Virtual synchronous generator asymmetric fault ride-through cooperative control method and system

Abstract

The invention relates to an asymmetric fault ride-through cooperative control method and system for a virtual synchronous generator, and belongs to the technical field of power electronics and new energy power generation. Aiming at the problems of three-phase current asymmetry, overcurrent, transient instability and the like faced by a virtual synchronous generator when a power grid has an asymmetric fault, the method comprises the following steps: firstly, setting a negative sequence current reference to be zero through positive and negative sequence separation control so as to keep output current symmetry, and freezing a reactive power ring; secondly, comprehensively considering current amplitude limiting, voltage supporting capacity and system capacity, and dynamically calculating optimal solutions of positive sequence excitation reference amplitudes under different fault depths; finally, positive and negative sequence power components and excitation dynamic changes are considered, an active reference instruction real-time dynamic adjustment method is provided, and a power angle is controlled to be stabilized at a target value by adjusting a power reference value in real time. The method can effectively improve the transient stability during the fault ride-through period, maximally supports the power grid voltage while ensuring that the current is not out of limit, and improves the system capacity utilization rate.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of power electronics and new energy power generation, and relates to a virtual synchronous generator asymmetric fault ride-through cooperative control method and system. BACKGROUND

[0002] With the centralized access of a large number of new energy to the power grid, the system faces the significant problem of damping and inertia loss, resulting in the decline of voltage and frequency regulation capability. The grid-connected inverter based on the virtual synchronous generator (VSG) can simulate the output characteristics of the traditional synchronous generator and has the ability to support the system voltage and frequency. However, when the power grid fails, the virtual synchronous generator will also face problems such as transient instability and output power quality decline.

[0003] Compared with the traditional synchronous generator, the inverter composed of power electronic devices has poorer overcurrent bearing capacity caused by faults during the period, and may trigger overcurrent protection and be disconnected from the grid, aggravating the system operation risk. Therefore, the virtual synchronous generator is required to operate without being disconnected from the grid under certain conditions, while outputting good power quality and maintaining the support capability for the grid voltage and frequency to promote the recovery of the stable operation of the power grid.

[0004] Current research on the transient stability and current limiting strategy of the virtual synchronous generator under asymmetric faults is relatively insufficient, and existing literature mainly focuses on the output current limiting and power angle stability under symmetric faults. The common way to suppress overcurrent during symmetric faults is to directly limit the current loop reference value, but this method will cause the virtual synchronous generator to switch from voltage source mode to current source mode when the current loop is saturated, resulting in insufficient system inertia and damping capability. Although some documents achieve fault current limiting by adaptively calculating the virtual impedance, this method changes the equivalent output impedance of the inverter, although it maintains the voltage source characteristics, but weakens the support capability for the grid.

[0005] Existing control strategies of the virtual synchronous generator under power grid faults are mostly limited to a single control target, and cannot consider both the output power quality and voltage support capability during asymmetric faults, nor establish the interaction mechanism between fault current limiting and voltage support. In addition, the research on the transient stability of the system under asymmetric faults is still insufficient, and the existing analysis does not consider the dynamic change of the excitation amplitude and the influence of the active power generated by the negative sequence component, and lacks multi-dimensional consideration of the given basis for the power angle when dynamically adjusting the active power reference value according to the power angle target value.

[0006] In view of the above, the application provides an asymmetric fault ride-through cooperative control strategy for a virtual synchronous generator, which realizes current symmetry through positive and negative sequence separation control, combines current limiting, voltage support and system capacity constraint dynamic adjustment of the excitation reference amplitude, and innovatively proposes a real-time dynamic adjustment method for active reference instructions, and finally verifies the effectiveness of the strategy through a semi-physical experiment. SUMMARY

[0007] Therefore, the application aims to provide a virtual synchronous generator asymmetric fault ride-through cooperative control method and system.

[0008] To achieve the above-mentioned purpose, the application provides the following technical solutions.

