CN109687494B - Operation architecture and control method based on urban multilayer direct current power grid - Google Patents
Operation architecture and control method based on urban multilayer direct current power grid Download PDFInfo
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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
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J1/00—Circuit arrangements for dc mains or dc distribution networks
- H02J1/10—Parallel operation of dc sources
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- 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 discloses an operation framework and a control method based on an urban multilayer direct current power grid, which comprises two layers of direct current power grids with voltage levels and a DC-DC converter, wherein each layer of direct current power grid comprises a plurality of VSC (voltage source converter) stations distributed in a direct current ring network topological form, and a DC-DC converter is arranged between every two adjacent VSC stations; the high-voltage direct-current power grid adopts direct-current voltage deviation control, and a control target of the DC-DC converter adopts a free exchange power strategy; a plurality of VSC converter stations of a low-voltage direct-current power grid are connected with a central load area and suburban new energy, direct-current voltage inside the low-voltage direct-current power grid is controlled by the VSC converter stations at the lower end of a DC-DC converter, fixed frequency control is adopted, and complete acceptance of the new energy and quick response to fluctuating load are guaranteed.
Description
Technical Field
The invention relates to an operation architecture and a control method based on an urban multilayer direct current power grid.
Background
The power grid construction of the city, which is the center of social and economic development and population gathering, is very important. The operation reliability of the urban power grid is seriously influenced by the scarcity of power transmission corridor resources and the standard exceeding of the short-circuit current of the alternating current line. On the other hand, renewable energy is rapidly developing. The renewable energy sources have the characteristics of intermittence and randomness, the traditional power grid structure and operation technology are increasingly careless in solving the problem of clean energy access, and the adoption of a new power transmission mode in an urban power grid is concerned more and more. A multi-terminal flexible direct current transmission (VSC-MTDC) technology based on a voltage source converter will be an effective technical means to solve the above problems.
The multi-terminal flexible direct-current transmission technology develops rapidly, and many countries establish a demonstration project of multi-terminal flexible direct-current transmission. The multi-end flexible direct current has the characteristics of precise control of power flow, flexible power transmission mode and the like, and has remarkable advantages in capacity increase and transformation of urban power grids in the future. With the continuous development of the VSC-MTDC in the future, the multi-terminal flexible direct current transmission applied to the urban power grid may be similar to the development of the alternating current power grid, and develops into a multi-voltage-level loop-type multi-layer direct current power grid (MDCG). Referring to the structure and the function of the alternating current power grid, the future multilayer direct current power grid has the following characteristics: (1) having a plurality of voltage levels; (2) the network is expanded from a radioactive topology to a mesh topology; (3) the direct current power grids have a power exchange function. The multilayer direct current power grid has obvious advantages in the aspects of improving the power transmission capacity of the urban power grid, improving the flexibility of power flow control, reducing the level of short-circuit current, reducing power transmission loss and the like, and is an effective means for solving the problems of renewable energy access of the urban power grid, difficulty in building power transmission corridors of the urban power grid and improvement of the electric energy quality of the urban power grid in the future.
At present, much research is carried out on the topological structure and the control strategy of a single-layer direct-current power grid. The project which is put into operation is mostly radial multi-terminal flexible direct current transmission with a single voltage class. However, the concept gap between single-layer multi-terminal flexible direct-current transmission and a multilayer direct-current power grid is large, and the topological structure and the operation control problem of the multilayer direct-current power grid are relatively few at home and abroad. The application of multilayer direct current power grids to topology and control strategy problems of urban power grid power supply is rarely researched at home and abroad.
Disclosure of Invention
The invention provides an operation architecture and a control method based on an urban multilayer direct-current power grid, which can be suitable for multilayer direct-current power grid control of different voltage classes and different topological forms and are suitable for direct-current network transformation and direct-current power grid operation of different types of power grids such as a future provincial power grid, an urban power grid, a regional power grid and the like.
