Bridge arm reactor parameter selection method, system, equipment and medium

Abstract

The invention discloses a bridge arm reactor parameter selection method, system and device and a medium, and the method comprises the steps: carrying out the reasoning analysis of a three-phase simplified equivalent circuit of an MMC converter, and obtaining a basic electrical quantity relation rule of the MMC converter, including the differential-mode voltage and common-mode voltage of an upper bridge arm and a lower bridge arm of the MMC converter; reasoning analysis is carried out according to the differential mode voltage and the common mode voltage, and a calculation method of the second harmonic current is determined; on the basis of a second harmonic current calculation method, the second harmonic current of the MMC converter bridge arm is obtained through calculation; and determining bridge arm reactor parameters of the MMC according to the second harmonic current. According to the method, the second harmonic circulation value is calculated accurately and simply, and the value of the bridge arm reactor is determined on the basis of the second harmonic circulation value, so that the purpose that the phase unit series resonance angular frequency is far away from the frequency doubling circulation resonance rate is achieved, and the problem that an existing bridge arm reactor parameter selection method is poor in accuracy is solved.

Description

Technical Field

[0001] The present application relates to the technical field of flexible direct current transmission, and in particular to a method, system, device and medium for selecting parameters of a bridge arm reactor. Background Art

[0002] The basic unit of flexible DC transmission is called Modular Multilevel Converter (MMC), which is composed of multiple cascaded sub-modules (SM) with the same structure. The structure of the sub-module can be divided into three types of components: half-H bridge type, full H bridge type and double clamp sub-module type. The bridge arm reactor is also called phase reactor or valve reactor. A bridge arm reactor is connected in series in each bridge arm. The phase reactor and the leakage reactance of the connecting transformer together form a group belt for transmitting power between the converter and the AC system. Together with the valve side voltage of the connecting transformer, it determines the power transmission capacity of the converter, realizes the regulation and control of active power and reactive power, and suppresses short-circuit current.

[0003] The method for selecting the parameters of the bridge arm reactor in the prior art mainly analyzes and calculates the three factors of the bridge arm reactor as a part of the connection reactor, the relationship between the bridge arm reactance value and the ring current resonance, and the bridge arm reactor used to suppress the rise rate of the DC side fault current. However, the analysis and calculation methods of the prior art are complicated and the physical meaning is not obvious. The capacitance of the phase unit equivalent circuit does not consider the influence of the modulation effect, resulting in a large difference between the calculated value of the double frequency ring current resonance rate and the actual value. The bridge arm reactor value determined by this method is very different from the expected value. Therefore, it is urgent to design a method for selecting the parameters of the bridge arm reactor with higher accuracy. Summary of the invention

[0004] The present application provides a method, system, device and medium for selecting parameters of a bridge arm reactor, which are used to solve the problem of poor accuracy of the existing method for selecting parameters of a bridge arm reactor.

[0005] In view of this, the first aspect of the present application provides a method for selecting parameters of a bridge arm reactor, the method comprising:

[0006] The simplified equivalent circuit of the three-phase MMC converter is analyzed and the basic electrical quantity relationship of the MMC converter is obtained, including the differential mode voltage and common mode voltage of the upper and lower bridge arms of the MMC converter.

[0007] A calculation method for the second harmonic current is obtained by performing inference analysis based on the differential mode voltage and the common mode voltage. The inference analysis process includes: analyzing the influence of the bridge arm fundamental wave and the second harmonic current on the module capacitor voltage, and proposing the concept of modulation impedance;

[0008] Based on the second harmonic current calculation method, the second harmonic current of the MMC converter bridge arm is calculated;

[0009] The arm reactor parameters of the MMC converter are determined according to the second harmonic current.

[0010] Optionally, the method for calculating the second harmonic current is obtained by performing reasoning analysis based on the differential mode voltage and the common mode voltage. The process of the reasoning analysis includes: analyzing the influence of the bridge arm fundamental wave and the second harmonic current on the module capacitor voltage, and proposing the concept of modulation impedance, including:

[0011] The first analysis conclusion is obtained by reasoning and analyzing the variation mechanism of the submodule capacitor voltage of the MMC converter;

[0012] According to the first analysis conclusion and in combination with the differential mode voltage, a reasoning analysis is performed on the submodule capacitor voltage variation rule of the MMC converter to obtain a second analysis conclusion;

[0013] According to the second analysis conclusion, the effect of the periodic component of the common mode voltage of the bridge arm of the MMC converter is analyzed by reasoning to obtain a third analysis conclusion;

[0014] According to the third analysis conclusion, the effect of the second harmonic circulating current component of the bridge arm of the MMC converter is inferred and transformed to obtain a calculation formula for the second harmonic current.

[0015] Optionally, the calculation formula of the second harmonic current is expressed as:

[0016] ;

[0017] In the formula, is the second harmonic current, is the original component of the second harmonic bridge arm voltage, for, is the ratio of the second harmonic impedance value of the phase unit reactance to the second harmonic impedance value of the phase unit capacitance, is the modulation ratio of the electromotive force of the MMC converter, is the amplitude of the alternating current.

