Island protection method for power distribution network containing distributed power supply in consideration of power grid disturbance

Abstract

The invention relates to the field of power system protection, in particular to a distributed power supply-containing power distribution network islanding protection method considering power grid disturbance, which comprises the following steps of: acquiring power grid data of a distributed power supply access point; when the power grid data meets the voltage starting judgment data, the distributed power supply injects periodic reactive current into the grid-connected point, and reactive power flowing through a local load is sampled; carrying out wavelet decomposition on the sampled reactive power, judging whether a decomposed vector is power grid disturbance by adopting a support vector machine, and if not, injecting disturbance current into distributed power supply output current; judging whether the power grid data exceed a protection threshold value or not, if yes, judging that a distributed power supply island appears, and sending out a tripping signal by the protection device to disconnect the distributed power supply; otherwise, performing self-adaptive adjustment on the disturbance current injected into the output current of the distributed power supply, and performing power grid data re-detection; according to the method, the detection blind area is eliminated by adaptively adjusting the frequency deviation and adaptively adjusting the injected disturbance current, and the island can be identified more accurately.

Description

Technical Field

[0001] The present invention relates to the field of power system protection, and in particular to an island protection method for a distribution network containing distributed power sources taking into account power grid disturbances. Background Art

[0002] With the global energy shortage and pollution intensification and the response to the sustainable development goals, distributed power sources such as photovoltaics and wind power are one of the important forms of renewable energy utilization. Their access to the power system not only helps to alleviate energy pressure, but also improves the power supply reliability and economy of the power system. Therefore, the access ratio of such distributed power sources in the distribution network is getting higher and higher. Distributed power sources usually have the characteristics of small capacity and large volatility, so their access usually causes voltage fluctuations. At the same time, when distributed power sources form islands, it means that they are not clamped by the main power grid, which may cause a certain degree of damage to the equipment and lines in the island area. Therefore, the island protection of distributed power grid connection faces severe challenges.

[0003] At present, inverter-side island protection includes two methods: active and passive. Although the passive detection method will not have any impact on the power quality at the grid connection point, there is a large detection blind spot; the active detection method reduces the detection blind spot of the passive detection and improves the detection efficiency, but it will have a certain impact on the power quality. Therefore, it is still necessary to conduct more in-depth research on the island detection method. At the same time, the traditional island protection method is often interfered by the grid disturbance during the detection process, which will cause the misjudgment of the island detection and fail to determine the island effect in time, which will affect the power quality and grid protection.

[0004] At present, the research on island protection focuses on the rapid and stable identification of islands while avoiding protection misoperation caused by various disturbances. Some technicians have proposed a detection method based on the combination of active and passive detection methods based on harmonic analysis, which has the advantages of sensitive response and small detection blind area, but does not consider the disturbance effect from the grid voltage. Some technicians have proposed a coordinated operation method for island protection and fault crossing of inverter-type distributed power sources based on the impedance characteristics of the common connection point, which can distinguish islands from voltage transient disturbances, but still has a certain detection blind area.

[0005] To sum up, the existing island protection methods have detection blind spots, and the reliability of island protection is affected by power grid disturbances. Therefore, eliminating the detection blind spots and improving the detection sensitivity, and exploring island protection methods that take into account the impact of power grid disturbances have become issues that technical personnel in this field urgently need to solve. Summary of the invention

[0006] In order to solve the above problems in the prior art, the present invention proposes a method for island protection of a distribution network containing distributed power sources considering power grid disturbances, the method comprising:

[0007] S1, real-time collection of grid data from distributed power access points;

[0008] S2, setting voltage start judgment criteria, when the grid data meets the voltage start judgment criteria, executing step S3, otherwise returning to step S1;

[0009] S3, the distributed power source injects periodic reactive current into the grid connection point and samples the reactive power flowing through the local load;

[0010] S4, performing wavelet decomposition on the sampled reactive power, and using a support vector machine to determine whether the decomposed vector is a power grid disturbance, if it is a power grid disturbance, returning to step S1, otherwise executing step S5;

[0011] S5, injecting disturbance current into the output current of the distributed power supply;

[0012] S6 sets a protection threshold, determines whether the power grid data exceeds the protection threshold, if it exceeds the protection threshold, executes step S8, otherwise obtains the disturbance current adjustment times, sets the disturbance current adjustment times threshold, if the disturbance current adjustment times is greater than or equal to the disturbance current adjustment times threshold, returns to step S1, otherwise executes step S7;

[0013] S7, adaptively adjusting the disturbance current injected into the output current of the distributed power supply, and returning to step S5;

[0014] S8. It is determined that a distributed power source island has occurred, and the protection device sends a tripping signal to disconnect the distributed power source.

