Abstract:
A multi-functional automatic switching circuit for direct current ice melting and a switching method thereof are provided. The automatic switching circuit comprises at least one sub-switching circuit. The sub-switching circuit comprises a six-pulse current converter (R) with no saturable reactor, six reactors (L 1   a , L 1   b , L 1   c , L 2   a , L 2   b , and L 2   c ), three three-phase knife switches (Sac 1 , Sac 2 , and Sac 3 ), and five single-phase knife switches (SV 1 , SV 2 , SV 3 , SV 4 , and SV 5 ). The sub-switching circuits in series connection or parallel connection, four direct current side switching knife switches (Sdc 1 , Sdc 2 , Sdc 3 , and Sdc 4 ), an isolation knife switch (K), a breaker (QF) and a control and protection system (CP) may form an automatic switching circuit for six-pulse or twelve-pulse direct current ice melting. The automatic switching circuit with no saturable reactor reduces noise during operation, achieves mutually automatic switching among modes of reactor switching, ice melting and test, and may complete the through-flow test of the automatic switching circuit without switching in a power transmission line, thereby solving the problem of daily operation and maintenance.

Description:
FILED OF THE INVENTION 
       [0001]    The present invention relates to a multi-functional automatic switching circuit for direct current ice melting and a switching method thereof, and in particular, to a circuit able to achieve mutually automatic switching among a Thyristor Controlled Reactor (TCR), a Thyristor Switched Reactor (TSR), direct current ice melting and equivalent test functions, and a switching method thereof, which belong to an innovative technology applied by direct current ice melting for power transmission lines of high voltage grids and ultra-high voltage grids. 
       BACKGROUND OF THE INVENTION 
       [0002]    Icing of power transmission lines caused by low temperature and rain or snow constitutes one of serious threats to power systems in a lot of countries. Severe icing will cause large-area power outage due to grid disconnection and tower collapse. It also leads to difficulties to resume power supply. The threat of ice disaster is always one technical difficulty fought by the industry of power systems for a long time. 
         [0003]    In 1998, storm in North America had a serious effect on power grids in America and Canada, which caused large-area power outage. In 2005, low temperature and rain or snow caused large damage to Central China and North China Power Grids. In January-February, 2008, low temperature and rain or snow attacked south China, central China, and east China, which caused large-area and long-term outage of power transmission lines in Guizhou province, Hunan province, Guangdong province, Yunnan province, Guangxi province, Jiangxi province, and so on and further caused enormous losses to national economy and people&#39;s livelihood. 
         [0004]    Many deicing/ice melting techniques were researched in industrial circles and academic circles to reduce the serious threat of more frequent ice disaster to the infrastructures of power systems. Wherein, the basic principle of direct current ice melting is as follows: alternating current electric power available from the power system is converted into direct current electric power by high-power rectifier devices, then the direct current electric power is input to wires of lines to be subjected to ice melting, the wires are heated with the action of the direct current to ensure that the ice covered thereon is melted, and therefore, the risks of line disconnection and tower collapse are eliminated. The direct current ice melting technique overcomes the limitation of alternating current ice melting, and inductive components of line resistance do not function during direct current ice melting, which greatly reduces the volume necessary for direct current ice melting and increases the ice melting efficiency. Further, during direct current ice melting, the direct current voltage is continuously adjustable, ice melting requirements of lines with different lengths may be met through adjusting the direct current output voltage, and no impedance matching is needed, thus the strict demand on operation modes of the power system is greatly lowered. Moreover, a direct current ice melting device located in a load-center substation may be used to melt ice covered on all the lines through the substation. 
         [0005]    In the world, the former Soviet Union started using diode rectifier devices to melt ice since 1972, and later adopted silicon controlled rectifier devices. The direct current research institute of Russia successfully developed silicon controlled rectifier ice melting devices of two voltage classes: 14 kV (powered by a 11 kV alternating current bus) and 50 kV (powered by a 38.5 kV alternating current bus). The rated power of the 14 kV device is 14 MW, and the rated power of the 50 kV device is 50 MW. The 50 MW device was put into operation in a substation in 1994, and successfully applied to deicing of a 110 kV power transmission line of 315 km. After ice disaster in North America in 1998, the Quebec Electricity And Water Authority and the AREVAT&amp;D company invested 25 millions EUR to cooperatively develop a direct current ice melting device, and one set of direct current ice melting device was mounted in the Lvis substation of the Quebec power grid, with the volume of 250 MW and the direct current output voltage of ±17.4 kV, and its design purpose is to melt ice covered on four 735 kV lines and two 315 kV lines. This device was subjected to field test in 2008, but so far has not been implemented in practical ice melting. 
