A method for lockout equivalent simulation of dual active bridge converter

Abstract

The application relates to an equivalent simulation method under blocking modes of a dual active bridge (DAB) converter, which comprises the following steps: obtaining phase unit operation parameters of the dual active bridge converter, and performing blocking mode identification; when the identification result is a complete blocking mode, four actual diode branches for judging current directions are inserted into each phase unit; when the identification result is a partial blocking mode, the diode branches are bypassed, and the Thevenin / Norton equivalent parameters of the power module when the DAB is not electrified are calculated; and when the identification result is an unlocking mode, the diode branches are bypassed, and the Thevenin / Norton equivalent parameters of each power module when the DAB is electrified are calculated. The application utilizes state variable multiplexing and the internal mechanism of electromagnetic simulation, and the model can accurately simulate various blocking modes while improving simulation efficiency.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of power system simulation, and particularly relates to a blocking equivalent simulation method of a dual active bridge converter. BACKGROUND

[0002] The cascaded H-type power electronic transformer (CHB-PET) can flexibly provide a medium-voltage alternating-current port, a low-voltage alternating-current port, a low-voltage direct-current port and a low-voltage alternating-current port, and has a modular structure that is easy to expand, and has obvious advantages in microgrids, photovoltaic systems and other occasions that do not require a medium-voltage direct-current port. The isolation stage widely adopts a dual active bridge converter (DAB) composed of fully controlled devices.

[0003] The dual active bridge converter (CHB-PET) with a cascaded H-bridge front stage is usually combined in an input-series-output-parallel (ISOP) connection mode, and the power sub-module usually includes two-stage conversion links. System simulation modeling is the basis for studying the control characteristics of the dual active bridge converter. This type of topology has three typical characteristics: "high frequency", "isolated type" and "multi-module". The "high frequency" and "multi-module" make the simulation efficiency of the electromagnetic transient detailed model relatively low, which brings a great burden to the simulation calculation of the system. Therefore, some scholars have proposed equivalent simulation models based on the node splitting method or Thevenin / Norton theorem to speed up the simulation, but it is difficult to accurately simulate the interpolation of diodes, which makes it impossible to simulate the transient process in the blocking stage or obtain the initial value of the network state variable after the blocking state. In addition, current research generally only targets a one-stage power conversion link, and there is little research on multi-stage conversion PET topologies. CHB-PET is different from the one-stage conversion topology composed of cascaded H-bridges, and its blocking characteristics and blocking modes are also quite different. Therefore, it is necessary to propose a fast equivalent simulation method for the dual active bridge converter considering multiple blocking modes. SUMMARY

[0004] In view of the above technical problems existing in the prior art, the application provides an equivalent simulation method for a dual active bridge converter considering multiple blocking modes. The method equivalent the power module (PM) of this type of power electronic transformer to Thevenin / Norton equivalent branches whose values change with the blocking mode, and introduces actual diode elements, which can improve the simulation efficiency while accurately simulating multiple blocking modes.

[0005] The application discloses a lockout equivalent simulation method for a dual active bridge converter, and the dual active bridge converter comprises two-stage power conversion links, namely a cascaded H-bridge (CHB) stage and a dual active bridge (DAB) stage, the CHB stage and the DAB stage jointly form a CHB-DAB phase unit, an input port of the CHB stage adopts a connection form in which a plurality of full-controlled H-bridges are connected in series, an output port of the CHB stage is connected with the DAB stage through a capacitor, an input port of the DAB stage is connected with a capacitor, and an output port of the DAB stage is connected in parallel; wherein the full-controlled H-bridge comprises four IGBTs with anti-parallel diodes, the DAB comprises two full-controlled H-bridges, a high-frequency transformer and an additional inductor L1; and the equivalent modeling method comprises the following steps:

[0006] (1) obtaining phase unit operation parameters of the dual active bridge converter, wherein the phase unit operation parameters comprise switching states of IGBT switching tubes in each power module and a 2-bit binary signal indicating whether the cascaded H-bridge stage and the DAB stage in the power module are locked, and performing lockout mode identification;

[0007] (2) if it is identified that the mode is a complete lockout mode (the cascaded H-bridge and the dual active bridge are both locked), then four actual diode branches for judging current directions are added in each phase unit, then the capacitors in the power module are discretized by using a trapezoidal integral method, and Thevenin / Norton equivalent parameters of each power module when the DAB is not powered are calculated.

