Network construction type photovoltaic energy storage control system and method for improving supporting capacity of power grid

Abstract

The invention provides a network construction type photovoltaic energy storage control system and method for improving the supporting capacity of a power grid. The system comprises a data sampling module, a reference power generation module, a VSG module, a voltage feed-forward compensation and virtual impedance control module, a voltage and current closed-loop control module and a PWM generation module. According to the invention, modeling is carried out on a network construction type photovoltaic energy storage control model, then virtual impedance is introduced into VSG reactive voltage control, and a power grid voltage feed-forward compensation model is provided according to a vector diagram, so that the value of the virtual impedance is determined to weaken the influence of voltage drop when a power grid fails and suppress overcurrent. The problem that a photovoltaic energy storage face is insufficient in low-voltage ride-through capacity is solved.

Description

Technical Field

[0001] The present invention relates to the field of grid-forming photovoltaic energy storage, and specifically to a grid-forming photovoltaic energy storage control system and method for enhancing the grid support ability. Background Art

[0002] In recent years, new energy power generation technologies have developed rapidly. The proportion of wind power and photovoltaic power in the power grid has increased significantly, while the proportion of traditional synchronous generators has gradually decreased. Due to the intermittent and fluctuating characteristics of new energy power generation, the large-scale grid connection of new energy power generation will affect the power quality and grid stability. Therefore, it is necessary to combine energy storage units to make up for the above deficiencies. In addition, the large-scale replacement of synchronous generators makes it impossible for them to continue to provide inertia for the power grid, resulting in a low strength of the power grid system, easy fluctuations, weak anti-interference ability, and the risk of overall collapse and large-scale power outages. Therefore, the grid-forming technology using virtual synchronous generators has emerged. In terms of operating principle, it provides a voltage source with built-in power synchronization to the system connection point through an inverter via an impedance, realizing the support of equivalent inertia and system strength. However, the short-circuit characteristics of traditional virtual synchronous generator technology under grid voltage sag and voltage recovery conditions are still not comparable to those of traditional synchronous generators.

[0003] The existing technologies have limited functions in aspects such as power synchronization, overcurrent suppression, and handling asymmetric grid faults, lack complete functions, cannot provide transient voltage support for the power grid, do not have the ability of low voltage ride-through, and are prone to equipment disconnection when symmetric or asymmetric faults occur on the grid side. Therefore, a solution is needed in the existing technologies to solve these problems. Summary of the Invention

[0004] The present invention provides a grid-forming photovoltaic energy storage control system and method for enhancing the grid support ability. It models the grid-forming photovoltaic energy storage control model, then introduces virtual impedance in the VSG reactive power voltage control, and proposes a grid voltage feed-forward compensation model based on the vector diagram, thereby determining the value of the virtual impedance to weaken the influence of voltage drop during grid faults and suppress overcurrent, for solving the problem of insufficient low voltage ride-through ability of photovoltaic energy storage.

[0005] A grid-forming photovoltaic energy storage control system for enhancing the grid support ability includes: a data sampling module, a reference power generation module, a VSG module, a voltage feed-forward compensation and virtual impedance control module, a voltage-current closed-loop control module, and a PWM generation;

[0006] The data acquisition module is used to collect the AC bridge arm voltage of the grid-side inverter 、the connection point voltage of the grid-side inverter and the connection point current of the grid-side inverter ;

[0007] The reference power generation module is used to, when a grid fault occurs, correct based on the active power reference value required by the grid and the reactive power reference value to obtain the updated active power reference value and the reactive power reference value ;

[0008] The VSG module is used to establish a VSG model of the grid-side converter, process the updated active power reference value and the reactive power reference value to obtain the reference voltage through the active power and reactive power equations;

[0009] The voltage feed-forward compensation and virtual impedance control module is used to generate a feed-forward compensation voltage according to the collected grid-side inverter connection point voltage and the grid-side inverter connection point current . The feed-forward compensation voltage is added to the reference voltage to generate a new reference voltage ;

[0010] The voltage and current closed-loop control module is used to construct a voltage and current closed-loop control, and calculate the duty ratio value according to the generated new reference voltage , the grid-side inverter connection point current collected by the data acquisition module , and the grid-side inverter AC bridge arm voltage , and input it to the PWM generator;

[0011] The PWM generator is used to generate a PWM signal according to the duty ratio value input by the voltage and current closed-loop control module, and input the PWM signal into the grid-side inverter.

[0012] Furthermore, when a grid fault occurs, the reference power generation module corrects based on the active power reference value required by the grid and the reactive power reference value to obtain the updated active power reference value and the reactive power reference value , including:

[0013] The required active and reactive power reference values of the system are and respectively, and the reactive power to be corrected is as shown in the following formula:

[0014]

[0015] where, is the reactive power correction coefficient, is the change in voltage sag, is the rated voltage; the active power to be corrected is as shown in the following formula:

[0016]

[0017] The reference value of the corrected active power and the reference value of the reactive power are calculated by the following formula:

[0018]

[0019]

[0020] where 、 are the reference values of the corrected active and reactive powers.

