CN111398685B - Impedance measurement type island detection method suitable for annular flexible direct current power distribution network - Google Patents
Impedance measurement type island detection method suitable for annular flexible direct current power distribution network Download PDFInfo
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- G01—MEASURING; TESTING
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- G01R27/00—Arrangements for measuring resistance, reactance, impedance, or electric characteristics derived therefrom
- G01R27/02—Measuring real or complex resistance, reactance, impedance, or other two-pole characteristics derived therefrom, e.g. time constant
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- G01R31/081—Locating faults in cables, transmission lines, or networks according to type of conductors
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Abstract
The invention discloses an impedance measurement type island detection method suitable for an annular flexible direct current power distribution network, which comprises the steps of firstly, carrying out high-frequency impedance modeling based on the switching states of a DC/DC converter and an MMC in an annular flexible direct current power distribution system and electrical elements contained in the DC/DC converter and the MMC; periodically locking a full-control bridge at the low-voltage side of a certain submodule of the DC/DC converter, and injecting high-frequency harmonic disturbance with specific frequency into the annular flexible direct-current power distribution system; in the time period of injecting high-frequency harmonic disturbance into the DC/DC converter of each unit, obtaining the harmonic intensity and the measured impedance of an injection frequency band by utilizing a Fourier algorithm; and then, identifying the occurrence of the island operation state according to the obtained amplitude variation of the measured impedance, and judging the unit entering the island operation state according to the harmonic intensity. According to the method, the complex island working condition comprising a plurality of adjacent units is effectively identified according to the measured reactance and harmonic amplitude value changes, and accurate and rapid island detection is realized.
Description
Technical Field
The invention relates to the technical field of power system analysis, in particular to an impedance measurement type island detection method suitable for an annular flexible direct current power distribution network.
Background
At present, compared with the traditional alternating current power distribution system, the flexible direct current power distribution system based on the MMC has the advantages of few current conversion links, high energy conversion rate, high waveform quality and the like, and gradually becomes a hot spot of domestic and foreign research. In a direct current power distribution network containing a Photovoltaic (PV) unit, safety hazards are brought to personnel and equipment by unplanned island operation, and an island detection method in the direct current power distribution network needs to be researched urgently.
Island detection in the prior art can be mainly divided into remote type detection and local type detection, and the dependence of the remote type on communication equipment and channels limits the application of the remote type detection in a direct current system; the local detection can be further divided into a passive method and an active method, which have relatively complete development in the ac power grid, but most of them cannot be directly applied to the dc power grid. The passive method detects the island by means of the change of the electrical quantity caused by power mismatching after the island is detected, the direct current power grid does not transmit reactive power and cannot acquire frequency and phase information, so that the passive method relying on the frequency, the phase and the reactive power cannot be used, the direct current island can be detected by using voltage and current signals, but a large detection blind area exists when a power supply in an island system is approximately matched with load power, and the passive method is not suitable for being independently applied to a direct current system; the active method is characterized in that disturbance components are artificially injected into a system, and the system is rapidly destabilized by disturbance after an island so as to detect the island.
At present, research aiming at an anti-islanding strategy of a direct current system is less, a detection method based on voltage positive feedback and using a DC/DC converter to inject active disturbance is mainly focused, in a multi-terminal direct current network, the requirement on disturbance intensity is improved due to the increase of system capacity, and detection time is increased or even detection failure may be caused by dispersed low-intensity disturbance of each unit. However, due to the difference of the electrical parameters of each DC/DC converter, the multi-machine synchronous injection disturbance is difficult to realize, and the application of the power disturbance method in a multi-terminal system has certain difficulty; meanwhile, in the annular direct-current power distribution network, besides a single-machine island, a complex island operation state formed by a plurality of adjacent photovoltaic stations may occur, and a new challenge is provided for island detection, so that an island detection technology suitable for an annular flexible direct-current power distribution network with multi-machine access needs to be researched.
Disclosure of Invention
The invention aims to provide an impedance measurement type island detection method suitable for an annular flexible direct current power distribution network.
The purpose of the invention is realized by the following technical scheme:
an impedance measurement type island detection method suitable for an annular flexible direct current power distribution network, the method comprising the following steps:
and 4, identifying the occurrence of the island operation state according to the obtained amplitude variation of the measured impedance, and judging the unit entering the island operation state according to the harmonic intensity.
