Gas could be the most expensive, least reliable path to grid stability

The discussion on inertia and synchronous generation is rather confused. Inertia is necessary because it provides stability to the grid. “Strength” and “stiffness” are terms that are used to show that lots of rotating inertia is “good’ or even essential to a stable grid. Because it was free it is often assumed that the system needs as much generator inertia as it always had.

But inertia is just one way to supply stability and it can be argued that beyond more than a certain minimum level, generator inertia is an expensive and anachronistic way of providing stability.

Stability is required to protect timing circuits, minimise mechanical loads on motors and generators caused by changing speed, prevent overheating of inductive loads like AC motors and most of all to prevent voltage/frequency oscillations after fast changes in load or generation e.g. from a loss of a connection or generator or even start-up of all the hot water services at 10PM.

Inertia is one simple way of providing stability because the rotating mass of the generators absorbs or disburses energy by small changes in speed.

While the inertia of turbines is large, it is only useful as a store of energy if you can use it.  Most of the energy stored in a rotating turbo-generator is unavailable because the energy is 1/2Jω² where ω is the angular velocity and J the rotary inertia. As the angular velocity is only supposed to vary by 0.15Hz in 50Hz you can only use 0.6% (49.85/50)² of the inertia in the system to stabilise the load. Even if a 1Hz short term deviation is allowed it is still only 4% of the system inertia.

The key to stability is not so much the inertia itself but the synchronous nature of an AC system which locks all the turbines and loads together at the same frequency, thus inertia is not just that of one generator but all the synchronous generators, the capacitance of the transmission and distribution network and even all the AC motors and loads on the load side. These later contributors are still there, even if some of the generation is no longer synchronous.

The downside of inertia is that once it is given up it must be replaced. So, if system frequency falls by 1Hz, to recover the frequency a large fraction of the output response from the remaining generators is used just to spin all the generators and loads back up to speed rather than just supply lost power to the grid. In the best case, it will prolong the frequency disturbance. In worst case the extended frequency deviation will trigger protection circuits and more widespread faults.

In a conventional system inertia provides the first 0.1-10 seconds of load disturbance response and it was free. A steam plant is quite good for the next 3-6 seconds after a disturbance because there is a quantity of steam in the steam chest which can be released quickly.

If the lost generation stays off line steam is then limited because it has slow ramping after that first steam dump. Hydro comes up after 20-150 seconds but has excellent stability and very fast ramps. The combination of inertia of water in the penstock and rotary inertia of the generator gives very stable ramping and for large scale power changes, hydro seems to offer the best combination of ramp rate and stability.

Gas turbines respond quite well after 8-30 seconds, then ramp quickly if they don’t stall or oscillate which they are prone to do at low loads. It is clear that “the straw that broke the camel’s back” in the SA blackout was the failure of gas turbine generators at the Quarantine station to respond properly to rapidly increasing demand.

However, even if inertia is seen as desirable at the plant level, gas turbine plants have no more inertia per MW than wind and many of them are operated slaved to the largest generator(s) because it is simpler and more efficient.

But if the key large generator(s) are for some reason isolated from the grid, the gas turbines will sag under the increased load and they will have limited mechanism or perhaps, if they are already at full load, even capacity, to respond. So, within fractions of a second their frequency will start to fall just as quickly as a group of wind turbines.

Even if governor response is fast, maximum stable ramp rates are around 5-10% per minute usually starting at less than that (they tend to have S shaped response curves) Gas turbines have another weakness which means that their inertia is of less value to the grid.

If frequency falls the compressors slow down reducing compression ratio and thus power so even more so more of the governor response is needed just to compensate for reduced air flow.

Finally, gas turbines are relatively low inertia probably about 1/3^rd of an equivalent steam generator. Typically, the rotating inertia constant is around 3 which means that the total rotary inertia is around 3 seconds of maximum generation capacity.

