The effects of voltage irregularities are more sorely felt by owners of PV systems the farther they are along a line, writes GSES.
Photovoltaic power systems have been deployed in record numbers across the globe and Australia is leading the charge. From insignificant levels in the beginning of the 21st century there is now over 12.9GW of combined capacity across Australia. Most of this is small-scale grid-connected distributed resources (2-5kW systems) installed by end users. As of 2018, rooftop PV accounted for 19.6% of the total clean energy generation across Australia.
This amazing uptake of distributed generation has occurred in such a short time that there simply has not been any coordinated planning for adverse system effects. Hence, new complexities in the power system have emerged, mainly in the distribution network. In some communities where rooftop solar penetration is above 45%, power system reliability and voltage control are being challenged.
Times of peak solar generation typically correspond to periods of low energy consumption, and the exported energy in the distribution network has led to reverse power flows in a system that was not designed for it, which can cause over voltages, brown outs and black outs.
Always in mode
Australian technical and network standards stipulate that grid-connected inverters must respond to voltage quality limitations. There are two main types of voltage response modes: power curtailment, where the PV system’s exported real power is proportionately decreased in response to the voltage detected by the inverter; and reactive power control, whereby the inverter will absorb or inject reactive power according to an over or under voltage on the network. Depending on the local distribution network service provider, there are different requirements for the type of voltage control response mode and voltage set points.
The current standards do not take into account the position of the customer along a power line, which can drastically change how much voltage the customer is experiencing. The physics of electricity mean that the customers who are furthest away from the transformer experience the highest voltage during times of reverse power flow.
Customers who experience the highest voltages on a line will be the ones whose PV system export is curtailed the most. The curtailment of their solar export will have a real impact on their energy costs from the utility, and thus they may experience a higher bill just for being on the end of an LV line when compared to their neighbours for similar behaviour.
Since customers seldom understand their position on the line, or how far away they are from the feeder, there is a possibility that they may not even be able to realise the level of their curtailment.
How effective are voltage response modes at controlling voltage? And how does an inverter’s response mode and the whole system impact a customer’s daily costs?
GSES performed a very rudimentary study based on a uniform feeder in OpenDSS, a distribution network modelling program. It consisted of an 11kV slack bus voltage source and an 11kV to 400V 3-phase transformer. All houses were modelled to have a 5kW PV system, with seasonal load profiles derived from real solar home data from Ausgrid. During times of net import, the household was modelled as a load, and during times of net export, the household was modelled as a negative load, or generator.
The curtailment of real power is theoretically the most effective way of reducing voltage in resistive networks, such as LV feeders, and this was confirmed by comparing the effects of volt-watt and volt-var mode. Nevertheless, when deployed individually, the voltage was not kept under the reference voltage at the end of the line. It was only when the two inverter modes were deployed together that the voltage was actually kept under the reference voltage.
While the voltage was effectively controlled, the dual deployment means that the disadvantages of both methods are also experienced, namely the loss of excess solar energy and the increased overall reactive power demand, which can cause network strain and increase the demand from large-scale fossil fuel generators.
The effects of the inverter response across the line was stark. It was determined that voltage control disproportionately affected customers that are the furthest away from the transformer. Due to these inverters detecting the highest line voltage, their curtailment of real power and absorption of reactive power was the greatest.
The customers with the most curtailment experienced the greatest increase on their daily electricity bill. Customers with a flat tariff can experience a bill 12% higher than their neighbours in autumn, 38% in summer and 94% in spring.
Under the time of use (ToU) tariff, the differences are even more stark, with daily charges 21% more expensive for customers at the end of the line in autumn, 55% in summer and a whopping 372% in spring. That is an effective increase in daily charge of almost five times greater for the customer on the end of the uniform feeder.
Decisions about the future of the grid and standards and policy changes could more effectively incorporate the quantifiable financial impacts of inverter voltage control responses. Future work should focus on applying the concept of a financial analysis of inverter curtailment to model what is more representative of real conditions.
Also, battery energy storage systems have immense potential to act like sponges in the grid, not only soaking up excess solar energy and deploying it in times of peak demand but helping to regulate voltage in the network.