Improved power standby control method and system based on frequency power angle feedback

Abstract

The invention provides an improved power standby control method and system based on frequency power angle feedback, and relates to the technical field of power electronic power generation conversion. The method comprises the following steps: a preceding stage controls the output power of the photovoltaic system through a power control loop; the rear stage controls the grid-connected synchronization, the direct current side voltage and the output power of the converter through a synchronous ring, a direct voltage ring and a voltage and current inner ring respectively; in the improved control, the reference power of a post-stage synchronous ring is adjusted by feeding back the difference value between the output frequency of the synchronous ring and the rated frequency of the power grid to the input of the synchronous ring; the difference value between the output frequency of the synchronous ring and the rated frequency of the power grid is fed back to the preceding stage, and the output power of the preceding-stage photovoltaic system is adjusted; the power angle is fed back to the preceding stage during a fault, the output power of the photovoltaic system during the fault is adjusted, the power angle is fed back to the input of the post-stage synchronous ring, and the output power of the converter is adjusted. The transient stability of the system can be improved, the damage risk of the converter is reduced, and the service life of the system is prolonged.

Description

Technical Field

[0001] The present invention relates to the technical field of power electronics power generation conversion, and particularly relates to an improved power reserve control method and system based on frequency and power angle feedback. Background Art

[0002] With a large number of wind and solar power sources connected to the power system through converters. The grid-forming virtual synchronous control adopts the power synchronization method, simulates the second-order swing equation of the synchronous machine to match the grid frequency; tracks the grid voltage through the reactive power droop control. In order to support the grid frequency, the energy storage-free photovoltaic grid-connected system adopts the power reserve control to achieve grid-connected operation, so as to reduce costs and improve the service life of equipment.

[0003] However, the energy storage-free photovoltaic grid-connected system adopting the power reserve control ignores the transient stability of the system. Therefore, during a fault, such as a voltage dip fault, the synchronization loop is prone to lose synchronization, causing the system to experience transient instability, which poses challenges to the stability of the energy storage-free photovoltaic grid-connected system operation and the ability to support the grid. Summary of the Invention

[0004] The purpose of the present invention is to provide an improved power reserve control method and system based on frequency and power angle feedback, which is used to solve the problems that the energy storage-free photovoltaic grid-connected system adopting the power reserve control ignores the transient stability of the system, so that the synchronization loop is prone to lose synchronization during a fault, causing the system to experience transient instability, affecting the stability of grid-connected operation and the ability to support the grid, etc. It can improve the synchronization ability of the VSG during a fault, enhance the transient stability of the energy storage-free photovoltaic grid-connected system, and reduce the risk of converter damage.

[0005] To achieve the above purpose, in the first aspect, the present invention provides an improved power reserve control method based on frequency and power angle feedback, including: The front stage controls the output power of the photovoltaic system through a power control loop; The rear stage controls the grid connection synchronization, DC side voltage, and output power of the converter through a synchronization loop, a DC voltage loop, and a voltage and current inner loop respectively; The improved control feeds back the difference between the output frequency of the synchronization loop and the rated grid frequency to the input of the synchronization loop to adjust the reference power of the rear stage synchronization loop; feeds back the difference between the output frequency of the synchronization loop and the rated grid frequency to the front stage to adjust the output power of the front stage photovoltaic system; feeds back the power angle to the front stage during a fault to adjust the output power of the photovoltaic system during the fault, and also feeds back the power angle to the input of the rear stage synchronization loop to adjust the output power of the converter.

