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Electrical System Design and Selection of Electrical Equipment

May 03, 2018

Electrical System Design and Selection of Electrical Equipment


With the development of the country, the electrical system is also becoming more and more complicated, and the design of the electrical system has become very cumbersome. The electrical main wiring design, component selection, electrical equipment selection, and power distribution device design are all required. Comprehensive consideration of site conditions and user needs, calculations in electrical systems, and relay protection devices are all very common problems in electrical systems. The quality of electrical systems will directly affect the safety and economic operation of the entire equipment.


1, Electrical System Design

Electrical system design is the component selection of the electrical system, circuit calculation, and finally draw out the electrical circuit diagram. The electrical circuit diagram is divided into main line and auxiliary line. The main line diagram is the most important part of the electrical system.


The requirement for the main electrical connection line is to be reliable, flexible, economical, and convenient for expansion. The reliable operation of the electrical circuit diagram is the top priority. The equipment loss and personal safety are the greatest losses. In the electrical system, it is necessary to add an electric leakage protector for the entire circuit to prevent people from injuring at the time of leakage. The working principle of the leakage protector is shown in Fig.1.

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Figure 1 Leakage protector working principle diagram


Description: In Figure 1, 1 is the power transformer, 2 is the main switch, 3 is the test circuit, 4 is the zero-sequence current transformer, and 5 is the electromagnetic leakage release.


The working principle of the leakage protector is that when the equipment is working normally, the phasor of the three-phase current of the zero-sequence current transformer detection circuit is zero. When the equipment leaks, the person touches the electrical equipment and the three-phase current is not equal to zero. The transformer will be transferred back through people and the earth. This will cause the leakage trip to have a voltage, and the leakage protector will quickly and automatically sense and disconnect the main switch to avoid electric shock.


The three most important items in the design of electrical systems are electrical schematics, electrical component layouts, and electrical installation wiring diagrams. These three can be used as guidance documents. Electrical schematics are drawn on the basis of working principles. The structure needs to be simple and clear. The layout of the electrical components is used to indicate the actual location of the components on the electrical equipment. The electrical installation wiring diagram is the actual wiring of the installed electrical components.


The electrical schematic diagram is composed of electrical symbols and numbers. The characteristics of the elements or lines in the circuit are to be marked. The digital symbols distinguish the different line segments in the circuit diagram. For example, the three power supplies are denoted by L1, L2, L3, and some also represent the components, as shown in FIG. 2 .

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Figure 2 Contactor chain positive and negative control circuit


The layout of electrical components needs to draw the outline of the mechanical equipment and all visible electrical components and equipment. The outline is drawn with thick solid lines. The electrical components are consistent with the relevant drawings. The electrical installation wiring diagram uses graphic and text symbols, the same electrical components should be drawn in the same position, and at the same time with the actual installation location, the electrical components need to use the terminal board connection on different control cabinets.


2, Electrical Equipment

The choice of electrical equipment is one of the contents of design. In electrical systems, electrical equipment has different working roles. At the same time, their selection methods are also different. Some of them are based on voltage selection, some are selected through the circuit, and the stability is as stable as possible through electrical schematics. High, economical, safe and reliable electrical components.


Common electrical components include contactors, relays, fuses, low-voltage circuit breakers, switches, and buttons. The choice of these devices is through voltage and current selection, through the voltage selection, to ensure that the maximum allowable operating voltage of the electrical equipment is 1.1-1.15 times the rated voltage of the device, and the current selection must ensure that the long-term current is not less than the maximum duration of the circuit Working current. The fuse selection is based on the current of the circuit. Its selection formula is as follows: IRN ≥ (1.5-2.5) IN, where IRN is the fuse current and IN is the circuit current.


3, Distribution Equipment

The power distribution device is an important part of the electrical control system. Its role is to receive and distribute electrical energy, mainly outside the house and inside the house. The power distribution device is mainly composed of switches, protection and measurement. Its basic requirements are to ensure reliable operation, personal safety and economical efficiency. At the same time, it is also convenient for personnel to operate and maintain. Figure 3 shows the outdoor power distribution device.

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Figure 3 Diagram of outdoor power distribution unit


External power distribution outside the house is generally connected to the factory through external lines. At this time, it is necessary to ensure the minimum safety net distance of high-voltage power, that is, A1, B1 and B2 in the figure. In the figure above, the power distribution device and the electrical equipment are The height of the fence should not be less than 1200mm, and the mesh of the electrical equipment should not be less than 1700mm.


In the continuous development today, human beings have become more and more functional and quality products. This requires companies to continue to innovate, research and development must continue to keep up with new, electrical systems as the main driving force of equipment, the core system, need With long-term stability and high standards, the stable operation and production efficiency of the system are relatively contradictory in some large-volume production equipment. Therefore, how to combine the electrical system with the production requirements is a problem that designers are constantly updating and challenging. It is also an important factor in the stable operation of the equipment.