Inverters to play a big role as rooftop PV sails past Australia’s biggest power stations

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The Australian energy landscape is undergoing one of the most significant transformations in history. In the last decade, Australians have embraced rooftop solar enthusiastically, with over two million rooftop solar PV systems installed across the country. This has led to one of the highest levels of distributed energy resource (DER) penetration in the world with some low voltage networks exhibiting up to 36% DER penetration at the state level. As a result, solar PV capacity has sailed past that of the largest NEM generators supplying power to the states of Queensland and South Australia with Western Australia very close (see figure 1) and it’s only a matter of time that the same trend replicates itself across other states.  

Figure 1 – Comparison of BTM solar PV capacity against the leading NEM power station in each state (AEMO,CER, 2019)

Whilst, ‘behind-the-meter’ solar PV capacity has surpassed some of the largest generators in the country its important to recognise that the limited capacity factor of solar PV limits the overall energy delivered to the power system. As a result, the actual output from behind the meter solar PV is still well behind the largest generators, except in South Australia (see figure 2). However, growing consumer discontent with rising electricity bills, generous state government subsidies for both solar PV and household battery systems as well as key regulatory rule changes are likely to see the continued growth of distributed energy resources (DERs) in Australia.

Figure 2 – Comparison of BTM solar PV output against the leading NEM power station in each state (AEMO, CER, 2019)

While the output from the large, scheduled NEM generators is heavily controlled though market and dispatch mechanisms run by AEMO, the BTM solar fleet is largely invisible and uncontrolled. This creates a number of challenges that must be resolved to fully harness the value of Australia’s rapidly expanding DER fleet.

One of the key challenges is the increase to network voltage. Increasing PV penetration has led to some areas of the low voltage network operating outside defined voltage limits. This causes some inverters to trip off completely or scale back exports, reducing value for customers and creating unutilised capital for the system. It also leads to operating and capital costs for networks to rectify the problem by manually changing the settings on transformers or augmenting the network.  However, advances in DER technology have meant that the cause of this problem can also be its solution.

The power of inverters

‘Networks Renewed’, an ARENA funded project led by the University of Technology, Sydney, has illustrated the impact that smart inverters and batteries can have in managing local fluctuations on the low voltage network (see figure 3). In their own words “Networks Renewed is a smart inverter demonstration project. It aims to understand the extent to which residential solar panels, battery storage can manage voltage in distribution networks – and increase their PV hosting capacity. Thereby turning the problem into a solution”.[1] The research conducted by this project illustrates that whilst in the short to medium term, solar PV won’t be largest generator of electricity, solar PV and batteries supported by smart inverters will have a leading role in supporting the impending transition in our power system to a two-way system for our distribution network businesses.

Figure 3 – The impact of inverter reactive power response from solar and battery systems on the grid (Networks Renewed, 2019)

The key question that begs from this research, is how does more solar PV systems on the network benefit and improve network power quality without significant network augmentation? The answer is the ability for smart inverters to manage power quality by providing volt-watt and volt-VAR response to manage the voltage on the local network. In less technical terms, the inverter works much like a shock absorber when fluctuations in power quality occur, the inverter responds by either injecting reactive power to the grid or absorbing reactive power. This response helps the local distribution network achieve optimal levels of power quality with no impact on the energy delivered to the supporting system and surrounding households. The exciting thing about this trial is that batteries are not required (although they help!). Customers with just a solar PV system and a smart inverter can support the power quality of their local grid by injecting reactive power. (see figure 4)

Figure 4 – The impact of inverter reactive power response, from solar systems only, on the grid (Networks Renewed, 2019)

Smart inverters are the answer, yes! But they’re only half the answer. Solar PV and batteries are at present unscheduled generating units with minimal visibility available to distribution networks and the market operator (hence the establishment of the DER register by AEMO). Ultimately, the real impact will be uncovered through coordinated voltage and frequency response. As GreenSync CEO Phil Blythe highlights, smart inverters are “immensely powerful but are lacking the digital backbone by which they can be linked together and linked to central grid operators to utilise these capabilities”[2].

Overcoming this challenge is buried in the detail, specially product standards. At present, these smart inverter capabilities are non-mandatory items of product standard AS/NZS 4777.2. AEMO is currently progressing a fast-tracked revision of the standard to ensure that they are mandatory, but the industry has already surpassed this requirement with almost 80% of the CEC list already complying with capabilities that are required to be identified as a ‘smart inverter’. Ultimately, the real change will come with efforts that focus on asset coordination, aggregation and dynamic communication, facilitated by dynamic connection agreements. This change would substantially shift the role of DER assets to not being Australia’s largest generator but to be the largest enabler of distributed clean energy generation in the world.