Corona & tracking conditions in MV Switchgear
"Partial discharge activity has long been accepted as a major cause of failure of medium voltage switchgear…" Testing Distribution Switchgear for PD in the Laboratory and Field, IEEE 2008
The advice that all power systems should be operated discharge-free is a very good one and so in the past 10 years industries worldwide came to the understanding that this advise is even practicable with the correct type of instrument and form of inspection. Over the past years the need for corona free maintenance has increased especially in switchgears due to their elaborated design and the repair limitations and thus corona camera begun to have a major role in all predictive maintenance of corona in switchgears.
The medium voltage switchgear is the core "component" of electrical networks and of industrial users. In catastrophic occurrence, an internal failure may have a significant impact on power availability to supplied loads in particular, as restoration time can be long.
While mechanical issues of the switching equipment are often addressed by manufacturer's prescribed maintenance and related conditions are easier to check; insulating parts, surface degradation, especially if triggered from environmental or electrical stresses may become critical in the equipment operating life when aging phenomena starts and standard maintenance procedure are not likely to prevent or detect them.
In this article we try to explain the process of corona on switchgears components, its factors and the importance of corona inspection with a corona camera.
Partial discharge (PD) activity associated to an insulation aging and degradation process typically increases with the defect's evolution as in a positive loop. Therefore, PD activity measurement can be used as a diagnostic tool to assess equipment condition and to locate defect source, enabling selective intervention to remove aging component prior to complete failure.
Once corona becomes active, it leaves behind a conductive "tracking" (Read more about "tracking") path on surfaces and also creates a very conductive cloud of air around itself. A flash-over can occur once a tracking pathway is completed from phase to phase or phase to ground. It can also occur from the conductive cloud of surrounding air once it finds a path to ground.
Electrical discharge in the form of corona and tracking has caused many failures in switchgears with little advanced warning or understanding of the cause. This is especially frustrating for the end-user when infrared technology is being utilized as a predictive tool to prevent such occurrences. Corona and tracking conditions are voltage problems that rarely produce heat thus, usually go undetected during a typical infrared inspection.
Influencing Factors in Corona Formation
Geometric factors, spatial factors and environmental conditions are the main reasons for corona formation in switchgear; These 3 main factors have a major influence on the electrical field intencity. Once the electrical field rise above a threshold value (usually less than 30kV/cm), corona and tracking may appear. Due to the design on switchgears, once corona starts in a certain component it begins feeding of itself, deteriorating the insulation material of the component until reaches to its core.
Geometric factorsSharp edges on conductors, connections and switchgear cabinet components can include sharp and squared surfaces, tag ends on conductors, sloppy tape wraps and corners and other sharp edges on cabinet bracing and support shelves.
Spatial factorsSmall air gap spaces between conductors and switchgear cabinet components can be a reason for corona formation;
- Conductors being tie-wrapped together
- Conductors touching insulators, conduit, and edges of cabinets
- Non-shielded cables in contact with grounded surfaces
- Bus bars in close proximity to insulation board
Environmental conditionsHave a great affect on the presence of corona and tracking. Contamination in the forms of dust, oils/fluids, and other particulates on conductors and insulators will create corona. Also, switchgear rooms that are hot because of poor air circulation and cabinet enclosures that are subject to wet/ humid conditions are more likely of have corona and tracking activity over those cabinets that are cool and dry.
Early stages of corona may not show any visible or thermal signs. Typically, advanced cases of corona and tracking will be heard, If no noise is heard, it cannot be concluded that a problem does not exist. The interior geometric design of the cabinet may attenuate the noise wave and signal. Likewise, mild cases of corona may have a very weak discharge signal that is not heard until the cabinet is open.
"Attempting to measure and trend levels of observed ultrasonic activity and trying to relate them to the potential seriousness of surface partial discharge or end of life is not valid and could potentially lead to misleading conclusion" [Testing Distribution Switchgear for Partial Discharge in the Laboratory and the Field", by EA Technology, IEEE2008 ].
By nature the corona camera is most suitable solution for internal switchgear corona inspection. It can pinpoint the exact location of the problem and give you an estimation on the severity by counting the UV events. The below corona evidences, as were captured by Ofil's corona camera, are just few examples to express Ofil's corona camera added values.
Condition assessment of MV substation equipment can play a significant role in supporting asset management decisions and drive maintenance, retrofit or refurbishment actions. On the long run it can help to limit loss of supply events due to equipment failures and relevant personnel safety concerns and thus it is imperative to perform a complete inspection with IR equipment as well as with a corona camera in order to achieve a full predictive maintenance.