A petal-shaped distribution network inverse-time overcurrent protection method based on adaptive correction of characteristic parameters

Abstract

The present invention relates to a petal-shaped distribution network inverse-time overcurrent protection method based on adaptive correction of characteristic parameters. An adaptive correction inverse-time operating characteristic equation applicable to the petal network is constructed based on the characteristic parameters, aiming to make full use of the variation law of the fault current with the movement of the fault location when a fault occurs in the main trunk line of the petal network. The proposed method has good quick-acting performance, and the upper and lower level protections can maintain a constant operating time difference. It has wide applicability, reliable performance, and is simple and easy to implement.

Description

Technical Field

[0001] The invention belongs to the field of relay protection of distribution networks in power systems. Based on the unique fault characteristics of petal-shaped distribution networks, a method for inverse-time overcurrent protection of petal-shaped distribution networks based on adaptive correction of characteristic parameters is proposed. Background Art

[0002] As an important part of the power grid, the distribution network shoulders the important responsibility of ensuring the electricity demand for people's livelihood and social development in the region. It is one of the most important infrastructure in the city, and the grid structure is the key factor affecting the power supply reliability level of the distribution network. Traditional distribution networks mostly adopt a radial grid structure with open-loop operation. However, with the gradual improvement of the grid automation level, the disadvantages of open-loop operation of the distribution network have gradually emerged. During line faults or maintenance, the open-loop power supply mode cannot avoid short-term power outages of some loads and cannot meet the stringent requirements of users in high-tech areas for power supply. In contrast to the radial distribution network with open-loop operation, the petal-shaped distribution network (petal network) with closed-loop operation has certain advantages in terms of power supply reliability and power quality. Drawing on the power supply mode of the distribution network in Singapore, which is at the world leading level in power supply reliability, petal networks have been put into trial operation in some areas such as Suzhou and Guangzhou in China, and the Xiongan New Area is also researching and constructing petal networks.

[0003] Different from traditional radial distribution networks, the petal network with closed-loop operation has the characteristics of bidirectional flow of power flow and fault current. The fault current on the main line of the petal network is correlated with the line impedances on both sides of the fault point. The change in the line impedance on one side of the fault point will affect the fault current on the other side. Moreover, when the line impedance on one side of the fault point approaches zero, the fault current flowing through the line on the other side will also approach zero, posing a severe challenge to the overcurrent protection of the petal network. There are many limitations when existing overcurrent protection methods are applied to the petal network. The definite-time overcurrent protection has problems such as a long protection operation time and being greatly affected by the "weak feed" characteristic of the petal network. Although the inverse-time overcurrent protection with a standard inverse-time operating characteristic can reduce the protection operation time to a certain extent, the protection quick-acting performance has not been significantly improved, and there are still problems such as the protection operation time being greatly affected by the fault type and the operation time increasing significantly when acting as a remote backup. Therefore, for the petal network with closed-loop operation, the invention proposes a method for inverse-time overcurrent protection of petal-shaped distribution networks based on adaptive correction of characteristic parameters, which greatly improves the protection quick-acting performance on the premise of ensuring protection selectivity. The invention does not require communication and only needs to collect the steady-state current values flowing through each protection point of the petal network, without installing voltage transformers, and is easy to implement in engineering. Summary of the Invention

[0004] The present invention aims to make full use of the variation law of the fault current with the movement of the fault location when a fault occurs in the main line of the petal network, construct an adaptive correction inverse-time operating characteristic equation applicable to the petal network based on characteristic parameters, and propose an improved inverse-time overcurrent protection method for the petal network with good speed performance and a constant operating time difference between upstream and downstream protections.

