KR20190008733A - Method of reclosing distribution system - Google Patents

Abstract

Disclosed is a method for reclosing a distribution system. The method for reclosing a distribution system includes a step of determining a load current as fault clearing if the absolute value of a slope of a fault line load current of a line, in which a fault current is generated, is increased to a setting value or more. The method for reclosing a distribution system can reduce a blackout time in comparison with an existing reclosing method which adopts a fixed zero voltage time. In addition, the method can prevent damage to a load and shortening of a lifespan in comparison with an existing reclosing method which performs reclosing after a set time even if a fault current is not cleared, since the method does not perform reclosing when the fault current is not cleared.

Description

{Method of reclosing distribution system}

The present invention relates to a reclosing method of a power distribution system that utilizes a neutral line current and an electrostatic induction phenomenon in a distribution line to shorten the blackout time.

Most failures in the power distribution system are temporary failures. Therefore, reclosing after constant voltage-free time is performed for quick recovery after fault current interruption. A recloser is a device used in the distribution system to shut down the fault current and perform reclosing. As shown in Fig. 1, the reclosing circuit breaker of Korea Electric Power Corporation adopts the no-voltage time of 0.5 second and 15 seconds. When the voltage-free time elapses, the recloser circuit breaker automatically performs the reclosing operation. If the primary reclosing fails, it is automatically reopened and the secondary reclosing operation is performed.

It is very important to distinguish between temporary failure and permanent failure in reclosing operation. In the case of the existing reclosing operation, the reclosing operation is carried out irrespective of whether the failure is eliminated or not. Various methods have been proposed to detect fault removal and reduce non-voltage time in the transmission system. However, these methods can not be applied to the distribution system due to the configuration of the distribution system and the short length. There are very few studies on adaptive reclosing in the distribution system.

'Jun-Hee Park; Hun-Chul Seo; Chul-Hwan Kim; Sang-Bong Rhee. Development of Adaptive Reclosing Scheme Using Wavelet Transform of Neutral Line Current Distribution System. Electric Power Components and Systems 2016, 44 (4), 426-433. ', A wavelet transform is performed and a total harmonic distortion coefficient is calculated to determine fault elimination. However, the calculation process is complicated.

Patent Registration No. 10-0343122 (Date of Notification: July 05, 2002)

A problem to be solved by the present invention is to shorten the power failure time in comparison with the prior art when a temporary failure occurs in the power distribution system.

As a solution to the above problem, the present invention proposes a reclosing method as described below.

A method for reclosing a power distribution system, comprising the steps of: determining that a fault is eliminated if an absolute value of a slope of a fault line load current of a line in which a fault current is generated is increased to a set value or more.

In such a reclosing method, the failure removal determination step may be performed in a state where a breakdown current is generated and the breaker is opened.

In addition, after the fault removal determination step, it may include a reclosing step.

It is preferable that the load current is a current flowing from the neutral wire to the load.

It is preferable that the set value is equal to or larger than the absolute value of the slope of the load current to the normal line.

The present invention can shorten the blackout time compared to the conventional reclosing method employing a fixed voltage-free time.

In addition, since the reclosing is not performed in the state where the fault current is not removed, damage such as damage to the load and shortening of the life can be prevented.

1 is a conceptual view of a conventional jigger operation,
2 shows an equivalent circuit after a breakdown of a fault current,
3 is a schematic diagram of a power distribution system for explaining a reclosing method according to the present invention,
4 is a flow chart of the reclosing method according to the present invention,
5 is a variation of the i _{load_rms} │Δi _{load_rms} │,
Fig. 6 is a graph _showing the relationship _{between the} time duration of the failure, i.e.,? I load_rms,
7 is a graph showing the relationship between the grid current,
FIG. 8 is a graph _showing the relationship _{between the} time duration of the failure, that is, .DELTA.i _{load_rms} ,
Fig. 9 is a graph showing the relationship between the grid current,
FIG. 10 shows a case _where |? I _{load_rms} |
Figure 11 shows the grid current when in a permanent fault.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings.

