CN111682583A - Power grid reactive power equal division control method based on self-adaptive virtual impedance - Google Patents

Power grid reactive power equal division control method based on self-adaptive virtual impedance Download PDF

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CN111682583A
CN111682583A CN202010442156.0A CN202010442156A CN111682583A CN 111682583 A CN111682583 A CN 111682583A CN 202010442156 A CN202010442156 A CN 202010442156A CN 111682583 A CN111682583 A CN 111682583A
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power
reactive power
control method
virtual impedance
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于惠钧
龚星宇
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Hunan University of Technology
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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
    • 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
    • H02J3/388—Islanding, i.e. disconnection of local power supply from the network
    • 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
    • H02J3/46—Controlling of the sharing of output between the generators, converters, or transformers
    • H02J3/466—Scheduling the operation of the generators, e.g. connecting or disconnecting generators to meet a given demand
    • 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
    • H02J3/46—Controlling of the sharing of output between the generators, converters, or transformers
    • H02J3/50—Controlling the sharing of the out-of-phase component
    • 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
    • H02J2203/10—Power transmission or distribution systems management focussing at grid-level, e.g. load flow analysis, node profile computation, meshed network optimisation, active network management or spinning reserve management
    • 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
    • H02J2203/20—Simulating, e g planning, reliability check, modelling or computer assisted design [CAD]

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Abstract

The invention provides a power grid reactive power equal division control method based on self-adaptive virtual impedance, which comprises the following steps of: 1. analyzing a traditional droop control method to obtain expressions of active power and reactive power of the distributed power supply; 2. obtaining a traditional P-f droop control expression and a traditional Q-E droop control expression from the 1; 3. introducing self-adaptive virtual impedance, and self-adaptively adjusting the reactive power of each DG unit of the distributed power supply; 4. the effectiveness and the feasibility of the control method are verified through simulation. The control method accurately realizes the reactive power sharing of the distributed micro-sources, only uses local information for control, does not need information transmission among the micro-sources, and ensures the plug-and-play characteristic of the micro-grid; the impedance value of the feeder line does not need to be estimated or measured online in advance, so that the complexity and the cost of the system are reduced; the invention has lower requirement on the structure of the micro-grid and can be suitable for a complex impedance network.

Description

Power grid reactive power equal division control method based on self-adaptive virtual impedance
Technical Field
The invention relates to the field of micro-grids, in particular to a power grid reactive power equipartition control method based on self-adaptive virtual impedance.
Background
In recent years, micro-grids have become popular research focuses at home and abroad due to diversified energy supply modes and flexible control modes. When the micro-grid isolated island operates, the power sharing is realized by adjusting the amplitude and the frequency of the voltage. In the traditional droop control method, because the frequencies of the system lines are consistent, the respective active powers can be distributed in a balanced manner. And reactive output cannot be reasonably distributed according to a conventional droop coefficient due to mismatching of feeder line impedance, so that reactive circulation current is generated among distributed power supplies, and the electric energy quality and the stability of a system are greatly influenced. Therefore, how to realize the reactive power sharing among the DGs is the first problem to be solved by the microgrid in the running process.
In order to achieve the uniform reactive power, a series of researches are carried out by scholars at home and abroad. From the perspective of compensating for line impedance mismatching, a feeder voltage drop online estimator is designed in literature, and real-time estimated voltage is incorporated into a power control scheme to realize accurate control of reactive power equalization in a microgrid during island operation. The disadvantage is that the microgrid should be operated in a grid-connected state before the islanding operation for the purpose of correctly estimating the voltage drop. Impedance mismatch information between lines is obtained in advance by injecting harmonics in the lines, but the harmonic waves introduced by the method can cause the electric energy quality of the microgrid to be reduced. In droop control, the phenomenon of power coupling can be decoupled and eliminated by introducing certain virtual impedance, and meanwhile, the reactive power sharing precision is improved. In order to adjust the output impedance of the micro-source, the voltage drop at two ends of the virtual impedance is reduced from the voltage closed-loop instruction by using a virtual impedance method. The method effectively reduces the influence of the impedance of the line on the output power, and has the defect that the voltage drop problem occurs at the output end of the micro source. By introducing appropriate virtual impedance values, the equivalent unit impedance of the DG is designed to be inversely proportional to its nominal value to eliminate reactive power distribution errors. However, the control method of the virtual impedance is based on the fact that the physical feed line is known, which is not easily available in the usual case.
