An energy acquisition system for the ground wire of an overhead transmission line can provide a continuous and stable power supply for an on-line monitoring device. Its key issue is how to obtain enough power. To solve this problem, an energy acquisition scheme based on the double-insulated ground wire of an overhead transmission line has been investigated in this study. Three energy acquisition schemes were proposed, equivalent circuit analysis models of the three energy acquisition schemes were established, and the maximum power acquired was theoretically analyzed. The energy acquisition power of the three energy acquisition schemes for different tower-type sizes was also analyzed. A simulation model was built in PSCAD. The effects of load impedance, length of energy acquisition wire, grounding resistance, and load current on the power acquired were analyzed. The research results of this paper provide theoretical guidance for choosing an energy acquisition scheme and for designing key parameters in practice.
With the increasing dependence of society on electric energy, the reliability requirements of stable power grid operation are becoming ever more demanding, and hence, a large number of on-line monitoring devices have been put into operation on transmission lines. Due to the lack of a continuous and stable low-voltage power supply, on-line transmission line monitoring devices are usually powered by solar cells, small wind turbines, and batteries. The stability of this power supply is greatly affected by the weather, and the output power is small, the volume of the device is large, and the economy is poor. The need for a power supply with sufficient power and continuous stability has become the development bottleneck of on-line transmission line monitoring devices. According to the principle of electromagnetic induction, there is induction voltage on the overhead ground wire [
At present, some researchers have carried out studies on energy acquisition technology for overhead transmission lines. References [
However, none of the above studies addressed the selection of a ground wire or the installation scheme of a ground wire-based energy acquisition system, and all ignored the influence of tower size on the power of the energy acquisition system. References [
This study assumes that the key to the feasibility of ground wire energy acquisition lies in the adequacy of the power acquired. In this paper, the energy acquisition scheme and the parameter selection for an energy acquisition system for double-insulated ground wire were studied. Three energy acquisition schemes were proposed, and their equivalent circuit analysis models were established. The maximum power of the energy acquisition system was theoretically analyzed. The energy acquisition power of the three energy acquisition schemes for different tower-type sizes was compared and analyzed. A simulation model of a double-insulated ground wire energy acquisition system was built in PSCAD, and a simulation analysis was carried out. The effects of load impedance, length of energy acquisition wire, grounding resistance, and load current on the power acquired and the energy acquisition scheme were analyzed.
The alternating current in the transmission line conductor generates an alternating electromagnetic field in space, and the ground wire in the alternating electromagnetic field generates an induced electromotive force. Induced current can be formed on the ground wire when the ground wire forms a closed loop with the earth or another ground wire. There is also a coupling capacitor between the ground wire and the conductor, and an electrostatic induction voltage can be generated on the ground wire. Because the ground wire is grounded, the electrostatic induction voltage is 0. Therefore, the electrostatic induction effect can be ignored.
Figure
Equivalent circuit diagram of a double-segmented insulated ground wire.
The formulae for calculating the induced electromotive force on the ground wire are
The equivalent impedance of the ground wire can be calculated as
For a system where the two ground wires are both segmented insulated ground wires, there are three schemes for installing the energy acquisition device.
As shown in Figure
Schematic diagram of energy acquisition scheme I.
Equivalent circuit diagram of energy acquisition scheme I.
As shown in Figure
Schematic diagram of energy acquisition scheme II.
Equivalent circuit diagram of energy acquisition scheme II.
As shown in Figure
Schematic diagram of energy acquisition scheme III.
Equivalent circuit diagram of energy acquisition scheme III.
To perform a theoretical analysis of the power acquired by an energy acquisition device, a mathematical model of the energy acquisition circuit must be established. According to the equivalent circuit of each energy acquisition scheme described earlier, its mathematical model can be established using the Thevenin equivalence principle.
According to Figure
The power acquired by the energy acquisition device is
According to the impedance matching principle, when the load impedance
According to Figure
The maximum acquisition power is derived, and the formula for the maximum power is as follows:
According to Figure
The maximum acquisition power is derived, and the formula for the maximum power of energy acquisition scheme III is as follows:
To obtain the maximum power from the energy acquisition device, this paper studies the influence of the spatial position relationship between the ground wire and the conductor, the load impedance, the length of energy acquisition wire, the grounding resistance, and the load current on the energy acquisition scheme.
In different tower-structure types and sizes, the spatial relationship between the ground wire and the conductor is different, resulting in different induced voltages on the ground wire. By comparing the power of different energy acquisition schemes under different positional relationships between the ground wire and the conductor, a position space can be divided into four regions, as shown in Figure
Spatial position relationship between ground wire and conductor and the power acquired.
To study the influence of load impedance, length of energy acquisition wire, grounding resistance, and load current on the energy acquisition scheme, four kinds of tower with different head size were selected in this study, with the spatial positions of ground wire and conductor belonging to the four regions in Figure
The voltage level of the transmission line is 110 kV. The two ground wires are segmented insulated ground wires with the same parameters. The resistance per unit length of ground wire
Simulation model of transmission line.
Simulation model of ground wire energy acquisition.
