CN114362575B - Method for starting cascaded H-bridge grid-connected converter - Google Patents
Method for starting cascaded H-bridge grid-connected converter Download PDFInfo
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Abstract
The invention provides a starting method of a cascading H-bridge grid-connected converter, and belongs to the technical field of power electronics. The method aims to solve the problems of surge voltage, surge current and overmodulation in the starting process of the traditional starting method. According to the method, the cascade H-bridge grid-connected converter is enabled to display the external characteristics of the virtual boost resistor by means of current control, and the direct-current capacitor voltage of the H-bridge module is enabled to be boosted to an expected value stably. The method can adjust the active power absorbed by the cascaded H-bridge grid-connected converter from the power grid only by adjusting the value of the virtual boost resistor in the starting process, and has certain feasibility.
Description
Technical Field
The invention relates to a starting method of a cascading H-bridge grid-connected converter, and belongs to the technical field of power electronics.
Technical Field
The cascade H-bridge grid-connected converter is widely used for systems such as static var compensators (STATCOM), active Power Filters (APF) and the like due to the advantages of high modularization, high power capacity and the like. The cascade H-bridge grid-connected converter should charge the direct-current side capacitor of the H-bridge module when being started so as to provide a certain direct-current side capacitor voltage supporting system to stably operate. However, in the starting process, since the inductance value of the filter inductor is smaller and the voltage difference between the capacitor voltage at the direct current side and the reference value is larger, the system generates impulse voltage and impulse current at the direct current side and the alternating current side respectively, which affects the safe operation of the system. Therefore, the research on the starting method of the cascade H-bridge grid-connected converter has important engineering significance.
Because the direct-current side capacitors of all the H bridge modules of the cascade H bridge grid-connected converter are mutually independent, when the number of the cascade modules is large, the separate excitation starting method of the external direct-current power supply increases the system cost. Therefore, the self-excitation starting method for taking electricity from the power grid becomes a research hot spot. The existing self-excitation starting method often utilizes a voltage-current double-closed-loop self-excitation starting method based on a proportional-integral regulator to increase the capacitor voltage at the direct current side, and still has limitations. For example:
1) Liu Bo, cardia Hong Ji and silver dragon are published in "slow setting method for inhibiting starting impulse current of PWM rectifier" at 12 th stage of volume 33 of the journal of electrotechnical science, 6 of 2018, which proposes a starting control method for slow setting of capacitor voltage at dc side: the reference value of the DC side capacitor voltage is gently changed to a desired value so as to limit the magnitude and the change rate of the instantaneous value of the DC side capacitor voltage and restrain the impact current. The starting method cannot effectively solve the problem of impact current generated in the first switching cycles of the system in the starting process.
2) Yang Jianfeng, wang Shuai and Xie Yankai are published in 2 nd month of 2014, "power electronics technology," 48 nd phase 2 "active power filter capacitor voltage start control study, which proposes a start control method for a multiple proportional integral controller: and a plurality of groups of different proportional-integral regulator parameters are used at different starting stages, so that the rising rate and overshoot of the capacitor voltage at the direct current side are effectively controlled. The starting method has the problems of difficult parameter setting and excessive time of parameter switching.
In summary, the existing starting method has the following problems:
1. when the number of cascade modules of the cascade H-bridge grid-connected converter is large, the separately excited starting method of the externally added direct current power supply increases the system cost;
2. in the voltage-current double-closed-loop self-excitation starting method based on the proportional-integral regulator researched in the prior art, the reduction of the voltage difference between the capacitor voltage at the direct current side and the reference value of the capacitor voltage can not effectively inhibit the impact current generated at the initial stage of system starting;
3. in the voltage-current double-closed-loop self-excitation starting method based on the proportional-integral regulator studied in the prior art, the parameter setting difficulty and the time transition problem of parameter switching exist in the parameter modification of the proportional-integral regulator.
