Distributed unbalanced current compensation method, device and medium for photovoltaic power distribution network

Abstract

The present application is a distributed unbalanced current compensation method, device and medium for photovoltaic-participated distribution network, which belongs to the field of power quality compensation and distributed photovoltaic converter control technology. In view of the limitations of low efficiency of large-scale data processing in existing centralized control and point-to-point communication control methods, it proposes: a distributed unbalanced current compensation method for photovoltaic-participated distribution network, including: setting up an inverter participating in compensation, and enabling it to inject negative-sequence and zero-sequence current into the power grid according to compensation instructions; calculating the active power and reactive power of each phase flowing through the substation transformer in the current cycle, as well as the ideal compensation instruction; calculating the compensation coefficient to obtain the actual compensation instruction; obtaining the compensation current of each phase to perform unbalanced current compensation. The present invention has lower communication bandwidth requirements and the ability to reliably cope with communication failures; it does not require the configuration of additional energy storage equipment, effectively alleviates the three-phase unbalanced current flowing through the substation transformer, and can also alleviate voltage imbalance.

Description

Technical Field

[0001] The present invention belongs to the technical field of power quality compensation and distributed photovoltaic inverter control, and relates to a distributed unbalanced current compensation method, device and medium for a photovoltaic-participated distribution network. Background Art

[0002] With the massive increase of three-phase and single-phase equipment in the three-phase four-wire (3P-4W) low-voltage distribution network, especially when the load and distributed power sources are unevenly distributed, the unbalanced phase current has brought great challenges to the efficient and safe operation of the distribution network. The problems caused by unbalanced current include impaired power quality, voltage imbalance, overheating of distribution transformers, neutral line overload, additional line loss, reduced distribution network capacity utilization, and malfunction of protection relays.

[0003] Therefore, in order to improve the power supply quality and reliability, the low-voltage distribution network should be equipped with an unbalanced compensation function. Among them, the photovoltaic inverter, as a common distributed power source, can use the surplus capacity of photovoltaic power generation to achieve unbalanced compensation and other power quality management functions, thereby reducing the dependence on special power quality management equipment (such as STATCOM, APF, SVC).

[0004] In a three-phase four-wire system, if single-phase photovoltaics are used to compensate for unbalanced current, it is necessary to have photovoltaic inverters that are connected to both phases and ground in the distribution network. The three-phase inverter of the three-phase four-wire system provides an additional zero-sequence current path and is an ideal unbalanced current compensation device. Most existing technologies focus on compensating for unbalanced local loads. However, the equipment points that generate unbalanced currents in the distribution network are numerous and widespread. Compensating for local imbalances alone cannot alleviate the problems of transformer overheating, neutral line overload, and low capacity utilization. Therefore, the photovoltaic inverters in the distribution network need to be coordinated and participated in the governance. Summary of the invention

[0005] In view of the limitations of low efficiency of large-scale data processing in existing centralized control and point-to-point communication-based control methods, the present invention provides a distributed unbalanced current compensation method, device and medium for photovoltaic-participated distribution network. Combining the characteristics of distributed controller and unidirectional communication, the three-phase active and reactive power measured at the outlet of the substation transformer in the distribution network is used to calculate the compensation instructions in the distributed controller, and the compensation instructions are further allocated according to the capacity of different photovoltaic inverters. The compensation instructions are sent to the downstream photovoltaic inverters for execution through a low-bandwidth unidirectional communication link, thereby alleviating the three-phase unbalanced current flowing through the substation transformer.

[0006] To this end, the present invention is a method for compensating distributed unbalanced current in a photovoltaic power distribution network, which comprises the following steps:

[0007] S1, setting the inverters participating in the compensation so that they can inject negative-sequence and zero-sequence currents into the power grid according to the compensation instructions; the compensation instructions are the compensation instructions distributed by the substation transformer to the photovoltaic inverters participating in the compensation task in the distribution network through low-bandwidth unidirectional communication;

[0008] S2, sampling the three-phase voltage and current of the current cycle and the active compensation instruction and reactive compensation instruction of the previous cycle, calculating the active power and reactive power of each phase flowing through the substation transformer of the current cycle, and the ideal compensation instruction that can achieve full compensation of the three-phase current of the substation transformer;

[0009] S3, calculating the compensation coefficient of each photovoltaic inverter based on the result of step S2, and associating the actual compensation instruction with the ideal compensation instruction by adding the compensation coefficients of all photovoltaic inverters and obtaining a total compensation coefficient;

[0010] S4, obtaining the compensation current of each phase according to the compensation instruction, so that the photovoltaic can ensure the maximum available power output while using the remaining capacity to compensate for the unbalanced current.

[0011] The present invention proposes a distributed unbalanced current compensation method for a photovoltaic-participated distribution network, which can enable the photovoltaic inverter in the distribution network to perform unbalanced current compensation through a low-bandwidth unidirectional communication link, thereby reducing the three-phase unbalanced current flowing through the substation transformer and improving the system power quality.

