Equivalent modeling method for half-bridge MMC (Modular Multilevel Converter) containing energy release branch

Abstract

The invention discloses an equivalent modeling method for a half-bridge MMC (Modular Multilevel Converter) containing an energy release branch. By analyzing the topological structure and the working principle of the half-bridge MMC containing the energy release branch, the function of suppressing capacitor voltage ripples by using the energy release branch is expanded, a half-bridge MMC model is established, and meanwhile, related system key parameters of the energy release branch are determined. On the basis, a Thevenin equivalent method is adopted, the half-bridge type sub-module is equivalent to an adjoint circuit by adopting a binary resistor and a trapezoidal integral method, then the adjoint circuit is equivalent to a Thevenin circuit by adopting a Thevenin theorem, and finally the Thevenin circuits of all the sub-modules on the bridge arm are connected in series to obtain a bridge arm equivalent circuit.

Description

Technical Field

[0001] The present invention relates to the technical field of high-voltage flexible direct current power transmission, and in particular to an equivalent modeling method of a half-bridge modular multilevel converter (MMC) with an energy dissipation branch. Background Art

[0002] In recent years, in order to achieve the "dual carbon" goal and realize my country's energy and power transformation, a new power system with new energy as the main body has developed rapidly. Modular multilevel converter (MMC) has been widely used in the field of flexible DC transmission projects because of its advantages such as low output voltage harmonic content, low current distortion rate, convenient expansion, low switching loss, and strong fault ride-through capability. Half-bridge MMC uses fewer components, is highly economical, and is simple to control. At present, most of the modular multilevel converters used in most of the flexible DC transmission projects put into operation in my country are half-bridge structures.

[0003] In a flexible DC transmission system, when a ground fault occurs in the AC grid at the receiving end, the voltage on the AC side of the receiving end drops, the transmission capacity of active power is greatly reduced, and the sending end cannot be adjusted quickly, resulting in a power surplus in the system, which in turn increases the bus voltage on the DC side. If not resolved in a timely manner, the flexible DC transmission system will not be able to operate normally, which may cause a large area of ​​wind turbines to be disconnected from the grid, further causing the entire DC transmission system to fail, thereby greatly reducing the availability of the flexible DC transmission system. Therefore, the system needs to have a flexible and controllable energy-consuming device that consumes the surplus power for fault crossing.

[0004] Different from the classic half-bridge submodule, after the new topology with energy dissipation branch is proposed, the traditional equivalent models in simulation platforms such as PSCAD and RTDS cannot be simulated, and the simulation of the new submodule cannot be achieved by changing the external parameters. Therefore, it is necessary to re-target the new topology, according to the equivalent principle, starting from the port, derive a detailed analytical model of the equivalent circuit of the half-bridge submodule with energy dissipation branch, and then establish a simulation model, so as to build the corresponding two-terminal, bipolar half-bridge MMC model. Summary of the invention

[0005] The purpose of the present invention is to provide a half-bridge MMC equivalent modeling method containing an energy dissipation branch. The method is simple and effective, the modeling method is flexible, and the number of modules is easy to modify. While successfully realizing the energy dissipation branch function, the simulation accuracy is high and the simulation efficiency can be significantly improved, saving simulation time.

[0006] The objective of the present invention is achieved through the following technical solutions:

[0007] A half-bridge MMC equivalent modeling method including an energy dissipation branch, the method comprising:

[0008] Step 1, studying the topological structure of the half-bridge submodule with the energy dissipation branch, studying the working principle of the energy dissipation branch, and establishing a detailed model of the half-bridge MMC with the energy dissipation branch;

[0009] Step 2: According to the model established in step 1, determine the model parameters such as the submodule capacitance and the energy dissipation branch control threshold in the system;

[0010] Step 3: Using the Thevenin theorem to perform equivalence, firstly, the half-bridge submodule is equivalent to the adjoint circuit by using binary resistors and the trapezoidal integration method, and then the Thevenin equivalent method is used to convert it into the Thevenin equivalent circuit;

[0011] Step 4: Based on the Thevenin equivalent circuit of the submodule obtained in step 3, N submodules are connected in series to obtain the Thevenin equivalent circuit of the bridge arm.

[0012] It can be seen from the technical solution provided by the present invention that the method is simple and effective, the modeling method is flexible, the number of modules is easy to modify, and while successfully realizing the energy discharge branch function, the simulation accuracy is high and the simulation efficiency can be significantly improved, saving simulation time. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without paying creative work.

