A method for dual-terminal differential protection of a distributed power generation distribution network with T-connected inverters

Abstract

The application provides a kind of T-connected inverter type distributed power supply distribution network double-end differential protection method containing T, for the scene that new energy inverter type distributed power supply IIDG is connected in T mode in feeder and is incorporated into distribution network, i.e. the scene of line T-connected DG, the protection method utilizes the characteristics that IIDG does not output negative sequence current when distribution network is asymmetric fault, and whether fault occurs is judged by comparing the sum of negative sequence current at the two ends of feeder with setting threshold current;The protection method is for two-phase short-circuit fault type, and whether current differential protection action needs to be executed is determined by positive and negative sequence current decomposition;The application is for the fault type of two-phase short-circuit and two-phase ground fault of large-scale T-connected distributed power supply distribution network, only needs current information at the two ends of feeder, without T-connected distributed power supply branch electric quantity information.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of relay protection, and particularly relates to a double-end differential protection method for a distribution network containing T-connected inverter-interfaced distributed power sources. BACKGROUND

[0002] With the gradual depletion of traditional fossil energy and the improvement of people's environmental awareness, the power industry of various countries has begun to turn its attention to clean, efficient and flexible power generation - distributed generation (DG). However, the large-scale connection of inverter-interfaced distributed generation (IIDG) to the distribution network has changed the power supply network topology of the traditional distribution network, thereby affecting the direction and size of the fault current. At the same time, the output characteristics of the inverter-interfaced distributed generation are more complex than those of the traditional power source, and the output current is determined by the control strategy of the controller. In view of the above problems, the traditional current protection is prone to misoperation and refusal after the connection of the IIDG.

[0003] According to the position of the IIDG connected to the feeder of the distribution network, several fault scenarios can be analyzed. The IIDG is connected to the downstream or upstream of the feeder or T-connected to the feeder, which will affect the fault current of different positions and different types.

[0004] The control strategy of the IIDG connected to the inverter can be divided into two types: grid forming (GFM) and grid following (GFL). The grid following PQ control is commonly used, which controls the active power and reactive power by adjusting the active current and reactive current. According to the domestic grid connection requirements, the IIDG needs to have low voltage ride through (LVRT) capability during the fault of the distribution network. At the same time, in order to improve the output characteristics of the IIDG during the asymmetric short-circuit fault of the distribution network, the IIDG adopts the negative sequence current elimination control, and the IIDG can be equivalent to a current source controlled by the positive sequence voltage at the grid connection point.

[0005] The existing multi-end differential protection method for the distribution network containing T-connected distributed power sources compares the sum of the currents of each branch with the set threshold current or compares other electrical quantity information to determine whether a fault occurs.

[0006] The existing distribution network containing T-connected distributed power sources has two types of multi-end and double-end differential protection methods. The multi-end differential protection method needs to collect the electrical quantity information of all branches including the T-connected distributed power sources, which has high communication requirements. The existing double-end differential protection method needs to set the threshold according to the parameters of the distribution network.

[0007] The application discloses a double-end differential protection method for a distribution network containing T-connected distributed power supplies, which utilizes the characteristic that IIDG (Inverter Interfaced Distributed Generator) does not output negative sequence current during asymmetric fault of the distribution network, and compares the sum of the negative sequence currents at two ends of a feeder with a set threshold current to determine whether a fault occurs. SUMMARY

[0008] The application discloses a double-end differential protection method for a distribution network containing T-connected inverter-type distributed power supplies, which is suitable for two-phase short-circuit and two-phase ground-fault types of the distribution network containing large-scale T-connected distributed power supplies, and only needs current information at two ends of a feeder without needing electric quantity information of a T-connected distributed power supply branch.

[0009] The application adopts the following technical scheme.

[0010] The application discloses a double-end differential protection method for a distribution network containing T-connected inverter-type distributed power supplies, which is suitable for a scenario that new energy inverter-type distributed power supplies IIDG are connected to a feeder in a T-connected mode and then enter a distribution network, namely a scenario of line-in T-connected DG, and the protection method utilizes the characteristic that IIDG does not output negative sequence current during asymmetric fault of the distribution network, and compares the sum of the negative sequence currents at two ends of the feeder with a set threshold current to determine whether a fault occurs.

