Parallel fault-tolerant converter bus short circuit current limiting method and system

Abstract

The invention discloses a parallel fault-tolerant converter bus short-circuit current limiting method and system, and relates to the technical field of wind power generation, and the method comprises the steps: building a differential equation of a wind power generation voltage in a normal state, and solving a periodic component and a non-periodic component of the differential equation when a wind power generation converter has a short-circuit fault; calculating the relation between the periodic component and the normal current before the fault through the reactive power characteristic when the converter is short-circuited, obtaining the result expression of the periodic component, introducing a virtual resistor and a virtual inductor, orienting the reference voltage of the converter, and calculating the current of the converter based on the reference voltage, the current d-axis component and the current q-axis component of the converter. And calculating a virtual resistance value and a virtual inductance value which meet reactive current injection requirements. According to the invention, the short-circuit current of the converter bus can be effectively limited, and corresponding protection measures can be taken to protect the converter and other equipment in wind power generation when the wind power generation converter has a short-circuit fault.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of wind power generation, in particular to a parallel fault-tolerant converter bus short-circuit current limiting method and system, which is suitable for parallel converters in wind power generation. BACKGROUND

[0002] In wind power generation systems, parallel converters are widely used for power conversion and power transmission. However, when a bus short-circuit fault occurs, the converter may be subjected to excessive short-circuit current, leading to equipment damage and system instability. Existing current limiting methods have problems such as slow response speed and unsatisfactory current limiting effect.

[0003] Once a short-circuit fault occurs in a converter, the fault current reaches a peak value in a very short time, which not only increases line loss, but also may make protection fail to act reliably and make it difficult to clear the fault. Therefore, taking necessary current limiting measures after a fault occurs is an important transitional means to achieve fault clearance. Existing fault current limiting methods can be divided into two categories: one is to use physical devices to suppress short-circuit current, and the other is to change the control strategy of the converter to limit the fault current.

[0004] A network-type flexible DC system current limiting method, system and device based on power matching (publication number: CN119891342A) discloses that by converting the power limiting condition into an angular frequency judgment condition, the system determines that when overcurrent occurs under large disturbance, the power matching module is connected, the converter output power is limited to the maximum value, and the current is limited to the maximum value allowed by the converter.

[0005] The present technology introduces the concept of real-time angular frequency, converts real-time power into real-time angular frequency, and inevitably has errors and omissions in the conversion and adjustment process, which cannot timely judge the short-circuit fault, so a parallel fault-tolerant converter bus short-circuit current limiting method and system with faster response speed and more accurate judgment are needed. SUMMARY

[0006] The purpose of the present application is to provide a parallel fault-tolerant converter bus short-circuit current limiting method to solve the problems in the prior art.

[0007] To achieve the above purpose, the present application provides the following technical scheme: a parallel fault-tolerant converter bus short-circuit current limiting method, comprising:

[0008] S1: establishing a differential equation of wind power voltage under normal conditions, solving the differential equation when a short-circuit fault occurs in the converter of wind power generation, obtaining the periodic component i gp (t) and the non-periodic component i gnp (t) of the differential equation, calculating the non-periodic component ignp The maximum value of (t)i gnpm ;

[0009] S2: Based on the characteristic that the reactive power Q can be ignored when the converter is short-circuited, the periodic component i is calculated. gp The relationship between the normal current before the fault and the calculation result is converted into a phasor representation to obtain the periodic component i gp The result expression of

[0010] S3: Introduce virtual resistance and virtual inductance to calculate the d-axis component I of the wind power generation current d and the q-axis component of the wind power generation current I q , based on the d-axis component I d and the q-axis component I q Calculate the converter short-circuit current;

[0011] S4: Set the reference voltage U c Directed according to the wind power generation voltage, a reference voltage value is calculated, and virtual resistance and virtual inductance values ​​that meet the reactive current injection requirements are calculated through the reference voltage, the converter current d-axis component and the converter q-axis component.

[0012] Preferably, the periodic component i gp (t) and the non-periodic component i gnp The calculation method of (t) is:

[0013]

[0014] Among them: U c is the converter output voltage, ω0 is the rated angular frequency, is the impedance angle, t0 is the fault time, E g is the single-phase wind power generation voltage, L t is the circuit inductance, R t is the circuit resistance, k is the voltage drop coefficient of wind power generation;

[0015] Preferably, the relationship between the impedance angle and the circuit inductance and circuit resistance is:

[0016]

[0017] Preferably, for the non-periodic component i gnp The maximum value of (t)i gnpm The calculation method is:

[0018]

[0019] Preferably, when the converter is in a short-circuit state, it mainly transmits active power to the wind power generation, and transmits less reactive power. The wind power generation voltage is calculated as follows:

[0020]

[0021] Wherein: U is wind power voltage, and δ is phase difference.

