CN107947213B - Starting method of multi-terminal flexible direct-current power transmission system with full-bridge module - Google Patents
Starting method of multi-terminal flexible direct-current power transmission system with full-bridge module Download PDFInfo
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- CN107947213B CN107947213B CN201610891100.7A CN201610891100A CN107947213B CN 107947213 B CN107947213 B CN 107947213B CN 201610891100 A CN201610891100 A CN 201610891100A CN 107947213 B CN107947213 B CN 107947213B
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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
- H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M1/00—Details of apparatus for conversion
- H02M1/36—Means for starting or stopping converters
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/60—Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]
Abstract
The invention provides a starting method of a multi-terminal flexible direct-current power transmission system with a full-bridge module, which comprises the following steps: closing the direct current bus breaker; closing the AC circuit breakers connected in series with the starting resistance branches of each phase of the flexible DC power transmission system at each end to access the starting resistance of each phase of the flexible DC power transmission system at each end, so that the flexible DC power transmission system at each end enters an uncontrolled rectifying charging state; cutting off the capacitors of all full-bridge modules on each bridge arm with current flowing from bottom to top in the flexible direct-current power transmission system at least one end, and keeping the cut-off state until all sub-modules are charged; and disconnecting the alternating current circuit breaker on each phase starting resistance branch circuit to cut off each phase starting resistance of the flexible direct current transmission system at each end, and unlocking the flexible direct current transmission system at each end after the voltage of each submodule of the flexible direct current transmission system at each end is stable. The invention can solve the problem of large voltage mutation between the positive direct current bus and the negative direct current bus after the system is unlocked.
Description
Technical Field
The invention relates to the technical field of flexible direct current transmission, in particular to a starting method of a multi-terminal flexible direct current transmission system with a full-bridge module.
Background
The flexible direct current transmission technology is an important component for constructing the smart grid. Compared with the traditional power transmission mode, the flexible direct current power transmission has stronger technical advantages in aspects of island power supply, capacity increasing transformation of an urban power distribution network, interconnection of alternating current systems, large-scale wind power plant grid connection and the like, and is a strategic choice for changing the development pattern of a large power grid.
The topology structure of the traditional multi-terminal flexible direct current transmission system generally adopts a half-bridge module. However, with the increase of the application occasions of the overhead lines and the improvement of the requirement on the reliability of the multi-terminal flexible direct current transmission system, the topological structure of the multi-terminal flexible direct current transmission system needs to have the direct current fault blocking capability, the topological structure formed by the half-bridge module cannot effectively block the direct current fault, once the direct current fault occurs, the switching devices in the topological structure are inevitably burnt, and thus great loss is caused.
In order to solve the above problems, in the prior art, a full-bridge module is applied to a topology structure of a multi-terminal flexible direct-current power transmission system, the formed full-bridge topology structure is widely applied to application occasions such as SVG, and the application of the full-bridge module to the multi-terminal flexible direct-current power transmission system becomes a research hotspot in the field.
However, since the full-bridge module has good symmetry, its charging characteristics are bidirectional, i.e.: no matter the current flows from top to bottom or from bottom to top, the capacitor in the full-bridge module can be charged, so that a large voltage sudden change can be generated between a positive direct current bus and a negative direct current bus after the system is unlocked, and the problem cannot be effectively solved by the conventional starting method of the multi-terminal flexible direct current transmission system comprising the full-bridge module.
Disclosure of Invention
The technical problem to be solved by the present invention is to provide a starting method for a multi-terminal flexible dc power transmission system including a full-bridge module, so as to solve the problem of large voltage jump between a positive dc bus and a negative dc bus after unlocking the system, and the problem of module voltage imbalance when the full-bridge module is in series-parallel connection with other sub-modules.
The technical scheme adopted for solving the technical problem of the invention is as follows:
the invention provides a starting method of a multi-end flexible direct current transmission system containing a full-bridge module, wherein each end of the flexible direct current transmission system is connected through a positive direct current bus and a negative direct current bus, and the starting method comprises the following steps:
closing the direct current bus breaker;
closing the AC circuit breakers connected in series with the starting resistance branches of each phase of the flexible DC power transmission system at each end to access the starting resistance of each phase of the flexible DC power transmission system at each end, so that the flexible DC power transmission system at each end enters an uncontrolled rectifying charging state;
in the charging process, the capacitors of all full-bridge modules on each bridge arm, the current of which flows from bottom to top in the flexible direct-current power transmission system at least at one end, are cut off, and the cut-off states of the capacitors of all the full-bridge modules on the bridge arm are kept until all the sub-modules are charged;
and disconnecting the alternating current circuit breakers connected in series with the starting resistance branches of the phases of the flexible direct current transmission system at each end to cut off the starting resistance of each phase of the flexible direct current transmission system at each end, and unlocking the flexible direct current transmission system at each end after the voltage of each submodule of the flexible direct current transmission system at each end is stable.
