Households can hold grid together when big coal units fail

At 8:28pm on Wednesday last week the Kogan Creek coal fired generator in Queensland tripped.

This is Australia’s largest generator and the event instantly withdrew 750MW from the grid causing the frequency to drop below the minimum operating threshold of 49.85Hz.

This frequency disturbance was felt all across the mainland National Electricity Market in an instant, from large generating units to individual houses.

Because frequency changes are felt instantaneously across the NEM, generators and demand response – regardless of location – can be used to help rebalance the system following a disturbance.

By not having to be positioned in key locations on the grid, support can come in the form of smaller, distributed systems acting together.

Such distributed resources offer to add increased stability for lower costs – no single utility or government has to pay for the generator and individuals can lower their power bills by providing grid support.

It is not usually easy to visualise just how “in-sync” frequency is across the grid and therefore why location is (mostly) not important.

Reposit Power provides consumer technology and services for joining, building and operating Virtual Power Plants. As such, Reposit has devices across the NEM at customers’ homes that are constantly monitoring the grid.

The furthest distance between Reposit systems is nearly 2000 kms – from Townsville to Adelaide.

By looking at just the South Australian and Queensland systems over a one minute interval surrounding the trip you can easily identify that the frequencies measured are almost identical for all 1-second samples regardless of location.

The following graphs shows the average frequency at each interval per state – this is second-by-second data across hundreds of systems spanning thousands of kilometres giving near perfect frequencies.

When a trip like Kogan Creek occurs other generators and enabled demand response resources on the grid providing frequency control ancillary services (FCAS) start responding to recover the frequency back to the normal operating range.

Allan O’Neil has compiled a graph of the responses of the various FCAS generators that responded to the Kogan Creek disturbance. The locations of which can be seen below.

Despite the trip happening in Queensland ,it’s clear that the responses came from across the country. A generator in South Australia has the potential to detect and respond just as fast as one in Queensland.

If you’ve read the news lately you will have seen that the South Australia “Tesla big battery” (officially known as the Hornsdale Power Reserve) is doing just that.

Because location is not the most important factor, distributed systems are starting to play a key role in frequency response.

Distributed recovery mostly comes in two forms – Demand Response (DR) from companies such as EnerNOC which was enabled to provide 13MW of demand response during the Kogan Creek trip; and Distributed Energy Resources (DER) which are usually smaller, at-home systems grouped together to form a virtual power plant (VPP) such as Reposit Power’s.

Whilst Reposit didn’t respond to this event, changes in regulation and an ever-expanding fleet will allow them to participate in the future passing on the financial reward to their customers.

Individual systems can monitor the frequency and respond independently – the aggregated response appears as if it were a single generator with significant capacity. This is the premise on which VPPs are built and proven to work.

With DR and DER participation across the NEM rising, FCAS responses from smaller distributed systems can be just as powerful as using large generators but with added benefits.

Home-owners and small businesses get to earn money off their power bills for providing such services and selling their power back to the grid or reducing load in times of need; and the quality of FCAS response from these system is better – being able to respond in milliseconds rather than the several seconds required for a coal unit to spin up and respond.

This also lessens the impact of a single-point-of-failure as we are seeing with the Kogan Creek trip – each of the grid-connected devices is able to operate on their own and hundreds of thousands of these devices would have to simultaneously fail in order to see the type of sudden energy withdrawal that we have seen with the Kogan Creek trip.

The grid of the past – made of just a few large centralised generators – is already beginning its transition out as “big batteries”, DR and DER comes online and are already well established.

We were promised a smart grid. It has arrived.

Mike Leonard is Lead Developer at Reposit Power