Maximum transmission efficiency method

Abstract

It is proposed a method of operating a power line:-the power line comprises a transmitting end and a receiving end; -at least one shunt reactive power compensation device for regulating the reactive power in the power line; means for measuring the line voltage of the power line at the receiving end; means for measuring the reactive power Qload of the load; -means for determining the reactive power required for the shunt reactive power compensation device, said method comprising:-receiving information about parameters of the power line; determining a maximum transmission efficiency parameter H for the power line as a function of the parameters of the power line; measuring the line voltage at the receiving end of the power line; -determining the required reactive power Qreq from the parameter H and the measured line voltage at the receiving end of the power line; measuring the reactive power Qload of the load; -determining the required shunt reactive power, Q < shunt >, as a function of Q < req > and the measured reactive power, Q < load >, of the load; -adjusting the shunt reactive power compensation device to deliver the Qshunt to the power line to achieve a maximum transmission efficiency of the power line.

Description

Technical Field

[0001] The present invention relates to a method of operating a power line in a system wherein the power line will operate at its respective maximum efficiency under any applied load. Background Art

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

[0003] Modern life is inseparable from electricity. Once electricity is generated, it must be transmitted from the source to the consumer through the power transmission and distribution system. The power lines in the power transmission and distribution system can be overhead lines, underground cables, submarine cables, or a combination thereof. Power losses occur at every stage of the power transmission and distribution system, and line losses are the main cost in the delivery of electrical energy. Therefore, improving the transmission efficiency of the power system is of great significance for saving energy, saving costs, and overall reducing emissions associated with delivering electricity.

[0004] Electrical energy is primarily generated, transmitted, distributed, and utilized as alternating current (AC). With respect to the transmission of alternating current, there are losses associated with both active power P and reactive power Q. Changes in reactive power affect the voltage, current, and phase shift between voltage and current on the power transmission line, which in turn affects power losses.

[0005] There are methods to improve the efficiency of AC power transmission lines. Power factor control is the most common and proven technique. The goal of this technique is to improve the power factor (reduce reactive power) and thus improve the efficiency of power line transmission. However, a high power factor is not always the best way to improve line 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 easy to determine how much reactive power a system needs. The demand for reactive power changes as the system load changes.

[0007] Reactive power compensation technology is widely used to improve power system transmission efficiency and performance. In practice, the goal is to improve the power factor (reduce reactive power), which is considered to improve power line transmission efficiency.

[0008] Based on the reasons explained above, a solution for improving the power line transmission efficiency is proposed, wherein the power line will always operate at the corresponding maximum transmission efficiency under any given load condition.

[0009] Theoretical Background of the Invention

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

[0011] In theory, energy loss only occurs in resistors, not pure inductors or capacitors. However, all power lines have shunt capacitance that generates capacitive currents that result in higher resistive losses. The following equation explains the power loss P L , the relationship between current and resistance.

[0012] P L =I 2 R

[0013] The voltage, current, and phase shift between the voltage and current on the power line depend on both the load (including active power P and reactive power Q) and the power line characteristics. That is, the power line loss is affected by both the load and the power line characteristics. Therefore, changing the reactive power on the output side of the power line affects the power line loss for a given active power P delivered to the consumer (load).

[0014] To illustrate the calculation of the maximum transmission efficiency of this method, this chapter will describe some power line calculation and modeling methods.

[0015] There are four basic parameters for power line modeling and analysis: series resistance (R), series inductance (L), shunt capacitance (C), and shunt conductance (G). Figure 1 As shown, a passive component model of both ends of an AC power line has been used to illustrate the power transfer characteristics.

[0016] The model represents a single-phase power line. The sending end represents the power source and the receiving end represents the consumer, U s and I s is the voltage and current at the sending end, and U r and I r is the voltage and current at the receiving end. The relationship between the number of transmitters and the number of receivers can be written as:

[0017]

[0018] as well as

[0019] U s =A*U r +B*I r

[0020] I s =C*U r +D*I r

[0021] The parameters A, B, C, D depend on the power line constants R, L, C and G, which are all complex values.

[0022] The line constants R, L, C and G are usually per-length values ​​with units of Ω / km, H / km, f / km and S / km, and are evenly distributed along the length of the line. Several important parameters can be calculated from these four line constants.

