A method and device for determining IGBT voltage and current of a flexible DC transmission engineering submodule

Abstract

The invention discloses a method and device for determining voltage and current of an IGBT of a flexible direct current transmission engineering submodule, and a method for determining voltage and current of an IGBT of a flexible direct current transmission engineering submodule, by obtaining the topological structure and system parameters of a converter to be analyzed, based on the differential equation of a capacitor element, a discretized adjoint model of a capacitor in a half-bridge submodule is obtained; according to the number of submodules currently put into use, the Thevenin model equivalent resistance of the half-bridge submodule in the converter to be analyzed is calculated, and a controlled voltage source is calculated using the voltage value at the last moment, the current value at the last moment, and the Thevenin model equivalent resistance; based on the discretized adjoint model, a Thevenin equivalent model of a bridge arm is obtained, and according to the topological structure and system parameters, the Thevenin equivalent model of the bridge arm is simulated to obtain the voltage stress and current stress of the IGBT in a normal state. By building an accurate transient model equivalent to MMC, the accuracy of the voltage and current stress of the actual working condition of the IGBT of a single submodule obtained is improved.

Description

Technical Field

[0001] The present invention relates to the field of flexible high-voltage direct current power transmission, and in particular to a method and device for determining voltage and current of an IGBT device. Background Art

[0002] In recent years, with the access of a large number of renewable energy sources far from the load center to the grid, especially the increasing demand for island power supply projects, the power system is facing the most important transformation since its birth. In the field of flexible high-voltage direct current transmission, the converter has experienced the application from mercury arc valve to thyristor to insulated gate bipolar transistor. Flexible direct current transmission technology can quickly, independently and accurately control the active power and reactive power of the system, and has the advantages of flexible and reliable operation. It is an effective means to solve the problem of renewable energy in my country. At the same time, modular multilevel converter (MMC), as a typical device in flexible direct current transmission technology, has always been a research hotspot in the field of direct current transmission.

[0003] However, the statistical results of domestic converter valve failures show that the main cause of converter valve failure is converter valve component failure, accounting for 84% of the total converter valve failures. Among them, the statistical results of component failures show that the main cause of its failure is power device failure, accounting for 34% of the total component failures. At present, the power devices used in flexible DC converter valves are mainly IGBT devices. Therefore, the reliability of IGBT devices is crucial to the safe and reliable operation of flexible DC transmission converter valves. In order to better analyze the failure mechanism and study the state monitoring of the crimped IGBT devices for sub-modules, it is necessary to obtain the voltage and current stress of the sub-module IGBT under various working conditions. In the current MMC transient model using the Thevenin equivalent, there is a lack of accurate methods to obtain the actual working condition voltage and current of a single sub-module IGBT, and the voltage and current stress before and after the IGBT failure under extreme working conditions is not considered. Summary of the invention

[0004] In order to solve the above technical problems, an embodiment of the present invention provides a method and device for determining the voltage and current of an IGBT of a flexible DC transmission engineering submodule, so as to solve the problem of low accuracy in obtaining the voltage and current stress of an IGBT device in the prior art.

[0005] A first aspect of an embodiment of the present invention provides a method for determining IGBT voltage and current of a flexible DC transmission engineering submodule, the method comprising:

[0006] The topological structure and system parameters of the converter to be analyzed are obtained, and based on the differential equation of the capacitor element, the discretized adjoint model of the capacitor in the half-bridge submodule is obtained;

[0007] According to the number of submodules currently put into use, the Thevenin model equivalent resistance of the half-bridge submodule in the converter to be analyzed is calculated, and the controlled voltage source is calculated using the voltage value at the previous moment, the current value at the previous moment and the Thevenin model equivalent resistance;

[0008] Based on the discretized adjoint model, the voltage value at the current moment is updated by using the current value at the current moment, the controlled voltage source and the Thevenin model equivalent resistance to obtain an updated voltage value, and the current value at the current moment is updated to obtain an updated current value;

[0009] The updated voltage value and the updated current value are stored in the storage array, and the updated voltage value is determined to be the voltage value at the previous moment, and the updated current value is determined to be the current value at the previous moment, and the step of calculating the equivalent resistance of the Thevenin model of the half-bridge sub-module in the converter to be analyzed according to the number of sub-modules currently put into use continues to update until the preset step length is reached to obtain the Thevenin equivalent model of the bridge arm, and the Thevenin equivalent model of the bridge arm is simulated according to the topological structure and system parameters to obtain the voltage stress and current stress of the IGBT under normal conditions.

[0010] In a possible implementation manner of the first aspect, a discretized adjoint model of the capacitor in the half-bridge submodule is obtained based on a differential equation of the capacitor element, including:

[0011] Based on the differential equation of capacitance and the trapezoidal formula, the discretized adjoint model is obtained, where the calculation formula of the discretized adjoint model is:

[0012]

[0013] In the formula, u n+1 is the voltage value at time n+1, u n is the voltage value at time n, i n+1 is the current value at time n+1, i n is the current value at time n, C is the capacitance value, Δt represents the step size, R Ceq is the equivalent resistance of the Thevenin model, E Ceq is a controlled voltage source.

[0014] In a possible implementation manner of the first aspect, the number of sub-modules currently put into use is obtained by modulating a bridge arm reference voltage value, including:

[0015] Modulate the reference voltage value of the bridge arm to obtain the current number of submodules put into use by the bridge arm;

[0016] Determine whether the number of submodules currently put into operation in the bridge arm has changed. If so, determine whether the current bridge arm current is greater than or equal to 0. If so, put into operation the submodules whose voltage is less than or equal to the preset voltage value among the submodules currently put into operation in the bridge arm. If so, put into operation the submodules whose voltage is greater than the preset voltage value among the submodules currently put into operation in the bridge arm to obtain the number of submodules currently put into operation.

[0017] If there is no change, the sub-modules put into operation at the previous moment are used to obtain the number of sub-modules put into operation currently.

[0018] In a possible implementation manner of the first aspect, modulating the bridge arm reference voltage value to obtain the current number of sub-modules put into use by the bridge arm includes:

[0019] According to the voltage whose difference with the bridge arm reference voltage value is less than the preset value, the number of submodules currently put into the upper bridge arm and the number of submodules currently put into the lower bridge arm are calculated using the submodule number calculation formula, where the submodule number calculation formula is:

[0020]

[0021] Where N p N is the number of submodules used in the upper bridge arm. n is the number of submodules put into the lower bridge arm, N is the number of submodules included in the upper bridge arm and the lower bridge arm, round(x) represents the integer closest to x, u0 is the phase voltage reference value, U c is the average DC voltage of the submodule;

[0022] According to the number of sub-modules currently put into use in the upper bridge arm and the number of sub-modules currently put into use in the lower bridge arm, the current number of sub-modules put into use in the bridge arm is obtained.

[0023] In a possible implementation manner of the first aspect, the method further includes:

[0024] The Thevenin equivalent model of the bridge arm is simulated under system fault to obtain the voltage stress and current stress of the IGBT under system fault;

[0025] The number of upper bridge arm submodules and lower bridge arm submodules in the bridge arm Thevenin equivalent model is adjusted to obtain an adjusted bridge arm Thevenin equivalent model, and according to the adjusted bridge arm Thevenin equivalent model, an IGBT failure simulation under a system fault is performed to obtain the voltage stress and current stress of the IGBT after the IGBT fails;

[0026] The IGBT failure state is obtained based on the voltage stress and current stress of the submodule in the normal state, the voltage stress and current stress of the submodule under the system fault, and the voltage stress and current stress of the submodule after the IGBT fails, so that personnel can give fault warning to the converter to be analyzed according to the failure state.

