Maximum Transmission Efficiency Method

Abstract

A method for operating a power line is presented, the power line comprising: a sending end and a receiving end; at least one shunt reactive power compensation device for regulating reactive power in the power line; means for measuring the line voltage of the power line at the receiving end; and a load Q. load and - means for determining a required reactive power of the shunt reactive power compensation device, the method comprising: - receiving information about parameters for the power line; - determining a maximum transmission efficiency parameter H on the power line based on the parameters for the power line; - measuring a line voltage at a receiving end of the power line; - determining a required reactive power Q based on the parameter H and the measured line voltage at the receiving end of the power line. req and -load Q load By measuring the reactive power to -Q req and load Q load Based on the measured reactive power to shunt and adjust the shunt reactive power compensation device to achieve the maximum transmission efficiency of the power line. shunt and distributing the power to the power line.

Description

[Technical Field]

[0001] The present invention relates to a method of operating a power line in a system in which the power line is operated at its corresponding maximum efficiency for any applied load.

[0002] The present invention relates to a method for improving the transmission efficiency of any type of ac power line so that the power line can operate at its corresponding maximum transmission efficiency under any load condition.

[0003] Electricity is an essential part of modern life. After electricity is generated, it must be transmitted from the source to the consumer through a transmission and distribution system. The power lines in a transmission and distribution system can be overhead lines, underground cables, undersea cables, or a combination. Power losses occur at each stage of the transmission and distribution system, and power line losses represent a major cost in the distribution of electrical energy. Therefore, improving power system transmission efficiency is important for energy conservation, cost savings, and the overall reduction of emissions associated with the distribution of electrical power.

[0004] Electrical energy is mostly generated, transmitted, distributed, and utilized as alternating current (AC). When connected to the transmission of AC current, there are losses in both real power P and reactive power Q. Changes in reactive power affect the voltage, current, and phase shift between voltage and current along the transmission line, which in turn affects power losses.

[0005] There are methods to improve the efficiency of AC transmission lines. Power factor control is the most common and proven technique. The purpose of this technique is to improve the power factor (reduce reactive power) and thereby improve the efficiency of power line transmission. However, a high power factor is not always the best way to improve power transmission efficiency, especially when the line is operating under low load conditions.

[0006] Constant reactive power control is also used in some applications, but it is not always easy to gauge how much reactive power a system requires. The demand for reactive power will change as the load on the system changes.

[0007] Reactive power compensation techniques are widely used to improve power system transmission efficiency and performance. In practice, the goal is power factor correction (reactive power reduction), which is believed to improve power line transmission efficiency.

[0008] For the reasons explained above, solutions have been developed to improve power line transmission efficiency, whereby the power line will always operate at its corresponding maximum power efficiency under any given load condition.

[0009] The following five publications relate to prior art devices for operating power lines: -US2011 / 031936 A1, -Pradhan ak et al: "Maximum efficiency of flexible ac transmission systems", International Journal of Power and Energy Systems, Jordan Hill, Oxford, GB, vol.28, no.8, 1 2006-10-01, pp.581-588, xp025096410, ISSN:0142-0615, -JP S63 7140 A, -Thales m papazoglou: "Benchmarking double-circuit OHL for power transmission efficiency", University Power Engineering International Conference (upec), 2010, 45th International, IEEE, Piscataway, NJ, USA, August 31, 2010, pages 1-3, ISBN: 978-1-4244-7667-1, -Papazoglou tm: "Maximum Efficiency of Interconnected Transmission Lines", Proceedings of the IEE: Generation, Transmission, and Distribution of Electric Power, Institution of Electrical Engineers, GB, vol. 141, no. 4, part c, July 1, 1994 (1994-07-01), pp. 353-356, ISSN: 1350-2360

[0010] Theoretical background of the invention In theory, a power line can be represented by an infinite series of resistors, inductors in series, and capacitors in parallel (there is also a shunt conductance G, which is neglected for many types of power lines in 50 and 60 Hz systems).

[0011] In theory, energy loss occurs only in electrical resistance, not in pure inductors or capacitors. However, every power line has shunt capacitance that generates capacitive currents and results in higher resistive losses. The following equation describes the power loss P L , explain the relationship between current and resistance. P L =I 2 R

[0012] The voltage, current, and phase shift between the voltage and current along a power line depend on both the load (including the real power P and reactive power Q) and the power line properties. This means that the power line losses are affected by both the load and the power line properties. Therefore, changing the reactive power at the power line output side will affect the power line losses for a given real power P (load) delivered to the consumer.

