阅读说明：本技术 一种滤波器断路器开断容性负荷过电压的计算方法及装置 (Method and device for calculating switching-off capacitive load overvoltage of filter circuit breaker ) 是由 刘凯 彭在兴 王颂 黄克捷 易林 赵林杰 李锐海 于 2019-10-21 设计创作，主要内容包括：本发明公开了一种滤波器断路器开断容性负荷过电压的计算方法及装置,该方法包括：根据设定的交流母线模块、断路器模块和滤波器模块中各元件的参数,对对应的且预先建立的元件进行参数设置,以构建用于计算滤波器断路器开断容性负荷过电压的等效电路模型；其中,所述断路器模块中预先建立的元件包括若干个断口电阻和若干个断口并联电容,每一所述断口电阻的参数是根据断路器灭弧室的体绝缘电阻的阻值和断路器瓷套表面电阻的阻值配置的；通过所述等效电路模型进行滤波器断路器开断容性负荷过电压的仿真计算,以得到所述滤波器断路器开断容性负荷过电压。采用本发明能对滤波器断路器开断容性负荷过电压进行准确计算。(The invention discloses a method and a device for calculating the switching-off capacitive load overvoltage of a filter circuit breaker, wherein the method comprises the following steps: setting parameters of corresponding and pre-established elements according to the set parameters of each element in the alternating current bus module, the breaker module and the filter module so as to construct an equivalent circuit model for calculating the switching-off capacitive load overvoltage of the filter breaker; the circuit breaker comprises a circuit breaker module, a plurality of resistors, a plurality of capacitors and a plurality of capacitors, wherein the elements pre-established in the circuit breaker module comprise a plurality of fracture resistors and a plurality of fracture parallel capacitors, and the parameters of each fracture resistor are configured according to the resistance value of a body insulation resistor of a circuit breaker arc extinguish chamber and the resistance value of a surface resistor of a circuit breaker porcelain bushing; and carrying out simulation calculation on the switching-on and switching-off capacitive load overvoltage of the filter circuit breaker through the equivalent circuit model so as to obtain the switching-on and switching-off capacitive load overvoltage of the filter circuit breaker. The invention can accurately calculate the switching-off capacitive load overvoltage of the filter circuit breaker.)

1. A method for calculating the overvoltage of an open capacitive load of a filter circuit breaker is characterized by comprising the following steps:

setting parameters of corresponding and pre-established elements according to the set parameters of each element in the alternating current bus module, the breaker module and the filter module so as to construct an equivalent circuit model for calculating the switching-off capacitive load overvoltage of the filter breaker; the circuit breaker comprises a circuit breaker module, a plurality of resistors, a plurality of capacitors and a plurality of capacitors, wherein the elements pre-established in the circuit breaker module comprise a plurality of fracture resistors and a plurality of fracture parallel capacitors, and the parameters of each fracture resistor are configured according to the resistance value of a body insulation resistor of a circuit breaker arc extinguish chamber and the resistance value of a surface resistor of a circuit breaker porcelain bushing;

and carrying out simulation calculation on the switching-on and switching-off capacitive load overvoltage of the filter circuit breaker through the equivalent circuit model so as to obtain the switching-on and switching-off capacitive load overvoltage of the filter circuit breaker.

2. The method for calculating the overvoltage of the filter circuit breaker to open the capacitive load according to claim 1, wherein the simulation calculation of the overvoltage of the filter circuit breaker to open the capacitive load is performed through the equivalent circuit model to obtain the overvoltage of the filter circuit breaker to open the capacitive load, and specifically comprises:

running the equivalent circuit model; the initial state of a breaker module in the equivalent circuit model is a closed state;

sending a first switching-off signal to the circuit breaker module to control the circuit breaker module to be switched to a switching-off state when the current time reaches a first time; the first moment is the moment when the current flowing through the circuit breaker module crosses zero for the first time after the circuit breaker module receives the first switching-off signal;

sending a closing signal to the breaker module to control the breaker module to be switched to a closed state when the accumulated time length of the breaker module in the open state reaches a preset time length; the preset time length is equal to one-half period time length of alternating current;

sending a second switching-off signal to the circuit breaker module to control the circuit breaker module to be switched to a disconnection state when the current time reaches a second time; the second moment is the moment when the current flowing through the circuit breaker module crosses zero for the first time after the circuit breaker module receives the second switching-off signal;

and after the circuit breaker module is disconnected for the second time, acquiring a voltage peak value born by the circuit breaker module, and taking the voltage peak value born by the circuit breaker module as the switching-off capacitive load overvoltage of the filter circuit breaker.

3. The method for calculating the overvoltage of a filter breaker to open a capacitive load as claimed in claim 2, wherein said breaker module comprises a plurality of breaking units;

when the number of the fracture units in the breaker module is one, one end of each fracture unit is connected with the alternating current bus module, and the other end of each fracture unit is connected with the filter module;

when the number of the fracture units in the breaker module is more than one, after the fracture units are connected in series, one end of each fracture unit is connected with the alternating current bus module, and the other end of each fracture unit is connected with the filter module;

each fracture unit comprises one fracture resistor, one fracture parallel capacitor and a plurality of switches positioned on a trunk line;

the switches are connected in series and then connected in parallel with the fracture resistor, and the switches are connected in series and then connected in parallel with the fracture parallel capacitor.

4. The method of calculating an overvoltage of a filter circuit breaker open capacitive load of claim 3, wherein said ac bus module comprises an ac voltage source, a system inductance, and a bus stray capacitance to ground;

one end of the alternating current voltage source is grounded, the other end of the alternating current voltage source is connected with one end of the system inductor, the other end of the system inductor is connected with the circuit breaker module, the other end of the system inductor is further connected with one end of the bus-to-ground stray capacitor, and the other end of the bus-to-ground stray capacitor is grounded.

5. The method of calculating an overvoltage of a filter circuit breaker to open a capacitive load of claim 3, wherein said filter module comprises a first capacitor, a first inductor, a second inductor, a first resistor and a second capacitor;

the circuit breaker comprises a circuit breaker module, a first inductor, a second inductor, a first resistor, a second inductor and a second capacitor, wherein one end of the first capacitor is connected with the circuit breaker module, the other end of the first capacitor is connected with one end of the first inductor, the other end of the first inductor is connected with one end of the second capacitor, the other end of the second capacitor is grounded, the first resistor is connected with the first inductor in parallel, and the second inductor is connected with the second capacitor in parallel.