[0023] Series impedance per unit length

[0024] Series impedance per unit length

[0025] Characteristic impedance

[0026] Propagation Constant

[0027] The propagation constant γ is complex valued and can be expressed as follows:

[0028] γ=α+jβ, propagation constant

[0029] α-decay constant

[0030] β-phase constant

[0031] The parameters A, B, C, and D for a two-port network are:

[0032] A = cosh(yl), per unit

[0033] B=sinh(yl)Z c ,Ω

[0034] Siemens

[0035] D = A, per unit

[0036] Where l is the length of the electric line.

[0037] The voltage and current in complex quantity form are:

[0038]

[0039] in, is the absolute or RMS value of the voltage, and is the voltage phase angle

[0040]

[0041] in is the absolute or RMS value of the voltage, and is the voltage phase angle

[0042]

[0043] is the phase difference between voltage and current

[0044] as well as

[0045]

[0046] The complex power (apparent power) in the AC system is calculated according to the following formula:

[0047]

[0048] where V and I are complex-valued voltage and current, and and is the conjugate of the current and voltage (complex value).

[0049] Prior art disclosure

[0050] EP2093855A2 teaches systems and methods for dynamic reactive power support of power transmission systems. In one embodiment, the method for dynamic reactive power support of a power transmission system includes connecting at least one circuit breaker of a shunt capacitor bank, the shunt capacitor bank comprising at least one shunt capacitor connected in parallel with a metal oxide varistor (MOV). The controller is used to detect a voltage drop at a power transmission system (such as a substation bus). In response to detecting a voltage drop, the controller closes the circuit breaker connecting the shunt capacitor to the power transmission system. The controller then monitors at least one MOV to detect current flow. After detecting current flow 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 can be disconnected simultaneously, sequentially, in groups, or otherwise.

[0051] CN113346511A teaches a reactive power compensation correction method. The method includes the following steps: measuring the impedance value at different frequencies on both sides of the power frequency; and comparing the impedance value measured in the previous step to perform compensation correction. In the whole process, there is no need to measure the phase angle difference between voltage and current. This correction method is different from the existing traditional compensation method, and does not require the measurement of the phase angle difference between voltage and current, which simplifies the measurement scheme, reduces equipment cost, reduces failure rate, and improves stability.

[0052] EP3220503A1 teaches a power cable assembly for transmitting high voltage alternating current over long distances. The assembly comprises a continuous power cable, wherein each power cable comprises a core wire, a conductor and a cable insulation. It also comprises a shunt inductor connected to each conductor at regular intervals along each conductor. The present invention also comprises a method for providing a power cable assembly for transmitting high voltage alternating current over long distances by means of a continuous power cable, wherein each power cable comprises a core wire, a conductor and a cable insulation. The method is characterized in that a shunt inductor is connected to each conductor at regular intervals.

[0053] US2005040655A1 teaches real and reactive power control for a wind turbine generator system. The technology described herein provides the possibility of providing dynamic VAR (reactive power support) using the total capacity of a wind turbine generator system (such as a wind farm). The VAR support provided by a single wind turbine generator in the system can be dynamically changed to suit application parameters.

[0054] CN110829455A teaches a reactive power compensation method for capacitors in an electric power distribution network, comprising the following steps: determining the optimal compensation point in a reactive power margin manner, online monitoring the voltage at the compensation point, and controlling the switching of capacitors with the voltage as a constraint, thereby dynamically adjusting the compensation capacity; and performing capacitor compensation for the voltage drop caused by the impact load, analyzing the harmonic impact caused by the lag of the parallel capacitor when the capacitor is connected to the electric power distribution network, and connecting a reactor in series with the capacitor to suppress the amplification effect of the parallel capacitor on the harmonics, so as to reduce the impact of the load impact on the power grid.

[0055] US2013134789A1 teaches a reactive power compensation method, comprising generating a variable power factor curve for at least one electric generator based on information about network parameters; obtaining a value of an active output power parameter from at least one generator; calculating reactive power based on the variable power factor curve and the value of the active power output parameter of at least one generator; generating a reactive power compensation instruction based on the calculated reactive power; and transmitting the reactive power compensation instruction to at least one electric generator to control the operation of the at least one electric generator.

