CN109951093B - Hybrid parameter-based midpoint voltage control system and method - Google Patents

Hybrid parameter-based midpoint voltage control system and method Download PDF

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CN109951093B
CN109951093B CN201910196663.8A CN201910196663A CN109951093B CN 109951093 B CN109951093 B CN 109951093B CN 201910196663 A CN201910196663 A CN 201910196663A CN 109951093 B CN109951093 B CN 109951093B
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level grid
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CN109951093A (en
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吕建国
解艳宇
王纪东
阎亦然
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Nanjing University of Science and Technology
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Abstract

The invention discloses a system and a method for controlling midpoint voltage based on hybrid parameters. The system comprises a three-level grid-connected inverter, a digital processing control module and a driving circuit, wherein the digital processing control module comprises a sampling unit, a closed-loop control unit, a network side information calculation unit, a working condition judgment unit, a mixed parameter PI controller unit and a sine pulse width modulation unit. The method comprises the following steps: the sampling unit collects a power grid voltage signal and a power grid side current signal, and after the power grid side voltage signal and the power grid side current signal are processed by the power grid side information calculating unit, a calculation result is sent to the working condition judging unit to divide the working conditions; the hybrid parameter PI controller unit switches corresponding control parameters according to working conditions, calculates a zero-sequence component according to the direct-current side capacitor voltage, adds the zero-sequence component and the three-phase modulation signal, obtains a pulse width modulation signal through processing of the sine pulse width modulation unit, and drives a three-level grid-connected inverter switching tube to work. The invention has low hardware cost, accurate control and wide application range, and reduces the distortion rate of the network access current.

Description

Hybrid parameter-based midpoint voltage control system and method
Technical Field
The invention belongs to the technical field of control in power electronic conversion technology, and particularly relates to a system and a method for controlling midpoint voltage based on hybrid parameters.
Background
The NPC three-level grid-connected inverter has the advantages of mature topological structure, low voltage bearing of a switching device, low output harmonic content and the like, and is widely applied to medium and high power occasions. However, due to the inherent characteristics of the NPC three-level grid-connected inverter, the dc side of the grid-connected inverter has a problem of midpoint voltage fluctuation. At present, three solutions are mainly provided for the problem of neutral point voltage fluctuation on the direct current side of a three-level grid-connected inverter: (1) an independent direct current voltage source is adopted to supply power to a direct current side capacitor; (2) a midpoint balance control circuit is externally connected; (3) a specific midpoint voltage control method is employed. The first two methods are generally not considered because they increase hardware costs.
Under ideal grid conditions, the existing midpoint voltage control method is relatively mature, such as a DPWM method based on zero sequence component injection, an SVPWM method based on redundant small vector adjustment, a method based on hybrid modulation, and the like. However, in actual conditions, a grid fault can cause unbalance of three-phase voltages on a grid side, the amplitude of the midpoint voltage fluctuation can be increased in a state of unbalance of the three-phase voltages, the frequency is changed from three times of power frequency in the three-phase balance to power frequency, and odd harmonic components of 3, 5, 7 and the like are contained, so that difficulty is brought to control of the midpoint voltage.
Disclosure of Invention
The invention aims to provide a midpoint voltage control system and a midpoint voltage control method, which can realize the balance of upper and lower capacitor voltages on the direct current side of a grid-connected inverter under the condition of a non-ideal power grid and effectively balance midpoint voltage under the condition of high unbalance degree.
The technical solution for realizing the purpose of the invention is as follows: a mid-point voltage control system based on Hybrid parameters comprises a three-level grid-connected inverter, a digital processing control module and a driving circuit, wherein the digital processing control module comprises a sampling unit, a closed-loop control unit, a grid-side information calculation unit, a working condition judgment unit, a Hybrid Parameter PI controller unit (HP-PI) and a sine pulse width modulation unit;
the sampling unit respectively collects an upper capacitor voltage signal and a lower capacitor voltage signal on the direct current side of the three-level grid-connected inverter, a three-phase voltage signal on the alternating current side of the three-level grid-connected inverter and a three-phase current signal on the alternating current side of the three-level grid-connected inverter and sends the three-phase voltage signals to the closed-loop control unit and the grid-side information calculation unit; the network side information calculation unit calculates the current unbalance degree lambda and the power factor angle according to the sampled power grid voltage signal and the sampled network side current signal
Figure BDA0001993847680000011
And net side voltage negative sequence component phase angle
Figure BDA0001993847680000012
The calculation result is sent to a working condition judgment unit for judgmentProcessing, namely dividing the operating conditions of the three-level grid-connected inverter into different working conditions; the mixed parameter PI controller unit switches corresponding control parameters according to different working conditions, and then calculates the zero-sequence component u according to the upper and lower capacitor voltages at the direct current side0(ii) a And adding the zero sequence component and the modulation wave signal obtained by the closed-loop control unit, and sending the added signal to a sinusoidal pulse width modulation unit, wherein the output end of the sinusoidal pulse width modulation unit is connected to each switching tube of each phase of bridge arm in the three-level grid-connected inverter through a driving circuit.