6. The method for calculating an overvoltage across a capacitive load being switched off by a filter circuit breaker according to claim 3, wherein the step of sending a first switching signal to the circuit breaker module to control the circuit breaker module to switch to the off state when the current time reaches a first time is further characterized by:

and sending a first switching-off signal to the circuit breaker module to control at least one switch in each fracture unit of the circuit breaker module to be switched off at the first moment, so that the circuit breaker module is switched to a switching-off state at the first moment.

7. The method for calculating the overvoltage of the capacitive load of the filter circuit breaker opened according to claim 3, wherein the step of sending a closing signal to the circuit breaker module to control the circuit breaker module to switch to the closed state when the accumulated time length of the circuit breaker in the open state reaches a preset time length comprises the following steps:

and sending a closing signal to the breaker module to control the switch which is opened in each fracture unit of the breaker module to be closed when the accumulated time length in the off state reaches the preset time length, so that the breaker module is switched to the closed state when the accumulated time length in the off state reaches the preset time length.

8. The method for calculating an overvoltage across a capacitive load to be switched off by a filter circuit breaker according to claim 3, wherein the step of sending a second switching signal to the circuit breaker module to control the circuit breaker module to switch to the off state when the current time reaches a second time comprises:

and sending a second switching-off signal to the circuit breaker module to control at least one switch in each breaking unit of the circuit breaker module to be switched off at the second moment when the current moment reaches, so that the circuit breaker module is switched to a disconnected state when the current moment reaches the second moment.

9. The method for calculating the overvoltage of the filter circuit breaker to open the capacitive load according to claim 3, wherein the step of obtaining the peak voltage value born by the circuit breaker module comprises:

acquiring a voltage peak borne by each fracture unit in the breaker module;

and calculating the voltage peak value borne by the breaker module according to the voltage peak value borne by each fracture unit.

10. A device for calculating an overvoltage of a capacitive load to be disconnected by a filter circuit breaker, comprising:

the model building module is used for carrying out parameter setting on corresponding and pre-established elements according to the set parameters of each element in the alternating current bus module, the breaker module and the filter module so as to build an equivalent circuit model for calculating the switching-off capacitive load overvoltage of the filter breaker; the circuit breaker comprises a circuit breaker module, a plurality of resistors, a plurality of capacitors and a plurality of capacitors, wherein the elements pre-established in the circuit breaker module comprise a plurality of fracture resistors and a plurality of fracture parallel capacitors, and the parameters of each fracture resistor are configured according to the resistance value of a body insulation resistor of a circuit breaker arc extinguish chamber and the resistance value of a surface resistor of a circuit breaker porcelain bushing;

and the overvoltage calculation module is used for carrying out simulation calculation on the switching-on and switching-off capacitive load overvoltage of the filter circuit breaker through the equivalent circuit model so as to obtain the switching-on and switching-off capacitive load overvoltage of the filter circuit breaker.

Technical Field

The invention relates to the technical field of electricity, in particular to a method and a device for calculating the overvoltage of a cut-off capacitive load of a filter circuit breaker.

Background

The switching circuit breaker of the alternating current filter needs to cut off capacitive load due to special application occasions, switching is frequent, the switching circuit breaker needs to bear alternating current and direct current mixed voltage, the working condition is severe, and the switching circuit breaker has higher technical requirements than a common circuit breaker. In recent years, a plurality of filter circuit breaker accidents occur successively, the basic phenomena of the accidents are that the filter circuit breaker is re-ignited or flashover after being disconnected, and a plurality of explosion accidents occur, so that the reliable and safe operation of a power grid is greatly challenged. Whether the filter breaker can be successfully cut off or not depends on the relation between overvoltage and insulation recovery strength generated in the cutting process, and when the overvoltage is higher than the insulation recovery strength, the breaker can be re-ignited or broken down. After the circuit breaker leaves the factory, under the condition that parameters such as structural form, charging pressure and the like are fixed, the insulation recovery strength is almost unchanged, so whether the filter circuit breaker can be successfully switched on or off depends on overvoltage in the switching-on and switching-off process. Therefore, the accurate calculation of the switching-off capacitive load overvoltage of the filter circuit breaker has important significance for ensuring the reliable and safe operation of the power grid.

At present, a model for calculating the switching-off capacitive load overvoltage of a filter circuit breaker is generally constructed according to a circuit breaker model provided by PSCAD (power system computer aided design) or EMTDC (electro-magnetic transient DC) software, and the circuit breaker is equivalent to an element with certain arc impedance by the PSCAD or EMTDC software provided with the circuit breaker model so as to meet the constraint conditions of arc voltage and arc current. However, in the process of implementing the present invention, the inventor finds that, since the circuit breaker is composed of an arc-extinguishing chamber, an outer porcelain bushing, a parallel capacitor, and the like, these components have certain influence on the calculation of the overvoltage of the filter circuit breaker for opening and closing the capacitive load, and the existing model for calculating the overvoltage of the filter circuit breaker for opening and closing the capacitive load does not consider the influence of these components on the overvoltage, which may make the calculation of the overvoltage of the filter circuit breaker for opening and closing the capacitive load inaccurate.

Disclosure of Invention

The embodiment of the invention provides a method and a device for calculating the overvoltage of a cut-off capacitive load of a filter circuit breaker, which can accurately calculate the overvoltage of the cut-off capacitive load of the filter circuit breaker.

In order to achieve the above object, an embodiment of the present invention provides a method for calculating an overvoltage of a filter circuit breaker when a capacitive load is opened, including:

setting parameters of corresponding and pre-established elements according to the set parameters of each element in the alternating current bus module, the breaker module and the filter module so as to construct an equivalent circuit model for calculating the switching-off capacitive load overvoltage of the filter breaker; the circuit breaker comprises a circuit breaker module, a plurality of resistors, a plurality of capacitors and a plurality of capacitors, wherein the elements pre-established in the circuit breaker module comprise a plurality of fracture resistors and a plurality of fracture parallel capacitors, and the parameters of each fracture resistor are configured according to the resistance value of a body insulation resistor of a circuit breaker arc extinguish chamber and the resistance value of a surface resistor of a circuit breaker porcelain bushing;

and carrying out simulation calculation on the switching-on and switching-off capacitive load overvoltage of the filter circuit breaker through the equivalent circuit model so as to obtain the switching-on and switching-off capacitive load overvoltage of the filter circuit breaker.

