A photovoltaic grid-connected control method under terminal voltage imbalance of three-phase distribution line

Abstract

The application relates to a photovoltaic grid-connected control method under three-phase distribution line end voltage imbalance, voltage Hall sensors are used to sample grid voltage, and 3 / 2 conversion is used to obtain voltage signals in a stationary coordinate system. The voltage signals in the stationary coordinate system are respectively shifted forward and backward by 1 / 4 cycle to obtain delay voltage signals, current reference instructions are generated by using active power reference values, reactive power reference values, voltage components in the stationary coordinate system and the delay voltage signals, current Hall sensors are used to sample photovoltaic grid-connected inverter output currents, 3 / 2 conversion is used to obtain current signals in the stationary coordinate system. Error signals are obtained by subtracting the current signals from the current reference instructions, the error signals are sent into a PR regulator, the obtained signals are sent into an SVPWM modulation module after 2 / 3 conversion, and finally, driving signals of switching tubes are generated. The application can simultaneously realize active / reactive power constancy of a system under three-phase distribution line end voltage imbalance and sinusoidal output of grid-connected current.

Description

Technical Field

[0001] This invention relates to a control method for photovoltaic grid connection in the field of power electronics, and more particularly to a control method for photovoltaic grid connection under voltage imbalance at the end of a three-phase distribution line. Background Technology

[0002] With the increasing prevalence of industrial and residential electricity consumption, power distribution lines are becoming increasingly sensitive to power quality, especially voltage quality. Therefore, real-time voltage quality control is crucial for both industrial and residential electricity use. In recent years, countries worldwide have accelerated their new energy strategy deployments and vigorously developed distributed generation technologies, such as solar energy. In practical applications, grid-connected control of photovoltaic (PV) power sources can solve power quality problems related to voltage imbalance in distribution networks and voltage at the end of lines. However, existing PV grid-connected controllers can only achieve grid connection under ideal conditions where the voltage imbalance at the connection point is within acceptable limits. Under non-ideal grid conditions, this can lead to severe distortion of the grid current, causing potential power supply hazards to sensitive loads at the end of three-phase distribution lines. Therefore, researching PV grid-connected control methods under voltage imbalance at the end of three-phase distribution lines is of great significance. Early research on photovoltaic grid-connected control methods under voltage imbalance at the end of three-phase distribution lines largely focused on traditional instantaneous power theory. Professor Pedro Rodriguez of Spain proposed five control methods in his paper "Flexible active power control of distributed power generation systems during grid faults," laying an important theoretical foundation for research on photovoltaic grid-connected control methods under voltage imbalance at the end of three-phase distribution lines. However, this approach neglected the coordinated control of power / current quality. In response, the paper "Flexible Control Strategy for Grid-Connected Inverter under Unbalanced Grid Faults without PLL" proposed a phase-locked loop-free control method, quantitatively analyzing the instantaneous power fluctuation and grid-connected current distortion mechanism under voltage imbalance at the end of three-phase distribution lines. However, it still failed to simultaneously achieve the control objectives of zero power fluctuation and zero grid-connected current distortion. The paper "Power Controllability of Three-Phase Converter with Unbalanced AC Source" points out that the three-wire structure of a three-phase grid-connected inverter has limited control freedom, making it impossible to simultaneously achieve constant power and sinusoidal grid current. Therefore, a four-wire structure is adopted to introduce zero-sequence components for control. However, this strategy has a complex control structure and is prone to causing system response delays. Furthermore, the paper "Evaluation of IRP, FBD, SD, and generalized non-active power theories" provides a theoretical derivation of virtual instantaneous power, but the derivation process in a rotating coordinate system is complex and requires a phase-locked loop to extract the positive and negative sequence components of voltage / current, making it inconvenient for practical applications. Summary of the Invention

[0003] The purpose of this application is to overcome the shortcomings of the prior art and provide a photovoltaic grid-connected control method under voltage imbalance at the end of a three-phase distribution line. This method can meet the requirements of constant power and non-distorted grid-connected current control, reduce the potential power supply risks to sensitive loads in the system, and ensure the reliable operation of the photovoltaic grid-connected system under voltage imbalance at the end of a three-phase distribution line.