[0014] Figure 1 It is a flow chart of the equivalent modeling method of a half-bridge MMC including an energy dissipation branch;

[0015] Figure 2 It is a half-bridge MMC system topology with an energy dissipation branch;

[0016] Figure 3 It is a topological structure diagram of the traditional half-bridge and the half-bridge sub-module with energy dissipation branch;

[0017] Figure 4 This is the flow chart of the control principle of the energy release branch;

[0018] Figure 5 It is the equivalent circuit of a half-bridge submodule including an energy dissipation branch;

[0019] Figure 6 is the Thevenin equivalent circuit of the half-bridge arm including the energy dissipation branch; DETAILED DESCRIPTION

[0020] The following is a clear and complete description of the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings 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, which does not constitute a limitation of the present invention. 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.

[0021] like Figure 1 The figure shows a flow chart of a method for equivalent modeling of a half-bridge MMC with an energy dissipation branch provided by an embodiment of the present invention, the method comprising:

[0022] Step 1: Study the topological structure of the half-bridge submodule with the energy dissipation branch, study the working principle of the energy dissipation branch, and establish a detailed model of the half-bridge MMC with the energy dissipation branch:

[0023] In this step, if Figure 2 The figure shows the topological structure of the half-bridge MMC system according to the embodiment of the present invention. The converter is composed of three phase units. Each phase includes two bridge arms, an upper bridge arm and a lower bridge arm. Each bridge arm is composed of N half-bridge sub-modules connected in series. At the same time, the bridge arm also includes a bridge arm reactor L s ;like Figure 3 (a) shows a schematic diagram of the conventional half-bridge submodule topology structure, which consists of two IGBTs, an anti-parallel diode and a DC capacitor C. Figure 3 (b) shows a schematic diagram of the topological structure of a half-bridge submodule with an energy dissipation branch. Compared with the traditional structure, the new topology adds a branch consisting of an IGBT switch group and an energy-consuming resistor in series, which is connected in parallel on both sides of the capacitor. At the same time, the submodule in the project will have a large resistor connected in parallel on both sides of the capacitor. The large resistor is generally tens of kilo-ohms.

[0024] The working principle of the energy discharge branch is that when the capacitor voltage is too high, the IGBT of the energy discharge branch is turned on and the energy discharge branch is put into operation. Because it is connected in parallel at both ends of the DC capacitor, the DC capacitor will discharge, the energy discharge resistor begins to absorb power, and the capacitor voltage gradually decreases; when the capacitor voltage drops to a certain level, the IGBT of the energy discharge branch is turned off, the energy discharge branch exits operation, and the MMC resumes normal operation.

[0025] When a fault occurs on the receiving AC side of the offshore wind power flexible direct current transmission system, the surplus power cannot be transmitted, causing the DC side voltage to increase, and the sub-module voltage also increases accordingly. When the sub-module voltage increases to a certain threshold, the IGBT of the energy dissipation branch is turned on, and the energy dissipation resistor begins to consume energy. Therefore, in the transient stage of the fault, the sub-module voltage can be limited to below a set threshold to prevent the sub-module from overvoltage and causing serious faults such as device damage and wind turbine disconnection from the grid.

[0026] Limiting the transient overvoltage of the submodule is the starting point of the new topology. However, because it can realize the "peak clipping" function of the submodule capacitor voltage, its function can be extended to the steady-state operation stage. When the MMC is in steady-state operation, the ripple of the capacitor voltage is an important factor affecting the operating characteristics and parameter design of the MMC. In order to limit the capacitor voltage ripple within a safe range, the commonly used method is to increase the capacitance of the submodule capacitor, but this method will increase the cost and volume of the MMC. Another method is to inject appropriate bridge arm circulating current or third harmonic voltage to reduce the power fluctuation of the bridge arm. In the topology containing the energy discharge branch, the role of the energy discharge branch can well limit the amplitude of the capacitor voltage ripple. Therefore, in steady state, the control of the energy discharge branch is put into use, which can limit the capacitor voltage ripple within the desired range, so as to reduce the capacitance of the capacitor, save the manufacturing cost of the MMC, and reduce the volume.

[0027] Based on the above functions of the energy release branch under transient and steady-state conditions, the control principle flow chart of the energy release branch is as follows: Figure 4 As shown, U set It is the user set value. It can be set to a lower value in steady state to limit the upper limit of capacitor voltage ripple. At this time, there is still U set >U N , can be set to a higher value in transient state to limit capacitor overvoltage and achieve fault ride-through; U N is the submodule capacitor voltage rating.

