PLL-free negative sequence current suppression method based on cluster new energy station control

Abstract

The application discloses a PLL-free negative sequence current suppression method based on cluster new energy station control, which comprises the following steps: step 1: based on the positive sequence power output priority of a grid-connected inverter, a feasible solution domain of the negative sequence current suppression of the current positive sequence power output is determined; step 2: through a geometric analysis method, an optimal suppression reference quantity in the feasible solution domain of the negative sequence current suppression is determined, and the suppression capacity of the new energy station is maximally utilized; and step 3: a control method without a PLL link is adopted to realize rapid suppression of the negative sequence current. The method does not need additional devices, has small reconstruction investment, is easy to control, effectively reduces the negative sequence current invading the generator, and can better solve the problem of the over-standard negative sequence current during single operation of a large-scale power system.

Description

Technical Field

[0001] This invention belongs to the field of negative sequence current suppression technology during non-full-phase operation of power systems, and specifically relates to a PLL-free negative sequence current suppression method based on clustered new energy power station control. Background Technology

[0002] The penetration rate of new energy sources in power systems is increasing year by year. Under the background of uneven power generation and load distribution, large-scale power systems often form with multiple clusters of new energy sources transmitting power. Large-scale power systems are generally equipped with reclosing mechanisms to prioritize system power transmission; however, large-scale power systems have low equivalent impedance, resulting in a significant increase in negative sequence current during non-full-phase operation. The second-harmonic power fluctuations caused by negative sequence current intrusion into generators can not only severely damage the generator itself, but also, under the background of negative sequence current inverse-time protection, may cause multiple generators to trip, seriously threatening the safe and stable operation of the system. Therefore, the necessity of negative sequence current management in large-scale power systems is extremely important.

[0003] In addition to the significant characteristic of low equivalent impedance, large-scale power systems also feature a high penetration rate of renewable energy. While renewable energy grid-connected inverters possess control potential, current research is limited to output control during faults or normal operation, neglecting relevant control during non-full-phase operation.

[0004] Negative sequence current control is mainly considered to be carried out through additional devices, that is, to generate a negative sequence current with the opposite polarity to the negative sequence current of the system through additional devices to achieve the suppression effect. From this perspective, some scholars have studied negative sequence current suppression methods based on additional devices. For example, using STATCOM devices to suppress negative sequence current, refer to the record in reference [1]: Lu D, Wei M, Shen S, et al. A Coordination Control With Extra Active Power Exchange Way to Extend Negative Sequence Current Compensation Range for STATCOM Based on Hybrid Cascaded Converter[J].IEEE Transactions on Power Electronics,2022,37(12):15442-15456. Adding STATCOM devices to suppress system negative sequence current has high flexibility. Only the STATCOM body needs to be considered during control. However, the investment in additional STATCOM devices is huge, and the suppression capacity required in large-scale power systems is large. From an economic point of view, it is difficult to apply additional device strategies on a large scale.

[0005] Controlling grid-connected inverters to participate in system power quality management mainly involves low voltage ride-through control, harmonic control, reactive power compensation, and suppression of negative sequence current in the inverter itself. For example, the positive and negative sequence components of the grid-connected inverter are separated and controlled, and the negative sequence current reference of the negative sequence inverter is set to 0 to avoid the inverter outputting negative sequence current. According to the record in reference [2]: Jiang Weidong, Wu Zhiqing, Li Wangmin, She Yangyang, Hu Yang. Control strategy for grid-connected inverters to suppress negative sequence current when the grid is unbalanced [J]. Journal of Electrical Engineering, 2015, 30(16): 77-84., this scheme can effectively suppress the internal unbalanced current of the inverter because the suppression is based on the inverter itself, but the suppression effect on the negative sequence current that originally exists in the system is very small. Under the premise of satisfying the normal positive sequence power output, the inverter control can realize functions such as reactive power compensation and harmonic current control, as recorded in reference [3]: Li Jiahang. Research on photovoltaic grid-connected system with comprehensive power quality optimization [D]. Harbin University of Science and Technology, 2022.

[0006] In summary, highly controllable inverters have the potential to participate in power quality management, but strategies for suppressing negative sequence current during non-full-phase operation of large-scale power systems remain lacking. Summary of the Invention

[0007] To address the problem of increased negative-sequence current during non-full-phase operation of large-scale power systems, which threatens generator negative-sequence current protection and leads to cluster generator disconnection, this invention proposes a PLL-free negative-sequence current suppression method based on clustered renewable energy power plant control. This method, based on clustered renewable energy power plants in large-scale power systems, considers the non-full-phase operation characteristics of the system, determines the feasible solution domain for inverter negative-sequence current suppression while ensuring priority delivery of positive-sequence power, and determines the optimal negative-sequence current suppression reference value through geometric analytical methods. It employs PLL-free control to achieve the relevant control of positive-sequence power output and negative-sequence current suppression, reducing the negative-sequence current intruding into the generator without the need for additional devices, thus contributing to the safe and stable operation of the system.

