A grid-tied solar PV system is a solar PV system that, as its name suggests, is connected or “tied” to the grid.
The figure above shows what a grid-tied solar PV system looks like.
- Solar PV modules generate DC electricity.
- The grid-tied solar inverter converts the DC electricity generated by the solar PV modules into AC and feeds it into the grid. In order to do that, the solar inverter also has to ensure that its output is in phase with the grid.
- The sell meter (or the export meter) measures the electricity fed by the grid-tied solar inverter into the grid.
- The purchase mater (or the import meter) measures the electricity drawn from the grid.
Big, utility scale solar PV plants (> 1 MW in capacity) are also grid-tied solar PV systems. However, the only difference with respect to the figure above is that there are typically no local loads which consume electricity. And therefore, there is no purchase meter (or the import meter) either; there is only one meter and it is the sell meter (or the export meter).
The grid-tied solar PV system shown in the figure above needs support from the state electricity boards in two ways:
- The state electricity boards (SEBs) need to give you permission to feed electricity into the grid.
- The SEBsneed to pay you and pay you well for the electricity fed into the grid. Otherwise, it is not economically viable/feasible/attractive.
Point no. 1 is significant because the SEBs need to take some action at their end when many consumers start feeding electricity during the day. To elaborate on it a bit more, consider a city like Mumbai whose consumption is 3000MW. If many consumers go solar and start feeding 10% of the total consumption into the grid, it would mean that 300MW is injected into the grid every day (or on most days) during a 10 hour period. MSEB would then have to compensate for that and generate that much less from its other power generation sources during the day.Now imagine that the MSEB has done that and the next day happens to be very cloudy, which means that the solar electricity will not be produced at all or not as much as expected. This situation would land MSEB in a big soup and it would have to scramble to arrange for the required power to cover for the shortfall in solar PV energy.So, in general, SEBs need to do a lot at their end to allow renewable energy sources (which are inherently “infirmed”) to feed electricity into the grid and yet ensure that the grid remains stable and people get power when they want.
As far as point no. 2 and paying consumers is concerned, typically, SEBs calculate the “net” energy consumed by the consumer (purchase meter reading – sell meter reading) and charge him for that, which is why it is called “net metering”. This effectively means that the SEBs are buying electricity from you at the rate at which they are selling to you. That is a god deal in most cases.
If the net consumed by the consumer happens to be negative, which means that the consumer fed more into the grid than he consumed, then the SEBs pay him at a pre-defined rate. This rate is Rs.9 per unit (or kWh) in the state of Uttarakhand (at least when its policy was declared) which is quite good. But some SEBs might not be that generous. In fact, they might not even allow consumers to feed more into the grid than they consume. For that matter, they might also put an upper limit on how much consumers could feed into the grid and link it to their monthly consumption, say 50% for example. In general, a particular SEB can frame its net metering in many ways and will vary from state to state.
As of now, only a few states have come up with a net metering policy; many states are still framing their solar and net metering policy. In general, rooftop solar is yet to pick in India in a big way. But the day that it does, grid stability will become an issue, as it has in the Western countries.
However, despite the fact that net metering policy is not there in most states of India, grid-tied solar PV systems are popular in India. But they are slightly different as compared to the figure above; the output of the grid-tied solar inverter is connected to the grid “behind” the purchase meter. So with respect to the above figure, it implies the following changes:
- Replace the net metering meter with the purchase meter (or the import meter).
- Get rid of the sell meter (or the export meter).
In such systems, the grid-tied solar inverter is sized such that it takes care of 85% of the minimum loads during the day or less. So the grid-tied solar inverter takes care of the load current requirement partially; the remaining current is drawn from the grid.
However, even if the inverter sizing is done as described above, after a thorough analysis of the daily taking all seasonal variation into consideration, the situation where the solar PV modules produce more electricity than what is required by the loads arises, and it is on holidays or off days. When this happens:
- The excess electricity produced by the solar PV modules will flow into the grid.
- In states where the net metering policy is not there, this is not allowed, technically and legally speaking, without taking prior permission from the SEB. That being said, the SEB wouldn’t mind if they get free electricity.
- The meter might do one of the following things:
- It will count backwards. However, it is highly unlikely that states that do not have a net metering policy will have a bi-directional meter. But this is good from the consumer’s standpoint.
