PHAETON ENERGY: FUTURE STABILITY NEEDS REMOTE CONTROL RENEWABLES ON THE GRID.
Australia is amidst an electricity network stability crisis, with significant impacts from “system strength” issues. These impacts include curtailment of the output of pre-existing solar farms, extraordinary delays in connecting new generators and essential players leaving the market due to uncertainty. At present, the solution appears to be painstaking, time-consuming startup procedures for new generators and the deployment of multi-million dollar “synchronous condensers” to provide “inertia”. Unfortunately, this situation is fast becoming a national crisis, with necessary investment in renewables stalled, recently completed projects bleeding money and only a few solutions in sight.
A Bit of History
So, what is the fundamental system strength or stability issue? We first need to review how the grid is controlled to understand this.
Historically, electricity was generated by steam-driven generators. Steam is generated by boiling water using heat from burning coal or gas or even nuclear decay. Then the steam is fed through a turbine to drive a generator. These generators are large spinning machines and have a fundamental responsibility to load regardless of the energy source. If the electrical load is too high and exceeds the power supplied by the steam, then the generator slows down, and the output frequency of the electrical power decreases. Conversely, if the generator is under-loaded and has more steam power than electrical load, the generator speeds up, and the output frequency increases.
For historical reasons, frequency is used as a proxy for over-or under-load. Therefore, the fundamental method developed for controlling the grid has been to control the frequency. Note that these steam-driven turbines and generators are enormous machines — the rotor in a 1000MW generator weighs thousands of tons. The nature of a heavy object to resist speed changes is called inertia. When we talk about grid inertia, we are talking about the tendency of massive spinning machines to resist changes in speed as the electrical load changes. The large inertia of huge spinning generators has allowed a relatively “hands-off” approach to controlling.
In addition to the primary market for energy, AEMO operates secondary markets to ensure grid stability. These secondary markets allow generators (and large loads) to bid to provide frequency modifications — that is, to raise or lower the frequency. AEMO accepts these bids as required to ensure the grid frequency remains close to 50Hz. There are raise and lower markets for minor adjustments (so-called frequency regulation) for three different periods — 6 seconds, 60 seconds, and 5 minutes. In control systems, these periods are prolonged — for example, anti-lock brakes update the control signals up to 10 times a second. For the enormous mass of traditional, large generators, 6-second updates work. However, things are changing.
Renewables and Grid Stability
Unlike traditional generators, renewable generators connect to the grid via an inverter. The inverter consists of computer-controlled transistors that switch and modulate the generators’ constant voltage (DC) into an alternating, sine-wave signal (AC) compatible with the grid. First, the computer estimates the current frequency of the grid (and the time offset or phase). Then, it matches the inverter output to the grid. There are two critical things to note here: grid-following inverters have no inertia, and inverters are directly controlled by a computer and can achieve cycle-by-cycle control.
The lack of inertia contributes to grid stability issues — inertia-less generators do not fit the historical model. As the fraction of renewable generation has risen and the fraction of the grid without inertia has risen, grid stability issues inevitably arise. Control of an inertia-less grid via frequency cannot work.
Growth of Smaller Generators
Another factor contributing to grid stability issues is the number of generators — particularly smaller ones. Due to the changing generation landscape, recent fossil-fuel projects have been smaller, more agile plants. Also, renewable projects are much smaller than traditional GW-scale generators. The result is that there’s been considerable growth in smaller generators over the last decade.
Rooftop solar is another significant driver: every rooftop solar installation has its inverter. There are over 2 million rooftop solar installations in Australia. Australia’s NEM has gone from 200 generators late last century to millions of generators now. This explosion in a smaller-scale generation is a classic example of “too many cooks spoil the broth”. These smaller generators attempt to estimate the current frequency, phase, and voltage and control them. The smaller generators (cooks) fight against each other without effective overall management (head chef).
We have “too many cooks playing Chinese whispers to spoil the broth” to extend the analogy further. The generators don’t have good information in the grid — they are connected via long transmission lines that introduce delay and uncertainty.