[0008] The technical solution adopted in this invention is as follows:

[0009] A PLL-free negative sequence current suppression method based on clustered renewable energy power plant control includes the following steps:

[0010] Step 1: Based on the priority of positive sequence power output of the grid-connected inverter, determine the feasible solution domain for suppressing the negative sequence current of the current positive sequence power output;

[0011] Step 2: Determine the optimal suppression reference value within the feasible solution domain of negative sequence current suppression using geometric analysis methods, so as to maximize the suppression capability of new energy power plants.

[0012] Step 3: Use a control method without PLL to achieve rapid suppression of negative sequence current.

[0013] In step 1, based on the current three-phase output current vector of the grid-connected inverter... Single-phase current limit I of grid-connected inverter lim The current constraint is constructed as shown in equation (1). The feasible solution domain for suppressing negative sequence current under the premise of ensuring priority output of positive sequence power is obtained by solving equation (1):

[0014]

[0015] In formula (1): I lim This indicates the maximum allowable amplitude of single-phase current in a grid-connected inverter; The vector represents the three-phase output current of the grid-connected inverter, x = a, b, c, where the superscripts '+' and '-' represent the positive and negative sequence components, respectively.

[0016] a represents the rotation factor, a = e j120° , j represents the imaginary unit in a complex number;

[0017] With inverter current The endpoints A, B, and C are the three centers of a circle, with I as the center. lim Draw a circle with radius , and the shaded area where the three circles intersect represents the feasible solution domain for suppressing negative sequence current.

[0018] In step 2, let the negative sequence current to be suppressed be... Endpoint N;

[0019] When N is within the feasible solution domain, it has complete suppression capability. In this case, the optimal suppression reference quantity for negative sequence current suppression is the phasor formed by connecting the center and point N.

[0020] When N is outside the feasible solution domain, considering the constraints of the feasible solution domain, the system divides the system into 6 regions based on the line segment length and included angle: Regions I, II, III, IV, V, and VI. Using the geometric analytical method, the optimal suppression reference value is solved, as shown in Table 1.

[0021] Table 1 Optimal Solution of Negative Sequence Current Suppression Reference Values

[0022]

[0023] Where: ε A ε B ε C ε X ε Y ε Z These represent the angles between line segments AN, BN, CN, XN, YN, and ZN and the x-axis, respectively; A X A Y Represent the x-coordinate and y-coordinate of point A, respectively; B XB Y Let B represent the x-coordinate and y-coordinate of point B, respectively; C X C Y Let x and y represent the x and y coordinates of point C, respectively; θ represents the angle between the positive and negative sequence currents.

[0024] I is the positive sequence current magnitude; lim This represents the maximum allowable amplitude of single-phase current in the grid-connected inverter; |NA|, |NB|, and |NC| represent the line segment lengths, respectively.

[0025] Step 3 includes the following steps:

[0026] Step 3.1: Calculate the control parameters without PLL, including the virtual fixed phase angle and the sequence components under the dq coordinate axis;

[0027] Step 3.2: Substitute the relevant parameters obtained in Step 3.1 into the PLL-less control circuit to achieve PLL-less fast control of the grid-connected inverter.

[0028] In step 3.1, considering the low phase-locking accuracy and long delay of conventional PLL control methods when the grid voltage is unbalanced, a virtual fixed phase angle is used to complete the control without a PLL link. The virtual fixed phase angle θ... n The calculation is shown in equation (2):

[0029] θ n =ω n t = 2πf n t (2);

[0030] In equation (2): f n The system's rated frequency is 50Hz; t is time; ω n Indicates the rated electrical angular velocity;

[0031] With virtual phase angle θ n As a reference for the grid voltage e abc and current i abc Perform a dq coordinate transformation to generate the voltage dq component e. dq and current dq component i dq ;

[0032] The positive and negative sequence components in the dq component are separated using a delay algorithm to obtain the positive and negative sequence components in the voltage and current dq axis coordinate system: Where: the subscripts d and q represent the d-axis components and the superscripts + and - represent the positive and negative order components, respectively.

[0033] In step 3.2, considering the high priority of positive sequence power output during single-phase reclosing of the high-voltage power grid, the positive sequence current command value in the current control loop is selected as DC active power. reactive power As a constraint equation, the positive sequence current reference value is solved according to equation (6), and a PI regulator can be used to form a double closed-loop control circuit without the need for a phase-locked loop.

[0034]

[0035] All coordinate transformations use a virtual phase angle θ as the reference phase angle. n Completed, ensuring control response speed and accuracy under voltage imbalance; in PLL-less control loops, all coordinate transformations are based on a virtual fixed impedance angle θ. n For reference only.

[0036] Detect the status of circuit breakers and the control mode of grid-connected inverters. If the large-scale power system is in a non-full-phase operation state and the grid-connected inverter exits the low voltage ride-through control during the fault period, the PLL-less negative sequence current suppression strategy is put into operation.

