A generalized direct-current electromotive force control method for an interface converter in an alternating-direct-current hybrid microgrid

Abstract

The application provides a generalized direct-current electromotive force control method of an interface converter in an AC / DC hybrid microgrid. The direct-current side voltage deviation and the alternating-current side frequency deviation of the interface converter are subjected to a generalized direct-current electromotive force control link to obtain a slowly changed active power reference value, which controls the active power of the interface converter to slowly reach a new steady state. In the generalized direct-current electromotive force control, the active power between the AC / DC subnets can be judged and distributed through a virtual resistance, which is relatively simple compared with the traditional bi-directional droop control. Meanwhile, a virtual inductance is introduced, which adds an inertia link to the system and improves the dynamic performance of the bi-directional interface converter, effectively weakening the influence of the power fluctuation between the subnets.

Description

Technical Field

[0001] This invention relates to microgrid operation control, and more particularly to a bidirectional interface converter power transmission control method applicable to AC / DC hybrid microgrids. Background Technology

[0002] Hybrid AC / DC microgrids are gradually becoming the future trend of microgrid development due to their ability to effectively combine the advantages of both AC and DC microgrids. The bidirectional interface converter, connecting the AC and DC buses, controls the energy conversion and transmission between the AC and DC subgrids, serving as the hub of the hybrid AC / DC microgrid and playing a crucial role in maintaining system stability. In controlling the bidirectional interface converter, to achieve power sharing between the AC and DC subgrids, the impact of both AC frequency and DC voltage on power transmission must be considered. The power sharing control strategy of the interface converter tightly connects the AC and DC subgrids. Due to the generally insufficient inertia of microgrids, power fluctuations within one subgrid can easily affect the stability of the entire hybrid microgrid through the interface converter. Some control strategies improve the stability of the hybrid microgrid by combining distributed power sources in the subgrid to stabilize the DC bus voltage and AC bus frequency. However, these strategies do not consider the impact of the interface converter on the hybrid microgrid when the DC bus voltage or AC bus frequency is unstable. Summary of the Invention

[0003] To address the above problems, this invention provides a generalized DC electromotive force control method for an interface converter in an AC / DC hybrid microgrid, comprising the following steps:

[0004] S1: Calculate the DC-side voltage deviation and AC-side frequency deviation of the interface converter;

[0005] S2: Calculate the electromotive force coefficient for generalized DC electromotive force control based on the allowable fluctuation range of DC side voltage and the allowable fluctuation range of AC side frequency;

[0006] S3: Input the DC voltage difference and AC frequency difference into the generalized DC electromotive force controller to obtain the equivalent DC current deviation;

[0007] S4: Calculate the reference value of active power of the interface converter;

[0008] S5: Control the bidirectional power transmission of the interface converter based on the active power reference value.

[0009] Furthermore, in S1, the DC-side voltage deviation of the interface converter is denoted by ΔU. dc The frequency deviation on the AC side of the interface converter is represented by Δf. ac It means that ΔU dc and Δf ac They are represented as follows:

[0010] ΔU dc =U dc -U dcN

[0011] Δf ac =f ac -f acN

[0012] Among them, U dc U is the DC-side voltage of the interface converter. dcN f is the rated voltage on the DC side of the interface converter. ac f is the AC side frequency of the interface converter. acN The rated frequency for the AC side of the interface converter.

[0013] Furthermore, in S2, let K be the electromotive force coefficient for generalized DC electromotive force control. E K indicates E Represented as:

[0014]

[0015] Where, Δω ac =2πΔf ac For AC side angular frequency deviation, M u M represents the maximum permissible fluctuation value of the DC side voltage. f M represents the maximum permissible fluctuation value of the AC side frequency. u and M f Represented as:

[0016] |U dc -U dcN |≤M u

[0017] |f ac -f acN |≤M f

[0018] Furthermore, in S3, the equivalent DC current deviation output by the generalized DC electromotive force controller is denoted by ΔI, which is expressed as:

[0019]

[0020] Among them, R a L is the virtual resistance in the generalized DC electromotive force control. a R is the virtual inductance in the generalized DC electromotive force control. a Represented as:

[0021]

[0022] Among them, S NThis refers to the rated capacity of the interface converter.

[0023] Furthermore, in S4, the active power reference value P of the interface converter ref Represented as:

[0024]

[0025] Where P0 is the initial active power of the interface converter.

[0026] Furthermore, the active power reference value of the interface converter described in S5 is used to achieve bidirectional power coordination control of the AC / DC hybrid microgrid system through a dual closed-loop power and current control loop.

