An improved power backup control method and system based on frequency power angle feedback

Abstract

The application provides an improved power backup control method and system based on frequency power angle feedback, and relates to the technical field of power electronic generation and conversion. The method comprises the following steps: a front stage controls the output power of a photovoltaic system through a power control loop; a rear stage controls the grid synchronization, DC side voltage and output power of a converter through a synchronization loop, a direct voltage loop and a voltage and current inner loop respectively; the improved control adjusts the reference power of the rear stage synchronization loop by feeding back the difference between the output frequency of the synchronization loop and the rated frequency of the power grid to the input of the synchronization loop; the output power of the photovoltaic system is adjusted by feeding back the difference between the output frequency of the synchronization loop and the rated frequency of the power grid to the front stage; the output power of the photovoltaic system during a fault is adjusted by feeding back the power angle to the front stage during the fault, and the output power of the converter is adjusted by feeding back the power angle to the input of the rear stage synchronization loop. The application can improve the transient stability of the system, reduce the damage risk of the converter and improve the service life of the system.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of power electronic power conversion technology, and particularly relates to an improved power reserve control method and system based on frequency power angle feedback. BACKGROUND

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

[0003] However, the energy storage-free photovoltaic grid-connected system with power reserve control ignores the transient stability of the system, so that the synchronous ring is easy to lose synchronization during a fault, such as a voltage dip fault, causing transient instability of the system, which challenges the stability of the energy storage-free photovoltaic grid-connected system and the ability to support the grid. SUMMARY

[0004] The present application aims to provide an improved power reserve control method and system based on frequency power angle feedback, which can solve the problem that the energy storage-free photovoltaic grid-connected system with power reserve control ignores the transient stability of the system, so that the synchronous ring is easy to lose synchronization during a fault, causing transient instability of the system, affecting the stability of grid-connected operation and the ability to support the grid, and can improve the synchronization ability of 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-mentioned purpose, in a first aspect, the present application provides an improved power reserve control method based on frequency power angle feedback, comprising:

[0006] The front stage controls the output power of the photovoltaic system through the power control loop;

[0007] The rear stage controls the grid synchronization, DC side voltage and output power of the converter through the synchronous ring, direct voltage loop and voltage current inner loop, respectively;

[0008] The improved control adjusts the reference power of the rear stage synchronous ring by feeding back the difference between the output frequency of the synchronous ring and the rated frequency of the grid to the input of the synchronous ring; adjusts the output power of the front stage photovoltaic system by feeding back the difference between the output frequency of the synchronous ring and the rated frequency of the grid to the front stage; adjusts the output power of the photovoltaic system during a fault by feeding back the power angle to the front stage during a fault, and also feeds back the power angle to the input of the rear stage synchronous ring to adjust the output power of the converter.

[0009] According to the improved power backup control method based on frequency power angle feedback provided by the application, the difference between the output frequency of the synchronization ring and the rated frequency of the power grid is fed back to the input of the synchronization ring to adjust the reference power of the post-stage synchronization ring; the difference between the output frequency of the synchronization ring and the rated frequency of the power grid is fed back to the front stage to adjust the output power of the front-stage photovoltaic system, comprising:

[0010] The output frequency of the synchronization ring and the rated frequency of the power grid are obtained, and the difference between the output frequency of the synchronization ring and the rated frequency of the power grid is calculated as a frequency input value;

[0011] The frequency input value is fed back to the input of the synchronization ring through a proportional link to adjust the reference power of the post-stage synchronization ring; the frequency input value is fed back to the input of the synchronization ring through an integral link through a power angle feedback switch to adjust the reference power of the post-stage synchronization ring;

[0012] The frequency input value is fed back to the front stage through a proportional link to adjust the output power of the front-stage photovoltaic system; the frequency input value is fed back to the front stage through an integral link through a power angle feedback switch to adjust the reference power of the front-stage photovoltaic system;

[0013] The trigger flag and the switch state of the power angle feedback switch are determined according to the voltage at the point of common coupling.

[0014] According to the improved power backup control method based on frequency power angle feedback provided by the application, the relationship between the reference power of the synchronization ring and the frequency input value is:

[0015]

[0016] In the formula, is the actual reference power of the post-stage synchronization ring 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 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 synchronization ring, is the rated frequency of the power grid, is the power angle, is the Laplace operator.