[0011] The protection method is suitable for two-phase short-circuit fault types, and determines whether current differential protection action needs to be performed through positive and negative sequence current decomposition.

[0012] The positive and negative sequence current decomposition method is specifically as follows.

[0013] Current instantaneous values and fault component instantaneous values at two ends of a line M and N are obtained, positive, negative and zero sequence current amplitudes and phase values of the current and the fault component at the two ends of the line M and N are obtained through sequence current decomposition according to the current instantaneous values and the fault component instantaneous values at the two ends of the line M and N, and the sequence current decomposition algorithm is as follows.

[0014]

[0015]

[0016] In the formula, I k (ωt) is a three-phase current instantaneous value, is a three-phase current instantaneous value with a phase of 90°, is an m sequence current phasor instantaneous value.

[0017] According to the positive, negative and zero sequence current amplitudes and phase values of the current and the fault component at the two ends of the line M and N, a condition of current differential protection action is as follows.

[0018]

[0019] In the formula, I positive, negative, zero sequence components of line M terminal current positive, negative, zero sequence components of line N terminal current ΔMN2 the phase angle difference between fault negative sequence component of line M terminal current and fault negative sequence component of line N terminal current k1, k2 are action coefficients.

[0020] When there is no fault in the protection zone or outside the protection zone, the phase difference value θ ΔMN2 of fault component negative sequence current at both ends of the line is 180°, at this time the differential protection does not act;

[0021] When a fault occurs in the protection zone, the phase difference value of fault component negative sequence current at both ends of the line is:

[0022]

[0023] wherein, Z MS2 is the negative sequence impedance of the power supply side, Z Mf , Z Nf , Z NL are respectively the line impedance from fault point f to M terminal, from fault point f to N terminal, and from N terminal to load, Z load is the load impedance, is the line impedance angle, is the load impedance angle;

[0024] If then

[0025] When there is no T-connected DG in the line in the distribution network protection zone, the positive and negative sequence current ratio is expressed by the formula as:

[0026]

[0027]

[0028]

[0029]

[0030] According to the superposition principle, the downstream equivalent independent current source in the protection zone its individual action is expressed by the formula as follows:

[0031]

[0032]

[0033] ​​According to the superposition principle, the equivalent independent current source in the upstream of the protection zone The single action is expressed as follows:

[0034]

[0035]

[0036]

[0037] The amplitude is not more than Or much smaller than From the above formula:

[0038]

[0039]

[0040] From the above equivalent inductive network

[0041]

[0042]

[0043]

[0044] In the criterion for determining whether the current differential protection action needs to be performed by positive and negative sequence current decomposition, after the T-connected DG in the line of the distribution network protection zone, the positive and negative sequence current ratio is slightly larger than that without DG, and due to the fact that the DG does not output negative sequence current, the phase difference of the fault component negative sequence current at both ends of the line is not affected by the access of the DG, and the differential protection can reliably act,

[0045] The protection range of the criterion for the interphase short-circuit resistance is expressed as

[0046]

[0047] In the above formula, U is the voltage level of the feeder, and S is the load capacity of the line.

[0048] The current differential protection action is executed by a relay protection device.

[0049] The advantages of the present application are:

[0050] 1. The present application is aimed at two-phase short-circuit and two-phase ground short-circuit types of distribution network containing large-scale T-connected distributed power supply, only the current information at both ends of the feeder is needed, and the branch power information of the T-connected distributed power supply is not needed.