[0056] Purpose of the invention

[0057] The object of the present invention is to propose a method for improving the transmission efficiency of any type of AC power line, so that the power line can always operate at the corresponding maximum transmission efficiency under any load condition. Therefore, compared with the prior art, the present invention has the advantage of enabling the AC power transmission line to operate continuously at the optimal operating point under different load conditions. This will increase the transmission capacity, reduce the transmission costs, and reduce the emissions related to the delivery of electricity to the consumer. Summary of the invention

[0058] One aspect of the invention is a method for operating a power line:

[0059] -The power line includes: a transmitting end and a receiving end;

[0060] - at least one shunt reactive power compensation device for regulating reactive power in the power line;

[0061] - means for measuring the line voltage of the power line at the receiving end;

[0062] - Used to measure the reactive power Q of the load load Devices;

[0063] - means for determining the reactive power required to shunt reactive power compensation equipment;

[0064] The method comprises:

[0065] - receiving information about parameters of the power line;

[0066] - determining a maximum transmission efficiency parameter H for the power line according to the parameters of the power line;

[0067] - measuring the line voltage at the receiving end of the power line;

[0068] - Determine the required reactive power Q based on the parameter H and the measured line voltage at the receiving end of the power line req ;

[0069] -Measure the reactive power Q of the load load ;

[0070] -According to Q req and the measured reactive power Q of the load load , determine the required shunt reactive power Q shunt ;

[0071] -Regulate the shunt reactive power compensation device to deliver Q to the power line shunt , in order to achieve the maximum transmission efficiency of the power lines.

[0072] In an embodiment of the present invention, H is determined according to a set of power line parameters: resistance R per unit length, capacitance C per unit length, inductance L per unit length, length, and frequency.

[0073] In an embodiment of the present invention, the shunt reactive power compensation device is connected to the power line receiving end.

[0074] In an embodiment of the present invention, the parameter H is calculated based on a set of power line parameters ABCD for a power line two-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, where H is determined by the following formula:

[0075]

[0076] In an embodiment of the present invention, the parameter H is calculated based on a set of power line parameters, namely, the attenuation constant α, the attenuation constant β of the propagation constant γ, the series resistance R of the power line and the length l of the power line, wherein H is determined by the following formula:

[0077]

[0078] In an embodiment of the present invention, the parameter H for the short-distance power line is based on a set of power line parameters, namely, the operating frequency f, the power line shunt capacitance C per unit length, line and the power line length l, where H is determined by:

[0079] H=π*f*c line *l

[0080] In the embodiment of the present invention, the reactive power Q required at the receiving end of the power line req is determined from the parameter H and the measured line voltage at the receiving end of the power line, where Q req Determined by the following formula:

[0081]

[0082] In an embodiment of the invention, the device comprises a connection to a power line receiving end, the device comprising:

[0083] - at least one shunt reactive power compensation device for injecting reactive power into the power line;

[0084] - means for measuring the line voltage of the power line at the receiving end;

[0085] - Used to measure the reactive power Q of the load load Devices;

[0086] - means for determining the required reactive power to be injected by the shunt reactive power compensation device;

[0087] The device for determining the required reactive power is determined by the method described in the embodiment of the present invention. In the embodiment, the shunt reactive power compensation device is connected to the receiving end of the power line. BRIEF DESCRIPTION OF THE DRAWINGS

[0088] Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:

[0089] Figure 1 A two-port passive component model of a single-phase power line is shown.

[0090] Figure 2 An arrangement according to the method is shown.

[0091] Figure 3 The overall transmission efficiency is shown for a 20 km, 33 kV underground distribution cable with active power between 5-20 MW and power factor between 0.2 and 1. A color plot of the two variables is shown, along with the solution corresponding to the maximum transmission efficiency for the best power factor at a given load.

[0092] Figure 4 The theoretical maximum efficiency graph is shown, as well as the Figure 3 Efficiency diagram of the method for 33kV underground distribution cables.

[0093] Figure 5 The overall transmission efficiency of a 150 km, 150 kV submarine transmission cable is shown for active powers between 20-100 MW and power factors between 0.2 and 1. A colored graph of the two variables is shown, along with the curve corresponding to the maximum transmission efficiency for the best power factor at a given load.