[0028] After completing the study of the topological structure and working principle of the energy dissipation branch, a detailed model of the half-bridge MMC is established based on the topological structure and working principle.

[0029] Step 2: According to the model established in step 1, determine the model parameters such as the submodule capacitance and the energy dissipation branch control threshold in the system:

[0030] In order to make the model parameters closer to the engineering, the rated voltage of the submodule capacitor is set to 2.1kV, and the capacitance is set to 18mF. At this time, in order to suppress the overvoltage of the submodule capacitor in transient state, the upper threshold of the energy discharge branch is set to 2.8kV, the lower rated voltage is set to 2.1kV, the energy discharge resistance of the energy discharge branch is 2Ω, and the resistance value of the parallel large resistance branch is 30kΩ.

[0031] Step 3: Use Thevenin theorem to equalize. First, use binary resistors and trapezoidal integration method to equalize the half-bridge submodule, convert it into a companion circuit, and then use Thevenin equivalent method to equalize it into a Thevenin equivalent circuit:

[0032] The topology of the half-bridge submodule (HBSM) with energy dissipation branch is as follows: Figure 3As shown in (b), in order to perform Thevenin equivalence, the circuit can only contain resistors, voltage sources or current sources. Therefore, the IGBT switch group and capacitor need to be treated equivalently.

[0033] Each IGBT switch group in the topology can be regarded as a binary resistor that switches between high and low resistance values. When the switch group is turned on, the resistance value is low resistance R ON (Usually 10 -3 Ω); when the switch group is turned off, the resistance value is high resistance R OFF (Usually 10 6 Ω). At this time, the three IGBT switch groups can be equivalent to two resistors R 1 , R 2 , R 3 The resistance value of the resistor is determined by the corresponding IGBT control signal, and its formula is shown in formula (1), where T i It is the IGBT control signal.

[0034]

[0035] While making the IGBT switch group equivalent, the capacitor also needs to be discretized so that it can be modeled in the electromagnetic transient simulation environment. This paper uses the trapezoidal integration method to discretize the capacitor. Finally, the accompanying circuit of a single submodule is obtained as follows Figure 5 (a) where the equivalent resistance R C And the equivalent voltage source U CEQ is the result after the capacitor is discretized, and the calculation formula is shown in equation (2) and equation (3). The current I of the capacitor equivalent branch C_HB The calculation formula of (t) is as shown in formula (4), which is used to update the submodule capacitor voltage in each simulation step. ch_HB The calculation formula of (t) is as shown in formula (5), which can be used to detect the working condition of the energy discharge branch and calculate the consumed energy data.

[0036]

[0037] U CEQ (t-ΔT)=U C (t-ΔT)+R C I C (t-ΔT) (3)

[0038]

[0039] After obtaining the companion circuit of the half-bridge submodule, the circuit only contains resistors and voltage sources, which meets the conditions for Thevenin equivalence. The companion circuit is subjected to Thevenin equivalence and converted into Figure 5(b) The Thevenin equivalent circuit of the submodule shown in Figure 1, where the equivalent resistance R SMEQ_HB And the equivalent voltage source U SMEQ_HB Calculated by formula (6) and formula (7). In formula (6), R 1 and R 2 is a variable with two states, so R SMEQ_HB It is also a time variable.

[0040]

[0041] For equations (4) to (7), we have:

[0042]

[0043] Step 4: Based on the Thevenin equivalent circuit of the submodule obtained in step 3, N submodules are connected in series to obtain the Thevenin equivalent circuit of the bridge arm:

[0044] The N in a single bridge arm of the half-bridge MMC 1 The equivalent circuits of the sub-modules are connected in series to obtain the Thevenin equivalent circuit of the entire bridge arm as follows: Figure 6 As shown, U C_HB and T SM_HB They are the N outputs of the bridge arms. 1 The voltage of the submodule capacitor and the input bridge arm N 1 The switch group control signal of each sub-module. Figure 6 Thevenin equivalent resistance R of the middle bridge arm ARMEQ_HB and U ARMEQ_HB The calculation formulas are shown in equations (8) and (9).

[0045]

[0046] In the formula, R SMEQ_HB_i and U SMEQ_HB_i They are respectively Figure 5 (b) shows the Thevenin equivalent resistance and voltage of the i-th submodule, namely Figure 6 The bridge arm equivalent circuit in depends on N 1 The conduction state and instantaneous voltage information of each sub-module.

