Distributed Photovoltaic Inverter Inertia Support Control Method and System

Abstract

The present invention relates to a distributed photovoltaic converter inertia support control method and system. Based on the improved perturbation observation method control, the method of the present invention utilizes the DC side bus voltage signal and realizes fast power response under the power fluctuations of the source and load in a weak DC receiving-end power grid through a feed-forward control algorithm, provides partial inertia support for the system, and effectively reduces the fluctuations in the DC bus voltage dynamic process. While ensuring the stable operation of the distributed photovoltaic in the right half plane of the power curve, the present invention provides fast power response for the DC bus during the dynamic process and reduces the amplitude of the DC bus voltage fluctuations.

Description

Technical Field

[0001] The invention belongs to the technical field of distributed photovoltaic converter control, and relates to a method and system for inertial support control of a distributed photovoltaic converter in a weak DC receiving grid. Background Art

[0002] In recent years, with the wide application of power electronics technology, the access ratio of DC distributed power sources and DC loads has been increasing year by year. The weak DC receiving grid can be compatible with the access of DC type sources, storage, and loads, reducing system losses, and is an ideal solution for realizing the local consumption of renewable energy. However, due to the access of a high proportion of power electronic devices, the weak DC receiving grid itself has the characteristic of low inertia. During the process of power fluctuation, it is easy to cause sudden changes in the bus voltage, affecting the power supply quality, increasing the risk of misoperation of the protection devices in the weak DC receiving grid, and may even lead to equipment tripping and faults in the weak DC receiving grid in severe cases.

[0003] Therefore, how to effectively utilize the resources in the weak DC receiving grid to achieve inertial support for the DC bus and reduce the voltage fluctuation of the bus is one of the difficulties in the popularization and application of the weak DC receiving grid. Summary of the Invention

[0004] The technical problem to be solved by the invention is to overcome the defects of the above-mentioned prior art, and provide a method and system for inertial support control of a distributed photovoltaic converter in a weak DC receiving grid. By using the characteristics of the fast response of the distributed photovoltaic converter itself, an improved perturbation observation method and an inertial support control strategy based on a feed-forward link are adopted to provide fast power response for the DC bus during the dynamic process while ensuring the stable operation of the distributed photovoltaic in the right half plane of the power curve, and reducing the amplitude of the voltage fluctuation of the DC bus.

[0005] For this purpose, a technical solution adopted by the invention is: a method for inertial support control of a distributed photovoltaic converter, which includes the following steps:

[0006] (1) Using the voltage and current sensors at the output port of the photovoltaic converter, respectively sample to obtain the current i2 output by the photovoltaic converter at present, the DC bus voltage v b and the current i of the filter capacitor c ; Using the voltage and current sensors at the input port of the photovoltaic converter, respectively sample the current port voltage v pv and the current i pv of the photovoltaic array at present;

[0007] (2) Calculate the output power P of the photovoltaic converter according to the sampled voltage and current values pv ;

[0008] (3) Calculate the reference power P of the photovoltaic output according to the DC bus voltage ref, while calculating the differences between the power, voltage at the input port of the PV inverter and the control sampling of the previous beat respectively, to obtain ΔP pv and Δv pv , and judge the sign of ΔP pv ·Δv pv . If ΔP pv ·Δv pv is positive, then the voltage reference increases by a perturbation value based on the previous beat; if ΔP pv ·Δv pv is negative, then further judge whether P ref is greater than P pv . If P ref is greater than P pv , then the voltage reference decreases by a perturbation value based on the previous beat. If P ref is less than or equal to P pv , then the voltage reference increases by a perturbation value based on the previous beat;

[0009] (4) Input the difference between the voltage reference and the port voltage sampling value v pv obtained in step (1) into the voltage control loop PI control module to obtain the output current reference value of the PV inverter

[0010] (5) According to the DC bus voltage, calculate the inertial response current reference value using the feedforward control link. Add the result obtained in step (4) and to obtain the input current reference of the current control loop Take the difference between and the output current i2 of the PV inverter as the input of the current control loop PI control module. The output of the current control loop PI control module is subtracted from the output of the damping loop to obtain the converter control duty cycle d, and after PWM modulation, control the PV inverter to achieve the inertial support control of the DC receiving-end weak grid.