[0017] According to the improved power backup control method based on frequency power angle feedback provided by the application, the relationship between the reference power of the photovoltaic system and the frequency input value is:

[0018]

[0019] In the formula, is the actual reference power of the photovoltaic system after feedback, is the initial reference power of the photovoltaic system.

[0020] According to the improved power backup control method based on frequency power angle feedback provided by the application, the triggering mark and the switch state of the power angle feedback switch are:

[0021]

[0022] In the formula, Flag is the triggering mark of the power angle feedback switch, is the point of common coupling voltage, is 0.9 times the rated voltage of the power grid; When the value is greater than 0, the switch state is closed; When the value is less than 0, the switch state is open.

[0023] According to the improved power backup control method based on frequency power angle feedback provided by the application, the frequency feedback proportional coefficient of the improved control is:

[0024]

[0025] In the formula, K f represents the active frequency modulation coefficient.

[0026] According to the improved power backup control method based on frequency power angle feedback provided by the application, the design process of the power angle feedback integral coefficient of the improved control is:

[0027] Step SB1, set the power angle as 0 as input at the time of fault;

[0028] Step SB2, calculate the point of common coupling current based on the following formula:

[0029]

[0030]

[0031] In the formula, I t is the point of common coupling current, V t is the point of common coupling voltage, V g is the power grid voltage, X g is the line impedance, k Q is the reactive droop coefficient, V 0 is the reactive loop voltage reference value, Q ref is the reactive reference value;

[0032] Step SB3, judging the size relation between the common coupling point current calculated in step SB2 and the maximum current value, if the common coupling point current is less than the maximum current value, then the power angle is increased by a first preset value and returns to step SB2 for recalculation I max I t , if the common coupling point current is greater than the maximum current value, then the power angle is decreased by a second preset value and returns to step SB2 for recalculation I max I t , if the common coupling point current is greater than or equal to the maximum current value, then the power angle at this time is determined I t I max

[0033] Step SB4, judging whether the power angle is decreased at the time of fault, if the power angle is decreased, then the reactive power droop coefficient is increased by a second preset value and returns to step SB1, if the power angle is not decreased, then the reactive power droop coefficient at this time is determined

[0034] Step SB5, after the power angle and the reactive power droop coefficient are determined, the power angle feedback integral coefficient of the improved control is calculated as:

[0035]

[0036] In the formula, P is the active power output by the converter. P

[0037] According to the improved power backup control method based on frequency power angle feedback provided by the application, the first preset value is 0.001 and the second preset value is 0.0001.

[0038] According to the improved power backup control method based on frequency power angle feedback provided by the application, the fault includes voltage drop.

[0039] In the second aspect, the application provides an improved power backup control system based on frequency power angle feedback, which comprises a photovoltaic assembly, a direct current boosting circuit, a converter, a common coupling point and a control module, the output of the converter is connected to the common coupling point through a filtering inductor, a filtering capacitor and a line impedance and then connected to the grid; the control module is configured to execute the method of the first aspect.

[0040] The technical scheme of the application at least has the following technical effects:

[0041] ​​​​​The application provides an improved power backup control method and system based on frequency power angle feedback. BRIEF DESCRIPTION OF DRAWINGS

[0042] In order to more clearly illustrate the technical solutions in the application or the prior art, the drawings needed to be used in the embodiments or the prior art description will be briefly introduced below. Obviously, the drawings in the following description are some embodiments of the application, and other drawings can be obtained by those skilled in the art without any creative effort on the basis of these drawings.

[0043] In the drawings:

[0044] Figure 1 The flow chart of the improved power backup control method based on frequency power angle feedback of the application;

[0045] Figure 2 The structural schematic diagram of a non-energy storage photovoltaic grid-connected system before the improved power backup control is adopted;

[0046] Figure 3 The principle diagram of the improved control of the application;

[0047] FIGS. 4(a) and 4(b) are area method diagrams for improving transient stability according to the application;

[0048] Figure 5 The frequency support simulation diagram of the application;

[0049] FIG. 6(a) is a simulation diagram of a non-improved control under fault according to the application;

[0050] FIG. 6(b) is a simulation diagram of an improved control under fault according to the application. DETAILED DESCRIPTION

[0051] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0052] The following detailed description of some embodiments of the present invention will be provided in conjunction with the accompanying drawings. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0053] Please see Figure 1 This invention provides an improved power reserve control method based on frequency power angle feedback, comprising:

[0054] Step 1: The front-end controls the output power of the photovoltaic system through a power control loop;

[0055] Step 2: The subsequent stage passes through a synchronization loop, a direct voltage loop, and an inner voltage-current loop (including...). Figure 2 The current loop and voltage loop in the converter control the grid synchronization, DC side voltage and output power of the converter, respectively.

