CN116961116B - Transient stability lifting method for grid-built inverter based on self-adaptive q-axis voltage feedback - Google Patents
Transient stability lifting method for grid-built inverter based on self-adaptive q-axis voltage feedback Download PDFInfo
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02P—CONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
- H02P21/00—Arrangements or methods for the control of electric machines by vector control, e.g. by control of field orientation
- H02P21/12—Stator flux based control involving the use of rotor position or rotor speed sensors
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02P—CONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
- H02P21/00—Arrangements or methods for the control of electric machines by vector control, e.g. by control of field orientation
- H02P21/14—Estimation or adaptation of machine parameters, e.g. flux, current or voltage
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02P—CONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
- H02P21/00—Arrangements or methods for the control of electric machines by vector control, e.g. by control of field orientation
- H02P21/22—Current control, e.g. using a current control loop
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Abstract
The invention discloses a self-adaptive q-axis voltage feedback-based grid-built inverter transient stability lifting method, which relates to the field of grid-built inverter fault ride-through. The invention can not influence the output of the active power of the grid-formed inverter under the normal operation condition, can automatically adjust the equivalent active power reference according to the voltage drop depth of the power grid under the short circuit fault condition, ensures that the system has a stable balance point, realizes the fault ride-through of the system under different voltage drop conditions of the power grid, and can improve the transient stability of the system.
Description
Technical Field
The invention relates to the field of fault ride-through of grid-formed inverters, in particular to a transient stability lifting method of a grid-formed inverter based on self-adaptive q-axis voltage feedback.
Background
As the power system is developing toward high-proportion new energy and high-proportion power electronic equipment, the strength of the power grid is reduced, and active support of system voltage and frequency is difficult to realize with the grid-type inverter. The grid-built inverter realizes active support of system voltage and frequency, and is therefore an important research object. However, the grid-structured inverter is extremely easy to cause an overcurrent problem during short-circuit faults, and is often limited by adopting a current limiting link, however, the addition of the current limiting link makes a transient process of the inverter during short-circuit faults more complex, a transient stability margin is reduced, and a transient instability is easily caused by losing a stable balance point.
At present, aiming at the problem that a grid-connected system of a grid-built inverter lacks a stable balance point after faults, main solutions comprise switching control strategies, virtual impedance increase, additional control loops and the like. The switching control strategy is to switch the grid-connected control into the grid-connected control when the power grid fails, and switch the grid-connected control back to the grid-connected control by the grid-connected control after the failure is removed so as to meet the reactive power demand of the system during the failure, but the method needs a standby phase-locked loop, is complex in operation in practical application, is difficult to ensure the smoothness and stability of the switching process, and has the stability problem under the weak network. Increasing the virtual impedance can limit transient over-current during a fault, thereby improving the fault ride through capability of the system, but is not applicable when the voltage drops substantially. Part of researches are to introduce a grid-connected point q-axis voltage feedback branch in an active power synchronous loop of a grid-structured inverter, and the introduction of the branch can reduce equivalent active power reference and increase system damping so as to improve transient stability. However, the q-axis voltage feedback coefficient is a fixed value, so that the output power is smaller than the active reference under the normal operation condition, and the selection principle of the q-axis voltage feedback coefficient is not given, so that the q-axis voltage feedback coefficient is difficult to be directly used for practical engineering application. Therefore, a control method capable of realizing that the output of the active power of the grid-formed inverter is not affected under the normal operation working condition, and simultaneously, the equivalent active power reference can be automatically adjusted according to the voltage drop depth of the power grid under the short circuit fault condition is needed, so that the system is ensured to have a stable balance point, and the transient stability of the system is improved.