[0004] The technical solution adopted by the embodiments of this application to solve the technical problem is:

[0005] A method for controlling photovoltaic grid connection under voltage imbalance at the end of a three-phase distribution line, the method comprising the following steps:

[0006] Step 1: Sample the grid voltage e using a voltage Hall sensor. a ,e b ,e c The voltage signal e in the stationary coordinate system is obtained through a 3 / 2 transformation. α ,e β ;

[0007] Among them, e a e b e c These are the sampled A-phase, B-phase, and C-phase grid voltage signals, respectively; e α ,e β These are the voltage signals along the α-axis and β-axis in the stationary coordinate system, respectively.

[0008] Step 2, convert the voltage signal e in the stationary coordinate system α Shifting forward 1 / 4 cycle, the voltage signal e along the β axis in the stationary coordinate system β Shifting back 1 / 4 cycle yields the delayed voltage signal.

[0009] in, They are ahead of e respectively α A 1 / 4-cycle delayed voltage signal lags behind e β A voltage signal delayed by 1 / 4 cycle;

[0010] Step 3, using the active power reference value P * Reactive power reference value Q * Voltage signals e along the α and β axes in a stationary coordinate system α ,e β and delayed voltage signal Generate current signal reference command Its expression is:

[0011]

[0012] in, These are the reference commands for the generated α-axis and β-axis current signals, respectively. This is the reference command for the active power current signal under the α axis. This is the reference command for the reactive power current signal under the α axis. This is the reference command for the active power current signal under the β axis. This is the reference command for the reactive power current signal under the β axis.

[0013] Step 4: Sample the output current signal i of the photovoltaic grid-connected inverter using a current Hall sensor. abc The current signal i in the stationary coordinate system is obtained through a 3 / 2 transformation. α i β The output expressions for the virtual instantaneous active and reactive power of the photovoltaic grid-connected inverter are defined as follows:

[0014]

[0015] Among them, i α i β α and β are the current signals in the stationary coordinate system, respectively, where p is the defined virtual instantaneous active power and q is the defined virtual instantaneous reactive power.

[0016] Step 5, transfer the current signal i α i β Reference command for current signal The error signal is obtained by subtraction and sent to the PR controller. The resulting adjustment signal S α ,S β After a 2 / 3 transformation, the signal is sent to the SVPWM modulation module, which finally generates the drive signals S1 to S6 for the switching transistor.

[0017] S α ,S β These are the adjustment signals for the α-axis and β-axis obtained after adjustment by the PR regulator, respectively. S1 to S6 are the six drive signals for turning the drive switch on and off, respectively.

[0018] Furthermore, the method for generating the delayed signal in step 2 is as follows:

[0019]

[0020] Where the phase shift filter coefficient ω c Both ω and ω are taken as the angular frequency of the grid voltage signal. c ω and ω are both coefficients of the phase-shift filter, and s is the Laplace operator.

[0021] Furthermore, the reference current generation method described in step 3 is as follows:

[0022]

[0023] Furthermore, the transfer function of the PR regulator in step 5 is:

[0024]

[0025] k p k is the proportional coefficient in the proportional resonant PR regulator. r This is the resonance coefficient in the proportional resonance PR regulator.

[0026] The advantages and positive effects of the embodiments of this application are:

[0027] This invention proposes a photovoltaic grid-connected control method under voltage imbalance at the end of a three-phase distribution line. This method can simultaneously achieve constant active / reactive power and sinusoidal grid-connected current output under voltage imbalance at the end of the three-phase distribution line. Furthermore, from a control structure perspective, this invention eliminates the need for a four-wire structure in the three-phase grid-connected inverter and eliminates the need for a phase-locked loop (PLL) structure to extract the positive and negative sequence components of voltage and current, simplifying the system control structure and facilitating practical engineering applications. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of a photovoltaic grid-connected control method under voltage imbalance at the end of a three-phase power distribution line according to the present invention.

[0029] Figure 2 This is a diagram showing the grid voltage waveform of the photovoltaic grid-connected inverter system of the present invention.

[0030] Figure 3 The output power and grid-connected current waveforms are for traditional photovoltaic grid-connected inverter control methods.

[0031] Figure 4 The output power and grid-connected current waveforms of the photovoltaic grid-connected inverter control method of the present invention are shown below.

