CN113673099A - Active MMC time domain analysis modeling method based on modal division - Google Patents
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Abstract
The invention establishes an active MMC time domain analysis modeling method based on modal division, wherein the active MMC stores energy for a battery and is connected in parallel to the capacitor side of a half-bridge submodule through a bidirectional DC-DC converter. The method comprises the steps of firstly analyzing working modes of the active MMC under different working states according to topology, working principle and energy storage charging and discharging mechanism of the active MMC, further introducing a switching function model, establishing a relation between bridge arm current and inflow sub-module current, introducing an average value method, establishing a relation between energy storage current and output current of a bidirectional DC-DC converter, finally writing a differential equation set according to a sub-module topology structure, KCL and KVL columns, deducing a time domain analysis expression of capacitance voltage, capacitance current and energy storage current of the active MMC, and establishing a time domain analysis model of the active MMC based on modal division. The establishment of the active MMC time domain analysis model provides a theoretical basis for analyzing relevant electric quantity influence factors, inhibiting the capacitance voltage fluctuation and reducing the harmonic content in the current, and has important significance for the work of equipment model selection, parameter optimization and the like.
Description
Technical Field
The invention relates to the technical field of flexible direct current transmission and energy storage and saving, in particular to an active MMC time domain analysis modeling method based on modal division.
Background
The new energy has many problems, for example, solar energy and wind energy are influenced by factors such as weather change and radiation intensity change, and the new energy has the characteristics of volatility, intermittence and the like, the intermittence and uncertainty of the power generation output of the new energy can cause grid-connected power fluctuation, the stable operation and the electric energy quality of a power grid are adversely affected, and the large-scale grid-connected operation can bring potential safety hazards to the power grid.
In response to the above challenges, the introduction of energy storage technology has become an effective solution. The energy storage technology is a technology that converts electric energy into other forms of energy to be stored and releases the electric energy when needed. The energy storage technology can solve the problems of fluctuation, intermittence and the like of new energy power generation to a great extent. The problem that the traditional power grid structure and operation technology are difficult to meet system requirements of energy multidirectional flow, active regulation and control of power flow and the like when large-scale new energy power grid connection is accepted is solved. The energy storage technology is utilized to safely and efficiently consume a high proportion of new energy to generate electricity, and the power generation is widely concerned. The active MMC has become one of the most potential energy storage schemes due to its significant advantages of high operating efficiency, high delivery capacity, high modularization degree, flexible energy storage capacity adjustment, and the like. Calculating time domain analysis expressions of the active MMC capacitor voltage, the capacitor current and the energy storage current has great significance for analyzing relevant electric quantity influence factors, equipment model selection, parameter optimization and other works, and therefore an active MMC time domain analysis model based on modal division needs to be built.
Disclosure of Invention
The invention provides an active MMC time domain analysis modeling method based on modal division, aiming at solving the challenges and problems of the existing large-scale new energy grid connection.
In order to achieve the purpose of the invention, the invention adopts the following technical scheme:
1. an active MMC time domain analysis model based on modal division. It is characterized by comprising:
the active MMC is still a three-phase voltage source converter essentially, and the topological structure of the submodule of the active MMC is that a battery energy storage unit is connected in parallel to the capacitor side of a half-bridge submodule through a bidirectional DC-DC converter, and the active MMC is an extension structure of the MMC. An active MMC system belongs to a three-port converter and is simultaneously connected with an alternating current end, a direct current end and a battery energy storage end.
