Why does a higher than nominal voltage supply waste power and energy?

Multiple independent surveys have shown that the average voltage supplied to consumers is usually above 245V, well above the Australian standard of 230V [1].  This has several negative impacts on consumers such as limiting the amount of rooftop solar generation, shortening appliance lifetimes and also wasting power and energy.  Many people have asked us why high voltages waste power and energy.  This article aims to answer that question for the technically oriented and curious readers.

Figure 1: University of NSW voltage survey conducted for the Energy Security Board (and reported by ABC News) [1]

The relationship of voltage to the power and energy consumed depends on the type of load. 

For resistive type loads such as the old incandescent light bulbs it’s pretty easy to understand: the electrical power consumption is given by P = V²/R (where R is the resistance of the bulb), so increasing the voltage will cause a (squared) increase in power consumption. Since Energy is the accumulation of power over time the impact on energy is similar.   So increasing voltage causes the light to glow brighter and consume more energy – simple right?  Yes, but there are not many incandescent bulbs around anymore.  There are, however, many other resistive loads (eg. electric water heaters, kettles, toasters, ovens etc) but their response is a bit more subtle.  Some loads like kettles will consume more power at higher voltage but will be quicker to get to boiling point with the total energy consumed (to boil a fixed amount of water) remaining constant.  Loads like stovetop cookers & ovens are more complex since not all the power consumed by the appliance gets transferred to the food (a significant portion gets wasted).  The power wastage is related to the temperature of the element; the higher the voltage the higher the power consumption (according to P = V²/R) and the hotter the element and the higher the loss to the environment.  To complicate matters further ovens and stovetops are temperature controlled by thermostats in a switched manner.  At higher voltages, while the element is on, it consumes more power and burns hotter, which increases the temperature of the element faster but also wastes more energy to the surrounding environment.  So again, for these loads there is some increase in energy consumption with increased voltage. 

Another very common type of load is a motor load.  Motors are used in a wide range of appliances and machines including fans, pumps, refrigerators, washers, air-conditioners etc.  We can separate them into those controlled by variable speed drives (or inverters) such as inverter air conditioners, and those connected directly to the mains such as fans.  Note that most consumer grade motors with stepped speed response like fans, fridges etc are direct connected and use techniques like pole switching to change their speed, not variable speed drives.  Direct connected motors (generally induction machines) are often considered to be constant power loads.  This is because they are generally driving fixed mechanical loads requiring constant power.  So for example if you decrease the voltage on a pump motor it will not slow down much and its current will increase, meaning its power consumption (given by P = V * I * cosθ) will remain roughly the same.  This assumption is common in the electrical power industry.  However closer inspection reveals that this assumption is not true for lightly loaded motors or as we increase voltage beyond the nameplate rating of the motor.  The magnetic flux in an induction motor is proportional to the voltage divided by the frequency (λ ∝ v/f) and since the frequency is fixed at 50Hz it is proportional to the voltage.  As we raise voltage above the nameplate value the magnetic flux increases beyond its nominal value and begins to saturate the iron cores in the motor.  This increases the losses of the motor i.e. increasing the voltage of the motor beyond its rating doesn’t make the motor run faster it just drives it into magnetic saturation, increasing the losses and power and energy consumption.  This effect can be clearly seen in a test done at the University of Wollongong on a bench grinder.  The figure below shows the temperature profile (measured with an infrared camera) at different voltages.  It can be clearly seen that the motor runs considerably hotter at higher voltages, burning extra energy as heat.

Figure 2: University of Wollongong  Thermal test results of a bench grinder run at different voltages

Power electronic loads such as computers, TVs etc are more complex to analyse and their energy consumption versus voltage depends on their design.  Some are essentially constant power devices while others show some change in power consumption with increased voltage.  The interested reader is referred to reference [8] which contains some test results of typical electronic loads.

