Differential protection method for distribution network based on harmonic characteristics of photovoltaic power supply

Abstract

This invention relates to a differential protection method for distribution networks based on the harmonic characteristics of photovoltaic power sources. The method includes: collecting three-phase voltage and current at both ends of a protected section; determining whether a fault has occurred based on fault occurrence criteria; calculating the second and third harmonic amplitudes at both ends of the protected section using Fourier transform when a fault occurs; exchanging information between the protected section ends and the common grid connection point; and determining whether action is required based on the photovoltaic system's position relative to the protected section using appropriate fault identification criteria. This application designs fault identification criteria using the vector sum of harmonic components of the short-circuit current at both ends and, outside the protected section, designs fault identification criteria based on the ratio of harmonic component amplitudes at both ends, ensuring the selectivity and sensitivity of the differential protection when a line fault occurs under different fault scenarios.

Description

Technical Field

[0001] This invention relates to the field of photovoltaic power distribution network protection technology, and specifically to a differential protection method for power distribution networks based on the harmonic characteristics of photovoltaic power sources. Background Technology

[0002] After photovoltaic (PV) power is connected to an inverter, the characteristics of short-circuit faults in the distribution network change due to factors such as the inverter's control strategy. Traditional power frequency distribution network protection methods are insufficient to address these new challenges. When the PV capacity is large, the operating current during normal system operation may exceed the braking current, causing maloperation of the protection system. Conversely, if the braking coefficient is set too high, the sensitivity of the protection system will decrease, and if the transition resistance is high, it may fail to operate. Although methods such as active injection can utilize high-frequency harmonic information to improve the shortcomings of power frequency protection methods, they still face challenges due to difficulties in initiating harmonic injection in some cases and the impact of the introduced high-order harmonics on grid stability. Furthermore, the application of PV output current harmonic characteristics in relay protection is still relatively lacking. Therefore, it is necessary to propose a novel differential protection scheme that addresses the problem of maloperation or failure to operate in power frequency protection systems caused by the output characteristics of PV power sources, taking into account the PV output harmonic characteristics. Summary of the Invention

[0003] This invention aims to provide a differential protection method for distribution networks based on the harmonic characteristics of photovoltaic power sources. It constructs fault initiation criteria using voltage dips and second- and third harmonic content, determines the fault type based on the second- and third harmonic content, and designs a fault identification criterion based on the vector sum of harmonic components of the short-circuit current at both ends when photovoltaic power is present within the protected section. Outside the protected section, a fault identification criterion is designed based on the amplitude ratio of harmonic components at both ends, ensuring the selectivity and sensitivity of the differential protection when line faults occur under different fault scenarios. Validation was performed in the MATLAB / Simulink environment, and simulation results show that it can meet the relay protection requirements of distribution networks containing photovoltaic power, has strong resistance to transition resistance, and can operate effectively.

[0004] The technical solution adopted by this invention to solve its technical problem is: a differential protection method for distribution networks based on the harmonic characteristics of photovoltaic power sources, comprising: S1. Collect the three-phase voltage and current at both ends of the protected section, and determine whether a fault has occurred based on the fault occurrence criteria. S2. When a fault occurs, detect the three-phase voltage and current at both ends of the protected section and the common grid connection point for one power frequency cycle, and calculate the second and third harmonic amplitudes at both ends of the protected section through Fourier transform. S3. Information from both ends of the protected section and information from the public grid connection point are sent to the other end for information exchange. The information includes the second harmonic amplitude and the third harmonic amplitude at both ends of the protected section, as well as the second harmonic amplitude and the third harmonic amplitude at the public grid connection point. S4. Based on the location of the photovoltaic system relative to the protected area, use the corresponding fault identification criteria to determine whether action is required.

[0005] Specifically, the fault occurrence criteria in step S1 include: The fault occurrence criterion is based on the voltage drop amplitude.

