In efforts to reduce greenhouse gas emissions, control energy costs, limit smog and/or enhance energy security, a significant number of nations have established renewable energy targets as a share of electricity production or primary energy. These include the EU (27% of energy consumption by 2030) [1
], Germany (40–45% by 2025) [2
], the USA (20% of electricity generation by 2020 nationally; 30% by 2025 for federal government consumption) [3
], China (15% of primary energy from non-fossil sources by 2020) [4
], India (35%, no date) and Australia (20% of electricity generation by 2020) [5
]. Globally, renewable generation was reported to account for 22% of world electricity generation in 2012 [6
]. In some countries grid-connected photovoltaic (PV) systems are seen as part of this transition, including utility scale (e.g., >1 MWp) systems connected to the medium voltage network and medium scale (10 kWp–1 MWp) or small scale (<10 kWp) systems connected to the low voltage (LV) network [7
]. The majority of existing low voltage distribution networks (LVDN), however, are radial and have been designed for unidirectional power flow from high voltage down to low voltage [8
When rooftop PVs first emerged, many network operators were reluctant to ‘allow’ such systems to connect to their networks, citing safety and reliability of supply as the main concerns. Academic and industry research since the 1980s has reported numerous ‘issues’ that PVs create for distribution networks: islanding, harmonic distortion, electromagnetic interference, reactive power/power factor, reverse power flow and frequency and voltage regulation [8
]. The nature and extent of such problems for LVDNs has been examined in relation to the penetration level of PVs (for example as reported in [14
]). Perhaps the most reported issue relates to voltage quality (over voltage, under voltage and voltage unbalance) [7
]. Academic reviews of PV/network issues tend to focus on specific issues and/or solutions, such as smoothing the output power from PVs to account for variability [19
], improving PV converter technology [20
], the dynamic use of PV and distributed storage to mitigate load and PV unbalance [21
], islanding techniques [22
] and the transformation from passive to active distribution networks [23
]. Most, if not all, of this research has been conducted from the perspective of the Distribution Network Service Provider (DNSP).
Despite the fact that power quality issues have always existed in electrical networks [24
], current public discourse gives the impression that PV systems are the only or main cause of power quality problems in LV networks. This is evidenced in online media [25
] and print/online news in multiple countries [28
]. Much of the language of both academic and industry publications and political and regulatory rhetoric, creates an impression (deliberately or inadvertently) that LV networks were effectively managed before the rise of PVs and that somehow this technology is causing problems that did not previously exist and do not exist in parts of the networks without rooftop PV systems. An example of public misinformation is provided below:
“...the variations in solar production are causing all sorts of problems with local electrical infrastructure ... it causes the voltage at the transformer to increase ... this excessive voltage is causing problems, such as power trips and appliance failure, for all consumers on the electrical network as electrical appliances are designed to use 240v.”
But is this perception that PVs are the main or sole cause of LVDN problems the whole truth? Research literature on LV network monitoring is very limited and under-explored [32
] and to the authors’ knowledge, there have been no published reports on power quality examination from the perspective of end-use solar households. The purpose of this paper is to report on measured power quality (voltage, frequency and power factor) at the customer service point—the point of common coupling (PCC) or customer point of attachment (PoA). Power quality, household consumption (major services and total) and PV generation from four random households are evaluated at different time intervals (12 months, monthly, daily) to investigate the likelihood of any of these factors contributing to identified power quality issues on their respective networks. The analysis raises questions of whether DNSPs’ current public focus on managing and regulating rooftop PV systems is masking underlying and pre-existing power quality issues that have not previously been identified, managed, regulated and/or publically communicated.
2. Materials and Methods
A mixed methods approach using both quantitative and qualitative data was used. This section compares Australian power quality requirements with international practice (Section 2.1
), describes the context of the four case study solar houses where power quality was measured (in 4 different LVDNs) and four additional case studies (113 residential properties in the same LVDN) (Section 2.2
) and the methods used for collecting and analysing this data (Section 2.3
and Section 2.4
). A mixed methods approach is deemed suitable for examining complex relationships between different elements [33
], enhancing data validity and reducing the chance of bias.
2.1. Power Quality Standards
Deviations in voltage, current and frequency are a means of quantifying power quality [24
]. This section summarises international and Australian power quality requirements, against which the case study data will be compared.
2.1.1. Frequency Standards
50 Hz is the international standard for supply voltage of 230/400 volts. In electricity networks designed around large centralised power stations, the frequency of the network is typically ‘managed’ by the synchronous generators, provided that the generation capacity and load are relatively matched and that there are no fast rates of change in either state. In Australia, the Frequency Operating Standards define allowable ranges under normal operating conditions on the interconnected national network (i.e., the range shown in Table 1
must be kept 99% of the time and any deviations stabilised within 5 min) as well as containment and stabilisation ranges and recovery timeframes for other conditions such as failures in transmission lines or supply scarcity. Its definitions apply to “all power system equipment, including generation equipment, transmission and distribution equipment and consumer equipment” [36
]. Table 1
shows that despite 50 Hz being the international standard, there are slight variations in the upper and lower tolerances between countries, for ‘normal’ operating conditions. There may also be differences in the tolerance time (e.g., 99.5% under EN50160 but 99% in Australia). These local variations are important to understand before analysing frequency data in particular jurisdictions.
