CN111049189A - Grid-connected impact current prediction and virtual inertia selection method containing virtual inertia model - Google Patents

Grid-connected impact current prediction and virtual inertia selection method containing virtual inertia model Download PDF

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CN111049189A
CN111049189A CN201911319161.6A CN201911319161A CN111049189A CN 111049189 A CN111049189 A CN 111049189A CN 201911319161 A CN201911319161 A CN 201911319161A CN 111049189 A CN111049189 A CN 111049189A
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grid
virtual inertia
impact current
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voltage
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易皓
张枭
王振雄
卓放
龚培娇
唐启迪
王瑞
李建伟
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Xian Jiaotong University
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    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
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Abstract

The invention discloses a grid-connected impact current prediction and virtual inertia selection method containing a virtual inertia model, which aims at a microgrid sag control model introduced with virtual inertia, gives a prediction calculation formula of the impact current influence of the virtual inertia based on mathematical analysis of a grid-connected transient model, and gives a method for restraining impact current by reasonably selecting a virtual inertia value. Under the method, the dynamic performance of the system is good, the grid connection process is smooth and has no impact, and the method has important guiding significance and practical value for practical engineering design.

Description

Grid-connected impact current prediction and virtual inertia selection method containing virtual inertia model
Technical Field
The invention belongs to the technical field of micro-grid connection control, and particularly relates to a method for predicting grid-connected impact current and selecting virtual inertia, wherein the method comprises a virtual inertia model.
Background
With the rapid development of new energy, distributed power generation is widely introduced, thereby drawing attention of many researchers to the microgrid. The micro-grid can work in an island mode and a grid-connected mode, and droop control is mainly adopted. To obtain better dynamic performance, a virtual inertia (J) is introduced into the control loop. The virtual inertia has a positive effect on improving the dynamic property of the micro-grid under the island operation, but some adverse effects on grid-connected impact current are not solved, and how to solve the impact caused by the virtual inertia is a subject to be researched.
Disclosure of Invention
The invention provides a grid-connected impact current prediction and virtual inertia selection method containing a virtual inertia model.
In order to achieve the purpose, the invention provides a grid-connected impact current prediction and virtual inertia selection method comprising a virtual inertia model, which comprises the following steps:
step 1, carrying out equivalent circuit transformation on the microgrid circuit, and solving the equivalent circuit voltage v of the microgrid circuitdAnd an output impedance Zo，ZojX, X being the virtual reactance value introduced in the control strategy;
step 2, obtaining the circuit voltage v according to the calculation in the step 1dObtaining the amplitude V of the voltage sourcedAnd phase angle thetad(ii) a Measuring and obtaining voltage amplitude V of public connection point of micro-grid and main gridpAnd phase angle thetap；
Step 3, selecting a virtual inertia value J;
step 4, obtaining the amplitude V of the voltage source according to the step 2dAnd phase angle thetadVoltage amplitude V of common connection pointpAnd phase angle thetapAnd step 3, calculating and combining the virtual inertia value J selected in the stepAmplitude of net impulse current IgAnd the attenuation speed v of the grid-connected impact current;
step 5, obtaining output impedance Z according to the step 1oAnd the amplitude V of the voltage source obtained in step 2dVoltage amplitude V to common connection pointpCalculating a virtual inertia critical value JcAnd according to the virtual inertia critical value JcA virtual inertia value is determined.
Further, in step 1, the equivalent circuit voltage v of the microgrid circuitdThe calculation formula of (2) is as follows: v. ofd＝vref，vrefThe calculated reference voltage phasor is used for the droop equation.
Further, in step 4, the amplitude I of the grid-connected impact currentgThe calculation formula of (a) is as follows:
Figure BDA0002326678290000021
wherein, the calculation formula of Δ P is as follows:
Figure BDA0002326678290000022
wherein the content of the first and second substances,
Figure BDA0002326678290000023
K＝3VdVp/2X；
Vgthe amplitude of the voltage of the main power grid is defined, and delta Q is a reactive power impact value generated by a system when grid connection occurs; delta P is an active power impact value k generated by the system when grid connection occurspThe active droop coefficient.
Further, in step 4, the calculation formula of the grid-connected impact current attenuation speed v is as follows:
Figure BDA0002326678290000024
kpthe active droop coefficient.
Further, in step 5, the virtual inertia critical value JcThe calculation formula of (2) is as follows:
Figure BDA0002326678290000025
wherein k ispThe active droop coefficient.
Further, in step 5, the selected virtual inertia value is J c50 to 60 percent of the total weight of the composition.
Further, in step 5, the selected virtual inertia value is J c50% of the total.
Compared with the prior art, the invention has at least the following beneficial technical effects:
aiming at the problems, the influence of the virtual inertia on the grid-connected impact current is analyzed in detail by establishing a grid-connected transient model, and a mathematical expression is established to predict the influence of the virtual inertia on the initial amplitude, the attenuation trend and the attenuation time of the grid-connected impact current.
