CN107480875A - A kind of marine frequency dividing cable transmission systems allocation plan choosing method - Google Patents
A kind of marine frequency dividing cable transmission systems allocation plan choosing method Download PDFInfo
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Abstract
A kind of marine frequency dividing cable transmission systems allocation plan choosing method, belongs to Power System Planning, flexible transmission technical field.The transmission voltage grade of marine frequency dividing cable transmission systems is determined according to the offshore wind farm place transmission line capability to be transmitted, and designs AC/AC (alternating current) variable-frequency station configuration parameter；Power frequency current-carrying capacity according to needed for transmission line capability and voltage class determine cable, primary election is carried out to submarine cable according to power frequency current-carrying capacity；The Frequency Of Optimum Transmission of current system is chosen based on optimal frequency choosing method, it is determined that the frequency dividing current-carrying capacity under optimal transmission frequency.If divide the power frequency current-carrying capacity that current-carrying capacity is less than 1.1 times, it is determined that the transmission frequency and submarine cable of Fractional Frequency Power Transmission System, otherwise need again to select submarine cable.Advantage is that Upgrade Problem can not be optimized by solving different Fractional Frequency Power Transmission System conveying capacitys and economy so that the defeated electric energy power of frequency dividing cable transmission systems obtains dividing cable transmission systems optimal system configuration scheme with finding balance in system economy.
Description
Technical Field
The invention belongs to the technical field of power system planning and flexible power transmission, and particularly relates to a method for selecting a configuration scheme of an offshore frequency division cable power transmission system.
Background
Offshore crossover cable transmission of electricity system structure includes: offshore wind power plants, submarine cables, AC/AC frequency conversion stations, offshore transformers and the like. Therefore, the configuration scheme of the frequency division cable power transmission system comprises a voltage grade selection method, an optimal power transmission frequency selection method, a submarine cable selection method and an AC-AC frequency conversion station configuration parameter design method.
The traditional high-voltage alternating-current transmission voltage grades are selected according to different lengths of transmission lines, and the voltage grades to be selected are different. The optimal combination point of the two is generally determined through theoretical calculation and empirical data to finally determine the transmission voltage grade, the maximum transmission power and the transmission distance of the transmission line. The transmission voltage grade selection principle of the frequency division transmission system is different from that of the traditional high-voltage alternating-current transmission. The frequency division power transmission system mostly uses a cable as a transmission medium. During power frequency transmission, the transmission capacity and the transmission distance are severely limited by capacitive charging power generated during cable transmission. When the cable line is used for power frequency transmission, the transmission distance is 50km at most. When frequency division technology is used, the economic transmission distance can reach 250km. Therefore, the transmission voltage class of the frequency division cable transmission system cannot be selected by the traditional method.
The transmission frequency discussed by the traditional frequency division transmission system is mostly 50/3Hz, and for different transmission systems, transmission capacities and transmission distances, the transmission frequency cannot maximize the transmission capacity and the transmission distance. The optimal transmission frequency of the frequency division transmission system is provided by combining the factors of transmission capacity, transmission distance, voltage grade and the like of frequency division transmission, and the relation between transmission capacity and transmission frequency is researched by selecting proper indexes to constrain the working state of the line, so that the optimal transmission frequency is determined, and the transmission capacity of the line is improved to the maximum extent under the condition of safe and stable operation of the line.
The frequency division cable transmission system transmits electric energy by using a submarine cable, and the cost of the submarine cable determines the overall cost of the offshore wind power system. The frequency division power transmission system can improve the power transmission current-carrying capacity, and can use a submarine cable with a smaller cross section to transmit the same power transmission capacity. Thus, the choice of divided power transmission system cable determines the economics of the divided cable power transmission system.
The AC-AC frequency conversion station is used as a core frequency conversion electrical device in the frequency division power transmission system, and the design of the configuration parameters of the AC-AC frequency conversion station is related to the stability and the economy of the whole frequency division power transmission system. The modular multilevel matrix converter (M3C) can be used as a frequency conversion device of a frequency division power transmission system and can rapidly control the reactive power of an input end and an output end, therefore, the flexible low-frequency cable power transmission system adopting the M3C does not need to additionally install large-capacity reactive compensation equipment, the reliability of system operation is improved, and the investment is saved. Based on the method, the AC-AC frequency conversion station adopts the modular multilevel matrix converter as a frequency conversion device of the frequency division power transmission system.