[0051] 2. The present application adopts a new fault criterion, which can adapt to unbalanced faults of distribution networks with different grid parameters, and does not need to set the action threshold value according to the grid parameters. Attached Figure Description

[0052] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments:

[0053] Appendix Figure 1 Simplified circuit diagram of the present invention under normal operation or external fault conditions:

[0054] Appendix Figure 2 This is a simplified circuit diagram of the fault upstream of the DG grid connection point of the present invention:

[0055] Appendix Figure 3 This is a simplified circuit diagram illustrating a fault downstream of the DG grid connection point of the present invention:

[0056] Appendix Figure 4 This is a schematic diagram of the integrated sequence network when there is an upstream fault at the DG grid connection point of the present invention:

[0057] Appendix Figure 5 This is a schematic diagram of the integrated sequence network when a fault occurs downstream of the DG grid connection point of the present invention:

[0058] Appendix Figure 6 This is a schematic diagram of the negative sequence network of fault components during line faults according to the present invention. Detailed Implementation

[0059] As shown in the figure, a dual-terminal differential protection method for a distribution network with a T-connected inverter-type distributed power source is provided for scenarios where a new energy inverter-type distributed power source (IIDG) is connected to the feeder and integrated into the distribution network in a T-connection manner, i.e., a T-connected DG within the line. The protection method utilizes the characteristic that the IIDG does not output negative sequence current when there is an asymmetrical fault in the distribution network, and determines whether a fault has occurred by comparing the sum of the negative sequence currents at both ends of the feeder with the set threshold current.

[0060] The protection method, for two-phase short-circuit faults, determines whether current differential protection needs to be performed by decomposing positive and negative sequence currents.

[0061] The method for decomposing the positive and negative sequence currents is as follows:

[0062] Obtain the instantaneous current values ​​and fault component values ​​at both ends of lines M and N. Perform sequence current decomposition based on these values ​​to obtain the positive, negative, and zero sequence current amplitudes and phase values ​​of the current and fault components at both ends of lines M and N. The sequence current decomposition algorithm is as follows:

[0063]

[0064]

[0065] In the above formula: I k(ωt) represents the instantaneous value of the three-phase current. The instantaneous value of the three-phase current leading by 90°. This represents the instantaneous value of the m-sequence current phasor.

[0066] Based on the current at both ends of line M and N, and the amplitude and phase values ​​of the positive, negative, and zero-sequence currents of the fault component, the conditions for the current differential protection to operate are as follows:

[0067]

[0068] In the above formula: These are the currents at the M end of the line. The positive, negative, and zero-order components, These are the currents at the N end of the line. The positive, negative, and zero order components, θ ΔMN2 For the negative sequence component of the current fault at the M terminal of the line With the negative sequence component of the N-terminal current fault The difference in phase angles, k1 and k2 are the action coefficients.

[0069] When there is no fault within the protected area or a fault occurs outside the protected area, the phase difference θ between the negative sequence currents of the fault components at both ends of the line is... ΔMN2 =180°, at this time The differential protection did not operate;

[0070] When a fault occurs within the protected area, the phase difference of the fault component negative sequence current at both ends of the line is as follows:

[0071]

[0072] Among them, Z MS2 Z is the negative sequence impedance on the power supply side. Mf Z Nf Z NL Z represents the line impedance from fault point f to terminal M, from fault point f to terminal N, and from terminal N to the load, respectively. load For load impedance, The line impedance angle. This is the load impedance angle;

[0073] like but

[0074] like Figure 4 , Figure 5 As shown, when there is no T-connection DG in the distribution network protection zone, the ratio of positive and negative sequence currents is expressed by the formula:

[0075]

[0076]

[0077]

[0078]

[0079] like Figure 4 As shown, based on the superposition principle, the equivalent independent current sources in the middle and lower reaches of the protected area are... Its individual function can be expressed by the following formula:

[0080]

[0081]

[0082] like Figure 5 As shown, according to the superposition principle, the upstream equivalent independent current source in the protected area... Its individual function can be expressed by the following formula:

[0083]

[0084]

[0085]

[0086] The amplitude does not exceed or much smaller From the above formula, we get:

[0087]

[0088]

[0089] like Figure 4 , Figure 5 As shown, from the above equivalent inductive network, we obtain

[0090]

[0091]

[0092]

[0093] In the criterion for determining whether to perform differential current protection by decomposing positive and negative sequence currents, after a DG is connected to the line within the distribution network protection zone, the ratio of positive to negative sequence currents is slightly larger than when there is no DG. Furthermore, since the DG does not output negative sequence current, the phase difference of the negative sequence current of the fault component at both ends of the line is not affected by the DG connection, and the differential protection can operate reliably.

