CN109951093A - A kind of mid-point voltage control system and method based on hybrid parameter - Google Patents
A kind of mid-point voltage control system and method based on hybrid parameter Download PDFInfo
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Abstract
The invention discloses a kind of mid-point voltage control system and method based on hybrid parameter.The system includes three level grid-connected inverters, digital processing control module and driving circuit, and wherein digital processing control module includes sampling unit, Closed Loop Control Unit, net side information calculating unit, operating condition judging unit, hybrid parameter PI controller unit and sinusoidal pulse width modulation unit.Method are as follows: sampling unit acquires mains voltage signal and current on line side signal, and after the processing of net side information calculating unit, the calculation result is sent to operating condition judging units to divide operating condition；Hybrid parameter PI controller unit switches corresponding control parameter according to operating condition, and zero-sequence component is calculated according to DC capacitor voltage, then zero-sequence component is added with three-phase modulations signal, by sinusoidal pulse width modulation cell processing, obtain pulse-width signal, driving three level grid-connected inverter switching tubes work.Hardware cost of the present invention is low, precise control, applied widely, reduces the aberration rate of networking electric current.
Description
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 (hybrid parameter PI, 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 signalAnd net side voltage negative sequence component phase angleThe 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.
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 a relation factor delta between the zero-sequence component and the average midpoint current:
when delta is greater than 0, the three-level grid-connected inverter works under the working condition 1;
when delta 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 of δmaxThe maximum value of the relation factor delta under the working condition 1 is the proportionality coefficient kpComprises the following steps:
when the three-level grid-connected inverter works under the working condition 2:
definition of δminThe minimum value of the relation factor delta under the working condition 2 is the proportionality coefficient kpComprises the following steps:
wherein C is a capacitance, fsThe relation factor delta is the unbalance degree lambda and the power factor angle for the switching frequencyAnd negative sequence voltage phase angleRespectively calculating the maximum value delta of the relation factor by adopting a Lagrange multiplier methodmaxAnd minimum value deltaminFinally, 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 unitAnd net side voltage negative sequence component phase angle
Step 6, the working condition judging unit judges the sign of the relation factor delta 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 relation factor δ between the zero sequence component and the average midpoint current in step 1 is specifically as follows:
in the formula，ImpIs the amplitude of the positive sequence component of the net side current, lambda is the degree of imbalance,in order to be the power factor angle,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 of δmaxThe maximum value of the relation factor delta under the working condition 1 is the proportionality coefficient kpComprises the following steps:
solving for the maximum value delta of the relationship factor using the Lagrange multiplier methodmaxDefining the lagrange function:
wherein v isiIs lagrange multiplier, i is 1,2, 3;to constrain the equation, it is expressed as:
definition ofIs a Lagrangian functionWith respect to the variables λ, The first order gradient of (A) is known from the Karush-Kuhn-Tucher requirementThe first order requirement that the maximum point should satisfy is:
definition ofIs a Lagrangian functionWith respect to the variables λ, Second order gradient, function ofThe second order sufficiency condition that the maximum point should satisfy is:
wherein,expressed as:
according to a functionThe 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 conditionThe maximum point of (2) is substituted into the functionI.e. to find the maximum value deltamaxAnd the proportionality coefficient kp；
When the three-level grid-connected inverter works under the working condition 2:
definition of δminThe minimum value of the relation factor delta under the working condition 2 is the proportionality coefficient kpComprises the following steps:
solving for the minimum value delta of the relationship factor delta using the Lagrange multiplier methodminDefining the lagrange function:
in the formulaExpressed as:
solving a function under a constraint conditionSubstituting the minimum point into the functionI.e. to find the minimum value deltaminAnd 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 delta surface plot of the relationship factor for different imbalance and power factor angle conditions in an embodiment of the present invention, in which (a) is the current negative sequence voltage phase angleThe relationship factor delta curved surface diagram under the condition of different unbalance degrees and power factor angles, (b) is the negative sequence voltage phase angleAnd in time, the relationship factor delta curved surface diagram 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 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 signalAnd net side voltage negative sequence component phase angleThe working condition judgment unit judges the sign of the relation factor delta 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 a relation factor delta between the zero-sequence component and the average midpoint current:
when delta is greater than 0, the three-level grid-connected inverter works under the working condition 1;
when delta is less than or equal to 0, the three-level grid-connected inverter works under the working condition 2;
δ is a relation factor between the zero sequence component and the average midpoint current, and δ can be expressed as:
in the formula ImpIs the amplitude of the positive sequence component of the net side current, lambda is the degree of imbalance,in order to be the power factor angle,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:
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:
setting the inflection frequency of the PI controller to be far greater than the cut-off frequency thereof, and calculating the transfer function GolCut-off frequency of(s)Neglecting time constant T when rate is highiIs then the cut-off frequency omegacThe expression of (a) is:
at design scale factor kpThen, the following conditions must be satisfied:
working condition 1: delta >0
Definition of δmaxIs the maximum value of the relation factor delta under the working condition 1 according to kpThe condition at the time of design is to make a proportional gain kpComprises the following steps:
wherein the relation factor delta is the unbalance degree lambda and the power factor angleAnd negative sequence voltage phase angleThe function of (d), noted as δ (λ,) Solving the maximum value delta of the relation factor by using a Lagrange multiplier methodmaxDefining the lagrange function:
wherein v isiIn order to be a lagrange multiplier,to constrain the equation, it can be expressed as:
definition ofIs a Lagrangian functionWith respect to the variables λ, The first order gradient of (A) is known from the Karush-Kuhn-Tucher requirementThe first order requirement that the maximum point should satisfy is:
definition ofIs a Lagrangian functionWith respect to the variables λ, Second order gradient, function ofThe second order sufficiency condition that the maximum point should satisfy is:
wherein,can be expressed as:
can obtain the function under the constraint conditionMaximum point of (d):
substituting the maximum point into the functionThen the maximum value deltamaxComprises the following steps:
δmax＝1.94
when negative sequence voltage phase angleThe relationship factor delta surface diagram under different unbalance and power factor angle conditions is shown in FIG. 4(a), and the maximum value deltamaxThe position of (a) is marked in fig. 4 (a).