In summary, the offshore divided cable transmission system is a comprehensive system composed of a plurality of parts, and therefore, the system transmission capacity and the economy are determined by the system parameter configuration.
Disclosure of Invention
The invention aims to provide a method for selecting a configuration scheme of an offshore frequency division cable power transmission system, which solves the problem that the transmission capacity and economy of different frequency division power transmission systems cannot be optimally improved, so that the power output of the frequency division cable power transmission system is balanced with the economy of the system, and the optimal system configuration scheme of the frequency division cable power transmission system is obtained.
In order to achieve the above purpose, the invention provides a method for selecting the transmission voltage grade, the transmission frequency, the submarine cable and the AC-AC frequency conversion station configuration parameters of a frequency division power transmission system, so that the offshore frequency division cable power transmission system configuration scheme is optimized. According to the economic power transmission distance of the frequency division cable power transmission system, when the offshore wind farm is 30km-250km from the shore, the frequency division power transmission technology can be used.
Determining the transmission voltage grade of the offshore frequency division cable transmission system according to the transmission capacity to be transmitted by the offshore wind farm, and designing configuration parameters of an AC-AC frequency conversion station; secondly, determining power frequency current-carrying capacity required by the cable according to the power transmission capacity and the voltage grade, and primarily selecting the submarine cable according to the power frequency current-carrying capacity; and then, selecting the optimal transmission frequency of the current system based on an optimal frequency selection method, thereby determining the frequency-division current-carrying capacity under the optimal transmission frequency. And if the frequency-dividing current-carrying capacity is less than 1.1 times of the power frequency current-carrying capacity, determining the power transmission frequency of the frequency-dividing power transmission system and the submarine cable, otherwise, re-selecting the submarine cable, and re-calculating the optimal transmission frequency of the frequency-dividing power transmission system. The specific process steps and the controlled technical parameters are as follows:
step 1: voltage grade selection method
The method comprises the steps of firstly selecting a transmission voltage grade of the offshore frequency division cable system, and determining the transmission voltage grade of the frequency division transmission system according to the transmission capacity required by an offshore wind farm, as shown in a table 1.
TABLE 1 Range of fractional transmission voltages and transmission capacities
| Voltage class/kv | Transport volume/MVA |
| 35 | 2-10 |
| 110 | 10-80 |
| 220 | 100-500 |
| 330 | 200-800 |
| 500 | 1000-1500 |
And 2, step: when the power transmission capacity and the power transmission voltage grade of the system are determined, the current-carrying capacity I required by the submarine cable during power transmission frequency transmission can be obtained according to the formula 1 50Hz . And selecting a proper submarine cable as the alternative submarine cable of the frequency division power transmission system according to the current-carrying capacity of the alternative submarine cable.
And step 3: the optimal frequency selection method comprises the following steps:
after the alternative submarine cable is selected, a cable equivalent model is established for the submarine cable according to the submarine cable structure parameters, and the head-end voltage difference and the head-end current ratio are selected as limiting conditions, so that the optimal transmission frequency f of the current frequency-division cable transmission system is obtained FFTS 。
And 4, step 4: the submarine cable selection method comprises the following steps:
after the optimal transmission frequency is determined, recalculating the optimal transmission frequency f of the alternative submarine cable FFTS Current carrying capacity I FFTS . If I FFTS >1.1I 50Hz Returning to the step 4, reselecting the submarine cable of the submarine cable, reducing the cross-sectional area of the alternative submarine cable by one level, and recalculating; if I FFTS <1.1I 50Hz Then it is determined that the currently selected alternative submarine cable is a power transmission cable of the current divided cable power transmission system.
