A π-type virtual synchronous generator control method for interface converter in AC / DC hybrid microgrid

Abstract

The present invention provides a π-type virtual synchronous generator control method for an interface converter in an AC / DC hybrid microgrid. The virtual synchronous generator control equation for the AC frequency is adjusted, and the inertia control equation for the DC voltage is introduced. This control strategy can provide voltage and frequency support for the AC / DC hybrid microgrid, and introduce inertia for the AC frequency and DC voltage, solving the problem that the traditional virtual synchronous motor control technology can only provide inertia for the AC side in the AC / DC hybrid microgrid. While effectively improving the anti-disturbance characteristics of the AC frequency and DC voltage in the AC / DC hybrid microgrid, the AC / DC subnets can jointly bear the impact of the sudden change in load disturbance.

Description

Technical Field

[0001] The invention relates to microgrid operation control, and in particular to a bidirectional interface converter power transmission control method suitable for an AC / DC hybrid microgrid. Background Art

[0002] AC / DC hybrid microgrids are becoming the future development trend of microgrids because they can effectively combine the advantages of AC microgrids and DC microgrids. The bidirectional interface converter connecting the AC bus and the DC bus controls the energy conversion and transmission between the AC subgrid and the DC subgrid. It is the hub of the AC / DC hybrid microgrid and plays a vital role in maintaining the stable operation of the system. In the control of the bidirectional interface converter, if power sharing between the AC and DC subgrids is to be achieved, the influence of AC frequency and DC voltage on power transmission should be considered at the same time. The power sharing control strategy of the interface converter makes the AC subgrid and the DC subgrid closely connected. Due to the general lack of inertia of microgrids, power fluctuations within a 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 the distributed power sources in the subgrid to stabilize the DC bus voltage and AC bus frequency. However, the influence of the interface converter on the hybrid microgrid when the DC bus voltage or AC bus frequency is unstable is not considered. Summary of the invention

[0003] To solve the above problems, the present invention provides a π-VSG control method for an interface converter in an AC / DC hybrid microgrid, which comprises the following steps:

[0004] S1: Calculate the voltage deviation of the DC side of the interface converter and the frequency deviation of the AC side of the interface converter, and calculate the electromotive force coefficient of the DC motor according to the allowable fluctuation range of the DC side voltage and the allowable fluctuation range of the AC side frequency;

[0005] S2: Introduce the DC motor electromotive force equation into the virtual synchronous generator control strategy, propose a DC side voltage control method, and build a π-VSG control model;

[0006] S3: Input the rated voltage of the DC side and the transmission power of the interface converter into the active loop controlled by the π-VSG to obtain the equivalent current deviation;

[0007] S4: Using the active loop and reactive loop in the π-VSG control algorithm, the voltage reference value e is calculated. abc ;

[0008] S5: According to the voltage reference value e abc Control interface converter for bidirectional power transfer.

[0009] Furthermore, in S1, the DC side voltage deviation of the interface converter is ΔUdc The frequency deviation of the AC side of the interface converter is represented by Δω ac Indicates that ΔU dc and Δω ac Respectively expressed as:

[0010] ΔU dc =U dc -U dcN

[0011] Δω ac =ω ac -ω acN

[0012] Among them, U dc is the DC side voltage of the interface converter, U dcN is the rated voltage of the DC side of the interface converter, Δω ac is the AC side frequency of the interface converter, Δω acN is the rated frequency of the AC side of the interface converter.

[0013] Assume that the electromotive force coefficient of the DC motor is K E Indicates that K E It is expressed as:

[0014]

[0015] Among them, M u is the maximum allowable fluctuation value of the DC side voltage, M ω M is the maximum allowable frequency fluctuation value on the AC side. u and M ω It is expressed as:

[0016] |U dc -U dcN |≤M u

[0017] |ω ac -ω acN |≤M ω

[0018] Furthermore, in S2, the mechanical equation of the virtual synchronous motor is compared

[0019]

[0020] Where J is the moment of inertia, P m is the mechanical power, P e is the electromagnetic power.

