A method and system for regulating inverse time overcurrent relay protection

Abstract

The application provides a reverse time-lag overcurrent relay protection regulation method and system, comprising: when the system resonates based on the outlet current monitoring of the APF: based on the pre-constructed optimal relay protection action table, the APF of the optimal relay protection action value is cut off; continue to monitor the outlet current of the APF, if the current harmonic content is less than the action value but the power grid quality does not meet the set power supply quality requirement, based on the power quality requirement in the micro grid, the active damping control is added in the selected APF. When the micro grid island APF parallel system operation resonates, first, the connection between the APF and the large power grid is cut off by the reverse time-lag overcurrent relay protection to suppress the inter-machine resonance, then the active damping control is added to improve the system power quality, the problem that the phase deviation exists between the LCL type APF output compensation current and the detection current is solved, the system loss is reduced, the effective suppression of the APF parallel inter-machine resonance is realized, and the power quality is improved.

Description

Technical Field

[0001] The present invention relates to an inverse time overcurrent relay protection technology for an islanded microgrid, and in particular to an inverse time overcurrent relay protection control method and system when multiple active power filters are connected in parallel and resonate under the condition of islanded microgrid operation. Background Art

[0002] When operating in an isolated microgrid, lacking the voltage support of the broader system, one or more of the various renewable energy generation devices (inverters) within the microgrid, previously operating in current source mode, must switch to voltage source mode. Due to the limited capacity of renewable energy generation, they cannot provide the same sinusoidal voltage support as the broader grid. Furthermore, various power electronic rectifiers and inverters within the microgrid, such as electric vehicle charging stations and photovoltaic generators, generate significant harmonics, which can degrade power quality within the grid and, in severe cases, cause converter equipment to disconnect from the grid. To ensure system power quality during isolated operation, active power filters (APFs), as a dynamic harmonic mitigation device, have begun to be used in microgrid power quality control. Compared to traditional compensation devices consisting of passive LC filter circuits, APFs detect harmonic currents from the source and generate a compensating current of equal magnitude but opposite polarity, thereby ensuring that the grid current contains only the fundamental component. This active filter has gained widespread attention due to its ability to track and compensate for harmonics that vary in both amplitude and frequency.

[0003] However, with the development and expansion of microgrids, the increasing number of power electronic devices has led to a growing demand for APF equipment to ensure that the power quality of microgrids meets standards. APFs have begun to be connected to the power system in large numbers and in a disorderly manner, especially on the user load side. This is bound to cause negative interactions between APF devices. For example, when APFs need to work in parallel with multiple machines, the coupling interaction between the APF modules cannot be ignored. The most serious problem is the parallel resonance problem that occurs during the parallel connection of multiple APFs. Once such resonance occurs, it will not only affect the APF's ability to suppress microgrid system harmonics, but may also cause a chain reaction of devices within the network to disconnect from the grid.

[0004] Therefore, when operating in an islanded microgrid, insufficient transformer capacity causes the microgrid to exhibit weak grid operation characteristics. The system reference voltage quality cannot meet the power supply standards of the larger system. Consequently, as the number of parallel APFs increases, inter-machine resonance is highly likely to occur. While the proposed improved control strategy based on differential current can improve the compensation performance of multi-machine parallel APF systems, the resonant overcurrent problem of multi-machine parallel APF systems remains fundamentally unresolved when LCL filters are used in microgrids. Further proposals have been made to suppress the resonance of the LCL filters by adding passive damping resistors. While this effectively suppresses the resonance of the LCL filters and improves control system stability, the increased passive damping resistors can affect filter performance and increase equipment losses, reducing system efficiency. In summary, the problem of parallel resonance caused by multiple APFs operating in parallel, which generates current harmonics and degrades power quality, remains unresolved. Summary of the Invention

[0005] In order to address the above-mentioned deficiencies in the prior art, the present invention proposes a method and system for controlling inverse time overcurrent relay protection. Based on the inverse time overcurrent relay protection technology, some APFs with severe resonance are selectively cut off, and active damping control is added to the remaining APF devices, thereby eliminating the impact of inter-machine resonance on the power quality of the microgrid and improving the safety of microgrid operation.

[0006] The present invention provides a technical solution for an inverse time overcurrent relay protection control method, including:

[0007] When the APF-based outlet current monitoring detects system resonance:

[0008] Based on the pre-built optimal relay protection action table, the APF of the optimal relay protection action value is cut off;

[0009] Continue to monitor the APF outlet current. If the current harmonic content is less than the action value but the grid quality does not meet the set power quality requirements, add active damping control to the selected APF based on the power quality requirements in the microgrid.

[0010] Preferably, the APF-based outlet current monitoring detecting that the system is resonating includes:

[0011] Extract the current harmonic content from the monitored APF outlet current;

[0012] When the current harmonic content is not less than the action value within the inverse time overcurrent relay protection action time difference, the system resonates.

[0013] Preferably, extracting the current harmonic content from the monitored APF outlet current includes:

[0014] Based on the pre-built microgrid model, the system resonant frequency expression when APF is running in parallel is constructed;

[0015] Obtaining a resonant frequency range based on an expression of the system resonant frequency when the APFs are operated in parallel;

[0016] A bandpass filter is used to extract the current harmonic content within the resonant frequency range from the monitored APF outlet current.

[0017] Preferably, the system resonant frequency expression when the APFs are operated in parallel is as shown in the following formula:

[0018]

[0019] Where: N: number of APFs in parallel; L S : grid equivalent inductance; L1: APF AC valve side inductance; L2: grid side inductance; C: filter capacitor.

[0020] Preferably, the cutting off of the APF of the optimal relay protection action value based on the pre-built optimal relay protection action table includes:

[0021] S201, obtaining a set of optimal relay protection action values ​​corresponding to the current working condition from the pre-built optimal relay protection action table;

[0022] S202: based on a set of optimal relay protection action values ​​corresponding to the current working condition, disconnecting the APF that first reaches the optimal relay protection action value;

[0023] S203: Continue to monitor the outlet current of the APF. If the system still resonates, execute S202 until the current harmonic content is less than the action value.

