Influence of distributed generation on fault characteristics and relay protection of rural distribution network

High proportion distributed generation affect traditional rural distribution network. In the paper, a typical line model of rural distribution network with high proportion distributed generation was constructed, and the current and voltage characteristics of rural distribution network after fault were analyzed. At the same time, the compound voltage blocking element and autoreclosure strategy of current protection of rural distribution network were analyzed. The results showed that the short circuit current at the protection location was lower than that without distributed generation, and the voltage of the whole line after the fault was higher than that without distributed generation. In addition, if the autoreclosure strategy of the circuit breaker at the head of the line adopts the non-voltage detection mode, it may cause failure of the breaker reclosing.


Introduction
The new power system has a high proportion of new energy, which brings new challenges to the reliability, security, stability and economic operation of the power grid.Rural distribution network has the characteristics of radial lines, long lines and low load density.The high proportion of distributed generation (DG) connected to the rural distribution network through distributed, multi-point and decentralized forms.It has a great impact on the traditional relay protection and leads to the corresponding changes in the structure and characteristics of the rural distribution network.As the power supply changes from single-terminal to multi-terminal, the fault characteristics of the distribution network will also change [1][2] .The traditional fault calculation method and the setting value of relay protection may no longer be applicable, which may lead to false operation or refused operation of protection relay, affecting the safety of the distribution network [3][4] .

Construction of typical line model for rural distribution network
In order to study of the fault characteristics of the rural distribution network when a high proportion of distributed generation is connected, a typical model of 10kV lines of the rural distribution network is established, as shown in Figure .1.The grid structure of the line is a single radiation line, which has thirteen 400V distribution area, the rated load capacity is 3610kVA.The line has two photovoltaic (PV) power areas (PV1 and PV2, respectively connected to node 15 and node 16).The total rated capacity of the photovoltaic power is 1566kVA.The circuit breaker at the head of the line (node 1) is equipped with instantaneous current protection and definite time current protection.

Analysis of typical line fault characteristics of rural distribution network
The electromagnetic transient simulation software of power system is used for simulation.The transition resistance and fault location were changed respectively, and the fault current characteristics of rural distribution network with and without PV were compared and analyzed.The fault current characteristics of the distribution network were analyzed when the PV position and capacity change.
The fault voltage characteristics of rural distribution network with and without PV were compared and analyzed in the typical scene.On the condition of various transition resistors, when PV is present, the overall trend of fault current flowing through the protection location after fault is reduced compared with the situation which is without PV.When the three-phase short-circuit fault whose transition resistance is 10 ohms occurred, the fault current reduced the most, by 42A, or 8.6%.On the condition of various fault locations, when PV is present, the overall trend of fault current flowing through the protection location after fault is reduced compared with the situation which is without PV.When the two-phase short-circuit fault occurred at node 10, the current reduced the most, by 23A, or 6.4%.When the PV output reduced and the PV location was farther away from the location of relay protection, more fault current flows through the protection location.When a two-phase short-circuit fault occurs, the fault current increases the most, increasing by 15A, or 1.2%.

Voltage characteristic after fault.
A three-phase short-circuit fault was set to occur at node 3 and the transition resistance was 5 ohms.The voltage characteristic of the whole line after the fault is calculated, as shown in Figure .8.When PV is present, there is a voltage rise in the whole line after the fault compared with the situation without PV, and the voltage rise from the fault point to the end of the line is more obvious.The maximum voltage rise occurred at node 9, from 1.834kV to 2.443kV, with a rise of 33.2%.

Influence on the compound voltage blocking element of current protection
The compound voltage blocking element is composed of an interphase low voltage element and a negative sequence overvoltage element with the "OR" logic.The negative sequence overvoltage element is used to judge the asymmetric short circuit, and the phase low voltage element is used to judge the three-phase short circuit.The operation criterion of the compound voltage blocking element is: the minimum value of the three-phase voltage is less than the fixed value of the complex voltage, or the negative sequence voltage is greater than the negative sequence voltage set value.If any of the above conditions are met, the composite voltage blocking element operates.The simulation was carried out under various short-circuit faults at node 3, node 5 and node 10 respectively.The voltage and negative sequence voltage after the fault at the location of the protection installation (node1) were calculated.The simulation results are shown in Table 1.The low voltage setting value of the compound voltage blocking element for current protection at the head of the line was 70V, so the primary side is 7000V (line voltage), thus the corresponding phase voltage is 4041V.The negative sequence voltage setting value was set to 6V, so the primary side is 600V.Combined with the above simulation results, it can be seen that when symmetrical short circuit (three-phase short circuit) occurred, the minimum value of three-phase voltage was less than the low voltage setting value.When asymmetric short circuit (two-phase short circuit, two-phase short circuit grounding) occurred, the negative sequence voltage is greater than the negative sequence voltage setting value.Therefore, the distributed PV has no influence on the compound voltage blocking element of the current protection.