[0008] (3) if it is identified that the mode is a partial lockout mode (the cascaded H-bridge is unlocked, and the dual active bridge is still locked), then the actual diode branches are bypassed in each phase unit, the IGBT switching tubes (and the anti-parallel diodes thereof) in the cascaded H-bridge are replaced by a variable conductance G, the IGBT is high in resistance when it is turned on, and low in resistance when it is turned off, then the capacitors in the power module are discretized by using a trapezoidal integral method, and Thevenin / Norton equivalent parameters of each power module when the DAB is not powered are calculated.

[0009] (4) if it is identified that the mode is an unlock mode (the cascaded H-bridge and the dual active bridge are both unlocked), then the actual diode branches are bypassed in each phase unit, the IGBT switching tubes (and the anti-parallel diodes thereof) in the cascaded H-bridge and the DAB stage are replaced by a variable conductance G respectively, the IGBT is high in resistance when it is turned on, and low in resistance when it is turned off, then the capacitors and the inductors in the power module are discretized by using a trapezoidal integral method, and Thevenin / Norton equivalent parameters of each power module when the DAB is powered are calculated.

[0010] (5) the left sides of single sub-modules are connected in series, the right sides of the single sub-modules are connected in parallel to form a phase unit equivalent model, the phase unit equivalent model is added to the whole system, the whole circuit network is solved by using an electromagnetic transient simulation software, and phase unit current values of each phase unit at a next moment are obtained.

[0011] (6) From the obtained phase unit current value, the internal node voltage is inversely solved, and the update of the capacitor voltage, transformer current and inductor current information of each sub-module is completed. BRIEF DESCRIPTION OF DRAWINGS

[0012] Fig. 1 is a structure diagram of a typical three-phase system of a dual active bridge type converter topology with cascaded H-bridge in front stage.

[0013] Fig. 2 is a current flowable loop of a dual active bridge type converter when the DAB stage is locked.

[0014] Fig. 3 is an equivalent model of a dual active bridge type converter single phase unit integrated with a lock simulation function.

[0015] Fig. 4 is a decoupling companion circuit of a transformer. DETAILED DESCRIPTION

[0016] In the embodiment, the CHB-PET is a three-phase cascaded H-bridge type power electronic transformer (CHB-PET), which is a dual active bridge type converter. The modeling steps and related principles of the present application will be further described in detail below in combination with the drawings.

[0017] As shown in Fig. 1, the CHB-PET includes three phases, each phase has one phase unit, and each phase unit is composed of multiple power modules. As shown in the small box in Fig. 1, the power unit is composed of 12 IGBTs (S1-S12), 12 diodes (D1-D12), 2 capacitors (C1, C2), a high-frequency transformer (T) and its additional inductance (L). 12 12

[0018] The present application considers two lock modes of the CHB-PET: 1) the CHB stage and the DAB stage are in the locked state at the same time, which is referred to as "complete lock" below; 2) the CHB stage is unlocked and the DAB stage is locked, which is referred to as "partial lock" below. Taking the starting charging condition with active on both sides as an example, before the DAB is put into operation, the capacitor C1 and the capacitor C2 need to be charged to the rated value through the CHB stage and the DC / AC converter. The charging process of the capacitor C1 includes two stages of uncontrolled charging (i.e. "complete lock") and controllable charging (i.e. "partial lock"), and then the equivalent modeling method introduced in the present application can be used for simulation.

[0019] The present application provides an equivalent modeling simulation method of a CHB-PET considering multiple lock modes, including the following steps:

[0020] ​​(1) Obtain the phase unit operating parameters of the dual active bridge converter, the phase unit operating parameters including the switching state of the IGBT switch tube in each power module and the 2-bit binary signal indicating whether the cascaded H-bridge stage and DAB stage in the power module are locked, and perform lock mode identification;

[0021] The operating parameters of the phase unit include the switching signals of each power unit. In normal operating conditions, the condition of bridge arm shoot-through (the trigger signals of S1 and S3 are both 1) does not occur for each fully controlled H-bridge, so each set of bridge arms corresponds to a set of control signals, and a total of 6 switching signals need to be obtained. In the locked operating condition, the 2-bit binary signal indicating whether the cascaded H-bridge stage and DAB stage in the power module are locked can be directly added to determine the result, that is, the switching signals of the fully controlled H-bridge are all 0.