Has the advantages that:
the starting method of the multi-terminal flexible direct-current transmission system with the full-bridge module is not only suitable for the multi-terminal flexible direct-current transmission system with the full-bridge module, but also suitable for the multi-terminal flexible direct-current transmission system with the full-bridge module in series-parallel connection with other sub-modules (such as a half-bridge module, a clamping bi-sub module, a diode clamping module and the like), and can effectively solve the problem that large voltage mutation is generated between a positive direct-current bus and a negative direct-current bus after the system is unlocked and the problem that when the full-bridge module is in series-parallel connection with other sub-modules, the voltages of the modules are unbalanced.
Drawings
Fig. 1 is a schematic diagram of a single-ended flexible dc power transmission system in which a full-bridge module and a half-bridge module are connected in series and parallel according to an embodiment of the present invention;
fig. 2 is a schematic diagram of an uncontrolled rectification path of a single-ended flexible direct current power transmission system in which a full-bridge module and a half-bridge module are connected in series and parallel according to an embodiment of the present invention;
fig. 3 is a flowchart of a starting method of a single-ended flexible dc power transmission system in which a full-bridge module and a half-bridge module are connected in series and in parallel for an external power module according to an embodiment of the present invention;
fig. 4 is a flowchart of a starting method of a single-ended flexible dc power transmission system in which a full-bridge module and a half-bridge module are connected in series and connected to each other, for a self-powered module according to an embodiment of the present invention;
FIG. 5a is a schematic waveform diagram of a phase voltage provided by an embodiment of the present invention;
FIG. 5b is a schematic diagram illustrating a waveform of a charging current according to an embodiment of the present invention;
fig. 6 is a flowchart of a starting method of the multi-terminal flexible dc power transmission system including the full-bridge module according to the embodiment of the present invention.
Detailed Description
In order to make the technical solutions of the present invention better understood, the present invention is further described in detail below with reference to the accompanying drawings and examples.
The embodiment of the invention provides a starting method of a multi-terminal flexible direct-current transmission system with a full-bridge module. The starting method reasonably conducts the switch devices of partial full-bridge modules in the charging process to cut off the capacitors of the full-bridge modules, so that the bidirectional charging of the full-bridge modules is controlled to be unidirectional charging, the direct-current bus voltage is prevented from suddenly changing after the system is unlocked, and meanwhile, the charging number of the full-bridge modules is adjusted to be consistent with the voltage of other sub-modules after the sub-modules are charged. The method can better solve the problem that the direct-current voltage caused by the full-bridge module charging is suddenly changed and the problem that the charging voltage of each submodule in the series-parallel system is inconsistent, and can reduce the impact in the charging process and avoid the failure of starting the machine. It should be noted that the multiple ends refer to more than two single ends. The start-up method of the present invention is described in detail below.
Each single-ended flexible direct current transmission system comprises three phase units, namely an A phase unit, a B phase unit and a C phase unit, each phase unit comprises an upper bridge arm and a lower bridge arm, the upper bridge arm and the lower bridge arm of each phase unit are identical in structure, and each phase unit comprises a reactor and a plurality of sub-modules which are sequentially connected in series. Specifically, the total number of the submodules included in each bridge arm is determined by the factors such as the direct-current voltage passing through the positive and negative direct-current buses at the beginning of system design, the voltage-resistant grade of the electronic device, the types of the submodules and the like. In this embodiment, the number of sub-modules included in each bridge arm is Udc/USMWhere Udc is the DC voltage between the positive and negative DC buses, USMIs the capacitor voltage of each sub-module.
In the embodiment of the present invention, all of the sub-modules in each phase unit may be full-bridge modules, or some of the sub-modules may be full-bridge modules, and the rest may be other modules, such as half-bridge modules, clamping bi-sub-modules, diode clamping modules, and the like.
If the middle part of the sub-modules in each phase unit is a full-bridge module and the rest is a half-bridge module, a single-ended flexible direct current power transmission system in which the full-bridge module and the half-bridge module are in series-parallel connection as shown in fig. 1 can be formed. As shown in FIG. 1, each bridge arm comprises reactors L which are connected in series in sequence0M full-bridge modules and n half-bridge modules. It should be noted that, if the "bridge arm" mentioned in the present invention is not indicated as "upper bridge arm" or "lower bridge arm", it may be "upper bridge arm" or "lower bridge arm", and those skilled in the art can deduce the situation according to the specific situation.
The half-bridge module comprises a transistor VT11 and a diode D11 connected in inverse parallel with the transistor VT11, a transistor VT12 and a diode D12 connected in inverse parallel with the transistor VT12, and a capacitor C1. The collector of the transistor VT11 is connected to the cathode of the diode D11 and the anode of the capacitor C1, respectively, the emitter of the transistor VT11 is connected to the anode of the diode D11 and the collector of the transistor VT12, respectively, the collector of the transistor VT12 is also connected to the cathode of the diode D12, the emitter of the transistor VT12 is connected to the anode of the diode D12 and the cathode of the capacitor C1, the output a of the half-bridge module is connected to the connection point of the emitter of the transistor VT11 and the collector of the transistor VT12, and the output B of the half-bridge module is connected to the connection point of the emitter of the transistor VT12 and the cathode of the capacitor C1.