Disclosure of Invention
The technical problems to be solved by the invention are as follows: the output of the active power of the grid-formed inverter is not affected under the normal operation working condition, and meanwhile, the equivalent active power reference can be automatically adjusted according to the dropping depth of the power grid voltage under the short circuit fault condition, so that the system is ensured to have a stable balance point, and the transient stability of the system is improved. In order to solve the technical problems, the invention provides a self-adaptive q-axis voltage feedback-based transient stability lifting method for a grid-built inverter, which comprises the following steps:
a method for improving transient stability of a grid-built inverter based on self-adaptive q-axis voltage feedback comprises the following steps:
step 1: constructing a grid-formed inverter control loop based on q-axis voltage feedback
Determining a grid-built inverter control based on q-axis voltage feedback: collecting inverter output voltageV PCC And current flowI g After park conversion, the output active power is obtained by transmitting the output active power to a power calculation modulePAnd output reactive powerQIs transmitted to a power control link through a first-order low-pass filter, and is controlled through active-angular frequency droop and inverter output voltageV PCC Generating angular frequency by q-axis component feedback leg of (2)ωIntegrating to obtain output phase angle of power synchronous unitθThen generates voltage modulation signal after passing through the voltage outer ring and the current inner ringm d Andm q modulating the voltage into a signalm d Andm q transmitting to an inverse Peak conversion module to obtain a modulated wavem abc Finally, generating a driving signal through a pulse width modulation module to control the on and off of the IGBT;
step 2: calculating self-adaptive q-axis voltage feedback coefficient based on transient stability constraint
Step 2.1: calculating output phase angle of power synchronization unit of grid-structured inverter based on q-axis voltage feedbackθAnd define virtual power angleδIs thatdShaft and grid voltageV g Phase angle difference between;
step 2.2: calculating the q-axis component of the output voltage of the grid-connected inverter during non-trigger current limiting according to the circuit relation and the phasor relationV qu And active powerP u Thereby obtaining the equivalent active power reference when the current limiting is not triggeredP refueq To cause the fault to be instantaneousP refueq Is equal to zero, thereby solving the q-axis voltage feedback coefficient when the current limiting is not triggeredk u ；
Step 2.3: calculating the q-axis component of the output voltage of the grid-connected inverter when triggering current limiting according to the circuit relation and the phasor relationV qlim And active powerP lim Thereby obtaining the equivalent active power reference when triggering current limitingP reflimeq To cause the fault to be instantaneousP reflimeq Is equal to zero, thereby solving the q-axis voltage feedback coefficient when triggering current limitingk lim ；
Step 3: self-adaptive control of grid-built inverter based on q-axis voltage feedback
Monitoring the voltage of a power grid and the output current of an inverter in real time, and when the system is in a normal running state, enabling the q-axis voltage feedback coefficient to bek=0, the inverter employs droop control with low pass filter;
when a short circuit fault occurs in the system, switching to a grid-formed inverter control based on q-axis voltage feedback, and detecting whether the system triggers current limiting or not;
if the current limiting is triggered, the q-axis voltage feedback coefficient is calculatedkSet to the q-axis voltage feedback coefficient when triggering current limitingk lim ；
If the current limiting is not triggered, the q-axis voltage feedback coefficient is calculatedkSet to q-axis voltage feedback coefficient when current limiting is not triggeredk u ；
After the fault is removed, the voltage feedback coefficient is calculatedkReset to 0, i.e. revert to droop control with low pass filter.
Further, in the step 1, the power calculation module calculates and outputs active powerPAnd output reactive powerQThe method of (1) is as follows:
（1）
wherein,Pin order to output the active power,Qin order to output the reactive power,V d and (3) withV q Is the output side voltage of the grid-structured inverterV PCC Voltage generated after Park conversiondqA component;I d and (3) withI q Is the output side current of the grid-structured inverterI g Current generated after Park conversiondqA component;
angular frequencyωVoltage phase angle of (2)θThe calculation method of (1) is as follows:
（2）
（3）
（4）
wherein,L(s) Is a transfer function of a first order low pass filter,ω c in order to cut-off the angular frequency,ωin order to be of an angular frequency,ω ref for the angular frequency to be rated,K p for the active-angular frequency dip coefficient,P ref as a reference value for the active power,θthe phase angle is output for the power synchronization unit,kfor the q-axis voltage feedback coefficient,sis a laplace operator.
Further, in the step 1, the voltage modulation signalm d Andm q the calculation method of (1) is as follows:
1) Calculating a voltage reference value obtained by a virtual impedance loopV dref And (3) withV qref ；
2) According to the voltage reference valueV dref And (3) withV qref Calculating an initial current reference value obtained by a voltage loopI d ref1 And (3) withI q ref1 ；
3) According to the initial current reference valueI d ref1 And (3) withI q ref1 Calculating a current reference value under the current limiting condition obtained by the current limiting loopI dref And (3) withI qref ；
4) According to the current reference value in the current limiting conditionI dref And (3) withI qref Calculating the acquisition of the voltage modulation signal by the current loopm d And (3) withm q ：
（5）
Wherein,V dc to input the dc voltage of the grid-formed inverter,m d and (3) withm q For the voltage modulation signal generated by the current loop,I fd and (3) withI fq As unfiltered currentdqThe component(s) of the composition,k pc and (3) withk ic The current loop ratio coefficient and the integral coefficient are respectively,L f is the filter inductance value.
