CN114825999B - Modularized multi-level converter loss optimization method based on optimal control - Google Patents
Modularized multi-level converter loss optimization method based on optimal control Download PDFInfo
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
- 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
- H02M7/483—Converters with outputs that each can have more than two voltages levels
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Abstract
The invention discloses a modularized multi-level converter loss optimization method based on optimal control, which relates to the technical field of multi-level power electronic converters and is used for monitoring the number of sub-modules to be put into each bridge arm, bridge arm current and sub-module capacitor voltage in the current control period of the modularized multi-level converter in real time; establishing an objective function of loss optimization, and setting constraint conditions; solving a submodule input/cut state capable of enabling an objective function to reach minimum; according to the solution obtained by solving, the corresponding submodules are put into to finish the balance optimization of the capacitance voltage and the loss of each submodule of the current converter, the invention not only can effectively reduce the loss difference between each submodule in the modularized multi-level current converter while balancing the capacitance voltage of the submodule, but also can not bring extra voltage and current stress to equipment devices, does not need to increase the cost of the modularized multi-level current converter, and has stronger practical value in the aspects of prolonging the service life and the reliability of the modularized multi-level current converter.
Description
Technical Field
The invention relates to the technical field of multi-level power electronic converters, in particular to a modular multi-level converter loss optimization method based on optimal control.
Background
In recent years, with the rapid development of power electronic technology, a modularized multi-level converter has become the most attractive converter topology in a high-voltage high-power system, and by virtue of the advantages of high modularization degree, high energy transmission efficiency, low harmonic content of output voltage at an alternating side, realization of redundancy control and the like, the modularized multi-level converter is widely applied in the fields of flexible direct current transmission, medium-high voltage motor driving, large-scale new energy grid connection and the like, the modularized multi-level converter is composed of a large number of sub-modules, in the actual operation process, uneven phenomena often occur in loss distribution of each sub-module, service life difference of the sub-module of the MMC is caused by thermal stress difference caused by uneven loss, and therefore reliability of the MMC is reduced, and loss optimization is one of important challenges for improving the reliability of the MMC.
The loss optimization of the modularized multi-level converter is usually realized by adopting a method for changing the topological structure of a sub-module or injecting circulation, so that the problems of increased construction cost, increased device current stress and the like of the modularized multi-level converter are solved, the application of the method in actual engineering is limited, and therefore, the modularized multi-level converter loss optimization method based on optimal control is provided.
Disclosure of Invention
In order to solve the defects in the background art, the invention aims to provide a modularized multi-level converter loss optimization method based on optimal control.
The aim of the invention can be achieved by the following technical scheme: a modular multilevel converter loss optimization method based on optimal control, the modular multilevel converter loss optimization method comprising the steps of:
Step one: monitoring the number N (k) of sub-modules to be put into each bridge arm, the bridge arm current i arm (k) and the sub-module capacitance voltage u c_i (k) in the current control period k of the modularized multi-level converter in real time, wherein i=1, 2, …, N and N are the number of the bridge arm sub-modules;
Step two: according to the existing circuit mathematical model, establishing a loss optimized objective function J, and setting constraint conditions phi of a submodule switching function;
Step three: solving a switching state capable of enabling an objective function to be minimum through an optimal control theory, wherein the obtained switching state S i (k) is a solution in the control period;
Step four: according to the solution input corresponding sub-modules, the balance optimization of the capacitance voltage and the loss of each sub-module of the modular multilevel converter can be completed.
Further, the constraint Φ of the objective function J and the submodule switching function is as follows:
Where u c_i (k+1) is a predicted value of capacitance voltage of the ith sub-module at k+1, u c_ref is a nominal value of capacitance voltage of the sub-module, λ is a weight factor, P sm_i (k+1) is a predicted value of loss of the ith sub-module from k to k+1, P sm_ave (k) is an average value of loss of each sub-module from k-1 to k, and S i (k) is a put-in state of each sub-module at k.
Further, the Si (k) is expressed as:
The input state is as follows: the first power switch T 1 is in an on state, and the second power switch T 2 is in an off state; the excision status is: the first power switch T 1 is in an off state and the second power switch T 2 is in an on state.