As an improvement of the above scheme, the performing simulation calculation of the filter circuit breaker open-close capacitive load overvoltage through the equivalent circuit model to obtain the filter circuit breaker open-close capacitive load overvoltage specifically includes:

running the equivalent circuit model; the initial state of a breaker module in the equivalent circuit model is a closed state;

sending a first switching-off signal to the circuit breaker module to control the circuit breaker module to be switched to a switching-off state when the current time reaches a first time; the first moment is the moment when the current flowing through the circuit breaker module crosses zero for the first time after the circuit breaker module receives the first switching-off signal;

sending a closing signal to the breaker module to control the breaker module to be switched to a closed state when the accumulated time length of the breaker module in the open state reaches a preset time length; the preset time length is equal to one-half period time length of alternating current;

sending a second switching-off signal to the circuit breaker module to control the circuit breaker module to be switched to a disconnection state when the current time reaches a second time; the second moment is the moment when the current flowing through the circuit breaker module crosses zero for the first time after the circuit breaker module receives the second switching-off signal;

and after the circuit breaker module is disconnected for the second time, acquiring a voltage peak value born by the circuit breaker module, and taking the voltage peak value born by the circuit breaker module as the switching-off capacitive load overvoltage of the filter circuit breaker.

As an improvement of the above solution, the breaker module comprises a plurality of breaking units;

when the number of the fracture units in the breaker module is one, one end of each fracture unit is connected with the alternating current bus module, and the other end of each fracture unit is connected with the filter module;

when the number of the fracture units in the breaker module is more than one, after the fracture units are connected in series, one end of each fracture unit is connected with the alternating current bus module, and the other end of each fracture unit is connected with the filter module;

each fracture unit comprises one fracture resistor, one fracture parallel capacitor and a plurality of switches positioned on a trunk line;

the switches are connected in series and then connected in parallel with the fracture resistor, and the switches are connected in series and then connected in parallel with the fracture parallel capacitor.

As an improvement of the above scheme, the alternating current bus module comprises an alternating current voltage source, a system inductor and a bus stray capacitance to ground;

one end of the alternating current voltage source is grounded, the other end of the alternating current voltage source is connected with one end of the system inductor, the other end of the system inductor is connected with the circuit breaker module, the other end of the system inductor is further connected with one end of the bus-to-ground stray capacitor, and the other end of the bus-to-ground stray capacitor is grounded.

As an improvement of the above solution, the filter module includes a first capacitor, a first inductor, a second inductor, a first resistor, and a second capacitor;

the circuit breaker comprises a circuit breaker module, a first inductor, a second inductor, a first resistor, a second inductor and a second capacitor, wherein one end of the first capacitor is connected with the circuit breaker module, the other end of the first capacitor is connected with one end of the first inductor, the other end of the first inductor is connected with one end of the second capacitor, the other end of the second capacitor is grounded, the first resistor is connected with the first inductor in parallel, and the second inductor is connected with the second capacitor in parallel.

As an improvement of the above scheme, the sending of the first switching signal to the circuit breaker module to control the circuit breaker module to switch to the off state when the current time reaches the first time specifically includes:

and sending a first switching-off signal to the circuit breaker module to control at least one switch in each fracture unit of the circuit breaker module to be switched off at the first moment, so that the circuit breaker module is switched to a switching-off state at the first moment.

As an improvement of the above scheme, the sending of the closing signal to the circuit breaker module to control the circuit breaker module to switch to the closed state when the accumulated time length in the open state reaches the preset time length specifically includes:

and sending a closing signal to the breaker module to control the switch which is opened in each fracture unit of the breaker module to be closed when the accumulated time length in the off state reaches the preset time length, so that the breaker module is switched to the closed state when the accumulated time length in the off state reaches the preset time length.

As an improvement of the above scheme, the sending of the second switching signal to the circuit breaker module to control the circuit breaker module to switch to the off state when the current time reaches the second time specifically includes:

and sending a second switching-off signal to the circuit breaker module to control at least one switch in each breaking unit of the circuit breaker module to be switched off at the second moment when the current moment reaches, so that the circuit breaker module is switched to a disconnected state when the current moment reaches the second moment.

As an improvement of the above scheme, the obtaining of the voltage peak value borne by the breaker module specifically includes:

acquiring a voltage peak borne by each fracture unit in the breaker module;

and calculating the voltage peak value borne by the breaker module according to the voltage peak value borne by each fracture unit.

Another embodiment of the present invention provides a device for calculating an overvoltage of a capacitive load when a filter circuit breaker is opened, including:

the model building module is used for carrying out parameter setting on corresponding and pre-established elements according to the set parameters of each element in the alternating current bus module, the breaker module and the filter module so as to build an equivalent circuit model for calculating the switching-off capacitive load overvoltage of the filter breaker; the circuit breaker comprises a circuit breaker module, a plurality of resistors, a plurality of capacitors and a plurality of capacitors, wherein the elements pre-established in the circuit breaker module comprise a plurality of fracture resistors and a plurality of fracture parallel capacitors, and the parameters of each fracture resistor are configured according to the resistance value of a body insulation resistor of a circuit breaker arc extinguish chamber and the resistance value of a surface resistor of a circuit breaker porcelain bushing;

and the overvoltage calculation module is used for carrying out simulation calculation on the switching-on and switching-off capacitive load overvoltage of the filter circuit breaker through the equivalent circuit model so as to obtain the switching-on and switching-off capacitive load overvoltage of the filter circuit breaker.

Compared with the prior art, the method and the device for calculating the switching-on and switching-off capacitive load overvoltage of the filter circuit breaker provided by the embodiment of the invention have the advantages that the corresponding and pre-established elements are subjected to parameter setting according to the set parameters of each element in the alternating current bus module, the circuit breaker module and the filter module so as to construct an equivalent circuit model for calculating the switching-on and switching-off capacitive load overvoltage of the filter circuit breaker; the circuit breaker comprises a circuit breaker module, a plurality of resistors, a plurality of capacitors and a plurality of capacitors, wherein the elements pre-established in the circuit breaker module comprise a plurality of fracture resistors and a plurality of fracture parallel capacitors, and the parameters of each fracture resistor are configured according to the resistance value of a body insulation resistor of a circuit breaker arc extinguish chamber and the resistance value of a surface resistor of a circuit breaker porcelain bushing; and performing simulation calculation on the switching-on and switching-off capacitive load overvoltage of the filter circuit breaker through the equivalent circuit model to obtain the switching-on and switching-off capacitive load overvoltage of the filter circuit breaker, so that the switching-on and switching-off capacitive load overvoltage of the filter circuit breaker is accurately calculated. In the process of constructing the equivalent circuit model, the influence of the body insulation resistance, the surface resistance of the porcelain bushing and the parallel capacitor of the arc extinguish chamber in the circuit breaker on the on-off capacitive load overvoltage of the filter circuit breaker is considered, so that the equivalent circuit model is more accurate, and the on-off capacitive load overvoltage of the filter circuit breaker can be accurately simulated and calculated.

