With the acceleration of China's urbanization process, more and more high-rise buildings have entered our lives, and the design of high-rise buildings is also increasing. The electrical design of high-rise buildings is very diverse, including: load calculation, power supply selection, high and low voltage. Distribution system, Lighting system, weak current system. Among them, the distribution system is the main content in the design. The reliability and grounding of the power distribution system has always been a hot topic for everyone. The author has been engaged in electrical design for many years and now share some opinions with you.
1. Selection of power distribution system
(1) Load grading of high-rise buildings
Primary load: fire-fighting equipment, emergency lighting, fire elevator
Secondary load: passenger elevator, water supply system, public lighting
Three-level load: residential electricity and other
(2) Power distribution system
Because high-rise buildings have primary or secondary loads, the power supply for high-rise building distribution systems should have two separate loops or be powered by a single loop power supply and a backup power source (generator).
a, high-voltage power distribution system: modern high-rise buildings are powered by two independent 10kV power supply. Generally, the high voltage adopts single busbar segmentation, which is automatically switched and alternates with each other. The number of busbar segments is adapted to the number of power supply loops. Only when the power supply is a master and a backup, consider the use of a single bus without a segmentation. Almost all of the power inlets are cabled.
B. In order to reduce the number of transformers, the capacity selection of a single transformer is generally greater than 1000kVA. In order to limit the short-circuit current on the low-voltage side, the transformer is disengaged during normal operation, and a tie switch is provided in the middle. Lighting and power are divided into transformers. When the power consumption capacity is too small, the power transformers may not be installed separately, and the power load shall be classified and charged on the low-voltage side.
C, the high-voltage system and the low-voltage mains distribution methods basically use a radial system. Floor distribution is a hybrid system. The main part of the distribution equipment is the main line. Plug-in busbars are often used in the shafts of modern high-rise buildings. Due to the difficulty of wiring, the horizontal trunk line is mostly connected with the shaft of the shaft. Each floor of the shaft is set up to ask questions about power distribution. The interlayer distribution box receives power from the shaft parent line through the plug-in automatic air switch. When the number of layers is large, the power is generally divided into layers, or the transformer is dispersed in the base layer, the middle layer or the top layer.
D. All the switches of the low-voltage power distribution system adopt automatic air switch (circuit breaker) to set three-stage overcurrent protection device with instantaneous, short delay and long delay. For the protection and setting of automatic air switches at all levels, attention should be paid to selective coordination to prevent over-level tripping.
E. The distribution voltage is 220/380V. The distribution system uses a single-phase (a total of three lines: L line, N line, PE line) 220V power distribution or three-phase (five lines: L1 L2 L3 line, N line, PE line) according to the load size. The vertical trunk line of high-rise residential buildings should adopt power cables, branch cables or busway distribution, trunk lines should be laid in electrical shafts, and cable sections should be selected according to heating conditions (ie, the allowable current carrying capacity is not less than the calculated current through the phase line). ), and then verify the voltage loss and mechanical degree.
Only the primary and secondary loads in high-rise residential buildings are powered by dual power supplies. Care should be taken when designing the system.
Each residential unit shall be provided with a house distribution main box, a floor meter box and a household distribution box, and the floor meter box and the household distribution box shall be separately set. The lighting load of public corridors and stairwells shall be separately measured by a public meter. Residential energy metering systems should use a bus-type centralized meter reading or automatic meter reading system for property management.
The household distribution box shall be provided with lighting circuit and general power socket circuit, kitchen socket circuit, bathroom socket circuit and air conditioning socket circuit. In addition to the selection of the lighting circuit conductor section, the remaining loop conductor sections are selected with reference to the cable section previously described.
