Modern engineering has very high requirements for all aspects, especially in the area of ​​power distribution systems. It requires high adaptability and flexibility of power distribution equipment to meet the increasingly complex engineering needs. As an industrial and mining enterprise, it is crucial to have a stable and reliable transmission and distribution system, which provides a solid guarantee for the steady development of the company.
As part of the low-voltage power supply system, the busway system can meet the needs of modern engineering, and its advantages mainly reflect the following aspects:
- The system is flexible and can be staggered;
- Save installation space;
- Facilitate temporary changes to the project, the busway system can be reassembled and reused;
- High short circuit strength and high degree of protection for a variety of harsh environments. [1]
1 Structural characteristics of the busway The busway system is mainly used for the transmission and distribution of transformers to low-voltage distribution systems.
In this process, the various units of the system play a very important role. Free papers. Compared with the cable, the system is more flexible and convenient. When the load is distributed, the setting of the plug-in point is not fixed. The plug-in interface can be reserved according to the needs of the site, so that it can be re-added when the device is added or the device is changed. Plug in to meet your needs.
The busway system consists of the following units: (1) Feeder busbar or plug-in busbar straight section unit (2) Transformer incoming line unit, power distribution cabinet incoming line unit or cable entry unit (3) Reversing unit : L-type unit, T-type unit, Z-type unit (four) plug-in unit.
For all busway systems, safety performance is particularly important. With reference to the national standard GB7251.2, we can understand the three technical indicators of temperature rise, short circuit strength and electrical performance of the busbar trunking system. With the universal application of computers, The influence of harmonics on system equipment is getting more and more attention, and the busway system is analyzed in combination with the technical specifications of the products and the specific tests we are engaged in [1].
2. The key point of the temperature rise of the busway is the rated current of the product. The calculation and selection should pay attention to the following points when arranging the layout of the busbar system:
- the location, number and connection of the load or distribution system;
- dispersion coefficient;
- set the short circuit level;
- Connection to the power distribution cabinet must be provided: model of the power distribution cabinet, incoming line (top, bottom or back);
- Location and conditions of installation: space size, building construction (for suspension and installation), busway orientation, environmental conditions (temperature, humidity, air quality, etc.), wall through which the system passes;
- Compatible with other systems - for example, the part supporting the installation of the busway system: the plan of the power supply line, the position of the ventilation duct, and the plan of the lighting system;
- The number and location of the docking units are required.
The busway system is designed in strict accordance with the points mentioned above and the rated current is calculated by the following formula.
Ie rated current (A);
Ue rated working voltage (V);
COSΦ power factor;
Pinst installation power;
Alpha dispersion coefficient;
b feed coefficient b=1 single-sided feed b=0.5 double-sided feed and intermediate feed unit
The rated current Ie is based on a certain ambient temperature (the average temperature of 24 hours is 35 ° C, not exceeding 40 ° C), as shown in Figure 1. The load capacity should be corrected as the ambient temperature is different.
Figure 1. Load capacity versus ambient temperature
3. Short-circuit strength of the busway The industrial and mining enterprise project has higher requirements for the busway system, especially in the short-time withstand current, and the requirements for short-time withstand current are higher and higher, when the delivery current is high, for example The weld line on the production floor requires a high short-time withstand current for the busbar system, as is the case with plug-in units.
I2sh three-phase short-circuit current impact value (kA)
l parallel conductor length (m)
a two parallel conductor center line spacing (m)
Figure 2. Ks conductor shape factor graph
4. The electrical performance of the busbar trunk system reference standard GB7251.2 busbar electrical performance mainly includes two parts, one is the system's resistance, reactance and impedance, the performance can be obtained through the test at the reference temperature of 20 ° C and the operating temperature impedance. The second is the relative N, PE, PEN resistance, reactance and impedance values ​​under fault conditions. These parameters allow the designer to calculate the system voltage drop and the short circuit current under different short circuit conditions.
If the busway system is long-distance transmission and distribution, the impact of the voltage drop on the system must be considered. The following is the specific calculation formula for the voltage drop:
ΔU = voltage drop (V);
Ib = rated current (A);
L = total length of the system (m);
a = load distribution coefficient;
Ri = resistance (mΩ/m);
Xi=reactance (mΩ/m);
Cosφ = power factor.
The load distribution factor must be based on the type of load distribution. The chart below reflects the different ways in which the load is distributed with a given rated current. Figure 3. Load distribution coefficient change graph
Through the impedance of the entire busway system, it is easy to estimate the system short-circuit current according to the following formula.
The busway system must be overloaded and short-circuit protected during operation. Generally, fuses or circuit breakers are widely used as protection devices in the system. Free papers.
In practical applications, since the sensitivity of the fuse is very high, and when the current slightly exceeds the rated current, the fuse starts to melt, but the melting time is relatively long, so the fuse is not very suitable for use as an overload protection device in the system. If the overload protection device uses a fuse, in order to ensure that the protection device provides appropriate protection to the busway system, the rated current of the fuse must be one level lower than the rated current of the busway system. If a circuit breaker is used for protection, the protection unit can be adjusted according to the rated current of the busway system, which means that the busway system can reach 100% of the current carrying capacity.
If it is decided to use fuses and circuit breakers for short-circuit protection, the current of the selected model should not exceed the protection current specified by the busway system. Also consider the strength of the short-circuit current, whether a current-limiting protection device and a short circuit of the selected protection device are required. What is the switch capacity and so on.
5. Neutral and protective conductor design 5.1 Neutral conductor electronic equipment is susceptible to electromagnetic interference, especially in the vicinity of the power supply system, the interference phenomenon will be more obvious, how to reduce or eliminate this part of the interference to the busway system is higher Claim. Because the magnetic fields and harmonics generated by currents in the busway transmission system affect computer control systems, servo systems, and advanced automation system devices. Many automatic equipment on the main line have a high-capacity neutral line to eliminate the influence of harmonics on the system equipment. The commonly used 100% neutral busway, the higher design is the two N-conductor busway. System, because the 200% neutral busbar system can eliminate the influence of harmonics on electronic equipment, it is more and more widely used in engineering projects.
5.2 PE Conductor PE conductor has a large cross section, which guarantees the safety performance of the system. At the same time, the system has good grounding, and can also improve the electrical continuity of the system and the ability to withstand short-circuit current, without the transient of the protection device. Disconnection caused the entire system to stop working.
5.3 Exposed grounding wire Because the PE conductor is completely insulated from the busbar housing, this conductor configuration is safe and reliable when powering electronic equipment in the building. The short circuit between the phase line and the outer casing is not caused by the PE conductor or the exposed protective ground, but by the bare part between the phase lines. When the busway system is energized, a certain magnetic field is generated around it, but the generated magnetic field does not affect the performance of the grounding wire. Therefore, it is also an ideal choice to use a bare grounding wire when powering electronic equipment.
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