According to the technical scheme provided by the invention, the method can effectively identify the complex island working condition comprising a plurality of adjacent units according to the measured reactance and the harmonic amplitude value, and can realize accurate and rapid island detection in the annular flexible direct current power distribution network with multi-unit access.
Drawings
In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings needed to be used in the description of the embodiments are briefly introduced below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on the drawings without creative efforts.
Fig. 1 is a schematic diagram of an impedance measurement type island detection method suitable for an annular flexible direct current power distribution network according to an embodiment of the present invention;
FIG. 2 is a schematic diagram of a DC/DC boost converter according to an embodiment of the present invention;
FIG. 3 is a schematic diagram of a topology of a DC/DC buck converter according to an embodiment of the present invention;
FIG. 4 illustrates two possible paths for the presence of injected high frequency harmonics according to the present invention;
FIG. 5 is a schematic diagram of an equivalent circuit of a sub-module according to an embodiment of the present invention;
FIG. 6 is a schematic diagram of an exemplary MMC structure according to an embodiment of the present invention;
FIG. 7 is a schematic diagram of a MMC high-frequency impedance model according to an embodiment of the present invention;
FIG. 8 is a schematic diagram of simulation of different island operating conditions in a flexible direct current distribution network simulation system according to an example of the present invention;
FIG. 9 is a graph illustrating measured reactance versus injected harmonic intensity under a single machine island in an example of the present invention;
FIG. 10 is a graph illustrating measured reactance versus injected harmonic intensity for an islanding PV34 unit according to an exemplary embodiment of the present invention;
fig. 11 is a schematic diagram of the measured reactance and injected harmonic intensity curves under the multi-machine island condition in the example of the present invention.
Detailed Description
The technical solutions in the embodiments of the present invention are clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments of the present invention without making any creative effort, shall fall within the protection scope of the present invention.
The following will describe an embodiment of the present invention in further detail with reference to the accompanying drawings, and as shown in fig. 1, a schematic diagram of an impedance measurement type island detection method suitable for an annular flexible dc power distribution network provided by the embodiment of the present invention is shown, where the method includes:
in this step, in order to set an operation threshold of the island detection algorithm, a high-frequency impedance of the system needs to be modeled in advance to obtain a theoretical value of the high-frequency impedance, and specifically, a high-frequency impedance model which is simple and convenient to calculate and has high accuracy at high frequency is obtained based on the switching states of a DC/DC Converter and a Modular Multilevel Converter (MMC) in the annular flexible direct-current power distribution system and on-off states of included electrical elements.
Specifically, two types of DC/DC converters exist in the annular flexible direct current power distribution system, one type is a DC/DC boost converter for connecting a photovoltaic power source and a direct current system, and fig. 2 is a schematic structural diagram of the DC/DC boost converter according to the embodiment of the present invention; the other type is a DC/DC buck converter for connecting a DC system and a DC load, the buck converter generally adopts an input parallel connection and an output series connection, and the topology structure is similar to that of the boost converter, except that the input side and the output side of the sub-module are reversed, as shown in fig. 3, the topology structure of the DC/DC buck converter according to the embodiment of the present invention is schematically illustrated.
The harmonic analysis is carried out by taking the DC/DC boost converter as an example, when the harmonic is injected from the DC system side, the harmonic firstly passes through the uncontrolled rectifying circuit at the output side of the converter, and the amplitude of the injected square wave does not exceed the amplitude of the output voltage of the high-frequency transformer under the normal operation state of the converter. Therefore, the switching-on condition of a diode in the uncontrolled rectifying circuit cannot be changed by the harmonic injection, the path of the harmonic circulation is controlled by a fully controlled inverter bridge at the input side of the converter, and two possible paths of the injected high-frequency harmonic exist are shown in fig. 4, wherein;
fig. 4(a) and (b) show harmonic paths when the bridge arms controlled by G11 and G12 are turned on, respectively, the two harmonic paths contain completely the same electrical components, and the same condition can be considered when impedance analysis is performed, the whole DC/DC converter and its submodules can be analyzed as a two-port network, and the G parameter of the two-port network is defined as:
firstly, analyzing the G parameter of a single sub-module, replacing the high-frequency transformer by using a T-type equivalent circuit, and calculating all elements in the sub-module to the high-voltage side to obtain the sub-module equivalent circuit shown in fig. 5, wherein the impedances of three branches on the T-type equivalent circuit of the transformer in fig. 5 are recorded as: z1＝a2*r+jXL1，Z2＝r+jXL2，Z3＝a*jXL12。CHAnd CL/a2Is respectively denoted as ZCHAnd ZCL. The specific parameters in the G parameters can be expressed as:
g22＝(Z1//Z3+Z2)//ZCL (6)
where// represents the parallel connection of two impedances.