Further, the digital processing control module adopts TMS320F2808 and EPM1270T chips.
A method for controlling midpoint voltage based on hybrid parameters comprises the following steps:
step 1, dividing the operation condition of the three-level grid-connected inverter into two different working conditions according to the relation factor between the zero-sequence component and the average midpoint current:
when the voltage is greater than 0, the three-level grid-connected inverter works under the working condition 1;
when the voltage is less than or equal to 0, the three-level grid-connected inverter works under the working condition 2;
step 2, respectively designing control parameters under different working conditions:
when the three-level grid-connected inverter works under the working condition 1:
definition ofmaxThe maximum value of the relation factor under the working condition 1 is the proportionality coefficient kpComprises the following steps:
Figure BDA0001993847680000021
when the three-level grid-connected inverter works under the working condition 2:
definition ofminThe minimum value of the relation factor under the working condition 2 is the proportionality coefficient kpComprises the following steps:
Figure BDA0001993847680000022
wherein C is a capacitance, fsIs a switching frequency, a relation factorIs the angle of the power factor of the unbalance degree lambda
Figure BDA0001993847680000023
And negative sequence voltage phase angle
Figure BDA0001993847680000024
Respectively calculating the maximum value of the relation factor by adopting a Lagrange multiplier methodmaxAnd minimum valueminFinally, the corresponding proportionality coefficient k is obtainedp；
Step 3, sampling three-phase voltage e at the AC sidea、eb、ecAlternating side three-phase current ia、ib、icCapacitor voltage U on the DC sideC1Lower capacitor voltage U on the DC sideC2；
Step 4, obtaining a three-phase modulation wave u through a closed-loop control unita、ub、uc；
Step 5, calculating the current unbalance degree lambda and the power factor angle through the network side information calculation unit
Figure BDA0001993847680000025
And net side voltage negative sequence component phase angle
Figure BDA0001993847680000026
Step 6, the working condition judging unit judges the sign of the relation factor according to the calculation result of the step 5, and determines the working condition of the three-level grid-connected inverter;
7, switching parameters of the PI controller according to the working condition of the three-level grid-connected inverter, and then calculating a zero-sequence component u according to the capacitance voltage at the direct current side0；
Step 8, zero sequence component u0Adding the three-phase modulation wave obtained in the step 4 to obtain a three-phase modulation signal:
and 9, generating a pulse width modulation signal by the three-phase modulation signal through a sine pulse width modulation unit, and controlling the work of a switching tube of the three-level grid-connected inverter through a driving circuit.
Further, the relationship factor between the zero sequence component and the average midpoint current in step 1 is as follows:
Figure BDA0001993847680000031
in the formula ImpIs the amplitude of the positive sequence component of the net side current, lambda is the degree of imbalance,
Figure BDA0001993847680000032
in order to be the power factor angle,
Figure BDA0001993847680000033
is the net side voltage negative sequence component phase angle.
