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A Quantification of the Energy Savings by Conservation Voltage Reduction Wendy Ellens, Adam Berry, Sam West Abstract-The introduction of 'Smart grid' technologies in the electricity supply industry has attracted new attention to Conservation Voltage Reduction (CVR). CVR is a method that the minimum voltage level, without exposing customers at the end of the lines to under-voltage conditions [4]. aims to save energy by reducing the voltage level of the electrical distribution network. However, not all devices consume less electricity when exposed to lower voltage levels; some devices keep energy consumption constant by extending their operating time, while others increase the current, leading to higher line losses. This paper describes a model designed to calculate the energy savings that can be reached through the use of CVR. The model has been applied to the Australian residential sector, showing that 0.4% of energy savings can be obtained for 1.0% of voltage reduction. Besides a description of the model and the results, the paper contains a thorough sensitivity analysis. Furthermore, we present a simple but effective formula to predict the energy savings by CVR. Index Terms-energy conservation, power distribution, smart grids, voltage control I. IN TRODUCTION (CVR), also or Conservation Voltage Control (CVC), has been proposed as a method to save energy by reducing the voltage level of the electrical distribu tion system [1], [2]. CVR is expected to have considerable environmental benefits, because the reduction of the energy consumption will lead to fewer CO2 emissions associated with energy production'. CVR is not a new energy conservation measure; tests have been performed as early as in 1973 [1]. However, recent developments in the field of smart grids have drawn new attention to CVR [4], now also called voltage optimisation. Integrated Volt-Var Control (also kwnown as VoltlVar manage ment), which is an important feature of the future grid, enables the application of CVR. Traditionally, the substation voltage is set to the maximum allowed voltage in order to guarantee a minimum voltage level to customers at the end of the distribution lines [4], [5]. In future energy networks, it will be possible to control the voltage level along the distribution lines in order to supply all customers with approximately the same voltage, such that low-voltage substations can operate close to ONSERVATION VOLTAGE REDUCTION C called Conservative Voltage Reduction w. Ellens is with TNO Performance of Networks and Systems, P.O. Box 5050, 2600 GB Delft, The Netherlands (e-mail: [email protected]). A. Berry and S. West are with CSIRO Energy Technology, PO Box 330, Newcastle NSW 2300, Australia (e-mail: [email protected] sam. [email protected]). 1 Australia's greenhouse emissions totaled about 540 million tonnes of carbon dioxide in 2010. Electricity generation accounts for 36 percent of emissions [3]. A. The Effects of Conservation Voltage Reduction Assuming that household devices require no reactive power, the effect of CVR on energy consumption can be explained as follows. By Joule's law, the power P, voltage V and current 1 in a resistive circuit satisfy P VI. It follows from Ohm's law V 1R that lowering the voltage level reduces the power when the load consists of pure resistors with constant 2 resistance R, because in that case we have P V / R. In reality, this is true if loads are constant-resistance (hot water system, fridge, oven, incandescent lighting, pool pump, etc.), but not if loads are constant-power (computer, TV, etc.). If we have constant-power loads, dropping the voltage will mean that the current has to increase, which leads to higher energy loss in the lines, according to the power loss formula llines 12 Rlines. Also, many constant-resistance devices (hot-water system, fridge, oven, etc.) have a feedback loop - mostly measuring the temperature - that extends the operating time, leading to constant energy consumption (the energy consumption U satisfies U PT, where T is the duration of the power consumption). Some lighting technologies (compact fluorescent lighting) keep the current constant. The power consumption of these devices decreases linearly with the voltage according to P VI. The savings are therefore smaller than for constant-resistance devices, for which the power consumption is quadratic in V. Because CVR effectiveness depends on the type of device, we categorise electrical devices into four load categories: = = = = = = 1) Constant-resistance loads without a feedback loop, which we call constant-resistance loads, reduce the energy consumption for both the loads and the lines; 2) Constant-resistance loads with a feedback loop, which we call constant-energy loads, have a constant energy consumption (for time-scales that are longer than the duration of the feedback loop); 3) Constant-power loads increase the energy consumption, because of increasing line losses due to increased current draw caused by the lowered voltage. 