[0005] The present invention solves its technical problems through the following technical solutions:

[0006] An inverse-time overcurrent protection method for a petal-shaped distribution network based on adaptive correction of characteristic parameters, specifically including the following aspects:

[0007] (1) Protection devices and circuit breakers are configured at both ends of each line of the main line of the petal network operating in a closed loop. The lines and protections are numbered sequentially in the clockwise direction. For the line where a certain numbered protection is located, the line where the protection device with a smaller number is located is defined as the upstream line, and vice versa as the downstream line; the positive direction of the protection is from the adjacent switch station bus or substation bus to the line. All overcurrent protections are equipped with additional direction elements, and the protections are divided into two groups. Protections numbered 1, 3, 5, 7, 9 are the first group of inverse-time overcurrent protections, which only respond to the fault current upstream of the fault point; protections numbered 2, 4, 6, 8, 10 are the second group of inverse-time overcurrent protections, which only respond to the fault current downstream of the fault point.

[0008] (2) When a three-phase short-circuit fault occurs in the main line of the petal network, the magnitudes of the fault currents I1 and I2 upstream and downstream of the fault point can be obtained as shown in the following formula:

[0009]

[0010]

[0011] In the formula: E S is the equivalent electromotive force on the system side

[0012] r and x are the resistance and reactance per unit length of the line respectively; r s , x s are the equivalent resistance and reactance on the system side respectively;

[0013] L is the total length of the main line of the petal network;

[0014] l is the distance from the fault point through the upstream fault line to the 10 kV bus A, and this distance is defined as the fault distance;

[0015] To further analyze the variation law of I1 and I2 with the movement of the fault location, the derivatives of formula (1) and formula (2) are taken respectively, and the variation law of the gradients of I1 and I2 with the movement of the fault location is obtained as shown in the following formula:

[0016]

[0017] Since \(0 \lt l \lt L\), it is easy to know that \(\frac{dI_1}{dl} \lt 0\) and \(\frac{dI_2}{dl} \gt 0\). Therefore, equation (1) shows a monotonically decreasing characteristic in the feasible region, that is, the fault current \(I_1\) upstream of the fault point decreases as the fault distance \(l\) increases; equation (2) shows a monotonically increasing characteristic in the feasible region, that is, the fault current \(I_2\) downstream of the fault point increases as the fault distance \(l\) increases. At the same time, it can be known that this monotonic characteristic does not change with the change of the full length \(L\) of the petal network line;

[0018] Similarly, when a two-phase short-circuit fault occurs on the main trunk line of the petal network, it can be deduced that the magnitudes of the positive and negative sequence fault currents are half of those in the three-phase short-circuit; the magnitude of the fault phase current is The fault current characteristics when an interphase short-circuit fault occurs on the main trunk line of the petal network can be summarized as follows: The fault currents on both sides of the fault point are related. The ratio of the fault currents upstream and downstream of the fault point is equal to the inverse ratio of the line impedances upstream and downstream of the fault point; when the line impedance on one side of the fault point approaches zero, the fault current flowing through the other side of the line will also approach zero. There must be a situation where the fault current on the main trunk line of the petal network is less than the maximum load current, resulting in the refusal of the overcurrent protection to operate; the fault current upstream of the fault point decreases monotonically as the fault distance increases, and the fault current downstream of the fault point increases monotonically as the fault distance increases; the fault current characteristics in two-phase short-circuit faults and three-phase short-circuit faults are the same, and the fault currents in the two fault types only have a numerical multiple relationship;

[0019] (3) To solve the problems of long operating time and possible refusal of protection when the existing overcurrent protection is applied to the petal network, a method for constructing an inverse-time operating characteristic equation based on adaptive correction of characteristic parameters is proposed to improve the quick-acting performance of the overcurrent protection while ensuring the selectivity of the overcurrent protection.