[0037] PLL-free negative sequence current suppression strategies include:

[0038] ① High-priority control guarantees:

[0039] Non-full-phase operation is considered an abnormal system operation, and the low-voltage ride-through control of the grid-connected inverter may be activated due to voltage triggering. The system's operating status is assessed by detecting the circuit breaker status to determine if the system is operating in a non-full-phase manner: if it is operating in a non-full-phase manner and the grid-connected inverter's low-voltage ride-through control is not engaged, then negative-sequence current suppression is implemented while ensuring positive-sequence power output; otherwise, low-voltage ride-through control is prioritized.

[0040] ② Determination of the optimal suppression reference value:

[0041] Based on the determined positive-sequence power output reference value, determine the current three-phase output current vector of the grid-connected inverter. x = a, b, c, and the single-phase current limit I of the grid-connected inverter. lim ;

[0042] According to equation (1), if the grid-connected inverter has available remaining capacity, the optimal negative sequence current suppression reference value is calculated using a combined analytical method. If the grid-connected inverter has no available capacity, the negative sequence current suppression strategy should be abandoned to ensure priority delivery of positive sequence power.

[0043] ③: The grid-connected inverter adopts a PLL-free positive and negative sequence separation control, with positive and negative sequence control in parallel:

[0044] During normal operation, the negative sequence suppression circuit does not start when the positive sequence power is output. The positive sequence power output adopts a PLL-less fast control scheme. The relevant control parameters are calculated from the control parameters of the PLL-less circuit, including: virtual fixed phase angle θ calculation, Parker transformation, and DSC delay algorithm solution.

[0045] The reference quantity for negative sequence current suppression is the negative sequence current vector obtained by the geometric analytical method. The relevant parameters required for the geometric analytical method are obtained by the DSC delay algorithm.

[0046] In the dual closed-loop control of the grid-connected inverter, a feedforward decoupling algorithm is used to determine the voltage command value of the grid-connected inverter, as shown in equation (7):

[0047]

[0048] In equation (7): u * Indicates the voltage command value; e represents the grid connection point voltage; i represents the inverter current; ω represents the angular frequency; L represents the filter inductance; '*' represents the reference value; superscripts '+' and '-' represent the positive and negative sequence components respectively; subscripts d and q represent the d-axis and q-axis components respectively; K P K represents the proportionality coefficient. I This represents the integral control parameter.

[0049] By superimposing the positive-sequence voltage reference and the negative-sequence voltage reference vectors, the reference value for PWM modulation can be obtained. The reference angle for dq transformation in the strategy adopts a given phase angle θ. Combined with the DSC algorithm, the positive and negative sequence components are separated to ensure the high performance of the strategy when the grid voltage is unbalanced.

[0050] This invention discloses a PLL-free negative sequence current suppression method based on clustered renewable energy power plant control, with the following beneficial effects:

[0051] 1) The method of the present invention is based on the available remaining capacity of the new energy inverter to suppress negative sequence current. It can achieve system negative sequence current suppression without additional devices, which can effectively save investment in additional devices.

[0052] 2) This invention adopts a control method without PLL links, which requires minimal modification to the control system and has a fast response speed.

[0053] 3) The negative sequence current suppression strategy described in this invention is of medium priority and does not affect the high priority control of the system, effectively contributing to the safe and stable operation of the system.

[0054] 4) The method of this invention addresses the problem of significantly increased negative sequence current during single-phase reclosing caused by the small equivalent impedance of large-scale power systems. It uses the remaining available capacity of the grid-connected inverter of new energy power plants to determine the optimal reference value for suppressing negative sequence current, and adopts a PLL-free method for rapid control to reduce the negative sequence current intruding into the generator terminal and avoid the phenomenon of multiple generators being disconnected due to excessive negative sequence current.

[0055] 5) The method of this invention fully considers the requirements of low voltage ride-through and priority of positive sequence power transmission during single-phase reclosing. First, it analyzes the available capacity for negative sequence current suppression under positive sequence power transmission of the grid-connected inverter; then, it determines the optimal suppression reference value through geometric analysis; and finally, it adopts a control method without PLL links to form a negative sequence current suppression strategy based on grid-connected inverter control, thus gaining time for safe and stable system control.

[0056] 6) The method of the present invention requires no additional equipment, requires little investment in modification, is easy to control, effectively reduces the negative sequence current intruding into the generator, and can better solve the problem of excessive negative sequence current during single operation of large-scale power systems. Attached Figure Description

[0057] Figure 1 This is a schematic diagram of the feasible solution domain for negative-order suppression under positive-order power priority.

[0058] Figure 2 A schematic diagram illustrating the determination of the optimal suppression reference value within the solution domain using the geometric method.

[0059] Figure 3 This is the overall flowchart of the negative sequence current suppression strategy.