In order to achieve the purpose, the invention adopts the following technical scheme:
an operation architecture based on an urban multilayer direct current power grid comprises two layers of direct current power grids with voltage levels and DC-DC converters, wherein each layer of direct current power grid comprises a plurality of VSC converter stations distributed in a direct current ring network topology form, and a DC-DC converter is arranged between every two adjacent VSC converter stations;
the high-voltage direct-current power grid adopts direct-current voltage deviation control, a control target of the DC-DC converter adopts a free exchange power strategy, a VSC converter station at the upper end of the DC-DC converter adopts constant alternating-current voltage control, and a VSC converter station at the lower end of the DC-DC converter switches between free exchange power of the DC-DC converter and limited exchange power of the DC-DC converter according to the control target;
a plurality of VSC converter stations of a low-voltage direct-current power grid are connected with a central load area and suburban new energy, direct-current voltage inside the low-voltage direct-current power grid is controlled by the VSC converter stations at the lower end of a DC-DC converter, fixed frequency control is adopted, and complete acceptance of the new energy and quick response to fluctuating load are guaranteed.
Furthermore, the number of the VSC converter stations of the high-voltage direct-current power grid is determined according to urban power supply requirements and power grid construction planning.
Furthermore, the VSC converter station in the high-voltage direct-current power grid is provided with a fixed direct-current voltage station, M fixed direct-current voltage standby stations and N-M-1 fixed power stations, wherein N is the total number of the VSC converter stations of the high-voltage direct-current power grid.
Further, under the steady-state condition, in order to ensure the free flow of the internal power of the two-layer direct-current power grid, a free exchange power strategy is adopted by a control target of the DC-DC converter, and in order to ensure that the DC-DC converter has the capability of responding to power fluctuation in real time, the local control strategy of the VSC converter station at the lower end of the DC-DC converter is controlled by the fixed direct-current voltage.
Furthermore, when the high-voltage direct-current power grid fails or the high-voltage alternating-current power grid needs the high-voltage direct-current power grid to provide emergency power support, the DC-DC converter adopts a strategy of limiting exchange power, and the aim of limiting power exchange between two layers of power grids is achieved by current-limiting and locking the exchange current of the VSC converter station at the lower end.
Furthermore, the converter stations of the low-voltage direct-current power grid except the converter station connecting the urban central load area and the suburban new energy adopt constant-power control of the additional active signals.
The operation modes based on the above architecture include a normal operation mode, a current-limiting operation mode and a hierarchical operation mode.
Under the steady state condition, the urban multilayer direct current power grid adopts a normal operation mode;
when the high-voltage alternating-current power grid encounters an emergency, the high-voltage direct-current power grid supports the power of the high-voltage alternating-current power grid in response to a scheduling requirement, and the urban multi-layer direct-current power grid enters a current-limiting operation mode;
when the DC-DC converter breaks down and stops running, the power exchange capacity between two layers of direct current power grids is lost, and the urban multilayer direct current power grid enters a layered running mode.
When the low-voltage direct-current power grid connected to the lower end of the DC-DC converter is in power shortage, the low-voltage direct-current power grid acquires the shortage power from the high-voltage direct-current power grid in real time through the DC-DC converter; when power redundancy occurs in the low-voltage direct-current power grid, redundant power can be transmitted to the high-voltage direct-current power grid in real time through the DC-DC converter.
The combined control strategy of the normal operation mode comprises a local control strategy and an advanced control strategy, wherein the advanced control strategy calculates an active power reference value based on a scheduling requirement and the operation state of an alternating current power grid at intervals in the normal operation mode of the multilayer direct current power grid, after a new active power reference value is obtained, the advanced control strategy issues the new active power reference value to a fixed active power station in the multilayer direct current power grid by utilizing communication, and each converter station keeps operating the reference value until the next adjustment.
In the current-limiting operation mode, the multilayer direct current power grid will limit the power transmitted by the high voltage direct current power grid to the low voltage power grid via the DC-DC:
and in a normal operation state, the active upper limit and the active lower limit of the converter station at the lower end of the DC-DC converter are respectively equal, when the multilayer direct current power grid enters a current-limiting operation mode, the direct current voltage station of the high-voltage direct current power grid is determined as a reserved power margin, the current limit value of the converter station at the lower end of the DC-DC converter is adjusted through scheduling, and the active upper limit is adjusted to be the same, so that the purpose of limiting the DC-DC unidirectional power is realized.