Given that only 4% maximum is available as inertia services it means that a 250MW generator can compensate for about 3*250*.04 MW seconds or 30MW for 1 second or 6MW for 5 seconds. A 10MW battery system can provide 10MW for 40-200 minutes

In summary, a gas turbine based system (without spare backup in the form of interconnectors or hydro), which loses 30% of generation quickly will crash just as surely as SA did. In fact there is a good argument to suggest that a system with a large proportion of gas turbines is possibly more vulnerable to short term instability than a modern renewable based system with storage.

Wind plants are also run in slave mode because they were originally minor contributors to the system and had to follow the big generators.  However modern wind plants can operate in a synchronous fashion; it is a matter of supplying them with advanced power converters which have that capacity, as can smart inverters in solar plants.

They can in millisecond time frames adjust power output to keep frequency and voltage constant. Of course, if there is not enough power, the system will crash just as it will if you overload a gas turbine or a coal plant.

However, a key advantage of modern wind (and hydro) turbines is that they can be variable speed machines so they can give up perhaps 10% of their mechanical rotational energy while still maintaining electrical frequency. As we have seen synchronous gas or steam turbine can only give up 0.6-4% of its rotational energy before frequency falls well outside the allowable band.

Figure 1 old high inertia Diesel vs new low inertia 230kg reduced to 53kg but the new unit has much better speed stability under changing loads

Then there is the question of how much inertia is needed anyway. In the past, it was free so the more the better.

Just as early steam engines had large flywheels because their response to load changes was poor, many other systems from power grids to diesel engines, machine tools and even aircraft engines used inertia to compensate for the inadequacies of the control systems.

Modern fast response controls mean that in all these applications inertias have been reduced because a) inertia costs money and b) while it does damp unwanted disturbance, it also slows response to real load changes, so modern machinery is faster and more accurate because inertia has been reduced. With proper engineering the same can apply to a power system.

The alternative to conventional “spinning reserves” (i.e. turbines running just in case) is storage. Batteries, flywheels, pumped hydro storage and power to heat.

All these have far better ramp rates than gas turbines, have fixed costs not subject to wide fluctuations in fuel price and because of their rapid response rates, provide a much better bridge to hold up voltage and frequency while larger generators build up power. Of course, gas or steam turbines are much better to cover long outages because they can run for days, weeks or months if necessary.

Therefore, a modern system with batteries, smart inverters and synthetic inertia on wind turbines and solar plants can reach full power in about 50-100ms so inertia only needs to cover the period from t₀ (the time where the incident occurs) to t₀+ 50-150msec.

In a conventional, mainly gas turbine system like SA, if it was without interconnects, inertia needs to cover most of the response for at least 10 seconds and up to 100 so the system needs 50-200 times as much inertia as in a fully battery supported system. This observation has been verified by a successful trial in Ireland where a 10MW battery is providing equivalent short term response to a 100MW gas turbine.

Batteries can also be supplemented by supercapacitors, rotary condensors spinning at synchronous speed, or asynchronous flywheels which can supply 20-100 times the power to inertia ratio that a spinning generator can, so it is clear that in a modern grid the amount of generator inertia required is orders of magnitude less than that assumed when the current grid was built.

This is not to stay longer term support/backup of renewables is not required, it is simply that grid stability can be provided at far less cost than from additional synchronous generators. The point is not to eliminate power supply from gas, it is to give gas generators time to ramp up safely so the generators are more efficient and fuel is not wasted on spinning reserves.

This is why most of the recently contracted battery storage systems in the UK have been located at gas and biofuel power stations. It means the power stations can sell more power at peak demand and turn off altogether at times of low demand saving fuel and wear and tear because they don’t need to run at very low loads “just in case”. They can also arbitrage excess renewables, coal or nuclear when prices are low to recharge their batteries and resell the power at double or triple the price hours or days later.

In conclusion, probably the most expensive, least reliable way to supply grid stability is by adding gas turbines, if the primary purpose is to add inertia. Inertia is not the goal, frequency and voltage stability is. Arguing for more inertia is a bit like saying heavy cars are safer than small cars in some accidents so all cars should be as heavy as possible.