[0009] A virtual synchronous generator asymmetric fault ride-through cooperative control method comprises the following steps:

[0010] S1: When an asymmetric fault occurs in the power grid, the negative sequence current reference value is set to zero through positive and negative sequence separation control to realize three-phase symmetry of the output current and freeze the reactive power ring;

[0011] S2: According to the current limiting value of the virtual synchronous generator VSG, the voltage support capacity and the system capacity constraint, the optimal solution of the positive sequence excitation reference amplitude under different fault depths is calculated;

[0012] S3: Based on the optimal solution of the positive sequence excitation reference amplitude, and taking into account the dynamic changes of the active power generated by the positive and negative sequence components and the excitation amplitude, the active power reference value is dynamically adjusted in real time to control the power angle to reach the target value and improve the transient stability of the system.

[0013] Further, the calculation of the optimal solution of the positive sequence excitation reference amplitude in S2 comprises:

[0014] When the grid voltage is shallowly dropped, if the following condition is met Then the positive sequence excitation reference amplitude is set to the rated value E N ;

[0015] When the grid voltage is deeply dropped, the positive sequence excitation reference amplitude is set to

[0016] Wherein, is the positive sequence component amplitude of the grid voltage, I N is the rated current, Z is the line impedance, and E N is the rated excitation amplitude.

[0017] Further, in the transient regulation process in S2, the positive sequence excitation reference amplitude E satisfies the constraint condition:

[0018]

[0019] wherein, δ + is the real-time positive sequence power angle value output by the active power loop during the fault.

[0020] Further, the calculation formula of the real-time dynamic adjustment of the active power reference value P ref in S3 is:

[0021]

[0022] wherein, P ref_N is the rated active power reference value, is the post-fault positive sequence excitation reference amplitude, is the pre-fault grid voltage positive sequence component amplitude, is the target positive sequence power angle, E N is the rated excitation amplitude, is the post-fault grid voltage positive sequence component amplitude, δ N is the rated power angle, is the active power generated by the negative sequence component.

[0023] Further, the target positive sequence power angle is determined according to the optimal solution of the positive sequence excitation reference amplitude and the system capacity utilization, so that the positive sequence excitation amplitude is maximized under the premise of meeting the current amplitude limiting and voltage support.

[0024] Further, in S3, the acceleration area S Δ+ is controlled by suppressing the increase of the power angle, and the calculation formula is:

[0025]

[0026] wherein, δ x is the power angle after the acceleration ends, is the active power generated by the positive sequence component.

[0027] An asymmetric fault ride-through collaborative control system for a virtual synchronous generator, the system comprising:

[0028] a fault detection module for detecting a grid asymmetric fault and triggering a control strategy switching;

[0029] a positive and negative sequence separation control module for setting the negative sequence current reference value to zero to realize three-phase symmetry of the output current and freezing the reactive power loop;

[0030] an excitation reference calculation module for calculating an optimal solution of the positive sequence excitation reference amplitude according to the current amplitude limiting value, the voltage support capability and the system capacity constraint;

[0031] The active reference adjustment module is configured to dynamically adjust the active power reference value in real time based on the optimal solution of the positive sequence excitation reference amplitude, so as to control the power angle to reach the target value.

[0032] Further, the excitation reference calculation module sets the positive sequence excitation reference amplitude to the rated value E N , and sets it to

[0033] Further, the active reference adjustment module dynamically adjusts the active power reference value P ref , so that the positive sequence power angle is stabilized at the target value and the active power generated by the negative sequence component has an impact on the transient stability.

[0034] Further, the system further comprises a hardware-in-the-loop experiment platform for verifying the effectiveness of the control strategy, the platform comprising a control chip with DSP TMS320F28377D as the core and a StarSim HIL real-time simulator based on NI-PXIe-786R.

[0035] The present application has the following advantages:

[0036] (1) The present application can effectively eliminate the negative sequence component in the output current by adopting the positive and negative sequence separation control technology, ensure the symmetry of the three-phase current output by the inverter during the fault, and significantly improve the power quality. At the same time, the proposed real-time dynamic adjustment method of the active power reference instruction fully considers the influence of the dynamic change of the excitation amplitude and the negative sequence power component, can accurately control the power angle change, effectively suppress the increase of the power angle, and thus greatly improve the transient stability of the system and prevent the system from being unstable.