In the layered operation mode, the direct current voltage in the high-voltage direct current power grid is controlled by a fixed direct current voltage station, the direct current voltage in the low-voltage direct current power grid is controlled by a fixed direct current voltage converter station at the lower end of a DC-DC converter, and after the DC-DC converter quits operation due to faults, the direct current voltage in the low-voltage direct current power grid is directly controlled by the fixed power station with the additional active signals.
When the internal power of the low-voltage direct-current power grid is unbalanced, the direct-current voltage is increased or decreased, when the voltage is increased or decreased and exceeds the voltage limit value of the additional active signal, the output of the fixed power station is changed under the direct-current voltage-active power droop control action of the fixed power station with the additional active signal, the internal power of the low-voltage direct-current power grid is gradually balanced, the direct-current voltage is stabilized near the voltage limit value of the additional signal, and the low-voltage direct-current power grid can still stably operate after the DC-DC converter quits operation due to faults.
Compared with the prior art, the invention has the beneficial effects that:
1) the operation mode and the control method of the urban multilayer direct-current power grid solve the existing problems of the urban power grid in China, conform to the development trend of the future direct-current power grid, and have technical prospect.
2) The normal operation mode, the current-limiting operation mode and the layered operation mode provided by the invention can be suitable for the control of multilayer direct-current power grids with different voltage grades and different topological forms, and are suitable for the direct-current network transformation and the direct-current power grid operation of different types of power grids such as a future provincial power grid, an urban power grid, a regional power grid and the like.
3) The operation mode and the control method of the urban multilayer direct-current power grid are simple in control structure and comprehensive in control target, and provide appropriate architecture reference and actual operation control strategies for the multilayer direct-current power grid which may appear in a future city.
Drawings
The accompanying drawings, which are incorporated in and constitute a part of this application, illustrate embodiments of the application and, together with the description, serve to explain the application and are not intended to limit the application.
Fig. 1 is a schematic diagram of an urban multi-layer dc power grid according to the present invention;
FIG. 2 is a schematic diagram of a high-voltage direct-current power grid in an urban multi-layer direct-current power grid according to the present invention;
FIG. 3 is a schematic diagram of a DC-DC converter in an urban multi-layer DC network according to the present invention;
fig. 4 is a schematic diagram of a low-voltage dc power grid in an urban multi-layer dc power grid according to the present invention;
FIG. 5 is a schematic diagram of three operation modes of the urban multi-layer DC power grid according to the present invention;
FIG. 6 is a schematic diagram of a combined control strategy corresponding to a normal operation mode of an urban multi-layer direct-current power grid;
FIG. 7 is a schematic diagram illustrating a comparison between a current-limiting operation mode and a normal operation mode of an urban multi-layer DC power grid;
fig. 8 is a schematic diagram of a combined control strategy corresponding to a current-limiting operation mode of an urban multi-layer direct-current power grid;
fig. 9 is a schematic diagram of a simulation model for verifying urban multilayer direct-current power grid control.
The specific implementation mode is as follows:
the invention is further described with reference to the following figures and examples.
It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the disclosure. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.
It is noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments according to the present application. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, and it should be understood that when the terms "comprises" and/or "comprising" are used in this specification, they specify the presence of stated features, steps, operations, devices, components, and/or combinations thereof, unless the context clearly indicates otherwise.
In the present invention, terms such as "upper", "lower", "left", "right", "front", "rear", "vertical", "horizontal", "side", "bottom", and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the drawings, and are only terms of relationships determined for convenience of describing structural relationships of the parts or elements of the present invention, and are not intended to refer to any parts or elements of the present invention, and are not to be construed as limiting the present invention.
In the present invention, terms such as "fixedly connected", "connected", and the like are to be understood in a broad sense, and mean either a fixed connection or an integrally connected or detachable connection; may be directly connected or indirectly connected through an intermediate. The specific meanings of the above terms in the present invention can be determined according to specific situations by persons skilled in the relevant scientific or technical field, and are not to be construed as limiting the present invention.