[0011] The present invention proposes a method for inertial support control of a distributed PV inverter in a DC receiving-end weak grid, which can provide partial inertial support for the DC bus, thereby reducing the voltage fluctuation caused by the power fluctuation of the source and load and improving the power quality of the system.

[0012] Furthermore, in step (3), calculate the PV output reference power P ref according to formula (1):

[0013]

[0014] where r eis the redundancy coefficient, P * is the rated peak power of the photovoltaic, is the reference voltage of the weak power grid bus at the DC receiving end, is the maximum allowable voltage of the DC bus,

[0015] Further, in step (4), calculate according to formula (2)

[0016]

[0017] where, K vp is the proportional coefficient of the PI control module of the voltage control loop, K vi is the integral coefficient of the PI control module of the voltage control loop.

[0018] Further, in step (5), calculate the reference value of the inertia response current according to formula (3)

[0019]

[0020] where, r e is the redundancy coefficient, P * is the rated peak power of the photovoltaic, is the reference voltage of the weak power grid bus at the DC receiving end, is the minimum allowable voltage of the DC bus.

[0021] Further, in step (5), calculate the converter control duty cycle d according to formula (4):

[0022]

[0023] where, K ip is the proportional coefficient of the PI control module of the current control loop, K ii is the integral coefficient of the PI control module of the current control loop, r D is the damping coefficient.

[0024] Another technical solution adopted by the present invention is: a distributed photovoltaic converter inertia support control system, which includes:

[0025] Data sampling unit: Using the voltage and current sensors at the output port of the photovoltaic converter, respectively sample the current output i2 of the photovoltaic converter, the DC bus voltage v b and the filter capacitor current i c ; Using the voltage and current sensors at the input port of the photovoltaic converter, respectively sample the current port voltage v of the photovoltaic array pv and the current i pv ;

[0026] Photovoltaic inverter output power calculation unit: Calculate the output power P of the photovoltaic inverter according to the sampled voltage and current values pv ;

[0027] Voltage reference calculation unit: Calculate the photovoltaic output reference power P according to the DC bus voltage ref , and calculate the differences between the power, voltage at the input port of the photovoltaic inverter and the control sampling of the previous beat respectively to obtain ΔP pv and Δv pv , and judge the positive and negative of ΔP pv ·Δv pv . If ΔP pv ·Δv pv is positive, then the voltage reference increases by a perturbation value on the basis of the previous beat; if ΔP pv ·Δv pv is negative, then further judge whether P ref is greater than P pv . If P ref is greater than P pv , then the voltage reference decreases by a perturbation value on the basis of the previous beat. If P ref is less than or equal to P pv , then the voltage reference increases by a perturbation value on the basis of the previous beat;

[0028] Output current reference value calculation unit: Input the difference between the voltage reference and the port voltage sampling value v pv obtained in step (1) into the voltage control loop PI control module to obtain the output current reference value of the photovoltaic inverter

[0029] Inertial support control unit: Calculate the inertial response current reference value according to the DC bus voltage by using the feedforward control link Add the result obtained in step (4) to to obtain the input current reference of the current control loop Take the difference between and the output current i2 of the photovoltaic inverter as the input of the current control loop PI control module. The output of the current control loop PI control module is subtracted from the output of the damping loop to obtain the converter control duty cycle d, which is used to control the photovoltaic inverter through PWM modulation to achieve the inertial support control of the DC receiving-end weak grid.

[0030] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0031] (1) The present invention only needs to collect local electrical information, does not require additional communication cables, nor a centralized controller, and can achieve decentralized control.

[0032] (2) By improving the traditional perturbation observation method, the present invention can ensure that the photovoltaic operates on the right side of the maximum power point of the power / voltage curve, enhancing the stability of the distributed photovoltaic operation itself while ensuring power margin.

