Method for modeling input impedance of isop-dab dc transformer

Abstract

The application discloses an input impedance modeling method of an ISOP-DAB type direct-current transformer, and belongs to the field of direct-current transformer modeling. The method comprises the following steps: taking the power of each DAB sub-module in the transformer as a parameter, establishing a direct-current bus input current small signal model and a transformer total output current small signal model; establishing a first small signal model of the sum of power instructions of all DAB sub-modules and a second small signal model of the sum of input voltages of all DAB sub-modules, so as to update the transformer total output current small signal model, and further establishing a transformer total output voltage small signal model, so as to deduce a direct-current bus input voltage small signal model; and calculating the ratio of the direct-current bus input voltage small signal model and the direct-current bus input current small signal model, so as to obtain an input impedance model of the transformer. The established model is compatible with all modulation methods of the DAB.

Description

Technical Field

[0001] This invention belongs to the field of DC converter modeling, and more specifically, relates to a method for modeling the input impedance of an ISOP-DAB type DC transformer. Background Technology

[0002] With the rapid development of power electronics technology and the soaring demand for new energy grid connection, medium and low voltage DC distribution systems are gradually becoming a hot topic in future power grid architecture. DC transformers are widely used in DC distribution networks, primarily to connect medium-voltage and low-voltage DC buses to achieve bidirectional power flow conversion. However, in long-distance distribution lines, the interaction between the input impedance of the DC transformer and the non-negligible equivalent impedance on the distribution line can lead to weak damping or oscillations in the system. In other words, the input impedance characteristics of the DC transformer during operation are crucial to the stable operation of the DC distribution network under steady-state conditions.

[0003] The input impedance model of a DC transformer is the basis for analyzing system stability and the theoretical support for system stability optimization. For input-series output-parallel dual-active bridge (ISOP-DAB) DC transformers, existing input impedance modeling methods still have the following shortcomings: ① The actual control quantity of the DAB is the phase shift angle. Existing models of ISOP-DAB are all based on the phase shift angle, only considering the impedance modeling problem of each submodule operating under single-phase-shift modulation. Different modulation methods and operating conditions will correspond to different phase shift angles, failing to address impedance modeling problems under other modulation methods such as double-phase shift and extended phase shift, thus lacking model universality; ② The generalized average modeling method using state variables involves complex matrix transformations, resulting in large computational loads and a complex process; ③ The modeling process only retains the first-order components after Fourier decomposition, making it difficult to guarantee the accuracy of the model. Therefore, the input impedance modeling problem of ISOP-DAB DC transformers urgently needs to be solved. Summary of the Invention

[0004] To address the shortcomings and improvement needs of existing technologies, this invention provides an input impedance modeling method for ISOP-DAB type DC transformers. The purpose is to provide an accurate input impedance model compatible with all modulation methods of DAB, including single phase shift, double phase shift, and extended phase shift.

[0005] To achieve the above objectives, according to one aspect of the present invention, an input impedance modeling method for an ISOP-DAB type DC transformer is provided, comprising: establishing a small-signal model of the DC bus input current and a small-signal model of the total output current of the transformer, using the power of each DAB submodule in the transformer as a parameter; establishing a first small-signal model of the sum of power commands of all DAB submodules and a second small-signal model of the sum of input voltages of all DAB submodules; substituting the first small-signal model and the second small-signal model into the small-signal model of the total output current of the transformer to update it, and establishing a small-signal model of the total output voltage of the transformer based on the updated small-signal model of the total output current of the transformer; deriving a small-signal model of the DC bus input voltage based on the small-signal model of the total output voltage of the transformer; and calculating the ratio of the small-signal model of the DC bus input voltage to the small-signal model of the DC bus input current to obtain the input impedance model of the transformer.

[0006] Furthermore, the small-signal model of the DC bus input current is as follows:

[0007]

[0008] in, For the small signal of the DC bus input current, G vi G is the transfer function between the output voltage and input current of the DAB submodule. piin This is the transfer function between the per-unit transmitted power and the input current of the DAB submodule. This is the transfer function of the output voltage closed-loop controller. This is the small signal of the transformer's total output voltage. The input voltage to the DC bus is a small signal, M is the number of DAB sub-modules in the transformer, and Z is the number of sub-modules in the transformer. C This is the equivalent impedance of the input capacitor.