According to the calculation rule of the G parameter, the n submodules adopt the DC/DC converter formed by the structure shown in FIG. 2 or FIG. 3, and the G parameter of the two ports is as follows:
since only the case where harmonics are injected from outside the DC/DC converter is considered, the impedance model obtained by equation (7) is only established when the DC/DC converter is not an injection source, and when a plurality of DC/DC converters having the same carrier frequency (i.e., the same injection frequency) exist in the system, it is necessary to ensure that only one converter is injected at the same time.
Fig. 6 is a schematic structural diagram of a typical MMC according to an embodiment of the present invention, where each phase includes two bridge arms, and a single bridge arm includes N sub-modules and a bridge arm reactance L0Are connected in series. The sub-module structure has small influence on the harmonic impedance model analysis, the half-bridge sub-module with the simplest structure is taken as an example for analysis in the invention, the half-bridge sub-module mainly comprises a switch tube (and an anti-parallel diode thereof) and a sub-module capacitor, as shown in fig. 6, a high-frequency harmonic path of the sub-module in the input state and the cut-off state comprises one switch tube, and the harmonic additionally flows through the sub-module capacitor in the input state.
For convenient solution, the following parameters of each bridge arm are assumed to be equal under an ideal condition: bridge arm inductance L0Sum R of on-resistances of switching tubes of submodules on single bridge armarmSingle sub-module capacitor C0. As shown in fig. 7, which is a schematic diagram of the MMC high-frequency impedance model according to the embodiment of the present invention, on this basis, the MMC may be simplified into the RLC series-parallel circuit shown in fig. 7(a), where n is the number of sub-modules put into r-phase (ABC three-phase) k-bridge arms (k ═ p, n, respectively representing positive and negative bridge arms) in the figurekr. Under the influence of the actual running state of the system, the number of submodules put into each bridge arm is different and time-varying, but the sum of the number of submodules in the putting state of the same-phase two bridge arms is always constant, and the sum of the number is equal to the total number of submodules of a single bridge arm, namely, the following conditions are met:
npr+nnr＝N (8)
the impedances of the six legs in the MMC can be expressed as:
in fig. 7(a), a path of the harmonic injected from the dc side at A, B is drawn by a dotted line, four arms through which the harmonic flows form a wheatstone bridge, and considering that the inductive reactance provided by the arm capacitance under the high-frequency injection will be much larger than the capacitive reactance provided by the sub-module capacitance, equation (9) can be:
therefore, the high-frequency impedances of the four bridge arms of the Wheatstone bridge are approximately equal, the high-frequency bridge is balanced, and high-frequency harmonic waves cannot flow into the alternating current side through the MMC bridge arms of the AB two phases. By popularizing the conclusion to three phases, it can be known that the MMC impedance model is the in-phase positive and negative bridge arm series connection shown in fig. 7(a), and the three-phase bridge arm parallel connection structure does not contain alternating-current side system components. Considering the relation of the input quantity of the submodules given by the formula (6), the sum of the capacitors of the submodules input by the two bridge arms of the single phase is constant C0and/N. Fig. 7(a) can be further simplified to the series circuit shown in fig. 7(b), so that the high-frequency impedance model of the MMC viewed from the dc side can be expressed as:
in this step, the DC/DC converter module is operated by a plurality of sub-modules with input connected in parallel and output connected in series, and the process of periodically locking the fully controlled bridge at the low-voltage side of a certain sub-module of the DC/DC converter is equivalent to: injecting a row of square waves with frequency as carrier frequency to the high-voltage side of the DC/DC converter, wherein the square waves can be decomposed into direct-current components with amplitude in direct proportion to duty ratio and harmonic waves with times of switching frequency, the amplitude of the harmonic waves is in negative correlation with the times, the harmonic waves with times of switching frequency have the highest amplitude, and the frequency of the harmonic waves is not overlapped with the characteristic harmonic waves of the converter;
and selecting the harmonic wave of which the frequency is multiple times of the switching frequency of the DC/DC converter as the injected high-frequency harmonic wave disturbance.
