Renewables and tottering networks

Wind farm at sunset with transmission towers in the background (aemo report)
Image: Lucy Nicholson/Reuters

There is the belief that we will get to 100% renewables with the current operating and regulatory resources at our disposal but that belief is not warranted by realities, writes Phil Kreveld.

Related article: AEMO publishes Engineering Roadmap to 100% Renewables

The need for a secure integration path requires an overarching engineering plan encompassing distributed generation, distribution and transmission networks and generators. We are a long way off from having this. Based on the media responses from the electricity energy regulators, AEMO’s integrated systems plan, and the ‘rewiring the nation’ program, we would be led to believe that a secure and stable electricity supply is ‘in the bag’. However, the facts are:

  1. High electricity prices in spite of close to zero marginal cost of solar and wind;
  2. Distribution networks with unpredictable load profiles because of distributed generation, principally rooftop solar;
  3. Little enthusiasm for community batteries, virtual power plants, fast frequency ancillary services;
  4. Transmission lines operating close to maximum power and subject to voltage collapse;
  5. The need for demand response, load shedding and islanding in order to keep the remaining grid stable and secure;
  6. Overly complicated access for connection of generators to renewable energy zones
  7. The disinvestment signals to more generation at renewable energy zones because of network congestion;
  8. Inexperience with the operation of an increasingly asynchronous electricity system.

High prices for electrical energy

Although there are accusations levelled at synchronous generators for ‘gaming’ the market, they are for the present, indispensable as they are required to support basically all renewable generation. Control and the ability to mimic synchronous generation with battery-energised grid forming inverters i.e., smoothly replace conventional synchronous generation, still has to be tested in networks where they might predominate. Meanwhile conventional generation makes use of its still unique place in the energy market.

Distribution networks with unpredictable load profiles

Climate dependent generation, difficult to control solar inverters, variable frequency drives, constant power loads, large induction motor loads, coupled with no live monitoring at zone substations (power, reactive power, voltage angle) are a major headache for AEMO in its effort to prevent voltage collapse and high transmission voltages when demand drops and/or reverse power flow occurs during periods of high insolation.

Community batteries, virtual power plants, frequency support

A priory, attempting to control the national electricity system by ‘harnessing’ three million plus rooftop inverters is a bad idea. No doubt, it is politically popular to get the mums and dads of Australia involved and rewarded for supporting the grid—but as demonstrated in South Australia, the loss of synchronous support from other States, saw to the shutdown of solar inverters.

Transmission lines and voltage collapse

Long transmission lines with large voltage angle differences, i.e., operating close to the ‘knee’ of power-voltage curves are a very worrying feature of the South Eastern grid. Preventing voltage collapse is provided by load shedding and islanding, although with synchronised phasor measurements and appropriate control systems, instances of load shedding, demand control and islanding could be minimised.

Complications with accessibility of generators in renewable energy zones

The zones are characterised by low fault levels and often require reactive support by way of synchronous condensers but no effort appears to be made to provide far more efficient reactive support al load centres. A whole-of-system design approach, instead of the current piecemeal approach would improve accessibility of new generation capacity.

Inexperience in the operation of asynchronous systems

No amount of modelling including AEMO’s latest PSCAD-V5 will actually provide operational security. At best it might provide ‘entrance tickets’ to the main game for new generators. The incompleteness of inverter models, and the limitations of aggregating dozens of busbars into suitable equivalent circuits for electromagnetic transient modelling poses a major problem. One has to wonder about its value as the stand-in for real-time control.

Brett Redman of Transgrid has written “there’s no transition without transmission” and there must be large degree of commercial comfort with a $20 billion ‘Rewiring the Nation’ budget. However, we can fully expect electricity prices to reflect the massive grid extension projects. Yet is there any thought given to the fact that zone substations are becoming more and more independent of external electricity supply for large parts of the day and with AEMO having no second-by-second visibility of their power and reactive power flows? Is it not conceivable that with more distributed generation in low voltage networks plus battery storage, some transmission lines will be carrying uneconomic, low power flows? Leading UK energy consultants Mott MacDonald pose questions, “is it becoming too complex to allow markets alone to operate?” and “is there a need for increased government intervention (as has always been the case—think large-scale renewable energy target and small-scale renewable target target?”

Related article: New report reveals energy management foresights

If our national priority is to have 83% renewables by 2030 AND lower electricity prices than presently, a national engineering plan is essential. In a nutshell, our problem is that we think in terms of last century’s electricity systems in which we replace synchronous generation with asynchronous generation on the basis of the most elementary forecast power flow considerations—and expect this complex machine to work smoothly and economically.

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