[0018] Optionally, determining the arm reactor parameters of the MMC converter according to the second harmonic current includes:

[0019] The percentage of the second harmonic current to the bridge arm fundamental current of the MMC converter is used as a selection criterion, and the bridge arm reactor parameters of the MMC converter are determined according to the selection criterion.

[0020] A second aspect of the present application provides a bridge arm reactor parameter selection system, the system comprising:

[0021] The first analysis unit is used to perform reasoning analysis on the simplified equivalent circuit of the three phases of the MMC converter to obtain the basic electrical quantity relationship law of the MMC converter, including: the differential mode voltage and the common mode voltage of the upper and lower bridge arms of the MMC converter;

[0022] A second analysis unit is used to perform reasoning analysis based on the differential mode voltage and the common mode voltage to obtain a calculation method for the second harmonic current, wherein the reasoning analysis process includes: analyzing the influence of the bridge arm fundamental wave and the second harmonic current on the module capacitor voltage, and proposing a concept of modulation impedance;

[0023] A calculation unit, configured to calculate the second harmonic current of the bridge arm of the MMC converter based on the second harmonic current calculation method;

[0024] The selection unit is used to determine the arm reactor parameters of the MMC converter according to the second harmonic current.

[0025] Optionally, the second analysis unit is specifically used to:

[0026] The first analysis conclusion is obtained by reasoning and analyzing the variation mechanism of the submodule capacitor voltage of the MMC converter;

[0027] According to the first analysis conclusion and in combination with the differential mode voltage, a reasoning analysis is performed on the submodule capacitor voltage variation rule of the MMC converter to obtain a second analysis conclusion;

[0028] According to the second analysis conclusion, the effect of the periodic component of the common mode voltage of the bridge arm of the MMC converter is analyzed by reasoning to obtain a third analysis conclusion;

[0029] According to the third analysis conclusion, the effect of the second harmonic circulating current component of the bridge arm of the MMC converter is inferred and transformed to obtain a calculation formula for the second harmonic current.

[0030] Optionally, the calculation formula of the second harmonic current is expressed as:

[0031] ;

[0032] In the formula, is the second harmonic current, is the original component of the second harmonic bridge arm voltage, for, is the ratio of the second harmonic impedance value of the phase unit reactance to the second harmonic impedance value of the phase unit capacitance, is the modulation ratio of the electromotive force of the MMC converter, is the amplitude of the alternating current.

[0033] Optionally, the selection unit is specifically configured to:

[0034] The percentage of the second harmonic current to the bridge arm fundamental current of the MMC converter is used as a selection criterion, and the bridge arm reactor parameters of the MMC converter are determined according to the selection criterion.

[0035] A third aspect of the present application provides a bridge arm reactor parameter selection device, the device comprising a processor and a memory:

[0036] The memory is used to store program code and transmit the program code to the processor;

[0037] The processor is used to execute the steps of the bridge arm reactor parameter selection method as described in the first aspect according to the instructions in the program code.

[0038] A fourth aspect of the present application provides a computer-readable storage medium, wherein the computer-readable storage medium is used to store program codes, and the program codes are used to execute the bridge arm reactor parameter selection method described in the first aspect above.

[0039] It can be seen from the above technical solutions that this application has the following advantages:

[0040] The present application provides a method for selecting the parameters of a bridge arm reactor. Through the study of the equivalent circuit of the DC part, it can be known that the second harmonic current circulates among the three phases, and the related electrical quantities have a modulation impedance relationship. The influence of the secondary harmonic electrical quantities is eliminated, and the second harmonic circulating current value is accurately and simply calculated according to the percentage of the second harmonic circulating current in the bridge arm fundamental current. Based on this, the value of the bridge arm reactor is determined, that is, the purpose of achieving the purpose of making the phase unit series resonant angular frequency away from the double frequency circulating current resonance rate. This solves the problem of poor accuracy of the existing bridge arm reactor parameter selection method. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 A schematic diagram of a flow chart of a method for selecting parameters of a bridge arm reactor provided in an embodiment of the present application;

[0042] Figure 2 A simplified equivalent circuit diagram of three phases provided in an embodiment of the present application;

[0043] Figure 3 A structural schematic diagram of a bridge arm reactor parameter selection method provided in an embodiment of the present application. DETAILED DESCRIPTION

[0044] In order to enable those skilled in the art to better understand the solution of the present application, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0045] See also Figure 1 A method for selecting parameters of a bridge arm reactor provided in an embodiment of the present application includes:

[0046] Step 101: perform reasoning analysis on a simplified equivalent circuit of three phases of the MMC converter to obtain basic electrical quantity relationship rules of the MMC converter, including: differential mode voltage and common mode voltage of the upper and lower bridge arms of the MMC converter.

