Following two grid disturbances on July 30th and 31st, the National Load Dispatch Centre earlier this month came out with an action plan that would be followed till both incidents are analyzed in detail and the shortcomings identified.
The grid disturbance on July 30th had affected almost the entire northern grid, while during the disturbance on July 31st, northern, eastern and north eastern grids were affected.
As per the action plan prepared by Power System Operation Corporation Limited, a wholly owned subsidiary of Power Grid, NLDC and Regional Load Dispatch Centres in consultation with Central Electricity Authority and Central Transmission Utility would review transfer capability of inter-regional and other critical links keeping in view Central Electricity Regulatory Commission regulations and the experience during two grid collapses. The Short Term Open Access approvals by RLDCs/NLDC would be considered based on the transfer limits worked out. During emergency in the system, RLDCs would take all actions necessary to ensure system security including curtailment of transactions. NLDC/RLDCs would review and impose congestion charges as per CERC regulation for inter-regional links for any drawal beyond the limits specified.
The transfer limits considering Surge Impedance Loading limits on each trunk transmission line in the inter-regional corridor and intra-regional systems have been worked out by NLDC.
In the past few years, a number of concerns have come to the fore in several disturbances involving high antecedent line loadings in the system. Among them are protective relaying mis-operations either due to incorrect settings, load encroachment or use of distance relays for power swing blocking that result in cascading failures, sustained high loading for a period of ten minutes or more in daytime high ambient temperature conditions leading to line tripping on transient faults, possibly due to increase in sag, and high reactive power consumption by the transmission line under heavy loading conditions causing voltage dips in the system.
On the two grid disturbances that occurred last month, NLDC said that in the antecedent grid conditions there was heavy power flow on the 400 kV Bina-Gwalior-Agra single circuit section crossing 1,000 MW on the single circuit available. The second circuit was under outage since July 28th for up-gradation to 765 kV level. The Quad Bersimis conductor on the section has a SIL of 691 MW but was loaded much above the SIL. Likewise, the loading on many circuits in the eastern region was also of the order of 550 MW and above against the SIL of 515 MW for a twin Moose configuration line.
The planning criteria issued by CEA in June 1994 stipulates that the permissible loading on transmission lines would either be the stability limit evaluated from St. Clair curve or thermal limit depending on the line length. It states that while SIL gives a general idea of the loading capability of the line, it is usual to load the short lines above SIL and long lines below SIL because of stability considerations.
The thermal limit of a Quad Bersimis line at 40 degree centigrade ambient temperature is of the order of 3200 amps which correspond close to 2200 MVA at 400 kV voltage level in contrast to the SIL of 691 MW.