CN115995984A - DC transformer and control method thereof - Google Patents
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- CN115995984A CN115995984A CN202310026596.1A CN202310026596A CN115995984A CN 115995984 A CN115995984 A CN 115995984A CN 202310026596 A CN202310026596 A CN 202310026596A CN 115995984 A CN115995984 A CN 115995984A
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Abstract
The invention provides a direct current transformer and a control method thereof, wherein the direct current transformer comprises the following components: the low-voltage direct-current alternating-current conversion boosting unit, the alternating-current isolation boosting unit and the high-voltage alternating-current direct-current conversion boosting unit are sequentially connected. The invention utilizes the current source type converter and the modularized multi-level voltage source converter to form a three-stage boosting scheme, namely, two conversion links of a low-voltage side and a high-voltage side also have boosting capacity, and the three stages are added with the middle boosting transformer, so that the boosting capacity is stronger.
Description
Technical Field
The invention relates to the technical field of flexible direct current transmission, in particular to a direct current transformer and a control method thereof.
Background
Compared with the traditional alternating current transmission, the direct current transmission technology has the advantages of long transmission distance, large transmission capacity and the like, and is an important means for east-west electric transmission. The flexible direct current transmission technology taking the voltage source type converter as core equipment has the advantages of flexibility and controllability and adaptation to a weak power grid, and plays an important role in renewable energy grid-connected transmission scenes.
The direct current power grid is an effective solution means for large-scale long-distance transmission and wide-area consumption of new energy, and along with the base large-scale development of new energy, the full direct current and multi-voltage-level new energy collecting power grid can accommodate friendly access of new energy stations with different capacity scales, is suitable for wide-area collection and centralized transmission scenes of new energy, has better stability and flexibility, and is one of important development directions of future power grids. The direct current transformer is core equipment for constructing a full direct current new energy collection power grid and a multi-voltage-class direct current power grid, and has the functions of direct current power grid interconnection, power flow control, fault current suppression and the like.
At present, the DC transformer generally has the problem of low transformation ratio, and can not meet the requirements of large-scale new energy source sending scenes on the performance and good economy of the DC transformer.
Disclosure of Invention
Therefore, the technical problem to be solved by the invention is to overcome the defect of low transformation ratio of the direct current transformer in the prior art, thereby providing the direct current transformer and the control method thereof.
In order to achieve the above purpose, the present invention provides the following technical solutions:
in a first aspect, an embodiment of the present invention provides a dc transformer, where the dc transformer is connected in series to a dc power grid, and the dc transformer includes: the low-voltage direct-current alternating-current conversion boosting unit, the alternating-current isolation boosting unit and the high-voltage alternating-current direct-current conversion boosting unit are sequentially connected.
Optionally, the low-voltage direct-current alternating-current conversion boost unit comprises: a current source type current converting component, an alternating current filter capacitor and a direct current smoothing reactor, wherein,
the direct current port of the current source type current conversion component is connected with the positive direct current port and the negative direct current port of the direct current power grid, and the middle alternating current port of the current source type current conversion component is connected with the alternating current isolation boosting unit;
the alternating current filter capacitor is connected to the middle alternating current port of the current source type current conversion component;
the direct current smoothing reactor is connected in series with the low-voltage direct current side of the current source type current conversion component.
Optionally, the ac isolation boost unit includes: at least one alternating current transformer, a plurality of alternating current transformers are connected in series, in parallel or in cascade.
Optionally, the high-voltage ac/dc conversion boost unit includes: the middle alternating current port of the voltage source type conversion assembly is connected with the alternating current isolation boosting unit, and the direct current port of the voltage source type conversion assembly is connected with the positive direct current port and the negative direct current port of the direct current power grid.
Optionally, the current source type current converting assembly includes: the first bridge arms are connected in parallel to positive and negative direct current ports of a direct current power grid, each first bridge arm is divided into a first upper bridge arm and a first lower bridge arm, connection points of the first upper bridge arms and the first lower bridge arms are connected with primary winding leads of the alternating current transformer, and the first upper bridge arms and the first lower bridge arms comprise a plurality of power electronic assemblies which are connected in series.