[0047] Step 102: Perform reasoning analysis based on the differential mode voltage and the common mode voltage to obtain a calculation method for the second harmonic current. The reasoning analysis process includes: analyzing the influence of the bridge arm fundamental wave and the second harmonic current on the module capacitor voltage, and proposing the concept of modulation impedance.

[0048] Step 103: Based on the calculation method of the second harmonic current, the second harmonic current of the bridge arm of the MMC converter is calculated.

[0049] Step 104: Determine the arm reactor parameters of the MMC converter according to the second harmonic current.

[0050] Regarding step 101, it should be noted that:

[0051] First, a simplified equivalent circuit of the three-phase MMC converter is constructed. Specifically, the basic electrical quantity relationship of the MMC converter is analyzed using Kirchhoff's voltage-current-voltage law (KCL, KVL). The output voltage of the MMC bridge arm is composed of the output voltages of each submodule. All submodules in the upper and lower bridge arms are controlled to be switched on and off to obtain the desired controllable AC output voltage. The cascaded submodules of each bridge arm are directly replaced with a controllable voltage source to obtain a simplified equivalent circuit of the three phases, as shown in the figure. Figure 2 shown. Figure 2 Medium:i xp is the upper arm current (x=a, b, c represent phases A, B, and C respectively, the same below); i xn is the lower arm current, u cpx is the upper bridge arm voltage, u cnx is the voltage of the lower bridge arm, p and n represent the upper and lower bridge arms respectively (the same below); point O is the neutral point of the DC side, and its potential to the ground is u0; u dc is the DC voltage, i dcis the DC current, i cir is the common mode current of the upper and lower bridge arms. is the L0 bridge arm reactor; R s and L s are the equivalent loss resistance and equivalent connection reactance on the AC side respectively; u sx is the AC system voltage, i x For alternating current.

[0052] Then, the simplified equivalent circuit of the three-phase MMC converter is analyzed and the basic electrical quantity relationship of the MMC converter is obtained, including the upper and lower bridges of the MMC converter. Specifically, according to Kirchhoff's voltage and current equations, it can be obtained:

[0053] (3-1-1)

[0054] (3-1-2)

[0055] (3-1-3)

[0056] Adding formula (3-1-1) to formula (3-1-2) yields:

[0057] (3-1-4)

[0058] In formula (3-1-4), the differential mode voltage of the upper and lower bridge arms of the MMC is defined as:

[0059] (3-1-5)

[0060] At the same time, make the following variable substitutions:

[0061] (3-1-6)

[0062] After that, formula (3-1-4) can be simplified to:

[0063] (3-1-7)

[0064] Subtracting formula (3-1-1) from formula (3-1-2) yields:

[0065] (3-1-8)

[0066] In formula (3-1-8), the 2 times common mode voltage or phase unit voltage of the upper and lower bridge arms of the MMC is defined as:

[0067] (3-1-9)

[0068] (3-1-10)

[0069] Through the above analysis, the basic electrical quantity relationship law of the MMC converter is obtained.

[0070] In one embodiment, step 102 includes:

[0071] Step 1021: Perform reasoning analysis on the variation mechanism of the submodule capacitor voltage of the MMC converter to obtain a first analysis conclusion.

[0072] It should be noted that the physical basis of the (MMC) type flexible DC transmission system is a circuit composed of circuit elements, and its control is based on the characteristics of the circuit. When analyzing the basic modulation principle, it is assumed that the module capacitor voltage remains unchanged. By switching the bridge arm module capacitor, the controllable AC voltage output by the converter can be obtained, which can quickly change the active and reactive exchange between the converter and the power system. Through the control method of dq coordinate transformation and current feedback, the output of the converter can track the reference value well, which belongs to the fast control process of the flexible DC transmission system.

[0073] In fact, due to the limited value of module capacitance, the power exchange between the (MMC) type converter and the power system will cause changes in the module capacitor voltage. Reactive exchange only causes periodic changes in the module capacitor voltage, while active exchange causes periodic and non-periodic changes in the module capacitor voltage. The DC current component of the bridge arm causes periodic and non-periodic changes in the module capacitor voltage. The change in the module capacitor voltage in turn affects the power exchange, which requires continuous tracking of control feedback to ultimately make the power exchange reference value reach a stable state. At this time, the internal relationship between the various electrical quantities of the flexible DC transmission system is its steady-state circuit characteristics.

[0074] If the non-periodic changes in the module capacitor voltage caused by the AC and DC current components of the converter bridge arm cancel each other out, the injected power and output power of the converter are equal, and the DC transmission system is in a stable operating state. Otherwise, the DC voltage of the converter changes, and the DC current between the connected converters changes. This is the natural circuit characteristic of the DC current generated by the (MMC) converter type DC transmission system. When the converter at the sending end injects power from the power system and outputs power to the DC side, and the injected power is greater than the output power to the DC side, the module capacitor has a non-periodic voltage increase, which increases the voltage and current values ​​of the DC side, thereby increasing the output power of the DC side and tending to balance with the injected power. The process of the converter at the receiving end is the opposite.

