The safe and reliable operation of a distribution network (DN) is closely related to the development of national economy and the improvement of people's living standards. At present, the design principle of a distribution network is still based on a primary system and primary equipment, without considering the relay protection on the secondary side, which is an important guarantee for the safe and reliable operation of a power grid. The authors theoretically proved the influence of network structures on the main protection (instantaneous overcurrent protection and time-delay instantaneous overcurrent protection) of conventional stage-type current protection, and main influencing factors are analysed and verified in an example. The result shows that reasonable planning and design are an important premise for overcurrent protection to meet sensitivity requirements, and it shows a theoretical and practical guiding significance for the planning of a distribution network. INTRODUCTIONElectricity consumers are directly connected to a distribution network (DN), where a failure rate is high, so the quality of the construction and reformation of a DN directly affects the development of national economy and the improvement of people's material and cultural living standards. DN refers to the network directly connected with users in the power system. Document [1-3] only improves the primary side of the system in the construction of DN, and relay protection, as the secondary side, is relatively weak in the early stage of design. In document [4], it is proposed that the development of relay protection technology should adhere to four performance principles: reliability, rapidity, selectivity and sensitivity. At the same time, it is pointed out that we should abide by this principle in all links of design, manufacturing, construction and operation from the perspective of power grid. Literature [5,6] only considers the influence of reliability unilaterally, and makes decisions on variables such as new lines and upgraded lines of distribution network by analysing the influencing factors. Document [7] designs a protection scheme, which can quickly and accurately judge the fault line, and can improve the selectivity and rapidity of the traditional phased current protection when applied to different scenarios.

No abstract

No abstract

Electrical switching operation in a substation which locates in a high-voltage transmission system alters operating modes of main wiring in either the substation or the system. Major alterations may have negative influences on the switchgear of main wiring in a short time. The quantitative study of this problem has to be based on establishing equivalent circuits of main wiring, when there rarely are formulas to calculate the reactance of tubular busbars. In this paper on the basis of the electromagnetic field theory, the magnetic induction and flux linkages outside and inside tubular conductors are obtained from the Ampere Loop Theorem, and then the formulas to calculate approximately the reactance of tubular busbars with a three-phase parallel arrangement are derived. From the process and results of the calculation in an example it may be seen that the formulas are applied simply, conveniently and rapidly, and may be valuably spread in practical electrical engineering.

No abstract