CN108075487B - Hierarchical control method for island micro-grid with combination of self-adaptive droop and consistency - Google Patents
Hierarchical control method for island micro-grid with combination of self-adaptive droop and consistency Download PDFInfo
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Abstract
The invention discloses a hierarchical control method for self-adaptive droop control and consistency in an island micro-grid, which comprises the following steps: constructing a layered control scheme; constructing a local stratum and improving a control method of the local stratum; to improve droop control as primary control of voltage and angular frequency; constructing a network layer and designing a consistency algorithm with a pilot and considering the problem of self-adaptive packet loss in the data communication process to complete secondary control on voltage and angular frequency; designing a control method of reactive power distribution; in a micro-grid, due to the difference of the impedance of each line, the reactive power output by each DER cannot be reasonably distributed according to the rated capacity ratio of each DER; controlling the output current ratio of each DER through a consistency algorithm so as to reasonably distribute reactive power; and setting a reasonable experimental scene to verify the effectiveness of the hierarchical control method. The invention improves the reliability of the system and obviously improves the anti-interference capability of the system.
Description
Technical Field
The invention belongs to the field of intelligent power grid control, and relates to a hierarchical control method of an island micro-grid with self-adaptive droop and consistency combined, in particular to a distributed hierarchical coordination control method of an alternating current micro-grid with a plurality of intelligent agents.
Background
In recent years, with rapid development of the world economy and improvement of the living standard of people, the global demand level for energy is rapidly rising. The proposal of Distributed Energy Resource (DER) well solves the related problems and will be a powerful supplement and an effective support for future large-scale power grids. Although distributed power supplies have a series of advantages of high flexibility, low investment, environmental friendliness and the like, DER has some disadvantages due to the restriction of natural environmental conditions. The microgrid concept was subsequently derived in order to integrate DER into the main grid while attenuating its negative impact on the grid. For micro-grid systems, voltage and angular frequency are two important power quality indicators. In practical operation of the micro-grid, a reliable control method must be adopted to maintain the voltage and angular frequency of the system to be stable near the rated values, so that secondary control is widely applied to the micro-grid system. Meanwhile, the problem of power equalization is also an important direction for research of many scholars.
The conventional secondary control adopts a centralized control structure based on a central controller, and needs to collect all information of each DER and then send a control command to each DER. This results in a complex communication network, a large amount of communication traffic, a complex control structure and high economic cost. The system often contains a plurality of unit individuals, so that dimension disaster is easily generated due to overlarge dimension of a system mathematical model, and the stability of the system is reduced. In order to reduce the communication traffic of the system and the economic cost of the system, and to increase the stability of the system, in recent years, a distributed coordination control method is widely applied to secondary control of a microgrid. In distributed coordination control, the controller of each DER only communicates with adjacent DER, and a central controller is not needed, so that the communication traffic of the system is reduced, the control efficiency of the system is improved, and the robustness of the system is improved.
Disclosure of Invention
In order to realize the stability of the voltage and the angular frequency of the microgrid, the invention provides a hierarchical control method for the microgrid with self-adaptive droop control and consistency on an island. The method is based on a distributed hierarchical control technology platform of the microgrid, and mainly realizes the adjustment of voltage, angular frequency and reactive power.
In order to achieve the above object, the present invention is realized by the following technical solutions:
a hierarchical control method for adaptive droop control and consistency on an island microgrid comprises the following steps:
(1) constructing a layered control scheme;
(2) constructing a local stratum and improving a control method of the local stratum; to improve droop control as primary control of voltage and angular frequency;
(3) constructing a network layer and designing a consistency algorithm with a pilot and considering the problem of self-adaptive packet loss in the data communication process to complete secondary control on voltage and angular frequency;
(4) designing a control method of reactive power distribution; in a micro-grid, due to the difference of the impedance of each line, the reactive power output by each DER cannot be reasonably distributed according to the rated capacity ratio of each DER; controlling the output current ratio of each DER through a consistency algorithm so as to reasonably distribute reactive power;
(5) and setting a reasonable experimental scene to verify the effectiveness of the hierarchical control method.