[0037] (2) The control strategy can adaptively calculate the optimal positive sequence excitation reference amplitude under strict current amplitude constraints according to different fault depths. This not only ensures that the virtual synchronous generator does not run off the grid during the fault, realizes reliable fault ride-through, but more importantly, can maximize its support ability for the grid voltage. By dynamically adjusting the excitation and active output, the strategy enables the virtual synchronous generator to provide the most effective voltage support for the grid recovery within the allowable range of system capacity.

[0038] (3) The present application innovatively considers the current amplitude, voltage support ability and system capacity utilization rate in coordination, and gives the optimal solution under different working conditions. This method avoids the waste of system capacity and ensures the maximum utilization of capacity. By freezing the reactive ring and coordinating the control of the active ring, a complete cooperative control architecture is formed, so that the control objectives of the system during the fault can be balanced and considered, and the control process is smooth and stable.

[0039] Additional advantages, objects, and features of the application will be apparent to those skilled in the art upon examination of the following specification. It is intended that the application not be limited by any of the recited embodiments, but that the recited embodiments be considered exemplary of the working of the application and that the scope of the application be measured by the broadest interpretation of the claims that follow. BRIEF DESCRIPTION OF DRAWINGS

[0040] In order to make the objectives, technical solutions and advantages of the present application clearer, the preferred embodiments of the present application will be described in detail below with reference to the accompanying drawings, in which:

[0041] Figure 1 Control block diagram of VSG under asymmetric fault based on positive and negative sequence separation;

[0042] Figure 2 Geometric relationship diagram between positive sequence phasors at steady state under system constraints;

[0043] Figure 3 Power angle curve under optimized active reference instruction;

[0044] Figure 4 StarSim hardware-in-the-loop experimental platform;

[0045] Figure 5 The proposed low voltage ride through control strategy;(a) is the output current;(b) is the output excitation electromotive force;

[0046] Figure 6 The result diagram of the control strategy of directly giving asymmetric excitation reference;(a) is the positive sequence excitation electromotive force;(b) is the active power reference value;(c) is the system power angle and the positive sequence power angle;(d) is the system apparent power;(e) is the output power;

[0047] Figure 7 The result diagram of the control strategy of directly giving asymmetric excitation reference when the grid voltage drop is deep;(b) is the output positive sequence excitation electromotive force;(c) is the system power angle and the positive sequence power angle;(d) is the system apparent power;(e) is the output power;

[0048] Figure 8 The method of not using real-time dynamic adjustment of active reference instruction;(a) is the output current instability;(b) is the output power angle instability;

[0049] Figure 9 The method of applying real-time dynamic adjustment of active reference instruction value;(a) is the output current stability;(b) is the output power angle stability. DETAILED DESCRIPTION

[0050] The present application will be further described by way of specific embodiments, and the skilled in the art will readily obtain a number of other advantages and effects from the disclosure of the specification. The present application can also be implemented or applied in other different specific embodiments, and various modifications or changes can be made to the details based on different views and applications without departing from the spirit of the present application. It should be noted that the drawings provided in the following embodiments only illustrate the basic concept of the present application in a schematic manner, and the following embodiments and features in the embodiments can be combined with each other without conflict.

[0051] The drawings are only used for illustrative description, and the representation is only a schematic diagram, not a physical diagram, and cannot be understood as a limitation of the present application; in order to better illustrate the embodiments of the present application, some components in the drawings are omitted, enlarged or reduced, and do not represent the actual product size; for those skilled in the art, it is understandable that some well-known structures and their descriptions in the drawings can be omitted.

[0052] The same or similar reference numerals in the drawings of the embodiments of the present application correspond to the same or similar components; in the description of the present application, it should be understood that if the terms "upper", "lower", "left", "right", "front", "back" and the like indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, therefore the terms describing the positional relationship in the drawings are only used for illustrative description, and cannot be understood as a limitation of the present application, for those skilled in the art, the specific meaning of the above terms can be understood according to the specific circumstances.