[0056] Step 3: Improved control by feeding back the difference between the output frequency of the synchronization loop and the rated frequency of the grid to the input of the synchronization loop to adjust the reference power of the subsequent synchronization loop; by feeding back the difference between the output frequency of the synchronization loop and the rated frequency of the grid to the preceding stage to adjust the output power of the preceding photovoltaic system; by feeding back the power angle to the preceding stage during a fault to adjust the output power of the photovoltaic system during a fault, and also by feeding back the power angle to the input of the subsequent synchronization loop to adjust the output power of the converter.

[0057] It should be noted that this invention provides an improved control method based on frequency power angle feedback, building upon existing power reserve control methods, and also presents a design method for control parameters. Specifically, the front-end controls the output power of the photovoltaic system to achieve power reserve; the rear-end uses a direct current loop to control the DC-side voltage of the converter, employing virtual synchronous generator (VSG) technology to achieve grid synchronization through power synchronization. The improved control utilizes feedback frequency difference to change the reference power of the front-end photovoltaic system, thereby enhancing system stability under transient conditions. Simultaneously, power angle feedback under fault conditions is introduced to adjust the output power of the photovoltaic system and the converter during faults, further improving system transient stability. Furthermore, the synchronous adjustment of power in both the front and rear stages ensures the stability of the direct current loop, maintaining the DC-side voltage at a normal level.

[0058] Specifically, in the improved control, the reference power of the post-stage synchronous loop is adjusted by feeding back the difference between the output frequency of the synchronous loop and the rated frequency of the power grid to the input of the synchronous loop; the output power of the pre-stage photovoltaic system is adjusted by feeding back the difference between the output frequency of the synchronous loop and the rated frequency of the power grid to the pre-stage, including the following steps:

[0059] Step SA1, the output frequency of the synchronous loop and the rated frequency of the power grid are obtained, and the difference between the output frequency of the synchronous loop and the rated frequency of the power grid is calculated as a frequency input value.

[0060] Step SA2, the frequency input value is fed back to the input of the synchronous loop through a proportional link to adjust the reference power of the post-stage synchronous loop; the frequency input value is fed back to the input of the synchronous loop through an integral link through a power angle feedback switch to adjust the reference power of the post-stage synchronous loop.

[0061] The relationship between the reference power of the synchronous loop and the frequency input value is as follows:

[0062]

[0063] In the formula, is the actual reference power of the post-stage 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 frequency of the power grid, is the power angle, is the Laplace operator.

[0064] Step SA3, the frequency input value is fed back to the pre-stage through a proportional link to adjust the output power of the pre-stage photovoltaic system; the frequency input value is fed back to the pre-stage through an integral link through a power angle feedback switch to adjust the reference power of the pre-stage photovoltaic system.

[0065] The relationship between the reference power of the photovoltaic system and the frequency input value is as follows:

[0066]

[0067] In the formula, is the actual reference power of the photovoltaic system after feedback, is the initial reference power of the photovoltaic system.

[0068] Step SA4: Determine the trigger flag (Flag) and switch state of the power angle feedback switch based on the common coupling point voltage. When Flag=1, the switch is closed; otherwise, the switch is open. The Flag states are as follows:

[0069]

[0070] In the formula, Flag is the trigger flag of the power angle feedback switch. The voltage at the common coupling point, It is 0.9 times the rated voltage of the power grid.

[0071] Specifically, the design process for the control parameters is as follows.

[0072] Improved control frequency feedback proportional coefficient k d The design is shown in the following formula:

[0073]

[0074] In the formula, K f This represents the active frequency regulation coefficient, which is typically taken as 10 to 50.

[0075] Improved control power angle feedback integral coefficient k i The design process is as follows:

[0076] Step SB1: In case of a fault, such as various voltage drop faults, set the power angle. Use 0 as input.

