Dynamic adjustment method, stabilization method and system for power reference value of network-forming converter

Abstract

The invention discloses a method for dynamically adjusting a power reference value of a network-forming converter, which is used for adjusting the power reference value of an active power control loop by feeding back the difference value between the output voltage of the reactive power control loop and the rated voltage of a power grid to the input of the active power control loop. By adding a voltage difference value feedforward link, the reference power of an active power control loop is changed, the frequency supporting capacity of the system is enhanced, the deceleration area is equivalently increased, the acceleration area is equivalently reduced, the transient stability of the system is improved, the damage risk of the converter is reduced, and the service life of the system is prolonged.

Description

Technical Field

[0001] This invention relates to the field of power electronic power generation and conversion technology, specifically to a method, stabilization method, and system for dynamically adjusting the power reference value of a grid-type converter. Background Technology

[0002] With the introduction of dual-carbon targets, a large number of wind and solar power sources are being connected to the power system via converters. Grid-based virtual synchronous control adopts a power synchronization method, matching the grid frequency by simulating the second-order swing equation of a synchronous machine; and tracks the grid voltage through reactive power droop control. It can achieve almost the same steady-state performance as a synchronous machine, and the power electronic devices have the advantage of flexible control, even surpassing traditional synchronous machines in some aspects. Therefore, it is considered a forward-looking technology choice for grid-connected converter control in power electronic power systems, and can provide strong support for future 100% renewable energy power systems.

[0003] However, when the grid voltage drops significantly, the imbalance between the input active power and the reference active power leads to a continuous increase in the power angle, causing the converter to generate surge currents several times its rated current. Since the converter's components have limited overcurrent capacity, failure to address this issue promptly can damage the power generation equipment. Therefore, during faults, such as voltage dips, the active power loop is prone to loss of synchronization, leading to system transient instability and affecting the stability of the grid-connected converter system and its ability to support the grid. Summary of the Invention

[0004] This invention provides a method for dynamically adjusting the power reference value of a grid-connected converter. This method can improve the transient stability under grid voltage dip faults and reduce the risk of converter damage.

[0005] This method adjusts the power reference value of the active power control loop by feeding back the difference between the output voltage of the reactive power control loop and the rated voltage of the grid. The relationship between the actual reference power of the active power control loop after feedforward and the difference feedback input is as follows:

[0006]

[0007] In the formula, This is the actual reference power for the feedforward active power control loop. This is the initial reference power for the active power control loop. Forward coefficients, The output voltage of the converter. This is the rated voltage of the power grid.

[0008] Furthermore, the feedforward coefficients The following conditions must be met:

[0009]

[0010] In the formula, The output voltage at the stable equilibrium point of the converter. This is the voltage value after the grid voltage drops. The power angle L at the stable equilibrium point of the converter g This refers to the line inductance.

[0011] Furthermore, this method adjusts the power reference value of the active power control loop when a grid voltage dip fault occurs.

[0012] This invention also provides a fault transient stabilization method for a grid-connected system of a grid-connected converter. When a fault occurs in the grid-connected system of the grid-connected converter, the power reference value of the active power control loop of the grid-connected system is adjusted by the aforementioned dynamic adjustment method for the power reference value of the grid-connected converter.

[0013] The present invention also provides a dynamic adjustment system for the power reference value of a grid-type converter, comprising a grid-type converter, a common coupling point, and a control module. The output of the grid-type converter is connected to the common coupling point and then grid-connected after passing through a filter inductor, a filter capacitor, and a line impedance. The control module is configured to execute the dynamic adjustment method for the power reference value of the grid-type converter provided by the present invention.

[0014] The technical solution of the present invention has at least the following technical effects:

[0015] This invention enhances the frequency support capability of the system by adding a voltage difference feedforward link, thereby changing the reference power of the active power control loop, effectively increasing the deceleration area and decreasing the acceleration area, improving the transient stability of the system, reducing the risk of converter damage, and extending the system lifespan. Attached Figure Description

[0016] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention. It is obvious that the drawings described below are merely some embodiments of the invention, and those skilled in the art can obtain other drawings based on these drawings without any inventive effort. In the drawings:

[0017] Figure 1 A flowchart of the dynamic adjustment method provided by the present invention;

[0018] Figure 2 This is a schematic diagram of the structure of a converter grid-connected system before the dynamic adjustment method provided by this invention was adopted for control.