[0053] 1VSG system description

[0054] Figure 1 is the traditional positive and negative sequence separation control strategy of VSG under asymmetric conditions of power grid, the output current three-phase balance is realized by directly setting the inner loop negative sequence current reference value to zero, the control strategy switching during fault is realized by fault detection, the reactive power ring is frozen during fault, the positive sequence excitation amplitude reference value is obtained by system capacity limitation and output current amplitude constraint. The power outer ring of VSG under normal working condition is composed of formula (3) and (4).

[0055]

[0056] In the formula: E ref is the reactive ring output excitation reference amplitude, D q is the reactive droop coefficient, k1 is the integral coefficient of the reactive ring; U n and U are the terminal voltage reference value and the root mean square value of the grid connection point voltage of VSG respectively; Q ref and Qe These are the reference value and actual value of reactive power for VSG, respectively. P ref P is the active power reference value. e ω represents the actual active power output of the inverter; J and D are the virtual moment of inertia and damping coefficient of the system, respectively; n This is the reference angular frequency for the power grid.

[0057] 2 Fault Ride Control Methods

[0058] 2.1 Constraints of current limiting value and system capacity on excitation

[0059] When an asymmetrical fault occurs in the power grid, the fault ride-through control strategy of the VSG must not only ensure that the output current does not exceed the limit, but also provide maximum voltage support within system capacity constraints to help the power grid recover from the fault. This is because both the voltage support characteristics and the output current response can be described by the positive-sequence excitation phasor, and after the negative-sequence voltage components in the circuit are canceled out, the positive-sequence excitation E... + It has the same dynamic response characteristics as the complete excitation E, so the positive sequence excitation component of the VSG output can be analyzed directly.

[0060] Assuming that the VSG has a certain margin under normal operating conditions, in the event of a fault, the VSG is allowed to output at 1.2 times its rated capacity, and the current limit value is also 1.2 times the rated value.

[0061] During a typical fault, the voltage support capability of a VSG is directly proportional to the positive sequence excitation amplitude. Therefore, to maximize the voltage support capability of the VSG, it is necessary to determine the maximum output positive sequence excitation amplitude. Figure 2 As shown, U g , and These represent the positive-sequence phasors of the grid voltage under normal operating conditions, shallow voltage dips, and deep voltage dips, respectively. Considering that the VSG output current is within the limiting range, the endpoint of the line impedance voltage drop IZ will fall on the limiting circle. At this time, the smaller the output positive-sequence power angle, the larger the positive-sequence excitation amplitude. For example... Figure 6 The blue line indicates the excitation amplitude. If the power grid voltage drops further, such as Figure 2 As shown by the red line, even if the positive sequence power angle is zero, the maximum excitation amplitude is obtained. It will also be less than the excitation rating E N Therefore, the ability of the VSG to support the grid voltage during a fault is limited by both the degree of grid voltage drop and the output current limit.

[0062] In addition, it should be considered whether the output capacity of the VSG is being fully utilized during a fault.

[0063] The rated capacity S of the systemN may be expressed as:

[0064] S N = E N I N (3)

[0065] Therefore, to maximize the utilization without exceeding the maximum capacity of the system during the fault, there must be:

[0066] S = E + I + ≤ 1.2E N I N (4)

[0067] Figure 2 In the case of shallow grid drop, the excitation amplitude is taken as At this time, the positive sequence excitation is greater than the required rated value E N , and the output current is 1.2I N , which obviously exceeds the maximum capacity of the system, and its support for the positive sequence excitation has exceeded the required rated value. If the positive sequence excitation amplitude is exactly equal to the rated value E N , and the output current is less than 1.2I N , the system capacity will not be maximized. In Figure 2 , only when the excitation amplitude is taken as , can the output current limiting of the VSG, the support capacity and the capacity utilization rate be considered at the same time.