[0075] The change process of the submodule capacitor voltage is the key to analyzing the change of electrical quantity of the (MMC) converter type DC transmission system. The results given in the relevant literature are very concise, but the principles contained therein are relatively complex, involving two aspects: modulation mode and module capacitor voltage equalization control. Therefore, the present invention assumes that the voltage of all submodules of a bridge arm is added as the sum of the submodule voltages of the bridge arm, and the electric quantity of all submodules of a corresponding bridge arm is added as the sum of the electric quantity, and the sum of the electric quantity divided by the submodule capacitance value is equal to the sum of the voltage. The electric quantity of the submodule capacitor put into operation in the bridge arm changes under the action of the bridge arm current, while the electric quantity of the submodule capacitor in the cut-off state remains unchanged. The change in the sum of the electric quantity of the bridge arm is equal to the time integral of the product of the number of submodules put into operation and the bridge arm current.

[0076] The voltage balance between the submodule capacitors of the bridge arm is ensured by a selection mechanism. The process of switching on or off the module in each sampling period is based on the current direction of the corresponding bridge arm. If the current flow is to charge the submodule capacitor of the bridge arm, the submodule capacitor with the highest voltage in the switched-on state will be selected to be switched off, or the submodule capacitor with the lowest voltage in the switched-off state will be selected to be switched on. If the current flow is to discharge the capacitor of the bridge arm, the operation is reversed.

[0077] The ratio of the number of submodule capacitors in each bridge arm in operation to the total number of bridge arm modules N is called the modulation function, which is determined by the output of the modulation control. The number of submodule capacitors in operation can be determined by the modulation function. When the modular multilevel flexible DC converter is applied to high voltage and high power applications, the modulation function is approximately a continuous function.

[0078] Under the combined effect of modulation control and submodule capacitor voltage equalization, the voltage of all submodules in the bridge arm tends to be the same within a certain time interval, and ideally is equal at any time. The change in the amount of electricity obtained by the working submodule capacitor under the action of the bridge arm current is also instantaneously distributed to the submodule capacitor in the cut-off state to obtain the average amount of electricity of the submodule capacitor. Through this idealized equivalent method, there is no need to directly analyze a specific submodule capacitor, but to obtain the average voltage of the submodule capacitor of the bridge arm at any time.

[0079] Therefore, under the above-mentioned equivalent principle, a partial current is obtained by multiplying the modulation function and the actual current of the bridge arm, then the change in the sum of the voltages of the bridge arm submodules is equal to the sum of the voltage changes caused by the partial current passing through all submodules, or the change in the average value of the submodule capacitor voltage is equivalent to the partial bridge arm current passing through a submodule capacitor.

[0080] Step 1022: perform reasoning analysis on the variation law of the submodule capacitor voltage of the MMC converter according to the first analysis conclusion and in combination with the differential mode voltage to obtain a second analysis conclusion.

[0081] It should be noted that in order to realize the function of the flexible DC transmission system of modular multi-level converter type, it is necessary to control the input and removal of all sub-modules in the upper and lower bridge arms, so as to obtain the desired controllable AC output voltage to realize the power exchange between the converter and the power system, and this fast modulation control. The change of the sub-module capacitor voltage is caused by the integration of the bridge arm current, and it is about an order of magnitude slower than the fast modulation control when reaching a stable state.

[0082] To this end, it is necessary to analyze the law of the change of the module capacitor voltage caused by the bridge arm current, and then further analyze the impact of the change of the module capacitor voltage on the bridge arm current. Based on the conclusion of the analysis of the sub-module capacitor voltage change mechanism, we do not pay too much attention to the specific automatic control implementation method and process. An automatic control method called basic modulation is proposed. The average value of the sub-module capacitor voltage is used as a reference value to control the input and removal of all sub-modules in the upper and lower bridge arms, and obtain the differential mode voltage or internal virtual synthetic electromotive force e of the upper and lower bridge arms of the MMC. ref

[0083] The following assumptions are made: (1) The parameters of all sub-modules are exactly the same; (2) Due to the effect of sub-module voltage balancing, it is assumed that the voltages of all sub-modules within a bridge arm are exactly the same.

[0084] Theoretical derivation is carried out using single-phase MMC as an example. Assuming that the AC system voltage u s 、The reference value of the synthetic electromotive force output by MMC is e ref , AC current i and reference value u of 2nd harmonic bridge arm voltage component 2f They are:

[0085] (3-3-1)

[0086] (3-3-2)

[0087] (3-3-3)

[0088] (3-3-4)

[0089] Where: U s , E, I and U 2f They are the AC system voltage, the output synthetic electromotive force, the amplitude of the AC current and the original component of the 2nd harmonic bridge arm voltage; , and are their initial phase angles respectively. The reference value of the synthetic controllable electromotive force is e ref The phase angle of the AC system voltage is obtained by actually controlling the phase angle of the reference phase. , this is a theoretical description.