[0006] An improved power reserve control method based on frequency and power angle feedback provided by the present invention adjusts the reference power of the subsequent stage synchronous loop by feeding back the difference between the output frequency of the synchronous loop and the rated grid frequency to the input of the synchronous loop, and adjusts the output power of the previous stage photovoltaic system by feeding back the difference between the output frequency of the synchronous loop and the rated grid frequency to the previous stage, including: Obtain the output frequency of the synchronous loop and the rated grid frequency, and calculate the difference between the output frequency of the synchronous loop and the rated grid frequency as the frequency input value; Feed the frequency input value back to the input of the synchronous loop through a proportional link to adjust the reference power of the subsequent stage synchronous loop; feed the frequency input value back to the input of the synchronous loop through an integral link via a power angle feedback switch to adjust the reference power of the subsequent stage synchronous loop; Feed the frequency input value back to the previous stage through a proportional link to adjust the output power of the previous stage photovoltaic system; feed the frequency input value back to the previous stage through an integral link via a power angle feedback switch to adjust the reference power of the previous stage photovoltaic system; Determine the trigger flag and switch state of the power angle feedback switch according to the voltage at the point of common coupling.

[0007] For an improved power reserve control method based on frequency and power angle feedback provided by the present invention, the relationship between the reference power of the synchronous loop and the frequency input value is:

[0008] In the formula, is the actual reference power of the synchronous loop after feedback, is the initial reference power of the synchronous loop, is the power value changed by frequency feedback, is the power value changed by power angle feedback, is the frequency feedback proportional coefficient of the improved control, is the power angle feedback integral coefficient of the improved control, is the output frequency of the synchronous loop, is the rated grid frequency, is the power angle, is the Laplace operator.

[0009] For an improved power reserve control method based on frequency and power angle feedback provided by the present invention, the relationship between the reference power of the photovoltaic system and the frequency input value is:

[0010] In the formula, is the actual reference power of the photovoltaic system after feedback, is the initial reference power of the photovoltaic system.

[0011] An improved power reserve control method based on frequency and power angle feedback provided by the present invention, the trigger flag and switch state of the power angle feedback switch are:

[0012] In the formula, Flag is the trigger flag of the power angle feedback switch, is the voltage at the point of common coupling, is 0.9 times the rated grid voltage; When, the switch state is closed; When, the switch state is open.

[0013] An improved power reserve control method based on frequency and power angle feedback provided by the present invention, the frequency feedback proportional coefficient of the improved control is:

[0014] In the formula, K f represents the active frequency modulation coefficient.

[0015] An improved power reserve control method based on frequency and power angle feedback provided by the present invention, the design process of the power angle feedback integral coefficient of the improved control is: Step SB1: During a fault, set the power angle to 0 as the input; Step SB2: Calculate the current at the point of common coupling based on the following formula:

[0016]

[0017] In the formula, I t is the current at the point of common coupling, V t is the voltage at the point of common coupling, V g is the grid voltage, X g is the line impedance, k Q is the reactive power droop coefficient, V 0 is the reference value of the reactive power loop voltage, Q ref is the reactive power reference value; Step SB3: Judge the magnitude relationship between the current at the point of common coupling calculated in Step SB2 and the maximum current I max If I t is less than I max, then increase the power angle by the first preset value and return to step SB2 for recalculation I t , if I t is greater than or equal to I max , then determine the power angle at this time; Step SB4: Determine whether the power angle decreases during a fault. If the power angle decreases, increase the reactive power droop coefficient by the second preset value and return to step SB1. If the power angle does not decrease, determine the reactive power droop coefficient at this time; Step SB5: After determining the power angle and the reactive power droop coefficient, calculate the power angle feedback integral coefficient of the improved control as:

[0018] In the formula, P is the active power output by the converter.

[0019] According to an improved power reserve control method based on frequency and power angle feedback provided by the present invention, the first preset value is 0.001 and the second preset value is 0.0001.

[0020] According to an improved power reserve control method based on frequency and power angle feedback provided by the present invention, the fault includes a voltage dip.

[0021] In a second aspect, the present invention provides an improved power reserve control system based on frequency and power angle feedback, including a photovoltaic module, a DC boost circuit, a converter, a point of common coupling, and a control module. The output of the converter is connected to the point of common coupling through a filter inductor, a filter capacitor, and a line impedance and then connected to the grid; the control module is configured to execute the method of the first aspect.