[0027] Compared with the prior art, the control method proposed in this invention has the following advantages:

[0028] 1. This invention provides a generalized DC electromotive force (EMF) control method for an interface converter in an AC / DC hybrid microgrid. This method can provide inertia for the interface converter and reduce the impact of power fluctuations between AC and DC subgrids. When the DC bus voltage or AC bus frequency changes abruptly, a slowly changing active power reference value is obtained through the generalized DC EMF control loop, controlling the active power of the interface converter to slowly reach a new steady state, thereby reducing the mutual influence of power fluctuations between subgrids.

[0029] 2. The power sharing method proposed in this invention is simpler than the methods mentioned in other literature, especially when multiple interface converters are connected in parallel. Attached Figure Description

[0030] Figure 1 This is a circuit diagram of the DC subnet in an embodiment of the present invention;

[0031] Figure 2 This is a control block diagram of the AC subgrid inverter in an embodiment of the present invention;

[0032] Figure 3 This is a control block diagram of the bidirectional interface converter in an embodiment of the present invention;

[0033] Figure 4 This is the generalized DC electromotive force control model of the present invention;

[0034] Figure 5 This invention provides a generalized DC electromotive force control model for parallel multi-interface converters under steady-state conditions.

[0035] Figure 6 The above is a simulation waveform diagram of the interface converter in an embodiment of the present invention. Detailed Implementation

[0036] The principles and features of the present invention are described below with reference to the accompanying drawings. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0037] The AC / DC hybrid microgrid mainly consists of an AC subgrid, a DC subgrid, and interface converters connecting the AC and DC buses. In this embodiment of the invention, to simulate the abrupt changes in AC bus frequency and DC bus voltage, an ideal DC voltage source is connected in series with a resistor in the DC subgrid to implement PU droop control in the DC subgrid, such as... Figure 1 As shown; the AC subnetwork uses an inverter as an equivalent distributed power source to implement Pf droop control in the AC subnetwork. The inverter measures the required active and reactive power output, obtains the reference voltage through droop control, and finally maintains the stability of the AC bus voltage and frequency through a voltage-current loop. Its control block diagram is shown below. Figure 2 As shown; the bidirectional interface converter obtains the active power reference value by controlling the measured AC frequency and DC voltage values ​​through generalized DC electromotive force, and realizes the power transfer between subgrids through the power current loop. Its control block diagram is shown in the figure. Figure 3 As shown.

[0038] The PU droop relationship of the DC subnet and the Pf droop relationship of the AC subnet can be expressed as:

[0039]

[0040] ΔP ac =k ac Δf ac

[0041] Where, k dc U is the droop factor of the DC subgrid PU. dcN for Figure 1 The rated voltage of an ideal DC voltage source is also the rated voltage of the DC subgrid, R. dc for Figure 1 The resistance value in series, k ac Let Pf be the droop coefficient of the AC subnet.

[0042] Step 1: When the AC bus frequency or DC bus voltage suddenly changes, calculate the DC side voltage deviation ΔU of the interface converter. dc Frequency deviation Δf on the AC side of the interface converter ac ΔU dc and Δf ac They are represented as follows:

[0043] ΔU dc =U dc -U dcN

[0044] Δf ac =f ac-f acN

[0045] Among them, U dc U is the DC-side voltage of the interface converter. dcN f is the rated voltage on the DC side of the interface converter. ac f is the AC side frequency of the interface converter. acN The rated frequency for the AC side of the interface converter.

[0046] Step 2: Calculate the electromotive force coefficient K for generalized DC electromotive force control based on the allowable fluctuation range of DC-side voltage and AC-side frequency. E K E Represented as:

[0047]

[0048] Where, Δω ac =2πΔf ac For AC side angular frequency deviation, M u M represents the maximum permissible fluctuation value of the DC side voltage. f M represents the maximum permissible fluctuation value of the AC side frequency. u and M f Represented as:

[0049] |U dc -U dcN |≤M u

[0050] |f ac -f acN |≤M f

[0051] Step 3: Input the DC voltage difference and AC frequency difference into the generalized DC electromotive force controller. The generalized DC electromotive force control model is as follows: Figure 4 As shown. Therefore, the equivalent DC current deviation ΔI can be obtained, which is expressed as:

[0052]

[0053] Among them, R a L is the virtual resistance in the generalized DC electromotive force control. a R is the virtual inductance in the generalized DC electromotive force control. a Represented as:

[0054]

[0055] Among them, S N This refers to the rated capacity of the interface converter.

[0056] In the generalized DC electromotive force control method, KE The value is determined by the rated DC-side voltage and rated AC-side frequency of the interface converter; only R... a The value of the interface converter rated capacity S N This relates to the generalized DC electromotive force control method under steady-state conditions when multiple interface converters are connected in parallel. This method is equivalent to multiple virtual internal resistances R. ai Parallel connection, such as Figure 5 As shown, this will effectively simplify the construction and calculation of the entire system model. And the virtual inductor L... a The existence of this improves the dynamic response characteristics of the interface converter. When the AC frequency or DC voltage changes abruptly, the equivalent DC current deviation ΔI obtained by the generalized DC electromotive force controller will not change abruptly, but will slowly reach a new steady-state value.

