A Current Differential Protection Method and System for an Active Distribution Network

Abstract

The present disclosure belongs to the technical field of power systems, and particularly relates to a current differential protection method and system for an active distribution network, including: obtaining the current of the protected section of the active distribution network; judging whether to start the current differential protection for the protected section of the active distribution network according to the error of the obtained current and a preset current phase margin; and when the current differential protection is started, selecting a differential protection criterion according to whether the current transformer is saturated to implement the differential protection of the active distribution network. The present disclosure combines the fault current characteristics and measurement errors of the active distribution network, gives the braking area of the current differential protection in the amplitude-phase plane, and constructs an auxiliary criterion by using the harmonic content ratio and amplitude ratio of the secondary current when the CT is saturated. Compared with the conventional protection criterion, the present disclosure has higher sensitivity, stronger ability to withstand transition resistance, and can cope with CT saturation on the premise of ensuring reliability, and is more applicable to the active distribution network.

Description

Technical Field

[0001] The present disclosure belongs to the technical field of power systems, and particularly relates to a current differential protection method and system for an active distribution network. Background Art

[0002] The statements in this part merely provide background technical information related to the present disclosure, and do not necessarily constitute prior art.

[0003] The decentralized access of distributed power sources makes the distribution network present the characteristics of "multiple sources", and the power flow and fault current flow bidirectionally, which causes the traditional three-stage overcurrent protection to lose selectivity. The pilot protection is an effective means to solve the problem of multi-source network protection, and the current differential protection based on Kirchhoff's current law is undoubtedly the most effective protection principle.

[0004] The current differential protection regards the protected line as a node and identifies faults by judging the current flowing into the node. Therefore, for a distribution line where the influence of distributed capacitance can be ignored, the current differential protection principle can solve its protection problem in principle, and the key lies in the design of the protection criterion. In a traditional power grid, the internal potential of synchronous generators on both sides of the line remains unchanged before and after a fault, and the phases of the currents on both sides are similar during an in-zone fault and opposite during an out-of-zone fault. Based on this characteristic, a criterion that takes into account both sensitivity and reliability is designed. However, in an active distribution network, most distributed power sources are connected to the grid through inverters. Affected by the control strategy, the fault characteristics of distributed power sources are different from those of synchronous motors, and the amplitude of the fault current output is limited and the phase is controlled. At this time, the adaptability of the traditional differential protection criterion is challenged.

[0005] In practical engineering, the saturation of current transformers (CTs) is a key factor affecting the reliability of current differential protection. CT saturation blurs the boundary between in-zone faults and out-of-zone faults, and it is difficult for conventional differential protection criteria to identify faults. At present, there are two methods to solve the problem of relay protection failure caused by CT saturation: researching protection methods with strong anti-CT saturation ability and researching CT saturation identification methods; the former mainly uses the natural anti-CT saturation ability of current sampling values and makes judgments by making full use of the linear transfer stage; the latter designs saturation identification methods through the analysis of the waveform characteristics of secondary currents, including time difference method, harmonic method, etc., and a locking and opening scheme needs to be designed to cooperate with it. The above methods put higher requirements on the hardware equipment of the distribution network, and transmitting sampling values increases the communication burden of the distribution network. Therefore, it is difficult to take into account the situation of no refusal to operate for in-zone faults and no misoperation for CT saturation by relying on conventional current differential protection criteria. Summary of the Invention

[0006] To solve the above problems, the present disclosure proposes a current differential protection method and system for an active distribution network. Based on the analysis of the relationship between the amplitudes of the fault currents in the active distribution network, a braking region applicable to the active distribution network is given in the amplitude-phase plane. At the same time, considering the common CT saturation situation, a high-performance current differential protection scheme is proposed. The invention has higher sensitivity on the premise of ensuring reliability, can cope with CT saturation, and is more applicable to the active distribution network; on the basis of ensuring reliability, the adaptability of the differential protection in the active distribution network is effectively improved.