[0012] Furthermore, the specific process of step S2 is as follows:

[0013] The sampled active power of each phase in the current cycle is subtracted from the three-phase average active power and then added to the active power compensation instruction of the previous cycle to obtain the ideal active power compensation instruction of each phase;

[0014] The ideal reactive power compensation instruction for each phase is obtained by subtracting the three-phase average reactive power from the sampled reactive power of each phase in the current cycle and adding the reactive power compensation instruction of the previous cycle.

[0015] The specific formula used in step S2 is:

[0016] (1);

[0017] In the formula, is the measured active power of three phases A, B and C, is the three-phase reactive power, and They are respectively the active and reactive compensation instructions of the previous control cycle, , are the sampled active power and reactive power respectively, , are the ideal active compensation instruction and ideal reactive compensation instruction for each phase respectively, and m can be A, B, or C.

[0018] Furthermore, the specific process of step S3 is as follows:

[0019] S3.1, based on the minimum voltage in the distribution network , calculate the maximum amplitude of active and reactive current instructions corresponding to the power instruction under ideal conditions and ;

[0020] (2);

[0021] In the formula, and They are respectively the maximum amplitudes of active and reactive current instructions corresponding to the power instructions under ideal conditions.

[0022] S3.2, based on the positive sequence voltage amplitude of the photovoltaic converter (numbered PV-1) closest to the transformer and positive sequence current reference and the minimum voltage in the distribution network to correct the positive sequence current reference value;

[0023] (3);

[0024] In the formula, is the positive sequence current reference value of the photovoltaic converter closest to the transformer in the substation area, is the amplitude of the positive sequence voltage of the PV inverter; is the corrected positive sequence current reference value;

[0025] S3.3, construct the equation group consisting of the current limit constraint conditions of the photovoltaic inverter, and obtain the unbalanced compensation coefficients corresponding to the three-phase current limit by solving the equation group Unbalance compensation coefficient corresponding to neutral line current limitation , and The minimum value in is set to i Compensation coefficient of photovoltaic inverter (The subscript i represents the variable associated with the i-th PV inverter);

[0026] The equations of the constructed photovoltaic inverter current limit constraint conditions are expressed as follows:

[0027] (4);

[0028] in, Respectively i The amplitude of the ABC three-phase and neutral line current of a photovoltaic inverter should be less than or equal to the maximum allowable current of the photovoltaic inverter. , is the rated current of the photovoltaic inverter and capacity factor The product of = ; , Represent the ideal active compensation current of phase A and phase B respectively, , Represent the ideal reactive power compensation current of phase A and phase B respectively, is the compensation coefficient corresponding to the maximum allowable current of the three-phase and neutral line, that is, the positive root of the quadratic equation in formula (4), Take the minimum value of the compensation coefficient corresponding to the maximum allowable current of the three-phase and neutral line, that is, min( );

[0029] S3.4, add up the compensation coefficients of each photovoltaic inverter to obtain the sum of the unbalanced compensation coefficients of all photovoltaic inverters , based on the sum of the unbalanced compensation coefficients of all PV inverters Assign total compensation coefficient k , according to the ideal power compensation instruction ( and ) and the total compensation coefficient k The actual power compensation command is calculated by multiplying and ).

[0030] Furthermore, the specific process of step S3.4 is as follows:

[0031] When the sum of the compensation coefficients of all PV inverters When greater than 0 and less than 1, the total compensation coefficient k equal , then calculate the actual power compensation instruction;

[0032] When the sum of the compensation coefficients of all PV inverters When greater than 1, the total compensation coefficient is k =1, the sum of the compensation coefficients of all photovoltaic inverters The reciprocal of is taken as the ratio, and the ratio is adopted to change the unbalanced compensation coefficient of each photovoltaic inverter, and the changed unbalanced compensation coefficients of each photovoltaic inverter are obtained and added up, and then the proportion of the changed unbalanced compensation coefficients of each photovoltaic inverter to the total of the changed unbalanced compensation coefficients is calculated, and the three-phase power instructions of each photovoltaic inverter are allocated from the actual power compensation instruction according to the corresponding proportion of each photovoltaic inverter.

[0033] The formula used is:

[0034] (5);

[0035] Proportionally change the unbalance compensation coefficient of each photovoltaic inverter :

[0036] (6);

[0037] (7);

[0038] (8);

[0039] In the formula, is the changed imbalance compensation coefficient.

[0040] Furthermore, the specific process of step S4 is as follows:

[0041] Based on the actual power compensation command and the positive sequence voltage of each photovoltaic inverter, the amplitude of the three-phase active and reactive current of each photovoltaic inverter is calculated. Then, the three-phase compensation current command of the photovoltaic inverter is calculated according to the amplitude of the active and reactive current. The current reference value of each phase is obtained by adding it to the positive sequence current reference value obtained by DC voltage control. The reference current of each phase is then tracked. At the same time, the disturbance observation method is used on the DC side to maximize the available power of the photovoltaic output.