[0021] Furthermore, the VSG module establishes a VSG model for the grid-side converter, and processes the updated reference value of the active power and the reference value of the reactive power to obtain the reference voltage V vref , including:

[0022] Model the virtual synchronous generator of the grid-side converter of the photovoltaic energy storage system Set the series inductance impedance on the transmission line as , is the amplitude of the AC bridge arm voltage of the grid-side inverter, is the amplitude of the grid connection point voltage of the grid-side inverter, and the active power of the grid-side inverter and the reactive power are respectively:

[0023]

[0024] Define and represent the command values of the active power and the reactive power respectively, then the equation of the virtual synchronous generator is:

[0025]

[0026] where, is the moment of inertia, is the damping coefficient, and are the rated angular velocity and the actual angular velocity respectively, is the phase angle output by the VSG, and the reactive power-voltage regulation equation of the virtual synchronous generator is expressed as:

[0027]

[0028] Among them, is the reactive power equivalent inertia coefficient, is the reactive power voltage regulation coefficient, is the rated value of the output voltage, is the actual value of the output voltage, is the voltage amplitude output by the VSG; the output of the VSG is the vector , whose amplitude is and the phase angle is .

[0029] Furthermore, the voltage feed-forward compensation and virtual impedance control module generates the feed-forward compensation voltage and the grid-side inverter grid-connected current based on the collected grid-side inverter grid-connected voltage . The feed-forward compensation voltage is added to the reference voltage to generate a new reference voltage , including:

[0030] Step 4-1: Voltage feed-forward compensation

[0031] According to the requirements of the VSG technical rules, the dynamic reactive current generated by the VSG should track the AC-side voltage change in real time and meet the requirements of the following formula:

[0032]

[0033] Among them is 's incremental value, is the effective value of the dynamic reactive current, represents the effective value of the dynamic reactive current before the VSG enters the fault, , are the reactive current coefficients respectively, is between 1.5 and 2.5, is between 0 and 1.5, is the ratio of the actual positive sequence component on the AC side of the VSG to the rated voltage, is the current value of the rated current generated on the AC side of the VSG;

[0034] The grid current decreases with the change of and under the control of the VSG, 's phase is denoted as . The phase difference between is denoted as ; when suddenly changes, the VSG will act as a controllable voltage source, while the grid-connected inverter will maintain the magnitude of the internal voltage , so the current will change suddenly, and the fault current will pass through and transfer to ; all quantities during the voltage dip are denoted by adding the subscript ; in order to provide reactive power support to the grid, needs to meet the required by the grid code, so ; the transmission current between and is controlled by the following factors:

[0035]

[0036] Transforming the above equation into the dq coordinate system, we get:

[0037]

[0038] where and are the dq components of , and are the dq-axis components of ; the fault surge current and in the dq reference frame are obtained from the following equation:

[0039]

[0040] According to the obtained by the data acquisition module, add a feedforward compensation to the at the output end of the control diagram to cancel out the rightmost term in the above equation; the compensation voltage of the d-axis is calculated by the following equation:

[0041]

[0042] The compensation voltage of the q-axis is calculated by the following equation:

[0043]

[0044] Step 4-2: Virtual impedance control

[0045] Introduce virtual impedance:

[0046]

[0047] Among them, and are the virtual resistance and virtual inductance respectively;

[0048] Set the voltage drop in the virtual impedance to , then:

[0049]

[0050] Where is 's rated value, and use the feed-forward compensation voltage to compensate , then the compensated expression is:

[0051]

[0052] Then derive the virtual impedance value:

[0053]

[0054] Step 4-3: Reference voltage update

[0055] The reference voltage generated in Step 3 After dq transformation, it is obtained:

[0056]

[0057] Add the compensation amounts of the reference voltages generated in Steps 4-1 and 4-2 to the above formula, and get

[0058]

[0059] , That is the updated reference voltage;

[0060] After the reference voltage is updated, the change amount of the original reference phase angle output by the VSG is as follows:

[0061]

[0062] The corrected phase angle is:

[0063] .

[0064] Furthermore, the voltage-current closed-loop control module constructs a voltage-current closed-loop control, based on the generated new reference voltage , the grid-side inverter grid-connected point current collected by the data acquisition module, and the grid-side inverter AC bridge arm voltage Calculate the duty cycle value and input it into the PWM generator, including:

[0065] According to the structure topology diagram of the photovoltaic energy storage system, obtain:

[0066]

[0067] Among them , ; 、 、 After transformation, they are respectively 、 、 、 、 、 ; The mathematical model of the energy storage grid-side inverter after transformation is:

[0068]

[0069] The inverter adopts closed-loop control, and its voltage outer-loop equation is:

[0070]

[0071] The current inner-loop equation is:

[0072]

[0073] Among them 、 、 are respectively the proportional coefficient and integral coefficient of the voltage outer-loop and current inner-loop control, 、 are the current reference values; According to 、 in the above formula, and the phase angle after step correction, through the modulation of the PWM module, generate the duty cycle information of the inverter, and then control the grid-side inverter.

[0074] A grid-forming photovoltaic energy storage control method for improving the grid support ability, including:

[0075] Collect the AC bridge arm voltage of the grid-side inverter, the grid connection point voltage of the grid-side inverter, and the grid connection point current of the grid-side inverter;

[0076] When a grid fault occurs, based on the active power reference value required by the grid, the reactive power reference value Make corrections to obtain the updated active power reference value and the reactive power reference value ;

[0077] Establish a grid-side converter VSG model and process the updated active power reference value and the reactive power reference value to obtain the reference voltage through the active power and reactive power equations ;

[0078] Generate a feed-forward compensation voltage based on the collected grid-side inverter connection point voltage and the grid-side inverter connection point current . The feed-forward compensation voltage is added to the reference voltage to generate a new reference voltage ;

[0079] Construct a voltage-current closed-loop control. Based on the generated new reference voltage , the grid-side inverter connection point current collected by the data acquisition module , and the grid-side inverter AC bridge arm voltage calculate the duty cycle value and input it into the PWM generator;

[0080] Generate a PWM signal according to the duty cycle value input by the voltage-current closed-loop control module and input the PWM signal into the grid-side inverter.