In addition, in order to identify the unit entering an island operation state in the annular network, the DC/DC converters of the units in the annular flexible direct current power distribution system are subjected to harmonic injection in turn, and it is ensured that only the DC/DC converter of one unit injects high-frequency harmonic disturbance into the system at the same time.
in the step, in a period of time when the DC/DC converter of each unit injects high-frequency harmonic disturbance, the port voltage and current signals of the DC/DC converter contain full-frequency-band information, the voltage and current signals of the injection frequency band of the DC/DC converter are extracted by utilizing a Fourier algorithm, and the measured impedance under the injection frequency band is obtained by calculation. Because the direct current system contains a large amount of reactance components, reactance information in the measured impedance is extracted and used as a main criterion for island protection, so that the influence of direct current load change in the system on island detection can be avoided, and the detection precision is improved.
And 4, identifying the occurrence of the island operation state according to the obtained amplitude variation of the measured impedance, and judging the unit entering the island operation state according to the harmonic intensity.
In the step, the system structure is changed before and after the island operation, and the high-frequency measured reactance measured at the port of the DC/DC converter is changed along with the change of the system structure, so that the island operation can be accurately detected according to the change of the high-frequency measured reactance.
Furthermore, because each unit is injected intermittently in different time periods, the injection harmonic frequency of the unit in the corresponding time period after the grid disconnection is approximately reduced to zero, and therefore, after the island of multiple units is formed, the injection harmonic intensities in the corresponding time periods of the grid-connected unit and the island unit at the MMC end are obviously different, and the unit entering the island operation state is judged according to the harmonic intensities.
Specifically, a grid-connected state threshold value X is set for identifying whether an island unit exists in a systemth0Comprises the following steps:
Xth0＝1.10*XGrid (12)
in the formula XGridThe harmonic reactance theoretical value of the disturbance frequency in the grid-connected state is obtained. Compared with the original system, the number of parallel branches in the island subsystem and the grid-connected subsystem is reduced, so that the reactance amplitudes measured by all measuring points are increased when an island exists. Equation (12) indicates that when the measured reactance value exceeds 1.1 times of the theoretical reactance value in grid-connected operation, an island unit exists in the system, and at this time, the number of units entering island operation needs to be further judged.
Defining protection action threshold value X of single islandth1Comprises the following steps:
Xth1＝XGrid+kth(XIsland-XGrid) (13)
wherein XIslandThe harmonic reactance theoretical value of the disturbance frequency in the island state is shown. k is a radical ofthSetting k for sensitivity coefficient of single island threshold value and considering calculation error possibly occurring in practicethIs 0.9. Equation (13) indicates that the stand-alone islanding operation is considered to occur when the measured reactance varies from the grid-connected state theoretical value to the islanding state theoretical value by an amplitude exceeding 90% of the difference.
When the measured reactance of each unit exceeds a grid-connected threshold and does not exceed a single-machine island threshold, a multi-machine island state is considered to exist, and the harmonic intensity of the injection frequency is detected at the MMC end. Considering possible transient disturbance and measurement errors, when harmonic voltages in a disturbance period of a unit are all lower than 10% of the lowest amplitude of disturbance harmonic voltages in grid-connected operation, the corresponding unit is considered to have island operation.
In this example, as shown in fig. 8, a schematic diagram of simulation of different island operating conditions in a flexible direct current distribution network simulation system according to an example of the present invention is shown, a rated voltage of the system is ± 10kV (20kV), a rated output power of a PV set is 2MW, carrier frequencies of DC/DC converters in the system are all 3kHz, that is, disturbance harmonics are also 3 kHz. And harmonic injection with the duration of 0.125s is respectively carried out in every 0.5s by the four PV sets, so that the injection time periods are completely staggered, the direct current load in the simulation is completely matched with the output of the PV sets, and the flat wave reactance value set at the outlet of each converter is 10 mH.
