CN112671034A - Island type alternating current micro-grid frequency recovery and power optimal distribution method based on distributed two-layer control - Google Patents
Island type alternating current micro-grid frequency recovery and power optimal distribution method based on distributed two-layer control Download PDFInfo
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Abstract
An island type alternating current micro-grid frequency recovery and power optimal distribution method based on distributed two-layer control models a micro-grid system to obtain a system dynamic equation; determining a control target; and constructing a distributed controller; and finally, analyzing the steady state condition of the system to realize frequency recovery and active power distribution as required. The method of the invention considers the problem of frequency deviation caused by bottom layer droop in the alternating-current microgrid, and the provided distributed two-layer controller can not only realize frequency recovery, but also distribute active power as required, thereby further improving the economic benefit of the microgrid system.
Description
Technical Field
The invention belongs to the field of micro-grid control, and particularly relates to an island type alternating current micro-grid frequency recovery and power optimal distribution method based on distributed two-layer control.
Background
With the rapid development of new energy sources including Renewable Energy (RES), wind energy (WS), Photovoltaic (PV), and the like, and the improvement of power utilization reliability, a Microgrid (MG) combining an energy storage unit, a load and a related control device is paid more and more attention on the basis of a Distributed Generation (DG) with high efficiency and low carbon. Unlike fossil-fueled thermal generators in conventional power systems, the DG in microgrid systems is typically powered by clean energy sources, which typically produce intermittent power fluctuations and are also weak in generating capacity. This, in turn, results in microgrid system control that is more challenging than conventional power system control.
The micro-grid work modes mainly include two types: the isolated island power supply can be operated in parallel with a large power grid, can also be operated in an isolated island mode under the condition of power grid faults, independently supplies power to a local load, and has high power supply safety and reliability. At present, inspired by traditional power system control, isolated island microgrid mainly adopts the hierarchical control structure: the distributed droop control of the bottom layer realizes the power distribution problem, the two-layer control aims at eliminating the voltage and frequency deviation problem caused by the droop control of the bottom layer, and the third-layer control is the economic scheduling and optimization problem. For two-layer control, there are three main types of control strategies: centralized, decentralized, and distributed. The centralized control adopts an integrated controller to control the whole network; in the distributed control, a plurality of sub-controllers are adopted to control each DG, and no information interaction exists among the sub-controllers; the distributed control uses the information of the sub-controllers and the neighbors thereof for control, and has higher flexibility and reliability.
In recent years, a microgrid control system, an internet of things and the internet have a high convergence tendency, distributed control of a microgrid is becoming more and more diversified, but for most distributed two-layer control, although frequency and voltage recovery is considered, the related power distribution problem is not researched too much. In the conventional microgrid distributed control, the distribution ratio of the active power is determined by droop gains, and the droop gains are usually selected according to the rated power of the DG, and in an actual microgrid system, the ratio cannot achieve the optimal economic benefit.
Disclosure of Invention
Based on the problems, the invention provides an island type alternating current micro-grid frequency recovery and power optimal distribution method based on distributed two-layer control, which can not only solve the problem of frequency deviation caused by a droop function and enable the frequency to be synchronously tracked to a reference value, but also realize the distribution of the output active power of a micro-grid system according to any proportion.