[0022] The technical solution of the present invention at least has the following technical effects: An improved power reserve control method and system based on frequency and power angle feedback provided by the present invention change the reference power of the front-stage photovoltaic system and the reference power of the rear-stage VSG synchronization loop by adding a link for feedback of the frequency difference, thereby enhancing the frequency support ability of the system. By adding a power angle feedback link, the reference power of the front-stage photovoltaic system and the reference power of the rear-stage VSG synchronization loop are changed during a fault, equivalently increasing the deceleration area and decreasing the acceleration area, and improving the transient stability of the system. And by selecting reasonable control parameters (frequency feedback proportional coefficient and power angle feedback integral coefficient), while outputting as much active power as possible, the transient stability performance of the system is ensured. Compared with the original power reserve control method, the transient stability of the system is effectively improved, the risk of converter damage is reduced, and the system life is increased. Description of the Drawings

[0023] To more clearly illustrate the technical solutions in the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0024] In the drawings: Figure 1 It is a flowchart of the improved power reserve control method based on frequency power angle feedback of the present invention; Figure 2 It is a schematic structural diagram of a photovoltaic grid-connected system without energy storage before adopting the improved power reserve control; Figure 3 It is a schematic diagram of the principle of the improved control of the present invention; Figures 4(a) and 4(b) are schematic diagrams of the equal area method for improving transient stability of the present invention; Figure 5 It is a simulation diagram of frequency support of the present invention; Figure 6(a) is a simulation diagram of the present invention without improved control under a fault; Figure 6(b) is a simulation diagram of the present invention with improved control under a fault. Detailed implementation manners

[0025] To make the objectives, technical solutions and advantages of the present invention clearer, the following will clearly and completely describe the technical solutions in the present invention in conjunction with the drawings in the present invention. Obviously, the described embodiments are some but not all of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art without creative efforts based on the embodiments in the present invention belong to the scope of protection of the present invention.

[0026] The following will elaborate on some embodiments of the present invention in conjunction with the drawings. Without conflict, the following embodiments and the features in the embodiments can be combined with each other.

[0027] Please refer to Figure 1 , an embodiment of the present invention provides an improved power reserve control method based on frequency power angle feedback, including: Step 1, the front stage controls the output power of the photovoltaic system through a power control loop; Step 2, the rear stage controls the grid connection synchronization, DC side voltage and output power of the converter through a synchronization loop, a direct voltage loop and an inner voltage current loop (including Figure 2 the current loop and voltage loop in Step 3. Improved control: By feeding back the difference between the output frequency of the synchronization loop and the rated grid frequency to the input of the synchronization loop, the reference power of the subsequent synchronization loop is adjusted; by feeding back the difference between the output frequency of the synchronization loop and the rated grid frequency to the previous stage, the output power of the previous-stage photovoltaic system is adjusted; by feeding back the power angle to the previous stage during a fault, the output power of the photovoltaic system during the fault is adjusted, and the power angle is also fed back to the input of the subsequent synchronization loop to adjust the output power of the converter.

[0028] It should be noted that the present invention provides an improved control method based on frequency and power angle feedback on the basis of the original power reserve control, and also gives the design method of control parameters. Among them, the previous stage controls the output power of the photovoltaic system to realize the power reserve of the photovoltaic system; the subsequent stage uses a direct voltage loop to control the DC side voltage of the converter, and uses the virtual synchronous generator (VSG) technology to achieve grid connection synchronization in a power synchronization manner; the improved control uses the feedback frequency difference to change the reference power of the previous-stage photovoltaic system to enhance the stability of the system under transient conditions. At the same time, the power angle feedback under a fault is introduced to adjust the output power of the photovoltaic system and the output power of the converter during the fault to improve the transient stability of the system. At the same time, the synchronous adjustment of the power of the previous stage and the subsequent stage ensures the stability of the direct voltage loop and maintains the DC side voltage at a normal level.

[0029] Specifically, in the improved control, by feeding back the difference between the output frequency of the synchronization loop and the rated grid frequency to the input of the synchronization loop, the reference power of the subsequent synchronization loop is adjusted; by feeding back the difference between the output frequency of the synchronization loop and the rated grid frequency to the previous stage, the output power of the previous-stage photovoltaic system is adjusted, including the following steps: Step SA1. Obtain the output frequency of the synchronization loop and the rated grid frequency, and calculate the difference between the output frequency of the synchronization loop and the rated grid frequency as the frequency input value.