Further, the droop control of the band-low pass filter specifically includes: collecting inverter output voltageV PCC And current flowI g After Park conversion, the output active power is obtained by transmitting the converted power to a power calculation modulePAnd output reactive powerQThe first order low pass filter is used for transmitting the signals to a droop control link, and the active-angular frequency droop control generates angular frequencyωIntegrating to obtain voltage phase angleθAfter passing through the voltage outer loop and the current inner loop, a modulation signal is generatedm d Andm q transmitting to an inverse Peak conversion module to obtain a modulated wavem abc And then the pulse width modulation module generates a driving signal to control the on and off of the IGBT.
Further, the power synchronization unit output phase angle of the grid-formed inverter based on q-axis voltage feedback in the step 2.1θExpressed as:
（6）
wherein,ω c in order to cut-off the angular frequency,K p is thatP-fThe coefficient of sagging is set to be the same,P ref in order to output the reference value of the active power,θ g is the voltage phase angle of the power grid;
defining virtual power angleδIs thatdShaft and grid voltageV g Phase angle difference between:
（7）
output Voltage q-axis component when Current limiting is not triggered in step 2.2V qu And active powerP u Expressed as:
（8）
（9）
wherein,V qu andP u the q-axis component of the output voltage and the active power when the current limiting is not triggered,X v for the virtual reactance value,X g is the line impedance;Ethe amplitude of the internal voltage source of the inverter;δ u the virtual power angle is the virtual power angle when the current limiting is not triggered;
equation (8) and equation (9) represent the q-axis component of the output voltage when current limiting is not triggeredV qu And active power at non-triggered current limitP u With the formula (6), the virtual power angle is calculated according to the formula (7) when the current limiting is not triggeredδ u Expressed as:
（10）
wherein,k u the q-axis voltage feedback coefficient is the q-axis voltage feedback coefficient when the current limiting is not triggered;
simultaneously deriving and simplifying the two sides to obtain the equivalent active power reference when the current limiting is not triggeredP refueq The expression of (2) is:
（11）
active power reference at fault momentP ref And virtual power angle when current limiting is not triggeredδ u Expressed as:
（12）
（13）
wherein,δ 0 the virtual power angle of the system when the fault occurs;
active power reference expressed by equation (12) and equation (13)P ref And virtual power angle when current limiting is not triggeredδ u Substituting formula (11) and lettingP refueq =0, give
（14）
Wherein,athe voltage drop coefficient is the ratio of the power grid voltage to the rated voltage after the fault;
solving the above to obtain the q-axis voltage feedback coefficient when the current is limited in an un-triggered wayk u The method comprises the following steps:
（15）
the q-axis component of the output voltage at the time of triggering current limiting in step 2.3V qlim And active powerP lim Expressed as:
（16）
（17）
wherein,V qlim andP lim q-axis division of output voltage when triggering current limitingThe amount and active power when triggering current limiting;δ lim the virtual power angle is used for triggering the current limiting;
the q-axis component of the output voltage when triggering current limiting is expressed by the formula (16) and the formula (17)V qlim And active power when triggering current limitingP lim With the formula (6), the virtual power angle in the current limiting is triggered according to the formula (7)δ lim Expressed as:
（18）
wherein,k lim the q-axis voltage feedback coefficient is used for triggering current limiting;
simultaneously deriving and simplifying the two sides to obtain the equivalent active power reference when triggering current limitingP reflimeq The expression of (2) is:
（19）
virtual power angle during fault instant triggering current limitingδ lim Expressed as:
（20）
active power reference expressed by equation (12) and equation (20)P ref And virtual power angle when triggering current limitingδ lim Substituting formula (19) and lettingP reflimeq =0, resulting in:
（21）
solving the above to obtain the q-axis voltage feedback coefficient when triggering current limitingk lim The method comprises the following steps:
（22）
wherein the line impedanceX g The impedance measurement is obtained in advance or based on an online impedance estimation method.
Compared with the prior art, the invention has the following beneficial effects:
the invention provides a transient stability lifting method of a grid-constructed inverter based on self-adaptive q-axis voltage feedback, which is used for realizing fault ride-through of the grid-constructed inverter when a power grid has short-circuit fault, and the self-adaptive q-axis voltage feedback coefficient can adaptively change equivalent active power references under different power grid voltage drop degrees, so that the existence of a stable balance point of a system is ensured no matter whether current amplitude limiting is triggered or not. In addition, under normal operation conditions, the value of the q-axis voltage feedback coefficient is equal to zero, and the output of active power of the system is not influenced.
