Technological breakthrough of miniature circuit breakers

Selective coordination and backup coordination

Selective coordination and backup coordination have been discussed a lot in various magazines and literature. Here we only discuss a few points from the perspective of application:

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

2) The means of implementing selective coordination and backup protection are different. If we only look at the characteristics of changing the upper overcurrent protection device (circuit breaker for example):

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

B) Usually speed up the action of the upper circuit breaker to limit the short-circuit energy, thereby providing backup protection for the lower circuit breaker

Of course, the means to improve the selective coordination of superiors and subordinates are not limited to the coordination of action time, but can also be based on current coordination, energy coordination, logic coordination, etc.

Of course, it is best if the circuit breaker itself can provide good selectivity and backup protection for other overcurrent protection devices.

3) The purpose of requiring selective coordination is to reduce the scope of power outage due to faults; the purpose of requiring backup coordination is to reduce investment costs.

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

Note: Low-voltage overcurrent protection devices are mainly circuit breakers. If not specified below, circuit breakers will be used to refer to overcurrent protection devices.

Current status of terminal power distribution continuity

From the current situation of terminal power distribution, the degree of specialization of cable laying and equipment installation is not high, but the distribution range of electrical equipment is wide, and random wiring is common; there are many non-professional users, overloading is common, and the probability of overload or short circuit failure is very high. Especially during peak load periods, the power supply department is busy dealing with various tripping and power outage accidents. Because terminal power distribution rarely considers selective coordination, a single short circuit failure may cause a power outage on the entire floor or building, which makes the power supply department's fault location and power restoration work even more difficult.

From the perspective of electrical design, there are also misunderstandings about the protection against electric shock injuries inside general residential units. Some designers only consider installing a residual current protection device at the main switch of the house, or the entire residential unit is powered by only one circuit, or the time difference between the upper and lower circuit breakers is not considered. These may cause power outages in the whole house due to short circuits or grounding faults, including failure of security system power outages, power outages of refrigerators and life support systems (fish tanks), network interruptions (router power outages), etc.

Outside of a general residential unit, designers may not consider the selective coordination of overcurrent protection devices on the power supply side. There are household fuses or circuit breakers installed at the user's entrance, but they are under the jurisdiction of the power supply department. If these overcurrent protection devices malfunction due to poor selective coordination (the lower-level circuit breaker has cut off the fault), it will obviously unnecessarily increase the workload of the power supply department for on-site maintenance and greatly increase the power outage time due to faults.

Research approaches to selective coordination

Since selective coordination is becoming increasingly important in terminal power distribution, this article mainly discusses selective coordination. Generally speaking, there are two research approaches to study selective coordination, but they support each other and can be used together:

A) Find ways to improve selectivity by studying the coordination between upper and lower circuit breakers

B) Find ways to improve selectivity by studying improvements to the circuit breaker structure itself

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

The third or fourth level is terminal distribution, where selectivity or continuity of power supply is rarely considered due to cost reasons. The load distribution at this level is complex and the users are not professionals. A cost-effective and easy-to-implement solution is needed to ensure the growing continuity of power supply. In these cases, we often adopt the B) research approach, that is, to develop a new generation of circuit breakers to meet the selectivity requirements.

As early as 2004, at the "China Intelligent Building Qingdao Electric Salon" exchange meeting organized by China Intelligent Building Technology Information Network, China Building Design and Research Institute and ABB China Co., Ltd., Mr. Bernd Siedelhofer, a senior expert from ABB Germany, introduced the structure, principle and application of S700 selective overcurrent protection circuit breaker. The experts at the meeting gave positive affirmation to ABB's S700 SMCB with full selective function, and emphasized that the requirements for selective coordination should be improved in domestic terminal distribution, and the mature application of ABB's S700 SMCB is a good reference.

The confusion of improving the selective matching ability of terminal appliances

According to the IEC60898/GB10963 technical standard, the miniature circuit breaker (MCB) for terminal power distribution generally needs to act quickly to cut off the short-circuit fault as soon as possible. In other words, it does not need to withstand the "rated short-time withstand current Icw" as specified in IEC60947/GB14048, so the volume of MCB with the same current level is much smaller than that of MCCB.

If you want to improve the selective coordination ability of the upper and lower MCBs, you need to increase the time that the upper MCB can withstand the short-circuit current so that the lower MCB can quickly cut off the short-circuit current in the fault area; at the same time, in order to ensure that the "grandfather" short-circuit protection device does not malfunction due to excessive short-circuit current and long duration, it is also necessary to control the current limiting characteristics and action time of the upper MCB. In other words, the selective overcurrent protection circuit breaker SMCB should solve the selectivity problem between multi-level protection devices. The S700/S750 SMCB developed by ABB has made a successful breakthrough in technology, solving the problem of "you can't have your cake and eat it too".

Functions of S700/S750 SMCB - Selective + Backup Protection

SMCB is the acronym for the English definition of "selective overcurrent protection circuit breaker".

The design objective of the SMCB is to allow the use of a load-side circuit breaker with a breaking capacity lower than the expected short-circuit current at the installation point, while ensuring full selectivity, when it is coordinated with an ordinary circuit breaker.

