CN109861233B - Decoupling power flow determination method for AC/DC hybrid power distribution network - Google Patents
Decoupling power flow determination method for AC/DC hybrid power distribution network Download PDFInfo
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Abstract
The invention discloses a decoupling load flow calculation method for an alternating current-direct current hybrid power distribution network. The method mainly comprises the following steps: 1) decoupling the power distribution network into an alternating current subsystem and a direct current subsystem by taking the AC/DC converter as a boundary, and decoupling the direct current subsystem by taking the DC/DC converter as a boundary; 2) searching an alternating current-direct current electric island in the electric network according to the connection relation of the electric network equipment and the opening and closing state of the electric network equipment, and classifying the electric island of the alternating current-direct current electric network based on a DC side control mode and a connection equipment condition in AC/DC, and a control mode and a connection equipment condition in DC/DC; 3) and iteratively calculating various electric island currents according to the electric island classification. According to the method, the AC/DC hybrid power distribution network is subjected to electric island division, and the virtual DC bus is used for decoupling the control power of the high-voltage side and the low-voltage side of the power electronic transformer, so that the problem that the current load flow calculation is poor in applicability in the AC/DC hybrid power distribution network is solved, and the load flow calculation result of the AC/DC hybrid power distribution network can be quickly obtained.
Description
Technical Field
The invention relates to the technical field of power grids, in particular to a decoupling power flow determination method for an alternating-current and direct-current hybrid power distribution network.
Background
The direct current system in the existing alternating current-direct current hybrid power grid is generally an end-to-end system without a direct current network, and the power flow calculation method adopted by the alternating current-direct current hybrid power grid power flow is a PQ decoupling method or a Newton Raphson method without the need of direct current power grid power flow calculation.
The development of a direct-current power distribution technology, a multi-end flexible direct-current system and a power electronic transformer technology has the disadvantages that a direct-current network is continuously simple to complex, and the interaction relation of an alternating-current system and a direct-current system is more and more complex, so that the safety analysis of an alternating-current and direct-current hybrid power distribution network cannot be performed through simple calculation analysis. The current various load flow calculation methods cannot meet the load flow calculation requirements of complex alternating current and direct current hybrid power distribution networks.
Disclosure of Invention
In order to solve the problems, the invention provides a method for determining decoupling power flow of an alternating-current and direct-current hybrid power distribution network, which comprises the following steps:
(1) decoupling the power grid into an alternating current subsystem and a direct current power grid by taking the AC/DC converter as a boundary; decoupling a direct current power grid into a direct current subsystem by taking the DC/DC converter as a boundary;
(2) decomposing each alternating current subsystem and each direct current subsystem into an active electric island and a dead island according to the connection relation of power grid equipment and the opening and closing state of the power grid equipment, and classifying the active electric islands based on the condition of DC side connection equipment in an AC/DC converter and the condition of connection equipment in the DC/DC converter;
(3) and performing load flow calculation on the active electric islands, wherein different active electric islands calculate the load flow by using different load flow calculation methods in a classified manner.
According to the invention, according to the composition structure of the AC/DC hybrid power distribution network, the AC/DC hybrid power distribution network is subjected to electric island division, and the virtual DC bus is used for decoupling the control power of the high-voltage side and the low-voltage side of the power electronic transformer to perform decoupling calculation on the AC/DC power distribution network. The method solves the problem that the current power flow calculation is poor in applicability in the AC/DC hybrid power distribution network, can quickly obtain the power flow calculation result of the AC/DC hybrid power distribution network, reasonably calculates the power loss of the power electronic converter, and adapts to the change of the control mode of the power electronic converter. The invention provides an efficient and reliable method for load flow calculation of the alternating current and direct current hybrid power distribution network.
Drawings
The above features and technical advantages of the present invention will become more apparent and readily appreciated from the following description of the embodiments thereof taken in conjunction with the accompanying drawings.