[0033] (3) Utilizing the characteristics of fast power response of distributed photovoltaic, the present invention provides inertial support for the DC receiving-end weak grid bus, which can effectively reduce the voltage fluctuation during the dynamic regulation process and improve the power quality. Description of the Drawings

[0034] Figure 1 is the main topology diagram of the DC receiving-end weak grid;

[0035] Figure 2 is the topology diagram of the distributed photovoltaic inertial support system of the present invention;

[0036] Figure 3 is the control strategy block diagram of the distributed photovoltaic inertial support system of the present invention;

[0037] Figure 4 is the control flow chart of the improved equal area method of the present invention;

[0038] Figure 5 is the DC bus voltage diagram obtained by using the traditional MPPT control in the application example of the present invention;

[0039] Figure 6 is the DC bus voltage diagram obtained by using the method of the present invention in the application example of the present invention. Detailed Embodiments

[0040] Next, in combination with the drawings of the embodiments of the present invention, the technical solutions of the embodiments of the present invention will be explained and described. However, the following embodiments are only the preferred embodiments of the present invention and not all of them. Based on the embodiments in the embodiments, other embodiments obtained by those skilled in the art without creative efforts all fall within the protection scope of the present invention.

[0041] Embodiment 1

[0042] This embodiment provides a method for controlling the inertial support of a distributed photovoltaic converter in a DC receiving-end weak grid.

[0043] The main topology of the DC receiving-end weak grid is as Figure 1As shown, it includes energy storage, photovoltaic, load and bidirectional power flow controller (BPC). The working modes of the DC receiving-end weak power grid can be divided into grid-connected operation and island operation. In the grid-connected operation mode, the DC receiving-end weak power grid is connected to the AC distribution network through the BPC, and exchanges power with the AC grid through the BPC. At this time, the BPC can work in power source mode, or DC voltage source mode to help energy storage support the DC bus voltage. In the island operation mode, the DC receiving-end weak power grid is disconnected from the AC distribution network, and the BPC does not work in this mode.

[0044] The DC side energy storage works in voltage source mode as the main power supply to support the DC bus voltage. If only a single energy storage is configured in the DC receiving end weak grid, and the BPC does not participate in the DC bus support, the energy storage can work in constant voltage mode, and the controller adopts classic voltage and current dual-loop control. If there are multiple energy storages in the DC receiving end weak grid, or the BPC participates in the DC bus support, the energy storage and BPC both work in DC droop mode, and the voltage loop reference value It is calculated by DC droop control, that is,

[0045]

[0046] in is the rated voltage of the DC bus, m is the DC droop coefficient, i dc is the output current of the energy storage or BPC DC port.

[0047] The voltage and current sensors at the output port of the photovoltaic inverter are used to sample and obtain the current output current i2 and DC bus voltage v of the photovoltaic inverter. b And the filter capacitor current i c ; Use the voltage and current sensors at the input port of the photovoltaic inverter to sample the current port voltage v of the photovoltaic array pv With current i pv .

[0048] Calculate the output power P of the photovoltaic converter based on the sampled voltage and current values pv .

[0049] The overall topology diagram and control strategy block diagram of the distributed photovoltaic inertial support system are shown in the figure. Figure 2 , Figure 3 The photovoltaic inverter first collects the voltage and current of the input and output ports, and calculates the output reference power P according to the DC bus voltage. ref , and then through Figure 4 The improved perturbation and observation method shown in the figure determines the photovoltaic input voltage reference The improved perturbation observation method first calculates the difference between the current photovoltaic power and the converter input port voltage and the previous control cycle, and determines ΔP pv ·Δv pvThe symbol, if positive, indicates that the PV is operating in the left half-plane of the power curve MPPT point. At this time, the stability of the PV itself is weak, so v needs to be increased pv to make it operate in the right half-plane, so let If ΔP pv ·Δv pv is negative, then further judge the magnitudes of P ref and P pv If P ref > P pv , the controller is the same as the traditional MPPT mode, let Otherwise, it indicates that the current PV power is too high and the output needs to be limited, so let ε is the perturbation value.