[0015] Beneficial effects of the present invention:

[0016] After detecting the voltage fluctuation at the common coupling point, the prior art injects periodic reactive current disturbance to force the voltage frequency at the common coupling point to fluctuate accordingly to identify the grid disturbance, but the injection of reactive current will inevitably have a certain impact on the power quality. The present invention transmits training sample data to the support vector machine and uses its own self-penalty correction algorithm to achieve deep learning, which can avoid the false operation of island protection as much as possible.

[0017] The existing technology uses fixed criteria to detect islands, which has detection blind spots and may cause protection failure. The blind spots vary with the nature of the load. The present invention eliminates the detection blind spots by adaptively adjusting the frequency offset and injecting disturbance current, and can more accurately identify islands. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to make the purpose, technical solution and advantages of the invention more clear, the present invention will be further described in detail below with reference to the accompanying drawings, in which:

[0019] Figure 1 This is a flow chart of a method for island protection of a distribution network containing distributed power sources taking into account power grid disturbances according to the present invention;

[0020] Figure 2 This is a structural diagram of a distribution network containing distributed power sources. DETAILED DESCRIPTION

[0021] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0022] A method for island protection of a distribution network with distributed generation considering power grid disturbances is proposed. This method preliminarily determines the possibility of islanding by establishing voltage start-up criteria, and then identifies power grid disturbances through the self-punishment correction mechanism of the support vector machine. Then, an improved active frequency offset method is proposed, which eliminates the detection blind area and improves the accuracy of island protection. The specific process is as follows: Figure 1 As shown, the method includes:

[0023] S101: collecting the amplitude and effective value of the power frequency voltage and mth harmonic voltage of the distributed power supply access point in real time;

[0024] S102: If the mth harmonic voltage meets the start criterion, execute step S103, otherwise execute step S101;

[0025] S103: The distributed generation injects periodic reactive current into the grid connection point and samples the reactive power flowing through the local load;

[0026] S104: performing wavelet decomposition on the sampled reactive current, and using a support vector machine to check the decomposed detail components. If it is determined that the grid is in a disturbance state, executing step S101; otherwise, executing step S105;

[0027] S105: injecting a disturbance current into the output current of the distributed power source to cause a frequency shift;

[0028] S106: collecting the voltage and frequency of the distributed power access point in real time to determine whether they exceed the protection threshold. If the voltage and frequency exceed the protection threshold, executing step S108; otherwise, determining whether the number of times the disturbance current is adjusted is greater than or equal to 3 times. If so, returning to step S101, otherwise executing step S107;

[0029] S107: Adaptively adjust the disturbance current injected into the output current of the distributed power source, and execute step S105;

[0030] S108: It is determined that a distributed power source island is formed, and the protection device sends a tripping signal to disconnect the distributed power source.

[0031] As Figure 2 The analysis is carried out taking the distributed power distribution network structure schematic diagram shown as an example.

[0032] The amplitude and effective value of the power frequency voltage and mth harmonic voltage at the distributed power access point are collected in real time, where m is selected as follows:

[0033]

[0034] Z s =R s +jL s

[0035] ω0=2πf N

[0036] Among them, Z s is the equivalent impedance of the power grid, ω0 is the power frequency angular frequency, L and C are the capacitance and inductance parameters of the local load, and Rs is the impedance. When an island occurs, according to the analysis, the mth harmonic voltage that satisfies the above formula will increase, so these harmonics can be used as a preliminary criterion for identifying whether an island occurs.

[0037] According to the above formula, select the appropriate harmonic order m and sample it.