As shown in fig. 8, in the above system, a stand-alone island operation test is performed on PV3 and PV4 units, a reactance value measured at an outlet of a DC/DC boost converter of the PV3 unit and a change situation of a 3kHz harmonic voltage amplitude measured at a DC port of an MMC1 are shown in fig. 9, in the figure, the system enters an island at a time point of 3s, in order to compare a difference between a reactance measurement value before and after the island and a theoretical value, the system continues to operate after entering an island state, and a system start-up phase (0-1s) is not shown in the figure.
Fig. 9(a) and (b) show the variation curves of measured reactance and harmonic voltage before and after the PV3 single-machine island, in fig. 9(a), the harmonic reactance variation of corresponding measuring points before and after the PV3 island is the largest, and depends on the single-machine island threshold value Xth1And the single machine island operation state of the unit where the measuring point is located can be correctly judged. The system enters island at 3s, but the injection section of the last PV3 unit is finished at the moment, and 3s-3.125s are the next injection section. The reactance value measured by the PV3 unit therefore jumps at 3.125 s. Under the injection strategy, each unit is injected in a period of 0.5s, so that the maximum time delay of 0.5s exists in island detection, but the time delay does not exceed the limit of relevant regulations on island detection time.
Fig. 9(c) and (d) show the reactance measured at the PV3 point and the harmonic voltage measured by MMC1 of a PV4 single-machine island, which can be directly detected by the reactance measured by a PV4 unit. The lowest harmonic amplitude in the grid-connected state is indicated by a gray dashed line in the figure. After the PV4 unit enters an island state, due to the change of the system structure, the harmonic reactance measured by the PV3 unit changes and exceeds the grid-connected state threshold value Xth0. But the 3kHz harmonic does not drop significantly during the injection period of PV3 and rises somewhat due to the reduced system size, so the PV3 unit is not cut off. It can be seen in fig. 9(d) that the period injected harmonic amplitude corresponding to PV4 after islanding is reduced to approximately 0.
FIG. 10 shows the switches 32 and 45 of FIG. 8 open, resulting in an island comprising PV34 units before and after the PV3-6 units operating, measured reactance measured at the DC port of MMC1 and 3kHz harmonic measured at the DC port of the MMC1 unitThe graph is schematic. Due to the adoption of the alternate injection strategy, the reactance change delay measured by each unit is different. And because the positions of the units are different, the grid-connected reactance theoretical value and the grid-connected threshold value X of each unitth0And are also different. And measuring reactances of the four sets after the island exceed a grid-connected threshold in one injection period of 0.5s, and considering that a non-single-machine island state occurs in the system at the moment, but only the harmonic intensities of the PV3 and PV4 sets at corresponding moments are remarkably reduced to be close to 0, so that the composite island containing the PV34 set is detected.
Fig. 11 is a schematic diagram of reactance measured at a PV3 measuring point and harmonic voltage measured at an MMC1 under the condition of a multi-machine composite island of PV45, PV345, PV456, and PV3456 units, where an island reactance calculation error does not exceed 10% under each working condition in fig. 10 and 11, and injected harmonic intensities can correctly reflect an island/grid-connected operation state of a corresponding photovoltaic station.
It is noted that those skilled in the art will recognize that embodiments of the present invention are not described in detail herein.
In summary, the method of the embodiment of the present invention has the following advantages:
(1) an impedance measurement method is adopted to detect the island operation, the measured impedance directly reflects the structural change of the system, and the response to the island operation state in the system is sensitive;
(2) the required harmonic injection intensity is low, and the influence on the electric energy quality of the system is small;
(3) the DC/DC boost converter of the photovoltaic power direct current grid connection is used for harmonic injection, and external injection equipment is not relied on;
(4) the measured reactance is used as a main criterion for identifying the island operation, has a high value at high frequency, is not influenced by local load change, and can accurately reflect the occurrence of the island operation state;
(5) the harmonic intensity of the MMC port is used as an auxiliary criterion, and the unit entering the island operation state can be accurately judged through the harmonic intensity of the unit corresponding to the injection time period, so that the complex island operation state in the annular network can be accurately identified.
The above description is only for the preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present invention are included in the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.
