Ice melting device for power distribution network line

Abstract

The invention belongs to the technical field of power distribution network line ice melting, and particularly relates to a power distribution network line ice melting device. A power distribution network line ice melting device comprises a common direct current bus, an AC / DC unit used for converting alternating current into direct current and a DC / AC unit used for adjusting alternating current output voltage. The direct current side of the AC / DC unit is connected with the direct current side of the DC / AC unit through the common direct current bus; the alternating current side of the AC / DC unit and the alternating current side of the DC / AC unit are respectively used for connecting three-phase alternating current at two ends of a line to be de-iced in a power distribution network line, so that when the line to be de-iced is iced, the output current of the de-icing device is adjusted to enable the current of the line to be de-iced to be not lower than the current of the critical de-icing current. The technical problems that in the prior art, the ice melting effect of a power distribution network line is poor, and the equipment utilization rate is low are solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of ice melting of distribution network lines, and in particular relates to an ice melting device for distribution network lines. Background Art

[0002] In recent years, the frequency and prevalence of icing incidents on distribution network conductors have been on the rise, driven by extreme weather and the expansion of distribution networks. Existing distribution network de-icing primarily relies on manual methods. As the problem of line icing becomes increasingly severe, some networks have introduced transformer de-icing or added de-icing devices. However, the de-icing effect and overall improvement of the distribution network remain insufficient.

[0003] Transformer ice melting is to connect the AC transformer in series to the head end of the ice melting line to provide a suitable voltage source for the line and perform AC short circuit ice melting. Figure 1 AC transformers used for ice melting include dedicated ice melting transformers, modified transformers, and distribution transformers. When ice is detected on a line, the circuit breaker is closed at the line headend, and the upper system, acting as a power source, provides a high short-circuit current to heat the conductors and remove the ice. Furthermore, in low-voltage distribution networks, the A, B, and C lines can be connected in series and grounded at their ends. One phase on the power supply side is directly grounded, while the other phase operates at no load. This creates a ground short circuit between the two phases, using the earth as a path. This method reduces short-circuit current by increasing line impedance.

[0004] In the scheme of using ice melting device to melt ice, SVG (Static Var Generator) is often used to achieve it, such as Figure 2 As shown in the figure, the SVG-based live ice melting method uses a method of emitting reactive power at one end of the line and absorbing reactive power at the other end. The transmission current of the line is increased according to the degree of ice coverage, and the line impedance is used to achieve the purpose of heat and ice melting. At the same time, the grid voltage is maintained unchanged, and user power consumption is not affected, thus achieving ice melting.

[0005] However, using transformers for de-icing presents significant challenges in controlling short-circuit currents. Prolonged short-circuit currents can significantly impact the transformers themselves, leading to higher transformer failure rates and increased grid operation and maintenance costs. Furthermore, dedicated de-icing transformers are expensive but have low utilization rates, further increasing costs. Using SVGs for de-icing, while improving equipment utilization, remains ineffective in heavily iced environments due to the limited capacity of the SVG's DC side and the distance to the distribution network. Summary of the Invention

[0006] The object of the present invention is to provide a distribution network line de-icing device to solve the technical problems in the prior art of poor de-icing effect and low equipment utilization of distribution network lines.

[0007] To solve the above-mentioned technical problems, the present invention provides a technical solution for a distribution network line de-icing device: a distribution network line de-icing device comprising a common DC bus, an AC / DC unit for converting AC to DC, and a DC / AC unit for adjusting the AC output voltage; the DC side of the AC / DC unit and the DC side of the DC / AC unit are connected via the common DC bus; the AC side of the AC / DC unit and the AC side of the DC / AC unit are respectively used to connect to three-phase AC power at both ends of a line to be de-iced in the distribution network line, so that when the line to be de-iced is covered with ice, the output current of the de-icing device is adjusted to ensure that the current of the line to be de-iced is not less than the critical de-icing current.