[0027] A second aspect of an embodiment of the present invention provides a device for determining IGBT voltage and current of a flexible DC power transmission engineering submodule, comprising:

[0028] An acquisition module is used to acquire the topological structure and system parameters of the converter to be analyzed, and obtain a discretized adjoint model of the capacitor in the half-bridge submodule based on the differential equation of the capacitor element;

[0029] The first calculation module is used to calculate the Thevenin model equivalent resistance of the half-bridge submodule in the converter to be analyzed according to the number of submodules currently put into use, and calculate the controlled voltage source using the voltage value at the previous moment, the current value at the previous moment and the Thevenin model equivalent resistance;

[0030] The second calculation module is used to update the voltage value at the current moment based on the discretized adjoint model, using the current value at the current moment, the controlled voltage source and the Thevenin model equivalent resistance to obtain an updated voltage value, and to update the current value at the current moment to obtain an updated current value;

[0031] The first simulation module is used to store the updated voltage value and the updated current value in the storage array, determine that the updated voltage value is the voltage value at the previous moment, the updated current value is the current value at the previous moment, and proceed to calculate the Thevenin model equivalent resistance step of the half-bridge sub-module in the converter to be analyzed according to the number of sub-modules currently put into use, and continue to update until the preset step length is reached to obtain the Thevenin equivalent model of the bridge arm, and simulate the Thevenin equivalent model of the bridge arm according to the topological structure and system parameters to obtain the voltage stress and current stress of the IGBT under normal conditions.

[0032] In a possible implementation manner of the second aspect, the acquisition module includes a discretized adjoint model calculation unit, wherein:

[0033] The discretized adjoint model calculation unit is used to discretize based on the differential equation of capacitance and the trapezoidal formula to obtain a discretized adjoint model, wherein the calculation formula of the discretized adjoint model is:

[0034]

[0035] In the formula, u n+1 is the voltage value at time n+1, u n is the voltage value at time n, i n+1 is the current value at time n+1, i nis the current value at time n, C is the capacitance value, Δt represents the step size, R Ceq is the equivalent resistance of the Thevenin model, E Ceq is a controlled voltage source.

[0036] In a possible implementation manner of the second aspect, the first calculation module includes a modulation unit, a first judgment unit, and a second judgment unit, wherein:

[0037] The modulation unit is used to modulate the reference voltage value of the bridge arm to obtain the current number of submodules put into use by the bridge arm;

[0038] The first judgment unit is used to judge whether the number of submodules currently put into the bridge arm has changed. If it has changed, it is judged whether the current bridge arm current is greater than or equal to 0. If it is greater than or equal to 0, the submodules of the current submodules put into the bridge arm that are less than or equal to the preset voltage value are put into use. If it is less than 0, the submodules of the current submodules put into the bridge arm that are greater than the preset voltage value are put into use to obtain the number of submodules currently put into use;

[0039] The second judgment unit is used to obtain the number of sub-modules currently put into use by using the sub-modules put into use by the bridge arm at the previous moment if there is no change.

[0040] In a possible implementation manner of the second aspect, the modulation unit includes a submodule quantity calculation subunit and a bridge arm input submodule calculation subunit, wherein:

[0041] The submodule quantity calculation unit is used to calculate the number of submodules currently put into the upper bridge arm and the number of submodules currently put into the lower bridge arm according to the voltage whose difference with the bridge arm reference voltage value is less than the preset value, using the submodule quantity calculation formula, wherein the submodule quantity calculation formula is:

[0042]

[0043] Where N p N is the number of submodules used in the upper bridge arm. n is the number of submodules put into the lower bridge arm, N is the number of submodules included in the upper bridge arm and the lower bridge arm, round(x) represents the integer closest to x, u0 is the phase voltage reference value, U c is the average DC voltage of the submodule;

[0044] The bridge arm submodule calculation subunit is used to obtain the current number of submodules put into the bridge arm according to the current number of submodules put into the upper bridge arm and the current number of submodules put into the lower bridge arm.

[0045] In a possible implementation manner of the second aspect, the flexible direct current transmission engineering submodule IGBT voltage and current determination device further includes a second simulation module, a third simulation module and a fourth simulation module.

[0046] The second simulation module is used to simulate the bridge arm Thevenin equivalent model under system faults to obtain the voltage stress and current stress of the IGBT under system faults;

[0047] The third simulation module is used to adjust the number of upper bridge arm submodules and lower bridge arm submodules in the bridge arm Thevenin equivalent model to obtain an adjusted bridge arm Thevenin equivalent model, and simulate the failure of the IGBT under system fault according to the adjusted bridge arm Thevenin equivalent model to obtain the voltage stress and current stress of the IGBT after the IGBT fails;

[0048] The fourth simulation module is used to obtain the IGBT failure state according to the voltage stress and current stress of the submodule under normal state, the voltage stress and current stress of the submodule under system fault, and the voltage stress and current stress of the submodule after IGBT failure, so that personnel can give fault warning to the converter to be analyzed according to the failure state.

[0049] Compared with the prior art, the beneficial effects of the present invention are:

[0050] The method for determining the voltage and current of the IGBT of the flexible direct current transmission engineering submodule provided by the embodiment of the present invention obtains the topological structure and system parameters of the converter to be analyzed, and obtains the discretized adjoint model of the capacitor in the half-bridge submodule based on the differential equation of the capacitor element; calculates the Thevenin model equivalent resistance of the half-bridge submodule in the converter to be analyzed according to the number of submodules currently put into use, and calculates the controlled voltage source using the voltage value at the previous moment, the current value at the previous moment, and the Thevenin model equivalent resistance; based on the discretized adjoint model, the voltage value at the current moment is updated using the current value, the controlled voltage source, and the Thevenin model equivalent resistance at the current moment, and obtains The updated voltage value is obtained, and the current value at the current moment is updated to obtain the updated current value; the updated voltage value and the updated current value are stored in the storage array, and the updated voltage value is determined to be the voltage value at the previous moment, and the updated current value is determined to be the current value at the previous moment, and the step of calculating the equivalent resistance of the Thevenin model of the half-bridge sub-module in the converter to be analyzed according to the number of sub-modules currently put into use continues to update until the preset step length is reached, and the Thevenin equivalent model of the bridge arm is obtained. According to the topological structure and system parameters, the Thevenin equivalent model of the bridge arm is simulated to obtain the voltage stress and current stress of the IGBT under normal conditions. By building an accurate MMC equivalent transient model as described above, the accuracy of the voltage and current stress of the actual working condition of the IGBT of a single sub-module is improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0052] Figure 1 A method flow chart of an embodiment of a method for determining IGBT voltage and current of a flexible DC power transmission engineering submodule provided by the present invention;

[0053] Figure 2 A topological structure diagram of the MMC of the method for determining the voltage and current of the IGBT of the flexible DC power transmission engineering submodule provided by the present invention;

[0054] Figure 3 A frequency domain model of the MMC of the method for determining the voltage and current of the IGBT of the flexible DC power transmission engineering submodule provided by the present invention;

[0055] Figure 4 The original circuit diagram of the capacitor of the method for determining the voltage and current of the IGBT of the flexible DC power transmission engineering submodule provided by the present invention;

[0056] Figure 5 The Thevenin associated equivalent circuit diagram of the capacitor of the method for determining the voltage and current of the IGBT of the flexible DC power transmission engineering submodule provided by the present invention;

[0057] Figure 6 A discretized half-bridge submodule equivalent model of the flexible DC power transmission engineering submodule IGBT voltage and current determination method provided by the present invention;

[0058] Figure 7 The Thevenin equivalent model of the bridge arm of the method for determining the voltage and current of the IGBT of the flexible DC power transmission engineering submodule provided by the present invention;

[0059] Figure 8 A submodule switching flow chart of the method for determining the voltage and current of the IGBT submodule of the flexible DC power transmission project provided by the present invention;

[0060] Fig. 9 The lowest level modulation diagram of the method for determining the voltage and current of the IGBT of the flexible DC power transmission engineering submodule provided by the present invention;

[0061] Fig.10 The MMC working state diagram of the method for determining the voltage and current of the IGBT of the flexible DC power transmission engineering submodule provided by the present invention;