Furthermore, a calculation formula for reasonably designing the virtual inertia value is provided, so that the generation of grid-connected impact current is effectively inhibited, and the micro-grid can smoothly finish the grid-connected process without impact. The calculation formula of the critical value is simple and clear, and the virtual inertia value selected by the formula can inhibit grid-connected impact current and realize smooth and impact-free grid connection. The virtual inertia value selected by the formula has very obvious beneficial influence on the stability and the dynamic property of the system, and has higher application value and engineering guidance significance. In addition, a system simulation model is set up, simulation verification is carried out on the proposed impact current suppression method, and the correctness and reliability of the method are proved.
Drawings
FIG. 1 is a control block diagram of a microgrid inverter power supply;
FIG. 2 is a simplified schematic diagram of a grid-connected circuit model;
FIG. 3 is a control block diagram of a microgrid active power control loop;
fig. 4a is a simulation result of grid-connected impact current when the virtual inertia J is 1;
fig. 4b is a simulation result of grid-connected impact current when the virtual inertia J is 10;
fig. 4c is a simulation result of grid-connected impact current when the virtual inertia J is 50;
fig. 4d shows the virtual inertia J-102Then, a simulation result of grid-connected impact current is obtained;
fig. 4e shows the virtual inertia J being 5 × 102Then, a simulation result of grid-connected impact current is obtained;
fig. 4f shows the virtual inertia J-103Then, a simulation result of grid-connected impact current is obtained;
fig. 4g shows the virtual inertia J-104Then, a simulation result of grid-connected impact current is obtained;
fig. 4h shows the virtual inertia J105And (5) a simulation result of grid-connected impact current.
Detailed Description
In order to make the objects and technical solutions of the present invention clearer and easier to understand. The present invention will be described in further detail with reference to the following drawings and examples, wherein the specific examples are provided for illustrative purposes only and are not intended to limit the present invention.
The method aims at predicting and inhibiting the grid-connected impact current based on the virtual inertia in the micro-grid-connected transient process adopting a control strategy combining droop control and the virtual inertia.
Before describing the specific implementation steps, the variables in the formula used are explained as follows:
(1) j: a virtual inertia value;
(2) the omega inverter outputs angular frequency;
(3) v: the amplitude of the output voltage of the inverter power supply;
(4)ω*and V*: rated angular frequency of the inverter;
(5)V*: the rated voltage amplitude of the inverter;
(6)kp: an active droop coefficient;
(7)kq: reactive droop coefficient;
(8) q: average reactive power output by the inverter power supply;
(9)Pref: an active power reference value;
(10)Qref: a reactive power reference value;
(11) x: a virtual reactance value introduced in the control strategy;
(12)vref: reference voltage phasor, v, calculated by droop equationref＝Vsinωt；
(13)vs: voltage magnitude of thevenin equivalent circuit;
(14)Vs: voltage amplitude of thevenin equivalent circuit;
(15)Δθ0: the phase angle difference between the output voltage of the microgrid and the voltage of the main grid;
(16) Δ ω: angular frequency difference between the output voltage of the microgrid and the voltage of the main grid;
(17) Δ P: the active power impact value generated by the system when grid connection occurs;
referring to fig. 1, a grid-connected impact current prediction and virtual inertia selection method including a virtual inertia model includes the following specific implementation steps:
aiming at the microgrid sag control model with the virtual inertia introduced, the strategy is adopted to predict and restrain grid-connected impact current.
Step 1, the microgrid control equation is as follows or a variant thereof:
Figure BDA0002326678290000051
step 2, according to the attached diagram 1, carrying out Thevenin equivalent circuit transformation on the micro-grid circuit, and solving the equivalent circuit voltage v of the micro-grid circuit according to the following formuladAnd an output impedance ZoThe value of (c):
Zo＝jX,vd＝vref(b)；
step 3, simplifying the grid-connected model of the microgrid into a circuit topology shown in the attached figure 2 by using the Thevenin equivalent circuit, and calculating the circuit voltage v obtained in the step 2dThe amplitude V of the voltage source can be obtaineddAnd phase angle thetad. Measuring a voltage amplitude V of a Point of Common Coupling (PCC) of a microgrid to a main gridpAnd phase angle thetap。
And 4, selecting the virtual inertia value J as a variable quantity according to actual requirements.
And 5, predicting the grid-connected impact current as follows:
amplitude I of grid-connected impact currentg：
Figure BDA0002326678290000052
The calculation formula of the active power impact value delta P generated by the system when grid connection occurs is as follows:
Figure BDA0002326678290000061
wherein
Figure BDA0002326678290000062
K＝3VdVp/2X，VgIs the main grid voltage amplitude. The impact amplitude and the attenuation speed of the grid-connected impact current can be predicted and calculated according to the formula. Coefficient of index through e
Figure BDA0002326678290000063
Qualitatively estimating the attenuation speed v, wherein when the absolute value of the number is greater than or equal to 1, the larger the impact current is, the faster the attenuation speed is; when the absolute value of the number is less than 1, the larger the rate of decay is, the slower it is.
Figure BDA0002326678290000064
Step 6, calculating a virtual inertia critical value as follows:
Figure BDA0002326678290000065
selecting a virtual inertia value J when designing a control strategy c50% -60%, the grid-connected impact current can be effectively inhibited, and a smooth and impact-free grid-connected effect is obtained, as shown in fig. 4 c.
And (3) theoretical derivation process:
the invention is further detailed below with reference to the accompanying drawings, which include the establishment of a grid-connected transient model and the theoretical derivation process of a prediction formula.
The droop control equation after the virtual inertia is introduced is as follows:
Figure BDA0002326678290000066
in a practical microgrid, an inverter power supply is mostly controlled as a voltage source, and the control block diagram thereof is shown in fig. 1. Under the control strategy, the micro-grid inverter power supply model can be equivalent to a Thevenin circuit, and the parameter value of the equivalent circuit is obtained through calculation.
From FIG. 1, it can be seen that:
Figure BDA0002326678290000071
wherein: v. ofoThree-phase output voltage v after three-phase inversion of DC power supplyrefFor reference voltage phasors, v, obtained by droop control equationsrFor voltage drop caused by virtual impedance, vcIs the filter capacitor terminal voltage; i.e. iLFor the current flowing through the filter inductor, ioThe output current of the micro-grid is the output current of the micro-grid; zc(s) is the transfer function of the filter capacitance, ZLL(s) is the transfer function of the filter inductance, Zr(s) is the transfer function of the virtual impedance; gV(s) is the transfer function of the voltage outer loop, GI(s) is the transfer function of the current inner loop, GPWM(s) is the equivalent transfer function of the inverter.
Considering the bandwidth problem of the dual-loop controller and the current sharing problem of the circuit in the actual design process, the virtual impedance is generally designed to be purely inductive, and after irrelevant variables are eliminated, the following results can be obtained:
Zo＝jX,vs＝vref(3)
a grid-connected circuit model as shown in fig. 2 can then be obtained. Because the virtual inertia only influences the control loop of active power, and the voltage amplitude difference between the microgrid and the main power grid is generally very small, the influence caused by reactive power change can be ignored. From fig. 2, the phase angle difference between the inverter supply voltage and the common junction Point (PCC) voltage is generally small, so that the active power P output by the microgrid can be calculated:
Figure BDA0002326678290000072
combining the obtained power expression with the droop control equation, namely equation (5), a power control block diagram as shown in fig. 3 can be obtained, wherein the dotted line part is a step response generated by the system when grid connection occurs, namely, a reason for generating an impact current, and a prediction equation of the impact current can be obtained:
Figure BDA0002326678290000073
from fig. 3, a closed loop transfer function of the system when grid connection occurs is obtained, and according to an automatic control principle, a step signal delta theta introduced by grid connection can be obtained0Step response generated by the step response and delta omega, namely an active power impact value delta P generated by a system when grid connection occurs, wherein the expression of the step response in a frequency domain is as follows:
Figure BDA0002326678290000081
where Δ θ0And Δ ω is the phase angle difference and angular frequency difference between the microgrid output voltage and the mains grid voltage, respectively, K ═ 3VdVp/2X，VdAnd VpRespectively, the inverter output voltage amplitude and the common junction Point (PCC) voltage amplitude. After inverse laplace transform is carried out on the formula, a final impulse current prediction calculation formula can be obtained:
Figure BDA0002326678290000082
wherein the content of the first and second substances,
Figure BDA0002326678290000083
according to the expression of a, when
Figure BDA0002326678290000084
When a is approximately equal to 1/2, the formula (7) becomes a constant expression, and the impact current reaches the minimum; and (6) combining the results of the frequency domain analysis to derive a calculation formula of the critical virtual inertia:
Figure BDA0002326678290000085
selecting a virtual inertia value J when designing a control strategycAnd (5) multiplied by (50% -60%), namely, grid connection impact current can be effectively inhibited, and impact-free smooth grid connection is realized. The simulation results shown in fig. 4 a-4 h demonstrate the accuracy and reliability of this method.
Critical value J calculated by parameters used in simulationcAbout 100. As shown in fig. 4a and 4b, when the virtual inertia value is selected to be too small, it has no influence on the performance of the system, which is equivalent to that this control strategy is not introduced; FIG. 4c shows the smooth grid-connected effect obtained by the system when the appropriate virtual inertia value is selected to be 50; fig. 4d to 4h show that as the virtual inertia value is further increased, the system falls into an unstable state, and the inrush current becomes very large, so that the electrical equipment is damaged, which corresponds to the theoretical estimation result of the prediction calculation formula.
In conclusion, the method provides a prediction calculation formula of the influence of the virtual inertia on the impact current based on the mathematical analysis of the grid-connected transient model aiming at the microgrid sag control model with the virtual inertia introduced, and provides a method for reasonably selecting the virtual inertia value to inhibit the impact current. Under the method, the dynamic performance of the system is good, the grid connection process is smooth and has no impact, and the method has important guiding significance and practical value for practical engineering design. The method for suppressing the impact current is practical and feasible, and has important guiding significance for engineering design.
The above-mentioned contents are only for illustrating the technical idea of the present invention, and the protection scope of the present invention is not limited thereby, and any modification made on the basis of the technical idea of the present invention falls within the protection scope of the claims of the present invention.