Further, step 2 is to design the control parameters respectively under different working conditions, specifically as follows:
when the three-level grid-connected inverter works under the working condition 1:
definition ofmaxThe maximum value of the relation factor under the working condition 1 is the proportionality coefficient kpComprises the following steps:
Figure BDA0001993847680000034
solving for maximum values of relationship factors using lagrange multiplier methodmaxDefining the lagrange function:
Figure BDA0001993847680000035
wherein v isiIs lagrange multiplier, i is 1,2, 3;
Figure BDA0001993847680000036
to constrain the equation, it is expressed as:
Figure BDA0001993847680000037
definition of
Figure BDA0001993847680000041
Is a Lagrangian function
Figure BDA0001993847680000042
With respect to the variables λ,
Figure BDA0001993847680000043
Figure BDA0001993847680000044
The first order gradient of (A) is known from the Karush-Kuhn-Tucher requirement
Figure BDA0001993847680000045
The first order requirement that the maximum point should satisfy is:
Figure BDA0001993847680000046
definition of
Figure BDA0001993847680000047
Is a Lagrangian function
Figure BDA0001993847680000048
With respect to the variables λ,
Figure BDA0001993847680000049
Figure BDA00019938476800000410
Second order gradient, function of
Figure BDA00019938476800000411
The second order sufficiency condition that the maximum point should satisfy is:
Figure BDA00019938476800000412
wherein the content of the first and second substances,
Figure BDA00019938476800000413
expressed as:
Figure BDA00019938476800000414
according to a function
Figure BDA00019938476800000415
The first order necessary condition and the second order sufficient condition which should be satisfied by the maximum value point are obtained to obtain the function under the constraint condition
Figure BDA00019938476800000416
The maximum point of (2) is substituted into the function
Figure BDA00019938476800000417
I.e. to find the maximum valuemaxAnd the proportionality coefficient kp；
When the three-level grid-connected inverter works under the working condition 2:
definition ofminThe minimum value of the relation factor under the working condition 2 is the proportionality coefficient kpComprises the following steps:
Figure BDA00019938476800000418
solving for the minimum of the relationship factor using the lagrange multiplier methodminDefining the lagrange function:
Figure BDA00019938476800000419
in the formula
Figure BDA00019938476800000420
Expressed as:
Figure BDA00019938476800000421
solving a function under a constraint condition
Figure BDA00019938476800000422
Substituting the minimum point into the function
Figure BDA00019938476800000423
I.e. to find the minimum valueminAnd the proportionality coefficient kp。
Compared with the prior art, the invention has the remarkable advantages that: (1) the control parameters of the power grid are switched according to different power grid conditions and output power factors, and neutral point voltage balance control can be realized under a non-ideal power grid; (2) the neutral point voltage can be effectively balanced under the condition of high unbalance degree, and the control system is simple and easy to realize.
Drawings
FIG. 1 is a schematic diagram of a hybrid parameter-based midpoint voltage control system according to the present invention.
FIG. 2 is a control block diagram of the hybrid parameter based midpoint voltage control system of the present invention.
Fig. 3 is a schematic diagram of a main circuit structure of the NPC three-level grid-connected inverter.
FIG. 4 is a graph of the relationship factor curve under different imbalance and power factor angle conditions in an embodiment of the present invention, in which (a) is the negative sequence voltage phase angle
Figure BDA0001993847680000051
When the voltage is in negative sequence, the relationship factor curve diagram under different unbalance degrees and power factor angle conditions is shown in (b)
Figure BDA0001993847680000052
And (4) a relation factor surface graph under different unbalance degrees and power factor angle conditions.
Fig. 5 is a parameter selection flowchart of the miscellaneous parameter PI control unit in the present invention.
Fig. 6 is a voltage waveform diagram of upper and lower capacitors on the dc side before and after the control method of the present invention is used for 0.1s when the degree of unbalance is 0.1 and the power factor is 1 in the embodiment of the present invention.
Fig. 7 is a graph of waveforms of upper and lower capacitors on the dc side before and after the control method of the present invention is used for 0.1s when the degree of unbalance is 0.1 and the power factor is 0.707 in the embodiment of the present invention.
Fig. 8 is a graph of waveforms of upper and lower capacitors on the dc side before and after the control method of the present invention is used for 0.1s when the degree of unbalance is 0.2 and the power factor is 1 in the embodiment of the present invention.
Fig. 9 is a graph of waveforms of upper and lower capacitors on the dc side before and after the control method of the present invention is used for 0.1s when the degree of unbalance is 0.6 and the power factor is 0 in the embodiment of the present invention.
FIG. 10 shows the net side current i before and after the control method of the present invention is used in the embodiment of the present inventionbHarmonic distribution contrast diagram, wherein (a) is that the unbalance degree is 0.1 and the power factor is 1, the network side current i before and after the control method of the invention is usedbHarmonic distribution contrast diagram, (b) when the unbalance degree is 0.1 and the power factor is 0.707, the network side current i before and after the control method of the invention is usedbHarmonic distribution contrast diagram, (c) when the unbalance degree is 0.2 and the power factor is 1, the network side current i before and after the control method of the invention is usedbHarmonic distribution contrast diagram, (d) is the network side current i before and after the control method of the invention is used when the unbalance degree is 0.6 and the power factor is 0bHarmonic distribution versus plot.
Detailed Description
The invention is described in further detail below with reference to the figures and specific examples.
