CN111725835A - Method for restraining loop closing operation impact current of power distribution network containing distributed power supply - Google Patents

Method for restraining loop closing operation impact current of power distribution network containing distributed power supply Download PDF

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CN111725835A
CN111725835A CN202010368535.XA CN202010368535A CN111725835A CN 111725835 A CN111725835 A CN 111725835A CN 202010368535 A CN202010368535 A CN 202010368535A CN 111725835 A CN111725835 A CN 111725835A
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current
voltage
output
control
loop closing
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黄文焘
宋海涛
邰能灵
马洲俊
王勇
张明
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Shanghai Jiaotong University
Nanjing Power Supply Co of State Grid Jiangsu Electric Power Co Ltd
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Shanghai Jiaotong University
Nanjing Power Supply Co of State Grid Jiangsu Electric Power Co Ltd
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    • H—ELECTRICITY
    • 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
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H02J3/381—Dispersed generators
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J2203/00—Indexing scheme relating to details of circuit arrangements for AC mains or AC distribution networks
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Abstract

A method for restraining impulse current of loop closing operation of a power distribution network with a distributed power supply comprises the steps of modeling the power distribution network with a distributed power supply, estimating impulse current after loop closing of a voltage source type DG and a current source type DG, further calculating loop closing impulse current based on DG with different output external characteristics, adopting a unified strategy suitable for the voltage source type DG and the current source type DG to restrain the loop closing impulse current, and adaptively adjusting control parameters to enable the loop closing impulse current not to exceed a threshold value and switching when the impulse current is minimum so as to remarkably reduce influence of control parameter change on DG output. The invention can self-adaptively adjust the control parameter to ensure that the loop closing current does not exceed the threshold, and simultaneously, the adjustment process is selected to be switched when the impact current is minimum, thereby reducing the influence of the change of the control parameter on the DG output.

Description

Method for restraining loop closing operation impact current of power distribution network containing distributed power supply
Technical Field
The invention relates to a technology in the field of intelligent power grid control, in particular to a method for restraining loop closing operation impact current of a power distribution network with distributed power supplies.
Background
A Distributed Generator (DG) is supported by a power electronic device, has high response speed and low rotational inertia, can transfer loads through closed-loop operation of a power distribution network, and improves the power supply reliability. At present, current calculation and closed loop current regulation before and after closed loop of a power distribution network containing a distributed power supply are mainly based on steady-state current calculation, and DG transient state and dynamic characteristics are ignored. And the analysis of instantaneous impact current after loop closing is less. Meanwhile, no solution is provided for reducing the impact influence of the closed-loop operation of the DG-containing power distribution network.
Disclosure of Invention
Aiming at the defect that the prior art can not suppress closed-loop impact through a control strategy, the invention provides a method for suppressing impact current of closed-loop operation of a power distribution network containing a distributed power supply, which can adaptively adjust control parameters to ensure that the closed-loop current does not exceed a threshold value, and meanwhile, the adjustment process is switched when the impact current is minimum, so that the influence of the change of the control parameters on DG output is reduced.
The invention is realized by the following technical scheme:
the invention relates to a method for restraining loop closing operation impact current of a power distribution network containing a distributed power supply, which estimates the impact current after loop closing of a voltage source type DG and a current source type DG by modeling the power distribution network containing the DG, further adopts a unified strategy suitable for the voltage source type DG and the current source type DG to restrain the loop closing impact current based on calculation of the DG loop closing impact current with different output external characteristics, and adaptively adjusts control parameters to ensure that the loop closing impact current does not exceed a threshold value and switches when the impact current is minimum, thereby obviously reducing the influence of the change of the control parameters on the DG output.
The invention relates to a system for realizing the method, which comprises the following steps: a current calculation unit and an inrush current suppression unit, wherein: the current calculation unit is connected with the power distribution network measurement device and receives DG grid-connected bus voltage information, the impact current suppression unit is connected with the current calculation unit and transmits DG output current information under different control parameters, and the impact current suppression unit is respectively connected with each DG controller and transmits control parameter information.
Technical effects
The invention integrally solves the technical problems that: calculating and restraining the impact current of the DG after the loop closing operation of the power distribution network; compared with the prior art, the method and the device do not need to be configured with extra equipment, can adaptively switch the control parameter when the impact current is minimum, reduce the influence of the change of the control parameter on the DG output, and are suitable for voltage source type DGs and current source type DGs.