[0030] Step SA2. Feed the frequency input value back to the input of the synchronization loop through a proportional link to adjust the reference power of the subsequent synchronization loop; feed the frequency input value back to the input of the synchronization loop through an integral link via a power angle feedback switch to adjust the reference power of the subsequent synchronization loop.

[0031] The relationship between the reference power of the synchronization loop and the frequency input value is as follows:

[0032] In the formula, is the actual reference power of the synchronization loop after feedback, is the initial reference power of the synchronization loop, is the power value changed by the frequency feedback, is the power value changed by the power angle feedback, is the frequency feedback proportional coefficient of the improved control, To improve the power angle feedback integral coefficient of the control, is the output frequency of the synchronization loop, is the rated grid frequency, is the power angle, is the Laplace operator.

[0033] Step SA3: Feed the frequency input value back to the previous stage through a proportional link to adjust the output power of the previous-stage photovoltaic system; feed the frequency input value back to the previous stage through an integral link via a power angle feedback switch to adjust the reference power of the previous-stage photovoltaic system.

[0034] The relationship between the reference power of the photovoltaic system and the frequency input value is as follows:

[0035] In the formula, is the actual reference power of the photovoltaic system after feedback, is the initial reference power of the photovoltaic system.

[0036] Step SA4: Determine the trigger flag Flag and the switch state of the power angle feedback switch according to the common coupling point voltage. When Flag = 1, the switch is closed; otherwise, the switch is open. The Flag state is as follows:

[0037] In the formula, Flag is the trigger flag of the power angle feedback switch, is the common coupling point voltage, is 0.9 times the rated grid voltage.

[0038] Specifically, the design process of the control parameters is as follows.

[0039] The frequency feedback proportional coefficient of the improved control k d is designed as shown in the following formula:

[0040] In the formula, K f represents the active frequency modulation coefficient, generally taking values from 10 to 50.

[0041] The power angle feedback integral coefficient of the improved control k i has the following design process: Step SB1: During a fault, such as various voltage sag faults, set the power angle to 0 as the input.

[0042] Step SB2: Calculate the common coupling point current based on the following formula:

[0043]

[0044] Wherein, I t is the common coupling point current, V t is the common coupling point voltage, V g is the grid voltage, X g is the line impedance, k Q is the reactive power droop coefficient, V 0 is the reactive power loop voltage reference value, Q ref is the reactive power reference value. Step SB3: Judge the magnitude relationship between the common coupling point current calculated in step SB2 and the current maximum value I max . If I t is less than I max , then increase the power angle by a first preset value, for example = +0.001, and return to step SB2 to recalculate I t . If I t is greater than or equal to I max , then determine the power angle at this time.

[0045] Step SB4: Determine whether the power angle decreases during a fault. If the power angle decreases, then increase the reactive power droop coefficient by a second preset value, for example k Q = k Q +0.0001, and return to step SB1. If the power angle does not decrease, then determine the reactive power droop coefficient k Q at this time.

[0046] Step SB5: After determining the power angle and the reactive power droop coefficient k Q , calculate the final power angle feedback integral coefficient k i as follows:

[0047] Wherein, Pis the active power output by the converter.

[0048] Based on the same inventive concept, another embodiment of the present invention provides an improved power reserve control system based on frequency and power angle feedback, including a photovoltaic module, a DC boost circuit, a converter, a point of common coupling, and a control module. The output of the converter is connected to the point of common coupling and then grid-connected through a filter inductor, a filter capacitor, and a line impedance; the control module is configured to execute the improved power reserve control method of the foregoing embodiment.