[0020] According to the IEC 60255 standard, the standard form of the inverse-time operating characteristic equation is shown as follows:

[0021]

[0022] Where: \(t\) is the operating time of the protection;

[0023] \(I\) f is the fault current flowing through the protection;

[0024] \(I\) p is the starting current of the protection;

[0025] TDS is the time coefficient of the protection;

[0026] \(A\) and \(B\) are the characteristic parameters of the inverse-time operating characteristic equation;

[0027] Considering the characteristic parameter A in the standard inverse time action characteristic equation as a function of the fault current, the general form of the inverse time action characteristic equation based on adaptive correction of the characteristic parameter can be obtained as shown in the following formula:

[0028]

[0029] However, the existing invention does not clearly point out the function of the characteristic parameter A with respect to the fault current (i.e., A(I f ) equation), only mentioning that the A(I) equation that is suitable for the specific distribution network can be constructed by “data fitting” based on commonly used basic mathematical functions (such as exponential function, logarithmic function, polynomial function, etc.). f ) equation, therefore, the present invention proposes a new A(I f ) equation construction method is proposed, and based on this, a new method of petal network inverse time overcurrent protection based on characteristic parameter adaptive correction of inverse time action characteristic equation is proposed.

[0030] (4) By comparing equation (4) with equation (5), it can be seen that the key to constructing the characteristic equation based on characteristic parameter adaptive correction inverse time action is to construct the inverse time characteristic parameter A and the fault current I f The functional relationship between them is A(I f ) equation; construct A(I f ) equation is as follows: First, according to the fault characteristics analysis in Section 1, the functional relationship between fault current and fault distance can be obtained, that is, I f (l) equation; then, by analyzing the time coordination relationship between the upper and lower protections, the functional relationship between the action time of each protection and the fault distance can be constructed, that is, the t(l) equation; finally, the combined I f The general form of the improved inverse time action characteristic equation shown in equation (5) can be obtained by combining the equation (I) and the equation (t) to obtain A(I f )equation;

[0031] Next, we take protection group 1 (protection 1, 3, 5, 7, 9) as an example to explain the construction process of the functional relationship between its action time and fault distance (i.e., t(l) equation). According to the time coordination relationship between the above protections, the corresponding relationship between the protection action time and the fault distance can be listed point by point. For the convenience of understanding and deduction, it is assumed that the length of the petal network line L1~L5 is l e , the inherent action time of the protection device itself is t min , the action time difference of adjacent protections in the same group is Δt, and a more general t(l) equation can be obtained as shown below:

[0032]

[0033] Similarly, for protection group two, the corresponding t(l) equation can also be constructed by first listing and then writing the functional relationship, as shown in the following equation:

[0034]

[0035] Next, taking protection 1 as an example, the construction process of the functional relationship equation (i.e., the A(I f ) equation) between the inverse time characteristic parameter A and the fault current is introduced. By combining the I f (l) equation shown in Equation (1), the t1(l) equation in Equation (6), and the improved inverse time operating characteristic equation shown in Equation (5), the following system of equations can be obtained:

[0036]

[0037] By solving the system of equations shown in Equation (8), the A1(I f ) equation of protection 1 can be obtained as shown in the following equation:

[0038]

[0039] where I p1 , TDS1 are the setting values of the starting current and the time coefficient setting value of protection 1 respectively, both of which are constants and need to be set by the user; B is a preset inverse time parameter; t min , Δt are preset protection operating time parameters; l e is a known system parameter;

[0040] Similarly, for protections 2 to 10, their corresponding A(I f ) equations can be constructed through the same process;

[0041] (5) Substitute the A(I f ) equation representing the relationship between the inverse time characteristic parameter A and the fault current in each protection into the general form of the improved inverse time operating characteristic equation shown in Equation (5), and the adaptive correction operating characteristic equation based on characteristic parameters applicable to the petal network can be obtained as shown in Equation (10), where the subscript i represents protection i, i = 1, 2, 3,..., 10.

[0042]

[0043] Adopting the improved inverse time overcurrent protection with the above-mentioned adaptive correction operating characteristic equation based on characteristic parameters in the petal network can significantly reduce the protection operating time when faults occur in each line of the main trunk line of the petal network and improve the quick-acting performance of the overcurrent protection.

