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Harald Mayer |

On-site condition assessment - How to prevent unplanned outages?

Siemens offers comprehensive on-site assessment solutions to keep its customer’s switchgear systems up and running.


Air-insulated (AIS) and gas-insulated switchgear (GIS) are extremely important elements in transmission networks. It’s crucial to evaluate and understand their current condition in order to know when and where maintenance activities may be required before a fault causes a disruption in the grid. With its new and economical Onsite Condition Assessment Solutions, Siemens is ensuring the smooth operation of its customer’s assets.


There are many challenges for high-voltage switchgear: increasing load requirements, high demands from customers and processes, and limited backup power. But the number of these high-voltage components over 30 years old is increasing, making defects or failures more likely. In order to prevent major damage due to aging devices, it’s crucial to properly monitor and service high-voltage switchgear regularly with as little downtime as possible.


Analyze the past, assess the present, and predict the future

Siemens offers a broad range of proactive services for all phases of GIS and AIS lifecycle management. Deviations from the specified conditions are identified by condition assessments, and maintenance and spare-part services bring the asset back to optimal condition. “Siemens’ Customer Services have a strong heritage of expertise across transmission and distribution equipment, with highly skilled technical experts globally available,” says Armand Tsague, Field Service Engineer at Siemens. “With our Onsite Condition Assessment packages for GIS and AIS, we analyze the past, assess the present, and can predict the future of the asset’s condition. This significantly and economically enhances the reliability and availability of the switchgear.”


High flexibility and accuracy with portable equipment

Siemens’ Service Technicians use a variety of assessment methods. Of course, each method can stand alone, but the company prefers a combination of assessments in order to provide a holistic view of the condition of the GIS or AIS and help increase the accuracy of the diagnostics. “Performing measurements frequently and using trend analyses can also help predict the future condition of the asset,” says Tsague. Using portable measurement equipment, the Siemens experts take an in-depth look at its customer’s high-voltage equipment wherever necessary.


Levels of condition assessment

The company’s condition assessment portfolio is split into three levels. The first and second levels are performed while the asset is running (no switch-off required). The most important is the partial-discharge measurement. For temporary monitoring, the experts prefer to use ultra-high-frequency (UHF) methods to detect partial discharges in the switchgear (see info boxes on the right for more information). They might combine this method with acoustic partial-discharge detection, a visual inspection that employs the audit tool SAFE, a thermography scanning of the entire substation, and a gas analysis to measure moisture, SF6 percentage, dew point, sulfur dioxide (SO2) content, and other values. The third level comprises additional electrical measurements that can only be performed when the asset is out of operation: for example, dynamic contact resistance measurement (DCRM) of the breaker, timing tests, outdoor bushing measurement, and transformer current measurement.


Benefits to operators

“Our on-site partial-discharge monitoring offers a number of advantages to operators even if they aren’t using Siemens’ equipment,” Tsague explains. These benefits include precise failure prediction, access to real-time condition data by installing UHF sensors, an early warning of malfunctions, and keeping the asset healthy and in service for as long as possible – which delays the need for major investments in new equipment and avoids the high cost of repairing serious damage.


What is a partial discharge (PD)?

PD is a partial dielectric breakdown in an electrical insulation system under high voltage. It causes continuous deterioration of the insulating material that could lead to a complete breakdown and outage. Therefore, it’s important to detect PD at an early stage in order to prevent unplanned downtime. The main causes of PD in gas-insulated switchgear (GIS) include moving particles, voids in solid insulation, floating electrodes, defective insulators, and protrusions or corona.


Sensors for ultra-high-frequency (UHF) measurement

Partial discharges in a GIS generate current and acoustic signals, and they also produce ultra-high-frequency (UHF) electromagnetic waves. These UHF signals can be detected using special sensors. If the GIS isn’t pre-equipped with embedded sensors, external ones can sometimes be installed. Studies have confirmed that UHF measurement is very sensitive to partial discharge signals, and it’s becoming the dominant method for both commissioning tests and temporary and permanent monitoring.


Onsite Condition Assessment can be used as a trigger for condition-based maintenance and asset management



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