Fig. 1 is a schematic diagram of an ac/dc hybrid power distribution network according to an embodiment of the present invention;
FIG. 2 is a schematic power flow diagram of a power electronic transformer according to an embodiment of the present invention;
fig. 3 is a schematic view of a power electronic transformer load flow calculation model according to an embodiment of the invention;
fig. 4 is a schematic diagram of a dc grid-connected dc electrical island according to an embodiment of the present invention;
fig. 5 is a schematic diagram of a dc-grid ac electrical island according to an embodiment of the present invention;
fig. 6 is a schematic diagram of an ac grid-connected dc electrical island according to an embodiment of the present invention;
FIG. 7 is a schematic diagram of a DC island according to an embodiment of the present invention;
FIG. 8 is a schematic diagram of an AC island according to an embodiment of the present invention;
FIG. 9 is a schematic diagram of a flexible DC island according to an embodiment of the present invention;
fig. 10 is a schematic diagram of forward-backward substitution calculation of the dc power grid according to the embodiment of the present invention;
FIG. 11 is a schematic diagram of forward-backward substitution calculation of the AC power grid according to the embodiment of the present invention;
FIG. 12 is a schematic diagram of a radiating power grid in an AC power grid in accordance with an embodiment of the present invention;
fig. 13 is a step chart of a decoupling power flow determination method for an ac/dc hybrid power distribution network according to an embodiment of the present invention.
Detailed Description
Embodiments of the decoupling power flow determination method for the alternating current-direct current hybrid power distribution network according to the present invention will be described below with reference to the accompanying drawings. Those of ordinary skill in the art will recognize that the described embodiments can be modified in various different ways, or combinations thereof, without departing from the spirit and scope of the present invention. Accordingly, the drawings and description are illustrative in nature and not intended to limit the scope of the claims. Furthermore, in the present description, the drawings are not to scale and like reference numerals refer to like parts.
The electric islands are electric islands, and one electric island is a division of a calculation network area in the load flow calculation. The passive electric island is called as a dead island, and the active electric island is classified into: a dc grid-connected dc electrical island, a dc grid-connected ac electrical island, an ac grid-connected dc electrical island, a dc electrical island, an ac electrical island, a flexible dc, an electrical transformer electrical island, a dc electrical island, an ac electrical island, the following description is provided with reference to fig. 4 to 9:
1) the definition of the direct current grid-connected direct current island is as follows: the number of direct current lines in the electric island is equal to 0, the number of DC/DC converters is equal to 1, and the number of loads is greater than 0 or the number of power generation equipment is greater than 0 or the number of energy storage equipment is greater than 0. The direct current grid connection refers to a behavior of power exchange between direct current power generation equipment or direct current energy storage equipment or direct current power utilization equipment and a direct current power grid or an alternating current power grid.
2) The definition of the direct current grid-connected alternating current island is as follows: the number of direct current lines in the electric island is equal to 0, the number of AC/DC converters is equal to 1, and the number of loads is greater than 0 or the number of power generation equipment is greater than 0 or the number of energy storage equipment is greater than 0.
3) The definition of the AC grid-connected DC electric island is as follows: the number of direct current lines in the electric island is equal to 0, the number of AC/DC converters is equal to 1, and the number of loads is greater than 0 or the number of generated electricity is greater than 0 or the number of stored energy is greater than 0; the alternating current grid connection refers to a behavior of power exchange between alternating current power generation equipment or alternating current energy storage equipment or alternating current electric equipment and a direct current power grid or an alternating current power grid.
4) The definition of the direct current island is as follows: the number of direct current circuits in the electric island is greater than 0 or the number of direct short-circuit current limiters is greater than 0, the number of AC/DC converters is 1 or the number of DC/DC converters is 1;
5) the definition of the alternating current island is as follows: the number of alternating current lines or transformers or series reactors in the electric island is more than 0, and the number of AC/DC converters is 1;
6) the definition of a flexible dc island (dc based voltage source converter) is: the direct current bus and the direct current line of the flexible direct current are integrated, and for back-to-back direct current, the electric island only has the direct current bus;
7) the definition of the power electronic transformer electric island is as follows: as shown in fig. 2 and 3, a virtual DC bus is provided for the power electronic transformer, and each port of the power electronic transformer is connected to the virtual bus through an AC/DC converter or a DC/DC converter according to the conversion type, so as to convert the multi-port power electronic transformer into a combination of a plurality of two ports, where the power electronic transformer island only has the virtual bus of the power electronic transformer;
8) the definition of the direct current electric island is as follows: the number of direct current lines in the electric island is greater than 0, the number of generators plus the number of AC/DC converters plus the number of DC/DC converters is greater than 1;
9) the definition of an ac island is: the number of AC lines in the electric island is greater than 0, and the number of generators + the number of AC/DC converters is greater than 1.