Further, the calculation formulas of the capacitance voltage nominal value of the submodule and the capacitance voltage predicted value of the ith submodule at the moment k+1 are as follows:
Wherein U dc is the DC side voltage of the modular multilevel converter, T s is the control period, and C sm is the capacitance value of the submodule.
Further, the calculation formulas of the loss average value of each sub-module from the k-1 moment to the k moment and the loss predicted value of the ith sub-module from the k moment to the k+1 moment are as follows:
Wherein, P sm_i_t1 is the total loss of the first power switch T 1 of the ith sub-module, P sm_i_t2 is the total loss of the second power switch T 2 of the ith sub-module, P sm_i_d1 is the total loss of the first diode D 1 of the ith sub-module, and P sm_i_d2 is the total loss of the second diode D 2 of the ith sub-module.
Further, the calculation formula of P sm_i_t1、Psm_i_t2、,Psm_i_d1、Psm_i_d2 is:
Wherein P sm_i_t1_con (k+1) is the conduction loss of the first power switch T 1 of the submodule, P sm_i_t1_on (k+1) is the turn-on loss of the first power switch T 1 of the submodule, P sm_i_t1_off (k+1) is the turn-off loss of the first power switch T 1 of the sub-module, P sm_i_t2_con (k+1) is the turn-on loss of the second power switch T 2 of the sub-module, p sm_i_t2_on (k+1) is the on-loss of the second power switch T 2 of the sub-module, P sm_i_t2_off (k+1) is the off-loss of the second power switch T 2 of the sub-module, P sm_i_d1_con (k+1) is the conduction loss of the sub-module first diode D 1, P sm_i_d1_rec (k+1) is the reverse recovery loss of the sub-module first diode D 1, P sm_i_d2_con (k+1) is the conduction loss of the second diode D 2 of the sub-module, and P sm_i_d2_rec (k+1) is the reverse recovery loss of the second diode D 2 of the sub-module.
Further, the loss calculation formula of each part of the power device is as follows:
The conduction losses of the power switch T 1/T2 and the diode D 1/D2 are as follows:
Wherein V T is zero current state voltage drop of the power switch, R T is on state resistance of the power switch, V D is zero current state voltage drop of the diode, and R D is on state resistance of the diode;
the switching losses of the power switch T 1/T2 are as follows:
Wherein E on () is a power switch on energy function, E off () is a power switch off energy function, and U ref is a test voltage in a power switch data table;
the reverse recovery loss of diode D1/D2 is as follows:
Where E rec () is the diode reverse recovery energy function.
The invention has the beneficial effects that:
In the using process, the quantity of sub-modules to be put into each bridge arm, the bridge arm current and the capacitance voltage of the sub-modules in the current control period of the modularized multi-level converter are monitored in real time; establishing an objective function of loss optimization, and setting constraint conditions; solving a submodule input/cut state capable of enabling an objective function to reach minimum; according to the solution obtained by solving, corresponding submodules are put into, so that balance optimization of capacitor voltage and loss of each submodule of the converter is completed, the balance of capacitor voltage of the submodules can be ensured, meanwhile, the loss difference among the submodules in the modularized multi-level converter is effectively reduced, so that the whole operation life and reliability of the modularized multi-level converter are improved; the invention controls the sub-modules according to the optimal solution obtained in each control period, and can ensure that each control period has an optimal control effect, thereby meeting extremely high control requirements.
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In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described, and it will be obvious to those skilled in the art that other drawings can be obtained according to these drawings without inventive effort;
Fig. 1 is a flow chart of the present invention.
Detailed Description
The following description of the embodiments of the present invention will be made clearly and completely with reference to the accompanying drawings, in which it is apparent that the embodiments described are only some embodiments of the present invention, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
As shown in fig. 1, a modular multilevel converter loss optimization method based on optimal control includes the following steps:
Step one: monitoring the number N (k) of sub-modules to be put into each bridge arm, the bridge arm current i arm (k) and the sub-module capacitance voltage u c_i (k) in the current control period k of the modularized multi-level converter in real time, wherein i=1, 2, …, N and N are the number of the bridge arm sub-modules;
Step two: establishing a loss optimized objective function J according to a circuit mathematical model, and setting constraint conditions phi of a submodule switching function;
Step three: according to an optimal control theory, solving a switching state capable of enabling an objective function to reach the minimum, wherein the obtained switching state S i (k) is a solution in the control period;
Step four: according to the solution input corresponding sub-modules, the balance optimization of the capacitance voltage and the loss of each sub-module of the modular multilevel converter can be completed.