The full-bridge module comprises a transistor VT21 and a diode D21 connected in inverse parallel with the transistor VT21, a transistor VT22 and a diode D22 connected in inverse parallel with the transistor VT23 and a diode D23 connected in inverse parallel with the transistor VT23, a transistor VT24 and a diode D24 connected in inverse parallel with the transistor VT24, and a capacitor C2. The collector of the transistor VT21 is connected with the cathode of the diode D21 and the anode of the capacitor C2 respectively, the emitter of the transistor VT21 is connected with the anode of the diode D21 and the collector of the transistor VT23 respectively, the collector of the transistor VT23 is also connected with the cathode of the diode D23, the emitter of the transistor VT23 is connected with the anode of the diode D23 and the cathode of the capacitor C2 respectively, and the output end A of the full-bridge module is connected with the connection point of the emitter of the transistor VT21 and the collector of the transistor VT 23; the collector of the transistor VT22 is connected to the cathode of the diode D22 and the anode of the capacitor C2, respectively, the emitter of the transistor VT22 is connected to the anode of the diode D22 and the collector of the transistor VT24, respectively, the collector of the transistor VT24 is also connected to the cathode of the diode D24, the emitter of the transistor VT24 is connected to the anode of the diode D24 and the cathode of the capacitor C2, respectively, and the output terminal B of the full-bridge module is connected to the connection point of the emitter of the transistor VT22 and the collector of the transistor VT 24.
For the single-ended flexible direct current transmission system of the full-bridge module and the half-bridge module in series-parallel connection, because only the full-bridge module can effectively block the direct current fault and the half-bridge module cannot block the direct current fault, the number ratio of the half-bridge module and the full-bridge module in the system needs to use blocking of direct current fault current as a principle, and redundancy and design problems need to be considered. In the embodiment of the invention, the number of the half-bridge modules and the full-bridge modules on each bridge arm is the same, namely, the number ratio of the half-bridge modules to the full-bridge modules on each bridge arm is 1: 1.
Fig. 2 is a schematic diagram of an uncontrolled rectification path of a single-ended flexible direct current power transmission system in which a full-bridge module and a half-bridge module are connected in series and parallel. The charging current path of the series-parallel system of the half-bridge module and the full-bridge module during uncontrolled rectifying charging is shown by a dotted line and a dashed line in fig. 2. As can be seen from the specific charging path, the half-bridge module has a unidirectional charging characteristic, that is, only when the current flowing through the bridge arm is from top to bottom, the capacitor C1 in the half-bridge module can be charged; when the current flowing through the bridge arm is from bottom to top, the capacitor C1 in the half-bridge module is bypassed and cannot be charged. Unlike the half-bridge module, the full-bridge module has a bidirectional charging characteristic because of its good symmetry, that is, the capacitor C2 in the full-bridge module can be charged no matter whether the current flowing through the bridge arm is from top to bottom or from bottom to top.
The bidirectional charging characteristic of the full-bridge module causes the following two problems:
the problem is that for all sub-modules in each phase unit, the full-bridge module will be charged twice as much as the half-bridge module, and then the sum of the charging voltages of all full-bridge modules in the phase unit will also be twice as much as the sum of the charging voltages of all half-bridge modules in the case of uncontrolled rectifying charging. Of course, the premise is that the capacitance values of the capacitor C2 of the full-bridge module and the capacitor C1 of the half-bridge module are the same, and the capacitance values are usually selected to be the same in consideration of design convenience and capacitor cost in engineering. For each phase unit, the charging voltage of the half-bridge module is inconsistent with that of the full-bridge module, which brings certain problems to subsequent control.
The second problem is that the full-bridge module has a bidirectional charging characteristic, so that the direct-current voltage of the positive and negative direct-current buses of the flexible direct-current transmission system with the full-bridge module has a sudden change. Specifically, taking the a-phase unit as an example, the positive dc bus to ground voltage Udc + ═ Ua-m × Uc, and the negative direct current bus is connected with the earth voltage Udc-m and Uc-n, wherein Udc + and Udc-are the voltages to earth of the positive and negative buses respectively, Ua is A alternating current voltage, Uc is the capacitance voltage of the full bridge module/half bridge module, m is the total number of the full bridge modules on the upper bridge arm/lower bridge arm in the A phase unit, n is the total number of the half bridge modules on the upper bridge arm/lower bridge arm in the A phase unit, so the total number of the submodules on each bridge arm is (m + n), under the uncontrolled rectifying charging state, the direct current voltage of a positive direct current bus and a negative direct current bus of the flexible direct current transmission system is n × Uc, in other words, the direct current voltage of the positive direct current bus and the negative direct current bus is the sum of the capacitance voltages of half-bridge modules on bridge arms through which current flows from top to bottom in each phase unit; once the vehicle enters the unlocking state, the direct-current voltage of the positive and negative direct-current buses becomes (m × Uc + n × Uc), so that a large voltage sudden change is generated between the positive and negative direct-current buses, the sudden change is m × Uc, namely the sum of the capacitor voltages of all full-bridge modules on the upper bridge arm or the lower bridge arm of each phase unit, and if a cable or an overhead line bears such a large voltage change, some equipment may be damaged. The unlocked state refers to a state in which a control signal of a switching device (transistor) in each sub-module changes from an all-zero state (uncontrolled rectifying state) to a normal state.