Optionally, the voltage source type conversion assembly includes: the second bridge arms are connected in parallel to positive and negative direct current ports of the direct current power grid, each second bridge arm is divided into a second upper bridge arm and a second lower bridge arm, connection points of the second upper bridge arm and the second lower bridge arm are connected with secondary winding leads of the alternating current transformer, and each second upper bridge arm and each second lower bridge arm comprise a bridge arm reactor and a plurality of submodules.
In a second aspect, an embodiment of the present invention provides a dc transformer control method, which is based on the dc transformer in the first aspect of the embodiment of the present invention, and includes:
obtaining the voltage of a direct current port of a low-voltage direct current alternating current conversion boosting unit;
calculating to obtain d-axis current and q-axis current according to the direct current port voltage;
converting the d-axis current and the q-axis current into alternating current;
generating a first instruction signal for controlling power electronic components in the low-voltage direct-current alternating-current conversion boosting unit according to the alternating current;
the first instruction signal is connected into the driving of the power electronic component, and the power electronic component is controlled to be turned on and off, so that the low-voltage direct-current voltage is converted into alternating-current voltage and the first-stage boosting is performed;
performing secondary boosting on the converted alternating voltage by using an alternating current isolation boosting unit;
obtaining a reference direct current voltage and a reference alternating current voltage at a high voltage side of a high-voltage alternating current-direct current conversion boosting unit;
generating second bridge arm voltage instructions in the high-voltage alternating-current-direct-current conversion boosting unit according to the reference direct-current voltage and the reference alternating-current voltage;
generating a second instruction signal for controlling the submodule of the high-voltage alternating-current direct-current conversion boosting unit through a modulation algorithm according to each second bridge arm voltage instruction;
and the second instruction signal is connected into the driving of the submodule to control the on and off of the submodule, so that the alternating current voltage is converted into the direct current voltage and three-stage boosting is performed.
Optionally, the direct current transformer control method further comprises:
obtaining the capacitance average voltage of each submodule;
and adjusting the reference direct current voltage according to the average voltage of the capacitance of each submodule.
The technical scheme of the invention has the following advantages:
the invention provides a direct current transformer, which is connected in series with a direct current power grid, and comprises: the low-voltage direct-current alternating-current conversion boosting unit, the alternating-current isolation boosting unit and the high-voltage alternating-current direct-current conversion boosting unit are sequentially connected. The invention utilizes the current source type converter and the modularized multi-level voltage source converter to form a three-stage boosting scheme, namely, two conversion links of a low-voltage side and a high-voltage side also have boosting capacity, and the three stages are added with the middle boosting transformer, so that the boosting capacity is stronger.
The invention provides a control method of a direct current transformer, which is characterized in that a low-voltage direct current-alternating current conversion boosting unit is controlled to convert low-voltage direct current voltage into alternating current voltage and perform primary boosting, an alternating current isolation boosting unit is controlled to perform secondary boosting on the converted alternating current voltage, and finally a high-voltage alternating current-direct current conversion boosting unit is controlled to convert the alternating current voltage into direct current voltage and perform tertiary boosting. By adopting a three-stage boosting scheme, the boosting capacity is improved.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings that are needed in the description of the embodiments or the prior art will be briefly described, and it is obvious that the drawings in the description below are some embodiments of the present invention, and other drawings can be obtained according to the drawings without inventive effort for a person skilled in the art.
FIG. 1 is a topology of one specific example of a DC transformer in an embodiment of the invention;
FIG. 2 is a series-parallel topology of a low voltage DC/AC conversion boost unit in an embodiment of the invention;
FIG. 3 is a series-parallel topology of a high voltage AC/DC conversion boost unit in an embodiment of the present invention;
FIG. 4 is a topology of an embodiment of the present invention having a plurality of independent low voltage DC/AC conversion boost units or a plurality of independent high voltage AC/DC conversion boost units;
fig. 5 is a topology of another specific example of a dc transformer in an embodiment of the present invention;
fig. 6 is a topology of a first bridge arm reverse connection in an embodiment of the present invention;
FIG. 7 is a power electronics component topology in an embodiment of the invention;
FIG. 8 is a sub-module topology in an embodiment of the invention;
fig. 9 is a flowchart of a specific example of a dc transformer control method according to an embodiment of the present invention.