[0022] Preferably, the wind power current in the d-axis component I d and the wind power current in the q-axis component I q The calculation method is as follows:

[0023]

[0024] Wherein, I gd is the current of the converter in the d-axis, and I gq is the current of the converter in the q-axis.

[0025] Preferably, the calculation method of the converter current in the d-axis component I gd and the converter current in the q-axis component I gq The calculation method is as follows:

[0026]

[0027] I gd = I gN

[0028] Wherein, I gN is the voltage stabilization current.

[0029] Preferably, the calculation method of the reference voltage U c of the converter oriented according to the wind power voltage is as follows:

[0030]

[0031] Wherein, R v is a virtual resistance, and L v is a virtual inductance.

[0032] Preferably, the calculation method of the virtual resistance and the virtual inductance value meeting the requirement of reactive current injection is as follows:

[0033]

[0034] A computer device, the device comprising one or more processors and one or more memories, at least one program code is stored in the one or more memories, the program code is executed by the one or more processors, and a parallel fault-tolerant type converter bus short circuit current limiting method is realized.

[0035] Compared with the prior art, the beneficial effects of the present application are:

[0036] 1、The method can effectively limit the short-circuit current of the converter bus. When a short-circuit fault occurs in the converter of the wind power generation, the differential equation of the wind power generation voltage under normal conditions is established, and the periodic component and the non-periodic component are accurately solved. The calculation of the maximum value of the non-periodic component can predict the peak value of the short-circuit current in advance, thereby providing key data support for taking corresponding protection measures. This helps to avoid damage to the converter itself and other equipment in the wind power generation caused by excessive short-circuit current, and improves the safety of the entire power system.

[0037] 2、Based on the characteristic that the reactive power can be ignored when the converter is short-circuited, the relationship between the periodic component and the normal current before the fault is converted into a phasor representation, which makes the current calculation more accurate and intuitive. The virtual resistance and virtual inductance are introduced to calculate the wind power generation current component, which further refines the control of the converter short-circuit current. By orienting the reference voltage of the converter according to the wind power generation voltage, the virtual resistance and virtual inductance values that meet the requirement of reactive current injection are calculated, which can realize accurate control of the converter current, so that the converter can still operate according to the expected current characteristics under short-circuit fault conditions.

[0038] 3、The present application provides a parallel fault-tolerant solution. In a parallel operation converter system, when one of the converters experiences a bus short-circuit fault, this current limiting method can effectively limit the scope of the fault and prevent the fault from spreading to other parallel converters, thereby improving the fault tolerance of the entire system when facing a single converter fault and enhancing the reliability and stability of the system. BRIEF DESCRIPTION OF DRAWINGS

[0039] Figure 1 A virtual impedance principle diagram is introduced for the converter of the present application;

[0040] Figure 2 An equivalent circuit diagram after introducing virtual impedance for the converter of the present application;

[0041] Figure 3 A VSG control block diagram based on virtual impedance for the present application;

[0042] Figure 4 A power angle characteristic curve of the grid-connected converter during wind power generation voltage drop for the present application. DETAILED DESCRIPTION

[0043] The technical solutions in the embodiments of the present application will be described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present application, not all. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0044] Embodiment 1

[0045] As Figures 1-2 shown, the application provides a technical solution of a parallel fault-tolerant type converter bus short circuit current limiting method, comprising:

[0046] S1: establishing a differential equation of wind power generation voltage under normal state, solving the differential equation when short circuit fault occurs in the converter of wind power generation, obtaining periodic component i gp (t) and non-periodic component i gnp (t) of the differential equation, and calculating the maximum value i gnp (t) of the non-periodic component i gnpm ;

[0047] S2: based on the characteristic that reactive power Q can be ignored when the converter is short-circuited, calculating the relationship between the periodic component i gp and normal current before fault, converting the calculation result into a phasor representation, and obtaining the result expression of the periodic component i gp ;

[0048] S3: introducing virtual resistance and virtual inductance, calculating d-axis component I d of wind power generation current and q-axis component I q of wind power generation current, and calculating converter short-circuit current based on the d-axis component I d and q-axis component I q ;

[0049] S4: orienting the reference voltage U c of the converter according to wind power generation voltage, calculating reference voltage value, and calculating virtual resistance and virtual inductance values meeting the requirement of reactive current injection through the reference voltage, d-axis component of converter current and q-axis component of converter.