[0019] Figure 3This is a control principle diagram of the dynamic adjustment method provided by the present invention;

[0020] Figure 4 A schematic diagram of the equal-area method for improving transient stability using the dynamic adjustment method provided by this invention;

[0021] Figure 5(a) is a simulation diagram of the system without the dynamic adjustment method provided by the present invention under the condition of system failure;

[0022] Figure 5(b) is a simulation diagram of the dynamic adjustment method provided by the present invention used to control the system when a fault occurs. Detailed Implementation

[0023] Exemplary embodiments will now be described more fully with reference to the accompanying drawings. However, these exemplary embodiments can be implemented in many forms and should not be construed as limited to the examples set forth herein; rather, they are provided so that the invention will be more thorough and complete, and will fully convey the concept of the exemplary embodiments to those skilled in the art.

[0024] Furthermore, the described features, structures, or characteristics can be combined in any suitable manner in one or more embodiments. Numerous specific details are provided in the following description to give a full understanding of embodiments of the invention. However, those skilled in the art will recognize that the technical solutions of the invention can be practiced without one or more of the specific details, or other methods, components, apparatuses, steps, etc., can be employed. In other instances, well-known methods, apparatuses, implementations, or operations are not shown or described in detail to avoid obscuring various aspects of the invention.

[0025] The block diagrams shown in the accompanying drawings are merely functional entities and do not necessarily correspond to physically independent entities. That is, these functional entities can be implemented in software, in one or more hardware modules or integrated circuits, or in different network and / or processor devices and / or microcontroller devices.

[0026] The flowcharts shown in the accompanying drawings are merely illustrative and do not necessarily include all content and operations / steps, nor do they necessarily need to be performed in the described order. For example, some operations / steps can be broken down, while others can be combined or partially combined; therefore, the actual execution order may change depending on the specific circumstances.

[0027] Terminology Explanation:

[0028] The system refers to the system consisting of a grid converter, a common coupling point, a control module, filter inductors, filter capacitors, and line impedance.

[0029] Please see Figure 1The dynamic adjustment method provided in this embodiment of the invention includes:

[0030] Step S10: The active power control loop controls the output active power of the converter;

[0031] Step S20: The reactive power control loop controls the output reactive power of the converter;

[0032] Step S30: The dynamic adjustment method adjusts the power reference value of the active power control loop by feeding back the difference between the output voltage of the reactive power control loop and the rated voltage of the grid to the input of the active power loop.

[0033] This invention provides a method for dynamically adjusting the power reference value of a grid-type converter based on voltage feedforward control, building upon existing virtual synchronous control methods, and also specifies the range of control parameters. Specifically, it includes the following steps:

[0034] Step SA1: Obtain the converter output voltage and the grid rated voltage, and calculate the difference between the converter output voltage and the grid rated voltage as the voltage input value.

[0035] Step SA2: The voltage input value is fed back to the active power loop input through a proportional circuit, adjusting the reference power of the active power control loop; the relationship between the active power reference power and the voltage input value is as follows:

[0036]

[0037] In the formula, This is the actual reference power for the feedforward active power control loop. This is the initial reference power for the active power control loop. Forward coefficients, The output voltage of the converter. This is the rated voltage of the power grid.

[0038] To prevent the power angle from reversing due to an excessively large feedforward coefficient during a fault, the maximum value of the feedforward coefficient is:

[0039]

[0040] In the formula, K max E represents the maximum value of the feedforward coefficients. sep U is the output voltage at the stable equilibrium point of the converter. gf δ represents the voltage value after the grid voltage drop. sep The power angle L at the stable equilibrium point of the converter g This refers to the line inductance.

[0041] To ensure the existence of the converter's equilibrium point, the minimum value of the feedforward coefficient is:

[0042]

[0043] In the formula, K min This represents the minimum value of the feedforward coefficient.

[0044] A dynamic adjustment system for implementing this adjustment method includes a grid-type converter, a common coupling point, and a control module. The output of the converter is connected to the grid via a filter inductor, a filter capacitor, and a line impedance through the common coupling point. The control module is configured to execute the aforementioned dynamic adjustment method.

