Five kinds of solar curtailment

Western Australia has recently joined South Australia in implementing a ‘Distributed Generation Management’ scheme, whereby some roof-top solar systems can be remotely turned off by the electricity system operator.

Not surprisingly, that has raised eyebrows in the community. But when you look at the bigger picture, the occasional solar curtailment is okay, and actually economically and environmentally desirable, because it allows us to maintain the rapid pace of renewable deployment safely, and allows us to complete the clean energy transition.

The solar curtailment recently announced is just one kind of solar curtailment, and one part of an increasingly complex story about how we integrate more renewable energy into the power system.

Here’s an attempt at briefly describing five different kinds of solar curtailment, and how they fit together.

1. Distributed Generation Management

Our traditional model of electricity supply is large centralised generators with spinning turbines supplying power over high-voltage transmission lines through to sub-stations and distribution feeders and ultimately the use of that electricity by homes and businesses.

But these large generators aren’t just supplying energy, they are also supplying ‘system strength’; that is, they set and regulate the frequency of the electricity system (manage fluctuations in supply or demand) while maintaining a stable voltage level on the transmission network.

This system strength is delivered as a natural by-product of the way these systems generate electricity with a large spinning turbine that has physical inertia, which, amongst other things can slow the rate of change of frequency in the electricity system just through its physical mass.

As more utility-scale renewable power systems are deployed, they economically displace traditional utility-scale thermal generators because renewables have zero marginal cost. Todays renewable power systems (mostly) can’t deliver system strength.

So the transmission system operator needs to schedule a minimum amount of traditional thermal generation to maintain system strength, and so has the ability to curtail these large transmission-connected solar farms and wind farms to achieve this when load on the system is low, even when the cost of running the thermal generators is higher.

The transmission system operator has visibility and control over the generators connected to the transmission system, but they have had (until now) no ability to reach into, or manage, what is happening within distribution systems where roof-top solar is deployed.

On mild clear sky days, typically on weekends, often in Spring and Autumn, we are seeing increasing events with very high output from roof-top solar systems and very low underlying demand for electricity (on mild days we don’t use as much air-conditioning or heating).

From the transmission system operators point of view this shows up as very low load (down to zero). So low that there isn’t enough load to schedule the thermal generators with spinning turbines that provide system strength.

Enter Distributed Generation Management, which enables the Network Operator and Transmission System Operator to work together to manage roof-top solar systems connected to the distribution network to ensure there is enough load on the system to keep the traditional thermal generators running, which are essential (at least for now) for a strong and stable electricity system.

2. Automated Inverter Settings

A solar inverter converts the the output from photo-voltaic solar arrays to a usable form. Inverters are required to conform to standards to ensure the stability and safety of the power grid.

Where Distributed Generation Management is concerned with the state of the transmission system, Automated Inverter Settings are concerned with the state of the local low-voltage distribution network where the solar system is connected.

Inverter standards include a requirement for a number of autonomous functions, where the inverter senses the state of the network and adjusts its output automatically (in some cases turning down the active power output, and in some cases absorbing or injecting reactive power).

A common consequence of these settings is that in parts of the distribution network with a high-penetration of roof-top solar, we will sometimes see high voltage at times of high solar output, and solar systems will automatically switch off, or curtail their output, to ensure that voltage doesn’t exceed safe levels.

This effect is highly localised, and is related to local network infrastructure, and might just effect certain streets in a suburb, or even just houses on one side of the street.

3. Static Export Limits

When you connect a solar PV system or some other kind of embedded energy system to the distribution network you need a connection agreement with the distribution network operator.

This connection agreement can include a limit, or constraint, on the amount of energy you can import or export to and from the distribution network.

These constraints have generally applied to commercial and industrial customers, but residential customers trying to connect new solar systems have been impacted too. The actual constraint that is applied by the network operator is based on the capacity of the local network infrastructure, often in relation to the thermal limits of this infrastructure.