         [0006]    After ice disaster in 2008, power scientific and technical workers in China autonomously performed research and development of the direct current ice melting technique, and successfully researched and developed high-power direct current ice melting devices possessing completely independent intellectual property rights, mainly including many models such as a model with special rectifier transformers, a model with no special rectifier transformers, and a vehicle-mounted movable model, and further they were popularized and applied across the China, so far about 20 sets of direct current ice melting devices in total have been put into operation, where 19 sets of direct current ice melting devices have been provided in the China Southern Power Grid. 
         [0007]    In January, 2009, the Guizhou Power Grid Corporation performed direct current ice melting on a 500 kV FuShi II line, a 220 kV FuJiu line, a 110 kV FuNiu line, and a 110 kV Shuishumei line, the Yunnan Power Grid Corporation performed direct current ice melting on a 220 kV ZhaoDa I line, and the Guangdong Power Grid Corporation performed direct current ice melting on a 110 kV TongMei line. In November, 2009, the Yunnan Power Grid Corporation performed direct current ice melting on a 110 kV DaZhong T line. Initial practical applications indicate that the direct current ice melting technique is an effective means of deicing the power grids. 
         [0008]    In January, 2011, large-area icing attacked the China Southern Power Grid again, 19 sets of direct current ice melting devices which have been installed in the China Southern Power Grid all played a major role, and performed 217 times of direct current ice melting in total on the lines of above 110 kV, where more than 40 times of direct current ice melting on alternating current lines of 500 kV, which fully played a role of the direct current ice melting devices. 
         [0009]    In view of the practical application effect of the direct current ice melting devices, the power grid enterprises in China started a new round of large-scale popularization and application since 2011 again. 
         [0010]    However, it is found that in the practical application in the icing periods from 2009 to 2011, the existing direct current ice melting devices needs to be optimized in some aspects. For example, in one aspect, a saturable reactor of a converter has a large noise. In another aspect, current interruption occurs during short line ice melting. In a further aspect, the current-through test can only be done when the ice melting device is switched in the power transmission line, which is limited by the operation mode of power grids and also affects its normal operation. 
       SUMMARY OF THE INVENTION 
       [0011]    In consideration of the above problem, an object of the present invention is to provide a multi-functional automatic switching circuit for direct current ice melting, which greatly reduces the noise during the operation of the direct current ice melting device, is able to achieve mutually automatic switching among multiple modes and complete the current-through test without being switched in the power transmission line, and effectively solves the problem of daily operation and maintenance. 
         [0012]    A further object of the present invention is to provide a switching method of the multi-functional automatic switching circuit for direct current ice melting, which is simple to operate and convenient to use. 
         [0013]    According to the technical solution of the present invention, a multi-functional automatic switching circuit for direct current ice melting is provided, including at least one sub-switching circuit composed of the following members, wherein the sub-switching circuit includes a six-pulse converter R with no saturable reactor, reactors L 1   a , L 1   b , and L 1   c , reactors L 2   a , L 2   b , and L 2   c , three-phase knife switches Sac 1 , Sac 2 , and Sac 3 , single-phase knife switches SV 1 , SV 2 , SV 3 , SV 4 , and SV 5 . The six-pulse converter R with no saturable reactor of the sub-switching circuit is connected to the reactors L 1   a , L 1   b , and L 1   c , and connected to the reactors L 2   a , L 2   b , and L 2   c . The reactors L 2   a , L 2   b , and L 2   c  are correspondingly connected to the reactors L 1   c , L 1   b , and L 1   a  through the three-phase knife switch Sac 1 , separately. The three-phase knife switch Sac 3  is connected to one end of the six-pulse converter R at one end and short circuited at the other end. The three-phase knife switch Sac 2  is connected to the reactors L 2   a , L 2   b , and L 2   c  at one end and short circuited at the other end. The single-phase knife switch SV 1  is connected, at two ends, to a negative pole of a valve arm V 1  and a positive pole of a valve arm V 4  of the six-pulse converter R, separately. The single-phase knife switch SV 2  is connected, at two ends, to a negative pole of a valve arm V 3  and a positive pole of a valve arm V 6  of the six-pulse converter R, separately. The single-phase knife switch SV 3  is connected, at two ends, to a negative pole of a valve arm V 5  and a positive pole of a valve arm V 2  of the six-pulse converter R, separately. The single-phase knife switch SV 4  is connected between phases of the reactors L 2   b  and L 2   c , and the single-phase knife switch SV 5  is connected between phases of the reactors L 2   a  and L 2   c.    