Disclosure of Invention
The invention aims at the problems that the reactive power is not accurately evenly divided, the application range is single, the stability is not high, and the virtual impedance is not easy to obtain in the prior art. In order to achieve the purpose, the invention provides a power grid reactive power equal division control method based on self-adaptive virtual impedance.
A power grid reactive power uniform control method based on self-adaptive virtual impedance comprises the following steps:
s1, analyzing the traditional droop control method to obtain an expression of active power and reactive power of the distributed power supply; the expressions of active power and reactive power of the distributed power supply are respectively as follows:
Figure BDA0002504560230000021
Figure BDA0002504560230000022
wherein: eiRepresenting the output voltage amplitude, V, of the ith distributed power supplyPCCIs the amplitude of the ac voltage on the common bus,iis a power angle, Rli，XliResistance and reactance of the line respectively;
s2, obtaining a traditional P-f droop control expression and a traditional Q-E droop control expression from 1;
the conventional P-f droop control expression is:
ωi＝ω0-mp；
m＝Δω/pimax；
wherein: omega0And ωiRespectively the nominal angular frequency and the reference angular frequency of the ith DG unit; m is the droop coefficient of the frequency; Δ ω is the maximum frequency deviation allowed by the inverter; p and PmaxActual and maximum actual output power of the ith DG unit, respectively;
s3, introducing self-adaptive virtual impedance, and self-adaptively adjusting the reactive power of each DG unit of the distributed power supply;
and S4, verifying the effectiveness and feasibility of the control method through simulation.
Further, in step S1, when the line impedance is mainly inductive, there is X > > R, and at this time, the resistance is negligible, and the expressions of the active power and the reactive power of the distributed power supply are:
Pi＝EiVpcc i/Xli；
Qi＝(EiVpcc-V2 pcc)/Xli；
wherein: in the formula, EiRepresenting the output voltage amplitude, V, of the ith distributed power supplyPCCIs the amplitude of the ac voltage on the common bus,iis a power angle, Rli，XliRespectively the resistance and reactance of the line.
Further, before the virtual impedance is adjusted in step S3, the reactive power Q and the reactive power demand Q are acquired*The value of (c).
Further, in step S3, the adaptive virtual impedance is expressed as:
Figure BDA0002504560230000031
wherein:
Figure BDA0002504560230000032
fixed virtual inductance, kiQIs to adjust the integral gain of the virtual inductor.
Further, the conventional Q-E droop control expression in step S2 is:
Ei＝E0-nQ；
n＝ΔE/Qimax；
wherein E0And E is the rated voltage amplitude and the reference voltage amplitude of the DG unit respectively; Δ E is the maximum voltage deviation allowed for the inverter; n is the droop coefficient of the voltage amplitude; q and QmaxThe actual and maximum reactive power output of the ith DG unit, respectively.
Further, the inverter reactive power requirement in a distributed power supply is expressed as:
Figure BDA0002504560230000033
wherein Q isratedFor the total rated reactive power, Q, of the invertertotalSum of the reactive powers, Q, of all inverters received for a microgrid inverter*Is the reactive demand of each DG unit.
Further, in step S4, a simulation verification is performed using a microgrid microstructure simulation model including two DG units.
Further, in step S4, the power performance diagram is obtained by simulating the simulation model by using the conventional droop control method and the adaptive virtual impedance-based grid reactive power sharing control method, respectively.
The invention has the beneficial effects that:
1. the reactive power sharing of the distributed micro sources is accurately realized, only local information is used for control, information transmission among the micro sources is not needed, and the plug and play characteristic of the micro grid is guaranteed.
2. The impedance value of the feeder line does not need to be estimated or measured online in advance, and the complexity and the cost of the system are reduced.