The present invention provides the following solutions to solve the above technical problems:
an island type alternating current micro-grid frequency recovery and power optimal distribution method based on distributed two-layer control comprises the following steps:
step 1), establishing an alternating current micro-grid system model
Based on a droop control strategy, obtaining an output frequency expression of the alternating-current micro-grid as shown in a formula (1):
wherein tau isPFor the filter time constant, ω is the output frequency, ωdTo set the frequency, kPIs droop coefficient, P is output active power, PdOutputting an active power set value;
step 2), when the micro-grid operates in an island mode, acquiring operation data of a current system, and determining a control target, wherein the process is as follows:
2.1) firstly, in order to solve the frequency deviation caused by the droop function and realize the frequency tracking target, defining the frequency neighborhood tracking error as shown in the formula (2):
wherein ω isiIs the output frequency, ω, of the ith DGjIs the output frequency, ω, of the jth DGrefAs a reference frequency, a frequency of the reference frequency,
fixed gain coefficient for frequency of ith DG;
2.2) in order to optimize output active power distribution, realize accurate distribution of power as required, define the optimal active power distribution error, as shown in formula (3):
wherein P isiIs the output active power of the ith DG, PjIs the output active power of the jth DGiThe active power distribution ratio, χ, of the ith DGjDistributing the ratio for the active power of the jth DG;
and 3) constructing a distributed two-layer controller based on the analysis, wherein the distributed two-layer controller is shown in a formula (4):
eω＝(Lc+G)(ω-ωref)，eP＝Lcχ-1P
wherein
Is a controller gain factor, LcThe method comprises the following steps that a Laplace matrix is communicated with a micro-grid network, G is a frequency error coefficient, and χ is an active power distribution ratio;
and 4) taking the bottom layer droop model as a controlled object, and applying the proportional integral-based distributed two-layer controller to the controlled object to realize frequency recovery.
In the step 3), a proportional-integral controller is constructed by adopting a frequency error term and a power distribution error term, so that the aims of eliminating frequency deviation and distributing power as required are fulfilled. Under the condition of changing the load or DG and other disturbances, the control system can still better respond to the disturbances and eliminate the influence of the disturbances.
The inventionFirstly, sampling micro-grid operation data to obtain a current frequency value and further obtain a frequency error amount eωThe power error term e is used for controlling an integral link to realize frequency compensation and ensuring that the active power of the system is output according to the requirementPAnd the power control target is realized as a proportional link of the controller.
The working principle of the invention is as follows: firstly, carrying out system modeling on an alternating current micro-grid; constructing a frequency error term and a power error term; and further obtaining a distributed two-layer proportional-integral controller, and applying compensation of the distributed two-layer proportional-integral controller to a system model to realize frequency recovery and power distribution as required.
The invention has the advantages that: by modeling the alternating-current microgrid, a brand-new distributed two-layer controller is constructed to recover the frequency of the microgrid and accurately distribute power. Compared with the prior art, the control is realized only by the information communication between the neighbors, the frequency recovery can be realized, the output active power of the DG can be output according to any proportion instead of being distributed according to the droop coefficient ratio, the economic benefit can be greatly improved, and the practical value is high.
Drawings
FIG. 1 is a flow chart of an implementation of the present invention;
FIG. 2 is a control strategy block diagram of the present invention
FIG. 3 is a drawing of a graph when given
Before and after the control strategy, frequency and output active power experiment comparison graphs;
FIG. 4 is a drawing when given
Before and after the control strategy, frequency and output active power experiment comparison graphs;
FIG. 5 is a drawing showing when given
Before and after extra load is connected into the system, frequency and output active powerA power comparison graph;
FIG. 6 is a block diagram of a given block diagram
Before and after the extra load removal system, the frequency and the output active power are compared with each other;
FIG. 7 is a drawing when given
Changing the active power distribution ratio, the frequency and the output active power comparison chart;
FIG. 8 is a drawing showing when given
Changing the active power distribution ratio, the frequency and the output active power comparison chart;
fig. 9 is a graph comparing the change of the number of DGs in the microgrid, the change of the frequency and the change of the output active power;
fig. 10 is a diagram of a DG physical connection and communication topology.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in detail with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
Referring to fig. 1 to 10, an island-type alternating current microgrid frequency recovery and power optimal distribution method based on distributed two-layer control includes the following steps:
step 1), establishing an alternating current micro-grid system model
Based on a droop control strategy, obtaining an output frequency expression of the alternating-current micro-grid as shown in a formula (1):
wherein tau isPFor the filter time constant, ω is the output frequency, ωdTo set the frequency, kPIs droop coefficient, P is output active power, PdOutputting an active power set value;
step 2), when the micro-grid operates in an island mode, acquiring operation data of a current system, and determining a control target, wherein the process is as follows:
2.1) firstly, in order to solve the frequency deviation caused by the droop function and realize the frequency tracking target, defining the frequency neighborhood tracking error as shown in the formula (2):
wherein ω isiIs the output frequency, ω, of the ith DGjIs the output frequency, ω, of the jth DGrefAs a reference frequency, a frequency of the reference frequency,
fixed gain coefficient for frequency of ith DG;
2.2) in order to optimize output active power distribution, realize accurate distribution of power as required, define the optimal active power distribution error, as shown in formula (3):
wherein P isiIs the output active power of the ith DG, PjIs the output active power of the jth DGiThe active power distribution ratio, χ, of the ith DGjDistributing the ratio for the active power of the jth DG;
and 3) constructing a distributed two-layer controller based on the analysis, wherein the distributed two-layer controller is shown in a formula (4):
eω＝(Lc+G)(ω-ωref)，eP＝Lcχ-1P
wherein
Is a controller gain factor, LcThe method comprises the following steps that a Laplace matrix is communicated with a micro-grid network, G is a frequency error coefficient, and χ is an active power distribution ratio;
and 4) taking the bottom layer droop model as a controlled object, and applying the proportional integral-based distributed two-layer controller to the controlled object to realize frequency recovery.