[0049] The control process of the entire system is as follows: The front stage takes the difference between the initial reference power of the photovoltaic system and the calculated instantaneous photovoltaic power as the input, and through the power control loop, obtains the PWM control signal for controlling the DC / DC boost circuit, thereby controlling the photovoltaic output power to be the reference power; the latter stage collects the voltage and current at the point of common coupling, and through dq transformation, obtains , , , , After calculation, the active power and reactive power output by the converter P , Q are obtained, and the input DC voltage loop is controlled so that the DC voltage is the reference voltage , and the VSG input reference power P ref is obtained, P and P ref After passing through the VSG synchronization loop, the output phase is obtained, which tracks the phase of the detected voltage, thereby controlling the phase of the output voltage; the reactive power Q passes through the droop control to output the d-axis reference voltage V dref , while the q-axis reference voltage V qref is 0. After passing through the voltage and current loop, the modulation signal for controlling the output voltage of the converter is obtained, thereby controlling its output voltage and output power. When the grid load increases and the frequency drops, the Flag signal is not triggered, the power angle feedback is invalid, and the frequency feedback acts alone. At this time will be less than , and it is fed back to the initial reference power of the front stage, and the reference power of the photovoltaic system in the front stage is controlled to be , and at this time, active power is increased to maintain the system frequency stability. When a voltage dip fault occurs in the grid, at this time, due to the triggering of the Flag signal, the system introduces power angle feedback. Since the system accelerates during the voltage dip, the power angle increases. According to the power relationship under improved control, the reference active power P ref decreases. At this time, the equivalent deceleration area is increased and the acceleration area is reduced, improving the transient stability of the system.

[0050] For better understanding of the technical solution of the present invention, the following description is provided: The improved power reserve control system based on frequency and power angle feedback described in the present invention refers to a system composed of a photovoltaic module, a DC (DC / DC) boost circuit, and a grid-connected inverter (hereinafter, the system mentioned hereinafter is the abbreviation of the improved power reserve control system based on frequency and power angle feedback herein). The source side is a photovoltaic module PV without energy storage. The inverter refers to a power device that converts DC into AC. The power reserve control can effectively support the grid frequency. By improving the power reserve control through frequency and power angle feedback, the transient stability of the inverter can be improved.

[0051] As Figure 2 shown, the output of the photovoltaic module PV is connected in parallel with a large capacitor C after passing through the DC / DC boost circuit dc and then in parallel with the grid-connected inverter. The output of the inverter passes through a filter inductor , a filter capacitor and a line impedance and is connected to the common coupling point and then connected to the grid.

[0052] To support the grid frequency, a power reserve control is adopted in the photovoltaic grid-connected system without energy storage. However, due to the neglect of the transient stability of the system, in case of a fault, such as a voltage dip fault, the synchronization loop is prone to lose synchronization, leading to a situation of transient instability of the system, which will affect the service life of the inverter and weaken the stability of the photovoltaic module without energy storage connected to the grid.

[0053] Therefore, the present invention provides an improved control method. Based on the original power reserve control, namely the front stage and the rear stage, a link for feedback frequency difference and a power angle feedback link are added. During a fault, the reference power of the front-stage photovoltaic system is controlled, and a control parameter design method is provided. The front stage uses power control to adjust the photovoltaic output power to reach the reference value. The rear stage uses a direct voltage loop to control the DC side voltage of the inverter and uses VSG (virtual synchronous generator) control to achieve grid connection synchronization in a power synchronization manner; the improved control changes the reference power of the front-stage photovoltaic system and the active power output of the inverter by feedback frequency difference and power angle under voltage dip fault to enhance the control effect.

[0054] The front stage of the present invention continuously detects the voltage and current output by the photovoltaic module to calculate the photovoltaic output power, and compares the initial reference power of the photovoltaic system with the photovoltaic instantaneous power The difference is used as the input, and through the power control loop, the PWM control signal of the DC / DC boost circuit is obtained to control the output power of the photovoltaic module to the reference power. Combined with Figure 2 , the voltage and current are collected from the output terminal of the photovoltaic module. The difference between the initial reference power and the calculated instantaneous photovoltaic power is used as the input. After passing through the PI link, the obtained value is then subtracted from . The difference is then passed through another PI link to obtain the PWM control signal of the DC / DC boost circuit, thereby controlling the photovoltaic output power to the reference power.