In the reclosing method of the power distribution system according to the present invention, it is judged that the fault is eliminated when the absolute value of the slope of the fault line load current of the line in which the fault current is generated is increased to a set value or more. In such a reclosing method, the failure removal determination step may be performed in a state where a breakdown current is generated and the breaker is opened. In addition, after the fault removal determination step, it may include a reclosing step. It is preferable that the load current is a current flowing from the neutral wire to the load. It is preferable that the set value is equal to or larger than the absolute value of the slope of the load current to the normal line.

In detail, the distribution system of the Korean distribution system is a three-phase four-wire system and a 22.9 kV multiple-ground system. Most of the distribution systems are operating in unbalanced conditions. As a result, the neutral current always flows in the steady state. Which means that the neutral current will always flow even if a symmetrical fault occurs. Therefore, in the present invention, a method utilizing this current is proposed.

Therefore, in the present invention, to propose a new adaptive reclosing method in an unbalanced power distribution system, an electrostatic induction phenomenon between a neutral line and a distribution line is analyzed based on an equivalent circuit. Based on this analysis, we will show the configuration and algorithm presentation of the distribution system, the simulation, and the results to propose a new reclosing method.

≪ Induction of electrostatic induction between a neutral wire and a power distribution line &

The electrostatic induction between the neutral line and the distribution line is analyzed. Fig. 2 shows an equivalent circuit after a fault current interruption. Figure 2 assumes that the fault phase is A and the dry phase is B and C phases. The meaning of each parameter is as follows.

Even if the faulty part is disconnected from the distribution line, the neutral current due to the unbalanced state still flows because the area other than the fault is supplied with the normal current. Electrostatic induction phenomenon occurs between the neutral line and the distribution line due to the current flowing in the neutral line. In FIG. 2, the current flowing in the neutral line is divided into three phase currents due to the induction of static electricity.

The current flowing on A is expressed by Equation (2).

The currents flowing in B phase and C phase are as follows.

이 수학식 3의

보다 작기 때문에 수학식 2의 값은 수학식 3의 값보다 크다. 고장이 제거된다면, 수학식 2의 값과 수학식 3의 값은 같아지게 된다.Since the mutual capacitances C _Aneu , C _Bneu , and C _Cnu are very small, the difference in reactance in equations 2 and 3 is also very small. Therefore, it is the characteristic impedance of the line and the fault resistance that determine the relative magnitude of the current flowing in each phase in equations (2) and (3). The fault resistance is usually less than the characteristic impedance of the line. Thus, during a failure, in equation (2)

In Equation (3)

The value of Equation (2) is larger than the value of Equation (3). If the fault is eliminated, the value of Equation 2 and the value of Equation 3 become equal.

In other words, the value of Equation 2 has a large value during a failure, but it is changed to a small value due to failure elimination. This characteristic is utilized to detect fault removal.

<Adaptive reclosing method in unbalanced distribution system>

The present invention proposes a new adaptive reclosing method. In this method, the grid current (i _sys ) and the load current (i _load ) are input to the protection relay as shown in Fig. A communication method is also required. Since the existing recloser circuit breaker can not perform adaptive reclosing, it is replaced by a circuit breaker and a protective relay.

A flow chart of the proposed method is shown in FIG. First, calculate the effective value after receiving the grid current and the load current. If the grid current is greater than a certain value (α), the circuit breaker is opened to disconnect the distribution line. As shown in Fig. 2, the current flowing in the distribution line due to electrostatic induction will flow to the load side. Therefore, the load current is input and the rms value is calculated. The absolute value of the derivative of the rms value is then calculated.

Figure 5 shows a variation of the i _{load_rms} │Δi _{load_rms} │ failure to remove from the failure. During a fault occurrence, i _{load_rms} does not change, and therefore, | i _{load_rms} | However, if the fault is removed, the value of i _{load_rms} changes to a very small value, so that | i _{load_rms} | If | i _{load_rms} | increases above a certain value (?), The duration threshold increases. If the sustainability factor increases above ε, it is determined that the fault is finally eliminated and the re-close is performed immediately.

<Simulation and Review of Results>

System model:

The proposed method is proved in the distribution system of Fig. The capacities of loads 1 and 2 are 1000 Kw and 2000 Kw. The length and type of track 1 and 2 are 10 km and ACSR 95 mm2.

