CN115102173A - Layered distributed control method for flexible diamond type power distribution flexible interconnection converter - Google Patents
Layered distributed control method for flexible diamond type power distribution flexible interconnection converter Download PDFInfo
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- H—ELECTRICITY
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- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/04—Circuit arrangements for ac mains or ac distribution networks for connecting networks of the same frequency but supplied from different sources
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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
- H02J3/24—Arrangements for preventing or reducing oscillations of power in networks
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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
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- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
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- H—ELECTRICITY
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- H02J2300/00—Systems for supplying or distributing electric power characterised by decentralized, dispersed, or local generation
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Abstract
The invention discloses a layered distributed control method for a flexible diamond distribution flexible interconnection converter, which comprises the following steps: establishing a grid structure of a flexible interconnected power distribution network based on a power distribution interconnection converter; and constructing a layered distributed control strategy based on the grid structure and according to the structure and the principle of three-level control respectively, so as to realize the power flow control of the flexible interconnected power distribution network system. The invention realizes the multi-network coordination control and the optimized operation of the power distribution interconnection converter on the flexible interconnection power distribution network, can improve the voltage quality of a multi-region power distribution feeder line, reduce the line loss, optimize the operation level of the power distribution network, and can realize the seamless flexible switching between system-level control and local control.
Description
Technical Field
The invention relates to the technical field of power electronic technology and automatic control in a power system, in particular to a layered distributed control method for a flexible diamond distribution flexible interconnection converter.
Background
With the innovation and change of the energy and power field, the limitations of the traditional power distribution network are increasingly prominent, and the challenges are increasingly serious. Among them, the main problems include: the distributed renewable energy sources are merged into a power grid in a large scale, power of a power distribution network flows in two directions, loads among feeders are unbalanced, and node voltage is out of limit; the load types are diversified, and the structure and the operation mode of the traditional power distribution network cannot meet the requirements of various flexible loads; users have higher requirements on the reliability of power supply and the quality of electric energy.
Line voltage low voltage violations in power distribution networks, which often occur and occur in urban power distribution networks and other regional power distribution networks, have a serious impact on the stable operation of power systems and the use of industrial, commercial, and residential power. For a long time, the attention of the academic world and electric power companies lies in the power quality problem of medium and high voltage systems, but does not consider the power quality problem of the power distribution network. Along with the development of economy in China, the living standard of people is continuously improved, and the demand on electric energy is higher and higher. The load of the power distribution network is heavier and heavier, the load types are more and more, and the capacity is larger and larger, so that the voltage quality of the power distribution network is difficult to guarantee.
With the development of power electronic devices, the flexible direct current power distribution technology is continuously perfected. The flexible direct-current power distribution technology based on the voltage source converter is applied to the middle and low voltage flexible interconnection power distribution technology in a demonstration mode, an alternating-current and direct-current mixed flexible interconnection power distribution network is formed, more and more distributed power sources and various loads can be expected to exist in the future power distribution network, and the flexible direct-current power distribution network is a challenge to the stable operation of the power distribution network.
Disclosure of Invention
The technical problem solved by the invention is as follows: the power distribution network is increasingly heavy in load, increasingly in load type and increasingly large in capacity, so that the voltage quality of the power distribution network is difficult to guarantee, more and more distributed power sources and various types of loads exist in the future power distribution network, and the power distribution network is challenging to stably operate.
In order to solve the technical problems, the invention provides the following technical scheme: establishing a grid structure of a flexible interconnected power distribution network based on a power distribution interconnection converter; and constructing a layered distributed control strategy based on the grid structure and according to the structure and the principle of three-level control respectively, so as to realize the power flow control of the flexible interconnected power distribution system.
Preferably, the grid structure includes multiple interconnected converters disposed at the end of a distribution feeder, and interconnected by a dc bus between the multiple interconnected converters.
Preferably, the three levels comprise a converter, a local level and a system level.
