With the development of the country, electrical systems have also changed and become increasingly complex. The design of electrical systems has also become very complicated. The design of main electrical wiring, component selection, electrical equipment selection, and power distribution device design all require comprehensive consideration of the site conditions and user needs. Calculations and relay protection devices in electrical systems are very common problems. The quality of the electrical system directly affects the safe and economical operation of the entire equipment.
1. Electrical system design
Electrical system design involves selecting components, performing circuit calculations, and finally drawing electrical circuit diagrams. These diagrams are divided into main circuits and auxiliary circuits, with the main circuit diagram being the most important part of the electrical system.
The requirements for main electrical circuits are reliable operation, flexibility, economy, and easy expansion. Reliability of the electrical circuit is paramount, as power outages and personal injury pose the greatest risks. Therefore, a residual current device (RCD) needs to be added to the entire electrical system to prevent injury from leaks during operation. The working principle of an RCD is shown in Figure 1.
Figure 1. Working principle diagram of leakage current protection device
Explanation: 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 current trip unit.
The working principle of a residual current device (RCD) is that when the equipment is working normally, the phasor of the three-phase current in the zero-sequence current transformer detection circuit is zero. When the equipment leaks current, if a person touches the electrical equipment, the three-phase current is not equal to zero. The current will be transmitted back to the transformer through the person and the ground. This will cause the leakage trip to have voltage, and the RCD will quickly and automatically sense this and disconnect the main switch, thereby preventing electric shock.
The three most important components in electrical system design are the electrical schematic diagram, the electrical component layout diagram, and the electrical installation wiring diagram. These three can serve as guiding documents. The electrical schematic diagram is drawn based on the working principle and needs to be simple, clear, and hierarchical. The electrical component layout diagram is used to show the actual position of the components on the electrical equipment. The electrical installation wiring diagram is a drawing showing the actual wiring of the electrical components at their actual positions.
An electrical schematic diagram consists of electrical symbols and numbers. It should indicate the characteristics of the components or lines in the circuit. The numerical symbols distinguish different line segments in the circuit diagram. For example, three-phase power supply is represented by L1, L2, and L3. Some symbols also represent components, as shown in Figure 2.
Figure 2 Contactor Interlocking Forward and Reverse Control Circuit
The electrical component layout diagram needs to show the outline of the mechanical equipment and all visible electrical components and equipment, using thick solid lines to draw the outlines. The electrical components should be consistent with the relevant drawings. The electrical installation wiring diagram uses graphic and text symbols. The same electrical component should be drawn in the same location, and it should also be the same as the actual installation location. Electrical components need to be connected using terminal blocks on different control cabinets.
2. Electrical equipment
The selection of electrical equipment is one of the design contents. In an electrical system, electrical equipment has different working functions, and their selection methods are also different. Some are selected based on voltage, while others are selected based on circuit. By using electrical schematic diagrams, electrical components that are as stable, economical, reasonable, safe and reliable as possible can be selected.
Common electrical components include contactors, relays, fuses, low-voltage circuit breakers, switches, and pushbuttons. These devices are selected based on both voltage and current. For voltage selection, the maximum allowable operating voltage of the electrical equipment must be 1.1-1.15 times its rated voltage. For current selection, the continuous operating current must not be less than the maximum continuous operating current of the circuit. Fuse selection is based on the circuit current, and the selection formula is as follows: IRN ≥ (1.5-2.5)IN, where IRN is the fuse current and IN is the circuit current.
3. Power distribution equipment
Power distribution equipment is an important component of electrical control systems. Its function is to receive and distribute electrical energy, primarily addressing the basic requirements for indoor and outdoor power distribution. Power distribution equipment mainly consists of switches, protection devices, and measuring devices. Its basic requirements are to ensure reliable operation, personal safety, and economy, while also facilitating operation and maintenance. Figure 3 shows a diagram of an outdoor power distribution system.
Figure 3. Outdoor power distribution device diagram
Outdoor power distribution typically involves connecting external points to the factory via lines. In this case, it is necessary to ensure the minimum safe clearance for high-voltage electricity, which is A1, B1, and B2 in the diagram. In the power distribution device shown above, the fence height of electrical equipment should not be less than 1200mm, and the mesh barrier of electrical equipment should not be less than 1700mm.
In today's ever-evolving world, people have increasingly higher demands for product functionality and quality. This requires companies to continuously innovate and update their research and development. As the main power source and core system of equipment, the electrical system requires long-term stability and high standards. In some mass production equipment, the stable operation of the system and production efficiency are relatively contradictory. Therefore, how to perfectly combine the electrical system with production needs is a topic that designers are constantly updating and challenging, and it is also an important factor in the stable operation of equipment.
