CN116365580A - Method for establishing short-circuit current calculation model of offshore wind power transmission system - Google Patents
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Abstract
The invention relates to a method for establishing a short-circuit current calculation model of an offshore wind power transmission system, which is used for researching a transient fault current short-circuit calculation model considering the influence of a positive and negative sequence decomposition link, and obtaining a circuit equation under a dq axis coordinate system and a relation formula of positive/negative sequence calculation components of inverter outlet voltage and positive/negative sequence modulation signals by considering an SOGI transfer function in the positive and negative sequence decomposition link; and obtaining a short circuit calculation model by using a circuit equation, a control equation and a difference relation. The invention has scientific and reasonable design, can realize accurate calculation of the transient fault current, is suitable for different short circuit fault types, and accurately reveals the transient characteristic of the fault current.
Description
Technical Field
The invention belongs to the technical field of power systems and automation, relates to relay protection, and in particular relates to a method for establishing a short-circuit current calculation model of an offshore wind power transmission system.
Background
When different types of short-circuit faults occur in the offshore alternating current sending-out system, transient response of positive and negative sequence decomposition links in the control system can have significant influence on transient current output by the wind power plant. At present, the effect of low voltage ride through control is generally considered for steady-state fault current analysis, and a steady-state current value is obtained through sequential network analysis and calculation. Most of the documents equivalent a wind farm in a steady state after a short circuit fault to a controlled voltage source or a controlled current source, and establish a modeling program for simplifying fault analysis and calculation of a composite network under different fault conditions; the influence of links such as inverter control and phase-locked loop is considered in a calculation model aiming at transient fault current, and a transient current value is calculated through a train-write control link equation and an equivalent circuit differential equation.
However, the influence of the positive and negative sequence decomposition links on the short circuit calculation model is not considered in the prior art, so that the transient characteristics of the fault current cannot be accurately revealed. Aiming at the problem, the method fully considers the influence of positive and negative sequence decomposition links, establishes a fault current calculation model, and can reveal the transient characteristics of short-circuit current in the fault transient period in detail.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provides a method for establishing a short-circuit current calculation model of an offshore wind power transmission system.
The invention solves the technical problems by the following technical proposal:
a method for establishing a short-circuit current calculation model of an offshore wind power transmission system is characterized by comprising the following steps of: the method comprises the following steps:
s1, analyzing and deducing a fan grid-connected system circuit equation under a positive/negative sequence dq axis coordinate system in a positive/negative sequence decomposition link based on a positive/negative sequence decomposition process of a DSOGI-PLL
(1) Circuit equation in positive sequence dq axis coordinate system
Grid-connected point voltage u is written in array based on offshore wind power transmission system topological structure a 、u b 、u c To inverter outlet voltage v a 、v b 、v c The three-phase circuit equation is:
the three-phase circuit equation (1) is obtained by the Clark transformation to an alpha beta coordinate system:
wherein: v' α 、v' β For transforming to converter outlet voltage components under a two-phase stationary coordinate system;
i' α 、i' β outputting a current component for the converter under the transformation to the two-phase stationary coordinate system;
the SOGI element uses a transfer function in the complex frequency domain to represent the relationship between the input signal and the output signal:
carrying out Laplace transformation on the formula (2) to obtain a circuit equation under a complex frequency domain of a two-phase stationary coordinate system:
wherein: i' α (t 0 )、i' β (t 0 ) The current initial value is the current initial value under a two-phase static coordinate system during fault;
the SOGI link of (4) is performed by adopting a coordinate transformation mode to obtain positive sequence components under a two-phase static coordinate system
The method comprises the following steps:
transfer function G in equation (3) 1 (s)、G 2 (s) bringing into formula (5) and simplifying it to be available:
wherein:
the current initial value of the positive sequence component under the two-phase static coordinate system at the fault moment;
f 1 (s)、f 2 (s) is an oscillation damping component generated in the simplification process, and the specific expression is shown in the formula (7):
transforming equation (6) to the time domain yields:
finally, performing park transformation on the formula (8) to obtain a circuit equation under a positive sequence dq axis coordinate system:
wherein:
calculating a component for the inverter outlet positive sequence dq axis voltage after the decomposition of the DSOGI-PLL;
(2) Circuit equation under negative sequence dq axis coordinate system
The negative sequence circuit equation obtained according to the transformation process of (1) is:
wherein:
calculating a component for the negative sequence dq axis voltage of the inverter outlet after DSOGI decomposition;
h 1 (t)、h 2 and (t) is an attenuation component generated in the simplification process, and the specific expression is shown in the formula (11):
s2, writing a control loop equation according to the equivalent model of the positive and negative sequence control loop
(1) Positive sequence control loop equivalent calculation model
The positive sequence control adopts a low voltage ride through control scheme, and the basic principle is as follows: when the voltage u is the grid-connected point abc After falling, detecting the positive sequence voltage u of the grid-connected point pcc Falling amplitude, and determining an active reactive current reference value according to the voltage falling degree
The final control objective is: according to the short-circuit current tolerance capability of the converter, realizing the fault current i output after external fault abc Not more than 1.2 times the rated current;
when the voltage drop is less than 90%, the current control strategy is maintained unchangedI.e. generating an inner loop controlled dq-axis current reference value by outer loop control in accordance with a control strategy in normal operation
The mathematical equation of the outer loop control in the dq coordinate system is:
wherein: q (Q) * 、
Is the reference value of the reactive power and the direct current voltage of the outer ring;
Q、u dc is to actually send out reactive power and direct current side voltage;
when u is pcc After falling to 90%, the network side converter is required to provide a certain reactive support function, so as to improve the reactive response speed,
switching the outer loop reactive control to direct current control without the outer loop control generation, +.>
Will change to:
wherein: u (u) pcc Positive sequence voltage of dq axis with grid-connected point
The relation is:
at the position of
In the case of a determination, ->
By->
And->
Determining as shown in formula (16):
order the
And->
Equality, the voltage drop degree u 'of the grid-connected point when the d-axis is switched to direct current control can be obtained' pcc ：
And when u pcc When the voltage drops below 10%, the network-side converter only outputs reactive current, and the formulas (14) and (16) are modified as follows:
based on the low voltage ride through control strategy, when the handle
Regarded as u pcc Can be used to drop the voltage before and after +.>
The variation is represented by formula (19):
wherein: epsilon (u) pcc ) Is a step function;
finally, a control loop equation is obtained by utilizing the current reference value and the voltage current positive sequence component, wherein the control loop equation is as follows:
(2) Equivalent calculation model of negative sequence control loop
Because the direct current side capacitance of the grid-connected double PWM converter is larger, the double frequency pulsation of the direct current bus capacitor voltage is smaller under unbalanced voltage of smaller degree, and a large amount of negative sequence current is required to be injected into the power grid while active or reactive power fluctuation is restrained, which can aggravate unbalanced degree of the grid-side current, lead to uneven heating of the grid-connected converter and control wind of converter operationRisk increases; therefore, a control mode of controlling the negative sequence current of the power grid to be zero is generally adopted in the actual engineering at present, according to a negative sequence current inhibition control strategy of the wind turbine generator after the occurrence of faults, a negative sequence control current loop is often designed by combining a steady-state circuit equation of a main loop, and when a positive and negative sequence decomposition link is added in a control system, the decomposed negative sequence dq axis current is usually adopted
Grid-connected point negative sequence voltage->
Adding a negative sequence current loop and calculating to obtain a negative sequence dq axis inverter voltage modulation signal +.>
The negative sequence current reference is typically set to i d * 、i q * The available negative sequence suppression current loop control response equation for 0 is shown in equation (21):
wherein: l is the equivalent inductance of the filter between the network side converter port and the grid-connected point;
obtained by a negative-sequence current loop in a control system
Positive sequence voltage modulation signal +.>
Modulating an inverter by using a park inverse transformation to obtain a three-phase voltage modulation signal;
s3, analyzing the relation between the circuit equation decomposition quantity of the inverter outlet voltage and the modulation signal under the dq axis coordinate system
According to the coordinate transformation principle of the outlet voltage of the inverter, in a control system, the generation method of the modulating wave of the outlet voltage of the inverter comprises the following steps: the positive and negative sequence dq axis voltage obtained by the control of the inner loop positive and negative sequence current loop