[0062] Fig.11The on-off current variation waveform diagram of the 200-level lower half-bridge submodule 1 of the flexible DC power transmission engineering submodule IGBT voltage and current determination method provided by the present invention;

[0063] Fig.12 The on-off current variation waveform diagram of the 400-level lower half-bridge submodule 1 of the flexible DC power transmission engineering submodule IGBT voltage and current determination method provided by the present invention;

[0064] Fig.13 A structural diagram of a flexible direct current transmission system for a method for determining the voltage and current of an IGBT in a flexible direct current transmission engineering submodule provided by the present invention;

[0065] Fig.14 The distribution of fault current in the flexible DC power grid after a single-phase grounding fault on the converter transformer valve side of the flexible DC power transmission engineering submodule IGBT voltage and current determination method provided by the present invention;

[0066] Fig.15 The fault current distribution in the converter after a single-phase grounding fault on the converter transformer valve side of the flexible DC power transmission engineering submodule IGBT voltage and current determination method provided by the present invention;

[0067] Fig.16 The voltage and current stress of the submodule when the phase A is grounded in the method for determining the voltage and current of the IGBT of the flexible DC power transmission project provided by the present invention;

[0068] Fig.17 A voltage variation diagram of a submodule with 50% failure in the method for determining the voltage and current of the IGBT of a flexible DC power transmission project provided by the present invention;

[0069] Fig.18 A voltage variation diagram of a submodule with 33.3% failure in the method for determining the voltage and current of the IGBT submodule of the flexible DC power transmission project provided by the present invention;

[0070] Fig.19 A voltage variation diagram of a submodule with 25% failure in the method for determining the voltage and current of the IGBT of a flexible DC power transmission project provided by the present invention;

[0071] Fig. 20 A voltage variation diagram of a 10.7% failure of a submodule of the method for determining the voltage and current of the IGBT of a flexible DC power transmission project provided by the present invention;

[0072] Fig.21 This is a structural block diagram of another embodiment of the method for determining the voltage and current of the IGBT of the flexible DC power transmission engineering sub-module provided by the present invention. DETAILED DESCRIPTION

[0073] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0074] It should be understood that the step numbers used in this document are only for convenience of description and are not intended to limit the order in which the steps are executed.

[0075] The terms “include” and “comprising” indicate the presence of described features, integers, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or combinations thereof.

[0076] Therefore, a flow chart of an embodiment of a method for determining IGBT voltage and current of a flexible DC power transmission engineering submodule provided by an embodiment of the present invention is as follows: Figure 1 , including steps S101 to S104, each step is as follows:

[0077] S101, obtaining the topological structure and system parameters of the converter to be analyzed, and obtaining a discretized adjoint model of the capacitor in the half-bridge submodule based on the differential equation of the capacitor element.

[0078] In this embodiment, as shown in 2, Figure 2 It is an MMC topology structure. Each bridge arm is composed of a reactor and N sub-modules in series. There are 6 bridge arms in total for three phases. The upper and lower two bridge arms of each phase constitute a phase unit. Figure 2 The structure of each submodule is also given. Each submodule has three switching states: 1) Both insulated gate bipolar thyristors (IGBTs) are locked, which is generally used at startup and fault, and is called the locked state; 2) The upper IGBT (T1) is turned on and the lower IGBT (T2) is locked. At this time, the output voltage of the submodule is the capacitor voltage, which is called the input state; 3) The upper IGBT (T1) is locked and the lower IGBT (T2) is turned on. At this time, the output voltage of the submodule is 0, which is called the cut-off state. Therefore, the output voltage of the submodule can be controlled by a trigger pulse. According to the switching state and current direction of the upper and lower bridge arm IGBTs of the submodule, there are 6 working modes, as shown in Table 1.

[0079] Table 1 Working status of half-bridge submodule

[0080]

[0081] according to Figure 2 , the differential equation mathematical model of MMC can be derived:

[0082]

[0083] In the formula, u sj is the voltage source, U dc is the DC side voltage, L0 is the bridge arm equivalent inductance, L ac is the equivalent inductance on the AC side, R0 is the equivalent resistance, i pj is the upper arm current, i nj is the lower arm current, u oo‘ is the voltage output by MMC, i vj is the current output by MMC, u pj Upper bridge arm voltage, u nj is the lower bridge arm voltage.

[0084] The differential mode voltage and common mode voltage of the upper and lower bridge arms are defined as u diffj and u comj :

[0085]

[0086] In the formula, u pj Upper bridge arm voltage, u nj is the lower bridge arm voltage, u djffj is the differential voltage between the upper and lower bridge arms, u comj is the differential voltage between the upper and lower bridge arms.

[0087] Combine the differential equations of MMC and substitute u diffj and u comj Then the mathematical expression characterizing the dynamic performance of the AC and DC sides of the MMC is obtained:

[0088]

[0089] In the formula, u djffj is the differential voltage between the upper and lower bridge arms, u comj is the differential voltage of the upper and lower bridge arms, R0 is the equivalent resistance, U dc is the DC side voltage, i vj is the current output by the MMC, L ac is the equivalent inductance on the AC side, u oo‘ is the voltage output by MMC, u sj is a voltage source, i cirj is the j-phase circulation,

[0090] For the controller design of MMC, we only focus on the fundamental component on the AC side. Under the symmetrical condition of the AC system, u oo ‘ The fundamental component of is zero. Therefore, it can be simplified to:

[0091]

[0092] In the formula, u djffj is the differential voltage between the upper and lower bridge arms, L ac is the equivalent inductance of the AC side, L0 is the equivalent inductance of the bridge arm, R0 is the equivalent resistance, u sj is a voltage source, i vj is the current output by the MMC.

[0093] The fundamental frequency dynamic equation of the MMC AC side in the abc coordinate system can be obtained as follows:

[0094]

[0095] In the formula, i va 、i vb and i vc The A phase current, B phase current and C phase current of the AC side are u sa 、u sb and u sc The A phase voltage, B phase voltage and C phase voltage of the AC side are respectively, u djffa 、u djffb and u ajffc They are the A-phase differential mode voltage, B-phase differential mode voltage and C-phase differential mode voltage of the upper and lower bridge arms respectively.

[0096] Generally, a three-phase power system provides power by a set of interacting three-phase voltages and three-phase currents. However, an actual three-phase system cannot be regarded as a simple addition of three independent single-phase subsystems. Therefore, the control system of the three-phase grid-connected power converter needs to use the variables obtained by Park transformation to control it.

[0097] The time domain formula of MMC in the dq coordinate system obtained by Park transformation is:

[0098]

[0099] In the formula, i vd and i vp are the direct axis current and quadrature axis current, u sd and u sq are the direct-axis voltage and quadrature-axis voltage, u djffd and u djffq are the direct-axis differential mode voltage and the quadrature-axis differential mode voltage respectively, and ω is the synchronous rotation angular velocity.

[0100] Using the Lassen transformation, the frequency domain form of the state equation of MMC in the dq coordinate system can be obtained:

[0101]

[0102] In the formula, i vd and ivp are the direct axis current and quadrature axis current, u sd and u sq are the direct-axis voltage and quadrature-axis voltage, u djffd and u djffq are the direct-axis differential mode voltage and the quadrature-axis differential mode voltage respectively, and ω is the synchronous rotation angular velocity.

[0103] Therefore, the frequency domain model of MMC can be obtained through the frequency domain equation, such as Figure 3 shown.

[0104] In some embodiments, in step S101, "obtaining a discretized adjoint model of the capacitor in the half-bridge submodule based on the differential equation of the capacitor element" includes but is not limited to the following steps:

[0105] Based on the differential equation of capacitance and the trapezoidal formula, the discretized adjoint model is obtained, where the calculation formula of the discretized adjoint model is:

[0106]

[0107] In the formula, u n+1 is the voltage value at time n+1, u n is the voltage value at time n, i n+1 is the current value at time n+1, i n is the current value at time n, C is the capacitance value, Δt represents the step size, R Ceq is the equivalent resistance of the Thevenin model, E Ceq is a controlled voltage source.