[0081] Furthermore, when a grid fault occurs, based on the required active power reference value and the reactive power reference value make corrections to obtain the updated active power reference value and the reactive power reference value , including:

[0082] The required active and reactive power reference values of the system are respectively and . The reactive power to be corrected is as shown in the following formula:

[0083]

[0084] where is the reactive power correction coefficient, is the change in voltage dip, is the rated voltage; the active power to be corrected is as shown in the following formula:

[0085] ​​​​

[0086] The corrected active power reference value and the reactive power reference value are calculated by the following formula:

[0087]

[0088]

[0089] where 、 are the reference values of the corrected active and reactive powers.

[0090] Furthermore, the virtual synchronous generator (VSG) model of the grid-side converter is established, and the updated active power reference value and the reactive power reference value are processed to obtain the reference voltage V vref , including:

[0091] Model the virtual synchronous generator of the grid-side converter of the photovoltaic energy storage system, and set the series inductance impedance on the transmission line as , , is the amplitude of the AC bridge arm voltage of the grid-side inverter, is the amplitude of the grid connection point voltage of the grid-side inverter. The active power and the reactive power of the grid-side inverter are respectively:

[0092]

[0093] Define and to represent the command values of active power and reactive power respectively. Then the equation of the virtual synchronous generator is:

[0094]

[0095] where is the moment of inertia, is the damping coefficient, and are the rated angular velocity and the actual angular velocity respectively, is the phase angle output by the VSG. The reactive power-voltage regulation equation of the virtual synchronous generator is expressed as:

[0096]

[0097] where is the reactive power equivalent inertia coefficient, is the reactive power voltage regulation coefficient, is the rated value of the output voltage, is the actual value of the output voltage, is the voltage amplitude output by the VSG; the output of the VSG is a vector , and its amplitude is and the phase angle is .

[0098] Furthermore, the feed-forward compensation voltage Vc is generated according to the grid-side inverter connection point voltage and the grid-side inverter connection point current . The feed-forward compensation voltage Vc is added to the reference voltage V vref to generate a new reference voltage , including:

[0099] Step 4-1: Voltage feed-forward compensation

[0100] According to the requirements of the VSG technical rules, the dynamic reactive current generated by the VSG should track the AC-side voltage change in real time and meet the requirements of the following formula:

[0101]

[0102] where is 's incremental value, is the effective value of the dynamic reactive current, represents the effective value of the dynamic reactive current before the VSG enters the fault, , are the reactive current coefficients respectively, is between 1.5 and 2.5, is between 0 and 1.5, is the ratio of the actual positive-sequence component of the VSG AC side to the rated voltage, is the current value of the rated current generated by the VSG AC side;

[0103] Grid current decreases with and changing in the control of the VSG, The phase of is denoted as , The phase difference between ; when suddenly changes, the VSG will act as a controllable voltage source, while the grid-connected inverter will maintain the amplitude of the internal voltage , so the current will change suddenly, and the fault current will pass through and transfer to ; All quantities during the voltage dip are added with a subscript on the original basis​ It is shown that, in order to provide reactive power support to the power grid, the magnitude of should meet the requirements of the power grid specifications. Therefore, and the transmission current between is controlled by the following factors:

[0104]

[0105] Transforming the above formula into the dq coordinate system, we get:

[0106]

[0107] where and are the dq components of ; and are the dq-axis components of ; the fault surge current in the dq reference system and are obtained from the following formula:

[0108]

[0109] According to obtained by the data acquisition module, a feed-forward compensation is added to

[0110]

[0111] at the output end of the control diagram to cancel out the rightmost term in the above formula; the compensation voltage of the d-axis is calculated by the following formula:

[0112]

[0113] Step 4-2: Virtual impedance control

[0114] Introduce virtual impedance:

[0115]

[0116] where and are the virtual resistance and virtual inductance respectively;

[0117] Set the voltage drop in the virtual impedance to , then:

[0118]

[0119] where For the rated value, compensate with the feed-forward compensation voltage compensation , then the compensated expression is:

[0120]

[0121] Then derive the virtual impedance value:

[0122]

[0123] Step 4-3: Reference voltage update

[0124] The reference voltage generated in Step 3 After dq transformation, it is obtained:

[0125]

[0126] Add the compensation amounts of the reference voltages generated in Steps 4-1 and 4-2 to the above formula, and we get

[0127]

[0128] 、 That is the updated reference voltage;

[0129] After the reference voltage is updated, the change amount of the original reference phase angle output by the original VSG is as follows:

[0130]

[0131] The corrected phase angle is:

[0132] .