In order to solve the above problem, the inventor proposes the following starting method for the single-ended flexible dc power transmission system in which the full-bridge module and the half-bridge module are connected in series and parallel as shown in fig. 1.
If the full-bridge module is an external power taking module, a switching device (transistor) of the full-bridge module can be triggered in the charging process, so that for each bridge arm of each phase unit, when current flows from bottom to top, the capacitor of the half-bridge module on the bridge arm is bypassed and cannot be charged, the charging states of the full-bridge module and the half-bridge module on the bridge arm can be kept consistent by cutting off the capacitors of all the full-bridge modules on the bridge arm at one time, and at the moment, the capacitors of the full-bridge module and the half-bridge module on the bridge arm are bypassed and are not charged; when the current flows from top to bottom, the full-bridge module and the half-bridge module on the bridge arm can be charged without extra interference.
As shown in fig. 3, the specific starting method is as follows:
s101, closing the direct current bus breaker.
And S102, closing the alternating current circuit breakers connected with the starting resistor branches of all phases in series to access the starting resistors of all phases, so that the system enters an uncontrolled rectification charging state.
S103, in the charging process, all the capacitors of all the full-bridge modules on each bridge arm, of which the current flows from bottom to top, in the system are cut off at one time, and the cut-off states of the capacitors of all the full-bridge modules on the bridge arm are kept until all the sub-modules are charged. The external power module can be triggered when the voltage of the module capacitor is 0, so that sudden voltage change can not be caused by cutting off the capacitors of the full-bridge modules at one time.
In this step, the method for cutting off the capacitance of the full-bridge module on each bridge arm through which current flows from bottom to top includes: for the bridge arm with current flowing from bottom to top, the transistor T1 or the transistor T4 of the full bridge module on the bridge arm is turned on.
Further, the transistor T1 and the transistor T4 may be turned on alternately to balance the loss.
And S104, disconnecting the alternating current circuit breakers connected with the starting resistance branches of all phases in series to cut off the starting resistance of each phase.
And S105, when the voltage of each submodule is stable, unlocking the system.
The starting method can reduce or even avoid large voltage sudden change between the positive direct current bus and the negative direct current bus after the system is unlocked.
If the full-bridge module is a self-powered module, the voltage of the full-bridge module is low at the beginning of charging and cannot be triggered, so that the system can only be in an uncontrolled rectification charging state before the voltage of the full-bridge module can be triggered. The power-taking mode of the self-power-taking module is to take power from a module capacitor, specifically, after the module capacitor is charged to a certain voltage value, a switching power supply module connected in parallel to the module capacitor can start working, take power from the module capacitor, and provide driving for a switching device in the module. At this time, for each phase unit, due to the bidirectional charging characteristics of the full-bridge modules, the sum of the charging voltages of all the full-bridge modules in the phase unit is nearly twice of the sum of the charging voltages of all the half-bridge modules in the phase unit, and the dc voltages of the positive and negative dc buses only represent the capacitance voltages of the half-bridge modules on the bridge arm through which current flows from top to bottom. When the charging voltage of the full-bridge module reaches the voltage which can be triggered, the capacitors of the full-bridge modules on each bridge arm, the current of which flows from bottom to top, can be cut off step by step, so that the direct current voltage of the positive and negative direct current buses is increased step by step until the capacitors of all the full-bridge modules on the bridge arm are cut off, thereby avoiding the voltage mutation, keeping the charging states of the full-bridge modules and the half-bridge modules on the bridge arm consistent, and keeping the direct current voltage of the positive and negative direct current buses at the moment (m + Uc + n + Uc).
As shown in fig. 4, the specific starting method is as follows:
s201, closing the direct current bus breaker.
S202, closing the alternating current circuit breakers connected with the starting resistor branches of all phases in series to access the starting resistors of all phases, and enabling the system to enter an uncontrolled rectification charging state.
S203, in the charging process, after the charging voltage of each full-bridge module reaches a voltage capable of being triggered, the capacitors of the full-bridge modules on each bridge arm, of which the current flows from bottom to top, are cut off successively until the capacitors of all the full-bridge modules on the bridge arm are cut off, and the cut-off states of the capacitors of all the full-bridge modules on the bridge arm are maintained until all the sub-modules are charged. The method for gradually increasing the cutting number of the full-bridge module capacitor can minimize the voltage sudden change of the direct current voltage of the positive and negative direct current buses. Moreover, the number of the added full-bridge module capacitors in each time can be one or more.
In this step, the method for cutting off the capacitance of the full-bridge module on each bridge arm through which current flows from bottom to top includes: for the bridge arm with current flowing from bottom to top, the transistor T1 or the transistor T4 of the full bridge module on the bridge arm is turned on.
