Abstract : This article elaborates on the energy-saving principles and methods in building electrical design. It discusses several energy-saving methods in electrical design from aspects such as transformer capacity selection, power factor compensation, lighting dimming equipment, and motor starting equipment selection. Keywords : Electrical energy saving, transformer loss, power factor, lighting energy saving, variable frequency speed control, soft starter. Due to population growth, industrial development, and improved living standards, energy consumption has increased dramatically, making the energy crisis imminent. Therefore, various industries have put forward energy-saving requirements, and saving secondary energy—electricity—has become the focus of civil building electrical design. Principles of Energy Saving in Building Electrical Design Building electrical energy saving should adhere to the following three principles: 1. Meet the building's functions, i.e., meet the requirements for lighting illuminance, color temperature, and color rendering index; meet the requirements for comfort air conditioning temperature and fresh air volume, i.e., comfort and hygiene; ensure unobstructed vertical and horizontal transportation channels; and meet special process requirements, such as the electricity consumption of some electrical facilities in entertainment venues, and the process lighting and power consumption of exhibition halls. 2. Consider actual economic benefits. Energy saving should be based on actual economic benefits according to national conditions. Investment should not be excessively increased in the name of energy saving, thus increasing operating costs. Instead, the increased investment should be recovered within a few years or a shorter period through the reduced operating costs from energy conservation. 3. Saving Unnecessary Energy Consumption The focus of energy conservation should be on saving unnecessary energy consumption. First, identify where energy consumption is irrelevant to the building's function, and then consider what measures to take to save energy. For example, the power loss of transformers and the active power loss on power transmission lines are useless energy losses. Similarly, for large-scale lighting, advanced technologies should be used to reduce energy consumption. Therefore, energy conservation measures should also adhere to the principles of practicality, economic rationality, and advanced technology. Ways to Save Energy in Building Electrical Systems 1. Reducing the Active Power Loss of Transformers The active power loss of a transformer is expressed as follows: ΔPb = Po + Pkβ² Where: ΔPb—Active power loss of the transformer (kW); Po—No-load loss of the transformer (kW); Pk—Load loss of the transformer (kW); β—Load rate of the transformer. The Po component represents no-load loss, also known as iron loss. It consists of eddy current loss and leakage flux loss in the iron core and is a fixed component, the magnitude of which depends on the performance of the silicon steel sheets and the manufacturing process of the iron core. Therefore, energy-saving transformers should be selected, such as oil-immersed transformers or dry-type transformers like the S9, SL9, and SC8 types. These all use high-quality cold-rolled grain-oriented silicon steel sheets. Due to the "orientation" treatment, the magnetic domains of the silicon steel sheets are nearly aligned, reducing eddy current loss in the iron core. The 45° fully oblique joint structure ensures good joint tightness, reducing leakage flux loss. Pk is the power transmission loss, i.e., the transformer's line loss, which is determined by the resistance of the transformer windings and the magnitude of the current flowing through the windings, i.e., proportional to the square of the load factor β. Therefore, windings with lower resistance should be selected, and copper core transformers can be used. The extreme value of Pkβ² can be obtained by differentiating it. At β = 50%, the transformer's energy consumption is minimized per kilowatt of load. Therefore, in civil buildings designed in the mid-1980s, the load rate of transformers was mostly around 50%, meaning that half of the transformers were not actually in operation. Some designers still follow this practice today. However, this was solely for energy saving, without considering economic value. The following example illustrates its undesirability. An SC3-2000KVA transformer, when β=50%, can save P=16.01×(0.852-0.52)=7.56kW compared to β=85%. Assuming a shopping mall's peak electricity usage is 12 hours a day, 365 days a year, the total energy saved is W=7.56×12×365=33113KW·h. At a commercial electricity price of 0.78 yuan per kilowatt-hour, the annual savings are 33113×0.78=25828 yuan. Based on the initial installation cost per kilowatt: A 2000 kVA transformer would typically be used in a large residential building with dual power supply lines, resulting in an initial installation cost of 2240 yuan per kVA. The annual energy savings would only cover the initial installation cost of (25828/2240 = 11.53) kVA. There's also the additional initial installation cost of 988.5 kVA, plus the higher transformer price due to the increased capacity, the increased equipment costs for outgoing switchgear and bus tie cabinets, and the increased construction costs due to the increased building area. This represents a considerable investment, not even considering depreciation and maintenance costs. Therefore, using a 50% transformer load rate is not worthwhile. In fact, a 50% load rate only reduces line losses, not iron losses, and is therefore not the most energy-efficient measure. Taking into account initial installation costs, transformer, low-voltage switchgear, civil engineering investment, and various operating expenses, and ensuring adequate capacity is reserved for the transformer during its service life, the transformer load rate should ideally be between 75% and 85%. This also ensures optimal