Research for the Influence of Distribution Network Line Reclosing Current on Line Protection

According to the distribution network line structure and reclosing control strategy, the system simulation model of distribution network lines has established based on the realtime digital simulation RTDS. Based on this, distribution network switching impulse current characteristic has researched with different capacity, different distribution and different load power factor under the different voltage switching angles. The results of the study provide a scientific basis for distribution network line protection setting, in order to further lay the foundation for improvement the validity and reliability of distribution network line protection action.


Introduction
With the continuous random access of residential load and commercial unit into distribution network, the load current of distribution network increases constantly.Meanwhile, grid-connected operation of new distribution transformers which come into service recently has to some extent changed the system structure and power flow distribution.Therefore, a higher demand of safe and reliable operation of power network protection and control equipment is proposed.For example, the reclosing impact current of distribution network line will change, and the amplitude and fall time of impact current will change to a certain extent [1,2].Post-acceleration protection setting of distribution network was studied deeply and comprehensively in literatures [3][4][5], however, only the determination of postacceleration protection set point is studied from the single aspect of setting calculation, but the calculation did not involve the distribution situation of distribution transformer capacity of distribution line, as well as the parameters and load character of distribution line.Obviously, the accuracy level of post-acceleration protection set point of setting is poor, which then lead to the reduction of the success rate of reclosing of distribution line.
The establishment of technology test platform for distribution network line based on real-time digital simulation technology and line protective device is an efficient way to solve above problem [6].In this paper, 10kV system distribution line is taken as research object, a distribution line real-time simulation platform based on RTDS is established, and on this bases, the characteristics of closing impact current of distribution line under different distribution of distribution transformer capacity, different load power factor of low voltage side of the distribution transformer and different voltage closing angle is studied, and the main factors which influence the characteristics of closing impact current is sorted out.

Modular Design of Real-time Simulation
RTDS model includes 110kV infinity system model, 110kV-10kV step-down transformer model, 10kV switch model, 10kV distribution network line model, 10kV transformer model and 0.4kV low voltage load model.When modeling, simulation precision requirement of single transformer's magnetizing rush current should be met, meanwhile, limits on the number of compute nodes by hardware of simulation system should be considered.Therefore, it is best to use the modularized design method, and to optimize the allocation of RTDS processor cards.

Modeling Progress of RTDS
Real-time simulation modeling of distribution network lines can be divided into four parts: main loop of the distribution network wiring system, breaker analog output model, breaker digital input and output model, and fault control strategy model.Main loop of the distribution network wiring system uses infinite power to replace 110kV power network system.The control strategy model is established according to corresponding expressions and control structure diagram and based on the component model library of RTDS control system.

Modular Design of Real-time Simulation
In the analysis of the influence from distribution network line reclosing impact current to the line protection, two important parameters should be mainly considered, which are over current ration in acceleration section and delay time ration of independent setting in acceleration section.In simulation modeling, the lines and transformer model which determine the current current ration can be built correctly, but fall time of distribution transformer magnetizing rush current, which is related to time setting of protection in acceleration section, is unable to be obtained from known parameters of distribution transformer.It is required to measure the distribution transformer on field to obtain the fall time of magnetizing rush current and achieve the parameter identification of distribution transformer in simulation experiment.In the experiment, the field measurement is according to capacity of 4MW, 8MW and 10MW.The constant identification [7] of fall time of distribution transformer is approximated as 0.3.

Scheme on Real-time Simulation Test
To research the influence of distribution network line's transformer capacity, distribution, load power factor and closing angel to distribution network line closing impact current, bracketing method is used in the real-time simulation experiment, as shown in Table 1, where: (1) "136" denotes that the distribution of transformers of distribution network line is roughly that the transformer capacity close to exit section (forepart) of transformer substation is low, the capacity of distribution transform at end of line is high, and the capacity of distribution transform at mid-piece of line is between two parties.
(2) "334" denotes that the distribution of capacity of distribution transform at exit section of transformer substation, mid-piece and end of line is uniform.
(3) "631" denotes that the distribution of transformers of distribution network line is roughly that the transformer capacity close to exit section (forepart) of transformer substation is high, the capacity of distribution transform at end of line is low, and the capacity of distribution transform at mid-piece of line is between two parties.