2.1.2. Voltage Standards
Some international standards for voltage are summarized in Table 2
, providing a context for the Australian standard and practices in different jurisdictions in Australia. It shows the change in the Australian Standard in 2000 from 240 V to 230 V (row 6) but that this Standard has not been adopted in each jurisdiction yet (row 8). It also shows that those states that have adopted the 230 V standard (row 7) have a tighter preferred voltage tolerance (+6.1%, −2.2%) than the nominal tolerances applied by the 230 V standard.
An example voltage distribution compared with lower, preferred and upper operating zones required by the Australian 230 V standard is shown in Figure 1
. In contrast to the European adopted EN50160 that requires 230 V ±10% for 95% of time, in Australia the upper (+10%) and lower (−6%) of the 230 V standard is based on the 99th percentile. This means that voltages (over a 10 min average) can exceed the upper and lower limits only 1% of the time under normal network operation. In addition to these point-of-supply voltage standards, AS/NZS 3000 Wiring Rules allow for an additional 5% voltage drop to occur between the point of supply and the point of use within a customer’s premises. Exported energy from rooftop PVs is widely reported to cause voltage rise, particularly at times of high solar radiation and low load, due to the export of real power and the impedance of the system.
2.1.3. Power Factor Standards
In an alternating current circuit, power factor (pf) is the cosine of real power (kW) and apparent power (kVA). Unity power factor (pf = 1) occurs when there is no difference between real power and apparent power and any measurable difference then becomes a measure of energy wastage and an indication of network line losses and decreased network utilization [46
]. Reactive power is needed for equipment such as electric motors, fluorescent light ballasts and transformers and can cause current to lag voltage. Conversely, current leads voltage in capacitors and hence they are often used near load centres to support voltage profiles. Table 3
shows the measures different Australian states utilize to address power factor (as reported in 2009). Only the states represented by the case studies in this paper are presented. Note that to meet these power factor standards, most DNSPs in Australia implement power factor ‘management’ strategies (e.g., kW or kVA demand charges) for non-residential customers. No power factor correction/management strategies are applied to residential customers in any Australian jurisdiction.
2.2. Case Studies
2.2.1. Case Studies for Quantitative Data
This paper reports on data measured at four detached single-family homes in four different LVDNs as characterized in Table 4
. Three of the homes have a rooftop PV system and all homes are connected to a radial distribution network. These homes were part of a study evaluating innovations in energy efficiency and net zero energy (NZE) housing in Australia [47
]. It is coincidental that they are in four different LVDNs but the data collected for energy efficiency/NZE analysis inadvertently provided an opportunity to evaluate power quality measured at these premises.
2.2.2. Case Studies—Supplementary Qualitative Data
To examine issues arising with grid-connect PVs and LVDNs in more depth, information (historical, anecdotal and quantitative) was gathered from four additional sites (Table 5
). Each of these sites has roof-mounted PVs in different configurations and each is connected to the same LVDN, a radial network.
2.3. Quantitative Data Collection and Validation Methods
A Home Energy Management System (HEMS)—AuziMAX by Enopte (www.enopte.com
)—was customized for the four case study homes to record minute data from all electrical circuits (via current transformer (CT) clamps; 1% accuracy) as well as voltage, real power, power factor and frequency (via a Hiking DDS238-2 Energy Meter, a Class 1 meter compliant with IEC 61036: Alternating Current Static Watt-Hour Meters for Active Energy (Classes 1 and 2)). The equipment was installed at the PCC/PoA, meaning that all measurements are actual power quality experienced by the household at the point of connection to the network. The occupants have access to the online dashboard supplied with the HEMS unit, whilst the research team utilized the raw data to conduct their own data evaluation. The accuracy of the energy meter data was first validated through analysis of frequency (e.g., Tables 7 and 8 show that the measured frequency for all case study houses was within the required standard (49.85–50.15 Hz) for 99% of time) and later through the analysis software (refer to the following section).
Data from the additional four case studies was supplied by the relevant homeowners, communities or other bodies as indicated in Table 9 (e.g., written records and correspondence, equipment technical specifications, site inspections, measured data, DNSP documents etc.).