[0042] Furthermore, the formula used in step S4 is:

[0043] (9);

[0044] (10);

[0045] (11);

[0046] In the formula, For the i The amplitude of the three-phase active current of a photovoltaic inverter, For the i The magnitude of the reactive current of each photovoltaic inverter, For the i The amplitude of the positive sequence voltage obtained by the phase-locked loop of a photovoltaic inverter; is the three-phase compensation current instruction of the photovoltaic inverter, For the i The actual three-phase active power compensation instructions of each photovoltaic inverter. For the i Actual three-phase reactive power compensation instructions of photovoltaic inverters; is the phase difference between the current vector and the voltage vector, Corresponding to the three phases A, B, and C respectively, The phase of the local positive sequence voltage obtained by the phase-locked loop;

[0047] The present invention also provides a distributed unbalanced current compensation device for a photovoltaic power distribution network, comprising:

[0048] DC-DC converter, which is located on the PV side and is used to regulate the output of the PV array;

[0049] A DC-AC inverter, which is located on the grid side and is used to transmit power to the grid;

[0050] The data sampling unit includes a voltage sensor and a current sensor at the output port of the DC-AC inverter and a voltage sensor and a current sensor at the input port of the photovoltaic converter. The voltage sensor and the current sensor at the output port of the DC-AC inverter are sampled respectively to obtain the grid connection point (PCC) voltage of the DC-AC inverter. and the current output current ;Use the voltage sensor and current sensor at the input port of the photovoltaic inverter to sample the current port voltage and current of the photovoltaic array respectively;

[0051] Phase-locked loop unit, which determines the angle of the point of common coupling (PCC) voltage through a phase-locked loop (PLL) to keep the inverter synchronized with the grid;

[0052] The DC voltage and current control module maintains the voltage on the DC capacitor through voltage and current double closed-loop control Constant, the reference current is tracked by a proportional resonant (PR) controller;

[0053] Photovoltaic power conditioning module, using the perturbation and observation method for maximum power point tracking (MPPT);

[0054] Photovoltaic voltage ( ) control module, through the voltage and current double closed loop control, the voltage of the photovoltaic array ( ) is adjusted to its reference value ;

[0055] The compensation current generation module receives the power command from the distributed controller through one-way communication and generates the corresponding three-phase compensation current command .

[0056] A computer-readable storage medium stores a program, which, when executed by a processor, implements the above-mentioned method for compensating distributed unbalanced current in a photovoltaic-participated distribution network.

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

[0058] (1) The present invention can overcome the limitations of low efficiency of large-scale data processing in centralized control and point-to-point communication control methods. It only needs to utilize a low-bandwidth unidirectional communication link between the substation transformer and the photovoltaic power plant, has low communication bandwidth requirements and the ability to reliably cope with communication failures.

[0059] (2) The present invention does not require the configuration of additional energy storage equipment. While ensuring that the photovoltaic system generates the maximum available power, the compensation instructions are distributed according to the capacity of different photovoltaic inverters, which can effectively alleviate the three-phase unbalanced current flowing through the substation transformer.

[0060] (3) The present invention controls the current of each phase to be below the maximum allowable current of the photovoltaic inverter, thereby maximizing the utilization of the remaining capacity of the photovoltaic inverter.

[0061] (4) The present invention can reduce the three-phase unbalanced current flowing through the transformer of the substation while alleviating the voltage imbalance, and can be applied to distribution networks of various types of topologies. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] Figure 1 A comparison chart of the present application and the traditional distributed control method;

[0063] Figure 2 Flow chart for distributed unbalanced current compensation for photovoltaic participation in distribution network;

[0064] Figure 3 It is a schematic diagram for calculating the unbalanced compensation coefficient of photovoltaic inverters with different capacity factors;

[0065] Figure 4 It is a schematic diagram of the change process of power command and compensation coefficient when communication failure occurs;

[0066] Figure 5 This is the structural diagram of the distributed unbalanced current compensation device for photovoltaic power distribution network;

[0067] Figure 6 The phase-locked link and voltage and current inner loop control flow chart of the photovoltaic inverter involved in compensation;

[0068] Figure 7 This is the topology diagram of the distribution network in the application example;

[0069] Figure 8 It is a curve diagram of the total compensation power instruction actually issued by the distributed controller in the application example;

[0070] Fig. 9 The three-phase current and output power curves of PV-1 and PV-2 in the application example are shown;

[0071] Fig.10It is a comparison chart of the changes in the negative sequence, zero sequence current amplitude and voltage of the transformer in the substation in the application example;

[0072] In the figure: 1-phase locked loop unit; 2-DC voltage and current control module; 3-photovoltaic power regulation module; 4-photovoltaic voltage ( ) control module; 5- compensation current generating module. DETAILED DESCRIPTION

[0073] The following is an explanation and description of the technical solutions of the embodiments of the present invention in conjunction with the accompanying drawings of the embodiments of the present invention, but the following embodiments are only preferred embodiments of the present invention, not all. Based on the embodiments in the implementation mode, other embodiments obtained by those skilled in the art without creative work are all within the protection scope of the present invention.