[0013] To illustrate the calculation of maximum power transmission efficiency of the method, several power line calculation and modeling techniques are described in this chapter.

[0014] There are four basic parameters for power line modeling and analysis: series resistance (R), series inductance (L), shunt capacitance (C), and shunt conductance (G). A two-terminal passive element model of an AC power line is used to illustrate the power transfer characteristics, as shown in Figure 1.

[0015] The model represents a single-phase power line. The sending end corresponds to the power source, the receiving end corresponds to the consumer, and U s and I S are the sending end voltage and current, and U r and I r are the receiving end voltage and current. The relationship between the sending end quantity and the receiving end quantity is

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[0016] The parameters A, B, C, and D depend on the power line constants R, L, C, and G, which are complex-valued.

[0017] The wire constants R, L, C, and G are usually per-length values ​​with units of Ω / km, H / km, f / km, and S / km, and they are uniformly distributed along the length of the wire. Several important parameters can be calculated based on these four wire constants:

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[0018] The propagation constant γ is a complex value and can be expressed as:

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[0019] The parameters A, B, C, and D in a two-port network are

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[0020] Voltages and currents in complex form are

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[0021] Complex power (apparent power) in an AC system is

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[0022] Prior Art Disclosure EP 2093855 A2 teaches a system and method for dynamic reactive power support for a power transmission system. In one embodiment, the method for dynamic reactive power support for a power transmission system includes at least one circuit breaker connecting a shunt capacitor bank including at least one shunt capacitor with a metal oxide varistor (MOV) connected in parallel. A controller is used to detect a voltage drop in the power transmission system (e.g., a substation bus). In response to detecting the voltage drop, the controller closes circuit breaker(s) connecting the shunt capacitor to the power transmission system. The controller then monitors the at least one MOV to detect current. Upon detecting current in the MOV, the controller disconnects one or more shunt capacitors from the power transmission system by opening one or more circuit breakers. The shunt capacitors may be disconnected simultaneously, sequentially, in groups, or otherwise.

[0023] CN 113346511 A teaches a reactive power compensation correction method. The method comprises the following steps: measuring impedance values ​​on two sides of a power frequency under different frequencies; and comparing the impedance values ​​measured in the previous step to perform compensation correction. Throughout the process, the phase angle difference between voltage and current does not need to be measured. Unlike existing traditional compensation methods, the compensation method does not need to measure the phase angle difference between voltage and current, which simplifies the measurement scheme, reduces equipment costs, lowers the fault rate, and improves stability.

[0024] EP 3220503 A1 teaches a power cable assembly for transmitting high voltage alternating current over long distances. The assembly comprises a continuous power cable, each power cable comprising a core, conductors, and cable insulation. The assembly further comprises shunt inductances connected to each conductor at equal intervals along the conductors. The invention further comprises a method of providing a power cable assembly for transmitting high voltage alternating current over long distances with a continuous power cable, each power cable comprising a core, conductors, and cable insulation. The method comprises connecting shunt inductances to each conductor at equal intervals along the conductors.

[0025] US 2005040655 A1 teaches active and reactive power control for a wind turbine generator system. The techniques described herein provide the ability to utilize the full capacity of a wind turbine generator system (e.g., a wind farm) to provide dynamic VAR (reactive power support). The VAR support provided by individual wind turbine generators in the system can be dynamically changed to suit application parameters.

[0026] CN 110829455 A teaches a reactive compensation method for capacitors in a power distribution network, which includes the following steps: determining an optimal compensation point using a reactive margin method, monitoring the voltage of the compensation point on the line, and controlling the switched capacitor by using the voltage as a constraint so as to dynamically adjust the compensation capacity; performing capacitor compensation for voltage drops caused by shock loads, analyzing the harmonic impact caused by the lag of parallel-connected capacitors when the capacitors are connected to a power distribution network, and connecting a reactor in series with the capacitor to suppress the amplifying effect of the parallel-connected capacitors on harmonics, so as to reduce the impact of load shock on the power grid.

