WO2014079124A1  Model prediction control method for voltage sourcetype rectifier when grid voltage is unbalanced  Google Patents
Model prediction control method for voltage sourcetype rectifier when grid voltage is unbalanced Download PDFInfo
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 WO2014079124A1 WO2014079124A1 PCT/CN2012/087218 CN2012087218W WO2014079124A1 WO 2014079124 A1 WO2014079124 A1 WO 2014079124A1 CN 2012087218 W CN2012087218 W CN 2012087218W WO 2014079124 A1 WO2014079124 A1 WO 2014079124A1
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Classifications

 H—ELECTRICITY
 H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
 H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
 H02J3/00—Circuit arrangements for ac mains or ac distribution networks
 H02J3/18—Arrangements for adjusting, eliminating or compensating reactive power in networks

 H—ELECTRICITY
 H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
 H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
 H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
 H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
 H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
 H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
 H02M7/53—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
 H02M7/537—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters
 H02M7/5387—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration
 H02M7/53871—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration with automatic control of output voltage or current
 H02M7/53875—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration with automatic control of output voltage or current with analogue control of threephase output

 H—ELECTRICITY
 H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
 H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
 H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
 H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
 H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
 H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
 H02M7/53—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
 H02M7/537—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters
 H02M7/5387—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration
 H02M7/53871—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration with automatic control of output voltage or current
 H02M7/53875—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration with automatic control of output voltage or current with analogue control of threephase output
 H02M2007/53876—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration with automatic control of output voltage or current with analogue control of threephase output based on synthetising a desired voltage vector via the selection of appropriate fundamental voltage vectors, and corresponding dwelling times

 Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSSSECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSSREFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
 Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
 Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
 Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
 Y02E40/30—Reactive power compensation
Abstract
Description
Voltage source type rectifier model predictive control method when grid voltage is unbalanced
The invention relates to a control method of a voltage source type rectifier under an unbalanced grid voltage, and belongs to the field of power converter control.
Background technique
The voltage source type pulse width modulation rectifier has the advantages of grid side voltage sinusoidal, unit power factor, energy bidirectional flow and constant DC voltage control, which can realize "green conversion" of electric energy, so in industrial DC power supply, variable frequency speed control system, reactive power Power compensation, new energy (such as solar energy, wind power) and other fields have broad application prospects. The advantages of the above rectifiers are realized by the double closedloop linear control strategy and the nonlinear control strategy of the current inner loop and the voltage outer loop on the premise of the grid voltage balance. However, in engineering practice, the voltage source rectifier works in an unbalanced state (such as amplitude and phase imbalance, voltage dip, parametric imbalance, etc.), the presence of AC negative sequence current and negative sequence voltage, resulting in DC voltage appearing 2 4, ... subharmonics, 3, 5, ... harmonics will appear in the AC current, which will adversely affect the performance of the rectifier. In severe cases, it will cause a sharp deterioration of the entire system.
At present, the control strategy of the voltage source type rectifier when the grid voltage is unbalanced at home and abroad is mainly to try to eliminate or suppress the fundamental negative sequence component and the finite harmonic component of the grid side current and the harmonic component in the DC voltage. In the case of neglecting the power exchange of the line reactor, suppressing the fluctuation of the active power of the grid side output can keep the DC side capacitor voltage constant and without double frequency fluctuation; When the voltage source type rectifier is applied in a high power occasion, due to power electronics The limitation of the device's own loss, its switching frequency is generally low, the power fluctuation on the line reactor can not be ignored. At this time, if the control strategy of suppressing the active power fluctuation on the grid side is adopted, the active power fluctuation on the reactor needs DC side power. The fluctuations are offset, causing the DC side capacitor voltage fluctuations to be eliminated, causing frequent charging and discharging of the capacitors, and even affecting the stable operation of the entire system in severe cases. At present, the traditional method is to control the AC terminal power of the voltage source type rectifier, which can theoretically eliminate the DC voltage fluctuation, but there is a complicated solution of the current command, and more variables are introduced to make the control system difficult to implement, so it is necessary to seek a kind of Simple and effective way to enhance DC side voltage stability control.