[0090] First, we analyze the case where the bridge arm current of the MMC contains only a DC component and half of the AC current, that is, the second harmonic bridge arm current component is ignored. Therefore, the bridge arm currents of the upper and lower bridge arms can be expressed as:

[0091] (3-3-5)

[0092] Where: I d is the DC component of the bridge arm current. When the MMC operates symmetrically in three phases, I d One third of the DC current.

[0093] Under the basic modulation mode, the reference voltage of the upper and lower bridge arms of the MMC can be expressed as:

[0094] (3-3-6)

[0095] According to formula (3-3-6), the proportion of sub-modules put into the upper and lower bridge arms of the MMC to the total number of modules N in the bridge arm, that is, the modulation function is:

[0096] (3-3-7)

[0097] In formula (3-3-7): m is the modulation ratio of the output synthetic electromotive force, defined as According to the analysis conclusion of the submodule capacitor voltage change mechanism, the sum of the submodule capacitor charges of the upper and lower bridge arms under the action of the bridge arm current is:

[0098] (3-3-8)

[0099] According to the analysis conclusion of the submodule capacitor voltage change mechanism, the change of the average value of the submodule capacitor voltage is equivalent to the result of part of the bridge arm current passing through a submodule capacitor. Combining equations (3-3-5) and (3-3-7) or based on equation (3-3-8), we can get:

[0100] (3-3-9)

[0101] (3-3-10)

[0102] In formula (3-3-9) and formula (3-3-10): u cp and u cn are the submodule voltages of the upper and lower bridge arms of the MMC, and integrating both sides yields:

[0103] (3-3-11)

[0104] (3-3-12)

[0105] Where U cp0 and U cn0 They are the initial values ​​of the upper and lower bridge arm submodule voltages, generally U cp0 =U cn0 =U c0 ; The value of Q1 is

[0106] (3-3-13)

[0107] In the stable operation state, Q1 is equal to 0, that is, the average power of the AC and DC sides of the MMC must be equal. Therefore, there are the following constraints:

[0108] (3-3-14)

[0109] Therefore, by comparing equations (3-3-11) and (3-3-12), it can be found that: 1) Under ideal working conditions, the sub-module voltage fluctuation of the MMC only contains the fundamental frequency fluctuation component and the 2nd frequency fluctuation component; 2) Between the upper and lower bridge arms, the fundamental frequency component of the sub-module voltage fluctuation has the same amplitude, but opposite phase; 3) Between the upper and lower bridge arms, the 2nd frequency component of the sub-module voltage fluctuation has the same amplitude and the same phase.

[0110] Step 1023: Based on the second analysis conclusion, perform reasoning analysis on the periodic component effect of the common mode voltage of the bridge arm of the MMC converter to obtain a third analysis conclusion.

[0111] It should be noted that the phase unit voltage value can be obtained from (3-3-10), (3-3-11) and the modulation function:

[0112]

[0113] (3-4-1)

[0114] (3-4-2)

[0115] Substituting into (3-14), we get:

[0116] (3-4-3)

[0117] set up , ,but , (3-4-3) is transformed into,

[0118] (3-4-4)

[0119] u2f is the initial second harmonic voltage generated by the fundamental AC current,

[0120] (3-4-5)

[0121] (3-4-6)

[0122] (3-4-7)

[0123] The second harmonic voltage is a negative sequence quantity of three-phase balance, which will generate circulating current between phases, i 2g is the initial second harmonic circulating current, and the steady-state value is calculated as follows:

[0124] (3-4-8)

[0125] (3-4-9)

[0126]

[0127] Therefore, the MMC output synthesizes a controllable voltage e ref is the reference phase, the second harmonic voltage u 2f The reference direction is the same as the submodule voltage and DC voltage, and the second harmonic circulating current i 2g The reference direction and second harmonic voltage u 2f Then the second harmonic voltage u 2f Ahead of the synthetic controllable voltage e ref or hysteresis , second harmonic circulation i 2g Leading the second harmonic voltage u 2f 90° electrical angle, and the lag behind the synthetic controllable voltage e ref Electrical angle.

[0128] A set of actual parameters are as follows:

[0129] ;

[0130] ;

[0131] ;

[0132] , ;

[0133] ;

[0134] , (A);

[0135] Step 1024: Based on the third analysis conclusion, the effect of the second harmonic circulating current component of the MMC converter bridge arm is inferred and transformed to obtain a calculation formula for the second harmonic current.

[0136] Among them, the calculation formula of the second harmonic current is expressed as:

[0137] ;

[0138] In the formula, is the second harmonic current, is the original component of the second harmonic bridge arm voltage, for, is the ratio of the second harmonic impedance value of the phase unit reactance to the second harmonic impedance value of the phase unit capacitance, is the modulation ratio of the electromotive force of the MMC converter, is the amplitude of the alternating current.