[0032] Figure 5 The grid-connected current THD analysis diagram for traditional photovoltaic grid-connected inverter control methods;

[0033] Figure 6 The output power and grid-connected current THD analysis diagrams for the photovoltaic grid-connected inverter control method of the present invention are shown. Detailed Implementation

[0034] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0035] A photovoltaic grid-connected control method under voltage imbalance at the end of a three-phase distribution line. Figure 1The diagram shown is a schematic of a photovoltaic grid-connected control method under voltage imbalance at the end of a three-phase distribution line according to the present invention. The control method includes the following steps:

[0036] Step 1: Sample the grid voltage e using a voltage Hall sensor. a ,e b ,e c The voltage e in the stationary coordinate system is obtained by the 3 / 2 transformation. α ,e β The specific transformation process is as follows:

[0037]

[0038] Among them, e a e b e c These are the sampled A-phase, B-phase, and C-phase grid voltage signals, respectively; e α ,e β These are the voltage signals along the α-axis and β-axis in the stationary coordinate system, respectively.

[0039] Step 2, convert the voltage signal e in the stationary coordinate system α Shift forward 1 / 4 cycle, voltage signal e β Shift back 1 / 4 cycle to generate a delayed voltage signal The specific generation process is as follows:

[0040]

[0041] Where the phase shift filter coefficient ω c Both ω and ω are taken as the angular frequency of the grid voltage signal.

[0042] in, They are ahead of e respectively α A 1 / 4-cycle delayed voltage signal lags behind e β A 1 / 4-cycle delayed voltage signal; where the phase shift filter coefficient ω c Both ω and ω are taken as the angular frequency of the grid voltage signal. c ω and ω are both coefficients of the phase-shift filter, and s is the Laplace operator.

[0043] Step 3, using the active power reference value P * Reactive power reference value Q * Voltage component e in stationary coordinate system α ,e β and delayed voltage signal Generate current reference command The specific process for generating the current reference is as follows:

[0044]

[0045] in, These are the reference commands for the generated α-axis and β-axis current signals, respectively. This is the reference command for the active power current signal under the α axis. This is the reference command for the reactive power current signal under the α axis. This is the reference command for the active power current signal under the β axis. This is the reference command for the reactive power current signal under the β axis.

[0046] Step 4: Sample the output current i of the photovoltaic grid-connected inverter using a current Hall sensor. abc The current signal i in the stationary coordinate system is obtained through a 3 / 2 transformation. α i β Define the output expressions for the virtual instantaneous active and reactive power of a three-phase photovoltaic grid-connected inverter. The specific 3 / 2 conversion process and the virtual instantaneous power expression are as follows:

[0047]

[0048]

[0049] Among them, i α i β α and β are the current signals in the stationary coordinate system, respectively, where p is the defined virtual instantaneous active power and q is the defined virtual instantaneous reactive power.

[0050] Step 5, transfer the current signal i α i β With current reference command The difference is used to obtain the error signal, which is then fed into the PR regulator. The resulting signal S α ,S β After a 2 / 3 transformation, S is obtained. a ,S b ,S c The signals are then fed into the SVPWM modulation module, which finally generates the drive signals S1 to S6 for the switching transistors. The transfer functions of the 2 / 3 conversion process and the PR regulator are as follows:

[0051]

[0052]

[0053] S α ,S β These are the adjustment signals for the α-axis and β-axis obtained after adjustment by the PR regulator, respectively. S1 to S6 are the six drive signals for turning the drive switch on and off, respectively. p k is the proportional coefficient in the proportional resonant PR regulator. rThis is the resonance coefficient in the proportional resonance PR regulator.

[0054] The effectiveness of the present invention will now be verified.

[0055] The system parameters are as follows: DC side voltage of the photovoltaic grid-connected inverter is 120V, peak phase voltage of the grid is 50V, imbalance occurs at the end of the three-phase distribution line at 0.15s, voltage imbalance degree is 0.3, active power reference value is set to 300W, reactive power reference value is set to 225Var. Simulation results are as follows. Figures 2-4 As shown. Figure 2 This is a diagram showing the grid voltage waveform of the photovoltaic grid-connected inverter system of the present invention. Figure 3 The output power and grid-connected current waveforms are for traditional photovoltaic grid-connected inverter control methods. Figure 4 The output power and current waveforms of the photovoltaic grid-connected inverter control method of the present invention are shown. (Comparison) Figure 3 and Figure 4 It can be seen that using traditional methods to maintain a constant output of active / reactive power will lead to severe distortion of the grid-connected current, causing potential power supply hazards to sensitive loads in the distribution line system and affecting the safe operation of the photovoltaic grid-connected inverter. The solution proposed in this invention can simultaneously achieve constant output of active / reactive power and sinusoidal, distortion-free grid-connected current. A more detailed analysis of the grid-connected current THD is provided below. Figure 5 and Figure 6 It can be seen that the grid-connected current THD of the traditional control method is 31.68%, while the grid-connected current THD of the control method of the present invention is 1.86%, which meets the IEEE Std929-2000 international grid connection standard.