With reference to fig. 1, a midpoint voltage control system based on hybrid parameters includes a three-level grid-connected inverter, a digital processing control module and a driving circuit, wherein the digital processing control module includes a sampling unit, a closed-loop control unit, a grid-side information calculation unit, a working condition judgment unit, a hybrid parameter PI controller unit and a sine pulse width modulation unit;
the sampling unit respectively collects upper and lower capacitance voltage signals on the direct current side of the three-level grid-connected inverter and three-level grid-connected inversionThree-phase voltage signals at the alternating current side of the device and three-phase current signals at the alternating current side of the three-level grid-connected inverter are sent to the closed-loop control unit and the grid-side information calculation unit; the network side information calculation unit calculates the current unbalance degree lambda and the power factor angle according to the sampled power grid voltage signal and the sampled network side current signal
Figure BDA0001993847680000064
And net side voltage negative sequence component phase angle
Figure BDA0001993847680000065
The working condition judgment unit judges the sign of the relation factor according to the calculation result to obtain the working condition of the three-level grid-connected inverter, at the moment, the hybrid parameter PI controller unit is switched to the corresponding control parameter, and then the zero-sequence component u is calculated according to the upper and lower capacitor voltages at the direct current side0(ii) a And adding the zero sequence component and the modulation wave signal obtained by the closed-loop control unit, and sending the added signal to the sinusoidal pulse width modulation unit, wherein the output end of the sinusoidal pulse width modulation unit is connected to each switching tube of each phase of bridge arm in the three-level grid-connected inverter through a driving circuit.
As a specific example, the digital processing control module adopts TMS320F2808 and EPM1270T chips.
A method for controlling midpoint voltage based on hybrid parameters comprises the following steps:
step 1, dividing the operation condition of the three-level grid-connected inverter into two different working conditions according to the relation factor between the zero-sequence component and the average midpoint current:
when the voltage is greater than 0, the three-level grid-connected inverter works under the working condition 1;
when the voltage is less than or equal to 0, the three-level grid-connected inverter works under the working condition 2;
is a relation factor between the zero sequence component and the average midpoint current, and can be expressed as:
Figure BDA0001993847680000061
in the formula ImpIs a netThe amplitude of the positive sequence component of the side current, λ is the degree of imbalance,
Figure BDA0001993847680000062
in order to be the power factor angle,
Figure BDA0001993847680000063
is the net side voltage negative sequence component phase angle.
Step 2, designing control parameters according to the working conditions of the three-level grid-connected inverter:
the transfer function of the hybrid parameter PI controller is:
Figure BDA0001993847680000071
wherein k ispIs a proportionality coefficient, TiIs an integration time constant;
with reference to fig. 2, the open loop transfer function of the midpoint voltage control system is:
Figure BDA0001993847680000072
setting the inflection frequency of the PI controller to be far greater than the cut-off frequency thereof, and calculating the transfer function GolNeglecting the time constant T when the cut-off frequency of(s)iIs then the cut-off frequency omegacThe expression of (a) is:
Figure BDA0001993847680000073
at design scale factor kpThen, the following conditions must be satisfied:
Figure BDA0001993847680000074
working condition 1: 0
Definition ofmaxIs the maximum value, root, of the relation factor under the working condition 1According to kpThe condition at the time of design is to make a proportional gain kpComprises the following steps:
Figure BDA0001993847680000075
wherein the relation factors are the unbalance degree lambda and the power factor angle
Figure BDA0001993847680000076
And negative sequence voltage phase angle
Figure BDA0001993847680000077
The function of (c), noted as (a,
Figure BDA0001993847680000078
) Solving the maximum value of the relation factor by using a Lagrange multiplier methodmaxDefining the lagrange function:
Figure BDA0001993847680000079
wherein v isiIn order to be a lagrange multiplier,
Figure BDA00019938476800000710
to constrain the equation, it can be expressed as:
Figure BDA00019938476800000711
definition of
Figure BDA00019938476800000712
Is a Lagrangian function
Figure BDA00019938476800000713
With respect to the variables λ,
Figure BDA00019938476800000714
Figure BDA0001993847680000081
The first order gradient of (A) is known from the Karush-Kuhn-Tucher requirement
Figure BDA0001993847680000082
The first order requirement that the maximum point should satisfy is:
Figure BDA0001993847680000083
definition of
Figure BDA0001993847680000084
Is a Lagrangian function
Figure BDA0001993847680000085
With respect to the variables λ,
Figure BDA0001993847680000086
Figure BDA0001993847680000087
Second order gradient, function of
Figure BDA0001993847680000088
The second order sufficiency condition that the maximum point should satisfy is:
Figure BDA0001993847680000089
wherein the content of the first and second substances,
Figure BDA00019938476800000810
can be expressed as:
Figure BDA00019938476800000811
can obtain the function under the constraint condition
Figure BDA00019938476800000812
Maximum point of (d):
Figure BDA00019938476800000813
substituting the maximum point into the function
Figure BDA00019938476800000814
Then maximum valuemaxComprises the following steps:
max＝1.94
when negative sequence voltage phase angle
Figure BDA00019938476800000815
The relationship factor curve under different unbalance and power factor angle conditions is shown in FIG. 4(a), with the maximum valuemaxThe position of (a) is marked in fig. 4 (a).