Due to the adoption of the technical scheme, compared with the prior art, the layering control method provided by the invention has the beneficial effects that:
(1) each DER only exchanges information with the DER connected with the DER when exchanging information, and does not need to communicate with all other DERs, and the communication mode is essentially different from the traditional centralized communication mode; all DER in the micro-grid can be regarded as peer-to-peer nodes, so that the condition that the stability of the system is seriously influenced when a central node fails in a traditional mode is avoided, the reliability of the system is improved, and the anti-interference capability of the system is obviously improved;
(2) in the local layer control, a self-adaptive droop control method combining discrete time consistency is adopted, so that the anti-interference performance of the system is greatly improved; therefore, the problem of packet loss in the information exchange process is considered in the consistency protocol, and the problem of packet loss is compensated, so that the state of each DER can still follow the state of a virtual navigator even if the problem of information loss exists in the information exchange process of each DER;
(3) introducing a self-adaptive packet loss consistency theory with a virtual navigator into the microgrid; in a network layer, each DER enables the state of all DERs to follow the state of a virtual navigator through the consistency protocol according to local information and adjacent DER information of the DER, the state of the virtual navigator is kept unchanged, and the stabilization of the voltage and angular frequency of the whole network is realized through a feedback adding mode;
(4) improving the structure of a network layer; through improvement of a network layer, a part of secondary control output is offset from the output of a local layer, so that the output state of the DER follows the state of a virtual navigator;
(5) distributing reactive power by adjusting the output current of the DER; when each DER is connected with a large power grid through an inverter and operates in an island mode, each inverter is equivalently connected in parallel; when the system is stable, the output voltage of each DER is consistent, and at the moment, the output current of each DER is in a certain proportion by using a consistency algorithm on the current, so that the reactive power can be reasonably distributed according to the rated capacity ratio of each DER.
Drawings
FIG. 1 is a block diagram of a hierarchical control architecture for a microgrid;
FIG. 2 is a view of a hierarchical control architecture of an ith DER;
FIG. 3 is a simplified block diagram of a local control layer;
FIG. 4 is an analytical illustration of improved P- ω droop control;
FIG. 5 is an analytical illustration of improved Q-U droop control.
Detailed Description
The invention will be described in further detail with reference to the following detailed description and accompanying drawings:
the invention relates to a hierarchical control method for self-adaptive droop control and consistency in an island microgrid, which comprises the following steps:
the method includes the steps that a hierarchical control scheme is constructed based on a consistency theory with a virtual pilot and considering the problem of information packet loss;
the invention provides a novel distributed hierarchical coordination control method based on a consistency theory with a virtual pilot and considering the problem of information packet loss. Control can be performed on each DER by dividing into a local stratum and a network layer; in the local layer, each DER can output voltage and angular frequency information by using local information and combining with a droop control method; because the stratum control only needs the self related information and does not need the information of other DER, the control method is simple and quick, but the droop control is the poor adjustment, so the network layer and the local layer are needed to eliminate the system error together; in the network layer, each DER can be regarded as an agent and has two functions of communication and calculation; that is, each DER can not only use local information but also exchange information with the connected DER to obtain information of other DERs; the DER utilizes the information to generate a signal through a corresponding control method, and the signal is fed back to the local stratum, so that the cooperative cooperation of the local stratum and the network layer is realized. This is explained below with reference to fig. 1.