[0008] The beneficial effect of the above technical solution is that the technical solution of the distribution network line de-icing device of the present invention is a conversion-type invention. The present invention applies the existing AC / DC-DC / AC topology structure to the distribution network line de-icing, which can not only de-ice the line in the ice-covered state, but also realize the interconnection of different distribution network lines in the un-iced state to realize the function of load transfer or flow control. Moreover, compared with the existing method of SVG emitting reactive power at one end of the line and absorbing reactive power at the other end to melt the ice, the de-icing device of the present invention can be directly connected in series in the line instead of at both ends of the line, is not limited by the DC side capacity, and can achieve better de-icing effect. When the line is in the ice-covered state, the de-icing device of the present invention can quickly respond to output a de-icing current that is not less than the critical de-icing current, and uses the line impedance to generate heat to achieve de-icing. The present invention solves the technical problems of poor de-icing effect and low equipment utilization in the existing distribution network line.

[0009] Furthermore, the critical ice-melting current is determined according to parameters of the line to be ice-melted and parameters of an external environment of the line to be ice-melted.

[0010] Furthermore, the external environmental parameter includes at least one of ice layer surface temperature, ambient temperature and wind speed.

[0011] Furthermore, the critical ice melting current is determined according to the following formula:

[0012]

[0013] Among them, I C is the critical ice melting current; R L is the equivalent resistance of the line to be de-iced; E is the radiation heat dissipation coefficient; S is the Stefan-Boltzmann number; A is the convection cross-sectional area; h is the convection coefficient determined according to the wind speed; T i is the surface temperature of the ice layer; T e is the ambient temperature.

[0014] Furthermore, the critical ice melting current is determined according to the following formula:

[0015]

[0016]

[0017] Among them, I C is the critical ice melting current; R L is the equivalent resistance of the line to be de-iced; T i is the surface temperature of the ice layer; K i is; β is the cylindrical shape coefficient; r i is the radius of the line to be melted after ice cover; r c The radius of the line to be melted before ice accumulation.

[0018] Furthermore, the AC / DC unit and / or the DC / AC unit is a three-level structure.

[0019] Furthermore, each bridge arm on the AC side of the AC / DC unit and / or the DC / AC unit includes four fully-controlled devices and two diodes, namely a first fully-controlled device, a second fully-controlled device, a third fully-controlled device, a fourth fully-controlled device, a first diode, and a second diode; the first fully-controlled device, the second fully-controlled device, the third fully-controlled device, and the fourth fully-controlled device are connected in series in sequence; the anode of the first diode is connected to the cathode of the second diode to form a diode branch; the diode branch is connected in parallel at both ends of the circuit formed by the second fully-controlled device and the third fully-controlled device; the direction of the first diode and the second diode is consistent with the direction of the anti-parallel diodes in the fully-controlled devices;

[0020] The series connection point of the diode branch of each bridge arm on the AC side of the AC / DC unit and / or the DC / AC unit and the neutral point of the DC bus are connected together; a first capacitor is connected between the positive electrode of the DC side of the AC / DC unit and / or the DC / AC unit and the neutral point, and a second capacitor is connected between the negative electrode and the neutral point.

[0021] Furthermore, it also includes a soft start circuit arranged at the incoming end and / or outgoing end of the distribution network line; the soft start circuit includes a circuit breaker connected in series to the corresponding incoming end or outgoing end of the distribution network line and a soft start branch connected in parallel at both ends of the circuit breaker; the soft start branch includes a current limiting resistor and a soft start switch connected in series.

[0022] Furthermore, the DC / AC unit adopts a V / F control method. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is the topology of the transformer de-icing solution in the existing technology;

[0024] Figure 2The topology of the SVG ice melting device in the prior art;

[0025] Figure 3 The topology of the ice melting device of the distribution network line ice melting device embodiment of the present invention;

[0026] Figure 4 This is an operating logic diagram of an ice melting device according to an embodiment of the present invention. DETAILED DESCRIPTION

[0027] The present invention applies the existing AC / DC-DC / AC topology to de-icing distribution network lines. This not only allows for de-icing of lines in ice-covered conditions, but also enables interconnection of different distribution network lines in un-iced conditions to achieve load transfer or flow control. Furthermore, compared to the prior art method of SVG emitting reactive power at one end of a line and absorbing reactive power at the other end to achieve de-icing, the de-icing device of the present invention can be connected directly in series within the line rather than at both ends, unrestricted by DC side capacity, and thus achieves better de-icing results. When the line is ice-covered, the de-icing device of the present invention can rapidly respond to output a de-icing current no less than the critical de-icing current, utilizing line impedance to generate heat to achieve de-icing. The present invention addresses the prior art technical issues of poor de-icing results and low equipment utilization in distribution network lines.