[0007] According to some embodiments, the first solution of the present disclosure provides a current differential protection method for an active distribution network, adopting the following technical solutions:

[0008] A current differential protection method for an active distribution network includes:

[0009] Obtain the current of the protected section of the active distribution network;

[0010] According to the error of the obtained current and the preset current phase margin, determine whether to start the current differential protection for the protected section of the active distribution network;

[0011] When the current differential protection is started, select the differential protection criterion according to whether the current transformer is saturated to implement the differential protection of the active distribution network.

[0012] As a further technical limitation, obtain the current of the protected section of the active distribution network based on the current transformer arranged in the protected section of the active distribution network; according to the model of the current transformer arranged in the protected section of the active distribution network, determine the maximum current error caused by the current transformer, and the current error includes current amplitude error and current phase error.

[0013] As a further technical limitation, determine the maximum phase error caused by data synchronization according to the data synchronization method on both sides of the section; the data synchronization method includes GPS-based data synchronization method and self-synchronization at the fault moment

[0014] Furthermore, determine the current amplitude and phase relationship on both sides of the line when a fault occurs in the protected section, and draw the determined current amplitude and phase relationship in the unit circle.

[0015] As a further technical limitation, when the current transformer is not saturated, determine whether the main criterion is satisfied. If it is satisfied, perform differential protection; the design of the main criterion is as follows:

[0016] Considering a certain margin, draw the current transformer and the synchronization error range in the unit circle of the amplitude-phase plane;

[0017] Draw the current relationship on both sides of the line when a fault occurs in the zone in the unit circle of the amplitude-phase plane;

[0018] Draw a curve in the unit circle to separate the above two regions. One side of the curve is the braking area, and the other side of the curve is the action area.

[0019] Specifically, a current transformer and a synchronous error range draw a region in the unit circle of the amplitude-phase plane. When a fault occurs in the region, the current relationship between the two sides of the line is drawn as another region on the unit circle of the amplitude-phase plane. Then draw a curve to separate the above two regions.

[0020] Furthermore, the auxiliary criterion is specifically as follows: Where the subscript b represents the side with a larger harmonic content on both sides of the section; the subscript s represents the side with a smaller harmonic content on both sides of the section; I represents the fundamental wave current, λ represents the current harmonic content, and Where I x is the amplitude of the x-th current harmonic, and y is the highest-order harmonic.

[0021] According to some embodiments, the second solution of the present disclosure provides a current differential protection system for an active distribution network, adopting the following technical solution:

[0022] A current differential protection system for an active distribution network, comprising:

[0023] An acquisition module configured to acquire the current of the protected section of the active distribution network;

[0024] A judgment module configured to judge whether to start current differential protection for the protected section of the active distribution network according to the error of the acquired current and a preset current phase margin;

[0025] A differential protection module configured to, after the current differential protection is started, select a differential protection criterion according to whether the current transformer is saturated to implement the differential protection of the active distribution network.

[0026] According to some embodiments, the third solution of the present disclosure provides a computer-readable storage medium, adopting the following technical solution:

[0027] A computer-readable storage medium, on which a program is stored, and when the program is executed by a processor, it implements the steps in the current differential protection method for an active distribution network as described in the first aspect of the present disclosure.

[0028] According to some embodiments, the fourth solution of the present disclosure provides an electronic device, adopting the following technical solution:

[0029] An electronic device, comprising a memory, a processor, and a program stored on the memory and executable on the processor. When the processor executes the program, it implements the steps in the current differential protection method for an active distribution network as described in the first aspect of the present disclosure.