The method for determining the decoupling power flow of the alternating current-direct current hybrid power distribution network comprises the following steps:
(1) decoupling the power grid into an alternating current subsystem and a direct current power grid by taking the AC/DC converter as a boundary; decoupling a direct current power grid into a direct current subsystem by taking the DC/DC converter as a boundary;
(2) decomposing each alternating current subsystem and each direct current subsystem into an active electric island and a dead island according to the connection relation of power grid equipment and the opening and closing state of the power grid equipment, and classifying the active electric islands based on the condition of DC side connection equipment in an AC/DC converter and the condition of connection equipment in the DC/DC converter;
(3) and performing load flow calculation on the active electric islands, wherein different active electric islands calculate the load flow by using different load flow calculation methods in a classified manner.
Furthermore, preferably, a breadth-first search algorithm is employed to partition the active electrical island, wherein:
the method comprises the following steps that an AC island search is carried out by taking an AC side node of a generator or an AC/DC converter as an initial node, all node sets communicated with the initial node are searched, and equipment in an open state is brought into a search range in the search process;
searching a node set communicated with a starting node from a DC side of a generator or an AC/DC converter or any side node of the DC/DC converter as the starting node by using the direct current electric island search, and bringing equipment in an on state into a search range in the search process;
after the alternating current electric island and the direct current electric island are searched, nodes without the attributive electric island existing in the power grid are dead island nodes, and the dead islands do not participate in power flow calculation of the power grid.
In addition, preferably, in the power flow calculation method for the alternating current/direct current hybrid power distribution network, the calculation formulas of the losses of the DC/DC converter and the AC/DC converter are as follows:
wherein: p is input active power of the DC/DC or AC/DC converter; pLRFor DC/DC or AC/DC converter duty,
PNrated capacity for DC/DC or AC/DC converter; a. b and c are constant coefficients of the fitting relation between the efficiency of the converter and the conversion load rate.
Furthermore, preferably, the sequence of load flow calculation of the ac/dc hybrid power distribution network is as follows:
(1) counting the power of the direct current grid-connected direct current island: taking a direct-current bus in the DC/DC converter island as a balance node, namely, taking the voltage of the direct-current bus as a set value, taking the power of each branch as the product of the voltage U of the balance node and the current I of the branch, counting the power value in the island, and calculating the loss of the DC/DC converter in the island and the power of two sides by combining a calculation formula of the loss of the DC/DC converter and the AC/DC converter;
(2) counting the power of the direct-current grid-connected alternating-current electric island: taking a direct-current bus in an AC/DC converter island as a balance node, taking the power of each branch as the product of a balance node voltage U and a branch current I, counting the power value in the island, and calculating the loss of the AC/DC converter in the island and the power of two sides by combining with a calculation formula of the loss of the DC/DC converter and the AC/DC converter;
(3) counting the power of the alternating current grid-connected direct current island: taking an alternating current bus in an AC/DC converter island as a balance node, taking the power of each branch as the product of a voltage U of the balance node and a current I of the branch, counting the power value in the island, and calculating the loss of the AC/DC converter in the island and the power of two sides by combining with a calculation formula of the loss of the DC/DC converter and the AC/DC converter;
(4) calculating the flow of the alternating current island: calculating the loss and the power on two sides of an AC/DC converter of the AC island;
(5) calculating the direct current isolated island load flow: calculating the loss and the power on two sides of an AC/DC converter or a DC/DC converter of the direct current island;
(6) calculating flexible direct current power flow: obtaining power values of AC sides at two sides of the flexible direct current;
(7) calculating the current of the alternating current island and the direct current island: calculating the power of an AC/DC converter and two sides in the AC electric island by adopting an AC forward-backward substitution method, and calculating the loss of the DC/DC converter and the power of two sides of the DC electric island by adopting a DC forward-backward substitution method;
(8) calculating the power electronic transformer island tide: calculating the loss and power values at two sides of all AC/DC converters and DC/DC converters forming the power electronic transformer;
(9) and (5) repeating the steps 7 and 8, and calculating until the calculation result of the system is converged.