[0044] Compared with the prior art, the positive effects that the present invention can produce include the following points:

[0045] 1. The inverse-time overcurrent protection of the petal network can be achieved only by using the steady-state value of the fault current at the protection installation location, without the need for communication or the installation of voltage transformers, avoiding expensive communication costs and equipment installation costs, and being easy to implement in engineering.

[0046] 2. Based on the numerical multiple relationship of the fault current when a two-phase short-circuit fault and a three-phase short-circuit fault occur in the petal network, only by multiplying the corresponding numerical multiple in the I f equation in the construction of the A(I f )(l) equation, the protection can be made immune to the influence of the phase-to-phase short-circuit fault type, and has good protection effects when a three-phase short-circuit fault and a two-phase short-circuit fault occur in the petal network.

[0047] 3. When a fault occurs in each line of the main trunk line of the petal network, the present invention can have a relatively short operating time, and the operating time limit characteristics presented by each protection change from "step type" to "sawtooth type", greatly improving the quick-acting performance of the protection. In addition, the present invention can maintain a basically constant operating time interval between the upper and lower level protections to ensure that the operating time will not increase significantly when it is used as a remote backup.

[0048] 4. The method for constructing the inverse-time operating characteristic equation based on the adaptive correction of characteristic parameters proposed by the present invention can not only be applied to the petal network in closed-loop operation, but also to the radial distribution network in open-loop operation, with wide applicability, reliable performance, and being simple and easy to implement. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 is the topological structure diagram of the 10kV petal network;

[0050] Figure 2 is the equivalent circuit diagram when a three-phase short-circuit fault occurs in the main trunk line of the petal network;

[0051] Figure 3 is the construction flow chart of the inverse-time operating characteristic equation based on the adaptive correction of characteristic parameters;

[0052] Figure 4 is the equivalent split topological diagram of the petal network during the inverse-time overcurrent protection setting process;

[0053] Figure 5 is the flow chart of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0054] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Among them, the described embodiments are some, but not all, of the embodiments of the present invention. Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the present invention.

[0055] As Figure 1 shown in the topology structure diagram of the 10kV petal network, in the figure, the main line contains a total of 5 lines, numbered L1 - L5 in clockwise order; protection devices are configured at both ends of each line, numbered Protection 1 - 10 in sequence. For a line where a certain numbered protection is located, the line where the protection device with a smaller number is located is defined as the upstream line, and vice versa as the downstream line. In addition, the system contains a total of 5 sections of busbars, numbered A - E in clockwise order. Busbar A is the 10kV busbar at the outlet of the petal network substation, and busbars B - E are all switch station busbars. Each switch station contains multiple feeder lines.

[0056] Figure 2 is the equivalent circuit diagram when a three-phase short-circuit fault occurs in the main line of the petal network. In the figure, and ZS are the phase electromotive force and impedance on the system side respectively, is the phase voltage at 10kV busbar A, Z1 is the line impedance from 10kV busbar A to the fault point in clockwise direction, and Z2 is the line impedance from the fault point through the downstream line to 10kV busbar A; and are the fault phase currents flowing through the upstream and downstream of the fault point respectively.

[0057] A method for inverse-time overcurrent protection of petal-type distribution network based on adaptive correction of characteristic parameters specifically includes the following steps:

[0058] First, the starting current I p and the time coefficient TDS of each protection need to be set. The starting current of each protection is determined according to the principle of avoiding the maximum load current. The setting value of the starting current is shown in the following formula:

[0059]

[0060] In the formula: K rel is the reliability coefficient, generally taking 1.25 - 1.5; K Ms is the self-starting coefficient, with a value greater than 1, determined by the specific wiring of the network and the nature of the load; K re is the return coefficient of the current relay, generally taking 0.85 - 0.9.