[0022] (2) If it is identified as a complete lock mode (both the cascaded H-bridge and the dual active bridge are locked), four actual diode branches for judging the current direction are put into each phase unit, then the trapezoidal integration method is used to discretize the capacitor in the power module, and the Thevenin / Norton equivalent parameters of each power module when the DAB is not electrified are calculated.

[0023] As shown in FIG. 2, after the DAB stage is locked, the current flowing through the switch module S5-S 12 will quickly drop to 0, so that it is respectively decoupled from the previous stage (CHB) and the subsequent stage (DC-AC converter), and therefore when the CHB-DAB module is equivalently modeled, the DAB stage can be simplified as a non-electrified state.

[0024] FIG. 3 is an equivalent circuit of a single phase unit of a dual active bridge converter integrated with a lock simulation function, wherein the voltage source V SEQ and the resistor R SEQ are Thevenin equivalent parameters on the series side of the phase unit, the current source J PEQ and the conductance G PEQ are Norton equivalent parameters on the parallel side of the phase unit; the equivalent diodes D1-D4 are used to judge the zero-crossing point of the current of the phase unit, and the accuracy of the equivalent diode device in judging the zero-crossing point can be ensured by interpolation algorithm; Brk1-Brk4 are virtual switches for controlling the switching of the diode branches, which can be associated with the lock signal in simulation. Corresponding to the complete lock mode, Brk1 and Brk4 are disconnected, and Brk2 and Brk3 are connected.

[0025] The conduction resistance (turn-off resistance) of D1-D4 in the equivalent circuit of the phase unit is the sum of the conduction resistance (turn-off resistance) of the actual diodes in each power module, which can be expressed as follows:

[0026] R ON_D_EQ =N*2*R ON_D (1)

[0027] ROFF_D_EQ = N * 2 * R OFF_D (2)

[0028] Where: N is the number of power modules, 2 * R ON_D is the on-state resistance of the diode branch in the current circulation loop of a single power module, 2 * R OFF_D is the off-state resistance of the diode branch in the current circulation loop of a single power module.

[0029] The capacitor C1 in each power module can be represented by an equivalent historical voltage source and an equivalent resistance after being discretized by trapezoidal integration method. If the resistance of the off-state diode branch is assumed to be infinite, the equivalent voltage source V SEQ and the equivalent resistance R SEQ on the series side of the phase unit can be represented as:

[0030]

[0031] In the formula: ΔT is a simulation step, N is the number of power modules in each phase unit. The superscript "Blk" indicates that the parameter corresponds to the fully locked state mode.

[0032] The capacitor C2 in each power module can be represented by an equivalent historical current source in parallel with an equivalent conductance (or an equivalent historical voltage source in series with an equivalent resistance) after being discretized by trapezoidal integration method. If the resistance of the off-state diode branch is assumed to be infinite, the equivalent current source J PEQ and the equivalent conductance G PEQ on the parallel side of the phase unit can be represented as:

[0033]

[0034]

[0035] In the formula: ΔT is a simulation step, N is the number of power modules in each phase unit. The superscript "Blk" indicates that the parameter corresponds to the fully locked state mode.

[0036] (3) Identify as a partial lock mode (cascaded H-bridge unlocked, double active bridge still locked), bypass the actual diode branch in each phase unit, replace the IGBT switch (and its anti-parallel diode) in the cascaded H-bridge as a whole with a variable conductance G, which is high resistance when the IGBT is on and low resistance when the IGBT is off. Then discretize the capacitor in the power module by trapezoidal integration method, and calculate the Thevenin / Norton equivalent parameters of each power module when the DAB is not electrified.

[0037] As shown in Figure 3, in the fully locked-out mode, Brk2 and Brk3 are closed, while Brk1 and Brk4 are open. The capacitor C1 in each power module, after being discretized using the trapezoidal integration method, can be represented by an equivalent historical voltage source and an equivalent resistance.