The distribution system is modeled using EMTP / ATPDraw, and the proposed method is implemented with EMTP / MODELS. The sampling frequency is 120 samples / cycle.

Simulation conditions:

In the present invention, simulation was performed while changing the unbalance rate and the failure condition. The unbalance rate is calculated as shown in Equation (5).

Currently, the distribution system of KEPCO limits the unbalance rate to 20%. Therefore, the unbalance rates of 5% and 10% were selected. 1-wire ground fault and 3-phase ground fault are simulated. In addition, as shown in Table 1, various fault resistance, failure duration, and fault location are simulated. In all cases, failure occurs in 0.1 second. If the fault duration is 20 cycles, the fault is removed in 0.433 seconds, and for 40 cycles, it is removed in 0.766 seconds.

Simulation condition Unbalance rate Failure type Fault resistance Failure duration Fault location
5%
10%

1 pre-lock failure
3 upper limb lock failure
0.1Ω
10Ω
100Ω 20cycle
40cycle
pemanent 10%
50%
90%

Simulation results:

Table 2 shows the simulation results. Fault elimination is judged according to the proposed algorithm regardless of the unbalance rate and fault condition, and the reclosing is also successfully performed. The reclosing is performed after 1/2 cycle without waiting for the non - voltage time after fault clearing. Even at 5% unbalance rate (ie, when the neutral current is very small), the reclosing is performed adaptively after the fault removal judgment. It can also be seen that the reclosing is successful regardless of the type of failure since the neutral current always flows regardless of the symmetrical and asymmetrical faults. In addition, the reclosing is successfully performed not only in the high-resistance 100-ohm fault resistance but also in the very low fault resistance of 0.1-ohm. In addition to the failure near the power supply end, . It can also be seen that the reclosing is performed at the same time if the failure duration is the same regardless of the unbalance rate, fault type, fault resistance, and fault location. In a permanent failure, reclosing is not performed.

Simulation results 1 Unbalance rate (%) Failure type Fault resistance (Ω) Failure duration (cycles) Failure location (%) Reclosing time (s) 5 1 Prelude 0.1 20 10 0.441 50 0.441 90 0.443 40 10 0.775 50 0.775 90 0.776 everlasting 10 - 50 - 90 - 10 20 10 0.441 50 0.441 90 0.443 40 10 0.775 50 0.775 90 0.776 everlasting 10 - 50 - 90 - 100 20 10 0.441 50 0.441 90 0.443 40 10 0.775 50 0.775 90 0.776 everlasting 10 - 50 - 90 -

Simulation results 2 Unbalance rate (%) Failure type Fault resistance (Ω) Failure duration (cycles) Failure location (%)
Reclosing time (s)
5 3-phase ground fault 0.1 20 10 0.441 50 0.441 90 0.443 40 10 0.775 50 0.775 90 0.776 everlasting 10 - 50 - 90 - 10 20 10 0.441 50 0.441 90 0.443 40 10 0.775 50 0.775 90 0.776 everlasting 10 - 50 - 90 - 100 20 10 0.441 50 0.441 90 0.443 40 10 0.775 50 0.775 90 0.776 everlasting 10 - 50 - 90 -

Simulation results 3 Unbalance rate (%) Failure type Fault resistance (Ω) Failure duration (cycles) Failure location (%)
Reclosing time (s)
10 1 Prelude 0.1 20 10 0.440 50 0.440 90 0.441 40 10 0.774 50 0.774 90 0.775 everlasting 10 - 50 - 90 - 10 20 10 0.440 50 0.440 90 0.441 40 10 0.774 50 0.774 90 0.775 everlasting 10 - 50 - 90 - 100 20 10 0.440 50 0.440 90 0.441 40 10 0.773 50 0.773 90 0.774 everlasting 10 - 50 - 90 -

Simulation results 4 Unbalance rate (%) Failure type Fault resistance (Ω) Failure duration (cycles) Failure location (%)
Reclosing time (s)
10 3-phase ground fault 0.1 20 10 0.440 50 0.440 90 0.441 40 10 0.774 50 0.774 90 0.775 everlasting 10 - 50 - 90 - 10 20 10 0.440 50 0.440 90 0.441 40 10 0.774 50 0.774 90 0.775 everlasting 10 - 50 - 90 - 100 20 10 0.440 50 0.440 90 0.441 40 10 0.773 50 0.773 90 0.774 everlasting 10 - 50 - 90 -

In order to demonstrate the superiority of the proposed method, the simulation results of the proposed method and the simulated results of the existing reclosing method are compared. The conditions for comparison are a 10% unbalance rate, 1 line fault, 0.1 ohm fault resistance, and 10% fault location. For the existing reclosure method, wait for 0.5 and 15 seconds and then perform two reclosing. In order to shorten the simulation time, simulation was performed by setting 1.5 seconds instead of 15 seconds. The meaning of the numbers in each waveform is as follows.