Respectively through park inverse transformation and superposition to generate three-phase modulation wave, its value and converter outlet voltage v a 、v b 、v c Equal, there is thus a relationship as shown in the formula (22):
the equation (22) is subjected to coordinate transformation including positive and negative sequence decomposition links, and a decoupled positive and negative sequence equation can be obtained by taking the positive sequence equation into consideration when the right part of the equal sign of the equation (22) is subjected to positive and negative sequence decomposition links
Relation formula:
wherein: ", represents convolution;
"." represents the product;
g 1 (t)、g 2 (t) SOGI transfer functions G respectively 1 (s)、G 2 (s) reducing to a time domain expression;
from the formulas (23) and (24), the left-end circuit equation obtains the positive sequence component identical to the formula (9) after the coordinate transformation including the positive and negative sequence decomposition links
Thereby obtaining the positive sequence component +.>
Is shown in the specific relation of ∈and ∈>
The difference relation of the negative sequence components of the outlet voltage of the inverter can be obtained by the same method:
from the formulae (25) and (26), it can be seen that
S4, simultaneous equations are used for obtaining a transient short-circuit current time domain calculation model
The formula (9), the formula (20), the formula (23) and the formula (24) are combined to obtain a compound containing six unknowns
Second order differential equation set shape of six equationsThe positive sequence time domain short circuit model of the formula is shown in the formula (27):
the formula (10), the formula (21), the formula (25) and the formula (26) are combined to obtain a compound containing six unknowns
The negative sequence time domain short circuit model in the form of a second order differential equation set of six equations is shown in equation (28):
for simplified calculation, the difference between the component and the modulation signal is calculated for the inverter outlet voltage
Fitting is performed, and the fitting result is substituted as a known quantity to be calculated, and then the formula (27) and the formula (28) can be respectively simplified as:
the invention has the advantages and beneficial effects that:
according to the method for establishing the short-circuit current calculation model of the offshore wind power transmission system, a transient fault current short-circuit calculation model considering the influence of a positive and negative sequence decomposition link is researched, and an SOGI transfer function in the positive and negative sequence decomposition link is considered, so that a circuit equation under a dq axis coordinate system and a relation formula of positive/negative sequence calculation components of an inverter outlet voltage and positive/negative sequence modulation signals are obtained; the invention can realize accurate calculation of transient fault current, is applicable to different short-circuit fault types, and accurately reveals the transient characteristics of the fault current.
Drawings
FIG. 1 is a flow chart of the present invention;
FIG. 2 is a topology of the offshore wind turbine export system of the present invention;
FIG. 3 is a block diagram of a DSOGI-PLL of the present invention;
FIG. 4 is a diagram of a low voltage ride through control scheme in accordance with the present invention;
FIG. 5 is a diagram of a negative sequence current suppression scheme in accordance with the present invention;
FIG. 6 is a schematic diagram of the inverter outlet voltage coordinate transformation of the present invention;
FIG. 7 is a graph of the difference between the output of the inner loop and the output of the positive sequence dq-axis inverter according to the present invention;
fig. 8a is a graph of the calculation and simulation of the positive sequence d-axis current of the present invention, and fig. 8b is a graph of the calculation and simulation of the positive sequence q-axis current of the present invention.
Detailed Description
The invention is further illustrated by the following examples, which are intended to be illustrative only and not limiting in any way.
As shown in fig. 1, a method for establishing a short-circuit current calculation model of an offshore wind power transmission system is innovative in that: the method comprises the following steps:
step S1, as shown in FIG. 3, analyzing and deducing a fan grid-connected system circuit equation under a positive/negative sequence dq axis coordinate system in a positive/negative sequence decomposition link based on a positive/negative sequence decomposition process of a DSOGI-PLL
(1) Circuit equation in positive sequence dq axis coordinate system
As shown in FIG. 2, the grid-connected point voltage u is written based on the topology of the offshore wind power transmission system a 、u b 、u c To inverter outlet voltage v a 、v b 、v c The three-phase circuit equation is:
the three-phase circuit equation (1) is obtained by the Clark transformation to an alpha beta coordinate system:
wherein: v' α 、v' β For transforming to converter outlet voltage components under a two-phase stationary coordinate system;
i' α 、i' β outputting a current component for the converter under the transformation to the two-phase stationary coordinate system;