In addition, preferably, in the method for calculating forward-backward flow of a dc electrical island, the dc electrical island has a voltage control point, the given value of the voltage control point is used as a starting end voltage, the voltage of each node of the dc electrical island is set as a rated voltage, and the forward-backward flow of the dc electrical island is calculated by using the method for calculating forward-backward flow of the dc electrical island, and the calculation flow is described below with reference to fig. 10 as follows:
(1) forming a hierarchical network structure by adopting a breadth-first search method;
(2) the back substitution method comprises the following steps: according to the load power, from the end node, the following node j replaces the previous node i layer by layer, so as to obtain each branch current, and the formula is as follows:
Iij＝∑Ijk，
wherein, IjkThe current of the kth branch in all branch currents taking the node j of the lower layer as a power supply;
Iijis the current flowing from the node i of the upper layer to the node j of the lower layer;
(3) the forward pushing method comprises the following steps: using the head end node as the initial node, the node i in the previous layer is pushed forward to the node j in the next layer by layer to obtain the voltage of each node, and the formula thereofThe following were used: u shapej＝Ui-IijRijWherein, UiFor the voltage of the node i of the previous layer, UjIs the next level of node j voltage, IijFor the current flowing from node i of the previous layer to node j of the next layer, RijThe branch resistance between the node i of the upper layer and the node j of the lower layer is the branch resistance;
(4) and repeating the forward-backward process to make the calculated values of the voltage of each node and the current of the branch converge and obtain the current calculation result of the direct current island.
In addition, preferably, the direct current isolated island power flow calculation method is as follows: obtaining each node voltage and branch current by adopting a forward-backward-pushing tidal current calculation method of a direct-current power grid, thereby calculating each bus voltage in the direct-current island, each equipment current, power and loss of the direct-current island, and the loss and the power of two sides of an AC/DC converter or a DC/DC converter of the direct-current island;
the alternating current isolated island load flow calculation method comprises the following steps: and calculating the voltage of each bus in the alternating current island, the active power, the reactive power and the loss of each device of the alternating current island, and the loss and the power at two sides of the AC/DC converter of the alternating current island by adopting a forward-backward-flow power flow calculation method of the alternating current power grid.
In addition, preferably, the flexible direct current island load flow calculation method is as follows:
the control mode of the flexible direct current island rectification link AC/DC converter comprises DC side constant power, AC side constant voltage or no control;
the control mode of the inversion link AC/DC converter comprises DC side constant voltage, AC side constant voltage or no control, the AC side power of the rectification link AC/DC converter and the inversion link AC/DC converter is calculated through the DC side setting power of the rectification link AC/DC converter, the DC bus voltage in the flexible direct current is the setting voltage of the DC side of the inversion link AC/DC converter, wherein the AC side power of the flexible direct current rectification link AC/DC converter
The calculation is as follows:
wherein: pDCThe power is set for the flexible direct current DC side,
power loss for the rectification link;
wherein, the AC side power of the flexible DC inversion link AC/DC converter
The calculation is as follows:
wherein:
power loss in the inversion link.
Furthermore, preferably, the power electronic transformer load flow calculation method is as follows:
(1) acquiring node voltages and branch currents of a low-voltage end AC/DC converter and a low-voltage end DC/DC converter of the power electronic transformer based on a grid-connected direct current island load flow calculation method and an island power grid load flow calculation method, so as to calculate virtual direct current bus side power values of the low-voltage end AC/DC converter and the low-voltage end DC/DC converter of the power electronic transformer;
(2) power of virtual DC bus of power electronic transformer
Comprises the following steps:
wherein,
injecting the power of a virtual direct current bus into a kth low-voltage port, wherein m is the number of the low-voltage ports of the power electronic transformer;
(3) and calculating the loss of the high-voltage end AC/DC converter of the power electronic transformer by using the DC bus power, and calculating the AC side power of the AC/DC converter.
Furthermore, preferably, the dc electrical island calculation method is:
only one balance node is arranged in the direct current electric island, and the voltage of each bus in the direct current electric island, the current, the power and the loss of each device of the direct current electric island, the loss of an AC/DC converter or a DC/DC converter of the direct current electric island and the power at two sides are calculated by adopting a direct current power grid forward-backward substitution load flow calculation method;
further, preferably, the ac island load flow calculation method selects different algorithms according to a network model, the network model including a radial grid and a ring grid, wherein:
(1) radiation type power grid (as shown in fig. 12, power supply wiring method spreading to multiple lines of peripheral i1, i2, … in with bus power supply as center):
calculating the voltage of each bus in the alternating current island, the active power, the reactive power, the power and the loss of each device of the alternating current island, the loss of an alternating current/direct current (AC/DC) converter for grid connection of the alternating current island and the power at two sides by adopting an alternating current power grid forward-backward substitution load flow calculation method;
(2) an annular power grid:
and calculating the voltage of each bus in the alternating current island, the active power, the reactive power and the loss of each device of the alternating current island by adopting a Newton Raphson method.
