Low-voltage power distribution equipment selection

Main electrical parameters of low-voltage power distribution equipment

The main electrical parameters of low-voltage power distribution equipment are rated current, rated short-time withstand current (c, 1s), rated peak withstand current (k, 0.1s), rated dispersion coefficient, degree of protection, internal isolation and so on. The rated current, short-time withstand current, peak withstand current and protection level are found in the product specification, but the internal isolation form specification is generally not given. The dispersion coefficient is a parameter of the product test, although most of these parameters are Frequent contact, but it is not easy to understand accurately.

1. Rated current

The rated current generally refers to the rated current of the incoming switch of the power distribution equipment. The rated current of the power distribution cabinet generally refers to the maximum current of the horizontal busbar, which is often equal to the rated current of the main switch of the power receiving cabinet. The sum of the rated currents of all feeders is multiplied by the simultaneous factor to obtain the main busbar rated current. The rated current of the feeder cabinet generally refers to the current of the vertical busbar, and the rated current of the vertical busbar should be greater than the rated current of the feeder loop multiplied by the dispersion coefficient. In the power distribution design, the rated current of each cabinet should generally not exceed the rated current specified in the manual. Because the current is too large, the temperature rise in the cabinet is greatly affected. If the conditions are limited, the cabinet temperature can be increased and the conductor size can be increased to ensure that the temperature rise is within the standard range. As for how much to increase the temperature rise to ensure that the temperature rise does not exceed the standard, this uses the extrapolation method to calculate the temperature rise. You can refer to IEC 890, “Extending the temperature rise of the low-voltage switchgear and control equipment part type test equipment (PTTA) by extrapolation method”. The temperature rise involves the installation process, the degree of protection of the enclosure, the power consumption of the components in the cabinet, etc., so the extrapolation method is used to obtain a relatively complicated temperature rise value. But there is an easy way - analogy. For example, the XL-21 power cabinet has options A, B, and C:

Option A: cabinet width 400 mm, circuit breaker 400A, copper mother} TMY-5 (40mm × 4mm)

Option B: cabinet width 350 mm, circuit breaker 200A, copper busbar TMY-5 (40mm × 4mm).

Option C: cabinet width 600 mm, circuit breaker 800A, copper busbar TMY-5 (50mm × 5mm).

All the above schemes passed the type test and the temperature rise was qualified. The current calculation current is 600A, but the space limitation cannot put down the two-sided power cabinet, that is, the scheme A and the scheme B cannot be selected. The solution C can meet the requirements in space. However, the current of the circuit breaker frame is 800A, the setting value is 600A, and the busbar is TMY-5 (50mm×5mm). This ensures that the external terminals of the circuit breaker are consistent with the scheme C. Obviously, through the 800A current temperature rise, under the same conditions, the temperature rise of 600A will definitely not exceed the standard.

It should be pointed out that the analogy method should be used to ensure that the external conditions of the adopted scheme are unchanged. If the external conditions change, it is difficult to ensure that the temperature rise does not exceed the standard. If the copper bar is changed to TMY-5 (50mm×4mm) in the above conditions, the temperature rise will definitely increase due to the smaller contact surface between the busbar and the circuit breaker; the current will drop from 800A to 700A, and the temperature rise will decrease. It is difficult to say that the increase and decrease values ​​are large and small, and the temperature rise results are difficult to say. Secondly, the relationship between temperature rise and current, the relationship with the protection level, and the relationship between the mating surfaces of the busbars are not simple linear relationships. Foreign companies have their own extrapolation formula to calculate the temperature rise of non-standard schemes, which greatly meets the needs of power supply and distribution design. Here, the analogy method is obviously a big horse-drawn car, but in the absence of extrapolation. It is a simple way to verify that a non-standard solution is feasible.

2. Rated short-time withstand current (/c )

The rated short-time withstand current means that a circuit in a complete set of equipment can safely carry a short-term withstand current rms value under specified conditions, and the time is generally 1 s. It has a considerable relationship with the cabinet structure and the busbar fixings.