[0133] Furthermore, for the constructed voltage-current closed-loop control, according to the generated new reference voltage , the grid-side inverter connection point current collected by the data acquisition module, the grid-side inverter AC bridge arm voltage, calculate the duty cycle value and input it into the PWM generator, including:

[0134] According to the photovoltaic energy storage system structure topology diagram, we get:

[0135]

[0136] where , ; 、 、 After the transformation, they are respectively , , , , , ; The mathematical model of the energy storage grid-side inverter after transformation is:

[0137]

[0138] The inverter adopts closed-loop control, and its outer voltage loop equation is:

[0139]

[0140] The inner current loop equation is:

[0141]

[0142] Where , , are respectively the proportional coefficient and integral coefficient of the outer voltage loop and inner current loop control, , is the current reference value; According to the , in the above formula, and the phase angle after step correction, through the modulation of the PWM module, the duty cycle information of the inverter is generated, and then the grid-side inverter is controlled.

[0143] Compared with the existing technology, the present invention has the following beneficial effects::

[0144] 1. The present invention proposes a virtual impedance and voltage feed-forward control strategy for the energy storage converter, which corrects the voltage control signal in real time, determines the virtual impedance parameters, and realizes the redistribution of active power and reactive power, and finally improves the low voltage ride-through ability of the energy storage device.

[0145] 2. The grid-forming type photovoltaic energy storage control method for improving the grid support ability proposed by the present invention, aiming at the problems of uncertain output of photovoltaic power generation and inability to provide support for the grid, adopts a DC-side energy storage scheme, and adopts a virtual synchronization and control strategy in the energy storage converter, which greatly improves the grid support ability of the photovoltaic energy storage system. BRIEF DESCRIPTION OF THE DRAWINGS

[0146] Figure 1 is the topological structure diagram of the photovoltaic energy storage system of the present invention.

[0147] Figure 2 is the flow chart of the grid-side converter control system of the present invention.

[0148] Figure 3 is the control block diagram of the grid-side converter control system of the present invention.

[0149] Figure 4 is the power loop control block diagram.

[0150] Figure 5 is the voltage feed-forward compensation and virtual impedance control block diagram of the present invention.

[0151] Figure 6 is the voltage vector diagram before and during the voltage dip fault.

[0152] Figure 7 is under fault conditions the equivalent circuit diagrams before (top) and after (bottom) being replaced. Specific embodiments

[0153] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of 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 based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention.

[0154] Please refer to Figure 1 , a photovoltaic energy storage system of the present invention, whose topology includes a photovoltaic power generation device, a DC-side energy storage unit, a grid-side converter, and a grid-side converter. Among them, the DC-side energy storage unit is connected in parallel with the photovoltaic power generation device through the grid-side converter, and then connected to the grid-side converter. After being inverted by the grid-side converter, it is connected to the power grid through the point of common coupling.

[0155] The DC-side energy storage unit is composed of energy storage batteries and is used to provide power support when the photovoltaic power generation device generates insufficient power;

[0156] The grid-side converter adopts VSG control, which can make the photovoltaic energy storage system have the characteristics of a synchronous generator and provide inertia and damping support for the power grid;

[0157] The grid-side converter is a DC-DC boost converter and is used to control the DC voltage;

[0158] The grid-side converter adopts grid-forming control to provide voltage, inertia, and damping support for the power grid.

[0159] Such as Figure 3As shown in the figure, it is a grid-forming photovoltaic energy storage control system for enhancing the power grid support ability and connected to the grid-side converter according to an embodiment of the present invention, including a data sampling module, a PWM generator, a voltage and current closed-loop control module, a voltage feed-forward compensation and virtual impedance control module, a reference power generation module, and a VSG module;

[0160] The data acquisition module is used to acquire the AC bridge arm voltage of the grid-side inverter , the grid connection point voltage of the grid-side inverter and the grid connection point current of the grid-side inverter ;

[0161] The reference power generation module is used to correct the power value that needs to be adjusted in real time when a grid fault occurs. When a grid fault occurs, the reference power generation module is based on the active power , reactive power required by the power grid to perform correction, and obtain the updated active power reference value and reactive power reference value , so as to reduce the influence of inrush current during voltage sag and provide voltage support for the power grid. The updated active power reference value and reactive power reference value of the reference power generation module are input to the VSG module.

[0162] The VSG module is used to process the updated active power reference value ]>and reactive power reference value , and obtain the reference voltage V vref through the active power and reactive power equations, and the process reference Figure 4 . This reference voltage V vref will be combined with the output Vc of the voltage feed-forward compensation and virtual impedance control module and then input to the voltage and current closed-loop control module;

[0163] The voltage feed-forward compensation and virtual impedance control module is as Figure 5 shown, and is used to generate the feed-forward compensation voltage Vc according to the acquired grid connection point voltage of the grid-side inverter and the grid connection point current of the grid-side inverter. The feed-forward compensation voltage Vc is added to the reference voltage V vref to generate a new reference voltage , so as to weaken the influence during voltage sag and suppress overcurrent;

[0164] The voltage and current closed-loop control module is used to ensure the stable operation of the grid-connected inverter. This module can be based on the generated new reference voltage , the grid connection point current of the grid-side inverter acquired by the data acquisition module , the AC bridge arm voltage of the grid-side inverter Calculate the duty cycle value and input it into the PWM generator;

[0165] The PWM generator is used to generate a PWM signal according to the duty cycle value input by the voltage-current closed-loop control module and input the PWM signal into the grid-side inverter.

[0166] As Figure 2 shown, an embodiment of the present invention provides a grid-forming photovoltaic energy storage control method for enhancing the grid support ability, including the following steps:

[0167] Step 1: Data acquisition

[0168] Collect the AC bridge arm voltage of the grid-side inverter , the grid connection point voltage of the grid-side inverter and the grid connection point current of the grid-side inverter .