The invention can not influence the output of the active power of the grid-formed inverter under the normal operation condition, can automatically adjust the equivalent active power reference according to the voltage drop depth of the power grid under the short circuit fault condition, ensures that the system has a stable balance point, realizes the fault ride-through of the system under different voltage drop conditions of the power grid, and can improve the transient stability of the system.
Drawings
Fig. 1 is a schematic diagram of a network inverter model with current limiting links and q-axis voltage feedback connected to a three-phase ac power grid.
Fig. 2 is a control loop of the grid-formed inverter based on q-axis voltage feedback of the present invention.
Fig. 3 is a q-axis voltage feedback-based adaptive control flow of a grid-formed inverter according to the present invention.
Fig. 4 (a) shows conventional droop control of the experimental waveform when the grid voltage drops to 0.2 pu.
Fig. 4 (b) shows experimental waveform adaptive control when the grid voltage drops to 0.2 pu.
FIG. 5 (a) shows the adaptive control at different levelsK p Experimental waveform of the lower grid voltage dip to 0.2 pu:K p =0.00002。
FIG. 5 (b) shows the adaptive controlDifferent fromK p Experimental waveform of the lower grid voltage dip to 0.2 pu:K p =0.001。
FIG. 6 shows an adaptive control of on-line inductanceL g Experimental waveforms for grid voltage dip to 0.2pu when=10mh.
Detailed Description
The technical scheme of the invention is described in detail below with reference to the accompanying drawings and the embodiments. So that those skilled in the art may better understand the present invention.
Fig. 1 is a schematic diagram of a network inverter model with current limiting links and q-axis voltage feedback connected to a three-phase ac power grid, wherein a control module mainly comprises an active power calculation module, an active power synchronization unit, a virtual impedance link, a voltage control loop, a current limiter, a current control loop and PWM.
The invention discloses a self-adaptive q-axis voltage feedback-based method for improving transient stability of a grid-built inverter, which comprises the following steps: 1) Constructing a grid-formed inverter control loop based on q-axis voltage feedback; 2) A calculation method of a self-adaptive q-axis voltage feedback coefficient based on transient stability constraint.
1. Constructing a grid-formed inverter control loop based on q-axis voltage feedback
Fig. 2 is a control block diagram of a grid-formed inverter power synchronization unit based on q-axis voltage feedback in accordance with the present invention. Mainly comprises a sagging control loop with a filter and a q-axis voltage feedback branch.
1) A droop control-based grid-formation inverter control loop: collecting inverter output voltageV PCC And current flowI g After Park conversion, the output active power is obtained by transmitting the converted power to a power calculation modulePAnd output reactive powerQThe first order low pass filter is used for transmitting the signals to a droop control link, and the active-angular frequency droop control generates angular frequencyωIntegrating to obtain voltage phase angleθAfter passing through the voltage outer loop and the current inner loop, a modulation signal is generatedm d Andm q delivering to a reverse Park conversion module to obtain a modulated wavem abc Then is produced by pulse width modulation moduleThe generated driving signal controls the on and off of the IGBT.
2) Grid-formed inverter control loop based on q-axis voltage feedback: collecting inverter output voltageV PCC And current flowI g After park conversion, the output active power is obtained by transmitting the output active power to a power calculation modulePAnd output reactive powerQIs transmitted to a power control link through a first-order low-pass filter, and is controlled and controlled through active-angular frequency droopV PCC Generating angular frequency by q-axis component feedback leg of (2)ωIntegrating to obtain voltage phase angleθAfter passing through the voltage outer loop and the current inner loop, a modulation signal is generatedm d Andm q delivering to a reverse Park conversion module to obtain a modulated wavem abc And then the pulse width modulation module generates a driving signal to control the on and off of the IGBT.
The specific implementation steps of the grid-formed inverter control based on q-axis voltage feedback are as follows:
(1) The output active power is obtained by the power calculation modulePAnd output reactive powerQThe specific formula is as follows:
（1）
wherein,Pin order to output the active power,Qin order to output the reactive power,V d and (3) withV q Is the output side voltage of the grid-structured inverterV PCC Voltage and current generated after Park conversiondqA component;I d and (3) withI q Is the output side current of the grid-structured inverterI g Voltage and current generated after Park conversiondqA component.
(2) Acquisition of angular frequency by droop controlωPhase angle with voltageθThe specific formula is as follows:
（2）
（3）
（4）
wherein,L(s) Is a transfer function of a first order low pass filter,ω c in order to cut-off the angular frequency,ωin order to be of an angular frequency,ω ref for the angular frequency to be rated,K p for the active-angular frequency dip coefficient,P ref as a reference value for the active power,θthe phase angle is output for the power synchronization unit.