[0074] Example 1

[0075] This embodiment provides a method for compensating distributed unbalanced current in a photovoltaic power distribution network, which includes the following steps:

[0076] S1, setting the inverters participating in the compensation so that they can inject negative-sequence and zero-sequence currents into the power grid according to the compensation instructions; the compensation instructions are the compensation instructions distributed by the substation transformer to the photovoltaic inverters participating in the compensation task in the distribution network through low-bandwidth unidirectional communication;

[0077] S2, sampling the three-phase voltage and current of the current cycle and the active compensation instruction and reactive compensation instruction of the previous cycle, calculating the active power and reactive power of each phase flowing through the substation transformer in the current cycle, and the ideal compensation instruction that can achieve full compensation of the three-phase current of the substation transformer; the specific process is:

[0078] The sampled active power of each phase in the current cycle is subtracted from the three-phase average active power and then added to the active power compensation instruction of the previous cycle to obtain the ideal active power compensation instruction of each phase;

[0079] The ideal reactive power compensation instruction for each phase is obtained by subtracting the three-phase average reactive power from the sampled reactive power of each phase in the current cycle and adding the reactive power compensation instruction of the previous cycle.

[0080] The specific formula used is:

[0081] (1);

[0082] In the formula, is the measured active power of three phases A, B and C, is the three-phase reactive power, and They are respectively the active and reactive compensation instructions of the previous control cycle, , are the sampled active power and reactive power respectively, , are the ideal active compensation instruction and ideal reactive compensation instruction for each phase respectively, and m can be A, B, and C phases.

[0083] S3, based on the result of step S2, the compensation coefficient of each photovoltaic inverter is calculated, and the actual compensation instruction and the ideal compensation instruction are associated by adding the compensation coefficients of all photovoltaic inverters and obtaining the total compensation coefficient; the specific process is:

[0084] S3.1, based on the minimum voltage in the distribution network , calculate the maximum amplitude of active and reactive current instructions corresponding to the power instruction under ideal conditions and ;

[0085] (2);

[0086] In the formula, and They are respectively the maximum amplitudes of active and reactive current instructions corresponding to the power instructions under ideal conditions.

[0087] Minimum voltage A conservative estimate can be made based on the relationship between the voltage at the node where PV-1 is located and the lowest voltage at the node (for example, using the lowest ratio of the minimum meter voltage over a period of time to the voltage at the node where PV-1 is located, or using the minimum value of all meter readings over a period of time).

[0088] S3.2, based on the positive sequence voltage amplitude of the photovoltaic converter (numbered PV-1) closest to the transformer and positive sequence current reference and the minimum voltage in the distribution network to correct the positive sequence current reference value;

[0089] (3);

[0090] In the formula, is the positive sequence current reference value of the photovoltaic converter closest to the transformer in the substation area, is the amplitude of the positive sequence voltage of the PV inverter; is the corrected positive sequence current reference value;

[0091] S3.3, construct the equation group consisting of the current limit constraint conditions of the photovoltaic inverter, and obtain the unbalanced compensation coefficients corresponding to the three-phase current limit by solving the equation group Unbalance compensation coefficient corresponding to neutral line current limitation , and The minimum value in is set toi Compensation coefficient of photovoltaic inverter ;

[0092] The equations of the constructed photovoltaic inverter current limit constraint conditions are expressed as follows:

[0093] (4);

[0094] in, Respectively i The amplitude of the ABC three-phase and neutral line current of a photovoltaic inverter should be less than or equal to the maximum allowable current of the photovoltaic inverter. , is the rated current of the photovoltaic inverter and capacity factor The product of = ; , Represent the ideal active compensation current of phase A and phase B respectively, , Represent the ideal reactive power compensation current of phase A and phase B respectively, is the compensation coefficient corresponding to the maximum allowable current of the three-phase and neutral line, that is, the positive root of the quadratic equation in formula (4), Take the minimum value of the compensation coefficient corresponding to the maximum allowable current of the three-phase and neutral line, that is, min( );

[0095] S3.4, add up the compensation coefficients of each photovoltaic inverter to obtain the sum of the unbalanced compensation coefficients of all photovoltaic inverters , based on the sum of the unbalanced compensation coefficients of all PV inverters Assign total compensation coefficient k , according to the ideal power compensation instruction ( and ) and the total compensation coefficient k The actual power compensation command is calculated by multiplying and );