[0108] In this embodiment, the solution process of the discretized associated model is as follows: Step 1: Use appropriate numerical integration formulas to discretize the differential equations describing the characteristics of a single component to form a discretized model of a single component; Step 2: Establish a discretized associated component network of the entire power grid or a given structure based on the discretized model of the single component; Step 3: Obtain a solution at a discrete time point by solving the entire associated network; Step 4: Use the solution obtained at the current moment to recursively infer the discretized associated model of the next discrete moment. Repeating steps 2 to 4 continuously can obtain the solution of the system at a series of discrete time points.

[0109] The discrete model of the half-bridge submodule can be regarded as consisting of the discrete adjoint model of the capacitor and the variable resistor. Therefore, we need to construct the discrete adjoint model of the capacitor first. Figure 4 For the original capacitor circuit, the differential equation describing the capacitor element is:

[0110]

[0111] In the formula, i represents current and C represents capacitance.

[0112] Integrate both sides of the equation and discretize it using the trapezoidal formula of numerical integration. The discretization formula is:

[0113]

[0114] Among them, in the formula, u n+1 is the voltage value at time n+1, u n is the voltage value at time n, i n+1 is the current value at time n+1, i n is the current value at time n, C is the capacitance value, and Δt represents the step size.

[0115] The discrete companion model of capacitance can be obtained by the discretization formula. The formula of the discrete companion model of capacitance is:

[0116]

[0117] In the formula, u n+1 is the voltage value at time n+1, u n is the voltage value at time n, i n+1 is the current value at time n+1, i n is the current value at time n, C is the capacitance value, Δt represents the step size, R Ceq is the equivalent resistance of the Thevenin model, E Ceq is a controlled voltage source.

[0118] The Thevenin form of the discrete associated capacitor model can be derived from the discrete associated capacitor model formula, as follows: Figure 5 As shown. After determining the associated equivalent circuit of the capacitor, the Thevenin equivalent model of the half-bridge submodule can be derived. The half-bridge submodule consists of two IGBTs with anti-parallel diodes and a capacitor. Based on the difference between their switched-on and switched-off states, the combination of the IGBT and the diode can be equivalent to two variable resistors. When the combination is turned on, the variable resistor is a very small on-state resistance; when the combination is turned off, the variable resistor is a very large reverse resistance. Figure 5 The proposed capacitor associated equivalent circuit can obtain the discretized half-bridge submodule equivalent model, such as Figure 6 As shown. Looking into the circuit from the port, it can be equivalent to a submodule Thevenin equivalent circuit consisting of an equivalent voltage source and an equivalent resistor, as shown Figure 7 As shown. The values ​​of the equivalent voltage source and equivalent resistance can be obtained as:

[0119]

[0120] In the formula, R smeq is the equivalent resistance of the half-bridge submodule, E smeqis the equivalent voltage source of the half-bridge submodule, R1, R2 and R c are the resistance values ​​of each submodule, t is the time, E c It is the voltage source of the submodule.

[0121] Ideally, the on-state resistance can be considered to be 0 and the reverse resistance to be infinite, which will bring convenience to our bridge arm equivalent model.

[0122] S102, according to the number of sub-modules currently put into use, calculate the Thevenin model equivalent resistance of the half-bridge sub-module in the converter to be analyzed, and calculate the controlled voltage source using the voltage value at the previous moment, the current value at the previous moment and the Thevenin model equivalent resistance.

[0123] In this embodiment, during the model building process, the parameters of the previous time step are required when calculating the parameters in the current time step, for example: the discretization model construction of the capacitor. At this time, it is necessary to define a storage space to store the old value. This purpose can be achieved in PSCAD by relying on the STORF (NSTORF) storage array. The data stored in STORF (x) can be called in the entire model and play the role of a global variable. Therefore, in the model, an "Initial" module is created, in which the parameters are initialized (initial values ​​are assigned), the size of the STORF (x) storage array is defined, and the storage space required for the three phases A, B, and C is allocated to facilitate the construction of the later model. At the same time, an "Nsmcompute" module is created: this module inputs a voltage reference wave to determine the number of sub-modules that need to be invested in the upper and lower bridge arms.

[0124] According to the number of sub-modules required for the upper and lower bridge arms, the Thevenin model equivalent resistance R of the overall capacitor is first calculated based on the number of current working capacitors. Ceq ,according to The calculation formula is: The capacitor voltage u at the previous moment can be used n and the capacitor current i n Calculate the value of the controlled voltage source in the Thevenin model of a capacitor

[0125] In some embodiments, in step S102, “the number of sub-modules currently put into use is obtained by modulating the bridge arm reference voltage value” includes but is not limited to the following steps:

[0126] Modulate the reference voltage value of the bridge arm to obtain the current number of submodules put into use by the bridge arm;

[0127] Determine whether the number of submodules currently put into operation in the bridge arm has changed. If so, determine whether the current bridge arm current is greater than or equal to 0. If so, put into operation the submodules whose voltage is less than or equal to the preset voltage value among the submodules currently put into operation in the bridge arm. If so, put into operation the submodules whose voltage is greater than the preset voltage value among the submodules currently put into operation in the bridge arm to obtain the number of submodules currently put into operation.

[0128] If there is no change, the sub-modules put into operation at the previous moment are used to obtain the number of sub-modules put into operation currently.

[0129] In this embodiment, the normal operation of MMC has two conditions: 1) the maintenance of DC voltage, Figure 1 .1.1 For example, the number of submodules in the three phase units that are in the on-state is equal and unchanged, so that the voltage of each phase satisfies the formula: A1 +u A2 =u B1 +u B2 =u C1 +u C2 ; 2) The output of three-phase AC voltage is achieved by allocating the number of sub-modules in the upper and lower bridge arms of the three phase units to adjust the output voltage of the converter.

[0130] Taking phase A as an example, without considering the margin of submodules, each phase has 2N submodules. When all N submodules of the upper bridge arm of phase A are removed, u A1 =0, the voltage at point a is the DC positive voltage. At this time, all N submodules in the lower bridge arm of phase A must be put into operation to obtain the maximum DC voltage, thereby improving the device utilization rate. Because the number of submodules in the phase unit that are in the put-in state is a constant, in general, the number of submodules in the put-in state in each phase unit is N in theory, which is half of the total number of submodules in the phase unit (2N). In this way, the number of submodules in the put-in state in a single bridge arm can be 0, 1, 2...N, which means that the maximum number of levels that the MMC can output is N+1.

[0131] After obtaining the number of sub-modules to be put into or removed by modulating the bridge arm reference voltage, it is necessary to select specific and appropriate sub-modules to be put into or removed to ensure the balance of capacitor voltages inside each sub-module, which is generally achieved through a sub-module capacitor voltage sorting and equalization algorithm.

[0132] like Figure 8 As shown, the implementation process of the algorithm is: first determine N p If it has not changed, continue to invest in the submodule invested last time. If it has changed, reselect the submodule to invest. p Reselecting the submodule to be put into operation when changes occur is helpful to reduce switching losses.p When the bridge arm current i changes, first determine whether it is greater than 0, and then determine whether the current submodule should be charged or discharged. pa ≥0, the capacitor voltages of the submodules need to be sorted in ascending order, so the capacitors with smaller voltages are in the front, and U C (N p ) is the threshold value, capacitor voltage ≤ U C (N p ) p All sub-modules are put into operation, and the capacitor voltage U C (N p ) p All sub-modules are bypassed, so that the capacitor voltage N with smaller p The sub-modules are charged to increase the voltage. pa <0 is similar, except that we need to sort in descending order to find the threshold U C (N p ), the capacitor voltage should be ≥U C (N p ) p The submodule discharges, the capacitor voltage C (N p ) p Submodule bypass.