(3) Obtaining voltage reference values from virtual impedance loopsV dref And (3) withV qref The specific formula is as follows:
（5）
wherein,V dref and (3) withV qref For the voltage reference value generated by the virtual impedance loop,E dref and (3) withE qref For a given voltage reference value,R v and (3) withX v For the virtual resistance value and the virtual reactance value,I d and (3) withI q To output currentdqA component.
(4) Obtaining initial current reference value by voltage loopI d ref1 And (3) withI q ref1 The specific formula is as follows:
（6）
wherein,I d ref1 and (3) withI q ref1 For the current reference value generated by the voltage loop,V d and (3) withV q To output voltagedqThe component(s) of the composition,k pv and (3) withk iv The proportional coefficient and the integral coefficient of the voltage ring are respectively,C f is the filter capacitance value.
(5) Obtaining a current reference value under the current limiting condition by a current limiting loopI dref And (3) withI qref The specific formula is as follows:
（7）
wherein,I dref and (3) withI qref For the current reference value generated by the current limiter loop,I max for injecting the maximum allowable value of the grid current.
(6) Acquisition of voltage modulated signals by current loopm d And (3) withm q The specific formula is as follows:
（8）
wherein,m d and (3) withm q For the voltage modulation signal generated by the current loop,I fd and (3) withI fq As unfiltered currentdqThe component(s) of the composition,k pc and (3) withk ic The current loop ratio coefficient and the integral coefficient are respectively,L f is the filter inductance value.
Finally, willm d And (3) withm q Sending the modulated wave into a park inverse transformation module to obtain a modulated wavem abc And modulate the wavem abc Sending the signals to a pulse width modulation module to generate driving signals to control the on/off of the IGBT in the inverter.
2. And calculating a self-adaptive q-axis voltage feedback coefficient based on transient stability constraint.
The specific implementation steps are as follows:
(1) Power synchronization unit output phase angle of grid-structured inverter based on q-axis voltage feedbackθExpressed as:
（9）
wherein,ω c in order to cut-off the angular frequency,K p is thatP-fThe coefficient of sagging is set to be the same,PandP ref respectively outputting active power and reference values thereof;θ g is the grid voltage phase angle.
Defining virtual power angleδIs thatdShaft and grid voltageV g Phase angle difference between:
（10）
(2) According to the circuit relation and the phasor relation, the q-axis component of the output voltage when the current limiting is not triggeredV qu And active powerP u Expressed as:
（11）
（12）
wherein,V qu andP u the q-axis component of the output voltage and the active power when the current limiting is not triggered respectively.
Substituting the formula (9), the formula (11) and the formula (12) into the formula (10) to obtain a virtual power angle when the current limiting is not triggeredδ u The method comprises the following steps:
（13）
wherein,k u for non-triggered current limitingqShaft voltage feedback coefficient.
The method comprises the steps of simultaneously seeking a derivative and simplifying and arranging the two sides of the device to obtain:
（14）
（15）
wherein,P refueq ，H ueq andD ueq the equivalent active power, the equivalent inertia time constant and the equivalent damping coefficient are respectively used when the current limiting is not triggered.
(3) According to the circuit relation and the phasor relation, triggering the q-axis component of the output voltage during current limitingV qlim And active powerP lim Expressed as:
（16）
（17）
wherein,V qlim andP lim the q-axis component of the output voltage at the time of triggering current limiting and the active power at the time of triggering current limiting are respectively,δ lim to trigger a virtual power angle at the time of current limiting.
Substituting the formula (9), the formula (16) and the formula (17) into the formula (10) to obtain a virtual power angle when triggering current limitingδ lim The method comprises the following steps:
（18）
wherein,k lim for triggering current limitingqShaft voltage feedback coefficient.
The transient model after triggering current limiting is obtained by simultaneously seeking and simplifying the two sides of the model is as follows:
（19）
（20）
wherein,P ref eqlim ，H eqlim andD eqlim the equivalent active power, the equivalent inertia time constant and the equivalent damping coefficient when the current limiting is triggered are respectively.
(4) Equivalent active power reference when current limiting is not triggeredP refueq The expression of (2) is:
（21）
active power reference at fault momentP ref And virtual power angle when current limiting is not triggeredδ u Expressed as:
（22）
（23）
wherein,δ 0 is the virtual power angle of the system when the fault occurs.