[0021] Combined with the DC motor electromotive force equation, we get:

[0022]

[0023] Among them, U ω is the virtual electromotive force on the AC side, C ω is the virtual capacitance on the AC side. ω and C ω Respectively expressed as:

[0024] U ω =K ω ω

[0025]

[0026] The traditional VSG control strategy takes into account the dynamic response of AC frequency, but does not consider the change of DC voltage. In view of this, a DC voltage control method is proposed. u is the DC side virtual capacitor:

[0027]

[0028] Combined with the AC side control equation, a π-VSG control model is proposed.

[0029] Furthermore, in S3, the equivalent current deviation output by the π-VSG controller is denoted by ΔI u and ΔI ω Indicates that ΔI u and ΔI ω It is expressed as:

[0030]

[0031] Among them, P i is the output power of the interface converter, P0 is the initial active power of the interface converter, U dcN is the rated voltage of the DC bus.

[0032] Furthermore, in S4, the output phase angle θ is calculated based on the second-order mathematical model in the active loop:

[0033]

[0034]

[0035] The reactive loop of the interface converter adopts reactive power-voltage droop control:

[0036]

[0037] In the formula, E m is the AC voltage amplitude reference value, K Q is the reactive inertia coefficient, D q is the reactive power-voltage droop coefficient, V nand V are the given value and actual value of the AC voltage amplitude, respectively. Q0 and Q bic are the initial value and actual value of the reactive power of the interface converter respectively.

[0038] The frequency and phase signal of the active loop output and the voltage amplitude signal of the reactive loop output are combined into a reference voltage e abc :

[0039]

[0040] Furthermore, the π-VSG control output voltage reference value in S5 is controlled through a voltage and current dual closed-loop control link to achieve bidirectional power coordination control of the AC / DC hybrid microgrid system.

[0041] Compared with the prior art, the control method proposed by the present invention has the following beneficial effects:

[0042] 1. The present invention provides a π-VSG control method for an interface converter in an AC / DC hybrid microgrid. The control strategy can provide voltage and frequency support for the AC / DC hybrid microgrid, and introduce inertia for the AC frequency and DC voltage, thereby solving the problem that the traditional virtual synchronous motor control technology can only provide inertia for the AC side in an AC / DC hybrid microgrid, and effectively improving the anti-disturbance characteristics of the AC frequency and DC voltage in the AC / DC hybrid microgrid.

[0043] 2. Compared with the methods mentioned in other literatures, the power sharing method proposed in the present invention can adjust C u and C ω The impact of load power mutation is distributed and borne by the AC and DC subnetworks according to their respective carrying capacities. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 is a circuit diagram of a DC subnet in an embodiment of the present invention;

[0045] Figure 2 is a control block diagram of an AC subnet inverter in an embodiment of the present invention;

[0046] Figure 3 is a control block diagram of a bidirectional interface converter in an embodiment of the present invention;

[0047] Figure 4 It is the π-VSG control model of the present invention;

[0048] Figure 5 It is a simulation waveform diagram of the interface converter in the embodiment of the present invention. DETAILED DESCRIPTION

[0049] The principles and features of the present invention are described below in conjunction with the accompanying drawings. The examples given are only used to explain the present invention and are not used to limit the scope of the present invention.

[0050] The AC / DC hybrid microgrid is mainly composed of an AC subnet, a DC subnet, and an interface converter connected to the AC / DC bus. In the embodiment of the present invention, in order to simulate the sudden change of the AC bus frequency and the DC bus voltage, an ideal DC voltage source is connected in series with a resistor in the DC subnet to realize the PU droop control in the DC subnet, such as Figure 1 As shown; the AC subnet uses an inverter as an equivalent distributed power source to realize the Pf droop control in the AC subnet. Its control block diagram is as follows Figure 2 As shown; the control block diagram of the bidirectional interface converter is as follows Figure 3 shown.

[0051] The PU droop relationship of the DC subnet and the Pf droop relationship of the AC subnet can be expressed as:

[0052]

[0053] ΔP ac =k ac Δω ac

[0054] Among them, k dc is the droop coefficient of the DC subnet PU, U dcN for Figure 1 The rated voltage of the ideal DC voltage source is also the rated voltage of the DC subnet, R dc for Figure 1 The series resistance value, k ac is the Pf droop coefficient of the AC subnetwork.