4) Constant-current loads reduce the energy consumption for the loads. Table I lists some experimental results in the US. The list shows that CVR savings for residential areas are generally 0.6% to 0.8% for each percent of voltage reduction, the savings being higher for commercial areas and lower for industrial areas. The average savings across the three market sectors 978-1-4673-2868-5/12/$3l.00 ©2012 IEEE 2 TABLE I RESULTS OF CV R TESTS IN TERMS OF THE CV R FACTOR (RELATIVE ENERGY SAVINGS PER PERCENT OF VOLTAGE REDUCTION) Year Source Residential Commercial Industrial Overall 0.62% 1973 [1] 0.61 % 0.89% 0.35% 1977 [9] 0.76% 0.99% 0.41% 1979 [2] 0.73% 0.84% 0.49% 1989 [10] 0.71% 0.62% with equal source voltage Vsource and line resistance Rlines for all devices. The energy consumption of a device consists of energy consumed by the load and line losses. The calculation of these for the different load categories is explained in the subsections that follow. For an in depth discussion on the assumptions that have been made during the modelling proces, and also for the derivations of the formulas in the next subsections, see [11]. A. Energy Consumption of Constant-Resistance Devices are approximately 0.6% . The experimentally determined CVR factors (relative savings per 1% voltage reduction) differ substantially from utility to utility. As a result 'most utilities feel that another utility's voltage regulation results are not necessarily transferable to their own service territory' [6]. Al though the need for models assessing the factors that determine CVR effectiveness is clear, no such models have been found in the literature. For example [7], [8] describe results of CVR modelling in their own region (Taiwan and Pacific Northwest respectively), but do not discuss the sensitivity to the model parameters necessary to verify the validity of the results for other regions. The load resistance of constant-resistance devices is derived by applying the following formula to the power consumption data: Rload = 2302 y.;:;- , load where we have assumed that the nameplate power FI�ad is measured at a voltage of 230 V. For constant-resistance devices the actual power consumption Road at 230 V is somewhat lower than the nameplate power, because the actual voltage over the load is slightly less than 230 V (as the line resistance is non-zero). It is B. The Scope of this Research As pointed out in Subsection I-A, long term benefits of CVR are only obtained for constant-resistance and constant-current loads. For constant-energy loads there are no energy savings when reducing the voltage level, while reducing the voltage for constant-power loads leads to higher energy consumption. Therefore the question 'How effective is Conservation Voltage Reduction; does it really save energy?' needs to be addressed. Based on the breakdown of constant-resistance, constant energy, constant-power, constant-current loads and residential consumption statistics, we will verify whether the energy consumption in Australian households would be reduced when setting the voltage to a lower level. The paper is organised as follows. The next section, Section II, explains how the model is built. Section III describes the data used to calculate the energy savings by CVR in the Australian residential sector. It is followed by Section IV , which contains the modelling results. Section V describes the sensitivity analysis that is performed to point out the depen dency of the results on the modelling parameters. Finally, Section VI summarises the main modelling results and some theoretical results; describes the relevance of the research to industry, science and society; and suggests subjects for further research. II. DESCRI PTION OF T H E MODEL The model takes the part of the electrical network into account that is affected by CVR, i.e. the part of the network from the low voltage substations to the consumers. The energy consumption for a given voltage level is calculated as the sum of the energy consumption of the devices. For all devices, the circuit looks like a load in series with a resistor representing the line resistance, together connected to the source voltage, (1) n .Lload = J2Rload = ��urce R -R -- load tot = 2302 (Rload + Rlines )2 Rload, (2) by which we divide the yearly energy consumption in order to calculate the operating time of a constant-resistance device (yearly, for all Australian households together). We can use (2) also to calculate the power consumption at different voltage levels: .Fload = ��urce (Rload + Rlines )2 Rload, (3) Similarly we have for the line loss and the total power consumption: T> .Llines = ��urce T> (Rload + Rlines )2 Rlines and .Ltot = ��urce Rload + Rlines (4) The energy consumption of the load, the lines and the total energy consumption of the device are now easily calculated by multiplication by the operating time. For the standard voltage of 230 V, the calculated energy consumption of the load will be equal to the yearly energy consumption given in the data. B. Energy Consumption of Constant-Energy Devices The load consumption and line losses for constant-energy devices are easier to calculate. For every voltage level the load consumption is equal to the yearly energy consumption in the data. The line loss can be calculated by Ulines = UloadRlines Rload (5) The load resistance is derived by the nameplate power in the same way as for constant-resistance devices. TABLE II RESIDENTIAL DEVICES AND ELECTRICITY CONSUMPTION IN AUSTRALIA, 2007. AIR-CONDITIONING GIVES THE SUM OF THE ENERGY USED FOR SPACE HEATING AND SPACE COOLING. Constant-resistance Constant-energy 1 Water heater 21.7% constant-energy 2 Lighting 13.6% mostly constant-resistance 3 Air-conditioning 11.6% constant-energy or constant-resistance 4 Refrigerator 11.0% constant-energy 5 Television 7.0% constant-power Constant-current Unknown Constant-power C. Energy Consumption of Constant-Power Devices As for constant-energy devices, for constant-power devices the energy consumed by the load is equal to the yearly energy consumption data, regardless of the voltage level. The power consumed by the lines is llines = 12Rlines = ( Vsource - J"V;;�urce - 41loadRlines ) 2 . 