Projects:2021s1-13013 Investigation of voltage control modes and strategy for large embedded generators connected to the distribution network with high penetration of Distributed Energy Resources

South Australia’s energy landscape is transitioning quickly from fossil fuel generation to renewable resources. Considering this, an increasing number of South Australians are opting to generate their power through Distributed Energy Resources (DERs). With a distribution network with one of the highest penetrations of DERs in the world, concerns about voltage control arise for the increasing popularity of connecting these DERs. All kinds of DERs contribute to this concern, and domestic rooftop solar can contribute to voltage rise issues as well as Large Embedded Generation (LEGs) which generate power in the range 200KVA to 5MVA. This project only seeks to mitigate to adverse affects of LEGS, and not domestic rooftop PV, although the impact of rooftop PV and the voltage complications caused by them must be considered. The connection of LEGs to a distribution network which is already saturated with DERs like rooftop PV means that there is more chance for voltage rise events to occur which, at the bottom line, leaves customers in that region without power. If LEGs are to be installed to the distribution network such that that the total electricity demand may be met, all customers who are wanting to connect are able to, and voltage remains at an acceptable level, then alternative hardware and control options will be necessary to support this. This interim report presents the project group’s initial literature review, current methods for voltage control in South Australia and method for testing alternative voltage control techniques such as reactive power absorption, injection and droop control by using the SA Power Networks (SAPN) distribution network as a case study. This interim thesis proposes that fixed voltage control, fixed reactive power control and fixed power factor control may be used as voltage control methods for categorised types of LEGs on a distribution network with a high penetration of DERs.

Introduction

The connection of embedded generating systems to the distribution network may impact on distribution utilities’ ability to regulate network voltages. For this reason, distribution utilities require embedded generating systems to consider voltage, power factor and reactive power control system as described below.

• Voltage droop control which varies by a fixed percentage to manage network stability.
• Operate at an agreed power factor such that voltage variations are maintained within prescribed limits.
• Control reactive power output, within their capability, to maintain the connection point voltage to anagreed target.Evaluate the ideal voltage control and methodology for large embedded generators categories connected to the distribution network.
• >30kVA & ≤200kVA (NER Chapter 5A assessment)
• >200kVA & <5MVA (NER Chapter 5A assessment)
• >5MVA (NER Chapter 5 assessment) (optional scope)

Objectives

1. Investigate current voltage regulation strategies in other utilities in Australia
2. Investigate if these alternative strategies offer significant advantages over current SAPN practice
3. Investigate enhancements to and/or design approaches to achieve enhanced voltage regulation with the objective of increasing feeder DER hosting capacity as constrained by inadequate voltage regulation.
4. Formulate a robust method from which an optimal method for voltage control can be selected based on scenarios

Project team

Project students

• Ishika Ghosh
• Georgia Kappos

Supervisors

• Mr David Vowles
• Dr Ali Pourmousavi Kani
• Mr Andrew Lim (SA Power Networks)

Context & Significance

South Australia’s renewable energy generation has grown from 0% to over 50% since 2004 [1], and the state government says that they aim for South Australia to be “net 100% renewables by 2030”[2]. With the uptake of renewables in South Australia, comes the engineering problem of facilitating this transition using infrastructure that already exists. The project is being undertaken as SAPN have identified that there are many customers who are wanting to connect LEGs directly to the distribution grid. Many of the proposed LEGs, which exist in metro and regional areas of South Australia, are not feasible to connect to the distribution network as it exists currently, due to the voltage rise that will occur as a result of the LEG. SAPN want this study to take place so that these customers will one day be able to generate and use their own renewable energy resources. This study is being done to support equal financial and energy access opportunities to all South Australians and to further the State’s goals for renewable energy generation and use [1]. This study will also produce information that can be applied to currently existing LEGs that are connected to the distribution network and improve areas that are already at risk of experiencing poor power quality or blackouts due to voltage rise as a result of DERs.

As outlined in McGreevy’s paper, figuring out how to effectively manage voltage control for LEGs is paramount South Australia’s transition to renewable energy, which is characterised by ‘sociotechnical system transition’[1]. McGreevy describes this as a change in ‘technology, policy, markets, consumer practices, infrastructure, cultural meaning and scientific knowledge’, and this report will contribute to the technology, consumer practices and infrastructure aspects of this comment. It is currently not a feasible option to connect all proposed LEGs to the network from a DNSP’s perspective without incurring a major cost to upgrade or add on infrastructure due to the poor power quality it would create for every customer connected to that feeder. It is also not currently feasible economically for customers who want to connect to feeders with a high penetration of DERs to incur such a large cost for connection of renewable generation. Finding a set of rules which optimises voltage control methods for categories of feeders and minimising the need for extra costs will be important to ensure that customers want to and are able to connect and that they will experience reliable power quality. The transition to renewable energy is moving forwards in South Australia to meet the targets set by policy makers. As a result, and as more distributed energy technologies become available to customers, the economic and accessibility gap between socioeconomic classes gets smaller regarding access to reasonably priced and renewable power. In addition to this, optimising voltage control methods is significant to the engineering community for reliability, condition monitoring and application onto other networks in places that also strive to be completely renewably resourced. With emerging Battery Energy Storage Systems (BESS) becoming an increasingly affordable and popular option, soon it is forecasted that the types of LEGs for connections requests will not only consist of Photovoltaic systems (PV) but large BESS, and this engineering application will become more important as voltage rise will remain an issue at this stage.