Working condition 2: less than or equal to 0
Definition ofminFor the minimum value of the relation factor under the working condition 2 and for meeting the design condition, the proportional gain k is setpComprises the following steps:
Figure BDA00019938476800000816
solving for the minimum of the relationship factor using the lagrange multiplier methodminDefining the lagrange function:
Figure BDA00019938476800000817
in the formula
Figure BDA00019938476800000818
Can be expressed as:
Figure BDA0001993847680000091
solving a function under a constraint condition
Figure BDA0001993847680000092
Minimum value point of (c):
Figure BDA0001993847680000093
substituting the maximum point into the function
Figure BDA0001993847680000094
Then the minimum valueminComprises the following steps:
min＝-21.63
when negative sequence voltage phase angle
Figure BDA0001993847680000095
The relationship factor curve under different unbalance and power factor angle conditions is shown in FIG. 4(b), with the minimum valueminThe position of (c) is marked in fig. 4 (b).
Integration time TiThe inverse number of the inflection point frequency of the mixed parameter PI controller, and the inflection point frequency of the mixed parameter PI controller should be smaller than the crossing frequency omega to ensure that the crossing frequency of the mixed parameter PI controller is not influenced by the integral link of the mixed parameter PI controllercAnd in order to ensure that the hybrid parameter PI controller has larger phase margin under each working condition of the grid-connected inverter, the inflection point frequency of the PI controller can be selected at the crossing frequency of 1/10, and the integral time constant T is obtainediThe expression of (a) is:
Figure BDA0001993847680000096
the zero sequence component is then:
Figure BDA0001993847680000097
step 3, sampling three-phase voltage e at the AC sidea、eb、ecAlternating side three-phase current ia、ib、icCapacitor voltage U on the DC sideC1Lower capacitor voltage U on the DC sideC2；
Step 4, obtaining a three-phase modulation wave u through a closed-loop control unita、ub、uc；
Step 4.1, the voltage e at the alternating current side is measured by using a symmetrical component methoda、eb、ecAC side current ia、ib、icThe NPC three-level grid-connected inverter in the embodiment adopts a three-phase three-wire system connection method, so that zero-sequence components of voltage and current on the grid side are not considered, and only positive and negative sequence components are considered;
step 4.2, putting the three-phase stationary coordinate system under ea、eb、ec、ia、ib、icThe positive and negative sequence alternating current quantities are converted into direct current quantities under a positive and negative sequence synchronous rotating coordinate system, and the conversion matrixes of the positive and negative sequence synchronous rotating coordinate system are respectively as follows:
Figure BDA0001993847680000101
Figure BDA0001993847680000102
obtaining d and q axis components e of the voltage and the current under the positive sequence synchronous rotating coordinate system through conversiondp、eqp、idp、iqpAnd d and q axis components e of voltage and current under negative sequence synchronous rotation coordinate systemdn、eqn、idn、iqn；
4.3, under the condition of a non-ideal power grid, the active and reactive instantaneous power of the grid-connected inverter can contain alternating current quantity of twice power frequency, and according to the instantaneous reactive power theory, the direct current quantity and the alternating current quantity in the instantaneous power are as follows:
Figure BDA0001993847680000103
wherein i* dpGiven the current of the positive sequence d-axis, i* qpGiven the current of the positive sequence q-axis, i* dnGiven the current of the negative sequence d-axis, i* qnGiven for negative sequence q-axis current, P0Is the direct component of instantaneous active power, Pc2、Ps2Being an alternating component of instantaneous active power, Q0Being the direct component of instantaneous reactive power, Qc2、Qs2An alternating current component that is instantaneous reactive power;
when the control target is to eliminate the active power fluctuation amount Pc2、Ps2Then, the expression given for the current can be obtained:
Figure BDA0001993847680000104
Figure BDA0001993847680000105
Figure BDA0001993847680000106
Figure BDA0001993847680000107
wherein E is1、E2The expression of (a) is:
Figure BDA0001993847680000111