According to the power conservation principle, the sum of the power flowing into the three-port converter is zero, so that the power of two ends of the three ends is controlled, namely the power of the three ends of the converter can be controlled, and the freedom degree of controlling the power is increased. The active MMC can be divided into direct current side power control, alternating current side power control and energy storage battery charging and discharging power control. The active MMC alternating current side power control adopts classical inner and outer ring power decoupling control, and a bidirectional DC-DC converter is added in each submodule, so that the degree of freedom in control is increased. The energy storage side can set different power reference values to independently control the charging and discharging of each energy storage module. When the energy storage module is in a charging state, T is conducted4And D3And a filter inductance LbThe boost circuit is configured to operate the bidirectional DC-DC converter in a boost mode. When the energy storage module is in a discharging state, T in the bidirectional DC-DC converter is conducted3And D4And a filter inductance LbAnd forming a buck circuit to enable the bidirectional DC-DC converter to work in a voltage reduction mode. The energy storage module in the active MMC adopts a Shepherd model, simplifies the charge and discharge of the energy storage module into a reversible process, and divides the discharge process into three regions: an index zone, a rated working zone and a deep discharge zone. The discharge voltage value under the rated state is the rated voltage Eb。
2. A modeling method of an active MMC time domain analytical model based on modal division is disclosed. The method is characterized in that the model establishment comprises the following steps:
and finally, writing a differential equation set according to the sub-module topological structure, the KCL and the KVL, deducing a time domain analysis expression of the capacitance voltage, the capacitance current and the energy storage current of the active MMC, and establishing an active MMC time domain analysis model based on modal division.
Step 1: and analyzing the working modes of the active MMC submodule under different working states according to the topology of the active MMC submodule, the battery energy storage charge-discharge mechanism and the IGBT state of the half-bridge submodule.
Step 2: and deducing an upper bridge arm voltage time domain analytical expression and a lower bridge arm current mathematical time domain analytical expression which consider the second harmonic circulation according to the active MMC equivalent circuit and the mathematical principle.
And step 3: introducing a switching function model, establishing a relation between bridge arm current and current flowing into the sub-module, introducing an average value method, and establishing a relation between energy storage current and output current of the bidirectional DC-DC converter.
And 4, step 4: according to different working modes and kirchhoff's law of the active MMC, a differential equation set is written in series according to an energy storage equivalent model and capacitor characteristics, a time domain analytical expression of capacitance voltage, capacitance current and energy storage current of the active MMC is deduced, and the time domain analytical model of the active MMC based on modal division is established.
The active MMC time domain analysis model modeling method provided by the invention fills a technical blank suitable for the active MMC time domain analysis modeling method, can analyze relevant electric quantity influence factors of the active MMC with a battery energy storage unit connected in parallel to a capacitor side through a DC-DC converter aiming at submodule topology, and provides a theoretical basis for inhibiting capacitor voltage fluctuation, reducing harmonic content in current and optimizing equipment type selection and parameters.
Drawings
FIG. 1 is a flowchart of a modeling method of an active MMC time domain analytic model in the embodiment of the present invention;
FIG. 2 is a diagram of an active MMC topology in an embodiment of the present invention;
FIG. 3 is a diagram of an active MMC control strategy in an embodiment of the present invention;
FIG. 4 is a topology structure diagram of an active MMC sub-module in the embodiment of the present invention;
FIG. 5 is a diagram of the operation state and operation mode of an active MMC in accordance with an embodiment of the present invention;
Detailed Description
The drawings are for illustrative purposes only and are not to be construed as limiting the patent; for the purpose of better illustrating the embodiments, certain features of the drawings may be omitted, enlarged or reduced, and do not represent the size of an actual product;
it will be understood by those skilled in the art that certain well-known structures in the drawings and descriptions thereof may be omitted. The technical solution of the present invention is further described below with reference to the accompanying drawings and examples.
The embodiment of the invention provides a modeling method of an active MMC time domain analytical model, a specific flow chart is shown in figure 1, and the specific process is as follows:
s101: and analyzing the working modes of the active MMC submodule under different working states according to the topology of the active MMC submodule, the battery energy storage charge-discharge mechanism and the IGBT state of the half-bridge submodule.
S102: and deducing an upper bridge arm voltage time domain analytical expression and a lower bridge arm current mathematical time domain analytical expression which consider the second harmonic circulation according to the active MMC equivalent circuit and the mathematical principle.
S103: introducing a switching function model, establishing a relation between bridge arm current and current flowing into the sub-module, introducing an average value method, and establishing a relation between energy storage current and output current of the bidirectional DC-DC converter.