The lighting circuit requires a higher voltage. According to the provisions of GB50034-92, the terminal voltage of the lamp should not be higher than 105% of its rated voltage, and should not be lower than 95% of its rated voltage. Therefore, the cross section of the lighting circuit conductor should be selected as follows:
A=∑M+∑αM' /CΔUal%
∑M——sum of the power moment (M=PL) for calculating the line segment and its subsequent segments (referring to the segment with the same number of wires as the calculated line segment)
∑αM′——the sum of the power moments (M′=PL) of all the branch lines that are powered by the calculated line segment and whose number of wires is different from the calculated line segment. These power moments should be multiplied by the corresponding power moment conversion coefficients and then phased again. plus
Α—— is the power moment conversion factor (as shown in the following table)
ΔUal%' - the percentage of the allowable voltage drop from the head end of the calculated line segment to the end of the line to the rated voltage of the line
C - is the coefficient of calculation.
When calculating, the wire cross section of each line segment should be calculated from the first section of the main line close to the power supply. After calculating the cross section, select a similar and large standard section. The cross-section of each section of the conductor shall be verified according to mechanical strength and heating conditions.
Residential distribution lines should be provided with short-circuit protection, overload protection, grounding protection and leakage protection. In order to prevent the influence of power supply voltage fluctuation on household appliances, it is advisable to install a surge suppression protection device in the household distribution box, and the indoor wiring of the house should be darkly coated with PVC pipe.
2. Setting of protective earthing system, equipotential bonding and grounding protection for high-rise buildings
(1) If the high-rise building is powered by the urban public transformer, the protection grounding form of the low-voltage distribution system should adopt the TT grounding system and a special protection line. If the power is supplied to the transformer in the residential area or unit, the protection grounding form of the low-voltage distribution system should be in the form of TN-S.
(2) The equipotential bonding is an electrical connection in which the exposed conductive portion of the electrical device and the electrically conductive portion of the device are substantially equal in potential. The role of equipotential bonding is to reduce the contact voltage to ensure personnel safety.
According to the provisions of GB50054-95 "Low-Voltage Power Distribution Design Specification", when the ground fault protection is adopted, the total equipotential bonding should be made in the building. When the ground fault protection of the electrical device or a part thereof cannot meet the specified requirements, the local equipotential bonding should be performed in the local range. Therefore, the bathroom, bathroom, kitchen, etc. in the high-rise building should be connected to the local equipotential bonding.
(3) Grounding protection
a. Grounding protection in TN-S system: measures for total equipotential bonding;
If the distribution line only supplies the end line of the stationary power equipment, the ground fault protection action time should not be greater than 5s, ie top(E)≤5s
If power is supplied to the end lines of hand-held and mobile electrical equipment, then
Top(E)≤0.4s
The operating current Iop(E) of the system distribution line ground fault protection shall satisfy:
Iop(E)≤Uφ|Z∑φ|
Uφ_ system phase voltage
|Z∑φ|_Ground fault loop total impedance mode
Ground protection can be achieved by overcurrent protection or zero sequence current protection. If the protection requirements are not met, leakage current protection should be used.
b. Grounding protection in TT system
If the total equipotential bonding measure has been adopted, the grounding protection satisfies the following formula:
Iop(E)RE≤50v
Where: Iop(E)_ground fault protection action current
RE_Electrical equipment exposed grounding resistance and PE line resistance of conductive parts
When overcurrent protection is used, the inverse time characteristic overcurrent protection device Iop(E) shall ensure that the ground fault loop is cut within 5s; and when the transient action characteristic overcurrent protection is used, Iop(E) shall ensure that the ground fault loop is instantaneously cut off.
If the overcurrent protection does not meet the above requirements, then leakage current protection is adopted.
Summary: High-rise buildings often combine multiple functions, of which the staff and society have great influence. The reliability of the power supply of the building is very important, and it is also the main content of the electrical design of high-rise buildings. Therefore, the importance of this aspect should be exceptional. Note that only the scientific selection of power distribution schemes and the selection of diesel generators can effectively solve the problem of reliable power supply.
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