[0077] Step SB2: Calculate the common coupling point current based on the following formula:

[0078]

[0079]

[0080] In the formula, I t For the common coupling point current, V t The voltage at the common coupling point, V g This is the grid voltage. X g For line impedance, k Q This is the reactive power droop factor. V 0 represents the reference value for the reactive power loop voltage. Q ref This is the reactive power reference value.

[0081] Step SB3, judging the size relation between the common coupling point current calculated in step SB2 and the maximum current value I max If I t is less than I max , the power angle is increased by a first preset value, for example = +0.001, and the step SB2 is returned to recalculate I t If I t is greater than or equal to I max , the power angle at this time is determined .

[0082] Step SB4, judging whether the power angle is reduced at the time of fault, if the power angle is reduced, the reactive power droop coefficient is increased by a second preset value, for example k Q = k Q +0.0001, and the step SB1 is returned, if the power angle is not reduced, the reactive power droop coefficient at this time is determined k Q .

[0083] Step SB5, the power angle and the reactive power droop coefficient k Q are determined, and the final power angle feedback integral coefficient k i is calculated as follows:

[0084]

[0085] In the formula, P is the active power output by the converter.

[0086] Based on the same inventive concept, another embodiment of the present application provides an improved power backup control system based on frequency power angle feedback, comprising a photovoltaic assembly, a DC boost circuit, a converter, a common coupling point and a control module, 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; the control module is configured to perform the improved power backup control method of the foregoing embodiment.

[0087] The control process of the entire system is as follows:

[0088] The initial reference power of the photovoltaic system and the calculated photovoltaic instantaneous power The difference is input, through the power control loop, the PWM control signal of the DC / DC boost circuit is obtained, so that the photovoltaic output power is controlled as the reference power; the voltage and current of the grid-connected point are collected and current , through dq transformation, the active power and reactive power of the converter output are obtained 、 、 、 , , P 、 Q , The input DC voltage loop is controlled to be the reference voltage , and the VSG input reference power is obtained P ref , P and P ref The output phase of the VSG synchronization loop , the phase of the detected voltage is tracked, so as to control the phase of the output voltage; the reactive power Q The d-axis reference voltage is output through the droop control V dref , and the q-axis reference voltage V qref is 0, through the voltage and current loop, the modulation signal for controlling the output voltage of the converter is obtained, so as to control the output voltage and output power. When the grid load increases and the frequency decreases, the Flag signal will not trigger, the power angle feedback is invalid, and the frequency feedback acts alone, at this time will be less than , the initial reference power of the front stage is fed back to , the reference power of the photovoltaic system in the front stage is controlled as , at this time, the active power is increased to maintain the system frequency stable. When the grid voltage drops, the Flag signal is triggered, the system introduces the power angle feedback, and the power angle increases, according to the power relationship of the improved control, the reference active power P ref is reduced, at this time, the equivalent deceleration area is increased, the acceleration area is reduced, and the transient stability of the system is improved.

[0089] In order to better understand the technical scheme of the present application, the following description is made:

[0090] The improved power backup control system based on frequency power angle feedback described in the application refers to a system composed of a photovoltaic assembly, a DC / DC boost circuit and a grid-connected converter (hereinafter referred to as the system based on frequency power angle feedback of the improved power backup control system). The source side is a non-energy storage photovoltaic assembly PV, the converter refers to a power device for converting DC into AC, and the power backup control can effectively support the grid frequency. Through the improvement of the power backup control by frequency and power angle feedback, the transient stability of the converter can be improved.

[0091] As shown in Figure 2 , the photovoltaic assembly PV output is connected in parallel with a large capacitor C dc through a DC / DC boost circuit, and then connected in parallel with a grid-connected converter, and the converter output is connected in parallel with a filter inductor , a filter capacitor and a line impedance to access the point of common coupling and then connected to the grid.

[0092] In order to support the grid frequency, the non-energy storage photovoltaic grid-connected system adopts power backup control, but due to its neglect of system transient stability, the synchronous ring is easy to lose synchronization and cause system transient instability in the event of a fault, such as voltage sag fault, which will affect the service life of the converter and weaken the stability of the non-energy storage photovoltaic assembly grid-connected system.