S104: according to different working modes and kirchhoff's law of the active MMC, a differential equation set is written in series according to an energy storage equivalent model and capacitor characteristics, a time domain analytical expression of capacitance voltage, capacitance current and energy storage current of the active MMC is deduced, and the time domain analytical model of the active MMC based on modal division is established.
1. In the above S101, according to the topology of the active MMC submodule, the battery energy storage and charge-discharge mechanism, and the IGBT state of the half-bridge submodule, the operating modes in different operating states are analyzed, as shown in fig. 4 and 5.
The active MMC is still a three-phase voltage source converter in nature, and the sub-module topology structure of the active MMC is that a battery energy storage unit is connected in parallel to the capacitor side of a half-bridge sub-module through a bidirectional DC-DC converter, which is an extension structure of the MMC, as shown in fig. 2. An active MMC system belongs to a three-port converter and is simultaneously connected with an alternating current end, a direct current end and a battery energy storage end.
Psref-Pbref＝Pdcref (1)
Wherein, PsrefIs the AC side power, PdcrefFor direct side power, PbrefIs the energy storage side power.
According to the power conservation principle, the sum of the power flowing into the three-port converter is zero,therefore, the power of two ends of the three ends is controlled, namely the power of the three ends of the converter can be controlled, and the freedom degree of controlling the power is increased. The active MMC can be divided into direct current side power control, alternating current side power control and energy storage battery charging and discharging power control. The active MMC alternating current side power control adopts classical inner and outer ring power decoupling control, and a bidirectional DC-DC converter is added in each submodule, so that the degree of freedom in control is increased. The energy storage side can set different power reference values to independently control the charging and discharging of each energy storage module. When the energy storage module is in a charging state, T is conducted4And D3And a filter inductance LbThe boost circuit is configured to operate the bidirectional DC-DC converter in a boost mode. When the energy storage module is in a discharging state, T in the bidirectional DC-DC converter is conducted3And D4And a filter inductance LbAnd forming a buck circuit to enable the bidirectional DC-DC converter to work in a voltage reduction mode. The energy storage module in the active MMC adopts a Shepherd model, simplifies the charge and discharge of the energy storage module into a reversible process, and divides the discharge process into three regions: an index zone, a rated working zone and a deep discharge zone. The discharge voltage value under the rated state is the rated voltage Eb。
2. In the above S102, according to the active MMC equivalent circuit and mathematical principles, an upper and lower bridge arm voltage time domain analytic expression and an upper and lower bridge arm current mathematical time domain analytic expression considering the second harmonic circulation are derived.
The power is in the positive direction of flowing into the converter station, and when the power loss of a bridge arm is ignored, the power of an alternating current side, a direct current side and an energy storage side is conserved, so that the power is obtained
Psref-Pbref＝Pdcref (2)
Upper and lower bridge arm voltage uau、ualCan be expressed as
Upper and lower bridge arm current i considering second harmonic circulating currentau、ialCan be expressed as
Wherein, PsrefIs the AC side power, PdcrefFor direct side power, PbrefFor storing side power, UdcIs a DC voltage value, UmIs the amplitude of the alternating voltage, ImIs the amplitude of the fundamental current, ubFor the battery energy storage cell port voltage, ibIn order to store the energy current, the energy storage device is provided with a power supply,
in order to be the power factor angle,
is the initial phase angle of the second harmonic circulating current,
is the initial phase angle of the bridge arm voltage on the A phase, I2The amplitude of the second harmonic circulating current, omega is the fundamental angle frequency, and m is the modulation ratio.
3. In the above step S103, a switching function model is introduced to establish a relationship between the bridge arm current and the current flowing into the sub-module, and an average value method is introduced to establish a relationship between the energy storage current and the output current of the bidirectional DC-DC converter.