[0045] The control process of the entire system is as follows:

[0046] The initial reference power P0 of the active power control loop was set, and the voltage E (i.e., the output voltage of the converter) and current I at the grid connection point were collected. abc U is obtained through dq transformation d U q I d I q The active and reactive power P and Q of the converter output are calculated. P and P0 output phase θ through the VSG synchronization loop to track the phase of the detected voltage, thereby controlling the phase of the output voltage. The reactive power Q outputs the d-axis reference voltage U through droop control. dref The q-axis reference voltage U qref When the voltage is 0, the voltage-current loop generates a modulation signal that controls the output voltage of the converter, thereby controlling its output voltage and output power.

[0047] When the grid voltage is at a steady-state value, the converter controls the output voltage E to match the grid voltage command value E0 through the reactive power control loop. The difference between the two is zero, voltage feedforward fails, and the active power reference value remains P0, allowing the system to maintain stable operation. When a voltage dip fault occurs in the grid, the converter will experience a drop in output voltage and an increase in the power angle. At this time, the output voltage E decreases, and the difference between E and E0 increases. The system then introduces voltage feedforward, adjusting the active power reference value to P0-K(E0-E). The reference active power decreases, which effectively increases the deceleration area and reduces the acceleration area, improving the transient stability of the system.

[0048] To facilitate a better understanding of the technical solution of this invention, the following description is provided:

[0049] The dynamic adjustment system described in this invention refers to a system consisting of a grid-connected converter, a common coupling point, a control module, a filter inductor, a filter capacitor, and line impedance (hereinafter, "system" is the abbreviation for the dynamic adjustment system). A grid-connected converter is a power device that inverts DC to AC. Employing power reserve control, it can effectively support the grid frequency. By adjusting the active power reference value through voltage feedforward, the transient stability of the converter can be improved.

[0050] like Figure 2 As shown, the converter output passes through a filter inductor. Filter capacitor and line impedance After connecting to the common coupling point, it is connected to the grid.

[0051] When the grid voltage drops significantly, the imbalance between the input active power and the reference active power leads to a continuous increase in the power angle. This can cause the synchronization loop to lose synchronization, resulting in system transient instability. Furthermore, transient instability causes the converter to generate surge currents several times its rated current. Since the converter's components have limited overcurrent capacity, failure to address this issue promptly can damage the power generation equipment.

[0052] To address this, the present invention adds a voltage difference feedforward link to the virtual synchronization control, which dynamically adjusts the active power reference value during a fault.

[0053] The front-end of this invention calculates the photovoltaic output power by continuously detecting the voltage and current output by the converter, and continuously detects the output voltage and voltage command value to calculate the difference between the two. The output voltage and voltage command value are adjusted through a proportional circuit as input to dynamically adjust the active power reference value of the converter.

[0054] Combination Figure 2 The voltage E and current I at the grid connection point were collected. abc U is obtained through dq transformation d U q I d I q The active and reactive power P and Q of the converter output are calculated. P and P0 output phase θ through the VSG synchronization loop to track the phase of the detected voltage, thereby controlling the phase of the output voltage. The reactive power Q outputs the d-axis reference voltage U through droop control. dref The q-axis reference voltage U qref When the voltage is 0, the voltage-current loop generates a modulation signal that controls the output voltage of the converter, thereby controlling its output voltage and output power.

[0055] Combination Figure 2 The output phase θ is obtained by simulating the second-order rotor equation of the synchronous machine, which satisfies the following relationship:

[0056]

[0057] In the formula, θ is the output phase of the VSG, θ g For grid phase, Where J is the rated frequency of the power grid, and J is the moment of inertia. For damping, For output reference power, The active power output by the converter. Let s be the output power and let s be the Laplace operator.

[0058] The control principle of this invention is as follows: Figure 4 As shown, based on the range of the calculated feedforward coefficient K, the difference between the output voltage E and the voltage command value E0 is used as input and fed forward to the active power control loop through a proportional circuit to achieve the purpose of dynamically adjusting the active power reference value. Under fault conditions, the difference between E and E0 increases, the active power reference value is adjusted to P0-K(E0-E), and the reference active power decreases. At this time, the deceleration area is effectively increased and the acceleration area is reduced, thereby improving the transient stability of the system.

[0059] In this invention, the improvement in system transient stability can be described using the equal area method. Curves I and II represent the power angle curves before and after the fault, respectively. After a grid voltage dip fault occurs, the system state moves from point A to point B. At this point, since P0 is greater than P, the system enters an acceleration state until the fault is cleared at point C, at which point P0 is less than P, and deceleration begins. Before adopting the voltage feedforward control strategy, the reference active power during the fault is P0, the acceleration area is S1+S2, the deceleration area is S3, and the stability criterion is that S1+S2 is less than or equal to S3. After adopting the voltage feedforward control strategy, the reference active power during the fault is P0-K. p As the acceleration area decreases from E0 to S2, the stability criterion that the acceleration area is smaller than the deceleration area is easily satisfied, thus enhancing the system's transient stability. It is worth noting that changes in the reference active power only occur when the output voltage is not E0; the system state will still return to the equilibrium point before the fault, i.e., point A.