Figure BDA0001993847680000112
step 4.4, obtaining 4 paths of modulation wave signals u under the synchronous rotating coordinate system through a closed-loop control unitdp、uqp、udn、uqnThe control equation is as follows:
Figure BDA0001993847680000113
Figure BDA0001993847680000114
Figure BDA0001993847680000115
Figure BDA0001993847680000116
step 4.5, firstly, modulating wave signal u under negative sequence two-phase rotating coordinate systemdn、uqnConverting the negative sequence component under the three-phase static coordinate system into a positive sequence synchronous rotating coordinate system, wherein the conversion matrixes are respectively as follows:
Figure BDA0001993847680000117
Figure BDA0001993847680000118
obtaining negative sequence component under positive sequence rotating coordinate system through conversion, adding positive and negative sequence modulated wave components under the same coordinate system to obtain modulated wave component u under positive sequence synchronous rotating coordinate systemd、uq；
Step 4.6, converting the modulation wave signal under the synchronous rotating coordinate system into a three-phase modulation wave signal ua、ub、ucThe conversion formula is as follows:
Figure BDA0001993847680000119
step 5, calculating the current unbalance degree lambda and the power factor angle through the network side information calculation unit
Figure BDA0001993847680000121
And net side voltage negative sequence component phase angle
Figure BDA0001993847680000122
Step 6, the working condition judging unit judges the sign of the relation factor according to the calculation result of the step 5, and determines the working condition of the three-level grid-connected inverter;
7, switching parameters of the hybrid parameter PI controller according to working conditions of the three-level grid-connected inverter, and then calculating a zero-sequence component u according to the direct-current side capacitor voltage0；
Step 8, zero sequence component u0Adding the three-phase modulation wave obtained in the step 4 to obtain a three-phase modulation signal:
u'a＝ua+u0
u'b＝ub+u0
u'c＝uc+u0
step 9, adding the modulated wave signal u 'with the zero sequence component'a、u'b、u'cAnd the pulse width modulation signals are sent to a sine pulse width modulation unit to generate pulse width modulation signals, and a driving circuit controls the work of a switching tube of the three-level grid-connected inverter to realize the control of the neutral point voltage balance.
The NPC three-phase three-level grid-connected inverter has the modulation rule as follows: as shown in FIG. 3, taking the a-phase bridge arm as an example, in uarefPositive half cycle of (d), when uarefWhen greater than the carrier, order Sa1、Sa2When the a-phase bridge arm is conducted, the a-phase bridge arm outputs high level when u isarefWhen smaller than the carrier, order Sa2、Sa3Conducting, and outputting zero level by the a-phase bridge arm; at uarefNegative half cycle of (d), when uarefWhen smaller than the carrier, order Sa3、Sa4On, a phase bridge arm outputLow level, when uarefWhen greater than the carrier, order Sa2、Sa3And (4) conducting, and outputting zero level by the a-phase bridge arm. b. The modulation rules of the c-phase bridge arms are the same.
FIG. 5 is a flow chart of the parameter selection of the miscellaneous parameter controller, wherein
Figure BDA0001993847680000123
And λ(k)Respectively the phase angle of the negative sequence component of the network side voltage
Figure BDA0001993847680000124
Angle of power factor
Figure BDA0001993847680000125
The calculated value of the unbalance degree lambda in the kth power frequency period,(k)and expressing the value of the relation factor in the kth power frequency period, wherein the specific implementation process is as follows:
s1, sampling three-phase voltage and current signals at the network side;
s2, detecting the negative sequence component phase angle of the current power frequency cycle
Figure BDA0001993847680000126
Angle of power factor
Figure BDA0001993847680000127
With degree of unbalance lambda(k)；
S3, judging the current power frequency period relation factor according to the network side information(k)Symbol of (A), if
Figure BDA0001993847680000128
The mixed parameter PI controller applies the control parameter designed under the working condition 1, if so
Figure BDA0001993847680000129
The confounding parameter PI controller applies the control parameters designed under condition 2.