[0027] US 2013134789 A1 teaches a reactive power compensation method, including: generating a variable power factor curve for at least one generator based on information about network parameters; obtaining values ​​of active output power parameters from the at least one generator; calculating reactive power based on the variable power factor curve and the values ​​of the active power output parameters of the at least one generator; generating a reactive power compensation command based on the calculated reactive power; and sending the reactive power compensation command to the at least one generator to control operation of the at least one generator.

[0028] Object of the invention The object of the present invention is to provide a method for improving the transmission efficiency of any type of AC power line, so that the power line can always operate at its corresponding maximum transmission efficiency under any load condition. Thus, the advantage of the invention compared to the prior art is that it allows the AC transmission line to be continuously operated at its optimum operating point under various load conditions. This leads to higher transmission capacity, lower transmission costs, and reduced emissions related to distribution to consumers. Summary of the Invention

[0029] An aspect of the present invention is a method of operating a power line, comprising: The power line has a sending end and a receiving end, at least one shunt reactive power compensation device for regulating reactive power in the power line; - means for measuring the line voltage of the power line at the receiving end; - a means for measuring the reactive power into the load Qload; means for determining the required reactive power of the shunt reactive power compensation device, The method comprises: receiving information about parameters for a power line; determining a maximum transmission efficiency parameter H for the power line based on the parameters for the power line; - measuring the line voltage at the receiving end of the power line; - determining the required reactive power Qreq based on the parameter H and the measured line voltage at the receiving end of the power line; - measuring the reactive power into the load Qload; - determining a required shunt reactive power Qshunt based on Qreq and the measured reactive power into the load Qload; - adjusting the shunt reactive power compensation device to distribute Qshunt to the power line to reach maximum transmission efficiency of the power line.

[0030] In an embodiment of the present invention, H is determined based on a set of power line parameters: resistance per unit length R, capacitance per unit length C, inductance per unit length L, length, and frequency.

[0031] In an embodiment of the present invention, the shunt reactive power compensation device is connected to a power line receiving end.

[0032] In an embodiment of the present invention, the parameter H is calculated based on a power line parameter set ABCD for a power line two-port model, where the ABCD parameters are determined based on a series resistance R, a shunt capacitance C, a series inductance L, and a shunt conductance G, and H is calculated as follows:

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[0033] In an embodiment of the present invention, the parameter H is calculated based on a set of power line parameters, an attenuation constant α, an attenuation constant β of the propagation constant γ, a series resistance R of the power line, and a length I of the power line; H is

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[0034] In an embodiment of the present invention, a parameter H for a short-distance power line is estimated based on a power line parameter set of operating frequency f, power line shunt capacitance per unit length C, and power line length I, and H is given by:

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[0035] In the embodiment of the present invention, the required reactive power Q at the power line receiving end is req is determined based on the parameter H and the measured line voltage at the receiving end of the power line, and Q req teeth,

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[0036] In an embodiment of the invention, the device comprises a connection to a receiving end of a power line; at least one shunt reactive power compensation device for injecting reactive power into the power line; - means for measuring the line voltage of the power line at the receiving end; -Load Q load a means for measuring reactive power to the means for determining the required reactive power injected by the shunt reactive power compensation device, The means for determining the required reactive power is determined by the method described in the embodiment of the invention. In the embodiment, a shunt reactive power compensation device is connected to the power line receiving end.

[0037] Embodiments of the present invention will now be described, by way of example only, with reference to the following figures: [Brief explanation of the drawings]

[0038] [Figure 1] This is a diagram of a two-port passive element model of a single-phase power line. [Figure 2] FIG. 1 is a diagram of the apparatus setup according to the method. [Figure 3] Diagram of overall transmission efficiency in a 20 km 33 kV underground distribution cable for active powers between 5 and 20 MW and power factors between 0.2 and 1. The figure shows color plots for both variables and curves corresponding to maximum transmission efficiency for optimal power factor at a given load. [Figure 4] 4 is a plot of the efficiency of the theoretical maximum efficiency and using the method of FIG. 3 for the 33 kV underground distribution cable. [Figure 5] Diagram of overall power transmission efficiency in a 150 km 150 kV submarine transmission cable for active powers of 20 to 100 MW and power factors of 0.2 to 1. The figure shows color plots for both variables and curves corresponding to maximum power transmission efficiency for optimal power factor at a given load. [Figure 6] 6 is a plot of the efficiency of the theoretical maximum efficiency and using the method of FIG. 5 for the 150 kV submarine transmission cable. Detailed Description of the Invention