When a short circuit occurs, the contacts of the circuit breaker on the power supply side are temporarily separated to achieve the purpose of current limiting. When the protection release has not yet been actuated but the lower circuit breaker is "guaranteed" to be disconnected, the contacts are connected again to maintain power supply. At the same time, the action mechanism of the circuit breaker on the power supply side should have a delay function.

In addition, the selective coordination of the SMCB and the grandfather-level short-circuit protection device (fuse) must also be verified.

Obviously, specially designed contact systems and tripping systems are required to achieve the above objectives.

The picture on the right is a simplified diagram of the operating principle of the S750 SMCB.

If the short circuit occurs at the load end of the lower MCB, due to the line impedance, the short circuit current cannot be too large, but it is enough to trip the lower MCB (C special tripping characteristics: 5~7 In). Under the electromagnetic force of this short circuit current, the main contacts of the SMCB will produce a repulsive current limiting phenomenon, but it is not enough to trip the tripping mechanism of the SMCB; 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 operate but not to trip the SMCB substantially. After the lower MCB completely cuts off the short circuit current, the main contacts of the SMCB return to the closed position and continue to provide power to other loads. In the event that the MCB refuses to move or cannot be disconnected, the selective thermal element causes the SMCB to trip after a slight delay (<300mS).

If a short circuit occurs at the load end of the SMCB, the expected short-circuit current is quite large (depending on the short-circuit capacity), which may reach tens of kiloamperes. At this time, the SMCB can bear its maximum breaking capacity, and the short-circuit current electromagnetic force drives the tripping mechanism to break quickly.

The figure on the right is a schematic diagram of the three-step action process of SMCB when the lower-level MCB is disconnected.

1. Before the failure: Normal operation

2. Fault: Load short circuit, SMCB+MCB current limiting, MCB action

3. After the fault: SMCB is still connected (satisfying selectivity), MCB is disconnected

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

Similarly, S700/750 also improves the selectivity with grandfather-level short-circuit protection devices. When grandfather-level fuses are properly configured, the selectivity current limit between it and SMCB can reach up to the Icn value of SMCB.

S700/S750 Tripping Characteristics Selection

According to VDE0645/GB24530 technical standards, S700/S750 recommends using E tripping characteristics because it has a relatively sensitive overload protection tripping curve, which can effectively extend the service life of the cable and reduce the possibility of fire caused by cable overheating. Another advantage is that the compromise short-circuit protection tripping curve can avoid being forced to enlarge the cable due to insufficient short-circuit current value at the far end of the cable; it can also avoid malfunctioning due to interference from the starting current of the motor load.

In circuits with very high starting current loads, it is also possible to consider using SMCBs with Cs tripping characteristics. Fortunately, in general terminal distribution circuits, very high starting current loads are not common.

S700/S750 Other performance requirements

SMCB is mainly used for residential terminal MCB pre-stage protection, or in situations where both selectivity and backup protection are required, to achieve the purpose of selectivity + backup protection. Abroad, SMCB is often installed on the incoming line side of low-voltage users, and should comply with the overvoltage classification of IV to ensure the safety of circuit isolation, which is a requirement that general MCB (overvoltage classification of III) cannot meet.

With the expansion of low-voltage power grid capacity and the increase of expected short-circuit current, SMCB is also required to have a higher breaking capacity, such as 25kA. According to the requirements of IEC60439, the breaking capacity of the circuit breaker at the same installation point cannot be lower than the expected short-circuit current, which inevitably requires the MCB installed at the same location to have a high breaking capacity. From the perspective of MCB manufacturing technology and distribution system cost, this is also difficult to achieve. However, because SMCB can provide backup protection, it allows the use of MCBs with Icn less than the expected short-circuit current, providing an economical and reasonable solution for this.

S700/S750 adopts the special internal structure of SHU which is independent of voltage. The selectivity function it provides is very stable and is not affected by power supply voltage fluctuations or interference. In addition, it does not contain sensitive electronic components and has good electromagnetic compatibility. In terminal power distribution, there is almost no possibility of false operation.

The S700/S750 developed by ABB meets the installation and use environment requirements of pollution level 3 and -25℃~+55℃, which is more severe than the installation and use environment of MCB. It is particularly suitable for installation and use in cold and high temperature areas in China, and is also suitable for installation and use in industrial environments.

S700/S750 30 years of operating experience

In Europe, the first main circuit breaker S700 was introduced in 1982 and has been in operation for 30 years. Bavaria in Germany was the first state to specify the use of SMCB. According to incomplete statistics, more than 300 utilities in Germany have specified the use of S700, with more than 10 million units installed in the market.

Therefore, the German power supply department has long formulated TAB2000 as the technical standard for selective main circuit breakers, and promulgated the national standard E DIN VDE 0645 for selective main circuit breakers in 1996. This standard was revised in 2000 due to technological development. As SMCB is widely recognized by countries around the world, the IEC Standardization Committee is currently considering formulating corresponding IEC standards for this product.