Examples
In the embodiment, an NPC three-level grid-connected inverter model based on a Digital Signal Processor-Complex Programmable Logic Device (DSP-CPLD) control framework is built, after direct current passes through a direct current bus capacitor, a three-level inverter circuit inverts to output three-phase voltage, and smooth three-phase sinusoidal voltage is output through an LC filter circuit. The electrical parameter settings during the simulation are as in table 1:
TABLE 1
Figure BDA0001993847680000131
FIG. 6 shows a DC bus capacitor C when the grid-side imbalance of the three-level grid-connected inverter is 0.1 and the power factor is 11、C2Instantaneous voltage Uc1、Uc2The control method of the present invention is used at 0.1 s. The instantaneous voltage of the upper and lower capacitors on the dc side has a voltage fluctuation of about 40V in magnitude before the control method of the present invention is not used, and the fluctuation of the voltage of the upper and lower capacitors on the dc side is limited to within 2V after the control method of the present invention is used.
Fig. 7, 8 and 9 show that the degree of unbalance is 0.1, the degree of unbalance is 0.2 when the power factor is 0.707, the degree of unbalance is 0.6 when the power factor is 1, and the dc bus capacitor C when the power factor is 01、C2Instantaneous voltage Uc1、Uc2The other conditions are unchanged, and the fluctuation of the upper and lower capacitor voltages on the direct current side is limited within 2V by using the control method of the invention at the time of 0.1 s. It can be seen that the control method of the present invention still has a desirable effect under the conditions of high unbalance degree and low power factor. Fig. 10 (a), (b), (c), and (d) are comparisons of total harmonic distortion of the grid-side current before and after the midpoint control method is used under the above four conditions, respectively, and it can be seen that the method for controlling the midpoint voltage based on the hybrid parameters effectively suppresses odd harmonics such as 3, 5, 7, and 9 in the grid-side current and reduces the total harmonic distortion of the current.

Claims (5)

1. A mid-point voltage control system based on hybrid parameters is characterized by comprising a three-level grid-connected inverter, a digital processing control module and a driving circuit, wherein the digital processing control module comprises a sampling unit, a closed-loop control unit, a grid-side information calculation unit, a working condition judgment unit, a hybrid parameter PI controller unit and a sine pulse width modulation unit;
the sampling unit is used for respectively acquiring an upper capacitor voltage signal and a lower capacitor voltage signal on the direct current side of the three-level grid-connected inverter, a three-phase voltage signal on the alternating current side of the three-level grid-connected inverter and a three-phase current signal on the alternating current side of the three-level grid-connected inverter and sending the three-phase voltage signal on the alternating current side of the three-level grid-connected inverter and the three-phase current signal on the alternating current side of the three-level grid-connected inverter to the closed-loop control unit and the grid-; the network side information calculation unit calculates the current unbalance degree lambda and the power factor angle according to the sampled power grid voltage signal and the sampled network side current signal
Figure FDA0002743384950000013
And net side voltage negative sequence component phase angle
Figure FDA0002743384950000014
The calculation result is sent to a working condition judgment unit for judgment processing, and the operation conditions of the three-level grid-connected inverter are divided into different working conditions; the mixed parameter PI controller unit switches corresponding control parameters according to different working conditions, and then calculates the zero-sequence component u according to the upper and lower capacitor voltages at the direct current side0(ii) a And adding the zero sequence component and the modulation wave signal obtained by the closed-loop control unit, and sending the added signal to a sinusoidal pulse width modulation unit, wherein the output end of the sinusoidal pulse width modulation unit is connected to each switching tube of each phase of bridge arm in the three-level grid-connected inverter through a driving circuit.
2. The miscellaneous parameter-based midpoint voltage control system of claim 1, wherein the digital processing control module employs TMS320F2808 and EPM1270T chips.