Preferably, the control principle of the converter layer includes that a three-phase voltage type PWM rectifier is defined as an ac/dc converter, and a mathematical model of the converter layer after coordinate transformation is:
wherein e is d ,e q For the component of the alternating voltage source voltage dq axis, i d ,i q For the three-phase AC current dq axis component, the resistance R and the inductance L are the inductance and the resistance in the AC-side main circuit, v dc Is a DC side voltage, s d ,s q As a function of switchingdq axis component, C is DC capacitance, R L A load resistance;
defining the inductance L at the AC side and the capacitance C at the DC side as follows:
in the formula, E m For mains phase electromotive force, T s Is a switching period, v dc Is a direct voltage, I m For AC fundamental phase current, Δ i max For maximum allowable pulsating quantity of current, 20% I is taken m ，T imax Is the maximum time constant of inertia, Δ P L,max For maximum variation of load power, Δ v dc,max Is the maximum variation of the direct-current voltage,
to limit the rise time of the DC voltage, R Le For a rated DC load resistance, I dm For rated direct current, v d0 Is the lowest steady-state value of the DC voltage, v de Is a DC voltage rating;
introducing a PI controller in the control, and defining the gain parameter of the PI controller as:
wherein, T s For PWM switching period, K PWM For equivalent gain of the rectifier bridge, depending on the modulation mode, K P 、K I Respectively obtaining the gain coefficients of a proportional link and an integral link of the PI regulator;
the closed loop transfer function of the control loop is simplified into an inertia link:
preferably, the control principle of the local layer includes primary voltage regulation control and secondary voltage regulation control.
Preferably, the primary voltage regulation control includes,
establishing a droop equation, wherein the normalization equation and the droop equation after normalization are as follows:
P ac1 ＝P 1 +P dc1
controlling the normalized voltage at the AC side to be equal to the normalized voltage at the DC side, namely:
......
the dc normalized voltage of each line is defined to be equal as:
after the primary voltage regulation controlled by the local layer, the following formula can be obtained:
wherein N represents N distribution network flexible interconnections, Z 1 ,…,Z n Line impedances, P, of the 1 st, … th, n distribution networks, respectively 1 ,…,P n Load power, V, of the 1 st, … th, n distribution networks, respectively ac1 ,…,V acn Line end voltages, P, of the 1 st, … th, n distribution networks, respectively c1 ,…,P cn Active power, Q, transmitted by the 1 st, … th, n distribution network AC-DC converters c1 ,…,Q cn The reactive power, P, transferred by the AC-DC converters of the 1 st, … th, n distribution networks dc Is a direct current source or charged power.
Preferably, the secondary voltage regulation includes, on the basis of the primary voltage regulation, defining the transmission of active power and reactive power as given values, that is, controlling the power transmission of each feeder line as given values
The droop equation for each line becomes:
......
at steady state, the power and voltage equation satisfies:
the system meets the following requirements:
after the circuits are flexibly interconnected, the direct-current normalized voltage of each circuit is defined to be equal, and the direct-current normalized voltage is as follows:
by combining the above equations, one can obtain:
preferably, the control principle of the system layer includes that the optimal operation state of the system is obtained through retrieval of an intelligent algorithm according to voltage information and power information of each node acquired by the system.
Preferably, the method further comprises establishing a mathematical model of the actual problem according to a general mathematical model, wherein the general mathematical model is as follows:
minf＝f(x,u)
{s.t.g(x,u)＝0
h(x,u)≤0
where f (x, u) is the objective function, g (x, u) and h (x, u) are equality constraints and inequality constraints, respectively, and x and u are control variables and state variables, respectively.
The invention has the beneficial effects that: the invention realizes the multi-network coordination control and the optimized operation of the distribution interconnection converter on the flexible interconnection distribution network, can improve the voltage quality of a multi-region distribution feeder line, reduce the line loss, optimize the operation level of the distribution network, and can realize the seamless flexible switching of system-level control and local control.