[0009] Furthermore, the small-signal model of the total output current of the transformer is as follows:

[0010]

[0011]

[0012] in, This is the small signal of the transformer's total output current. Let G be the small signal of the output current of the j-th DAB submodule, M be the number of DAB submodules in the transformer, and G be the small signal of the output current of the j-th DAB submodule. pio_sub This is the transfer function between the per-unit transmitted power and the output current of the DAB submodule. G is the small signal of the per-unit transmission power of the j-th DAB submodule. viThe transfer function between the output voltage and input current of the DAB submodule. denoted as the small signal of the input voltage of the j-th DAB submodule.

[0013] Furthermore, the small-signal model of the power command for each DAB submodule is as follows:

[0014]

[0015] The first small-signal model is:

[0016]

[0017] in, For the j-th DAB submodule, the small signal represents the per-unit transmission power. This is the transfer function of the output voltage closed-loop controller. The transfer function of the input equalization closed-loop controller. This is the small signal of the transformer's total output voltage. Let be the small signal of the input voltage of the j-th DAB submodule, and M be the number of DAB submodules in the transformer.

[0018] Furthermore, the small-signal model of the total output voltage of the transformer is as follows:

[0019]

[0020] in, Z represents the small signal of the transformer's total output voltage. out For the open-loop output impedance, G pio This is the transfer function between the per-unit transmitted power and the total output current of the DAB submodule. G is the transfer function of the output voltage closed-loop controller. vi The transfer function between the output voltage and input current of the DAB submodule. This is a small signal representing the DC bus input voltage.

[0021] Furthermore, the updated small-signal model of the total output current of the transformer is as follows:

[0022]

[0023] in, For the small signal of the updated transformer total output current, G pio_sub Let M be the transfer function between the per-unit transmitted power and the output current of the DAB submodule, and M be the number of DAB submodules in the transformer. This is the transfer function of the output voltage closed-loop controller. G is the small signal of the transformer's total output voltage.vi The transfer function between the output voltage and input current of the DAB submodule. G is the small signal of the DC bus input voltage. pio This is the transfer function between the per-unit transmitted power and the total output current of the DAB submodule.

[0024] Furthermore, the small-signal model of the DC bus input voltage is as follows:

[0025]

[0026] in, Z is the small signal of the DC bus input voltage. out For the open-loop output impedance, G pio This is the transfer function between the per-unit transmitted power and the total output current of the DAB submodule. G is the transfer function of the output voltage closed-loop controller. vi The transfer function between the output voltage and input current of the DAB submodule. This is the small signal of the transformer's total output voltage.

[0027] Furthermore, the input impedance model is as follows:

[0028]

[0029] Among them, Z in_ISOP Z represents the input impedance of the transformer, M represents the number of DAB submodules in the transformer, and Z represents the input impedance of the transformer. C G is the equivalent impedance of the input capacitor. vi G is the transfer function between the output voltage and input current of the DAB submodule. piin This is the transfer function between the per-unit transmitted power and the input current of the DAB submodule. Z is the transfer function of the output voltage closed-loop controller. out For the open-loop output impedance, G pio This is the transfer function between the per-unit transmitted power and the total output current of the DAB submodule.

[0030] Furthermore, it also includes: establishing an equivalent small-signal control model for the transformer based on the small-signal model of the transformer's total output voltage and the small-signal model of the DC bus input current.

[0031] In summary, the above-described technical solutions conceived in this invention can achieve the following beneficial effects:

[0032] (1) A method for modeling the input impedance of ISOP-DAB type DC transformer is provided. The method is based on power modeling. Since the per-unit power of ISOP-DAB only represents the power level of the circuit, any specific per-unit power can correspond to a combination of multiple phase shift angles, making the per-unit power backward compatible with the modulation method. The input impedance model based on power will not have the problem of model incompatibility and can be applied to all modulation methods of DAB such as single phase shift, double phase shift, and extended phase shift.