[0047] In summary, the purpose of the present invention is to provide a half-bridge MMC equivalent modeling method with an energy dissipation branch. The method is simple and effective, the modeling method is flexible, and the number of modules is easy to modify. While successfully realizing the energy dissipation branch function, the simulation accuracy is high and the simulation efficiency can be significantly improved, saving simulation time.

[0048] It is worth noting that the contents not described in detail in the embodiments of the present invention belong to the prior art known to professional and technical personnel in the field.

[0049] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by any technician familiar with the technical field within the technical scope disclosed in the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims. The information disclosed in the background technology section of this article is only intended to deepen the understanding of the overall background technology of the present invention, and should not be regarded as an admission or in any form that the information constitutes prior art known to those skilled in the art.

Claims

1. A half-bridge MMC equivalent modeling method with an energy dissipation branch, the method comprising: Step 1, studying the topological structure of the half-bridge submodule with the energy dissipation branch, studying the working principle of the energy dissipation branch, and establishing a detailed model of the half-bridge MMC with the energy dissipation branch; Step 2: According to the model established in step 1, determine the model parameters such as the submodule capacitance and the energy dissipation branch control threshold in the system; Step 3: Using the Thevenin theorem to perform equivalence, firstly, the half-bridge submodule is equivalent to the adjoint circuit by using binary resistors and the trapezoidal integration method, and then the Thevenin equivalent method is used to convert it into the Thevenin equivalent circuit; Step 4: Based on the Thevenin equivalent circuit of the submodule obtained in step 3, N submodules are connected in series to obtain the Thevenin equivalent circuit of the bridge arm.

2. The equivalent modeling method of the half-bridge MMC with energy dissipation branch according to claim 1 is characterized in that: In step 1, the half-bridge MMC system topology diagram, the converter consists of three phase units, each phase includes two bridge arms, upper and lower, each bridge arm is composed of N half-bridge sub-modules in series, and the bridge arm also includes a bridge arm reactor L s ; As shown in Figure 3(a), it is a schematic diagram of the traditional half-bridge sub-module topology structure. The sub-module consists of two IGBTs, an anti-parallel diode and a DC capacitor C; as shown in Figure 3(b), it is a schematic diagram of the half-bridge sub-module topology structure with an energy dissipation branch. Compared with the traditional structure, the new topology adds a branch consisting of an IGBT switch group and an energy-consuming resistor in series, which is connected in parallel on both sides of the capacitor. At the same time, the sub-module in the project will have a large resistor connected in parallel on both sides of the capacitor. The large resistor is generally tens of kilo-ohms. The working principle of the energy discharge branch is that when the capacitor voltage is too high, the IGBT of the energy discharge branch is turned on and the energy discharge branch is put into operation. Because it is connected in parallel at both ends of the DC capacitor, the DC capacitor will discharge, the energy discharge resistor begins to absorb power, and the capacitor voltage gradually decreases; when the capacitor voltage drops to a certain level, the IGBT of the energy discharge branch is turned off, the energy discharge branch exits operation, and the MMC resumes normal operation. When a fault occurs on the receiving AC side of the offshore wind power flexible direct current transmission system, the surplus power cannot be transmitted, causing the DC side voltage to increase, and the sub-module voltage also increases accordingly. When the sub-module voltage increases to a certain threshold, the IGBT of the energy dissipation branch is turned on, and the energy dissipation resistor begins to consume energy. Therefore, in the transient stage of the fault, the sub-module voltage can be limited to below a set threshold to prevent the sub-module from overvoltage and causing serious faults such as device damage and wind turbine disconnection from the grid. Limiting the transient overvoltage of the submodule is the starting point of the new topology. However, because it can realize the "peak clipping" function of the submodule capacitor voltage, its function can be extended to the steady-state operation stage. When the MMC is in steady-state operation, the ripple of the capacitor voltage is an important factor affecting the operating characteristics and parameter design of the MMC. In order to limit the capacitor voltage ripple within a safe range, the commonly used method is to increase the capacitance of the submodule capacitor, but this method will increase the cost and size of the MMC. Another method is to inject appropriate bridge arm circulating current or third harmonic voltage to reduce the power fluctuation of the bridge arm. In the topology with the energy-draining branch, the energy-draining branch can effectively limit the amplitude of the capacitor voltage ripple. Therefore, in the steady state, the control of the energy-draining branch is put into use to limit the capacitor voltage ripple within the desired range, so as to reduce the capacitance value, save the manufacturing cost of the MMC, and reduce the volume. Based on the above functions of the energy release branch under transient and steady-state conditions, the control principle flow chart of the energy release branch is shown in Figure 4, where U set It is the user set value. It can be set to a lower value in steady state to limit the upper limit of capacitor voltage ripple. At this time, there is still U set >U N , can be set to a higher value in transient state to limit capacitor overvoltage and achieve fault ride-through; U N is the submodule capacitor voltage rating. After completing the study of the topological structure and working principle of the energy dissipation branch, a detailed model of the half-bridge MMC is established based on the topological structure and working principle.