[0096] Specifically: When the sum of the compensation coefficients of all PV inverters When greater than 0 and less than 1, the total compensation coefficient k equal , then calculate the actual power compensation instruction;

[0097] When the sum of the compensation coefficients of all PV inverters When greater than 1, the total compensation coefficient is k =1, the sum of the compensation coefficients of all photovoltaic inverters The reciprocal of is taken as the ratio, and the ratio is adopted to change the unbalanced compensation coefficient of each photovoltaic inverter, and the changed unbalanced compensation coefficients of each photovoltaic inverter are obtained and added up, and then the proportion of the changed unbalanced compensation coefficients of each photovoltaic inverter to the total of the changed unbalanced compensation coefficients is calculated, and the three-phase power instructions of each photovoltaic inverter are allocated from the actual power compensation instruction according to the corresponding proportion of each photovoltaic inverter.

[0098] The formula used is:

[0099] (5);

[0100] Proportionally change the unbalance compensation coefficient of each photovoltaic inverter :

[0101] (6);

[0102] (7);

[0103] (8).

[0104] S4, obtain the compensation current of each phase according to the compensation instruction, so that the photovoltaic can ensure the maximum available power output while using the remaining capacity to compensate for the unbalanced current. The specific process is:

[0105] Based on the actual power compensation command and the positive sequence voltage obtained by the phase-locked link of each photovoltaic inverter, the amplitude of the three-phase active and reactive current of each photovoltaic inverter is calculated, and then the three-phase compensation current command of the photovoltaic inverter is calculated according to the amplitude of the active and reactive current. , and the positive sequence current reference value obtained by DC voltage control is added to obtain the current reference value of each phase, and then the reference current of each phase is tracked. At the same time, the disturbance observation method is used on the DC side to make the photovoltaic output maximum available power.

[0106] The formula used is:

[0107] (9);

[0108] (10);

[0109] (11);

[0110] In the formula, For the i The amplitude of the three-phase active current of a photovoltaic inverter, For the i The magnitude of the reactive current of each photovoltaic inverter, For the iThe amplitude of the positive sequence voltage obtained by the phase-locked loop of a photovoltaic inverter; is the three-phase compensation current instruction of the photovoltaic inverter, For the i The actual three-phase active power compensation instructions of each photovoltaic inverter. For the i Actual three-phase reactive power compensation instructions of photovoltaic inverters; is the phase difference between the current vector and the voltage vector, Corresponding to the three phases A, B, and C respectively, The phase of the local positive sequence voltage obtained by the phase-locked loop;

[0111] In order to balance the three-phase current flowing through the substation transformer, the unbalanced current can be compensated by a three-phase four-wire photovoltaic inverter. Compared with the traditional distributed control, the results are as follows: Figure 1 As shown, the photovoltaic inverter with unidirectional communication function receives compensation instructions from the distributed controller according to its own different residual capacity, and compensates for the unbalanced current of the substation transformer.

[0112] The flowchart of the distributed unbalanced current compensation method of the photovoltaic power distribution network of the present invention is as follows: Figure 2 As shown in the figure, the distributed controller executes the process from START to RETURN TO START in one control cycle. First, the three-phase voltage and current at the outlet of the substation transformer are collected, and the active power of each phase flowing through the substation transformer is calculated. and reactive average power , and the fully compensated power command for each phase and ; Then according to the minimum voltage in the distribution network , calculate the maximum amplitude of active and reactive current instructions corresponding to the fully compensated power instruction and , and the corrected positive sequence current reference value of PV-1 ; Then, according to formula (4), the unbalanced compensation coefficient of the photovoltaic inverter numbered i is calculated For the remaining PV inverters (taking number j as an example), the unbalanced compensation coefficient of PV inverters with the same capacity factor but different rated current is proportional to the rated current; the unbalanced compensation coefficient of PV inverters with the same rated current but different capacity factor can be calculated by formula (4) and Figure 3 The calculation method shown is used to obtain the unbalanced compensation coefficient of each inverter.

[0113] Add up the unbalanced compensation coefficients of all PV inverters to get ,like Between 0 and 1, the total compensation coefficient kequal ,like , indicating that full compensation can be achieved, then the total compensation coefficient k is set to 1, and the unbalance compensation coefficient of each photovoltaic inverter is changed proportionally accordingly , the final actual compensation instruction and Designed to and take The compensation instruction is sent to the photovoltaic inverter through one-way communication according to the unbalanced compensation coefficient of each photovoltaic inverter, and is updated for the next control cycle iteration. and , is the compensation instruction of the previous control cycle and .

[0114] At the same time, in the photovoltaic inverter that performs the compensation task, the compensation current generation module receives the power command from the distributed controller through unidirectional communication and generates the corresponding three-phase compensation current command. .