[0050] After passing through the voltage loop PI control module, the current loop reference is obtained. Since the bandwidth of the current loop PI module is higher than that of the voltage loop, a feed-forward link is introduced in the current loop to provide a fast power response for the DC bus to achieve inertial support. The difference between the DC bus voltage and the reference value is multiplied by the inertia coefficient H j to obtain which is added to the current loop to obtain the duty cycle d, and then the converter switch tubes are controlled through PWM modulation.

[0051] Application example

[0052] Application of Example 1: It is illustrated by a single energy storage DC receiving-end weak grid operating in islanding mode. The rated value of the DC receiving-end weak grid bus voltage is set to 700V, the initial load power is 15kW, and the PV power is 10kW. The maximum PV power is 15kW, that is, there is a power margin of 5kW. The perturbation amount ε of the perturbation observation method is 0.1V, the parameters of the voltage loop PI control module are k vp = 2, k vi = 100, the parameters of the current loop PI control module are k ip = 12, k ii = 5000, the inertia coefficient H of the feed-forward link j = 2, the damping coefficient r D = 0.01.

[0053] The application example is tested through the RT-LAB experimental platform, and the DC bus voltage waveform is obtained as shown in Figures 5-6 . At the moment of 0.6s, a 45kW DC load is connected and removed at 1.6s. Under the traditional MPPT control, during the two perturbation processes, the DC bus voltage fluctuates greatly. At 0.6s, the bus voltage drops to 624V, as shown in Figure 5As shown, it has already fallen below the allowable range of 10% of the normal DC bus voltage. Such a drop is very likely to cause the tripping of voltage-sensitive equipment. However, with the control algorithm described in the present invention, the DC bus voltage drops to 636V during the first disturbance, as Figure 6 shown, significantly reducing the voltage fluctuation, improving the power quality of the system, and ensuring the normal operation of sensitive equipment. During the second disturbance process, the DC bus voltage rises by 56V, which is also significantly less than that of the system using the traditional MPPT control method.

[0054] Embodiment 2

[0055] This embodiment provides a distributed photovoltaic inverter inertia support control system, which consists of a data sampling unit, a photovoltaic inverter output power calculation unit, a voltage reference calculation unit, an output current reference value calculation unit, and an inertia support control unit.

[0056] Data sampling unit: Using the voltage and current sensors at the output port of the photovoltaic inverter, respectively sample the current output i2 of the photovoltaic inverter, the DC bus voltage v b and the filter capacitor current i c ; Using the voltage and current sensors at the input port of the photovoltaic inverter, respectively sample the current port voltage v pv and current i pv of the photovoltaic array;

[0057] Photovoltaic inverter output power calculation unit: Calculate the output power P of the photovoltaic inverter according to the sampled voltage and current values pv ;

[0058] Voltage reference calculation unit: Calculate the photovoltaic output reference power P according to the DC bus voltage ref , and at the same time calculate the differences between the input port power, voltage of the photovoltaic inverter and the control sampling of the previous beat, to obtain ΔP pv and Δv pv , and judge the positive and negative of ΔP pv ·Δv pv . If ΔP pv ·Δv pv is positive, then the voltage reference increases by a disturbance value on the basis of the previous beat; if ΔP pv ·Δv pv is negative, then further judge whether P ref is greater than P pv . If P ref is greater than P pv , then the voltage reference decreases by a disturbance value on the basis of the previous beat. If P ref is less than or equal to P pv , then the voltage reference Add a perturbation value based on the previous beat;

[0059] Output current reference value calculation unit: Input the difference between the voltage reference and the port voltage sampling value v obtained in step (1) into the voltage control loop PI control module to obtain the output current reference value of the photovoltaic converter pv

[0060] Inertia support control unit: Calculate the inertia response current reference value using the feed-forward control link according to the DC bus voltage Input the result obtained in step (4) and Add them to obtain the input current reference of the current control loop Input The difference between and the output current i2 of the photovoltaic converter is used as the input of the current control loop PI control module. The output of the current control loop PI control module is subtracted from the output of the damping loop to obtain the converter control duty cycle d, which is used to control the photovoltaic converter through PWM modulation to achieve inertia support control for the weak grid at the DC receiving end.