Simulation results show that the method can effectively inhibit the impact influence of the DG in the loop closing operation, the strategy does not need to change the configuration of relevant equipment of the power distribution network, and the grid connection friendliness of the DG is greatly enhanced.
In conclusion, the method for analyzing and suppressing the impact current of the distributed power supply after the loop closing operation of the power distribution network is suitable for the DGs of the voltage source type and the current source type, can effectively suppress the impact influence of the DGs in the loop closing operation, does not need to change the configuration of relevant equipment of the power distribution network, and greatly enhances the grid-connected friendliness of the DGs.
Drawings
Fig. 1 is a diagram illustrating a typical access manner of a DG;
FIG. 2 is a schematic diagram of a basic equivalent model of loop closing current analysis;
FIG. 3 is a diagram of a PQ control block;
FIG. 4 is a schematic diagram of a VSG control block;
FIG. 5 is a schematic view of a droop control block;
FIG. 6 is a schematic diagram of a rush current suppression process;
FIG. 7 is a schematic diagram of inrush current control adjustment;
FIG. 8 is a schematic view of a simulation model;
FIG. 9 is a diagram illustrating simulation results;
in the figure: (a) bus 1 voltage (b) bus 2 voltage schematic;
FIG. 10 is a diagram illustrating the results of IIDG output current simulation before the suppression strategy is adopted;
FIG. 11 is a diagram illustrating the variation of the IIDG output current;
FIG. 12 is a schematic diagram of bus voltage simulation results;
in the figure: (a) bus 1 voltage (b) bus 2 voltage;
FIG. 13 is a graph illustrating the results of IIDG output current simulation using PQ control before the suppression strategy;
FIG. 14 is a schematic diagram of bus voltage simulation results;
in the figure: (a) bus 1 voltage (b) bus 2 voltage;
FIG. 15 is a schematic diagram of droop control IIDG output current before applying the suppression strategy;
fig. 16 is a schematic diagram of the simulation result of the IIDG output current using the front and rear droop control of the suppression strategy, respectively.
Detailed Description
As shown in fig. 1, an application environment of this embodiment, that is, a scheme in which a DG connected in a conventional power distribution network is connected to a 10kV or 35kV bus of a 110kV substation, includes: 1) a private line is directly connected to a low (medium) voltage side bus of a public power grid; 2) the low (middle) voltage side bus of the system transformer substation is merged after passing through a system switching station; 3) after passing through 35kV and 110kV transformer substations, the high-voltage side of the transformer substation is looped in and out, and finally the transformer substation is merged into a low (medium) voltage side bus of a system transformer substation.
In the embodiment, the impact current of the DGs with the voltage source type and the current source type after ring closing is estimated by modeling the power distribution network containing the DGs, the ring closing impact current is suppressed by adopting a unified strategy suitable for the DGs with the voltage source type and the current source type based on the calculation of the DG ring closing impact current with different output external characteristics, and the control parameters are adaptively adjusted to ensure that the ring closing impact current does not exceed a threshold value and at the same time, the switching is performed when the impact current is minimum, so that the influence of the change of the control parameters on the DG output is remarkably reduced.
As shown in fig. 2, the modeling of the DG-containing power distribution network refers to: according to three typical connections of a DG access distribution network system, a closed loop current analysis basic model is provided, which comprises the following steps: two power branch roads that lie in the both sides that close the ring and DG branch road, the branch road that closes the ring and the equivalent load branch road that link to each other with the power branch road respectively, wherein: in the model, a branch 1 and a branch 2 are power supply branches on two sides of a loop, impedance comprises transformer reactance and line impedance, a branch 3 is a branch where a DG is located, and the DG is equivalent to a controlled power supply. The branch 4 is a loop closing branch. Branch 5 and branch 6 are equivalent load branches.