[0028] Example of a distribution network line ice melting device:

[0029] A distribution network line de-icing device includes a common DC bus, an AC / DC unit for converting AC to DC, and a DC / AC unit for regulating the AC output voltage; the DC side of the AC / DC unit is connected to the DC side of the DC / AC unit via the common DC bus; the AC side of the AC / DC unit and the AC side of the DC / AC unit are respectively connected to three-phase AC power at both ends of a line to be de-iced in the distribution network, so that when the line to be de-iced is covered with ice, the current of the line to be de-iced is controlled to be no less than a critical de-icing current.

[0030] The AC / DC unit and the DC / AC unit of this embodiment both adopt a three-level topology, and the overall structure is a two-level three-level topology. Figure 3 As shown in the figure, each bridge arm of the system's front-stage three-level AC / DC unit includes four fully controlled devices and two diodes. Figure 3Taking the leftmost bridge arm (corresponding to line terminal A) as an example, it comprises a first fully-controlled device T11, a second fully-controlled device T12, a third fully-controlled device T13, a fourth fully-controlled device T14, a first diode D1, and a second diode D2. The first, second, third, and fourth fully-controlled devices T11, T12, T13, and T14 are connected in series. The anode of the first diode D1 is connected to the cathode of the second diode D2 to form a diode branch. This diode branch is connected in parallel across the circuit formed by the second and third fully-controlled devices T12 and T13. The other bridge arms of the preceding AC / DC unit and the bridge arms of the subsequent DC / AC unit are the same as described above.

[0031] The common DC bus includes capacitors C1 and C2 connected in series. The neutral point of the DC bus (i.e., between capacitors C1 and C2) is connected to the neutral point of the diode branch of each bridge arm on the AC side of the AC / DC unit and the DC / AC unit to form a diode-clamped three-level topology.

[0032] In other implementations, other methods may be used to implement the three-level topology, such as replacing two diodes with a flying capacitor to form a flying capacitor-type three-level topology.

[0033] Specifically, in this embodiment, both the front and rear stages use IGBT fully-controlled devices, with high switching frequency, fast system response rate, and bidirectional power flow. At the same time, both the front and rear stages of the system use a three-level topology. Compared with the traditional two-level topology, the voltage withstand requirement of the power device is only 1 / 2 of the bus voltage, which reduces the difficulty and cost of device selection and facilitates the high power density integration and miniaturization design of the equipment.

[0034] A soft start circuit at the incoming and / or outgoing end of the distribution network line; the soft start circuit includes a circuit breaker connected in series to the corresponding incoming and outgoing ends of the distribution network line (i.e., QF1 at the incoming end and QF2 at the outgoing end) and a soft start branch connected in parallel at both ends of the circuit breaker; the soft start branch includes a current limiting resistor connected in series (i.e., R1, R2, R3 at the incoming end and R4, R5, R6 at the outgoing end) and a soft start switch (i.e., K1 at the incoming end and K2 at the outgoing end).

[0035] Each phase of the line between the AC side of the AC / DC unit and the soft-start circuit at the incoming line is also connected to a capacitor (C1, C2, C3) and an inductor (L1, L2, L3), with a delta connection between the capacitors. Each phase of the line between the AC side of the DC / AC unit and the soft-start circuit at the outgoing line of the distribution network is also connected to a capacitor (C4, C5, C6) and an inductor (L4, L5, L6), with a delta connection between the capacitors.

[0036] The front and rear stages of the ice melting device in this embodiment adopt a common DC bus connection method. The input side of the complete device is connected to the low-voltage switch cabinet of the distribution network. The AC / DC power unit of the front stage realizes AC / DC conversion. The rear stage three-level DC / AC operates in V / F (voltage / frequency) mode to achieve adjustable AC output voltage and frequency.

[0037] When the line is ice-covered, the ice-melting device of the present invention can respond quickly and adjust the output current according to the thickness of the ice, using the line impedance to generate heat to achieve ice melting; when the line is normal, the device can realize grid voltage regulation, substation interconnection and load transfer as an in-grid device.