Drawings

Fig. 1 is a schematic flowchart of a method for calculating an overvoltage of a filter circuit breaker when the capacitive load is opened according to an embodiment of the present invention.

Fig. 2 is a schematic structural diagram of an equivalent circuit model for calculating an overvoltage of a filter circuit breaker when the filter circuit breaker opens a capacitive load according to an embodiment of the present invention.

Fig. 3 is a diagram of the current waveform of a circuit breaker module for calculating an equivalent circuit model of a filter circuit breaker for breaking a capacitive load overvoltage.

Fig. 4 is a graph of the voltage waveform of the first capacitance of the filter module of the equivalent circuit model for calculating the filter circuit breaker open capacitive load overvoltage.

Fig. 5 is a graph of the voltage waveform of the parallel portion of the first inductance and the first resistance of the filter module of the equivalent circuit model for calculating the filter circuit breaker opening capacitive load overvoltage.

Fig. 6 is a graph of the voltage waveform of the parallel portion of the second inductance and the second capacitance of the filter module of the equivalent circuit model for calculating the filter circuit breaker opening capacitive load overvoltage.

Fig. 7 is a voltage waveform diagram of an ac bus module of an equivalent circuit model for calculating filter breaker open capacitive load overvoltage.

Fig. 8 is a graph of the voltage waveform of the first breaking unit of the breaker module of the equivalent circuit model for calculating the breaking of the capacitive load overvoltage by the filter breaker.

Fig. 9 is a voltage waveform diagram of a second breaking unit of the breaker module for calculating an equivalent circuit model of the filter breaker for breaking the capacitive load overvoltage.

Fig. 10 is a partial enlarged view of the voltage waveform of the ac busbar module of the equivalent circuit model for calculating the filter breaker open capacitive load overvoltage.

Fig. 11 is a partial enlarged view of the current waveform of the circuit breaker module of the equivalent circuit model for calculating the filter circuit breaker opening capacitive load overvoltage.

Fig. 12 is a graph of the voltage waveforms of the filter modules of the equivalent circuit model for calculating the filter circuit breaker open capacitive load overvoltage.

Fig. 13 is a graph of the recovery voltage waveforms of two breaking units of the breaker module at a breaking resistance non-uniformity coefficient of 10:1 for an equivalent circuit model for calculating the breaking capacitive load overvoltage of the filter breaker.

Fig. 14 is a graph of the recovery voltage waveforms of two breaking units of the breaker module at a breaking resistance non-uniformity coefficient of 100:1 for an equivalent circuit model for calculating the breaking capacitive load overvoltage of the filter breaker.

Fig. 15 is a schematic structural diagram of a calculating device for opening and closing the capacitive load overvoltage by the filter circuit breaker according to an embodiment of the present invention.

Detailed Description

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without any inventive step, are within the scope of the present invention.

Fig. 1 is a schematic flow chart of a method for calculating an overvoltage of a filter circuit breaker when a capacitive load is opened according to an embodiment of the present invention.

The method for calculating the overvoltage of the capacitive load of the filter circuit breaker, provided by the embodiment of the invention, is used for calculating the overvoltage of the capacitive load of the filter circuit breaker in a power system, and comprises the following steps from S110 to S120:

s110, setting parameters of corresponding and pre-established elements according to the set parameters of each element in the alternating current bus module, the breaker module and the filter module to construct an equivalent circuit model for calculating the switching-off capacitive load overvoltage of the filter breaker; the circuit breaker comprises a circuit breaker module, a plurality of breaker resistors, a plurality of breaker parallel capacitors, a circuit breaker porcelain bushing and a circuit breaker porcelain bushing, wherein the elements pre-established in the circuit breaker module comprise a plurality of breaker resistors and a plurality of breaker parallel capacitors, and parameters of each breaker resistor are configured according to the resistance value of the body insulation resistor of the circuit breaker arc extinguish chamber and the resistance value of the surface resistor of the circuit breaker porcelain bushing.

Specifically, an operator correspondingly creates and connects elements forming the ac bus module, the circuit breaker module and the filter module on an operation interface of simulation software (the simulation software of this embodiment is preferably PSCAD software or EMTDC software), so as to create the ac bus module, the circuit breaker module and the filter module, and then inputs corresponding parameters, such as a voltage parameter of an ac voltage source, a resistance parameter of a fracture resistance, a capacitance parameter of a fracture parallel capacitor and the like, for each element of the ac bus module, the circuit breaker module and the filter module, so as to perform parameter setting on each element, so that the simulation software can construct an equivalent circuit model for calculating the open-closed capacitive load overvoltage of the filter circuit breaker according to the corresponding parameters.

The device comprises an alternating current bus module, a circuit breaker module, a filter, a circuit breaker and a power system, wherein the elements, the parameters and the connection relation among the elements, which form the alternating current bus module, are obtained by performing equivalent analysis on an alternating current bus connected with the circuit breaker in the power system, the elements, the parameters and the connection relation among the elements, which form the circuit breaker module, are obtained by performing equivalent analysis on the circuit breaker in the power system, and the elements, the parameters and the connection relation among the elements, which form the filter module, are obtained by performing equivalent analysis on a filter connected with the circuit breaker in the power system.

It should be noted that, because the body insulation resistance, the surface resistance of the porcelain bushing and the parallel capacitance of the arc extinguish chamber in the circuit breaker have a certain influence on the switching-on/off capacitive load overvoltage of the filter circuit breaker, when the circuit breaker in the power system is subjected to equivalent analysis, the body insulation resistance and the surface resistance of the porcelain bushing of the arc extinguish chamber of the circuit breaker are equivalent to resistors to serve as the break resistance of each break in the circuit breaker module, and the parallel capacitance is equivalent to the break parallel capacitance of each break in the circuit breaker module, so that the constructed equivalent circuit model is more accurate, and thus, accurate simulation calculation can be performed on the switching-on/off capacitive load overvoltage of the filter circuit breaker.