the SOGI element uses a transfer function in the complex frequency domain to represent the relationship between the input signal and the output signal:
carrying out Laplace transformation on the formula (2) to obtain a circuit equation under a complex frequency domain of a two-phase stationary coordinate system:
wherein: i' α (t 0 )、i' β (t 0 ) The current initial value is the current initial value under a two-phase static coordinate system during fault;
the SOGI link of (4) is performed by adopting a coordinate transformation mode to obtain positive sequence components under a two-phase static coordinate system
The method comprises the following steps:
in the formula (3)Transfer function G 1 (s)、G 2 (s) bringing into formula (5) and simplifying it to be available:
wherein:
the current initial value of the positive sequence component under the two-phase static coordinate system at the fault moment;
f 1 (s)、f 2 (s) is an oscillation damping component generated in the simplification process, and the specific expression is shown in the formula (7):
transforming equation (6) to the time domain yields:
finally, performing park transformation on the formula (8) to obtain a circuit equation under a positive sequence dq axis coordinate system:
wherein:
calculating a component for the inverter outlet positive sequence dq axis voltage after the decomposition of the DSOGI-PLL;
(2) Circuit equation under negative sequence dq axis coordinate system
The negative sequence circuit equation obtained according to the transformation process of (1) is:
wherein:
calculating a component for the negative sequence dq axis voltage of the inverter outlet after DSOGI decomposition;
h 1 (t)、h 2 and (t) is an attenuation component generated in the simplification process, and the specific expression is shown in the formula (11):
s2, writing a control loop equation according to the equivalent model of the positive and negative sequence control loop
(1) Positive sequence control loop equivalent calculation model
As shown in fig. 4, a low voltage ride through control scheme is adopted for positive sequence control, and the basic principle is as follows: when the voltage u is the grid-connected point abc After falling, detecting the positive sequence voltage u of the grid-connected point pcc Falling amplitude, and determining an active reactive current reference value according to the voltage falling degree
The final control objective is: according to the short-circuit current tolerance capability of the converter, realizing the fault current i output after external fault abc Not more than 1.2 times the rated current;
when the voltage drop is less than 90%, the current control strategy is maintained unchanged, namely the current reference value of the internal ring control dq axis is generated by the external ring control according to the control strategy in normal operation
The mathematical equation of the outer loop control in the dq coordinate system is:
wherein: q (Q) * 、
Is the reference value of the reactive power and the direct current voltage of the outer ring;
Q、u dc is to actually send out reactive power and direct current side voltage;
when u is pcc After falling to 90%, the network side converter is required to provide a certain reactive support function, so as to improve the reactive response speed,
switching the outer loop reactive control to direct current control without the outer loop control generation, +.>
Will change to:
wherein: u (u) pcc Positive sequence voltage of dq axis with grid-connected point
The relation is:
at the position of
In the case of a determination, ->
By->
And->
Determining as shown in formula (16):
order the
And->
Equality, the voltage drop degree u 'of the grid-connected point when the d-axis is switched to direct current control can be obtained' pcc ：
And when u pcc When the voltage drops below 10%, the network-side converter only outputs reactive current, and the formulas (14) and (16) are modified as follows:
based on the low voltage ride through control strategy, when the handle
Regarded as u pcc Can be used to drop the voltage before and after +.>
The variation is represented by formula (19):
wherein: epsilon (u) pcc ) Is a step function;
finally, a control loop equation is obtained by utilizing the current reference value and the voltage current positive sequence component, wherein the control loop equation is as follows:
(2) Equivalent calculation model of negative sequence control loop
Because the direct current side capacitance of the grid-connected double PWM converter is larger, the double frequency pulsation of the direct current bus capacitor voltage is smaller under unbalanced voltage of smaller degree, and a large amount of negative sequence current needs to be injected into the power grid while active or reactive power fluctuation is restrained, so that the unbalanced degree of the grid-side current can be aggravated, the grid-connected converter is nonuniform in heating, and the running control risk of the converter is increased; therefore, in the actual engineering at present, a control mode of controlling the negative sequence current of the power grid to be zero is generally adopted, according to a negative sequence current suppression control strategy of the wind turbine after the occurrence of faults, as shown in fig. 5, a negative sequence control current loop is often designed by combining a steady-state circuit equation of a main loop, and when a positive and negative sequence decomposition link is added in a control system, the decomposed negative sequence dq axis current is usually designed