[0055] In the latter stage of the present invention, the voltage and current of the point of common coupling (PCC) are collected and transformed through dq transformation to obtain , , , , . Through calculation, the active power and reactive power P , Q output by the converter are obtained. And the input DC voltage loop is controlled so that the DC voltage is the reference voltage , and the input reference power P ref of the VSG is obtained. P And P ref output the phase through the VSG synchronization loop, track the phase of the detected voltage, and thus control the phase of the output voltage. The reactive power Q outputs the d-axis reference voltage V dref through droop control. And the q-axis reference voltage V qref is 0. Through the voltage loop and current loop, the PWM modulation signal for controlling the output voltage of the converter is obtained, thereby controlling its output voltage and output power. Combined with Figure 2 , in the DC voltage loop, is used as the input, and after passing through a PI link and multiplying by the DC voltage coefficient , the active power reference value P ref is obtained. P ref The difference between P and the actual active power passes through the synchronization loop, and the output phase is obtained by simulating the second-order rotor equation of the synchronous machine, satisfying the following relationship:

[0056] In the formula, is the output phase of the VSG, is the rated frequency of the power grid, J is the inertia, is the damping, is the output reference power, is the active power output by the converter, is the output frequency of the synchronization loop.

[0057] The improved control principle of the present invention is as shown in Figure 3 Based on the optimal control parameters obtained from the aforementioned cyclic calculation k i and k d . By taking the difference between the output frequency of the synchronization loop and the rated frequency of the power grid as the input, and feeding it back to the front-stage control part and the rear-stage control part through a proportional link, the reference power of the photovoltaic system and the output power of the converter are changed to achieve the purpose of enhancing the system frequency support ability; under faults, the integral power angle of the difference between the output frequency of the synchronization loop and the rated frequency of the power grid is used as the input and fed back to the front-stage control part and the rear-stage control part to adjust the reference power of the photovoltaic system and the output power of the converter. The equivalent deceleration area is increased and the acceleration area is reduced, improving the transient stability of the system.

[0058] In the present invention, the improvement of the system transient stability can be described by the equal area method.

[0059] As shown in Fig. 4(a), when a small fault occurs in the system, if the system does not have improved control, when the fault occurs, the maximum acceleration area is S 1 in Fig. 4(a), and the maximum deceleration area is S 4 in Fig. 4(a). The fault is removed at , and the system returns to the stable equilibrium point SEP 1 in Fig. 4(a); if the system adopts improved control, the maximum acceleration area is S 1 minus S 2 in Fig. 4(a). There is a stable equilibrium point SEP 1 when the fault is removed at . At this time, due to the failure of the fault removal power angle feedback, the reference power returns to the initial value, and the system returns to the stable equilibrium point SEP 1 after stabilization. The whole process equivalently reduces the maximum acceleration area, improves the transient stability of the system, increases the stable margin of the system, and the increased margin can be expressed by the following formula.

[0060]

[0061] As shown in Figure 4(b), if the fault severity increases and the fault is not removed, without improved control of the system, when a fault occurs, the system will accelerate throughout the process and experience transient instability; if the system adopts improved control, the maximum accelerating area is S 1 , and the maximum decelerating area is S 2 at this time. The whole process equivalently increases the maximum decelerating area and decreases the maximum accelerating area. Moreover, there is no equilibrium point without improved control, while there are stable equilibrium SEP 2 and unstable equilibrium point USEP 1 after adopting improved control. The system will stabilize at the new equilibrium point SEP 2 , improving the transient stability of the system and increasing the stability margin of the system. The increased stability margin can be expressed by the following formula.

[0062]

[0063] In the formula, V 1 is the voltage amplitude at the PCC point, is the voltage amplitude of the power grid after the fault, is the line impedance.