[0169] Step 2: Active and reactive power correction: When a grid fault occurs, based on the required active power reference value and reactive power reference value of the grid, perform correction to obtain the updated active power reference value and reactive power reference value .

[0170] To cope with low voltage ride-through, the grid needs to obtain additional reactive power to support the grid voltage. Therefore, the reference power generation module needs to perform correction based on the required active and reactive powers of the grid. The required active and reactive power reference values of the system are respectively and , and the reactive power to be corrected is shown in the following formula:

[0171]

[0172] where is the reactive power correction coefficient, is the change in voltage dip, is the rated voltage. The active power to be corrected is shown in the following formula:

[0173]

[0174] The corrected active power reference value and reactive power reference value are calculated by the following formula:

[0175]

[0176]

[0177] where and are the reference values of the corrected active and reactive powers.

[0178] Step 3: Establish the grid-side converter VSG model and process the updated active power reference value and reactive power reference value to obtain the reference voltage V vref .

[0179] Model the virtual synchronous generator (VSG) of the grid-side converter in the Figure 1 photovoltaic energy storage system. Assume that the series inductance impedance on the transmission line is , is the amplitude of the AC bridge arm voltage of the grid-side inverter, is the amplitude of the grid-connected point voltage of the grid-side inverter. The active power and reactive power of the grid-side inverter are respectively: Define

[0180]

[0181] and as the command values of active power and reactive power respectively. Then the equation of the virtual synchronous generator is: where

[0182]

[0183] is the moment of inertia, is the damping coefficient, and are the rated angular velocity and the actual angular velocity respectively, is the phase angle output by the VSG. The reactive power-voltage regulation equation of the virtual synchronous generator can be expressed as: where

[0184]

[0185] is the reactive equivalent inertia coefficient, is the reactive power voltage regulation coefficient, is the rated value of the output voltage, is the actual value of the output voltage, is the amplitude of the voltage output by the VSG. Therefore, the output of the VSG is the vector , whose amplitude is , and the phase angle is .​

[0186] Step 4: Voltage feedforward compensation based on virtual impedance: According to the grid-side inverter connection point voltage collected and the grid-side inverter connection point current generate a feedforward compensation voltage . The feedforward compensation voltage is added to the reference voltage to generate a new reference voltage .

[0187] The specific steps of Step 4 are as follows:

[0188] Step 4-1: Voltage feedforward compensation

[0189] According to the requirements of the VSG technical rules, the dynamic reactive current generated by the VSG should track the AC-side voltage change in real time and meet the requirements of the following formula:

[0190]

[0191] where is 's increment value, is the effective value of the dynamic reactive current, represents the effective value of the dynamic reactive current of the VSG before entering the fault, , are the reactive current coefficients respectively, is between 1.5 and 2.5, is between 0 and 1.5, is the ratio of the actual positive-sequence component of the VSG AC side to the rated voltage, is the current value of the rated current generated by the VSG AC side.

[0192] Figure 6 is the voltage vector diagram before and after the voltage dip, revealing the relationship between , and the impedance voltage before and during the fault. The grid current changes with and under the control of the VSG. The phase of is denoted as , and the phase difference between is denoted as . When suddenly changes, the VSG will act as a controllable voltage source, and the grid-connected inverter will maintain the amplitude of the internal voltage , so the current will change suddenly, and the fault current will pass through and transfer to All quantities during the voltage dip are subscripted on the original basis. To provide reactive power support to the power grid, the magnitude of needs to meet the specified by the grid code. Therefore, and the transmission current between is controlled by the following factors:

[0193]

[0194] Transforming the above equation to the dq coordinate system gives

[0195]

[0196] where and are the dq components of , and and are the dq-axis components of . The fault surge current and in the dq reference frame are obtained from the following equation:

[0197]

[0198] From the rightmost term of the above equation, it can be seen that the fault surge current is related to . Therefore, the fault current response can be intervened by adding a virtual impedance and voltage compensation. Here, according to the obtained by the data acquisition module, a feedforward compensation is added to the at the output of the control diagram to cancel out the rightmost term of the above equation. Therefore, the compensation voltage on the d-axis is calculated by the following equation

[0199]

[0200] The compensation voltage on the q-axis is calculated by the following equation

[0201]

[0202] Step 4-2: Virtual impedance control

[0203] Introduce a virtual impedance:

[0204]

[0205] where and are the virtual resistance and virtual inductance respectively.

[0206] Assume that the voltage drop in the virtual impedance is , then

[0207]

[0208] where is 's rated value. Use the feed - forward compensation voltage to compensate , then the compensated expression is:

[0209]

[0210] Then the virtual impedance value can be derived:

[0211]

[0212] Step 4 - 3: Reference voltage update

[0213] The reference voltage generated in Step 3, after dq transformation, gets the following result

[0214]

[0215] Add the compensation amounts of the reference voltages generated in Step 4 - 1 and Step 4 - 2 to the above formula, we get

[0216]

[0217] , which is the updated reference voltage.

[0218] After the reference voltage is updated, the original reference phase angle output by the original VSG will also change, and its change amount is as follows:

[0219]

[0220] Therefore, the corrected phase angle is:

[0221]

[0222] In this step, the equivalent circuits before and after voltage feed - forward compensation and virtual impedance control are as Figure 7 shown.