Characteristic analysis of distribution network line no-load closing impact current
In the simulation, the total capacity of distribution transformer is 8MVA, and after cutting of the switch while short circuit fault happens, the residual magnetic of distribution transformer is kept in a low level, the experiment data are shown in Table 2. Table 2 shows that under same closing angle, different distribution will lead to different closing impact current, and the higher capacity away from power side (outlet switch of transformer substation), the higher closing impact current is.For same distribution, the amplitude of closing impact current is different under different closing angle.The amplitude of closing impact current decreases with increase of closing angle, and the amplitude of closing impact current is maximum when 0°.

Characteristic analysis of impact current under different power factor and load feature
In the simulation, the total capacity of distribution transformer is 8MVA, and the distribution is "334", the experiment data are shown in Table 3. Table 3 shows that, considering the error of simulation, under same closing angle, the amplitude of impact current under different distribution, same capacity load and different power factor keeps essentially constant.It can be confirmed that under same load capacity, different power factor will not cause large change of amplitude of closing impact current.

Characteristic analysis of impact current under different distribution of distribution transformers
In the simulation, the total capacity of distribution transformer is 8MVA, and the power factor is 0.5, the experiment data are shown in Table 4. Table 4 shows that under same closing angle and different distribution of distribution transformers, the closing impact current is different.The characteristic is represented as that under the same closing angle, the higher distribution transformer capacity away from power side, the higher closing impact current is.

Characteristic analysis of impact current under different capacity of distribution transforms
In the simulation, the power factor of low voltage of distribution transformer is 0.5, and the experiment data are shown in Table 5.Table 5 shows that, under same closing angle and same distribution of distribution transforms, for different total distribution transform capacity, the closing impact current is different.The characteristic is represented as that under same closing angle, the higher total distribution transform capacity, the higher closing impact current is.Explanation: in the above four tables (Table 6-9), the first column denotes the maximum load current value of the line; the second column denotes post-acceleration protection setting value, which is 1.4 times of maximum load current; the first row denotes different typical distribution transformer total capacity, where 4M denotes the total capacity of distribution transformer is 4MW.
Through the simulation analysis of four typical lines, we find that there are some regular pattern of fall time of impact current under same capacity but different distribution, which is represented as that the higher capacity away from power source, the longer fall time of impact current is.

Figure 1 .
Figure 1.Distribution transformer no-load test(Left is measured waveform, right is simulation waveform)

Table 1 .
Table of experimental factors

Table 2 .
The maximum peak of no-load current

Table 3 .
The maximum peak of current under different power factor

Table 4 .
The maximum peak of current under different distribution

Table 5 .
The maximum peak of current under different capacity

Analysis of Fall Time of Closing Impact Current On
the basis of above studies, we select four typical lines with different distribution transformer capacity, under different closing angles, to analyze the needed time that closing impact current falls to selected current value.(1)"631"distribution:

Table 6 .
Fall time of impact current under "631" distribution

Table 7 .
Fall time of impact current under "136" distribution

Table 8 .
Fall time of impact current under "334" distribution

Table 9 .
Fall time of impact current under "361" distribution

Table 10 .
Fall time of impact current under different setting value and different capacityIn this paper, a distribution network line reclosing simulation model is established based on real-time digital simulation technology.Through packaging design of submodule, the advantages of RTDS modeling, such as expansibility and easy to be used in closed-loop test, are made good use.The realtime simulation experiment platform is established, then on this base, characteristics of reclosing impact current under different distribution transformer capacity distribution and different operation mode are researched, and magnetizing rush current fall time reference table under four typical capacity distribution is proposed, which provide the basis for post-acceleration setting of distribution network line relay protection device, ensure the protection device to avoid the magnetizing rush current and mistrip, and improve the success rate of distribution network line reclosing and the power supply reliability.