U when the submodule is put insm＝UcS1, U when the submodule is removedsm0, and 0. The port voltage of the ith sub-module of the A-phase upper bridge arm can be expressed as
usm,au_i＝Sau_i·uc,au_i (8)
Wherein, UsmFor sub-module port voltage, UcIs the sub-module capacitance voltage, Sau_iAnd the switching function of the ith submodule of the bridge arm on the phase A is taken as the switching function of the ith submodule of the bridge arm on the phase A. The average switching function represents the average input ratio of the submodules in the bridge arm, and the average switching function of the bridge arm on the A phase is
The relation between the bridge arm voltage on the A phase and the sub-module capacitor voltage is obtained
By substituting formula (4) for formula (9), it is possible to obtain
IGBT T in bidirectional DC-DC converter3And T4Has higher frequency which is 60 times of the power frequency. Thus will T3And T4The average value is equivalent, and the duty ratio is defined as alpha. From the control strategy of FIG. 3, it is readily apparent that
According to the PWM control principle of the bidirectional DC-DC converter
According to the proportional relation between the modulated wave and the triangular carrier wave
α＝1-d (14)
Wherein, Kp1、Kp2As a controller scaling factor, Ks1、Ks2As an integral coefficient, Pbat_refStoring power reference values, P, for individual sub-modulesbatActual value of stored energy power for individual submodules, ibirefStoring a current reference for a single submodule, ibiD is a PWM modulation wave for the actual value of the energy storage current of the single sub-module.
The obtained capacitance current, the capacitance current and the energy storage current of the active MMC satisfy the following relations:
ic-αib＝Sau·iau (15)
4. in S104 described above, the final active-type MMC mathematical model is determined by the following equation.
According to different working modes and kirchhoff's law of the active MMC, a differential equation set is written in series according to an energy storage equivalent model and capacitor characteristics, a time domain analytical expression of capacitance voltage, capacitance current and energy storage current of the active MMC is deduced, and the time domain analytical model of the active MMC based on modal division is established.
The modulation strategy fully plays a role, and the capacitor voltage u of each submodulecIn accordance with the capacitor voltage ucThe voltage u of the inductorLResistance voltage URVoltage of battery energy storage unit EbThe KVL equation can be written as follows.
The relation between the capacitor voltage and the capacitor current is
By substituting formula (14) and formula (16) for formula (15), it is possible to obtain
Solving linear differential equation to obtain active MMC capacitor voltage analytical expression uc。
Wherein, C1And C2Is a constant number, a* i、b* i(i is 1-10) is a coefficient parameter. When the active MMC operates normally, the sub-module capacitor voltage keeps constant, but the sub-module capacitor voltage fluctuates due to the energy of charging and discharging the converter valve in one period of reactive power and energy storage power.
According to the relation (16) and (15) of the capacitor voltage and the capacitor current, an energy storage current analytical expression i can be obtained through derivationb。
According to the KCL equation (14), a capacitance current analytic expression i can be obtained through derivationc。
Equations (18), (29) and (35) constitute a sub-module time domain analytical model of the active-type MMC represented by a switching function. All the time domain analytic expressions form an active MMC time domain analytic model based on modal division.
As will be appreciated by one skilled in the art, embodiments of the present application may be provided as a method, system, or computer program product. Accordingly, the present application may take the form of an entirely hardware embodiment, an entirely software embodiment or an embodiment combining software and hardware aspects. Furthermore, the present application may take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, and the like) having computer-usable program code embodied therein.
The present application is described with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the application. It will be understood that each flow and/or block of the flow diagrams and/or block diagrams, and combinations of flows and/or blocks in the flow diagrams and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.
These computer program instructions may also be stored in a computer-readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including instruction means which implement the function specified in the flowchart flow or flows and/or block diagram block or blocks.
These computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.
Finally, it should be noted that: the above embodiments are only intended to illustrate the technical solution of the present invention and not to limit the same, and a person of ordinary skill in the art can make modifications or equivalents to the specific embodiments of the present invention with reference to the above embodiments, and such modifications or equivalents without departing from the spirit and scope of the present invention are within the scope of the claims of the present invention as set forth in the claims.
Claims (3)
1. An active MMC time domain analysis model based on modal division. It is characterized by comprising:
the active MMC is still a three-phase voltage source converter essentially, and the topological structure of the submodule of the active MMC is that a battery energy storage unit is connected in parallel to the capacitor side of a half-bridge submodule through a DC-DC converter, and the active MMC is an extension structure of the MMC. An active MMC system belongs to a three-port converter and is simultaneously connected with an alternating current end, a direct current end and a battery energy storage end.
