A Brief Discussion on Low-Voltage Switchgear Design
Release time:
2022-08-29
Source:
Low-voltage switchgear components and complete sets of equipment are widely used, with enormous demand, low technical barriers, and a large number of manufacturers. This has led to product replication, resulting in significant waste of resources and energy. We should not be blindly complacent; the existing gaps must be thoroughly studied by electrical engineers, who should deepen their technical expertise and design superior products.
Low-voltage switchgear Its components and complete sets of equipment are widely used, with enormous demand, low technological barriers, and a large number of manufacturers, leading to product replication. This situation is well understood, yet it results in massive waste of resources and energy consumption. We should not be blindly complacent; the existing gaps must be thoroughly studied by electrical engineers, who should delve deeply into technology and design superior products.
Low-voltage switchgear Design goes beyond simply connecting switches according to the system diagram; low-voltage switchgear also requires thorough analysis and calculation to ensure a rational and well-optimized design.
In frame circuit breaker prototypes, heating power and the high-temperature derating factor serve as the foundation for low-voltage switchgear design. By calculating based on thermal power parameters, the internal temperature of the switchgear can be determined; meanwhile, the high-temperature current derating parameter allows for the specification of the switchgear’s rated current. Design should not rely solely on experience or on making blind, intuition-driven trial-and-error adjustments.
According to IEC 61439-1, Clause 10.10.4.3.1, the following methods may be applied for evaluating the ambient temperature Ti of circuit breakers:
Power-loss data for all internal components of low-voltage switchgear can be obtained from the component manufacturers, and the power loss is distributed approximately uniformly throughout the enclosure.
The rated current of the component circuit to be tested shall not exceed 80% of the rated conventional free-air thermal current (Ith), if specified, nor shall it exceed the rated current (In) of the switching devices and electrical components incorporated in the circuit.
The arrangement of mechanical components and installation equipment shall not significantly impede air circulation.
To calculate the temperature rise of the air inside the enclosure, the following data are required:
Cabinet dimensions: height/width/depth;
Cabinet installation type;
Cabinet design, with or without ventilation openings.
Number of internal horizontal partitions
Active power loss of equipment installed in the low-voltage switchgear
Effective power loss (Pn) installed on the cabinet’s main conductor.
Calorific value
Power consumption is the total loss measured at the enclosure current Inm for circuit breakers, as determined from frame circuit breaker samples provided by various manufacturers.
Calculating the contact pressure requires exceeding 100 N; such robust contact can meet the requirements of machine life testing, demonstrating an excellent design and high-quality product.
As evidenced by the high-temperature performance, the rated current under load is generally higher for this product than for domestic brands. At an ambient temperature of 70°C, the 6300 A frame circuit breaker can sustain a current of 5800 A, whereas domestic brands can better maintain 5100 A; the difference is only 4600 A, which corresponds to 73% of the rated current at 40°C.
Low-voltage switchgear Acoustic Issues During Operation
Capacitors generally operate silently; however, under certain conditions, discharge noises may occur during operation. If the capacitor bushings are exposed to the elements for an extended period, rainwater can enter the space between the two bushings, and when voltage is applied, discharge noise will result. When the oil level inside the capacitor is insufficient, the oil surface at the bottom of the bushing may become exposed, also leading to discharge noise. If there are virtual or detached solder joints inside the capacitor, flashover discharges may occur within the oil. Furthermore, if the capacitor core has poor contact with the casing, floating voltages can develop, causing discharge noise. In any of these discharge-noise scenarios, appropriate corrective measures must be taken: first, de-energize the capacitor, remove the outer bushing assembly, clean it thoroughly, and reinstall it; then, add capacitor oil of the same specification. If the discharge noise persists, the capacitor must be disassembled and overhauled. Before proceeding, ensure that the capacitor is de-energized and fully discharged, and verify that the core maintains good electrical contact with the casing.
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