3. The equivalent modeling method of a half-bridge MCC with an energy dissipation branch according to claim 2 is characterized in that: The process of step 2 is specifically as follows: In order to make the model parameters closer to the engineering, the rated voltage of the submodule capacitor is set to 2.1kV, and the capacitance is set to 18mF. At this time, in order to suppress the overvoltage of the submodule capacitor in transient state, the upper threshold of the energy discharge branch is set to 2.8kV, the lower rated voltage is set to 2.1kV, the energy discharge resistance of the energy discharge branch is 2Ω, and the resistance value of the parallel large resistance branch is 30kΩ.

4. The equivalent modeling method of a half-bridge MCC with an energy dissipation branch according to claim 2 is characterized in that: The process of step 3 is specifically as follows: The topology of the half-bridge submodule (HBSM) with the energy dissipation branch is shown in Figure 3(b). In order to perform the Thevenin equivalence, the circuit can only contain resistors, voltage sources or current sources. Therefore, the IGBT switch group and capacitor need to be treated equivalently. Each IGBT switch group in the topology can be regarded as a binary resistor that switches between high and low resistance values. When the switch group is turned on, the resistance value is low resistance R ON (Usually 10 -3 Ω); when the switch group is turned off, the resistance value is high resistance R OFF (Usually 10 6 Ω). At this time, the three IGBT switch groups can be equivalent to two resistors R1, R2, and R3 respectively. The resistance value of the resistor is determined by the corresponding IGBT control signal. The formula is shown in formula (1), where T i It is the IGBT control signal. While making the IGBT switch group equivalent, the capacitor also needs to be discretized so that it can be modeled in the electromagnetic transient simulation environment. This paper uses the trapezoidal integration method to discretize the capacitor. The accompanying circuit of a single submodule is finally obtained as shown in Figure 5(a), where the equivalent resistance R C And the equivalent voltage source U CEQ is the result after the capacitor is discretized, and the calculation formula is shown in equation (2) and equation (3). The current I of the capacitor equivalent branch C_HB The calculation formula of (t) is as shown in formula (4), which is used to update the submodule capacitor voltage in each simulation step. ch_HB The calculation formula of (t) is as shown in formula (5), which can be used to detect the working condition of the energy discharge branch and calculate the consumed energy data. U CEQ (t-ΔT)=U C (t-ΔT)+R C I C (t-ΔT) (3) After obtaining the companion circuit of the half-bridge submodule, the circuit only contains resistors and voltage sources, which meets the conditions for Thevenin equivalence. The companion circuit is subjected to Thevenin equivalence and converted into the Thevenin equivalent circuit of the submodule as shown in FIG5(b), where the equivalent resistance R SMEQ_HB And the equivalent voltage source U SMEQ_HB Calculated by equation (6) and equation (7). In equation (6), R1 and R2 are variables of two states, so R SMEQ_HB It is also a time variable. For equations (4) to (7), we have:

5. The equivalent modeling method of a half-bridge MCC with an energy dissipation branch according to claim 2 is characterized in that: The process of step 4 is specifically as follows: The N1 submodule equivalent circuits in a single bridge arm of the half-bridge MMC are connected in series to obtain the Thevenin equivalent circuit of the entire bridge arm as shown in Figure 6, where U C_HB and T SM_HB The Thévenin equivalent resistance R of the bridge arm in Figure 6 is respectively the capacitor voltage of the N1 submodules output by the bridge arm and the switch group control signal of the N1 submodules input by the bridge arm. ARMEQ_HB and U ARMEQ_HB The calculation formulas are shown in equations (8) and (9). In the formula, R SMEQ_HB_i and U SMEQ_HB_i They are respectively the Thevenin equivalent resistance and voltage of the i-th submodule as shown in FIG5( b ), that is, the bridge arm equivalent circuit in FIG6 depends on the conduction state and instantaneous voltage information of each of the N1 submodules.

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