[0033] (2) When modeling current based on power, the expression is very simple and the modeling result is clear. There are no approximation operations in the modeling process, which greatly ensures the high accuracy of the impedance model and makes the established input impedance model both accurate and simple. Attached Figure Description

[0034] Figure 1 A flowchart illustrating the input impedance modeling method for an ISOP-DAB type DC transformer provided in an embodiment of the present invention;

[0035] Figure 2 A schematic diagram of the topology of the ISOP-DAB type DC transformer provided in an embodiment of the present invention;

[0036] Figure 3 A closed-loop control block diagram of an ISOP-DAB type DC transformer provided in an embodiment of the present invention;

[0037] Figure 4 This is a schematic diagram of the equivalent small-signal control model of the ISOP-DAB type DC transformer provided in an embodiment of the present invention. Detailed Implementation

[0038] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0039] In this invention, the terms "first," "second," etc. (if present) in the invention and the accompanying drawings are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence.

[0040] Figure 1 A flowchart illustrating the input impedance modeling method for an ISOP-DAB type DC transformer provided in this embodiment of the invention. (See also...) Figure 1 , combined Figures 2-4The input impedance modeling method of the ISOP-DAB type DC transformer in this embodiment is described in detail. The method includes operations S1-S5.

[0041] Operation S1 establishes a small-signal model of the DC bus input current and a small-signal model of the total output current of the transformer, using the power of each DAB submodule in the transformer as a parameter.

[0042] See Figure 2 The topology of the ISOP-DAB type DC transformer (hereinafter referred to as the transformer) includes: a medium-voltage side DC voltage source V DC It has an equivalent reactance L BUS and equivalent resistance R BUS The medium-voltage side input bus, M (M≥2) DAB submodules (SM) connected by ISOP type connection structure, and the low-voltage side filter capacitor C o and equivalent load resistance R o Among them, M DAB submodules are connected in series at the medium-voltage side port and in parallel at the low-voltage side load port.

[0043] The switching frequency of ISOP-DAB is f s The per-unit transmission power is P pu On the series side, the bus voltage is v. bus The bus current is i bus On the parallel side, the output voltage is v. o All DAB submodules maintain the same input capacitors, turns ratios, and transfer inductors, and they are respectively C in N, L t In the j-th DAB submodule, the per-unit transmission power is p. pu_j The input voltage is V in_j The input current is i in_j The output current is i o_j See also Figure 3 The power command of each DAB submodule is generated by the cooperation of one output voltage controller and M-1 input voltage equalization controllers.

[0044] Based on the direct power control principle of DAB and the current source characteristics of DAB, the input current i of each DAB submodule of the ISOP-DAB type DC transformer is... in_j and output current i o_j The per-unit transmission power p of this submodule pu_j Proportional. For each DAB submodule in the transformer, based on its per-unit transmission power p pu_j Establish its input current i in_j Output current i o_j The model:

[0045]

[0046]

[0047] Furthermore, small-signal perturbation processing is applied to formulas (1)-(2) to obtain the input current i. in_j Output current i o_j Small signal model:

[0048]

[0049]

[0050] Since the output sides of the ISOP-DAB are connected in parallel, its total output current is the sum of the output currents of each DAB submodule. Based on formula (4), a small-signal model of the total output current of the transformer is established:

[0051]

[0052] in, This is the small signal of the transformer's total output current. Let G be the small signal of the output current of the j-th DAB submodule, M be the number of DAB submodules in the transformer, and G be the small signal of the output current of the j-th DAB submodule. pio_sub This is the transfer function between the per-unit transmitted power and the output current of the DAB submodule. G is the small signal of the per-unit transmission power of the j-th DAB submodule. vi The transfer function between the output voltage and input current of the DAB submodule. Let G be the small signal of the input voltage of the j-th DAB submodule. It should be noted that G... vi It is also the transfer function between the input voltage and output current of the DAB submodule.