utilization, as the service life of transformer insulation at full load is 20 years. After 20 years, better transformers may become available, allowing for replacement and maintaining the building's overall technological leadership. To reduce transformer losses, when multiple transformers are needed for a large capacity, the number of transformers should be minimized while rationally allocating the load, and larger capacity transformers should be selected. For example, if an installed capacity of 2000 kVA is required, two 1000 kVA transformers can be selected instead of four 500 kVA transformers. This is because using the former saves energy: ΔP = 4 × (1.6 + 4.44) - 2 × (2.45 + 7.45) = 4.36 kW (assuming β = 100%, under the same conditions, SC3 transformer). In transformer selection, adhering to the above three principles satisfies both energy conservation and economic rationality. Reducing energy loss on the line is crucial because the presence of resistance in the line results in active power loss when current flows. The formula is as follows: △P = 3IΦ²R × 10⁻³ (kW) Where: IΦ——phase current (A) R——line resistance (Ω) For example, when transmitting 60kW of electrical energy with cosφ=0.8 on a VV-3×50+2×25 cable with L=100m, the active power loss can be obtained by the following steps: IΦ = 60×10³/(×380×0.8) = 113.6A The resistance per kilometer of a 50mm² copper core wire with a core temperature of 70℃ is R0 = 0.44, then R = 0.1×0.44 = 0.044 (Ω) △P = 3×113.6²×0.044×10⁻³ = 1.704kW As can be seen from the above, the power loss on the line is equivalent to installing a 100W light bulb on every 6m of line. In a construction project, power lines crisscross horizontally and vertically. Small projects often have lines exceeding 10,000 meters in length, while large projects have countless lines. Therefore, the total active power loss along these lines is considerable, making energy reduction a crucial design consideration. Since the current in a line cannot be changed, reducing line losses requires reducing line resistance. Line resistance R = P × L/s, meaning it is directly proportional to conductance P, inversely proportional to cross-sectional area S, and directly proportional to line length L. Therefore, reducing line losses should focus on the following aspects: First, use conductors with low conductivity. Copper core is ideal, but the principle of copper conservation must be followed. Therefore, copper conductors are used in Class II and Class I buildings with higher loads, while aluminum core conductors are used in Class III buildings or buildings with lower loads. Second, reduce conductor length. First, power lines should be routed as straight as possible, minimizing bends to reduce conductor length. Second, low-voltage lines should avoid or minimize backhauls to reduce energy loss on the return lines. Third, transformers should be placed as close as possible to the load center to reduce power supply distance; when each floor of a building is approximately 10,000 m², at least two substations should be installed to reduce trunk line length. Fourth, in high-rise buildings, low-voltage distribution rooms should be located close to shafts, and the trunk lines should be supplied directly from the low-voltage distribution room to each shaft, preventing branch lines from backhauling along the trunk line. In other words, the layout of the low-voltage distribution room and shafts should ensure that all lines supply power forward, minimizing backhauls. Increasing conductor cross-section is also important. First, for longer lines, in addition to meeting the requirements of current carrying capacity, thermal stability, protection coordination, and voltage drop, increasing the conductor cross-section by one level will increase the cost by M. The annual operating cost reduction due to energy savings is m. Therefore, M/m represents the payback period. If the payback period is a few months or one or two years, the conductor cross-section should be increased by one level. Generally, increasing the conductor cross-section by one level is relatively easy to achieve for lines with a cross-section less than 70 mm² and a length exceeding 100 m. Second, lines with seasonal loads can be utilized. When these users are not in use, the lines can be supplied to permanent users to reduce line length and resistance. For example, air conditioning fans, fan coil units, and other loads with the same billing rate as lighting and hot water can be grouped together and supplied through the same main line. This facilitates the cutting off of non-fire-fighting power with a single fire alarm command and allows the same large main line cross-section to carry a smaller current during the spring and autumn seasons when air conditioning is not in use, thus reducing line losses. This is equivalent to fully utilizing seasonal load lines. By carefully implementing the above three measures in the design, energy loss on the lines can be reduced, achieving the goal of energy conservation. Improving the system's power factor reduces reactive power transmission on lines, thus achieving energy conservation. The formula for line loss, expanded, yields the following calculation: ΔP = 3IΦ²R × 10⁻³ = (RP²/UL² + RQ²/UL²)¹⁰⁻³ (kW) Where: UL—line voltage (V) P—active power (kW) Q—reactive power (kVAr) The first term, RP²/UL², represents the power loss caused by active power transmission on the line, and the second term, RQ²/UL², represents the power loss caused by reactive power transmission on the line. Active power is essential for meeting the building's functional requirements and is therefore immutable. Electrical equipment in the system, such as motors, transformers, lines, and rectifiers in gas discharge lamps, all have inductance, which generates lagging reactive power. This needs to be offset by introducing leading reactive power from the system. This leading reactive power is then transmitted from the system through high and low voltage lines to the electrical equipment, resulting