[0133] In some embodiments, the step S102 of "modulating the bridge arm reference voltage value to obtain the current number of sub-modules put into use by the bridge arm" includes but is not limited to the following steps:

[0134] According to the voltage whose difference with the bridge arm reference voltage value is less than the preset value, the number of submodules currently put into the upper bridge arm and the number of submodules currently put into the lower bridge arm are calculated using the submodule number calculation formula, where the submodule number calculation formula is:

[0135]

[0136] Where N p N is the number of submodules used in the upper bridge arm. n is the number of submodules put into the lower bridge arm, N is the number of submodules included in the upper bridge arm and the lower bridge arm, round(x) represents the integer closest to x, u0 is the phase voltage reference value, U c is the average DC voltage of the submodule;

[0137] According to the number of sub-modules currently put into use in the upper bridge arm and the number of sub-modules currently put into use in the lower bridge arm, the current number of sub-modules put into use in the bridge arm is obtained.

[0138] ​In this embodiment, the modulation of the bridge arm voltage reference value can usually be performed by the nearest level approximation method. The nearest level approximation modulation (NLM) is to approximate the sine wave by a step wave. It has a better fitting effect when applied to high-level situations and is currently a widely used MMC modulation strategy. When using the nearest level modulation, the most critical step is to select a suitable submodule to switch on the MMC main circuit so that its output is as close to the modulated wave as possible. The modulation process is as follows Fig. 9 shown.

[0139] The step wave output waveform can be obtained according to the following formula:

[0140]

[0141] Select the level output that is closest to the reference voltage per unit value, such as When u0=U C It can be seen that the error range of the step wave and the modulation wave is maintained at half of the submodule capacitor voltage, so the more the number of output levels, the smaller the error and the lower the harmonic content.

[0142] If the number of submodules put into use in the upper and lower bridge arms at a certain moment is N p and N n , then the output voltage u of the upper and lower bridge arms is p and u n for

[0143]

[0144] Because the output voltage is a step wave, as shown in the following formula. Thus, the number of modules N put into the upper and lower bridge arms can be obtained p and N n They are:

[0145]

[0146] Where N p N is the number of submodules used in the upper bridge arm. n is the number of submodules put into the lower bridge arm, N is the number of submodules included in the upper bridge arm and the lower bridge arm, round(x) represents the integer closest to x, u0 is the phase voltage reference value, U c is the average DC voltage of the submodule.

[0147] The above formula can be used to calculate the number of sub-modules put into use in the upper and lower bridge arms of the sub-module at any time.

[0148] S103. Based on the discretized adjoint model, the voltage value at the current moment is updated using the current value at the current moment, the controlled voltage source and the Thevenin model equivalent resistance to obtain an updated voltage value, and the current value at the current moment is updated to obtain an updated current value.

[0149] In this embodiment, after calculating the equivalent resistance and controlled voltage source of the Thevenin model, the capacitor voltage u at the current moment is then updated. n+1 and the capacitor current i n+1 , for use in the next time step. Finally, according to Using the currently measured capacitor current i n+1 and the controlled voltage source voltage obtained in the previous step The capacitor voltage u at the current moment can be calculated n+1 , and the capacitor current i at the current moment should also be updated n+1 , and repeat this cycle until the preset step length is reached.

[0150] S104, storing the updated voltage value and the updated current value in the storage array, determining that the updated voltage value is the voltage value at the previous moment, and the updated current value is the current value at the previous moment, and proceeding to the step of calculating the Thevenin model equivalent resistance of the half-bridge sub-module in the converter to be analyzed according to the number of sub-modules currently put into use, and continuing to update until the preset step length is reached to obtain the Thevenin equivalent model of the bridge arm, and simulating the Thevenin equivalent model of the bridge arm according to the topological structure and system parameters to obtain the voltage stress and current stress of the IGBT under normal conditions.

[0151] In this embodiment, the bridge arm submodule discrete model is equivalent to an equivalent voltage source and an equivalent resistor in series. It can be seen from the formula that there is an algebraic relationship between the voltage value of the equivalent voltage source at any certain moment and the equivalent voltage value of the submodule capacitor discrete model, and the equivalent resistor is a constant value that is only related to the operating state of the submodule at that moment. Therefore, the discrete model construction of the submodule capacitor in PSCAD can further obtain the bridge arm submodule discrete model, and finally realize the discrete model construction of the bridge arm.

[0152] Build the Arm module: The inputs of the Arm module are Nap, Nan (the number of submodules put into operation in the upper and lower bridge arms respectively) and StartNSTORF_a (providing the initial value of the pointer), and the outputs are Eap, Ean (equivalent voltage sources of the upper and lower bridge arms), Rap, Ran (equivalent resistances of the upper and lower bridge arms). In this module, the function of "calculating the equivalent resistance and controlled voltage source of the capacitor Thevenin model at the current moment" is implemented, and the equivalent voltage value of each submodule at the current moment can be further obtained. Then, in the Arm module, each submodule is sorted according to the size of the submodule voltage, and then the operating status (put into operation / removal) of each submodule is determined according to the direction of the bridge arm current at this time. The overall output is displayed through the bridge arm voltage of the upper and lower bridge arms.

[0153] After the Arm module is completed, the bridge arm current is updated and input into the Feedback module, in which the function of "updating the capacitor voltage and capacitor current at the current moment" is implemented. The results obtained in the Feedback module will be stored in the STORF(x) storage array. When the next simulation step comes, the Arm module will read the results in STORF(x), thus forming a closed loop and realizing the construction of a half-bridge MMC Thevenin discrete model in PSCAD.

[0154] On this basis, based on the multi-level simulation model of the capacitor Thevenin equivalent circuit, the voltage and current passing through the capacitor in any module at hundreds of levels can be calculated.

[0155] Now set the simulation conditions: the total number of levels is 200 / 400. The DC voltage is 320kV, the AC voltage is 135kV*1.732 / 1.414 (effective value of line voltage), and the phase voltage peak is 135kV. At 200 levels, the voltage of a single submodule is 1.6kV. At 400 levels, the voltage of a single submodule is 0.8kV.

[0156] The voltage that the IGBT bears during the shutdown process is the capacitor voltage on the submodule. Its value will fluctuate during the operation of the MMC and can be considered as an approximate average value, that is, 1.6kV or 0.8kV. Therefore, in the simulation, we mainly focus on the change law of the IGBT on-off current. The working state of the MMC is as follows: Fig.10 shown.

[0157] The simulation results at level 200 are as follows Fig.11 As shown in the analysis results, the current direction is specified to be from top to bottom. When the simulation current is positive, it corresponds to mode 2, that is, the module current passes through diode D1 at this time, but the IGBT T1 in mode 2 is in the on state. Therefore, when the simulation current is negative, it corresponds to mode 5, that is, the module current passes through the IGBT at this time. The simulation results at the 400 level are as follows Fig.12 shown.

[0158] In some embodiments, the “simulating the bridge arm Thevenin equivalent model” in step S104 includes but is not limited to the following steps:

[0159] The Thevenin equivalent model of the bridge arm is simulated under system fault to obtain the voltage stress and current stress of the IGBT under system fault;

[0160] The number of upper bridge arm submodules and lower bridge arm submodules in the bridge arm Thevenin equivalent model is adjusted to obtain an adjusted bridge arm Thevenin equivalent model, and according to the adjusted bridge arm Thevenin equivalent model, an IGBT failure simulation under a system fault is performed to obtain the voltage stress and current stress of the IGBT after the IGBT fails;

[0161] The IGBT failure state is obtained based on the voltage stress and current stress of the submodule in the normal state, the voltage stress and current stress of the submodule under the system fault, and the voltage stress and current stress of the submodule after the IGBT fails, so that personnel can give fault warning to the converter to be analyzed according to the failure state.

