By Phil Kreveld
Inverter stability and grid strength were the main themes of the CIGRE Workshop held in Melbourne this week. These were discussed in great detail and, for those who do not have these subjects as their daily fare, it is only too easy to fail to see the close links with the renewable transition.
The workshop afforded interviews with some key engineers on the proviso of anonymity. A shortage of engineering talent, not only in electrical engineering, and the need for a comprehensive ‘end to end’ engineering plan was the observation of senior CIGRE people. To quote from a recent CIGRE international report, “From an Australian perspective, this aligns strongly with current NEM priorities. As renewable penetration continues to increase, the challenge is no longer simply building more renewable generation, but ensuring the system remains secure, resilient, and affordable while delivering the energy transition at pace. Australia’s leadership in inverter integration, system strength frameworks and grid-forming technology places it at the centre of this global discussion.”
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Another quote is that “one of the strongest themes, reinforced by CIGRE’s One Grid initiative, [is] the increasing interdependence of transmission networks, distribution systems, distributed energy resources, electrification and emerging large loads [data centres]. The concept of “One Grid” emphasises planning, operating and governing the electricity system as a single integrated system rather than separate transmission and distribution domains.” (Note that the underscoring is this writer’s choice.)
Although Australia is a stand-out in engineering complexity, there are other parts of the world experiencing similar challenges. Conversation with a senior consulting engineer indicated that Scotland, Texas (ERCOT) and Hawaii have much in common with Australia. The physical attributes of the NEM, being long and radial, perhaps provide additional challenges as noted in a presentation from AEMO on historical oscillation modes between South Australia and Queensland. Grid strength as already noted is a major concern being addressed—and perhaps not in an entirely satisfactory manner with one AEMO presentation begging the question whether provision of grid strength ought to be a commercial service.
The contest (my choice of words) between synchronous condensers and voltage forming inverters was an important subject with one presenter pointing out that the great capital outlay required for thirty or more syncons, required examination of the alternate technology. The advantage presented by voltage forming inverters was pointed out as providing both grid-strengthening and on-demand energy provision. This subject will be discussed in a future article as there are interesting consequences associated with the interaction of grid following inverters and synchronous condensers.
A further comment on CIGRE’s ‘One Grid’ philosophy was amplified by a presentation of ElectraNet and the preservation of grid stability given South Australia’s frequently occurring excess generation capacity. The state’s four synchronous condensers, together with grid-forming inverter capacity at Hornsdale, are material assistance but do not avoid the necessity to curtail rooftop solar energy production (again, my words).
Battery connections at solar and wind farms was an interesting topic, explored from the engineering rather than the commercial operation aspect. Economies of construction can be achieved by evaluating AC or DC connection options as well as the opportunity to have both grid-following and grid-forming inverter connections to the grid, the latter not being a standard form of connection. When a battery is co-located with renewable energy generation (like a solar farm), they may use separate DUIDs (Dispatchable Unit Identifier) under an aggregate setup. For instance, the Quorn Park Solar Hybrid system splits its operations into operational modes.
The exploration of system restart utilising battery storage energy systems, in pursuance of a rule change by the AEMC coming into effect July 2027 was a very interesting topic given the retirement of synchronous generation with the closure of coal-fired power stations. Transformer inrush current, inrush mitigation by means of ‘point on wave’ monitoring and control, and state-of-charge information are still at an early stage but of critical importance. On of the most important features of restart procedures is the location of stabilising load support as this has become a challenge with the growth of rooftop solar capacity. Takeways from the presentation was that we are still very much in early days of locking in cranking paths with reliable loads.
The question on the influence of reduction in fault current through grid weakness remains an open one though critically important. Differential relaying over short distances is well-established technology but for example it relies on highly reliable communication technology for long transmission lines. Replacement of synchronous machines by grid-forming inverters leaves this as an as yet unanswered question. However, the interpretation of commentary by engineers is that considerably new capital investment in protection engineering is in prospect. In one presentation from Queensland, hardware-in-the-loop (HIL) testing of voltage forming inverters was discussed as a reliable means of establishing the fitness of present protection relays, for example in over and under-reach of impedance relays.
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Data centre connection and load profiles were an important topic and complicated requiring multi-parameter evaluations of load profiles in terms RMS and transient voltages, and frequency ride-through. An important factor in the behaviour of data centres as variable load depends on the interior architecture of their power supplies (compute, and ancillary services). In discussion with engineers involved in connection studies, it is obvious that technical things are kept close to the chest.
In all, CIGRE workshops are an invaluable meeting place for the engineering community, and the recently concluded event in Melbourne was no exception.