[0223] Step 5: Construct a voltage - current closed - loop control. According to the generated new reference voltage , the grid - connected inverter grid - side current collected by the data acquisition module, and the grid - side inverter AC bridge arm voltage , calculate the duty cycle value and input it into the PWM generator.

[0224] According to Figure 1 the structural topology diagram of the photovoltaic energy storage system, it can be obtained that:

[0225]

[0226] Among them , . 、 、 After passing through transformation, they are respectively 、 、 、 、 、 . The mathematical model of the energy storage grid-side inverter after transformation is:

[0227]

[0228] The inverter adopts closed-loop control, and its voltage outer-loop equation is:

[0229]

[0230] The current inner-loop equation is:

[0231]

[0232] Among them 、 、 are respectively the proportional coefficient and integral coefficient of the voltage outer-loop and current inner-loop control, 、 is the current reference value; According to 、 in the above formula, as well as the corrected phase angle obtained in step 4, the duty cycle information of the inverter can be generated through the modulation of the PWM module, and then the grid-side inverter can be controlled.

[0233] Step 6: Energy storage reference power value selection

[0234] In order to achieve the purpose of improving the power grid support ability, the photovoltaic energy storage system needs to ensure a stable active power output. When the sunlight is sufficient, the output power of the photovoltaic panel is greater than the active power output of the photovoltaic energy storage system VSG. The excess power generated by the photovoltaic will be stored in the energy storage system. Therefore, the input power of the energy storage device is , and at this time .

[0235] When the sunlight is insufficient, the power output of the photovoltaic system will drop significantly. At this time, the energy storage system needs to cooperate with the photovoltaic device to maintain the active power output during the low voltage ride-through period. , at this time, the output power of the photovoltaic panel is less than , that is .

[0236] Users should establish appropriate control strategies according to factors such as the specific hardware topology of the energy storage converter and the energy storage capacity, and as the reference power of the energy storage device, input it into the control strategy of the energy storage device.

[0237] The present invention introduces virtual impedance and voltage feedforward control to the energy storage converter to improve the low voltage ride-through ability of the energy storage device; it realizes the coordinated control of photovoltaic energy storage and the improvement of the grid support ability of the grid-connected photovoltaic energy storage.

[0238] According to the simulation results, when the three-phase voltage drops to 60% of the rated voltage, the transient inrush current and steady-state current are suppressed after using this method. After using this method during the transient period, the overshoot of reactive power is greatly reduced, and the time required to return to the normal state after the fault is cleared is shorter.

[0239] As mentioned above, it is only the specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed by the present invention should be covered by the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims.

Claims

1. A grid-forming photovoltaic energy storage control system for enhancing the grid support capacity, characterized in that, Including: A data sampling module, a reference power generation module, a VSG module, a voltage feedforward compensation and virtual impedance control module, a voltage-current closed-loop control module, and a PWM generation; The data acquisition module is used to acquire the AC bridge arm voltage of the grid-side inverter , the grid connection point voltage of the grid-side inverter and the grid connection point current of the grid-side inverter ; The reference power generation module is used to correct based on the active power reference value and the reactive power reference value required by the power grid when a power grid fault occurs, so as to obtain the updated active power reference value and the reactive power reference value ; The VSG module is used to establish the VSG model of the grid-side converter and process the updated active power reference value and the reactive power reference value to obtain the reference voltage through the active power and reactive power equations ; The voltage feedforward compensation and virtual impedance control module is used to generate a feedforward compensation voltage according to the collected grid-connected voltage of the grid-side inverter and the grid-connected current of the grid-side inverter ; the feedforward compensation voltage is added to the reference voltage to generate a new reference voltage ; ; The voltage-current closed-loop control module is used to construct a voltage-current closed-loop control, and according to the generated new reference voltage , the grid-side inverter connection point current collected by the data acquisition module , the grid-side inverter AC bridge arm voltage calculate the duty cycle value and input it into the PWM generator; The PWM generator is used to generate a PWM signal according to the duty cycle value input by the voltage-current closed-loop control module and input the PWM signal into the grid-side inverter.

2. The grid-forming type photovoltaic energy storage control system for enhancing the grid support capacity according to claim 1, wherein: When a grid fault occurs, the reference power generation module corrects based on the active power reference value and the reactive power reference value to obtain the updated active power reference value and the reactive power reference value , including: The active and reactive power reference values required by the system are respectively and , and the reactive power to be corrected is as shown in the following formula:​ ； Among them, is the reactive power correction coefficient, is the change in voltage sag, is the rated voltage; the active power to be corrected is shown in the following formula: ； The corrected active power reference value and the reactive power reference value are calculated by the following formula: ； ； wherein and are the reference values of the corrected active and reactive powers.

3. The network-forming photovoltaic energy storage control system for enhancing the power grid support capacity according to claim 1, wherein: The VSG module establishes a VSG model of the grid-side converter and processes the updated active power reference value and the reactive power reference value to obtain the reference voltage V vref through the active power and reactive power equations, including: Model the virtual synchronous generator of the grid-side converter in the photovoltaic energy storage system and set the series inductance impedance on the transmission line to be , is the amplitude of the AC bridge arm voltage of the grid-side inverter, is the amplitude of the grid-connected voltage of the grid-side inverter, and the active power and reactive power of the grid-side inverter are respectively:[[]] ； Definition and represent the command values of active power and reactive power respectively, then the equation of the virtual synchronous generator is as follows: ； Among them, is the moment of inertia, is the damping coefficient, and are the rated angular velocity and the actual angular velocity respectively, is the phase angle output by the VSG. The reactive power-voltage regulation equation of the virtual synchronous generator is expressed as: ； Among them, is the reactive power equivalent inertia coefficient, is the reactive power voltage regulation coefficient, is the rated value of the output voltage, is the actual value of the output voltage, is the voltage amplitude output by the VSG; the output of the VSG is a vector , whose amplitude is and the phase angle is .