[0139] It should be noted that the analysis of the effect of the second harmonic circulating current component of the bridge arm is as follows: (3-4-8) calculated i 2g is the initial second harmonic circulating current. This second harmonic current i 2g The resulting module voltage change generates a second harmonic voltage again, affecting the second harmonic circulating current. 2g The resulting module voltage change is calculated as follows:

[0140] (3-5-1)

[0141] (3-5-2)

[0142] Integrate both sides of equation (4-10) and equation (4-11) to obtain:

[0143] (3-5-3)

[0144] (3-5-4)

[0145] From the above two equations and the modulation function, we can get i 2g The resulting phase unit voltage change is calculated as follows:

[0146] (3-5-5)

[0147] (3-5-6)

[0148] (3-5-7)

[0149] (3-5-8)

[0150] The secondary phase unit voltage generated by the second harmonic current in formula (3-5-8) contains the second and fourth harmonic voltages and a constant voltage. It can be seen from formula (3-5-8) that the fourth harmonic voltage is less than 1 / 10 of the second harmonic voltage, and the bridge arm reactance of the fourth harmonic voltage is twice that of the second harmonic, so the current ratio generated by the two is less than 1 / 20, and its influence can be ignored.

[0151] byi 2g The ratio of the second harmonic voltage and current of the secondary phase unit generated is:

[0152] (3-5-9)

[0153] , the phase relationship is,

[0154] (3-5-10)

[0155] Visible 2g The generated second harmonic voltage of the phase unit lags the current phase by 90°, showing a capacitive impedance property. This is true for any set second harmonic current. This capacitive impedance is called common-mode modulation capacitive reactance.

[0156] According to the conclusion drawn from the analysis of the effect of the second harmonic circulating current component of the bridge arm, the equivalent circuit and formula for calculating the second harmonic current in one go can be obtained without the need to use the method of multiple stepwise approximation corrections. The solution of the second harmonic current becomes very simple. The final second harmonic current calculation is as follows:

[0157] ;

[0158] set up , ;

[0159] They are the second harmonic impedance of the phase unit reactance, the second harmonic impedance of the capacitors of all submodules in series in the bridge arm, and the second harmonic impedance of the phase unit capacitor under modulation.

[0160] The phasor equation of the relationship between the second harmonic voltage and current of the phase unit can be obtained.

[0161] ;

[0162] set up ,have to , substituting the actual parameters into the calculation, we get:

[0163] , ;

[0164] , ;

[0165] Comprehensive second harmonic circulation i 2f Leading the second harmonic voltage u 2f 90°, and the lag behind the synthetic controllable voltage e ref .

[0166] Using the equivalent circuit and formula of the second harmonic current, it is easy to evaluate the results after changing the bridge arm reactance value. As in the above example, if the phase current amplitude and phase remain unchanged, the second harmonic voltage remains unchanged. After increasing and decreasing the bridge arm reactance value by 30%, the calculation is as follows:

[0167] ,

[0168]

[0169] ,

[0170]

[0171] The present invention selects the bridge arm reactance value based on the accurate and simplified calculation result of the second harmonic circulating current. The bridge arm reactance value is selected based on the percentage of the second harmonic circulating current to the bridge arm fundamental current as a criterion, such as 20%, so as to achieve the purpose of keeping the phase unit series resonance angular frequency away from the double frequency circulating current resonance rate.

[0172] Considering equation (3-4-6) and the definition of G, let ,but:

[0173] ;

[0174] The calculation formula of the second harmonic circulating current is:

[0175] In one embodiment, step 104 includes:

[0176] The percentage of the second harmonic current to the bridge arm fundamental current of the MMC converter is used as a selection criterion, and the parameters of the bridge arm reactor of the MMC converter are determined according to the selection criterion.

[0177] It should be noted that the percentage of the second harmonic circulating current to the bridge arm fundamental current is used as the criterion, for example, 20%. ,have to:

[0178] ;

[0179] Depend on , have to:

[0180] , according to the judgment, , we calculate the following for the above example:

[0181] ;

[0182] ;

[0183] Change the judgment, ,have to:

[0184] ;

[0185] .

[0186] In summary, the method for selecting the parameters of the bridge arm reactor of the present invention can be known through the study of the equivalent circuit of the DC part that the second harmonic current circulates among the three phases, and the related electrical quantities have a modulation impedance relationship. The influence of the secondary harmonic electrical quantities is eliminated, and the second harmonic circulating current value is accurately and simply calculated according to the percentage of the second harmonic circulating current in the bridge arm fundamental current. The value of the bridge arm reactor is determined based on this, that is, the purpose of achieving the purpose of making the phase unit series resonant angular frequency away from the double frequency circulating current resonance rate. Thereby solving the problem of poor accuracy of the existing bridge arm reactor parameter selection method.