Drawings
Fig. 1 is a schematic diagram of an overall flow structure of a layered distributed control method for a flexible diamond-type power distribution flexible interconnection converter according to an embodiment of the present invention;
fig. 2 is a schematic system structure diagram of a layered distributed control method for a flexible diamond distribution flexible interconnection converter according to an embodiment of the present invention;
fig. 3 is a block diagram of a current inner loop control based on feed-forward decoupling for a hierarchical distributed control method of a flexible diamond distribution flexible interconnection converter according to an embodiment of the present invention;
fig. 4 is an equivalent circuit diagram of a single distribution network line of a layered distributed control method for a flexible diamond distribution flexible interconnection converter according to an embodiment of the present invention;
fig. 5 is a block diagram of a communication-free primary voltage regulation control of a local level control of a hierarchical distributed control method for a flexible diamond distribution flexible interconnection converter according to an embodiment of the present invention;
fig. 6 is an equivalent circuit diagram of a flexible interconnected power distribution network of a layered distributed control method of a flexible diamond-type power distribution flexible interconnected converter according to an embodiment of the present invention;
fig. 7 is a schematic diagram illustrating a principle of a primary voltage regulation without communication in a local-level control of a hierarchical distributed control method for a flexible diamond distribution flexible interconnection converter according to an embodiment of the present invention;
fig. 8 is a block diagram of a local-level controlled communicating secondary voltage regulation control method for a hierarchical distributed control method for a flexible diamond distribution flexible interconnection converter according to an embodiment of the present invention;
fig. 9 is a schematic diagram illustrating a principle of communicating secondary voltage regulation of local level control in a hierarchical distributed control method for a flexible diamond distribution flexible interconnection converter according to an embodiment of the present invention;
fig. 10 is a block diagram illustrating an algorithm of a system level control of a hierarchical distributed control method for a flexible diamond distribution flexible interconnection converter according to an embodiment of the present invention;
fig. 11 is a circuit diagram of an example of a simulation of a hierarchical distributed control method for a flexible diamond distribution flexible interconnection converter according to an embodiment of the present invention;
fig. 12 is a schematic diagram illustrating simulation results of active power transmission of the lines 1 and 2 in a layered distributed control method for a flexible diamond distribution flexible interconnection converter according to an embodiment of the present invention;
fig. 13 is a schematic diagram illustrating simulation results of reactive power transfer of the lines 1 and 2 in a layered distributed control method for a flexible diamond distribution flexible interconnection converter according to an embodiment of the present invention;
fig. 14 is a schematic diagram of simulation results of voltages at nodes of a line 1 in a layered distributed control method for a flexible diamond distribution flexible interconnection converter according to an embodiment of the present invention.
Detailed Description
In order to make the aforementioned objects, features and advantages of the present invention comprehensible, embodiments accompanied with figures are described in detail below, and it is apparent that the embodiments described are some, but not all embodiments of the present invention. All other embodiments, which can be obtained by a person skilled in the art without making creative efforts based on the embodiments of the present invention, shall fall within the protection scope of the present invention.
In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention, but the present invention may be practiced in other ways than those specifically described and will be readily apparent to those of ordinary skill in the art without departing from the spirit of the present invention, and therefore the present invention is not limited to the specific embodiments disclosed below.
Furthermore, reference herein to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one implementation of the invention. The appearances of the phrase "in one embodiment" in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments.
Example 1
Referring to fig. 1 to 10, for an embodiment of the present invention, there is provided a hierarchical distributed control method for a flexible diamond distribution flexible interconnection converter, including:
s1: establishing a grid structure of a flexible interconnected power distribution network based on a power distribution interconnection converter; it should be noted that:
the grid structure comprises a plurality of interconnected converters which are configured at the tail end of a distribution feeder line and are interconnected through direct current buses among the interconnected converters.
The three levels comprise a current converter, a local level and a system level.