[0039] Figure 1 shows a two-terminal passive element model of an AC power line as described earlier in this application. From this model, mathematical equations are derived that are used to calculate the value of the maximum transmission efficiency parameter H. Equation from the model

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[0040] By considering the operating voltage and the receiving end real power as constant, the power loss in the power line can be calculated as the reactive power Q at the receiving end. r can be expressed as a function of

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[0041] Parameter H is taken as the maximum transmission parameter for the power line. Because power line parameters B, C, and D are constant for a particular power line operating at a particular operating frequency, parameter H is constant for a particular power line operating at a specified operating frequency.

[0042] Following the above derivation, the required reactive power to control the power line to operate at the corresponding maximum efficiency point is given by the following equation:

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[0043] An equation for H can also be given in terms of the damping α and phase β constants.

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[0044] The above equation can be rewritten as follows:

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[0045] For short power lines, the magnitude of the parameter H is very close to half the total shunt susceptance of the power line. Therefore, we can use the following equation to define it as

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[0046] The reactive power required at the receiving end of the power line is given by the following equation:

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[0047] The required value of shunt reactive power is

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[0048] Thus, the method comprises the steps of determining the parameter H based on the power line constant as described. Further, the required reactive power at the receiving end is calculated based on H and the measured voltage. The shunt reactive power is then determined based on the calculated required reactive power and the measured reactive power of the load. Thereafter, the method adjusts the shunt reactive power compensation device to reach the corresponding maximum transmission efficiency on the power line.

[0049] There are many available techniques to regulate the reactive power of the power line, such as shunt reactors, shunt capacitors, SVCs, STATCOMs, synchronous condensers, etc. Depending on the reactive power of the load and the capacitive power of the power line, the reactive power (Q shunt ) can be capacitive or inductive or even variable during operation. The selection of a preferred reactive compensation technique should be possible for one skilled in the art depending on the operating requirements of the consumer (load) and application.

[0050] 2 shows a setup for carrying out the steps of the method. A power line 1 has a shunt reactive power compensation device 2, a voltage measurement device 3, a reactive load measurement device 4, and a load 5 connected thereto. In this example, the shunt reactive power compensation device, the voltage measurement device, and the reactive load measurement device are connected at the end of the power line at the receiving end near the load. This is one embodiment of the invention; these components may be connected anywhere along the power line, and more sets of these components may also be connected at various sections of the power line.

[0051] In Figures 3-6, two examples are given to demonstrate the effectiveness of the method. Both examples are three-phase AC power lines; the first is a 33 kV underground cable in an urban distribution system, and the second is a 150 kV submarine cable in a transmission system. The power line data and parameters are listed in Table 1. [Table 1]

[0052] Numerical analysis is carried out to study the theoretical power transmission efficiency of the power cable under various load conditions. The power transmission efficiency of the invented method is also further analyzed.

[0053] In Figure 3, a 20 km underground distribution cable is simulated. In the simulation, the load has a constant operating voltage (Un), which is 33 kV. The figure shows the overall transmission efficiency, a color plot of the transmission efficiency, and the corresponding theoretical maximum transmission efficiency curve for loads between 5 MW and 25 MW and power factors between 0.2 and 1.

[0054] FIG. 4 shows simulated transmission efficiency curves, as well as the corresponding theoretical maximum transmission efficiency curves, which confirm that the power line operates at the corresponding maximum transmission efficiency point under any load condition in a manner consistent with the simulated transmission efficiency curves.

[0055] Table 2 shows the corresponding theoretical maximum power transmission efficiency and the values ​​of the power transmission efficiency by the invented method, with loads ranging from 5 MW to 25 MW, and 1 MW is used in each step in the table. [Table 2]

[0056] In Figure 5, a 150 km submarine power distribution cable is simulated. In the simulation, the load has a constant operating voltage (Un), which is 150 kV. The figure shows the overall transmission efficiency, a color plot of the transmission efficiency, and the corresponding theoretical maximum transmission efficiency curve for loads of 20 MW to 100 MW and power factors of 0.2 to 1.

[0057] FIG. 6 shows simulated transmission efficiency curves, as well as the corresponding theoretical maximum transmission efficiency curves, which confirm that the power line operates at the corresponding maximum transmission efficiency point under any load condition in a manner consistent with the simulated transmission efficiency curves.