[0068] When the grid drop is deep, in the output current limiting range, Figure 2 the excitation amplitude obviously cannot support to the rated value, at this time, when the excitation amplitude is taken as , the support capacity and the capacity utilization rate of the VSG can be maximized.

[0069] In summary, when the grid drops shallowly, there is , at this time, the steady-state positive sequence excitation target value of the system is:

[0070]

[0071] And when the grid drops deeply, the steady-state positive sequence excitation target value of the system is:

[0072]

[0073] However, since the VSG output positive sequence power angle is obtained by the active power ring output, which has inertia and cannot be immediately controlled to the steady-state target value, if the excitation reference amplitude is directly set to the steady-state target value, overcurrent will occur during transient regulation. Therefore, during the transient regulation process, according to the above Figure 2The given amplitude E of the positive sequence excitation, i.e. the frozen reactive power loop, should satisfy:

[0074]

[0075] Therefore, the given excitation reference amplitude during the fault, i.e. the positive sequence excitation amplitude E, can be solved as:

[0076]

[0077] where I N Z needs to be calculated using the electrical quantities before the fault, and δ + is the real-time value of the power angle output by the active power loop during the fault.

[0078] 2.2 Real-time dynamic adjustment method of active reference instruction

[0079] During the symmetrical fault of the power grid, some documents suppress the increase of the power angle by dynamically adjusting the active power reference value. Since the bandwidth of the VSG voltage inner loop is significantly larger than that of the power outer loop, it can be considered that the VSG output excitation can quickly track the excitation reference value. Since there is an approximate equality constraint relationship between the inverter output excitation and the active power, the setting of the active power reference value can be guided according to the required excitation reference value of the VSG.

[0080] Assuming that the line impedance is approximately inductive, the active power output of the inverter before and after the fault can be represented as:

[0081]

[0082] where the subscripts be and af respectively represent the electrical quantities before and after the fault, N represents the rated value, k is the drop coefficient, and the subscript r is the abbreviation of the reference value ref.

[0083] Assuming that during the transient process, the active power reference value of the inverter is adjusted in real time to achieve a constant output power angle reference, i.e. δ r_af = δ N , then the ratio of the active power reference values of the inverter before and after the fault can be represented as:

[0084]

[0085] Therefore, the power reference value after the fault can be set as:

[0086]

[0087] The existing literature usually assumes that the system is strong excitation when analyzing the transient stability of VSG system, but the excitation amplitude varies during the actual transient period, which cannot be ignored. When the active power reference value is dynamically adjusted through the relationship between the output power and the power angle, the increase or stability of the power angle is suppressed, which reduces the acceleration area of the system, but few people discuss the influence of the power output by the interaction of negative sequence components on the calculation of acceleration and deceleration areas and the power angle change, and give the basis for setting the target power angle size during the fault period combined with other limiting conditions. This will make the analysis and calculation of the equal-area rule different from the traditional way, so it is necessary to carry out analysis.

[0088] Firstly, unlike symmetrical faults, during the transient period of unsymmetrical faults, when the negative sequence excitation does not completely offset the negative sequence components of the power grid, the output active power P e The power generated by the positive sequence component P and the power generated by the negative sequence component P together constitute the expression as follows:

[0089]

[0090] Since is zero at steady state, it cannot be represented in the power angle curve, which will cause a deviation between the acceleration and deceleration areas calculated according to the power angle curve and the actual value. In order to eliminate this deviation, an additional power reference term needs to be added to formula (12), and in order to make the positive sequence power angle output by the VSG finally stabilize at the target power angle the active power reference value needs to be dynamically adjusted in real time during the fault period as follows:

[0091]

[0092] where is the real-time value of the output excitation reference value calculated by the symmetrical component method, and the target power angle during the fault period The analysis and calculation process has been derived from formulas (5) and (6) above. The acceleration and deceleration area between the power reference generated by the negative sequence components of the system and the actual output power of the negative sequence components can be obtained as follows:

[0093]

[0094] where δ1 is the initial negative sequence power angle, and the negative sequence power angle is 0 at steady state. If the negative sequence excitation electromotive force can quickly track the negative sequence component of the excitation reference, the integral value will be approximately zero. In order to analyze and draw the curve conveniently, the integral calculation and power angle curve drawing of the negative sequence power part will be omitted in the following.

