Thevenin equivalent modeling method for full-bridge modular multilevel converter with energy release branch

Abstract

The invention relates to the technical field of electric power simulation, and discloses a Thevenin equivalent modeling method for a full-bridge modular multilevel converter with an energy release branch. According to the invention, trapezoidal integral discretization is carried out on capacitors in sub-modules, and binary resistance is carried out on a switch group formed by IGBT power tubes and anti-parallel diodes, so that an adjoint circuit only containing resistors and a voltage source is established; performing Thevenin equivalence on the adjoint circuit to obtain a Thevenin equivalent model of the sub-module; and performing algebraic superposition on the equivalent models of the N sub-modules of one bridge arm to finally obtain a Thevenin equivalent circuit of the whole bridge arm. The method is characterized in that aiming at the problem of curse of dimensionality during simulation of hundreds of levels of modular multilevel converters, the method can greatly reduce the number of nodes and improve the simulation efficiency, internal information is not lost, and the simulation precision is high.

Description

Technical Field

[0001] The invention relates to a Thevenin equivalent modeling method for a full-bridge modular multi-level converter including an energy dissipation branch, and belongs to the technical field of power system simulation. Background Art

[0002] In recent years, with the continuous breakthroughs in technologies such as floating wind power, deep-sea wind power is gradually promoting the development of offshore wind power from nearshore to offshore with its advantages of abundant renewable energy, high utilization hours, and no occupation of traffic channels. Flexible DC transmission technology has become the optimal technical solution for large-scale, long-distance offshore wind power to achieve grid connection due to its key technical advantages such as no commutation failure, active and reactive decoupling, and fast and flexible response.

[0003] When offshore wind power is connected to the grid using flexible DC transmission technology, there is a problem of overvoltage on the DC bus due to faults in the AC system at the receiving end. When a temporary grounding or short circuit fault occurs in the AC system at the receiving end, the active power transmission capacity at the receiving end will be greatly reduced. Since wind power has the characteristics of a large inertia power source, the power sent by the wind farm cannot be cut off in a short time, and there will be a power difference between the transmission power at the sending end and the grid-connected power at the receiving end. A large amount of power surplus accumulates in the flexible DC system, which will charge the sub-module capacitors and DC cable parasitic capacitors in the modular multilevel converter, thereby causing the DC bus voltage to increase. If measures are not taken in time, it will lead to overvoltage, and even serious faults such as converter valve locking and wind farm decomposition.

[0004] In order to maintain the stability of the DC voltage of the offshore wind power transmission system during the AC side fault at the receiving end and achieve smooth crossing under power grid faults, a DC energy dissipation device with flexible and controllable power surplus absorption has become a key technical equipment. The DC energy dissipation device is composed of an energy dissipation valve and an energy dissipation resistor. The energy dissipation valve contains a large number of power devices. According to the action characteristics of the power devices and the arrangement of the energy dissipation resistors, it can be divided into three categories: centralized, distributed and hybrid. Among them, some scholars have proposed a full-bridge sub-module topology with an energy dissipation branch. A switch group composed of an IGBT power tube and an anti-parallel diode and an energy dissipation resistor in series is added to the sub-module. The energy dissipation device is added to the sub-module of the converter, which can use the same cooling system as the sub-module, and can effectively control the capacitor voltage ripple, further reduce the capacitor volume and construction cost, and has good engineering application prospects.

[0005] Different from the classic submodules in modular multilevel converters, after the new topology of full-bridge submodules with energy dissipation branches is proposed, the original equivalent modeling method cannot be applied to the new topology. As the new energy power system shows a development trend of high voltage and large capacity, modular multilevel converters in actual projects need to cascade a large number of submodules, and their number has a trend of increasing. When building its detailed simulation model on simulation software platforms such as PSCAD / EMTDC, it is necessary to process a large number of switching devices, the model matrix has a high order, and the calculation is heavy, which seriously affects the simulation efficiency and makes it difficult to meet the needs of early project design and debugging.

[0006] In summary, there is an urgent need to conduct efficient electromagnetic transient modeling research on full-bridge modular multilevel converters with energy dissipation branches. Summary of the invention

[0007] The present invention provides a Thevenin equivalent modeling method for a full-bridge modular multi-level converter including an energy dissipation branch, and the establishment of the method comprises the following steps:

[0008] Step 1: Starting from a single full-bridge submodule with an energy dissipation branch, a companion circuit containing only resistors and voltage sources is constructed. The switch group consisting of IGBT power tubes and reverse parallel diodes is equivalent to a binary resistor, and the submodule capacitance is discretized using the trapezoidal integration method.