Background

Distributed Energy Resources

A Distributed Energy Resource (DER) is characterised by being a generation source that is connected directly to the distribution network. Large Embedded Generators (LEGs) are a of DER that generates power above 30kW, according to SAPN’s technical standards and operating procedure [3]. It is also important to remember that rooftop solar systems that generate less than 30kW are also categorised as DERs but are not considered LEGs. This project will only consider the voltage control methods associated with LEGs and not rooftop systems that generate less than 30kW, but will take into account the voltage rise contribution that a large amount of small rooftop solar in an area will contribute.

Reactive Power and Voltage relationship

Voltage is controlled by the injection or absorption of reactive power into a network. Voltage control must occur at the point of common coupling, the PCC, due to reactive power losses occurring very quickly [9]. Voltage Rise at the PCC must not exceed 0.05% according to SAPN’s rules [10]. SAPN essentially allows no voltage rise at the PCC from any customer connected generation source. If the voltage is not delivered at the required standards, not only does the customer experience the interruption, but the DNSP responsible for distribution of power to those customers are fined by the Australian Energy Regulator for not hitting the service performance targets that are outlined in the Service Target Performance Incentive Scheme [11]. Voltage control methods must be used to keep the voltage within prescribed limits and prevent unplanned sustained customer interruptions.

Voltage Droop Control

Voltage droop control uses a closed loop feedback mechanism, in which the inverter senses its own voltage output and manipulates the output injected or absorbed reactive power to keep the voltage at a setpoint.

Fundamental Principles

Consider the high-level diagram in Figure 1 of a SAPN feeder plan. Reading from right to left power arrives via Electranet’s high voltage transmission network and is stepped down to lower voltage appropriate to SAPN’s distribution network at TF2. The low voltage power is then distributed to customers connected to a specific feeder. This describes typical customer import of power. Reading from right to left, this scenario describes the customer connecting a LEG with capacity to export power back into the distribution grid. This may occur when the load of the customer is less than the magnitude of generation of the LEG and so the power is exported back into the distribution network, creating reverse power flow. The exporting of large amounts of power from a customer’s LEG when the total load of the feeder system is small results in voltage rise on the feeder. According to the National Electricity rules, clause S5.1a.4 states that “the voltage of supply at a connection point should not vary more than 10 percent above or below it’s normal voltage ”[7]. SAPN allow only 0.05% voltage rise at the point of common coupling due to any connected LEG.

Results

Voltage Droop for a country feeder

References

1. 1. McGreevy, D.M., et al., Expediting a renewable energy transition in a privatised market via public policy: The case of south Australia 2004-18. Energy Policy, 2021. 148: p. 111940.
   2. Parkinson, G. South Australia’s stunning aim to be “net” 100 per cent renewables by 2030. Renew Economy 2019.
   3. Networks, S.P., NICC270 Connection of Large Embedded Generation. 2020, SA Power Networks: SApowernetworks.com.au.
   4. Carter, C.C., Martina & Lu, Pengcheng & Crocker, Julian., An evaluation of options to mitigate voltage rise due to increasing PV penetration in distribution networks. Renewable Energy and Environmental Sustainability, 2017. 2(39).
   5. Babacan, O., W. Torre, and J. Kleissl, Siting and sizing of distributed energy storage to mitigate voltage impact by solar PV in distribution systems. Solar Energy, 2017. 146: p. 199-208.
   6. Croker, J., Transactive Energy for Voltage Support Within Residential grids with a high penetration of photovoltaics: a co-simulation analysis. Masters Dissertation, Murdoch University, 2016.
   7. Commision, A.E.M., National Electricity Rules, in System Standards. 2021, AEMO: AEMO.
   8. AER, SAPN Voltage Monitoring in the LV network, in Attachment G.12a. 2015, AER: https://www.aer.gov.au/system/files/SA%20Power%20Networks%20-%20G.12a_PUBLIC_Voltage%20Monitoring%20in%20the%20LV%20network%20v1.1.pdf.
   9. AEMO, Power Systems Requirements, in 3.3 Voltage Management. 2020, AEMO: https://www.aemo.com.au/-/media/Files/Electricity/NEM/Security_and_Reliability/Power-system-requirements.pdf.
   10. Networks, S.P., Technical Standard - TS130 in Inverter Energy Systems (IES) above 30kW and up to or equal to 200kW. 2017, SA Power Networks: https://www.sapowernetworks.com.au/public/download.jsp?id=9563.
   11. Regulator, A.E., Electricity Distribution Network Service Providers, in Service Target Performance Inventive Scheme. 2018, Commonwealth of Australia.
   12. Standard, A.N.Z., AS/NZS 4777 (2) Grid Connection of Energy Systems via Inverters, in Part 2: Inverter Requirements. 2020.