In addition, current imbalance control strategies generally use proportional integral (PI), proportional resonance (PR) controllers or current inner loops of some nonlinear controllers. The above methods have the following problems: 1) need to pass filters or delay algorithms Independent detection of positive and negative sequence current, there is steady state error or time delay; 2) using phaselocked loop to obtain synchronization signal, there is phase shift deviation and time delay; 3) PI or PR controller parameter design is more complicated, it is difficult to achieve Good current tracking accuracy and response speed; 4) The nonlinear controller has large parameter dependence, and the large amount of calculation results in poor realtime performance, which limits the use of the strategy. Therefore, when the grid voltage transient imbalance occurs, in order to enable the voltage source type rectifier to operate stably and reliably, the performance requirements of the control system such as response speed and precise tracking are higher, and the current inner loop control should be able to provide a relatively high Bandwidth, ensuring fast and accurate tracking of current, minimizing transient tracking time. Model predictive control can achieve accurate tracking of reference values due to its good dynamic characteristics. It has the advantages of small computational complexity and easy digital implementation. Therefore, a model predictive control method can be designed and applied to grid voltage imbalance control. Medium, thereby simplifying the system control algorithm and improving the control performance of the system.
Summary of the invention The invention aims to solve the existing problems of voltage source type rectifier control when the grid voltage is unbalanced, and provides a simple and easy to implement model predictive current control method for enhancing DC side voltage stability control, which can ensure fast and accurate current in stationary coordinates. While tracking, it effectively eliminates the fluctuation of the DC side voltage and improves the power quality of the system. At the same time, it realizes the reliable gridconnected operation of the voltage source rectifier in the transient grid voltage imbalance, improving the dynamic and stability of the whole system. In order to achieve the above object, the voltage source type rectifier model predictive control method for grid voltage unbalance proposed by the present invention adopts the following technical solutions:
A voltage source type rectifier model predictive control method for grid voltage imbalance, comprising the following steps:
(1) Let the threephase grid voltages be e _{a} , e _{b} , and e _{c} , respectively, and the threephase grid currents are _{a} , i _{b} , and the DC side voltage is . , respectively transforming the threephase grid voltage and current into a grid voltage e _{a} , e _{p} and current i _{a} , ip in _{a} twophase stationary coordinate system via abc/αβ coordinates;
(2) The positive and negative sequence components of the grid voltage e _{a} , _{ep} in the twophase stationary coordinate system are separated to obtain the positive sequence voltage of the grid (o, e _{p} ^{p} (o and negative sequence voltage (t), e; (t) _{;}
(3) Use the digital notch filter to filter out the actual DC voltage f/ _{d} . The second harmonic interference present in, then calculate f / _{d} . The error between the reference value U _{Ac ref} and the error is converted into the system active power reference value by the PI regulator operation.
(4) Rectifier reference current calculation: Let the PWM rectifier system average reactive power reference value _{av} , _{ref} be 0, calculate the positive and negative sequence components of the reference current in twophase stationary coordinates by the following expression: ref mnea ^{p}
=
Ref n m
'a.ref —m n
'ftref n —m — ―
2P av're ,f
m:
, llcoL
3[( ) ^{2} + ( ) ^{2}  «) ^{2}  (e; fr where ep ^{p} is the positive sequence voltage of the grid in the twophase αβ stationary coordinate system;
, e is the negative sequence voltage of the grid in the twophase αβ stationary coordinate system;
Zf 'ref .ref is a voltage source type positive sequence current reference value in a twophase αβ stationary coordinate system;
?a, ref .ref is a voltage source type negative sequence current reference value in a twophase αβ stationary coordinate system;
J is the network side incoming line filter inductor;
The positive sequence t and the negative sequence component of the above reference current are respectively added to obtain the system electrical reference values ^{Z} a, _{ref} , 3⁄4ref in the twophase αβ stationary coordinate system;
(5) Perform model prediction current control as follows: (a) Calculate the current value at time t _{k+1} from the following prediction model based on the grid voltage and current detected at the current time:
Where R is the internal resistance of the incoming inductor; r _{s} is the sampling period;
iM, is the actual current value at the time of the twophase αβ stationary coordinate system;
i _{a} (t _{M} ), z _{p} 3⁄4 _{+1} ) is the current estimate at the time t _{k+1} in the twophase αβ stationary coordinate system;