The method for estimating the impact current after the loop closing of the voltage source type DG and the current source type DG comprises the following specific steps:
1) calculating the closed loop current of the power distribution network: when the power distribution network is closed, due to the existence of the inductance element in the network, the closed loop current not only has periodic components, but also has non-periodic components for keeping the current from sudden change. Closed loop branch full current after closed loop
Figure BDA0002477327230000031
Wherein: i iscRepresenting the amplitude of steady-state loop current caused by the voltage difference between two sides of the interconnection switch at the loop closing point, α is the voltage angle difference between two sides of the interconnection switch during loop closing, β is the loop closing loop impedance angle, tcFor closing the ring time, TcIs the decay time constant of the non-periodic component. I ism,4And
Figure BDA0002477327230000032
showing the steady-state current amplitude and the initial phase angle after the loop closing at the loop closing position. Omega is the power frequency angular velocity, and t is time.
After the network is closed, the impact current flowing through the closed loop branch can be distributed to the non-closed loop branch, and the current i of the non-closed loop branchkEqual to the superposition of the original branch current and the impact current:
Figure BDA0002477327230000033
Figure BDA0002477327230000034
wherein: i ism,kAnd
Figure BDA0002477327230000035
showing the steady-state current amplitude and phase of branch k after loop closingAnd (4) an angle. Omega is the power frequency angular velocity, and t is time. I ism,k,0And
Figure BDA0002477327230000036
and representing the initial value of the steady-state current amplitude and the initial phase angle of the branch k after the loop closing. T iskAnd ckThe attenuation time constant and the initial value of the non-periodic component are obtained; so the voltage v of the bus 1 after the loop closing1Comprises the following steps:
Figure BDA0002477327230000037
wherein: e1And theta1The steady state amplitude and the initial phase angle, L, of the power supply at the front section of the loop closing position1，R1，Z1And
Figure BDA0002477327230000038
is the inductive resistance impedance and the impedance angle, V, of branch 1m,1And1the steady-state amplitude and the initial phase angle of the bus 1 voltage after loop closing are shown.
2) DG loop closing current analysis and calculation: unlike a general non-loop-closed branch, the DG output characteristic is determined by a control strategy. Depending on the external characteristics, DG can be divided into a controlled current source and a controlled voltage source. Hereinafter, the loop closing current analysis calculation is performed by taking a typical current source DG under PQ control, a typical droop control, and a typical voltage source DG under virtual synchronous motor control as examples. Neglecting the retarding effect of DG outlet inductance on current in analysis, consider that DG passes through constant reactance Z3Grid connection; at the same time. The control system is considered to have good tracking performance, and the transient process of voltage and current inner loop control is ignored.
When the DG adopts PQ control, as shown in FIG. 3, it is a typical PQ control block diagram, and its control equation is
Figure BDA0002477327230000041
Wherein: u. ofdAnd idAs d-axis voltage current measurements, iqIs a q-axis current measurement. PrefAnd QrefIs the DG output power reference. When the DG is PQ controlled, the DG can be equivalent to a controlled current source, i.e., its output active power remains unchanged and its output isThe current is determined by a control mode and is controlled by the output voltage; the DG output voltage is determined by the circuit equation.
In the circulation flow of calculating the output current, when the circulation number k is 1, the controlled current source is used, and the DG output current does not generate sudden change, namely i3(t1)＝i3,0(t1) According to circuit theory, then, the DG output voltage v3(t1)＝i3,0(t1)Z3+v1(t1)。tkThe time corresponding to k cycles.
The DG output voltage is fed back to the control system, and because the DC component can be filtered when the ABC coordinate axial direction DQ0 coordinate axis conversion is carried out on the voltage and current measured value in the control, each feedback quantity introduced in the DG control does not contain the non-periodic component, and the voltage under the DQ0 coordinate
Figure BDA0002477327230000042
Wherein: theta is the conversion angle when the ABC coordinate axis is converted to the DQ0 coordinate axis. Vm,1And1the steady-state amplitude and the initial phase angle of the bus 1 voltage after loop closing are shown. i.e. i3a，i3bAnd i3cThree-phase currents in each case of branch 3
Number of cycles k>1, the voltage value measured at the last moment is used as feedback to enter a control system to obtain a DG output current instantaneous value
Figure BDA0002477327230000043
Wherein: i.e. i3The current of branch 3, here phase a current.