[0057] Step 4: Calculate the active power reference value P of the interface converter based on the equivalent DC current deviation ΔI. ref , is represented as:

[0058]

[0059] Where P0 is the initial active power of the interface converter.

[0060] The slow change in the equivalent DC current deviation ΔI also affects the active power reference value P of the interface converter. ref It slowly reaches a new steady-state value.

[0061] Step 5: Slowly changing active power reference value P of the interface converter ref Through a dual closed-loop control system for power and current, the active power of the interface converter is controlled to gradually reach a new steady state according to the power demand of the subgrid.

[0062] The rated voltage U of the DC subgrid in the embodiment dcN The maximum allowable fluctuation value of DC voltage is 750V, M. u For 75V, the DC subgrid PU droop coefficient k dc It is 2.67MW / kV; the rated voltage amplitude V of the AC subgrid is... N 380V, rated frequency f acN The maximum permissible frequency fluctuation value M is 50Hz. f The droop coefficient k of the AC subnet Pf is 0.5Hz. ac The rated capacity is 0.4 MW / Hz. The rated capacity of the interface converter is S. N The virtual resistance R in the generalized DC electromotive force control is 0.2MW. a The Ω is 0.5625Ω, and the virtual inductance L is... a It is 0.2H.

[0063] The simulation results of the embodiment are shown in Figure 6When the AC subgrid load power suddenly increases, the AC bus frequency drops sharply. However, the transmission current of the interface converter under generalized DC electromotive force (GMMF) control does not change rapidly due to the sudden drop in AC frequency, but slowly reaches a new steady-state value. The DC bus voltage also slowly changes to a new steady-state value. GMMF control provides a certain inertia to the bidirectional interface converter, preventing the DC bus voltage from being affected by sudden changes in AC bus frequency. Similarly, when the DC subgrid load power suddenly increases, the DC bus voltage drops sharply. The interface converter using GMMF control ensures that the AC bus frequency changes slowly, unaffected by sudden changes in DC bus voltage.

[0064] As can be seen from this embodiment, generalized DC electromotive force control can provide inertia for the bidirectional interface converter, effectively improving system stability. When the power of the AC or DC subgrid changes, generalized DC electromotive force control can adjust the parameter R... a The value of L is used to achieve power distribution between subnets, and this is achieved by adjusting the parameter L. a The value controls the response speed of the interface converter and reduces the impact of fluctuations between subnets.

Claims

1. A method for controlling the generalized DC electromotive force of an interface converter in an AC / DC hybrid microgrid, characterized in that, Includes the following steps: S1: Calculate the DC-side voltage deviation and AC-side frequency deviation of the interface converter; S2: Calculate the electromotive force coefficient for generalized DC electromotive force control based on the allowable fluctuation range of DC side voltage and the allowable fluctuation range of AC side frequency; S3: Input the DC voltage difference and AC frequency difference into the generalized DC electromotive force controller to obtain the equivalent DC current deviation; S4: Calculate the reference value of active power of the interface converter; S5: Control the bidirectional power transmission of the interface converter based on the active power reference value; In S1, the DC-side voltage deviation of the interface converter is used for... This indicates that the frequency deviation on the AC side of the interface converter is... express, and They are represented as follows: , ; in, For the DC side voltage of the interface converter, The rated voltage of the DC side of the interface converter. For the AC side frequency of the interface converter, The rated frequency of the AC side of the interface converter; In S2, let the electromotive force coefficient for generalized DC electromotive force control be represented by... express, Represented as: ； in, For AC side angular frequency deviation, This represents the maximum permissible fluctuation value of the DC-side voltage. This represents the maximum permissible fluctuation value of the AC side frequency. and Represented as: ， ； In S3, the equivalent DC current deviation output by the generalized DC electromotive force controller is used as... express, Represented as: ; in, This refers to the virtual resistance in the generalized DC electromotive force control. This refers to the virtual inductance in the generalized DC electromotive force control. Represented as: ; in, This refers to the rated capacity of the interface converter; In S4, the active power reference value of the interface converter Represented as: ; in, This represents the initial active power of the interface converter.

2. The generalized DC electromotive force control method for the interface converter in an AC / DC hybrid microgrid according to claim 1, characterized in that, The active power reference value of the interface converter described in S5 is passed through a power and current dual closed-loop control loop to realize bidirectional power coordination control of the AC / DC hybrid microgrid system.

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