By Phil Kreveld
The vibes from the recent CIGRE Australia presentations in Adelaide from September 2-4 make it evident that ‘bad vibrations’, i.e., voltage and frequency stability, and oscillations are real risks if the time to sort them out—long term operational experience gathering, not theoretical projections—is not taken because of haste to achieve renewable energy source targets.
There are sufficient grounds for confidence that battery-sourced, voltage forming inverters can eventually take over the role of synchronous condensers and generators. To argue on those grounds that no investment need be made in synchronous condensers is unwise and therefore transmission companies are choosing well-established technology for voltage support, essential for the proper operation of phase-locked loop (i.e., grid following) inverter-based resources.
Currently most IBR in our grids, whether solar, wind or battery-based, are grid-following, including those classified as frequency-supporting (i.e., so-called ‘grid firming’). Voltage forming (i.e., grid forming, as opposed to ‘firming’) IBR were discussed and there is a general opinion that grids with 30% grid forming IBR in place of synchronous machines may well become a reality. However, we are not well served by advancing the clock based on impatience, or for that matter by assurances from inverter and battery suppliers who bear no direct responsibility for grid stability.
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Oscillations through the very dynamic character of renewables
Minimising oscillations focusses on power, i.e., watts, kilowatts, megawatts, rather than on energy because it is fluctuation in power generated and demanded consumption that gives rise to them. Whereas energy can be stored (batteries, flywheels, super capacitors, water level, etc.), power is always a zero sum, generation having to equal power demand at any instant. Any attempt to overcome this results in frequency changes and oscillations. Oscillations in electricity systems have always existed, and it isn’t so that renewables are inherently more troublesome than spinning machines—though they are generally faster to respond to power changes. The internal workings of inverters are mostly considered proprietary information. Grid connection approvals of IBR are usually based on mathematical models (so called black box models) in lieu of the ‘white box’ models, which reveal the actual operational features and circuitry details. This lack of detail, in black box models, can result in surprises when a modelled generator, approved for connection, is physically connected and gives rise to oscillations, not predicted.
Transmission lines as ‘springs’
A mechanical analogy will assist in understanding the origin of a large class of oscillations in grids by using a spring as example. It represents a transmission line and the attached weight (M) is equivalent to the power absorbed by the electrical load attached to the line. Imagine that suddenly, an additional weight (m) is added. Ultimately the spring will be further extended to a new, stable position (there being built-in damping for a physical spring) but before that occurs, the spring will experience a vibration mode with a frequency given by the equation below.
Spring oscillation frequency:
where k is the stiffness the spring, and M the attached load and m the additional load suddenly imposed.
One can think of the spring’s stiffness in electrical terms, i.e., grid strength. The point of attachment of the spring, can be another much stiffer spring, representing a generator, which would also experience vibrations, but of a different frequency. The mechanical spring, dissipates the energy of oscillation into heat, therefore assuring that the oscillations die down. The analogy won’t serve if pushed too far because in the electrical engineering environment this helpful damping is frequently absent or limited and therefore can lead to instability. Suffice it to say that weakening grid strength increases the tendency to develop oscillations of voltage and power under conditions of rapidly changing electrical loads.
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Faster response to load and generator power demand
Inverter-based resources are not the ‘devil incarnate’ for stability. But as many presenters on IBR stability showed, the more rapid response of output to changes in input (i.e., responding to commands for more or less power), and the influences caused by line voltage and power demand from connected loads and phase angle, are subtly different from synchronous machines. Like the latter, IBR use feedback to make sure that inputs result in desired outputs. IBR are electronic devices, and inherently fast (signals inside the IBR can travel at close the velocity of light). Synchronous machines use mechanical links and hydraulics—and need time, supplied by their inertia, for feedback to make the needed adjustments to inputs, be it steam, gas or water flow. Inter alia: inertia is often held out as an essential ingredient in the reliable operation of grids—and that IS the case for those with steam and water-driven synchronous generation.
The many as yet transmission line projects on the drawing board are an invitation to rethink the nation’s renewable path. The absence of tie lines between generators, and long, radial networks (without the voltage stability offered by meshing) provide a fertile ground for re-engineering terminal stations. Take as an example an N-1 contingency transmission line (a single malfunction will ‘take out’ the line) connecting a renewable energy zone to a terminal station. By placing battery storage and dual function (grid following/grid forming) Inverters in sub transmission a much greater degree of stability and security would be achieved, in addition to black-start capability in distribution networks.
Therefore, in conclusion, while Australia’s engineering talent has been actively engaged in analyses of large, high voltage systems, the time seems ripe for that talent to also re-examine the expansion plans for transmission, their maximum power capacities and the associated investments in syncons, var compensation, phase-shifting transformers, etc. This would see to an active design effort in storage opportunities at sub-transmission terminals, and in general, a thorough review of AEMO’s Integrated System Plan.