[0050] Preferably, the calculation method of the periodic component i gp (t) and the non-periodic component i gnp (t) is:

[0051]

[0052] Wherein, U c is the output voltage of the converter, ω0 is the rated angular frequency, is the impedance angle, t0 is the fault time, E g is the single-phase wind power generation voltage, L t is the circuit inductance, R t is the circuit resistance, and k is the wind power generation voltage drop coefficient.

[0053] Preferably, the relationship between the impedance angle and the circuit inductance and the circuit resistance is:

[0054]

[0055] Preferably, for the non-periodic component i gnp (t) is calculated as: gnpm

[0056]

[0057] Preferably, the current transformer mainly delivers active power and less reactive power to the wind power generation in the short-circuit state, and the wind power generation voltage is calculated as:

[0058]

[0059] wherein U is the wind power generation voltage and δ is the phase difference.

[0060] Preferably, the wind power generation current in the d-axis component I d and the wind power generation current in the q-axis component I q are calculated as:

[0061]

[0062] wherein I gd is the current of the current transformer in the d-axis and I gq is the current of the current transformer in the q-axis.

[0063] Preferably, the current transformer current in the d-axis component I gd and the current transformer current in the q-axis component I gq are calculated as:

[0064]

[0065] I gd = I gN

[0066] wherein I gN is the voltage stabilizing current.

[0067] Preferably, the calculation method for orienting the reference voltage U c of the current transformer according to the wind power generation voltage is:

[0068]

[0069] wherein R v is the virtual resistance and L v is the virtual inductance.

[0070] ​Preferably, the calculation method of the virtual resistance and the virtual inductance value meeting the reactive current injection requirement is as follows:

[0071]

[0072] A computer device, the device comprising one or more processors and one or more memories, at least one program code is stored in the one or more memories, the program code is executed by the one or more processors, realize a kind of parallel fault-tolerant type current transformer bus short-circuit current limiting method.

[0073] After the failure of wind power generation, the voltage of wind power generation drops to different degrees. In order to maintain the stability of the converter system, the instability mechanism of the converter after the voltage drop of wind power generation should be analyzed. First, by analyzing the power angle curve of the active power P and the power angle δ of the converter output, the power angle characteristics of the grid-connected converter after the voltage drop of wind power generation are studied, and then the influence of the voltage drop of wind power generation on the stability of the grid-connected system is analyzed. Due to the low bandwidth and slow regulation speed of the power control loop, the reference voltage generated by the power control loop will not change significantly within milliseconds after the failure of wind power generation, so when analyzing the transient impact current, it is approximately considered that the reference voltage does not change, and the influence of the power control loop is ignored.

[0074] Therefore, the present application needs to establish a differential equation after the voltage drop of wind power generation, solve the differential equation, and calculate the solution of the equation containing periodic components and non-periodic components. In order to accurately solve the maximum value of the transient impact current during the low voltage short circuit of wind power generation, the maximum values of the non-periodic component and the periodic component of the transient impact current need to be analyzed.

[0075] First, the maximum value of the non-periodic component of the transient impact current is analyzed. The expression of the non-periodic component is an exponential function, which decays in the form of an exponential decay coefficient. The value of the non-periodic component depends on the impedance angle φ of the system and the fault time t0, and its accurate value is difficult to determine, but its maximum value can be expressed as:

[0076]

[0077] In order to analyze the maximum value of the transient impact current under different voltage drops of wind power generation, it is considered that the non-periodic component is an exponential function with maximum value at the fault time t0. The non-periodic component of the transient impact current under the conditions of wind power generation voltage drop 100% (k=0), wind power generation voltage drop 50% (k=0.5) and wind power generation voltage drop 30% (k=0.7) can be obtained. With the deepening of the voltage drop of wind power generation, the value of the non-periodic component of the transient impact current is also larger.

[0078] Then the transient current periodic component needs to be derived. Considering that the grid-connected converter mainly transmits active power to the wind power generation and less reactive power, the active power Q can be calculated. To derive the relationship between the transient current periodic component and the normal current before the fault, the line resistance is ignored, and the vector form of the short-circuit current is obtained:

[0079]

[0080] The transient current periodic components of the wind power generation voltage drop of 100% (k=0), the wind power generation voltage drop of 50% (k=0.5), and the wind power generation voltage drop of 30% (k=0.7) can be obtained. Like the aperiodic component, the more serious the wind power generation fault is, the greater the value of the transient current periodic component is.

[0081] In the converter system, the transient current can be up to more than 7 times the rated current, and the introduction of virtual impedance in the control strategy can limit the transient current. It can be known that the virtual impedance is introduced into the system in the dq coordinate system. After the introduction of virtual voltage and virtual inductance, during the wind power generation fault, the current produces a phase shift relative to the voltage, so the voltage reference vector needs to be expressed.