And S120, performing simulation calculation on the switching-on and switching-off capacitive load overvoltage of the filter circuit breaker through the equivalent circuit model to obtain the switching-on and switching-off capacitive load overvoltage of the filter circuit breaker.

After an equivalent circuit model used for calculating the switching-on and switching-off capacitive load overvoltage of the filter circuit breaker is built, simulation software runs the equivalent circuit model and simulates the restriking process of the capacitive load of the circuit breaker module, and then the switching-on and switching-off capacitive load overvoltage of the filter circuit breaker is analyzed and calculated, so that the switching-on and switching-off capacitive load overvoltage of the filter circuit breaker is obtained.

Specifically, the step S120 includes steps S121 to S125, which are as follows:

s121, operating the equivalent circuit model; and the initial state of the breaker module in the equivalent circuit model is a closed state.

And after an equivalent circuit model for calculating the switching-off capacitive load overvoltage of the filter circuit breaker is constructed, the simulation software runs the equivalent circuit model.

S122, sending a first switching-off signal to the circuit breaker module to control the circuit breaker module to be switched to a switching-off state when the current time reaches a first time; the first time is the time when the current flowing through the circuit breaker module crosses zero for the first time after the circuit breaker module receives the first switching-off signal.

The operator can preset the operation time sequence of the breaker module on the simulation software so as to realize the on-off process of the breaker module, and can also realize the on-off process of the breaker module through manual control by online control. Taking the example of presetting the operation time sequence of the breaker module to realize the on-off process of the breaker module, the simulation software sends a first opening signal to the breaker module in the equivalent circuit model according to the preset operation time sequence so as to control the breaker module to be switched to the off state when the current time reaches the first time.

S123, sending a closing signal to the breaker module to control the breaker module to be switched to a closed state when the accumulated time length of the breaker module in the open state reaches a preset time length; and the preset time length is equal to one-half period time length of the alternating current.

After the fractures are separated from the 1/2 alternating current period, 2Um recovery voltage occurs between the fractures, the breaker is prone to re-breakdown at the moment, and the simulation software sends a closing signal to the breaker module according to the operation time sequence so as to control the breaker module to be switched to a closed state when the accumulated time length in the open state reaches the preset time length, and therefore the re-striking passing process of the breaker is simulated.

S124, sending a second switching-off signal to the circuit breaker module to control the circuit breaker module to be switched to a disconnection state when the current time reaches a second time; and the second moment is the moment when the current flowing through the circuit breaker module crosses zero for the first time after the circuit breaker module receives the second switching-off signal.

And after the circuit breaker module is subjected to re-breakdown, the simulation software sends a second switching-off signal to the circuit breaker module according to the operation time sequence so as to control the circuit breaker module to be switched to a switching-off state when the current time reaches a second time.

And S125, after the circuit breaker module is disconnected for the second time, acquiring a voltage peak value born by the circuit breaker module, and taking the voltage peak value born by the circuit breaker module as the switching-off capacitive load overvoltage of the filter circuit breaker.

After the circuit breaker module is disconnected for the second time, the simulation software calculates the voltage born by the circuit breaker module so as to obtain the peak value of the voltage born by the circuit breaker module, and the peak value of the voltage born by the circuit breaker module is used as the on-off capacitive load overvoltage of the filter circuit breaker so as to obtain the on-off capacitive load overvoltage of the filter circuit breaker.

On the basis of the above embodiment, as a preferred embodiment, the breaker module includes a plurality of breaking units;

when the number of the fracture units in the breaker module is one, one end of each fracture unit is connected with the alternating current bus module, and the other end of each fracture unit is connected with the filter module;

when the number of the fracture units in the breaker module is more than one, after the fracture units are connected in series, one end of each fracture unit is connected with the alternating current bus module, and the other end of each fracture unit is connected with the filter module;

each fracture unit comprises one fracture resistor, one fracture parallel capacitor and a plurality of switches positioned on a trunk line;

the switches are connected in series and then connected in parallel with the fracture resistor, and the switches are connected in series and then connected in parallel with the fracture parallel capacitor.

The number of the fracture units in the breaker module is determined according to the number of the fractures of the circuit breakers in the power system, for example, when the circuit breakers in the power system are double-fracture circuit breakers, the breaker module comprises two fracture units. The circuit breaker module 20 of the equivalent circuit model shown in fig. 2 includes two breaking units, wherein the first breaking unit 21 includes a first breaking resistor R1, a first breaking parallel capacitor C2, a first switch BRK1 and a second switch BRK2, the first switch BRK1 and the second switch BRK2 are connected in series and then connected in parallel with a first breaking resistor R1, and the first switch BRK1 and the second switch BRK2 are connected in series and then connected in parallel with a first breaking parallel capacitor C2; the second fracture unit 22 comprises a second fracture resistor R2, a second fracture parallel capacitor C3, a third switch BRK3 and a fourth switch BRK4 which are located on the trunk line, the third switch BRK3 and the fourth switch BRK4 are connected in series and then connected in parallel with the second fracture resistor R2, and the third switch BRK3 and the fourth switch BRK4 are connected in series and then connected in parallel with the second fracture parallel capacitor C3.

It should be noted that the arc resistance range when the arc burns stably in the breaker opening process is 0.5-5 Ω, but the influence of the arc resistance is ignored in the model because the change of the arc resistance in this range has little influence on the overvoltage. And because the equivalent resistance of the strut porcelain bushing of the circuit breaker is generally several times of the equivalent resistance of the fracture, the influence on the result is small, and the influence of the equivalent resistance of the strut porcelain bushing is ignored in the model for simplifying the model.

Referring to fig. 2, on the basis of the above embodiment, as a preferred embodiment, the AC busbar module 10 includes an AC voltage source AC, a system inductance L1, and a busbar-to-ground stray capacitance C1;

one end of the alternating current voltage source AC is grounded, the other end of the alternating current voltage source AC is connected to one end of the system inductor L1, the other end of the system inductor L1 is connected to the circuit breaker module 20, the other end of the system inductor L1 is also connected to one end of the bus-to-ground stray capacitor C1, and the other end of the bus-to-ground stray capacitor C1 is grounded.