Further, the transistor T1 and the transistor T4 may be turned on alternately to balance the loss.
And S204, disconnecting the alternating current circuit breakers connected with the starting resistance branches of all phases in series to cut off the starting resistance of each phase.
S205, after the voltage of each submodule is stable, for each phase unit, obtaining the difference value between the capacitor voltage average value of all full-bridge modules in the phase unit and the capacitor voltage average value of all other submodules.
S206, judging whether the difference value is smaller than a preset threshold value dV, if so, executing a step S208, and if not, executing a step S207.
The value of the threshold dV may refer to a range of a voltage difference of the steady-state operation module, and a typical value is 8%, and of course, a specific value of the threshold dV may also be set to other values by those skilled in the art according to actual situations.
And S207, for the bridge arm with the current flowing from top to bottom in the phase unit, successively cutting off the capacitance of the full-bridge module on the bridge arm until the difference value is less than or equal to a preset threshold value dV so as to solve the problem of unbalanced module voltage after charging, and then executing step S208.
In this step, the method for cutting off the capacitance of the full-bridge module on each bridge arm through which current flows from top to bottom includes: for the bridge arm through which the current flows from top to bottom, the transistor T2 or the transistor T3 of the full bridge module on the bridge arm is turned on.
Further, the transistor T2 and the transistor T3 may be turned on alternately to balance the loss.
In this step, before the capacitors of the full-bridge modules on the bridge arm are cut off each time, the capacitor voltages of all the full-bridge modules on the bridge arm can be sorted, and the full-bridge module with higher capacitor voltage is selected to be cut off according to the sorting result, or the capacitor voltages can be cut off alternately according to the cutting number.
And S208, unlocking the system.
The starting method can reduce or even avoid large voltage sudden change between the positive direct current bus and the negative direct current bus after the system is unlocked.
In the charging process, the judging method of the current direction flowing on each bridge arm can be used for acquiring the direction of the charging current on the bridge arm, and the direction of the current flowing on the bridge arm is the same as the direction of the charging current on the bridge arm.
However, the inventors have found that, when the charging current on the arm is small, it is difficult to determine by the direction of the charging current, and a determination error is easily caused, thereby disabling the control.
In order to solve the above problems, the inventor further provides a method for determining the direction of current flowing through each bridge arm, specifically, the direction of phase voltage of each phase unit is obtained, if the phase voltage of the phase unit is positive and the charging current is also positive, the direction of current flowing through the upper bridge arm of the phase unit is determined to be from bottom to top, and the direction of current flowing through the lower bridge arm of the phase unit is determined to be from top to bottom; if the phase voltage of the phase unit is negative and the charging current is also negative, the direction of the current flowing through the upper arm of the phase unit is determined to be from top to bottom, and the direction of the current flowing through the lower arm is determined to be from bottom to top, and the specific waveforms are shown in fig. 5a and 5 b. The method for judging the direction of the current flowing on the bridge arm by judging the direction of the phase voltage can effectively solve the problem of judgment error caused by over-small charging current, thereby avoiding the out-of-control charging process.
The above analysis only considers the starting method of the single-ended flexible direct current transmission system with the full-bridge module, however, in practical work, the situation of single-ended charging is not common, and more, the charging is double-ended, or even more.
Therefore, the inventor proposes a starting method of a multi-terminal flexible dc power transmission system including a full-bridge module in combination with the above analysis of the single-terminal charging condition, as shown in fig. 6, the starting method includes the following steps S301 to S305:
s301, closing the direct current bus breaker.
S302, in order to limit starting current, the alternating current circuit breakers connected with the starting resistance branches of the phases of the flexible direct current transmission system at each end in series are closed so as to be connected into the starting resistances of the phases of the flexible direct current transmission system at each end, and the flexible direct current transmission system at each end enters an uncontrolled rectifying charging state.
And S303, in the charging process, cutting off the capacitors of all full-bridge modules on each bridge arm, in which the current in the flexible direct-current power transmission system at least at one end flows from bottom to top, and keeping the cut-off states of the capacitors of all full-bridge modules on the bridge arm until all sub-modules are charged. After the voltage of the direct current bus is stabilized, the voltage of the direct current bus can rise to be close to the peak value of the line voltage of the alternating current system, and the module voltage of each bridge arm in the flexible direct current transmission system at least at one end is equal to the peak value of the line voltage of the alternating current system divided by the total number of the sub-modules on the bridge arm.
The method for judging the direction of the current flowing through each bridge arm may be to obtain the direction of the charging current flowing through the bridge arm, and the direction of the current flowing through the bridge arm is the same as the direction of the charging current flowing through the bridge arm.
Preferably, the method for determining the direction of the current flowing through each bridge arm includes acquiring the direction of the phase voltage of each phase unit in the at least one-end flexible direct-current transmission system, and if the phase voltage of the phase unit is positive, determining that the direction of the current flowing through the upper bridge arm of the phase unit is from bottom to top and the direction of the current flowing through the lower bridge arm of the phase unit is from top to bottom; if the phase voltage of the phase unit is negative, the direction of the current flowing through the upper bridge arm of the phase unit is judged to be from top to bottom, and the direction of the current flowing through the lower bridge arm is judged to be from bottom to top.