Detailed Description
The following description of the embodiments of the present invention will be made apparent and fully in view of the accompanying drawings, in which some, but not all embodiments of the invention are shown. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
In the description of the present invention, it should be noted that the directions or positional relationships indicated by the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc. are based on the directions or positional relationships shown in the drawings, are merely for convenience of describing the present invention and simplifying the description, and do not indicate or imply that the devices or elements referred to must have a specific orientation, be configured and operated in a specific orientation, and thus should not be construed as limiting the present invention. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance.
In the description of the present invention, it should be noted that, unless explicitly specified and limited otherwise, the terms "mounted," "connected," and "connected" are to be construed broadly, and may be either fixedly connected, detachably connected, or integrally connected, for example; can be mechanically or electrically connected; the two components can be directly connected or indirectly connected through an intermediate medium, or can be communicated inside the two components, or can be connected wirelessly or in a wired way. The specific meaning of the above terms in the present invention will be understood in specific cases by those of ordinary skill in the art.
In addition, the technical features of the different embodiments of the present invention described below may be combined with each other as long as they do not collide with each other.
The embodiment of the invention provides a direct current transformer, wherein the direct current transformer is connected in series to a direct current power grid. As shown in fig. 1, the dc transformer includes: the low-voltage direct-current alternating-current conversion boosting unit 1, the alternating-current isolation boosting unit 2 and the high-voltage alternating-current direct-current conversion boosting unit 3 are sequentially connected.
In one embodiment, the low-voltage dc-ac conversion boosting unit 1 is used for converting a low-voltage dc voltage into an ac voltage and performing a primary boosting. The ac isolation boosting unit 2 is used for performing secondary boosting on the converted ac voltage. The high-voltage ac/dc conversion boosting unit 3 is used for converting an ac voltage into a dc voltage and performing three-stage boosting. The invention adopts a three-stage boosting scheme, namely, two conversion links of a low-voltage side and a high-voltage side also have boosting capacity, and the boosting capacity is stronger because the three stages are adopted by the invention together with the middle boosting transformer.
In the embodiment of the invention, the direct current transformer can also comprise a low-voltage direct current-alternating current conversion boosting unit 1, an alternating current isolation boosting unit 2 and a high-voltage alternating current-direct current conversion boosting unit 3 which are connected in series, in parallel and in cascade. As shown in fig. 2, the low-voltage dc-ac conversion boost unit 1 is in a series connection and parallel connection topology. As shown in fig. 3, the high-voltage ac-dc conversion booster unit 3 is in a series connection and parallel connection topology. As shown in fig. 4, the topology includes a plurality of independent low-voltage ac/dc conversion step-up units 1 or a plurality of independent high-voltage ac/dc conversion step-up units 3.
In one embodiment, as shown in fig. 1, a dc transformer with a two-phase structure, a low-voltage dc-ac conversion boost unit 1 includes: the DC port of the current source type converter assembly 11 is connected with the positive DC port and the negative DC port of the DC power grid, and the middle AC port of the current source type converter assembly 11 is connected with the AC isolation boosting unit 2; the ac filter capacitor 12 is connected to the middle ac port of the current source type converter assembly 11.
In one embodiment, as shown in fig. 1, the low-voltage dc-ac conversion boost unit 1 further includes: the direct-current smoothing reactor 13, the direct-current smoothing reactor 13 is connected in series to the low-voltage direct-current side of the current source type converter assembly 11.
Fig. 5 is a schematic diagram of the invention extended to a three-phase structure. Each component can be expanded in series-parallel connection, so that higher voltage level and larger conveying capacity are realized, and the method has wide application scenes for new energy source delivery.