Preferably, the alternating voltage source AC is a single-phase RRL type alternating voltage source.

Referring to fig. 2, on the basis of the above embodiment, as a preferred implementation, the filter module 30 includes a first capacitor C4, a first inductor L2, a second inductor L3, a first resistor R3, and a second capacitor C5;

the circuit breaker module 10 is connected to one end of first electric capacity C4, the other end of first electric capacity C4 is connected the one end of first inductance L2, the other end of first inductance L2 is connected the one end of second electric capacity C5, the other end ground connection of second electric capacity C5, first resistance R3 is parallelly connected with first inductance L2, second inductance L3 with second electric capacity C5 is parallelly connected.

On the basis of the above examples, as a preferred embodiment, step S122 specifically is:

and sending a first switching-off signal to the circuit breaker module to control at least one switch in each fracture unit of the circuit breaker module to be switched off at the first moment, so that the circuit breaker module is switched to a switching-off state at the first moment.

Taking the equivalent circuit model shown in fig. 2 as an example, it may be that a first breaking signal is sent to the breaker module to control the first switch BRK1 in the first breaking unit 21 and the third switch BRK3 in the second breaking unit 22 of the breaker module to open when the current time reaches the first time, so that the breaker module is switched to the open state.

On the basis of the above examples, as a preferred embodiment, step S123 is specifically:

and sending a closing signal to the breaker module to control the switch which is opened in each fracture unit of the breaker module to be closed when the accumulated time length in the off state reaches the preset time length, so that the breaker module is switched to the closed state when the accumulated time length in the off state reaches the preset time length.

Taking the equivalent circuit model shown in fig. 2 as an example, a closing signal may be sent to the circuit breaker module to control the first switch BRK1 opened in the first breaking unit 21 and the third switch BRK3 opened in the second breaking unit 22 of the circuit breaker module to be closed when the accumulated time length in the open state reaches the preset time length, so that the circuit breaker module is switched to the closed state.

On the basis of the above examples, as a preferred embodiment, step S124 specifically is:

and sending a second switching-off signal to the circuit breaker module to control at least one switch in each breaking unit of the circuit breaker module to be switched off at the second moment when the current moment reaches, so that the circuit breaker module is switched to a disconnected state when the current moment reaches the second moment.

Taking the equivalent circuit model shown in fig. 2 as an example, it may be that a second breaking signal is sent to the breaker module to control the second switch BRK2 in the first breaking unit 21 and the fourth switch BRK4 in the second breaking unit 22 of the breaker module to open when the current time reaches the second time, so that the breaker module is switched to the open state.

On the basis of the above embodiment, as a preferred implementation, the step S125 obtains a voltage peak value borne by the breaker module, specifically:

acquiring a voltage peak borne by each fracture unit in the breaker module;

and calculating the voltage peak value borne by the breaker module according to the voltage peak value borne by each fracture unit.

Specifically, voltage peak values borne by all fracture units in the breaker module are obtained, and when only one fracture unit exists in the breaker module, the voltage peak value borne by the fracture unit is used as the voltage peak value borne by the breaker module; when two fracture units exist in the breaker module, the sum of the voltage peak values born by the two fracture units is used as the voltage peak value born by the breaker module, and the like, so that the voltage peak value born by the breaker module is calculated according to the voltage peak value born by each fracture unit.

An equivalent circuit model for calculating the overvoltage of the filter circuit breaker of the 500kV converter station when the converter station is switched on and off is described below.

The alternating current bus of the converter station can be equivalent to an ideal alternating current voltage source connected in series with a system inductor, and referring to fig. 2, an alternating current bus module in the equivalent circuit model consists of an alternating current voltage source AC, a system inductor L1 and a bus-to-ground stray capacitorC1, wherein the line voltage of the converter station is 550kV, the phase voltage of the bus is 318kV, and the system inductance L1 is 15e^-3H, bus-to-ground stray capacitance C1 is 3e^-3μ F. The circuit breaker of the converter station is a double-break circuit breaker, a circuit breaker module in the equivalent circuit model consists of two break units, two switches located in a trunk circuit are arranged in each break unit, the circuit breaker module comprises four switches including BRK1, BRK2, BRK3 and BRK4, the on-off operation of the circuit breaker module is realized through simulation of BRK1, BRK2, BRK3 and BRK4, and the resistance of each switch in an on state is 1e¹⁶Ω, resistance in the off state is 0.005 Ω; the circuit breaker module comprises two break resistors R1 and R2, and the resistance values of R1 and R2 are both 20e under the condition that the surface of a porcelain sleeve of the circuit breaker is dry and pollution-free⁹Omega; the breaker module comprises two fracture parallel capacitors of C2 and C3, and the capacitance values of C2 and C3 are 2e^-4μ F. The filter of the converter station is an A-type filter, and the filter is equivalent to a series-parallel circuit of a first capacitor C4, a first inductor L2, a second inductor L3, a first resistor R3 and a second capacitor C5, wherein the capacitance value of the first capacitor C4 is 1.839 muF, the inductance value of the first inductor L2 is 0.01507H, the resistance value of the first resistor R3 is 500 omega, the resistance value of the second capacitor C5 is 4.789 muF, and the inductance value of the second inductor L3 is 0.011156H. Because the lightning arresters are hung on the alternating current bus and the filter field of the converter station, the lightning arresters are arranged on the alternating current bus module and the filter module, the lightning arresters are respectively F1 and F2, the rated voltages of the lightning arresters F1 and F2 are 403kV, and the operation impact protection level is 780 kV.

The calculation process and principle of the overvoltage of the filter breaker of the converter station for breaking the capacitive load are described below with reference to the equivalent circuit model described above.

In the equivalent circuit model introduced above, the initial states of BRK1, BRK2, BRK3 and BRK4 are all at the closing position, and in order to simulate the recovery voltage when the switch-capacity load of the circuit breaker reignites, the circuit breaker opening and closing process is realized by setting the operation timings of BRK1, BRK2, BRK3 and BRK 4. By way of example, the timing of the operation of BRK1, BRK2, BRK3, BRK4 may be set such that BRK1 and BRK3 are open at 200ms, closed at 215ms, BRK2 and BRK4 are open at 215.1ms, and closed at 250 ms.