[0115] Assume that in the tth control cycle, a communication failure occurs between the inverter with unbalanced compensation coefficient and the substation transformer, which means that although the compensation power command has been calculated on the distributed controller, the PV inverter cannot receive the corresponding command and execute it.

[0116] make Representation variables In the tth control strategy execution cycle, the value (other variables are the same) is assumed to be constant in the system environment and load for convenience. , As an example, the same applies to reactive power instructions. The entire process from fault occurrence to recovery is shown in the attached figure. Figure 4 shown.

[0117] When satisfied When the communication failure occurs, in the next control cycle, due to the lack of compensation coefficient PV inverter development involved, There will be The increase of , Change to the original , remains unchanged, then , and k The same value will be maintained until the communication fault is restored. After one control cycle of fault recovery, all variables will be restored to the value before the fault.

[0118] When satisfied When a communication failure occurs, if the remaining capacity of the inverter is sufficient to fully compensate for the imbalance, , will continue to increase and gradually converge to times the original value, otherwise when the hth control cycle satisfies But the h+1th control cycle satisfies The iteration stops when , and k The same value will be maintained until the communication failure is restored. One cycle after the failure is restored, all variables will be restored to the value before the failure.

[0119] Therefore, during the fault period, the inverter with normal communication can execute the compensation power command as usual, so that the unbalanced current can still be alleviated, which shows that the control strategy has the ability to reliably respond to communication failures, and the compensation command can converge to a steady state when the communication status changes.

[0120] Example 2

[0121] A distributed unbalanced current compensation device for a photovoltaic power distribution network, comprising:

[0122] DC-DC converter, which is located on the PV side and is used to regulate the output of the PV array;

[0123] A DC-AC inverter, which is located on the grid side and is used to transmit power to the grid;

[0124] The data sampling unit includes a voltage sensor and a current sensor at the output port of the DC-AC inverter and a voltage sensor and a current sensor at the input port of the photovoltaic converter. The voltage sensor and the current sensor at the output port of the DC-AC inverter are sampled respectively to obtain the grid connection point (PCC) voltage of the DC-AC inverter. and the current output current ;Use the voltage sensor and current sensor at the input port of the photovoltaic inverter to sample the current port voltage and current of the photovoltaic array respectively;

[0125] Phase-locked loop unit 1, which determines the angle of the point of common coupling (PCC) voltage through a phase-locked loop (PLL) to keep the inverter synchronized with the grid;

[0126] DC voltage and current control module 2, maintains the voltage on the DC capacitor through voltage and current double closed-loop control Constant, the reference current is tracked by a proportional resonant (PR) controller;

[0127] Photovoltaic power regulation module 3, using the perturbation observation method for maximum power point tracking (MPPT);

[0128] Photovoltaic voltage ( ) control module 4, through the voltage and current double closed loop control to control the voltage of the photovoltaic array ( ) is adjusted to its reference value ;

[0129] The compensation current generating module 5 receives the power command from the distributed controller through one-way communication and generates the corresponding three-phase compensation current command .

[0130] The structure diagram of the distributed unbalanced current compensation device for photovoltaic power distribution network is as follows Figure 5 As shown, the photovoltaic inverter is connected to the PCC point through an LCL filter, which is used for power exchange between the photovoltaic array and the AC power grid. The DC-DC converter is used to adjust the output of the photovoltaic array, and the perturbation observation method is used to make the photovoltaic output have the maximum available power; the DC-AC inverter is used to deliver power to the power grid and compensate for the unbalanced current. In addition to the phase-locked loop, DC voltage and current control, and photovoltaic voltage control, the core of the control strategy involved in the present invention is the compensation current generation module. The photovoltaic inverter can generate compensation current for each phase according to the compensation current generation module. .

[0131] Specific control schemes such as Figure 6 As shown, the subscript , dq represent the variables in the stationary coordinate system and the rotating coordinate system respectively, where the position of the positive sequence voltage d-axis ( ) can be determined by the phase-locked loop (PLL), and the DC voltage is controlled to a constant value through the PI controller. After filtering the DC voltage fluctuation caused by the double frequency power fluctuation of the power grid through the double frequency notch filter, the positive sequence current reference value obtained by the DC voltage control is superimposed on the compensation current of each phase , which is the reference current of phases ABC, and the current inner loop uses a proportional resonant controller (PR) to track the reference current of each phase. In addition, the present invention uses a SOGI (second-order generalized integrator) filter and a sequence component extraction module based on a 90° phase shifter to extract the fundamental component and the sequence component respectively.

[0132] Example 3

[0133] A computer-readable storage medium stores a program, which, when executed by a processor, implements the method for compensating distributed unbalanced current in a photovoltaic-participated distribution network as described in Example 1.