Claims (7)

1. A grid-connected impact current prediction and virtual inertia selection method containing a virtual inertia model is characterized by comprising the following steps:
step 1, carrying out equivalent circuit transformation on the microgrid circuit, and solving the equivalent circuit voltage v of the microgrid circuitdAnd an output impedance Zo，ZojX, X being the virtual reactance value introduced in the control strategy;
step 2, calculating the equivalent circuit voltage v of the microgrid circuit obtained in the step 1dObtaining the amplitude V of the voltage sourcedAnd phase angle thetad(ii) a Measuring and obtaining voltage amplitude V of public connection point of micro-grid and main gridpAnd phase angle thetap；
Step 3, selecting a virtual inertia value J;
step 4, obtaining the amplitude V of the voltage source according to the step 2dAnd phase angle thetadVoltage amplitude V of common connection pointpAnd phase angle thetapAnd 3, calculating the amplitude I of the grid-connected impact current by the virtual inertia value J selected in the step 3gAnd the attenuation speed v of the grid-connected impact current;
step 5, obtaining output impedance Z according to the step 1oAnd the amplitude V of the voltage source obtained in step 2dVoltage amplitude V to common connection pointpCalculating a virtual inertia critical value JcAnd according to the virtual inertia critical value JcA virtual inertia value is determined.
2. The method for predicting grid-connected impact current and selecting virtual inertia based on virtual inertia model as claimed in claim 1, wherein in the step 1, equivalent circuit voltage v of microgrid circuitdThe calculation formula of (2) is as follows: v. ofd＝vref，vrefThe calculated reference voltage phasor is used for the droop equation.
3. The grid-connected impact current pre-stage comprising the virtual inertia model according to claim 1The method for selecting the measured and virtual inertia is characterized in that in the step 4, the amplitude I of the grid-connected impact currentgThe calculation formula of (a) is as follows:
Figure FDA0002326678280000011
wherein, the calculation formula of Δ P is as follows:
Figure FDA0002326678280000012
in the above formula, the first and second carbon atoms are,
Figure FDA0002326678280000021
K＝3VdVp/2X；
wherein, VgThe amplitude of the voltage of the main power grid is defined, and delta Q is a reactive power impact value generated by a system when grid connection occurs; delta P is an active power impact value k generated by the system when grid connection occurspThe active droop coefficient.
4. The method for predicting the grid-connected impact current and selecting the virtual inertia according to claim 1, wherein in the step 4, a calculation formula of a grid-connected impact current attenuation speed v is as follows:
Figure FDA0002326678280000022
kpthe active droop coefficient.
5. The method for predicting grid-connected impact current and selecting virtual inertia according to claim 1, wherein in the step 5, the virtual inertia critical value JcThe calculation formula of (2) is as follows:
Figure FDA0002326678280000023
wherein k ispThe active droop coefficient.
6. The method for predicting grid-connected impact current and selecting virtual inertia according to claim 1, wherein the selected virtual inertia value in the step 5 is Jc50 to 60 percent of the total weight of the composition.
7. The method for predicting grid-connected impact current and selecting virtual inertia according to claim 6, wherein the selected virtual inertia value in the step 5 is Jc50% of the total.
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