[0006] In the formula, U M Here is the line voltage of bus M; ρ = 0.9U M.N This is the threshold value; U M.N The rated voltage of bus M; When the first fault occurrence criterion is met, it is determined that a fault has occurred, and preparations are made to begin exchanging the information required for the fault identification criterion. When fault occurrence criterion one is not met, fault occurrence criterion two is designed by combining the harmonic characteristics of photovoltaic power supply output and utilizing the difference in harmonic content between normal and fault conditions:

[0007] In the formula, , The content of second and third harmonics in the current flowing through bus M; If the second fault occurrence criterion is met, a fault is determined to have occurred; if the criterion is not met, no fault is determined to have occurred, and the process returns to continue reading the voltage on the M side.

[0008] Specifically, step 3 involves determining the fault identification criterion when the photovoltaic power source is located within the protected area.

[0009] In the formula, I M.H 、I N.H The harmonic signal current amplitude measured by the protection devices at both ends of line M and N is denoted as , and the harmonic signal current K flowing through the fault point is denoted as . H =0.9. This coefficient is set to allow for some margin due to the inherent errors in current transformers and other devices. It can be adjusted based on the accuracy of the current transformer. The sum of the harmonic vectors of the currents at both ends. This is the action value corresponding to this value.

[0010] When the fault identification criteria are met, it is determined that a fault has occurred within the protected area, and the protection system operates. When the criteria are not met, it is determined that a fault has occurred outside the protected area, and the protection system does not operate.

[0011] Specifically, step 3 involves determining the fault identification criterion when the photovoltaic power source is located outside the protected area:

[0012] When the fault identification criteria are met, it is determined that a fault has occurred within the protected area, and the protection system operates. When the criteria are not met, it is determined that a fault has occurred outside the protected area, and the protection system does not operate.

[0013] The beneficial effects of this invention are as follows: The dual-start criterion proposed in this application combines voltage drop and harmonic content detection, effectively solving the problem of difficulty in starting a single voltage criterion under minor faults. Furthermore, the information exchange after local criterion start reduces communication resource requirements to some extent. The fault identification criterion constructed based on the vector sum and amplitude ratio of the second and third harmonic currents at both ends of the protected section exhibits good selectivity under different types of fault conditions, accurately distinguishing between faults inside and outside the protected area. Simulation verification results show that the proposed protection scheme has better adaptability and reliability than traditional methods, providing an effective approach to solving protection problems in photovoltaic power distribution networks. Attached Figure Description

[0014] Figure 1 This is the overall flowchart of this application.

[0015] Figure 2 This is a schematic diagram of a photovoltaic power distribution network.

[0016] Figure 3 This is a network diagram of fault current harmonic components inside and outside the protection zone when there is a photovoltaic power source within the protected area.

[0017] Figure 4 This is a network diagram of fault current harmonic components inside and outside the protection zone when there is a photovoltaic power source outside the protection zone.

[0018] Figure 5 Harmonic characteristic diagram of output current in a photovoltaic inverter under asymmetric fault; Figure 6 This is a diagram showing the harmonic characteristics of the output current during a symmetrical fault in a photovoltaic inverter.

[0019] Figure 7 The specific action flowchart of this application. Detailed Implementation

[0020] The invention will now be described in further detail with reference to the accompanying drawings.

[0021] like Figure 1 The method for differential protection of distribution networks based on the harmonic characteristics of photovoltaic power sources, as shown, includes: S1. Collect information from both ends of the protected section, and determine whether a fault has occurred based on the information from the first end and the fault occurrence criteria. S2. When a fault occurs, detect the three-phase current on the M side, N side, and the common grid connection point for one power frequency cycle, and calculate the second and third harmonic amplitudes on both sides of the protected section through Fourier transform. S3. Information from both ends of the protected section and information from the public grid connection point are sent to the other end for information exchange. The information includes the second harmonic amplitude and third harmonic amplitude from both ends of the protected section, as well as the second harmonic amplitude and third harmonic amplitude from the public grid connection point. That is, the second harmonic amplitude from side M, the second harmonic amplitude from side N, the third harmonic amplitude from side M, the third harmonic amplitude from side N, and the second harmonic amplitude and third harmonic amplitude from the public grid connection point. Information from side M, side N, and the public grid connection point is sent to the other end for information exchange. S4. Based on the location of the photovoltaic system relative to the protected area, use the corresponding fault identification criteria to determine whether action is required.