To study the influence of different load impedances on the electric power acquired under different schemes, it is assumed that the load impedance of the energy acquisition device varies between 1 and 50
Variation of power with load impedances in different cases.
The equivalent power supply internal resistances of scheme I and scheme III are small, but the equivalent power supply internal resistance of scheme II is larger. In different tower types, the load impedance has different effects on the choice of energy acquisition scheme. For example, in Case
For the case where the two ground wires are insulated ground wires, the power of the energy acquisition device is independent of the span between the two towers and is only related to the length of the wires between the grounding point and the power collection point (i.e., the length of the energy acquisition wire). Assuming that the length of the energy acquisition wire varies between 0.1 and 10 km, the maximum power curve for each energy acquisition scheme is as shown in Figure
Variation of power with length of energy acquisition wire in different cases.
In different tower types, the length of the energy acquisition wire has different effects on the choice of energy acquisition scheme. For example, in Case
To study the influence of the grounding resistance of the tower on the maximum power acquired by the energy acquisition device, it is assumed that the grounding resistance varies between 2 and 50
Variation of power with grounding resistance of the tower in different cases.
The grounding resistance of the tower has an influence on the choice of energy acquisition scheme. Because the influence of grounding resistance on each energy acquisition scheme is different, the optimal energy acquisition scheme under different grounding resistances is different. For example, in Case
To study the influence of transmission line current on the maximum power of the energy acquisition device, it is assumed that the load current varies between 30 and 300 A. The maximum power curve under each energy acquisition scheme is shown in Figure
Variation of power with load current in different cases.
According to the above analysis, the energy acquisition scheme for a double-insulated ground wire is affected by the spatial positional relationship between the ground wire and the conductor, the load impedance, the length of the energy acquisition wire, and the grounding resistance of the tower. Therefore, in practical application, according to these relationships, it is necessary to consider comprehensively the impedance of the energy acquisition system and the length of the energy acquisition wire and on this basis to select the installation scheme for the energy acquisition device.
In practical application, for a transmission line that has already been put into operation, its tower type, distance between wires, and operating parameters have been determined. Therefore, when designing an energy acquisition device, it is only necessary to consider the load impedance, the length of the energy acquisition wire, and the grounding resistance of the tower to select the energy acquisition scheme and parameters. They can be determined in turn according to the following steps.
First, select the length of the energy acquisition ground wire according to the required power and the curve of the power with the length of the energy acquisition wire. Assume that in Case
Secondly, according to the grounding resistance of each tower and the variation curve of the power with the grounding resistance of the tower, select another tower in the energy acquisition loop. It can be known from Figure
Finally, according to the variation curve of the power with the load impedance, the optimal load impedance of the energy acquisition device is selected. It can be known from Figure
A designed energy acquisition device was installed and operated on the actual transmission line. The main parameters of the energy acquisition system were shown in Table
The main parameters of the energy acquisition system.
Key parameters | Value |
---|---|
Spatial position relationship between ground wire and conductors | |
Average load current of the transmission line | 80 A |
Length of the energy acquisition ground wire | 1.187 km |
Grounding resistance of tower whose ground wire shorted | 9.4 |
Load resistance of the energy acquisition device | 8.6 |
Designed power | 2.08 W |
The field-installed energy acquisition device and its daily average power acquired.
In this study, an energy acquisition scheme and its parameter selection in the case of double-insulated ground wire have been considered. According to the characteristics of double-insulated ground wire, three energy acquisition schemes have been proposed, and equivalent circuit analysis models for the three schemes have been developed. The maximum power acquired by the three energy acquisition schemes has also been analyzed. The power acquired by the three energy acquisition schemes under different tower sizes has been compared and analyzed. The research results show that the relative spatial position of the ground wire and the conductor has a great influence on the power acquisition of the ground wire. Different spatial relationships between ground wires and conductors can change the optimal scheme for energy acquisition.
A simulation model of the double-insulated ground wire-based energy acquisition system was built in PSCAD, and a simulation analysis was carried out. The effects of load impedance, length of energy acquisition wire, grounding resistance, and load current on the power acquired were analyzed. The conclusions obtained are as follows:
When the load impedance is equal to the internal impedance of the equivalent power source, the power of the energy acquisition system reaches the maximum As the length of the energy acquisition wire increases, the maximum power increases, and the speed of increase is related to tower size The maximum power of scheme III is not affected by grounding resistance, and the maximum power of scheme II decreases with increasing grounding resistance. However, the maximum power variation of scheme I is complex and is related to tower size The maximum power of the three energy acquisition schemes increases with increasing load current, and the choice of energy acquisition scheme is not affected by the magnitude of the load current In different tower types, the load impedance, length of energy acquisition wire, and grounding resistance have different effects on the choice of energy acquisition scheme. In practical applications, it is necessary to consider comprehensively the factors described above to select the energy acquisition scheme and its parameters
The data used to support the findings of this study are included within the article.
The authors declare that there is no conflict of interest regarding the publication of this paper.
This research was funded by the science and technology project of State Grid Hunan Electric Power Company Limited (grand number: 5216AF18000A).