Understanding the Grid – Part II

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In the last article I gave you a brief introduction to the grid. In this article, I will explain a few more aspects related to the grid.

Understanding the Grid: Part II

Figure 5.1

The above figure shows the grid itself and what is connected to it. On the one end, is the generating station or the power plant which generates the power. This power is fed into a “step-up transformer” which converts the generating station’s output voltage to the high transmission voltage. To fall back on the analogy of humans and aliens in the previous article, the step-up transformer converts humans to aliens. The power is then carried long distances over the transmission line. It is then stepped down to lower voltages using a step-down transformer. In the human-alien analogy, it converts aliens back to humans. Heavy industry typically gets electricity at the 33 kV level, while light (as in “not heavy”) industry typically gets it at 11 kV. These industrial customers are called HT (short for High Tension) consumers. Homes and offices typically get electricity at single-phase 230V or 3-phase 440V. These consumers are called LT (short for Low Tension) consumers.

grid1It was this system that Westinghouse visualized. He backed it because he saw that it could scale very easily, and he has been proven right! There is virtually no limit to the amount of power that you can push through the grid, and there’s no limit to the size of the power plants either. Coal or gas fired plants, hydroelectric plants, and nuclear power plants tend to have huge capacities up to 1000 MW! And economies of scale come into play as the size of the power plants grows. That is why the AC grid has become ubiquitous.

grid2The company that manages the national grid in India is Power Grid Corporation of India, which is a Government of India enterprise. Back in the 1960s, every state had its own grid. Then state grids were inter-connected to form “regional” grids, and there were five such regional grids: Western, Northern, Eastern, North-Eastern, and Southern. The integration of these independent regional grids with each other was envisaged and conceptualized in the 1990s; the original motivation was to exchange operational surpluses between the regions. However, it evolved from regional self-sufficiency to having a “One Nation, One Grid, One Frequency”. Accordingly, the Eastern and North-Eastern grids were connected in October 1991 to form the E-NE grid. In March 2003, the Western grid was connected to it to form the W-E-NE grid. In August 2006, the Northern and Eastern grids were connected to form the W-N-E-NE grid. Finally, on 31st December 2013, the Southern grid was connected to the W-N-E-NE grid to form the single national grid operating at one frequency.

The thing about electricity in the grid is that whatever is generated has to be consumed right then and there; it can’t be stored. In other words, demand has to match the supply. But that is not possible all the time. So what happens if it doesn’t?

If the demand exceeds the supply, the frequency of the supply reduces. On the other hand, if the demand is less than the supply, the frequency increases. To understand this, think of a carriage being pulled by four horses. At the optimum weight, it runs at a certain speed. If we add more weight to the carriage, it will slow down, assuming that the horses work just as hard. On the other hand, if we remove some weight, then it will pick up speed.

Having one grid is great since a power plant in Ladakh can be used to power a home/office in Kanyakumari, and vice versa. While this is very advantageous, it also comes with its own risks and challenges.

grid3The main risk is that a problem in one part of the grid can affect the rest of the grid. To fall back on the analogy of the carriage, if one of the four horses simply stops pulling any weight, the other three horses will be excessively burdened and they will stop too! That is exactly what happened in 2012, when two severe power outages affected most of northern and eastern India on July 30th and 31st. The one on 30th July affected over 300 million people, and was the largest power outage in history till date. However, the one on 31st July broke that record by affecting over 620 million people, spread across 22 states! That’s the risk: the irresponsible behaviour of one state can affect other states which haven’t done anything wrong whatsoever.

The main challenge in having a single grid is that the planning – how much to generate and how to distribute it – has to be done meticulously, and there are big organizations, called State Load Dispatch Centres or SLDCs in short, that do just that! And they have to do this because electricity cannot be stored in the grid, as already mentioned earlier. But you may ask, “How can anybody accurately predict how much load is going to be there at a particular time? Even if we assume that the load follows certain patterns which can be predicted accurately based on historical data, what is the guarantee that the load will follow that pattern in the future? What happens if a million people decide to switch on/off certain loads at the same time? If they decide to do that, can the amount of generated power be changed and that too instantaneously?”

These are all valid questions and I will answer them one by one.

1. It is possible to predict the demand on a particular day at a particular time with a fair bit of accuracy. The degree of accuracy can be increased dramatically by using “smart” meters. We are not there yet, but we are certainly moving in that direction.

2. Theoretically, millions of people can decide to switch on certain loads at the same time. The probability of that happening depends on how contrived the scenario is. For example, the probability that a million people will switch on their lights during the day is very low. On the other hand, the probability that a million people will switch on their A/Cs on a hot summer day or switch on their TVs to watch the cricket world cup are very high!

3. It may or may not be possible to change the amount of power generated. Turbines – which are part of most types of power plants – are huge masses with an incredible amount of momentum and inertia. So they cannot be switched on/off at will. There are huge costs involved in doing that. They usually keep running and generating the “base” load. The output from hydro power plants, on the other hand, can be changed instantaneously. If the demand suddenly increases, it is taken care of by what is called as “spinning reserves”. In simpler terms, if one power plant goes down, the other plants pick up the slack. However, there are limits to how much slack they can pick up. If the demand far exceeds the supply in India, the SLDCs simply cut the power to some areas, which is why we have frequent power cuts in many parts of India; basically, we are a power-deficit country. In the developed countries, however, plants called “peaker” plants come online when the demand suddenly increases. Peaker plants might have to be called into action due to a sudden heat wave for example. So, from the grid’s standpoint, it is beneficial if the central air-conditioners work on gas. Alternatively, the central air-conditioners can produce chilled water at night, store it, and use it during the day for cooling; this is what is called a “demand-side solution” to the problem and is great because it balances out the load and takes it towards a “constant load scenario”, which is a lot easier to handle!

In general, the bigger the balancing region, the better it is. If all of India is used for balancing, all its diversities come into play. And I am not talking about the diversity that Jawaharlal Nehru talked about in his book The Discovery of India. What I mean is this. There is a time difference of one hour between India’s east and west coast. This naturally staggers the peaks of major cities on the east and west coast. Different parts of India can have vastly different weather. So for example, while it is blistering hot in Mumbai, it could be relatively a lot more pleasant in Pune. So not everybody will switch on their air conditioners at the same time. All the major festivals of India mean big peaks in the load, but thankfully Durga Puja isn’t at the same time as Ganesh festival; it that was the case, we would have had it!

grid4Long story short, load balancing isn’t child’s play, by any stretch of imagination. It is a fairly complex activity, especially if you put a condition that all consumers must get the required amount of power at all times. We are not there yet, but that is where we are heading. So load balancing is going to become a lot more complicated in the future, like it has already become in the developed countries. And to make matters worse, we are going to have renewable energy sources integrated into the grid, which are by their very nature variable, or “infirmed” as they are called in technical jargon. Solar PV is a lot less infirmed as compared to wind, at least in India, but it is variable nonetheless and will pose problems. And therefore, large-scale energy storage will become critical going forward. Much more on all these topics will be covered later in the series.

I hope you got a good understanding of the grid from this and the previous article. In the next article, I will talk about the electricity situation of India.

Sustainably yours,
Prashant Karhade.
Writer, Publisher, Entrepreneur

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