Grid-connected point negative sequence voltage->
Adding a negative sequence current loop and calculating to obtain a negative sequence dq axis inverter voltage modulation signal +.>
The negative sequence current reference is typically set to i d * 、i q * The available negative sequence suppression current loop control response equation for 0 is shown in equation (21):
wherein: l is the equivalent inductance of the filter between the network side converter port and the grid-connected point;
obtained by a negative-sequence current loop in a control system
Positive sequence voltage modulation signal +.>
Modulating an inverter by using a park inverse transformation to obtain a three-phase voltage modulation signal;
s3, analyzing the relation between the circuit equation decomposition quantity of the inverter outlet voltage and the modulation signal under the dq axis coordinate system
As shown in fig. 6, according to the inverter outlet voltage coordinate transformation principle, in the control system, the inverter outlet voltage modulation wave generation method is as follows: the positive and negative sequence dq axis voltage obtained by the control of the inner loop positive and negative sequence current loop
Respectively through park inverse transformation and superposition to generate three-phase modulation wave, its value and converter outlet voltage v a 、v b 、v c Equal, there is thus a relationship as shown in the formula (22):
the equation (22) is subjected to coordinate transformation including positive and negative sequence decomposition links, and a decoupled positive and negative sequence equation can be obtained by taking the positive sequence equation into consideration when the right part of the equal sign of the equation (22) is subjected to positive and negative sequence decomposition links
Relation formula:
wherein: ", represents convolution;
"." represents the product;
g 1 (t)、g 2 (t) SOGI transfer functions G respectively 1 (s)、G 2 (s) reducing to a time domain expression;
from the formulas (23) and (24), the left-end circuit equation obtains the positive sequence component identical to the formula (9) after the coordinate transformation including the positive and negative sequence decomposition links
Thereby obtaining the positive sequence component +.>
Is shown in the specific relation of ∈and ∈>
The difference relation of the negative sequence components of the outlet voltage of the inverter can be obtained by the same method:
from the formulae (25) and (26), it can be seen that
S4, simultaneous equations are used for obtaining a transient short-circuit current time domain calculation model
The formula (9), the formula (20), the formula (23) and the formula (24) are combined to obtain a compound containing six unknowns
The positive sequence time domain short circuit model in the form of a second order differential equation set of six equations is shown in equation (27): />
The formula (10), the formula (21), the formula (25) and the formula (26) are combined to obtain a compound containing six unknowns
The negative sequence time domain short circuit model in the form of a second order differential equation set of six equations is shown in equation (28):
for simplifying calculation, components are calculated for the outlet voltage of the inverterDifference of modulated signals
Fitting is performed, and the fitting result is substituted as a known quantity to be calculated, and then the formula (27) and the formula (28) can be respectively simplified as:
taking symmetric faults as an example to verify the accuracy of the proposed model
And setting a simulation model to generate three-phase faults in 2s, and dropping the voltage of the grid-connected point to 7%. Observing the inverter outlet voltage difference waveform is shown in figure 7,
the method can be approximately regarded as damping oscillation trend, and the damping oscillation trend tend to be stable after a transient process, namely the difference value is 0, and the transient process has larger fluctuation and longer duration, which is about 80ms. Fitting the difference of the outlet voltage of the inverter to obtain:
the dq-axis current transient calculation results obtained by substituting the formula (31) and the formula (32) as the known amounts into the formula (29) and calculating by the fourth-order longgrid-base method are shown in fig. 8 (a) and (b).
As can be seen from fig. 8 (a) and 8 (b), the difference between the positive sequence dq-axis transient current and the simulation current waveform solved by the formula is substantially small; simulation results prove that the transient current fault characteristic analysis method considering positive and negative sequence decomposition links provided by the patent application can effectively improve the accuracy of research results; and according to the waveform, positive and negative sequence decomposition links enable transient current to have obvious overshoot characteristics and longer duration.
Although the embodiments of the present invention and the accompanying drawings have been disclosed for illustrative purposes, those skilled in the art will appreciate that: various substitutions, changes and modifications are possible without departing from the spirit and scope of the invention and the appended claims, and therefore the scope of the invention is not limited to the embodiments and the disclosure of the drawings.