As described in the background art, there have been many researches on the topology and control strategy of single-layer dc power grid in the prior art. The project which is put into operation is mostly radial multi-terminal flexible direct current transmission with a single voltage class. However, the concept gap between single-layer multi-terminal flexible direct-current power transmission and the multilayer direct-current power grid is large, the topological structure and the operation control problem of the multilayer direct-current power grid are relatively few at home and abroad, and in order to solve the technical problems, the application provides the multilayer direct-current power grid structure suitable for urban peripheral renewable energy access and urban internal interconnection.
As shown in fig. 1, according to the voltage class and the power grid function characteristics of the power grid in the current large and medium cities, the multilayer direct current power grid topology is composed of a direct current power grid with two voltage classes and a DC-DC converter.
As shown in fig. 2, the high-voltage direct-current power grid is composed of a plurality of VSC converter stations, a direct-current ring network topology is adopted, and the specific number of converter stations is determined according to the urban power supply requirement and the power grid construction plan. In order to ensure accurate control of tide under normal working conditions and stability of direct-current voltage in the high-voltage direct-current power grid under abnormal working conditions, a local control strategy of the high-voltage direct-current power grid adopts direct-current voltage deviation control.
The high-voltage direct-current power grid is internally provided with a fixed direct-current voltage station, M fixed direct-current voltage standby stations and N-M-1 fixed power stations, and the specific network topology is shown in figure 2.
As shown in fig. 3, the DC-DC converter is the key to implementing multi-layer DC grid power exchange. The DC-DC converter studied in this document is mainly applied to exchange power in a high-power multi-layer DC grid, and therefore a voltage source type converter is used as the DC-DC converter. The specific topology is shown in fig. 3:
in order to ensure the voltage stability of an alternating current bus inside the DC-DC converter, a VSC converter station at the upper end of the DC-DC converter adopts constant alternating voltage control. The VSC converter station at the lower end of the DC-DC converter may be classified into two types according to its control target: one control objective is to ensure that the DC-DC converter exchanges power freely; another control objective is to limit the DC-DC converter exchange power.
Under the steady state condition, in order to ensure the free flow of the power in the two-layer direct current power grid, a free exchange power strategy is adopted by the control target of the DC-DC converter. In order to ensure that the DC-DC converter has the capability of responding to power fluctuation in real time, the local control strategy of the VSC converter station at the lower end of the DC-DC converter is constant direct-current voltage control.
When the high-voltage direct-current power grid fails or the high-voltage alternating-current power grid needs the high-voltage direct-current power grid to provide emergency power support, the DC-DC converter adopts a strategy of limiting the exchange power, and the purpose of limiting the power exchange between two layers of power grids is achieved by current-limiting and locking the exchange current of the VSC converter station at the lower end.
The low-voltage direct-current power grid consists of a plurality of VSC converter stations, and the specific topological structure is shown in FIG. 4: in a steady state situation the direct voltage inside the low voltage direct current grid is controlled by the VSC converter station at the lower end of the DC-DC converter. The local control strategy of the converter station connecting the urban central load area and the suburb new energy adopts fixed frequency control, so that complete admission of the new energy and quick response to fluctuating load are ensured.
In order to ensure that the low-voltage direct-current power grid does not lose the capability of controlling direct-current voltage after the DC-DC breaks down and quits operation, other converter stations in the low-voltage direct-current power grid adopt constant power control of an additional active signal.
As shown in fig. 5, the present invention provides three operation modes of the multilayer dc power grid applied to urban power supply and suburban new energy access based on the operation characteristics of the urban multilayer dc power grid: a normal mode of operation, a current limited mode of operation, and a tiered mode of operation.
Under the steady state condition, the urban multilayer direct current power grid adopts a normal operation mode;
when the high-voltage alternating current power grid encounters an emergency, the high-voltage direct current power grid supports the power of the high-voltage alternating current power grid in response to a scheduling requirement. At the moment, the internal fixed direct-current voltage station of the high-voltage direct-current power grid may not be capable of obtaining power from the alternating-current power grid. In order to ensure stable internal power, the urban multilayer direct-current power grid enters a current-limiting operation mode;
when the DC-DC converter breaks down and stops running, the power exchange capacity between two layers of direct current power grids is lost, and the urban multilayer direct current power grid enters a layered running mode.