[0094] The protection range of this criterion for phase-to-phase short-circuit resistance is expressed by the formula as follows:

[0095]

[0096] In the above formula, U is the feeder voltage level and S is the line load capacity.

[0097] The current differential protection action is executed by the relay protection device.

Claims

1. A dual-terminal differential protection method for a distribution network containing a T-connected inverter-type distributed generation (IIDG), used in scenarios where a new energy inverter-type distributed generation (IIDG) is connected to the distribution network via a feeder in a T-connection manner, i.e., a T-connected DG within the line, characterized in that: The protection method utilizes the characteristic of IIDG that no negative sequence current is outputted when the distribution network is asymmetrically faulted, and judges whether the fault occurs by comparing the sum of the negative sequence currents at both ends of the feeder with the set threshold current; The protection method judges whether the current differential protection action needs to be executed by positive and negative sequence current decomposition for two-phase short-circuit fault type; When there is no fault in the protected zone or a fault outside the protected zone, the phase difference value θ of the fault component negative sequence current at both ends of the line ΔMN2 = 180°, at this time the differential protection does not act. When the fault occurs in the protection zone, the phase difference of the fault component negative sequence currents at both ends of the line is: wherein, is the phase angle of the fault negative sequence component of the line M end current is the phase angle of the fault negative sequence component of the line N end current is the phase angle of the line impedance, MS2 is the negative sequence impedance of the power supply side, Mf is the negative sequence impedance of the load side, Nf is the negative sequence impedance of the line, NL is the line impedance from the fault point f to the M end, from the fault point f to the N end, and from the N end to the load, respectively, load is the load impedance, is the line impedance angle, is the load impedance angle. If then When there is no T-connected DG in the line in the distribution network protection zone, the ratio of the positive and negative sequence currents is expressed by the formula: positive and negative components of the line M end current positive and negative components of the line M end current, positive and negative components of the line N end current positive and negative components of the line N end current; According to the superposition principle, the equivalent independent current source of the downstream T junction DG in the protected area Its individual action is expressed in the formula as follows: According to the superposition principle, the equivalent independent current source in the upstream of the protected area Its individual action is expressed in the formula as follows: an amplitude of no more than From the above equation, it follows that The equivalent inductive network is obtained as follows:

2. The method according to claim 1, wherein the method is characterized in that: The method of positive and negative sequence current decomposition is specifically: The current instantaneous values and fault component instantaneous values at both ends of the line M and N are obtained, and the sequence current decomposition is performed according to the current instantaneous values and fault component instantaneous values at both ends of the line M and N, so that the positive, negative and zero sequence current amplitudes and phase values of the current and fault component at both ends of the line M and N are obtained, and the sequence current decomposition algorithm is: In the above formula: I k (ωt) is the three-phase current instantaneous value, is the three-phase current instantaneous value with a phase advance of 90°, is the m-sequence current phasor instantaneous value.

3. The method of claim 2, wherein the method is characterized by: According to the positive, negative and zero sequence current amplitudes and phase values of the current and fault component at both ends of the line M and N, the condition of the current differential protection action is: In the above formulae: are respectively positive, negative and zero sequence components of line M end current , are respectively positive, negative and zero sequence components of line N end current , ΔMN2 is the phase angle difference between line M end current fault negative sequence component and line N end current fault negative sequence component , k1, k2 are action coefficients.

4. The method of claim 3, wherein the method is characterized by: In the criterion for judging whether the current differential protection action needs to be executed by positive and negative sequence current decomposition, after the T-connected DG is connected in the line in the distribution network protection zone, the ratio of the positive and negative sequence currents is larger than that without the DG, and due to the fact that the DG does not output the negative sequence current, the phase difference of the fault component negative sequence currents at both ends of the line is not affected by the connection of the DG, so that the differential protection can reliably act, The protection range of the criterion for the inter-phase short-circuit resistance is expressed by the formula In the formula, U is the voltage level of the feeder, and S is the load capacity of the line.

5. The method of claim 4, wherein the method is characterized by: The current differential protection action is executed by the relay protection device.

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