In addition, fig. 11 is used to explain the method for calculating the forward-backward flow of the ac grid:
the i side is a power supply, the j side is a load, the backward process is j to i, and the forward process is i to j.
The calculation process of the forward-backward substitution load flow of the alternating-current power grid is as follows:
(1) the width is prior, and a layered network structure is formed;
(2) according to the load power, from the end node, replacing the node i on the next layer of node j layer by layer to obtain the current of each branch, wherein the purpose of the replacement is to obtain the power of each branch;
loss Δ S of branch ijijComprises the following steps:
Si＝ΔSij+Sj
wherein the apparent power S at the node i of the previous layeri＝Pi+iQi，Pi、QiFor active and reactive power, U, at node i of the previous layeriIs the voltage of the node i of the previous layer, ZijIs the impedance of branch ij, SL,jThe load apparent power of the next layer of node j (the generating power is a negative value); n is the node on the opposite side of all the branches connected with the node j of the next layer, SjkThe apparent power of the node branch except i connected with the next layer of node j.
(3) And (4) taking the head end node (balance node) as an initial node to push forward, and pushing the upper layer of node i forward and the lower layer of node j forward layer by layer, wherein the purpose of forward pushing is to obtain each bus voltage.
Ui＝Uj-IjZij
Wherein, UiIs the voltage of the node I of the previous layer, IjThe current on branch ij (the direction is from the node j of the next layer to the node i of the previous layer); vireFor the voltage U of the node i of the previous layeriReal part of, ViimFor the voltage U of the node i of the previous layeriImaginary part of, VjreFor the next layer of node j voltage UjReal part of, VjimFor the next layer of node j voltage UjAn imaginary part of (d); zij＝rij+jxij，rij、xijAre respectively a branchResistance and reactance of ij.
(4) And pushing back to a PV node in the generation (the PV node is a node with constant injection active power and given node voltage amplitude, and a power generation node with voltage regulation capability is generally selected).
In all the following formulas, positive directions of power and current are based on a bus, and positive and negative are output.
And (4) disconnecting at each PV node to form a network breakpoint and a power supply breakpoint, wherein the new network is still radial. By thevenin's theorem, looking into from two breakpoint, breakpoint voltage amplitude increment delta U and current amplitude increment delta I satisfy:
[Z][ΔI]＝[ΔU]
where Z is an impedance matrix, Z ═ R + jX. The above formula can be converted into:
[Z][ΔS*]＝[U][ΔU]
ΔS*Δ P-j Δ Q is the conjugate of the equivalent injection power at each PV node, and Δ U Δ E + j Δ F is the break voltage. The PV node port active mismatch Δ P is constantly 0, so the above equation is expanded as:
to obtain
Where RE represents the real part. And after each iteration of the load flow calculation, correcting the reactive power injected into the PV node by using the voltage amplitude deviation according to the formula.
The following describes the power flow determination method according to the present embodiment with a specific example.
Fig. 1 is a schematic diagram of an ac/dc hybrid power distribution network according to an embodiment of the present invention, fig. 13 is a diagram of a power flow calculation step of the ac/dc hybrid power distribution network according to the embodiment of the present invention, and a power flow determination method is specifically described below with reference to fig. 13 and fig. 1, where the specific steps include:
the communication subsystem comprises: a high-voltage alternating-current power grid A, a high-voltage alternating-current power grid B and a low-voltage alternating-current power grid.
The direct current subsystem includes: high voltage direct current power grids, medium voltage direct current power grids, low voltage direct current power grids, power electronic transformers and flexible direct current power grids.
(1) direct current grid-connected direct current electric island: photovoltaic H, photovoltaic M and energy storage M;
(2) direct current grid-connected alternating current island: a direct-drive fan;
(3) alternating current grid-connected direct current electric island: none;
(4) d, direct current island: none;
(5) and (3) alternating current island: a low-voltage alternating current grid (which is connected with an alternating current port of a power electronic transformer, and the alternating current port is equivalent to a DC/AC converter, so that the low-voltage alternating current grid can be regarded as an alternating current island);
(6) flexible direct current;
(7) a power electronic transformer;
(8) direct current electric island: a high-voltage direct-current power grid, a medium-voltage direct-current power grid and a low-voltage direct-current power grid (the direct-current power grid is connected to a direct-current port of a power electronic transformer and is equivalently a DC/DC port);
(9) an alternating current island: a high-voltage alternating-current power grid A, a high-voltage alternating-current power grid B and a medium-voltage alternating-current power grid.