e _{a} (t _{k} ), e _{p} (is the actual grid voltage value at the time of the twophase αβ stationary coordinate system;
u _{a} (t _{k} ), ) is the αβ component of the AC side voltage corresponding to the switching state applied in the first sampling period, and the initial time value can be set to 0;
(b) Calculate the AC side voltage _{+1} ) corresponding to each switch state in the HIth sampling period using the following formula,
"D β ^{l} k+\ , ie
Where &, s _{h} , & are the switching states of the three upper arms of the voltage source rectifier;
U _{ic} (t _{k+l} ) is the DC side voltage at time t _{k+l} ;
(c) From the abovementioned AC side voltage M _{a} (U, u _{p} (t _{k+l} ), further predict the current value _{a} (U, t _{k )} in the twophase stationary coordinate system at the time i _{+ 2} according to the aforementioned prediction model. _{+2} );
(d) Construct a value function
Where UU, _{ftref} (U is the current reference value of it _{+2} ), each switch state is evaluated by the value function g, and the switch state corresponding to the predicted current value that minimizes the value function is selected;
(e) According to the switch state selected in step (d), control the switches on the three upper arms of the voltage source rectifier to achieve stable operation of the rectifier. As a preferred embodiment, in step (2), the grid voltage e _{a} , the positive and negative sequence components are separated by the decomposition method shown in the following formula to obtain the grid positive sequence voltage (t), (t) and negative sequence voltage (0, e;(t) , ie (t) =→ _{a} (t) ^ ^[e _{p} (t )~ e _{p} (t) cos γ]
2 2 sin ω
(t) =→ _{p} (t) —^ [e _{a} (t  — e _{a} (t) cos γ]
2 2 sin ω
Where Z is the phase shift angle; ω is the grid voltage angular frequency; the superscripts ρ and η represent the positive and negative sequence components, respectively;
e _{a} (t), e _{p} (t) is the grid phase voltage in the twophase αβ stationary coordinate system at time t;
El (t), (t) is the positive sequence voltage of the grid in the twophase αβ stationary coordinate system at time t;
(t), e; (t) is the negative sequence voltage of the grid in the twophase αβ stationary coordinate system at time t;
e _{a} (t ^), et _ ) is the grid phase voltage in the twophase αβ stationary coordinate system at t  time.
ω ω ω
The invention has the following technical effects:
1. The control method proposed by the present invention proposes a method for enhancing the DC side voltage stability control by considering the power fluctuation on the incoming line inductance of the voltage source type rectifier under the unbalanced grid voltage, effectively solving the DC side voltage fluctuation problem. At the same time, the gridconnected operation of the voltage source rectifier under the unbalanced grid voltage is realized, the power quality of the system is improved, and the stability and reliability of the whole system are improved.
2. The control method of the present invention adopts a fast decomposition method of positive and negative sequence components, which performs positive and negative sequence separation compared with a filter or a quarter grid periodic delay algorithm, which reduces the decomposition error and time delay problem, and improves the problem. The stability and response speed of the control system is particularly suitable for transient grid imbalances.
3. The invention applies the model predictive current control to the grid voltage imbalance control, realizes the fast and accurate tracking of the reference current, has the advantages of good dynamic characteristics, small calculation amount, easy digitization, and the like, and is suitable for the transient grid voltage. Balance the situation.
4. The control method of the invention is implemented in twophase stationary coordinates, does not require a phaselocked loop to acquire the synchronization signal and positive and negative sequence decomposition of the current, and the control structure is simple and easier to implement, avoiding possible phase shift deviation and time delay. The problem is to improve the reliability of the system.
DRAWINGS
1 is a topological structural diagram of a main circuit of a voltage type voltage source type rectifier;
2 is a schematic diagram of the principle of rapid decomposition of voltage positive and negative sequence components;
Figure 3 is a flow chart of the model predictive current control algorithm;
Figure 4 is a block diagram of the control system of the voltage source rectifier under unbalanced grid voltage.
detailed description
The present invention will be further described in detail below with reference to the accompanying drawings and embodiments.
The invention is a novel control strategy proposed for gridconnected operation of a voltage source type rectifier under the condition of grid voltage imbalance, and can effectively suppress the second harmonic component of the DC side voltage in a twophase stationary coordinate system, simplifying the control system Algorithm to improve system response speed and reliability. The following is a further description of the power model under unbalanced grid voltage, the separation of positive and negative voltage components, the calculation of reference current value, and the design of model prediction control algorithm.