[0056] This invention discloses a photovoltaic grid-connected control method under voltage imbalance at the end of a three-phase distribution line. The method utilizes the grid voltage component, delayed voltage signal, and active / reactive power reference in a stationary coordinate system to generate a grid-connected reference current, and employs a PR regulator and SVPWM modulation module to generate the switching transistor drive signal. It uses virtual instantaneous power estimation to determine the instantaneous output power of the photovoltaic grid-connected inverter, simultaneously achieving constant instantaneous active / reactive power and sinusoidal grid-connected current output. Furthermore, this invention eliminates the need for a phase-locked loop structure to extract the positive and negative sequence components of voltage / current, simplifying the system control structure, ensuring reliable operation of the photovoltaic grid-connected inverter under voltage imbalance at the end of the three-phase distribution line, and improving the power supply quality to sensitive loads at the end of the three-phase distribution line.

[0057] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A photovoltaic grid-connected control method under voltage imbalance at the end of a three-phase distribution line, characterized in that: The method includes the following steps: Step 1: Sample the grid voltage e using a voltage Hall sensor. a ,e b ,e c The voltage signal e in the stationary coordinate system is obtained through a 3 / 2 transformation. α ,e β ; Among them, e a e b e c These are the sampled A-phase, B-phase, and C-phase grid voltage signals, respectively; e α ,e β These are the voltage signals along the α-axis and β-axis in the stationary coordinate system, respectively. Step 2, convert the voltage signal e in the stationary coordinate system α Shifting forward 1 / 4 cycle, the voltage signal e along the β axis in the stationary coordinate system β Shifting back 1 / 4 cycle yields the delayed voltage signal. in, They are ahead of e respectively α A 1 / 4-cycle delayed voltage signal lags behind e β A voltage signal delayed by 1 / 4 cycle; Step 3, using the active power reference value P * Reactive power reference value Q * Voltage signals e along the α and β axes in a stationary coordinate system α ,e β and delayed voltage signal Generate current signal reference command Its expression is: in, These are the reference commands for the generated α-axis and β-axis current signals, respectively. This is the reference command for the active power current signal under the α axis. This is the reference command for the reactive power current signal under the α axis. This is the reference command for the active power current signal under the β axis. This is the reference command for the reactive power current signal under the β axis. Step 4: Sample the output current signal i of the photovoltaic grid-connected inverter using a current Hall sensor. abc The current signal i in the stationary coordinate system is obtained through a 3 / 2 transformation. α i β The output expressions for the virtual instantaneous active and reactive power of the photovoltaic grid-connected inverter are defined as follows: Among them, i α i β α and β are the current signals in the stationary coordinate system, respectively, where p is the defined virtual instantaneous active power and q is the defined virtual instantaneous reactive power. Step 5, transfer the current signal i α i β Reference command for current signal The error signal is obtained by subtraction and sent to the PR controller. The resulting adjustment signal S α ,S β After a 2 / 3 transformation, the signal is sent to the SVPWM modulation module, which finally generates the drive signals S1 to S6 for the switching transistor. S α ,S β These are the adjustment signals for the α-axis and β-axis obtained after adjustment by the PR regulator, respectively. S1 to S6 are the six drive signals for turning the drive switch on and off, respectively.

2. The photovoltaic grid-connected control method under voltage imbalance at the end of a three-phase distribution line according to claim 1, characterized in that: The method for generating the delayed signal described in step 2 is as follows: Where the phase shift filter coefficient ω c Both ω and ω are taken as the angular frequency of the grid voltage signal. c ω and ω are both coefficients of the phase-shift filter, and s is the Laplace operator.

3. The photovoltaic grid-connected control method under voltage imbalance at the end of a three-phase distribution line according to claim 1, characterized in that: The reference current generation method described in step 3 is as follows:

4. The photovoltaic grid-connected control method under voltage imbalance at the end of a three-phase distribution line according to claim 1, characterized in that: The transfer function of the PR regulator in step 5 is: k p k is the proportional coefficient in the proportional resonant PR regulator. r This is the resonance coefficient in the proportional resonance PR regulator.

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