[0038] like Figure 4 As shown, the operation logic of the ice melting device of this embodiment is as follows:

[0039] 1) First, after receiving the ice melting instruction, the critical ice melting current under the current environmental conditions is calculated in real time using the external temperature, wind speed, humidity, and wire type as inputs, as shown in the following formula:

[0040]

[0041] Among them, I C is the critical ice melting current; R L is the equivalent resistance of the line to be de-iced; E is the radiation heat dissipation coefficient; S is the Stefan-Boltzmann number; A is the convection cross-sectional area; h is the convection coefficient; T i is the surface temperature of the ice layer, in °C; T e is the ambient temperature, in °C.

[0042] The above formula is derived in the following way:

[0043] When current passes through an ice-covered catenary (i.e., distribution network line), according to Joule's law, when the heat generated by the catenary is equal to the heat dissipated by the ice surface, and the interface temperature between the ice and the catenary is equal to 0°C, the critical icing state of the catenary can be maintained. At this time, the current flowing through the catenary is the critical ice-melting current Ic.

[0044] If the surface temperature of the ice layer is Ti, then the ice-catenary interface satisfies:

[0045]

[0046] Among them, I C is the critical ice melting current; R L is the equivalent resistance of the line to be de-iced; T i is the surface temperature of the ice layer; K i is the cylindrical shape coefficient; r i is the radius of the line to be melted after ice cover; r cThe radius of the line to be melted before ice accumulation.

[0047] Similarly, the ice-air interface satisfies:

[0048] T i K i β=-(Q R +Q C )

[0049]

[0050] Q C =Ah(T i -T e )

[0051]

[0052]

[0053]

[0054] Among them, Q R For radiation heat dissipation; Q C is the convective heat dissipation; E is the radiation heat dissipation coefficient, which is taken as 0.9; S is the Stefan-Boltzmann number, which is taken as 5.67×10 -8 W / m 2 ; A is the convection cross-sectional area; h is the convection coefficient; T e is the ambient temperature of the contact network; R e is the Reynolds number; ρ a is the air density, take 1.293kg / m3; V a is the wind speed; k a is the thermal conductivity of air, which is 0.0244W / (m·℃); C a is the specific heat capacity of air, which is 1005 J / kg·℃; μ is the dynamic viscosity coefficient of air, which is 1.88*10 -5 kg / (m·s), coefficients C and n are determined according to the Reynolds number. When 4000≤R e When ≤40000, coefficients C and n are 0.193 and 0.618 respectively. When 40≤R e When ≤4000, the coefficients C and n are 0.683 and 0.466 respectively. Combining the above formulas, the critical ice melting current is:

[0055]

[0056] It can be seen from the above formula that the critical ice melting current is mainly affected by the ice surface temperature T i and ambient temperature T eIf within a specific power supply section, it can be approximately assumed that the convection and radiation heat dissipation coefficients of the contact network-ice-air interface remain unchanged, and the contact network equivalent resistance remains unchanged. Since the surface temperature of the ice layer depends on the ambient temperature around the contact network, the magnitude of the ice melting current mainly depends on the ambient temperature T e .

[0057] The ice-melting command is issued by the superior monitoring system, and the real-time data is collected by the on-site monitoring terminal (such as sensors, image recognition, etc.). The background determines the current ice cover status, and then sends the ice-melting function enable command to the ice-melting device to melt the ice based on the judgment result.

[0058] In other embodiments, it is also possible to directly Get the critical ice melting current I C At this time, in addition to the ice surface temperature T i In addition, it is also necessary to detect the ice thickness of the line so that the ice thickness and the distribution network line radius r can be c Get the radius r of the ice-melting line after ice cover i , and further based on The cylindrical shape coefficient β is obtained, and then the Get the critical ice melting current I C .

[0059] 2) Secondly, determine the current I of the circuit L and critical ice melting current value I C Size, when I L ≥I C When I L <I C When I C The value is the minimum output current instruction of the device, which provides ice melting current to the line.

[0060] 3) Again, when the ice thickness of the line is relatively serious or the actual ice-melting current required is greater than the line load, load transfer or bidirectional power flow based on the device can be achieved to feed the surplus energy back to the grid.

[0061] 4) Finally, under normal line operating conditions, interconnection and mutual supply between different tie lines can be achieved according to the line load, thereby alleviating load peaks, balancing the load, and improving the utilization rate of the remaining capacity of the distribution network.