[0030] Compared with the prior art, the beneficial effects of the present disclosure are as follows:

[0031] The present disclosure combines the fault current characteristics and measurement errors of the active distribution network, gives the braking area of the current differential protection in the amplitude-phase plane, and constructs an auxiliary criterion by using the harmonic content ratio and amplitude ratio of the secondary current during CT saturation. Compared with the conventional protection criterion, the present disclosure has higher sensitivity, stronger ability to withstand transition resistance, and can cope with CT saturation on the premise of ensuring reliability, and is more suitable for the active distribution network. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The accompanying drawings forming a part of this disclosure are used to provide a further understanding of the present disclosure. The schematic embodiments and descriptions thereof of the present disclosure are used to explain the present disclosure and do not constitute an improper limitation of the present disclosure.

[0033] Figure 1 is a flowchart of the current differential protection method for the active distribution network in Embodiment 1 of the present disclosure;

[0034] Figure 2 is a diagram of the fault characteristics inside and outside the area of the active distribution network in Embodiment 1 of the present disclosure;

[0035] Figure 3 is a schematic diagram of a simple active distribution network in Embodiment 1 of the present disclosure;

[0036] Figure 4 is a schematic diagram of the high-performance main criterion in Embodiment 1 of the present disclosure;

[0037] Figure 5 is a schematic diagram of the active distribution network simulation model in Embodiment 1 of the present disclosure;

[0038] Fig. 6(a) is a schematic diagram of the performance verification result of the current differential protection criterion when the fault point is located at f1 in Embodiment 1 of the present disclosure;

[0039] Fig. 6(b) is a schematic diagram of the performance verification result of the current differential protection criterion when the fault point is located at f2 in Embodiment 1 of the present disclosure;

[0040] Fig. 6(c) is a schematic diagram of the performance verification result of the current differential protection criterion when the fault point is located at f3 in Embodiment 1 of the present disclosure;

[0041] Figure 7 is a structural block diagram of the current differential protection system for the active distribution network in Embodiment 2 of the present disclosure. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0042] The present disclosure will be further described below in conjunction with the accompanying drawings and embodiments.

[0043] It should be noted that the following detailed description is illustrative and is intended to provide further explanation of the present disclosure. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present disclosure pertains.

[0044] It should be noted that the terms used herein are merely for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present disclosure. As used herein, unless the context clearly indicates otherwise, the singular forms are also intended to include the plural forms. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they specify the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0045] In the case of no conflict, the embodiments in the present disclosure and the features in the embodiments can be combined with each other.

[0046] Embodiment 1

[0047] Embodiment 1 of the present disclosure introduces a current differential protection method for an active distribution network.

[0048] As Figure 1 shown, a current differential protection method for an active distribution network includes:

[0049] Step 1: Determine the maximum error (including current amplitude and current phase) that can be caused by the current transformer according to the type of current transformer configured for the protected section.

[0050] Step 2: Determine the maximum phase error caused by data synchronization according to the data synchronization method on both sides of the section.

[0051] Step 3: Considering a certain margin, draw the range of the obtained maximum phase error in the unit circle of the amplitude-phase plane.

[0052] Step 4: Determine the current amplitude and phase relationship on both sides of the line when a fault occurs in the section, and draw it in the unit circle

[0053] Step 5: Draw a curve in the unit circle to separate the above two regions

[0054] Step 6: After a fault occurs, calculate the harmonic content of the fault current on both sides; calculate the amplitude of the fault current on both sides; construct an auxiliary criterion through the current amplitude and harmonic content on both sides. If the criterion is satisfied, it is regarded as an in-zone fault.

[0055] In this embodiment, a 10P current transformer is adopted, and the current and phase errors caused by it on both sides are 10% and 6° respectively; the current phase error caused by data synchronization is 4°. Considering a certain margin, the current amplitude and phase offset caused by the error can be set to 20% and 15°. Represent this error range in the unit circle asFigure 2 as shown in the left grey area.

[0056] As Figure 3 shown, one side is selected as the system side, and the other side is the section where the inverter-type distributed generation DG is connected; the inverter-type DG adopts the conventional PQ control strategy. Affected by the control strategy, when a fault occurs in the zone, the phase difference of the currents on both sides can reach more than 90°. In this embodiment, it is considered according to 150°, that is Figure 2 the operating point of the fault in the zone is the right grey area in

[0057] In Figure 2 a curve is drawn in the white area in and closed with the unit circle. The unit circle is divided into two parts by this curve. The left part is the braking zone, and the right part is the operating zone.