Specifically, among the spatial grid structure of the flexible interconnection distribution network based on many interconnection converters (VSC), each VSC will dispose at the distribution feeder end, through the interconnection of the direct current bus between VSCs, can realize the flexible interconnection between adjacent feeders, and this spatial grid structure provides new power circulation route for between multizone net and the feeder, can realize the nimble regulation and the mutual confession of each other of distribution network net power.
Under the grid structure of a flexible interconnected power distribution network, each VSC needs to perform coordination control according to the voltage of a feeder node and load information, so that power flow interaction and flexible adjustment among the power distribution networks are realized.
S2: constructing a layered distributed control strategy based on a grid structure and according to the structure and the principle of three-level control respectively, and realizing power flow control of the flexible interconnected power distribution system; it should be noted that:
firstly, the control purpose of the converter layer is to process and convert an input current signal into an output AC/DC converter switch PWM modulation waveform, control the alternating current to measure the current and the voltage of the direct current side to be stable, realize the bidirectional power flow and control the power exchange of the alternating current and direct current side. The principle of the control of the converter level is as follows:
the three-phase voltage type PWM rectifier is widely applied to a distribution network due to the characteristics of low harmonic content, high power factor, quick dynamic response and the like, so that the three-phase voltage type PWM rectifier is selected as an AC-DC converter. The mathematical model after coordinate transformation is as follows:
wherein e is d ,e q For the voltage dq axis component of the AC voltage source, i d ,i q For the three-phase AC current dq axis component, the resistance R and the inductance L are the inductance and the resistance in the AC side main circuit, v dc Is a DC side voltage, s d ,s q Is the dq-axis component of the switching function, C is the DC capacitance, R L A load resistance;
in order to improve the operation performance of the system, it is necessary to select an appropriate ac side inductor and an appropriate dc side capacitor, and the ac side inductor L and the dc side capacitor C are defined as:
in the formula, E m For mains phase electromotive force, T s Is a switching period, v dc Is a direct voltage, I m For AC fundamental phase current, Δ i max For maximum allowable pulsating quantity of current, 20% I is taken m ，T imax Is the maximum time constant of inertia and is,ΔP L,max for maximum variation of load power, Δ v dc,max Is the maximum variation of the direct-current voltage,
to limit the rise time of the DC voltage, R Le For a rated DC load resistance, I dm Rated direct current, v d0 Is the lowest steady-state value of the DC voltage, v de Is a DC voltage rating;
a current inner loop control based on feedforward decoupling can be established according to the mathematical model, and the control block diagram is shown in FIG. 3; introducing a PI controller in the control, wherein the parameter design of the PI controller influences the dynamic performance of the system, and the gain parameter of the PI controller is defined as:
wherein, T s For PWM switching period, K PWM For equivalent gain of the rectifier bridge, depending on the modulation mode, K P 、K I Respectively obtaining the gain coefficients of a proportional link and an integral link of the PI regulator;
finally, the closed loop transfer function of the control loop can be simplified into an inertia link:
furthermore, the control purpose of the local layer is to control the power interaction between the feeders only according to the local voltage information when no communication line exists, so as to improve the problem of the voltage quality at the tail end of the line, which is called primary voltage regulation; and when a communication line exists, analyzing and calculating an optimal operation state according to the voltage and power information of each node of the power distribution network, and enabling the system to operate in the optimal state by controlling power interaction, namely secondary voltage regulation.
Specifically, the principle of the control at the local level is as follows:
suppose there are N distribution networks flexibly interconnected, Z 1 ,…,Z n Line impedances, P, of the 1 st, … th, n distribution networks, respectively 1 ,…,P n Load power, V, of the 1 st, … th, n distribution networks, respectively ac1 ,…,V acn Line end voltages, P, of the 1 st, … th, n distribution networks, respectively c1 ,…,P cn Active power, Q, transmitted by the 1 st, … th, n distribution network AC-DC converters c1 ,…,Q cn The reactive power, P, transferred by the AC-DC converters of the 1 st, … th, n distribution networks dc For dc source/load power, the equivalent circuit diagram of a single distribution network is shown in fig. 4.