It should be further described that, in the specific implementation process, the modularized multi-level converter circuit is composed of three-phase six bridge arms, each bridge arm is composed of N sub-modules with identical topology and a bridge arm inductance in series connection; the sub-module is of a half-bridge structure and consists of two power switches T1 and T2, two anti-parallel diodes D1 and D2 and an energy storage capacitor C.
It should be further noted that, in the implementation process, the constraint conditions Φ of the objective function J and the submodule switching function are as follows:
Where u c_i (k+1) is a predicted value of capacitance voltage of the ith sub-module at k+1, u c_ref is a nominal value of capacitance voltage of the sub-module, λ is a weight factor, P sm_i (k+1) is a predicted value of loss of the ith sub-module from k to k+1, P sm_ave (k) is an average value of loss of each sub-module from k-1 to k, and S i (k) is a put-in state of each sub-module at k.
It should be further noted that, in the implementation process, the Si (k) is expressed as:
The input state is as follows: the first power switch T 1 is in an on state, and the second power switch T 2 is in an off state; the excision status is: the first power switch T 1 is in an off state and the second power switch T 2 is in an on state.
It should be further noted that, in the implementation process, the calculation formulas of the nominal value of the capacitance voltage of the sub-module and the predicted value of the capacitance voltage of the ith sub-module at the time k+1 are as follows:
Wherein U dc is the DC side voltage of the modular multilevel converter, T s is the control period, and C sm is the capacitance value of the submodule.
It should be further noted that, in the implementation process, the calculation formulas of the loss average value of each sub-module from k-1 time to k time and the loss predicted value of the ith sub-module from k time to k+1 time are as follows:
Wherein, P sm_i_t1 is the total loss of the first power switch T 1 of the ith sub-module, P sm_i_t2 is the total loss of the second power switch T 2 of the ith sub-module, P sm_i_d1 is the total loss of the first diode D 1 of the ith sub-module, and P sm_i_d2 is the total loss of the second diode D 2 of the ith sub-module.
It should be further described that, in the implementation process, the calculation formula of P sm_i_t1、Psm_i_t2、,Psm_i_d1、Psm_i_d2 is as follows:
Wherein P sm_i_t1_con (k+1) is the conduction loss of the first power switch T 1 of the submodule, P sm_i_t1_on (k+1) is the turn-on loss of the first power switch T 1 of the submodule, P sm_i_t1_off (k+1) is the turn-off loss of the first power switch T 1 of the sub-module, P sm_i_t2_con (k+1) is the turn-on loss of the second power switch T 2 of the sub-module, p sm_i_t2_on (k+1) is the on-loss of the second power switch T 2 of the sub-module, P sm_i_t2_off (k+1) is the off-loss of the second power switch T 2 of the sub-module, P sm_i_d1_con (k+1) is the conduction loss of the sub-module first diode D 1, P sm_i_d1_rec (k+1) is the reverse recovery loss of the sub-module first diode D 1, P sm_i_d2_con (k+1) is the conduction loss of the second diode D 2 of the sub-module, and P sm_i_d2_rec (k+1) is the reverse recovery loss of the second diode D 2 of the sub-module.
It should be further described that, in the specific implementation process, the calculation formula of the loss of each part of the power device is as follows:
The conduction losses of the power switch T 1/T2 and the diode D 1/D2 are as follows:
Wherein V T is zero current state voltage drop of the power switch, R T is on state resistance of the power switch, V D is zero current state voltage drop of the diode, and R D is on state resistance of the diode;
the switching losses of the power switch T 1/T2 are as follows:
Wherein E on () is a power switch on energy function, E off () is a power switch off energy function, and U ref is a test voltage in a power switch data table;
the reverse recovery loss of diode D1/D2 is as follows:
Where E rec () is the diode reverse recovery energy function.
The foregoing has shown and described the basic principles, principal features and advantages of the invention. It will be understood by those skilled in the art that the present invention is not limited to the embodiments described above, and that the above embodiments and descriptions are merely illustrative of the principles of the present invention, and various changes and modifications may be made without departing from the spirit and scope of the invention, which is defined in the appended claims.