[0058] For the active distribution network, when CT saturation occurs, the fault point is generally located in the section closer to the substation outlet, and the transition resistance is extremely small. After a fault occurs, the DG downstream of the fault point outputs a small fault current due to the current limiting strategy and will not have the CT saturation phenomenon. The system power supply upstream of the fault point generates a large fault current due to the small transition resistance and may have the CT saturation phenomenon. Therefore, when CT saturation occurs in the short-circuit fault of the radial active distribution network, it has the following characteristics: when a fault occurs outside the zone, the fundamental wave amplitude of the current on the side with a larger harmonic content is smaller; when a fault occurs in the zone, the fundamental wave amplitude of the current on the side with a larger harmonic content is larger.

[0059] The auxiliary criterion is specifically:

[0060] where the subscript b represents the side with a larger harmonic content on both sides of the section; the subscript s represents the side with a smaller harmonic content on both sides of the section; I represents the fundamental wave current, λ represents the current harmonic content, and where I x is the amplitude of the x-th current harmonic, and y is the highest harmonic.

[0061] For the current differential protection method introduced in this embodiment, an actual example analysis is carried out. Specifically, an active distribution network model as Figure 5 shown is built using the electromagnetic transient simulation software PSCAD / EMTDC to verify the high-performance active distribution network differential protection criterion in this embodiment.

[0062] In Figure 5 S is the system power supply, B 0-6 is the ring main unit bus, K ij is the circuit breaker. L 1-5 is the load, and DG 1-3It is a distributed power source with a maximum output current of 1.5 times the rated current. The control strategy is conventional PQ control, and it has the ability of low voltage ride through. The system parameters and line parameters are shown in Table 1 and Table 2 respectively.

[0063] Table 1 Simulation system parameters

[0064]

[0065] Table 2 Line parameters

[0066]

[0067] a) Performance verification of current differential protection criterion

[0068] Set phase AB interphase faults, phase BC interphase grounding faults and ABC three-phase faults at points f1, f2 and f3 respectively, and set the transition resistance to change from 1Ω, 6Ω, 10Ω, 20Ω, 30Ω, 40Ω to 50Ω. The operating characteristics of the protection are shown in Figures 6(a), 6(b) and 6(c). In the figures, phase B is taken as an example for analysis. For comparison, the white area in the figure is the braking area of the conventional differential protection criterion, the dark area is the braking area proposed in this embodiment, and the arrow direction is the direction of increasing transition resistance.

[0069] It can be seen from Figures 6(a), 6(b) and 6(c) that under different fault sections and fault types, considering the control strategy of IIDG and the influence of load current, when a fault occurs within the zone it will run to the left half plane. At this time, the sensitivity of the conventional differential protection criterion decreases, and even refuses to operate. In contrast, the proposed scheme in this embodiment has higher sensitivity and stronger ability to withstand transition resistance. Since the criterion proposed in this embodiment takes into account the error situation in practical applications, under external faults it will operate reliably in the dark area.

[0070] b) Verification of CT saturation influence

[0071] Set a three-phase metallic short circuit fault of ABC at f1, and set K by changing the secondary impedance Rg of CT s1 、K 22 where the CT is saturated, and the current at K 21 、K 31 is transmitted normally. The operating conditions of sections B1B2 and B2B3 are shown in Table 3. The following simulation data takes the phase A current as an example.

[0072] Table 3 Judgment results of internal and external faults under CT saturation

[0073]

[0074] As can be seen from Table 3, the proposed auxiliary protection criterion can distinguish between internal and external faults. When Rg is 100 Ω, the proposed auxiliary criterion can still operate reliably. In contrast, the conventional current differential protection criterion can tolerate the influence of differential current caused by CT saturation to a certain extent. When R g is 20 Ω, the protection will malfunction.