[0093] Therefore, the application provides an improved control method, which adds a feedback frequency difference link and a power angle feedback link on the basis of the original power backup control, i.e. the front stage and the rear stage, controls the reference power of the front stage photovoltaic system in the event of a fault, and provides a control parameter design method. The front stage uses power control to adjust the photovoltaic output power to reach the reference value. The rear stage uses the VSG (virtual synchronous machine) control to control the converter DC side voltage and realize grid synchronization in the power synchronization mode; the improved control changes the reference power of the front stage photovoltaic system and the active power output by the converter through the feedback frequency difference and the power angle under voltage sag fault to enhance the control effect.

[0094] The front stage of the application calculates the photovoltaic output power by continuously detecting the voltage and current output by the photovoltaic assembly, takes the difference between the initial reference power of the photovoltaic system and the calculated photovoltaic instantaneous power as the input, obtains the PWM control signal of the DC / DC boost circuit through the power control loop, and controls the output power of the photovoltaic assembly to be the reference power. In combination with Figure 2 , the voltage and current are collected from the output end of the photovoltaic assembly, the initial reference power and the calculated photovoltaic instantaneous power The difference is input, and the value obtained after PI link is subtracted from The difference is input, and the value obtained after PI link is subtracted from

[0095] The latter stage of the application collects the voltage And the current Of the grid-connected point PCC 、 、 、 , The active power and reactive power output by the converter are obtained through calculation P 、 Q , The input direct voltage loop is controlled to be the reference voltage , and the VSG input reference power is obtained P ref , P And P ref The phase output by the VSG synchronization loop Traces the phase of the detected voltage, thereby controlling the phase of the output voltage. The reactive power Q The d-axis reference voltage output by the droop control V dref , V qref The q-axis reference voltage is 0, and the PWM modulation signal for controlling the output voltage of the converter is obtained through the voltage loop and the current loop, thereby controlling the output voltage and the output power. In combination Figure 2 , in the direct voltage loop, As input, through a PI link multiplied by the direct voltage coefficient The active power reference value P ref , P ref The difference between the actual active power P And the output phase Obtained through the synchronization loop, simulating the second-order rotor equation of the synchronous machine, satisfies the following relationship:

[0096]

[0097] In the formula, Is the VSG output phase, Is the rated frequency of the grid, J Is the inertia, Is the damping, Is the output reference power, Is the active power output by the converter, This is the output frequency of the synchronization loop.

[0098] The improved control principle of this invention is as follows: Figure 3 As shown, the optimal control parameters are obtained based on the aforementioned iterative calculations. k i and k d By increasing the output frequency of the synchronization loop and the rated frequency of the power grid The difference is used as input, fed back to the front-end and back-end control sections through a proportional circuit, to change the reference power of the photovoltaic system and the output power of the converter, thereby enhancing the system's frequency support capability; under fault conditions, the synchronous loop output frequency will be... and the rated frequency of the power grid The integral angle of the difference As input, this is fed back to the front-end and back-end control sections to adjust the reference power of the photovoltaic system and the output power of the converter. This effectively increases the deceleration area and decreases the acceleration area, improving the system's transient stability.

[0099] In this invention, the improvement of system transient stability can be described using the equal area method.

[0100] As shown in Figure 4(a), when a minor fault occurs in the system, if the system does not improve control, the maximum acceleration area at the time of the fault is S1 in Figure 4(a), and the maximum deceleration area is S4 in Figure 4(a). The system is fault-cleared and returns to the stable equilibrium point SEP1 in Figure 4(a); if the system adopts improved control, the maximum acceleration area is S1 minus S2 in Figure 4(a). When the fault is cleared, there is a stable equilibrium point SEP1. At this point, due to the failure of the power angle feedback during fault clearing, the reference power returns to its initial value, and the system returns to the stable equilibrium point SEP1 after stabilization. The entire process effectively reduces the maximum acceleration area, improves the transient stability of the system, and increases the stability margin of the system. The increased margin can be expressed by the following formula.