Disclosure of Invention
The invention aims to provide a starting method of a cascade H-bridge grid-connected converter, which utilizes current loop control to enable the cascade H-bridge grid-connected converter to display the external characteristic of a virtual boost resistor, and adjusts active power absorbed by the cascade H-bridge grid-connected converter from a three-phase power grid by adjusting the value of the virtual boost resistor so as to enable the direct-current capacitor voltage of an H-bridge module to be boosted to an expected value stably. According to the method, the overmodulation of the system is avoided by adjusting the partial pressure ratio between the virtual boost resistor and the series current limiting resistor in the starting process, the AC side surge current is effectively restrained, and the current reference value which is adaptively changed according to the sampling value of the DC side capacitor voltage of the H bridge module is effectively restrained.
The object of the present invention is thus achieved. The invention provides a starting method of a cascading H-bridge grid-connected converter, which relates to a topological structure of a circuit, wherein the topological structure comprises the cascading H-bridge grid-connected converter, a three-phase filter inductor L, a three-phase current-limiting resistor R, a three-phase grid-connected circuit breaker KM1, a three-phase current-limiting resistor circuit breaker KM2 and a three-phase power grid; the cascade H-bridge grid-connected converter is a three-phase cascade H-bridge grid-connected converter in star connection, wherein each phase of cascade H-bridge grid-connected converter comprises n identical H-bridge modules and n identical direct-current side capacitors, the n H-bridge modules are in cascade connection, and each H-bridge module is connected with one direct-current side capacitor in parallel; the output end of the three-phase cascade H-bridge grid-connected converter is connected with a three-phase filter inductance L, a three-phase current limiting resistor R and a three-phase grid-connected circuit breaker KM1 in series in sequence and then connected to a three-phase power grid; the three-phase current limiting resistor circuit breaker KM2 is connected with the three-phase current limiting resistor R in parallel;
the starting method comprises the following steps:
step 1, uncontrolled rectification
Closing a three-phase grid-connected circuit breaker KM1, keeping a three-phase current-limiting resistor circuit breaker KM2 in an open state, and boosting 3n direct-current side capacitor voltages corresponding to 3n H bridge modules to a steady state in an uncontrolled rectifying stage, wherein a circuit formed by a three-phase cascading H bridge type grid-connected converter, a three-phase filter inductor L, a three-phase current-limiting resistor R and a three-phase power grid is equivalent to three groups of single-phase bridge type uncontrolled rectifying charging circuits;
sampling voltages of 3n direct-current side capacitors corresponding to 3n H bridge modules at steady state moments, and recording the sampled values as initial direct-current side capacitor voltages u dcxk，0 Wherein x is a phase sequence, x=a, b, c, k is a serial number of each H-bridge module in each phase of the cascaded H-bridge grid-connected converter, k=1, 2.
Will 3n initial DC side capacitor voltages u dcxxk，0 The average value of (a) is recorded as an initial DC side capacitance voltage average value u dc_av0 The calculation formula is as follows:
step 2, virtual boost resistor and current control
N-round real-time sampling is carried out on the voltage of the 3N direct-current side capacitors corresponding to the 3N H-bridge modules, the current of the three-phase filter inductor L and the grid-connected voltage, and the 3N H-bridge modules are controlled so that the voltage of the 3N direct-current side capacitors corresponding to the 3N H-bridge modules is stably boosted to a set expected valueWherein, the voltages of the capacitors with N being 3N direct current sides all reach the expected value +.>The number of real-time sampling rounds is the number of real-time sampling rounds, and N is a positive integer;
the process of sampling and controlling in real time in one round is as follows:
step 2.1, recording any one of the N rounds of real-time sampling as a j-th round of real-time sampling, j being the number of real-time sampling rounds, j=1, 2, & gt, N; respectively sampling the voltages of 3n direct-current side capacitors corresponding to 3n H bridge modules in real time in the jth round, and recording the sampled values as direct-current side capacitor voltages u dcxk，j The method comprises the steps of carrying out a first treatment on the surface of the The j-th round of real-time sampling is carried out on the current of the three-phase filter inductor L, and the sampled value is recorded as an inductor current i x，j The j-th round of real-time is carried out on the grid-connected voltageSampling and recording the sampled value as grid-connected voltage u pccx，j ；x＝a，b，c，k＝1，2，...，n；