3. The requirement on the structure of the micro-grid is reduced, and the micro-grid impedance matching method is applicable to complex impedance networks.
Drawings
FIG. 1 is a simplified schematic diagram of a microgrid;
FIG. 2 is a graph of the active-frequency relationship;
FIG. 3 is a series virtual impedance control diagram;
FIG. 4 is a control diagram containing two DG units;
FIG. 5 is a graph of the active performance of a conventional droop control;
FIG. 6 is a graph of the reactive performance of the conventional droop control;
FIG. 7 is a diagram of adaptive virtual impedance control active performance;
fig. 8 is a graph of adaptive virtual impedance control reactive performance.
Detailed Description
The present invention will be further described with reference to the following embodiments.
A power grid reactive power average control method based on self-adaptive virtual impedance comprises the following steps:
the traditional droop control method comprises the following steps:
in a low voltage distribution network, each DG unit is connected to a microgrid bus by a feeder. A simplified architecture of a microgrid during islanding operation is shown in fig. 1.
In fig. 1, the active and reactive power flowing through the distributed power supply DG can be calculated and represented as:
Figure BDA0002504560230000041
Figure BDA0002504560230000042
in the formula, EiRepresenting the output voltage amplitude, V, of the ith distributed power supplyPCCIs the amplitude of the ac voltage on the common bus,iis a power angle, Rli，XliRespectively the resistance and reactance of the line. When the line impedance is mainly inductive, there is X>>And R is shown in the specification. At this time, the resistance can be ignored, and the power angleiUsually smaller, i.e. sin: (i)≈，cos(i) 1, then equations (1), (2) can be expressed as:
Pi＝EiVpcc i/Xli(3)
Qi＝(EiVpcc-V2 pcc)/Xli(4)
as can be seen from equations (3) and (4), the active output P and the reactive output Q depend on the power angle and the voltage difference Δ V ═ E, respectivelyi-VPCCAnd is in direct proportion to[7]. At the same time, both active and reactive power are equal to the line impedance XliIn inverse proportion. The traditional P-f droop control is expressed as follows:
ωi＝ω0-mp (5)
m＝Δω/pimax(6)
wherein ω is0And ωiRespectively the nominal angular frequency and the reference angular frequency of the ith DG unit; m is the droop coefficient of the frequency; Δ ω is the maximum frequency deviation allowed by the inverter; p and PmaxThe actual and maximum actual output power of the ith DG unit, respectively. Similarly, the conventional Q-E droop expression is:
Ei＝E0-nQ (7)
n＝ΔE/Qimax(8)
wherein E0And E is the rated voltage amplitude and the reference voltage amplitude of the DG unit respectively; Δ E is the maximum voltage deviation allowed for the inverter; n is the droop coefficient of the voltage amplitude; q and QmaxThe actual and maximum reactive power output of the ith DG unit, respectively.
Fig. 2 is a diagram of the relationship between active power and frequency. It can be seen that at the same frequency and voltage amplitude, a larger capacity DG unit provides more power due to its smaller droop slope. When the system operates in a steady state, each DG unit works under the same frequency, and each distributed unit can realize accurate active power sharing through P-omega droop control. However, due to the influences of the impedance mismatch of the feeder line and the large difference between the network ports corresponding to the ports, the voltage amplitudes of the DG units are difficult to unify, so that the reactive power cannot be reasonably distributed through the Q-V droop control, and the purpose of sharing is achieved.
Power grid reactive power equal division control method based on self-adaptive virtual impedance
And after receiving the reactive power information, the microgrid controller determines the total rated reactive power of the microgrid inverter. Finally, the feedback is sent back to all inverters, and each inverter determines the respective reactive power requirement (Q) according to the received total rated reactive power1 *、Q2 *…, and Qn *). Thus, the reactive power demand of each inverter can be expressed as:
Figure BDA0002504560230000051
wherein Q isratedFor the total rated reactive power, Q, of the invertertotalIs the sum of the reactive power of all inverters received by the microgrid inverter. Q*For each DG unitAnd (4) reactive power demand.