[0101]

[0102] As shown in Fig. 4(b), if the fault degree deepens and the fault is not removed, if the system has no improved control, when the fault occurs, the system is unstable in the whole acceleration transient state; if the system uses improved control, the maximum acceleration area is S1, and the maximum deceleration area is S2 at this time, the whole process is equivalent to increasing the maximum deceleration area and reducing the maximum acceleration area, and there is no equilibrium point in the case of not using improved control, and there is a stable equilibrium point SEP2 and an unstable equilibrium point USEP1 after using improved control, the system will be stable at the new equilibrium point SEP2, the transient stability of the system is improved, the stability margin of the system is increased, and the increased stability margin can be expressed by the following formula.

[0103]

[0104] In the formula, V 1 is the voltage amplitude of the PCC point, is the voltage amplitude of the power grid after the fault, is the line impedance.

[0105] Here, based on the control method of the application, the power control and VSG control of Figure 2 , and the improved control of Figure 3 are used in the MATLAB / Simulink simulation platform to build the non-energy storage photovoltaic grid-connected system shown in Figure 2 , and the simulation parameters are shown in Table 1, wherein k pv , k iv are the proportional coefficient and integral coefficient of the PI regulation on the photovoltaic side, and k p , k ip are the proportional coefficient and integral coefficient of the PI regulation on the converter side.

[0106] Table 1, system simulation parameters

[0107]

[0108] Based on the system simulation parameters in Table 1, the active load of the system is increased at 1 second, and the frequency drops, and compared with the photovoltaic grid-connected system using traditional VSG control and the photovoltaic grid-connected system using improved control, the former is shown as a red curve in Figure 5 , and cannot increase the power to support the frequency stability of the system; the latter is shown as a blue curve in Figure 5As shown by the blue curve, the power of the converter is increased to support the system frequency, which shows that the improved control can support the system frequency stability and is beneficial to maintain the system stability. At 1s, the voltage dip fault occurs, and the voltage drops to 0.35 p.u. and lasts for 0.1s. Compared with the photovoltaic grid-connected system adopting the conventional VSG control and the photovoltaic grid-connected system adopting the improved control, the former shown in Fig. 6(a) oscillates and transient instability occurs; the latter shown in Fig. 6(b) maintains the system stability, which shows that the improved control improves the transient stability of the system and reduces the risk of damage to the converter.

[0109] In summary, the improved power backup control method and system based on frequency power angle feedback provided by the application prevent transient instability caused by the loss of synchronization of the synchronization ring when a fault occurs, effectively improve the ability of the no-place energy photovoltaic grid-connected system to maintain the synchronization of the synchronization ring when a fault occurs, and improve the transient stability of the system. The specific control steps are as follows: the front-stage control calculates the power of the photovoltaic module PV by continuously detecting the output voltage and current of the photovoltaic module PV, and sets the output as the reference power through the power control loop; the rear-stage control realizes the grid synchronization, DC voltage control and output power control of the grid-connected converter through the synchronization ring, DC voltage ring and reactive power ring respectively, and feeds back the frequency difference between the synchronization ring and the power grid through the proportional link to the input of the front-stage and rear-stage, to control the reference power of the front-stage photovoltaic and the rear-stage converter; under a fault, the integral power angle of the frequency difference between the synchronization ring and the power grid is fed back to the input of the front-stage and rear-stage, to control the reference power of the front-stage photovoltaic and the rear-stage converter, thereby improving the transient stability and frequency support capability of the system.

[0110] Other embodiments of the application will be apparent to those skilled in the art from consideration of the specification and practice of the embodiments disclosed herein. It is intended that the application be limited only by the scope of the claims, a which encompass all variations that come within the scope of the present application. It is intended that the application not be limited to the specific examples described and shown in the above description and the accompanying drawings. The scope of the application is limited only by the claims.