Fig. 1 is a block diagram of a hierarchical control structure of a microgrid, where a DER processes local information and information of other DERs connected to the DER through a network layer, and then feeds the processed information back to the local layer, so that the local layer and the network layer cooperate with each other to maintain the stability of the system voltage and angular frequency. In the microgrid, each DER is connected with a primary control agent and a secondary control agent. Fig. 2 is a hierarchical control structure diagram of the ith DER. As shown in fig. 2, the ith DER uses the local information and the state information of the adjacent DER to pass through a consistency protocol with a virtual pilot, inputs the value calculated by the protocol into the secondary control, and feeds back the output value of the secondary control to the primary control (local layer) after the processing of the secondary control method. Therefore, the voltage and angular frequency output of the local layer droop control method and the voltage and angular frequency output of the secondary control are combined into a voltage signal through the voltage combining module, the sum of the voltage and the angular frequency is used as a reference value of the voltage-current double closed-loop module, a PWM waveform generated by the voltage-current double closed-loop module controls an inverter connected with the ith DER, and the voltage and angular frequency output by the DER can follow the reference value (the state value of a virtual pilot). The control method provided by the invention belongs to a distributed hierarchical coordination type control method, and can realize that the voltage and the angular frequency of the whole network follow a virtual pilot, maintain the stability of the voltage and the angular frequency of the system and improve the anti-interference capability of the system.
(2) Constructing a local stratum and an improved control method of the local stratum to improve droop control as primary control of voltage and angular frequency;
1. constructing a native layer
The local layer mainly comprises an energy calculation module, a droop control module, a voltage synthesis module and a voltage and current double closed-loop module; the energy calculation module is used for calculating output active power and reactive power by utilizing the voltage and current output by the DER; the droop control module is used for respectively obtaining the angular frequency and the voltage of droop output according to the calculated active power and reactive power through a droop algorithm; the voltage synthesis module synthesizes voltage and angular frequency output by the droop control and the secondary control into a voltage phasor, and outputs the voltage phasor to the voltage and current double-closed-loop module to be used as a reference signal of the voltage and current double-closed-loop module; in the voltage and current double-closed-loop module, a voltage loop is used as an outer loop, and a current loop is used as an inner loop; the voltage loop takes the synthesized voltage phasor as a reference signal, and takes a signal output by the voltage loop as a reference signal of the current loop; input signals of the voltage and current double closed-loop module are respectively the voltage and the current output by the DER; generating a PWM (pulse-width modulation) waveform signal through a voltage and current double closed-loop module to control an inverter connected with the DER, so that the voltage and angular frequency output of the DER follows a reference signal; in the local stratum, each DER has its own independent control unit, i.e. the DER only uses local information to perform local calculation, and does not exchange information with other DER. The expressions of the links are shown in fig. 2.
2. Control method for improving local stratum
In the method, a control method used by a local layer is a P-omega/Q-U droop control algorithm, and the method enables the output of an inverter to simulate the droop characteristics between the angular frequency and the terminal voltage of a synchronous generator in a high-voltage power system and the output active power and reactive power; since droop control is suitable for a high-voltage power grid, most of the existing micro-grids are low-voltage grids, virtual impedance needs to be added to reduce the impedance-inductance ratio of the micro-grid line, and a specific design method is disclosed in the published chinese patent CN 106877398A. However, the method is poor in adjustment, so that a consistency-combined adaptive droop control method is adopted to replace the traditional P-omega/Q-U droop control method.
The analysis and design process is as follows:
the droop control expression is as follows:
consider that when the load changes significantly, it can be represented as shown in fig. 4. The explanation of fig. 4 is as follows: when the micro-grid system runs stably, the micro-grid system works at a normal working point A, and a droop curve is L1When inWhen a load is added to the system, the system moves from the normal operating point A to the operating point B after the load changes along the droop direction, namely, the angular frequency is also from omega of the normal operating point AAOmega moved to operating point B after load changeBAt this time, if the angular frequency is too low, it will have serious consequences for the system. Restore the angular frequency to omegaAThere are generally two methods of (1): 1. mixing L with1Is translated to L2(ii) a 2. Mixing L with1Move to L3. But the first method will be to make ωrefConversion to ω'refThis is disadvantageous for the subsequent control steps. Therefore, the method of the present invention uses a second approach to design improved droop control.