In the embodiment of the present invention, as shown in fig. 1, a current source type converter assembly 11 includes: the first bridge arms are connected in parallel to positive and negative direct current ports of the direct current power grid, each first bridge arm is divided into a first upper bridge arm and a first lower bridge arm, the connection points of the first upper bridge arm and the first lower bridge arm are connected with primary winding leads of the alternating current transformer, and each first upper bridge arm and each first lower bridge arm comprise a plurality of power electronic assemblies 14 which are connected in series. Specifically, an alternating current port is led out at the connection point of the first upper bridge arm and the first lower bridge arm and is connected with a primary winding lead of an alternating current transformer. The first bridge arm can be connected in a positive way or in a reverse way. As shown in fig. 6, a topology of the first leg reverse connection.
Specifically, as shown in fig. 7, the power electronics assembly 14 includes a first power electronics device 15, a voltage equalizing circuit 16, an absorption loop 17, and an energy taking circuit 18. The first power electronic device 15 has a capability of blocking reverse current, and is controlled by an external control signal to turn on and off, including but not limited to a reverse resistance type IGCT, an IGBT and diode series assembly, and a reverse conduction type IGCT and diode series assembly.
Since the traditional scheme mainly has limited current tolerance of the low-voltage side power electronic device, the method is suitable for high-voltage and large-capacity, but the transformation ratio cannot be increased, and if the transformation ratio is increased, the capacity is correspondingly reduced. Compared with a modularized multi-level scheme, the low-voltage side of the invention adopts the IGCT current source type current converter, firstly, the current capacity of the device is higher, and secondly, under the same capacity, the current of the current source type current converter is smaller than the current of the direct current side, and the modularized multi-level current converter is opposite, so that the current source type current converter can have larger system direct current under the same device current level.
In addition, the low-voltage side adopts a current source type current conversion technology, so that the cost of the direct-current transformer is reduced, and better economy is achieved.
In one embodiment, as shown in fig. 1, an ac isolation boost unit 2 includes: at least one AC transformer, a plurality of AC transformers are connected in series, in parallel or in cascade.
In one embodiment, as shown in fig. 1, the ac isolated boost unit 2 is illustrated as including an ac transformer.
In one embodiment, as shown in fig. 1, the high-voltage ac/dc conversion boosting unit 3 includes: the voltage source type conversion assembly 31, the middle alternating current port of the voltage source type conversion assembly 31 is connected with the alternating current isolation boosting unit 2, and the direct current port of the voltage source type conversion assembly 31 is connected with the positive and negative direct current ports of the direct current power grid.
In the embodiment of the present invention, as shown in fig. 1, a voltage source type conversion assembly 31 includes: the second bridge arms are connected in parallel to positive and negative direct current ports of the direct current power grid, each second bridge arm is divided into a second upper bridge arm and a second lower bridge arm, connection points of the second upper bridge arm and the second lower bridge arm are connected with secondary winding leads of the alternating current transformer, and each second upper bridge arm and each second lower bridge arm comprise a bridge arm reactor 33 and a plurality of submodules 32. An alternating current port is led out at the connection point of the first upper bridge arm and the first lower bridge arm and is connected with a primary winding lead of an alternating current transformer.
Specifically, as shown in fig. 8, the sub-module 32 includes one or more capacitors and a plurality of second power electronics, and by controlling the on/off of the second power electronics, the capacitors can be put into and cut out of the series path. The second power electronic device has reverse current conduction characteristics including, but not limited to, IGBT and diode anti-parallel, IGCT and diode anti-series.
Because the traditional scheme has some schemes capable of realizing high transformation ratio, but the capacity and the voltage level are very low, if the traditional scheme is used at a higher voltage level, thousands of basic modules are required to be connected in series and parallel, which requires complex control and voltage equalizing and current equalizing, and cannot be realized by the current technology. The conversion parts at two sides of the invention respectively utilize the device series connection and the submodule series connection to boost the voltage, and the voltage equalizing and current equalizing design is simple and easy to realize. The natural complementary characteristics of the voltage source and the current source are combined, the control difficulty is lower, the response is faster, and the dynamic response performance of the direct-current transformer is improved.
The embodiment of the invention also provides a control method of the direct current transformer, which is based on the direct current transformer. As shown in fig. 9, the dc transformer control method includes the steps of:
step S1: and obtaining the DC port voltage of the low-voltage DC-AC conversion boosting unit.
