(Part-1: An insight into the curious case of the two black-outs)
Finally the warning bells chime at their horrific best, as Indians wake up to the biggest blackout ever in the world. Two blackouts on consecutive days (30th and 31st July, 2012) have exposed the covert fragility of one of the largest electricity grids in the world, with more than 94,185 circuit kilometers network of central transmission lines, 1,31,025 MVA transformation capacity, 154 nos. transmission level substations, more than 200 GW installed capacity of generation and an average daily load of approximately 137 GW. The buzz word of the day being smart grid, is the Indian Power System really ready for a complete cosmetic changeover with smart grid technologies or still it has a long way to go before taking the “smart” plunge? Although this is not the point to be discussed in detail in this story, but the reader is expected to retrospect over the relevance of smart grid technologies to prevent further occurrence of such incidents in the future.
About the Indian Grid Hierarchy:
Indian power system is operated as ﬁve Regional grids viz., Northern Regional grid (NR), Western Regional grid (WR), Eastern Regional grid (ER), Southern Regional grid (SR) and North Eastern Regional grid (NER). The control of the grid is planned to be done at four levels of hierarchy namely
- National Load Despatch Center (NLDC)
- Regional Load Despatch Center (RLDC)
- State Load Despatch Center (SLDC) and
- Area Load Despatch Center (ALDC)
In all, there are 5 regional grids, 31 state grids and 100 plus area-wise grids in India. Further, internal to area grids, local distribution area grids are also available. Each grid has its own dispatch center.
NR-ER-NER-WR (termed as NEW grid) – connected synchronously, thus maintains same frequency
SR – connected asynchronously with the NEW grid through a HVDC links
India has a mechanism of settlement of imbalances of the grid through a competitive market mechanism implemented through Unscheduled Interchanges, under Availability based tariff (ABT) based on frequency deviation since 2002.
The mechanism has been extended recently to intra-state level. At present the states are the participants in the central ABT scheduling mechanism. The RLDCs issue day-ahead schedules in coordination with the states and states have to conform to these fixed schedules on the day of operation. Any deviations are accounted for by the Unscheduled Interchange (UI) mechanism as a part of ABT. The UI rate is a function of system frequency at the time of operation, the definition of which is updated time to time by the Central Electricity Regulatory Commission (CERC).
The blackout-1 on 30-July-2012 (around 2:33hrs) occurred in the NR grid followed by blackout-2 on 31-July-2012 (around 13:00 hrs) which occurred in the NEW grid, after the NR was restored from the previous black-out and synchronized with the NEW grid. There was also some low significance tripping events during 30-July-2012 and 31-July-2012.
Most of the media coverage on the issue of the black-out-1 cite over-drawl of power by Northern states as the reason. The over-drawl may be due to the frequency approaching 50.46 Hz just before the event, which provides apparently free power to the over-drawing states as per the UI mechanism. This happened to overload the transmission lines, resulting in line tripping and power re-routing through the other connecting lines. This led to a cascading line trippings, thus isolating NR from the rest of the NEW grid. Whereas WR-ER-NER survived the situation, NR failed!
Around 300 million people were affected. The blackout had spread across Nine North Indian States, including the capital city, New Delhi.
Interpretations from NLDC Report and Graphs from IIT-Bombay
Download NLDC preliminary Report on blackout-1 (click here)–
As per NLDC report, the antecedent conditions confirm the inoperation of 400 kV Gwalior – Agra (ckt-2) and 400 kV Zerda – Kankroli. As explained above, overdrawl by northern states overloaded and thus tripped 400 kV Gwalior – Agra (ckt-1), and impedance between WR and NR suddenly increased. With no direct tie between NR and WR, the only link between the two is through ER. After about 3.35 seconds, the angular separation between WR and NR was gradually increasing which is confirmed by the recorded frequency differential of about 0.14 Hz (substantially constant) between Mumbai and Kanpur.
At this point the ER – NR ties would have snapped, owing to overloading with the re-routed power to separate the NR from the rest of the system.
(Contd… – Page 2)
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