Fig. 6 is a schematic diagram of a combined control strategy corresponding to a normal operation mode of an urban multi-layer direct-current power grid.
In the normal operation mode, the power exchanged by the multi-layer direct current power grid through the DC-DC converter is determined by the power change in the two layers of direct current power grids connected with the DC-DC converter.
When the low-voltage direct-current power grid connected to the lower end of the DC-DC converter is in power shortage, the low-voltage direct-current power grid acquires the shortage power from the high-voltage direct-current power grid in real time through the DC-DC converter; when power redundancy occurs in the low-voltage direct-current power grid, redundant power can be transmitted to the high-voltage direct-current power grid in real time through the DC-DC converter.
Under a normal operation condition, the power balance in the low-voltage direct-current power grid can be ensured constantly in a normal operation mode, and the low-voltage direct-current power grid can accept the active power output of the suburb new energy connected with the low-voltage direct-current power grid to the greatest extent due to the fact that the low-voltage direct-current power grid has sufficient power dispatching capacity. On the other hand, the low-voltage direct-current power grid can respond to the load change of the urban load center in real time, automatically adjust the output of a converter station connected with the load center and ensure the stable operation of the urban load center power grid.
The combined control strategy of the normal operation mode consists of a local control strategy and a high-level control strategy.
The local control strategy maintains the voltage and power stability in the multilayer direct current power grid under the condition of not using communication. In the normal operation mode of the multilayer direct current power grid, the advanced control calculates a new active power reference value at intervals based on the scheduling requirement and the operation state of the alternating current power grid:
wherein, Pi ref-inSetting a power rectification station reference value in a multilayer direct-current power grid; pj RESConnecting the converter station active power value of the suburb new energy to the multilayer direct current network; pcnst-vDetermining the power value of a direct-current voltage station for the high-voltage direct-current power grid; p is a radical ofi ref-outSetting a power inverter station reference value in a multilayer direct current power grid; pj CloadThe active power value of the convertor station connected with the urban central load area in the multilayer direct current network. N and N are the number of the internal rectifying stations of the multilayer direct-current power grid and the number of the constant-power rectifying stations respectively; m and s are the number of the internal inverter stations of the multilayer direct-current power grid and the number of the constant-power inverter stations respectively.
After obtaining the new active power reference value, the advanced control strategy issues the new active power reference value to the fixed active power station in the multilayer direct current network by using communication, and each converter station keeps the operation of the reference value until the next adjustment.
Fig. 7 is a schematic diagram comparing the current-limiting operation mode and the normal operation mode of the urban multi-layer dc power grid.
In the current-limiting operation mode, the multilayer direct current power grid will limit the power transmitted by the high voltage direct current power grid to the low voltage power grid via the DC-DC:
under the normal operation state, the active upper and lower limits of the converter station at the lower end of the DC-DC converter are respectively
And
when the multilayer direct current power grid enters a current-limiting operation mode, the direct current voltage station of the high-voltage direct current power grid is determined to reserve power margin, and the power margin is regulated under the DC-DC converter through schedulingCurrent limit of end converter station, upper limit of active power
Is adjusted to
The purpose of limiting the DC-DC unidirectional power is achieved.
When the multilayer direct current power grid enters a current-limiting mode to operate, for a low-voltage direct current power grid, the power transmitted from the high-voltage direct current power grid to the low-voltage direct current power grid through the DC-DC converter is limited, and the low-voltage direct current power grid may face power shortage. For a high-voltage direct-current power grid, although a certain power margin can be reserved by limiting the DC-DC, more support cannot be provided when more power is needed by an alternating-current side power grid connected to a fixed direct-current voltage station.
Aiming at the problems, a combined control strategy is provided and applied to a current-limiting operation mode of an urban multilayer direct-current power grid.
