THOUGHT LEADERSHIP

We built this synchronicity … but what now?

The alternating current (AC) transmission age started in 1891 through collaboration and perseverance. This transmission method has transformed the world, led us to the brink (or over the brink) of a climate disaster, and provides the backbone for a future founded on abundant renewable resources (in Australia, at least). Collaboration and perseverance are now required to ensure the continued utility of the grid.

Synchronous machines have always been the driving force in the grid. These elegantly simple electrical machines sit in powerhouses around the globe using their century-old technology (nearly 150 years, in fact) to convert mechanical energy into electrical energy to power the computer that I’m using to type this.

There’s a romance to these machines. It’s more than nostalgia. They spin, as the name suggests, in synchronism. If one of them falters, all the rest pick up the slack. The only connection they need is the power lines. This robustness is central to the way our electricity grids have operated for over 130 years – but the influence of synchronous machines is slowly being eroded.

The most obvious erosion of the influence of synchronous machines is their displacement by inverter-based renewable (IBR) technologies as solar, wind and battery energy storage (BESS) installations proliferate. A less obvious erosion of their influence is the diminishing understanding and appreciation of synchronous technology within the industry. This lack of understanding by engineers, planners and regulators of the fundamental building blocks of the electricity grid is starting to show. 

This might sound like the curmudgeonly ranting of yesterday’s engineer as technology passes them by. Maybe it is … but, rather than dwell on negatives, let’s look at what synchronous machines bring to the power system.

• System strength

The fundamental difference between synchronous machines and IBR is thermal inertia. Typical synchronous machine design can sustain high levels of over-current for a relatively long time (seconds) compared to IBR units (milliseconds). This difference allows synchronous machines to provide a strong ‘natural’ response to voltage variations in the power system without threat of overload and damage. Transmission protection systems – and therefore grid security and safety – rely on this characteristic.

There are alternatives. Overload capacity can be built into inverters, but this is expensive. Dedicated inverter-based devices such as static synchronous compensators (STATCOMs) can be used to provide fault response.

• Inertia

Synchronous machines spin. Their spinning bits (rotors) have mass, so they have mechanical inertia. This mechanical inertia doesn’t require a control system to provide an inertial response. The inertial response from a synchronous machine is a known, predictable quantity, regardless of voltage.

IBRs can provide synthetic inertia and, when voltage is healthy, can out-perform synchronous units. When voltage is not nominal, the same current limitations that affect the IBRs’ ability to deliver fault level can also restrict the effectiveness of synthetic inertia delivery.

• Robustness

The previous two characteristics – system strength and inertia – show the support that synchronous machines can provide to the power system during disturbances. The synchronous machine can deliver these supports across a wide range of power system disturbances to voltage and frequency.

IBRs typically rely on fast controls to manage their response to system disturbances. Under some extreme conditions these controls may not be fast enough or well enough tuned to manage. While tuning is important to synchronous machine performance, often it has a second-order effect or adds robustness over and above the natural response.

Displaced but not superseded

Understanding the inherent dynamics of synchronous machines gives power system engineers a better appreciation of these machines’ contribution to power system stability. Regulators should be mindful of the reduced risk (and mostly advantages) to system security that synchronous machines offer relative to IBRs. This is despite the uncontrollable nature of synchronous machines. That is, physics dictates the stabilising effects from synchronous machines whereas control algorithms determine whether an IBR can stabilise or destabilise. Controller model accuracy is therefore more important for IBRs than it is for synchronous machines where transient stability or electro-mechanical transient time frames are considered.

Better understanding of synchronous machines should lead to more appropriate rules relating to dynamic response to system events. Overly specific requirements for fault ride-through, power recovery post-fault, and maintenance of real and reactive power during voltage disturbances may all lead to needless protracted negotiations over access standards. This slows down the progress of the energy transition, frustrates otherwise helpful development, and diverts resources towards trivial considerations rather than focusing on issues of greater importance.

Synchronous machines will play a role in the energy grid of the future. Our industry needs to maintain expertise and regulatory frameworks that allow this technology to continue providing the grid with vital stabilisation and robustness.

ABOUT THE AUTHOR

Donald has over 20 years’ experience providing advice on regulatory and technical requirements for generators, substations and transmission systems. He has worked for all areas of the electrical industry, including generators, equipment suppliers, customers, NSPs and market operators. Donald specialises in the performance of power systems. His experience in generating units, governors and excitation systems provides a helpful perspective on how the physical electrical network behaves.

12 December, 2024