Claims (1)
1. A method for establishing a short-circuit current calculation model of an offshore wind power transmission system is characterized by comprising the following steps of: the method comprises the following steps:
s1, analyzing and deducing a fan grid-connected system circuit equation under a positive/negative sequence dq axis coordinate system in a positive/negative sequence decomposition link based on a positive/negative sequence decomposition process of a DSOGI-PLL
(1) Circuit equation in positive sequence dq axis coordinate system
Grid-connected point voltage u is written in array based on offshore wind power transmission system topological structure a 、u b 、u c To inverter outlet voltage v a 、v b 、v c The three-phase circuit equation is:
the three-phase circuit equation (1) is obtained by the Clark transformation to an alpha beta coordinate system:
wherein: v' α 、v' β For conversion to converter outlet electricity in two-phase stationary coordinate systemA pressing component;
i' α 、i' β outputting a current component for the converter under the transformation to the two-phase stationary coordinate system;
the SOGI element uses a transfer function in the complex frequency domain to represent the relationship between the input signal and the output signal:
carrying out Laplace transformation on the formula (2) to obtain a circuit equation under a complex frequency domain of a two-phase stationary coordinate system:
wherein: i' α (t 0 )、i' β (t 0 ) The current initial value is the current initial value under a two-phase static coordinate system during fault;
the SOGI link of (4) is performed by adopting a coordinate transformation mode to obtain positive sequence components under a two-phase static coordinate system
The method comprises the following steps:
transfer function G in equation (3) 1 (s)、G 2 (s) bringing into formula (5) and simplifying it to be available:
wherein:
the current initial value of the positive sequence component under the two-phase static coordinate system at the fault moment;
f 1 (s)、f 2 (s) is an oscillation damping component generated in the simplification process, and the specific expression is shown in the formula (7):
transforming equation (6) to the time domain yields:
finally, performing park transformation on the formula (8) to obtain a circuit equation under a positive sequence dq axis coordinate system:
wherein:
calculating a component for the inverter outlet positive sequence dq axis voltage after the decomposition of the DSOGI-PLL;
(2) Circuit equation under negative sequence dq axis coordinate system
The negative sequence circuit equation obtained according to the transformation process of (1) is:
wherein:
calculating a component for the negative sequence dq axis voltage of the inverter outlet after DSOGI decomposition;
h 1 (t)、h 2 and (t) is an attenuation component generated in the simplification process, and the specific expression is shown in the formula (11):
s2, writing a control loop equation according to the equivalent model of the positive and negative sequence control loop
(1) Positive sequence control loop equivalent calculation model
The positive sequence control adopts a low voltage ride through control scheme, and the basic principle is as follows: when the voltage u is the grid-connected point abc After falling, detecting the positive sequence voltage u of the grid-connected point pcc Falling amplitude, and determining an active reactive current reference value according to the voltage falling degree
The final control objective is: according to the short-circuit current tolerance capability of the converter, realizing the fault current i output after external fault abc Not more than 1.2 times the rated current;
when the voltage drop is less than 90%, the current control strategy is maintained unchanged, namely the current reference value of the internal ring control dq axis is generated by the external ring control according to the control strategy in normal operation
The mathematical equation of the outer loop control in the dq coordinate system is:
wherein: q (Q) * 、
Is the reference value of the reactive power and the direct current voltage of the outer ring;
Q、u dc is to actually send out reactive power and direct current side voltage;
when u is pcc After falling to 90%, the network side converter is required to provide a certain reactive support function, so as to improve the reactive response speed,
switching the outer loop reactive control to direct current control without the outer loop control generation, +.>
Will change to:
wherein: u (u) pcc Positive sequence voltage of dq axis with grid-connected point
The relation is:
at the position of
In the case of a determination, ->
By->
And->
Determining as shown in formula (16):
order the
And->
Equality, the voltage drop degree u 'of the grid-connected point when the d-axis is switched to direct current control can be obtained' pcc ：
And when u pcc When the voltage drops below 10%, the network-side converter only outputs reactive current, and the formulas (14) and (16) are modified as follows:
based on the low voltage ride through control strategy, when the handle
Regarded as u pcc Can be used to drop the voltage before and after +.>
The variation is represented by formula (19):
wherein: epsilon (u) pcc ) Is a step function;
finally, a control loop equation is obtained by utilizing the current reference value and the voltage current positive sequence component, wherein the control loop equation is as follows:
(2) Equivalent calculation model of negative sequence control loop