When the circuit breaker module is switched on and off, the rising rate of the recovery voltage is relatively low when the capacitor current is switched on and off due to the residual charge on the capacitor. This current is small compared to the short circuit current and is therefore easy to open. In this case, the opening will occur at the first current zero crossing. According to the operation sequence, the first switching signals are sent to BRK1 and BRK3 at the time of 200ms, and BRK1 and BRK3 receive the first switching signals, and the voltage on the bus side is zero, the current leads the voltage by 90 degrees, and the current is the maximum value. The first zero-crossing time after the BRK1 and the BRK3 receive the first switching signal is 205ms, and the BRK1 and the BRK3 are controlled to be switched off at the moment, so that the circuit breaker module is switched to an off state, and the current flowing through the circuit breaker module becomes zero. Specifically, the current flowing through the breaker module is shown in fig. 3.

Before the breaker module is opened, the voltage condition borne by the first capacitor C4, the voltage condition borne by the parallel portion of the first inductor L2 and the first resistor R3, and the voltage condition borne by the parallel portion of the second inductor L3 and the second capacitor C5 of the filter module are respectively as shown in fig. 4, 5 and 6. As can be seen from fig. 4, 5 and 6, the first capacitor C4 bears almost all of the power frequency voltage, because the impedances of the three parts are 1731 Ω, 4 Ω and 3.5 Ω respectively under the power frequency voltage, and the impedance of the first capacitor C4 is much greater than that of the other two parts, so that almost all of the power frequency voltage is borne.

At the moment of opening the breaker module, as shown in fig. 7, the voltage of the ac bus module is at the peak value Um, and as shown in fig. 4, the voltage on the first capacitor C4 is equal to the voltage before opening, and the dc voltage of Um will be maintained. The voltage of the alternating current bus module changes in a sine rule, the voltages borne by the two fracture units are the difference value between the power frequency voltage of the alternating current bus module and the direct current voltage of the first capacitor C4, and the voltage change rules of the first fracture unit 21 and the second fracture unit 22 are respectively shown in fig. 8 and fig. 9.

After 1/2 alternating current cycles (10ms) of breaker module disconnection, a recovery voltage of 2Um appears between fractures, the breaker module is easy to generate re-breakdown at the moment, BRK1 and BRK3 are controlled to simulate the re-breakdown process through closing (215ms) at the moment, BRK2 and BRK4 are controlled to open at the first zero crossing point of current (215.1ms receives a brake-off signal, and 215.5ms opens at the first zero crossing point of high-frequency oscillation current). The voltage across the alternating current bus module is impacted again to generate high-frequency oscillation until the power frequency is recovered, partial enlarged images of the voltage of the alternating current bus module and the current flowing through the breaker are respectively shown in fig. 10 and fig. 11, the circuit breaker is cut off at the first current zero crossing point of the high-frequency current flowing through the breaker module, and the voltage borne by the first capacitor C4 is approximately equal to the oscillation voltage value (about 800 kV) of the alternating current bus module at the cutting moment, as shown in fig. 4. Due to the existence of high-frequency current, as shown in fig. 5, the part of the filter module in which the first inductor L2 is connected in parallel with the first resistor R3 is subject to overvoltage and quickly attenuates to zero; as shown in fig. 6, the parallel portion of the second inductor L3 and the second capacitor C5 in the filter module is subject to an oscillating overvoltage of a specific frequency. The voltage borne by the filter module is the sum of the voltage borne by the first capacitor C4, the voltage borne by the parallel portion of the first inductor L2 and the first resistor R3, and the voltage borne by the parallel portion of the second inductor L3 and the second capacitor C5, as shown in fig. 12.

Under the action of the power frequency voltage of the alternating current bus module, the direct current voltage of the first capacitor C4 and the high-frequency voltage of the parallel connection part of the inductor and the second inductor L3 and the second capacitor C5, the change trend of the recovery voltage between fractures in the heavy-stroke penetration process is shown in fig. 8 and 9, the voltage amplitude borne by each fracture unit is 848.17kV, and the whole breaker module bears the recovery voltage of 1696.34kV, namely the breaking capacitive load overvoltage of the filter breaker of the converter station is 1696.34kV and exceeds the TRV specification requirement value of the breaker 1470 kV.

And controlling the BRK2 and the BRK4 to switch on at the time of 250ms, wherein the voltage of the alternating current bus module and the current flowing through the breaker module in the switching-on process generate high-frequency oscillation, the voltage of the filter module is equal to the voltage of the alternating current bus module, and the voltages of the two fracture units are zero.

Because the breaker module bears alternating current-direct current mixed voltage after being switched off, the surface of the porcelain sleeve of the arc extinguish chamber is influenced by dirt, humidity, dry belts and the like, the fracture resistance generates difference, and finally, the voltage division among fractures is uneven, so that the insulation faults such as external insulation flashover, re-burning, explosion and the like are caused, the influence of uneven fracture resistance on the fracture voltage distribution condition in the switching-off process can be researched by changing the sizes of R1 and R2 in the introduced equivalent circuit model, and the overvoltage calculation process and the principle under the condition of uneven fracture resistance of the breaker are introduced below.

As can be seen from the foregoing, after the breaker module is opened, the voltage of the fracture unit is the difference between the power frequency voltage of the ac bus module and the dc voltage of the first capacitor C4. After the breaker module is disconnected, under a stable state, the direct-current voltages born by the two fracture units are distributed according to the relation of the fracture resistance of the two fracture units, and the alternating-current voltages born by the two fracture units are distributed according to the relation of the fracture resistance and the port parallel capacitor in the two fracture units.

Under normal conditions, the measured value of the fracture resistance is 20G omega, and the fracture parallel capacitance value is 2e^-4μ F, impedance at power frequency of 1.59M Ω. The fracture resistance value is far larger than the capacitance impedance, so the power frequency voltage between the fractures is divided according to the magnitude relation of the parallel capacitance, the influence of dirt on the surface of the porcelain sleeve of the arc extinguish chamber on the capacitance value is small, and the nonuniformity comes from the allowable manufacturing deviation of the capacitance.