[0095] When the real-time dynamic adjustment method of the active reference instruction is applied, the power angle curve of the system is as follows: Figure 3The active reference value and the excitation electromotive force amplitude are considered to be unchanged during the period from fault occurrence to switching control strategy, and the VSG will accelerate, and the acceleration area is expressed as:

[0096]

[0097] Where the power angle δ x is the power angle after acceleration, and the specific value can be calculated by formula (2) combined with the fault response time.

[0098] When the control strategy is switched, assuming that the response speed of the voltage and current loop is fast enough, there are and generally , the set active reference value and the actual output satisfy:

[0099]

[0100] The VSG will immediately decelerate until reaching the steady state, and the deceleration area is:

[0101]

[0102] Where is the starting positive sequence excitation reference value after the control strategy is switched; is the steady-state value of the positive sequence excitation during the fault, which is also the calculated value obtained by formula (5) and (6).

[0103] It can be seen from formula (16) that the acceleration area S △+ is mainly determined by the fault response time and the fault depth, and because the time interval from fault occurrence to control strategy switching is short, the power angle δ x at the end of acceleration is infinitely close to the rated power angle δ N , so that the acceleration area is far less than the deceleration area, and the system power angle is finally stabilized at the ideal value, achieving the purpose of improving the transient stability of the system. In addition, the strategy takes into account the influence of negative sequence power and changing excitation amplitude on the acceleration and deceleration area, which will make the calculation of the limit parameters related to the acceleration and deceleration area more accurate.

[0104] 3 Experimental verification

[0105] In order to verify the effectiveness of the proposed control strategy, a semi-physical experimental platform based on StarSim is built with DSP TMS320F28377D as the control core, as shown in Figure 4 . The simulation signals of the simulator are converted into digital signals in the DSP in real time through a signal adapter board. The three-phase full-bridge VSG main circuit involved in the present invention is run in the StarSim HIL real-time simulator based on NI-PXIe-786R, as shown in Figure 1The fault-crossing control strategy shown is executed in the DSP control chip. The main experimental parameters involved are shown in Table 1.

[0106] Table 1 Experimental parameters

[0107] Parameter Parameter value Direct current bus voltage U dc ]]> 1000V Grid rated voltage amplitude U g ]]> 311 Filter capacitance C 35 μF Filter inductance L 3 mH Moment of inertia J 4 Kg / m 2 ]] Rated active power P e ]]> 56 kW Rated output voltage E 220V

[0108] Figure 5 The proposed low-voltage ride-through control strategy is shown in Figure 1; (a) represents the output current; and (b) represents the output magnetomotive force. Figure 5 As shown, the traditional control strategy based on positive and negative sequence separation can, through feedback regulation, make the negative sequence component in the output excitation electromotive force cancel the negative sequence component in the grid voltage when the A-phase voltage of the grid drops to 0.5pu. At this time, it can be clearly seen that the three phase amplitudes of the output excitation electromotive force are different, and the output current achieves three-phase symmetry within the limit range.

[0109] Figure 6 The control strategy results with a directly given asymmetrical excitation reference are shown in the diagram; (a) is the positive-sequence excitation electromotive force; (b) is the active power reference value; (c) is the system power angle and positive-sequence power angle; (d) is the system apparent power; and (e) is the output power. Using the proposed fault ride-through strategy, appropriate reactive power can be generated to support the grid connection point voltage. To ensure that the system capacity is not exceeded and to maximize system capacity utilization, the active power reference value is dynamically adjusted to reduce active power output and increase reactive power output. The active power reference value, system output apparent power, and active and reactive power output values ​​during the control period are shown in the diagram. Figure 6 As shown in (b), (d), and (e), the system's apparent power never exceeded the limit during the process, eventually stabilizing at 1.2 pu, and ensuring that the actively generated reactive power just met the maximum support requirements, thus supporting the amplitude of the positive sequence component of the grid connection voltage to its rated value. Figure 6 As shown in (a), the positive-sequence power angle and the system output power angle differ under real-time dynamic adjustment of the active power reference command value. The positive-sequence power angle stabilizes at the target value, and both the system power angle and the positive-sequence power angle change smoothly during control strategy switching. Figure 6 As shown in (c).

