July 2012 China's Miniature Circuit Breaker Technical Analysis

First, selective cooperation and backup cooperation

1) "Coordination" refers to the protection coordination between at least two upper and lower overcurrent protection devices, which does not exist without considering the cooperation of the other party.

2) The means of selective cooperation and backup protection are different. If only from the perspective of changing the characteristics of the upper-level overcurrent protection device (circuit breaker as an example):

A) Usually delay the action of the upper circuit breaker to wait for the action of the lower circuit breaker so as to obtain better selective cooperation

B) It is usually used to accelerate the action of the upper circuit breaker to limit the short circuit energy, so as to provide backup protection for the lower circuit breaker. Of course, the means for improving the upper and lower levels of selective cooperation is not limited to the action time coordination but may also be based on means such as current matching, energy matching, and logic control. Of course, it is better that the circuit breaker itself can provide good selectivity and backup protection for other overcurrent protection devices.

3) The purpose of requiring selective cooperation is to reduce the scope of fault blackouts; the purpose of requiring backup coordination is to reduce investment costs.

4) From the perspective of two different purposes and different implementation methods, there is a conflict between the two types of cooperation.

Second, the status of terminal power distribution continuity

According to the current status of terminal power distribution, the degree of specialization of cable laying and equipment installation is not high, but the distribution range of power equipment is wide, and the situation of random chaos is relatively common; there are many non-professional users, overload phenomenon is common, overload or short-circuit fault The probability is high. Especially during peak load periods, the power supply department is busy handling various types of trip power outages. Because terminal power distribution seldom considers selective cooperation, a single short-circuit fault may cause power outage on the entire floor or building, which in turn makes the power supply department's fault location and restoration work even worse.

From an electrical design point of view, there is also a misunderstanding of the protection against electric shock in the interior of a typical residential unit. Some designers only consider installing a residual current protection device at the main switch of the house, or the whole residential unit can be powered by only one circuit. Or not taking into account the action time difference of the upper and lower circuit breakers, etc., all of which may result in the failure of the entire house due to a short circuit or ground fault, including failure of the security system power failure, power failure of the refrigerator and the living system (fish tank), and network interruption (router power loss) .

Outside of a typical residential unit, the designer may not consider the selective fit of the supply-side over-current protection device. On the user's home side, an incoming fuse or circuit breaker is installed, but it belongs to the jurisdiction of the power supply department. If these overcurrent protection devices malfunction due to poor selective cooperation (the lower circuit breaker has cut off the fault), it obviously also unnecessarily increases the attendance and overhaul workload of the power supply department, and greatly increases the failure power-off time.

Third, selective cooperation research approach

Due to the increasing importance of selective cooperation in terminal distribution, selective coordination is mainly discussed in this paper. In general, there are two research approaches to study selective fit, but they support each other and work together:

A) Find ways to increase selectivity by studying the cooperation between breakers on the upper and lower levels

B) Finding ways to increase selectivity by studying the improvement of the structure of the circuit breaker itself

The low-voltage side of the distribution transformer often has three or four layers of power distribution (common voltage 220/380V, IEC voltage standard 230/400V), at the first or second distribution layer, the designer will consider the optional cooperation according to user requirements, but The number of associated circuit breakers is small and the distribution is concentrated. Intelligent circuit breakers can often be selected to meet the requirements for selective coordination; however, the selective setting is complex and requires professional management. In these occasions, the A research approach is often used.

The third or fourth tier is terminal power distribution, with little regard for selectivity or continuity of supply for cost reasons. The load of this layer is dispersed and the users are not professionals. There is a need for a cost-effective, simple-to-implement solution that guarantees ever-increasing power continuity requirements. In these occasions, we often use the B) research approach, which is to develop a new generation of circuit breakers to meet the selective requirements.

As early as 2004, at the “China Intelligent Building Qingdao Electric Salon” exchange organized by China Intelligent Building Technology Information Network, China Architecture Design and Research Institute and ABB China Co., Ltd., Mr. Mr. Bernd Siedelhofer introduced the structure, principle and application of the S700 with selective overcurrent protection circuit breaker. Experts at the meeting positively affirmed the S700 SMCB, which is fully selective by ABB, and emphasized that selective matching requirements should be improved in domestic terminal distribution. The mature application of ABB's S700 SMCB is a good reference.

Fourth, terminal appliances to improve the ability to match selective

According to the IEC60898/GB10963 technical standard, MCBs for terminal distribution are generally required to operate quickly to cut short circuit faults as soon as possible. In other words, it does not need to withstand the "rated short-term withstand current Icw" in accordance with IEC60947/GB14048, so the MCB volume of the same current level is much smaller than that of the MCCB.

If it is desired to increase the selectivity of the upper and lower MCBs, it is necessary to increase the time during which the upper MCB is subjected to the short-circuit current so that the lower MCB can quickly cut off the short-circuit current in the fault area; meanwhile, in order to ensure the “Grandfather” short-circuit protection device If the short-circuit current is too large and the time is too long to malfunction, it is necessary to control the current-limiting characteristics and operating time of the upper MCB. In other words, the selective overcurrent protection circuit breaker SMCB should solve the problem of selectivity between multiple protection devices. The S700/S750SMCB developed by ABB succeeded technically and solved the problem that "bears and fishes can't have both."