In total, there are a wide range of appliances and equipment connected to the grid.  Some will exhibit an increase in power and energy consumption with increased voltage and some will not.  The combined increase in power and energy with increased voltage will depend on the mix of loads on the network, but there will be a positive relationship. 

Conversely if we decrease the average voltage supplied to consumers from the present high values down closer to the standard voltage, it will decrease the power and energy consumed and the amount of this decrease will depend on the load mix.

The practice of reducing system voltage to save energy and power is known as Conservation Voltage Reduction (CVR). It is in fact well established and has been practiced in various parts of the world for the best part of 40 years.  Recently in the UK, Electricity Northwest conducted a widespread CVR test and found that a 1% reduction in energy for each 1% reduction in voltage occurred [2],[3].  This significant result has led to them planning a system wide roll-out with support from the UK energy regulator Ofgem.

In Australia, United Energy in Victoria did several system wide CVR tests where they reduced the average voltage delivered to consumers for periods ranging from 2 – 4 hours.  Across their whole network they observed a 0.7% reduction in energy for each 1% reduction in voltage [4], [5], [6]. 

These recent results are consistent with results reported from the US and elsewhere where the “CVR factor” (the ratio of energy reduction to voltage reduction) reported is typically in the range of 0.6 – 1 [8] – [21].  The Queensland government used a CVR factor of 0.65 as part of its decision to move to 230V [22].

Given our average voltage supplied is about 247V, if the voltage was decreased to 230V it would be a (247 – 230)/247 = 0.069 or 6.9% decrease in voltage; implying a possible energy decrease of 0.7*6.9% = 4.7% across Australia’s National Energy Market (NEM – consisting of the inter-connected eastern seaboard of QLD, NSW, VIC, Tasmania, the ACT and South Australia).  This represents the energy output of a significant coal fired power station. Even a more modest decrease in voltage of 4-5% would save 3.8TWh of energy, reduce bills by around $1 billion and save around 3 million tonnes of CO2 every year.  This is the motivation behind #fixthevoltage.

The interested reader is referred to the articles below as well as a recent presentation to the Electric Energy Society of Australia (EESA) [23].

References

[1] ABC 7.30 report (2020). Powerlines may be limiting savings Australians can make from solar, UNSW research suggests. https://www.abc.net.au/news/2020-08-17/solar-powerlines-already-over-voltage-limits-unsw-study-finds/12534332

[2] Electricity North West, UK, project website. Introducing the Smart Street project. https://www.enwl.co.uk/zero-carbon/innovation/key-projects/smart-street/

[3] B. Ingham (2018). Smart Street – Project Closedown Report. Electricity North West, UK.  https://www.enwl.co.uk/globalassets/innovation/smart-street/smart-street-key-docs/smart-street-closedown–report.pdf

[4] (2019). United Energy to test voltage control improving grid stability. Australian Renewable Energy Agency. https://arena.gov.au/news/united-energy-to-test-voltage-control-improving-grid-stability/

[5] (2019). United Energy Demand Response Project Performance Report – Milestone 6. United Energy. https://www.unitedenergy.com.au/wp-content/uploads/2020/01/Project-Performance-Report-Milestone-6.pdf

[6] (2018). United Energy Demand Response Project Performance Report – Milestone 3. Australian Renewable Energy Agency. https://arena.gov.au/assets/2018/08/united-energy-demand-response-project-performance-report-milestone-3.pdf

[7] C. Halliday. Conservation Voltage Reduction Case Study for Bathurst Community Climate Action Network. PowerLogic. http://www.bccan.org.au/files/conservation_voltage_reduction_paper_v2.1.pdf

[8] K. P. Schneider, J. C. Fuller, F. K. Tuffner, and R. Singh (2010), Evaluation of Conservation Voltage Reduction (CVR) on a National Level” Pacific Northwest National Laboratory (PNNL), Richland, WA (US), 2010 https://www.osti.gov/biblio/990131