[0024] Preferably, obtaining the optimal relay protection action table includes:

[0025] Based on the pre-built microgrid model, simulate all the operating conditions of APF multi-machine parallel operation in the microgrid;

[0026] Based on the outlet current of the APF under each working condition, the current harmonic content at the outlet of each APF under each working condition is obtained;

[0027] Based on the current harmonic content at the outlet of each APF under each working condition, the starting current of the inverse time overcurrent relay protection device on each APF under each working condition is obtained;

[0028] The starting current of the inverse time overcurrent relay protection device on each APF under each working condition is brought into the optimization model, and the optimization model is solved using the improved particle swarm algorithm to determine a set of optimal relay protection action values ​​corresponding to each working condition;

[0029] An optimal relay protection action table is generated based on a set of optimal relay protection action values ​​corresponding to all working conditions.

[0030] Preferably, the starting current of the inverse time overcurrent relay protection device is calculated as follows:

[0031]

[0032] Where: I set K is the starting current of the inverse time overcurrent relay protection device; k is the reliability coefficient; I th.max is the maximum current harmonic content; K f is the return coefficient.

[0033] Preferably, the starting current of the inverse time overcurrent relay protection device on each APF under each working condition is brought into the optimization model, and the optimization model is solved by using an improved particle swarm algorithm to determine a set of optimal relay protection action values ​​corresponding to each working condition, including:

[0034] The starting current of the inverse time overcurrent relay protection device on each APF under each working condition is used as the optimization variable of the optimization model;

[0035] Initializing the optimization variables based on the constructed constraints;

[0036] Setting an initial value for the change speed of the optimization variable based on preset upper and lower limits of the optimization variable update speed;

[0037] Setting an initial value for the change position of the optimization variable based on preset upper and lower limits of the optimization variable update position;

[0038] Calculating the change position of the optimization variable based on preset upper and lower limits of the optimization variable update speed;

[0039] Based on the optimization target, the initialized optimization variables, the change speed of the optimization variables and the change position of the optimization variables, a set of optimal relay protection action values ​​corresponding to each working condition is obtained through iteration.

[0040] Preferably, the change position of the optimization variable is calculated based on the preset upper and lower limits of the optimization variable update speed, as shown in the following formula:

[0041]

[0042] Where: is the position of the i-th particle at the n+1-th iteration; is the position of the i-th particle at the n-th iteration; v min is the lower limit of the variable update speed; v max is the upper limit of the optimization variable update speed; α is the first limiting coefficient; β is the second limiting coefficient.

[0043] Preferably, the adding of active damping control in the selected APF based on the power quality requirements in the microgrid includes:

[0044] Removing the optimal relay protection action value corresponding to the removed APF from a set of optimal relay protection action values ​​corresponding to the current working condition to obtain the optimal relay protection action value corresponding to each remaining APF;

[0045] Based on the difference between the optimal relay protection action value corresponding to each of the remaining APFs and the harmonic current at the outlet of each APF, the difference between the optimal relay protection action value and the harmonic current is obtained;

[0046] Active damping control is added to the APF with the smallest difference in sequence, and the power quality is monitored until the power quality in the microgrid meets the power supply quality requirements.

[0047] Preferably, the construction of the microgrid model includes:

[0048] Establish a microgrid model based on the operating topology of the microgrid under test;

[0049] Based on the electrical parameters of the equipment in the microgrid under test, parameter setting of the microgrid model is completed.

[0050] Based on the same inventive concept, the present invention provides an inverse time overcurrent relay protection control system, comprising:

[0051] The monitoring module is used to monitor the output current of the APF and detect the resonance of the system:

[0052] A cut-off module is used to cut off the APF of the optimal relay protection action value based on a pre-built optimal relay protection action table;

[0053] The active damping addition module is used to continue monitoring the outlet current of the APF. If the current harmonic content is less than the action value but the grid quality does not meet the set power quality requirements, active damping control is added to the selected APF based on the power quality requirements in the microgrid.

[0054] Preferably, the monitoring module includes:

[0055] The extraction submodule is used to extract the current harmonic content from the monitored APF outlet current;

[0056] The resonance judgment submodule is used to determine that the system resonates when the current harmonic content is not less than the action value within the inverse time overcurrent relay protection action time difference.

[0057] Compared with the prior art, the present invention has the following beneficial effects:

[0058] The technical solution provided by the present invention is based on the output current monitoring of the APF. When the system resonates, based on the pre-built optimal relay protection action table, the APF with the optimal relay protection action value is cut off; the output current of the APF is continued to be monitored. If the current harmonic content is less than the action value but the grid quality does not meet the set power supply quality requirements, active damping control is added to the selected APF based on the power quality requirements in the microgrid. When the microgrid island APF parallel system resonates during operation, the present invention first cuts off the connection between a certain APF and the large grid through inverse time overcurrent relay protection to suppress the inter-machine resonance, and then improves the system power quality by adding active damping control, thereby solving the problem of phase deviation between the output compensation current and the detection current of the LCL type APF. While reducing system losses, it achieves effective suppression of the resonance between the APF parallel machines and improves the power quality.