[0022] The following further explains the application, analyzing the adaptability of traditional differential protection in photovoltaic distribution networks. The typical system topology is as follows: Figure 2 As shown, in a traditional distribution network where photovoltaic power is not connected, the operating current setting value I is [not specified]. set Only the unbalanced current I in the system needs to be considered. unb That's sufficient. When photovoltaic power is connected to the distribution network, in addition to the unbalanced current I in the system... unb In addition, the impact of connecting to photovoltaic systems also needs to be considered.

[0023] Figure 2 In the middle, S G LD1 and LD2 are the load branches connected to busbars C and D, respectively; PV is the photovoltaic power source; f in f out These represent the locations of faults within and outside the protected area, respectively. When photovoltaic (PV) power is connected to the distribution network, the output power of the PV power source is affected by external factors (such as sunlight and wind speed), therefore, it is impossible to control the fault location within the protected area. set Timely setting can lead to protection malfunctions. Furthermore, existing differential protection methods often introduce restraining current to reduce the impact of unbalanced current. The final differential protection criterion is shown in the following formula:

[0024] In the formula, I op1 For operating current, I M.1 , I N.1 The power frequency current at both ends of the protected section MN, k r This is the braking coefficient. I rThis is the braking current.

[0025] when I op1 When it is larger, then I r The smaller the value, the lower the operating current when the photovoltaic capacity is large; when the system is operating normally, the operating current will be lower. I op1 It may be greater than the braking current. I r If the braking coefficient is set too high, the sensitivity of the protection will be reduced, and if the transition resistance is too high, the protection may fail to operate.

[0026] Under symmetrical fault conditions, the power grid operates symmetrically and does not contain negative sequence components. However, after a three-phase short-circuit fault occurs in the power grid, the grid voltage changes abruptly, causing a rapid increase in the fundamental frequency in the grid-connected current, along with a certain DC attenuation component. This attenuation typically occurs in tens of milliseconds, while the response speed of power electronic devices is on the order of microseconds. When this DC component is introduced into the control loop, it will trigger second harmonic components.

[0027] When the photovoltaic grid-connected inverter outputs active power, due to the influence of transient DC components, it contains a fundamental frequency fluctuation component, which in turn causes a fundamental frequency fluctuation component to appear in the DC voltage. The fundamental frequency fluctuation component in the DC bus voltage can be defined as:

[0028] In the formula , The amplitude and initial phase angle of the fundamental frequency fluctuation component are given.

[0029] Since the attenuation of the fundamental frequency fluctuation component is negligible at instantaneously, the component passing through the control loop is obtained according to the positive sequence control strategy as follows:

[0030] In the formula for Laplace transform.

[0031] Taking the inverse Laplace transform of the above equation and discarding the DC component, the time-domain expression of the fundamental harmonic component of the voltage reference is obtained as follows:

[0032] The fundamental frequency fluctuation attenuation component in DC voltage flows only along the d-axis, therefore the corresponding quantity along the q-axis can be considered as... =0, transforming the fundamental frequency fluctuation component to the three-phase stationary coordinate system yields:

[0033]

[0034]

[0035] in:

[0036] When the fundamental frequency fluctuation component passes through the control loop, it will introduce DC components and second harmonics into the generated voltage reference value. This will result in the final current generated by the photovoltaic inverter also containing second harmonic components.