[0060] Here, in the MATLAB / Simulink simulation platform, based on the method of this invention, respectively... Figure 2 Virtual synchronization control, and Figure 3 Improved control, and built Figure 2 The simulation parameters of the grid-connected converter system shown are shown in Table 1.

[0061] Table 1 Simulation Parameters

[0062] parameter numerical values parameter numerical values <![CDATA[U dc / V]]> 1000 <![CDATA[C dc / mF]]> 10 <![CDATA[E0 / V]]> 311 <![CDATA[X g / mH]]> 15 / Hz 50 J 20 <![CDATA[P0 / kW]]> 16 <![CDATA[K q ]]> 100 <![CDATA[D P ]]> 400 <![CDATA[Q ref ]]> 0

[0063] Based on the system simulation parameters in Table 1, the system experienced an 84% voltage drop fault lasting 0.3s at 0.5 seconds. Comparing the grid-connected system of the grid-type converter using traditional VSG control with the system of the present invention, the former, as shown in Figure 5(a), after the fault was cleared, the power angle continued to increase, the output voltage continued to decrease, and the output current continued to oscillate. At this time, the converter experienced transient synchronous instability. The latter, as shown in Figure 5(b), after adopting the voltage feedforward control strategy, achieved stable operation of the converter under the same fault impact.

[0064] In summary, the voltage-feedforward-based dynamic adjustment method and system for the power reference value of a grid-connected converter of the present invention can prevent transient instability caused by loss of synchronization of the synchronization loop when a grid voltage drop fault occurs. It effectively improves the ability of the grid-connected converter system to maintain synchronization of the synchronization loop during faults, thus improving the transient stability of the system. The specific control steps are as follows: acquire the converter output voltage and the grid rated voltage; calculate the difference between the converter output voltage and the grid rated voltage as the voltage input value; feed the voltage input value back to the active power loop through a proportional circuit to adjust the reference power of the active power control loop.

[0065] Other embodiments of the invention will readily occur to those skilled in the art upon consideration of the specification and practice of the embodiments disclosed herein. This invention is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein. It should be understood that the invention is not limited to the precise structures described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of the invention is limited only by the appended claims.

Claims

1. A method for dynamically adjusting the power reference value of a grid-type converter, characterized in that, This method adjusts the power reference value of the active power control loop by feeding back the difference between the output voltage of the reactive power control loop and the rated voltage of the grid. The relationship between the actual reference power of the active power control loop after feedforward and the difference feedback input is as follows: In the formula, This is the actual reference power for the feedforward active power control loop. This is the initial reference power for the active power control loop. Forward coefficients, The output voltage of the converter. This is the rated voltage of the power grid.

2. The method for dynamically adjusting the power reference value of a grid-type converter according to claim 1, characterized in that, The feedforward coefficient The following conditions must be met: In the formula, The output voltage at the stable equilibrium point of the converter. This is the voltage value after the grid voltage drops. The power angle L at the stable equilibrium point of the converter g This refers to the line inductance.

3. The method for dynamically adjusting the power reference value of a grid-type converter according to claim 1, characterized in that, This method adjusts the power reference value of the active power control loop when a grid voltage dip fault occurs.

4. A fault transient stabilization method for a grid-connected system with a grid-connected converter, characterized in that, When a fault occurs in the grid-connected system of the grid-connected converter, the power reference value of the active power control loop of the grid-connected system is adjusted by the dynamic adjustment method of the power reference value of the grid-connected converter as described in any one of claims 1-2.

5. The fault transient stabilization method for grid-connected systems with grid-connected converters according to claim 4, characterized in that, The fault is a power grid voltage drop fault.

6. A dynamic adjustment system for power reference value of a grid-type converter, characterized in that, The device includes a grid-type converter, a common coupling point, and a control module. The output of the grid-type converter is connected to the grid via a filter inductor, a filter capacitor, and a line impedance through the common coupling point. The control module is configured to perform the method described in any one of claims 1 to 3.

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