[0061] In the voltage reference calculation unit, calculate the photovoltaic output reference power P according to formula (1) ref :

[0062]

[0063] where r e is the redundancy coefficient, P * is the rated peak power of the photovoltaic, is the reference voltage of the weak grid bus at the DC receiving end, is the maximum allowable voltage of the DC bus,

[0064] In the output current reference value calculation unit, calculate

[0065]

[0066] where K vp is the proportional coefficient of the voltage control loop PI control module, and K vi is the integral coefficient of the voltage control loop PI control module.

[0067] In the inertia support control unit, calculate the inertia response current reference value according to formula (3)

[0068]

[0069] where, r​e is the redundancy coefficient, P * is the rated peak power of the photovoltaic, is the reference voltage of the DC receiving-end weak grid bus, is the minimum allowable voltage of the DC bus.

[0070] In the inertia support control unit, the converter control duty cycle d is calculated according to formula (4):

[0071]

[0072] where K ip is the proportional coefficient of the PI control module of the current control loop, K ii is the integral coefficient of the PI control module of the current control loop, r D is the damping coefficient.

[0073] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the technical principle of the present invention, several improvements and deformations can be made, and these improvements and deformations should also be regarded as the protection scope of the present invention.

Claims

1. A distributed photovoltaic inverter inertia support control method, characterized in that It includes the following steps: (1) By using the voltage and current sensors at the output port of the photovoltaic converter, sample the current i2 currently output by the photovoltaic converter and the DC bus voltage v respectively b and the current i of the filter capacitor c ; By using the voltage and current sensors at the input port of the photovoltaic converter, sample the current i and the port voltage v of the photovoltaic array currently pv respectively pv ; (2) Calculate the output power P of the PV inverter according to the sampled voltage and current values pv ; (3) Calculate the photovoltaic output reference power P based on the DC bus voltage ref , and simultaneously calculate the differences between the power, voltage at the input port of the photovoltaic converter and the control sampling of the previous beat to obtain ΔP pv and Δv pv , and determine the sign of ΔP pv ·Δv pv . If ΔP pv ·Δv pv is positive, then the voltage reference increases by a perturbation value on the basis of the previous beat; if ΔP pv ·Δv pv is negative, then further determine whether P ref is greater than P pv . If P ref is greater than P pv , then the voltage reference decreases by a perturbation value on the basis of the previous beat. If P ref is less than or equal to P pv , then the voltage reference increases by a perturbation value on the basis of the previous beat; (4) Input the difference between the voltage reference and the sampled port voltage value v pv obtained in step (1) into the voltage control loop PI control module to obtain the reference value of the output current of the photovoltaic converter (5) Calculate the reference value of the inertial response current using the feedforward control link according to the DC bus voltage The result obtained in step (4) And Add them together to obtain the reference input current of the current control loop Take The difference between and the output current i2 of the PV converter as the input of the PI control module of the current control loop. The output of the PI control module of the current control loop is subtracted from the output of the damping loop to obtain the converter control duty cycle d, which is used to control the PV converter through PWM modulation to achieve the inertial support control of the weak power grid at the DC receiving end; In step (5), calculate the inertial response current reference value according to formula (3). Among them, r e is the redundancy factor, and P * is the rated peak power of the photovoltaic, is the reference voltage of the DC receiving-end weak grid bus, is the minimum allowable voltage of the DC bus.