2.4. Quantitative Data Analysis and Validation Method
AuziMAX utilises its bespoke cloud based data storage and analysis system (Energy Maximiser: Enopte, Coolum Beach, Australia) to provide feedback to the customers’ dashboard. The raw data from the measurement devices is also accessible. Raw data for 1 year was extracted from the cloud server and uploaded in MS Excel and in MoniSoft, a Java-based platform-independent software that enables the analysis of energy data of buildings [51
]. This software was developed and utilised for Germany’s large energy efficiency in buildings project (EnOB) and utilises the database MarieDB (MariaDB Foundation: Espoo, Finland, 2016). This software includes analysis of data quality and plausibility. Data was analysed in 10 min averages (consistent with the Standard discussed in Section 2.1.2
) unless otherwise specified in the following sections.
Australia has over 1.5 million residential rooftop solar systems and the challenges (real or perceived) that these place on existing radial distribution networks are frequently in the news—in a technical, social, political or economic context. The general impression of this rhetoric is that PVs are causing immense problems on the network. In other jurisdictions, for example in California, there is at least an acknowledgement (sometimes) that a lack of ‘a clear policy to manage all the renewables being added to the grid’ is the issue, rather than PVs per se [52
The results presented in the previous section suggest that the low voltage distribution networks reported in this study do not have perfect networks and are not meeting power quality standards at PoA/PCC. Furthermore, the poor power quality cannot be attributed to the household rooftop PV systems reported here. This does not necessarily mean that PVs are not impacting on each of these LVDNs but rather that there may be other factors that contribute to power quality issues. Four such factors are discussed in the following sections: poor historical management and outdated risk management approach, missed opportunities to embrace PVs as a means of better network management, lack of acknowledgement of the emergence of the prosumer and lack of total quality management and systems thinking. Addressing these contributing factors, publicly, will help correct false and misleading public perceptions of the benefits and limitations of rooftop solar.
4.1. Historical Network Management
In his chairman’s address in 2016, billionaire and businessman Warren Buffet wrote:
Historically, the survival of a local electric company did not depend on its efficiency. In fact, a 'sloppy' operation could do just fine financially. That’s because utilities were usually the sole supplier of a needed product and were allowed to price at a level that gave them a prescribed return upon the capital they employed... That’s all changing.
The case studies presented in this study suggest poor network management of power quality in the low voltage part of the network. It is possible that a key factor in power quality at this level is the lack of balancing of residential loads and residential rooftop PV systems at a transformer level. The lack of knowledge about what is happening on their network at a LV level was recently reinforced by a Queensland DNSP [18
] that reported:
15% of three urban feeders studied were outside of the 240 V standard; and 29% were outside of the 230 V standard; this inferred that only 70% of the LV network was compliant with voltage requirements.
They have not managed/reset distribution transformer tap changers to manage voltage fluctuations at a transformer level even though their analysis suggests that 26.6% of currently non-compliant LV networks could be resolved through a transformer tap change. However, due to the high cost and small number of qualified and specialized manufacturers for on load tap changers, they consider it is not possible to equip on load tap changers to distribution transformers to actively manage voltage across a day or season. Therefore, load shedding is needed to reset the distribution transformer tap. This requires considerable planning and notifications to affected customers.
The company has significant gaps in their data/knowledge about their own network, particularly at a distribution transformer level and below. For example, they do not have accurate records of what tap position the transformer fleet is set to (although they are working to rectify this). They do not know what supply voltages are experienced by the customers. They have a poor understanding of customer loads such as the ratio of constant power loads to resistive loads at peak demand.
These deficiencies in their knowledge of their own system, at an LV level, call into question the assumptions made in their network models on which their business and operational decisions are made. It should also be noted, however, as evidenced by the experiences of Q17 and Q18 in Table 9
, that there is also a general lack of understanding in the electrical trades/electrical engineering sector about the need for phase balancing to optimize the performance of an electrical network that needs to manage both local loads and local generation.
The lack of attention to the low voltage network may also be a result of the way in which risk has been managed by the electricity generation and supply industry. With limited budgets and resources, risk management has focused on managing ‘high impact’ network events, such as loss of a centralised generator or collapse of a major transmission line. This was understandable as such events could impact on thousands or even millions of customers. In a centralised system voltage issues within a low voltage distribution network do not attract much attention from either the industry or the broader public. However, the increase in embedded generation at the low voltage level raises the question of whether there is a need to modify the risk management approach to be more in line with the changing network structure and conditions. These changes include not only the existence of embedded generation but also the sheer number of prosumers who are opting for rooftop PV systems. Voltage and other related power quality issues could become high impact events and could have a high likelihood of occurrence if not managed.