[0082] The reference voltage U c of the grid-connected inverter is oriented according to the wind power generation voltage, and the current I d and I q are calculated.

[0083]

[0084] I gd = I gN

[0085] Therefore, the values of the virtual impedance, i.e., the values of the virtual resistance and the virtual inductance, required to meet the dynamic reactive current injection requirement and the current limiting requirement are calculated, which are supplemented to the circuit to achieve the purpose of current limiting.

[0086] Example 2:

[0087] As shown in Figures 1-2 , for the virtual resistance and virtual inductance introduction method of the converter, the virtual resistance and virtual inductance constitute the virtual impedance in the circuit. The virtual resistance and virtual inductance are equivalent to the impedance in series with the line for current limiting. To simplify the analysis of the system after the introduction of the virtual resistance, the system line resistance is ignored, and the virtual resistance is equivalent to the line series impedance. The single-phase equivalent circuit of the system is analyzed, and then the d-axis component I d of the wind power generation current and the q-axis component I qIt can be seen from the calculation process that I d and I q are related to virtual resistance and virtual inductance respectively. Although adjusting virtual resistance or virtual inductance alone has certain inhibitory effect on transient impact current, it cannot adjust the reactive current injected by the system. Therefore, a reasonable solution is to adjust virtual resistance and virtual inductance simultaneously, to adjust the reactive and active currents synchronously to achieve transient impact current suppression and fast reactive compensation at the same time.

[0088] For fault current calculation when short circuit occurs, the following formula can be used:

[0089]

[0090] It can be seen that Id and Iq are related to virtual resistance and virtual inductance respectively. Although adjusting virtual resistance or virtual inductance alone has certain inhibitory effect on transient impact current, it cannot adjust the reactive current injected by the system. Therefore, a reasonable solution is to adjust virtual resistance and virtual inductance simultaneously, to adjust the reactive and active currents synchronously to achieve transient impact current suppression and fast reactive compensation at the same time.

[0091] When short circuit fault occurs in the converter, the output voltage of the grid-connected converter and the reference phase cannot change instantaneously, so virtual impedance must be introduced to achieve current limiting and reactive compensation when wind power drops. Under normal working conditions, the phase angle of current and voltage is consistent, but under the condition of introducing virtual impedance after fault, the current produces a phase shift relative to the voltage, so the reference voltage U c is introduced to change the phase of the reference voltage U

[0092] Example 3:

[0093] As Figure 3The control strategy is also applicable to a converter adopting VSG (Virtual Synchronous Generator) control. Under the control strategy, the controller first calculates active and reactive power output by the converter according to the sampled VSG output voltage and current, the active power outer ring simulates the synchronous generator rotor swing equation, the reactive power outer ring adopts droop control to adjust the amplitude of the VSG output voltage, according to the VSG output voltage reference value obtained by the power outer ring, the virtual impedance voltage outer ring is used to obtain the VSG output current reference value, and the zero steady-state error control of the output current is realized through the current closed-loop PI controller, and finally the switching signal of the power device can be obtained.

[0094] Although a single virtual inductance or virtual resistance can limit the transient impact current, increasing only the virtual inductance value will result in an increase in the time constant of the non-periodic component attenuation, thereby slowing down the non-periodic component attenuation speed. In addition, compared with the virtual inductance, the virtual resistance has higher stability. Therefore, the preset virtual resistance is selected to limit the transient impact current in the embodiment.

[0095] Since the resistive component in the system is much smaller than the virtual resistance, we can replace the actual system resistance Rt with the virtual resistance Rv, and represent the sum of the virtual inductance and the actual system inductance as Ln. After introducing the virtual resistance and the virtual inductance, the periodic component and the non-periodic component of the transient impact current are calculated as follows:

[0096]

[0097] When normal grid-connected operation, the active power and the reactive power are known, the unknown quantity δ in the wind power generation fault transient impact current can be solved, the output voltage in the wind power generation fault transient process is regarded as constant, the expression of the periodic component of the transient impact current can be obtained, and the wind power generation fault time t0 detected in the actual line impedance angle The expression of the non-periodic component of the transient impact current can be obtained. In order to make full use of the overcurrent capability of the hardware platform, as much inductive reactive current as possible is provided to the wind power generation voltage at the wind power generation drop moment to help the wind power generation voltage recover. According to the formula and the overcurrent capability of the converter, the virtual resistance and the virtual inductance values can be set to meet the requirements of limiting the current and quickly injecting the reactive current.