[0038] The voltage start judgment criterion is: calculate the difference between the maximum value of the mth harmonic voltage amplitude and the effective value of the mth harmonic voltage; divide the difference with the maximum value of the mth harmonic voltage amplitude. If the division result is between [0.5,1], the voltage start judgment criterion is met, otherwise it is not met. That is, the voltage start judgment criterion is:

[0039]

[0040] Among them, max(U mt ) is the maximum value of the mth voltage harmonic amplitude, U m0 It is the effective value of the mth harmonic voltage when the system is operating normally. That is, when the mth harmonic voltage amplitude at the current sampling point is twice or more than the effective value of the harmonic voltage during normal operation, the startup criterion is met.

[0041] In the specific implementation, in step S104, 40 groups of samples are taken for the reactive power flowing through the local load under the non-grid disturbance and grid disturbance states, and the first, second and third layer wavelet detail coefficients d1, d2 and d3 are obtained by wavelet decomposition. The 40 groups of d1, d2 and d3 obtained by decomposition are used as feature vectors as training samples of the support vector machine classifier, and the self-penalty correction algorithm of the support vector machine classifier is used to identify the non-grid disturbance and grid disturbance. After that, the first, second and third layer wavelet detail coefficients d1, d2 and d3 obtained by decomposition under the non-grid disturbance and grid disturbance states of another 50 groups are input into the support vector machine for classification detection to verify the accuracy.

[0042] In this embodiment, the self-penalty correction algorithm of the support vector machine classifier is used to identify non-grid disturbances and grid disturbances, including: the support vector machine performs classification learning through 40 sets of training samples previously input, finds the optimal classification hyperplane, and reduces the error influence caused by experience factors by introducing a relaxation factor. At the same time, the penalty factor can adjust the degree of penalty for misclassification in the case of misclassification, so that it can find the most appropriate data position between error rate and complexity to distinguish between grid and non-grid disturbances.

[0043] Wavelet transform is a localized analysis of time (space) and frequency, which gradually refines the signal (function) at multiple scales through scaling and translation operations. It ultimately achieves time subdivision at high frequencies and frequency subdivision at low frequencies, and can automatically adapt to the requirements of time-frequency signal analysis, thereby focusing on any details of the signal.

[0044] The disturbance current is injected into the output current of the distributed power supply to cause a frequency shift. The instantaneous value of the output current of the distributed power supply after the disturbance is injected is:

[0045] i=I m sin(ωt+θ Δ )

[0046] Among them, θ Δ is the active frequency shift angle, which is determined by the following formula:

[0047]

[0048] Where cf is the chopping factor. As shown in the above formula, the phase of the final output current is determined by the chopping factor cf.

[0049] In the specific implementation, in step S106, the voltage and frequency of the distributed power access point are collected in real time to determine whether they exceed the protection threshold. If the voltage and frequency exceed the protection threshold, the protection device sends a trip signal to disconnect the distributed power supply; otherwise, it is determined whether the number of adjustments of the disturbance current is greater than or equal to 3 times. If so, it returns to the starting step S101, otherwise, the injected current is readjusted to form a disturbance again and make a judgment.

[0050] In this embodiment, adaptive adjustment of the disturbance current is achieved by modifying the chopping factor and adaptively adjusting the positive feedback coefficient k.

[0051] The chopping factor of the uncorrected positive feedback active frequency shift algorithm is defined as follows:

[0052] (cf) 原 = cf0 + k·(f PCC -f0)

[0053] Where f0 is the rated frequency of the power grid, and k is the positive feedback coefficient introduced by the positive feedback active frequency shift algorithm during the shift process.

[0054] The chopping factor cf is corrected to:

[0055] (cf) 修 =sign(f PCC -f0)·cf0+k·(f PCC -f0)

[0056]

[0057] At the same time, the adaptive adjustment of the positive feedback coefficient k is realized in the above-mentioned modified chopping factor; taking Q f0 ×C norm Coordinate plane expression method, the detection blind area of ​​active frequency shift method can be summarized as:

[0058]

[0059] Among them C norm is the per-unit value relative to the resonant capacitor, Q f0 is the load quality factor at the rated frequency of the power grid and is defined as follows:

[0060]

[0061] Where R, L, C are local load parameters respectively; ω0 is the rated angular frequency of the power grid.