[0062] The present invention has the following characteristics:

[0063] The ice melting device of the present invention improves the existing ice melting scheme. The equipment can meet the application requirements under low-frequency, short-time ice melting and anti-icing conditions, and can also meet the application requirements under normal working conditions, thereby improving the reliability and utilization rate of the ice melting equipment. The equipment is connected as a flexible interconnection device, while optimizing the distribution network system structure and reducing investment costs.

[0064] In severe cold weather conditions, the device calculates the critical ice melting current based on external collected information and determines the current load current I L and critical ice melting current value I C size, making the output current flexible and controllable; under normal weather conditions, the equipment can automatically switch the operating mode and adjust the output current according to the current real-time load data and line status. At the same time, the equipment can receive upper-level scheduling instructions to achieve system coordinated control.

[0065] Finally, it should be noted that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art can still modify the technical solutions described in the aforementioned embodiments without inventive effort, or replace some of the technical features therein with equivalents. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A distribution network line ice melting device, characterized in that: The de-icing device comprises a common DC bus, an AC / DC unit for converting AC to DC, and a DC / AC unit for regulating the AC output voltage; the DC side of the AC / DC unit is connected to the DC side of the DC / AC unit via the common DC bus; the AC side of the AC / DC unit and the AC side of the DC / AC unit are respectively used to connect to three-phase AC power at both ends of a line to be de-iced in a distribution network, so that when the line to be de-iced is covered with ice, the output current of the de-icing device is adjusted to ensure that the current of the line to be de-iced is not lower than the critical de-icing current.

2. The ice melting device for distribution network lines according to claim 1, characterized in that: The critical ice-melting current is determined according to the parameters of the circuit to be ice-melted and the external environment parameters of the circuit to be ice-melted.

3. The ice melting device for distribution network lines according to claim 2, characterized in that: The external environmental parameter includes at least one of ice layer surface temperature, ambient temperature and wind speed.

4. The ice melting device for distribution network lines according to any one of claims 1 to 3, characterized in that: The critical ice melting current is determined according to the following formula: Among them, I C is the critical ice melting current; R L is the equivalent resistance of the line to be de-iced; E is the radiation heat dissipation coefficient; S is the Stefan-Boltzmann number; A is the convection cross-sectional area; h is the convection coefficient determined according to the wind speed; T i is the surface temperature of the ice layer; T e is the ambient temperature.

5. The ice melting device for distribution network lines according to claim 1, characterized in that: The critical ice melting current is determined according to the following formula: Among them, I C is the critical ice melting current; R L is the equivalent resistance of the circuit to be de-iced; T i is the surface temperature of the ice layer; K i is; β is the cylindrical shape coefficient; r i is the radius of the line to be melted after ice cover; r c The radius of the line to be melted before ice accumulation.

6. The ice melting device for distribution network lines according to claim 1, characterized in that: The AC / DC unit and / or the DC / AC unit is a three-level structure.

7. The ice melting device for distribution network lines according to claim 1 or 6, characterized in that: Each bridge arm of the AC side of the AC / DC unit and / or the DC / AC unit includes four fully-controlled devices and two diodes, namely a first fully-controlled device, a second fully-controlled device, a third fully-controlled device, a fourth fully-controlled device, a first diode, and a second diode; the first fully-controlled device, the second fully-controlled device, the third fully-controlled device, and the fourth fully-controlled device are connected in series in sequence; the anode of the first diode is connected to the cathode of the second diode to form a diode branch; the diode branch is connected in parallel to both ends of the circuit formed by the second fully-controlled device and the third fully-controlled device; the direction of the first diode and the second diode is consistent with the direction of the anti-parallel diodes in the fully-controlled devices; The series connection point of the diode branch of each bridge arm on the AC side of the AC / DC unit and / or the DC / AC unit and the neutral point of the DC bus are connected together; a first capacitor is connected between the positive electrode of the DC side of the AC / DC unit and / or the DC / AC unit and the neutral point, and a second capacitor is connected between the negative electrode and the neutral point.

8. The ice melting device for distribution network lines according to claim 1, characterized in that: It also includes a soft start circuit arranged at the incoming and / or outgoing end of the distribution network line; the soft start circuit includes a circuit breaker connected in series to the corresponding incoming or outgoing end of the distribution network line and a soft start branch connected in parallel at both ends of the circuit breaker; the soft start branch includes a current limiting resistor and a soft start switch connected in series.

9. The distribution network line ice melting device according to claim 1 or 6, characterized in that: The DC / AC unit adopts a V / F control method.

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