[0064] Here, in the MATLAB / Simulink simulation platform, based on the control method of the present invention, the power control and VSG control of Figure 2 , as well as the improved control of Figure 3 , are used to build the Figure 2 shown non-energy storage photovoltaic grid-connected system. The simulation parameters are shown in Table 1, where k pv , k iv are the proportional coefficient and integral coefficient of the PI regulation on the photovoltaic side respectively, and k p , k ip are the proportional coefficient and integral coefficient of the PI regulation on the converter side respectively.

[0065] Table 1. System simulation parameters

[0066] Based on the system simulation parameters in Table 1, when the active power load of the system increases and the frequency drops at 1 second, comparing the photovoltaic grid-connected system with traditional VSG control and the photovoltaic grid-connected system with improved control, the former is as shown by Figure 5 the red curve and cannot increase the power to support the system frequency stability; the latter is as shown by Figure 5As shown by the blue curve, the converter increases its output power to support the system frequency, indicating that the improved control can support the stability of the system frequency and is beneficial to maintaining the system stability. A voltage dip fault occurs at 1 second, with the voltage dropping to 0.35 p.u. and lasting for 0.1 s. Comparing the photovoltaic grid-connected system with traditional VSG control and the photovoltaic grid-connected system with improved control, the former oscillates and experiences transient instability as shown in Fig. 6(a), while the latter maintains system stability as shown in Fig. 6(b). This shows that the improved control enhances the transient stability of the system and reduces the risk of converter damage.

[0067] In summary, the improved power reserve control method and system based on frequency-power angle feedback provided by the present invention prevent transient instability caused by the loss of synchronization of the synchronization loop during a fault. The present invention effectively improves the ability of the off-grid photovoltaic grid-connected system to maintain the synchronization of the synchronization loop during a fault and enhances the transient stability of the system. The specific control steps are as follows: The pre-stage control continuously detects the output voltage and current of the photovoltaic module PV to calculate its power, and sets its output as the reference power through the power control loop; the post-stage control realizes the grid connection synchronization, DC side voltage control, and output power control of the grid-connected converter by means of the synchronization loop, DC voltage loop, and reactive power loop respectively. At the same time, the frequency difference between the synchronization loop and the power grid is fed back to the inputs of the pre-stage and post-stage through a proportional link to control the reference power of the pre-stage photovoltaic and post-stage converter; under a fault, the integral power angle of the frequency difference between the synchronization loop and the power grid is fed back to the inputs of the pre-stage and post-stage to control the reference power of the pre-stage photovoltaic and post-stage converter, improving the transient stability and frequency support ability of the system.

[0068] Those skilled in the art will readily conceive of other embodiments of the present invention after considering the specification and the practice of the embodiments disclosed herein. The present invention is intended to cover any variations, uses, or adaptations of the present invention that follow the general principles of the present invention and include known common knowledge or conventional technical means in the technical field not disclosed by the present invention. It should be understood that the present invention is not limited to the precise structures described above and shown in the drawings, and various modifications and changes can be made without departing from its scope. The scope of the present invention is only limited by the appended claims.

Claims

1. An improved power standby control method based on frequency power angle feedback, characterized in that: include: The front stage controls the output power of the photovoltaic system through a power control loop; The latter stage controls the grid-connected synchronization, DC side voltage and output power of the converter through the synchronization loop, direct voltage loop and voltage and current inner loop respectively; The improved control adjusts the reference power of the subsequent synchronous ring by feeding back the difference between the output frequency of the synchronous ring and the rated frequency of the power grid to the input of the synchronous ring; adjusts the output power of the preceding photovoltaic system by feeding back the difference between the output frequency of the synchronous ring and the rated frequency of the power grid to the preceding stage; adjusts the output power of the photovoltaic system when a fault occurs by feeding back the power angle to the preceding stage, and also feeds back the power angle to the input of the subsequent synchronous ring to adjust the output power of the converter.