[0053] On the input series side, the total input bus current i bus It can be expressed as the sum of the input current of any DAB submodule and the charging current of its input capacitor. Therefore, based on formula (3), a small-signal model of the DC bus input current is established:

[0054]

[0055] The small-signal model of the DC bus input current can be further extended to:

[0056]

[0057] By summing and averaging equation (7), an average small-signal model of the DC bus input current can be established, which serves as the final small-signal model of the DC bus input current:

[0058]

[0059] in, A small signal for the DC bus input current. Let be the small signal of the input current of the j-th DAB submodule. G represents the small signal of the current on the input capacitor of the j-th DAB submodule. piin This is the transfer function between the per-unit transmitted power and the input current of the DAB submodule. Z represents the small signal of the transformer's total output voltage. C This is the equivalent impedance of the input capacitor.

[0060] Operation S2 establishes the first small-signal model of the sum of power commands of all DAB submodules and the second small-signal model of the sum of input voltages of all DAB submodules.

[0061] In this embodiment, a small-signal model of the power command for each DAB submodule is established:

[0062]

[0063] Furthermore, based on formula (9), the first small signal model is established:

[0064]

[0065] in, This is the transfer function of the output voltage closed-loop controller. This is the transfer function of the input equalization closed-loop controller.

[0066] Since the input sides of the ISOP-DAB are connected in series, the total input voltage is the sum of the input voltages of all DAB submodules. The established second small-signal model is as follows:

[0067]

[0068] Operation S3 substitutes the first and second small-signal models into the transformer total output current small-signal model to update it, and establishes the transformer total output voltage small-signal model based on the updated transformer total output current small-signal model.

[0069] Substituting formulas (10)-(11) into formula (5) updates the small-signal model of the total output current of the transformer. The updated small-signal model of the total output current of the transformer is as follows:

[0070]

[0071] in, For the small signal of the updated transformer's total output current, G is the small signal of the DC bus input voltage. pio This is the transfer function between the per-unit transmitted power and the total output current of the DAB submodule.

[0072] Since the output voltage of ISOP-DAB is the sum of the output current and the open-loop output impedance Z, out The product of , then, according to formula (12), establish the small-signal model of the total output voltage of the transformer:

[0073]

[0074] Among them, Z out This is the open-loop output impedance.

[0075] Operation S4: Based on the small-signal model of the total output voltage of the transformer, derive the small-signal model of the input voltage of the DC bus.

[0076] Based on formula (13), the small-signal model of the DC bus input voltage is derived:

[0077]

[0078] Operation S5 calculates the ratio of the small-signal model of the DC bus input voltage to the small-signal model of the DC bus input current, thus obtaining the input impedance model of the transformer.

[0079] Based on the ratio of formula (14) and formula (8), the input impedance model of the transformer is obtained:

[0080]

[0081] Among them, Z in_ISOP G is the input impedance of the transformer. pio This is the transfer function between the per-unit transmitted power and the total output current of the DAB submodule.

[0082] The specific definitions of parameters such as the transfer function used in the above formula are as follows:

[0083] Z out = R o / (R o C o s+1) (16)

[0084] G piin = NV o / 8f s L t (17)

[0085] G vi = NP pu / 8f s L t (18)

[0086] G pio_sub = NV bus / 8Mf s L t (19)

[0087] G pio = NV bus / 8f s L t (20)

[0088] Z c = 1 / sC in (twenty one)

[0089] According to an embodiment of the present invention, the method further includes: establishing an equivalent small-signal control model of the transformer based on the small-signal model of the transformer's total output voltage and the small-signal model of the DC bus input current. Specifically, based on formulas (8) and (13), an equivalent small-signal control model of the transformer is established as follows: Figure 4 The equivalent small-signal control model shown is based on, for example, Figure 4 The equivalent small-signal control model shown controls the ISOP-DAB type DC transformer.

[0090] Furthermore, in this embodiment, the ISOP-DAB type DC transformer can be controlled based on the input impedance model obtained above to achieve DC-DC conversion under various modulation methods and operating conditions.