[9] D. Pinney (2014). Costs and Benefits of Conservation Voltage Reduction. US Department of EnergyCooperative Research Network. https://www.energy.gov/sites/prod/files/2016/10/f34/NRECA_DOE_Costs_Benefits_of_CVR_May_2014.pdf

[10] Wang et al (2013), Review on Implementation and Assessment of Conservation Voltage Reduction, IEEE Transactions on Power Systems, Volume 29, Issue 3, November 2013. https://ieeexplore.ieee.org/document/6670787

[11] M. McNamara, D. Feng, T. Pettit, D Lawlor (2017). Conservation Voltage Reduction/Volt VAR Optimization EM&V Practices. United States Environmental Protection Agency. https://www.energystar.gov/sites/default/files/asset/document/Volt%20Var%20and%20CVR%20EMV%20Best%20Practice%2006-01-17clean%20-%20508%20PASSED.PDF

[12] P Fairley,  IEEE Spectrum (2010). An Easy Smart-Grid Upgrade Saves Power – Conservation voltage reduction could be an easy win for smart grids. https://spectrum.ieee.org/energy/the-smarter-grid/an-easy-smartgrid-upgrade-saves-power

[13] P. K. Sen, K. H. Lee (2016). Conservation Voltage Reduction Technique: An Application Guideline for Smarter Grid. IEEE Transactions on Industry Applications. https://ieeexplore.ieee.org/document/7399736

[14] A. Singaravelan, M.Kowsalya (2017). A Practical Investigation on Conservation Voltage Reduction for its Efficiency with Electric Home Appliances. ScienceDirect. https://www.sciencedirect.com/science/article/pii/S1876610217324281

[15] S. Singh, S. P. Singh (2016). Opportunities and Challenges for Deployment of CVR/VVO Methodology in Indian Smart Energy Distribution System. ResearchGate. https://www.researchgate.net/publication/303374242_Opportunities_and_Challenges_for_Deployment_of_CVRVVO_Methodology_in_Indian_Smart_Energy_Distribution_System

[16] M. Castro, A. Moon, L. Elner, D. Roberts, B. Marshall (2016). The value of conservation voltage reduction to electricity security of Supply. Electric Power Systems Research. http://eatechnology.com/sea/wp-content/uploads/sites/9/2017/06/The-Value-of-Conservation-Voltage-Reduction-to-Electricity-Security-of-Supply.pdf

[17] K. Warner, R. Willoughby (2013). Voltage Management: A Hidden Energy Efficiency Resource. Green Tech Media. https://www.greentechmedia.com/articles/read/voltage-management-a-hidden-energy-efficiency-resource

[18] A. El-Shahat, R. J. Haddad, R. Alba-Flores, F. Rios, Z. Helton (2020). Conservation Voltage Reduction Case Study. IEEE Access. https://ieeexplore.ieee.org/document/9040616

[19] R. Ardis, R. Uluski (2015). CVR Is Here to Stay. T&D World. https://www.tdworld.com/grid-innovations/smart-grid/article/20965787/cvr-is-here-to-stay

[20] P. Anderson. Conservation Voltage Reduction (CVR). Idaho Power. https://assets.fiercemarkets.net/public/sites/energy/reports/cvrreport.pdf

[21] W. Ellens, A. Berry, S. R. West (2012). A quantification of the energy savings by Conservation Voltage Reduction. ResearchGate. https://www.researchgate.net/publication/261056101_A_quantification_of_the_energy_savings_by_Conservation_Voltage_Reduction

[22] Queensland Government (2017). Decision regulatory impact statement – Queensland’s statutory voltage limits https://cabinet.qld.gov.au/documents/2017/Sep/Voltage/Attachments/DecisionRIS.PDF

[23] Ty Christopher (2020), Conservation Voltage Reduction – Saving Customers, the Environment and the Network. Presentation to Electric Energy Society of Australia, Dec 2020, https://www.eesa.org.au/member-resources/event-recordings