Fig. 8 is a schematic diagram of a combined control strategy corresponding to a current-limiting operation mode of an urban multi-layer direct-current power grid.
After the multilayer direct current power grid enters a current limiting mode, determining a power margin reservation value P of a fixed direct current voltage station in the high-voltage direct current power grid according to the requirements of the high-voltage alternating current power gridholdAnd calculating the unidirectional current-limiting power P of the DC-DC converterMAX：
Wherein
Determining the rated active capacity of a direct-current voltage station for a high-voltage direct-current power grid;
the sum of the active reference values of the active power stations in the high-voltage direct-current power grid is determined.
When the low-voltage DC network is in shortage of power PshortageLess than PMAXLow pressure, low pressureThe operation of the direct current power grid is not affected. When the low-voltage DC network is in shortage of power PshortageGreater than PMAXDue to power limitation, the shortage power of the low-voltage direct-current power grid cannot be completely provided by the high-voltage direct-current power grid, and the internal voltage of the low-voltage direct-current power grid is reduced. When the voltage drop exceeds the voltage limit value of the additional active signal, the active power reference value of the fixed power station in the low-voltage direct-current power grid is adjusted through the additional active signal, and the internal power recovery balance of the low-voltage direct-current power grid is achieved.
When the power demand of the high-voltage alternating-current power grid is reduced, the system determines a new power margin reservation value
And recalculate PMAXAnd the influence on a low-voltage direct-current power grid is reduced. When the power requirement of a high-voltage alternating current power grid is increased, in order to not further increase the limit on a low-voltage direct current power grid, a high-level control is adopted to adjust a fixed power station in the high-voltage direct current power grid.
The change of the power reference value of the constant power station causes the power output or input of the constant power station to change, thereby influencing the stable operation of the alternating current side system connected with the constant power station. In order to reduce the influence of the reference value change on the alternating current system, the fixed power converter station with a stronger alternating current system is selected for reference value correction.
The strength of the alternating current system reflects the anti-interference capability of the system, the strong alternating current system does not have obvious voltage and power angle change when suffering from power fluctuation, but the weak alternating current system suffers from a small disturbance and can cause serious voltage or power angle deviation. At present, the industry and academia often measure the ac system with a Short Circuit Ratio (SCR), and in order to note the effects of ac filters and reactive compensation capacitors of converter stations, an Effective Short Circuit Ratio (ESCR) is used herein to compare the strength of the ac side system connected to the converter station:
wherein SSCFor converter station AC busShort circuit capacity at the line; qcNThe sum of the reactive power generated by the alternating current filter and the reactive compensation capacitor in a rated state; prefThe active power reference value is the constant power station active power reference value. It is generally accepted that the smaller the ESCR, the weaker the system and vice versa. The communication system is divided into:
ESCR < 2; extremely weak system
2< ESCR < 3; weak system (4)
ESCR > 3; strong system
In order to avoid great influence on the AC-side system, the connected constant power converter station does not participate in adjustment for the AC-side system with ESCR < 2. By comparing the short-circuit ratio of the alternating-current side system with ESCR >2 connected with the constant-power converter station, under the condition of considering the adjustable power margin of each converter station, the distribution coefficient alpha is utilized to ensure that the converter station with a larger short-circuit ratio of the connected alternating-current side system shares more unbalanced power and the converter station with a smaller short-circuit ratio shares less unbalanced power:
wherein Δ PiAdjusting the power value required by the constant-power converter station participating in adjustment; m is the number of the constant power stations participating in the adjustment; and n is the total number of the fixed power stations in the high-voltage direct-current power grid.
Will calculate the obtained Δ PiAnd sending the power to each fixed power converter station, adjusting the output of the fixed power converter station, and quickly eliminating unbalanced power to realize power balance of the multilayer direct current power grid.
When the DC-DC converter fails or quits operation due to a fault, five cases can be classified according to the type of quitting: the method comprises the following steps of firstly, quitting the operation of the VSC converter station at the upper end of the DC-DC converter, secondly, quitting the operation of the VSC converter station at the lower end of the DC-DC converter, thirdly, quitting the operation of the DC-DC converter due to the integral fault, fourthly, quitting the operation of the DC-DC converter due to the fault of a connecting line or a transformer, fifthly, automatically locking the DC-DC converter and quitting the operation.