Mixing L with1Move to L3In fact, it is the change in sag factor. The following formula holds:
wherein, ω isA,ωBThe angular frequencies of point A and point B respectively; m is1,m3Are respectively L1And L3The sag factor of (d); pBActive power at point B.
As can be seen from the equation (2), when Δ m → 0, the droop control is represented by L1Move to L3. From the above analysis, using the discrete consistency protocol, the following can be derived:
the self-adaptive coefficient of the P-omega droop control is as follows:
mi[k+1]＝(ωref-ωiB[k+1])/PB,k＝0,1,2... (4)
similarly, for Q-U droop, as shown in fig. 5, the adaptive change of the droop coefficient can also be implemented in the same manner, and the following formula holds:
wherein, UA,UBVoltages of a point A and a point B respectively; n is1,n3Are respectively L1And L3The sag factor of (d); qBAnd is reactive power at point B.
As can be seen from the equation (5), when Δ n → 0, the droop control is represented by L1Move to L3. From the above analysis, using the discrete consistency protocol, the following can be derived:
the self-adaptive coefficient of Q-U droop control is as follows:
ni[k+1]＝(Uref-UiB[k+1])/QB,k＝0,1,2... (7)
the gains in the above equations (3) and (6) are related to the packet loss rate, and when an appropriate gain is selected, the deviation of the droop coefficient to be rotated corresponding to each DER will eventually approach to 0, that is, the rotation of the droop curve will eventually be completed, thereby achieving improved droop control; although the droop control can be improved by the method, and the improved droop control can be used as primary control of the voltage and the angular frequency, the droop control still has difference adjustment and needs to be added with secondary control.
(3) Constructing a network layer and designing a consistency algorithm with a pilot and considering the problem of self-adaptive packet loss in the data communication process to complete secondary control on voltage and angular frequency;
1. building a network layer
In a network layer, each DER is equivalent to an agent and consists of a communicator and a decision maker; the communicator comprises a signal receiver and a signal transmitter; the decision maker comprises a sensor and a decision controller; information of a power supply, energy storage equipment, a load and other equipment in the physical layer is transmitted to a signal receiver through a sensor; the signal receiver can receive not only own information but also information of DER connected with the signal receiver; at this time, the signal receiver sends the information of the signal receiver and the information of other DER to the signal transmitter; the signal transmitter transmits the information of the signal transmitter to the neighbor DER, and transmits all the information gathered by the receiver to the decision controller, and the control signal generated by the decision controller controls the corresponding physical equipment.
2. Control method for designing network layer
In a network layer, each DER is regarded as an agent and has two functions of communication and calculation without a central node; meanwhile, each DER can collect local related information and can exchange information with neighbor DERs to obtain information of other DERs; the DER is fed back to the local stratum through the data processed by the consistency algorithm only through limited information, so that the cooperative cooperation of the local stratum and the network layer is realized; because a proper pilot cannot be selected in the actual industrial environment, a Virtual pilot (Virtual pilot) method is selected in the method to achieve the consistency effect; as long as the Virtual leader is a global reachable point, that is, information can be transmitted to any DER, the voltage and angular frequency states of each DER can be consistent with the corresponding state quantity of the Virtual leader; a with global reachable Point reasonable by designijAnd biTo be implemented.
According to the graph theory: the topology of a multi-agent network is usually represented by a directed graph G ═ (V, epsilon), which is composed of a set of vertices V ═ {1,2 …, n } and a set of edges
And (4) forming. The n nodes defining the directed graph represent n agents Σ 1, Σ 2, … Σ n. In the directed graph, if the ith node has information to be transmitted to the jth node, the ith node has an edge pointing to the jth node. If the directed graph adjacency matrix A with N node sets N {1,2 …, N } is aij∈Rn×nIs defined as if (x)i,yj) E epsilon, if the ith node has information to be transferred to the jth node, aij1, otherwise aij＝0。
Theory of continuous time consistency: the information state of an individual agent may be represented by the following equation:
wherein x isi∈RnIndicating the information state of the ith agent, ui∈RnRepresenting a control input. In a multi-agent network topology, the information states of a multi-agent system can be consistent, because the information states of n single-integral agents can be represented by an n-order linear system. A general coherence protocol can thus be derived:
if the consistency protocol of the pilot is considered, the consistency protocol is as follows:
wherein x isL(t) is the state quantity of the pilot, kiIs the gain, biIs the connection weight between the ith agent and the pilot, if there is a connection, bi> 0, otherwise bi0. The final state quantity of the ith intelligent agent can be consistent with the state quantity of the pilot through the formula (10).