Step 2.2, the DC side capacitor voltage u obtained according to step 2.1 dcxk，j Calculating the average value of 3n DC side capacitor voltages and recording as the average value u of the DC side capacitor voltages dc_av，j The formula is as follows:
step 2.3, according to the initial DC side capacitance voltage average value u obtained in step 1 dc_av0 And the average value u of the capacitor voltage at the direct current side obtained in the step 2.2 dc_av，j The virtual boost resistor R is obtained through a current proportional integral regulator vch，j The formula is as follows:
wherein k is p For the proportional coefficient, k, of the current proportional-integral regulator c For the virtual resistance adjustment coefficient, R ch The current limiting resistance value is the current limiting resistance value of the three-phase current limiting resistor R;
virtual resistance adjustment coefficient k c And a current limiting resistance value R ch The value ranges of (a) are respectively as follows:
step 2.4, the inductor current i obtained by sampling in real time in the step 2.1 is sampled x，j And grid-connected voltage u pccx，j Coordinate transformation is carried out to obtain d-axis inductance current i under the grid fundamental wave frequency synchronous rotation coordinate system d，j Q-axis inductor current i q，j Grid-connected voltage of d-axisu pccd，j And q-axis grid-connected voltage u pccq，j ；
Step 2.5, the virtual boost resistor R obtained according to step 2.3 vch，j And the d-axis inductance current i obtained in the step 2.4 d，j Q-axis inductor current i q，j Grid-connected voltage u of d-axis pccd，j And q-axis grid-connected voltage u pccq，j Calculating to obtain d-axis inductance current reference valueq-axis inductor current reference value->And a voltage feedforward coefficient k f，j The calculation formula is as follows:
step 2.6, the average value u of the capacitor voltage at the DC side obtained in the step 2.2 dc_av，j D-axis inductance current i obtained in step 2.4 d，j Q-axis inductor current i q，j Grid-connected voltage u of d-axis pccd，j And q-axis grid-connected voltage u pcq，j D-axis inductance current reference value obtained in step 2.5q-axis inductor current reference value->And a voltage feedforward coefficient k f，j The d-axis modulation signal m is obtained through a current proportional integral regulator d，j And q-axis modulated signal m q，j The formula is as follows:
wherein k is i The integral coefficient of the current proportional integral regulator is s is Laplacian, omega is fundamental wave angular frequency of grid-connected voltage, and L 0 The inductance value of the three-phase filter inductance L;
modulating the d-axis modulation signal m d，j And q-axis modulated signal m q，j The upper limit value of (1) and the lower limit value of (1) are set to-1, when the d-axis modulation signal m d，j Modulated signal m on q axis q，j When the upper limit value or the lower limit value is reached, the cascade H-bridge grid-connected converter is overmodulated;
step 2.7, the d-axis modulation signal m obtained in the step 2.6 is processed d，j And q-axis modulated signal m q，j Coordinate transformation is carried out to obtain a three-phase modulation signal m under a three-phase symmetrical static coordinate system x，j X=a, b, c; three-phase cascade H-bridge grid-connected converter modulates signal m in three phases x，j Controlling each H bridge module under modulation;
step 3, recording the interval time of each of the N rounds of real-time sampling as delta, respectively carrying out j+1st round of real-time sampling on the voltages of the 3N direct-current side capacitors corresponding to the 3N H bridge modules after delta time, and recording the sampled value as the direct-current side capacitor voltage u after the j round of adjustment dcxk，j+1 Examine the DC side capacitor voltage u after 3n jth round adjustment dcxk，j+1 Whether or not all of them meet the expected valueIs required by the following steps: if yes, go to step 4; if not, returning to the step 2.1, and carrying out the next round of real-time sampling and control;
and 4, closing the three-phase current limiting resistor circuit breaker KM2 to bypass the three-phase current limiting resistor R.
Compared with the prior art, the invention has the following beneficial effects:
1) The invention does not need to add extra direct current power supply, thereby greatly saving the system cost.
2) According to the invention, the active power absorbed by the cascaded H-bridge grid-connected converter from the power grid is dynamically adjusted according to the sampled DC side capacitor voltage of the H-bridge module, so that the system overmodulation problem caused by the DC side capacitor voltage at the lower starting control initial moment is avoided, and the impact current generated at the initial stage of system starting is effectively restrained.
