature also describes source segregation, noise separation and TDR-based single-end measurement for pinpointing PD location. The PD61 is a portable high-frequency analyzer for cable PD detection, diagnosis and location, including double-end location capability, while the PD71X and PD74i provide additional portable testing options.
ITL-UK, in technical collaboration with Synaptec, developed Distributed Electrical Sensing (DES) device as a passive, fibre-optic technology based instrumentation approach for the protection and condition monitoring in substations and wider power networks. The passive sensors do not require local data networks, control power or expensive remote civil works at each measurement point, which can be valuable where equipment is distributed across large or difficult-to-access sites.
For digital-substation applications, these sensors, work on centralized monitoring of multiple feeders using passive devices connected by standard single-mode optical fibre. It also describes synchronised waveform data, IEC 61850 Sampled Values, IEC 61850 GOOSE or dry-contact trip outputs, and automated condition monitoring of critical assets. Passive temperature and strain transducers are shown as part of the condition-monitoring architecture.
From a predictive-maintenance perspective, the important benefit is the ability to bring electrical and mechanical measurements into one centralised system. This can give maintenance teams earlier visibility of changes in asset behaviour and can help optimise scheduled maintenance rather than relying only on periodic site visits.
As networks become more complex through undergrounding, new connections, distributed energy resources and multiple branches, identifying the faulted section can become more difficult. The ITL/Synaptec line-differential passive remote sensors connected over optical fibre to a central Interrogator, allowing multiple measurement locations to support selective protection without the same level of remote power, telecoms and civil infrastructure used by conventional schemes
Mixed or hybrid circuits combine overhead lines with underground cable sections or transformer connections. These sections behave differently during faults: overhead faults are often suitable for auto-reclose, while an underground cable fault may require auto-reclose to be blocked. The supplied MCP Sensors are designed around this distinction, using multi-zone differential measurements to identify the faulted section quickly and inform the protection response.
The MCP system is described as monitoring differential currents on up to five cable sections from one system. It uses Passive Secondary Converters (PSC) with a Distributed Electrical Sensing (DES) Interrogator and can provide ANSI 50G earth-overcurrent and 87L differential-current protection logic. It is also described as compatible with conventional relays and capable of streaming Sampled Values and GOOSE messages for IEC 61850 / IEC 61869 systems.
Substation maintenance is increasingly moving beyond a simple repair-after-failure model. Predictive and condition-based maintenance can give operators better visibility of critical assets and allow inspection, testing and corrective work to be planned before a developing problem becomes an emergency.
CTs, PTs and VTs may seem like small components within a larger electrical system, but they perform a vital job. They act as the connection between high-power electrical circuits and the instruments that help engineers measure, monitor and protect those circuits.
Partial discharge monitoring provides targeted condition information for GIS, power transformers and HV cable systems. DES-based instrumentation can centralise electrical and mechanical measurements from remote locations, while cable fault detection, line differential protection and mixed circuit protection can improve faulted-section identification and protection decisions in complex networks.
Reactive maintenance will always have a role when unexpected failures occur. However, for critical assets, a balanced strategy that combines preventive maintenance, condition monitoring and predictive analysis can support safer planning, improved reliability and more efficient use of maintenance resources.
At Gleam International, electrical infrastructure requirements are supported by our principal & technology partner ITL-UK, offering a broad range of products including Current & Voltage Transformers for Low, Medium and High Voltage applications with standard electrical parameters as well as bespoke design to suit the exact project site requirement, Partial Discharge Sensors (PDS) for the protection of GIS substation, Power Transformers & Power Cables, DES, ACMT and Mixed circuit Protection sensors for overhead and underground power cable network in predictive maintenance of the smart Grid Substation.
