Two problems in the design and type selection of t

Two problems in the design and selection of low-voltage incoming circuit breaker

low voltage incoming circuit breaker is generally used at the low-voltage side of transformer, and it is a relatively important circuit breaker. If it is improperly selected, once failure or misoperation occurs, it may cause a large range of power failure. This paper will describe two practical problems and treatment methods encountered by the author in the design and selection of low-voltage incoming circuit breaker, for the reference of colleagues. If there is anything wrong, please correct it

1. About the short-circuit breaking capacity of the circuit breaker

the circuit breaker generally has two parameters reflecting the short-circuit breaking capacity of the circuit breaker: one is the rated limit short-circuit breaking capacity (hereinafter referred to as the limit breaking capacity) ICU, and its test sequence is o-t-co (o represents the breaking operation, CO represents the making operation followed by the breaking operation, T represents the time interval, generally 3min). After acting according to the specified test sequence, Do not consider that the circuit breaker continues to carry its rated current; The second is the rated operating short-circuit breaking capacity (hereinafter referred to as the operating breaking capacity) ics. Its test operation sequence is o-t-co-t-co. after acting according to the specified test sequence, it is necessary to consider that the circuit breaker continues to carry its rated current

which parameter should be used in the design and selection of circuit breakers is not clearly specified in the specification, nor in various manuals. Most manuals point out that the rated short-circuit making and breaking capacity of the circuit breaker is equal to or greater than the maximum short-circuit current that may occur in the line, which is generally calculated according to the effective value. It is not specified whether it is the ultimate breaking capacity or the operating breaking capacity. Only a few manuals indicate the use of limit breaking capacity, but I think that the operating breaking capacity should be used for the low-voltage incoming circuit breaker for the following reasons:

(1) from the perspective of reliability, the operating breaking capacity should be used to select the circuit breaker. After the circuit breaker disconnects the short-circuit current, it can also ensure that the circuit breaker can withstand its rated current, reduce the possibility of circuit breaker failure, and thus improve the reliability of circuit breaker operation

(2) × 50 from the perspective of feasibility

for new circuit breakers, the operating breaking capacity is generally large, which can meet the requirements of short-circuit current. Table 1 lists the operating breaking capacity of several representative new circuit breakers. Table 1

the effective value of short-circuit current in the table refers to the short-circuit current when the short-circuit capacity at 10kV side of the transformer is considered as 150-200mva, and a metallic short-circuit occurs at the busbar 3M away from the low-voltage outgoing end of the transformer. Cb11 is a frame circuit breaker launched by Changzheng electric 11th factory in recent years, tm30 is a molded case circuit breaker recently launched by Tianjin Low Voltage Switchgear Factory, and m and NS (c) are products of Meilan Rilan company. The values of ICs and ICW (short-time withstand current) listed in the table are the values of corresponding shell frame current level circuit breakers that meet the requirements of transformer capacity in each series of circuit breakers

it can be seen from table 1 that for frame circuit breakers, except for cb11, when the transformer is 2000kVA, its operating breaking capacity is slightly insufficient, other circuit breakers can meet the requirements; For molded case circuit breakers, as long as the capacity meets the requirements, the breaking capacity is generally no problem. For the old circuit breaker, the manufacturer did not provide the parameter of operating breaking capacity, but only the limit breaking capacity. Ics=icu50% can be considered in the design and type selection. In this way, when the rated current of the circuit breaker is appropriate, its operating breaking capacity is often insufficient, and it is necessary to meet the requirements of breaking capacity by improving the shell current level of the circuit breaker. For example, for 800kVA transformer, it can be seen from table 1 that its rated current is 1200A and short-circuit current is 23.5ka. Dw15-1500 circuit breaker can be selected according to the rated current, and its limit breaking capacity is 40ka. If it is selected according to the limit breaking capacity, it meets the requirements; If it is selected according to the operating breaking capacity, it does not meet the requirements, and it is necessary to select a circuit breaker with a larger shell current level. For example, dw15-2500 has a limit breaking capacity of 60kA. Therefore, for the old circuit breaker, it is also feasible to select according to the operating breaking capacity

(3) from the perspective of economy

for the old type circuit breaker, it is generally necessary to increase the shell current of the circuit breaker and take root in Hefei in more than 20 grades according to the operation breaking capacity to meet the requirements of breaking capacity; For the new circuit breaker, although its operating breaking capacity is high, it is generally not necessary to improve the current level of the circuit breaker shell in order to meet the requirements of breaking capacity, but its price is higher than the old circuit breaker of the same current level. Therefore, if the circuit breaker is selected according to the operating breaking capacity, the general investment will increase

however, due to the small number of low incoming circuit breakers, the investment increase caused by them accounts for a small proportion in the investment of the whole project, and its importance is high. Its failure or misoperation will cause a large area of power failure, bring inconvenience to production and life, and even cause large economic losses. Therefore, it is economically reasonable to increase the investment for the reliable operation of the low-voltage incoming circuit breaker

in addition, it is an inevitable trend for new circuit breakers to replace the old ones. When it completely replaces the old circuit breaker, the problem of increasing investment will no longer exist when the circuit breaker is selected according to the operating breaking capacity. At that time, it will be an inevitable choice to select the circuit breaker according to the operating breaking capacity