And 5: the design method of the configuration parameters of the AC-AC frequency conversion station comprises the following steps:
the AC-AC frequency conversion station is used as a core frequency conversion electrical device in the frequency division power transmission system, and the design of the configuration parameters of the AC-AC frequency conversion station is related to the stability and the economy of the whole frequency division power transmission system. The parameter design is mainly based on a modular multilevel matrix converter. The design parameters comprise cascade submodule number selection, submodule switch device selection, submodule capacitance parameter selection and link reactance parameter selection.
And (4) obtaining the optimal transmission voltage grade, transmission frequency and submarine cable of the frequency division cable transmission system according to the steps 1-5. The method can be suitable for determining the optimal configuration scheme of the offshore fractional frequency cable power transmission system with different power transmission capacities and voltage grades.
In conclusion, the method can be suitable for the offshore frequency division cable power transmission systems with different power transmission capacities, power transmission distances and voltage levels, so that the configuration scheme of the offshore frequency division cable power transmission system is optimized.
Drawings
Fig. 1 is a block diagram of an offshore divided power transmission system.
FIG. 2 is a flow chart of the steps of selecting a scheme for a power transmission system for a divided frequency cable at sea
Fig. 3 is a block diagram of a modular multilevel ac-base converter station topology.
FIG. 4 is a diagram of an equivalent model of a cable.
Fig. 5 is a graph of the ratio of the head and tail end currents, the frequency and the transmission distance.
FIG. 6 is a plot of head end voltage deviation versus frequency.
Fig. 7 is a diagram of the range of the frequency division power transmission frequency.
Detailed Description
The following describes in detail the optimization of the scheme of the power transmission system of the divided-up cable with reference to the accompanying drawings.
The topology structure of the offshore crossover cable power transmission system is shown in fig. 1, and comprises: offshore wind power plants, submarine cables, AC/AC frequency conversion stations, offshore transformers and the like. A method for selecting an optimal configuration scheme of an offshore fractional-n cable power transmission system, as shown in fig. 2, includes: the method comprises a power transmission voltage grade selection method of a frequency division power transmission system, an optimal power transmission frequency selection method, a submarine cable selection method and an AC-AC frequency conversion station parameter design method.
The economic power transmission distance of the frequency division cable power transmission system is determined, and if the offshore distance of the offshore wind power plant is greater than 250km, the direct current power transmission system is recommended to be used; and if the offshore wind farm is less than 30km away, recommending the high-voltage alternating-current transmission system. And when the offshore wind farm is in the distance of 30km-250km from the shore, the frequency division transmission technology can be used.
Step 1: a voltage class selection method, comprising:
the frequency division power transmission system still transmits power by alternating current, but the voltage grade method is different from the traditional high-voltage alternating current power transmission, the power transmission voltage grade of the traditional high-voltage alternating current power transmission system needs to be selected by considering the influence of power transmission distance, and the optimal combination point of power transmission capacity and power transmission grade is determined through theoretical calculation and empirical data. The frequency-division power transmission system does not need to consider the influence of the power transmission distance, and the frequency-division power transmission technology can be used in the economic interval (30 km-250km from the shore) of the frequency-division power transmission system. And if the offshore wind farm is between 30km and 250km, finding out the voltage level corresponding to the transmission capacity according to the required transmission capacity S as shown in the table I. And transmitting the voltage grade with the voltage grade system cable.
Step 2: according to the transmission capacity S and the transmission voltage level U required by the offshore wind farm, the current-carrying capacity I required by the offshore cable system during power frequency transmission can be obtained by the formula (1) 50Hz . From the obtained current carrying capacity I 50Hz And selecting a proper submarine cable as the alternative submarine cable of the frequency division power transmission system according to the current-carrying capacity of the alternative submarine cable.
And 3, step 3: the optimal power transmission frequency determination method comprises the following steps:
as shown in fig. 4, after the alternative submarine cable is selected, a cable equivalent model is established for the submarine cable according to the submarine cable structure parameters. Selecting a head-to-tail voltage difference delta U H And head-to-tail end current ratio n I As a limiting condition, obtaining the optimal transmission frequency f of the current frequency division cable transmission system FFTS 。
1. Calculating the current ratio n between the head and the tail I ：
The head end current effective value can be obtained by calculating from the tail end to the head end, and is expressed as:
wherein, r, x, b are the series resistance, reactance and the parallelly connected susceptance value that submarine cable unit pi corresponds respectively, and the assumption system is full-load operation, and cable head end phase current effective value is this frequency download flow promptly, expresses as:
the simultaneous (2) and (3) can obtain the current ratio n of the head end to the tail end I . Then n at this time I And f and the power transmission distance L are shown in fig. 5.