[0009] Step 2: According to the Thevenin theorem, perform Thevenin equivalent on the adjoint circuit to obtain the Thevenin model equivalent voltage source u of a single submodule SMEQ_k And the equivalent resistance R SMEQ_k value.

[0010] Step 3: Perform algebraic superposition on the N sub-module Thevenin equivalent models in each phase bridge arm to obtain the Thevenin circuit equivalent voltage source u of the bridge arm ARMEQ And the equivalent resistance R ARMEQ value.

[0011] Step 4: Connect the bridge arm Thevenin equivalent circuit in series into the modular multilevel converter bridge arm, and obtain the parameters of the current step-length external circuit through electromagnetic transient simulation software, which are provided for the next step-length calculation. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 The topological structure diagram of the full-bridge submodule with energy dissipation branch

[0013] Figure 2 For accompanying circuit diagram

[0014] Figure 3 Thevenin equivalent circuit diagram

[0015] Figure 4 The equivalent circuit diagram of the bridge arm

[0016] Figure 5 Schematic diagram of the simulation process DETAILED DESCRIPTION

[0017] The present invention provides a Thevenin equivalent modeling method for a full-bridge modular multi-level converter including an energy dissipation branch. The modeling steps of the present invention will be further described in detail below.

[0018] Step 1:

[0019] The full-bridge submodule topology with energy dissipation branch is as follows: Figure 1 As shown. The submodule consists of a DC energy storage capacitor C, 2 resistors and 5 switch groups consisting of IGBT power tubes and reverse parallel diodes. Among them, the energy dissipation branch is composed of a switch group consisting of IGBT power tube T5 and reverse parallel diode D5 in series with R6 energy dissipation resistor, which is connected in parallel on both sides of the capacitor. At the same time, a large resistor of tens of kilo-ohms will be connected in parallel on both sides of the capacitor in the project. The switch states of T1-T4 are jointly determined by the nearest level approximation modulation and capacitor voltage equalization control algorithm, and the switch state of T5 is independently determined by the energy dissipation branch control module. The working principle of the energy dissipation branch is: when it is detected that the capacitor voltage of the submodule exceeds the manually set upper limit value, T5 is turned on, R6 energy dissipation resistor is put into operation, absorbs the surplus energy of the DC energy storage capacitor, and the capacitor voltage begins to drop; when the voltage drops to the manually set lower limit value, T5 is turned off, R6 energy dissipation resistor exits operation, and the submodule resumes normal operation.

[0020] In order to perform Thevenin equivalence on the full-bridge submodule including the energy dissipation branch, the circuit can only contain resistors and voltage sources. Therefore, the switch group composed of the IGBT power tube and the anti-parallel diode and the capacitor need to be processed accordingly.

[0021] The switch group can be equivalent to a binary resistor, that is, it can be regarded as a variable resistor that switches between high and low resistance values. When the switch group IGBT control signal Ti (i = 1, 2, 3, 4, 5) is turned on, the resistance value R i Equal to the low resistance R ON (Usually 10 -3 Ω); when the switch group IGBT control signal T i When off, the resistance value R i Equal to the high resistance R OFF (Usually 10 6 Ω). Therefore, the five switch groups in the full-bridge submodule containing the energy dissipation branch can be equivalent to variable binary resistors, and their resistances are determined by the corresponding IGBT control signals, as shown in formula (1).

[0022]

[0023] The capacitor uses the widely used trapezoidal integration method to construct a discrete adjoint equivalent circuit. The differential equation of the capacitor is:

[0024]

[0025] Where: u c (t) is the voltage across the capacitor; i c (t) is the current flowing through the capacitor; C is the capacitance value.

[0026] Integrating formula (2), we can get:

[0027]

[0028] Where: Δt is the simulation step length.

[0029] Using the trapezoidal integration method to discretize equation (3), we can obtain:

[0030]

[0031] make:

[0032]

[0033] u CEQ (t-Δt)=u c (t)-R C ×i c (t)(6)

[0034] Substituting equation (5) and equation (6) into equation (4), we can get:

[0035] u CEQ (t-Δt)=u c (t-Δt)+R C ×i c (t-Δt) (7)

[0036] In summary, the accompanying circuit of the full-bridge submodule with energy dissipation branch can be obtained, as Figure 2 As shown. Among them, the variable resistors R1, R2, R3, R4, and R5 are equivalent to a switch group, and the equivalent resistor R C and the equivalent voltage source u CEQ They are collectively equivalent to the submodule capacitance, and their values ​​are all functions of time, as shown in equations (1), (5), and (7).