And (3) load flow calculation, comprising:
And step 304, calculating the load flow of the alternating current and direct current electric island.
The flexible direct current device adopts constant direct current voltage control on the high-voltage alternating current power grid A, adopts constant power control on the high-voltage alternating current power grid B, selects the high-voltage alternating current power grid A for final calculation, and firstly calculates the alternating current power grid B;
and (3) a high-voltage alternating-current power grid B, wherein the calculation result is as follows: (1) a high-voltage alternating-current power grid B power grid tide; (2) loss and power on both sides of the AC/DC converter of the high-voltage AC power grid B-low-voltage DC power grid.
Medium voltage alternating current network, the calculation result: (1) medium voltage ac grid power flow; (2) loss and both-side power of an AC/DC converter of a power electronic transformer-medium voltage alternating current power grid; (3) loss and power on two sides of a DC/DC converter of a high-voltage direct-current power grid-medium-voltage alternating-current power grid.
High voltage direct current electric wire netting, the calculated result: (1) high voltage direct current power grid flow; (2) loss and power on two sides of an AC/DC converter of a high-voltage alternating current power grid A-high voltage direct current power grid.
And (3) a medium-voltage direct-current power grid, wherein the calculation result is as follows: (1) medium voltage direct current grid power flow; (2) loss and power on both sides of a power electronic transformer-DC/DC converter of a medium voltage ac power grid.
Low-voltage direct current network, the calculated result: (1) low voltage dc grid power flow; (2) loss and power on both sides of a power electronic transformer-DC/DC converter of a low-voltage DC power grid.
305, calculating the load flow of the power electronic transformer
And (3) calculating the result: loss and power on both sides of the AC/DC converter of the power electronic transformer-high voltage AC grid a.
And (3) repeatedly utilizing the steps 304 and 305 until the system calculation structure is converged, obtaining the power of the flexible direct current device, the power of the power electronic transformer and the power of the AC/DC converter, and finally calculating the A load flow of the high-voltage alternating current power grid. And (3) calculating the result: high-voltage alternating current power grid A tide, high-voltage alternating current power grid B tide, high-voltage direct current power grid tide, medium-voltage direct current power grid tide and low-voltage direct current power grid tide.
And the whole power grid load flow calculation is completed.
The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (9)
1. The method for determining the decoupling power flow of the alternating-current and direct-current hybrid power distribution network is characterized by comprising the following steps of:
(1) decoupling the power grid into an alternating current subsystem and a direct current power grid by taking the AC/DC converter as a boundary; decoupling a direct current power grid into a direct current subsystem by taking the DC/DC converter as a boundary;
(2) decomposing each alternating current subsystem and each direct current subsystem into an active electric island and a dead island according to the connection relation of power grid equipment and the opening and closing state of the power grid equipment, and classifying the active electric islands based on the condition of DC side connection equipment in an AC/DC converter and the condition of connection equipment in the DC/DC converter;
(3) performing active electric island load flow calculation, wherein different active electric island classifications use different load flow calculation methods to calculate load flow,
the classification of active electrical islands includes: a DC grid-connected DC electric island, a DC grid-connected AC electric island, an AC grid-connected DC electric island, a DC island, an AC island, a flexible DC island, a power electronic transformer electric island, a DC electric island, an AC electric island,
1) the definition of the direct current grid-connected direct current island is as follows: the number of direct current lines in the electric island is equal to 0, the number of DC/DC converters is equal to 1, the number of loads is greater than 0 or the number of power generation equipment is greater than 0 or the number of energy storage equipment is greater than 0;
2) the definition of the direct current grid-connected alternating current island is as follows: the number of direct current lines in the electric island is equal to 0, the number of DC/AC converters is equal to 1, the number of loads is greater than 0 or the number of power generation equipment is greater than 0 or the number of energy storage equipment is greater than 0;
3) the definition of the AC grid-connected DC electric island is as follows: the number of direct current lines in the electric island is equal to 0, the number of AC/DC converters is equal to 1, the number of loads is greater than 0 or the number of power generation equipment is greater than 0 or the number of energy storage equipment is greater than 0;
4) the definition of the direct current island is as follows: the number of direct current circuits in the electric island is greater than 0 or the number of direct short-circuit current limiters is greater than 0, the number of AC/DC converters is 1 or the number of DC/DC converters is 1;
5) the definition of the alternating current island is as follows: the number of alternating current lines or transformers or series reactors in the electric island is more than 0, and the number of AC/DC converters is 1;
6) the flexible direct current island is defined as follows: the direct current bus and the direct current line of the flexible direct current are integrated, and for back-to-back direct current, the electric island only has the direct current bus;
7) the definition of the power electronic transformer electric island is as follows: connecting each port of the power electronic transformer with a virtual bus through an AC/DC converter or a DC/DC converter according to the conversion type, so that the multi-port power electronic transformer is converted into a combination of a plurality of double ports, and the power electronic transformer island only has the virtual bus of the power electronic transformer;
8) the definition of the direct current electric island is as follows: the number of direct current lines in the electric island is greater than 0, the number of generators plus the number of AC/DC converters plus the number of DC/DC converters is greater than 1;
9) the definition of an ac island is: the number of AC lines in the electric island is greater than 0, and the number of generators + the number of AC/DC converters is greater than 1.