[0059] The technical solution provided by the present invention utilizes a combination of an inverse time overcurrent relay protection strategy and active damping to replace the passive damping APF parallel system resonance suppression strategy. Under the topological structure of microgrid island operation, the improved particle swarm optimization algorithm proposed by the present invention is used. When the microgrid APF parallel resonance occurs, a set of optimal relay protection action values ​​are quickly called for the relay protection device, so that the relay that should act the fastest can act accurately and quickly, eliminating the damage to the power supply system and transformer caused by the APF inter-machine resonance. After the resonance is eliminated, if the system power quality still does not meet the power supply quality requirements, the system can call the improved particle swarm optimization algorithm under this working condition to store the ranking of the resonant current sizes at the outlets of each APF, and select the APF with the largest resonant current at this time to add active damping control, thereby solving the problem that the system power quality does not meet the power supply quality requirements. BRIEF DESCRIPTION OF THE DRAWINGS

[0060] Figure 1 A flow chart of an inverse time overcurrent relay protection control method provided by the present invention;

[0061] Figure 2 Schematic diagram of an equivalent circuit of a capacitor in series with active damping in an embodiment of the present invention;

[0062] Figure 3 A control block diagram of an alternative process in an embodiment of the present invention;

[0063] Figure 4Flowchart of coordinated control of inverse time overcurrent relay protection and active damping of the present invention;

[0064] Figure 5 This is a control block diagram of a parallel APF system in an embodiment of the present invention;

[0065] Figure 6 FIG1 is a structural diagram of an APF system using a PI-VPI controller in an embodiment of the present invention;

[0066] Figure 7 This is a block diagram of the voltage outer loop control in an embodiment of the present invention;

[0067] Figure 8 This is a structural diagram of harmonic current fractional detection in an embodiment of the present invention;

[0068] Figure 9 This is a schematic diagram of the harmonic detection and current shunting principle in an embodiment of the present invention;

[0069] Figure 10 This is a block diagram of the current inner loop control of the VPI controller in an embodiment of the present invention;

[0070] Figure 11 1 is a block diagram of an improved control of a PLL circuit according to an embodiment of the present invention;

[0071] Figure 12 This is a flow chart of an improved particle swarm optimization algorithm according to an embodiment of the present invention;

[0072] Figure 13 This is a topology diagram of the APF parallel system of the isolated microgrid in the prior art;

[0073] Figure 14 This is a diagram of the harmonic content of four APFs in parallel in an embodiment of the present invention;

[0074] Figure 15 4 is a Bode diagram of the transfer function from the detection current to the actual output current in the embodiment of the present invention. DETAILED DESCRIPTION

[0075] In order to better understand the present invention, the present invention is further described below with reference to the accompanying drawings and examples.

[0076] Example 1

[0077] This invention proposes a method that combines an overcurrent inverse-time relay protection strategy based on frequency component identification with active damping control to achieve resonance suppression in a multi-machine parallel APF system within a microgrid, thereby ensuring safe microgrid operation. When resonance occurs in the parallel APF system, the frequency component of the line current is detected. By removing some APFs and employing an active damping control structure on the remaining APF units, the impact of the resonance caused by inter-machine harmonic circulating current on the microgrid's power quality is eliminated, thereby improving the LCL-type APF's compensation effect on harmonic currents. This invention has been verified on an electromagnetic simulation model of an isolated microgrid.

[0078] like Figure 1 As shown, the present invention provides an inverse time overcurrent relay protection control method, comprising:

[0079] S1. When the APF-based outlet current monitoring detects system resonance:

[0080] S2. Based on the pre-built optimal relay protection action table, the APF of the optimal relay protection action value is cut off;

[0081] S3. Continue to monitor the outlet current of the APF. If the current harmonic content is less than the action value but the grid quality does not meet the set power quality requirements, add active damping control to the selected APF based on the power quality requirements in the microgrid.

[0082] The concept of the present invention is as follows: an overcurrent inverse time relay protection device based on frequency component identification is installed on the line connecting the APF and the large power grid.

[0083] S1. Based on the APF outlet current monitoring, the system is resonant, including:

[0084] First, get the resonant frequency range:

[0085] Based on the actual island operation topology of the tested microgrid, an offline simulation model of the microgrid including the APF group is established. Based on the actual system and equipment electrical parameters, the microgrid equivalent model parameters are set, and the system resonant frequency expression when the APF is operating in parallel is constructed. The system resonant frequency is calculated based on this expression.

[0086] The expression of the system resonant frequency is as follows:

[0087]

[0088] Where: N: number of APFs in parallel; L S : grid equivalent inductance; L1: APF AC valve side inductance; L2: grid side inductance; C: filter capacitor;

[0089] The calculated system resonant frequency is taken as the center frequency. The frequency higher than the center frequency until it attenuates to 0.707 times the center frequency is the upper sideband, and the opposite is the lower sideband. The resonant frequency range is between the upper and lower sidebands.

[0090] Then, the system harmonic current content is monitored in real time, the frequency components in the line current are calculated and analyzed, and the current components within the resonant frequency range are extracted using a bandpass filter;

[0091] When the system current harmonic content is detected to be less than the action value within the time difference ΔT of the inverse time overcurrent relay protection action, the remaining inverse time overcurrent relay protection devices remain in the locked state. If the system current harmonic content is still greater than the action value within the time difference ΔT of the inverse time overcurrent relay protection action, the system resonates.

[0092] S2. Based on the pre-built optimal relay protection action table, the APF of the optimal relay protection action value is cut off, including:

[0093] In order to make the relay action selective when the resonance between machines occurs, and to control the number of APF cutting machines to a minimum, and to minimize the overall action time of the relay, all working conditions when multiple APF machines are running in parallel in the microgrid are simulated through offline simulation, that is, when various types of multi-machine parallel resonance occur in the APF in the microgrid system, the resonant current at the outlet of each APF is calculated, and the starting current of the relay protection on each APF line under each working condition is calculated according to the inverse time overcurrent protection formula, and then the action time of the relay protection is determined, and then the improved particle swarm optimization algorithm proposed by the present invention is used to calculate a set of optimal action current values ​​under the corresponding working conditions and store them. When the size of the optimal action current value is determined, due to the different action current values ​​at the outlet of the APF line, the time when the resonant current reaches the action current value will also be different, so there is a corresponding order for cutting off the APF. An optimal relay protection action table is formed based on a set of optimal action current values ​​corresponding to all working conditions.