Because the problem of information loss is inevitable when information is transmitted among the intelligent agents, and the packet loss rate is random, in the method of the present invention, a novel adaptive consistency protocol related to the packet loss rate is designed as follows:
that is, the following equation holds:
laplace transform the above equation, including:
after simplification, the method comprises the following steps:
when two DER follow a pilot in the system, the system equation is assumed to be a first order equation, i.e.
Let the states of two DER be x1And x2. The state of the virtual navigator is xLAnd is constant. The errors of the follower and the pilot are respectively:
with respect to the above formula, the recombination (14) can be modified as follows:
wherein, c1＝∫x1(t)dt|t＝0,c2＝∫x2(t)dt|t＝0Both are constants.
Therefore gain k1(θ1,θ2,θL1) And k2(θ1,θ2,θL2) As long as it satisfies
The consistency can be achieved.
Discrete-time consistency theory: the discrete-time coherence protocol with a virtual navigator can be represented by the following equation:
wherein xi[k]Representing the system state, k is a discrete time series.
When two DER follow a pilot in the system, the states of the two DER are respectively set as x1And x2. The state of the virtual navigator is xLAnd is constant, the error between the follower and the pilot is the equation (15).
The recombination (15) can be modified as follows:
z is taken for conversion of the formula (15) to obtain:
therefore gain k1(θ1,θ2,θL1) And k2(θ1,θ2,θL2) As long as it satisfies
The consistency can be achieved.
In summary, with the change of the packet loss rate, the gain in the compliance protocol is also constantly changed, that is, a suitable gain value can be found according to the method provided by the present invention to overcome the random packet loss problem.
(1) The design process of the voltage controller is as follows:
in the local layer, the primary control method of the ith DER is droop control, and the mathematical expression is as follows:
wherein, ω isrefAnd UrefAngular frequency and voltage reference values, respectively; m isiAnd niIs a droop coefficient and is a constant; piAnd QiActive power and reactive power output for DER.
Since the magnitude of the DER output voltage is expressed in dq coordinate system as:
therefore, the voltage control method of the primary control can also be written as:
as described above, the effect of the microgrid secondary voltage coordinated controller is to synchronize the voltages of the distributed power sources to a given reference value taking into account the bounded control inputs. Differentiating the equation (22) and taking the auxiliary variable uvi：
This process is input-output feedback linearization. Therefore, the voltage synchronization problem of a micro-grid consisting of a plurality of DER can be converted into the synchronization tracking problem of a first-order linear multi-agent network.
By using the graph theory knowledge and combining a consistency method with a virtual pilot and considering the problem of packet loss, a voltage consistency protocol is designed as follows:
wherein N isi＝{1,2…,n}；Ui，UjAnd are respectivelyVoltage amplitude of ith, j DER in the microgrid system; u shapeLIs a voltage of a virtual pilot, and UL＝Uref。
As can be seen from fig. 2, the secondary voltage control output values are:
(2) the design process of the angular frequency controller is as follows:
similar to the design idea of secondary voltage control, the angular frequency in equation (20) is derived and the auxiliary variable u is takenωiThe following formula is obtained:
if the consistency method is provided with a virtual leader and considers the problem of packet loss, the angular frequency consistency protocol is as follows:
wherein, ω isi，ωjRespectively output angular frequency values of ith and j DER in the microgrid system; omegaLIs the angular frequency of the virtual pilot, and ωL＝ωref。
As can be seen from fig. 2, the angular frequency quadratic control output value is:
as can be seen from fig. 2, the output of the primary control and the output of the secondary control together determine the output of the DER voltage and the angular frequency, so:
and has the following components:
from the formula (30):
therefore, the requirement of the formula (26) can be realized through a designed consistency protocol, namely, the secondary control of each DER voltage and angular frequency can be completed.