Considering the state of the system without a communication line, applying primary voltage regulation, and analyzing that the active power transmitted by each distribution network and the line terminal voltage meet the natural droop characteristic, so that a droop equation can be established, wherein the normalization equation and the droop equation after normalization are as follows:
P ac1 ＝P 1 +P dc1
a primary voltage regulation control block diagram for controlling no communication in a local layer is shown in fig. 5, and the normalized voltage at the ac side is controlled to be equal to the normalized voltage at the dc side, that is:
......
after the lines are flexibly interconnected, an equivalent circuit diagram is shown in fig. 6, and the direct-current normalized voltages of the lines are defined to be equal, namely:
the above formula is established simultaneously, and after primary voltage regulation controlled by a local layer, the following can be obtained:
the control principle is shown in fig. 7.
When the system has no communication line, the local layer is adopted to control primary voltage regulation; in the actual operation of the power distribution network, when the line impedance is constant, the end voltage of the line is determined by the load power of the line, and the end voltage of the line is reduced more when the load power is higher; the load power is lower, and the terminal voltage of the line is reduced less; the result formula after primary voltage regulation shows that after the multiple feeder lines are flexibly interconnected, the normalized voltage at the alternating current side of each line is at the same voltage level, and the power of each line realizes complementary mutual aid.
Considering the state of the system with communication line, secondary voltage regulation is applied, and on the basis of primary voltage regulation, active power and reactive power transmission are defined as given values, i.e. the power transmission of each feeder line is controlled as given values
The droop equation for each line becomes:
......
the block diagram of the secondary voltage regulation control with communication controlled by the local layer is shown in fig. 8, and in a steady state, the power and voltage equation satisfies the following conditions:
the system meets the following requirements:
after the lines are flexibly interconnected, an equivalent circuit diagram is shown in fig. 5, and the direct-current normalized voltages of the lines are defined to be equal as follows:
by combining the above formulas, we can obtain:
the control principle is shown in fig. 9.
When the system has communication lines, the secondary voltage regulation controlled by the local layer is adopted to control the interaction of active power and reactive power to be given values, the power transmission of each feeder line is accurately controlled, the running state of each line is adjusted according to the requirement, and an interface is provided for the control of the layer of the layered distributed control system and the local layer.
Finally, the control purpose of the system level is to calculate and obtain the optimal operation state of the system through the retrieval of an intelligent algorithm according to the voltage information and the power information of each node acquired by the system, so that the power scheduling of each area is planned, the voltage level of each node is not out of limit, the two problems of minimum power loss of the whole system are simultaneously met, the whole operation condition of the system is optimized, and the whole operation economy of the system is improved. The principle of system level control is as follows:
essentially, the control at the system level solves an optimal power flow problem; the optimal power flow problem of the power system is a complex nonlinear programming problem with constraints, and the stable operation state of the system with the optimal preset target is realized by adjusting available control means in the system under the specific safety constraint condition; the method has the advantages of safety, economy, reliability and the like; wherein, a general mathematical model can be expressed as:
minf＝f(x,u)
{s.t.g(x,u)＝0
h(x,u)≤0
wherein f (x, u) is an objective function, g (x, u) and h (x, u) are equality constraints and inequality constraints, respectively, and x and u are control variables and state variables, respectively. :
from the mathematical model, a desired objective function can be generally set according to actual requirements, and constraint conditions suitable for the objective function are designed according to variables in the objective function and considering limitations of various aspects, so that the mathematical model of the actual problem is established.
