Many computers and electronic devices are now used in smart buildings. Computers and other automation equipment have the weaknesses of low dielectric strength, high quality requirements for power supplies, and poor tolerance to overvoltages, making these highly sensitive electronic systems often run with incorrect program operation due to power supply effects. Faults such as data errors, time errors, crashes, restarts without cause, and even the use of electrical equipment to cause permanent damage cause huge losses. For this reason, in smart buildings, it is necessary to study the power supply quality and implement effective protective measures.
Power quality specifications
The technical indicators for measuring power quality include voltage fluctuations, frequency fluctuations, harmonics, and three-phase imbalances. The quality of the power supply can be affected by many factors. Such as changes in the load, the use of a large number of non-linear loads, the influence of higher harmonics, the input and shutdown of power factor compensation capacitors, lightning and man-made faults, and public facilities. Such as motors, elevators, etc. will affect the quality of the power supply to reduce the quality of the power supply.
Voltage fluctuations
The waveform of the ideal supply voltage sine wave is continuous, smooth, and undistorted, and its amplitude and frequency are stable. When the load increases significantly, the amplitude of the sinusoidal wave of the power supply is often affected when the large equipment in the neighboring system starts up.
Generate low voltage. When the power supply voltage fluctuates beyond the allowable range, the computer and the sophisticated electronic equipment may malfunction and even cause the power outage detection circuit of the computer to be mistaken for a power outage and issue a power outage processing signal to affect the normal operation of the computer. The general computer allows the voltage fluctuation range to be AC380V, 220V±5%, when the voltage is reduced to 70% of the rated voltage. The computer is considered interrupted. For this reason, “Design Specification for Electronic Computer Room†GB50174-93 (hereinafter abbreviated as “Specificationâ€) clearly specifies voltage fluctuations and classifies voltage fluctuations as Levels A, B, and C. See Table 1.
The frequency fluctuation of the power supply is mainly caused by the change of the generator speed due to the overload operation of the power grid. Most of the computer's external devices use synchronous motors. The allowable frequency range of common computer frequencies is 50HZ ± 1%. When the frequency fluctuation of the power supply exceeds the allowable range, the frequency of the computer information storage may change, resulting in an error or even loss of information. The "Specifications" clearly stipulates frequency fluctuations. The frequency fluctuations are divided into A, B, and C levels. See Table 2.
The main reason for the distortion of the power supply voltage waveform is that the non-linear load in the power grid, especially the presence of some high-power controllable rectifiers, may affect the voltage waveform of the power supply. This kind of waveform distortion will cause the current on the filter capacitor in the computer's DC power supply circuit to significantly increase the heating of the capacitor, and will cause the power outage detection circuit of the computer to mistakenly consider the power outage and issue a power outage processing signal due to the appearance of the saw-like waveform, affecting the computer. normal work. The technical indicator for measuring waveform distortion is the waveform distortion ratio, which is the percentage of the ratio of the sum of all higher harmonics of the AC voltage at the input of the power equipment to the rms value of the fundamental wave. The “Specifications†stipulates that the waveform distortion rate is divided into three levels A, B, and C (see Table 3).
4 transient pulse
Transient pulse, also known as spike or voltage flicker, is a narrow pulse superimposed on the ideal sine wave of the power grid within a period of less than one half of the power grid. Peak voltages up to 20000V Pulse voltages with durations ranging from one millionth of a second to one ten-thousandth of a second, which not only cause electronic equipment to malfunction or cause electronic equipment to be disturbed. Loss of data, or temporary collapse. Severe It can also cause component breakdown and board burnout. There are many causes of transient pulses, which are generally internal overvoltage and lightning overvoltage.
5 Three-phase imbalance
Due to the unequal distribution of the three-phase loads, the three-phase load currents are asymmetrical. This results in three-phase negative-sequence components. The degree of unbalance is a measure of the three-phase load status. It mainly includes voltage imbalance, current imbalance, and phase angle imbalance. The degree to which the three-phase imbalances will affect the stable and reliable operation of the computer is not yet fully documented. Only reference to the manufacturer's specific requirements for three-phase unbalance to ensure the normal and stable operation of the computer and its equipment generally requires three-phase load unbalance to be less than 20%.