in active power losses on the lines. These losses can be mitigated through several measures, including: improving the natural power factor of the equipment to reduce the demand for leading reactive power. This can be achieved by using synchronous motors with higher power factors; using electronic ballasts with high harmonic distortion (HDC) below 15% for fluorescent lamps; and using inductive ballasts in gas discharge lamps, as well as installing capacitors in individual lamps. These methods can all increase the natural power factor to 0.85–0.95, thereby reducing the leading reactive power transmitted through the high and low voltage lines of the system. Since inductive reactance generates lagging reactive power, capacitor compensation can be used. Capacitors generate leading reactive power, and the two can cancel each other out, i.e., Q = QL - QC. Therefore, reactive power compensation can improve the power factor and thus reduce reactive power demand. Reactive power compensation devices should be installed locally for on-site compensation to reduce reactive power transmission on the lines and achieve energy saving. Currently, most civil building designs use centralized compensation on the low-voltage side of the transformer. This approach only reduces reactive power transmission on the high-voltage lines from the regional substation to the user, improving the power factor at the user's location and avoiding or minimizing fines from the power bureau. However, for the user, reactive power is still transmitted from the transformer's low-voltage bus to each user point via transmission lines. The reactive power transmission on the low-voltage lines is not reduced, so reactive power compensation fails to achieve energy saving. In Japan, Tokyo Electric Power Company regulations stipulate that 30μF electrostatic capacitors must be installed at the terminals of motors with a capacity of 0.75kW or higher to reduce active power losses caused by reactive power transmission on the lines. The forthcoming "Design Code for Industrial and Civil Power Supply" in my country stipulates that "reactive loads of electrical equipment with large capacity, stable load, and long-term use should be compensated locally...". The Shaanxi Provincial "Three-Electricity Office" stipulated in 1989 that "asynchronous motors with a relatively stable load of 10kV and above should use local compensation...". Currently, the minimum capacity of self-healing electrostatic capacitors produced in my country is 3kVar, which can provide local compensation for reactive power in motors of 7.5kW and above. The reason why local compensation is often mentioned for motors with stable loads is that the motor terminal voltage changes when the load fluctuates, causing the capacitor to recharge before fully discharging. This generates reactive surge current in the capacitor, making the motor susceptible to overvoltage and damage. Therefore, for intermittent loads, such as elevators, escalators, and moving walkways, compensation capacitors should not be installed at the motor terminals. Furthermore, asynchronous motors with star-delta starting should also not have compensation capacitors installed at the motor terminals because the instantaneous switching between open and closed circuits during starting causes the capacitor to recharge instantly during discharge, also leading to overvoltage and damage to the motor. In civil buildings, the practice of centralized capacitor installation should be changed. Local compensation devices should be installed at the motor ends of fans, pumps, conveyors, etc., with a capacity exceeding 10kW. Air conditioning units and chilled pumps are often placed in dedicated substations nearby for centralized compensation, but local compensation is preferred if the power supply distance exceeds 20m. There are two wiring methods for local motor compensation devices: one is to connect it in parallel after the primary line of the heating element, where the setting current of the heating element should be calculated based on the compensated motor operating current; this wiring is suitable for newly installed motors. The other is to install the compensation capacitor after the main contacts of the contactor and before the primary coil of the heating element; in this case, the setting current of the heating element is not affected by compensation, which is suitable for motors undergoing renovation. This allows the capacitor and motor to be switched on and off simultaneously. By properly handling the above three parts—reducing natural reactive power, reactive power compensation, and the installation location of compensation devices—the rational selection of reactive power compensation methods can be achieved to achieve energy conservation. Energy Saving in Lighting Because lighting consumption is large and widespread, the potential for energy saving in lighting is significant. Efforts should be made in the following aspects: Adopting high-efficiency light sources. Incandescent lamps were once the most widely used because they were inexpensive and easy to install and maintain. However, their fatal weakness was their low luminous efficiency, leading to their frequent replacement by various new light sources with high luminous efficiency, good color rendering, and excellent color rendering performance. Table 1 lists the luminous flux per watt (Lm) of various light sources. The table shows that low-pressure sodium lamps and high-pressure sodium lamps have the highest luminous efficiency, but due to their low color temperature and warm light, their color rendering index (CRI) is between 40 and 60, resulting in significant color distortion. They are only suitable for street lighting or plaza lighting. High CRI sodium lamps (CRI of 60) can be combined with mercury lamps to form mixed lamps for factory and stadium lighting, representing a large and widespread lighting application. Metal halide lamps, tri-phosphor fluorescent lamps, and rare-earth metal fluorescent