[0162] In this embodiment, during the operation of the flexible direct current system, various extreme working conditions will be encountered, among which short-circuit fault is a relatively common extreme working condition. Now, an offshore flexible direct current system is built on this basis, with a transmission power of 3000MW, and a wind farm bus voltage of 66kV. It is transformed to 290kV through a converter transformer, and then sent to the onshore converter station through a 100km submarine cable through a 500kV true bipolar transmission system, and finally transmitted to the 500kV AC power grid through overhead lines. The converter valves all adopt the MMC structure, the number of bridge arm submodules is 300, and the simulation model is as follows: Fig.13 shown.

[0163] The simulation model uses a current source as a substitute for the input end of the wind farm. The rated current of each current source is 13.3kA. The AC power supply on the right side is composed of two voltage sources, and the rated voltage of each voltage source is 525kV. Among them, VSC is a voltage source converter, including an MMC modular multilevel converter and a converter transformer. The rated voltage of each MMC converter valve is 500kV, the maximum DC power is 1500MW, the capacitance value of each submodule is 9mF, and the number of submodules in a single bridge arm is 300. The converter transformer is divided into offshore and onshore. The rated capacity of the offshore converter transformer is 1700MVA, the short-circuit impedance is 0.16pu, the grid-side rated voltage is 72.5kV, and the valve-side voltage is 520.5kV; the rated capacity of the onshore converter transformer is 1710MVA, the short-circuit impedance is 0.14pu, the grid-side rated voltage is 535kV, and the valve-side voltage is 520.5kV. The DC cable used is the DC submarine cable of the demonstration project, which is a 100-kilometer 500kV DC cable with a cross-sectional area of ​​3500mm2, an outer diameter of 35mm, and a conductor resistivity of 1.815e10-8.

[0164] In the flexible direct current system, various faults may occur. Here, we take the valve side short circuit of the converter transformer in the converter station as an example to analyze its mechanism: When a fault occurs in the converter station, it is generally a serious fault, and after the fault occurs, the station cannot operate normally and must be shut down. After the station fault occurs, the overcurrent protection of the converter valve bridge arm will be activated. The development process of the flexible direct current system fault after the station fault occurs is as follows: Fig.14 As shown in the figure above, when a single-phase grounding fault occurs on the valve side of the converter transformer, the fault point is equivalent to being directly connected to the AC side of the bridge arm reactor. As a result, the potential in the middle of the upper and lower bridge arms is forced to become zero potential. The fault point is located in the converter station, and the fault location and the DC system are connected by the converter valve and the bridge arm reactor.

[0165] Unlike AC system faults, after a valve-side fault occurs, the DC system and the fault point can directly establish contact, and the converter station will inject fault current to this point. Therefore, the transient fault current flowing through the fault point after the fault occurs in the station is very large. The current on the positive pole of the DC bus can be blocked by turning off the IGBT of the upper bridge arm submodule, but the fault current of the lower bridge arm is blocked by the anti-parallel diode in the IGBT. A symmetrical bipolar half-bridge MMC topology is used in the flexible DC system. When a valve-side fault occurs in the converter transformer, the converter fault current transformer is as follows: Fig.15 shown.

[0166] The symmetrical bipolar system consists of two MMCs, the neutral point of which is grounded or grounded through a metal return line, forming a "reference zero potential" on the DC side. However, there is a DC bias on the three-phase voltage on the valve side of the converter valve. Therefore, the three-phase voltage on the valve side of the converter valve in normal operation is:

[0167]

[0168] In the formula, u An 、u Bn and u Cn They are the voltage values ​​of phase A, phase B and phase C on the valve side of the bipolar MMC converter transformer to ground, respectively. A 、u B and u C are the A-phase voltage, B-phase voltage and C-phase voltage of the valve side of the bipolar MMC converter transformer, respectively. dc is the DC side voltage.

[0169] Therefore, the three-phase line voltage on the valve side of the converter transformer is still an AC voltage without bias. After a single-phase grounding fault occurs on the valve side, the voltage between phase C and the ground becomes zero, so the three-phase line voltage on the valve side becomes:

[0170] u' AB =u AB

[0171]

[0172] In the formula, u' AB is the voltage between phase A and phase B after the change, u' CA is the valve side voltage after the change.

[0173] It can be seen from the above formula that when a single-phase grounding fault occurs on the valve side, the three-phase line voltage on the valve side changes. There is a DC bias in the voltage of two phases, which will cause overcurrent in the bridge arm. In addition, at the moment of short circuit of phase C, the fault current value is very large, as shown in the figure. The IarmUF value is relatively large; in addition, after the fault occurs, the capacitor of the lower bridge arm submodule will discharge to the fault voltage instantly, which will cause the current IarmUF value of the lower bridge arm to increase. After the fault occurs, IarmUF can be shut down by locking, while the current IarmUF of the lower bridge arm cannot be interrupted by locking, and can only be shut down by tripping the AC circuit breaker. In summary, the instantaneous fault current value of a single-phase grounding fault on the valve side of the converter transformer is very large, which will inevitably trigger the overcurrent protection; and there is a path for the fault current in the lower bridge arm, so the fault is a serious permanent fault.

[0174] Based on this simulation analysis, when a phase A ground fault occurs on the valve side of the converter transformer, the voltage and current stress of the IGBT of its submodule is as follows: Fig.16 As shown in the figure, by analogy, the voltage and current stress of the sub-module under other extreme working conditions can be obtained, and then the failure mechanism analysis and state monitoring research of the IGBT can be carried out.

[0175] When the system is in extreme working conditions for a long time, the IGBT will inevitably fail due to overvoltage, overcurrent or overtemperature. When the IGBT is in a failed state, the impact on the external circuit is usually manifested in the following three situations: open circuit failure, in this case, the IGBT cannot be turned on, the circuit is interrupted, and the current cannot flow through the IGBT, and the downstream circuit cannot work normally; short circuit failure, in this case, the IGBT cannot be turned off in the on state, causing the current to pass directly through the IGBT and form a short circuit. This will cause the current in the circuit to rise sharply, which may cause the overcurrent protection device to operate, such as fuse melting or circuit breaker tripping; parameter drift, failure may cause changes in the electrical parameters of the IGBT, such as increased on-state voltage drop, extended switching time, increased leakage current, etc. This may lead to a decrease in circuit performance, such as reduced efficiency, increased heat generation, unstable control, etc.

[0176] Taking the short-circuit fault after IGBT failure as an example, the voltage and current characteristics of the submodule under the condition of partial IGBT failure are studied. Continue to improve the built flexible DC system, adjust the number of upper and lower bridge arm submodules to equalize the submodule IGBT failure ratio, and simulate the failure of some submodules in the upper bridge arm of the system. Set 4 groups of failure ratios: 50%, 33.3%, 25%, and 10.7%, that is, the number of upper bridge arm submodules is 150, 100, 75, and 32 respectively. At the same time, set the single-phase grounding fault on the valve side of the converter transformer for simulation. The simulation results are as follows: Figure 17-20 shown.

[0177] Through Fig.16 By comparing the voltage and current stress of the submodules under normal circumstances, the failure state of the submodule IGBT can be obtained: when the number of submodules is 150, under normal circumstances, the voltage of a single submodule rises from 1.7kV to 3.4kV. Due to the existence of the lower bridge arm, the current of the submodule basically does not change, but when a fault occurs, the submodule voltage rises from 2.7kV to 5.1kV, and the submodule current fluctuates more than that in the non-failure state, and finally drops to 0. However, both voltage and current will eventually tend to a stable state after the fault.

[0178] When the number of submodules is 100, after a fault occurs, the submodule voltage will rise in steps after a period of stabilization after a sharp rise, but the rise is small; the submodule current fluctuates more after the fault occurs, and there will be a small fluctuation after it drops to 0, that is, a small current appears. When the number is 75, the submodule voltage rises after stabilization for a period of time, and the amplitude is large; the submodule current fluctuates more and more after it drops to 0. When the number is 32, the submodule voltage stabilization time is greatly reduced, and the step-by-step rise is larger and more frequent; the submodule current fluctuates more after it drops to 0 compared to 75 submodules, and the maximum is close to 2kA, and the number of occurrences also increases accordingly.

