Rooftop solar and grid stability: Is there a solution that is good for consumers?

The push to decarbonise the Australian grid, combined with the insatiable demand for residential, commercial and industrial solar, means that a large number of distributed energy resources (DERs), connected via inverters will connect to the distribution grid at the low voltage level.

The consequence of high penetration of DERs on the electricity network is increased grid instability. So what is the solution?

Recent revisions of Australian Standards AS4777 attempt to address the issue with the introduction of new performance standards for inverters, while electricity network operators elect to introduce their own requirements over and above the minimum performance standards set out in AS4777.

Inverter power quality response mode

One solution is to regulate inverter performance and inverter interaction with the electricity grid, using guidelines and framework defined inside Australia Standard AS4777.

Inverters are required to provide support to the grid by functioning outside their normal operating window. These operating limits are flexible but ultimately set by the local electricity network operator.

In this mode the inverter power output is varied in response to the terminal voltage. In the case of Queensland DNSPs, Essential Energy (NSW), TasNetworks and Power and Water Corporation (NT) the Volt-VAR response mode settings are set at:

Reference        Voltage (V)        VAR % of Rated VA        Power Factor

V1            207            44%                0.9 leading

V2            220            0%                1.0

V3            240            0%                1.0

V4            258            60%                0.8 lagging

 

As the terminal voltage rises above 240V, the inverter output power starts to ramp down. Solar energy lost by the power curtailment process must be purchased from the grid. Power curtailment may occur even if the solar energy being produced is wholly consumed on site with no export.

A University of New South Wales report prepared for the Energy Security Board (ESB) investigated voltage across the National Electricity Market (NEM) to gauge the extent of losses to prosumers from high voltage curtailment of inverters.

The key finding of the report was that even in the absence of solar PV there was a significant high voltage across the NEM in all regions and states. The nominal voltage standard in the NEM is 230V +10% / -6% – 95% of readings were found to be higher than this.

The ESB report found that the average voltage in South Australia, Queensland, New South Wales and Victoria was between 244 and 248V and, as a result, a high number of inverters would operate in high voltage curtailment mode for most of the time, with reduced power output.

Export Limiting

Export limiting is determined by your local electricity network operator to throttle the inverter output if certain solar export limits are reached, usually coinciding with strong solar production days.

The impact of export limits is twofold: The first impact is that the income stream arising from the solar FIT is diminished; the second is that it changes the dynamics of the solar system design. With unrestricted export, the Net Present Value (NPV) is greatest for a larger solar system. With export controls, the solar system is optimised for self-consumption, leading to a small solar system, poor utilisation of available roof space and unrealised energy, cost and GHG savings.

Fixed Power Factor

Australian Standards require inverters to include the capability to set a power factor from 0.8 leading to 0.8 lagging, this provides the local network operator the opportunity to manage line voltage through power factor control.

At a power factor of 0.8, the real power output of an inverter is reduced by 20%, meaning your 100kW solar PV system now performs as an 80kW solar PV system. The net effect is that your solar system is over-capitalised and underperforming.

Impact on solar performance and investment return 

When the inverter performance is manipulated through power factor, export and power curtailment, while it may be advantageous to the local network operator, all the downside is with the asset owner.

A reduction in solar feed-in tariff (FiT) and the need to purchase more energy from the grid all impact on the financial performance of the solar system.

Under a Power Purchase Agreement model, the owner of the solar asset may need to employ software with sophisticated algorithms to predict the “lost performance” of the solar system and adjust the PPA price of electricity to the end user accordingly.

There will also be an impact on a system’s production of small-scale technology certificates (STCs), where a solar system is deemed to create a certain number of STCs (RECs) based on its expected generation to 2030.

Since the formula is based on assuming 100% generation, more STCs are being created than the actual amount of energy the solar system will generate. This means there is less renewable energy being generated than the scheme is targeting and that more STCs are being created than there should be, suppressing the STC price and thereby the price paid by retailers required to purchase and surrender STCs.

For large commercial and industrial systems, the investor or asset owner must write down the value of future income stream from LGCs, again testing the financial viability of the initial investment.

AI Technology to the Rescue

Ten years ago, eleXsys Energy recognised these problems would eventuate, which is why we developed the eleXsys technology. The eleXsys hardware and AI-based software manages the stability and resilience of distributed solar and wind generation.

It unlocks the full potential of electricity networks to integrate distributed solar and wind energy and battery storage, either standalone or in a microgrid, ensuring the most efficient, lowest-cost delivery of clean energy and grid stability services.

Unlike other software and hardware-based solutions, the eleXsys does this by addressing the root cause: the unstable grid. By managing the line voltage and power quality, inverters are free to operate in the 220-240V window with maximum power output, and without the need for export curtailment for any size solar PV system.

Incorporating an integrated solar, battery and eEMS, underutilised rooftops are transformed into revenue-generating, clean energy power stations. With grid-forming capability, eleXsys can also be configured for economically viable off-grid applications, replacing diesel-based microgrids.

Utilities leverage eleXsys to improve grid stability impacted by DER and increase network capacity without expensive grid upgrades.

Conclusion

The decarbonisation of electricity grid has created a new dynamic and a new set of problems – foreseen by some but misunderstood by others. Solar penetration has reached a tipping point, and our attempts to address an unstable grid are at the expense of solar production and challenges the financial viability of solar.

It is no longer feasible or desirable that utilities and network operators dictate terms to the detriment of renewable energy investment. There is no reason that high penetration of distributed energy resources and stable grid cannot co-exist, it is exactly this that eleXsys promises and delivers.

eleXsys addresses two problems, improving power quality at the point of connection and facilitating more DERs to be connected onto the low voltage network.

Planet Ark Power is the developer of award-winning eleXsys technology. To read more about the technology, click here.