[0038] As an optional technical solution of the present invention, if it is assumed that the resistance of the IGBT branch and the anti-parallel diode branch in the off state is infinite, then the equivalent voltage source V on the series side of the phase unit... SEQ and equivalent resistance R SEQ It can be represented as:

[0039]

[0040] In the formula: ΔT is a simulation step size, R ON This is the parallel equivalent resistance of the IGBT and its anti-parallel diode. Calculated using equations (3) and (4) respectively. Flag i This indicates the activation status of the i-th fully controlled H-bridge in the CHB stage. A positive level is applied when both S1 and S4 are simultaneously on. (Flag) i The value is 1; when S2 and S3 are both turned on, a negative level is applied, Flag. i =1; when S1, S2 or S3, S4 are simultaneously turned on, a zero level is applied, Flag. i =0. The superscript "Pblk" indicates the corresponding partial locking mode.

[0041] After discretization using the trapezoidal integration method, the capacitor C2 in each power module can be represented by an equivalent historical current source in parallel with equivalent conductance (or an equivalent historical voltage source in series with equivalent resistance). If we assume that the resistance of the turned-off diode branch is infinite, then the equivalent current source J on the parallel side of the phase unit... PEQ and equivalent conductance G PEQ It can be represented as:

[0042]

[0043] In the formula: ΔT is a simulation step size, and N is the number of power modules in each phase unit. The superscript "Pblk" indicates that this parameter corresponds to the fully locked-out state mode.

[0044] As an optional technical solution to this scheme, if we consider that the IGBT branch in the off state and its anti-parallel diode branch have an actual resistance R... OFF Then the equivalent current source J on the parallel side of the phase unit PEQ and equivalent conductance G PEQ It can be represented as:

[0045]

[0046] A, B, C, D in the formula can be represented as:

[0047]

[0048] E in the formula can be represented as:

[0049] E=R OFF *R OFF -R ON *R ON (20)

[0050] E in the formula can be represented as:

[0051] E=R ON *R ON -R OFF *R OFF (21)

[0052] E in the formula can be represented as:

[0053] E=0 (22)

[0054] As an optional technical solution of the present scheme, the equivalent circuit of the partial lockout mode can reuse the equivalent circuit in the unlock mode, and only the following processing is needed: when all the controlled H bridges of the CHB stage are in the controllable state, the binary resistance equivalence of the switch branch can be performed without the help of the interpolation algorithm, and the IGBTs on all the H bridges of the DAB stage are in the off state in the partial lockout mode, and the corresponding branch can be equivalent to a large resistance, and the resistance is the external equivalent off resistance of the IGBT and the antiparallel diode.

[0055] (4) Recognize as unlock mode (both cascaded H bridge and double active bridge are unlocked), then bypass the actual diode branch in each phase unit, as shown in Figure 3, corresponding to the unlock mode, Brk1 and Brk4 are disconnected, and Brk2 and Brk3 are closed. Replace each IGBT switch (and its antiparallel diode) in the cascaded H bridge and the DAB stage with a variable conductance G, which is high resistance when the IGBT is turned on, and low resistance when the IGBT is turned off. Then, the trapezoidal integration method is used to discretize each energy storage element in the power module, and finally the Ward equivalent method in the electrical network is used to obtain the Thevenin / Norton equivalent parameters of each power module when the DAB is electrified. Capacitor, inductor and transformer. The discretization method of capacitor and inductor has been given in many documents, here only the transformer discretization equivalent method adopted by the present application is supplemented.

[0056] Figure 4 is the transformer decoupling companion circuit adopted by the present application, here briefly introduces the process of obtaining the transformer decoupling companion circuit. Starting from the mutual coupling circuit equation, the port voltage-current equation of the transformer can be obtained:

[0057]

[0058] where L 11 , L 22 are the self-inductance parameters of the primary and secondary sides of the transformer, L 12 , L 21 are the mutual-inductance parameters between the primary and secondary sides of the transformer. The port characteristic equation can be expressed as equation (25) by trapezoidal discretization integration of the currents I1, I2.

[0059]

[0060] where V = [V1 V2] T , I = [I1 I2] T , Y MAT is the equivalent admittance matrix obtained by trapezoidal discretization integration, and ΔT is a simulation step.

[0061] The decoupling companion circuit of the transformer shown in Fig. 4 is constructed, and equation (26) is obtained. The decoupling integration algorithm uses V(t-ΔT) to partially replace V(t) on the basis of the trapezoidal integration method, so that is only related to the state quantity at the previous time, and I(1,1) is only related to V(1,1) and is not related to V(2,1), thereby realizing approximate decoupling of the electrical quantities of the primary and secondary sides of the transformer.