[0060] Figure 4 The diagram shows the control method for negative sequence current suppression strategy.

[0061] Figure 5 This is a simulation model of a large-scale power system incorporating clustered new energy sources, built using PSCAD software.

[0062] Figure 6 This is a waveform diagram of the grid connection point voltage during non-full-phase operation.

[0063] Figure 7 This diagram shows the effective voltage value and unbalance during non-full-phase operation.

[0064] Figure 8 The waveform diagram is shown to suppress the negative sequence current at the front end.

[0065] Figure 9 The graph shows the negative sequence current response curve without a PLL control.

[0066] Figure 10 To completely suppress (I p =0.6pu,I n Waveform of (=0.4pu).

[0067] Figure 11 Incomplete inhibition (I) p =0.8pu,I n Waveform of 0.4 pu.

[0068] Figure 12Incomplete inhibition (I) p =0.8pu,I n Waveform of (=0.6pu).

[0069] Figure 13 This is a diagram showing the output current of the grid-connected inverter in a cluster of new energy power plants. Detailed Implementation

[0070] A PLL-free negative-sequence current suppression method based on clustered renewable energy power plant control is proposed. This method considers the high-priority control of grid-connected inverters and the priority of positive-sequence power output during non-full-phase operation of large-scale power systems, and determines the available remaining capacity limit of renewable energy grid-connected inverters. A geometric analytical method is used to determine the optimal negative-sequence current suppression reference value within the feasible solution domain to maximize the utilization of the grid-connected inverter potential. Considering the requirement for rapid negative-sequence current suppression, the control response speed under unbalanced voltage is improved. Control-related parameters are calculated through virtual fixed phase angle calculation, and a PLL-free control strategy is adopted to achieve positive and negative sequence separation control, thereby realizing rapid suppression of system negative-sequence current.

[0071] Specifically, the following steps are included:

[0072] Step 1: Check the circuit breaker status and grid-connected inverter control mode:

[0073] Incomplete phase operation occurs after a single-phase ground fault in the system. During the fault, the low-voltage ride-through control of the renewable energy grid-connected inverter is required, and its control is subject to the voltage at the renewable energy grid connection point, thus having a high priority. The system operation mode is determined first by checking the circuit breaker status. If the system is incomplete phase operation, the system further checks whether the low-voltage ride-through control has exited. If the low-voltage ride-through control exits during incomplete phase operation, the negative sequence current suppression strategy is then implemented.

[0074] Step 2: Determining the feasible solution domain for negative-sequence suppression under positive-sequence power priority:

[0075] During non-full-phase operation, priority should be given to the delivery of positive-sequence power to ensure the power balance of the system. The feasible solution domain for negative-sequence current suppression under the positive-sequence power priority condition of the new energy grid-connected inverter is determined according to the constraints shown in Equation (1).

[0076]

[0077] Among them: I lim This indicates the maximum allowable amplitude of single-phase current in a grid-connected inverter. The three-phase output current vector of the grid-connected inverter is represented by x = a, b, c; the superscripts '+' and '-' represent the positive and negative sequence components, respectively, and a represents the rotation factor, a = e j120° .

[0078] Inverter current The endpoints correspond to points A, B, and C; with A, B, and C as centers respectively, I... lim Draw a circle with radius , and the shaded area where the three circles intersect represents the feasible solution domain for negative sequence current suppression, as shown below. Figure 1 The area shown is shaded in red.

[0079] Step 3: Determine the optimal suppression reference quantity within the solution domain using the geometric analytical method:

[0080] Based on the relative positions of the endpoints of the current to be suppressed in the feasible solution domain of the negative sequence current, the optimal suppression solution is determined in the following two cases:

[0081] (1): The negative sequence current to be suppressed lies within the solution domain. Let the phasor of the negative sequence current to be suppressed be... Where O represents the origin and N represents the endpoint of the negative sequence current to be suppressed. When N is within the solution domain, complete suppression is achieved. In this case, the optimal reference quantity for suppressing the negative sequence current is the phasor connecting the center and point N.

[0082] (2): When When located outside the solution domain, the optimal suppression reference value is determined using a geometric analytical method. The solution process is as follows: Figure 2 Let A, B, and C be the inverter currents. The three corresponding endpoints; X, Y, and Z are the three endpoints of the feasible solution domain; N is the endpoint of the negative sequence current to be suppressed. Based on the relative position of point N to the feasible solution domain, the feasible solution domain is divided into... Figure 2 The six regions shown are labeled I, II, III, IV, V, and VI. The optimal suppression solution is determined for each region. For example:

[0083] When point N is in region I, the optimal suppression reference value is Point R connects the intersection of points N and A with the feasible solution domain;

[0084] When point N is in region II, the optimal suppression solution is:

[0085] When point N is in region III, the R point of the optimal suppression solution is the intersection point connecting points N and C with the feasible solution domain;

[0086] When point N is in region IV, the optimal suppression solution is:

[0087] When point N is in region V, the R point of the optimal suppression solution is the intersection point connecting points N and B with the feasible solution domain;

[0088] When point N is in region VI, the optimal suppression solution is:

[0089] Furthermore, the region to which the negative sequence current to be suppressed belongs is determined by the criteria in Table 1. The optimal solutions for the six regions are shown in Table 1. In actual calculations, the solutions will be obtained directly according to the criteria shown in Table 1.