[0179] It can be seen from this that the failure state of the sub-module IGBT is: when no fault occurs, the voltage of the sub-module will increase significantly, and the current flowing through the sub-module will basically not change; after a fault occurs, when the number of sub-module failures is small, the sub-module voltage will enter a stable state after rising, and the sub-module current will fluctuate greatly before dropping to 0, but when the number of failures is large, the sub-module voltage will increase in steps after being stable for a period of time, and the sub-module current will have more and larger fluctuations after dropping to 0.

[0180] It should be understood that although Figure 1 The steps in the flowchart are shown in sequence as indicated by the arrows, but these steps are not necessarily executed in the order indicated by the arrows. Unless otherwise specified in this document, there is no strict order restriction for the execution of these steps, and these steps can be executed in other orders. Moreover, Figure 1 At least part of the steps may include multiple steps or multiple stages. These steps or stages are not necessarily performed at the same time, but can be performed at different times. The execution order of these steps or stages is not necessarily sequential, but can be performed in turn or alternately with other steps or at least part of the steps or stages in other steps.

[0181] In some embodiments, Fig.16 As shown, it shows a block diagram of a flexible DC transmission engineering submodule IGBT voltage and current determination device 1600 provided in an embodiment of the present application, including: an acquisition module 1601, a first calculation module 1602, a second calculation module 1603 and a first simulation module 1604, wherein:

[0182] An acquisition module 1601 is used to acquire the topological structure and system parameters of the converter to be analyzed, and obtain a discretized adjoint model of the capacitor in the half-bridge submodule based on the differential equation of the capacitor element;

[0183] The first calculation module 1602 is used to calculate the Thevenin model equivalent resistance of the half-bridge submodule in the converter to be analyzed according to the number of submodules currently put into use, and calculate the controlled voltage source using the voltage value at the previous moment, the current value at the previous moment and the Thevenin model equivalent resistance;

[0184] The second calculation module 1603 is used to update the voltage value at the current moment based on the discretized adjoint model, using the current value at the current moment, the controlled voltage source and the Thevenin model equivalent resistance to obtain an updated voltage value, and to update the current value at the current moment to obtain an updated current value;

[0185] The first simulation module 1604 is used to store the updated voltage value and the updated current value in the storage array, determine that the updated voltage value is the voltage value at the previous moment, the updated current value is the current value at the previous moment, and proceed to the step of calculating the Thevenin model equivalent resistance of the half-bridge sub-module in the converter to be analyzed according to the number of sub-modules currently put into use, and continue to update until the preset step length is reached to obtain the Thevenin equivalent model of the bridge arm, and simulate the Thevenin equivalent model of the bridge arm according to the topological structure and system parameters to obtain the voltage stress and current stress of the IGBT under normal conditions.

[0186] In some embodiments, the acquisition module includes a discretized adjoint model calculation unit, wherein:

[0187] The discretized adjoint model calculation unit is used to discretize based on the differential equation of capacitance and the trapezoidal formula to obtain a discretized adjoint model, wherein the calculation formula of the discretized adjoint model is:

[0188]

[0189] In the formula, u n+1 is the voltage value at time n+1, u n is the voltage value at time n, i n+1 is the current value at time n+1, i n is the current value at time n, C is the capacitance value, Δt represents the step size, R Ceq is the equivalent resistance of the Thevenin model, E Ceq is a controlled voltage source.

[0190] In some embodiments, the first calculation module includes a modulation unit, a first judgment unit and a second judgment unit, wherein:

[0191] The modulation unit is used to modulate the reference voltage value of the bridge arm to obtain the current number of submodules put into use by the bridge arm;

[0192] The first judgment unit is used to judge whether the number of submodules currently put into the bridge arm has changed. If it has changed, it is judged whether the current bridge arm current is greater than or equal to 0. If it is greater than or equal to 0, the submodules of the current submodules put into the bridge arm that are less than or equal to the preset voltage value are put into use. If it is less than 0, the submodules of the current submodules put into the bridge arm that are greater than the preset voltage value are put into use to obtain the number of submodules currently put into use;

[0193] The second judgment unit is used to obtain the number of sub-modules currently put into use by using the sub-modules put into use by the bridge arm at the previous moment if there is no change.

[0194] In some embodiments, the modulation unit includes a submodule quantity calculation subunit and a bridge arm input submodule calculation subunit, wherein:

[0195] The submodule quantity calculation unit is used to calculate the number of submodules currently put into the upper bridge arm and the number of submodules currently put into the lower bridge arm according to the voltage whose difference with the bridge arm reference voltage value is less than the preset value, using the submodule quantity calculation formula, wherein the submodule quantity calculation formula is:

[0196]

[0197] Where N p N is the number of submodules used in the upper bridge arm. n is the number of submodules put into the lower bridge arm, N is the number of submodules included in the upper bridge arm and the lower bridge arm, round(x) represents the integer closest to x, u0 is the phase voltage reference value, U c is the average DC voltage of the submodule;

[0198] The bridge arm submodule calculation subunit is used to obtain the current number of submodules put into the bridge arm according to the current number of submodules put into the upper bridge arm and the current number of submodules put into the lower bridge arm.

[0199] In some embodiments, the flexible DC power transmission engineering submodule IGBT voltage and current determination device further includes a second simulation module, a third simulation module and a fourth simulation module.

[0200] The second simulation module is used to simulate the bridge arm Thevenin equivalent model under system faults to obtain the voltage stress and current stress of the IGBT under system faults;

[0201] The third simulation module is used to adjust the number of upper bridge arm submodules and lower bridge arm submodules in the bridge arm Thevenin equivalent model to obtain an adjusted bridge arm Thevenin equivalent model, and simulate the failure of the IGBT under system fault according to the adjusted bridge arm Thevenin equivalent model to obtain the voltage stress and current stress of the IGBT after the IGBT fails;

[0202] The fourth simulation module is used to obtain the IGBT failure state according to the voltage stress and current stress of the submodule under normal state, the voltage stress and current stress of the submodule under system fault, and the voltage stress and current stress of the submodule after IGBT failure, so that personnel can give fault warning to the converter to be analyzed according to the failure state.

[0203] The technical features of the above embodiments may be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0204] The specific embodiments described above further illustrate the purpose, technical solutions and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. It is particularly pointed out that for those skilled in the art, any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for determining the voltage and current of an IGBT in a flexible DC transmission engineering submodule, characterized in that: include: The topological structure and system parameters of the converter to be analyzed are obtained, and based on the differential equation of the capacitor element, the discretized adjoint model of the capacitor in the half-bridge submodule is obtained; According to the number of submodules currently put into use, the Thevenin model equivalent resistance of the half-bridge submodule in the converter to be analyzed is calculated, and the controlled voltage source is calculated using the voltage value at the previous moment, the current value at the previous moment and the Thevenin model equivalent resistance; Based on the discretized adjoint model, the voltage value at the current moment is updated using the current value at the current moment, the controlled voltage source and the Thevenin model equivalent resistance to obtain an updated voltage value, and the current value at the current moment is updated to obtain an updated current value; The updated voltage value and the updated current value are stored in a storage array, the updated voltage value is determined to be the voltage value at the previous moment, the updated current value is determined to be the current value at the previous moment, and the step of calculating the Thevenin model equivalent resistance of the half-bridge sub-module in the converter to be analyzed according to the number of sub-modules currently put into use continues to be updated until a preset step length is reached to obtain the Thevenin equivalent model of the bridge arm, and the Thevenin equivalent model of the bridge arm is simulated according to the topological structure and the system parameters to obtain the voltage stress and current stress of the IGBT under normal conditions.