Claims (1)
1. The modularized multi-level converter loss optimization method based on optimal control is characterized by comprising the following steps of:
Step one: monitoring the number N (k) of sub-modules to be put into each bridge arm, the bridge arm current i arm (k) and the sub-module capacitance voltage u c_i (k) in the current control period k of the modularized multi-level converter in real time, wherein i=1, 2, …, N and N are the number of the bridge arm sub-modules;
Step two: according to the existing circuit mathematical model, establishing a loss optimized objective function J, and setting constraint conditions phi of a submodule switching function;
The constraint conditions Φ of the objective function J and the submodule switching function are as follows:
wherein u c_i (k+1) is a predicted value of the capacitance voltage of the ith sub-module at the moment k+1, u c_ref is a nominal value of the capacitance voltage of the sub-module, lambda is a weight factor, P sm_i (k+1) is a predicted value of the loss of the ith sub-module from the moment k to the moment k+1, P sm_ave (k) is an average value of the loss of each sub-module from the moment k-1 to the moment k, and S i (k) is the input state of each sub-module at the moment k;
The calculation formulas of the capacitance voltage nominal value of the sub-module and the capacitance voltage predicted value of the ith sub-module at the time k+1 are as follows:
Wherein U dc is the DC side voltage of the modularized multi-level converter, T s is the control period, and C sm is the capacitance value of the submodule;
the calculation formulas of the loss average value of each sub-module from the k-1 moment to the k moment and the loss predicted value of the ith sub-module from the k moment to the k+1 moment are as follows:
Wherein, P sm_i_t1 is the total loss of the first power switch T 1 of the ith sub-module, P sm_i_t2 is the total loss of the second power switch T 2 of the ith sub-module, P sm_i_d1 is the total loss of the first diode D 1 of the ith sub-module, and P sm_i_d2 is the total loss of the second diode D 2 of the ith sub-module;
The calculation formula of the P sm_i_t1、Psm_i_t2、Psm_i_d1、Psm_i_d2 is as follows:
Wherein P sm_i_t1_con (k+1) is the conduction loss of the first power switch T 1 of the submodule, P sm_i_t1_on (k+1) is the turn-on loss of the first power switch T 1 of the submodule, P sm_i_t1_off (k+1) is the turn-off loss of the first power switch T 1 of the sub-module, P sm_i_t2_con (k+1) is the turn-on loss of the second power switch T 2 of the sub-module, p sm_i_t2_on (k+1) is the on-loss of the second power switch T 2 of the sub-module, P sm_i_t2_off (k+1) is the off-loss of the second power switch T 2 of the sub-module, P sm_i_d1_con (k+1) is the conduction loss of the sub-module first diode D 1, P sm_i_d1_rec (k+1) is the reverse recovery loss of the sub-module first diode D 1, P sm_i_d2_con (k+1) is the conduction loss of the second diode D 2 of the sub-module, and P sm_i_d2_rec (k+1) is the reverse recovery loss of the second diode D 2 of the sub-module;
the calculation formula of the loss of each part of the power device is as follows:
The conduction losses of the power switch T 1/T2 and the diode D 1/D2 are as follows:
Wherein V T is zero current state voltage drop of the power switch, R T is on state resistance of the power switch, V D is zero current state voltage drop of the diode, and R D is on state resistance of the diode;
the switching losses of the power switch T 1/T2 are as follows:
Wherein E on () is a power switch on energy function, E off () is a power switch off energy function, and U ref is a test voltage in a power switch data table;
the reverse recovery loss of diode D1/D2 is as follows:
wherein E rec () is a diode reverse recovery energy function;
Step three: solving a switching state capable of enabling an objective function to be minimum by adopting an optimal control theory, wherein the obtained switching state S i (k) is a solution in the control period k;
Si (k) is expressed as:
The input state is as follows: the first power switch T 1 is in an on state, and the second power switch T 2 is in an off state; the excision status is: the first power switch T 1 is in an off state, and the second power switch T 2 is in an on state;
Step four: according to the solution input corresponding sub-modules, the balance optimization of the capacitance voltage and the loss of each sub-module of the modular multilevel converter can be completed.
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