6 Instant power failure
If an instantaneous power outage occurs in the power grid, it will directly affect the normal operation of the computer. When the power supply is interrupted within 1.5ms, the main capacitor of the computer can be discharged to maintain the continuous operation of the computer without affecting the system. When the power supply is interrupted for more than 1.5ms, the CMOS circuit is generally used in the memory. Once the power is cut off for a long time, the computer will lose memory and cause the loss of data in a large number of operation processes, causing the computer to operate with errors or even stop. Maintaining the normal operation of the computer generally requires that the power supply be interrupted within 10 ms. For the duration of momentary power outages, the provisions of the "Code" for the quality of power supply are divided into A, B, C three. A level: O ms a 4 ms; B level: 4ms a 200ms; C level: 200ms a 1500ms above is the main technical index that measures the quality of the power, these technical indicators reflect the situation of the power quality. Will directly affect the operation of the computer system. Therefore, depending on the degree of power pollution and the requirements of the computer system on the quality of the power supply, appropriate protective measures should be taken to prevent interference from other equipment in the power grid, improve the quality of power supply, and enable the computer system to operate stably and reliably.
2 Ways to Improve Power Quality
The factors that affect power quality are complex. When a contaminated power supply supplies power to computers and precision electronic equipment, it is extremely harmful to its operation. When the power quality of the power supply grid cannot meet the requirements, a reasonable power supply system and necessary technical measures should be adopted as required. The impact of the pollution power supply on computers and precision electronic devices should be targetedly eliminated. These measures include the use of isolation transformers, filters, voltage regulators, and uninterruptible power supplies. In order to filter out high-frequency noise, stabilize the voltage isolation from the urban grid, eliminate voltage and frequency deviations, and absorb surges and other disturbances to obtain an ideal power supply. Several commonly used computer power distribution systems mainly include direct power supply systems, isolation transformers, voltage regulators and filter combination systems, and uninterruptible power supply systems.
2.1 Direct power supply system
Direct power supply system is to connect the mains (usually AC38OV, 50HZ) directly to the distribution cabinet, and then distributed to the computer equipment. The direct power supply system is only applicable to the technical indicators of the power grid quality to meet the requirements of the computer, and there is no place where the starting and braking of the heavy load and the electromagnetic interference are small. The advantages of the direct power supply system are: simple power supply system, less equipment, low investment, low operating costs, and easy maintenance. Its disadvantages are high requirements on the quality of the power grid, no protection against power pollution, and vulnerability to variations in the load on the grid.
2.2 Isolation Transformer, Regulator, and Filter Combination System
The isolation transformer 'regulator and filter combination system is a kind of power distribution system used in the computer room. The system can eliminate transient disturbances in the power grid, startup and braking of large loads, voltage fluctuations and electromagnetic interference. The advantages of this system are low cost, reliable operation, easy maintenance and low operating costs. Its disadvantage is that there is no protection when the grid has large frequency fluctuations and sudden power outages.
2.3 Uninterrupted power supply
Uninterruptible power supply It is a power supply device that combines a power converter, an energy storage device (battery), and a switch. The uninterrupted power supply has functions such as regulation, frequency stabilization, anti-interference, and surge prevention. Moreover, when an unexpected power failure occurs, the uninterruptible power supply can continue to supply power to the power-consuming device for a period of time, so that people can timely process information in the memory of the computer or other devices, or immediately start the backup power supply, so that the computer and other devices continue to work.
2.4 Transient surge protectors
Transient overvoltage is the most common type of interference in a power distribution system. Lightning is only a main switch operation, reactive power compensation capacitors and elevators and other heavy load devices such as the input and removal, will have a transient overvoltage. Most of the overvoltages are generated with randomness and repetitiveness, which often accompany the occurrence of other disturbances in the power grid. The above several kinds of power supply systems, including regulated power supply and uninterruptible power supply can not eliminate the overvoltage because the regulated power supply and uninterruptible power supply can not respond to the fast pulse overvoltage in time, and even damage the regulated power supply and the uninterrupted power supply. Therefore, transient surge protectors must be used to protect electronic equipment from transient overvoltage interference and infringement. In summary, with the continuous development of smart construction, people have increasingly strict requirements on power quality. However, how to improve the power quality to eliminate the hidden dangers in the electrical. To ensure the normal operation of computers and precision electronic equipment, will be the engineering design staff has always been concerned about.
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