Because the direct current side capacitance of the grid-connected double PWM converter is larger, the double frequency pulsation of the direct current bus capacitor voltage is smaller under unbalanced voltage of smaller degree, and a large amount of negative sequence current needs to be injected into the power grid while active or reactive power fluctuation is restrained, so that the unbalanced degree of the grid-side current can be aggravated, the grid-connected converter is nonuniform in heating, and the running control risk of the converter is increased; therefore, a control mode of controlling the negative sequence current of the power grid to be zero is generally adopted in the actual engineering at present, according to a negative sequence current inhibition control strategy of the wind turbine generator after the occurrence of faults, a negative sequence control current loop is often designed by combining a steady-state circuit equation of a main loop, and when a positive and negative sequence decomposition link is added in a control system, the decomposed negative sequence dq axis current is usually adopted
Grid-connected point negative sequence voltage->
Adding a negative sequence current loop and calculating to obtain a negative sequence dq axisInverter voltage modulation signal
The negative sequence current reference is typically set to i d * 、i q * The available negative sequence suppression current loop control response equation for 0 is shown in equation (21):
wherein: l is the equivalent inductance of the filter between the network side converter port and the grid-connected point;
obtained by a negative-sequence current loop in a control system
Positive sequence voltage modulation signal obtained with positive sequence current loop
Modulating an inverter by using a park inverse transformation to obtain a three-phase voltage modulation signal;
s3, analyzing the relation between the circuit equation decomposition quantity of the inverter outlet voltage and the modulation signal under the dq axis coordinate system
According to the coordinate transformation principle of the outlet voltage of the inverter, in a control system, the generation method of the modulating wave of the outlet voltage of the inverter comprises the following steps: the positive and negative sequence dq axis voltage obtained by the control of the inner loop positive and negative sequence current loop
Respectively through park inverse transformation and superposition to generate three-phase modulation wave, its value and converter outlet voltage v a 、v b 、v c Equal, there is thus a relationship as shown in the formula (22):
the equation (22) is subjected to coordinate transformation including positive and negative sequence decomposition links, and a decoupled positive and negative sequence equation can be obtained by taking the positive sequence equation into consideration when the right part of the equal sign of the equation (22) is subjected to positive and negative sequence decomposition links
Relation formula:
wherein: ", represents convolution;
"." represents the product;
g 1 (t)、g 2 (t) SOGI transfer functions G respectively 1 (s)、G 2 (s) reducing to a time domain expression;
from the formulas (23) and (24), the left-end circuit equation obtains the positive sequence component identical to the formula (9) after the coordinate transformation including the positive and negative sequence decomposition links
And then obtainAnd the actual positive sequence component->
Is shown in the specific relation of ∈and ∈>
The difference relation of the negative sequence components of the outlet voltage of the inverter can be obtained by the same method:
from the formulae (25) and (26), it can be seen that
S4, simultaneous equations are used for obtaining a transient short-circuit current time domain calculation model
The formula (9), the formula (20), the formula (23) and the formula (24) are combined to obtain a compound containing six unknowns
The positive sequence time domain short circuit model in the form of a second order differential equation set of six equations is shown in equation (27):
the formula (10), the formula (21), the formula (25) and the formula (26) are combined to obtain a compound containing six unknowns
Negative sequence time domain short circuit in the form of a second order differential equation set of six equationsThe model is shown in formula (28):
for simplified calculation, the difference between the component and the modulation signal is calculated for the inverter outlet voltage
Fitting is performed, and the fitting result is substituted as a known quantity to be calculated, and then the formula (27) and the formula (28) can be respectively simplified as:
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| WO2015035892A1 (en) * | 2013-09-10 | 2015-03-19 | 国家电网公司 | Method for calculating transient short circuit current of dfig set |
| CN107069800A (en) * | 2017-02-21 | 2017-08-18 | 华北电力大学 | A kind of method for building up of double-fed fan motor short circuit current flow model |
| CN115313473A (en) * | 2022-07-06 | 2022-11-08 | 华北电力大学 | Fault current analysis method considering inverter positive and negative sequence decoupling control |
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| WO2015035892A1 (en) * | 2013-09-10 | 2015-03-19 | 国家电网公司 | Method for calculating transient short circuit current of dfig set |
| CN107069800A (en) * | 2017-02-21 | 2017-08-18 | 华北电力大学 | A kind of method for building up of double-fed fan motor short circuit current flow model |
| CN115313473A (en) * | 2022-07-06 | 2022-11-08 | 华北电力大学 | Fault current analysis method considering inverter positive and negative sequence decoupling control |
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| CN117240049A (en) * | 2023-09-08 | 2023-12-15 | 东南大学 | Quick voltage response and transient state ride through control method and system for converter |
| CN117240049B (en) * | 2023-09-08 | 2024-03-19 | 东南大学 | Quick voltage response and transient state ride through control method and system for converter |
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