[0090] Table 1 Optimal Solution of Negative Sequence Current Suppression Reference Values

[0091]

[0092] Where: ε A ε B ε C ε X ε Y ε Z These represent the angles between line segments AN, BN, CN, XN, YN, and ZN and the x-axis, respectively; A X A Y Let A and B represent the x and y coordinates respectively. X B Y Let B and C represent the x and y coordinates, respectively. X C Y θ represents the x and y coordinates of point C; θ represents the angle between the positive and negative sequence currents. I is the positive sequence current magnitude; lim This indicates the maximum allowable amplitude of single-phase current in the grid-connected inverter. |NA|, |NB|, and |NC| represent the corresponding line segment lengths.

[0093] Step 4: Calculation of control parameters for PLL-less components:

[0094] To implement control without PLL, relevant parameters need to be calculated to facilitate control implementation. These parameters include virtual fixed phase angles and sequence components under the dq coordinate axis.

[0095] (1): Virtual fixed phase angle calculation:

[0096] Virtual fixed phase angle θ n Calculate according to formula (2):

[0097] θ n =ω n t = 2πf n t (2);

[0098] Where f n The system's rated frequency is 50Hz, ω n This represents the rated electrical angular velocity, where t is time.

[0099] (2): Calculation of each order component under the dq coordinate axis:

[0100] The Park transform and DSC delay algorithm are used to calculate the order components under the dq coordinate axis.

[0101] Ordinal components include: In this context, the subscripts d and q represent the d-axis and q-axis components, respectively, and the superscripts + and - represent the positive and negative order components, respectively.

[0102] Collect inverter grid-side voltage e abc and current i abc Using a virtual fixed phase angle θ n As a reference angle for the Park transform, the voltage and current components e in the dq coordinate axis are generated. dq i dq Because the use of positive and negative Parker transforms will generate harmonic interference when the input is unbalanced, further reducing e dq i dq Separate the computational order components using the DSC delay algorithm: To prepare for subsequent control measures.

[0103] The Parker transformation matrix is ​​shown in equations (3) and (4):

[0104]

[0105]

[0106] Where: corresponding These represent the positive and negative output matrices of the Parker transform, taking current as an example; P + and P - Let i represent the Parker positive and negative order transformation matrices, respectively; abc This represents the three-phase current input matrix; θ represents the Parker transform reference angle, which is virtually fixed in this invention. n As the reference angle for the Park transformation, we have θ = θ n i a i b i c Let a, b, and c be the three-phase currents, respectively. When calculating the sequence components of voltage e on the dq coordinate axis, replace the input of equations (3) and (4) with e. abc .

[0107] The DSC delay algorithm is shown in equation (5):

[0108]

[0109] Among them U d (t) represents the output of the DSC algorithm; This represents the original vector value of the d-axis component, which in this invention corresponds to the positive and negative order components under the dq axis. and The d-axis component delay vector is represented by t; time is represented by T; period is represented by T. In this study of a 50Hz system, T = 0.02s is taken; n represents the harmonic order; this invention is used to separate positive and negative sequence components, and n is taken as 2.

[0110] Step 5: PLL-less rapid control of grid-connected inverter:

[0111] The PLL-less fast control simultaneously adapts to the requirements of both positive-sequence and negative-sequence control, i.e., the requirement for separate positive and negative sequence control. Overall, it still maintains a dual closed-loop control with an outer loop factor and an inner loop current, requiring minimal control modifications. The PLL-less implementation of the positive-sequence power delivery section is as follows:

[0112] The calculation of the current command for positive sequence power is based on the selection of DC active power. reactive power As a constraint equation, the positive sequence current reference value is solved according to equation (6), and a PI regulator can be used to form a double closed-loop control circuit without the need for a phase-locked loop.

[0113]

[0114] The PLL-less control for negative sequence current suppression is as follows:

[0115] The negative sequence current command is calculated by using the optimal suppression reference value determined by the geometric analytical method as the control input. The voltage control reference value is obtained through a double closed-loop control without a PLL. This voltage control reference value is then combined with the voltage control reference value generated by the positive sequence power control to obtain the total voltage control reference value, which is then modulated by PWM.

[0116] Step Six: Rapid Suppression of Negative Sequence Current

[0117] A negative-sequence current suppression strategy, prioritizing positive-sequence power, is constructed based on inverter control during non-full-phase operation. The overall implementation process of the negative-sequence current suppression strategy is as follows: Figure 3 As shown, it includes: high-priority control guarantee, determination of optimal suppression reference quantity, and control implementation.

