Contrary to reports, the grid collapse on two days was due to different factors – underdrawal on the first day and overdrawal on the second.
The Northern Grid failure that occurred on July 30 occasioned a flood of comment in the print and electronic media. Unfortunately, some of this comment was ill-informed and misleading. This could lead to people losing faith in one of the finest institutions in the country. A well-informed approach is necessary to fully understand the events that unfolded on July 30 and 31.
At the heart of our National Grid system is the vast network of Powergrid Corporation of India (PGCIL) — one of the world’s largest and most respected power transmission utilities, that transmits 50 per cent of the country’s power over its EHVAC & HVDC lines with an enviable record of 99.92 per cent availability.
On July 30, going by the statements made by the Power Minister, the first failure of the Northern Grid took place at 2.35 a.m. when the grid frequency was 50.46 Hz. The timing and the grid frequency are important. It was early morning and frequency was beyond the normal upper operating limit.
It is well known that power demand varies with the time of day — it is highest during evening peak hours (generally 6-10 p.m.) and lowest during the night hours (12 midnight-6 a.m.). In graphical terms, it could be called an S-curve.
During peak hours when demand is highest, the generators tend to slow down, and so power system frequency falls below 50 Hz (cycles per second).Fifty Hz corresponds to 3,000 rpm for a conventional two-pole generating set.
Likewise, during night (or off-peak) hours when offices, shops and many factories are shut, and most lights are off, the power demand is at its lowest. At this point, power system operators have to grapple with a different type of problem — one of excess generation.
While most power generators reduce their generation at night to the extent possible to match the reduced demand, for nuclear and thermal generation there is a lower limit called ‘technical minimum’ below which the generation cannot be reduced, and the unit will have to be shut down.
If a thermal unit is shut down, it is a costly and time-consuming exercise to re-start it. Hence generators are unwilling to reduce generation below the technical minimum.
When power generation exceeds the demand, the generating sets speed up beyond 3,000 rpm and system frequency exceeds 50 Hz. On 30 July, at 0235 hours the frequency had reached as high as 50.46 Hz, much above the grid code upper limit of 50.2 Hz. Further, there is a frequency limit beyond which it is not advisable to run a generator, and the same will trip if that limit is crossed.
THE FIRST DAY
With this background in mind, the reader will be better able to appreciate what happened in the early hours of July 30. By 0235 hours the load had dropped much below the generation and the frequency had already exceeded the upper operational limit of 50.2 Hz.
The excess supply could have been dealt with in two ways — underdrawing States (consumers) should have been ordered to increase drawal as per their schedule, or generating stations should have been asked to reduce generation in line with their schedule, by shutting down sets if need be.
This obviously did not happen to the extent required, and it is a matter for the investigating committee to determine which of the two, or whether both generators and States, infringed grid operating discipline.
Demand-supply mismatch has been endemic, going by the reports of hearings being conducted by the regulator on the subject of grid indiscipline. Another uneventful night would have passed, had it not been for a trigger — which in this case was the reported tripping of a loaded line (400 kV Bina/Agra/Gwalior).
Further loss of load in a scenario where there already exists oversupply, overfrequency, (and presumably over-voltage) proved to be the proverbial last straw. Frequency went beyond trip settings, resulting in a series of generator and line shutdowns, known as a cascade tripping, because each tripping worsens the operating parameters for the surviving plant and has a snowballing effect.
So before we proceed to the events of July 31 (which were of a different type) we first need to identify the issues from the grid failure of July 30. These are broadly as follows:
Underdrawal vis-a-vis schedule by the constituents (States) at the time of the incident.
Excess generation vs schedule by the generators
Actions if any taken by the Northern Load Despatch Centre (NLDC)/Regional Load Despatch Centre (RLDC) to correct the imbalance.
Reason for tripping of the line/equipment which triggered the grid failure.
For a demand/supply mismatch which resulted in a grid frequency of 50.4 Hz it is certain that there was violation of grid discipline by some or all sections of the constituents, and/or failure of the NLDC/RLDCs to correct it in time.
The event of July 30 was a matter of ‘oversupply’ and not ‘overdrawal’ as widely reported in the media and by Power Ministry spokesmen, though the eventual outcome was the same — a grid collapse.
…AND THE SECOND
Having said this we can now turn to the events of July 31. It is difficult to piece together with any degree of exactitude the events leading up to this failure, but one thing is clear — there was excess drawal vis-a-vis schedule by certain constituents at a time when the grid was in a fragile condition, and a trigger event like a line/equipment tripping took place, thus exacerbating an existing demand- supply mismatch.
The lessons from both events are the same. Persistent underdrawal or overdrawal in disregard of LDC instructions amounts to grid indiscipline and needs to be firmly dealt with. Likewise, oversupply by generators, or failure to meet schedules also amounts to failure of grid discipline.
The action or otherwise of the NLDC/RLDCs is the next contributory factor, and all of these combine to produce catastrophic failure when a crucial line trips due possibly to poor maintenance, or some other random cause.
Therefore, State and generator discipline, and grid maintenance and operation all need more attention. All this is well within the competence of our power sector professionals and regulators, provided, of course, that they are not subjected to political interference.
The intent of the Electricity Act, 2003, is quite clearly to distance the Government from day-to-day operations of the power sector. A beginning can be made by further empowering the load despatch centres and making them independent of their respective State governments.
Respected professionals like those from Powergrid can be fully relied upon to take care of our grid and ensure reliable power supply, as they have done for the past 11 years.
(The author is an independent consultant. firstname.lastname@example.org)