When the DG is equipped with an energy storage device, the Virtual Synchronous Generator (VSG) control strategy shown in fig. 4 is adopted, and the control equation is
Figure BDA0002477327230000044
Wherein: the virtual inertia constant is H, P is the active power value output by the inverter port under the control of VSG, omega is the DG output angular frequency value, omegarefThe reference value of the angular frequency is the angular frequency corresponding to the power frequency, and D is the active droop coefficient. Q is DG port output under VSG controlValue of reactive power of DQIs a reactive sag factor, EsetIs the DG terminal voltage reference under the VSG algorithm. PrefAnd QrefIs the DG output power reference. E is the three-phase voltage amplitude of the terminal voltage of the IIDG under the VSG algorithm.
When the DG is controlled by a virtual synchronous motor (VSG), unlike the PQ control, the DG under the control of the virtual synchronous motor is an equivalent controlled voltage source, i.e., the DG outlet voltage is controlled by an algorithm. Namely, the output voltage is determined by a control mode and is controlled by the output power and the angular frequency; the DG output current is determined by the circuit equation.
When k is 1, the VSG-DG outlet voltage does not generate sudden change as a controlled voltage source, and the voltage amplitude and the phase angle are still the values before loop closing, namely v3(t1)＝v3.0(t1). Then DG output current i3(t1)＝[v3,0(t1)-v1(t1)]/Z3Wherein: i.e. i3The current of branch 3, here phase a current. v. of3The voltage of branch 3, here the a-phase voltage.
The non-periodic component can be filtered by considering the power measurement value introduced by the control system, and the calculation formula of the active power and the reactive power after the direct-current component is filtered is as follows:
Figure BDA0002477327230000051
wherein: ' (t)k)-1
k>1, the power and phase angle measured at the last moment can be used as feedback to enter a control system, and the amplitude and phase angle of the DG outlet voltage at the moment can be obtained according to a control equation
Figure BDA0002477327230000052
Wherein: Δ t is the calculation time interval in the algorithm.
The final DG output current obtained is: i.e. i3(tk)＝[Vm,3(tk)sin((tk))-v1(tk)]/Z3Wherein: i.e. i3The current of branch 3, here phase a current. Vm,3Is DG outlet after loop closingThe steady state magnitude of the voltage.
As shown in FIG. 5, when the DG employs droop control, the control equation is
Figure BDA0002477327230000053
Wherein: ω is the DG output angular frequency value, ωrefIs an angular frequency reference value, i.e. the angular frequency corresponding to the power frequency, DPIs the active droop coefficient. P and Q are power values output by DG port, DQIs a reactive sag factor, PrefAnd QrefIs the DG output power reference. And E is the three-phase voltage amplitude of the terminal voltage of the DG.
Similar to the control of DG under the virtual synchronous motor, when k is 1, the DG outlet voltage does not generate sudden change as a controlled voltage source, namely v3(t1)＝v3,0(t1) Then the DG output current and the power value are obtained by the above method.
k>1, a last-time measured value is used as feedback to enter a control system, and the amplitude and the angular frequency of the DG outlet voltage at the moment can be obtained according to a control equation:
Figure BDA0002477327230000054
wherein: p (t)k-1) And Q (t)k-1) And (5) corresponding to the power value output by the DG port at the circulation moment by k-1, and similarly obtaining the DG output current.
In summary, the loop closing current solving idea is different for different output characteristics DG. Table 1 compares the solutions of the loop closing currents of the current source type and the voltage source type DG. Wherein, the DG loop closing instant DG output current of the current source type DG does not generate mutation, the output voltage is determined by a circuit equation, and the output current is determined by a control equation; for a voltage source type DG, because the DG output voltage cannot be suddenly changed at the loop closing moment, the solution thought of the loop closing current is as follows: and solving the DG output current according to a line equation, and solving the DG output voltage according to the bus 1 voltage and a control equation.
TABLE 1 Current Source type and Voltage Source type DG Loop-closing Current
Figure BDA0002477327230000061
According to the DG output current calculation method, an impact current buffer strategy is provided: at the moment of loop closing, the DG output current is suddenly increased to form impact current, so that the safe and stable operation of the power grid is influenced. The method is implemented by a unified DG loop-closing current buffer control strategy as shown in fig. 6 for DG with two different external characteristics of a current source type and a voltage source type, and includes specific steps.
3.1) DG output current calculation. And (3) calculating the DG impact current according to different control strategies and DG output characteristics, and if the instantaneous current value of a certain phase at a certain moment is larger than a threshold value, entering a step (2) to implement an impact current control algorithm.
