Technical Presentations

Single Pole Auto Reclosure and NGR Analysis 794

Abstract
Unsuccessful auto reclosure has been observed in 765 kV HVAC transmission lines connected with Generation Bus. Back up Impedance Protection of Line reactor triggered during AR dead time due to power f... see morerequency oscillations in faulty phase (disconnected phase). It triggered tripping of Non Switchable Line Reactor at one end, which resulted tripping of 765 kV HVAC Line and avoidable Generation loss.

These Unsuccessful AR operations triggered us to study and model entire phenomena in Power System Analysis software (EMTP).


Initially, two circuits of 765 kV Transmission Line commissioned between Generation Station and Transmission Station with Line Reactor and NGR in both line at either end. Successful auto reclosure observed during single-phase faults on these lines. Intermediate Switching station constructed and both lines were divided in 4 section as per new requirement. Same configuration (rating) of Line Reactor and NGR adopted at newly constructed station on each line. Unsuccessful AR observed during AR running cycle on these lines after new configuration. Backup Impedance protection of Line Reactor found operated during AR Dead time.
Different sets of simulations has been carried out to understand the phenomena:

- Single Phase fault and Auto reclosure in 765 kV Transmission line in different section of entire corridor with Varying Shunt compensation (Value of Line Reactor)
- Different NGR values and observations for above simulations
- Trip the Line Reactor Breaker during Auto Reclosure Phenomena to avoid operation of Backup Impedance protection and its consequences

After EMTP Simulations and study, it was concluded that:

- Line Reactor (Degree of compensation) should be adjusted with adequate value while line length altered from its original design.
- NGR re-sizing can address the issues at some extent. Suitable value of NGR (based on Line Length and amount of shunt compensation) should be employed
Appropriate Solution and mitigation suggested after detail simulation & study.

Tag(s): single pole, analysis, auto reclosure, line reactor

POWERGRID experience with EMTP793

Abstract
To meet the increasing power demand, a number of extra high voltage (EHV) transmission lines is being added to the power system network, which enhances the complexity of power system network. It is es... see moresential duty of power engineer to fulfill the growing demand ensuring the reliability, stability & security of GRID. Transient simulation tools like EMTP play a vital role to provide the flexibility to power engineer for simulating/studying the various case scenarios which couldn’t be simulate in physical power system.


Such tools eventually help power engineer to build a sustainable and secure GRID. In POWERGRID, being a central transmission utility, various studies are being carried out to arrive the technical particular of new transmission system and optimize or fine tuning of existing system. Some of them i.e. Transient Recovery Voltage (TRV) study of 1200kV Transmission network, Smart Power from insulated earth wire of EHV transmission line and 1200kV Transformer Inrush current study are discussed in presentation. 1200kV National Test Station was established & successfully charged along with the transmission line in state of BINA in India.


TRV study was carried out for assessment of various test duties of 1200kV Circuit Breaker. Presentation cover the detailed discussion of 1200kV TRV study and a brief about the facilities like HIL, HVDC control Replica setup, STATCOM replica setup available at Powergrid Advanced Research and Technology Centre (PARTeC).

Modeling and evaluation of overvoltage792

Abstract
A case study will be presented demonstrating modeling of overvoltage in a moderately sized system involving medium voltage vacuum breakers, cables and transformers. The focus is on simulating high fre... see morequency overvoltage generated by circuit breaker prestrikes and re-ignitions and evaluating the impacts on transformer insulation.
-Results obtained from a customized breaker TRV model which is capable of controlling the capability of switching high frequency current will be presented.
-The methodology of analyzing severity of overvoltage in the frequency domain was applied in this project. Results and discussions regarding frequency domain analysis will be shared.
-Simulation results regarding typical system configurations with variation of key parameters such as cable lengths, load power factors will be briefly presented.
-Application of EMTP study results in assisting equipment specification, as well as overvoltage mitigation methods including optimizing switching sequences, necessity of snubber circuits will be discussed.
-Case specific strategies to improve simulation speed and work flow with EMTP will be introduced and discussed.

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.