(4) Control method for designing reactive power distribution
In a micro-grid, due to the difference of the impedance of each line, the reactive power output by each DER cannot be reasonably distributed according to the rated capacity ratio of each DER; the method controls the output current ratio of each DER through a consistency algorithm so as to reasonably distribute the reactive power.
Because of the existence of
And each voltage is equal to the others during steady state operation by the uniformity control. And because once the line is determined, the impedance of the line is also determined, i.e.
(
Representing the power factor angle) is also determined. Therefore, proportional capacity allocation for reactive power can be achieved by properly allocating current.
Taking the case of two distributed power sources connected in parallel as an example, the formula for apportioning by capacity is expected to be as follows:
in the formula, Q1And Q2Respectively the reactive power output by the two distributed power supplies,
and
the rated reactive power of the two distributed power supplies is respectively.
And has the following components:
in the formula I1And I2Currents, U, of two distributed power supplies, respectively1And U2The voltages of the two distributed power supplies are respectively,
and
the power factor angles corresponding to the two distributed power supplies are respectively.
After elementary transformation, the method comprises the following steps:
according to the above formula, the control method of reactive power distribution is as follows:
the first step is as follows: when the system is stable, the voltage value is equal by the voltage control in the aforementioned hierarchical control, including the primary control and the secondary control, the parallel connection requirement voltage is equal, the circulating current is effectively restrained, and the circuit is determined,
is also determined;
the second step is that: designing a current consistency protocol, and realizing consistency of current values of all distributed power supplies;
the third step: and multiplying the current of each distributed power supply by the corresponding proportionality coefficient according to a formula (34) to realize the distribution of the reactive power.
In combination with the reality, considering the random packet loss problem, the current controller can be designed as follows:
wherein k isIi(θi,θj) Is the gain; a isijIs an element of the adjacency matrix; i is0iAnd I0jIs the DER value of the output current through the inverter and LC filter.
The currents are made to be consistent through a consistency protocol, and then the currents are multiplied by corresponding coefficients according to needs, so that the output currents of the DER are made to be in a certain proportion, and therefore reactive power distribution can be achieved.
(5) And setting a reasonable experimental scene to verify the effectiveness of the hierarchical control method.
The invention sets two experimental methods to verify the effectiveness of the hierarchical control method. In the first condition, under the normal condition, the method can maintain the voltage and angular frequency of the system at specified values; the second is to apply a large disturbance load at a certain point in time. In both cases, it is assumed that there is a problem of information loss and the packet loss rates are the same in the information exchange process. Experiments prove that the control method can keep the stability of the voltage and the angular frequency of the system and improve the anti-interference performance of the system.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (1)
1. A hierarchical control method of an island micro-grid combining self-adaptive droop and consistency is characterized by comprising the following steps: the method comprises the following steps:
step 1: constructing a layered control scheme based on a consistency theory with a virtual navigator and considering the problem of information packet loss;
based on a consistency theory with a virtual navigator and considering the problem of information packet loss, controlling each distributed power supply to be divided into a local layer and a network layer; in the local stratum, each distributed power supply can output voltage and angular frequency information by utilizing local information and combining a droop control method; the local control only needs the self related information and does not need the information of other distributed power supplies, the droop control is differential adjustment, and a network layer and a local layer are needed to jointly eliminate system errors; in a network layer, each distributed power supply can be regarded as an agent and has two functions of communication and calculation; that is, each distributed power supply can not only utilize local information but also exchange information with the connected distributed power supplies so as to obtain information of other distributed power supplies; the distributed power supply utilizes the information to generate a signal through a corresponding control method, and the signal is fed back to the local layer, so that the cooperative cooperation of the local layer and the network layer is realized;