2. Calculating the voltage difference between the head and the tail ends H
And introducing the end voltage difference as a standard for judging the stability of the system. From the line model, it can be derived:
the voltage difference across the cable lines depends on the resistance, reactance of the cables and their current flow, which in turn is related to the capacitive current of the cables. According to the formulas (2) and (4), head end voltageCan be expressed as:
the delta U can be obtained by the combined vertical type (4) and (5) H The relationship between f and L is shown in FIG. 6.
3. The determination of the optimal frequency comprises the following steps:
considering from two aspects of safe and stable operation of the system and system loss, let n I ≥0.9,ΔU H And less than or equal to +/-5 percent, and then carrying out projection processing on the graph 5 and the graph 6 to obtain the frequency selection range of the frequency division power transmission system, such as the graph 7. The blue envelope curve is the value range of the end voltage difference, and the red curve is the value range of the ratio of the effective values of the current at the head end and the tail end. Assuming that the power transmission distance of the current frequency division cable power transmission system is 150km, the frequency value range of the power transmission system is more than or equal to 19Hz and less than or equal to f under the distance FFTS ≤34Hz。
And 4, step 4: a submarine cable selection method comprising:
after the optimal power transmission frequency is determined, recalculating the optimal power transmission frequency f of the alternative submarine cable FFTS Current carrying capacity I FFTS . If I FFTS >1.1I 50Hz Returning to the step 3, reselecting the submarine cable, reducing the cross-sectional area of the alternative submarine cable by one level, and recalculating; if I FFTS <1.1I 50Hz Then it is determined that the currently selected alternative submarine cable is a power transmission cable of the current divided cable power transmission system.
And 5: the design method of the configuration parameters of the AC-AC frequency conversion station comprises the following steps:
the AC-AC frequency conversion station is used as a core frequency conversion electrical device in the frequency division power transmission system, and the design of the configuration parameters of the AC-AC frequency conversion station is related to the stability and the economy of the whole frequency division power transmission system. The parameter design of the offshore AC-AC frequency conversion station is a modular multilevel matrix converter, and the design parameters comprise the selection of the number of cascaded sub-modules, the selection of sub-module switching devices, the selection of sub-module capacitance parameters and the selection of link reactance parameters.
(1) Cascaded submodule number selection
As shown in fig. 2, the ac-to-ac converter station adopts a modular multilevel matrix converter structure, i.e., M3C. The number of levels of M3C is generally determined by the input and output voltage levels and the voltage withstand of the selected power electronic switching device. Meanwhile, in order to ensure the reliability of the whole device during operation, the practical problems such as redundancy margin of device parameters in the module and the like need to be considered in engineering.
The output voltage value of the bridge arm is larger than the voltage peak values of the input side line and the output side line:
U branch ＝nU cap (6)
wherein U is branch Is the maximum output voltage of the bridge arm, n is the number of the bridge arm cascade modules, U cap Is the module capacitor voltage.
The relation between the IGBT withstand voltage and the direct current capacitor voltage of the submodule is as follows:
U IGBT_LIM ＝k m_IGBT U cap (7)
wherein U is IGBT Is a U of an IGBT CE Withstand voltage value, U cap Is the sub-module DC capacitor voltage, K m_IGBT For the margin factor, 1.2-1.5 is generally selected. The sub-module dc capacitor voltage is:
assume again that the M3C output side line voltage is equal to the input line voltage, i.e., U in ＝U out . Then the number of bridge arm modules can be calculated as:
wherein the number n of modules is an integer.