The method specifically comprises the steps of cutting off the capacitance of all full-bridge modules on each bridge arm with the current flowing from bottom to top in any y-end flexible direct current transmission system in the x-end flexible direct current transmission system, keeping the cut-off state of the capacitance of all full-bridge modules on the bridge arm until all sub-modules are charged, and
in the process of cutting off the full-bridge module capacitor, the sub-modules in the flexible direct current transmission system at any y end charge the sub-modules in the flexible direct current transmission system at the other (x-y) ends, wherein y is more than or equal to 1 and less than x, x is the total end number of the flexible direct current transmission system, and x is more than 1, and x and y are integers.
Because the capacitors of all full-bridge modules on each bridge arm through which current flows from bottom to top in the any y-end flexible direct-current power transmission system are cut off, the direct-current voltage generates sudden change of m × Uc, wherein m is the total number of the full-bridge modules on each bridge arm, Uc is the capacitor voltage of the full-bridge module/half-bridge module, and the capacitors of the full-bridge modules are not cut off in the remaining (x-y) -end flexible direct-current power transmission system, a large voltage difference exists between corresponding ends of the direct-current bus, so that a large charging current flows through the direct-current bus, and the corresponding charging path specifically comprises: and the current is output from the alternating current side and passes through the bridge arm and the positive direct current bus in which the current of one phase unit with the largest alternating current voltage instantaneous value in the any y-end flexible direct current transmission system flows from bottom to top, and the bridge arm in which the current of two phase units with smaller alternating current voltage instantaneous values in the other (x-y) end flexible direct current transmission systems flows from top to bottom. In other words, the arbitrary y-side flexible dc power transmission system is charged from the ac side, i.e., active charging, and the remaining (x-y) -side flexible dc power transmission systems are charged from the dc side, i.e., passive charging.
Of course, if the terminals can be completely synchronized, the step may also be specifically implemented by cutting off the capacitors of the full-bridge modules on the bridge arms through which all the currents flow from bottom to top in the flexible direct-current transmission system at each terminal, and keeping the cut-off states of the capacitors of all the full-bridge modules on the bridge arms until all the sub-modules are charged, and
and in the process of cutting off the full-bridge module capacitor, the full-bridge module in the flexible direct-current power transmission system at each end is charged from the alternating-current side. In other words, the flexible direct current transmission system at each end is charged from the alternating current side, namely, actively charged, and the mode that each end is charged from the alternating current side has small impact and the starting process is mild.
In addition, in this step, if the full-bridge module is an external power taking module, the capacitors of all full-bridge modules on each bridge arm, of which the current flows from bottom to top in the at least one end of the flexible direct current transmission system, are all cut off at one time; and if the full-bridge module is a self-powered module, sequentially cutting off the capacitors of the full-bridge modules on each bridge arm with the current flowing from bottom to top in the flexible direct-current power transmission system at least one end until the capacitors of all the full-bridge modules on the bridge arm are cut off, and cutting off the capacitors of the full-bridge modules after the capacitor voltage of the full-bridge modules reaches a level capable of triggering a switching device therein so as to minimize sudden voltage change of the direct-current voltage of the positive and negative direct-current buses.
For each bridge arm with current flowing from bottom to top in the at least one end flexible direct current transmission system, if the capacitances of i x k full-bridge modules on the bridge arm are cut off successively, wherein i sequentially takes 1,2, … …, and s is m/k, k is not less than 1 and is less than m, m is the total number of the full-bridge modules on each bridge arm, and i, k, s and m are integers, before the capacitances of the i x k full-bridge modules on the bridge arm are cut off each time, the starting method further comprises the following steps:
sequencing the capacitor voltages of all the full-bridge modules on the bridge arm, and selecting i x k full-bridge modules with higher capacitor voltages according to a sequencing result;
and then, the capacitors of the i x k full-bridge modules with higher capacitor voltage are cut off, so that the charging opportunities of the full-bridge modules are reduced, and the voltage consistency of each sub-module is ensured.
Further, the capacitance of the i × k full-bridge modules cut off at each time on the bridge arm specifically is: and (4) alternately cutting off the capacitances of i x k full-bridge modules in the m full-bridge modules on the bridge arm. Therefore, the i × k full-bridge modules per cut are not fixed, but are rotated among the m full-bridge modules. And through multiple rotation, the voltage consistency of each submodule is fully ensured.
In this step, the method for cutting off the capacitance of the full-bridge module on each bridge arm through which current flows from bottom to top includes: for the bridge arm with current flowing from bottom to top, the transistor T1 or the transistor T4 of the full bridge module on the bridge arm is turned on.
Further, the transistor T1 and the transistor T4 may be turned on alternately to balance the loss.
S304, disconnecting the alternating current circuit breakers in series connection with the starting resistance branches of the phases of the flexible direct current transmission system at each end to cut off the starting resistance of each phase of the flexible direct current transmission system at each end.