In a fixed virtual inductor
Figure BDA0002504560230000061
An integration element is introduced to adjust the DG virtual impedance shown in fig. 3. Before that, the reactive power Q and the reactive power demand Q need to be obtained*And taking the difference between them to adaptively adjust the DG unit LvirThe virtual impedance of (2) is specifically expressed as:
Figure BDA0002504560230000062
wherein
Figure BDA0002504560230000063
Fixed virtual inductance, kiQIs to adjust the integral gain of the virtual inductor. When the load in the power grid changes, the virtual impedance value can be correspondingly changed according to the change value, and the reactive power sharing error is controlled within a small range.
Simulation verification:
a micro-grid microstructure simulation model containing two DGs as shown in FIG. 4 is built on a Matlab/Simulink software platform. The simulation model parameters are shown in the following table:
Figure BDA0002504560230000064
the microgrid model in fig. 4 is composed of two DG units with the same capacity and several linear loads, and is connected by a feeder line, and then is connected in parallel to the PCC and then to the large power grid. The control parameters of each DG unit used in the simulation are shown in the above table. The MGCC exchanges the required information with the DG local controller through low bandwidth communications. Under the condition of a certain rated power, the two DG units averagely share the reactive and active requirements of the load.
Simulation analysis of traditional droop control
Two DG units with the same capacity adopting the traditional droop control method are operated in parallel. As shown in fig. 5 and fig. 6, when the microgrid starts to operate, it is assumed that the load 1 and the load 2 are in the disconnected state, and only the load 3 is connected to the PCC. When t is 4s, the load 4 is switched in to the PCC to realize a step load change. When t is 7s, the local load (load 2) of DG2 is connected to the system, which keeps the local load (load 1) of DG1 still in the off state. After the operation, the active power of the two DG units has a short jitter, but quickly and consistently, and the equal division is realized. I.e., the line impedances of the two DG units do not match, the active power requirement controlled by the conventional droop method can be accurately shared between the two DG units. However, reactive power sharing shows very poor performance due to the effect of mismatched feeder impedances. Particularly, when a local load acts, the phenomenon of uneven reactive power distribution is more serious.
Simulation method for power grid reactive power equal division control based on self-adaptive virtual impedance
In the parallel operation process of two inverters with the same capacity, the improved virtual impedance control reactive power equipartition strategy is adopted, and the simulation process is as follows: the traditional control method is adopted for 0-1 s; the improved virtual impedance control method is adopted within 1-9s to compensate errors in the traditional control method. And when t is 4s, adding a load 4 to a PCC interface to research the distribution situation of the control strategy adopted when the impedance of the feeder line changes to the reactive power output. At t-7 s, keeping load 1 still connected to DG1, the local load 2 of local DG2 is ramped up to investigate the effectiveness of the proposed control strategy for the presence of a local load.
As shown in fig. 7, the proposed virtual impedance reactive sharing control scheme causes only small transient changes for the active power sharing between DG units, and the proposed control strategy is taken when t is 1-2s, causing small transient changes, but the sharing is restored very quickly. When t is 7s, the active power continuously fluctuates for about 0.5s, overshoot occurs in the traditional droop control method, the recovery time is long, overshoot also occurs in the proposed virtual impedance reactive power sharing control method, but the recovery time is fast, the system is more stable, and the active power sharing is well followed.
As shown in fig. 8, in the conventional droop control method with t being 0-1s, the reactive power sharing performance is poor, the proposed control strategy is adopted when t is 1s, the reactive power deviation between two DGs starts to be compensated, and the sharing effect is achieved when t is 2 s. After the sudden increase of the feeder impedance load 4, the reactive power distribution is not affected either, and the equal division is kept on. When t is 7s, when the local load 2 is added to DG2 alone, the reactive power continues to fluctuate for about 0.5s, overshoot occurs in the conventional droop control method, and the proposed virtual impedance reactive power share control method is smooth and stable rise, and finally share is achieved after 0.5 s. The results show that even if the load changes or local loads are applied, the reactive power of the two DG units is equally distributed after the transient duration of 1.5s, and the reactive average error is almost 0. Compared with the traditional droop control method, the virtual impedance reactive power equal-division control method has the advantages of better stability and better reactive power balance performance.