(1) Power model of voltage source rectifier under grid voltage imbalance
The voltage source type rectifier acts as a power converter, and its main function is to obtain specific active power and reactive power from the grid side to meet the load demand. Therefore, the control of the input current and the DC side voltage by the voltage source type rectifier is actually an input. And control of output power. If the instantaneous active and reactive powers are quickly and efficiently controlled, the voltage source rectifier Can get good dynamic and static characteristics. Therefore, it is necessary to analyze the operating characteristics of the voltage source type rectifier under voltage imbalance from the power point of view, and then effectively determine and design the control method. The power model of a voltage source rectifier under unbalanced grid voltage is described below.
Figure 1 is a schematic diagram of the main circuit topology of a voltagetype voltage source rectifier. In the figure, e _{a} , e _{b} , and ^ are the grid phase voltages; _{a} , _{b} , . For AC side phase current; u _{a} , u _{h} , is the AC side voltage of the rectifier; ^ is the DC side voltage; J, R is the line reactor and its internal resistance, P _{g} , ^ are the input power of the grid side and The active power is input to the AC side. The vector equation of the voltage source rectifier in twophase stationary coordinates is
E _{aP} =U _{aP} +L^ + RI _{ap} (1) where £ _{α} β, and / _{α} β are the combined vectors of the grid voltage, the AC side voltage of the rectifier, and the input current in the twophase αβ stationary coordinate system.
When the grid voltage is unbalanced, both the grid voltage £^ and the current/ _{α} β contain a positive sequence component, a negative sequence component, and a zero sequence component. For a threephase neutral line system, the zero sequence component can be disregarded. In this case, in the rotating coordinate system, in addition to the positive sequence component rotated counterclockwise, the negative sequence component rotated clockwise, then £αβ, and /αβ can Expressed as
< υ _{αΡ} = νι, + υι, =^( + Κ + j ) (2)
, ρ = 3⁄4 + 3⁄4
+ _{e}  ^{JiUi} ( _{d} ^{n} +j _{q} ^{n} ) where: the superscripts ρ and η represent the positive and negative sequence components respectively; the subscripts d and q represent the d and q axis components of the rotating coordinate system respectively; ω is the synchronization of the grid voltage Angular velocity.Therefore, when the grid voltage is unbalanced, the complex power S input on the grid side can be expressed as
S _{s} =E _{a} ^ =P _{s +} iQ _{s} (3)
Substituting equation (2) into equation (1)
P _{e}
+ P _{s} lsm(2 ot) _{(4)}3⁄4 ϋ cos (2 ) + _{2} sin (2 )
In the formula,
The input power of the AC terminal of the voltage source rectifier can be expressed as = + P: _{2} cos {2cot) + P _{2} sin [lcot)
(5)
Q _{c} = Ql + Qli cs (2 ) + Ql _{2} sin {2ωή
In the formula,
P = I.
=1.
It can be seen from equation (4) that when the grid voltage is unbalanced, in the positive and negative sequence doublerotation coordinate system, the active power / ^{5} _{g} and reactive power input on the grid side are composed of a direct current component and a double frequency component. Where /^ is the active power DC component; and /3⁄4 is the secondary power component amplitude of the active power; _{v} is the reactive power DC component; ρ and _{s} ^{g} _{2} are the reactive power double frequency component amplitude; The input power of the AC source of the voltage source rectifier is similarly defined by 3⁄4; e _{d} ^{p} is the dq axis component of the positive sequence voltage of the grid; e _{d} ^{n} is the dq axis component of the grid negative sequence voltage; and is the positive sequence voltage dq axis of the AC terminal Component; and is the grid negative sequence voltage dq axis component; _{d} ^{p} and is the AC terminal positive sequence current dq axis component; _{d} ^{n} and _{q} ^{n} are the AC terminal negative sequence current dq axis component.
(2) Separation of positive and negative sequence components of grid voltage
When the voltage of the threephase grid is unbalanced, especially the transient imbalance, in order to control the input current of the rectifier, the positive and negative sequence components of the grid voltage need to be separated in order to calculate the positive and negative sequence current reference values. The common method is to use the trap. The wave filter is separated from the quarter grid cycle delay method, and there is a problem of separation error and time delay. Therefore, the present invention adopts a fast positive and negative sequence component decomposition method, and the principle of positive and negative sequence component decomposition of the twophase stationary coordinate system can be expressed as follows:
1 γ
( = e _{a} ( ― ~ (t) cos γ]
2 sin o
A _{ea} (t)cos ] (6)
n ω
e;(t) = e _{p} (t)e; (t) is the phase shift angle;
e _{a} (t), e _{p} (0 is the grid phase voltage in the twophase αβ stationary coordinate system at time t; e _{a} (t ), e t_^) is the grid phase in the twophase αβ stationary coordinate system at ttime Voltage;
ω ω ω
El{t), eUt) is the positive phase sequence of the grid phase voltage in the twophase αβ stationary coordinate system at time t (t), e; {t) is the negative phase sequence of the phase voltage of the grid in the twophase αβ stationary coordinate system at time t.