[0118] The high priority is ensured according to step one, and after determining non-full-phase operation and voltage ride-through control exit, negative sequence current is rapidly suppressed.

[0119] Following steps two and three, the feasible solution domain is determined, and the optimal suppression reference quantity i is solved using the geometric analytical method. -* .

[0120] The control implementation is carried out according to steps four and five, including parameter calculation without PLL, and positive and negative sequence separation control (positive and negative sequence control in parallel). In the dual closed-loop control, the feedforward decoupling algorithm is used to determine the grid-connected inverter voltage command value. As shown in equation (7):

[0121]

[0122] Where: u * The voltage command value is represented by 'e', ​​the grid connection point voltage is represented by 'i', the inverter current is represented by 'ω', the angular frequency is represented by 'L', the filter inductance is represented by '*', the superscript '+' and '-' represent the positive and negative sequence components respectively, and the subscripts 'd' and 'q' represent the d-axis and q-axis components respectively. P K represents the proportionality coefficient. I This represents the integral control parameter.

[0123] The reference for large PWM modulation is obtained by vector superimposing the positive-sequence voltage reference and the negative-sequence voltage reference. The reference angle for the dq transformation in the strategy uses a given phase angle θ. Combined with the DSC algorithm, the positive and negative sequence components are separated, ensuring high performance of the strategy under grid voltage imbalance. The control part for negative-sequence current suppression is as follows... Figure 4 As shown.

[0124] Step 7: Use PSCAD software to build such a model Figure 5 The simulation model of a large-scale power system including clustered new energy sources shown is intended to verify the effectiveness of the aforementioned negative sequence current suppression strategy.

[0125] The system voltage level is 500kV, and the renewable energy source is a photovoltaic power cluster. Each photovoltaic array has a rated power of 250kW, with a total of 1700 arrays, resulting in a total installed capacity of 425MW. Synchronous generators have a total installed capacity of 800MW, representing a penetration rate of approximately 35%. Other relevant parameters are as follows:

[0126] Line length = 500km

[0127] Transmission line parameters:

[0128] r1=0.021Ω / km: r0=0.115Ω / km

[0129] l1=0.8984mH / km; l0=2.2892mH / km

[0130] c1=0.0129μF / km; c0=0.0052μF / km

[0131] A phase-A ground fault (AG) was simulated, and the negative sequence current at the generator terminals of the synchronous generator and the voltage at the grid connection point of the clustered new energy were monitored to verify the suppression effect.

[0132] 1): The voltage waveform at the grid connection point during non-full-phase operation is as follows: Figure 6 , Figure 7 As shown.

[0133] During non-full-phase operation, the voltage asymmetry at the grid connection point is relatively small. The calculated asymmetry VUF = (U + / U - )*100% is approximately 4%, the effective value of the grid connection point (U rms The voltage is kept above 0.9 pu. According to general low voltage ride-through control procedures, LVRT is only implemented when the grid connection point voltage is below 0.9 pu. Therefore, during non-full-phase operation, the grid-connected inverter can be used to suppress negative sequence current while prioritizing positive sequence power output, in accordance with the strategy proposed in this paper.

[0134] 2): Suppress negative sequence current at the front end, such as Figure 8 As shown.

[0135] The fault was cleared in 1.58 seconds, and the system entered non-full-phase operation. Before the negative sequence current compensation, the negative sequence current intruding into the synchronous generators at both ends during non-full-phase operation was 0.86kA (steady-state amplitude) at the new energy access end and 1.07kA (steady-state amplitude) on the opposite side of the new energy source. Such a large negative sequence current intruding into the synchronous generator will inevitably threaten the negative sequence protection configured for the synchronous generator, and it is necessary to suppress the negative sequence current.

[0136] 3): The negative sequence current suppression effect is as follows Figures 9-13 As shown.