4R hnes (6) where lload is the nameplate power of the device. The line energy consumption is a product of the line power consump tion and the operating time, which is simply the yearly energy consumption divided by the power consumption. D. Energy Consumption of Constant-Current Devices For constant-current devices, the current through the load and lines and the power consumption by the lines are constant: 1= ��ad 23 0 and Rlines = 12Rlines· (7) The total and load power consumption are given by Ptot= Vsource1 and lload= Ptot - llines' (8) The operating time of a constant-current device is calculated as follows: T= Ul�ad Ul�ad = Road 23 01 - 12Rlines ) (9) with Ul�ad the given yearly power consumption, which is used to find the energy consumption for the load, lines and the total energy consumption. III. RESEARCH DATA In this section we briefly describe the data that we have used in the application of our model to the Australian residential sector. For the complete set of data see [11]. The most important input for the model is information about the yearly energy consumption of devices, their power consumption and their load category. For data on the energy consumption of residential electrical devices we rely on the report Energy use in the Australian residential sector 1986-2020 of the Australian Department of the Environment, Water, Heritage and the Arts [12]. The residential devices that consume the largest amount of energy are given in Table II. Most of the typical power consumption values have been taken from report [12] as well. Information Fig. 1. Breakdown of the 2007 Australian residential electricity consumption by load category. Constant-current devices are 1.4% of the energy consump tion. The category 'Unknown' consists of a group of miscellaneous devices (8.6%) and pools (3.4%), which contain two load categories. This category has not been included in the model. about the reactive power of the devices was not available, therefore we have not included it in the model. The cat egorisation of the residential devices in constant-resistance, constant-energy, constant-power and constant-current devices is based on experimental data. The proportion (in yearly energy consumption, according to the 2007 energy data of [12]) of constant-resistance, constant-energy, constant-power and constant-current devices has been visualised in Fig. 1. For the line resistance we have chosen the values 0, 0.1, 0.2, 0.5, 1.0 and 2.0 n because they lead to reasonable values for the total line loss; between 0% and 5.5% of the total energy consumption. The distribution and transmission losses in reality are about 7.5% (in 2008 in Australia [13]). Therefore 5.5% of line loss for the modelled part of the network is almost certainly an upper bound. Also important for our model is the allowed lower and upper bound on the voltage. Since 2000 the standard voltage in Australia is 230 V with a tolerance of + 1 0% and -6% [14]. IV. MODELLING RESULT S In this section we discuss the outcome of the model. The energy consumption at different voltage levels is given. The results are analysed for different line resistance values and the absolute and relative savings by CVR are quantified. Fig. 2 gives the yearly Australian residential electricity consumption (for different voltage levels and different values of the line resistance). It turns out that the energy consumption is almost linear in the source voltage with almost the same slope for all line resistance values. The absolute electricity savings (in the Australian residential sector, per year) are approximately 0.09 . 109 kWh per 1 V voltage reduction (independently of the line resistance). The relative energy consumption (with respect to the con sumption at the standard of 230 V) can be found in Fig. 3. We see that the relation between the relative energy consumption and the relative voltage level (with respect to 230 V) is approximately linear. The slope is again almost independent of the line resistance. Hence, our conclusion does not depend on the chosen line resistance (in the 0.0 to 2.0 n interval), 4 Energy(109 kWh) Rlines 53 Rlines 52 Rlines 51 Rlines Rlines 50 = 2.00 = 1.00 = 0.50 = = 0.10 0.00 to determine the reference energy consumption, can be different from 230 V. While all of these modifications can be used to analyse the sensitivity of our results with respect to errors in the input data, all except for item 2) can also be used to model future changes in the input data. A. Modifying the Energy and/or Power Consumption 49 48 47 220 230 240 250 Voltage(V) Fig. 2. Yearly Australian residential electricity consumption for the allowed voltage levels and several values of the line resistance. Rlines Rlines relative energy 2% - = 0.00 = 2.00 1% relative voltage Fig. 3. Yearly Australian residential electricity consumption relative to the consumption at 230 V for the allowed voltage reduction levels relative to the standard of 230 Y. The consumption has been plotted for two values of the line resistance. nor will it change if some devices are slightly further from the substation (larger line resistance) than others. The savings are about 0.4% per 1% voltage reduction for the Australian households. Because of the linear relation, a 2.5% voltage reduction (from 230 V to 224 V ) gives up to 1% energy savings. V. SEN SITIV IT Y ANALY SIS In this section we perform a sensitivity analysis in order to point out dependencies of the described results on the modelling