[0061] Since the existing setting methods for inverse-time overcurrent protection are all aimed at radial distribution networks, while petal networks often operate in a closed-loop manner, when applying the setting method of inverse-time overcurrent protection to petal networks, the petal network needs to be first split into two radial networks as shown in the appendix Figure 4 Then, the setting is carried out for protection group 1 (protections 1, 3, 5, 7, 9) and protection group 2 (protections 2, 4, 6, 8, 10) respectively. Taking protection group 1 as an example, the setting starts from protection 9 which is the farthest from the power source. When a three-phase short-circuit fault occurs at the outlet of protection 9, set protection 9 to have the fastest operating time t min , then the time coefficient TDS9 of protection 9 can be solved. Next, set protection 7. When a three-phase fault occurs at the outlet of protection 9, set the operating time of protection 7 to increase by a Δt compared with protection 9 to meet the selectivity of the protection, and thus the time coefficient TDS7 of protection 7 can be solved. And so on, the time coefficient setting values of protections 5, 3, and 1 can be obtained. Similarly, the parameter setting values of each protection in protection group 2 can also be obtained.

[0062] Then, the starting current and time coefficient setting values obtained from the setting need to be input into each protection device, and at the same time, the inverse-time operating characteristic equation adaptively corrected based on characteristic parameters is input.

[0063] Next, when a fault occurs on the main line of the petal network, if the direction of the fault current flowing through the protection device is opposite to the preset positive direction of the protection, the protection is immediately blocked; if the direction of the fault current flowing through the protection device is the same as the preset positive direction of the protection, and the protection device detects that the flowing fault current is greater than the starting current setting value, the protection is immediately started and enters the fault identification mode.

[0064] Finally, when the protection is started and enters the fault identification mode, it is necessary to calculate the value of the current fault current characteristic parameter A according to the preset A(I f ) equation (Equation (11)) of each protection, and substitute it into the inverse-time operating characteristic equation adaptively corrected based on characteristic parameters (Equation (10)) to calculate the operating time of the protection. Each protection delays its operation according to the calculated operating time to cut off the fault.

[0065] The flowchart of the above inverse-time overcurrent protection method for petal-type distribution networks based on adaptive correction of characteristic parameters is as shown in the appendix Figure 5 as shown.

[0066] The above content is only an embodiment of the present invention, and its purpose is not to limit the systems and methods proposed by the present invention. The protection scope of the present invention is subject to the claims. Without departing from the spirit and scope of the present invention, various obvious modifications or changes in form and details made by those skilled in the art should fall within the protection scope of the present invention.