The regulation of the terminal voltage of DG.
The Chinese national standard [5] and several IEEE standards [6][7] required the response characteristics of distributed generation when the grid voltage is abnormal.Set the current grid voltage as U and the required distributed generation off-grid time as T, then the requirements for voltage protection and anti-island protection are as follows: If U≤0.5p.u., T≤0.2s.If 0.5p.u.<U≤0.85p.u,T≤2.0s.When the power grid voltage decreases after the fault, the anti-island protection action time T≤0.2s.At the same time, those standards recommends the requirements for low voltage traverse characteristics of distributed generation: when the voltage is continuously lower than 0.9p.u.The distributed generation can be taken off the grid within 2.0s.
According to the requirements of above standards, if the terminal voltage of the DG is lower than 0.5p.u., the DG should be off-grid within 0.2s.If the terminal voltage is between 0.5p.u. and 0.85p.u., the DG should be off-grid within 2.0s.

Analysis of typical line autoreclosure strategy.
The simulation is carried out under various transient and permanent short-circuit faults occurring at node 3, node 5 and node 10.After the fault, the terminal voltage of PV1 and PV2 was calculated within 0.2s after the circuit breaker was opened.The line is a 10kV feeder, and the terminal phase voltage of the distributed PV is 0.5p.u., corresponding to 2886V.The simulation results are shown in Table 2.The phase voltage of PV1 was named Upv1, the phase voltage of PV2 was named Upv2.The phase voltage of fault phase was named Ufp, the phase voltage of non-fault phase was named Unfp.The reclosing time of the circuit breaker at the head end of the line is 1s.Combined with the above simulation results, the analysis is as follows: In the case of permanent fault, after the circuit breaker at the head of the line tripped, the three-phase voltage of the DG did not exceed 0.5p.u. in the case of three-phase short circuit fault and two-phase short circuit grounding fault.According to the voltage protection requirements of the distributed power supply, the distributed DG will be off-grid within 0.2s, which has no impact on the reclosing strategy.In the case of two-phase short-circuit fault, the three-phase voltage of DG is greater than 0.5p.u..The DG will be off-grid within 2.0s, and the reclosing time setting valve is 1s, so the breaker will close before the DG is off-grid.There may be synchronization problems for the DG of motor type, but it has no impact on the DG of inverter type.
In the case of transient fault, after the short-circuit fault occurs at any position of the line and the circuit breaker at the head of the line trips, the voltage of the DG is greater than 0.5p.u..In this case, the DG will be off-grid within 2.0s.Therefore, the breaker will close before the DG is off-grid.There may be synchronization problems for the DG of motor type, but it has no impact on the DG of inverter type.

Discussion on the non-voltage detection method for autoreclosure.
If the autoreclosure adopts the non-voltage detection mode, when the circuit breaker at the head of the line is opened after the fault, the distributed DG is not off-grid, there is a certain voltage on the line side of the breaker, which will have a certain impact on the non-voltage detection of the autoreclosure.
The no-voltage setting value for autoreclosure was is set to 30V, so the corresponding primary side is 3000V.After the fault occurred, within 0.2s after the breaker at the head of the line was opened, the breaker voltage of the line side was calculated.The simulation results are shown in Table 3.It can be seen from the above results that when the transient fault occurred or the permanent twophase short-circuit or permanent two-phase short-circuit grounding fault occurred, after the breaker at the head of the line was opened, the breaker voltage of the line side exceeded the non-voltage setting value of autoreclosure.It will cause the failure of checking non-voltage and the breaker cannot be correctly reclosed.Countermeasures: the reclosing time can be set to more than 2s.

Conclusions
The high proportion of distributed generation has a certain impact on the fault characteristics of rural distribution network.Compared with the situation without DG, the short circuit current at the location of relay protection decrease, and the voltage of the whole line increase after the fault.In addition, if the autoreclosure of the circuit breaker adopts the non-voltage detection mode, the rise of voltage after the fault may cause failure of the breaker reclosing.It is suggested that in the situation where a high proportion of DG is connected to the rural distribution network, the applicability analysis of relay protection should be carried out in an appropriate way.

Figure 1 .
Figure 1.Typical 10kV lines in rural distribution network.

2. 2 . 1
Different transition resistances.The short-circuit fault occurred on node 3 and the transition resistances were set as 0.01 ohms, 5 ohms and 10 ohms respectively, the simulation of each fault situation is shown in Figure. 2 and Figure.3.

2. 2 . 2
Different fault locations.The transition resistance is set to 5 ohms and the fault points are located at node 3, node 5 or node 10 respectively, the simulation of each fault situation is shown in Figure.4 to Figure.6.

Figure 8 .
Figure 8. Voltage characteristic of the whole line after fault.

Table 1 .
Voltage at the Location of Protection After Fault.

Table 2 .
Distributed PV terminal voltage after the circuit breaker was opend under different fault types.

Table 3 .
Ubk-The breaker voltage of the line side after the breaker at the head of the line was opened.