1) Function of S700/S750 SMCB - Selective + Backup Protection

SMCB is an acronym for English definition of “selective over-current protection circuit breaker”.

The design goal of SMCB is: When it is matched with an ordinary circuit breaker, it allows to use the load side circuit breaker whose breaking capacity is lower than the expected short circuit current at the installation point, and guarantees complete selectivity.

When a short circuit occurs, the power-side circuit breaker contacts are briefly separated to achieve the purpose of current limitation. When the protection trip unit has not actuated yet but the lower circuit breaker “sures” that it has been broken, the contacts are switched on again to maintain the power supply. At the same time, the power-side circuit breaker action mechanism should have a delay function.

In addition, the selective fit of the SMCB and Grandfather short-circuit protection devices (fuses) must also be verified. Obviously, a specially designed contact system and tripping system are needed to achieve the above-mentioned purpose. If the short circuit occurs at the load side of the lower MCB, the short circuit current may not be too large because of the line impedance, but this is enough to trip the lower MCB (C-tripping characteristic: 5-7 In). Under the action of the electromagnetic force of this short-circuit current, the SMCB will cause the main contact to repel the current limiting phenomenon, but not enough to trip the SMCB trip mechanism; at the same time, the limiting resistor R and its thermal element will control the short-circuit current. : It is necessary to allow the MCB to act and not to substantially trip the SMCB.

After the lower MCB completely removes the short-circuit current, the SMCB main contact returns to the closed position and continues to supply power to other loads. In the unlikely event that the MCB rejects or cannot be switched off, the optional thermal element causes the SMCB to be delayed by a small delay (if the short circuit occurs at the load side of the SMCB, the expected short-circuit current is quite large (depending on the short-circuit capacity), which may reach several tens of thousands. At the level of safety, the SMCB can assume its maximum breaking capacity, and the short-circuit current electromagnetic force drives the tripping mechanism to break quickly.

1, before the failure: normal operation

2. In the event of a fault: Short-circuit at the load end, SMCB+MCB current limit, MCB action

3, after the failure: SMCB is still connected (to meet the selectivity), MCB disconnected

It should be emphasized that due to the special structure and current limiting function of the S700/750, it not only improves the selectivity of the lower MCB, but also provides backup protection. It has been verified that the selectivity current limit is even higher than the lower MCBIcn value.

By the same token, the S700/750 also improved the selective fit of grandfather-level short circuit protection appliances. In the proper configuration of the grandfather fuse, the selective current limit of the SMCB and its SMCB is up to the SMCB Icn value.

2) Selection of trip characteristics of S700/S750

According to the VDE0645/GB24530 technical standard, the S700/S750 recommends using the E trip feature because it has a relatively sensitive overload protection trip curve, which can effectively extend the service life of the cable and reduce the possibility of fire caused by the cable overheating. Another advantage is that the compromised short-circuit protection tripping curve can avoid being forced to amplify the cable due to insufficient short-circuit current at the remote end of the cable, and can also avoid malfunction due to the disturbance of the starting current of the motor load.

In circuits with high starting current loads, the SMCB with Cs tripping characteristics may also be considered. Fortunately, in a typical terminal distribution circuit, loads with very high starting currents are not common.

3) Other performance requirements of S700/S750

SMCB is mainly used for MCB front-end protection of residential terminals, or where selective and backup protection is required, to achieve selective + back-up protection. In foreign countries, SMCB is often installed on the line side of low-voltage users and should be classified as class IV to ensure the safety of circuit isolation. This is a requirement that cannot be met by a common MCB (overvoltage classification III).

With the expansion of low-voltage power grid capacity and the increase in the expected short-circuit current, SMCB is also required to have higher breaking capacity, such as 25 kA. According to the requirements of IEC60439, the breaking capacity of the circuit breaker at the same installation point must not be lower than the expected short-circuit current, which inevitably requires MCBs installed in the same place to have high breaking capacity. This is also difficult to realize from the MCB manufacturing technology and the cost of the distribution system. of. However, since the SMCB provides backup protection, allowing the use of MCBs with Icn less than the expected short-circuit current, this provides an economically sound solution.

The S700/S750 uses a voltage-independent SHU special internal structure, which provides a very selective function that is not affected by fluctuations in the supply voltage or interference. In addition, it does not contain sensitive electronic components and has excellent electromagnetic compatibility. In terminal distribution, there is almost no possibility of misoperation.

The S700/S750 developed by ABB meets the installation environment of Pollution Degree 3 and -25°C to +55°C, which is more severe than the installation and use environment of MCB. It is particularly suitable for installation in China's cold and high temperature areas, and also suitable for industrial environments. Install and use.

4) S700/S75030 operating experience

In Europe, since the birth of the first main circuit breaker S700 in 1982, there have been 30 years of operating experience. Germany's Bavaria is the first designated state to use the SMCB. According to incomplete statistics, there are more than 300 utilities in Germany that use the S700, with more than 10 million installed in the market.

Therefore, the German power supply department has long established the TAB2000 as a technical standard for selective main circuit breakers. In 1996, it also issued the national standard EDINVDE0645 for selective main circuit breakers. In 2000, this standard was revised due to technological development. Since SMCB is widely recognized by various countries in the world, the IEC standardization committee is currently considering the development of corresponding IEC standards for this product.