         [0014]    The six-pulse converter R of the abovementioned sub-switching circuit does not have a saturable reactor. 
         [0015]    The inductances of the reactors L 1   a , L 1   b , and L 1   c  of the abovementioned sub-switching circuit are determined based on a thyristor current changing ratio di/dt and a short circuit current limit. The inductances of the reactors L 1   a , L 1   b , and L 1   c  are 0-0.2 times than those of the reactors L 2   a , L 2   b , and L 2   c . The rated currents of the reactors L 1   a , L 1   b , and L 1   c  are designed as required for smoothing reactors in an ice melting mode. The rated currents of the reactors L 2   a , L 2   b , and L 2   c  are designed as required for the TCR or TSR. 
         [0016]    The abovementioned automatic switching circuit for six-pulse direct current ice melting includes a sub-switching circuit, direct current side switching knife switches Sdc 1 , Sdc 2 , Sdc 3 , and Sdc 4 , an isolation knife switch K, a breaker QF, and a control and protection system CP. The direct current side switching knife switches Sdc 1  and Sdc 2  are in parallel connection and then connected to a short circuited end of the three-phase knife switch Sac 3  of the sub-switching circuit. The single-phase knife switches Sdc 3  and Sdc 4  are in parallel connection and then connected to a short circuited end of the three-phase knife switch Sac 2  of the sub-switching circuit. The sub-switching circuit is connected to a 35 kV or 10 kV bus of a substation through the isolation knife switch K and the breaker QF. Position signals of the knife switches Sac 1 , Sac 2 , Sac 3 , K, SV 1 , SV 2 , SV 3 , SV 4 , SV 5 , Sdc 1 , Sdc 2 , Sdc 3 , and Sdc 4  and of the breaker QF, alternating current side current signals Iva, Ivb, and Ivc, direct current side current signals Idp and Idn, and direct current side voltage signals Udp and Udn of a converter and a monitoring signal of the six-pulse converter R are communicated to the control and protection system CP. The control and protection system CP sends on/off instructions to the knife switches and the breaker QF, and control/trigger instructions to the six-pulse converter R. 
         [0017]    The abovementioned series-connection type automatic switching circuit for twelve-pulse direct current ice melting includes two sub-switching circuits in series connection, which are respectively a sub-switching circuit TC 1  and a sub-switching circuit TC 2 , a Y/Y coupling transformer T 1 , a Y/Δ coupling transformer T 2 , direct current side switching knife switches Sdc 1 , Sdc 2 , Sdc 3 , and Sdc 4 , an isolation knife switch K, a breaker QF, and a control and protection system CP. A middle point of a connection between the two sub-switching circuits is grounded, the sub-switching circuit TC 1  is connected to the Y/Y coupling transformer T 1 , and the sub-switching circuit TC 2  is connected to the Y/Δ coupling transformer T 2 . The six-pulse switching converters R with no saturable reactors of the sub-switching circuits TC 1  and TC 2  are connected through a short circuited end of the three-phase knife switch Sac 3 . The direct current side switching knife switches Sdc 1  and Sdc 2  are in parallel connection and then connected to a short circuited end of the three-phase knife switch Sac 2  of the sub-switching circuit TC 1 . The single-phase knife switches Sdc 3  and Sdc 4  are in parallel connection and then connected to a short circuited end of the three-phase knife switch Sac 2  of the sub-switching circuit TC 2 . The Y/Y coupling transformer T 1  and the Y/Δ coupling transformer T 2  are connected to a 35 kV or 10 kV or 220 kV bus of a substation through the isolation knife switch K and the breaker QF. Position signals of the knife switches Sac 1 , Sac 2 , Sac 3 , K, SV 1 , SV 2 , SV 3 , SV 4 , SV 5 , Sdc 1 , Sdc 2 , Sdc 3 , and Sdc 4  and of the breaker QF, valve side current signals Iyva, Iyvb, Iyvc, Idva, Idvb, and Idvc, grid side currents Iya, Iyb, Iyc, Ida, Idb, and Idc, direct current side current signals Idp, Idn, and Idgn, and direct current side voltage signals Udp and Udn of rectifier transformers, and monitoring signals of the six-pulse converters R are communicated to the control and protection system CP. The control and protection system CP sends on/off instructions to the knife switches and the breaker QF, and control/trigger instructions to the six-pulse converters R. 