4. The grid-forming photovoltaic energy storage control system for enhancing the grid support capacity according to claim 1, wherein: The voltage feed-forward compensation and virtual impedance control module generates a feed-forward compensation voltage based on the collected grid-connected point voltage of the grid-side inverter and the grid-connected point current of the grid-side inverter The feed-forward compensation voltage is added to the reference voltage to generate a new reference voltage which includes: ​ Step 4-1: Voltage feedforward compensation According to the requirements of the VSG technical rules, the dynamic reactive current generated by the VSG should track the AC-side voltage change in real time and meet the requirements of the following formula: ； wherein is the incremental value, is the effective value of the dynamic reactive current, represents the effective value of the dynamic reactive current of the VSG before entering the fault, and are the reactive current coefficients respectively, is between 1.5 and 2.5, is between 0 and 1.5, is the ratio of the actual positive sequence component of the VSG AC side to the rated voltage, is the current value of the rated current generated by the VSG AC side; Grid current Decreases with the control of the VSG And Changes occur The phase of is denoted as , The phase difference between and is denoted as ; When Suddenly changes, the VSG will act as a controllable voltage source, and the grid-connected inverter will maintain the magnitude of the internal voltage Of the fault current Will pass through Transfer to All quantities during the voltage dip are denoted with a subscript On the original basis; To provide reactive power support to the grid The magnitude of needs to meet the requirements of the grid code , Therefore ; Between And The transmission current between is controlled by the following factors:​​ ； Transforming the above formula to the dq coordinate system, we get: ； Among them and are the dq components of and are the dq-axis components of; the fault surge current in the dq reference system and are obtained by the following formula: ； According to what is obtained by the data acquisition module Add a feedforward compensation to the output end of the control diagram to cancel out the rightmost term in the above formula; the compensation voltage of the d-axis is calculated by the following formula: , to cancel out the rightmost term in the above formula; the compensation voltage of the d-axis is calculated by the following formula: ； The compensation voltage of the q-axis is calculated by the following formula: ； Step 4-2: Virtual impedance control Introduce virtual impedance: ； Among them, and are a virtual resistor and a virtual inductor respectively; Set the voltage drop in the virtual impedance to be , then: ； wherein is rated value of, compensated by feedforward compensation voltage compensate , then the compensated expression is: ； Then derive the virtual impedance value: ； Step 4-3: Reference voltage update The reference voltage generated in Step 3 After dq transformation, it is obtained as follows: ； Adding the compensation amounts of the reference voltages generated in Step 4-1 and Step 4-2 to the above formula, we get ； , Namely, it is the updated reference voltage; After the reference voltage is updated, the change in the reference phase angle originally output by the VSG is as follows: is as follows: ； The corrected phase angle is: 。 5. The grid-forming photovoltaic energy storage control system for enhancing the grid support capacity according to claim 4, wherein: The voltage-current closed-loop control module constructs a voltage-current closed-loop control, and according to the generated new reference voltage , the grid-side inverter grid-connected point current collected by the data acquisition module , the grid-side inverter AC bridge arm voltage calculate the duty cycle value and input it into the PWM generator, including: According to the photovoltaic energy storage system structure topology diagram, we get: ； Among them , ; , , After transformation, they are respectively , , , , , ; The mathematical model of the energy storage grid-side inverter after transformation is: ； The inverter adopts closed-loop control, and its outer voltage loop equation is as follows: ； The current inner-loop equation is: ； Among them and and are the proportional coefficient and integral coefficient for the voltage outer loop and current inner loop controls respectively, and are the current reference values; according to and in the above formula, and the phase angle after step correction, through the modulation of the PWM module, the duty cycle information of the inverter is generated, and then the grid-side inverter is controlled.

6. A grid-forming photovoltaic energy storage control method for enhancing the grid support ability, characterized in that, Including: Collect the AC bridge arm voltage of the grid-side inverter , the grid connection point voltage of the grid-side inverter and the grid connection point current of the grid-side inverter ; When a power grid failure occurs, based on the active power reference value required by the power grid , the reactive power reference value is corrected to obtain the updated active power reference value and the reactive power reference value ; Establish the grid-side converter VSG model and process the updated active power reference value and the reactive power reference value to obtain the reference voltage V through the active power and reactive power equations vref ; According to the grid-side inverter connection point voltage collected and the grid-side inverter connection point current generate a feed-forward compensation voltage , the feed-forward compensation voltage is added to the reference voltage to generate a new reference voltage ; Construct a voltage-current closed-loop control, and according to the generated new reference voltage , the grid-connected inverter grid-side connection point current collected by the data acquisition module , the grid-side inverter AC bridge arm voltage calculate the duty cycle value and input it into the PWM generator; Generate a PWM signal according to the duty cycle value input by the voltage-current closed-loop control module and input the PWM signal into the grid-side inverter.