Active power reference expressed by equation (22) and equation (23)P ref And virtual power angle when current limiting is not triggeredδ u Substituting formula (21) and lettingP refueq =0, resulting in:
（24）
wherein,athe voltage drop coefficient is the ratio of the power grid voltage to the rated voltage after the fault.
Solving equation (24) to obtain q-axis voltage feedback coefficient when current limiting is not triggeredk u The method comprises the following steps:
（25）
(5) Equivalent active power reference when triggering current limitingP reflimeq The expression of (2) is:
（26）
virtual power angle during fault instant triggering current limitingδ lim Expressed as:
（27）
wherein,δ 0 is the virtual power angle of the system when the fault occurs.
Active power reference expressed by equation (22) and equation (27)P ref And virtual power angle when current limiting is not triggeredδ u Substitution (26) and orderP reflimeq =0, resulting in:
（28）
solving equation (25) to obtain q-axis voltage feedback coefficient when triggering current limitingk lim The method comprises the following steps:
（29）
wherein the line impedanceX g The impedance measurement may be obtained in advance or based on an online impedance estimation method.
(6) Self-adaptive control flow
As shown in fig. 3, the adaptive control flow of the grid-built inverter based on q-axis voltage feedback of the present invention is as follows:
first, monitoring the grid voltage and the inverter output current in real time. When the voltage of the power grid is more than 90% of the rated voltage, the system is considered to be in a normal running state, and then the system is controlled tok=0, the inverter is droop control with low pass filter. When the power grid voltageV g Drop to rated valueV gN If 90% or less of the total voltage is detected, the system is considered to have short circuit fault, and is switched to network control based on q-axis voltage feedback, and at this time, whether the system triggers current limiting is detected, and the triggering current limiting is based onk lim Setting upkOr otherwise according to the value ofk u Determining that after fault removalkReset to 0, i.e. revert to droop control with low pass filter.
The specific parameters of the examples are shown in table 1.
Table 1 inverter grid-tie system parameters
。
FIG. 4 (a) is a waveform of a conventional droop control experiment when the grid voltage drops to 0.2pu, the inverter is intWhen=0s, the system is connected to the power grid and operates in a steady state, whentWhen the power grid is in the range of 3s, 80% of phase-shift-free three-phase voltage drop occurs, the voltage source of the inverter is changed into a current source under the action of the current limiting link during the fault period, the amplitude of the power angle curve is greatly reduced, the output active power and the reference active power have no intersection point, and the system is in transient instability due to the loss of balance points. FIG. 4 (b) is an experimental waveform of the power grid voltage dropping to 0.2pu after the adaptive control strategy is adopted, the inverter is intWhen=0s, the system is connected to the power grid and operates in a steady state, whentWhen the power grid is subjected to 80% phase-shift-free three-phase voltage drop in the time of being=3s, the q-axis voltage feedback coefficientkAnd (3) adjusting the equivalent active power reference of the system to zero at the moment of failure along with the voltage drop condition of the power grid, and then increasing the equivalent active power reference along with the reduction of the power angle, so that the system operates at a stable balance point during the failure period. At the position oftWhen=6s, the fault clears, and the system resumes normal operation. Simulation results show that the transient stability of the system can be realized by adopting the self-adaptive control strategy.
Fig. 5 (a) and fig. 5 (b) are respectively adaptive controlK p =0.00002 sumK p Experimental waveform of grid voltage dip to 0.2pu when=0.001. Coefficient of sagK p The smaller the damping and inertia of the system, the better the dynamic performance, the system can remain transient stable during failure, but the oscillation conditions are exacerbated. The results verify that the proposed adaptive control strategy is at different droop coefficientsK p The system can maintain good fault ride-through capability.
FIG. 6 shows an adaptive control of on-line inductanceL g Experimental waveforms for grid voltage dip to 0.2pu when=10mh. During a fault, the system can be inL g Good transient stability performance is still maintained at =10mh. This result verifies that the adaptive control strategy presented herein enables the system to maintain good fault ride-through capability under weak networks.
In conclusion, the invention can not influence the output of the active power of the grid-formed inverter under the normal operation working condition, can automatically adjust the equivalent active power reference according to the voltage drop depth of the power grid under the short circuit fault condition, ensures that the system has a stable balance point, realizes the fault ride-through of the system under different voltage drop conditions of the power grid, and can improve the transient stability of the system.