parameters. In our model there are five types of parameters that can be modified: 1) The yearly energy consumption of a device can change, which means that the device is used more often; 2) The power consumption of a device can be modified, that is, the power consumption data used were incorrect, but the yearly energy consumption was correct; 3) The power and the yearly energy consumption of a device can change, which means that the device requires more power and thus consumes more energy; 4) Any device can change load category (e.g. from constant-resistance to constant-power); 5) The standard voltage, used to calculate the resistance of the constant-resistance and constant-energy devices, the current of constant-current devices and also used If the operating time (and thus the yearly energy consump tion) is increased by a certain factor, the energy consumption of the load, the line loss and the total energy consumption is increased by the same factor (for all load categories). Increasing the (nameplate) power (but not changing the yearly energy consumption) of a device increases the line loss linearly (approximately in the case of constant-power and constant current devices) but does not affect the energy consump tion of the loads for all categories. It follows directly that increasing the energy and power consumption by the same factor (modelling a possible increase of power consumption of devices) increases the load consumption and line consumption. The load consumption is linear in the described parameter, while the relation between the line consumption and the energy/power consumption is (approximately) quadratic. These observations show that, in order to give a good approximation of the energy consumption for different volt age levels, it is much more important to know the energy consumption of devices for the standard voltage, than to know their power consumption. B. Modifying the Load Category of a Device In Fig. 4 we have analysed the energy consumption in four extreme cases; the cases in which all loads are of the same category. The graph shows that the constant-resistance loads determine the change in energy consumption as the voltage varies. Although the energy consumption of constant-power devices increases by a small amount when the voltage is reduced, the decrease in energy consumption of the constant resistance devices is much larger for the same voltage reduc tion. The influence of constant-current devices is very limited because of their small share. As already stated, constant energy devices do not influence the effect of CVR. It is important to know how large the share of constant power devices can be in order to still have benefits when applying CVR. We have therefore taken linear combinations of the graphs in Fig. 4. The results for the actual mixture of load categories, the 10%/90% mixture and other mixtures have been drawn in Fig. 5. The graph for the actual mixture matches the graph in Fig. 2 (for a line resistance of 0.5 n). To generate this curve no data about the load category of specific devices are used. This fact shows again that the energy savings by CVR heavily depend on the energy consumption for the different load categories, while the power of the individual devices has less influence. It is interesting to note that Fig. 5 indicates that CVR will save energy if at least 2% of the constant-resistance and constant-power devices are constant-resistance. Energy(109 kWh) constant-resistance constant-current 54 TABLE III T HE ENERGY CONSUMPTION OF THE LOADS, LINES AND THE TOTAL ENERGY CONSUMPTION FOR SEVERAL VALUES OF THE SOURCE VOLTAGE Vsource AND A SOURCE VOLTAGE REDUCED BY 1 % vsr;,eirce. THE LINE RESISTANCE IS 1 0 . 53 52 Total consumption 51 Load consumption 50 constant-energy 49 Line consumption constant-power Voltage Vsource e Vs� 1rce 1l0V 201.6 200.8 0.39% 230 V 179.8 179.0 0.42% 0.42% Savings llOV 174.7 174.0 230 V 174.7 174.0 0.42% llOV 26.9 26.8 0.14% 230 V 5.1 5.1 0.14% 48 47 V I. DI SCU S SION A. Conclusions 46 ,I 'I 220 ' , 230 ,, I I , ' 240 250 Voltage(V) Fig. 4. Yearly Australian residential electricity consumption for the allowed voltage levels if all devices were of the same load category. The consumption has been calculated for a 0.5 0 line resistance. Energy(109 kWh) actual mix 51 10%/90% 50 -- 5%/95% 2%/98% 1%/99% 49 -- 48 220 230 240 250 Voltage(V) Fig. 5. Yearly Australian residential electricity consumption for the allowed voltage levels for several load category mixtures. The consumption has been calculated for a 0.5 0 line resistance. The actual mixture in Australia in 2007 was about 20% constant-resistance devices, 62% constant-energy, 17% constant-power, 2% constant-current. The results have also been calculated for other mixtures of constant-resistance and constant-power devices; x%/y% stands for x% of constant-resistance and y% of constant-power devices. C. Modifying the Standard Voltage Table III shows that the results of Fig. 3 are almost independent of the standard voltage (e.g. 240 V versus 110 V ). The savings on the load consumption are higher than the savings on the line consumption. Together with the fact that the proportion of line losses increases as the source voltage decreases, this leads to slightly smaller savings on the total energy consumption when a lower source voltage is applied. Our model indicates that applying CVR in Australia would indeed save energy. In the residential area, a 2.5% voltage reduction (from 230 V to 226 V ) gives 1% energy savings, implying up to 1% discount on the residential energy bills and 1% less