3) According to the invention, the virtual boost resistance value and the current reference value are adaptively modified according to the sampled direct-current side capacitor voltage of the H-bridge module, so that a plurality of groups of regulator parameters do not need to be set.
Drawings
Fig. 1 is a circuit topology diagram of the starting method of the present invention.
Fig. 2 is a single-phase equivalent circuit of a virtual boost resistor starting strategy according to the present invention.
Fig. 3 is a control block diagram of the cascaded H-bridge grid-connected converter according to the present invention.
Fig. 4 is a waveform diagram of a dc side capacitor voltage using a conventional start-up method.
Fig. 5 is a waveform diagram of inductor current using a conventional starting method.
Fig. 6 is a waveform diagram of a d-axis modulation signal using a conventional starting method.
Fig. 7 is a waveform diagram of a dc side capacitor voltage using the starting method of the present invention.
Fig. 8 is a waveform diagram of inductor current using the starting method of the present invention.
Fig. 9 is a waveform diagram of a d-axis modulation signal using the starting method of the present invention.
Detailed Description
Embodiments of the present invention will be described below by way of specific examples with reference to the accompanying drawings.
Fig. 1 is a circuit topology diagram of the starting method of the present invention. As can be seen from fig. 1, the topology structure of the circuit involved in the starting method includes a cascaded H-bridge grid-connected converter, a three-phase filter inductance L, a three-phase current-limiting resistor R, a three-phase grid-connected circuit breaker KM1, a three-phase current-limiting resistor circuit breaker KM2 and a three-phase power grid. The cascade H-bridge grid-connected converter is a three-phase cascade H-bridge grid-connected converter in star connection, wherein each phase of cascade H-bridge grid-connected converter comprises n identical H-bridge modules and n identical direct-current side capacitors, the n H-bridge modules are in cascade connection, and each H-bridge module is connected with one direct-current side capacitor in parallel. The output end of the three-phase cascade H-bridge grid-connected converter is connected with a three-phase filter inductance L, a three-phase current limiting resistor R and a three-phase grid-connected circuit breaker KM1 in series in sequence and then connected to a three-phase power grid; the three-phase current limiting resistor circuit breaker KM2 is connected with the three-phase current limiting resistor R in parallel. In fig. 1, C is a dc side capacitor connected in parallel to each H-bridge module. As can also be seen from fig. 1, in this embodiment n=7.
In an ideal case, the neutral point of the cascaded H-bridge grid-connected converter is equal to the neutral point potential of the grid-connected voltage, and a single-phase equivalent circuit of the virtual boost resistor starting strategy according to the invention is shown in fig. 2. The single-phase equivalent circuit is exemplified by a phase a in a three-phase system. The virtual boost resistor starting strategy enables the cascaded H-bridge grid-connected converter to present a virtual boost resistor R vch By adjusting the external characteristics of the virtual boost resistor R vch The size of the converter enables the cascaded H-bridge grid-connected converter to be started stably. In FIG. 2, u pcca Is the voltage value of the phase a parallel network voltage.
Fig. 3 is a control block diagram of the cascaded H-bridge grid-connected converter according to the present invention, which includes two parts, namely a virtual boost resistor and a current control. As can be seen from fig. 3, the starting method comprises the following steps:
step 1, uncontrolled rectification
The three-phase grid-connected circuit breaker KM1 is closed, the three-phase current-limiting resistor circuit breaker KM2 is kept in an open state, and a circuit formed by the three-phase cascade H-bridge grid-connected converter, the three-phase filter inductor L, the three-phase current-limiting resistor R and the three-phase power grid is equivalent to a three-group single-phase bridge type uncontrolled rectifying charging circuit, and 3n direct-current side capacitor voltages corresponding to 3n H-bridge modules are boosted to a steady state in an uncontrolled rectifying stage.
Corresponding 3n H bridge modulesThe voltage of the DC side capacitor at the steady state moment is sampled, and the sampled value is recorded as the initial DC side capacitor voltage u dcxk，0 Wherein x is a phase sequence, x=a, b, c, k is a serial number of each H-bridge module in each phase of the cascaded H-bridge grid-connected converter, k=1, 2.