[0187] The above is a method for selecting parameters of a bridge arm reactor provided in an embodiment of the present application, and the following is a system for selecting parameters of a bridge arm reactor provided in an embodiment of the present application.

[0188] See also Figure 3 , a bridge arm reactor parameter selection system provided in an embodiment of the present application includes:

[0189] The first analysis unit 201 is used to perform reasoning analysis on the simplified equivalent circuit of the three phases of the MMC converter to obtain the basic electrical quantity relationship law of the MMC converter, including: the differential mode voltage and the common mode voltage of the upper and lower bridge arms of the MMC converter.

[0190] The second analysis unit 202 is used to perform inference analysis based on the differential mode voltage and the common mode voltage to obtain a calculation method for the second harmonic current. The inference analysis process includes: analyzing the influence of the bridge arm fundamental wave and the second harmonic current on the module capacitor voltage, and proposing the concept of modulation impedance.

[0191] The calculation unit 203 is used to calculate the second harmonic current of the bridge arm of the MMC converter based on the calculation method of the second harmonic current.

[0192] The selection unit 204 is used to determine the arm reactor parameters of the MMC converter according to the second harmonic current.

[0193] Furthermore, an embodiment of the present application also provides a bridge arm reactor parameter selection device, the device comprising a processor and a memory:

[0194] The memory is used to store program code and transmit the program code to the processor;

[0195] The processor is used to execute the steps of the bridge arm reactor parameter selection method as described in the above method embodiment according to the instructions in the program code.

[0196] Furthermore, a computer-readable storage medium is provided in an embodiment of the present application, and the computer-readable storage medium is used to store program code, and the program code is used to execute the bridge arm reactor parameter selection method described in the above method embodiment.

[0197] Those skilled in the art can clearly understand that, for the convenience and brevity of description, the specific working processes of the systems and units described above can refer to the corresponding processes in the aforementioned method embodiments and will not be repeated here.

[0198] The terms "first", "second", "third", "fourth", etc. (if any) in the specification of the present application and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the data used in this way can be interchangeable where appropriate, so that the embodiments of the present application described herein can be implemented in an order other than those illustrated or described herein, for example. In addition, the terms "including" and "having" and any of their variations are intended to cover non-exclusive inclusions, for example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units that are clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0199] It should be understood that in this application, "at least one (item)" means one or more, and "plurality" means two or more. "And / or" is used to describe the association relationship of associated objects, indicating that three relationships may exist. For example, "A and / or B" can mean: only A exists, only B exists, and A and B exist at the same time, where A and B can be singular or plural. The character " / " generally indicates that the objects associated before and after are in an "or" relationship. "At least one of the following" or similar expressions refers to any combination of these items, including any combination of single or plural items. For example, at least one of a, b or c can mean: a, b, c, "a and b", "a and c", "b and c", or "a and b and c", where a, b, c can be single or multiple.

[0200] In the several embodiments provided in the present application, it should be understood that the disclosed systems, devices and methods can be implemented in other ways. For example, the device embodiments described above are only schematic. For example, the division of the units is only a logical function division. There may be other division methods in actual implementation, such as multiple units or components can be combined or integrated into another system, or some features can be ignored or not executed. Another point is that the mutual coupling or direct coupling or communication connection shown or discussed can be an indirect coupling or communication connection through some interfaces, devices or units, which can be electrical, mechanical or other forms.

[0201] The units described as separate components may or may not be physically separated, and the components shown as units may or may not be physical units, that is, they may be located in one place or distributed on multiple network units. Some or all of the units may be selected according to actual needs to achieve the purpose of the solution of this embodiment.

[0202] In addition, each functional unit in each embodiment of the present application may be integrated into one processing unit, or each unit may exist physically separately, or two or more units may be integrated into one unit. The above-mentioned integrated unit may be implemented in the form of hardware or in the form of software functional units.

[0203] If the integrated unit is implemented in the form of a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present application is essentially or the part that contributes to the prior art or all or part of the technical solution can be embodied in the form of a software product. The computer software product is stored in a storage medium, including several instructions to enable a computer device (which can be a personal computer, server, or network device, etc.) to execute all or part of the steps of the method described in each embodiment of the present application. The aforementioned storage medium includes: U disk, mobile hard disk, read-only memory (full name in English: Read-Only Memory, English abbreviation: ROM), random access memory (full name in English: Random Access Memory, English abbreviation: RAM), disk or optical disk and other media that can store program codes.

[0204] As described above, the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A method for selecting parameters of a bridge arm reactor, characterized in that: include: The simplified equivalent circuit of the three-phase MMC converter is analyzed and the basic electrical quantity relationship of the MMC converter is obtained, including the differential mode voltage and common mode voltage of the upper and lower bridge arms of the MMC converter. A calculation method for the second harmonic current is obtained by performing inference analysis based on the differential mode voltage and the common mode voltage. The inference analysis process includes: analyzing the influence of the bridge arm fundamental wave and the second harmonic current on the module capacitor voltage, and proposing the concept of modulation impedance; Based on the second harmonic current calculation method, the second harmonic current of the MMC converter bridge arm is calculated; The arm reactor parameters of the MMC converter are determined according to the second harmonic current.

