Technical Presentations

VFTO Induced TGPR Analysis for 400kV GIS Substation855

Abstract
Transient enclosure voltage (TEV) also known as Transient ground potential rise (TGPR) is a special case of VFTO. This phenomenon refers to short rise time, short duration high voltage transient which... see more appear on earthed enclosure of GIS. This is mainly through the coupling of internal transients on conductor refracted to the enclosure at enclosure discontinuities such as air terminations, insulated flanges at GIS/Cable interfaces and some current transformers. Primarily disconnector switch operations are causes of VFTO and SF6-Air bushing seems to be the most significant source of TEV.

Observation of sparking between grounded enclosure and support structure, failure of protective devices, inadvertent operation of relays etc are common malfunction of TEV. Despite proper grounding this phenomenon indicates presence of high potentials on GIS enclosures which raises the issue of equipment protection, migration of these transients to adjacent equipment and shock hazards.

TEV in 400kV GIS substation due to “disconnector switch operation” was studied and simulations performed for GIS model with the equivalent resistance of ground grid, resistance and inductance of grounding strip using EMTP. The presentation summarizes the results of study needed for a 400kV GIS substation in India to ascertain VFTO induced TGPR level. The study was carried out by modeling all GIS equipment, enclosure in EMTP and careful consideration of grounding system model. The results of the study were analyzed to decide on the requirement of mitigation methods to ensure the permissible level of TEV.

Tag(s): gis, vfto, very fast transient

VFTO Induced TGPR Analysis for 400kV GIS Substation856

Abstract
Transient enclosure voltage (TEV) also known as Transient ground potential rise (TGPR) is a special case of VFTO. This phenomenon refers to short rise time, short duration high voltage transient which... see more appear on earthed enclosure of GIS. This is mainly through the coupling of internal transients on conductor refracted to the enclosure at enclosure discontinuities such as air terminations, insulated flanges at GIS/Cable interfaces and some current transformers. Primarily disconnector switch operations are causes of VFTO and SF6-Air bushing seems to be the most significant source of TEV.

Observation of sparking between grounded enclosure and support structure, failure of protective devices, inadvertent operation of relays etc are common malfunction of TEV. Despite proper grounding this phenomenon indicates presence of high potentials on GIS enclosures which raises the issue of equipment protection, migration of these transients to adjacent equipment and shock hazards.

TEV in 400kV GIS substation due to “disconnector switch operation” was studied and simulations performed for GIS model with the equivalent resistance of ground grid, resistance and inductance of grounding strip using EMTP. The presentation summarizes the results of study needed for a 400kV GIS substation in India to ascertain VFTO induced TGPR level. The study was carried out by modeling all GIS equipment, enclosure in EMTP and careful consideration of grounding system model. The results of the study were analyzed to decide on the requirement of mitigation methods to ensure the permissible level of TEV.

Tag(s): gis, vfto, very fast transient

Analysis of voltage transformer failures at Jirau GIS with the EMTP790

Abstract
The presentation will discuss the analysis of successive dielectric failures of inductive voltage transformers (VT) at a 500 kV GIS of Jirau hydroelectric plant in Brazil. All the failures involved th... see moree same region of the high voltage winding and the cause was undetermined. These incidents motivated a series of actions to identify the cause and improve the reliability of these equipment in the field.
One possibility the effect of very fast transients (VFTO) expected to occur during frequent switchings of disconnector and/or circuit-breaker at GIS. It well known that VFTO may stress inductive equipment insulation leading to a dielectric failure.

In this context, different switching conditions were simulated with the EMTP to determine the transient overvoltage at the potential transformer terminals. A special high frequency model of the VT was developed based on its frequency admittance matrix measured in the field. The result of the simulations was discussed together with the VT manufacturer and a final diagnosis of the failures was achieved.

Insulation Coordination Study of 765kV GIS SUBSTATION788

Abstract
Gas Insulated Substations provide cost effective solution due to reduced land requirements particularly in metropolitan area where power demand is registering steep growth. With the use of GIS technol... see moreogy, it is required to assess the risk of failure due to overvoltage. The Insulation Coordination study forms a very important aspect in design stage. This presentation deals with the IC study carried out for a large – 765/400/220kV GIS substation in a SUPER THERMAL POWER STATION in UTTAR PRADESH.


Due to the decreasing ratio of lightning impulse withstand voltage to the system voltage, the difference between LIOV and VFTO decreases and this factor calls for VFTO performance analysis for 765kV GIS as well.
The discussion includes modeling of Extra High Voltage GIS, Temporary Overvoltage (TOV), Lightning Impulse Overvoltage (LIOV), Switching Overvoltage (SOV) and Very fast transient Overvoltage (VFTO) performance analysis on 765kV GIS substation.
TOV typically caused by fault to ground, load rejections and energization of long lines are analyzed to decide the rating of the surge arrester.
LIOV simulation studies are carried out for possible onerous paths including longer and short paths to decide the location of the arrester. SOV analysis deals with the overvoltage due to switching operations during line energization, re-energization and fault occurrence and clearance.


The LIOV and SOV results have been validated with reference to protective levels prescribed. The VFTO analysis is performed with various GIS disconnector operations in accordance with the methodology and guidelines as per CIGRE WG D1.03-TB 519 and the results are validated with the limits specified.
Keywords: Insulation Coordination, VFTO, EHV, GIS, 765kV, Protective ratio.