In the control of the system level, an optimization algorithm is the core of the control; for the optimization algorithm of the related flexible interconnection system, the problem that power interaction between distribution networks is controlled in the flexible interconnection system to realize overall global optimization of the system is solved; the problem is that the variable is a multi-variable problem, the independence among the variables is strong, and the monotonicity of the variables is not single; therefore, when the system is optimized, a plurality of local optimal solutions are generated, and the algorithm usually falls into a local optimal interval; therefore, a simulated annealing optimization algorithm is selected to be combined with a Newton-Raphson power flow algorithm to serve as a control algorithm of a system level; the algorithm has the advantages that the algorithm can jump out of a local optimal interval with a certain probability and is suitable for an optimization algorithm of a multi-terminal flexible interconnection system; the algorithm block diagram is shown in fig. 10.
The method is based on the flexible interconnected power distribution network, layered distributed control is established, and the operation of the flexible interconnected power distribution network is improved; on one hand, the scheme can be considered from the overall level of the system to carry out overall operation optimization control; on the other hand, the improvement of local voltage quality and power balance can be carried out only through local information in consideration of a local power distribution network level; and the control of the system layer and the control of the local layer can be flexibly and seamlessly switched, so that the flexible control of the power distribution system is realized, the line loss is reduced, the voltage quality of a power distribution feeder line is improved, and the economical efficiency and the flexibility of the operation of the power distribution network are improved.
Example 2
Referring to fig. 11 to 14, another embodiment of the present invention is shown, in order to verify and explain the technical effects adopted in the method, the present embodiment adopts specific examples to test the method of the present invention, and uses a scientific demonstration means to verify the actual effects of the method.
In order to verify the optimization effect of the flexible interconnection system level operation optimization control on the overall operation of the power distribution network system, the embodiment designs an example for comparative analysis, wherein the example is composed of two independent power distribution network lines, each feeder line is connected with a photovoltaic grid connection and a user load, the line impedances of the two lines are different, and the two lines are flexibly interconnected through a direct current side, as shown in fig. 11.
In the calculation example, the loads of three nodes of a line 1 are respectively 24+ j18 KVA, -50kW and 24+ j18 KVA, the loads of three nodes of a line 2 are respectively 20+ j15 KVA, -30kW and 20+ j15 KVA, the rated voltages of the line 1 and the line 2 are 220V, the upper limit and the lower limit of the voltage of each power distribution network are respectively set to be 110% and 90% of the rated voltage during normalization, and the line impedance of the line 1 is twice of that of the line 2.
After the retrieval of the intelligent optimization algorithm, the optimal working condition is obtained, and the output result is as follows:
line 1: p c1 ＝15960W,Q c1 ＝-20000Var；
Line 2: p is c2 ＝-15960W,Q c2 ＝-20000Var；
The line loss is reduced from 14584W to 5095W.
The control effect of the invention is verified through simulation; the simulation time sequence is as follows: when t is 0s, the simulation is started, the communication secondary voltage regulation controlled by the local layer is started according to the power instruction controlled by the receiving system layer, and the line 1 and the line 2 are both started to the communication secondary voltage regulation controlled by the local layer; when t is 1.5s, simulating the interruption of a communication line, closing the control of a system level, and switching the control of a local level to primary voltage regulation without communication; t is 2.5s, and the simulation is finished.
The simulation results are shown in FIGS. 12-14; fig. 12-14 are graphs of the active power and reactive power transfer waveforms for line 1 and line 2, respectively, the voltage levels at the nodes in line 1 and line 2, and the dq-axis components of the converter current in line 1 and line 2.
Analyzing the simulation result, as shown in the simulation result shown in fig. 12, when 0.5 s-1.5 s, the system puts into secondary voltage regulation control, the line 1 transmits 16kW active power to the direct current side, and the line 2 absorbs 16kW active power from the direct current side; when 1.5s, the communication between the feeder lines is interrupted, and the system is switched from secondary voltage regulation to primary voltage regulation; when the voltage is 1.5 s-2.5 s, the system does not receive a power instruction output by the system level control any more, but controls according to local voltage information at the tail end of the line; at this time, line 1 transmits 24kW of active power to the dc side, and line 2 absorbs 24kW of active power from the dc side.