The above-mentioned embodiments are merely preferred embodiments for fully illustrating the present invention, and the scope of the present invention is not limited thereto. The equivalent substitution or change made by the technical personnel in the technical field on the basis of the invention is all within the protection scope of the invention.

Claims (8)

1. A power grid reactive power uniform control method based on self-adaptive virtual impedance comprises the following steps:
s1, analyzing the traditional droop control method to obtain an expression of active power and reactive power of the distributed power supply; the expressions of active power and reactive power of the distributed power supply are respectively as follows:
Figure FDA0002504560220000011
Figure FDA0002504560220000012
wherein: eiRepresenting the output voltage amplitude, V, of the ith distributed power supplyPCCIs the amplitude of the ac voltage on the common bus,iis a power angle, Rli，XliResistance and reactance of the line respectively;
s2, obtaining a traditional P-f droop control expression and a traditional Q-E droop control expression from 1;
the conventional P-f droop control expression is:
ωi＝ω0-mp；
m＝Δω/pimax；
wherein: omega0And ωiRespectively the nominal angular frequency and the reference angular frequency of the ith DG unit; m is the droop coefficient of the frequency; Δ ω is the maximum frequency deviation allowed by the inverter; p and PmaxActual and maximum actual output power of the ith DG unit, respectively;
s3, introducing self-adaptive virtual impedance, and self-adaptively adjusting the reactive power of each DG unit of the distributed power supply;
and S4, verifying the effectiveness and feasibility of the control method through simulation.
2. The method according to claim 1, wherein in step S1, when the line impedance mainly exhibits inductive property, there is X > > R, and at this time, the resistance is negligible, and the expressions of the active power and the reactive power of the distributed power supply are:
Pi＝EiVpcc i/Xli；
Qi＝(EiVpcc-V2 pcc)/Xli；
wherein: in the formula, EiRepresenting the output voltage amplitude, V, of the ith distributed power supplyPCCIs the amplitude of the ac voltage on the common bus,iis a power angle, Rli，XliRespectively the resistance and reactance of the line.
3. The method for controlling the reactive power sharing of the power grid based on the adaptive virtual impedance of claim 1, wherein the reactive power Q and the reactive power demand Q are obtained before the virtual impedance is adjusted in step S3*The value of (c).
4. The method for controlling reactive power sharing of a power grid based on an adaptive virtual impedance as claimed in claim 1, wherein in step S3, the adaptive virtual impedance is expressed as:
Figure FDA0002504560220000021
wherein:
Figure FDA0002504560220000022
fixed virtual inductance, kiQIs to adjust the integral gain of the virtual inductor.
5. The method for controlling reactive power average of power grid based on adaptive virtual impedance of claim 1, wherein in step S2, the expression of the conventional Q-E droop control is:
Ei＝E0-nQ；
n＝ΔE/Qimax；
wherein E0And E is the rated voltage amplitude and the reference voltage amplitude of the DG unit respectively; Δ E is the maximum voltage deviation allowed for the inverter; n is the droop coefficient of the voltage amplitude; q and QmaxThe actual and maximum reactive power output of the ith DG unit, respectively.
6. The method for controlling reactive power of power grid based on adaptive virtual impedance of claim 1, wherein in step S3, the reactive power demand of inverter in distributed power supply is expressed as:
Figure FDA0002504560220000023
wherein Q isratedFor the total rated reactive power, Q, of the invertertotalSum of the reactive powers, Q, of all inverters received for a microgrid inverter*For each DReactive demand of the G unit.
7. The power grid reactive power average control method based on the adaptive virtual impedance as claimed in claim 1, wherein in step S4, a micro-grid micro-structure simulation model including two DG units is used for simulation verification.
8. The method according to claim 1, wherein in step S4, the conventional droop control method and the adaptive virtual impedance-based grid reactive power share control method are respectively used to simulate the simulation model, so as to obtain the power performance diagram.
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