7. The grid-forming type photovoltaic energy storage control method for enhancing the grid support ability according to claim 6, wherein: When a power grid failure occurs, based on the active power reference value required by the power grid , the reactive power reference value is corrected to obtain the updated active power reference value and the reactive power reference value , including: The active and reactive power reference values required by the system are respectively and , and the reactive power that needs to be corrected is as follows:​ ； Among them, is the reactive power correction coefficient, is the change in voltage sag, is the rated voltage; the active power to be corrected is shown in the following formula: ； Corrected active power reference value and reactive power reference value are calculated by the following formula: ； ； Among them and are the reference values of the corrected active and reactive powers.

8. The grid-forming type photovoltaic energy storage control method for enhancing the grid support ability according to claim 6, characterized in that: The establishment of the grid-side converter VSG model is carried out for the updated active power reference value and the reactive power reference value are processed, and the reference voltage is obtained through the active power and reactive power equations, including: Modeling of the virtual synchronous generator of the grid-side converter in a photovoltaic energy storage system Modeling is carried out, and the series inductance impedance on the transmission line is set to be , is the amplitude of the AC bridge arm voltage of the grid-side inverter, is the amplitude of the grid-connected point voltage of the grid-side inverter. The active power and reactive power of the grid-side inverter are respectively:[[]]END]] ； Definition and represent the command values of active power and reactive power respectively. Then, the equation of the virtual synchronous generator is as follows: ； Among them, is the moment of inertia, is the damping coefficient, and are the rated angular velocity and the actual angular velocity respectively, is the phase angle output by the VSG. The reactive power-voltage regulation equation of the virtual synchronous generator is expressed as: ； Among them, is the reactive equivalent inertia coefficient, is the reactive power voltage regulation coefficient, is the rated value of the output voltage, is the actual value of the output voltage, is the voltage amplitude output by the VSG; the output of the VSG is a vector , whose amplitude is and the phase angle is .

9. The grid-forming type photovoltaic energy storage control method for enhancing the power grid support ability according to claim 6, wherein: The feedforward compensation voltage is generated based on the collected grid-side inverter connection point voltage and the grid-side inverter connection point current to generate a feedforward compensation voltage , and the feedforward compensation voltage is added to the reference voltage to generate a new reference voltage , including: Step 4-1: Voltage feedforward compensation According to the requirements of the VSG technical rules, the dynamic reactive current generated by the VSG should track the AC-side voltage change in real time and meet the requirements of the following formula: ； wherein is the increment value, is the effective value of the dynamic reactive current, represents the effective value of the dynamic reactive current before the VSG enters the fault, , are the reactive current coefficients respectively, is between 1.5 and 2.5, is between 0 and 1.5, is the ratio of the actual positive sequence component on the AC side of the VSG to the rated voltage, is the current value of the rated current generated on the AC side of the VSG; Grid current Decreases under the control of the VSG as and changes occur,[ The phase of is denoted as and the phase difference between and is denoted as ; When changes suddenly, the VSG will act as a controllable voltage source, and the grid-connected inverter will maintain the magnitude of the internal voltage so that the current will change suddenly, and the fault current will transfer through ; All quantities during the voltage dip are denoted with a subscript added to the original; To provide reactive power support to the grid, needs to meet the required by the grid code, so ; The transmission current between and is controlled by the following factors: ； Transforming the above formula to the dq coordinate system, we get: ； wherein and are the dq components of and are the dq-axis components of; the fault surge current in the dq reference system and are obtained by the following formula: ； According to what is obtained by the data acquisition module At the output end of the control diagram Add a feedforward compensation , to cancel out the rightmost term in the above formula; the compensation voltage of the d-axis is calculated by the following formula: ； The compensation voltage of the q-axis is calculated by the following formula: ； Step 4-2: Virtual impedance control Introduce virtual impedance: ； Among them, and are a virtual resistor and a virtual inductor respectively; Set the voltage drop in the virtual impedance to be , then: ； where is the rated value, compensated by the feed-forward compensation voltage to compensate , then the compensated expression is: ； Then derive the virtual impedance value: ； Step 4-3: Reference voltage update The reference voltage generated in Step 3 After dq transformation, it is obtained as follows: ； Adding the compensation amounts of the reference voltages generated in Step 4-1 and Step 4-2 to the above formula, we get ； , is the updated reference voltage; After the reference voltage is updated, the change in the reference phase angle output by the original VSG is as follows: is as follows: ； The corrected phase angle is: 。 10. The grid-forming type photovoltaic energy storage control method for enhancing the grid support ability according to claim 6, characterized in that: The construction of the voltage-current closed-loop control is based on the generated new reference voltage , the grid-side inverter grid connection point current collected by the data acquisition module , the grid-side inverter AC bridge arm voltage Calculate the duty cycle value and input it into the PWM generator, including: According to the photovoltaic energy storage system structure topology diagram, we get: ； Among them , ; , , After transformation, they are respectively , , , , , ; The mathematical model of the energy storage grid-side inverter after transformation is: ； The inverter adopts closed-loop control, and its outer voltage loop equation is: ； The current inner-loop equation is: ； Among them and and are the proportional coefficient and integral coefficient for the voltage outer loop and current inner loop controls respectively, and is the current reference value; according to and in the above formula, as well as the phase angle after step correction, through the modulation of the PWM module, the duty cycle information of the inverter is generated, and then the grid-side inverter is controlled.

Images