By Phil Kreveld
“So one faulty photovoltaic inverter in an entire grid had a cascading effect that shut down the entire Iberian Peninsula, which had previously the market operator there had been saying, ‘Look, we can run this on renewables.”
The above statement is attributed to the Opposition Leader Sussan Ley from Sunday, November 16.
The quoted statement (Australian Financial Review and RenewEconomy) is without merit. The analyses of what happened in the lead-up to the black-out of the Iberian Peninsula on April 28 conducted by ENTSO-e (the European network of transmission system operators) and EPRI (Electrical power research institute-USA—European office) make no mention at all of an event Ley referred to.
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It is one thing to propose an energy policy in opposition to that of the Commonwealth Government, however, Ley’s statement appears as misinformed and futile as the motivation of the legendary Don Quixote de La Mancha, the Spanish nobleman, to tilt at windmills, mistaking them for malevolent giants (book by Miguel de Cervantes, 1605).
The analyses of ENTSO-e and EPRI have not yet been concluded. What follows here is this writer’s reading of both reports. In summary the incident of 28 April appears to have been caused by two important factors;(a) lightly loaded transmission lines—attributable to energy self-sufficiency in distribution networks, and (b) operational procedures of conventional, synchronous generator operators to restrain sub-synchronous voltage and power oscillations.
Like all attempts at summaries of multi-factorial events, they are open to interpretation. However, rather than only relying on the two above mentioned reports, the writer also accessed the heavily redacted Spanish Government Report, discussing this with Australian academics involved in stability analyses of electrical networks. In the following an explanation is provided, one that Australia can possibly benefit from, along with operators of electricity networks elsewhere.
Oscillations in the power system, possible imported from France, caused Spanish operators to attempt to dampen these by paralleling lightly loaded transmission lines. The effect of decreasing impedance is to dampen sub-synchronous oscillations. It is an important task because such oscillations can cause severe mechanical damage to turbines. As the transmission network was generally lightly loaded, the decreased impedance was mainly capacitive, therefore causing voltages to continue to rise by virtue of leading reactive power flow.
This effect, the so-called Ferranti-effect, had been anticipated by Spanish operators, who had instructed a number of synchronous generators to absorb reactive power from the transmission lines. This requires generators to operate in under-excited mode, and because of the diminishing of synchronising torque, the degree of absorption of reactive power was limited. Therefore, voltage rise became uncontrollable, thus causing protective measures to disconnect generators, resulting in a cascading blackout. Note: generally, to minimise sub-synchronous oscillation in synchronous generators, excitation-based power system stabilisers can be used—but that option was not available to Spanish operators.
There are lessons for Australia in that periods of lightly loaded transmission lines are increasing owing to energy self-sufficiency of distribution networks during periods of intense insolation (because of rooftop solar). Australian networks employ var compensators and synchronous condensers. The former respond very rapidly to voltage changes (whether cause by leading or lagging reactive power), and voltage support by synchronous condensers. Note: Metropolitan Spain does not employ synchronous condensers.
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One could argue therefore that if it weren’t for rooftop solar, being a culprit behind lightly loaded high voltage transmission lines, operational problems would be lessened. Inter alia: in the case of Spain, it is not rooftop solar but solar farms and wind farms connected to the medium voltage lines of distribution networks—Sussan Ley, please note! However, the effect is the same as for transmission line operators it results in lightly or even unloaded lines. Note: long transmission lines like EnergyConnect therefore require significant infrastructure to control voltage (var compensation in combination with synchronous condensers).
In general, Australian governments and their oppositions, whether state or commonwealth, should avail themselves of sound technical advice in formulating electrical energy policies. It is such a vital factor to the welfare of the population and the economy that there should be no wasting of precious time in essentially silly debates, fanned by the popular press.
