The diagram of the low-voltage distribution system is shown in the figure below
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Although the code for design of power supply and distribution system (GB 50052-2009) has provisions on distribution series, it does not give a reasonable definition of distribution series
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In order to maintain good selectivity and economy, generally speaking, the next switch in the same level can be replaced by an isolating switch, such as C1, C2, E1 and E2
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According to the specification, the same voltage of high-voltage distribution system should not be more than two levels of distribution, for example, from 10kV substation to 10kV substation is one level, and from 10kV substation to building transformer is one level, with a total of two levels
.
“If there are too many power distribution stages in the power supply and distribution system, not only the management is inconvenient and the operation is frequent, but also because of too many series components, the possibility of accidents caused by component failures and operation errors will increase
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That is to say, the incoming switch and outgoing switch of a power distribution device are called the first level together, and the power distribution level will not be changed because its incoming switch adopts circuit breaker or disconnector
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For example: low voltage distribution of substation: the incoming line is one level, and the outgoing line is one level, which is two levels
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Switch A1 and switches B1, B2, B3 and B4 constitute the primary distribution
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Generally speaking, to realize the selectivity between superior and subordinate circuit breakers, the setting value and delay of superior switch can be adjusted
.
This is often controversial in engineering design
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If B1 switch and C1 switch are circuit breakers, then B1 and C1 are circuit breakers of the same level, and the selectivity between them does not need to be considered, because the blackout range caused by B1 or C1 tripping is the same, so the selectivity between them does not need to be considered
.
In the low-voltage distribution system, for example, from the 380V substation on the low-voltage side to the low-voltage room of the building, from the low-voltage room to each distribution box, and from the distribution box to the electrical equipment, there are three levels in total
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The concept of distribution series and protection series
.
Therefore, protection series and distribution series can not be confused and must be understood separately
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For different types of loads, the selectivity of protection is also different
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In the figure, AP1, ap2 (AP3) and AP4 (AP5) distribution boxes are 380V primary distribution box, secondary distribution box and tertiary distribution box of a building respectively
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It is not allowed to increase the cost by unconditionally realizing the selectivity, and it is not allowed to reduce the reliability of power supply without considering the selectivity in important loads
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When meeting the requirements of the specification, the number of distribution stages should be reduced as much as possible
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The design of power supply and distribution system should strive for simple system, reasonable setting of protection level and distribution level, ensuring personal safety, reliable power supply, advanced technology and reasonable economy
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In AP1 distribution box, A1 switch is incoming switch and B1 ~ B4 is outgoing switch
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Similarly, it can be seen that the low-voltage distribution system is a tertiary distribution system
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When the terminal line of F1 switch is short circuited or overloaded, F1 switch should be turned off, and other switches will not be affected
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A definition of distribution series should be given, which is: “a power supply circuit is divided into several circuits through the distribution device, which is called primary distribution”
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Selectivity is achieved to a certain extent, it is difficult to ensure that the circuit breaker in the whole building can achieve 100% selectivity or can only increase the cost to achieve..
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There are basically two views, one is that the protection series is the distribution series
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The same is true for distribution boxes
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If the switch is circuit breaker, the selectivity between upper and lower circuit breakers should be considered
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The diagram of protection series and distribution series takes the low-voltage distribution system as an example to illustrate the distribution series and protection series
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If F1 switch can not act or turn off at the same time with E1, it will expand the scope of power failure and cause unnecessary economic loss
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According to the code for design of low voltage power distribution (GB 50054-2011), selectivity must be considered for important loads, and partial selectivity or no selectivity can be considered for non important loads
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As can be seen from the figure, since AP1: L1 is the incoming line of AP2 distribution box, B1 and C1 can be considered as switches of the same level
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The use of disconnectors can only reduce the number of protection stages, not the number of distribution stages
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Obviously, the design of low-voltage distribution system is difficult to meet the requirements
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In some large projects, if there are too many protection levels, it will cause more trouble in property management, and low-voltage distribution should consider economy and selectivity, so it is appropriate to reduce the protection level
.
Similarly, D1 and E1 are switches of the same level
.