2. The inertial support control method for a distributed photovoltaic converter according to claim 1, characterized in that In step (3), calculate the photovoltaic output reference power P according to formula (1) ref : where r e is the redundancy factor, P * is the rated peak power of the photovoltaic, is the reference voltage of the weak power grid bus at the DC receiving end, is the maximum allowable voltage of the DC bus, 3. The inertial support control method of the distributed photovoltaic inverter according to claim 1, characterized in that In step (4), calculate according to formula (2) Among them, K vp is the proportional coefficient of the PI control module of the voltage control loop, and K vi is the integral coefficient of the PI control module of the voltage control loop.

4. The distributed photovoltaic inverter inertia support control method according to claim 1, wherein In step (5), calculate the converter control duty cycle d according to formula (4): Among them, K ip is the proportional coefficient of the PI control module of the current control loop, and K ii is the integral coefficient of the PI control module of the current control loop, and r D is the damping coefficient.

5. Distributed photovoltaic inverter inertia support control system, characterized in that It includes: Data sampling unit: By using the voltage and current sensors at the output port of the PV inverter, the current i2 currently output by the PV inverter and the DC bus voltage v are sampled respectively b as well as the filter capacitor current i c ; By using the voltage and current sensors at the input port of the PV inverter, the current i and the port voltage v of the PV array are sampled respectively pv and the current i pv ; Photovoltaic inverter output power calculation unit: calculates the output power P of the photovoltaic inverter based on the sampled voltage and current values pv ; Voltage reference calculation unit: Calculate the photovoltaic output reference power P based on the DC bus voltage ref , and simultaneously calculate the differences between the power, voltage at the input port of the photovoltaic converter and the control sampling of the previous beat to obtain ΔP pv and Δv pv , and judge the sign of ΔP pv ·Δv pv . If ΔP pv ·Δv pv is positive, the voltage reference increases by a perturbation value on the basis of the previous beat; if ΔP pv ·Δv pv is negative, further judge whether P ref is greater than P pv . If P ref is greater than P pv , the voltage reference decreases by a perturbation value on the basis of the previous beat. If P ref is less than or equal to P pv , the voltage reference increases by a perturbation value on the basis of the previous beat; Output current reference value calculation unit: Input the difference between the voltage reference and the port voltage sampling value v obtained in step (1) pv into the voltage control loop PI control module to obtain the output current reference value of the photovoltaic converter Inertia support control unit: According to the DC bus voltage, use the feedforward control link to calculate the reference value of the inertia response current The result obtained in step (4) And Add them to obtain the reference input current of the current control loop Take The difference between and the output current i2 of the PV converter is used as the input of the PI control module of the current control loop. The output of the PI control module of the current control loop is subtracted from the output of the damping loop to obtain the converter control duty cycle d, which is used to control the PV converter through PWM modulation to achieve the inertia support control of the DC receiving-end weak grid; In the inertial support control unit, the inertial response current reference value is calculated according to formula (3). Among them, r e is the redundancy coefficient, and P * is the rated peak power of the photovoltaic, is the reference voltage of the weak AC grid bus at the DC receiving end, is the minimum allowable voltage of the DC bus.

6. The distributed photovoltaic inverter inertia support control system according to claim 5, wherein In the voltage reference calculation unit, the photovoltaic output reference power P is calculated according to formula (1). ref : where r e is the redundancy coefficient, P * is the rated peak power of the photovoltaic, is the reference voltage of the weak grid bus at the DC receiving end, is the maximum allowable voltage of the DC bus, 7. The inertial support control system of the distributed photovoltaic converter according to claim 6, characterized in that, In the output current reference value calculation unit, calculate according to formula (2) Among them, K vp is the proportional coefficient of the PI control module of the voltage control loop, and K vi is the integral coefficient of the PI control module of the voltage control loop.

8. The distributed photovoltaic inverter inertia support control system according to claim 1, characterized in that, In the inertial support control unit, calculate the converter control duty cycle d according to formula (4): Among them, K ip is the proportional coefficient of the PI control module of the current control loop, K ii is the integral coefficient of the PI control module of the current control loop, and r D is the damping coefficient.

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