It is not uncommon for this constraint to be zero (i.e. no export). This means every time a solar system exceeds the load at a site it needs to be automatically curtailed. This requires a local control system that can measure site load and use these measurements to control solar inverters.

Network operators need to model the power flows of the distribution network, and how these might change over time as load increases, and then they use these models to set constraints as part of the connection process, as well as to support planning for network augmentation.

In the past it has been difficult for network operators to justify network augmentation to support export services (i.e. to reduce these export constraints), but in Australia new rules will enable distribution networks to do this, and also allow them to charge network users for export services.

4. Dynamic Operating Envelopes

Static Export Limits are not efficient and can lead to unnecessary curtailment. They need to be set by the network based on a ‘worst case scenario’ and there can be times of the day or times of the year when there is capacity available on the distribution network, and even more, there can be times when more export (or injection of energy) to the distribution network is very desirable.

Rather than set constraints based on periodic power flow modelling and load forecasting activities, networks are attempting to move towards dynamic ‘state estimation’ models. If networks can determine the state of the network dynamically, they can set constraints dynamically (the dynamic envelope), allowing the distribution network to host more roof-top solar without expensive augmentation.

When Dynamic Operating Envelopes are in place, Solar PV systems (and/or the 3rd-party aggregators controlling large portfolios of DER assets) need control systems that can receive signals from the network operator to moderate solar system output, or more ideally co-optimise solar system output with other resources across the portfolio like like battery storage systems, EV chargers, flexible loads, and so on, perhaps as part of a Virtual Power Plant.

We’ve previously written about how you can model projects that are exposed to Dynamic Operating Envelopes with Gridcog.

5. Economic Curtailment

Australia, like many parts of the world, has wholesale electricity markets, including the Wholesale Energy Market (WEM) on the West Coast and the National Electricity Market (NEM) on the East Coast..

In fact, the NEM is the most volatile commodity market in the world. The price for electricity is set every 5-minutes and can range from a high of around $15,000/MWh down to a low of -$1,000/MWh. This physical electricity market is also complemented by a financial market where market participants can trade futures contacts to help manage price risk (for example allowing market participants to buy a ‘cap’ to limit their price exposure to very high prices from the owners of peaking gas plants and energy storage systems).

Most electricity consumers aren’t exposed to this volatility, but that doesn’t change the fact that significant value can be created by reducing load when prices are high and increasing load when prices are negative. This value can create incentives for economic curtailment of Solar PV Systems, because turning off your solar system can make you money.

If Wholesale Energy Prices are negative then exporting energy costs money, and turning down solar output to increase load makes money.

The way this value can efficiently flow through to end customers through new retail products and new aggregation models is still not super prevelant, but there are an increasing number of models out there and this is something you can model in Gridcog, along with the constraints reflected by the other four curtailment models mentioned above.

How does this all fit together?

While these mechanisms are all quite different, they are not independent of each other.

Ultimately they all arise from the huge amount of solar we are deploying and the fact that all of these systems, largely, produce energy at the same time. Whether we need to curtail solar output because of the limits of the transmission system, the limits of the distribution system, or because of the price of energy, the end game is the same.

We are moving from a power system where generation output was adjusted to match load, and we are moving to a system where load needs to be adjusted to match generation output.

During the times when our solar systems are producing and clean cheap energy is abundant we need to shift loads (heat, cool, pump, process, etc) and we need to store energy (batteries, vehicles, pumped storage, etc). This is a co-optimisation problem and something Gridcog is designed to do for distributed energy projects.

We also need to move to a system where system strength is no longer delivered from fossil-fuel based thermal generators with spinning turbines, to a system where system strength is delivered by power electronics and smart inverters, backed by energy from renewable sources.

So you can see that the occasional solar curtailment is okay, and economically and environmentally desirable as it allows us to maintain the rapid pace of renewable deployment safely, and achieve the ultimate objective: To complete the clean energy transition.

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