Psref-Pbref＝Pdcref (1)
Wherein, PsrefIs the AC side power, PdcrefFor direct side power, PbrefIs the energy storage side power.
According to the power conservation principle, the sum of the power flowing into the three-port converter is zero, so that the power of two ends of the three ends is controlled, namely the power of the three ends of the converter can be controlled, and the freedom degree of controlling the power is increased. The active MMC can be divided into direct current side power control, alternating current side power control and energy storage battery charging and discharging power control. The active MMC alternating current side power control adopts classical inner and outer ring power decoupling control, and a bidirectional DC-DC converter is added in each submodule, so that the degree of freedom in control is increased. The energy storage side can set different power reference values to independently control the charging and discharging of each energy storage module. When the energy storage module is in a charging state, T is conducted4And D3And a filter inductance LbThe boost circuit is configured to operate the bidirectional DC-DC converter in a boost mode. When the energy storage module is in a discharging state, T in the bidirectional DC-DC converter is conducted3And D4And a filter inductance LbAnd forming a buck circuit to enable the bidirectional DC-DC converter to work in a voltage reduction mode. Active type MThe energy storage module in the MC adopts a Shepherd model, the charging and discharging of the energy storage module are simplified into a reversible process, and the discharging process is divided into three regions: an index zone, a rated working zone and a deep discharge zone. The discharge voltage value under the rated state is the rated voltage Eb。
2. The modeling method of the modal-partitioning-based active-type MMC time-domain analytical model according to claim 1. The method is characterized in that the model establishment comprises the following steps:
and finally, writing a differential equation set according to the sub-module topological structure, the KCL and the KVL, deducing a time domain analysis expression of the capacitance voltage, the capacitance current and the energy storage current of the active MMC, and establishing an active MMC time domain analysis model based on modal division.
Step 1: and analyzing the working modes of the active MMC submodule under different working states according to the topology of the active MMC submodule, the battery energy storage charge-discharge mechanism and the IGBT state of the half-bridge submodule.
Step 2: and deducing an upper bridge arm voltage time domain analytical expression and a lower bridge arm current mathematical time domain analytical expression which consider the second harmonic circulation according to the active MMC equivalent circuit and the mathematical principle.
And step 3: introducing a switching function model, establishing a relation between bridge arm current and current flowing into the sub-module, introducing an average value method, and establishing a relation between energy storage current and output current of the bidirectional DC-DC converter.
And 4, step 4: according to different working modes and kirchhoff's law of the active MMC, a differential equation set is written in series according to an energy storage equivalent model and capacitor characteristics, a time domain analytical expression of capacitance voltage, capacitance current and energy storage current of the active MMC is deduced, and the time domain analytical model of the active MMC based on modal division is established.
3. The modal-partitioning-based active-type MMC time-domain analysis model according to claim 1 and the model building method according to claim 2, wherein the previous step is a basis for execution of the next step from step 1 to step 4, and the 4 modeling steps are executed in a loop-by-loop manner and sequentially with each other, and are an organic, indivisible whole. Meanwhile, the active MMC time domain analysis model can intuitively reflect waveform characteristics and influence factors of related electric quantity so as to provide theoretical support for equipment model selection, parameter optimization and the like by inhibiting capacitor voltage fluctuation and reducing harmonic content in energy storage current.
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| CN114884162A (en) * | 2022-05-06 | 2022-08-09 | 华北电力大学 | Battery state-of-charge prediction optimization control method suitable for distributed energy storage type MMC |
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| CN113489360A (en) * | 2021-08-16 | 2021-10-08 | 华北电力大学 | Energy storage type MMC accurate analysis modeling method based on modal division |
| CN114884162A (en) * | 2022-05-06 | 2022-08-09 | 华北电力大学 | Battery state-of-charge prediction optimization control method suitable for distributed energy storage type MMC |
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