Will 3n initial DC side capacitor voltages u dcxk，0 The average value of (a) is recorded as an initial DC side capacitance voltage average value u dc_av0 The calculation formula is as follows:
in the present example, u dc_av0 ≈37.7V。
Step 2, virtual boost resistor and current control
N-round real-time sampling is carried out on the voltage of the 3N direct-current side capacitors corresponding to the 3N H-bridge modules, the current of the three-phase filter inductor L and the grid-connected voltage, and the 3N H-bridge modules are controlled so that the voltage of the 3N direct-current side capacitors corresponding to the 3N H-bridge modules is stably boosted to a set expected valueWherein, the voltages of the capacitors with N being 3N direct current sides all reach the expected value +.>The number of real-time sampling rounds is the positive integer.
In the present example of the present invention,
the process of sampling and controlling in real time in one round is as follows:
and 2.1, recording any one of the N rounds of real-time sampling as a j-th round of real-time sampling, wherein j is the number of real-time sampling rounds, and j=1, 2. Respectively sampling the voltages of 3n direct-current side capacitors corresponding to 3n H bridge modules in real time in the jth round, and recording the sampled values as direct-current side capacitor voltages u dcxk，j . The j-th round of real-time sampling is carried out on the current of the three-phase filter inductor L, and the sampled value is recorded as an inductor current i x，j The j-th round of real-time sampling is carried out on the grid-connected voltage, and the sampled value is recorded as grid-connected voltage u pccx，j ；x＝a，b，c，k＝1，2，...，n。
Step 2.2, the DC side capacitor voltage u obtained according to step 2.1 dcxk，j Calculating the average value of 3n DC side capacitor voltages and recording as the average value u of the DC side capacitor voltages dc_av，j The formula is as follows:
step 2.3, according to the initial DC side capacitance voltage average value u obtained in step 1 dc_av0 And the average value u of the capacitor voltage at the direct current side obtained in the step 2.2 dc_av，j The virtual boost resistor R is obtained through a current proportional integral regulator vch，j The formula is as follows:
wherein k is p For the proportional coefficient, k, of the current proportional-integral regulator c For the virtual resistance adjustment coefficient, R ch The current limiting resistance value of the three-phase current limiting resistor R.
In the present example, k p ＝5.6，k c ＝2，R ch ＝80Ω。
Virtual resistance adjustment coefficient k c And a current limiting resistance value R ch The value ranges of (a) are respectively as follows:
virtual resistance adjustment coefficient k c And a current limiting resistance value R ch The above-mentioned determination of the value range is aimed at ensuring that the cascaded H-bridge grid-connected converter is not overmodulated during the start-up procedure.
Step 2.4, the inductor current i obtained by sampling in real time in the step 2.1 is sampled x，j And grid-connected voltage u pccx，j Coordinate transformation is carried out to obtain d-axis inductance current i under the grid fundamental wave frequency synchronous rotation coordinate system d，j Q-axis inductor current i q，j Grid-connected voltage u of d-axis pccd，j And q-axis grid-connected voltage u pccq，j 。
Step 2.5, the virtual boost resistor R obtained according to step 2.3 vch，j And the d-axis inductance current i obtained in the step 2.4 d，j Q-axis inductor current i q，j Grid-connected voltage u of d-axis pccd，j And q-axis grid-connected voltage u pccq，j Calculating to obtain d-axis inductance current reference valueq-axis inductor current reference value->And a voltage feedforward coefficient k f，j The calculation formula is as follows:
step 2.6, the average value u of the capacitor voltage at the DC side obtained in the step 2.2 dc_av，j D-axis inductance current i obtained in step 2.4 d，j Q-axis inductor current i q，j Grid-connected voltage u of d-axis pccd，j And q-axis grid-connected voltage u pccq，j D-axis inductance current reference value obtained in step 2.5q-axis inductor current reference value->And a voltage feedforward coefficient k f，j The d-axis modulation signal m is obtained through a current proportional integral regulator d，j And q-axis modulated signal m q，j The formula is as follows:
wherein k is i The integral coefficient of the current proportional integral regulator is s is Laplacian, omega is fundamental wave angular frequency of grid-connected voltage, and L 0 The inductance value of the three-phase filter inductance L.