3.2) impact current suppression: recording the maximum moment t when the DG impact current is greater than the threshold valuek *At tk *After the moment, searching for the moment t when the impact current reaches the minimum value for the first timeksWill tcTo tksInter-control parameter lambda (t)k) Decrease by 0.01, where λ (t)k) Is a power factor, Pref(tk)＝λ(tk)PrefAnd Qref(tk)＝λ(tk)QrefWhile adjusting tkλ (t) of the latter periodk) So that it rises smoothly.
3.3) when the parameter is adjusted, the control parameter P is causedrefOr QrefIf the value is less than zero, the control parameter is adjusted to zero, and the closed loop condition is still not met. This indicates that the difference between the DG steady-state current value and the threshold current value is small, and the small fluctuation in the system causes the DG output transient current to exceed the threshold value, so that in this case, loop closing is still not allowed; when the control parameter is not less than zero, the step returns to the step 3.1, namely, the DG impact current i is recalculated according to the updated control parameter3,p(tk) And (p ═ a, b, c). The control parameter is selected to be adjusted when the inrush current is minimized in order to minimize the influence of the parameter variation on the DG output and reduce the output ripple.
FIG. 7 shows an example of the regulation of the inrush current control, the initial inrush current being shown in FIG. 7(a) due to the presence of an inrushWhen the current is larger than the threshold, finding the maximum moment when the impact current is larger than the threshold, wherein the impact current reaches the minimum value for the first time, and reducing the power reference value in the time interval to be lambda (t)k) 0.99. The impulse current obtained by substituting the control parameter into the recalculation is shown in fig. 7 (b). Since there is a moment after which the control parameter is increased from 0.99 to 1, the surge current will be superimposed by a new ripple. Meanwhile, as can be seen from the figure, because the suppression of the impact current is insufficient, the situation that the impact current is larger than the threshold current still exists within a period of time, and therefore, the control parameter lambda (t) is continuously reducedk) The rush current is recalculated to 0.98 as shown in fig. 7 (c). In this case, although the inrush current is sufficiently suppressed, the current fluctuation due to the control parameter variation exceeds the threshold current, and therefore, it is necessary to lower the control parameter λ (t)k) 0.97. The impact current is obtained by recalculation as shown in fig. 7(d), and at the moment, the impact current is smaller than the threshold current at any moment, so that the loop closing impact is comprehensively controlled.
The threshold current needs to be set in consideration of the relationship between the closed-loop current and the maximum allowable load current, and the relay protection current setting value. For the influence of the closed-loop current on the current protection in the conventional power distribution network, the document [14] has performed detailed analysis, and indicates that the attenuation non-periodic component of the closed-loop current generally does not affect the current protection section ii, and the closed-loop current also does not affect the current quick-break protection as long as the effective value of the steady-state current after the closed-loop does not exceed the maximum allowable current-carrying capacity of the feeder line.
However, after considering DG grid connection, a bus outlet in actual engineering is generally provided with a pilot differential protection as a main protection, and when a line fails, switches on two sides are enabled to trip rapidly at the same time. Therefore, the influence of the closed-loop current on the pilot protection is considered, namely, the longitudinal differential protection ensures that the protection device does not trip in normal operation and avoids the maximum unbalanced current in normal operation. However, as can be seen from the above analysis, after the loop closing operation, the current on the line has an impact component, which may cause a short-time increase in the current value, which may cause an increase in the primary current flowing into the current transformers at both ends of the line. Because the current transformers at two ends of the line have different excitation characteristics, namely different ferromagnetic saturation degrees, two secondary currents are generatedThe larger difference, i.e. the increase in the unbalance current. In addition, the impact component contains a non-periodic component, and the amplitude of the exciting current required by the current transformer when the non-periodic component is transmitted is far larger than that required when the periodic component is transmitted. Thus, core saturation may be caused and the unbalance current may increase. Therefore, the influence of the loop-closing surge current on the longitudinal differential protection should be considered, and the threshold current I is set based on the considerationmax。
In this embodiment, simulation verification is performed specifically in a certain distribution network area in the shanghai songjiang area, and a simulation system is shown in fig. 8. The simulation is carried out by respectively adopting PQ control, VSG control and droop control inverter type DG, and the control parameters are shown in Table 2.