S305, when the voltage of each submodule of the flexible direct current power transmission system at each end is stable, unlocking the flexible direct current power transmission system at each end.
In this step, if the full-bridge module is a self-powered module, after the voltages of the sub-modules of the flexible direct current power transmission system at each end are stabilized and before the flexible direct current power transmission system at each end is unlocked, the starting method further includes the steps of:
for each phase unit in the flexible direct current transmission system at each end, judging whether the difference value between the average value of the capacitor voltages of all full-bridge modules in the phase unit and the average value of the capacitor voltages of all other sub-modules is less than a preset threshold value dV or not,
if yes, unlocking the flexible direct current transmission system at each end;
and if not, successively cutting off the capacitances of j x h full-bridge modules on the bridge arm for the bridge arm with the current flowing from top to bottom in the phase unit, wherein j is 1,2, … … and r in sequence, r is m/h, h is more than or equal to 1 and less than m, m is the total number of the full-bridge modules of each bridge arm, and j, h, r and m are integers until the difference value is less than or equal to a preset threshold value dV. The value of the threshold dV may refer to a range of a voltage difference of the steady-state operation module, and a typical value is 8%, and of course, a specific value of the threshold dV may also be set to other values by those skilled in the art according to actual situations.
The method for cutting off the capacitance of the full-bridge module on each bridge arm with the current flowing from top to bottom comprises the following steps: for the bridge arm through which the current flows from top to bottom, the transistor T2 or the transistor T3 of the full bridge module on the bridge arm is turned on.
Further, the transistor T2 and the transistor T3 may be turned on alternately to balance the loss.
In addition, before the capacitors of j × h full-bridge modules on the bridge arm are cut off each time, the starting method further comprises the following steps:
sequencing the capacitor voltages of all the full-bridge modules on the bridge arm, and selecting j x h full-bridge modules with higher capacitor voltages according to a sequencing result;
and then cutting off the capacitors of j x h full-bridge modules with higher capacitor voltage.
Further, the capacitance of the full-bridge module with j × h cut-off on the bridge arm each time is specifically: and (4) alternately cutting off the capacitances of j x h full-bridge modules in the m full-bridge modules on the bridge arm. Therefore, the j × h full-bridge modules per cut are not fixed, but are rotated among the m full-bridge modules.
In summary, the starting method of the multi-terminal flexible direct current transmission system with the full-bridge module can reduce or even eliminate the voltage jump of the direct current bus voltage after the system is unlocked. Therefore, the cable or the overhead line cannot bear large voltage change, the damage of equipment is avoided, and the problems of impact and incapability of starting the machine caused by the unlocking process can be avoided; when the full-bridge module is in series-parallel connection with other sub-modules, the charging voltage of the full-bridge module can be consistent with that of the other sub-modules, the problem of unbalanced voltage of the sub-modules after charging is solved, and therefore the performance of the system is improved; the direction of the current flowing on the bridge arm is judged by detecting the direction of the phase voltage, so that the problem of judgment error caused by over-small charging current is avoided.
It will be understood that the above embodiments are merely exemplary embodiments taken to illustrate the principles of the present invention, which is not limited thereto. It will be apparent to those skilled in the art that various modifications and improvements can be made without departing from the spirit and substance of the invention, and these modifications and improvements are also considered to be within the scope of the invention.
Claims (9)
1. A starting method of a multi-terminal flexible direct current transmission system with a full-bridge module is disclosed, wherein each terminal flexible direct current transmission system is connected through a positive direct current bus and a negative direct current bus, and the starting method is characterized by comprising the following steps:
closing the direct current bus breaker;
closing the AC circuit breakers connected in series with the starting resistance branches of each phase of the flexible DC power transmission system at each end to access the starting resistance of each phase of the flexible DC power transmission system at each end, so that the flexible DC power transmission system at each end enters an uncontrolled rectifying charging state;
in the charging process, the capacitors of all full-bridge modules on each bridge arm, the current of which flows from bottom to top in the flexible direct-current power transmission system at least at one end, are cut off, and the cut-off states of the capacitors of all the full-bridge modules on the bridge arm are kept until all the sub-modules are charged; for the upper bridge arm, the current flowing from bottom to top means that the current flows from the alternating current side to the direct current side, and the current flowing from top to bottom means that the current flows from the direct current side to the alternating current side; for the lower bridge arm, the current flowing from bottom to top means that the current flows from the direct current side to the alternating current side, and the current flowing from top to bottom means that the current flows from the alternating current side to the direct current side;
disconnecting the alternating-current circuit breakers connected in series with the starting resistance branches of the phases of the flexible direct-current power transmission system at each end to cut off the starting resistance of each phase of the flexible direct-current power transmission system at each end, and unlocking the flexible direct-current power transmission system at each end after the voltage of each submodule of the flexible direct-current power transmission system at each end is stable;
the method comprises the following steps of cutting off the capacitance of all full-bridge modules on each bridge arm with current flowing from bottom to top in the flexible direct current transmission system at least at one end, wherein the step comprises the following steps:
cutting off the capacitance of all full-bridge modules on each bridge arm with the current flowing from bottom to top in any y-end flexible direct current transmission system in the x-end flexible direct current transmission system, and
in the process of cutting off the full-bridge module capacitor, the sub-modules in the flexible direct current transmission system at any y end charge the sub-modules in the flexible direct current transmission system at the other (x-y) ends, wherein y is more than or equal to 1 and less than x, x is the total end number of the flexible direct current transmission system, and x is more than 1, and x and y are integers.