Fig. 13 shows that R1 is 20e⁹Ω、R2＝20e⁸And omega, the uneven coefficient is 10:1, wherein Eab1 (a part of wave form with darker color in the figure) is the recovery voltage change condition of the first fracture unit 21, and Eab2 (a part of wave form with lighter color in the figure) is the recovery voltage change condition of the second fracture unit 22. Fig. 14 shows that R1 is 20e⁹Ω、R2＝20e⁷And omega, the uneven coefficient is 100:1, wherein Eab1 (a part of wave form with darker color in the figure) is the recovery voltage change condition of the first fracture unit 21, and Eab2 (a part of wave form with lighter color in the figure) is the recovery voltage change condition of the second fracture unit 22. The impedance of the fracture parallel capacitor is far smaller than that of the resistor, the alternating voltage is evenly divided in the two fracture units, and the direct voltage is divided according to the fracture resistor. However, as can be seen from fig. 13 and 14, the difference of the recovery voltage between the two fracture units after disconnection is small, and the voltage of the two fractures is almost one under the non-uniform coefficient of 10:1In the case of 100:1, a little difference occurs, and the dc voltage is not divided by the non-uniform coefficient. The voltage borne by the first capacitor C4 of the filter module is analyzed, so that the capacitor voltage is changed from original alternating current to direct current at the moment of disconnection of the breaker module, and alternating current voltage and direct current voltage are respectively extracted from the voltage signal. After the breaker is disconnected, the capacitor provides a step voltage, the time constant tau is R1R 2/(R1+ R2) (C1+ C2), tau under the uneven coefficient of 100:1 is 0.8s, the voltage generally tends to be stable after 3 tau-5 tau, 1/2 periods (10ms) are needed when the recovery voltage reaches the maximum value after the breaker is disconnected, and the distribution of the alternating current and direct current voltage between fractures at the stage is in a transient state. Therefore, when the fracture resistance is large, the time constant is large, and the recovery voltage distribution between fractures is less affected by the fracture resistance and is uniformly distributed according to the capacitance value.

According to the method for calculating the switching-off capacitive load overvoltage of the filter circuit breaker, which is provided by the embodiment of the invention, the corresponding and pre-established elements are subjected to parameter setting according to the set parameters of each element in the alternating current bus module, the circuit breaker module and the filter module so as to construct an equivalent circuit model for calculating the switching-off capacitive load overvoltage of the filter circuit breaker; the circuit breaker comprises a circuit breaker module, a plurality of resistors, a plurality of capacitors and a plurality of capacitors, wherein the elements pre-established in the circuit breaker module comprise a plurality of fracture resistors and a plurality of fracture parallel capacitors, and the parameters of each fracture resistor are configured according to the resistance value of a body insulation resistor of a circuit breaker arc extinguish chamber and the resistance value of a surface resistor of a circuit breaker porcelain bushing; and performing simulation calculation on the switching-on and switching-off capacitive load overvoltage of the filter circuit breaker through the equivalent circuit model to obtain the switching-on and switching-off capacitive load overvoltage of the filter circuit breaker, so that the switching-on and switching-off capacitive load overvoltage of the filter circuit breaker is accurately calculated. In the process of constructing the equivalent circuit model, the influence of the body insulation resistance, the surface resistance of the porcelain bushing and the parallel capacitor of the arc extinguish chamber in the circuit breaker on the on-off capacitive load overvoltage of the filter circuit breaker is considered, so that the equivalent circuit model is more accurate, and the on-off capacitive load overvoltage of the filter circuit breaker can be accurately simulated and calculated.

Fig. 15 is a schematic structural diagram of a computing apparatus for switching on/off an overvoltage of a capacitive load by a filter circuit breaker according to an embodiment of the present invention.

The device for calculating the overvoltage of the capacitive load of the filter breaker, provided by the embodiment of the invention, comprises:

the model building module 10 is used for setting parameters of corresponding and pre-established elements according to the set parameters of each element in the alternating current bus module, the breaker module and the filter module so as to build an equivalent circuit model for calculating the switching-off capacitive load overvoltage of the filter breaker; the circuit breaker comprises a circuit breaker module, a plurality of resistors, a plurality of capacitors and a plurality of capacitors, wherein the elements pre-established in the circuit breaker module comprise a plurality of fracture resistors and a plurality of fracture parallel capacitors, and the parameters of each fracture resistor are configured according to the resistance value of a body insulation resistor of a circuit breaker arc extinguish chamber and the resistance value of a surface resistor of a circuit breaker porcelain bushing;

and the overvoltage calculation module 20 is configured to perform simulation calculation on the switching-on and switching-off capacitive load overvoltage of the filter circuit breaker through the equivalent circuit model to obtain the switching-on and switching-off capacitive load overvoltage of the filter circuit breaker.

For the specific working principle of each module of the calculating apparatus for switching on and off the capacitive load overvoltage by the filter circuit breaker according to the embodiment of the present invention, please refer to the related content of the calculating method for switching on and off the capacitive load overvoltage by the filter circuit breaker, which is not described herein again.

According to the device for calculating the capacitive load overvoltage of the filter circuit breaker, which is provided by the embodiment of the invention, the corresponding and pre-established elements are subjected to parameter setting according to the set parameters of each element in the alternating current bus module, the circuit breaker module and the filter module so as to construct an equivalent circuit model for calculating the capacitive load overvoltage of the filter circuit breaker; the circuit breaker comprises a circuit breaker module, a plurality of resistors, a plurality of capacitors and a plurality of capacitors, wherein the elements pre-established in the circuit breaker module comprise a plurality of fracture resistors and a plurality of fracture parallel capacitors, and the parameters of each fracture resistor are configured according to the resistance value of a body insulation resistor of a circuit breaker arc extinguish chamber and the resistance value of a surface resistor of a circuit breaker porcelain bushing; and performing simulation calculation on the switching-on and switching-off capacitive load overvoltage of the filter circuit breaker through the equivalent circuit model to obtain the switching-on and switching-off capacitive load overvoltage of the filter circuit breaker, so that the switching-on and switching-off capacitive load overvoltage of the filter circuit breaker is accurately calculated. In the process of constructing the equivalent circuit model, the influence of the body insulation resistance, the surface resistance of the porcelain bushing and the parallel capacitor of the arc extinguish chamber in the circuit breaker on the on-off capacitive load overvoltage of the filter circuit breaker is considered, so that the equivalent circuit model is more accurate, and the on-off capacitive load overvoltage of the filter circuit breaker can be accurately simulated and calculated.

The above description is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, several modifications and variations can be made without departing from the technical principle of the present invention, and these modifications and variations should also be regarded as the protection scope of the present invention.