The specific embodiment can be represented by FIG. 2, and the grid voltage e _{a} , ep in the twophase αβ stationary coordinate system is obtained by sampling the grid side voltage through Clark transform, and then the grid positive sequence voltage and negative sequence are calculated by using equation (6). Voltage e;, e _{p} ^{n} . The algorithm can be completed in («≥1) sampling periods. When the sampling period of the system is very short, this separation method can improve the stability and response speed of the control system.
(3) Calculation of reference current value in twophase stationary coordinate system
When the voltage of the threephase grid is unbalanced, the control of the voltage source rectifier is mainly to suppress the active power fluctuation of the system to eliminate the second harmonic of the DC side voltage. Under the influence of the degree of freedom of control, the system generally only controls _{v} , βαν ^ and Λ _{2} , but does not control G _{e2} and ft _{2} . The control system needs to select the appropriate power reference value to get the required reference current. In order to simplify the system control algorithm, the calculation of the reference current in the present invention is implemented in a twophase stationary coordinate system, so that the angular phase locking link and coordinate rotation transformation of the positive and negative sequence voltage vector of the grid are not needed, and the possible phase shift deviation is avoided. And time delay, which improves the reliability of the system.
Related to the flow side load, its 3⁄4T 3⁄4il :, the flow side outer ring PI is adjusted,
The difference between the measured value and the actual sampled value is input to the PI controller. The product of the PI controller output value and the DC voltage reference value is used as the system instantaneous active power reference value:
_{Where νρ} , , are the ratio and integral gain of the voltage outer loop PI regulator, respectively; [4 is the DC side voltage reference value. In addition, the voltage source type rectifier is generally required to operate with a unit power factor, and the average reactive power reference value 3⁄4 _{ν} ^ is set to 0.
Under the condition that the inductance of the line reactor is small, the power fluctuation of the line reactor can be ignored, and P _{C} =P _{S is considered} . Therefore, the fluctuation of the active power on the grid side can be suppressed, and the DC side capacitor voltage can be kept constant and there is no difference. Multiplier fluctuations. When the voltage source type rectifier is applied in high power applications, the inductance of the incoming line is relatively large. When the grid voltage is unbalanced, the power fluctuation on the line reactor cannot be ignored. At this time, if the control of the active power fluctuation on the grid side is adopted, In the strategy, the active power fluctuation on the reactor needs to be offset by the DC side power fluctuation, which causes the DC side voltage fluctuation to be eliminated. In order to effectively eliminate the double frequency fluctuation of the DC side voltage, it is necessary to consider the power fluctuation on the reactor. At this time, the input power of the AC end of the rectifier should be controlled instead of the input power of the control network side, that is, the /^ sum should be equal to the active power reference value respectively. And 0, while making / 3⁄4, equal
0. At this time, the system reactive power is still uncontrollable, and there will still be double frequency fluctuations, but the average value is 0. Therefore, the expression of the positive and negative sequence components of the reference current in the synchronous rotating coordinate system is obtained by the equation (5) when the DC side voltage is not fluctuating.
Where D _{dq} = (uf + (ulf  {u «) ^{2} ≠ 0;
d ^{p} _{ref} , _{q} ^{p} _{ref} is the positive sequence of the line current reference value in the positive sequence dq rotating coordinate system _{d} ^{n} _{ref} , _{q} ^{n} _{ref} is the negative sequence of the line current reference value in the negative sequence dq rotating coordinate system
Let coordinate rotation transformation matrix M _{dq+→aP} P 1⁄2^_ _{→}
Cos^ ^{p} sin^ ^{1}
M dq+ _{→a} + (10)
Sin^ ^{p} cos^ ^{p} cos θ ^{η} sin Θ ^{1}
M dq  * β (11)
sin^" cos<9'
In the formula, and ^ ^{1} are the positive and negative sequence rotation angles of the grid side voltage vector, respectively. Then, when both sides of equations (8) and (9) are multiplied by the matrices M _{dq+→aP} _ and M _{dq} _ _{→a} p_, the positive and negative sequence components of the reference current in the twophase αβ stationary coordinate system can be obtained.