[0110] When the voltage of phase A of the grid drops to 0.2 pu, the voltage drop is significant. The VSG (Voltage Sensing Grid) is no longer able to support the positive-sequence component of the grid-connected voltage to its rated value while meeting the current limiting condition. Even if the positive-sequence power angle is controlled to zero, the VSG cannot generate sufficient reactive power, and the system capacity cannot be utilized to its maximum extent. The relevant waveforms are as follows: Figure 7 As shown. The positive sequence power angle is zero, but the output active power is not zero because the line impedance does not meet the purely inductive condition for active power calculation. Figure 7(a) The result of directly applying the asymmetric excitation reference control strategy when the grid voltage drop is deep; (b) The output positive sequence excitation electromotive force; (c) The system power angle and positive sequence power angle; (d) The system apparent power; (e) The output power.

[0111] To verify the improvement of the proposed control strategy on the transient stability of the system, without changing other system parameters, the initial active power reference value was increased to 150 kW at the same fault depth, and experiments were conducted once each with the active power reference command value applied and with the active power reference value applied in real time.

[0112] Figure 8 (a) Output current instability; (b) Output power angle instability. For example, the following conditions were not met: (a) Output current instability; (b) Output power angle instability. Figure 8 As shown, without adopting the active power reference command real-time dynamic adjustment method, the output current can still be limited to within 1.2pu in the first 0.25s after the fault occurs. However, as the grid voltage drops, the output active power is less than the active power reference value. In order to increase the active power output, the system power angle increases. Finally, when the power angle increases to a certain value, both the output current and the system power angle become unstable.

[0113] Figure 9 To apply the active power reference command value, a real-time dynamic adjustment method is used; (a) for stable output current; (b) for stable output power angle. For example... Figure 9 As shown, when the active power reference command is used for real-time dynamic adjustment, the active power reference value is dynamically adjusted according to the fault drop depth and the excitation amplitude, so that the difference between the actual value and the reference value of active power caused by the grid voltage drop is rapidly reduced, the output power angle is quickly stabilized to a fixed value, and the transient stability of the system is obviously improved.

[0114] To improve the fault ride-through capability of VSGs under asymmetric faults, this invention proposes a VSG fault ride-through control strategy that simultaneously enhances power angle stability. The conclusions drawn from theoretical analysis and semi-physical experiments are as follows:

[0115] 1) The proposed fault ride-through strategy freezes the reactive power loop during the fault period. Based on current limiting, voltage support capability and system capacity, the optimal solution of excitation reference amplitude under different fault depths is given.

[0116] 2) In the asymmetric fault, considering that the active power output between negative sequence components will cause deviation in the calculation of acceleration and deceleration area, and the excitation amplitude also changes dynamically during the transient state, the application additionally combines the capacity utilization rate and the maximum voltage support capability to give a calculation method of the steady-state positive sequence excitation target power angle value, and finally gives a method of real-time dynamic adjustment of the active power reference value. Compared with the traditional active power reference value adjustment method, the method can avoid the calculation deviation caused by the application of the equal-area rule during the asymmetric fault, and enhance the power angle stability of the system.

[0117] Finally, it should be pointed out that the above embodiments are only used to illustrate the technical solutions of the present application and are not limiting. Although the present application has been described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that the technical solutions of the present application can be modified or replaced equivalently without departing from the purpose and scope of the technical solutions, which should be covered in the scope of the claims of the present application.