lamps have very high luminous efficiency and a wide color temperature range from 3200K to 4000K. Fluorescent lamps offer excellent color selectivity and a high color rendering index (CRI) of 80-95, with minimal color distortion. Metal halide lamps, in particular, exhibit exceptional color rendering for human skin. Therefore, besides being used in shopping malls and exhibition halls, they are widely used in waiting rooms at train stations, docks, airport terminals, and stage lighting. General fluorescent lamps and rare-earth metal fluorescent lamps are suitable for office and residential lighting. Fluorescent high-pressure mercury lamps, self-rectified high-pressure mercury lamps, sodium lamps, and combinations of these are commonly used in factory lighting. Incandescent lamps should be used sparingly or avoided altogether, except in localized artistic lighting or for lighting antique paintings and calligraphy to prevent high-frequency spectral exposure. Although they offer good color and the highest CRI, they do not achieve energy savings. Buildings should maximize natural lighting. Windows and doors near the exterior should be large and made of high-transmittance glass to fully utilize natural light. For lighting that utilizes natural light, automatic light adjustment can be implemented by measuring the illuminance according to standards. For gas discharge lamps, stepless automatic light adjustment is used, which adjusts the filament to achieve dimming. However, this is too costly. Each 36W lamp requires an additional investment of 2,000 to 3,000 yuan, while the energy savings are limited by electricity prices. This is because it only reduces artificial lighting during strong daylight hours (generally from 10:00 AM to 3:00 PM), saving at most 25% energy per lamp. Assuming 12 hours of operation per day and 365 days per year, the savings in operating costs would be: m = 36 × 0.25 × 12 × 365 × 0.78 × 10⁻³ = 30.7 yuan. Therefore, the investment in control would take 2,000–3,000 / 30.7 = 65–97 years to recover, making it impractical. Using this dimming scheme in work lighting is not advisable. This type of dimming equipment is only suitable for special conditions, such as small control rooms in weather stations and navigation stations, where the indoor illuminance needs to be in harmony with the outdoor natural light. Furthermore, the flicker effect of rare-earth metal fluorescent lamps used in this type of dimming equipment is difficult for the human eye to tolerate. For situations where natural light can be fully utilized and dimming is required, a grouped, segmented automatic on/off control scheme can be adopted. Although there may be abrupt changes, it will not affect eyesight or people's mood, making it a preferable approach. For situations requiring long-term on/off operation but needing automatic illuminance adjustment based on pedestrian traffic, with minimal additional investment, fluorescent lamps can be controlled using voltage regulation with fixed adjustment levels, as seen in the dimming equipment used in the Beijing subway from Australia. In lighting energy conservation, in addition to meeting illuminance, color temperature, and color rendering index requirements, high-efficiency light sources and high-efficiency luminaires should be used. Using grouped, segmented automatic control on/off methods for luminaires that can utilize natural light or for variable illuminance lighting can achieve energy-saving effects. Energy Saving During Electric Motor Operation Electric motors in building electrical systems are typically supplied by manufacturers and are used in conjunction with equipment from HVAC, plumbing, and construction. Therefore, energy-saving measures must be implemented during operation. Besides using local compensation capacitors to reduce active power losses caused by reactive power transmission delays, it's crucial to minimize light-load and no-load operation of the motor. In these conditions, motor efficiency is very low, and energy consumption is not proportional to the load reduction. Using a variable frequency drive (VFD) allows the motor to automatically adjust its speed to adapt to load changes as the load decreases. This improves motor efficiency under light loads, achieving energy savings. However, the price of such equipment remains relatively high, limiting its application. Another energy-saving method is using a soft starter. A soft starter gradually adjusts the conduction angle of the thyristor according to the starting time to control voltage changes. Because the voltage is continuously adjustable, starting is smooth, and full voltage is applied immediately after starting. This device can also employ speed feedback, voltage negative feedback, or current positive feedback to control the conduction angle of the thyristor using feedback information, thus allowing the speed to change with the load. It can be used in equipment with large motor capacity that requires frequent starts, as well as in situations where nearby electrical equipment has high voltage stability requirements. Because its current change from start-up to operation does not exceed three times, it ensures that grid voltage fluctuations remain within the required range. However, since it uses thyristor voltage regulation, all electrical energy in the non-conducting portion of the sine wave is consumed by the thyristor and does not return to the grid. Therefore, it requires adequate heat dissipation and ventilation. Its price is lower than that of frequency converters, making it suitable for use in the control equipment of large-capacity motors in water pump systems. Civil buildings have significant energy-saving potential, which should be carefully considered in the design. However, when selecting new energy-saving equipment, its principles, performance, and effects should be thoroughly understood, and a technical and economic comparison should be made before selecting energy-saving equipment to achieve true energy savings.