Power failure: How a grid works,and when it doesn’t

The working of electrical grids and how overdrawing from them can cause them to trip

Written by Anil Sasi | Published: July 31, 2012 12:15:24 am

What is an electrical grid?

A grid is an interconnected network of transmission lines and substations hooked on to generating stations on the one hand and load centres on the other. The generating stations,put together,supply the electricity demand through the transmission lines; the load centres or distribution companies then draw the power from the lines and wheel it to consumers. The basic premise for the stability of the grid is that load and generation must be balanced at all times to prevent a failure. The flow of electricity through the lines should ideally not exceed the rated capacity,otherwise the lines could trip due to an overload. For grid operators,one of the prerequisites is to ensure that there is adequate redundancy in the system so that the possible tripping of one line does not lead to a cascading event that can potentially impact operations of the grid as a whole.

What are a grid’s components?

A grid consists of three main components: power stations that produce electricity from fossil fuels (coal,gas) or non-combustible fuels (hydro,nuclear,wind,solar); transmission lines that carry electricity from power plants to demand centers; transformers that reduce the voltage so that distribution lines carry power for final delivery.

What happens in an ideal situation?

All generating stations are expected to inject power as per schedules declared by them to the grid operator while all load centres or distribution utilities are expected to draw power as per the drawal schedule given by them to load despatch centres.

How does one know if a grid is stable?

This is done by monitoring the grid frequency,an index that shows whether power is being supplied and drawn as per schedule. The optimal frequency in India is pegged at 50 hertz,or cycles per second. The permissible frequency band for grid operations in India is 49.5Hz to 50.2Hz,as per the Indian Electricity Grid Code. The larger the grid size,the more stable it is deemed to be. At the same time,if a grid disturbance does happen,its scale could be larger for a larger grid.

What is the disincentive against overdrawing?

There cannot be physical control over excess drawal,only a financial penalty. In India,the grid frequency-linked penal measure is called the Unscheduled Interchange or UI rate. If the grid frequency drops,the UI rate shoots up,acting as a disincentive for discoms to overdraw when the frequency dips.

Does the penalty serve its purpose?

Northern states are repeat violators of the grid frequency norms,especially Uttar Pradesh,Haryana,Punjab,Jammu and Kashmir. The ‘UI’ penalty has failed to deter some of these states,which overdraw and then default on the ‘UI’ payment itself with the grid operators. UP,for instance,is known to routinely run up ‘UI’ bills of several hundred crores and delay the payments. The state has also taken advantage of a High Court order under which it does not pay the full UI penal rate.

When does a grid collapse?

There can be two main reasons. One is equipment failure due to reasons such as fog and pollution,as had happened when the northern grid collapsed in 2002. The other,and more common,reason is when one or more constituents violate the grid code and overdraw in a big way from the grid,causing it to fail due to the imbalance in the power injection and drawal patterns,as seems to be the case in Monday’s northern grid collapse.

What is the network in India like?

At present the northern,western,eastern and northeastern regions are integrally connected through AC (alternating current) transmission links to form what is called the ‘NEW’ grid. There is a free flow of power between these four regions. The southern region is hooked up with the rest of the country through HVDC (high voltage direct current) transmission links,which have constraints on wheeling capacity,thereby limiting the free flow of power from and to the southern region. India’s current cumulative installed capacity is 205 gigawatts (1GW is 1,000MW).

Are grid failures an Indian phenomenon?

Not really. While India has seen two major grid failures in the last 10 years,there have been several instances of grid collapses in other parts of the world. These include the Brazil-Paraguay blackout of 2009 that affected 87 million people,the Northeast blackout of 2003 in North America that affected some 55 million residents,the 2003 Italy blackout that hit another 55 million people,and the 2005 Java-Bali blackout,by which close to 100 million people were affected in Indonesia.

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