Modulating the d-axis modulation signal m d，j And q-axis modulated signal m q，j The upper limit value of (1) and the lower limit value of (1) are set to-1, when the d-axis modulation signal m d，j Modulated signal m on q axis q，j When the upper limit value or the lower limit value is reached, the cascade H-bridge grid-connected converter is overmodulated.
In the present example, k i ＝10000，L 0 ＝0.6mH。
Step 2.7, the d-axis modulation signal m obtained in the step 2.6 is processed d，j And q-axis modulated signal m q，j Coordinate transformation is carried out to obtain a three-phase modulation signal m under a three-phase symmetrical static coordinate system x，j X=a, b, c; three-phase cascade H-bridge grid-connected converter modulates signal m in three phases x，j And control each H-bridge module under modulation.
In the above steps, step 2.3 is virtual boost resistance control, and steps 2.4-2.7 are current control.
Step 3, recording the interval time of each of the N rounds of real-time sampling as delta, respectively carrying out j+1st round of real-time sampling on the voltages of the 3N direct-current side capacitors corresponding to the 3N H bridge modules after delta time, and recording the sampled value as the direct-current side capacitor voltage u after the j round of adjustment dcxk，j+1 Examine the DC side capacitor voltage u after 3n jth round adjustment dcxk，j+1 Whether or not all of them meet the expected valueIs required by the following steps: if yes, go to step 4; if not, returning to the step 2.1, and carrying out the next round of real-time sampling and control.
And 4, closing the three-phase current limiting resistor circuit breaker KM2 to bypass the three-phase current limiting resistor R.
The starting method controls the switching devices in the H-bridge modules through the modulation unit to enable the three-phase cascade H-bridge grid-connected converter to stably absorb functional quantity and enable the direct-current side capacitor voltage of each H-bridge module to be stably boosted to an expected value
In the present embodiment, the interval time δ is 0.0001 seconds.
Uniformly marking the time in the whole starting process as t; the voltage values of 3n direct-current side capacitors in the whole starting process are uniformly recorded as u dc The method comprises the steps of carrying out a first treatment on the surface of the Uniformly marking the inductance current value in the whole starting process as i; the d-axis modulation signal in the whole starting process is uniformly recorded as m d 。
Fig. 4 is a waveform diagram of a capacitor voltage at a dc side using a conventional starting method, fig. 5 is a waveform diagram of an inductor current using a conventional starting method, and fig. 6 is a waveform diagram of a d-axis modulation signal using a conventional starting method. And closing the three-phase grid-connected circuit breaker KM1 in 0.01 second, and keeping the three-phase current-limiting resistor circuit breaker KM2 in an open state. The direct-current side capacitor voltage corresponding to each H bridge module is boosted to a steady state in an uncontrolled rectifying stage. At 0.8 seconds, the three-phase current limiting resistor circuit breaker KM2 is closed to bypass the three-phase current limiting resistor R and put into a voltage-current double closed loop control algorithm based on a proportional integral regulator. As can be seen from fig. 4, the dc side capacitor voltage develops a surge voltage after 0.8 seconds of the voltage-current double closed loop control algorithm based on the proportional-integral regulator. As can be seen from fig. 5, the inductor current shows up as a rush current after 0.8 seconds of input into the voltage-current double closed loop control algorithm based on the proportional-integral regulator. From fig. 6, after the voltage-current double closed-loop control algorithm based on the proportional-integral regulator is put in 0.8 seconds, the d-axis modulation signal reaches the upper limit value of 1, and the cascade H-bridge grid-connected converter is overmodulated.
