We are facing unknown unknowns
By Phil Kreveld
Going from old technology to new wind and solar electricity generation faces us with an interesting confrontation: how long still, can we rely on the subservient synchronous source of old?
Subservient—prepared to obey without questioning—implies for synchronous generators that they will behave themselves and continue to provide stable voltage and frequency irrespective of whatever else may be occurring in the grids. Stable voltage and frequency, as matters stand, are essential to the generation of wind and solar energy—and yet the two distinct means of generation can run headlong into each other. The 60% or so of coal, gas and hydro synchronous generation is the dominant force in national grids—but for how long?
What happens when a surplus of power is created by Renewable Energy sources?
So here is an interesting question. What would happen if all those renewable sources create a surplus of power over and above that required by the aggregate loads? Answer: some synchronous generators would turn into motors—or rather, attempt it and in the process wreck their turbines and much else. As the move to more renewables continues, and the proportion of synchronous capacity reduces, the opportunity for excess renewable generation reduces as well because the subservient synchronous generators still in the system cannot be turned into motors, yet they are essential for the operation of renewable generation!
The solution might be to take as many as possible synchronous sources out of service—or to charge batteries or pump water or compress air. But can we actually take synchronous generators out altogether? As technology stands at the moment, the answer is ‘no’!
Stable voltage and frequency
We rely on their provision of stable voltage and frequency in order that solar and wind can pump out power—and that means they must also supply power to the grid at all times. The question in part is how low can we go with synchronous generation as a proportion of total wind and solar, as well as battery generated power?
There is no reliable answer to the above question and it is one that the Australian Energy Security Board no doubt is considering in its advice to the Australian Federal Government to approve a capacity market for synchronous generation. Note that the ESB doesn’t mention ‘synchronous’ but it might as well because its advice relates to gas, coal and hydro generation—all synchronous.
Power for when the sun don’t shine
In point of fact, the popular interpretation of the capacity market recommendation is that ‘we need power when the sun doesn’t shine or the wind doesn’t blow’. However, that is a highly simplistic explanation.
Equally, the Australian energy minister, Chris Bowen’s light hearted comment that storage in batteries is the answer because ‘rain doesn’t always fall so we store water in dams’. Hydrologically that may be a good simplification but it doesn’t apply to electricity.
The interaction between synchronous and renewable (asynchronous) sources
Any attempt to try and model the effects of solar and wind in the Australian NEM grid for this article are obviously impossible. Nevertheless, the use of an example to illustrate the nature of the interaction between synchronous and renewable (asynchronous) sources at the very least points to an inescapable problem area to be resolved.
The above diagram shows on the left the ‘aggregate’ synchronous generator, in the middle the aggregate load (power P and reactive power Q) and aggregate renewable generation, on the right, shown as a current generator, I connected to the network of resistance R2 and reactance x2 (representing a higher impedance associated with renewable generation in renewable energy zones).
The load absorbs the currents i1 (from the synchronous source) and i2 from the current generator which is the simplified representation of an aggregate grid following inverter-based resource (IBR). Provided the load P + jQ can absorb power (i1*.V1Ð0) from the synchronous generator (EÐd1), i.e., current i1 is flowing, then voltage V1Ð0 will be available to allow the grid following IBR to operate. The voltage at the terminals of the IBR, required to synchronise it to the grid, is V1Ð0 + i2.(R2 + jx2). It is assumed that i2 in phase with V2Ðd2. It needn’t be but it doesn’t alter the situation. The phasor d2 is equal to tan-1x2/R2.
What if the aggregate current source, I, is more than required by the load, then excess energy will be lost in the network with R2 resistance (or be stored in batteries) but in any event there will be no voltage V1Ð0 with a strong synchronous component. In the highly simplified circuit, the frequency may change to the internal oscillator of the IBR current generator, I, rather than it following grid voltage at its terminals via its phase locked loop (PLL). In all this, it is assumed that the synchronous generator has been disconnected so as to prevent it from being ‘motored’.
When there are no Synchronous Sources present
But there is more, because in reality with no synchronous sources present, the individual circuits (n) with their grid-following IBRs will all have different phasors, dn (tan-1xn/Rn) and synchronicity is gone, with as consequence a no means of providing any form of stability. In the 100% renewable grid, proposed by ‘green’ enthusiasts, the role of synchronous generators has to be taken up by voltage forming IBR rather than having only current generating, grid-following IBR. The big problem is that the world has zero experience with running such a grid on the scale of the Australian NEM!
Conclusion – Keeping Renewable Generation subservient for now
In conclusion we had better treat the subservient synchronous sources that remain in the system as important and reverse roles for renewable generation making them subservient for the moment, notwithstanding the enthusiastic scenarios painted by Australia’s AEMO’s integrated systems plans. At present, there is work being done to prove the viability of grid forming inverters to take over from synchronous sources, and reliance is being placed on the use of synchronous condensers. The latter can provide voltage stability and inertia for a few seconds, and in theory synchronous generation could be devoted to reactive power generation for voltage support, making no financial return unless a capacity market were available. We are facing unknown unknowns!