2. The improved power standby control method based on frequency power angle feedback according to claim 1 is characterized in that: The reference power of the subsequent synchronous ring is adjusted by feeding back the difference between the synchronous ring output frequency and the grid rated frequency to the synchronous ring input; By feeding back the difference between the synchronous ring output frequency and the grid rated frequency to the front stage, the output power of the front stage photovoltaic system is adjusted, including: Obtaining the synchronous ring output frequency and the grid rated frequency, and calculating the difference between the synchronous ring output frequency and the grid rated frequency as the frequency input value; Feeding back the frequency input value to the input of the synchronization ring through the proportional link to adjust the reference power of the subsequent synchronization ring; Feeding back the frequency input value to the input of the synchronization ring through the integral link and the power angle feedback switch to adjust the reference power of the subsequent synchronization ring; Feeding back the frequency input value to the preceding stage through the proportional link to adjust the output power of the preceding stage photovoltaic system; Feeding back the frequency input value to the preceding stage through the integral link and the power angle feedback switch to adjust the reference power of the preceding stage photovoltaic system; The trigger mark and switch state of the power angle feedback switch are determined according to the voltage at the common coupling point.

3. The improved power standby control method based on frequency power angle feedback according to claim 2 is characterized in that: The relationship between the reference power and frequency input value of the synchronization ring is: In the formula, is the actual reference power of the synchronous loop after feedback, is the initial reference power of the synchronization ring, is the power value changed by frequency feedback, is the power value changed by the power angle feedback, To improve the frequency feedback proportionality factor of the control, To improve the power angle feedback integral coefficient of the control, is the synchronous ring output frequency, is the rated frequency of the power grid, is the power angle, is the Laplace operator.

4. The improved power standby control method based on frequency power angle feedback according to claim 3 is characterized in that: The relationship between the reference power and frequency input value of the photovoltaic system is: In the formula, is the actual reference power of the photovoltaic system after feedback, is the initial reference power of the photovoltaic system.

5. The improved power standby control method based on frequency power angle feedback according to claim 3 is characterized in that: The trigger flag and switch state of the power angle feedback switch are: Where Flag is the trigger flag of the power angle feedback switch. is the voltage at the common coupling point, 0.9 times the rated voltage of the grid; When , the switch state is closed; , the switch state is open.

6. The improved power standby control method based on frequency power angle feedback according to claim 4 is characterized in that: The frequency feedback proportional coefficient of the improved control is: In the formula, K f Indicates the active frequency modulation coefficient.

7. The improved power standby control method based on frequency power angle feedback according to claim 4 is characterized in that: The design process of the power angle feedback integral coefficient of the improved control is as follows: Step SB1, in case of a fault, set the power angle to 0 as input; Step SB2: Calculate the common coupling point current based on the following formula: In the formula, I t is the common coupling point current, V t is the voltage at the common coupling point, V g is the grid voltage, X g is the line impedance, k Q is the reactive power droop coefficient, V 0 is the reactive loop voltage reference value, Q ref is the reactive power reference value; Step SB3: Determine the common coupling point current calculated in step SB2 and the maximum current I max If I t Less than I max , then increase the power angle by the first preset value and return to step SB2 to recalculate I t ,like I t Greater than or equal to I max , then determine the power angle at this time; Step SB4, determining whether the power angle decreases during a fault. If the power angle decreases, the reactive power droop coefficient is increased by a second preset value and the process returns to step SB1. If the power angle does not decrease, the reactive power droop coefficient at this time is determined. Step SB5: After determining the power output angle and reactive power droop coefficient, the power angle feedback integral coefficient of the improved control is calculated as: In the formula, P is the active power output by the converter.

8. The improved power standby control method based on frequency power angle feedback according to claim 7, characterized in that: The first preset value is 0.001, and the second preset value is 0.0001.

9. The improved power standby control method based on frequency power angle feedback according to claim 1, characterized in that: The fault includes a voltage sag.

10. An improved power reserve control system based on frequency power angle feedback, characterized in that: The invention comprises a photovoltaic module, a DC boost circuit, a converter, a common coupling point and a control module, wherein the output of the converter is connected to the common coupling point through a filter inductor, a filter capacitor and a line impedance and then connected to the grid; and the control module is configured to execute the method according to any one of claims 1 to 9.

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