[0037] A 200kW photovoltaic power supply was simulated in MATLAB. Fourier analysis was performed on the inverter output current during the first power frequency cycle 20ms after a symmetrical fault occurs, under different transition resistances, to obtain the harmonic content of each harmonic. Figure 5 As shown.

[0038] When an asymmetrical fault occurs, compared with a symmetrical fault, the grid-connected voltage and current will have an additional negative sequence component due to the influence of the asymmetrical operation of the system, and the output active power will have an additional second harmonic fluctuation component.

[0039] When the active power output of a photovoltaic grid-connected inverter contains a second harmonic ripple component due to the influence of negative sequence components, this will also cause a second harmonic ripple component to appear in the DC voltage. The second harmonic ripple component in the DC bus voltage can be defined as:

[0040] In the formula , The amplitude and initial phase angle of the fundamental frequency fluctuation component are given.

[0041] The second harmonic component in DC voltage will cause The second harmonic component also appears, and the negative sequence current command value is... The function of this property causes it to also exhibit a second harmonic component. After passing through the positive sequence control loop, the second harmonic component generates the following component in the voltage reference value:

[0042] In the formula for Laplace transform.

[0043] After performing an inverse Laplace transform on the equation and discarding the DC component, the time-domain expression for the second harmonic component of the voltage reference quantity is obtained as follows:

[0044] The second harmonic ripple attenuation component in DC voltage flows only along the d-axis, therefore the corresponding quantity along the q-axis can be considered as... =0, transforming the second harmonic wave component to a three-phase stationary coordinate system yields:

[0045]

[0046]

[0047] in

[0048] It can be seen that after the second harmonic component of the DC voltage flows through the control circuit, in addition to generating the fundamental frequency negative sequence component, a non-zero sequence third harmonic component is added to the positive sequence voltage command value output by the inverter. This will result in the appearance of the third harmonic component in the output current of the photovoltaic inverter.

[0049] A 200kW photovoltaic power supply was simulated in MATLAB. The inverter output current was analyzed using Fourier analysis on the first power frequency cycle 20ms after an asymmetrical fault occurred, under different fault types and transition resistances, to obtain the harmonic content of each harmonic. Figure 6 As shown.

[0050] When photovoltaic power exists within the protected area, such as Figure 2 Taking a 10kV photovoltaic distribution network as an example, this paper analyzes the harmonic signal fault characteristics at both ends of the protected section. A 200KVA photovoltaic power source is configured within the MN section of the line. Considering the generally short length of distribution network lines, to more clearly reveal the fault characteristics of harmonic current signals, the relatively small line impedance can be ignored when analyzing its propagation path. The harmonic signal fault characteristics at both ends of the protected section are analyzed within the harmonic network corresponding to the harmonic signal. The equivalent circuit of the harmonic phase harmonics in the photovoltaic distribution network when faults occur inside or outside the protected area is shown below. Figure 3 As shown.

[0051] Figure 3 middle, I M.H 、I N.H The harmonic signal current amplitude measured by the protection devices at both ends of line MN is , and the harmonic signal current flowing through the fault point is . Z M.h Z is the equivalent harmonic impedance of the upstream system at point M; N.h The equivalent harmonic impedance of the downstream load at point N; R f For fault transition resistance, I PCC.H This refers to the harmonic amplitude injected into the distribution network at the photovoltaic point of common coupling.

[0052] According to Kirchhoff's current law, when a fault occurs within the protected section or outside the protected section, the vector sum of the harmonic signal currents at both ends... The following relationship must be satisfied:

[0053] The symbols in the formula and Figure 3 Consistent.