(2) Submodule switching device selection
The submodule switch devices of the modular multilevel matrix converter comprise a full-control switch device IGBT and an uncontrollable diode, and the selection of the submodule switch devices is generally obtained according to the withstand voltage of a main current switch device and the analysis of M3C level number. Because the M3C submodule is of an H-bridge structure, the IGBT and the diode are clamped by the direct current capacitor during the turn-off period of the device. Therefore, the withstand voltage of the IGBT and the diode is selected to satisfy the following conditions:
the selection of the rated current of the submodule switch device can be calculated according to the rated capacity of the device. Let the rated power of the converter be P N Rated output voltage of U out The reference value of the rated current is calculated as follows:
on the basis, a certain safety margin is considered, and an appropriate IGBT and a diode are selected as a switching device in the power module.
(3) Selection of sub-module capacitance parameters
The capacitance of the capacitor in the submodule is related to the voltage fluctuation of the capacitor. Therefore, a larger value is selected with due consideration to the cost and volume of the converter station. The sub-module capacitance parameter selection mainly considers the capacitance withstand voltage and the capacitance value. The voltage withstanding value of the capacitor needs to consider the selection of the switching elements and the number of the bridge arm cascades in the submodule. The capacitance value of a capacitor is the ratio of the total stored energy in the capacitor to the capacity of the device in seconds. The constant is defined as
In the formula:
c is the equivalent capacitance value of the equipment;
U dc -the operating voltage of the dc capacitor;
Q out -reactive power output by the device.
Usually U cc This amount is in the range of 30-120ms. The capacitance value of the module capacitor is
(4) Associative reactance parameter selection
The modular multilevel matrix converter is connected with a reactor to suppress harmonic components of high-frequency current at the input side and the output side. However, a larger inductance value will also bring some negative effects, such as reducing the response speed of the system, affecting the compensation effect of the system on the high frequency component command, increasing the loss and volume, and causing the cost to increase.
The inductance value is usually designed to be 5 to 12% (per system unit) by referring to the parameter selection method and experience in the prior art, and is calculated as follows
In the formula:
U l -input line voltage or output line voltage;
omega-operating angular frequency;
s-system capacity;
k L inductance selection factor, 5-12%. Where k is L Selecting as 10%;
the transmission frequency of the frequency-division power transmission system is low, namely the frequency is f FFTS (ii) a The grid-connected frequency being power frequency, i.e. frequency f 50Hz . Thus, the link reactance L of the AC-AC frequency conversion input and output in ，L out Respectively as follows:
step 6: and (4) obtaining the optimal transmission voltage grade, transmission frequency and submarine cable of the frequency division cable transmission system according to the steps 1-5. The method can be suitable for determining the optimal configuration scheme of the offshore fractional frequency cable power transmission system with different power transmission capacities and voltage grades.
The above description is only for the preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present invention are included in the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.
Claims (5)
1. A method for selecting a configuration scheme of an offshore frequency division cable power transmission system is used for an offshore wind farm between 30km and 250km from an shore; the method is characterized by comprising the following process steps and technical parameters:
step 1: selecting the transmission voltage grade of the offshore frequency division cable system, and determining the transmission voltage grade of the frequency division transmission system by looking up a table according to the transmission capacity required by the offshore wind farm;
step 2: when the power transmission capacity and the power transmission voltage grade of the system are determined, the current-carrying capacity I required by the submarine cable during power transmission frequency transmission can be obtained according to the formula 1 50Hz (ii) a Selecting a proper submarine cable as the alternative submarine cable of the frequency division power transmission system according to the current-carrying capacity of the alternative submarine cable;
and step 3: after the alternative submarine cable is selected, a cable equivalent model is established for the submarine cable according to the submarine cable structure parameters, and the voltage difference of the head end and the tail end are selectedThe current ratio is a limiting condition; considering from two aspects of safe and stable operation of the system and system loss, the ratio n of head current to tail current is used I Not less than 0.9, voltage difference between head end and tail end delta U H Less than or equal to +/-5 percent, namely obtaining the frequency selection range of the frequency division power transmission system, and obtaining the power transmission frequency f of the current frequency division cable power transmission system in the presence of power transmission distance FFTS ；
And 4, step 4: after the optimal power transmission frequency is determined, recalculating the optimal power transmission frequency f of the alternative submarine cable FFTS Current carrying capacity I FFTS . If I FFTS >1.1I 50Hz Returning to the step 4, reselecting the submarine cable of the submarine cable, reducing the cross-sectional area of the alternative submarine cable by one level, and recalculating; if I FFTS <1.1I 50Hz Determining that the currently selected alternative submarine cable is a transmission cable of the current frequency division cable transmission system;
and 5: the AC-AC frequency conversion station is used as the core frequency conversion electrical equipment in the frequency division power transmission system, and the design of the configuration parameters of the AC-AC frequency conversion station is related to the stability and the economy of the whole frequency division power transmission system; the parameter design mainly comprises a modular multilevel matrix converter; the design parameters comprise the selection of the number of cascaded submodule blocks, the selection of a submodule switch device, the selection of submodule capacitance parameters and the selection of link reactance parameters.