Step S2: d-axis current and q-axis current are calculated from the dc port voltage.
Step S3: the d-axis current and the q-axis current are converted and calculated as alternating currents.
Step S4: a first command signal for controlling power electronics in the low voltage DC-AC conversion boost unit is generated based on the AC current.
Step S5: and the first command signal is connected into the driving of the power electronic component to control the power electronic component to be turned on and off, so that the low-voltage direct-current voltage is converted into the alternating-current voltage and the first-stage boosting is performed.
Step S6: and performing secondary boosting on the converted alternating voltage by using an alternating current isolation boosting unit.
Step S7: and obtaining the reference direct current voltage and the reference alternating current voltage at the high voltage side of the high-voltage alternating current-direct current conversion boosting unit.
Step S8: and generating each second bridge arm voltage instruction in the high-voltage alternating-current direct-current conversion boosting unit according to the reference direct-current voltage and the reference alternating-current voltage.
Step S9: and generating a second instruction signal for controlling the submodule in the high-voltage alternating-current-direct-current conversion boosting unit through a modulation algorithm according to each second bridge arm voltage instruction.
Step S10: and the second instruction signal is connected into the driving of the submodule to control the submodule to be turned on and off, so that the alternating current voltage is converted into the direct current voltage and three-stage boosting is performed.
In a specific embodiment, in a specific low-voltage dc power grid and high-voltage dc power grid application scenario, the dc transformer adopts the following control method:
first, the low-voltage direct-current alternating-current conversion boost unit has two degrees of control freedom, and the voltage of the low-voltage direct-current port and the reactive power of the self alternating-current port are respectively controlled.
Second, the low-voltage direct current port voltage control method comprises the following steps: when the voltage of the low-voltage direct-current port is higher, the d-axis current of the low-voltage direct-current alternating-current conversion boosting unit is increased, otherwise, when the voltage of the low-voltage direct-current port is lower, the d-axis current of the low-voltage direct-current alternating-current conversion boosting unit is reduced.
Thirdly, an alternating current port reactive power control principle: the ac section q-axis current is controlled so that the reactive power is kept at 0.
Fourth, the dq-axis current obtained in the second and third is converted and calculated as an alternating current.
Fifthly, inputting the calculated alternating current command into a PWM signal generator to generate a gate signal of a first power electronic device for controlling the low-voltage direct-current alternating-current conversion boosting unit.
And sixthly, the gate electrode signal of the device is connected into the gate electrode driving of the first power electronic device, and the device is controlled to be turned on or off.
Seventh, the high-voltage ac-dc conversion boost unit is responsible for establishing ac-side ac voltage.
Eighth, each bridge arm voltage command of the high-voltage alternating-current direct-current conversion boosting unit is generated according to the high-voltage side reference direct-current voltage and the high-voltage side reference alternating-current voltage.
And ninth, generating gate signals of the second power electronic devices through a modulation algorithm according to the bridge arm voltage command in the eighth step.
And tenth, the gate electrode signal of the device is connected into the gate electrode drive of the second power electronic device, and the device is controlled to be turned on or off.
The invention provides a control method of a direct current transformer, which is characterized in that a low-voltage direct current-alternating current conversion boosting unit is controlled to convert low-voltage direct current voltage into alternating current voltage and perform primary boosting, an alternating current isolation boosting unit is controlled to perform secondary boosting on the converted alternating current voltage, and finally a high-voltage alternating current-direct current conversion boosting unit is controlled to convert the alternating current voltage into direct current voltage and perform tertiary boosting. By adopting a three-stage boosting scheme, the boosting capacity is improved.
In an embodiment, the direct current transformer control method further includes the following steps:
step S10: and obtaining the capacitance average voltage of each sub-module.
Step S11: and adjusting the reference direct-current voltage according to the average voltage of the capacitors of the submodules.
In a specific embodiment, the purpose of this step is to ensure that the capacitance average voltage stabilizes around the nominal value.
