The anatomy of a blackout

By Dr Tilak Siyambalapitiya

It is too early to come to conclusions on what caused the first event towards the blackout on Tuesday afternoon, why that event propagated all over the grid causing a national blackout, and why it took so long to restore electricity supply. The blackout set in around 1235; it was about 2230 when the last customer was reconnected. There is no official statement on whether the problem has been resolved, whether there were equipment damages and whether any such damages have imposed constraints to operate the grid in its normal state.

In the recent past, blackouts have occurred on 9th October 2009, 27th September 2015, 25th February 2016 and 13th March 2016. The blackout earlier this week on 17th August was the 5th blackout in recent memory.

Electric power systems are designed to receive electrical energy from power plants and deliver to customers. Unlike any other commodity, electricity cannot be stored in the form of electricity. It can be stored as water (in a reservoir), fuel (coal, oil or gas) and as chemical energy (in a battery). Wind and solar power generation have no storage whatsoever. Producing electricity from water, fuel or batteries has to be done at the same instant the customer requests the electricity supply to his light, air conditioner, water pump or the factory machine. Therefore, the key word is “dynamic equilibrium”.

That means the rate of electricity production at any moment (measured in megawatt) should be equal to the total customer demand plus the losses in the power transmission and distribution network. As long as there is a balance, all customers will get electricity supply, at the correct voltage and frequency.

Customer demand for electricity is not static. It varies all the time, based on time of day, weather, tea breaks and lunch breaks, and even as a result of TV programmes. Power system controllers also watch TV, particularly when extremely popular programmes and cricket matches are aired to raise power plant output when the match begins, be watchful during breaks, and reduce power generation when the match is over. Remember the production and the demand have to be the same all the time.

Then there are other causes. Sudden rain in a hydropower area would compel such power plants to be immediately brought into operation to save water from spilling over the reservoir. Fast moving clouds over a solar power generating area would cause electricity production from solar power to fluctuate. Electricity production from wind power plants fluctuate all the time, severely at times. These fluctuations of electricity production are somewhat predictable and can be managed, provided the amount of fluctuating hydro, solar and wind power are not very large portions of the supply. Remember the production and demand have to be the same all the time.

When any external event or an equipment failure causes that equilibrium to be lost, then we say the electricity system enters a transient state. The first reaction of the protection equipment would be to isolate the affected section of the network. Just like the fuse, the circuit breaker or the trip switch would isolate a section or all of your house, similar equipment would immediately detect the problem and isolate that faulty section. Electricity travels very fast, at the speed of light. So this isolation too, has to be done very fast, for two reasons: the faulty equipment has to be saved from damage and the fault should be prevented from causing secondary ones.

If the faulty section caused the loss of a power plant, or caused a sizeable share of customers to be disconnected, the matter will be serious, because now we have lost the balance between electricity production and demand. There would be either a shortage or a surplus of electricity production. In most situations, it is a shortage of electricity production because most problems occur within power plants or in the immediately vicinity of power plants. So, now we have less production, and it is not possible to meet the customer demand.

This is when the stored energy in the power system, in the form of rotating generators as well as rotating equipment owned by customers come to help. Any rotating mass has a stored energy. In the technical jargon (this is taught at A-levels too), the energy stored is the rotational kinetic energy. This stored energy is in the form of mechanical energy and is proportional to the size of the generator and to the square of the rotating speed. Large, fast-rotating generators (such as Norochcholai, Kerawalapititya and Kelanitissa) have larger stored energy. Large but slow rotating generators such as hydropower, have moderate stored energy. Small, slow rotating generators such as small hydro and wind power have a small amount of stored energy. Reciprocating engine-generators such as Sapugaskanda and Embilipitiya have very small stored energy. Finally, solar power has zero stored energy.

After the initial fault, such as a short circuit, the affected section is isolated by switches operating automatically and if that causes a power plant to be lost, then the remaining generators would immediately slowdown. Remember that the stored energy is proportional to the square of the speed? So when slowing down, they ‘release’ their stored energy, and convert that to electrical energy, to serve customers. This happens automatically; no operator intervention is required.

Remember these events happen all in a few seconds. Fault isolation may take about 0.1 seconds. Slowing down of generators will happen immediately and may go one for about 2 to 5 seconds. Now, slowing down of generators cannot be done all the time because they would then come to standstill and would not produce any electricity. In fact, this slowing down is allowed by about 5% of the rated speed. As the generators slow down, just like the heartbeat, the ‘frequency’ of the power supply also decreases. If the frequency, which is normally 50 cycles per second, reduces to 49 cycles per second, and stabilizes, then there will be no problem. The frequency stabilizes and then within 5 to 10 seconds, water or fuel valves of power plants will open and admit more energy into generators, which will raise the production of electricity. Then the frequency will also increase and again stabilizes at 50 cycles per second. All these happen automatically; no physical intervention is required, provided there are generators already connected to the grid, producing electricity, with spare capacity, and ‘fuel’ in store.

Sri Lanka’s power system is running with very little spare capacity, thanks to the two politicians who cancelled all the major power plants that were on the drawing boards in 2015. Politicians in Sri Lanka take pride in cancelling projects, but not for facilitating their construction. Then over 2016-2020, the country was compelled to run the existing oil power plants and purchase new oil power plants (much to the delight of some others), then the production costs went up. However, electricity prices cannot be increased because the same two politicians would not allow. So, most of the time, there is no extra fuel in the tanks to quickly raise the electricity production. In other words, spare capacity is not used, even if it is available, because keeping them spinning on partial production levels, hoping some emergency may occur, is costly. Such spare capacity, in the jargon, is known as ‘spinning reserve’.

So how does a grid go dead?

Assuming the short circuit is relieved, then if a power plant has shutdown, the ‘frequency’ drops, attempting to balance the supply and demand. What if it is unable to balance; if the gap between supply and demand is too high and if the frequency cross 49 cycles per second and goes down further? Then the second layer of protection comes into action, automatically. Customers are automatically removed from the grid in blocks, thus reducing the demand for electricity. This happens in several stages, automatically. If the gap between supply and demand is too large, up to 50% of customers may be automatically removed, in a desperate attempt to restore the balance. In most case this works, but for reasons yet to be investigated, it did not happen in this Tuesday’s blackout.

If the supply and demand cannot be balanced even after removing 50% of customers, the there is no hope. A blackout is inevitable. The ‘frequency’ may hit 47 cycles per second and then larger generators (Norochcholai, Kelanitissa) would trip automatically, for their own safety. Hydropower may hold on for a bit longer, but would not be allowed to reach even 46 cycles. One by one, all generations in the grid would shut down, automatically, for their own safety.

All this happens, typically within five seconds. For how long the grid struggled on Tuesday afternoon this week to recover is still unknown. In the 2009 blackout, it was all over in just over 3 seconds (yes seconds, not minutes).

In the 2015 blackout, the grid struggled for 3 ½ minutes before its collapse. In 2016 February blackout, the gird struggled for 8 minutes, before the final collapse.