(12)
Where n = (\  ^\  A ( Lm) ^{2} \ l2 L. From equations (16) and (17), it can be seen that after considering the power fluctuations on the reactor, the reference current positive and negative sequence components The calculation contains two parts. From the perspective of the input power of the grid side, the first part can be regarded as the average active power required to provide the DC side load, and the second part can be regarded as the active power fluctuation absorbed on the compensation reactor. The control method can well control the input power of the AC end of the rectifier to be constant, and ensure that the DC side voltage is stable and has no double frequency fluctuation. At the same time, it can be found that, in practical applications, since the voltage drop on the inductor is not large, the system power factor Approximate to 1, to meet the needs of the application.
(4) Model predictive control algorithm design
To achieve ripplefree control of the DCside voltage of the voltage source rectifier when the grid is unbalanced, the positive and negative sequence current reference values must be tracked without static tracking. At present, the current inner loop of PI, PR or some nonlinear controllers is generally used in the unbalanced control strategy, but the following problems exist: 1) The positive and negative sequence currents need to be independently detected by a filter or a delay algorithm, and there is a steady state error or Time delay; 2) Using phaselocked loop to obtain synchronization signal, there is phase shift deviation and time delay; 3) PI or PR controller parameter design is more complicated, it is difficult to achieve better current tracking accuracy and response speed; 4) The linear controller has large parameter dependence, and the large amount of calculation results in poor realtime performance. When the grid voltage is transiently unbalanced, in order to enable the voltage source rectifier to operate stably and reliably, the response speed and precise tracking of the control system are required to be higher. Therefore, the current inner loop control should be able to provide a relatively high bandwidth. , to ensure fast and accurate tracking of current, to minimize transient tracking time. Because the model predictive control has good dynamic characteristics, it can realize accurate tracking of reference values, and has the advantages of small calculation amount and easy digital implementation. Therefore, the present invention proposes a model predictive control method and applies it to the grid voltage. In the unbalanced control, the system control algorithm can be simplified and the control performance of the system can be improved.
A common model predictive control method is to establish a system discretization prediction model, and then construct a value function. In each sampling period, each switching voltage vector is evaluated by the prediction model, so that the minimum value of the switching vector of the value function is next. The sampling period is employed to achieve optimal tracking control.
In one sampling week 7, the discretization prediction model of the voltage source rectifier is obtained according to equation (1).
Where L and R are the incoming inductance and its resistance; 7; is the sampling period.
33⁄4 _{+} ,) is the current predicted value at the time of the twophase αβ stationary coordinate system; iM, is the actual current sample value at the time of the twophase αβ stationary coordinate system;
e _{a} (t _{k} ), e _{p} (is the actual grid voltage sample value at the time of the twophase αβ stationary coordinate system; u _{a} (t _{k} ) , ) is the AC end corresponding to the switch state applied during the first sampling period The αβ component of the voltage in a twophase stationary coordinate system can be obtained by the following equation: (t _{k} ) = U _{Ac} (t _{k} )[S _{a}   (S _{h}  S _{c} )] Where, &, S _{h} , S _{e} are the switching states of the three upper arms of the voltage source rectifier (a total of 8 switching states);
U _{Ac} (t _{k} ) is the DC side voltage at time t _{k} .
In the actual control system, there is usually the influence of calculation time and control delay. In order to improve the performance of the model prediction controller, delay compensation is required. Assuming that the selected switch state is applied during the A+1th sampling period, the current at the sampling instant must be predicted. Therefore, we need to calculate equation (18) forward one step, and we can get the following prediction model t _{k+2} ) =
Where i _{a} D, (U is the current predicted value at the time of the twophase αβ stationary coordinate system;
e _{a} (t _{k+l} ), e _{p} (t, _{+1} ) is the predicted value of the grid voltage in the twophase αβ stationary coordinate system at time t _{k+i} . Since the sampling frequency is much larger than the grid frequency, the grid voltage can be considered One sample period remains unchanged, ie
(t _{M} ), (U is the αβ component of the AC terminal voltage corresponding to the predicted switching state in the A+1 sampling period in the twophase stationary coordinate system, and the value can be based on the DC side voltage at time t _{k+1} And the switch states &, &, & (a total of 8 switch states) are obtained by equation (19).
