GIS Insulation Accident Concern

GIS (Gas-Insulated Switchgear) characterizes with many merits such as high reliability,easy-maintenance, small volume, and receives widely application in the world.
Even though, due to testing devices and techniques, there are still quite large amout of GIS never undergo strict fields test for earlier applications. it happens that insulation problem is still an improtant issue for fields maintenance, from which it aroses many methods to conduct insulation experiment, such as chemical analyzing method, pulse, ultrasonic and UHF method.

From the experience, there are certain kind of defect, which can not be anyharm and not easy to detect, will exemplify themselves in the later years, especially when CB operates, partial discharge occures that may result in large accident and maintenance has to be in power-off conditions so as to lead to large economic loss.

There are some insulation faults happened in Guangdong province, China:
Jiangmen 500kVGIS, DayaWan 400kVGIS, Shenzhen Huanggang station220kVGIS, ShajiaoA power plant220kVGIS.
Especially for Shajiao A plant, in the year of 1989, a 220kVGIS N.3 faults result in loss of 32GW.h.

Edited by J.Zh
IEEETM Center

On-site Sensitivity Verification for UHF PD Detection in GIS（NOTES）

Necessity of UHF Sensitivity Check
A article features with on-line check of Sensitivity, which rises an important issue.
S. Meijer1*, J.J. Smit11
Delft University of Technology, Mekelweg 4, 2628 CD, Delft, The Netherlands

In industry, partial discharge technology related to switchgear has been operational for many years. Partial discharge testing has been conducted on both new high voltage switchgear in the laboratory setup, as well as several older HV installations in the utility grid. The results clearly indicate that partial discharge analysis of HV switchgear is an excellent online tool to assess its dielectriccondition. “Dielectric windows” present on existing GIS offer the possibility to pick-up the signals by means of external couplers.In order to check the sensitivity or to prove the possibility to detect a certain PD level, a sensitivity check procedure has beenpublished by Cigre several years ago.

This procedure was mainly focusing on new equipment in which internal sensors are beinginstalled in the factory. In this case, it is possible to make comparison tests in the laboratory between the UHF technique andaccording to the IEC 60270 to get a relation between both readings.For older GIS with PD couplers retrofitted or using externalsensors, usually the laboratory step required in this sensitivity check procedure is not possible due to lack of spare parts. In this contribution, a solution to perform the laboratory test on-site is described.

Proceedings of the International Conference on
Electrical Engineering and InformaticsInstitut Teknologi Bandung, Indonesia
June 17-19, 2007

Why place more efforts on magnetizing characteristics for CT than VT?

Technical Notes of Instrument Transformers
The Role that Magnetizing Impedance Plays

Instrument Transformer is special transformer, due to its operation condition, the equivalent circuit differes with respect to tranfer parameters.

Difference in equivalent circuit between CT and VT
1.VT. due to its operation limit, the secondary voltage runs rarely out of the limit value (80%~120), even in fault conditions, protection equipment will prevent them long keep in the state of out-limit. so it is reasonable to assume that the magnetizing impedance during operation is constant with respect to protection, and linear with respect to metering.Also, as VT is connected in paralle with power lines, only voltage is considered. the magnetizing impedance has very little influence on the burden voltage.

2.CT, secondary current is considered, its operation range is (1%~120%,to say,S class), magnetizing impedance varies significantly thus bypass secondary current from the burden, and affect the state value from primary being transfered to secondary. especially in fault conditions, such as primary line short-circuit, magnetizing impedance is a very important parameters to be considered.

In consideration of metering accuracy:

Both CT and VT characterize their error graph by nonlinearity of magnetizing impedance (MI). In principle, MI bypasses burden current, which influence the voltage drop on secondary lines and thus affecting the signal supplied to the burden.

MI is an interesting parameter that still attracts big conern, both for metering and protections.

J.ZH
IEEETM Central Sources