Claims (5)
1. The method for improving transient stability of the grid-built inverter based on self-adaptive q-axis voltage feedback is characterized by comprising the following steps:
step 1: constructing a grid-formed inverter control loop based on q-axis voltage feedback
Determining a grid-built inverter control based on q-axis voltage feedback: collecting inverter output voltageV PCC And current flowI g After park conversion, the output active power is obtained by transmitting the output active power to a power calculation modulePAnd output reactive powerQIs transmitted to a power control link through a first-order low-pass filter, and is controlled through active-angular frequency droop and inverter output voltageV PCC Generating angular frequency by q-axis component feedback leg of (2)ωIntegrating to obtain output phase angle of power synchronous unitθThen generates voltage modulation signal after passing through the voltage outer ring and the current inner ringm d Andm q modulating the voltage into a signalm d Andm q transmitting to an inverse Peak conversion module to obtain a modulated wavem abc Finally, generating a driving signal through a pulse width modulation module to control the on and off of the IGBT;
step 2: calculating self-adaptive q-axis voltage feedback coefficient based on transient stability constraint
Step 2.1: calculating output phase angle of power synchronization unit of grid-structured inverter based on q-axis voltage feedbackθAnd define virtual power angleδIs thatdShaft and grid voltageV g Phase angle difference between;
step 2.2: calculating the q-axis component of the output voltage of the grid-connected inverter during non-trigger current limiting according to the circuit relation and the phasor relationV qu And active powerP u Thereby obtaining the equivalent active power reference when the current limiting is not triggeredP refueq To cause the fault to be instantaneousP refueq Is equal to zero, thereby solving the q-axis voltage feedback coefficient when the current limiting is not triggeredk u ；
Step 2.3: calculating the q-axis component of the output voltage of the grid-connected inverter when triggering current limiting according to the circuit relation and the phasor relationV qlim And active powerP lim Thereby obtaining the equivalent active power reference when triggering current limitingP reflimeq To cause the fault to be instantaneousP reflimeq Is equal to zero, thereby solving the q-axis voltage feedback coefficient when triggering current limitingk lim ；
Step 3: self-adaptive control of grid-built inverter based on q-axis voltage feedback
Monitoring the voltage of a power grid and the output current of an inverter in real time, and when the system is in a normal running state, enabling the q-axis voltage feedback coefficient to bek=0, the inverter employs droop control with low pass filter;
when a short circuit fault occurs in the system, switching to a grid-formed inverter control based on q-axis voltage feedback, and detecting whether the system triggers current limiting or not;
if the current limiting is triggered, the q-axis voltage feedback coefficient is calculatedkSet to the q-axis voltage feedback coefficient when triggering current limitingk lim ；
If the current limiting is not triggered, the q-axis voltage feedback coefficient is calculatedkSet to q-axis voltage feedback coefficient when current limiting is not triggeredk u ；
After the fault is removed, the voltage feedback coefficient is calculatedkReset to 0, i.e. revert to droop control with low pass filter.
2. The method for transient stability improvement of a grid-built inverter based on adaptive q-axis voltage feedback according to claim 1, wherein in step 1, the power calculation module calculates the output active powerPAnd output reactive powerQThe method of (1) is as follows:
；
wherein,Pin order to output the active power,Qin order to output the reactive power,V d and (3) withV q Is the output side voltage of the grid-structured inverterV PCC Voltage generated after park transformationdqA component;I d and (3) withI q Is the output side current of the grid-structured inverterI g Current generated after park transformationdqA component;
angular frequencyωOutput phase angle of power synchronization unit of (a)θThe calculation method of (1) is as follows:
；
；
；
wherein,L(s) Is a transfer function of a first order low pass filter,ω c in order to cut-off the angular frequency,ωin order to be of an angular frequency,ω ref for the angular frequency to be rated,K p for the active-angular frequency dip coefficient,P ref as a reference value for the active power,θthe phase angle is output for the power synchronization unit,kfor the q-axis voltage feedback coefficient,sis a laplace operator.
3. The method for transient stability improvement of a grid-built inverter based on adaptive q-axis voltage feedback according to claim 2, wherein in step 1, the voltage modulation signal ism d Andm q the calculation method of (1) is as follows:
1) Calculating a voltage reference value obtained by a virtual impedance loopV dref And (3) withV qref ；
2) According to the voltage reference valueV dref And (3) withV qref Calculating an initial current reference value obtained by a voltage loopI d ref1 And (3) withI q ref1 ；
3) According to the initial current reference valueI d ref1 And (3) withI q ref1 Calculating a current reference value under the current limiting condition obtained by the current limiting loopI dref And (3) withI qref ；
4) According to the current reference value in the current limiting conditionI dref And (3) withI qref Calculating the acquisition of the voltage modulation signal by the current loopm d And (3) withm q ：
；
Wherein,V dc network formation for inputThe dc voltage of the inverter is set to be,m d and (3) withm q For the voltage modulation signal generated by the current loop,I fd and (3) withI fq As unfiltered currentdqThe component(s) of the composition,k pc and (3) withk ic The current loop ratio coefficient and the integral coefficient are respectively,L f is the filter inductance value.