[0091] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for modeling the input impedance of an ISOP-DAB type DC transformer, characterized in that, include: Using the power of each DAB submodule in the transformer as a parameter, a small-signal model of the DC bus input current and a small-signal model of the total output current of the transformer are established. Establish a first small-signal model of the sum of power commands of all DAB submodules and a second small-signal model of the sum of input voltages of all DAB submodules; The first small-signal model and the second small-signal model are substituted into the small-signal model of the total output current of the transformer to update it, and the small-signal model of the total output voltage of the transformer is established based on the updated small-signal model of the total output current of the transformer. Based on the small-signal model of the total output voltage of the transformer, the small-signal model of the DC bus input voltage is derived. The input impedance model of the transformer is obtained by calculating the ratio of the small-signal model of the DC bus input voltage to the small-signal model of the DC bus input current. The input impedance model is as follows: in, This is the input impedance of the transformer. The number of DAB submodules in the transformer. This is the equivalent impedance of the input capacitor. The transfer function between the output voltage and input current of the DAB submodule. This is the transfer function between the per-unit transmitted power and the input current of the DAB submodule. This is the transfer function of the output voltage closed-loop controller. The open-loop output impedance is... This is the transfer function between the per-unit transmitted power and the total output current of the DAB submodule.

2. The input impedance modeling method for ISOP-DAB type DC transformer as described in claim 1, characterized in that, The small-signal model of the DC bus input current is as follows: in, A small signal for the DC bus input current. The transfer function between the output voltage and input current of the DAB submodule. This is the transfer function between the per-unit transmitted power and the input current of the DAB submodule. This is the transfer function of the output voltage closed-loop controller. This is the small signal of the transformer's total output voltage. The small signal is the DC bus input voltage. The number of DAB submodules in the transformer. This is the equivalent impedance of the input capacitor.

3. The input impedance modeling method for ISOP-DAB type DC transformer as described in claim 1, characterized in that, The small-signal model of the total output current of the transformer is as follows: in, This is the small signal of the transformer's total output current. For the first The small signal of the output current of each DAB submodule The number of DAB submodules in the transformer. This is the transfer function between the per-unit transmitted power and the output current of the DAB submodule. For the first Small signal of per-unit transmission power of each DAB submodule The transfer function between the output voltage and input current of the DAB submodule. For the first The small signal of the input voltage of each DAB submodule.

4. The input impedance modeling method for ISOP-DAB type DC transformer as described in claim 1, characterized in that, The small-signal model of the power command for each DAB submodule is as follows: The first small-signal model is: in, For the first Small signal of per-unit transmission power of each DAB submodule This is the transfer function of the output voltage closed-loop controller. The transfer function of the input equalization closed-loop controller. This is the small signal of the transformer's total output voltage. For the first The small signal of the input voltage of each DAB submodule, This represents the number of DAB submodules in the transformer.

5. The input impedance modeling method for ISOP-DAB type DC transformer as described in claim 1, characterized in that, The small-signal model of the total output voltage of the transformer is as follows: in, This is the small signal of the transformer's total output voltage. The open-loop output impedance is... This is the transfer function between the per-unit transmitted power and the total output current of the DAB submodule. This is the transfer function of the output voltage closed-loop controller. The transfer function between the output voltage and input current of the DAB submodule. This is a small signal representing the DC bus input voltage.

6. The input impedance modeling method for ISOP-DAB type DC transformer as described in claim 1, characterized in that, The updated small-signal model of the total output current of the transformer is as follows: in, For the small signal of the updated transformer's total output current, This is the transfer function between the per-unit transmitted power and the output current of the DAB submodule. The number of DAB submodules in the transformer. This is the transfer function of the output voltage closed-loop controller. This is the small signal of the transformer's total output voltage. The transfer function between the output voltage and input current of the DAB submodule. The small signal is the DC bus input voltage. This is the transfer function between the per-unit transmitted power and the total output current of the DAB submodule.

7. The input impedance modeling method for ISOP-DAB type DC transformer as described in claim 1, characterized in that, The small-signal model of the DC bus input voltage is as follows: in, The small signal is the DC bus input voltage. The open-loop output impedance is... This is the transfer function between the per-unit transmitted power and the total output current of the DAB submodule. This is the transfer function of the output voltage closed-loop controller. The transfer function between the output voltage and input current of the DAB submodule. This is the small signal of the transformer's total output voltage.

8. The input impedance modeling method for ISOP-DAB type DC transformers as described in any one of claims 1-7, characterized in that, Also includes: Based on the small-signal model of the total output voltage of the transformer and the small-signal model of the DC bus input current, an equivalent small-signal control model of the transformer is established.

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