GGD, GCK, MNS and other low-voltage power distribution cabinets are assembled by profiles. GCK cabinets use CF 28 profiles, MNS cabinets and imported cabinets mostly use C-profiles, GGD cabinets use 8 MF profiles, and profiles are made of cold-rolled steel sheets. The profile is punched with equidistant holes for mounting support. The busbar fixings are called busbar frames or busbar clamps, which are pressed with epoxy resin and are resistant to high temperatures and have a certain strength. The rated short-circuit current is divided into six gears of 10kA, 15kA, 30kA, 50kA, 80kA and 100kA in "Low-voltage switchgear and control equipment Part 1: Type test and partial type test equipment" (GB 7251.1-2005).

The main busbar of the withdrawable switchgear GCK and GCS cabinets has a short-time withstand current of 80kA and a vertical busbar of 50kA. The rated short-circuit withstand current of the MNS cabinet is larger, and the main busbar has a short-time withstand current of 100kA. It is used in key projects such as subways, airports, and petrochemicals where the short-circuit current is large and the power supply reliability is high.

The fixed cabinet GGD1 has a short short-time withstand current, generally 30kA, GGD2 is 50kA, and GGD3 is 80kA. It is generally used in small power supply and distribution projects, and can also be used in medium-sized supply with low reliability requirements. In electrical engineering.

The short-time withstand current of the power cabinet is generally below 30kA, which is generally used as a power field distribution in small power distribution projects. The distribution box is generally used for indoor power distribution. It is an end product. Because it is operated by non-professionals, its short-circuit withstand current is relatively low, generally below 10kA.

3. Rated peak withstand current (/k)

Rated peak withstand current is the peak current that the manufacturer specifies for the circuit to withstand satisfactorily under specified test conditions. The peak current time is typically 0.1S. To determine the strength of the electrodynamic force, the peak value of the withstand current (, D) is obtained by applying the short-time withstand current multiplied by the coefficient n.

4. Rated dispersion coefficient

Rated dispersion factor means that there is a number of main circuits in a plant or in one part of a plant (for example a rack unit or frame unit), the sum of the maximum expected currents of all main circuits at any one time and the selected part of the plant or its selected part The ratio of the sum of the rated currents of all main circuits. This parameter is given in GB 7251.1-2005.

The nominal dispersion factor is especially useful for power distribution designs. For example, the vertical busbar in the GCK cabinet has a rated current of 1600A, and a power distribution cabinet has three 400A loops, three 200A loops, and one 160A loop. The total installed current is 1960A, according to the provisions of GB 7251.1-2005. The loop dispersion coefficient is 0.7, the vertical busbar rated current is 1600A, the installation current can be 1600A/0.7=2285.7A, and the above total installation current is 1960A. From the perspective of the vertical busbar current, three 400A are installed in one cabinet. The circuit, three 200A loops, one 160A loop, under normal conditions will not exceed the vertical busbar capacity.

Low-voltage distribution equipment protection level and internal isolation form

Protection level

Refers to the degree of protection of the equipment enclosure. For the drawout power distribution cabinet, the test position, the separation position and the connection position protection level are the same, and the protection level does not change when moving from one position to another. The higher the degree of protection, the better the safety performance. However, the higher the degree of protection, the better the tightness of the casing, the worse the heat dissipation, and the heat generated during operation cannot be dissipated, resulting in an increase in the temperature inside the cabinet and a decrease in the insulation performance of the electrical appliance, which directly poses a threat to the safe operation of the equipment. Therefore, when selecting the cabinet type for power supply and distribution design, the high protection level should not be pursued blindly. If the protection level is too high, the operating temperature inside the cabinet is high, the switchgear may be derated, and the insulation should be specially treated. manufacturing cost. In the contradiction between the protection level and the temperature rise, the analysis should be based on the specific situation. The degree of protection of the power distribution cabinet in the power distribution room can be selected lower. In order to prevent small animals such as snakes and rats from entering the cabinet, it can be blocked at the cable trench and the entry and exit line. Due to the lower degree of protection, the operating conditions in the cabinet are good. In the distribution equipment in the power distribution room, the IP35 is generally sufficient for the protection level: the waterproof problem should be considered in the outdoor, and the protection level may reach IP54. The protection level of GCK and GCS withdrawable switchgear is generally IP30 and IP40. The protection level of MNS cabinet is generally IP40 and IP50.