No matter which kind of above-mentioned fault condition appears, all can cut off the power exchange between two-layer direct current electric wire netting, and city multilayer direct current electric wire netting will be from normal operating mode or current-limiting operation mode entering layering operation mode.
In the layered operation mode, the direct-current voltage in the high-voltage direct-current power grid is controlled by the fixed direct-current voltage station, and the operation state of the DC-DC converter is slightly influenced by exiting of the DC-DC converter.
The direct-current voltage in the low-voltage direct-current power grid is controlled by a fixed direct-current voltage converter station at the lower end of the DC-DC converter. After the DC-DC converter quits operation due to faults, the direct current voltage in the low-voltage direct current power grid is directly controlled by the fixed power station added with the active signal.
When the internal power of the low-voltage direct-current power grid is unbalanced, the direct-current voltage is increased or decreased. When the voltage rises or falls and exceeds the voltage limit value of the additional active signal, under the control action of the direct-current voltage-active power droop of the fixed power station with the additional active signal, the output of the fixed power station is changed, the internal power of the low-voltage direct-current power grid is gradually balanced, and the direct-current voltage is stabilized near the voltage limit value of the additional signal. And after the DC-DC converter quits operation due to faults, the low-voltage DC power grid can still stably operate.
In order to verify the effectiveness of the operation mode and the control strategy of the urban multilayer direct-current power grid, a multilayer direct-current power grid simulation model shown in fig. 9 is established on a PSCAD/EMTDC software platform.
The effectiveness of the multi-layer direct-current power grid cooperative scheduling control in three modes of normal operation, current-limiting operation and layered operation of the urban multi-layer direct-current power grid is verified through the simulation model of fig. 9. Under a normal operation mode, free exchange of internal power of two layers of direct current power grids can be realized through the DC-DC converter, and the internal power of each layer of direct current power grid is ensured to be in a balanced state; under the current limiting mode, the internal power and voltage fluctuation of the low-voltage direct-current power grid can be stabilized through the cooperation of local control and advanced control; under the layered operation mode, the multilayer direct current power grid can resist power fluctuation and maintain the layered reliable operation of the multilayer direct current power grid.
The above description is only a preferred embodiment of the present application and is not intended to limit the present application, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, improvement and the like made within the spirit and principle of the present application shall be included in the protection scope of the present application.
Although the embodiments of the present invention have been described with reference to the accompanying drawings, it is not intended to limit the scope of the present invention, and it should be understood by those skilled in the art that various modifications and variations can be made without inventive efforts by those skilled in the art based on the technical solution of the present invention.
Claims (9)
1. An operation architecture based on urban multilayer direct current power grid is characterized in that: the direct current power grid comprises two layers of direct current power grids with voltage levels and DC-DC converters, wherein each layer of direct current power grid comprises a plurality of VSC converter stations distributed in a direct current ring network topology form, and a DC-DC converter is arranged between every two adjacent VSC converter stations;
the high-voltage direct-current power grid is controlled by direct-current voltage deviation, and a VSC converter station at the upper end of the DC-DC converter controls a VSC converter station at the lower end of the DC-DC converter to switch between free exchange power of the DC-DC converter and limited exchange power of the DC-DC converter according to a control target of the VSC converter station;
a plurality of VSC converter stations of the low-voltage direct-current power grid are connected with a central load area and suburban new energy, the direct-current voltage in the low-voltage direct-current power grid is controlled by the VSC converter stations at the lower end of the DC-DC converter, and fixed frequency control is adopted to ensure complete acceptance of the new energy and quick response to fluctuating load;
the operation architecture based on the urban multilayer direct current power grid comprises a normal operation mode, a current-limiting operation mode and a layered operation mode;
under the steady state condition, the urban multilayer direct current power grid adopts a normal operation mode;
when the high-voltage alternating-current power grid encounters an emergency, the high-voltage direct-current power grid supports the power of the high-voltage alternating-current power grid in response to a scheduling requirement, and the urban multi-layer direct-current power grid enters a current-limiting operation mode;
when the DC-DC converter breaks down and stops running, the power exchange capacity between two layers of direct current power grids is lost, and the urban multilayer direct current power grid enters a layered running mode;
when the low-voltage direct-current power grid connected to the lower end of the DC-DC converter is in power shortage, the low-voltage direct-current power grid acquires the shortage power from the high-voltage direct-current power grid in real time through the DC-DC converter; when power redundancy occurs in the low-voltage direct-current power grid, redundant power can be transmitted to the high-voltage direct-current power grid in real time through the DC-DC converter.