[0094] When the system resonates, the next level of inverse time overcurrent relay protection device will be activated, and the operating current data under this working condition stored in the algorithm will be called up. The APF that should act fastest at this time will be cut off, and the system will monitor, control and adjust again, thereby eliminating the impact of resonance on the system power quality.

[0095] The propagation path of the resonant overcurrent targeted by the present invention is the power line. Considering the harm caused by the resonant overcurrent to the power equipment, the relay protection scheme of the present invention utilizes the super-inverse time overcurrent protection scheme, as shown in the following formula:

[0096]

[0097] Where, I pis the protection starting current, I is the fault current, t p It is the setting value of the time constant of the inverse time overcurrent protection, and its dimension is time.

[0098] Inverse time current protection starting current setting calculation formula:

[0099]

[0100] Where K k is the reliability coefficient, which is 1.2, I th.max is the maximum APF harmonic current flowing through this switch, K f is the return coefficient, which is 0.85.

[0101] S3. Continue to monitor the APF outlet current. If the current harmonic content is less than the action value but the grid quality does not meet the set power quality requirements, add active damping control to the selected APF based on the power quality requirements in the microgrid, including:

[0102] If the current harmonic content is less than the action value but the power quality still does not meet the power supply standard, the relay protection action current values ​​at the remaining APF outlets stored by the improved particle swarm optimization algorithm under the corresponding working condition can be called, and the active damping control method can be added to one or more APFs whose resonant current at the APF outlet is closest to the action current value under this working condition. The equivalent circuit of the LCL filter after active damping control is shown in Figure 2 , the control block diagram of the alternative process is as follows Figure 3 As shown, the stability of the system is strengthened. It should be noted that when implementing the technical solution provided by this application, there will not be a situation where there are no remaining APFs. Because when the number of APFs in the system decreases and the resonance is not effectively suppressed, active damping control will be added to the remaining APFs. Real-time simulation experiments have verified that after the remaining APFs are added with active damping control, the occurrence of inter-APF resonance can be effectively suppressed, thereby improving the power quality of the system.

[0103] Next, this embodiment further describes the inventive concept of the present invention in detail, including:

[0104] 1. Power quality improvement strategy of APF parallel system based on active damping

[0105] Active damping control differential link expression:

[0106]

[0107] Since the feedback variable of the current inner loop is the compensation current of the APF, Figure 4 The implementation of active damping is to add the active damping correction value Δi to the current feedback. d (s):

[0108]

[0109] After adding the active damping correction, the output voltage on the AC valve side is expressed as formula (4-4):

[0110] u PWM (s)=G0(s)(i Ls.abc (s)-Δi d (s)-i L2 (s)) (4-4)

[0111] according to Figure 5 When a single APF is running, the open-loop transfer function of the LCL filter is as follows:

[0112]

[0113] In the formula, V PWM AC valve side output voltage, V s is the grid-side voltage of the system.

[0114]

[0115]

[0116] By combining equations (4-4) to (4-5), we can obtain the output flow instructions of a single LCL filter, as shown in equation (4-6).

[0117] i L2 (s)=G(s)i Ls.abc (s)+Y(s)u s.abc (s) (4-6)

[0118] Let the formula

[0119]

[0120]

[0121] When multiple APFs are connected in parallel for harmonic compensation, assuming that the parameters of all parallel APFs are the same and they compensate the same proportion of load current, the system grid-side voltage can be obtained according to KVL as formula (4-7):

[0122]

[0123] According to formula (4-6) and formula (4-7), when N APFs are working in parallel, the output current of a single APF is as shown in formula (4-8):

[0124]

[0125] Assumptions Then G N (s) is the proportional coefficient from the APF detection current to the actual output current.

[0126]

[0127] Let s = jω to get formula (4-9), then |K| is the ratio of the effective value of the compensation current sent by the APF to the effective value of the detection current. When |K| is closer to 1 and the phase angle is close to 0°, the effect of active damping on resonance suppression is better; however, the left side of the equation (4-9) contains an imaginary part, so the compensation current sent by the APF will always have a phase difference with the detection current. Therefore, the active damping parameter K can be changed to d , T d To adjust the size and phase angle of the LCL type APF output current, while suppressing resonance and improving the operating efficiency of the entire parallel APF system.

[0128] 2. Overall control strategy based on PI-VPI controller

[0129] In order to simplify the control scheme of the APF parallel system and improve the response speed, a differential current control strategy based on the PI-VPI controller is proposed. Figure 6 As shown. Figure 6 In the current control strategy, the load current (i L,abc ) and the power supply current (i S,abc ), and the difference between the two i LS,abc As the command current signal of APF, it controls the APF output compensation current i f,abc Therefore, compared with the traditional PI control APF, the extraction of the load current harmonic components is omitted, thereby improving the dynamic response capability of the APF parallel system. The designed control strategy can be implemented through only two control loops: the voltage outer loop control loop and the current inner loop control loop. The voltage outer loop control loop maintains the DC side voltage stability through the PI controller, which helps the APF parallel system quickly adapt to load changes and grid fluctuations. The output of this control loop serves as the active power correction value i of the command. * sd and the difference current i LS,abc The sum of the two is used as the input signal of the PI-VPI controller. At the same time, the reactive power correction of the command current can be set to zero, which ensures that the reactive power provided by the power supply is zero and the reactive power consumed by the load is all provided by the APF. The PI-VPI control adopted by the current inner loop control loop is based on the active power correction i * sd and the difference current i LS,abcThe sum is used as the input of the controller, and the output is V * f.abc The PWM control signal of the VSI controls the APF to output the corresponding harmonic current. Because only the inner current control loop in the control strategy is related to high-frequency harmonic current, the compensation performance of the APF is only related to the PI-VPI controller.