Claims

1. A method for asymmetric fault ride-through cooperative control of a virtual synchronous generator, characterized in that: The method comprises the following steps: S1: when an asymmetric fault occurs in the power grid, setting a negative sequence current reference value to zero through positive and negative sequence separation control to realize three-phase symmetry of output current and freeze the reactive power loop; S2: calculating an optimal solution of a positive sequence excitation reference amplitude under different fault depths according to current limiting amplitude, voltage support capability and system capacity constraints of a virtual synchronous generator (VSG); S3: based on the optimal solution of the positive sequence excitation reference amplitude, and taking into account active power generated by the positive and negative sequence components and dynamic changes of the excitation amplitude, dynamically adjusting an active power reference value in real time to control a power angle to reach a target value and improve system transient stability.

2. The method of claim 1, wherein the method further comprises: The calculation of the optimal solution of the positive sequence excitation reference amplitude in S2 comprises: When the grid voltage is shallowly dropped, if the positive sequence excitation reference amplitude is set to the rated value E N ; When the grid voltage is deeply dropped, the positive sequence excitation reference amplitude is set to wherein is the grid voltage positive sequence component amplitude, I N is the rated current, Z is the line impedance, E N is the rated excitation amplitude.

3. The method for asymmetrical fault ride-through cooperative control of VSGs according to claim 2, characterized in that: In the transient regulation process in S2, the positive sequence excitation reference amplitude E satisfies the constraint condition: where δ + is the real-time value of the positive sequence power angle output by the active power loop during the fault.

4. The method of claim 1, wherein the method further comprises: The real-time dynamic adjustment of the active power reference value P described in S3 ref The calculation formula is: wherein P ref_N is the rated active power reference value, is the post-fault positive sequence excitation reference amplitude, is the post-fault grid voltage positive sequence component amplitude, is the target positive sequence power angle, E N is the rated excitation amplitude, is the pre-fault grid voltage positive sequence component amplitude, δ N is the rated power angle, is the active power generated by the negative sequence component.

5. The method of asymmetric fault ride-through coordinated control of a VSG according to claim 4, characterized in that: The target positive sequence power angle According to the optimal solution of the positive sequence excitation reference amplitude and the comprehensive determination of the system capacity utilization, the positive sequence excitation amplitude is maximized under the premise of meeting the current amplitude limiting and voltage support.

6. The method of asymmetric fault ride-through coordinated control of a virtual synchronous generator according to claim 1, characterized in that: The acceleration area S3 is controlled by suppressing the increase of the power angle, and the deceleration area S4 is controlled by suppressing the decrease of the power angle. Δ+ The calculation formula is as follows: where δ x is the power angle after acceleration is finished, is the active power generated by the positive sequence component.

7. A virtual synchronous generator asymmetrical fault ride-through coordination control system, characterized in that: The system comprises: a fault detection module for detecting an asymmetric fault of the power grid and triggering control strategy switching; a positive and negative sequence separation control module for setting a negative sequence current reference value to zero to realize three-phase symmetry of output current and freeze the reactive power loop; an excitation reference calculation module for calculating an optimal solution of a positive sequence excitation reference amplitude according to current limiting amplitude, voltage support capability and system capacity constraints; an active reference adjustment module for dynamically adjusting an active power reference value in real time based on the optimal solution of the positive sequence excitation reference amplitude to control a power angle to reach a target value.

8. The virtual synchronous generator asymmetrical fault ride-through coordinated control system of claim 7, wherein: The excitation reference calculation module sets the positive sequence excitation reference amplitude to the rated value E when the grid voltage is shallowly dropped N and is set to 9. The virtual synchronous generator asymmetrical fault ride-through coordinated control system of claim 7, wherein: The active power reference adjustment module uses the active power reference value P ref The dynamic adjustment ensures that the positive sequence work angle remains stable at the target value. And taking into account the active power generated by the negative sequence component. The impact on transient stability.

10. The virtual synchronous generator asymmetrical fault ride-through coordinated control system of claim 7, wherein: The system further comprises a hardware-in-the-loop experiment platform for verifying effectiveness of the control strategy, the platform comprising a control chip with DSP TMS320F28377D as a core and a StarSim HIL real-time simulator based on NI-PXIe-786R.

Images