step 2: constructing a local stratum and an improved control method in the local stratum to improve droop control as primary control of voltage and angular frequency;
1. constructing a native layer
The local layer mainly comprises an energy calculation module, a droop control module, a voltage synthesis module and a voltage and current double closed-loop module; the energy calculation module calculates the output active power and reactive power by using the voltage and current output by the distributed power supply; the droop control module is used for respectively obtaining the angular frequency and the voltage of droop output according to the calculated active power and reactive power through a droop algorithm; the voltage synthesis module combines the voltage and the angular frequency output by the droop control and the secondary control into a voltage phasor, and outputs the voltage phasor to the voltage and current double-closed-loop module to be used as a reference signal of the voltage and current double-closed-loop module; in the voltage and current double-closed-loop module, a voltage loop is used as an outer loop, and a current loop is used as an inner loop; the voltage loop takes the synthesized voltage phasor as a reference signal, and takes a signal output by the voltage loop as a reference signal of the current loop; the input signals of the voltage and current double closed-loop module are respectively the voltage and the current output by the distributed power supply; generating a PWM waveform signal through a voltage and current double closed-loop module to control an inverter connected with the distributed power supply, so that the voltage and angular frequency output by the distributed power supply follow a reference signal;
2. control method for improving local stratum
The control method used by the stratum is a P-omega/Q-U droop control algorithm, and a droop control expression is as follows:
a self-adaptive droop control method combining discrete time consistency is designed, and the design process is as follows:
by varying the droop coefficient, the angular frequency is restored to ωA，
The following formula holds:
wherein, ω isA,ωBThe angular frequency of the normal working point of the system and the angular frequency of the working point after the load change are respectively; m is1,m3The droop coefficients of the normal P-omega droop curve before load change and the ideal P-omega droop curve after load change are respectively obtained; pBThe active power of the working point after the load change;
as can be seen from the formula (2), when Δ m → 0, the angular frequency returns to ωA(ii) a According to the above analysis, the design is as follows:
the self-adaptive coefficient of the P-omega droop control is as follows:
mi[k+1]＝(ωref-ωiB[k+1])/PB,k＝0,1,2... (4)
similarly, for the Q-U droop, the adaptive change of the droop coefficient is realized in the same form, and the following formula holds:
wherein, UA,UBRespectively the voltage of the normal working point of the system and the voltage of the working point after the load changes; n is1,n3The droop coefficients of the normal Q-U droop curve before load change and the ideal Q-U droop curve after load change are respectively obtained; qBThe reactive power of the working point after the load change;
as shown in the formula (5), when Δ n → 0, the voltage returns to UA(ii) a According to the above analysis, the design is as follows:
the self-adaptive coefficient of Q-U droop control is as follows:
ni[k+1]＝(Uref-UiB[k+1])/QB,k＝0,1,2... (7)
the gains in the above equations (3) and (6) are related to the packet loss rate, and when an appropriate gain is selected, the deviation of the droop coefficient to be rotated corresponding to each distributed power supply will eventually approach to 0, that is, the rotation of the droop curve will eventually be completed, thereby achieving the improved droop control;
and step 3: constructing a network layer and designing a consistency algorithm with a pilot and considering the problem of self-adaptive packet loss in the data communication process to complete secondary control on voltage and angular frequency;
1. building a network layer
In a network layer, each distributed power supply is equivalent to an agent and consists of a communicator and a decision maker; the communicator comprises a signal receiver and a signal transmitter; the decision maker comprises a sensor and a decision controller; information of a power supply, energy storage equipment, a load and other equipment in the physical layer is transmitted to a signal receiver through a sensor; the signal receiver can receive not only own information but also information of the distributed power supply connected with the signal receiver; at the moment, the signal receiver sends the information of the signal receiver and the information of other distributed power supplies to the signal transmitter; the signal transmitter transmits the information of the signal transmitter to the neighbor distributed power supply outwards on one hand, and transmits all the information gathered by the receiver to the decision controller on the other hand; controlling the corresponding physical equipment by the control signal generated by the decision controller;