Fig. 7 is a waveform diagram of a dc side capacitor voltage using the starting method of the present invention. Fig. 8 is a waveform diagram of inductor current using the starting method of the present invention. Fig. 9 is a waveform diagram of a d-axis modulation signal using the starting method of the present invention. And closing the three-phase grid-connected circuit breaker KM1 in 0.01 second, and keeping the three-phase current-limiting resistor circuit breaker KM2 in an open state. The direct-current side capacitor voltage corresponding to each H bridge module is boosted to a steady state in an uncontrolled rectifying stage. And at 0.8 seconds, inputting a virtual boost resistance control algorithm related to the starting method of the invention. And when 1.6 seconds is needed, the three-phase current limiting resistor circuit breaker KM2 is closed to bypass the three-phase current limiting resistor R, and the system operates stably. As can be seen from fig. 7, the dc side capacitor voltage smoothly increases to a desired value during start-up. As can be seen from fig. 8, no rush current occurs in the inductor current during start-up. As can be seen from fig. 9, the d-axis modulation signal does not reach the upper limit value 1 or the lower limit value-1 during the starting process, and the cascaded H-bridge grid-connected converter is not modulated.
In summary, the calculation method is simple to implement, and the active power absorbed by the cascaded H-bridge grid-connected converter from the power grid can be adjusted by adjusting the value of the virtual boost resistor only by utilizing current control to enable the cascaded H-bridge grid-connected converter to display the external characteristic of the virtual boost resistor, so that the direct-current capacitor voltage corresponding to the H-bridge module is stably boosted to an expected value, and the method has certain feasibility.
Claims (1)
1. The topological structure of a circuit related to the starting method comprises the cascaded H-bridge grid-connected converter, a three-phase filter inductor L, a three-phase current limiting resistor R, a three-phase grid-connected circuit breaker KM1, a three-phase current limiting resistor circuit breaker KM2 and a three-phase power grid; the cascade H-bridge grid-connected converter is a three-phase cascade H-bridge grid-connected converter in star connection, wherein each phase of cascade H-bridge grid-connected converter comprises n identical H-bridge modules and n identical direct-current side capacitors, the n H-bridge modules are in cascade connection, and each H-bridge module is connected with one direct-current side capacitor in parallel; the output end of the three-phase cascade H-bridge grid-connected converter is connected with a three-phase filter inductance L, a three-phase current limiting resistor R and a three-phase grid-connected circuit breaker KM1 in series in sequence and then connected to a three-phase power grid; the three-phase current limiting resistor circuit breaker KM2 is connected with the three-phase current limiting resistor R in parallel;
the starting method is characterized by comprising the following steps of:
step 1, uncontrolled rectification
Closing a three-phase grid-connected circuit breaker KM1, keeping a three-phase current-limiting resistor circuit breaker KM2 in an open state, and boosting 3n direct-current side capacitor voltages corresponding to 3n H bridge modules to a steady state in an uncontrolled rectifying stage, wherein a circuit formed by a three-phase cascading H bridge type grid-connected converter, a three-phase filter inductor L, a three-phase current-limiting resistor R and a three-phase power grid is equivalent to three groups of single-phase bridge type uncontrolled rectifying charging circuits;
sampling voltages of 3n direct-current side capacitors corresponding to 3n H bridge modules at steady state moments, and recording the sampled values as initial direct-current side capacitor voltages u dcxk，0 Wherein x is a phase sequence, x=a, b, c, k is a serial number of each H-bridge module in each phase of the cascaded H-bridge grid-connected converter, k=1, 2.