[0054] In the event of a fault outside the protected section, since the protected section MN lacks obvious shunt branches such as transition resistance branches, therefore... It is approximately equal to the harmonic amplitude of the current injected by the photovoltaic power source; however, when an in-zone fault occurs, due to Z... M.h With Z N.h All exhibit inductive impedance, making and The phase angle difference between them is always less than 90°, therefore, Less than Furthermore, this difference is more pronounced when the transition resistance is low. Much smaller Therefore, it can be concluded that, under fault scenarios both inside and outside the protected area, the vector sum of the harmonic signal currents at both ends... It should have the following characteristics:

[0055] As can be seen from the equation, the harmonic signal current vectors at both ends can be used to determine the relationship between them. The value determines whether the fault occurs inside or outside the protected zone. When a harmonic signal current satisfies... < A fault within the zone can be determined immediately.

[0056] When there is a photovoltaic power source outside the protected area, the same applies. Figure 2 Taking a 10kV photovoltaic distribution network as an example, a 200KVA photovoltaic power source is configured outside the MN section of the line. The harmonic signal fault characteristics at both ends of the protected section are analyzed. Considering the similarity between upstream and downstream photovoltaic systems, the case of upstream photovoltaic system is taken as an example. Similar treatment is applied as when photovoltaic system exists within the protected section, ignoring the relatively small line impedance. The harmonic signal fault characteristics at both ends of the protected section are analyzed within the harmonic network corresponding to the harmonic signals. The equivalent circuit of the harmonic phase harmonics in the photovoltaic distribution network when a fault occurs inside or outside the protected section is shown below. Figure 4 As shown.

[0057] According to Kirchhoff's laws, when a fault occurs within the protected zone or outside the protected zone, the harmonic signal current vector at both ends... The following relationship will be satisfied:

[0058] When a fault occurs outside the protected section, the harmonic currents at both ends are approximately equal because there are no obvious shunt branches such as transition resistance branches inside the protected section MN. However, when a fault occurs inside the protected section, the harmonic currents at both ends will no longer be approximately equal because there are shunt branches inside the protected section. The harmonic current flowing through the end farther from the photovoltaic power source will be smaller, and this difference will be more significant when the transition resistance is small.

[0059] When a short-circuit fault occurs in the distribution network, a voltage drop will occur. Considering the fault ride-through control characteristics of photovoltaic power sources under low voltage, a fault occurrence criterion is designed based on the voltage drop magnitude:

[0060] In the formula, U M Here is the line voltage of bus M; ρ = 0.9U M.N This is the threshold value; U M.N This is the rated voltage of bus M.

[0061] When the criteria are met, it is determined that a fault has occurred, and preparations are made to begin exchanging the information required for the fault identification criteria. When the transition resistance of the distribution network is small, this criterion can operate normally. However, when the transition resistance of the distribution network is large during a fault, especially when a single-phase short circuit occurs, the voltage drop is low. Using this as the criterion alone may result in the protection device failing to operate.

[0062] Therefore, based on the above criteria and combined with the harmonic characteristics of the photovoltaic power supply output, the harmonic content in the output current of the photovoltaic power supply after a fault is quantitatively calculated according to the expression of the second and third harmonic characteristics of the photovoltaic output. Further considering different types of faults and changes in transition resistance, the criteria are designed using the difference in harmonic content between normal and fault conditions.

[0063] In the formula, K H =0.9. Because current transformers and other devices have a certain degree of error, a coefficient is set to allow for a certain margin. This coefficient can be adjusted according to the accuracy of the current transformer.

[0064] When the photovoltaic system is located outside the protected area, taking the photovoltaic system located upstream of the protected area as an example, when the fault initiation conditions are met in the distribution network, the second or third harmonic current information of a complete power frequency cycle is collected and exchanged on both sides of the protected area and at the common grid connection point. When the obtained second or third harmonic current vectors on both sides of the protected area meet the following criteria, the fault can be identified as being within the protected area:

[0065] In summary, as Figure 7 The specific action flowchart is shown. First, the local voltage and current information of the M bus is read. Based on the voltage drop amplitude, it is checked whether the fault occurrence criterion one is met. If it is met, a fault is determined to have occurred, and information exchange is performed. If the fault occurrence criterion one is not met, the harmonic characteristics of the photovoltaic power supply output are combined, and the difference in harmonic content between normal and fault conditions is used to determine whether the fault release criterion two is met. If it is met, a fault is determined to have occurred. The second or third harmonic current amplitude at each point is obtained through Fourier transform. After information exchange, the relevant fault identification criterion formula is used for calculation to complete the fault type determination. When the fault is determined to be within the protection zone, the action is performed. If the fault occurs outside the protection zone, the protection returns and the local information is read again. At this point, the information of the N side and photovoltaic can be omitted, and only the M terminal information can be used to determine whether the fault has occurred. Here, only the M terminal information can be read.

[0066] This invention is not limited to the described embodiments. Anyone should know that any structural changes made under the guidance of this invention, and any technical solutions that are the same as or similar to this invention, fall within the protection scope of this invention.

[0067] The technologies, shapes, and structures not described in detail in this invention are all known technologies.

Claims

1. A differential protection method for distribution networks based on the harmonic characteristics of photovoltaic power sources, characterized in that: include: S1. Collect the three-phase voltage and current at both ends of the protected section, and determine whether a fault has occurred based on the fault occurrence criteria. S2. When a fault occurs, detect the three-phase voltage and current at both ends of the protected section and the common grid connection point for one power frequency cycle, and calculate the second and third harmonic amplitudes at both ends of the protected section through Fourier transform. S3. Information exchange, wherein the information includes the second harmonic amplitude and the third harmonic amplitude at both ends of the protected section, as well as the second harmonic amplitude and the third harmonic amplitude at the common grid connection point. The information at both ends of the protected section and the information at the common grid connection point are sent to the other end for information exchange. S4. Based on the location of the photovoltaic system relative to the protected area, use the corresponding fault identification criteria to determine whether action is required.

2. The method for differential protection of distribution networks based on the harmonic characteristics of photovoltaic power sources according to claim 1, characterized in that: The fault occurrence criteria in step S1 include: The fault occurrence criterion is based on the voltage drop amplitude. ； In the formula, U M Here is the line voltage of bus M; ρ = 0.9U M.N This is the threshold value; U M.N The rated voltage of bus M; When the first fault occurrence criterion is met, it is determined that a fault has occurred, and preparations are made to begin exchanging the information required for the fault identification criterion. When fault occurrence criterion one is not met, fault occurrence criterion two is designed by combining the harmonic characteristics of photovoltaic power supply output and utilizing the difference in harmonic content between normal and fault conditions: ； In the formula, , The content of second and third harmonics in the current flowing through bus M; If the second fault occurrence criterion is met, a fault is determined to have occurred; if the criterion is not met, no fault is determined to have occurred, and the process returns to continue reading the voltage on the M side.

3. The method for differential protection of distribution networks based on the harmonic characteristics of photovoltaic power sources according to claim 1, characterized in that: Step 3 specifically involves determining the fault identification criterion when the photovoltaic power source is located within the protected area. ； In the formula, I M.H 、I N.H The harmonic signal current amplitude measured by the protection devices at both ends of line M and N is denoted as , and the harmonic signal current K flowing through the fault point is denoted as . H =0.9, because there are certain errors in devices such as current transformers, so a coefficient is set to leave a certain margin. In practice, it can be adjusted according to the accuracy of the current transformer. When the fault identification criteria are met, it is determined that a fault has occurred within the protected area, and the protection system operates. When the criteria are not met, it is determined that a fault has occurred outside the protected area, and the protection system does not operate.

4. The method for differential protection of distribution networks based on the harmonic characteristics of photovoltaic power sources according to claim 1, characterized in that: Specifically, step 3 involves determining the fault identification criterion when the photovoltaic power source is located outside the protected area: ； When the fault identification criteria are met, it is determined that a fault has occurred within the protected area, and the protection system operates. When the criteria are not met, it is determined that a fault has occurred outside the protected area, and the protection system does not operate.

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