2. The method of claim 1,
the number of the cascade submodules is selected as follows: the AC-AC frequency conversion converter station adopts a modular multilevel matrix converter structure, namely M3C. The level number of the M3C is generally determined by the voltage grades of the input side and the output side and the withstand voltage of the selected power electronic switching device; meanwhile, in order to ensure the reliability of the whole device during operation, the problem of redundancy margin of device parameters in the module needs to be considered in engineering;
the output voltage value of the bridge arm is larger than the voltage peak values of the input side line and the output side line:
U branch ＝nU cap (6)
wherein U is branch Is the maximum output voltage of the bridge arm, n is the number of the bridge arm cascade modules, U cap Is the module capacitor voltage;
the relation between the IGBT withstand voltage and the direct-current capacitor voltage of the submodule is as follows:
U IGBT_LIM ＝k m_IGBT U cap (7)
wherein U is IGBT Is U of IGBT CE Withstand voltage value, U cap Is the sub-module DC capacitor voltage, K m_IGBT Generally selecting 1.2-1.5 for the margin coefficient; the sub-module dc capacitor voltage is:
assume again that the M3C output side line voltage is equal to the input line voltage, i.e., U in ＝U out . Then the number of bridge arm modules can be calculated as:
wherein the number n of modules is an integer.
3. The method of claim 1,
the submodule switch device selection: the submodule switching devices of the modular multilevel matrix converter comprise a full-control switching device IGBT and an uncontrollable diode, and selection of the submodule switching devices is generally obtained according to the withstand voltage of a main-stream switching device and the analysis of M3C level number. Because the M3C sub-module is of an H-bridge structure, the IGBT and the diode are clamped by the direct current capacitor during the turn-off period of the device; therefore, the withstand voltage of the IGBT and the diode is selected to satisfy the following conditions:
the rated current of the submodule switch device can be selected according to the rated capacity of the device; let the converter rated power be P N Rated output voltage of U out The reference value of the rated current is calculated as follows:
on the basis, the IGBT and the diode are selected as the switching device in the power module.
4. The method of claim 1,
the selection of the sub-module capacitance parameters: the capacitance value of the capacitor in the submodule is related to the voltage fluctuation of the capacitor; selecting sub-module capacitance parameters considering capacitance withstand voltage and capacitance value; the voltage withstanding value of the capacitor needs to consider the selection of switching elements and the number of bridge arm cascades in the submodule; the capacitance value of the capacitor is the ratio of the total stored energy in the capacitor to the capacity of the equipment, and the unit is second; the constant is defined as
In the formula:
c is the equivalent capacitance value of the equipment;
U dc -the operating voltage of the dc capacitor;
Q out -reactive power output by the device;
usually U cc This amount is in the range of 30-120ms. The capacitance value of the module capacitor is
5. The method of claim 1,
the connection reactance parameter selection: the inductance value is designed to be 5-12% per unit of the system, and the calculation method is as follows:
in the formula:
U l -input line voltage or output line voltage;
omega-the operating angular frequency;
s-system capacity;
k L inductance selection factor, 5-12%. Where k is L Selecting 10 percent;
the transmission frequency of the frequency-division power transmission system is low, namely the frequency is f FFTS (ii) a The grid-connected frequency being power frequency, i.e. frequency f 50Hz (ii) a Thus, the link reactance L of the AC-AC frequency conversion input and output in ，L out Respectively as follows:
3
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