Claims

1. A petal - type distribution network inverse - time over - current protection method based on adaptive correction of characteristic parameters, specifically including the following steps: (1) Protection devices and circuit breakers are configured at both ends of each line of the main trunk line of the petal network in closed - loop operation. The lines and protections are numbered in a clockwise direction in sequence. For a line where a certain - numbered protection is located, the line where the protection device with a smaller number is located is defined as the upstream line, and vice versa as the downstream line; the positive direction of the protection is from the adjacent switch - station bus or sub - station bus to the line. All over - current protections are equipped with additional direction elements, and the protections are divided into two groups. Protections numbered 1, 3, 5, 7, 9 are the first - group inverse - time over - current protections, which only respond to the fault current upstream of the fault point; Protections numbered 2, 4, 6, 8, 10 are the second - group inverse - time over - current protections, which only respond to the fault current downstream of the fault point; (2) When a three - phase short - circuit fault occurs on the main trunk line of the petal network, the magnitudes of the fault currents I1 and I2 upstream and downstream of the fault point can be obtained as shown in the following formula: where: E S is the equivalent electromotive force on the system side r and x are the resistance and reactance per unit length of the line; r s and x s are the equivalent resistance and reactance on the system side respectively; L is the total length of the main trunk line of the petal network; l is the distance from the fault point through the upstream fault line to the 10 kV bus A, and this distance is defined as the fault distance; To further analyze the variation law of I1 and I2 with the movement of the fault position, the derivatives of equations (1) and (2) are taken respectively, and the variation laws of the gradients of I1 and I2 with the movement of the fault position are obtained as shown in the following formula: Since 0 < l < L, it is easy to know that dI1 / dl < 0 and dI2 / dl > 0, that is, the fault current I1 upstream of the fault point decreases with the increase of the fault distance l; that is, the fault current I2 downstream of the fault point increases with the increase of the fault distance l. At the same time, it can be seen that this monotonic characteristic does not change with the change of the total length L of the petal network line; Similarly, when a two-phase short-circuit fault occurs in the main line of Huaban.com, it is deduced that the magnitudes of the positive and negative sequence fault currents are half of those in the three-phase short circuit; the magnitude of the fault phase current is The fault current characteristics when an interphase short-circuit fault occurs in the main line of Huaban.com are summarized as follows: The fault currents on both sides of the fault point are related. The ratio of the fault currents upstream and downstream of the fault point is equal to the inverse ratio of the line impedances upstream and downstream of the fault point. When the line impedance on one side of the fault point approaches zero, the fault current flowing through the other side of the line will also approach zero. There must be a situation where the fault current on the main line of Huaban.com is less than the maximum load current, resulting in the refusal of the overcurrent protection to operate. The fault current upstream of the fault point decreases monotonically with the increase of the fault distance, and the fault current downstream of the fault point increases monotonically with the increase of the fault distance. The fault current characteristics in two-phase short-circuit faults and three-phase short-circuit faults are the same, and there is only a numerical multiple relationship between the fault currents in the two fault types. (3) Regarding the characteristic parameter A in the standard inverse - time operating characteristic equation as a function of the fault current, the general form of the inverse - time operating characteristic equation based on adaptive correction of characteristic parameters can be obtained as shown in the following formula: Where: t is the operating time of the protection; I f The fault current flowing through the protection; I p Starting current for protection; TDS is the time coefficient of the protection; A and B are the characteristic parameters of the inverse - time operating characteristic equation; (4) Next, taking Protection Group 1 as an example, the construction process of the functional relationship between its operating time and fault distance, that is, the t(l) equation, is described. According to the above time coordination relationship between protections, the corresponding relationship between protection operating time and fault distance is listed point by point. It is assumed that the lengths of the petal network lines L1 to L5 are all l e and the inherent operating time of the protection device itself is t min and the operating time difference between adjacent protections in the same group is Δt. A more general t(l) equation is obtained as shown below: Similarly, for protection group two, the corresponding t(l) equation is constructed by the method of first listing a table and then writing the function relationship, as shown in the following formula: Next, taking Protection 1 as an example, the functional relationship between the inverse-time characteristic parameter A and the fault current, that is, the construction process of the A(I f ) equation, is introduced. By combining the I f (l) equation shown in Equation (1), the t1(l) equation in Equation (6), and the improved inverse-time operating characteristic equation shown in Equation (5), the following system of equations can be obtained: The A1(I of Protection 1 can be obtained by solving the system of equations shown in Equation (8) f ) The equation is shown as follows: Where I p1 , TDS1 are respectively the starting current setting value and the time coefficient setting value of Protection 1, both of which are constants and need to be set by the user; B is the preset inverse time parameter; t min , Δt are the preset protection operation time parameters; l e is a known system parameter; Similarly, for protections 2 to 10, the corresponding A(I f ) equations can be constructed through the same process; (5) Substitute the A(I f ) equation that characterizes the relationship between the inverse-time characteristic parameter A and the fault current in each protection into the general form of the improved inverse-time operating characteristic equation shown in Equation (5), and the adaptive correction operating characteristic equation based on the characteristic parameters applicable to the petal network can be obtained as shown in Equation (10), where the subscript i represents protection i, and i = 1, 2, 3, …, 10;

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