[0055] Step 1: When the AC bus frequency or DC bus voltage changes suddenly, calculate the DC side voltage deviation ΔU of the interface converter dc The frequency deviation of the AC side of the interface converter is Δω ac , ΔU dc and Δω ac Respectively expressed as:

[0056] ΔU dc =U dc -U dcN

[0057] Δω ac =ω ac -ω acN

[0058] Among them, U dc is the DC side voltage of the interface converter, U dcN is the rated voltage of the DC side of the interface converter, ωac is the AC side frequency of the interface converter, ω acN is the rated frequency of the AC side of the interface converter.

[0059] According to the allowable fluctuation range of DC side voltage and AC side frequency, calculate the electromotive force coefficient K of the DC motor. E , K E It is expressed as:

[0060]

[0061] Among them, M u is the maximum allowable fluctuation value of the DC side voltage, M ω M is the maximum allowable frequency fluctuation value on the AC side. u and M ω It is expressed as:

[0062] |U dc -U dcN |≤M u

[0063] |ω ac -ω acN |≤M ω

[0064] Step 2: Compare the mechanical equations of the virtual synchronous motor

[0065]

[0066] Where J is the moment of inertia, P m is the mechanical power, P e is the electromagnetic power.

[0067] Combined with the DC motor electromotive force equation, we get:

[0068]

[0069] Among them, U ω is the virtual electromotive force on the AC side, C ω is the virtual capacitance on the AC side. ω and C ω Respectively expressed as:

[0070] U ω =K ω ω

[0071]

[0072] The traditional VSG control strategy considers the dynamic response of AC frequency, but does not consider the change of DC voltage. In view of this, a DC voltage inertia control method is proposed. uis the DC side virtual capacitor:

[0073]

[0074] Combined with the AC side control equation, the π-VSG control model is proposed as follows: Figure 4 As shown, virtual capacitors are added on the DC side and the AC side respectively, and the power distribution when multiple machines are connected in parallel is realized by controlling the virtual resistance. The power distribution of the AC and DC subnets is realized by controlling the virtual capacitors to increase the system inertia.

[0075] Step 3: Assume that the equivalent current deviation output by the π-VSG controller is ΔI u and ΔI ω Indicates that ΔI u and ΔI ω It is expressed as:

[0076]

[0077] Among them, P i is the output power of the interface converter, P0 is the initial active power of the interface converter, U dcN is the rated voltage of the DC bus.

[0078] Step 4: Calculate the output phase angle θ in the active loop based on the second-order mathematical model:

[0079]

[0080]

[0081] The reactive loop of the interface converter adopts reactive power-voltage droop control:

[0082]

[0083] In the formula, E m is the AC voltage amplitude reference value, K Q is the reactive inertia coefficient, D q is the reactive power-voltage droop coefficient, V n and V are the given value and actual value of the AC voltage amplitude, respectively. Q0 and Q bic are the initial value and actual value of the reactive power of the interface converter respectively.

[0084] The frequency and phase signal of the active loop output and the voltage amplitude signal of the reactive loop output are combined into a reference voltage e abc :

[0085]

[0086] Step 5: The π-VSG controls the output voltage reference value through the voltage and current dual closed-loop control link to realize the bidirectional power coordination control of the AC / DC hybrid microgrid system, controls the active power of the interface converter to slowly reach a new steady state according to the power demand of the subgrid, and provides support for the voltage and frequency of the AC side subgrid.

[0087] The rated voltage U of the DC subnetwork in the embodiment dcN The maximum allowable fluctuation value of DC voltage is 750V. u is 75V, the DC subnet PU droop coefficient k dc is 2.67MW / kV; the rated voltage amplitude of the AC subnet is V N 380V, rated frequency f acN The maximum allowable frequency fluctuation value is 50Hz. f is 0.5Hz, the AC subnet Pf droop coefficient k ac 0.4MW / Hz. Rated capacity of interface converter S N The virtual resistance R in π-VSG control is 0.5MW. a is 0.225Ω, the virtual capacitor C u is 0.3H, the virtual capacitor C w For 1H.