[0130] 3. Voltage outer loop control circuit

[0131] The voltage outer loop control block diagram is as follows Figure 7 As shown, its purpose is to maintain the stability of the APF DC side voltage through a simple PI controller, and its output is used as one of the components of the reference current in the synchronous rotating coordinate system, as shown in formula (4-10):

[0132]

[0133] where K pdc and K idc It is the proportional coefficient and integral coefficient of the PI controller, V * dc and V dc are the reference value and measured value of the APF DC side voltage respectively.

[0134] In practice, due to the presence of harmonic currents on the DC side, the DC voltage of the APF will contain small ripples at the harmonic components. Therefore, a low-pass filter needs to be added to the DC side voltage feedback loop of the APF to filter out the ripples of the feedback amount, so that the reference value of the active current i * sd Smoother.

[0135] 4. Harmonic detection and shunting

[0136] The harmonic current detection and shunting link of the APF multi-machine parallel system adopts the method of fractional detection and shunting, which can well detect each harmonic and control the compensation degree of each APF for each harmonic. Figure 8 The internal structure of Nth harmonic current detection and shunt is as follows Figure 9 As shown. Figure 9 In the harmonic current detection and shunting link, Park transformation is first used to convert each harmonic from abc stationary coordinates to dq synchronous rotating coordinates, and by giving the dq coordinate axis mθ s , we can get mθ in dq coordinate system through low-pass filter. s k (=1 / N or 0) is the compensation coefficient, N is the number of APF machines connected in parallel, and by changing k, the shunting and control of harmonic currents can be achieved.

[0137] 5. Current inner loop control circuit

[0138] The current inner loop control loop using the VPI controller is as follows Figure 10 As shown. Since the VPI current controller is designed in a synchronous rotating coordinate system, the measured difference i LS,abc The current signal i is converted into the synchronous rotating coordinate system through Park transformation LS,dq , and then the active correction value i output by the DC voltage control loop * L.dq Add them together to get the command current signal which is output to the PI-VPI controller. The PI-VPI controller outputs a voltage signal V * f.dq The command voltage signal V in the three-phase stationary coordinate system is obtained through Park inverse transformation * f.abc The command voltage signal passes through the PWM delay link G PWM (s)=1 / (T d s+1) is used as the control signal of the VSI main circuit in the APF.

[0139] 6. PLL controller

[0140] Since the VPI controller is designed in a synchronous rotating coordinate system, a phase-locked loop (PLL) must be added to the control system to detect the phase of the power supply fundamental voltage. In actual distribution networks, the power supply voltage is not a pure sinusoidal waveform. It may contain harmonic components, which will affect the measurement accuracy of the PLL circuit. Therefore, to solve this problem, a lead / lag correction link is usually added to the PLL controller to correct the power supply voltage angular frequency (ω s ), so that its output only contains the phase of the voltage fundamental component. The improved control block diagram of the PLL circuit is as follows Figure 11 shown.

[0141] 7. In the relay protection setting calculation, the relevant constraints are as follows:

[0142] (1) Time coordination between main protection and backup protection of relay:

[0143] T j,k -T i,k ≥ΔT (4-11) where T j,k It represents the operating time of the backup protection relay Rj configured for the relay Ri; ΔT represents the protection differential time.

[0144] (2) Relay coordination constraint coefficient M i,j

[0145]

[0146] Where δ is the actual system protection step time, and the total number of constraint violations for all relay coordination pairs in all faults is ∑∑M i,j If ∑∑M i,j The value of 0 indicates that the setting result can ensure the selectivity of the protection device action for all fault conditions, which is the best result of the setting calculation.

[0147] To ensure rapid action of the protection devices, the time difference between coordinated pairs should be minimized while ensuring that the time difference is greater than the time difference. The sum of the difference between the time differences between coordinated protection devices under various fault conditions and the time difference between coordinated pairs should be minimized as an objective function, which must be considered in relay protection setting calculations.

[0148] (3) Constraints on relay setting time coefficient and action time:

[0149]

[0150] Among them, T i Indicates the operating time of relay Ri when a fault occurs in area k, usually TDS of the time coefficient i , with the upper and lower limits being 0.1 and 11 respectively.

[0151] 8. If Figure 12 The basic process of the improved particle swarm optimization algorithm shown in the figure includes:

[0152] (1) Particle initialization. Under the premise of satisfying some constraints, initialize the appropriate solution of the particle part according to formulas (4-12) and (4-13), and determine the optimal relay setting time coefficient value according to formula (4-2). Determine the dimension of the optimization variable according to formula (4-14), set the particle swarm size and algorithm parameters, i.e., learning factor, speed limit, etc.; set the maximum number of iterations. The position of each particle represents a possible set of calculation results, that is, each particle contains all the determined calculation information. For each protection segment, its coordination status with the adjacent protection can be represented by a binary character of a certain length, and the characters are connected to form the basic information of a particle in the population.

[0153] (2) Calculate the fitness value of each particle according to formula (4-15).

[0154]

[0155] Where fit represents the particle's fitness, m represents the number of constraints, and n represents the threshold for the current constraint. When all constraints are satisfied, n is 0; when the solution is unsuitable, n is 1. The objective function must meet the minimum action time for the entire protection and maximize the satisfaction of all protection constraints.

[0156] (3) Calculate the fitness function value of each particle's historical optimal point and compare it with the current fitness function value of each particle. If the particle's current fitness function value is greater than the fitness function value of its historical optimal point, replace the particle's historical optimal point position with its current position, and replace the historical optimal fitness function value with the current fitness function value. Compare the fitness function values ​​of the current optimal point and the historical optimal point of the population. If the fitness function value of the current optimal point of the population is greater than the fitness value of the historical optimal point, replace the population's historical optimal point position with the position of the current population's optimal particle, and replace the population's historical optimal fitness function value with the fitness function value of the current optimal particle.