[0137] Among them, the control response without PLL is as follows Figure 9 As shown, the control method without a PLL can achieve a relatively fast response speed even when the grid voltage is unbalanced, reaching the theoretical control response in about 16ms. Complete suppression (I p =0.6pu, I n The waveform of (=0.4pu) is as follows Figure 10 As shown, where I p I represents the positive sequence current. n I represents the negative sequence current. p =0.6pu, I n In the 0.4kA scenario, there is a large negative sequence suppression range, and the simulated negative sequence current is completely suppressed. At the renewable energy access point, the current is 0.03kA (steady-state amplitude), and at the opposite renewable energy side, it is 0.05kA (steady-state amplitude), effectively suppressing the negative sequence current at the synchronous generator terminal. Considering that the negative sequence compensation capacity of the renewable energy grid-connected inverter varies under different positive sequence operating conditions, when the positive sequence current is large, I... p =0.8pu, I n =0.4 pu, corresponding to incomplete suppression, such as Figure 11 As shown. At this point, although the negative sequence current intruding into the generator terminal could not be completely suppressed to near zero, it was significantly weakened compared to before compensation, with 0.11kA (steady-state amplitude) at the new energy input end and 0.19kA (steady-state amplitude) on the opposite side of the new energy source. Considering a greater demand for negative sequence current suppression, I... p =0.8pu,I n=0.6 pu, which corresponds to the case of incomplete suppression, such as Figure 12 As shown. Compared to I p =0.8pu,I n Under the condition of 0.4 pu, the positive sequence current remains unchanged, while the negative sequence current to be suppressed increases by 0.2 pu, meaning the suppression capability is relatively reduced. The current at the renewable energy access end is 0.437 kA (steady-state amplitude), and the current on the opposite side of the renewable energy source is 0.552 kA (steady-state amplitude). Compared to the unsuppressed situation, the negative sequence current decreases by approximately 48.4%, indicating that the negative sequence current intruding into the synchronous generator is effectively suppressed. The current of a single converter in a cluster renewable energy power station is as follows... Figure 13 As shown, the converter arm current limit I lim Set to 120A, during non-full-phase operation, the amplitude of the output current in two phases is I. lim Furthermore, the three-phase current of the grid-connected inverter does not exceed I. lim This means that all grid-connected inverters have maximized their respective negative sequence compensation strategies.

Claims

1. A PLL-free negative sequence current suppression strategy, characterized in that... include: ①High-priority control: Non-full-phase operation is considered an abnormal system operation, and the low-voltage ride-through control of the grid-connected inverter may be activated by voltage triggering. The status of the circuit breaker is detected to determine the system operation status and whether the system is operating in a non-full-phase manner: if it is operating in a non-full-phase manner and the low-voltage ride-through control of the grid-connected inverter is not activated, then negative-sequence current suppression is performed on the premise of ensuring positive-sequence power output; otherwise, low-voltage ride-through control is ensured first. ② Determination of the optimal suppression reference value: Based on the determined positive-sequence power output reference value, determine the current three-phase output current vector of the grid-connected inverter. , x =a, b, c, and the single-phase current limit of the grid-connected inverter ; Determine if the grid-connected inverter has available remaining capacity, and then solve for the optimal reference value for suppressing negative sequence current. If the grid-connected inverter has no available capacity, it will be shut down to ensure priority delivery of positive sequence power. ③: The grid-connected inverter adopts a PLL-free positive and negative sequence separation control, with positive and negative sequence control in parallel: During normal operation, the negative sequence suppression circuit does not activate during positive sequence power output. The positive sequence power output adopts a PLL-less fast control scheme, and the relevant control parameters are calculated from the control parameters of the PLL-less circuit, including: virtual fixed phase angle. θ Calculation, Park transform, and DSC delay algorithm solution; The reference quantity for negative sequence current suppression is the negative sequence current vector obtained by the geometric analytical method. The relevant parameters required for the geometric analytical method are obtained by the DSC delay algorithm.

2. The PLL-free negative sequence current suppression strategy according to claim 1, characterized in that: Based on the priority of positive-sequence power output from the grid-connected inverter, determine the feasible solution domain for suppressing the negative-sequence current of the current positive-sequence power output; specifically including: Based on the current three-phase output current vector of the grid-connected inverter Single-phase current limit of grid-connected inverter The current constraint is constructed as shown in equation (1). The feasible solution domain for suppressing negative sequence current under the premise of ensuring priority output of positive sequence power is obtained by solving equation (1): （1）； In formula (1): This indicates the maximum allowable amplitude of single-phase current in a grid-connected inverter; This represents the three-phase output current vector of the grid-connected inverter. x =a, b, c, where the superscripts '+' and '-' represent the positive and negative order components, respectively; Indicates the rotation factor. , Represents the imaginary unit in complex numbers; With inverter current , , The endpoints A, B, and C are the three centers of the circle, with Draw a circle with radius , and the shaded area where the three circles intersect represents the feasible solution domain for suppressing negative sequence current.

3. The PLL-free negative sequence current suppression strategy according to claim 1, characterized in that: Using geometric analysis, the optimal suppression reference value within the feasible solution domain for negative sequence current suppression is determined; specifically, this includes: Based on the relative positions of the endpoints of the current to be suppressed in the feasible solution domain of the negative sequence current, the optimal suppression solution is determined in the following two cases: 1): The negative sequence current to be suppressed is located within the solution domain; let the phasor of the negative sequence current to be suppressed be... Where O represents the origin and N represents the endpoint of the negative sequence current to be suppressed; when N is within the solution domain, it has complete suppression capability, and the optimal reference quantity for suppressing the negative sequence current at this time is the phasor formed by connecting the center and point N. ; 2): When When located outside the solution domain; the optimal suppression reference value is determined by geometric analytical method; let A, B, and C be the inverter currents respectively. , , The three endpoints are X, Y, and Z, which are the three endpoints of the feasible solution domain; N is the endpoint of the negative sequence current to be suppressed; based on the relative position of point N with the feasible solution domain, the feasible solution domain is divided into 6 regions, namely regions I, II, III, IV, V, and VI, and the optimal suppression solution in each region is determined: When point N is in region I, the optimal suppression reference value is Point R is the intersection of points N and A with the feasible solution domain; When point N is in region II, the optimal suppression solution is: ; When point N is in region III, the R point of the optimal suppression solution is the intersection point connecting points N and C with the feasible solution domain; When point N is in region IV, the optimal suppression solution is: ; When point N is in region V, the R point of the optimal suppression solution is the intersection point connecting points N and B with the feasible solution domain; When point N is in region VI, the optimal suppression solution is: .