TABLE 2 control parameters
Figure BDA0002477327230000071
VSG controlled inverter DG: the phase angle difference between the power supply 1 and the power supply 2 is 5 degrees, the loop closing operation is carried out when the phase angle difference is 2 seconds, and the threshold current Imax0.473 kA. Fig. 9 shows the simulation results, and it can be seen that after loop closing, the voltage of the bus 1 rises and the voltage of the bus 2 falls.
For the inverter in the simulation model, the VSG control scheme is adopted, and fig. 10 shows the IIDG output current simulation result before the suppression strategy is adopted. It can be seen that the IIDG steady state current drops after loop closing, but the IIDG output current exceeds the threshold current between 2s and 2.15s, thus requiring a suppression strategy. Fig. 11 shows the variation of the IIDG output current, and it can be seen that the inrush current is suppressed between 2s and 2.15 s.
PQ-controlled inversion type DG: the phase angle difference between the power supply 1 and the power supply 2 is-10.5 degrees, loop closing operation is carried out when the phase angle difference is 2s, and the threshold current Imax0.471 kA. Fig. 12 shows the bus voltage simulation result.
Fig. 13 shows the results of the IIDG output current simulation with PQ control before the suppression strategy is applied. It can be seen that the steady-state current of the IIDG after loop closing is very close to the threshold current, which means that the control parameter λ < 0 can suppress the inrush current, and therefore the loop closing operation is not allowed.
Droop controlled inversionType DG: the phase angle difference between the power supply 1 and the power supply 2 is-3.5 degrees, the loop closing operation is carried out when the phase angle difference is 2s, and the threshold current Imax0.471 kA. Fig. 14 shows bus voltage simulation results.
Fig. 15 and 16 show the results of the IIDG output current simulation using the front and rear droop control with the suppression strategy, respectively. It can be seen that the rush current between 2s and 2.047s is completely suppressed.
The foregoing embodiments may be modified in many different ways by those skilled in the art without departing from the spirit and scope of the invention, which is defined by the appended claims and all changes that come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Claims (7)

1. A method for restraining loop closing operation impact current of a power distribution network with a distributed power supply is characterized in that impact current after loop closing of a voltage source type DG and a current source type DG is estimated by modeling the power distribution network with the DG, further based on calculation of the loop closing impact current of the DG with different output external characteristics, a unified strategy suitable for the voltage source type DG and the current source type DG is adopted to restrain the loop closing impact current, control parameters are adjusted in a self-adaptive mode to enable the loop closing current not to exceed a threshold value, and switching is carried out when the impact current is minimum, so that influence of control parameter change on DG output is reduced remarkably.
2. The method for suppressing the loop closing operation impact current of the power distribution network with the distributed power supplies according to claim 1, wherein the modeling of the power distribution network with the DGs is that: according to three typical connections of a DG access distribution network system, a closed loop current analysis basic model is provided, which comprises the following steps: two power branch roads that lie in the both sides that close the ring and DG branch road, the branch road that closes the ring and the equivalent load branch road that link to each other with the power branch road respectively, wherein: in the model, a branch 1 and a branch 2 are power supply branches on two sides of a loop closing circuit, the impedance comprises a transformer reactance and line impedance, a branch 3 is a branch where a DG is located, the DG is equivalent to a controlled power supply, a branch 4 is a loop closing branch, and a branch 5 and a branch 6 are equivalent load branches.
3. The method for suppressing the loop closing operation impact current of the power distribution network with the distributed power supplies according to claim 1, wherein the step of estimating the impact current after the loop closing of the voltage source type DG and the current source type DG comprises the following specific steps:
1) calculating the closed loop current of the power distribution network;
2) DG loop closing current analysis and calculation specifically comprises the following steps: analyzing and calculating loop closing currents of a current source type DG and a droop control under the PQ control and a voltage source type DG under the control of a virtual synchronous motor;
3) according to a DG output current calculation method, an impact current buffer strategy is provided: at the moment of loop closing, the DG output current is suddenly increased to form impact current, so that the safe and stable operation of a power grid is influenced; the method is realized by a unified DG loop-closing current buffer control strategy aiming at DGs with two different external characteristics of a current source type and a voltage source type.