Will 3n initial DC side capacitor voltages u dcxk，0 The average value of (a) is recorded as an initial DC side capacitance voltage average value u dc_av0 The calculation formula is as follows:
step 2, virtual boost resistor and current control
N-round real-time sampling is carried out on the voltage of 3N direct-current side capacitors corresponding to 3N H-bridge modules, the current of the three-phase filter inductor L and the grid-connected voltage, and the 3N H-bridge modules are controlled so as to enable the 3N H-bridge modules to correspond to each otherThe voltage of 3n direct-current side capacitors is smoothly boosted to a set expected valueWherein, the voltages of the capacitors with N being 3N direct current sides all reach the expected value +.>The number of real-time sampling rounds is the number of real-time sampling rounds, and N is a positive integer;
the process of sampling and controlling in real time in one round is as follows:
step 2.1, recording any one of the N rounds of real-time sampling as a j-th round of real-time sampling, j being the number of real-time sampling rounds, j=1, 2, & gt, N; respectively sampling the voltages of 3n direct-current side capacitors corresponding to 3n H bridge modules in real time in the jth round, and recording the sampled values as direct-current side capacitor voltages u dcxk，j The method comprises the steps of carrying out a first treatment on the surface of the The j-th round of real-time sampling is carried out on the current of the three-phase filter inductor L, and the sampled value is recorded as an inductor current i x，j The j-th round of real-time sampling is carried out on the grid-connected voltage, and the sampled value is recorded as grid-connected voltage u pccx，j ；x＝a，b，c，k＝1，2，...，n；
Step 2.2, the DC side capacitor voltage u obtained according to step 2.1 dcxk，j Calculating the average value of 3n DC side capacitor voltages and recording as the average value u of the DC side capacitor voltages dc_av，j The formula is as follows:
step 2.3, according to the initial DC side capacitance voltage average value u obtained in step 1 dc_av0 And the average value u of the capacitor voltage at the direct current side obtained in the step 2.2 dc_av，j The virtual boost resistor R is obtained through a current proportional integral regulator vch，j The formula is as follows:
wherein k is p For the proportional coefficient, k, of the current proportional-integral regulator c For the virtual resistance adjustment coefficient, R ch The current limiting resistance value is the current limiting resistance value of the three-phase current limiting resistor R;
virtual resistance adjustment coefficient k c And a current limiting resistance value R ch The value ranges of (a) are respectively as follows:
step 2.4, the inductor current i obtained by sampling in real time in the step 2.1 is sampled x，j And grid-connected voltage u pccx，j Coordinate transformation is carried out to obtain d-axis inductance current i under the grid fundamental wave frequency synchronous rotation coordinate system d，j Q-axis inductor current i q，j Grid-connected voltage u of d-axis pccd，j And q-axis grid-connected voltage u pccq，j ；
Step 2.5, the virtual boost resistor R obtained according to step 2.3 vch，j And the d-axis inductance current i obtained in the step 2.4 d，j Q-axis inductor current i q，j Grid-connected voltage u of d-axis pccd，j And q-axis grid-connected voltage u pccq，j Calculating to obtain d-axis inductance current reference valueq-axis inductor current reference value->And a voltage feedforward coefficient k f，j The calculation formula is as follows:
step 2.6, the average value u of the capacitor voltage at the DC side obtained in the step 2.2 dc_av，j D-axis inductance current i obtained in step 2.4 d，j Q-axis inductor current i q，j Grid-connected voltage u of d-axis pccd，j And q-axis grid-connected voltage u pccq，j D-axis inductance current reference value obtained in step 2.5q-axis inductor current reference value->And a voltage feedforward coefficient k f，j The d-axis modulation signal m is obtained through a current proportional integral regulator d，j And q-axis modulated signal m q，j The formula is as follows:
wherein k is i The integral coefficient of the current proportional integral regulator is s is Laplacian, omega is fundamental wave angular frequency of grid-connected voltage, and L 0 The inductance value of the three-phase filter inductance L;
modulating the d-axis modulation signal m d，j And q-axis modulated signal m q，j The upper limit value of (1) and the lower limit value of (1) are set to-1, when the d-axis modulation signal m d，j Modulated signal m on q axis q，j When the upper limit value or the lower limit value is reached, the cascade H-bridge grid-connected converter is overmodulated;
step 2.7, the d-axis modulation signal m obtained in the step 2.6 is processed d，j And q-axis modulated signal m q，j Coordinate transformation is carried out to obtain a three-phase modulation signal m under a three-phase symmetrical static coordinate system x，j X=a, b, c; three-phase cascade H-bridge grid-connected converter modulates signal m in three phases x，j Controlling each H bridge module under modulation;
step 3, recording the interval time of each of the N rounds of real-time sampling as delta, respectively carrying out j+1st round of real-time sampling on the voltages of the 3N direct-current side capacitors corresponding to the 3N H bridge modules after delta time, and recording the sampled value as the direct-current side capacitor voltage u after the j round of adjustment dcxk，j+1 Examine the DC side capacitor voltage u after 3n jth round adjustment dcxk，j+1 Whether or not all of them meet the expected valueIs required by the following steps: if yes, go to step 4; if not, returning to the step 2.1, and carrying out the next round of real-time sampling and control;
and 4, closing the three-phase current limiting resistor circuit breaker KM2 to bypass the three-phase current limiting resistor R.
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