2. About the selection of circuit breaker structure type

low voltage circuit breakers can be divided into plastic shell type and frame type according to the structure type

as the incoming switch, the frame circuit breaker is generally selected, but the frame circuit breaker has weaknesses such as large volume, high price and poor contact protection, so as the incoming circuit breaker, it is not the best choice

molded case circuit breaker has the characteristics of small volume, compact installation, beautiful appearance, low price, good contact protection and so on. In the past, it did not become the first choice of incoming line switch, which is mainly limited by its small capacity, low short-circuit breaking capacity, poor selectivity and short-term withstand capacity. However, with the development of technology and the introduction of new products, these problems have been improved to varying degrees. Now I will explain these aspects separately

2.1 capacity problem

for the old molded case circuit breaker, its maximum capacity is generally about 600A, which is too small as an incoming switch. However, the capacity of the new shell circuit breaker has been greatly improved. For example, the maximum capacity of DZ20 can reach 1250A, the maximum capacity of tm30 can reach 2000a, and some products can even reach 3000A. Therefore, for the new circuit breaker, its capacity is still insufficient for large transformers, but it is fully competent for small and medium-sized transformers

2.2 breaking capacity problem

in recent years, the short-circuit breaking capacity of circuit breakers produced in China, whether frame type or plastic shell type, has been greatly improved

as far as the plastic shell type is concerned, the improvement of its breaking capacity is amazing. For example, the peak value of the maximum short-circuit breaking capacity of dz10 circuit breaker, which was the most commonly used in the past, has only reached 40ka, while the effective value of the operating breaking capacity of the new circuit breaker has reached a very high level. It can be seen from table 1 that the maximum breaking capacity of DZ20 can reach 50kA, and that of tm30 can reach 75kA. The effective value of the limit breaking capacity of both models can reach 100kA

therefore, in terms of breaking capacity, the molded case circuit breaker can fully meet the requirements as a low-voltage incoming circuit breaker

2.3 selectivity problem

the old molded case circuit breaker generally does not have a short time-delay release, so it is difficult to achieve selective protection. But now some new circuit breakers have this function. For example, tm30 series intelligent release can realize short time delay function and complete selective protection; NS series electronic release can also achieve this function. According to the development trend of intelligent, modular, combined, electronic and multifunctional low-voltage circuit breakers, in the future, there will be more and more molded case circuit breakers with selective function, and the other end can only rotate around the axis of the sample. Selectivity will no longer be a problem for molded case circuit breakers

2.4 short time withstand capacity problem

short time withstand capacity is a weakness of molded case circuit breakers. At present, most manufacturers of molded case circuit breakers do not mention the parameter of rated short-time withstand current ICW. However, some manufacturers have provided this parameter

Table 1 lists the rated short-time withstand current of tm30 and c-series circuit breakers. According to the data in the table, the short-time withstand current of the molded case circuit breaker is far from that of the frame circuit breaker, and its value is far less than the value of the short-circuit current at the low-voltage side of the transformer. However, as an incoming switch, as long as the setting current of the instantaneous release of the circuit breaker is selected and the short-time delay current is limited within the withstand current range that the circuit breaker can withstand, the molded case circuit breaker can meet the requirements of withstand current

for example, there is a 630kVA transformer. According to table 1, the rated current on the low-voltage side of the transformer is 960A and the short-circuit current is 21ka. Tm30-1000 circuit breaker can be selected, and its operating breaking capacity is 50kA, which meets the requirements; Its short-time withstand current is 15ka, which does not meet the requirements of our service cycle. At this time, the action current of the instantaneous release of the circuit breaker can be set at 15ka. When the short-circuit current exceeds 15ka, the circuit breaker will trip instantaneously. The consequence of this is the loss of partial selectivity of protection. In order to greatly improve the short-term withstand capacity of molded case circuit breakers, we still need to wait for the further development of technology and the emergence of new materials

for occasions that do not need selective protection, the molded case circuit breaker can avoid its weaknesses and make its advantages more apparent

from the above analysis, it can be seen that as a low-voltage incoming switch, the molded case circuit breaker can replace the frame circuit breaker under certain conditions. (end)