4. The PLL-free negative sequence current suppression strategy according to claim 1, characterized in that: The Parker transformation matrix is ​​shown in equations (3) and (4): （3）； （4）； Where: corresponding , , represent the positive and negative order output matrices of the Parker transform, taking current as an example; and These represent the Parker positive order transformation matrix and the negative order transformation matrix, respectively. Represents the matrix of three-phase current input quantities; This represents the Parker transformation reference angle, which is virtually fixed in this invention. As a reference angle for the Park transformation, that is, ; , , Calculate the three-phase currents (a, b, and c) and the voltage. When calculating the order components under the dq coordinate axis, replace the input of equations (3) and (4) with... .

5. The PLL-free negative sequence current suppression strategy according to claim 4, characterized in that: A control method without PLL circuitry is employed to achieve rapid suppression of negative sequence current, specifically including: 3.1: Calculate the control parameters without PLL, including the virtual fixed phase angle and the sequence components under the dq coordinate axis; 3.2: Substitute the relevant parameters obtained in 3.1 into the PLL-less control circuit to achieve PLL-less fast control of the grid-connected inverter.

6. The PLL-free negative sequence current suppression strategy according to claim 5, characterized in that: In section 3.1, a virtual fixed phase angle is used to complete the control without a PLL. The virtual fixed phase angle... The calculation is shown in equation (2): （2）； In formula (2): The system's rated frequency is 50Hz; t For time; Indicates the rated electrical angular velocity; With virtual phase angle As a reference for grid voltage and current Perform a dq coordinate transformation to generate the voltage dq components. and current dq component ; The positive and negative sequence components in the dq component are separated using a delay algorithm to obtain the positive and negative sequence components in the voltage and current dq axis coordinate system: , , , , , , , Where: the subscripts d and q represent the d-axis components and the superscripts + and - represent the positive and negative order components, respectively.

7. The PLL-free negative sequence current suppression strategy according to claim 6, characterized in that: In section 3.2, considering the high priority of positive-sequence power output during single-phase reclosing in the high-voltage grid, the positive-sequence current command value in the current control loop is selected from the DC active power... reactive power As a constraint equation, the positive sequence current reference value is solved according to equation (3), and a PI regulator is used to form a double closed-loop control circuit.

8. The PLL-free negative sequence current suppression strategy according to claim 1, characterized in that: The DSC delay algorithm is shown in equation (5): （5）； in This represents the output of the DSC algorithm; This represents the original vector value of the d-axis component, corresponding to the positive and negative order components under the dq axis. and ; Represents the d-axis component delay vector; Indicates time; Indicates period, Indicates the harmonic order.

9. The PLL-free negative sequence current suppression strategy according to claim 1, characterized in that: a. The PLL-less implementation of the positive sequence power delivery section is as follows: The calculation of the current command for positive sequence power is based on the selection of DC active power. reactive power As a constraint equation, the positive sequence current reference value is solved according to equation (6), and a PI regulator can be used to form a double closed-loop control circuit without the need for a phase-locked loop. （6）； All coordinate transformations use virtual phase angles as the reference phase angle. Completed, ensuring control response speed and accuracy under voltage imbalance; in PLL-less control loops, all coordinate transformations are based on virtual fixed impedance angles. θ For reference only; b. The control without a PLL for negative sequence current suppression is as follows: The calculation of the negative sequence current command is based on the optimal suppression reference value determined by the geometric analytical method as the control input. The voltage control reference value is obtained through the double closed-loop control without PLL. This voltage control reference value is then combined with the voltage control reference value generated by the positive sequence power control to obtain the total voltage control reference value, which is then modulated by PWM. In the dual closed-loop control of the grid-connected inverter, a feedforward decoupling algorithm is used to determine the voltage command value of the grid-connected inverter, as shown in equation (7): （7）； In equation (7): Indicates the voltage command value; Indicates the voltage at the grid connection point; Indicates the inverter current; Indicates angular frequency; The symbol represents the filter inductance, '*' represents the reference value, the superscripts '+' and '-' represent the positive and negative sequence components respectively, and the subscripts d and q represent the d-axis components respectively; Represents the proportionality coefficient. This represents the integral control parameter.

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