- It will count forward although you are feeding electricity into the grid. This might happen if the meter is just a counter and measures the flow of electricity through it, irrespective of the direction. This is very bad from the consumer’s standpoint; he is actually feeding electricity into the grid but getting charged for it! It is good for the SEB though since they are getting some free electricity.
- It won’t count when feeding electricity into the grid. This is neither good nor bad for the consumer; he is feeding electricity into the grid for free but isn’t getting penalized for it. It is good for the SEB though since they are getting some free electricity.
To avoid this situation, EPC companies implement what is called a “zero export” solution.
- The simplest way to achieve this on off days, when the generation is guaranteed to be more than the consumption, is to trip the MCBs and disconnect the grid-tied solar PV system from the grid.However, this is a crude way of doing it. There is no guarantee that some electricity will not flow into the grid on working days. And on off days, there could be some non-zero load which could have been fed by the grid-tied solar PV system but will have to be fed from the grid instead.
- The other way is to implement a sophisticated system which constantly detects if energy is flowing back into the grid. This can be done with the help of a bidirectional meter, which is there only to detect reverse flow of energy and not for net metering. There are two ways of achieving this:
- The output of the inverter can be reduced.However, most inverters in the market do not have this feature.
- The system can be designed using many smaller sub-systems which can be connected to the grid or disconnected from it independently using MCBs. However, this design has cost implications; typically the larger the inverter, the less expensive it is.
Boon for residential consumers
Grid-tied solar PV systems supported by a net metering policy are essential to bring residential consumers in the solar PV play because their consumption pattern and the solar PV electricity generation pattern doesn’t match at all; solar PV electricity generation happens during the day, i.e. during the sunshine hours from 8 a.m. to 6 p.m., whereas most of the residential consumption happens in the non-sunshine hours, i.e. before 8 a.m. and after 6 p.m.
If residential consumers had to go solar without support from the SEBs, then they would necessarily need to go for solar PV systems with battery backup. But that increases the cost of the system. Solar PV systems also have to be maintained and incur recurring charges every few years when the batteries wear out and therefore have to be replaced. Grid-tied solar PV systems, on the other hand, enables residential consumers to go for solar PV systems without batteries. For them, the grid acts a “storage” or an “energy bank”, in a manner of speaking; they push power into the grid during sunshine hours and withdraw it during the non-sunshine hours. Of course, in reality the grid can’t actually store the energy. As explained earlier in the article, it does so by generating lesser energy during the sunshine hours.
What happens when the grid fails?
What happens to the grid-tied solar PV systems when the grid fails? Does the grid-tied inverter keep pushing power into the grid? The short answer to that question is: no. When the grid fails, the grid-tied does not keep pushing power into the grid. It cannot keep pushing power into the grid.
This answer is very surprising and counter-intuitive to most people. The first time they hear it they say, “That doesn’t make any sense. The time when the grid fails is the time when I want my solar PV system to work the most!”
While that may be right, grid-tied solar inverters cannot do that and here’s why. In the oden days, the grid and all the generators connected to it were owned by utility companies. When local generation started, be it from solar, wind, or from other renewable sources, it was decided that these generators would switch off when the grid failed, which is almost always due to some fault condition, and it is extremely hazardous to continue to keep feeding power in such conditions. This behaviour of grid-tied inverters is called anti-islanding protection; the grid-tied inverters are prohibited from feeding power into the grid when there is a grid fault and they are also prohibited from feeding the loads in an island separated from the grid. Actually, intentional islanding is allowed, although with a lot of restrictions; unintentional islands are completely forbidden.
The way grid-tied inverters implement anti-islanding is by continuously monitoring the grid and detecting one of the following conditions:
- A sudden change in system voltage magnitude.
- A sudden change in system frequency.
- A sudden change in the rate of change of system frequency.
- A sudden increase in the active output power (kW) well beyond the “normal” level.
- A sudden increase in the reactive output power (kVAR) well beyond the “normal” level.
When any one of the above conditions occurs, the grid-tied solar inverters stop pushing power into the grid. In other words, they go “offline”, and come back online only when the grid is stable again.
Grid-tied solar PV systems have become very popular all over the world and are on the verge of taking off big time in India as well. The SEBs need to do their part by coming up with good net metering policies. In response, the consumers should go for solar in a big way.