3. A method for controlling midpoint voltage based on hybrid parameters is characterized by comprising the following steps:
step 1, dividing the operation condition of the three-level grid-connected inverter into two different working conditions according to the relation factor between the zero-sequence component and the average midpoint current:
when the voltage is greater than 0, the three-level grid-connected inverter works under the working condition 1;
when the voltage is less than or equal to 0, the three-level grid-connected inverter works under the working condition 2;
step 2, respectively designing control parameters under different working conditions:
when the three-level grid-connected inverter works under the working condition 1:
definition ofmaxThe maximum value of the relation factor under the working condition 1 is the proportionality coefficient kpComprises the following steps:
Figure FDA0002743384950000011
when the three-level grid-connected inverter works under the working condition 2:
definition ofminThe minimum value of the relation factor under the working condition 2 is the proportionality coefficient kpComprises the following steps:
Figure FDA0002743384950000012
wherein C is a DC bus capacitor, fsThe relation factor is the unbalance degree lambda and the power factor angle for the switching frequency
Figure FDA0002743384950000023
And negative sequence voltage phase angle
Figure FDA0002743384950000025
Respectively calculating the maximum value of the relation factor by adopting a Lagrange multiplier methodmaxAnd minimum valueminFinally, the corresponding proportionality coefficient k is obtainedp；
Step 3, sampling three-phase voltage e at the AC sidea、eb、ecAlternating side three-phase current ia、ib、icCapacitor voltage U on the DC sideC1Lower capacitor voltage U on the DC sideC2；
Step 4, obtaining a three-phase modulation wave u through a closed-loop control unita、ub、uc；
Step 5, calculating the current unbalance degree lambda and the power factor angle through the network side information calculation unit
Figure FDA0002743384950000024
And net side voltage negative sequence component phase angle
Figure FDA0002743384950000026
Step 6, the working condition judging unit judges the sign of the relation factor according to the calculation result of the step 5, and determines the working condition of the three-level grid-connected inverter;
7, switching parameters of the PI controller according to the working condition of the three-level grid-connected inverter, and then calculating a zero-sequence component u according to the capacitance voltage at the direct current side0；
Step 8, zero sequence component u0Adding the three-phase modulation wave obtained in the step 4 to obtain a three-phase modulation signal:
and 9, generating a pulse width modulation signal by the three-phase modulation signal through a sine pulse width modulation unit, and controlling the work of a switching tube of the three-level grid-connected inverter through a driving circuit.
4. The method according to claim 3, wherein the relation factor between the zero sequence component and the average midpoint current in step 1 is as follows:
Figure FDA0002743384950000021
in the formula ImpIs the amplitude of the positive sequence component of the net side current, lambda is the degree of imbalance,
Figure FDA0002743384950000022
in order to be the power factor angle,
Figure FDA0002743384950000027
is the net side voltage negative sequence component phase angle.
5. The hybrid parameter-based midpoint voltage control method according to claim 3, wherein the control parameters are respectively designed in the step 2 under different working conditions, specifically as follows:
when the three-level grid-connected inverter works under the working condition 1:
definition ofmaxThe maximum value of the relation factor under the working condition 1 is the proportionality coefficient kpComprises the following steps:
Figure FDA0002743384950000031
solving for maximum values of relationship factors using lagrange multiplier methodmaxDefining the lagrange function:
Figure FDA0002743384950000032
wherein v isiIs lagrange multiplier, i is 1,2, 3;
Figure FDA00027433849500000317
to constrain the equation, it is expressed as:
Figure FDA0002743384950000033
definition of
Figure FDA00027433849500000313
Is a Lagrangian function
Figure FDA00027433849500000314
With respect to the variables λ,
Figure FDA00027433849500000315
Figure FDA00027433849500000321
The first order gradient of (A) is known from the Karush-Kuhn-Tucher requirement
Figure FDA00027433849500000316
The first order requirement that the maximum point should satisfy is:
Figure FDA0002743384950000034
definition of
Figure FDA0002743384950000036
Is a Lagrangian function
Figure FDA00027433849500000318
With respect to the variables λ,
Figure FDA00027433849500000319
Figure FDA00027433849500000320
Second order gradient, function of
Figure FDA0002743384950000037
The second order sufficiency condition that the maximum point should satisfy is:
Figure FDA0002743384950000038
wherein the content of the first and second substances,
Figure FDA0002743384950000039
expressed as:
Figure FDA0002743384950000035
according to a function
Figure FDA00027433849500000310
The first order necessary condition and the second order sufficient condition which should be satisfied by the maximum value point are obtained to obtain the function under the constraint condition
Figure FDA00027433849500000311
The maximum point of (2) is substituted into the function
Figure FDA00027433849500000312
I.e. to find the maximum valuemaxAnd the proportionality coefficient kp；
When the three-level grid-connected inverter works under the working condition 2:
definition ofminThe minimum value of the relation factor under the working condition 2 is the proportionality coefficient kpComprises the following steps:
Figure FDA0002743384950000041
solving for the minimum of the relationship factor using the lagrange multiplier methodminDefining the lagrange function:
Figure FDA0002743384950000042
in the formula
Figure FDA0002743384950000044
Expressed as:
Figure FDA0002743384950000043
solving a function under a constraint condition
Figure FDA0002743384950000045
Substituting the minimum point into the function
Figure FDA0002743384950000046
I.e. to find the minimum valueminAnd the proportionality coefficient kp。
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