4. The method for suppressing the loop closing operation impact current of the power distribution network with the distributed power supplies according to claim 3, wherein the step 1 specifically comprises: closed loop branch full current after closed loop
Figure FDA0002477327220000011
Wherein: i iscRepresenting the amplitude of steady-state loop current caused by the voltage difference between two sides of the interconnection switch at the loop closing point, α is the voltage angle difference between two sides of the interconnection switch during loop closing, β is the loop closing loop impedance angle, tcFor closing the ring time, TcIs the non-periodic component decay time constant; i ism,4And
Figure FDA0002477327220000012
representing the steady-state current amplitude and the initial phase angle after the loop closing at the loop closing position; omega is power frequency angular velocity, t is time;
after the network is closed, the impact current flowing through the closed loop branch can be distributed to the non-closed loop branch, and the current i of the non-closed loop branchkEqual to the superposition of the original branch current and the impact current:
Figure FDA0002477327220000021
wherein: i ism,kAnd
Figure FDA0002477327220000022
representing the steady-state current amplitude and phase angle of the branch k after loop closing; omega is power frequency angular velocity, t is time; i ism,k,0And
Figure FDA0002477327220000023
representing the initial value of the steady-state current amplitude and the initial phase angle of the branch k after the loop closing; t iskAnd ckThe attenuation time constant and the initial value of the non-periodic component are obtained; so the voltage v of the bus 1 after the loop closing1Comprises the following steps:
Figure FDA0002477327220000024
wherein: e1And theta1The steady state amplitude and the initial phase angle, L, of the power supply at the front section of the loop closing position1，R1，Z1And
Figure FDA0002477327220000025
is the inductive resistance impedance and the impedance angle, V, of branch 1m,1And1the steady-state amplitude and the initial phase angle of the bus 1 voltage after loop closing are shown.
5. The method for suppressing the loop closing operation impact current of the power distribution network with the distributed power supplies according to claim 3, wherein the step 2 specifically comprises: when the DG adopts PQ control, the control equation is
Figure FDA0002477327220000026
Wherein: u. ofdAnd idAs d-axis voltage current measurements, iqIs a q-axis current measurement; prefAnd QrefIs the DG output power reference; when the DG is controlled by PQ, the DG can be equivalent to a controlled current source, that is, the output active power of the DG remains unchanged, and the output current of the DG is determined by a control mode and is controlled by an output voltage; the DG output voltage is determined by a circuit equation;
in the circulation flow of calculating the output current, when the circulation number k is 1, the controlled current source is used, and the DG output current does not generate sudden change, namely i3(t1)＝i3,0(t1) According to circuit theory, then, the DG output voltage v3(t1)＝i3,0(t1)Z3+v1(t1)；tkIs the time corresponding to k cycles;
the DG output voltage is fed back to the control system, and because the DC component can be filtered when the ABC coordinate axial direction DQ0 coordinate axis conversion is carried out on the voltage and current measured value in the control, each feedback quantity introduced in the DG control does not contain the non-periodic component, and the voltage under the DQ0 coordinate
Figure FDA0002477327220000027
Wherein: theta is a conversion angle when the ABC coordinate axis is converted to the DQ0 coordinate axis; vm,1And1the steady-state amplitude and the initial phase angle of the voltage of the bus 1 after loop closing are obtained; i.e. i3a，i3bAnd i3cThree-phase currents in each case of branch 3
Number of cycles k>1, the voltage value measured at the last moment is used as feedback to enter a control system to obtain a DG output current instantaneous value
Figure FDA0002477327220000028
Wherein: i.e. i3The current of branch 3, here phase a current;
when the DG is provided with the energy storage equipment, a virtual synchronous motor (VSG) control strategy is adopted, and the control equation is
Figure FDA0002477327220000031
Wherein: the virtual inertia constant is H, P is the active power value output by the inverter port under the control of VSG, omega is the DG output angular frequency value, omegarefThe reference value of the angular frequency is the angular frequency corresponding to the power frequency, and D is the active droop coefficient; q is the reactive power value output by the DG port under VSG control, DQIs a reactive sag factor, EsetIs DG terminal voltage under VSG algorithmA reference value; prefAnd QrefIs the DG output power reference; e is the three-phase voltage amplitude of the terminal voltage of the IIDG under the VSG algorithm;