2. The method of claim 1, wherein the active electrical island is partitioned using a breadth-first search algorithm, wherein:
the method comprises the following steps that an AC island search is carried out by taking an AC side node of a generator or an AC/DC converter as an initial node, all node sets communicated with the initial node are searched, and equipment in an open state is brought into a search range in the search process;
searching all node sets communicated with the initial node by taking the nodes on the DC side of the generator or the AC/DC converter or any side of the DC/DC converter as the initial node, and bringing the equipment in an open state into a search range in the searching process;
after the alternating current electric island and the direct current electric island are searched, nodes without the attributive electric island existing in the power grid are dead island nodes, and the dead islands do not participate in power flow calculation of the power grid.
3. The method of claim 1,
in the load flow calculation method of the alternating current-direct current hybrid power distribution network, the calculation formula of the loss of a DC/DC converter and the loss of an AC/DC converter are as follows:
wherein: p is input active power of the DC/DC or AC/DC converter;
PLRfor DC/DC or AC/DC converter duty,
PNrated capacity for DC/DC or AC/DC converter;
a. b and c are constant coefficients of the fitting relation between the efficiency of the converter and the conversion load rate.
4. The method of claim 3, wherein the sequence of the load flow calculation of the AC/DC hybrid power distribution network is:
(1) calculating the current of the direct current grid-connected direct current island: taking a direct current bus in a DC/DC converter island as a balance node, counting power values in the island, and calculating loss of the DC/DC converter in the island and power on two sides by combining a calculation formula of loss of the DC/DC converter and the AC/DC converter;
(2) calculating the current of the direct current grid-connected alternating current island: taking a direct current bus in an AC/DC converter island as a balance node, counting power values in the island, and calculating AC/DC converter loss and power on two sides in the island by combining a calculation formula of DC/DC and AC/DC converter loss;
(3) calculating the power flow of the alternating current grid-connected direct current island: taking an alternating current bus in an AC/DC converter island as a balance node, counting power values in the island, and calculating AC/DC converter loss and power on two sides in the island by combining a calculation formula of DC/DC and AC/DC converter loss;
(4) calculating the flow of the alternating current island: calculating the loss and the power on two sides of an AC/DC converter of the AC island;
(5) calculating the direct current isolated island load flow: calculating the loss and the power on two sides of an AC/DC converter or a DC/DC converter of the direct current island;
(6) calculating the load flow of the flexible direct current island: obtaining power values of AC sides at two sides of the flexible direct current island;
(7) calculating the current of the alternating current island and the direct current island: calculating the power of an AC/DC converter and two sides in the AC electric island by adopting an AC forward-backward substitution method, and calculating the loss of the DC/DC converter and the power of two sides of the DC electric island by adopting a DC forward-backward substitution method;
(8) calculating the power electronic transformer island tide: calculating the loss and power values at two sides of all AC/DC converters and DC/DC converters forming the power electronic transformer;
(9) and (5) repeating the steps 7 and 8, and calculating until the calculation result of the system is converged.