[0094] Figure 6 The theoretical maximum efficiency graph is shown, as well as the Figure 5 Efficiency diagram of the approach of a 150kV submarine transmission cable. DETAILED DESCRIPTION

[0095] Figure 1 The passive component model of the two ends of the AC power line described earlier in this application is shown. The mathematical equation for calculating the value of the maximum transmission efficiency parameter H is derived from the model. By using the equation according to the model:

[0096]

[0097] And the apparent power S at the transmitter is obtained:

[0098] is the phase shift between voltage and current at the output side. Rearranged and simplified:

[0099]

[0100] The apparent power can be expressed as:

[0101] S s =P s +1j*Q s

[0102] And the active power:

[0103]

[0104] Use the following relationship:

[0105]

[0106] We express the active power at the sending end as:

[0107]

[0108] The power loss of the power line can then be expressed as:

[0109]

[0110] By assuming that the operating voltage and the actual power at the receiving end are constant, we can express the power loss of the power line as the reactive power Q at the receiving end r Function:

[0111] ΔP=f(Q r )

[0112]

[0113] Where K is a constant:

[0114]

[0115] For the power loss function Q r Find the derivative:

[0116]

[0117] When f′(Q r )=0, the minimum power loss ΔP occurs:

[0118]

[0119] We set H as the multiplier, where Vr abs 2 is a power, and Q r is the product:

[0120]

[0121] Parameter H is called the maximum transmission parameter for the power line. Since the power line parameters B, C, D are constant for a specific power line operating at a specific operating frequency, parameter H is constant for a specific power line operating at a specific operating frequency.

[0122] Based on the above derivation, we can conclude that the reactive power required to control the power line to operate at the corresponding maximum efficiency point is given by the following equation:

[0123]

[0124] An expression for H can also be given in terms of the decay α and phase β constants.

[0125]

[0126] Substituting the following formula into the original equation for H, we can get the above expression:

[0127]

[0128] as well as:

[0129]

[0130] For short power lines, the parameter H is very close to half of the total power line resistance. Therefore, the following formula H~πfC can be used cablel To approximate the parameter H, where f is the frequency and C cable is the cable capacitance per length, and l is the cable length. Therefore, the parameter H is given by the following equation:

[0131] For short lengths of power lines

[0132] The reactive power required at the receiving end of the power line is given by the following equation, where, is the square of the RMS value of the line voltage at the receiving end:

[0133]

[0134] The value of the required shunt reactive power is given by the following formula:

[0135] Q shunt =Q req -Q load

[0136] The reactive power provided by the shunt is equal to the reactive power required at the receiving end of the power line minus the reactive power of the load.

[0137] Therefore, the method comprises the following steps: determining a parameter H according to the power line constant. Then, calculating the required reactive power at the receiving end according to H and the measured voltage. Then, determining the shunt reactive power according to the calculated required reactive power and the measured load reactive power. Then, the method adjusts the shunt reactive power compensation device so that the power line reaches the corresponding maximum transmission efficiency.

[0138] There are many technologies for regulating the reactive power of power lines, such as shunt reactors, shunt capacitors, SVC, STATCOM, synchronous capacitors, 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 and can even change during operation. The technician can select the preferred reactive power compensation technology based on the operating requirements of the consumer (load) and the application.

[0139] Figure 2 The arrangement for carrying out the steps of the method is shown in FIG. 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 to the power line. In this example, the shunt reactive power compensation device, the voltage measurement device and the reactive load measurement device are connected at the receiving end of the power line near the load. This is an embodiment of the present invention, and these components can be connected at any position along the power line, and more groups of these components can also be connected at different sections of the power line.

[0140] Figures 3 to 6 Two examples are given in Figure 1 to demonstrate the effectiveness of this method. Both examples are three-phase AC power lines, the first one is a 33kV underground cable for urban power distribution system, and the second one is a 150kV submarine cable for power transmission system. Table 1 lists the data and parameters of the power lines.

[0141] Table 1: Example power cable data

[0142]

[0143]

[0144] Numerical analysis is performed to study the theoretical power transmission efficiency of the power cable under different load conditions. The power transmission efficiency using the method of the present invention is also analyzed.

[0145] Figure 3A 20km underground distribution power cable is simulated in FIG. In the simulation, the load has an operating voltage (Un) that remains constant at 33kV. The figure shows a graph of the overall transmission efficiency, a transmission efficiency color map, and a theoretically corresponding maximum power transmission efficiency curve for loads of 5MW to 25MW and power factors of 0.2 to 1.