4. The adaptive q-axis voltage feedback-based grid-tied inverter transient stability boost method of claim 1, wherein the droop control with low-pass filter is specifically: collecting inverter output voltageV PCC And current flowI g After park conversion, the output active power is obtained by transmitting the output active power to a power calculation modulePAnd output reactive powerQThe first order low pass filter is used for transmitting the signals to a droop control link, and the active-angular frequency droop control generates angular frequencyωIntegrating to obtain output phase angle of power synchronous unitθAfter passing through the voltage outer loop and the current inner loop, a modulation signal is generatedm d Andm q transmitting to an inverse Peak conversion module to obtain a modulated wavem abc And then the pulse width modulation module generates a driving signal to control the on and off of the IGBT.
5. The method for transient stability improvement of a grid-built inverter based on adaptive q-axis voltage feedback according to claim 1, wherein the power synchronization unit output phase angle of the grid-built inverter based on q-axis voltage feedback in step 2.1 isθExpressed as:
；
wherein,ω c in order to cut-off the angular frequency,K p is thatP-fThe coefficient of sagging is set to be the same,P ref a reference value for outputting active power;θ g is the voltage phase angle of the power grid;V q is output by a grid-structured inverterVoltage (V)V PCC Voltage generated after park transformationqA component;
defining virtual power angleδIs thatdShaft and grid voltageV g Phase angle difference between:
；
output Voltage q-axis component when Current limiting is not triggered in step 2.2V qu And active powerP u Expressed as:
；
；
wherein,V qu andP u the q-axis component of the output voltage and the active power when the current limiting is not triggered,X v for the virtual reactance value,X g is the line impedance;Ethe amplitude of the internal voltage source of the inverter;δ u the virtual power angle is the virtual power angle when the current limiting is not triggered;
substituting q-axis component of output voltage at non-triggered current limitV qu And active power at non-triggered current limitP u The virtual power angle when the current limit is not triggeredδ u Expressed as:
；
wherein,k u the q-axis voltage feedback coefficient is the q-axis voltage feedback coefficient when the current limiting is not triggered;
simultaneously deriving and simplifying the two sides to obtain the equivalent active power reference when the current limiting is not triggeredP refueq The expression of (2) is:
；
active power reference at fault momentP ref And virtual power angle of non-triggered current limitingδ u Expressed as:
；
；
wherein,δ 0 the virtual power angle of the system when the fault occurs;
substituting active power referencesP ref And virtual power angle of non-triggered current limitingδ u And let(s) express(s)P refueq =0, give
；
Wherein,athe voltage drop coefficient is the ratio of the power grid voltage to the rated voltage after the fault;
solving the above to obtain the q-axis voltage feedback coefficient when the current is limited in an un-triggered wayk u The method comprises the following steps:
；
the q-axis component of the output voltage at the time of triggering current limiting in step 2.3V qlim And active powerP lim Expressed as:
；
；
wherein,V qlim andP lim the q-axis component of the output voltage when the current limiting is triggered and the active power when the current limiting is triggered are respectively;δ lim the virtual power angle is used for triggering the current limiting;I max the maximum allowable value of the current of the injected power grid is;
substitution of q-axis component of output voltage at trigger current limitV qlim And active power when triggering current limitingP lim The expression of (2) triggers the virtual power angle in current limitingδ lim Expressed as:
；
wherein,k lim the q-axis voltage feedback coefficient is used for triggering current limiting;
simultaneously deriving and simplifying the two sides to obtain the equivalent active power reference when triggering current limitingP reflimeq The expression of (2) is:
；
virtual power angle for instantaneous triggering current limiting of faultδ lim Expressed as:
；
substituting active power referencesP ref And a virtual power angle triggering current limitingδ lim And let(s) express(s)P reflimeq =0, resulting in:
；
solving the above to obtain q-axis voltage inverse when triggering current limitingFeed coefficientk lim The method comprises the following steps:
；
wherein the line impedanceX g The impedance measurement is obtained in advance or based on an online impedance estimation method.
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