By Phil Kreveld
No, that’s not being done here, however, we’re listing as yet unresolved engineering items—ones that affect the speed of the renewables transition, connection of data centres, stability control—and possibly the functioning of the energy market. They determine the road ahead, and the timetable.
Related article: Inverter stability, grid strength themes at CIGRE workshop
The good things first
- The Australian Energy Market Operator (AEMO) is a critically important factor in NEM grid developments, and we are fortunate its capacity for technology development is world-class.
- The growth in customer energy resources and batteries in distribution networks.
The unresolved engineering things next
- Low grid strength
- Restarting from blackout
- The lack of standards for grid-forming inverters
- The overall control of a mainly renewable grid.
Absence of markets
- Fault current (needed to reliable operate protective gear)
- Inertia
- Reactive power support.
Discussion
Low grid strength and low fault current are the top engineering problems. As often mentioned, low grid strength is a problematic and endemic feature—it was the case already before the transition to renewables commenced in earnest. The withdrawal of synchronous generation due to closure of gas and coal-fired generation, and replacement with grid-forming inverters is causing:
- Low fault current
- Lack of reactive power.
Were there to be a market for capacity (MVA) as well as energy (MWh), both fault current and reactive power could be substantially increased. The impression gained from public media is that grid forming inverters are the same as synchronous generators. This is not so. The main handicap is the lack of over-current capacity in order to protect the electronic switches.
Increasing the capacity to meet MVA criteria beyond that needed for rated MWh is highly uneconomic. A financial reward would be needed by way of a capacity market.
Reactive power is required to support voltage. Reactive power does not provide tradeable energy. Increasing the MVA rating beyond active power rating would allow provision of reactive power over an above active power needed for energy supply.
Reactive power is now being provided by synchronous condensers, as well as inertia, as a result of the diminishing contribution from synchronous generation. Thus, in the absence of a market for both, we are adding network costs, although energy costs are declining in terms of wholesale pricing.
Synchronous condensers provide fault current but the planned investment, based on AEMO’s Integrated System Plan is probably insufficient, and therefore requires revision and additional investment in grid protection engineering.
Low grid strength, as mentioned, is an endemic feature because of the length of transmission lines. The decline of synchronous generation is seeing to the disappearance of the ‘infinite bus’, sources of constant voltage irrespective of power generated. This is creating a serious problem in power and voltage oscillation. It is the principal reason for very slow connection approval of new generation.
Grid-forming inverters are well-engineered products, and for grids where adequate fault level and reactive voltage support are available, they are an adequate replacement for synchronous generation. In addition to lack of over-current capacity, there is an absence of standard specifications caused by manufacturers not declaring the details of their control engineering. This implies major challenges in overall stability control of the national grids. Black start procedures are therefore also affected.
The replacement of conventional synchronous generation with inverters operating on a millisecond rather than a several seconds interval, has major implications for ultimate stability control. It imports the need for machine learning and cyber security. The recent events such as the interference with refrigeration control in Carrefour supermarkets in Europe sound an alarm that must not be ignored.
Related article: Climate wars and grid strength
To wrap up
There are, as mentioned, major unresolved engineering issues. The absence of central engineering design and execution is met by a very cautious approach to the integration of renewables and more recently, the connection of data centres. This is therefore necessary because the integrity of the electricity system is essential.
The remarkable feature is the growth in consumer energy resources, and has to be applauded. However, increasing energy independence causes more spilling of energy from large scale energy suppliers. We are not resolving this growing dilemma. Its resolution arises out a belief that the operation of ‘the market’ will take care of matters.