5. The method according to claim 4, wherein in the forward-backward flow calculation method for the dc island, the dc island has a voltage control point, the given value of the voltage control point is used as a starting voltage, the voltage of each node of the dc island is set as a rated voltage, and the forward-backward flow calculation method is used to calculate the dc island, and the calculation process is as follows:
(1) forming a hierarchical network structure by adopting a breadth-first search method;
(2) the back substitution method comprises the following steps: according to the load power, from the end node, the following node j replaces the previous node i layer by layer, so as to obtain each branch current, and the formula is as follows:
Iij＝∑Ijk，
wherein, IjkThe current of the kth branch in all branch currents taking the node j of the lower layer as a power supply;
Iijfor passing from node i of the upper layer to node j of the next layerCurrent flow;
(3) the forward pushing method comprises the following steps: taking a head end node as an initial node, and pushing the node i of the previous layer forward to the node j of the next layer by layer to obtain the voltage of each node, wherein the formula is as follows: u shapej＝Ui-IijRijWherein, UiFor the voltage of the node i of the previous layer, UjIs the next level of node j voltage, IijFor the current flowing from node i of the previous layer to node j of the next layer, RijThe branch resistance between the node i of the upper layer and the node j of the lower layer is the branch resistance;
(4) and repeating the forward-backward process to make the calculated values of the voltage of each node and the current of the branch converge and obtain the current calculation result of the direct current island.
6. The method of claim 5,
the direct current isolated island load flow calculation method comprises the following steps: obtaining each node voltage and branch current by adopting a forward-backward-pushing tidal current calculation method of a direct-current power grid, thereby calculating each bus voltage in the direct-current island, each equipment current, power and loss of the direct-current island, and the loss and the power of two sides of an AC/DC converter or a DC/DC converter of the direct-current island;
the alternating current isolated island load flow calculation method comprises the following steps: and calculating the voltage of each bus in the alternating current island, the active power, the reactive power and the loss of each device of the alternating current island, and the loss and the power at two sides of the AC/DC converter of the alternating current island by adopting a forward-backward-flow power flow calculation method of the alternating current power grid.
7. The method of claim 4, wherein the flexible direct current island power flow calculation method is as follows:
the control mode of the flexible direct current island rectification link AC/DC converter comprises DC side constant power control, AC side constant voltage control or no control;
the control mode of the inversion link AC/DC converter comprises DC side constant voltage control, AC side constant voltage control or no control,
calculating the power of the AC side of the rectification link AC/DC converter and the AC side of the inversion link AC/DC converter through the setting power of the DC side of the rectification link AC/DC converter, wherein the DC bus voltage in the flexible direct current is the setting voltage of the DC side of the inversion link AC/DC converter, wherein,
AC side power of flexible DC rectification link AC/DC converter
The calculation is as follows:
wherein: pDCThe power is set for the flexible direct current DC side,
power loss for the rectification link;
wherein, the AC side power of the flexible DC inversion link AC/DC converter
The calculation is as follows:
wherein:
power loss in the inversion link.
8. The method of claim 4, wherein the power electronic transformer load flow calculation method is:
(1) acquiring node voltages and branch currents of a low-voltage end AC/DC converter and a low-voltage end DC/DC converter of the power electronic transformer based on a grid-connected direct current island load flow calculation method and an island power grid load flow calculation method, so as to calculate virtual direct current bus side power values of the low-voltage end AC/DC converter and the low-voltage end DC/DC converter of the power electronic transformer;
(2) power of virtual DC bus of power electronic transformer
Comprises the following steps:
wherein,
injecting the power of a virtual direct current bus into a kth low-voltage port, wherein m is the number of the low-voltage ports of the power electronic transformer;
(3) and calculating the loss of the high-voltage end AC/DC converter of the power electronic transformer by using the DC bus power, and calculating the AC side power of the AC/DC converter.
9. The method according to claim 5, wherein the direct current island load flow calculation method is as follows:
only one balance node is arranged in the direct current electric island, and the voltage of each bus in the direct current electric island, the current, the power and the loss of each device of the direct current electric island, the loss of an AC/DC converter or a DC/DC converter of the direct current electric island and the power at two sides are calculated by adopting a direct current power grid forward-backward substitution load flow calculation method;
the method for calculating the load flow of the alternating current island selects different algorithms according to the situation of a network model, wherein the network model comprises a radiation type power grid and an annular power grid, and the method comprises the following steps:
(1) radiation type electric wire netting:
calculating the voltage of each bus in the alternating current island, the active power, the reactive power, the power and the loss of each device of the alternating current island, the loss of an alternating current/direct current (AC/DC) converter for grid connection of the alternating current island and the power at two sides by adopting an alternating current power grid forward-backward substitution load flow calculation method;
(2) an annular power grid:
and calculating the voltage of each bus in the alternating current island, the active power, the reactive power and the loss of each device of the alternating current island by adopting a Newton Raphson method.
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