[0062] Let the lower boundary of the detection blind area be greater than the upper boundary to obtain:

[0063]

[0064] After sorting, we can get the value range of the positive feedback coefficient:

[0065]

[0066] Finally, it is organized into this form:

[0067]

[0068] By real-time detection of the parameter f PCC , Q, P, to achieve adaptive adjustment of the positive feedback coefficient k under different load conditions.

[0069] The present invention provides an island protection method for a distribution network containing distributed power sources taking into account power grid disturbances. A voltage starting criterion is established on the basis of taking into account power grid disturbances to preliminarily judge that an island may be generated. A self-punishment correction mechanism of a support vector machine is proposed to identify power grid disturbances. An improved active frequency offset method is proposed to eliminate detection blind spots, improve the accuracy of island protection, and effectively solve the problem of insufficient reliability of existing methods.

[0070] The above embodiments further illustrate the purpose, technical solutions and advantages of the present invention in detail. It should be understood that the above embodiments are only preferred implementation modes of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made to the present invention within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for island protection of a distribution network containing distributed power sources considering power grid disturbance, characterized in that: include: S1, real-time collection of grid data from distributed power access points; S2, setting voltage start judgment criteria, when the grid data meets the voltage start judgment criteria, executing step S3, otherwise returning to step S1; S3, the distributed power source injects periodic reactive current into the grid connection point and samples the reactive power flowing through the local load; S4, performing wavelet decomposition on the sampled reactive power, and using a support vector machine to determine whether the decomposed vector is a power grid disturbance, if it is a power grid disturbance, returning to step S1, otherwise executing step S5; S5, injecting disturbance current into the output current of the distributed power supply; S6, set a protection threshold, determine whether the power grid data exceeds the protection threshold, if it exceeds the protection threshold, execute step S8, otherwise obtain the disturbance current adjustment times, set the disturbance current adjustment times threshold, if the disturbance current adjustment times is greater than or equal to the disturbance current adjustment times threshold, return to step S1, otherwise execute step S7; S7, adaptively adjusting the disturbance current injected into the output current of the distributed power supply, and returning to step S5; S8. It is determined that a distributed power source island has occurred, and the protection device sends a tripping signal to disconnect the distributed power source.

2. The method for island protection of a distribution network containing distributed power sources considering power grid disturbance according to claim 1, characterized in that: The grid data collected at the distributed power access point include: power frequency voltage, amplitude of mth harmonic voltage, effective value of mth harmonic voltage, voltage and frequency.

3. The method for island protection of a distribution network containing distributed power sources considering power grid disturbance according to claim 2, characterized in that: The value of the harmonic voltage order m is: Among them, Z s is the equivalent impedance of the power grid, ω0 is the angular frequency of the power frequency, and L and C are the capacitance and inductance parameters of the local load.

4. The method for island protection of a distribution network containing distributed power sources considering power grid disturbance according to claim 1, characterized in that: The voltage start judgment criterion is: calculate the difference between the maximum value of the mth harmonic voltage amplitude and the effective value of the mth harmonic voltage; divide the difference by the maximum value of the mth harmonic voltage amplitude. If the division result is between [0.5,1], the voltage start judgment criterion is met, otherwise it is not met.

5. The method for island protection of a distribution network containing distributed power sources considering power grid disturbance according to claim 1, characterized in that: The wavelet decomposition of the sampled reactive power includes: using a wavelet decomposition algorithm to decompose the reactive power into three layers of wavelet detail coefficients; using the three layers of wavelet coefficients as feature vectors, and inputting the feature vectors into a support vector machine model for training; and using a penalty correction algorithm of the support vector machine model to identify power grid disturbances.

6. The method for island protection of a distribution network containing distributed power sources considering power grid disturbance according to claim 1, characterized in that: The instantaneous value of the distributed power supply output current after the disturbance current is injected is: i=I m sin(ωt+θ Δ ) Among them, θ Δ is the active frequency shift angle, ω is the angular frequency, I m is the mth distributed current value, and cf is the chopping factor.

7. The method for island protection of a distribution network containing distributed power sources considering power grid disturbance according to claim 1, characterized in that: The adaptive adjustment of the disturbance current includes correcting the chopping factor and adaptively adjusting the positive feedback coefficient k, so as to realize the adaptive adjustment of the disturbance current.

Images