As a result of the simulation shown in fig. 13, the reactive power transfer conditions of line 1 and line 2 substantially coincide; when 0.5 s-1.5 s, the system adopts a local layer to control the non-communication secondary voltage regulation, and the line 1 and the line 2 absorb 20kVar reactive power; when 1.5s, the system is switched from secondary voltage regulation to primary voltage regulation; and when the time is 1.5 s-2.5 s, the system does not receive the power instruction output by the system level control any more, and the reactive power of the line 1 and the line 2 is stabilized at 0 Var.
In the voltage levels at the nodes of line 1 shown in fig. 14: when the system is not controlled, the voltages of the node 1, the node 2 and the node 3 are not out of limit, but the voltage of the node 3 is at a lower level, and the voltages of the node 1, the node 2 and the node 3 do not reach the rated voltage level; when the system is put into the non-communication primary voltage regulation controlled by the local layer, compared with the non-control system, the voltage of each node is lifted to a certain extent and is closer to the rated voltage level; the system receives a power instruction controlled by a system level, and when the system is put into communication secondary voltage regulation controlled by a local level, the voltage level of each node is greatly improved and is closest to a rated voltage. In particular the voltage at node 3, i.e. the voltage at the end of line 1, is significantly improved.
In the voltage levels of the nodes of the line 2: when the system is not controlled, the voltages of the node 1 and the node 2 are not out of limit and have larger deviation with a rated value, while the voltage of the node 3 is lower than 0.9p.u., is in low-voltage out of limit, and has serious voltage quality problem; when the system is put into the communication-free primary voltage regulation controlled by the local layer, the voltage of the node 3 is raised by 25.6V and is within the range of 0.9p.u. -1.1 p.u.; and the voltage level of each node is improved; when the system receives a power instruction controlled by the system level and puts the power instruction into secondary voltage regulation controlled by the local level, the voltage level of each node is similar to the voltage level controlled by primary voltage regulation, the problem of the voltage quality of the tail end of the line 2 is solved, and the overall voltage level is improved.
Therefore, in the flexible interconnected power distribution network, the problems of unbalanced feeder line load, low voltage quality at the tail end of a line, high line loss and the like in the power distribution network can be solved through layered distributed control, and the power supply reliability and the operation flexibility of the power distribution network are improved.
It should be noted that the above embodiments are only for illustrating the technical solutions of the present invention and not for limiting, and although the present invention is described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that modifications or equivalent substitutions may be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, which should be covered in the claims of the present invention.
Claims (9)
1. A layered distributed control method for a flexible diamond distribution flexible interconnection converter is characterized by comprising the following steps:
establishing a grid structure of a flexible interconnected power distribution network based on a power distribution interconnection converter;
and constructing a layered distributed control strategy based on the grid structure and according to the structure and the principle of three-level control respectively, so as to realize the power flow control of the flexible interconnected power distribution system.
2. The method of hierarchical distributed control of a flexible diamond power distribution flexible interconnection transformer according to claim 1, wherein: the grid structure comprises a plurality of interconnected converters which are configured at the tail end of a distribution feeder line and interconnected through direct current buses among the interconnected converters.
3. The method of hierarchical distributed control of a flexible diamond power distribution flexible interconnection transformer according to claim 1, wherein: the three levels comprise a current converter, a local level and a system level.