when the DG is controlled by the virtual synchronous motor, different from the PQ control, the DG under the control of the virtual synchronous motor is an equivalent controlled voltage source, namely the DG outlet voltage is controlled by an algorithm; namely, the output voltage is determined by a control mode and is controlled by the output power and the angular frequency; the DG output current is determined by a circuit equation;
when k is 1, the VSG-DG outlet voltage does not generate sudden change as a controlled voltage source, and the voltage amplitude and the phase angle are still the values before loop closing, namely v3(t1)＝v3.0(t1) (ii) a Then DG output current i3(t1)＝[v3,0(t1)-v1(t1)]/Z3Wherein: i.e. i3The current of branch 3, here phase a current; v. of3The voltage of branch 3, here the a-phase voltage;
the non-periodic component can be filtered by considering the power measurement value introduced by the control system, and the calculation formula of the active power and the reactive power after the direct-current component is filtered is as follows:
Figure FDA0002477327220000032
wherein: ' (t)k)-1
k>1, the power and phase angle measured at the last moment can be used as feedback to enter a control system, and the amplitude and phase angle of the DG outlet voltage at the moment can be obtained according to a control equation
Figure FDA0002477327220000033
Wherein: Δ t is the calculation time interval in the algorithm;
the final DG output current obtained is: i.e. i3(tk)＝[Vm,3(tk)sin((tk))-v1(tk)]/Z3Wherein: i.e. i3The current of branch 3, here phase a current; vm,3The steady-state amplitude of the DG outlet voltage after loop closing is obtained;
when DG adopts droop control, itThe control equation is
Figure FDA0002477327220000034
Wherein: ω is the DG output angular frequency value, ωrefIs an angular frequency reference value, i.e. the angular frequency corresponding to the power frequency, DPIs the active droop coefficient; p and Q are power values output by DG port, DQIs a reactive sag factor, PrefAnd QrefIs the DG output power reference; e is the three-phase voltage amplitude of the terminal voltage of the DG;
similar to the control of DG under the virtual synchronous motor, when k is 1, the DG outlet voltage does not generate sudden change as a controlled voltage source, namely v3(t1)＝v3,0(t1) If so, the DG output current and the power value are obtained by adopting the mode;
k>1, a last-time measured value is used as feedback to enter a control system, and the amplitude and the angular frequency of the DG outlet voltage at the moment can be obtained according to a control equation:
Figure FDA0002477327220000041
wherein: p (t)k-1) And Q (t)k-1) And (5) corresponding to the power value output by the DG port at the circulation moment by k-1, and similarly obtaining the DG output current.
6. The method for suppressing the impact current of the closed loop operation of the power distribution network with the distributed power supplies according to claim 3, wherein the impact current buffering strategy comprises the following specific steps;
3.1) computing the DG output current; calculating the DG impact current according to different control strategies and DG output characteristics, and if the instantaneous current value of a certain phase at a certain moment is larger than a threshold value, entering a step 2 to implement an impact current control algorithm;
3.2) impact current suppression: recording the maximum moment t when the DG impact current is greater than the threshold valuekAt tkAfter time t, searching for the time t when the impact current reaches the minimum value for the first timeksWill tcTo tksInter-control parameter lambda (t)k) Decrease by 0.01, where λ (t)k) Is a power factor, Pref(tk)＝λ(tk)PrefAnd Qref(tk)＝λ(tk)QrefWhile adjusting tkλ (t) of the latter periodk) So that the water is smoothly raised;
3.3) when the parameter is adjusted, the control parameter P is causedrefOr QrefWhen the voltage is less than zero, the difference between the DG steady-state current value and the threshold current is small, and the DG output transient current exceeds the threshold due to small fluctuation in the system, so that loop closing is not allowed; when the control parameter is not less than zero, the step returns to the step 3.1, namely, the DG impact current i is recalculated according to the updated control parameter3,p(tk),(p＝a,b,c)。
7. A rush current suppression system for implementing the method of any preceding claim, comprising: a current calculation unit and an inrush current suppression unit, wherein: the current calculation unit is connected with the power distribution network measurement device and receives DG grid-connected bus voltage information, the impact current suppression unit is connected with the current calculation unit and transmits DG output current information under different control parameters, and the impact current suppression unit is respectively connected with each DG controller and transmits control parameter information.
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