[0075] This embodiment combines the fault current characteristics and measurement errors of the active distribution network, gives the braking area of the current differential protection in the amplitude-phase plane, and constructs an auxiliary criterion by using the harmonic content ratio and amplitude ratio of the secondary current during CT saturation. Compared with the conventional protection criterion, the present disclosure has higher sensitivity, stronger ability to withstand transition resistance, and can cope with CT saturation under the premise of ensuring reliability, and is more suitable for active distribution networks.

[0076] Embodiment 2

[0077] Embodiment 2 of the present disclosure introduces a current differential protection system for an active distribution network.

[0078] As Figure 7 shown, a current differential protection system for an active distribution network includes:

[0079] An acquisition module configured to acquire the current of the protected section of the active distribution network;

[0080] A judgment module configured to judge whether to start current differential protection for the protected section of the active distribution network according to the error of the acquired current and a preset current phase margin;

[0081] A differential protection module configured to, when the current differential protection is started, select a differential protection criterion according to whether the current transformer is saturated, and implement the differential protection of the active distribution network.

[0082] The detailed steps are the same as those of the current differential protection method for the active distribution network provided in Embodiment 1, and will not be elaborated here.

[0083] Embodiment 3

[0084] Embodiment 3 of the present disclosure provides a computer-readable storage medium.

[0085] A computer-readable storage medium, on which a program is stored, and when the program is executed by a processor, it implements the steps in the current differential protection method for the active distribution network as described in Embodiment 1 of the present disclosure.

[0086] The detailed steps are the same as those of the current differential protection method for the active distribution network provided in Embodiment 1, and will not be elaborated here.

[0087] Embodiment 4

[0088] Embodiment 4 of the present disclosure provides an electronic device.

[0089] An electronic device includes a memory, a processor, and a program stored on the memory and executable on the processor. When the processor executes the program, it implements the steps in the current differential protection method for an active distribution network as described in Embodiment 1 of the present disclosure.

[0090] The detailed steps are the same as those of the current differential protection method for the active distribution network provided in Embodiment 1, and will not be elaborated here.

[0091] The above are only the preferred embodiments of the present disclosure and are not used to limit the present disclosure. For those skilled in the art, the present disclosure may have various changes and modifications. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present disclosure shall be included within the protection scope of the present disclosure.

[0092] Although the specific implementation manners of the present disclosure have been described above in conjunction with the accompanying drawings, they are not limitations on the protection scope of the present disclosure. Those skilled in the art should understand that, based on the technical solutions of the present disclosure, various modifications or deformations that can be made by those skilled in the art without creative efforts are still within the protection scope of the present disclosure.

Claims

1. A current differential protection method for an active distribution network, characterized in that, Including: Obtain the current of the protected section of the active distribution network; Judge whether the current differential protection of the protected section of the active distribution network is started according to the error of the obtained current and the preset current phase margin; When the current differential protection is started, select the differential protection criterion according to whether the current transformer is saturated to implement the differential protection of the active distribution network; When the current transformer is not saturated, judge whether the main criterion is satisfied. If it is satisfied, differential protection is carried out; The design of the main criterion is as follows: Considering a certain margin, draw the range of the current transformer and the synchronization error in the unit circle of the amplitude-phase plane; Draw the current relationship between the two sides of the line when a fault occurs in the zone in the unit circle of the amplitude-phase plane; Draw a curve in the unit circle to separate the two regions. One side of the curve is the braking zone, and the other side of the curve is the action zone; The two regions are: draw a region in the unit circle of the amplitude-phase plane for the range of the current transformer and the synchronization error, and draw another region in the unit circle of the amplitude-phase plane for the current relationship between the two sides of the line when a fault occurs in the zone, and then draw a curve to separate the above two regions; When the current transformer is saturated, judge whether the auxiliary criterion is satisfied. If it is satisfied, differential protection is carried out, otherwise, obtain the current of the protected section of the active distribution network again; When the current transformer is saturated, after a fault occurs in the active distribution network, calculate the harmonic content of the fault current on both sides, calculate the amplitude of the fault current on both sides, and construct an auxiliary criterion through the amplitude and harmonic content of the current on both sides. If the auxiliary criterion is satisfied, it is regarded as a fault occurring in the zone; The specific auxiliary criterion is as follows: Among them, the subscript b represents the side with a larger harmonic content on both sides of the section; the subscript s represents the side with a smaller harmonic content on both sides of the section; I represents the fundamental wave current, λ represents the current harmonic content, and where I x is the amplitude of the x-th current harmonic, and y is the highest harmonic.