[0157] (4) Update the particle's velocity according to equations (4-16), (4-17), and (4-18), determine the comparison benchmark value according to equation (4-19), and judge the particle's mobility according to the mobility strategy. If the particle mobility conditions are met, proceed to step (5); otherwise, enter the threshold calculation and evaluation function calculation phase and repeat step (2).

[0158] (5) Discretize the particle position according to formula (4-20), update the particle position according to formula (4-16), and meet the taboo algorithm detection. If necessary, calculate the average particle distance of the population according to formula (4-21) to make the particles search again in the solution space.

[0159] (6) According to the above process, the new position and velocity of each particle are calculated until the maximum number of iterations is reached or the expected fitness value is reached.

[0160] 9. Annotation of formulas used in the optimal particle swarm algorithm process:

[0161]

[0162] Among them, x max 、x min Respectively represent the maximum and minimum values ​​of particle position movement in each iteration, v max 、v min They represent the maximum and minimum values ​​of the particle speed in each iteration, and r is a random value between 0 and 1.

[0163]

[0164] The final position iteration formula through the improvement of empirical formula (4-16) is:

[0165]

[0166] The above equation represents the iterative relationship between particle positions, where α is the first constraint coefficient, β is the second constraint coefficient, d,i refers to the position of particle i in D-dimensional space, and n is the number of iterations. The present invention uses values ​​of 0.9 and 0.85, respectively, to prevent particles from falling into local optimal solutions.

[0167]

[0168] Where r1 and r2 are both random numbers in [0, 1].

[0169]

[0170] Formula (4-17) is the improved weight calculation formula, ω max 、ω min are the maximum and minimum values ​​of the weight, iter, iter max are the current and maximum number of iterations, respectively. As the number of iterations increases, the weight gradually decreases, and the algorithm tends to converge. Unlike iterations that use linear weight decreases, this paper uses a concave function decrease iteration method, which significantly improves the convergence speed of the particle swarm algorithm without affecting the convergence accuracy.

[0171] In order to limit the exploration speed of particles, the speed of each particle cannot exceed the maximum speed v max If the maximum speed is too large, the particles may miss the optimal solution; on the contrary, if the maximum speed is too small, the particles will fall into the local optimal solution and lose globality.

[0172]

[0173] Where p is the benchmark value for iterative comparison, and iter is the current number of iterations.

[0174]

[0175] The present invention uses the average population distance to describe the diversity of the population. Let L be the maximum diagonal length of the search space, S be the size of the population, and m' be the dimension of the solution space. represents the position of the d-dimensional coordinate value of the i-th particle at the n-th iteration, Represents the mean value of the d-dimensional coordinates of all particles, and the average particle distance D(n) of the population at the n-th iteration is defined as:

[0176]

[0177] In this invention, the harmonic current content at the APF line outlet is measured online based on the set value (i.e., the starting current) of the relay protection settings on each APF line. When the harmonic current content reaches the set value, the relay protection is activated. Because the harmonic current content at the outlet of each APF line varies, the relay protection activation time also varies, thus determining the order in which the relay protection actions are performed.

[0178] In this paper, the particle swarm optimization algorithm is improved in the particle update process in the following two aspects for the microgrid protection setting problem:

[0179] ① Improve the selection of initial values

[0180] Optimizing the selection of initial values ​​and randomly determining the particle's position a and velocity v has the advantage of expanding the initial search space and increasing the probability of the algorithm finding the optimal solution. This patented algorithm introduces the following rules when selecting initial values:

[0181] x max =(x max -x min )×r+x max

[0182] v max =(v max -v min )×r+v min

[0183] Among them, x max 、x min Respectively represent the maximum and minimum values ​​of particle position movement in each iteration, v max 、v min They represent the maximum and minimum values ​​of the particle speed in each iteration, and r is a random value between 0 and 1.

[0184] ②Improve the particle update process

[0185] This patented algorithm updates particle positions dimensionally during the iteration process. During the improved particle swarm algorithm, the particle's position in one-dimensional space is first updated. After the iteration, the particle's position is checked to see if it is within the permitted region. If so, the update is accepted; otherwise, the particle retains its original position. During the improved particle swarm algorithm, each particle can draw on the experience of all other particles, leveraging additional information to adjust its position.

[0186] The algorithm is based on the improved particle velocity formula and makes the following improvements to the position iteration formula:

[0187]

[0188] The above formula is the iterative relationship of particle position, where α is the first restriction coefficient; β is the second restriction coefficient, which are respectively set to 0.9 and 0.8 in the present invention, thereby preventing particles from falling into a local optimal solution.

[0189] The connection between the improved particle swarm algorithm and this application:

[0190] This application takes into account the role of relay protection in improving the power quality of the power system and issues such as the selection of operating current. Based on this, the relay protection problem is equivalent to a mixed integer nonlinear programming problem. Therefore, an optimization model is established with the system power quality as the constraint condition and the expected value of the relay protection operation time as the optimization target.

[0191] Then, the improved particle swarm optimization algorithm is used to solve the optimization model to achieve the shortest operation time of the APF circuit breaker;

[0192] In order to make the relay action selective when resonance occurs and minimize the number of APF relay protection actions, the difference in action time between the two relays of the relay protection device should be greater than the given time difference ΔT.

[0193] Therefore, during the optimization process, the total relay coordination time is as an indicator to consider.

[0194] Where n is the number of APF line relays; ΔT i is the action time difference of the relay.

[0195] The input value of the improved particle swarm optimization algorithm is the relay protection setting current value calculated offline under various operating conditions. The output value is the value of the input setting current value after being optimized by the particle swarm algorithm. The output value is stored in the central controller. When the system is running online and resonance occurs in any operating condition, the central processor can call a set of relay operating current values ​​optimized by the improved particle swarm algorithm under the corresponding operating condition for the relay. This will quickly eliminate the resonant APF and ensure the system's power quality.