4. The method of hierarchical distributed control of a flexible diamond power distribution flexible interconnection transformer according to claim 3, wherein: the control principle of the converter level comprises that,
a three-phase voltage type PWM rectifier is defined as an AC-DC converter, and a mathematical model of the AC-DC converter after coordinate transformation is as follows:
wherein e is d ，e q For the component of the alternating voltage source voltage dq axis, i d ，i q For the three-phase AC current dq axis component, the resistance R and the inductance L are the inductance and the resistance in the AC side main circuit, v dc Is a DC side voltage, s d ，s q As a function of the switchingD q-axis component of (A), C is a DC capacitor, R L A load resistance;
defining the inductance L at the AC side and the capacitance C at the DC side as follows:
in the formula, E m For mains phase electromotive force, T s Is a switching period, v dc Is a direct voltage, I m For AC fundamental phase current, Δ i max For maximum allowable pulsating quantity of current, 20% I is taken m ，T imax Is the maximum time constant of inertia, Δ P L，max For maximum variation of load power, Δ v dc，max Is the maximum variation of the direct-current voltage,
to limit the rise time of the DC voltage, R Le For a rated DC load resistance, I dm For rated direct current, v d0 Is the lowest steady-state value, v, of the DC voltage de Is a DC voltage rating;
introducing a PI controller in the control, and defining the gain parameter of the PI controller as:
wherein, T s For PWM switching period, K PWM For equivalent gain of the rectifier bridge, depending on the modulation mode, K P 、K I Respectively obtaining the gain coefficients of a proportional link and an integral link of the PI regulator;
the closed loop transfer function of the control loop is simplified into an inertia link:
5. the method of hierarchical distributed control of a flexible diamond power distribution flexible interconnection transformer according to claim 3, wherein: the control principle of the local layer comprises primary voltage regulation control and secondary voltage regulation control.
6. The method of hierarchical distributed control of a flexible diamond power distribution flexible interconnection transformer according to claim 5, wherein: the primary voltage regulation control comprises the following steps of,
establishing a droop equation, wherein the normalization equation and the droop equation after normalization are as follows: :
P ac1 ＝P 1 +P dc1
controlling the normalized voltage at the AC side to be equal to the normalized voltage at the DC side, namely:
……
defining the direct current normalized voltage of each line to be equal, and the direct current normalized voltage is as follows:
the above formula is established simultaneously, and after primary voltage regulation controlled by a local layer, the following can be obtained:
wherein N represents N distribution network flexible interconnections, Z 1 ，...，Z n Line impedances, P, of the 1 st, the right, the n distribution networks, respectively 1 ，...，P n Load power, V, of the 1 st, the ac1 ，...，V acn Line end voltages, P, of the 1 st, the right, the n distribution networks, respectively c1 ，...，P cn Active power, Q, transmitted by the AC-DC converters of the 1 st, the other, the n distribution networks respectively c1 ，...，Q cn The reactive power P transferred by the AC-DC converters of the 1 st, the other, the n distribution networks dc Is a direct current source or charged power.
7. The method of hierarchical distributed control of a flexible diamond power distribution flexible interconnection transformer according to claim 5, wherein: the secondary voltage regulation comprises the following steps of,
on the basis of the primary voltage regulation, active power and reactive power transmission are defined as given values, namely, the power transmission of each feeder line is controlled to be the given values
The droop equation for each line becomes:
……
at steady state, the power and voltage equation satisfies:
the system meets the following requirements:
after each line is flexibly interconnected, the direct-current normalized voltage of each line is defined to be equal, and the direct-current normalized voltage is as follows:
by combining the above equations, one can obtain:
8. the method of hierarchical distributed control of a flexible diamond power distribution flexible interconnection transformer according to claim 3, wherein: the control principle at the system level includes,
and calculating to obtain the optimal operation state of the system through retrieval of an intelligent algorithm according to the voltage information and the power information of each node acquired by the system.
9. The method of hierarchical distributed control of a flexible diamond power distribution flexible interconnection transformer according to claim 8, wherein: also comprises a step of adding a new type of additive,
establishing a mathematical model of the actual problem according to a general mathematical model, wherein the general mathematical model is as follows:
min f＝f(x，u)
{s.t.g(x，u)＝0
h(x，u)≤0
where f (x, u) is the objective function, g (x, u) and h (x, u) are equality constraints and inequality constraints, respectively, and x and u are control variables and state variables, respectively.
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