DMS (Partial Discharge Monitoring System) is an essential part of a EHV GIS Substation. What is partial Discharge? In High Voltage systems, Insulation strength of power cable/Equipment can be drastically reduced due to voids in the insulation of that particular Cable/Equipment. This will lead to progressive deterioration of insulation materials and ultimately resulting a sever electrical breakdown in the system. Partial Discharge can happen mainly in Power Cable Joints, Power Cable Terminations, GIS , Power Transformers and Reactors. So the substation should be well equipped to detect and alarm PD activities within the substation during the erection testings/commissioning testings stages as well as long term operation of the switchgear.
Different PDM systems are nowadays available and all systems operate on the same principle of UHF technology for GIS, which has not changed over the last 10 years.The difference between them lies in their PD detection sensitivity and the system features. The user-friendly display of the PD data, the accuracy of automatic PD identification and more importantly the reliability of the PDM system equipment itself are important facts which have to be taken into account.
The theory of the UHF technique was invented more than two decades ago, and is now well-known worldwide after being adopted by many leading switchgear manufacturers and utilities. However the theory leads to current pulse which forms the partial discharge has a comparatively insignificant short rise-time, which recent measurements have indicated can be less than 70 picoseconds. The rising edges of these pulses excite the GIS chambers into multiple resonances at frequencies of up to at least some GHz. Although the duration of the current pulse is less than a few nanoseconds, the microwave resonances persist for a relatively long time, typically a few microseconds. They may readily be picked up by UHF couplers/PD Sensors fitted either inside the GIS chambers, or over dielectric apertures in the chamber wall(Near by PRDs of the GIS housings).
There are two main PDMS systems which are adopted the same detection method as above(UHF) are used commonly. However, One system employees the same UHF wave by removing the noise while the other detects signal from the sensor, converts to optical signals using OCUs (Optical Converter Units), and transmitted via an optical fiber to the equipment cabinets.
Nowadays Clients are very keen to propose the reverted specification to the vendor for the extensive protection. specially in the switch gear. Different contractors are usually proposing slight changes to the systems (DSP speed,No. of Sensor units, Operating Temp/humidity levels) to go inline with the site/substation conditions. Commonly IEC 60270 is being used for the PDM Systems related erections/Testings. Sensor locations are proposed in coordination with the vendor. Typical locations are power cable termination points and Bus section points in switch gears whereas it can be located in Power Cable joints for transmission systems.
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