So how can we possibly solve these problems? To give a pointer to the solution, let us imagine a future grid that is 100% renewable. In such a grid, there are no machines to exhibit the frequency response. If we overload such a grid, the frequency will not drop. This is because the frequency of the inverters is computer-controlled and has no relationship to the load. Instead, for an overloaded renewable grid, the voltage will drop. So, it seems clear that we need to transition to new methods for managing the grid. The load will need to be controlled via voltage — if the system voltage drops, the grid is overloaded, and if the voltage rises, the grid is under-loaded.
One more consideration in this imaginary 100% renewable grid is synchronising the inverters. They must all be generating their 50Hz AC waveform precisely simultaneously. If they get out significantly of step, they will be driving different voltages — and “fighting” each other, rather than supplying power to grid loads. So, the inverters must be synchronised. Here we see the three core components of a future grid:
- Load control via voltage control
- Inverter time synchronisation
- Centralised control of frequency by broadcast to all generators
To ensure that generators are not fighting against one another and work together to deliver power to the system loads, they need to be closely synchronised. Traditional generators achieve synchronisation by following the grid (which is partly why the grid frequency varies). If we have an external synchronisation method, it needs to be accurate. For example, ten-millionths of a second (microsecond) of error would lead to at most 0.3% voltage mismatch at 50Hz. Aiming for 1-microsecond accuracy seems desirable. A cheap, ubiquitous accurate time source is available: the GPS delivers worldwide timing to see here in 25 nanoseconds (25 billionths of a second). It is 40 times better than is required. For robustness, a secondary time synchronisation method should be developed and deployed to protect the grid against cyber-attack. One option would be wired (or optical fibre) connections to each generator.
An Immediate Solution to Frequency Stability Issues
If renewable inverters have good time synchronisation, then a new tool for grid frequency stabilisation is available: remote control of inverter frequency. A quick, simple scheme for a stable renewable generation would be to place a sensor at the nearest conventional generator that transmits frequency control to the renewable inverter. Under this scheme, the inverter would be ideal in step with the traditional generator. It would be as if the conventional generator had increased size/inertia.
Note that both the sensor and the inverter need to share the same time reference (GPS). This shared time reference allows low-cost communication networks to transmit the frequency and phase. Alternatively, a dedicated, deterministic communications link could be used. The basic process is:
1. Periodically (let’s say once per second), the frequency and phase of the conventional generator are measured (concerning GPS time).
2. That information is transmitted to the renewable inverter controller.
3. The inverter generates AC outputs to match the conventional generator perfectly.
The conventional generator slows down if the grid is overloaded, and its output frequency decreases. Then the renewable inverter follows suit — the same with an under-loaded grid, except the frequency increase. Existing frequency control techniques can be employed to stabilise the grid.
The current grid stability issues with the new renewable projects being constructed without battery storage is a classic case of “too many cooks”. Predictably, grid stability issues will crop up first in distant regions with high renewables percentages. Here, the voltage fluctuates many times a second.
From our analogy, the solution is easy to see — if the cooks are fighting, bring in a head chef (or promote one). In this case, the multiple renewable projects should upgrade their inverters to receive remote commands (this is primarily a software change). A new computer should be deployed to control the five inverters in harmony.
Alternatively, one of the renewable projects could be nominated as the master, and it could provide commands to the other four projects. Unfortunately, the current solution of deploying a synchronous condenser will cost tens of millions and ignore the root cause of the problem. The synchronous condenser will slow down the fighting and reduce the magnitude, but there will still be fighting. (In our analogy, a synchronous condenser is akin to making the pot bigger — the inputs from each of the cooks will be diluted and averaged out).
As the grid evolves, it is expected that clustering will be increasingly used. A single cluster for the entire grid would be the most accurate from a control perspective. However, having the grid keep operating when communication lines are down would recommend multiple clusters from a reliability standpoint. State-wide clusters could be deployed, or clusters based on locality and interconnectedness.
Australia’s grid is in crisis, with stability issues cropping up with increasing frequency. However, by considering the future 100% renewable grid, we have proposed a scheme for stabilising the low-cost grid and can be deployed very rapidly. The cost will likely be less than a single day’s losses from the currently curtailed renewable projects! The grid of the future will have inverters that can be remote-controlled. The lack of system inertia can be addressed with remote-controlled inverters. The proliferation of small-scale inverters can be arrested (by clustering).
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