Claims

1. An improved power backup control method based on frequency power angle feedback, characterized in that, The method comprises: A front stage controls output power of a photovoltaic system through a power control loop; A back stage controls grid synchronization, DC side voltage and output power of a converter through a synchronization loop, a direct voltage loop and a voltage and current inner loop respectively; The improved control adjusts reference power of the back stage synchronization loop by feeding back a difference between an output frequency of the synchronization loop and a rated frequency of a power grid to an input of the synchronization loop, adjusts output power of the photovoltaic system by feeding back the difference between the output frequency of the synchronization loop and the rated frequency of the power grid to the front stage, and adjusts output power of the converter by feeding back an angle of attack to the front stage during a fault and feeding back the angle of attack to the input of the back stage synchronization loop; The improved control adjusts reference power of the back stage synchronization loop by feeding back a difference between an output frequency of the synchronization loop and a rated frequency of a power grid to an input of the synchronization loop; The improved control adjusts output power of the photovoltaic system by feeding back a difference between an output frequency of the synchronization loop and a rated frequency of a power grid to the front stage, comprising: An output frequency of the synchronization loop and a rated frequency of a power grid are obtained, and a difference between the output frequency of the synchronization loop and the rated frequency of the power grid is calculated as a frequency input value; The frequency input value is fed back to the input of the synchronization loop through a proportional link to adjust reference power of the back stage synchronization loop, and the frequency input value is fed back to the input of the synchronization loop through an angle of attack feedback switch through an integral link to adjust reference power of the back stage synchronization loop; The frequency input value is fed back to the front stage through a proportional link to adjust output power of the photovoltaic system, and the frequency input value is fed back to the front stage through an angle of attack feedback switch through an integral link to adjust reference power of the photovoltaic system; A trigger mark and a switch state of the angle of attack feedback switch are determined according to a voltage of a point of common coupling.

2. The improved frequency power angle feedback based power reserve control method of claim 1, wherein, A relationship between the reference power of the synchronization loop and the frequency input value is: wherein is the actual reference power of the feedback slave loop, is the initial reference power of the slave loop, is the power value of the frequency feedback change, is the power value of the power angle feedback change, is the frequency feedback proportional factor of the improvement control, is the power angle feedback integral factor of the improvement control, is the output frequency of the slave loop, is the grid rated frequency, is the power angle, is the Laplace operator.

3. The improved frequency power angle feedback based power reserve control method of claim 2, wherein, A relationship between the reference power of the photovoltaic system and the frequency input value is: wherein is the actual reference power of the feedback photovoltaic system, is the initial reference power of the photovoltaic system.

4. The improved frequency power angle feedback based power reserve control method of claim 2, wherein, The trigger mark and the switch state of the angle of attack feedback switch are: In the formula, Flag is the trigger mark of the power angle feedback switch, is the public coupling point voltage, is 0.9 times of the rated voltage of the power grid; when the power angle is greater than 0, the switch state is closed; when the power angle is less than 0, the switch state is open.

5. The improved frequency power angle feedback based power reserve control method of claim 3, wherein, A frequency feedback proportional coefficient of the improved control is: In the formula, K f represents the active frequency modulation coefficient.

6. The improved frequency power angle feedback based power reserve control method of claim 3, wherein, A design process of an angle of attack feedback integral coefficient of the improved control is: Step SB1, during a fault, an angle of attack is set as 0 as an input; Step SB2, a point of common coupling current is calculated based on the following formula: wherein, I t is the point of common coupling current, V t is the point of common coupling voltage, V g is the grid voltage, X g is the line impedance, k Q is the reactive droop coefficient, V 0 is the reactive loop voltage reference value, Q ref is the reactive reference value; Step SB3, judging the size relation between the common coupling point current calculated in step SB2 and the maximum current value I max If I t is less than I max , the power angle is increased by a first preset value and then the step SB2 is returned to recalculate I t , if I t is greater than or equal to I max , the power angle at this time is determined. Step SB4, whether the angle of attack decreases is determined, if the angle of attack decreases, a reactive droop coefficient is increased by a second preset value and then the step SB1 is returned, if the angle of attack does not decrease, the reactive droop coefficient at this time is determined; Step SB5, after the angle of attack and the reactive droop coefficient are determined, the angle of attack feedback integral coefficient of the improved control is calculated as: In the formula, P P is the active power output by the converter.

7. The improved frequency power angle feedback based power reserve control method of claim 6, wherein, The first preset value is 0.001, and the second preset value is 0.0001.

8. The improved frequency power angle feedback based power reserve control method of claim 1, wherein, The fault comprises voltage drop.

9. An improved power backup control system based on frequency power angle feedback, characterized by, The photovoltaic system comprises a photovoltaic component, a DC boost circuit, a converter, a point of common coupling and a control module, an output of the converter is connected to the point of common coupling through a filtering inductor, a filtering capacitor and a line impedance and then connected to a power grid, and the control module is configured to perform the method according to any one of claims 1 to 8.

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