Working condition 2: delta is less than or equal to 0
Definition of δminIs a relation factor under the working condition 2Delta, to satisfy the design conditions, let the proportional gain kpComprises the following steps:
solving for the minimum value delta of the relationship factor delta using the Lagrange multiplier methodminDefining the lagrange function:
in the formulaCan be expressed as:
solving a function under a constraint conditionMinimum value point of (c):
substituting the maximum point into the functionThe minimum value δminComprises the following steps:
δmin＝-21.63
when negative sequence voltage phase angleRelation factor delta under different unbalance and power factor angle conditionsThe minimum value δ is shown in FIG. 4(b)minThe 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:
the zero sequence component is then:
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:
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:
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、Ps2When a given current is obtainedExpression:
wherein E is1、E2The expression of (a) is:
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:
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:
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:
step 5, calculating the current unbalance degree lambda and the power factor angle through the network side information calculation unitAnd net side voltage negative sequence component phase angle
Step 6, the working condition judging unit judges the sign of the relation factor delta 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、Sa4When the a-phase bridge arm is conducted, the a-phase bridge arm outputs low level when u isarefWhen 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, whereinAnd λ(k)Respectively the phase angle of the negative sequence component of the network side voltageAngle of power factorCalculated value of the degree of unbalance lambda in the kth power frequency period delta(k)And (3) representing the value of the relation factor delta 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 cycleAngle of power factorWith degree of unbalance lambda(k)；
S3, judging the current power frequency period relation factor delta according to the network side information(k)Symbol of (A), ifThe mixed parameter PI controller applies the control parameter designed under the working condition 1, if soThe 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
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 respectively collects the upper and lower capacitance voltage signals of the DC side of the three-level grid-connected inverter, the three-phase voltage signal of the AC side of the three-level grid-connected inverter and the three-level grid-connected inverterThree-phase current signals at the alternating current side of the device are sent to the closed-loop control unit and the network 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 signalAnd net side voltage negative sequence component phase angleThe 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 a relation factor delta between the zero-sequence component and the average midpoint current:
when delta is greater than 0, the three-level grid-connected inverter works under the working condition 1;
when delta 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 of δmaxThe maximum value of the relation factor delta under the working condition 1 is the proportionality coefficient kpComprises the following steps:
when the three-level grid-connected inverter works under the working condition 2:
definition of δminThe minimum value of the relation factor delta under the working condition 2 is the proportionality coefficient kpComprises the following steps:
wherein C is a capacitance, fsThe relation factor delta is the unbalance degree lambda and the power factor angle for the switching frequencyAnd negative sequence voltage phase angleRespectively calculating the maximum value delta of the relation factor by adopting a Lagrange multiplier methodmaxAnd minimum value deltaminFinally, 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 unitAnd net side voltage negative sequence component phase angle
Step 6, the working condition judging unit judges the sign of the relation factor delta 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 δ between the zero sequence component and the average midpoint current in step 1 is as follows:
λ∈[0,0.65],
in the formula ImpIs the amplitude of the positive sequence component of the net side current, lambda is the degree of imbalance,in order to be the power factor angle,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 of δmaxThe maximum value of the relation factor delta under the working condition 1 is the proportionality coefficient kpComprises the following steps:
solving for the maximum value delta of the relationship factor using the Lagrange multiplier methodmaxDefining the lagrange function:
wherein v isiIs lagrange multiplier, i is 1,2, 3;to constrain the equation, it is expressed as:
definition ofIs a Lagrangian functionWith respect to the variables λ, The first order gradient of (A) is known from the Karush-Kuhn-Tucher requirementThe first order requirement that the maximum point should satisfy is:
definition ofIs a Lagrangian functionWith respect to the variables λ, Second order gradient, function ofThe second order sufficiency condition that the maximum point should satisfy is:
wherein,expressed as:
according to a functionThe 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 conditionThe maximum point of (2) is substituted into the functionI.e. to find the maximum value deltamaxAnd the proportionality coefficient kp；
When the three-level grid-connected inverter works under the working condition 2:
definition of δminThe minimum value of the relation factor delta under the working condition 2 is the proportionality coefficient kpComprises the following steps:
solving for the minimum value delta of the relationship factor delta using the Lagrange multiplier methodminDefining the lagrange function:
in the formulaExpressed as:
solving a function under a constraint conditionSubstituting the minimum point into the functionI.e. to find the minimum value deltaminAnd the proportionality coefficient kp。
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