2. The operation architecture based on the urban multilayer direct current power grid as claimed in claim 1, characterized in that: the number of VSC converter stations of the high-voltage direct-current power grid is determined according to urban power supply requirements and power grid construction planning.
3. The operation architecture based on the urban multilayer direct current power grid as claimed in claim 1, characterized in that: the VSC converter station in the high-voltage direct-current power grid is provided with a fixed direct-current voltage station, M fixed direct-current voltage standby stations and N-M-1 fixed power stations, wherein N is the total number of the VSC converter stations of the high-voltage direct-current power grid.
4. The operation architecture based on the urban multilayer direct current power grid as claimed in claim 1, characterized in that: under the steady state condition, in order to ensure the free flow of the internal power of the two-layer direct current power grid, a free exchange power strategy is adopted by a control target of the DC-DC converter, and in order to ensure that the DC-DC converter has the capability of responding to power fluctuation in real time, the local control strategy of the VSC converter station at the lower end is the constant direct current voltage control.
5. The operation architecture based on the urban multilayer direct current power grid as claimed in claim 1, characterized in that: when the high-voltage direct-current power grid fails or the high-voltage alternating-current power grid needs the high-voltage direct-current power grid to provide emergency power support, the DC-DC converter adopts a strategy of limiting the exchange power, and the purpose of limiting the power exchange between two layers of power grids is achieved by current-limiting and locking the exchange current of the VSC converter station at the lower end.
6. The operation architecture based on the urban multilayer direct current power grid as claimed in claim 1, characterized in that: the converter stations of the low-voltage direct-current power grid except for the converter station connecting the urban central load area and the suburban new energy adopt constant-power control of an additional active signal.
7. The operation architecture based on the urban multilayer direct current power grid as claimed in claim 1, characterized in that: the combined control strategy of the normal operation mode comprises a local control strategy and an advanced control strategy, wherein the advanced control strategy calculates an active power reference value based on a scheduling requirement and the operation state of an alternating current power grid at intervals in the normal operation mode of the multilayer direct current power grid, after a new active power reference value is obtained, the advanced control strategy issues the new active power reference value to a fixed active power station in the multilayer direct current power grid by utilizing communication, and each converter station keeps operating the reference value until the next adjustment.
8. The operation architecture based on the urban multilayer direct current power grid as claimed in claim 1, characterized in that: in the current-limiting operation mode, the multilayer direct current power grid will limit the power transmitted by the high voltage direct current power grid to the low voltage power grid via the DC-DC:
under the normal operation state, the active upper and lower limits of the converter station at the lower end of the DC-DC converter are respectively
And 1Pwhen the multilayer direct current power grid enters a current-limiting operation mode, the direct current voltage station of the high-voltage direct current power grid is determined to reserve power margin, the current limit value of the converter station at the lower end of the DC-DC converter is adjusted through scheduling, and the upper limit of active power is adjusted from the upper limit value
Is adjusted to
The purpose of limiting the DC-DC unidirectional power is achieved.
9. The operation architecture based on the urban multilayer direct current power grid as claimed in claim 1, characterized in that: in the layered operation mode, the direct current voltage in the high-voltage direct current power grid is controlled by a fixed direct current voltage station, the direct current voltage in the low-voltage direct current power grid is controlled by a fixed direct current voltage converter station at the lower end of a DC-DC converter, and after the DC-DC converter quits operation due to faults, the direct current voltage in the low-voltage direct current power grid is directly controlled by the fixed power station with the additional active signals.
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