4.2. Missed Opportunities for Better Network Management
Certainly, the addition of residential rooftop PVs into a radial distribution network may present challenges for the management of such networks. The networks were designed for unidirectional energy flows through a network from high to medium to low voltage. The previous section discussed how the focus of network management has been on the medium voltage part of the network with little attention paid to their low voltage conditions such as phase balancing. This lack of knowledge about LV networks appears to be a global problem [32
The introduction of residential rooftop PVs means that LV networks are now bi-directional in terms of energy flows and this may exacerbate pre-existing network management issues, such as phase balancing (Figure 11
). Connection of residential PV systems to the network (undertaken by the DNSP) may present an opportunity to rebalance the transformers and reset tap changers.
Poor management is also evidenced by DNSPs lack of involvement, as an industry, in a ‘systems thinking’ approach to energy service needs and electricity supply. DNSPs and the energy sector in general are very active in some technical, regulatory and market issues, as highlighted in column 1 of Table 10
. This information is collated from a preliminary scan of public documents and government/industry websites in Australia. Column 2 presents additional measures that could be used to address each issue but DNSP involvement in these opportunities is conspicuously absent. This suggests that DNSPs have a somewhat restricted approach to solving power quality issues and are not using their knowledge and influence to improve the total energy system.
One possible reason for this lack of involvement in the whole energy system, is the complexity of their own part of the system (the transmission and distribution network) and its different sub-systems. In Australia, the management of the national network receives a lot of attention, with computer modelling of the transmission network for example including approximately 2400 nodes. In contrast, distribution networks are more complex. For example, in south-east Queensland the distribution network has more than 9 million nodes in a single-phase model. This means modelling such a network, to understand power quality, energy demand and energy flows, will be more complex and time consuming (i.e., have a higher computational cost). Because this part of an energy system has been seen as a lower risk than the transmission network, the DNSPs may not have adequate communicable metering devices within the low voltage network, limiting their knowledge of what is happening at this LV level. Better understanding and operation of the network requires further research to fill gaps in areas such as data acquisition, communication, state estimation, computing, innovative control and embedded generation and energy storage technologies.
4.3. Redefining Relationships with Residential Prosumers
Australia has a significant and growing number of residential rooftop PV systems across all states and territories. Rooftop PV distribution ranges from a low of 10% of residences in the Northern Territory to almost 30% of dwellings in Queensland and South Australia. The rise of residential rooftop PV has already signalled two major challenges to the DNSP business model in terms of customer relations. First, these households are now prosumers—both consumers and producers of electricity—and therefore have different expectations and demands from DNSPs and electricity retailers [47
]. Prosumer goals may include, for example, self-sufficiency or reduced reliance on grid electricity, or reduced greenhouse gas emissions or reduced energy costs. Second, because of the inverter, these customers have a network power quality monitor of sorts at their disposal—and may therefore be more aware of when power quality is poor. In the absence of full reporting of power quality by the networks, these customers are likely to call them out on Quality of Service outages. Other opportunities are also emerging that are likely to continue to grow the role of prosumers in the energy market. These include the emergence of home energy storage systems, electric vehicles and the possible active participation of prosumers in wholesale, retail or informal electricity markets through peer-to-peer trading or other mechanisms.
DNSP businesses have always operated in a complex regulatory and technological system but the emergence of prosumers, their differing goals and the access to data about their consumption and generation arguably means that DNSPs now have to add relationship complexity to their business models. The rise of the prosumer has implications for the way that DNSPs have historically managed their assets and how they make business decisions. In essence managing prosumers will require a change in DNSP business culture.
4.4. Lessons from a Quality Management System
Total Quality Management (TQM) is a management approach for improving business culture, products, processes and services. TQM’s eight principles espouse an organisation that sees their culture, products, processes and services as an integrated system; an organisation that is process centred and deploys a strategic and systematic approach to business; an organisation that is both customer focused and actively engages all of their employees; an organisation that is based on fact based decision making, continual improvement and effective communication. Similarly, ISO9000 contains a set of standards for quality management and assurance, helping organisations to establish and document systems for continuous improvement whilst meeting business, regulatory and customer needs and requirements.
These quality management approaches provide a process that DNSPs could adopt to incorporate into their business culture and practices both the technical complexities that rooftop PV present and the relationship complexities that prosumers present. In particular a systematic approach to a thorough and comprehensive audit of LV networks appears to be warranted, to determine existing power quality and management settings and parameters. This will assist in providing evidence to support instances where specific power quality issues or constraints are, or are not, attributable to rooftop PV systems. Equally important would be the re-setting of the relationship with prosumers—from adversarial to collaborative and collegial. Moving from a ‘risk management’ approach to an ‘opportunity investigation’ approach would entail DNSPs actively engaging with prosumers to develop innovations in technology and business models to enable both prosumers and DNSPs to benefit from the increasing penetration of rooftop solar. The traditional approach to electricity supply and demand and the manner in which networks are managed, could be re-conceptualised to a supply, demand and management model applied at an LV or even neighbourhood level, identifying opportunities where the greatest value of rooftop PV can be realized—for prosumers and the network.