[0196] Example 2

[0197] Reference Figure 13 Based on the topology of an isolated microgrid with parallel APF systems, a simulation model of the isolated microgrid system was built using the RT-LAB / OP5600, consisting of a micro gas turbine, a battery, a wind turbine, a photovoltaic generator, and a four-parallel LCL-type APF cluster. Simulation experiments were also conducted to investigate the resonance characteristics and active damping suppression. The simulation design employed PI control for the outer-loop DC voltage controller, and a current controller combining one PI controller and three VPI controllers in parallel for the inner-loop current controller, capable of compensating for harmonics up to the 19th order.

[0198] The system parameters are as follows:

[0199] (1) The voltage of the isolated micro power supply is U = 380 V, and the equivalent inductance is Ls = 0.05 mH;

[0200] (2) APF1 to APF4 have the same capacity and parameters as shown in Table 1.

[0201] Table 1 Main circuit parameters of LCL-type APF in isolated island microgrid

[0202]

[0203] PI controller parameter K P =12, K I =100, Table 2 shows the parameters of the VPI controller, PWM switching frequency f PWM =10kHz.

[0204] Table 2 Controller parameters

[0205]

[0206] Implementation steps:

[0207] 1) The PCC switch connecting the microgrid to the main grid is disconnected, and the microgrid is in an islanded operation state. The microgrid system only has two distributed power sources, the micro gas turbine and the battery, from 0 to 0.4 seconds. At 0.4 seconds, due to the access of wind turbines, photovoltaic generators and corresponding loads, the system needs to perform APF multi-machine parallel compensation. The output current of a certain APF outlet is collected, and the operation results are as follows: Figure 14 As shown, observe Figure 14 (0.4s-0.43s waveform) It can be seen that when the four APF machines are connected in parallel for harmonic compensation, resonance occurs between the APF machines, resulting in a large 955Hz resonant overcurrent on the filter capacitor, thereby significantly increasing the content of the 955Hz resonant current on the microgrid side of the system.

[0208] 2) The relay protection device uses a bandpass filter to detect and extract the 955Hz harmonic overcurrent component in time, and then Figure 4 The relay action logic shown disconnects the APF from the microgrid and observes Figure 14 It can be seen that when APF parallel resonance occurs in the system, the relay on the fastest-acting APF line can quickly and reliably cut off the line within 0.03s, suppressing the system parallel resonance.

[0209] 3) After the inverse time overcurrent relay protection is activated, Figure 14 (0.43s-0.47s waveform) It can be seen that after the relay protection is activated, the system power quality has improved, but it is still not ideal. Therefore, active damping control is added to one or more APFs to improve the power quality. Figure 15 As shown, after many offline experiments, it can be seen that when the active damping parameter in the differential link expression of active damping control is K d =10 / 11, T d =1 / 100, the power quality of the microgrid system is the best; further observation Figure 14 (Waveform after 0.47s) It can be seen that when active damping control is added to the system, the 19th resonant current content on the filter capacitor is significantly reduced, and the system power quality is restored, thereby effectively protecting the filter capacitor, power supply system and transformer from damage.

[0210] In summary, Figure 14 It can be seen that when four APFs are connected in parallel for harmonic compensation, resonance occurs between the APFs, resulting in a large 955Hz resonant overcurrent on the filter capacitor, which significantly increases the content of 955Hz harmonic current in the isolated microgrid system. When a certain APF is removed using the inverse time overcurrent relay protection scheme proposed in this application, the resonance between the APFs in the system is effectively suppressed. Then, by adjusting the active damping parameters to change the amplitude and phase of the output current to detection current ratio in the APF control, the control of the remaining APFs is optimized, and the system power quality level is further improved, thereby solving the problem that the system power quality does not meet the power supply quality requirements.

[0211] Example 3

[0212] Based on the same inventive concept, an embodiment of the present invention further provides an inverse time overcurrent relay protection control system, comprising:

[0213] The monitoring module is used to monitor the output current of the APF and detect the resonance of the system:

[0214] A cut-off module is used to cut off the APF of the optimal relay protection action value based on a pre-built optimal relay protection action table;

[0215] The active damping addition module is used to continue monitoring the outlet current of the APF. If the current harmonic content is less than the action value but the grid quality does not meet the set power quality requirements, active damping control is added to the selected APF based on the power quality requirements in the microgrid.

[0216] In an embodiment, the monitoring module includes:

[0217] The extraction submodule is used to extract the current harmonic content from the monitored APF outlet current;

[0218] The resonance judgment submodule is used to determine that the system resonates when the current harmonic content is not less than the action value within the inverse time overcurrent relay protection action time difference.

[0219] Those skilled in the art will appreciate that the embodiments of the present application can be provided as methods, systems, or computer program products. Therefore, the present application can adopt the form of a complete hardware embodiment, a complete software embodiment, or an embodiment in combination with software and hardware. Moreover, the present application can adopt the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to magnetic disk storage, CD-ROM, optical storage, etc.) that contain computer-usable program code.

[0220] The present application is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each process and / or box in the flowchart and / or block diagram, as well as the combination of the processes and / or boxes in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the steps in the process. Figure 1 a process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0221] These computer program instructions may also be stored in a computer readable memory that can direct a computer or other programmable data processing device to work in a specific manner, so that the instructions stored in the computer readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 a process or multiple processes and / or boxes Figure 1 The function specified in one or more boxes.

[0222] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operational steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing the instructions executed on the computer or other programmable device for implementing the process. Figure 1 a process or multiple processes and / or boxes Figure 1 A step that specifies a function in one or more boxes.

[0223] The above are merely embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention are included in the scope of the claims of the present invention to be approved.