[0098] Embodiment 4:

[0099] As Figure 4As shown, during the low voltage period of wind power generation, there is a difference between the converter output power and the power reference value, which may cause the converter to have steady-state overload current and transient power angle instability problems, endangering the stable operation of the grid-connected converter system. When the wind power fault voltage drops, the power angle characteristic curve of the grid-connected converter will change according to the different wind power voltage drop levels, in turn from the normal operation I curve to the II, III and IV curves. However, after adjusting the active reference value according to the wind power drop level, the intersection point of the corresponding active power reference value and the power angle characteristic curve is always near δs1, and the intersection point does not change. When the wind power voltage drop level is deep, the active power given value and the power angle characteristic curve will always have an intersection point, and the system will not have instability phenomenon.

[0100] It will be apparent to those skilled in the art that the application is not limited to the details of the above-exemplified embodiment and that the present application can be implemented in other particular forms without departing from the spirit or essential characteristics of the present application. Therefore, the embodiments should be considered in all respects as illustrative and not restrictive, the scope of the application being indicated by the appended claims rather than by the above description, and therefore all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein. No reference signs in the claims should be considered as limiting the scope of the claims.

Claims

1. A busbar short-circuit current limiting method for a parallel fault-tolerant converter, characterized by: include: S1: Establish a differential equation for the wind power generation voltage under normal conditions. When a short circuit fault occurs in the wind power converter, solve the differential equation to obtain the periodic component i of the differential equation. gp (t) and the non-periodic component i gnp (t), calculate the non-periodic component i gnp The maximum value of (t)i gnpm ; S2: Based on the characteristic that the reactive power Q can be ignored when the converter is short-circuited, the periodic component i is calculated. gp The relationship between the normal current before the fault and the calculation result is converted into a phasor representation to obtain the periodic component i gp The result expression of S3: Introduce virtual resistance and virtual inductance to calculate the d-axis component I of the wind power generation current d and the q-axis component of the wind power generation current I q , based on the d-axis component I d and the q-axis component I q Calculate the converter short-circuit current; S4: Set the reference voltage U c Directed according to the wind power generation voltage, a reference voltage value is calculated, and virtual resistance and virtual inductance values ​​that meet the reactive current injection requirements are calculated through the reference voltage, the converter current d-axis component and the converter q-axis component.

2. The busbar short-circuit current limiting method for a parallel fault-tolerant converter according to claim 1, characterized in that: The periodic component i gp (t) and the non-periodic component i gnp The calculation method of (t) is: Among them: U c is the converter output voltage, ω0 is the rated angular frequency, is the impedance angle, t0 is the fault time, E g is the single-phase wind power generation voltage, L t is the circuit inductance, R t is the circuit resistance, and k is the voltage drop coefficient of wind power generation.

3. The busbar short-circuit current limiting method for a parallel fault-tolerant converter according to claim 2, characterized in that: The impedance angle is related to the circuit inductance and circuit resistance as follows:

4. The busbar short-circuit current limiting method for a parallel fault-tolerant converter according to claim 2, characterized in that: For the non-periodic component i gnp The maximum value of (t)i gnpm The calculation method is:

5. The busbar short-circuit current limiting method for parallel fault-tolerant converters according to claim 4, characterized in that: When the converter is in short-circuit state, it mainly transmits active power to the wind power generation, and transmits less reactive power. Calculate the wind power generation voltage: Where: U is the wind power generation voltage, δ is the phase difference.

6. A busbar short-circuit current limiting method for parallel fault-tolerant converters according to claim 5, characterized in that: The d-axis component of the wind power generation current I d and the q-axis component of the wind power generation current I q The calculation method is: Among them, I gd is the current of the converter on the d axis, I gq is the current of the converter in the q axis.

7. A busbar short-circuit current limiting method for a parallel fault-tolerant converter according to claim 6, characterized in that: The d-axis component of the converter current I gd and the converter in the q-axis component I gq The calculation method is: I gd =I gN Among them, I gN is the regulated current.

8. The busbar short-circuit current limiting method for parallel fault-tolerant converters according to claim 7, characterized in that: The reference voltage U c The calculation method for orientation according to wind power generation voltage is: Among them, R v is the virtual resistance, L v is the virtual inductor.

9. The busbar short-circuit current limiting method for parallel fault-tolerant converters according to claim 8, characterized in that: The calculation method of the virtual resistance and virtual inductance values ​​that meet the reactive current injection requirements is:

10. A computer device, comprising one or more processors and one or more memories, wherein the one or more memories store at least one program code, and when the program code is executed by the one or more processors, implements a parallel fault-tolerant converter bus short-circuit current limiting method according to any one of claims 1 to 9.

Images