[0037] Step 2:

[0038] Figure 2 is a full-bridge submodule accompanying circuit with an energy dissipation branch. According to Kirchhoff’s voltage and current laws, the capacitor current i can be obtained. C (t) and energy dissipation branch current i ch(t):

[0039]

[0040] The current i in the capacitor equivalent branch C (t) can be used to update the submodule capacitor voltage and energy dissipation branch current i in each simulation step. ch (t) can be used to detect the working condition of the energy release branch and calculate the energy consumed.

[0041] By converting the switch group into binary resistors and discretizing the capacitor trapezoidal integral, a full-bridge submodule companion circuit consisting of only resistors and voltage sources with an energy dissipation branch is obtained, which meets the conditions for performing Thevenin equivalence. Performing Thevenin equivalence on its companion circuit, the Thevenin equivalent circuit of the full-bridge submodule with an energy dissipation branch can be obtained, as shown in Figure 3 As shown. Among them, the equivalent resistance R SMEQ and the equivalent voltage source u SMEQ It can be calculated by formula (10) and formula (11).

[0042]

[0043] For equations (8) to (11), we have:

[0044]

[0045] Step 3:

[0046] Connect the N sub-module Thevenin equivalent models in series to obtain the Thevenin equivalent circuit of the entire bridge arm, as shown in Figure 4 As shown in the figure, u C and T SM They are the N submodule capacitor voltages output by the bridge arm and the submodule trigger signal input by the controller to the bridge arm. The overall Thevenin equivalent resistance of the bridge arm R ARMEQ and u ARMEQ From equations (13) and (14), we can get:

[0047]

[0048] Where: R SMEQ＿k and u SMEQ＿k (k=1, 2, ..., N) are the Thevenin equivalent resistance and voltage of the kth submodule respectively.

[0049] Step 4:

[0050] The solution process of the Thevenin equivalent model of the full-bridge modular multilevel converter with energy dissipation branch is divided into two parts: forward solution and inverse solution. The simulation process is as follows: Figure 5 shown.

[0051] When the answer is correct, check the kth submodule T1k -T4 k The conduction state of the switch group and the capacitor voltage u SM_k . When u SM_k Greater than the artificial upper limit value U max When u SM_k Less than the artificially set lower limit U min , turn off the energy dissipation branch switch group, and the energy dissipation resistor exits operation. According to the switch states of each submodule and formula (10) and formula (11), the Thevenin model equivalent voltage source u of a single submodule is obtained: SMEQ_k And the equivalent resistance R SMEQ_k Using equations (13) and (14) to perform algebraic superposition on the N submodules of a phase bridge arm, we can obtain the equivalent voltage source u of the bridge arm Thevenin model. ARMEQ And the equivalent resistance R ARMEQ value.

[0052] When inversely solving, the Thevenin equivalent circuit of the bridge arm is connected in series to the bridge arm of the modular multilevel converter, and the relevant parameters of the external circuit of the current step length are obtained through the electromagnetic transient simulation software, which provides the positive solution for the next step length.

[0053] The Thevenin equivalent modeling method of a full-bridge modular multi-level converter with an energy dissipation branch proposed in the present invention has high accuracy under steady-state and transient conditions and significant acceleration effect.

[0054] Finally, it should be noted that the described embodiments are only part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

Claims

1. Thevenin equivalent modeling method for full-bridge modular multilevel converter with energy dissipation branch. It is characterized by: Starting from a single submodule, the present invention adopts binary resistance equivalence for the switch group composed of IGBT power tube and reverse parallel diode, discretizes the submodule capacitance by trapezoidal integration method, constructs a companion circuit containing only resistance and voltage source, and obtains the Thevenin equivalent circuit according to Thevenin and superposition theorem. The proposed method has significant efficiency improvement while ensuring high accuracy. The method mainly includes the following steps: Step 1: Construct the accompanying circuit of the full-bridge submodule with the energy dissipation branch. The switch group composed of the IGBT power tube and the reverse parallel diode can be regarded as a variable resistor that switches between high and low resistance values, and the capacitor uses the widely used trapezoidal integration method to construct a discrete accompanying equivalent circuit. Step 2: According to the Thevenin theorem, perform Thevenin equivalence on the adjoint circuit to obtain the Thevenin equivalent model of a single submodule. Step 3: Perform algebraic superposition on the N sub-module Thevenin equivalent models in each phase bridge arm to obtain the Thevenin equivalent circuit of the bridge arm. Step 4: Connect the bridge arm Thevenin equivalent circuit in series into the modular multilevel converter bridge arm and solve it using electromagnetic transient simulation software to initialize the electrical network for the next simulation step.

2. The Thevenin equivalent modeling method for a full-bridge modular multilevel converter with an energy dissipation branch according to claim 1, characterized in that: From step 1 to step 4, the previous step is the basis for the execution of the next step. These four modeling steps are closely linked to each other and executed sequentially, forming an organic and indivisible whole.

Images