In the step 3), a proportional-integral controller is constructed by adopting a frequency error term and a power distribution error term, so that the aims of eliminating frequency deviation and distributing power as required are fulfilled. Under the condition of changing the load or DG and other disturbances, the control system can still better respond to the disturbances and eliminate the influence of the disturbances.
In the invention, firstly, the current frequency value is obtained by sampling the micro-grid operation data, and the frequency error quantity e is further obtainedωThe power error term e is used for controlling an integral link to realize frequency compensation and ensuring that the active power of the system is output according to the requirementPAnd the power control target is realized as a proportional link of the controller.
In order to visually verify the effect of the strategy provided by the invention, the following experiment effects are respectively explained by examples: table 1 gives the main electrical parameters and controller parameters.
TABLE 1
The invention provides a 220V, 50Hz island type alternating current micro-grid system consisting of 4 DGs, and the physical connection and the corresponding communication topological structure of the system are shown in figure 10. The experimental platform adopted mainly comprises two parts: the physical connections of the entire MG system and the main controller of the DG are modeled and simulated in a real-time simulator OP5600, while the distributed two-layer controller proposed herein is implemented on a Digital Signal Processor (DSP) controller board. The Modbus TCP/IP communication protocol is used for communication among the distributed DSP controllers.
Fig. 3 and 4 are experimental waveform diagrams obtained by controlling the normal operation of the ac microgrid by using the method disclosed herein. As shown in fig. 3, the system frequency cannot reach the reference value due to the droop function, but at a given time
Under the condition, after the control strategy provided by the invention is accessed, the frequency of 4 DGs can track to the reference frequency omega within the adjusting time of T being 0.45sref50Hz, and under the condition of a given power ratio chi, 6, 3 and 2, the output active power can be distributed according to the proportion. At a given point
After that, the system frequency can still reach the reference value synchronously, and only the adjusting time is increased to be 0.95 s.
After the system accesses the control strategy proposed herein and operates stably for a period of time, additional loads are connected to the system, and experimental waveforms are shown in fig. 5 and 6. As can be seen from fig. 5, when an extra load is connected to the network, the frequency can still be restored to the reference value of 50Hz after a very short regulation time, and the output active power of DG is increased by a certain margin, but after a regulation time T ═ 0.9s, the active power can still be distributed according to a predetermined ratio. Similarly, when the load is removed from the system, the corresponding power is also reduced proportionally, and after T-0.95 s, the frequency reaches a steady state and can still stably track the reference value.
After the load is removed, the system is stabilized again, and at this time, the active power ratio of the system is changed, and the obtained waveforms are shown in fig. 7 and 8. In FIG. 7, coefficients are given
The active power ratio of the system is changed from P1:P2:P3:P41:2:3:6 to P1:P2:P3:P4When the adjusting time T is 0.8s, the output active power ratio is changed, the distribution as required is realized, and meanwhile, the microgrid frequency can be stably maintained at the reference value of 50 Hz. Similarly, if a given coefficient is changed to
Then the power distribution ratio P is changed1:P2:P3:P4As can be seen from fig. 8, after a given coefficient is changed, the active power can still be distributed in a given proportion, but the regulation time is slightly increased, and at the same time, the system frequency can be always maintained at 50 Hz.