Claims

1. A method for controlling inverse time overcurrent relay protection, characterized in that: include: When the APF-based outlet current monitoring detects system resonance: Based on the pre-built optimal relay protection action table, the APF of the optimal relay protection action value is cut off; Continue to monitor the APF outlet current. If the current harmonic content is less than the action value but the grid quality does not meet the set power quality requirements, add active damping control to the selected APF based on the power quality requirements in the microgrid. The APF-based outlet current monitoring detects that the system resonates, including: Extract the current harmonic content from the monitored APF outlet current; When the current harmonic content is not less than the action value within the inverse time overcurrent relay protection action time difference, the system resonates; The step of extracting the current harmonic content from the monitored APF outlet current includes: Based on the pre-built microgrid model, the system resonant frequency expression when APF is running in parallel is constructed; Obtaining a resonant frequency range based on an expression of the system resonant frequency when the APFs are operated in parallel; Extracting the current harmonic content within the resonant frequency range from the monitored APF outlet current using a bandpass filter; The system resonant frequency expression when the APFs are operated in parallel is as shown below: Where: N: number of APFs in parallel; L S : grid equivalent inductance; L1: APF AC valve side inductance; L2: grid side inductance; C: filter capacitor.

2. The method according to claim 1, wherein The method of cutting off the APF of the optimal relay protection action value based on the pre-built optimal relay protection action table includes: S201, obtaining a set of optimal relay protection action values ​​corresponding to the current working condition from the pre-built optimal relay protection action table; S202: based on a set of optimal relay protection action values ​​corresponding to the current working condition, disconnecting the APF that first reaches the optimal relay protection action value; S203: Continue to monitor the outlet current of the APF. If the system still resonates, execute S202 until the current harmonic content is less than the action value.

3. The method according to claim 2, wherein The acquisition of the optimal relay protection action table includes: Based on the pre-built microgrid model, simulate all the operating conditions of APF multi-machine parallel operation in the microgrid; Based on the outlet current of the APF under each working condition, the current harmonic content at the outlet of each APF under each working condition is obtained; Based on the current harmonic content at the outlet of each APF under each working condition, the starting current of the inverse time overcurrent relay protection device on each APF under each working condition is obtained; The starting current of the inverse time overcurrent relay protection device on each APF under each working condition is brought into the optimization model, and the optimization model is solved using the improved particle swarm algorithm to determine a set of optimal relay protection action values ​​corresponding to each working condition; An optimal relay protection action table is generated based on a set of optimal relay protection action values ​​corresponding to all working conditions.

4. The method according to claim 3, wherein The starting current of the inverse time overcurrent relay protection device is calculated as follows: Where: I set K is the starting current of the inverse time overcurrent relay protection device; k is the reliability coefficient; I th.max is the maximum current harmonic content; K f is the return coefficient.

5. The method according to claim 3, wherein The starting current of the inverse time overcurrent relay protection device on each APF under each working condition is brought into the optimization model, and the optimization model is solved using the improved particle swarm algorithm to determine a set of optimal relay protection action values ​​corresponding to each working condition, including: The starting current of the inverse time overcurrent relay protection device on each APF under each working condition is used as the optimization variable of the optimization model; Initializing the optimization variables based on the constructed constraints; Setting an initial value for the change speed of the optimization variable based on preset upper and lower limits of the optimization variable update speed; Setting an initial value for the change position of the optimization variable based on preset upper and lower limits of the optimization variable update position; Calculating the change position of the optimization variable based on preset upper and lower limits of the optimization variable update speed; Based on the optimization target, the initialized optimization variables, the change speed of the optimization variables and the change position of the optimization variables, a set of optimal relay protection action values ​​corresponding to each working condition is obtained through iteration.

6. The method according to claim 5, wherein The change position of the optimization variable is calculated based on the preset upper and lower limits of the optimization variable update speed, as shown in the following formula: Where: is the position of the i-th particle at the n+1-th iteration; is the position of the i-th particle at the n-th iteration; v min is the lower limit of the variable update speed; v max is the upper limit of the optimization variable update speed; α is the first limiting coefficient; β is the second limiting coefficient.

7. The method according to claim 2, wherein The active damping control is added to the selected APF based on the power quality requirements in the microgrid, including: Removing the optimal relay protection action value corresponding to the removed APF from a set of optimal relay protection action values ​​corresponding to the current working condition to obtain the optimal relay protection action value corresponding to each remaining APF; Based on the difference between the optimal relay protection action value corresponding to each of the remaining APFs and the harmonic current at the outlet of each APF, the difference between the optimal relay protection action value and the harmonic current is obtained; Active damping control is added to the APF with the smallest difference in sequence, and the power quality is monitored until the power quality in the microgrid meets the power supply quality requirements.

8. The method according to any one of claims 1 or 3, wherein: The construction of the microgrid model includes: Establish a microgrid model based on the operating topology of the microgrid under test; Based on the electrical parameters of the equipment in the microgrid under test, parameter setting of the microgrid model is completed.

9. An inverse time overcurrent relay protection control system, characterized in that: include: The monitoring module is used to monitor the output current of the APF and detect the resonance of the system: A cut-off module is used to cut off the APF of the optimal relay protection action value based on a pre-built optimal relay protection action table; The active damping addition module is used to continue monitoring the outlet current of the APF. If the current harmonic content is less than the action value but the grid quality does not meet the set power quality requirements, active damping control is added to the selected APF based on the power quality requirements in the microgrid; The monitoring module includes: The extraction submodule is used to extract the current harmonic content from the monitored APF outlet current; A resonance judgment submodule is used to determine that the system resonates when the current harmonic content is not less than the action value within the inverse time overcurrent relay protection action time difference; The extraction submodule specifically includes: Based on the pre-built microgrid model, the system resonant frequency expression when APF is running in parallel is constructed; Obtaining a resonant frequency range based on an expression of the system resonant frequency when the APFs are operated in parallel; Extracting the current harmonic content within the resonant frequency range from the monitored APF outlet current using a bandpass filter; The system resonant frequency expression when the APFs are operated in parallel is as shown below: Where: N: number of APFs in parallel; L S : grid equivalent inductance; L1: APF AC valve side inductance; L2: grid side inductance; C: filter capacitor.

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