2. The method for determining the voltage and current of the IGBT of the flexible DC transmission engineering submodule according to claim 1, characterized in that: The differential equation based on the capacitor element is used to obtain a discretized adjoint model of the capacitor in the half-bridge submodule, including: Discretization is performed based on the differential equation of capacitance and the trapezoidal formula to obtain a discretized adjoint model, wherein the calculation formula of the discretized adjoint model is: In the formula, u n+1 is the voltage value at time n+1, u n is the voltage value at time n, i n+1 is the current value at time n+1, i n is the current value at time n, C is the capacitance value, Δt represents the step size, R Ceq is the equivalent resistance of the Thevenin model, E Ceq is a controlled voltage source.

3. The method for determining the voltage and current of the IGBT of the flexible DC power transmission engineering submodule according to claim 1, characterized in that: The number of submodules currently put into use is obtained by modulating the bridge arm reference voltage value, including: Modulate the reference voltage value of the bridge arm to obtain the current number of submodules put into use by the bridge arm; Determine whether the number of submodules currently put into the bridge arm changes. If so, determine whether the current bridge arm current is greater than or equal to 0. If so, put submodules whose voltage is less than or equal to a preset voltage value into the current submodules put into the bridge arm. If so, put submodules whose voltage is greater than a preset voltage value into the current submodules put into the bridge arm to obtain the number of submodules currently put into operation. If there is no change, the sub-modules put into operation at the previous moment are used to obtain the number of sub-modules put into operation currently.

4. The method for determining the voltage and current of the IGBT of the flexible DC transmission engineering submodule according to claim 3, characterized in that: The step of modulating the bridge arm reference voltage value to obtain the current number of submodules put into operation in the bridge arm includes: According to the voltage whose difference with the reference voltage value of the bridge arm is less than the preset value, the number of submodules currently put into the upper bridge arm and the number of submodules currently put into the lower bridge arm are calculated using the submodule number calculation formula, wherein the submodule number calculation formula is: Where N p N is the number of submodules used in the upper bridge arm. n is the number of submodules put into the lower bridge arm, N is the number of submodules included in the upper bridge arm and the lower bridge arm, round(x) represents the integer closest to x, u0 is the phase voltage reference value, U c is the average DC voltage of the submodule; The current number of submodules invested in the bridge arm is obtained according to the current number of submodules invested in the upper bridge arm and the current number of submodules invested in the lower bridge arm.

5. The method for determining the voltage and current of the IGBT of the flexible DC power transmission engineering submodule according to claim 1, characterized in that: Also includes: Simulating the bridge arm Thevenin equivalent model under system fault to obtain the voltage stress and current stress of the IGBT under system fault; The number of upper bridge arm submodules and lower bridge arm submodules in the bridge arm Thevenin equivalent model is adjusted to obtain an adjusted bridge arm Thevenin equivalent model, and according to the adjusted bridge arm Thevenin equivalent model, a simulation of IGBT failure under system fault is performed to obtain the voltage stress and current stress of the IGBT after the IGBT fails; The IGBT failure state is obtained according to the voltage stress and current stress of the submodule in the normal state, the voltage stress and current stress of the submodule under the system fault, and the voltage stress and current stress of the submodule after the IGBT fails, so that personnel can give a fault warning to the converter to be analyzed according to the failure state.

6. A device for determining IGBT voltage and current of a flexible DC transmission engineering submodule, characterized in that: include: An acquisition module is used to acquire the topological structure and system parameters of the converter to be analyzed, and obtain a discretized adjoint model of the capacitor in the half-bridge submodule based on the differential equation of the capacitor element; A first calculation module is used to calculate the Thevenin model equivalent resistance of the half-bridge submodule in the converter to be analyzed according to the number of submodules currently put into use, and calculate the controlled voltage source using the voltage value at the previous moment, the current value at the previous moment and the Thevenin model equivalent resistance; A second calculation module is used to update the voltage value at the current moment based on the discretized adjoint model, using the current value at the current moment, the controlled voltage source and the Thevenin model equivalent resistance to obtain an updated voltage value, and to update the current value at the current moment to obtain an updated current value; The first simulation module is used to store the updated voltage value and the updated current value in a storage array, determine that the updated voltage value is the voltage value at the previous moment, and the updated current value is the current value at the previous moment, and proceed to the step of calculating the Thevenin model equivalent resistance of the half-bridge sub-module in the converter to be analyzed according to the number of sub-modules currently put into use, and continue to update until a preset step size is reached to obtain the Thevenin equivalent model of the bridge arm, and simulate the Thevenin equivalent model of the bridge arm according to the topological structure and the system parameters to obtain the voltage stress and current stress of the IGBT under normal conditions.

7. The device for determining IGBT voltage and current of a flexible DC power transmission engineering submodule according to claim 6, characterized in that: The acquisition module includes a discretized adjoint model calculation unit, wherein: The discretized adjoint model calculation unit is used to perform discretization based on the differential equation of capacitance and the trapezoidal formula to obtain a discretized adjoint model, wherein the calculation formula of the discretized adjoint model is: In the formula, u n+1 is the voltage value at time n+1, u n is the voltage value at time n, i n+1 is the current value at time n+1, i n is the current value at time n, C is the capacitance value, Δt represents the step size, R Ceq is the equivalent resistance of the Thevenin model, E Ceq is a controlled voltage source.

8. The device for determining IGBT voltage and current of a flexible DC power transmission engineering submodule according to claim 6, characterized in that: The first calculation module includes a modulation unit, a first judgment unit and a second judgment unit, wherein: The modulation unit is used to modulate the reference voltage value of the bridge arm to obtain the current number of submodules invested in the bridge arm; The first judgment unit is used to judge whether the number of submodules currently put into the bridge arm has changed. If it has changed, it is judged whether the current bridge arm current is greater than or equal to 0. If it is greater than or equal to 0, the submodules of the current submodules put into the bridge arm that are less than or equal to the preset voltage value are put into use. If it is less than 0, the submodules of the current submodules put into the bridge arm that are greater than the preset voltage value are put into use to obtain the number of submodules currently put into use; The second judgment unit is used to obtain the number of sub-modules currently put into use by using the sub-modules put into use by the bridge arm at the previous moment if there is no change.

9. The device for determining IGBT voltage and current of a flexible DC power transmission engineering submodule according to claim 8, characterized in that: The modulation unit includes a submodule quantity calculation subunit and a bridge arm input submodule calculation subunit, wherein: The submodule quantity calculation unit is used to calculate the number of submodules currently put into the upper bridge arm and the number of submodules currently put into the lower bridge arm according to the voltage whose difference with the bridge arm reference voltage value is less than the preset value, using the submodule quantity calculation formula, wherein the submodule quantity calculation formula is: Where N p N is the number of submodules used in the upper bridge arm. n is the number of submodules put into the lower bridge arm, N is the number of submodules included in the upper bridge arm and the lower bridge arm, round(x) represents the integer closest to x, u0 is the phase voltage reference value, U c is the average DC voltage of the submodule; The bridge arm investment submodule calculation subunit is used to obtain the current number of submodules invested in the bridge arm according to the current number of submodules invested in the upper bridge arm and the current number of submodules invested in the lower bridge arm.

10. The device for determining IGBT voltage and current of a flexible DC power transmission engineering submodule according to claim 6, characterized in that: The flexible DC power transmission engineering submodule IGBT voltage and current determination device also includes a second simulation module, a third simulation module and a fourth simulation module. The second simulation module is used to simulate the bridge arm Thevenin equivalent model under system fault to obtain the voltage stress and current stress of the IGBT under system fault; The third simulation module is used to adjust the number of upper bridge arm submodules and lower bridge arm submodules in the bridge arm Thevenin equivalent model to obtain an adjusted bridge arm Thevenin equivalent model, and simulate the failure of the IGBT under system fault according to the adjusted bridge arm Thevenin equivalent model to obtain the voltage stress and current stress of the IGBT after the IGBT fails; The fourth simulation module is used to obtain the IGBT failure state based on the voltage stress and current stress of the submodule under the normal state, the voltage stress and current stress of the submodule under the system fault, and the voltage stress and current stress of the submodule after the IGBT fails, so that personnel can provide fault warning for the converter to be analyzed based on the failure state.

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