Claims (4)
1. An impedance measurement type island detection method suitable for an annular flexible direct current power distribution network is characterized by comprising the following steps:
step 1, performing high-frequency impedance modeling based on the switching states of a DC/DC converter and an MMC in the annular flexible direct current power distribution system and electrical elements contained in the annular flexible direct current power distribution system to obtain a high-frequency impedance model of the annular flexible direct current power distribution system;
step 2, periodically locking a full control bridge at the low-voltage side of a certain submodule of the DC/DC converter, and injecting high-frequency harmonic disturbance with specific frequency into the annular flexible direct current power distribution system;
step 3, measuring voltage and current signals of a port of the DC/DC converter in a time period when the DC/DC converter of each unit injects high-frequency harmonic disturbance, and obtaining the harmonic intensity and the measured impedance of an injection frequency band by utilizing a Fourier algorithm;
step 4, identifying the occurrence of an island operation state according to the obtained amplitude variation of the measured impedance, and judging the unit entering the island operation state according to the harmonic intensity;
the unit process of judging to enter the island operation state according to the harmonic intensity specifically comprises the following steps:
because each unit time interval intermittent injection, the injection harmonic frequency of the corresponding time interval of unit after off-line will be approximately reduced to zero, therefore at the MMC end after the multimachine island, there is apparent difference in the injection harmonic intensity of the corresponding time interval of grid-connected unit and island unit, so judge the unit that gets into island running state according to harmonic intensity, specifically:
setting a grid-connected state threshold value X for identifying whether an island unit exists in a systemth0Comprises the following steps:
Xth0＝1.10*XGrid (12)
in the formula XGridThe harmonic reactance theoretical value is the disturbance frequency in a grid-connected state; compared with the original system, the number of parallel branches in the island subsystem and the grid-connected subsystem is reduced, so that reactance amplitudes measured by all measuring points are increased when an island exists;
the expression (12) shows that when the measured reactance value exceeds 1.1 times of the theoretical reactance value in grid-connected operation, an island unit exists in the system, and the number of the units entering island operation needs to be further judged at the moment;
further defining the protection action threshold value X of the single islandth1Comprises the following steps:
Xth1＝XGrid+kth(XIsland-XGrid) (13)
wherein XIslandThe harmonic reactance theoretical value is the disturbance frequency in an island state; k is a radical ofthSetting k for sensitivity coefficient of single island threshold value and considering calculation error possibly occurring in practicethIs 0.9;
formula (13) represents: when the measured reactance changes from the grid-connected state theoretical value to the island state theoretical value by more than 90% of the difference value, the single-machine island operation is considered to occur;
when the measured reactance of each unit exceeds a grid-connected threshold and does not exceed a single-machine island threshold, a multi-machine island state is considered to exist, and the harmonic intensity of the injection frequency is detected at the MMC end; and considering possible transient disturbance and measurement errors, when the harmonic voltage in the disturbance time period of the unit is lower than 10% of the lowest amplitude of the disturbance harmonic voltage in grid-connected operation, the corresponding unit is considered to have island operation.
2. The impedance measurement type island detection method suitable for the annular flexible direct current power distribution network according to claim 1, wherein the process of the step 2 specifically comprises:
injecting a row of square waves with frequency as carrier frequency to the high-voltage side of the DC/DC converter, wherein the square waves can be decomposed into direct-current components with amplitude in direct proportion to duty ratio and harmonic waves with times of switching frequency, the amplitude of the harmonic waves is in negative correlation with the times, the harmonic waves with times of switching frequency have the highest amplitude, and the frequency of the harmonic waves is not overlapped with the characteristic harmonic waves of the converter;
and selecting the harmonic wave of which the frequency is multiple times of the switching frequency of the DC/DC converter as the injected high-frequency harmonic wave disturbance.
3. The method for detecting islanding by measuring impedance of an annular flexible direct current power distribution network according to claim 1, wherein in the step 2:
specifically, the DC/DC converters of all units in the annular flexible direct current power distribution system are subjected to harmonic injection in turn, and the fact that only the DC/DC converter of one unit injects high-frequency harmonic disturbance into the system at the same time is guaranteed.
4. The impedance measurement type island detection method suitable for the annular flexible direct current power distribution network according to claim 1, wherein the process of the step 3 specifically comprises:
in the period of injecting high-frequency harmonic disturbance into the DC/DC converter of each unit, the port voltage and current signals of the DC/DC converter contain full-frequency-band information, the voltage and current signals of the injection frequency band of the DC/DC converter are extracted by utilizing a Fourier algorithm, and the measured impedance under the injection frequency band is obtained through calculation.
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