2. The startup method of claim 1,
if the full-bridge module is an external power taking module, the step of cutting off the capacitance of all full-bridge modules on each bridge arm with the current flowing from bottom to top in the flexible direct current transmission system at least at one end comprises the following steps:
the capacitors of all full-bridge modules on each bridge arm, the current of which flows from bottom to top, in the flexible direct-current power transmission system at least at one end are all cut off at one time;
if the full-bridge module is a self-powered module, the step of cutting off the capacitance of all the full-bridge modules on each bridge arm with the current flowing from bottom to top in the flexible direct current transmission system at least at one end comprises the following steps:
and successively cutting off the capacitors of the full-bridge modules on each bridge arm in which the current flows from bottom to top in the flexible direct-current power transmission system at least one end until the capacitors of all the full-bridge modules on the bridge arm are cut off, and cutting off the capacitors of the full-bridge modules after the capacitor voltage of the full-bridge modules reaches a level capable of triggering the switching devices in the full-bridge modules.
3. A starting method according to claim 2, wherein for each bridge arm in the at least one end flexible direct current transmission system, if the capacitances of i x k full-bridge modules on the bridge arm are cut off successively, where i is 1,2, … …, s, m is m/k, k is greater than or equal to 1 and less than m, m is the total number of full-bridge modules on each bridge arm, and i, k, s, and m are integers, before the capacitances of i x k full-bridge modules on the bridge arm are cut off each time, the starting method further comprises the steps of:
sequencing the capacitor voltages of all the full-bridge modules on the bridge arm, and selecting i x k full-bridge modules with higher capacitor voltages according to a sequencing result;
and then cutting off the capacitors of the i x k full-bridge modules with higher capacitor voltage.
4. The starting method according to claim 3, wherein the step of cutting off the capacitances of the i x k full-bridge modules on the bridge arm each time is specifically as follows:
and (4) alternately cutting off the capacitances of i x k full-bridge modules in the m full-bridge modules on the bridge arm.
5. The method according to claim 2, wherein if the full-bridge module is a self-powered module, after voltages of sub-modules of the terminal-side flexible direct current power transmission system are stabilized and before the terminal-side flexible direct current power transmission system is unlocked, the method further comprises the steps of:
for each phase unit in the flexible direct current transmission system at each end, judging whether the difference value between the average value of the capacitor voltages of all full-bridge modules in the phase unit and the average value of the capacitor voltages of all other sub-modules is less than a preset threshold value dV or not,
if yes, unlocking the flexible direct current transmission system at each end;
and if not, successively cutting off the capacitances of j x h full-bridge modules on the bridge arm for the bridge arm with the current flowing from top to bottom in the phase unit, wherein j is 1,2, … … and r in sequence, r is m/h, h is more than or equal to 1 and less than m, m is the total number of the full-bridge modules of each bridge arm, and j, h, r and m are integers until the difference value is less than or equal to a preset threshold value dV.
6. The startup method according to claim 5, characterized in that before each time the capacitances of j x h full-bridge modules on the bridge leg are cut off, the startup method further comprises the steps of:
sequencing the capacitor voltages of all the full-bridge modules on the bridge arm, and selecting j x h full-bridge modules with higher capacitor voltages according to a sequencing result;
and then cutting off the capacitors of j x h full-bridge modules with higher capacitor voltage.
7. The method according to claim 5, wherein the step of cutting off the capacitors of the j x h full-bridge modules on the bridge arm each time is specifically as follows:
and (4) alternately cutting off the capacitances of j x h full-bridge modules in the m full-bridge modules on the bridge arm.
8. A starting method according to any one of claims 1-7, characterized in that the starting method further comprises the steps of:
the method for judging the direction of the current flowing on each bridge arm specifically comprises the step of acquiring the direction of the charging current on the bridge arm, wherein the direction of the current flowing on the bridge arm is the same as the direction of the charging current on the bridge arm.
9. A starting method according to any one of claims 1-7, characterized in that the starting method further comprises the steps of:
the method for judging the direction of the current flowing on each bridge arm specifically comprises the steps of acquiring the direction of phase voltage of each phase unit in the at least one-end flexible direct-current transmission system, and if the phase voltage of the phase unit is positive, judging that the direction of the current flowing on the upper bridge arm of the phase unit is from bottom to top and the direction of the current flowing on the lower bridge arm of the phase unit is from top to bottom; if the phase voltage of the phase unit is negative, the direction of the current flowing through the upper bridge arm of the phase unit is judged to be from top to bottom, and the direction of the current flowing through the lower bridge arm is judged to be from bottom to top.
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