2. The current differential protection method for an active distribution network as described in claim 1, characterized in that, Obtain the current of the protected section of the active distribution network based on the current transformer arranged in the protected section of the active distribution network; According to the model of the current transformer arranged in the protected section of the active distribution network, determine the maximum current error caused by the current transformer. The current error includes current amplitude error and current phase error.

3. The current differential protection method for an active distribution network as described in claim 2, wherein, Determine the maximum phase error caused by data synchronization according to the data synchronization method on both sides of the section; The data synchronization method includes GPS-based data synchronization method and self-synchronization at the fault moment.

4. The current differential protection method for an active distribution network according to claim 2, characterized in that Determine the current amplitude and phase relationship between the two sides of the line when a fault occurs in the protected section, and draw the determined current amplitude and phase relationship in the unit circle.

5. A current differential protection system for an active distribution network, characterized in that Including: An acquisition module configured to obtain the current of the protected section of the active distribution network; A judgment module configured to judge whether the current differential protection of the protected section of the active distribution network is started according to the error of the obtained current and the preset current phase margin; A differential protection module configured to select a differential protection criterion according to whether the current transformer is saturated to implement the differential protection of the active distribution network when the current differential protection is started; When the current transformer is not saturated, judge whether the main criterion is satisfied. If it is satisfied, differential protection is carried out; The design of the main criterion is as follows: Considering a certain margin, draw the range of the current transformer and the synchronization error in the unit circle of the amplitude-phase plane; Draw the current relationship between the two sides of the line when a fault occurs in the zone in the unit circle of the amplitude-phase plane; Draw a curve in the unit circle to separate the two regions. One side of the curve is the braking zone, and the other side of the curve is the action zone; The two regions are as follows: A region is drawn in the unit circle of the amplitude-phase plane for the current transformer and the synchronization error range. Another region is drawn on the unit circle of the amplitude-phase plane for the current relationship on both sides of the line during a fault within the region. Then, a curve is drawn to separate the above two regions; When the current transformer is saturated, it is judged whether the auxiliary criterion is satisfied. If it is satisfied, differential protection is carried out; otherwise, the current of the protected section of the active distribution network is obtained again. When the current transformer is saturated, after a fault occurs in the active distribution network, the harmonic contents of the fault currents on both sides are calculated, the amplitudes of the fault currents on both sides are calculated, and an auxiliary criterion is constructed through the current amplitudes and harmonic contents on both sides. If the auxiliary criterion is satisfied, it is regarded as an in-zone fault; The specific auxiliary criterion is as follows: Among them, the subscript b represents the side with larger harmonic content on both sides of the section; the subscript s represents the side with smaller harmonic content on both sides of the section; I represents the fundamental wave current, λ represents the current harmonic content, and Among them, I x is the amplitude of the x-th current harmonic, and y is the highest harmonic.

6. A computer-readable storage medium having a program stored thereon, characterized in that, When the program is executed by the processor, the steps in the current differential protection method of the active distribution network described in any one of claims 1-4 are implemented.

7. An electronic device, comprising a memory, a processor, and a program stored on the memory and executable on the processor, characterized in that, When the processor executes the program, the steps in the current differential protection method of the active distribution network described in any one of claims 1-4 are implemented.

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