[0134] Application Examples

[0135] In order to verify the effectiveness of the proposed control strategy, a distribution network containing two groups of photovoltaic inverters with different powers and three-phase unbalanced loads is selected, such as Figure 7 As shown, the rated frequency of the power grid is 50Hz and the voltage level is 220 / 380V. The rated powers of the two photovoltaic inverters involved in unbalanced current compensation are 15kW and 18kW respectively, and the maximum allowable phase currents are 42A and 50.4A respectively. In order to adapt to the rapid changes in environmental conditions, the control cycle cannot be too short. In addition, since the current inner loop responds very quickly (several milliseconds), the communication delay is usually tens of milliseconds, so 0.2s is selected as the control cycle. The application example is tested through the RTLAB semi-physical simulation platform. The control strategy considers 4 events, which occur at the following time points:

[0136] At t = 0s, the proposed distributed compensation control is put into operation;

[0137] t=2-3s, the irradiance of photovoltaic gradually increases to a total of 100W / m 2 ;

[0138] t=4s, phase A increases load by 1.5kW;

[0139] At t=5s, the 1.5kW load is removed from phase A.

[0140] Figures 8 to 10 The simulation results of multiple operating conditions in the application example of the present invention are as follows: Figure 8 (a) is the total active power compensation instruction The change graph of Figure 8 (b) is the total active power compensation instruction ; Fig. 9 (a) is the three-phase current curve of PV-1. Fig. 9 (b) is the three-phase current curve of PV-2. Fig. 9 (c) is the output power curve of PV-1 and PV-2. Fig.10 is the change of negative sequence, zero sequence current amplitude and voltage at the transformer outlet of the substation. Figures 8 to 10 It can be seen that the initial unbalanced current of the substation transformer is the negative sequence current , zero sequence current After the proposed control strategy was put into operation at the 0th second, the unbalanced current flowing through the substation transformer was greatly reduced (negative sequence dropped to 6.01A, zero sequence dropped to 4.82A). With the increase of irradiance, the remaining capacity of photovoltaic for unbalanced compensation decreased, and the unbalanced current of the substation transformer gradually increased. After the A-phase load was connected / removed, the unbalanced current of the substation transformer increased / decreased respectively. During the whole process, the three-phase currents of the photovoltaic inverters participating in the unbalanced compensation were all lower than their respective maximum allowable phase currents, and the photovoltaic inverters maintained the maximum available power unchanged during the whole process. In addition, after the unbalanced current of the substation transformer was compensated, the unbalanced condition of the three-phase voltage was also improved.

[0141] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for compensating distributed unbalanced current in a photovoltaic power distribution network, characterized in that: The steps include: S1, setting the inverters participating in the compensation so that they can inject negative-sequence and zero-sequence currents into the power grid according to the compensation instructions; the compensation instructions are the compensation instructions distributed by the substation transformer to the photovoltaic inverters participating in the compensation task in the distribution network through low-bandwidth unidirectional communication; S2, sampling the three-phase voltage and current of the current cycle and the active compensation instruction and reactive compensation instruction of the previous cycle, calculating the active power and reactive power of each phase flowing through the substation transformer of the current cycle, and the ideal compensation instruction that can achieve full compensation of the three-phase current of the substation transformer; S3, based on the result of step S2, the compensation coefficient of each photovoltaic inverter is calculated, and the actual compensation instruction and the ideal compensation instruction are associated by adding the compensation coefficients of all photovoltaic inverters and obtaining the total compensation coefficient; the specific process is: S3.1, based on the minimum voltage in the distribution network, calculate the maximum amplitude of the active and reactive current instructions corresponding to the power instruction under the ideal state; S3.2, correcting the positive sequence current reference value according to the positive sequence voltage amplitude and the positive sequence current reference value of the photovoltaic converter closest to the transformer and the minimum voltage in the distribution network; S3.3, construct an equation group consisting of the current limiting constraint conditions of the photovoltaic inverter, and obtain the unbalanced compensation coefficient corresponding to the three-phase current limiting and the unbalanced compensation coefficient corresponding to the neutral line current limiting by solving the equation group, and set the minimum value as the compensation coefficient of one of the photovoltaic inverters; the equation group expression is: ； in Respectively i The amplitude of the ABC three-phase and neutral line current of a photovoltaic inverter should be less than or equal to the maximum allowable current of the photovoltaic inverter. , is the rated current of the photovoltaic inverter and capacity factor The product of is the corrected positive sequence current reference value, , Represent the ideal active compensation current of phase A and phase B respectively, , Represent the ideal reactive power compensation current of phase A and phase B respectively, are the compensation coefficients corresponding to the maximum allowable currents of the three phases ABC and the neutral line, that is, the positive roots of the quadratic equation in the above formula, Take the minimum value of the compensation coefficient corresponding to the maximum allowable current of the three-phase and neutral line, that is, min( ). S3.4, add up the compensation coefficients of each photovoltaic inverter to obtain the sum of the unbalanced compensation coefficients of all photovoltaic inverters, assign a total compensation coefficient based on the sum of the unbalanced compensation coefficients of all photovoltaic inverters, and calculate the total compensation coefficient based on the ideal power compensation instruction and the total compensation coefficient. k The actual power compensation instruction is obtained by multiplying the product of S4, obtaining the compensation current of each phase according to the actual compensation instruction, so that the photovoltaic can ensure the maximum available power output while using the remaining capacity to compensate for the unbalanced current.

