By Phil Kreveld, Power Parameters
The recently concluded Electrical Engineering Forum at Energy Week brought into sharp focus the clash between classical, alternating-current electrical engineering and newer technologies. The discussions at the forum made it clear that addressing the power system instability, which is lying in wait as a result of increasing penetration of renewable sources, is not being adequately addressed. It is not that solar PV and wind generator inverters, to take one example, are the very latest in engineering—on the contrary, they have been part of the electricity distribution and transmission landscape for decades—rather, the rising level of inverter-generated power is approaching the same order of magnitude as traditional, synchronous generator power and therein lies a big problem. Pointing to the European example of successful integration, particularly of wind generation, misses one essential point; namely that those energy sources are part of highly meshed networks. By contrast the Australian NEM is serviced by a long, skinny transmission system—one of the longest in the world and one that is much more ‘springy’. It is the mechanical example of coiled springs that usefully illustrates the electrical stability features of networks.
Alternating current technology since its inception in the late 1800’s has relied on rotational inertia of synchronous generators to provide stability for varying electrical loads. Generators exchange power between each other to keep the overall system stable. The mechanical example of a spring, or interconnected springs (as in the case of meshed networks) is a very useful analogy. Thus, a conventional power system, four or five decades ago, could be pictured as interconnected springs with electrical loads attached to spring ends (see the illustration).
An increase in load results in a pulling force increase on a spring but for a strong spring that would be resisted with only a small amount of deflection resulting in a minimum of spring vibration for short term load variations, and with much less vibration in other interconnected springs (for string vibration read frequency oscillations).
As the number of interconnected springs is reduced (and we approach that condition by way of the Australian 5000-kilometre transmission network stretching from Queensland to South Australia), the fewer springs are stretched much more and with load variation, are far more subject to vibration. The resulting instability can cause disconnection of large parts of the transmission system. Load variation is caused by wind speed drops and scudding clouds. Employing battery backup reduces the problem but all power fed back through inverters lacks any inertia and thus cannot stop ‘vibration’ as explained in the analogous spring system.
None of the foregoing is surprising information. AEMO is well aware of the hazards increasing renewable penetration is posing but in essence not very much is actually being done to address the gradual development of a much less sound grid. In order to keep the ‘spring set’ stable there is a growing requirement to keep electrical loads at substations as level as possible and to prevent sudden changes in load. Notwithstanding the minimising of load variation, the introduction of more and more electronic power inverters is reducing basic protection provided by synchronicity because they have no inertia and are therefore can cause rapid changes in frequency.
The Electrical Engineering Forum did discuss the parameters leading to instability in Australian networks but did not address cohesive, overall system engineering requirements relevant to high penetration of renewables. To cut to the chase; one cannot ignore the ‘laws of physics’ as one contributor to the Forum discussion put it, when explaining what happens when the rotor of a synchronous generator is forced past its critical voltage angle, resulting in ‘runaway’ frequency instability. AEMO power system engineers are obviously well aware of lurking problems due to the reduction in synchronous generator participation and the effect of connecting wind and solar farms to transmission lines not designed to carry their maximum power flows. However, in ‘distribution land’ it is apparently not a matter of concern although it ought to be. On the other hand, voltage control in medium and low voltage networks is attracting attention.
At the most elementary level, for distribution systems to aid stability, the revisions of AS 4777 and AS4755-2 draft circulation provide a start, but only if a high degree of uniform, open standard communication is forced upon inverters that allows overriding control of power and reactive power export and demand management. Superimposed on this system of inverters and the communication network, has to be near contiguous real-time power parameters monitoring system feeding into Big Data providing push outputs for inverter communication systems. Ideally such a monitoring and communication system should tie in with transmission line monitoring via synchrophasor monitoring units (PMU) in order to provide the proper basis for network stability control in the NEM. Such a monitoring and communication system would also allow for future enhancements including islanding of distribution networks and resynchronisation.