Further, when the system operates stably, the number of DGs in the network is changed, and the frequency and power waveform diagrams thereof are given in fig. 10. In FIG. 10, power is expressed as P1:P2:P3:P4When the 4 th DG is disconnected from the network, the power of the other 3 DGs is increased by a certain amount, and after the adjustment time T of 0.6s, the output power of the 3 DGs reaches the predetermined ratio P again1:P2:P3When the 4 th DG is connected to the microgrid again, the other 3 DG devices perform power distribution again, and after T is 0.9s, the active power ratio of the 4 DG devices reaches the original set ratio P again1:P2:P3:P41:2:3: 6. In the whole process, except for short adjustment time, the frequency of the system can be always kept at a reference value of 50 Hz.
From the experimental results, the frequency and power recovery strategy of the island type alternating current micro-grid based on distributed two-layer control, provided by the invention, can effectively eliminate the frequency deviation caused by droop control, so that the system frequency reaches a reference value, and the active power of DG in the network can be output according to a given proportion, instead of outputting the active power according to a droop coefficient, so that the distribution of micro-grid energy can be greatly optimized, and the important practical significance is realized on the improvement of the economic benefit of the micro-grid.
The embodiments of the present invention have been described and illustrated in detail above with reference to the accompanying drawings, but are not limited thereto. Many variations and modifications are possible which remain within the knowledge of a person skilled in the art, given the concept underlying the invention.
Claims (2)
1. An island type micro-grid frequency recovery and power optimal distribution method based on distributed two-layer control is characterized by comprising the following steps:
step 1), establishing an alternating current micro-grid system model
Based on a droop control strategy, obtaining an output frequency expression of the alternating-current micro-grid as shown in a formula (1):
wherein tau isPFor the filter time constant, ω is the output frequency, ωdTo set the frequency, kPIs droop coefficient, P is output active power, PdOutputting an active power set value;
step 2), when the micro-grid operates in an island mode, acquiring operation data of a current system, and determining a control target, wherein the process is as follows:
2.1) firstly, in order to solve the frequency deviation caused by the droop function and realize the frequency tracking target, defining the frequency neighborhood tracking error as shown in the formula (2):
wherein ω isiIs the output frequency, ω, of the ith DGjIs the output frequency, ω, of the jth DGrefAs a reference frequency, a frequency of the reference frequency,
fixed gain coefficient for frequency of ith DG;
2.2) in order to optimize output active power distribution, realize accurate distribution of power as required, define the optimal active power distribution error, as shown in formula (3):
wherein P isiIs the output active power of the ith DG, PjIs the output active power of the jth DGiThe active power distribution ratio, χ, of the ith DGjDistributing the ratio for the active power of the jth DG;
and 3) constructing a distributed two-layer controller based on the analysis, wherein the distributed two-layer controller is shown in a formula (4):
eω＝(Lc+G)(ω-ωref)，eP＝Lcχ-1P
wherein
Is a controller gain factor, LcThe method comprises the following steps that a Laplace matrix is communicated with a micro-grid network, G is a frequency error coefficient, and χ is an active power distribution ratio;
and 4) taking the bottom layer droop model as a controlled object, and applying the proportional integral-based distributed two-layer controller to the controlled object to realize frequency recovery.
2. The island type alternating current microgrid frequency recovery and power optimal distribution method based on distributed two-layer control of claim 1 is characterized in that in the step 3), a proportional integral controller is constructed by adopting a frequency error term and a power distribution error term, so that the aims of eliminating frequency deviation and distributing power as required are fulfilled. Under the condition of changing the load or DG and other disturbances, the control system can still better respond to the disturbances and eliminate the influence of the disturbances.
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