greenhouse gas emissions associated with residential electricity use. If CVR is applied in all residential areas in Aus tralia, the absolute savings are approximately 0.09· 1 09 kWh, corresponding to 140 million Australian dollars (assuming a linear tariff of 28 cents per kWh [15]), or 470 million kg of CO2 (for electricity generated in Australia [16]) per year. The CVR factor we found is 0.4% , which is smaller than the CVR factors around 0.6% found during real-world tests. The most likely explanation for this is that the number of constant resistance devices is decreasing over time, while constant power devices are used more and more. For the same reason, it is expected that the savings by CVR will diminish in the future. As CVR is beneficial also for constant-current devices, but less than for constant-resistance devices, replacement of traditional lighting by compact fluorescent lighting will decrease but not cancel the positive effects of CVR. According to our model, the CVR factor is almost inde pendent of the values of the source voltage. This means that our results are valid also in countries with different standard voltages and along distribution lines where voltages vary between households. We found that savings do not depend strongly on the line resistance, therefore further specifications of the line lengths, material and diameter are not required. An important conclusion of our sensitivity analysis is that the savings by CVR are mainly determined by the load-mix, or more precisely by the percentage of constant-resistance devices. As a consequence (using (4)) the overall savings 6.u can be easily approximated by the following formula: 6.u = (1 - (1 - Cred)2 ) Cresist;::::; 2CredCresist. (10) In this formula, Cred is the relative voltage reduction and Cresist is the relative amount of constant-resistance devices (in terms of yearly energy consumption). When the voltage is reduced by 2.5% and 20% of the devices are constant resistance, the formula predicts a 1% energy reduction the same result as we found using the model. By taking 6 Gred 0.01 (a 1% voltage reduction) and Gresist 1 (100% constant-resistance devices), (10) provides an upper bound on the CVR factor that can be reached: = = CVRf:S: 2%. (11) A second consequence of the fact that the increase of energy consumption of constant-power devices is very small compared to the energy savings by constant-resistance devices is the following rule of thumb: as long as the energy consumed by constant-resistance devices is at most fifty times smaller than the energy consumed by constant-power devices, CVR will reduce the energy consumption. This suggests that CVR will continue to have benefits in the future and that savings can be obtained also for commercial and, potentially, for industrial areas - where more constant-power devices are used. B. The Contribution of this Research Conservation Voltage Reduction can be applied as a measure to reduce carbon dioxide emissions, by reducing the electricity consumption. The model described in this paper offers a tool to quantify the effectiveness of CVR. While tests have the same goal, the model is more sophisticated as it can predict the energy savings when devices change load category (for example a television becoming constant-power instead of constant-resistance), when the power consumption of a device (a television for instance) increases, or when consumers use a device (such as an air conditioner) more often. In addition to the calculation of future savings, the model can also used to analyse regional and seasonal differences. Aside from the numerical results (especially interesting for the energy industry) the model offers, it also gives a deeper theoretical understanding of CVR, showing how the results depend (or do not depend) on the parameters. Additionally, the modelling results have shown that the simple formula (10) gives a tight approximation of the CVR factor. Other theo retical results include the fact that the savings are essentially linear in the voltage reduction (see (10» - justifying the use of the CVR factor as a measure of CVR effectiveness - and the determination of an upper bound on the CVR factor (see (11». C. Further Research Further research in the field CVR modelling may include lab testing in order to determine the reactive power of household appliances. However, it is expected that the reactive power of residential devices is small compared to the real power, because residential consumers generally have a power factor close to 1 [17]. Nevertheless, quantifying the savings by CVR on reactive power might be interesting as CVR factors are shown to be larger for reactive power than for real power. Lab testing could also give the current profile of electrical devices and distribution lines (the load's response to voltage variations). Embedding these results in our model would provide a way to verify that the categorisation in ideal constant-resistance, constant-energy, constant-power and constant-current devices gives a good approximation. As predictions of future residential energy consumption in Australia are available [12], it would be interesting to use these data to predict future savings by CVR. Extension of the model to commercial and industrial areas would be necessary to get an approximation of the total energy savings that can be reached by applying CVR to the entire Australian electricity network. The model for industrial application of CVR should include an additional load category for induction motors. 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