[0062]

[0063] where λ is the diagonal matrix generated by Y MAT .

[0064] (5) The left side of a single sub-module is connected in series, and the right side is connected in parallel to form a phase unit equivalent model. The phase unit equivalent model is added to the entire system, and the electromagnetic transient simulation software is used to solve the entire circuit network to obtain the port current value and the voltage value of each phase unit at the next time.

[0065] (6) The internal node voltage is inversely solved from the obtained phase unit current value and voltage value, and the capacitor voltage, transformer current and inductor current information of each sub-module are updated.

[0066] When it is identified as the complete blocking mode, it is assumed that the off resistance of the diode is infinite, so the current on the capacitor C1 used to update the historical voltage source is the phase unit current value, and the current on the capacitor C2 can be obtained through the voltage on the parallel port, and can be expressed as equation (27). The transformer current and the inductor current are both set to zero in this mode.

[0067]

[0068] When the partial lock-up mode is identified, assuming the off-state resistance of IGBT is infinite, the relationship between the current on the capacitor C1 used to update the history voltage source and the phase current can be expressed as equation (28). The transformer current and the inductor current are both set to zero in this mode.

[0069]

[0070] wherein I1(t) is the phase current flowing from the input port of the CHB stage. Flag i represents the input situation of the i-th full-controlled H-bridge of the CHB stage, and the positive level is input when S1 and S4 are simultaneously turned on, Flag i is 1; the negative level is input when S2 and S3 are simultaneously turned on, Flag i is 1; the zero level is input when S1, S2 or S3, S4 are simultaneously turned on, Flag i is 0.

[0071] The current update expression on the capacitor C2 is consistent with equation (26).

[0072] When the unlock mode is identified, the external four-terminal circuit corresponding to the WARD equivalent method (i.e. the circuit in the small box in FIG. 4) is known, and the internal node voltage can be solved by combining the stored node admittance matrix.

Claims

1. A method for lock-up equivalent simulation of a dual active bridge converter; characterized in that, Based on the basic method of state variable multiplexing, the power module of power electronic transformer is equivalent to Thevenin / Norton equivalent branch with value changing with the blocking mode, and the actual diode element is introduced, which can improve the simulation efficiency while accurately simulating various blocking modes; The method comprises the following steps: Step 1: Obtain the phase unit operating parameters of the dual active bridge converter, the phase unit operating parameters include the switching state of the IGBT switch tube in each power module and the 2-bit binary signal indicating whether the cascaded H bridge stage and the DAB stage in the power module are blocked, and the blocking mode is identified; Step 2: Identify the full blocking mode, the full blocking mode is that the cascaded H bridge and the dual active bridge are both blocked, then four actual diode branches for judging the current direction are put into each phase unit, then the capacitors in the power module are discretized by using the trapezoidal integral method, and the Thevenin / Norton equivalent parameters of each power module when the DAB is not charged are calculated; Step 3: Identify the partial blocking mode, the partial blocking mode is that the cascaded H bridge is unlocked and the dual active bridge is still blocked, then the actual diode branch is bypassed in each phase unit, the IGBT switch tube and its antiparallel diode in the cascaded H bridge are replaced by a variable conductance G, the IGBT is high resistance when it is turned on, and low resistance when it is turned off, then the capacitors in the power module are discretized by using the trapezoidal integral method, and the Thevenin / Norton equivalent parameters of each power module when the DAB is not charged are calculated; Step 4: Identify the unlocking mode, the unlocking mode is that the cascaded H bridge and the dual active bridge are both unlocked, then the actual diode branch is bypassed in each phase unit, the IGBT switch tube and its antiparallel diode in the cascaded H bridge and the DAB stage are replaced by a variable conductance G, the IGBT is high resistance when it is turned on, and low resistance when it is turned off, then the capacitors and inductors in the power module are discretized by using the trapezoidal integral method, and the Thevenin / Norton equivalent parameters of each power module when the DAB is charged are calculated; Step 5: The left side of the single sub-module is connected in series, and the right side is connected in parallel to form a phase unit equivalent model, then the phase unit equivalent model is added to the whole system, and the electromagnetic transient simulation software is used to solve the whole circuit network to obtain the phase unit current value of each phase unit at the next time; Step 6: The internal node voltage is solved from the obtained phase unit current value, and the capacitor voltage, transformer current and inductor current information of each sub-module are updated.

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