[0058] Table 3 shows the corresponding theoretical maximum power transmission efficiency and the values ​​of the power transmission efficiency by the invented method, where the load is 20MW to 100MW, and 5MW is used for each step in the table. [Table 3]

Claims

1. A method for operating an AC power line (1) using a device, wherein the AC power line (1) has a sending end and a receiving end, the receiving end of the AC power line (1) is connected to an applied load (5), and the device is - At least one shunt reactive power compensation device (2) connected to the receiving end of the AC transmission line (1), the at least one shunt reactive power compensation device (2) configured to adjust the reactive power in the AC transmission line (1), - Means provided for measuring the line voltage of the AC transmission line (1) at the receiving end, - Reactive power Q from the AC transmission line (1) to the applied load (5) ｌｏａｄ A means for measuring the applied load (5) comprising an active power section and a reactive power section, - The shunt reactive power compensation device (2) comprises means for determining the required reactive power, The following steps apply to the applied load on the AC transmission line (1): - Receiving information regarding parameters for the AC transmission line (1), - Based on the parameters for the AC transmission line (1), determine the maximum transmission efficiency parameter H for the AC transmission line (1), - Measuring the line-to-line voltage at the receiving end of the AC transmission line (1), - Based on the maximum power transmission efficiency parameter H and the measured line voltage at the receiving end of the AC transmission line (1), the required reactive power Q ｒｅｑ To seek, - Reactive power Q applied to the above-mentioned load ｌｏａｄ Measuring and - Required reactive power Q ｒｅｑ and the measured reactive power Q applied to the applied load (5) ｌｏａｄ Based on this, the required shunt reactive power Q ｓｈｕｎｔ To seek, - Adjust the shunt reactive power compensation device (2) to reach the maximum power transmission efficiency of the AC transmission line (1) at the applied load (5), Q ｓｈｕｎｔ The method includes distributing the AC power to the AC transmission line (1).

2. The method according to claim 1, wherein the maximum power transmission efficiency parameter H is determined based on the AC power maximum power transmission efficiency parameter set: resistance R per unit length, capacitance C per unit length, inductance L per unit length, length, and frequency.

3. The maximum power transmission efficiency parameter H is calculated based on the AC power transmission efficiency parameter set ABCD for the AC transmission line 2-port model, where the ABCD parameters are determined based on the series resistance R, the shunt capacitance C, the series inductance L, and the shunt conductance G, and H is, [Math 1] The method according to claim 1, as determined by [the specified method].

4. The maximum power transmission efficiency parameter H is calculated based on the AC transmission line parameter set, the attenuation constant α, the attenuation constant β of the propagation constant γ, the series resistance R of the power line, and the length I of the power line, and the maximum power transmission efficiency parameter H is, [Math 2] The method according to claim 1, as determined by [the specified method].

5. The maximum power transmission efficiency parameter H for short-distance power lines is determined by the power line parameter set, the operating frequency f, and the power line shunt capacitance C per unit length. ｌｉｎｅ , and estimated based on the length I of the AC transmission line, the maximum transmission efficiency parameter H is, [Math 3] The method according to claim 1, as determined by [the specified method].

6. The required reactive power Q at the receiving end of the AC transmission line (1) ｒｅｑ is obtained based on the maximum power transmission efficiency parameter H and the measured line-to-line voltage at the receiving end of the AC transmission line (1), and Q ｒｅｑ is [Math 4] The method according to claim 1, as determined by [the specified method].

7. A device for operating an AC power line (1), wherein the receiving end of the AC power line is connected to an applied load (5), and the applied load (5) comprises an active power section and a reactive power section, and the device is - Connection to the receiving end of the AC transmission line (1), - At least one shunt reactive power compensation device (2) configured to inject reactive power into the AC transmission line (1), the at least one shunt reactive power compensation device (2) configured to be connected to the receiving end of the AC transmission line (1), - Means for measuring the line voltage of the AC transmission line at the receiving end, -Load Q ｌｏａｄ The system includes means for measuring reactive power to The device further, - Further comprising means for determining the required reactive power to be injected by the shunt reactive power compensation device (2), The required shunt reactive power Q ｓｈｕｎｔ The means for determining the applied load (5) is operated by the method described in any one of claims 1 to 6, wherein the device is configured to determine the applied load (5).

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