When the current at time _{+2} is predicted, a value function g is constructed to evaluate the voltage vectors of the rectifier, that is, the switching state corresponding to the current prediction value that minimizes the value function is selected in the next sampling period. This cycle allows for the ideal input current. Different control criteria will use different value functions g. Common methods use the sum of the absolute values of current errors as a function of value. The expression is:
S ( +2 ( +2 ( +2 ( +2 ) (21)
Where i _{a} , _{ief} (t _{k+2} ), _{ftref} (U is the current reference value in the twophase αβ stationary coordinate system at the time of it _{+2} , and its value can be passed from the first few moments of the reference current value through Euler Secondorder recursive method
, ref ( +2 ) = 3Z _{a} , _{ref} (3⁄4+ι )  3 _{a ef} (t _{k} ) + Z _{a} , _{ref} (j _{k} ― )
3⁄4ref ( +2 ) = 33⁄4 _{re} f (^+l )― 33⁄4 _{re} f ) + 3⁄4ref )
Where _{ref} (im;), β, π^ίπθ are the current reference values in the twophase αβ stationary coordinate system at the moment;
iaM, _{ref} (i is the current reference value in the twophase αβ stationary coordinate system at the moment;
a, ref( l), ^ef^W) is the current reference value in the twophase θ stationary coordinate system at the moment.
Based on the above analysis, the implementation of the model predictive control can be represented by the algorithm flow chart shown in Figure 3:
(a) First collect the threephase current i(tk) at time h, the threephase grid voltage 3⁄4, and the DCside voltage U '
(b) applying the switching state S{t _{k} , calculated at the previous moment, using the prediction model shown in equation (18) to estimate the current value (im) at time t _{k+1} ;
(c) Using eight kinds of switching states (&, &, and), and the prediction models shown in equations (19) and (20), respectively, to calculate the eight current prediction values i at t _{k+1} (t) _{k+2} );
(d) Construct a value function g as shown in equation (21) and calculate its eight results _{&} , and select the switch state corresponding to the current prediction value that minimizes the value of the value function to be used at the next moment, thus circulating the current. Precise tracking control System.
In summary, the preferred embodiment of the control method proposed by the present invention can be systematically represented as FIG. 4, and specifically includes the following steps:
1) Using the voltage sensor and the current sensor to detect the threephase grid voltage e _{a} , e _{h} , e _{e} and the grid side threephase input current i _{b} , respectively, and obtain the grid under the twophase stationary coordinate system via the abcαβ coordinate transformation module respectively. Voltage e _{a} , and input current
4, Hey,
2) taking the grid voltage e _{a} in the step (1) and the fast positive and negative sequence component decomposition method shown in FIG. 2 to obtain the grid positive sequence voltage and the negative sequence voltage e: in the twophase stationary coordinate system; e; ,
3) Using the voltage sensor to detect the DC side capacitor voltage U _{dc} , use the digital notch filter to filter out the second harmonic component, and then calculate the DC reference voltage f / _{de} , _{ref} and the filtered [4 difference, after PI control The product of the output value of the device and the filtered DC voltage f/ _{dc} is used as the reference active power of the DC side output. At this time, the reference reactive power is set to zero.
4) The grid positive sequence voltage, e _{p} ^{p} and negative sequence voltage e:;, e; and the average reference power 7^^ and 0 obtained in step (2) from the twophase stationary coordinates in step (3) „^, by the reference current calculation method shown in equations (20) and (21), calculate the reference current in twophase stationary coordinate system, _{re} f, _{P} , ref;
5) From the grid voltage e _{a} in step (1), and the input current, the current reference value in step (4), _{ref} , _{re} f and the DC side voltage f / _{dc} , the model is used to predict the current as shown in Figure 3. The control algorithm can obtain the switching signals &, & and & of the three upper arms of the voltage source type rectifier to realize the breaking of the control power device.
In summary, the control method of the invention can ensure the rapid and accurate tracking of the current while effectively eliminating the DC side voltage fluctuation, improving the power quality of the system, and realizing the voltage source type rectifier in the transient state. Reliable grid operation when the grid voltage is unbalanced. The proposed control system has simple algorithm and only needs to be realized in twophase stationary coordinate system. There is no coordinate rotation transformation, phaselocked loop and positive and negative sequence decomposition of current, which reduces the time delay and steadystate error of the control system and improves the whole. System dynamics, stability and reliability.
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