2. control method for designing network layer
2.1 the design process of the voltage controller is as follows:
in a local layer, the primary control method of the ith distributed power supply is droop control, and the mathematical expression of the droop control is as follows:
wherein, ω isrefAnd UrefAngular frequency and voltage reference values, respectively; m isiAnd niIs a droop coefficient and is a constant; piAnd QiActive power and reactive power output for the distributed power supply;
since the magnitude of the distributed power supply output voltage is expressed in dq coordinate system as:
therefore, the voltage control method of the primary control can also be written as:
differentiating the equation (8) and taking the auxiliary variable uvi：
By using the graph theory knowledge and combining a consistency method with a virtual pilot and considering the problem of packet loss, a voltage consistency protocol is designed as follows:
wherein, UiAnd UjThe voltage amplitudes of the ith and j distributed power supplies in the microgrid system are respectively; u shapeLIs a voltage of a virtual pilot, and UL＝Uref；
The design voltage secondary control feedback value is as follows:
2.2 the design process of the angular frequency controller is as follows:
similar to the design idea of secondary voltage control, the angular frequency in equation (8) is derived and the auxiliary variable u is takenωiThe following formula is obtained:
if the consistency method is provided with a virtual leader and considers the problem of packet loss, the angular frequency consistency protocol is as follows:
wherein, ω isiAnd ωjOutput angular frequency values of ith and j distributed power sources in the micro-grid system are respectively; omegaLIs the angular frequency of the virtual pilot, and ωL＝ωref；
Designing the angular frequency secondary control feedback value as follows:
the output of the distributed power supply voltage and the output of the angular frequency are determined by the output of the primary control and the output of the secondary control together, so that:
and has the following components:
from equation (18) it follows:
therefore, the secondary control of the voltage and angular frequency of each distributed power supply can be completed through a designed consistency protocol;
and 4, step 4: control method for designing reactive power distribution
Taking the case of two distributed power sources connected in parallel as an example, the formula for apportioning by capacity is expected to be as follows:
in the formula, QiAnd QjReactive power, Q, output by the ith and jth distributed power supplies respectively0iAnd Q0jRated reactive power of the ith and jth distributed power supplies respectively;
and has the following components:
in the formula IiAnd IjThe output currents of the ith and jth distributed power supplies are respectively; u shapeiAnd UjThe output voltages of the ith and jth distributed power supplies are respectively;
and
respectively corresponding power factor angles of the ith and jth distributed power supplies;
if so:
wherein,
if the output current of the distributed power supply satisfies equation (22), the desired goal of equation (20) may be achieved; according to the above equation (22), the reactive power distribution control method is as follows:
the first step is as follows: when the system is stable, the voltage value is equal by the voltage control in the aforementioned hierarchical control, including the primary control and the secondary control, the parallel connection requirement voltage is equal, the circulating current is effectively restrained, and the circuit is determined,
is also determined;
the second step is that: designing a current consistency protocol, and realizing consistency of current values of all distributed power supplies;
the third step: multiplying the current of each distributed power supply by the corresponding proportionality coefficient according to the formula (22) to realize the distribution of reactive power;
in combination with the reality, considering the random packet loss problem, the current controller can be designed as follows:
wherein k isIi(θi,θj) Is the gain; a isijIs an element of the adjacency matrix; i is0iAnd I0jThe output current value of the distributed power supply passing through the inverter and the LC filter is obtained;
enabling the currents to be consistent through a consistency protocol, and multiplying the currents by corresponding coefficients according to requirements to enable the output currents of all distributed power supplies to be in a certain proportion, so that reactive power distribution is achieved;
and 5: and setting a reasonable experimental scene to verify the effectiveness of the hierarchical control method.
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