Our Frequency

Welcome my esteemed readers to the introductory blog to express my point of view on a very important topic being discussed and debated amongst the power sector professionals in India these days.

What should be the standard limits of system frequency in Indian Power system? Is it time to modify the existing limits? Before that I would like to mention a few basis facts:

Our nominal supply system frequency is 50 cycles/second, also called as Hertz (Hz) while in the US, parts of Japan and elsewhere it is 60 Hz. If we want to achieve the system frequency of 50 Hz at all times (24 x 7), a basic requirement is that the sum of the power generations from all sources (thermal, nuclear, hydro, renewable etc.) should be equal to the sum of electrical Load in the system plus the system losses. If this balance is upset, we have either an under-frequency (< 50 Hz) or over-frequency (>50 Hz). There are strong reasons (to be discussed in another article) as to why the frequency is to be maintained at 50 Hz or close to this standard value. And hence the limits are specified in the Indian standards and the grid operating codes for the power system operators to keep these values within the specified permissible limits by suitable control measures.

Coming back to the core issue of modifying the existing limits on system frequency in order to ‘stiffen’ the frequency band, we need to understand how the Indian power system operates. The Indian power system that consists of 5 electrical regions (congruent with the geographical regions) namely, the South, West, East, North east and North, all interconnected as one synchronous grid since 31st December 2013. This allows import/export of power from surplus/deficit region(s) seamlessly. The interconnection of such large electrical regions with load demand ranging from 2700 MW(NE region) - 51000 MW (Northern region) is attendant with problems of stability & control both in steady state and under fault conditions. Since system frequency is an index of the balance between the generation and load demand as mentioned above, The Indian Electricity Grid Code (IEGC) inter alia specifies certain frequency bandwidth for the Indian power grid within which the system operator has to operate the power system. The original frequency range (49.0 - 50.5 Hz) in the first IEGC (of 2000) specified the desired (steady-state) frequency limits for secure grid operation (of all the five regional grids).

The proposed revision in the IEGC is ±1% (49.5 to 50.5 Hz) to the frequency band. This is primarily due to the reason that our power system controls have improved drastically over the years besides bringing in advanced techniques in short term load forecast, unit commitment, dispatch etc. to achieve a stiffer frequency regime. A comparison of annual frequency variations during two different periods given in the following figure would reveal this improvement in the Indian power system.

Source: POSOCO

One can confidently believe that the system operators will be able to achieve this new bandwidth restrictions with their modern control centres at the national and regional levels.

The issue is whether we should now get the Indian Standard also changed to include the above limits as against the current range which is ±3% (48.5-51.5 Hz). If the present value is retained to be the legal range in the Indian Standard, it will give no right to the consumers to complain as long as the steady-state frequency remains within this wide range. In order to provide quality power to the consumers, I feel that it is absolutely necessary to make changes in the Indian Standard specifications as well so as to bring the legal range of frequency of power supply in line with the new IEGC proposal. This will go a long way in assuring quality power supply to the consumers in India. In this context, it is worth mentioning here that the standard frequency range for Continental Europe (CE) is +/- 50 mHz. Although there has been a discussion on this important topic in the professional circuit, such discussions should be out in the open amongst the other stakeholders too.