It is apparent that the above examples are given by way of illustration only and are not limiting of the embodiments. Other variations or modifications in form will be apparent to persons skilled in the art upon the description hereinabove. It is not necessary here nor is it exhaustive of all embodiments. And obvious variations or modifications thereof are contemplated as falling within the scope of the present invention.
Claims (8)
1. A dc transformer, wherein the dc transformer is connected in series to a dc power grid, the dc transformer comprising: the low-voltage direct-current alternating-current conversion boosting unit, the alternating-current isolation boosting unit and the high-voltage alternating-current direct-current conversion boosting unit are sequentially connected.
2. The dc transformer of claim 1, wherein the low voltage dc-ac conversion boost unit comprises: a current source type current converting component, an alternating current filter capacitor and a direct current smoothing reactor, wherein,
the direct current port of the current source type current conversion component is connected with the positive direct current port and the negative direct current port of the direct current power grid, and the middle alternating current port of the current source type current conversion component is connected with the alternating current isolation boosting unit;
the alternating current filter capacitor is connected to the middle alternating current port of the current source type current conversion component;
the direct current smoothing reactor is connected in series with the low-voltage direct current side of the current source type current conversion component.
3. The dc transformer of claim 1, wherein the ac isolated boost unit comprises: at least one alternating current transformer, a plurality of alternating current transformers are connected in series, in parallel or in cascade.
4. The dc transformer of claim 1, wherein the high voltage ac-dc conversion boost unit comprises: the middle alternating current port of the voltage source type conversion assembly is connected with the alternating current isolation boosting unit, and the direct current port of the voltage source type conversion assembly is connected with the positive direct current port and the negative direct current port of the direct current power grid.
5. The dc transformer of claim 2, wherein the current source type commutation assembly comprises: the first bridge arms are connected in parallel to positive and negative direct current ports of a direct current power grid, each first bridge arm is divided into a first upper bridge arm and a first lower bridge arm, connection points of the first upper bridge arms and the first lower bridge arms are connected with primary winding leads of the alternating current transformer, and the first upper bridge arms and the first lower bridge arms comprise a plurality of power electronic assemblies which are connected in series.
6. The dc transformer of claim 4, wherein the voltage source converter assembly comprises: the second bridge arms are connected in parallel to positive and negative direct current ports of the direct current power grid, each second bridge arm is divided into a second upper bridge arm and a second lower bridge arm, connection points of the second upper bridge arm and the second lower bridge arm are connected with secondary winding leads of the alternating current transformer, and each second upper bridge arm and each second lower bridge arm comprise a bridge arm reactor and a plurality of submodules.
7. A direct current transformer control method, characterized in that it is based on the direct current transformer according to any one of claims 1-6, comprising:
obtaining the voltage of a direct current port of a low-voltage direct current alternating current conversion boosting unit;
calculating to obtain d-axis current and q-axis current according to the direct current port voltage;
converting the d-axis current and the q-axis current into alternating current;
generating a first instruction signal for controlling power electronic components in the low-voltage direct-current alternating-current conversion boosting unit according to the alternating current;
the first instruction signal is connected into the driving of the power electronic component, and the power electronic component is controlled to be turned on and off, so that the low-voltage direct-current voltage is converted into alternating-current voltage and the first-stage boosting is performed;
performing secondary boosting on the converted alternating voltage by using an alternating current isolation boosting unit;
obtaining a reference direct current voltage and a reference alternating current voltage at a high voltage side of a high-voltage alternating current-direct current conversion boosting unit;
generating second bridge arm voltage instructions in the high-voltage alternating-current-direct-current conversion boosting unit according to the reference direct-current voltage and the reference alternating-current voltage;
generating a second instruction signal for controlling the submodule of the high-voltage alternating-current direct-current conversion boosting unit through a modulation algorithm according to each second bridge arm voltage instruction;
and the second instruction signal is connected into the driving of the submodule to control the on and off of the submodule, so that the alternating current voltage is converted into the direct current voltage and three-stage boosting is performed.
8. The method of controlling a dc transformer according to claim 7, further comprising:
obtaining the capacitance average voltage of each submodule;
and adjusting the reference direct current voltage according to the average voltage of the capacitance of each submodule.
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