2. The method for compensating distributed unbalanced current in a photovoltaic power distribution network according to claim 1, characterized in that: The specific process of step S2 is: The sampled active power of each phase in the current cycle is subtracted from the three-phase average active power and then added to the active power compensation instruction of the previous cycle to obtain the ideal active power compensation instruction of each phase; The ideal reactive power compensation instruction for each phase is obtained by subtracting the three-phase average reactive power from the sampled reactive power of each phase in the current cycle and adding the reactive power compensation instruction of the previous cycle.

3. The method for compensating distributed unbalanced current in a photovoltaic power distribution network according to claim 1, characterized in that: The specific process of step S3.4 is as follows: When the sum of the compensation coefficients of all PV inverters When greater than 0 and less than 1, the total compensation coefficient k equal , then calculate the actual power compensation instruction; When the sum of the compensation coefficients of all PV inverters When greater than 1, the total compensation coefficient is k =1, the sum of the compensation coefficients of all photovoltaic inverters The reciprocal of is taken as the ratio, and the ratio is adopted to change the unbalanced compensation coefficient of each photovoltaic inverter, and the changed unbalanced compensation coefficients of each photovoltaic inverter are obtained and added up, and then the proportion of the changed unbalanced compensation coefficients of each photovoltaic inverter to the total of the changed unbalanced compensation coefficients is calculated, and the three-phase power instructions of each photovoltaic inverter are allocated from the actual power compensation instruction according to the corresponding proportion of each photovoltaic inverter.

4. The method for compensating distributed unbalanced current in a photovoltaic power distribution network according to claim 1, characterized in that: The specific process of step S4 is as follows: based on the actual power compensation instruction and the positive sequence voltage of each photovoltaic inverter, the amplitude of the three-phase active and reactive current of each photovoltaic inverter is calculated, and then the three-phase compensation current instruction of the photovoltaic inverter is calculated according to the amplitude of the active and reactive current, and the current reference value of each phase is added to the positive sequence current reference value obtained by DC voltage control, and then the reference current of each phase is tracked. At the same time, the disturbance observation method is used on the DC side to make the photovoltaic output maximum available power.

5. The method for compensating distributed unbalanced current in a photovoltaic power distribution network according to claim 4, characterized in that: The formula used in step S4 is: ； ； ； In the formula, For the i The amplitude of the three-phase active current of a photovoltaic inverter, For the i The magnitude of the reactive current of each photovoltaic inverter, For the i The amplitude of the positive sequence voltage obtained by the phase-locked loop of a photovoltaic inverter; is the three-phase compensation current instruction of the ith photovoltaic inverter, For the i The actual three-phase active power compensation instructions of each photovoltaic inverter. For the i Actual three-phase reactive power compensation instructions of photovoltaic inverters; is the phase difference between the current vector and the voltage vector, Corresponding to the three phases A, B, and C respectively, It is the phase of the local positive sequence voltage obtained by the phase-locked loop.

6. A distributed unbalanced current compensation device for photovoltaic power distribution network, which can adopt the method described in any one of claims 1 to 5, characterized in that: include: DC-DC converter, which is located on the PV side and is used to regulate the output of the PV array; A DC-AC inverter, which is located on the grid side and is used to transmit power to the grid; A data sampling unit, including a voltage sensor and a current sensor at the output port of the DC-AC inverter and a voltage sensor and a current sensor at the input port of the photovoltaic inverter, sampling the voltage sensor and the current sensor at the output port of the DC-AC inverter respectively to obtain the grid-connected point voltage and the current output current of the DC-AC inverter; The voltage sensor and current sensor at the input port of the photovoltaic inverter are used to sample the current port voltage and current of the photovoltaic array respectively; Phase-locked loop unit, which determines the angle of the grid connection point voltage through the phase-locked loop to keep the inverter synchronized with the grid; The DC voltage and current control module keeps the voltage on the DC capacitor constant through voltage and current double closed-loop control, and tracks the reference current through a proportional resonant controller; Photovoltaic power regulation module, using the perturbation and observation method for maximum power tracking; The photovoltaic voltage control module adjusts the voltage of the photovoltaic array to its reference value through voltage and current double closed-loop control; The compensation current generation module receives the power instruction from the distributed controller through unidirectional communication and generates corresponding three-phase compensation current instructions.

7. A computer-readable storage medium, characterized in that: A program is stored thereon, and when the program is executed by a processor, the distributed unbalanced current compensation method for a photovoltaic-participated distribution network as described in any one of claims 1 to 5 is implemented.

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