2. The method for selecting parameters of a bridge arm reactor according to claim 1, characterized in that: The method for calculating the second harmonic current is obtained by performing inference analysis based on the differential mode voltage and the common mode voltage. The process of inference analysis includes: analyzing the influence of the bridge arm fundamental wave and the second harmonic current on the module capacitor voltage, and proposing the concept of modulation impedance, including: The first analysis conclusion is obtained by reasoning and analyzing the variation mechanism of the submodule capacitor voltage of the MMC converter; According to the first analysis conclusion and in combination with the differential mode voltage, a reasoning analysis is performed on the submodule capacitor voltage variation rule of the MMC converter to obtain a second analysis conclusion; According to the second analysis conclusion, the effect of the periodic component of the common mode voltage of the bridge arm of the MMC converter is analyzed by reasoning to obtain a third analysis conclusion; According to the third analysis conclusion, the effect of the second harmonic circulating current component of the bridge arm of the MMC converter is inferred and transformed to obtain a calculation formula for the second harmonic current.

3. The method for selecting parameters of a bridge arm reactor according to claim 2, characterized in that: The calculation formula of the second harmonic current is expressed as: ； In the formula, is the second harmonic current, is the original component of the second harmonic bridge arm voltage, for, is the ratio of the second harmonic impedance value of the phase unit reactance to the second harmonic impedance value of the phase unit capacitance, is the modulation ratio of the electromotive force of the MMC converter, is the amplitude of the alternating current.

4. The method for selecting parameters of a bridge arm reactor according to claim 1, characterized in that: The method of determining the arm reactor parameters of the MMC converter according to the second harmonic current comprises: The percentage of the second harmonic current to the bridge arm fundamental current of the MMC converter is used as a selection criterion, and the bridge arm reactor parameters of the MMC converter are determined according to the selection criterion.

5. A bridge arm reactor parameter selection system, characterized in that: include: The first analysis unit is used to perform reasoning analysis on the simplified equivalent circuit of the three phases of the MMC converter to obtain the basic electrical quantity relationship law of the MMC converter, including: the differential mode voltage and the common mode voltage of the upper and lower bridge arms of the MMC converter; A second analysis unit is used to perform reasoning analysis based on the differential mode voltage and the common mode voltage to obtain a calculation method for the second harmonic current, wherein the reasoning analysis process includes: analyzing the influence of the bridge arm fundamental wave and the second harmonic current on the module capacitor voltage, and proposing a concept of modulation impedance; A calculation unit, configured to calculate the second harmonic current of the bridge arm of the MMC converter based on the second harmonic current calculation method; The selection unit is used to determine the arm reactor parameters of the MMC converter according to the second harmonic current.

6. The bridge arm reactor parameter selection system according to claim 5, characterized in that: The second analysis unit is specifically used for: The first analysis conclusion is obtained by reasoning and analyzing the variation mechanism of the submodule capacitor voltage of the MMC converter; According to the first analysis conclusion and in combination with the differential mode voltage, a reasoning analysis is performed on the submodule capacitor voltage variation rule of the MMC converter to obtain a second analysis conclusion; According to the second analysis conclusion, the effect of the periodic component of the common mode voltage of the bridge arm of the MMC converter is analyzed by reasoning to obtain a third analysis conclusion; According to the third analysis conclusion, the effect of the second harmonic circulating current component of the bridge arm of the MMC converter is inferred and transformed to obtain a calculation formula for the second harmonic current.

7. The bridge arm reactor parameter selection system according to claim 6, characterized in that: The calculation formula of the second harmonic current is expressed as: ； In the formula, is the second harmonic current, is the original component of the second harmonic bridge arm voltage, for, is the ratio of the second harmonic impedance value of the phase unit reactance to the second harmonic impedance value of the phase unit capacitance, is the modulation ratio of the electromotive force of the MMC converter, is the amplitude of the alternating current.

8. The bridge arm reactor parameter selection system according to claim 5, characterized in that: The selection unit is specifically used for: The percentage of the second harmonic current to the bridge arm fundamental current of the MMC converter is used as a selection criterion, and the bridge arm reactor parameters of the MMC converter are determined according to the selection criterion.

9. A bridge arm reactor parameter selection device, characterized in that: The device comprises a processor and a memory: The memory is used to store program code and transmit the program code to the processor; The processor is used to execute the bridge arm reactor parameter selection method according to any one of claims 1 to 4 according to the instructions in the program code.

10. A computer-readable storage medium, characterized in that: The computer-readable storage medium is used to store program codes, and the program codes are used to execute the method for selecting parameters of a bridge arm reactor according to any one of claims 1 to 4.

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