(1): Failure occurred

(2): Breaker open due to fault

(3): the reclosing time of the proposed method

(4): The first reclosing time of the existing reclosing method

(5): Second reclosing time of existing reclosing method

Figs. 6 and 7 show the effective value waveform of the current and the current _i i _{load_rms} when the failure duration is 20 _cycles . After the fault has been removed at 0.433 _s , │Δi _{load_rms} │ rapidly increases, and fault elimination is therefore judged. As shown in FIG. 7, the reclosing of the proposed method is performed 0.44 seconds after the fault removal judgment. However, in the case of the existing reclosing, reclosing is performed at 0.65 seconds after the fixed voltage-free time of 0.5 seconds. Therefore, the blackout time can be shortened by the proposed method.

8 shows? I load_rms when the failure duration is 40 _cycles . │Δi _{load_rms} │ increases sharply after the fault is removed at 0.766 sec. According to the proposed method, the fault clearance is determined at 0.773 seconds and the reclosing is performed immediately. The rms value waveform of the system current is shown in Fig. In the proposed reclosing method, reclosing was successfully performed at 0.773 seconds and normal current was supplied to the load. In the case of the existing recloser, the recloser is attempted at 0.65 seconds before the failure is removed. Since the fault has not been removed, a large fault current flows again, and thus the circuit breaker is opened again at 0.7 seconds. The second recloser runs at 2.2 seconds after 1.5 seconds and a steady current flows through the load. It can be seen that the blackout time is shortened by 1.45 seconds by the proposed method. If the non-voltage time of 15 seconds is applied in KEPCO, the blackout time can be further reduced by the proposed method.

10 shows a │Δi _{load_rms} │ when permanent failure. This value remains 0 because the fault has not been removed. Figure 11 shows the rms value waveform of the grid current when it is a permanent fault. For the proposed method, reclosure is not performed. In the case of an existing reclosure, two reclosures are performed but all fail and the breaker is reopened. In case of reclosing, a large fault current flows in the electrical equipment such as line, insulator, etc., which may cause damage such as equipment damage and shortened life span.

As described above, the power system is operated in an unbalanced state due to the load imbalance. Therefore, the neutral current will always flow. In the present invention, an adaptive reclosing method based on this neutral current and the electrostatic induction in a distribution line is proposed. Based on the equivalent circuit analysis of the electrostatic induction phenomenon, the difference between the failure phase and the healthy phase is analyzed. Based on this result, we propose a method to determine the failure elimination using the increase of the absolute value of the differential current of the load current and to perform the reclosing.

In order to prove the proposed method, various simulations based on the unbalance rate, fault type, fault resistance, failure duration, and fault location were performed. It was confirmed that the reclosing was successful in all the results. In addition to this, we have demonstrated the superiority of the proposed method by comparing it with the existing reclosing method.

While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. This is possible.

Therefore, the scope of the present invention should not be limited to the described embodiments, but should be determined by the equivalents of the claims, as well as the claims.

Claims (5)

A method of reclosing a power distribution system,
Determining that the fault is eliminated when the absolute value of the slope of the fault line load current of the line where the fault current is generated is increased beyond the set value;
Wherein the reclosing of the power distribution system comprises:
The method according to claim 1,
Wherein said step is performed in a state in which a fault current is generated and the circuit breaker is opened.
The method according to claim 1,
After this step,
Reclosing;
Further comprising the steps of:
The method according to claim 1,
Wherein the load current is a current flowing from the neutral line to the load.
The method according to claim 1,
Wherein the set value is equal to or greater than an absolute value of the slope of the load current to the normal line.

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