[0088] The simulation results of the embodiment are shown in Figure 5 When the AC subnet load power increases suddenly, the DC voltage drop decreases and then slowly rises to a stable state, while the AC frequency first decreases slightly and then slowly reaches a stable state. The π-VSG control is a bidirectional interface converter that provides a certain inertia for both the AC and DC sides, and C u The existence of makes the DC voltage and AC frequency bear the impact of the sudden increase in load power together. Similarly, when the DC subnet load power increases suddenly, the AC frequency drops and then slowly rises to a stable state, and the DC voltage first drops slightly and then slowly reaches a stable state.

[0089] It can be seen from this embodiment that π-VSG control can provide inertia for the bidirectional interface converter and can effectively improve the stability of the system. u Value and C ω The value is used to distribute the impact of load power mutation on subnets and the adjustment capability between subnets.

Claims

1. A π-VSG control method for an interface converter in an AC / DC hybrid microgrid, characterized in that: The following steps are involved: S1: Calculate the voltage deviation of the DC side of the interface converter and the frequency deviation of the AC side of the interface converter, and calculate the electromotive force coefficient of the DC motor according to the allowable fluctuation range of the DC side voltage and the allowable fluctuation range of the AC side frequency; S2: Introduce the DC motor electromotive force equation into the virtual synchronous generator control strategy, propose a DC side voltage control method, and build a π-VSG control model; S3: Input the rated voltage of the DC side and the transmission power of the interface converter into the active loop controlled by the π-VSG to obtain the equivalent current deviation; S4: Using the active loop and reactive loop in the π-VSG control algorithm, the voltage reference value e is calculated. abc ; S5: According to the voltage reference value e abc bidirectional power transfer of control interface converters; In S1, the DC side voltage deviation of the interface converter is ΔU dc The frequency deviation of the AC side of the interface converter is represented by Δω ac Indicates that ΔU dc and Δf ac Respectively expressed as: ΔU dc =U dc -U dcN Give ac =ω ac -oh acN Among them, U dc is the DC side voltage of the interface converter, U dcN is the rated voltage of the DC side of the interface converter, ω ac is the AC side frequency of the interface converter, ω acN is the rated frequency of the AC side of the interface converter; Assume that the electromotive force coefficient of the DC motor is K E Indicates that K E It is expressed as: Among them, M u is the maximum allowable fluctuation value of the DC side voltage, M ω M is the maximum allowable frequency fluctuation value on the AC side. u and M ω It is expressed as: |In dc -IN dcN |≤M u |oh ac -oh acN |≤M ω ; In S2, the mechanical equation of the virtual synchronous motor is compared Where J is the moment of inertia, P m is the mechanical power, P e is the electromagnetic power; Combined with the DC motor electromotive force equation, we get: Among them, U ω is the virtual electromotive force on the AC side, C ω is the virtual capacitance on the AC side, K ω is the electromotive force coefficient of the virtual DC motor, I m is the input current on the AC side, I e is the output current on the AC side, U ω and C ω Respectively expressed as: U ω =K ω ω The traditional VSG control strategy considers the dynamic response of AC frequency, but does not consider the change of DC voltage. In view of this, a DC voltage inertia control method is proposed; in is the DC side output current, I out is the input current on the AC side, C u is the DC side virtual capacitor: Combined with the control equations on the AC side, a π-VSG control model is proposed; In S3, the equivalent current deviation output by the π-VSG controller is denoted by ΔI u and ΔI ω Indicates that ΔI u and ΔI ω It is expressed as: Among them, P i is the output power of the interface converter, P0 is the initial active power of the interface converter, U dcN is the rated voltage of the DC bus; In S4, the output phase angle θ is calculated based on the second-order mathematical model in the active loop: The reactive loop of the interface converter adopts reactive power-voltage droop control: In the formula, E m is the AC voltage amplitude reference value, K Q is the reactive inertia coefficient, D q is the reactive power-voltage droop coefficient, V n and V are the given value and actual value of the AC voltage amplitude, respectively. Q0 and Q bic are the initial value and actual value of the reactive power of the interface converter respectively; The frequency and phase signal of the active loop output and the voltage amplitude signal of the reactive loop output are combined into a reference voltage e abc :

2. The π-VSG control method of the interface converter in the AC / DC hybrid microgrid according to claim 1, characterized in that: In S5, the π-VSG controls the output voltage reference value through the voltage and current dual closed-loop control link to achieve bidirectional power coordination control of the AC / DC hybrid microgrid system.

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