[0146] Figure 4 The transmission efficiency curve according to the simulation and the theoretical corresponding maximum transmission efficiency curve are shown, which confirms that by this method, under any load condition, the power line operates at the corresponding maximum transmission efficiency point.

[0147] Table 2 shows the values ​​of the theoretically corresponding maximum transmission efficiency and the values ​​of the transmission efficiency according to the method of the present invention, for loads ranging from 5 MW to 25 MW, wherein 1 MW is used in each step in the table.

[0148] Table 2: 33 kV underground distribution cable, theoretical maximum transmission efficiency and transmission efficiency according to this method

[0149]

[0150]

[0151] Figure 5 A 150 km submarine transmission power cable has been simulated in Figure 1. In the simulation, the load has a rated operating voltage (Un) of 150 kV. The figure shows the overall transmission efficiency, the transmission efficiency color map and the theoretically corresponding maximum power transmission efficiency curve for loads from 20 MW to 100 MW and power factors from 0.2 to 1.

[0152] Figure 6 The transmission efficiency curve according to the simulation and the theoretical corresponding maximum transmission efficiency curve are shown, which confirms that by this method, under any load condition, the power line operates at the corresponding maximum transmission efficiency point.

[0153] Table 3 shows the values ​​of the theoretically corresponding maximum transmission efficiency and the values ​​of the transmission efficiency according to the method of the present invention, for loads ranging from 20 MW to 100 MW, wherein 5 MW is used in each step in the table.

[0154] Table 3: 150 kV submarine transmission cable, theoretical maximum transmission efficiency and transmission efficiency according to this method

[0155]

[0156]

Claims

1. A method for operating a power line: - The power line comprises: The sender and receiver; - at least one shunt reactive power compensation device for regulating reactive power in said power line; - means for measuring the line voltage of said power line at said receiving end; - Used to measure the reactive power Q of the load load Devices; - means for determining the reactive power required by the shunt reactive power compensation device; The method comprises: - receiving information about parameters of said power line; -determining a maximum transmission efficiency parameter H of the power line according to the parameters of the power line; - measuring the line voltage at the receiving end of the power line; - Determine the required reactive power Q based on the parameter H and the measured line voltage at the receiving end of the power line req ; -Measure the reactive power Q of the load load ; -According to Q req and the measured reactive power Q of the load load , determine the required shunt reactive power Q shunt ; - Adjust the shunt reactive power compensation device to deliver Q to the power line shunt , in order to achieve the maximum transmission efficiency of the power line.

2. The method according to claim 1, wherein: The parameter H is determined according to a set of power line parameters: resistance R per unit length, capacitance C per unit length, inductance L per unit length, length and frequency.

3. The method according to claim 1, wherein: The shunt reactive power compensation device is connected to the receiving end of the power line.

4. The method according to claim 1, wherein: The parameter H is calculated based on a set of power line parameters ABCD for the power line two-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, where H is determined by the following formula:

5. The method according to claim 1, wherein: The parameter H is calculated based on a set of power line parameters, namely, the attenuation constant α, the attenuation constant β of the propagation constant γ, the series resistance R of the power line and the length l of the power line, where H is determined by the following formula:

6. The method according to claim 1, wherein: The parameter H for short-distance power lines is based on a set of power line parameters, namely, the operating frequency f, the power line shunt capacitance C per unit length line and the length l of the electric line, where H is determined by the following formula: H=π*f*c line *l 7. A method according to any one of the preceding claims, wherein: Reactive power required at the receiving end of the power line Q req is determined based on the parameter H and the measured line voltage at the receiving end of the power line, where Q req Determined by the following formula:

8. A device for operating a power line, wherein: The device comprises a connection to a receiving end of the power line, the device comprising: - at least one shunt reactive power compensation device for injecting reactive power into said power line; - means for measuring the line voltage of said power line at said receiving end; - Used to measure the reactive power Q of the load load Devices; - means for determining the required reactive power to be injected by said shunt reactive power compensation device; The device for determining the required reactive power is determined according to the method according to any one of claims 1 to 7.

9. The device according to claim 8, wherein: The power line comprises a receiving end, wherein the shunt reactive power compensation device is connected to the power line receiving end.

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