         [0018]    The abovementioned parallel-connection type automatic switching circuit for direct current ice melting includes two sub-switching circuits in parallel connection, which are respectively a sub-switching circuit TC 1  and a sub-switching circuit TC 2 , a Y/Y coupling transformer T 1 , a Y/Δ coupling transformer T 2 , direct current side switching knife switches Sdc 1 , Sdc 2 , Sdc 3 , and Sdc 4 , an isolation knife switch K, a breaker QF, and a control and protection system CP. The sub-switching circuit TC 1  is connected to the Y/Y coupling transformer T 1 , and the sub-switching circuit TC 2  is connected to the Y/Δ coupling transformer T 2 . A short circuited end of the three-phase knife switch Sac 2  of the sub-switching circuit TC 1  is connected to a short circuited end of the three-phase knife switch Sac 3  of the sub-switching circuit TC 2 . A short circuited end of the three-phase knife switch Sac 3  of the sub-switching circuit TC 1  is connected to a short circuited end of the three-phase knife switch Sac 3  of the sub-switching circuit TC 2 . The direct current side switching knife switches Sdc 1  and Sdc 2  are in parallel connection and then connected to a short circuited end of the three-phase knife switch Sac 2  of the sub-switching circuit TC 1 . The side-phase knife switches Sdc 3  and Sdc 4  are in parallel connection and then connected to a short circuited end of the three-phase knife switch Sac 2  of the sub-switching circuit TC 2 . The Y/Y coupling transformer T 1  and the Y/Δ coupling transformer T 2  are connected to a 35 kV or 10 kV or 220 kV bus of a substation through the isolation knife switch K and the breaker QF. Position signals of the knife switches Sac 1 , Sac 2 , Sac 3 , K, SV 1 , SV 2 , SV 3 , SV 4 , SV 5 , Sdc 1 , Sdc 2 , Sdc 3 , and Sdc 4  and of the breaker QF, valve side current signals Iyva, Iyvb, Iyvc, Idva, Idvb, and Idvc and grid side currents Iya, Iyb, Iyc, Ida, Idb, and Idc, direct current side current signals Idp and Idn, and direct current side voltage signals Udp and Udn of rectifier transformers, and monitoring signals of six-pulse converters R are communicated to the control and protection system CP. The control and protection system CP sends on/off instructions to the knife switches and the breaker QF, and control/trigger instructions to the six-pulse converters R. 
         [0019]    When the abovementioned automatic switching circuit for six-pulse direct current ice melting operates in an ice melting mode, a direct current side is not grounded. 
         [0020]    When the abovementioned series-connection type automatic switching circuit for twelve-pulse direct current ice melting operates in an ice melting mode, a connecting point of the two six-pulse converters R at the direct current side is directly grounded. The abovementioned series-connection type automatic switching circuit for twelve-pulse direct current ice melting keeps twelve pulses both when operating in the ice melting mode and in a TCR mode. The inductances of the reactors L 1   a , L 1   b , and L 1   c  of the abovementioned series-connection type automatic switching circuit for twelve-pulse direct current ice melting may be 0, that is, the reactors L 1   a , L 1   b , and L 1   c  are not provided. 
         [0021]    When the abovementioned parallel-connection type automatic switching circuit for twelve-pulse direct current ice melting operates in an ice melting manner, a direct current side is not grounded. The abovementioned parallel-connection type automatic switching circuit for twelve-pulse direct current ice melting keeps twelve pulses both when operating in the ice melting mode and in a TCR mode. The inductances of the reactors L 1   a , L 1   b , and L 1   c  of the abovementioned parallel-connection type automatic switching circuit for twelve-pulse direct current ice melting may be 0, that is, the reactors L 1   a , L 1   b , and L 1   c  are not provided. 
         [0022]    A switching method of a multi-functional automatic switching circuit is provided by the present invention, including the following switching modes: 
         [0023]    1) a one-go and one-return direct current ice melting mode, that is, A and B phase lines are in series connection to melt ice, wherein the three-phase knife switch Sac 1  is turned off, the three-phase knife switches Sac 2  and Sac 3  are turned on, the single-phase knife switches SV 1 , SV 2 , and SV 3  are turned off, the single-phase knife switches SV 4  and SV 5  are turned on, the direct current side switching knife switches Sdc 1  and Sdc 2  are turned on, the direct current side switching knife switches Sdc 3  and Sd 4  are turned off, and wherein the alternating current side isolation knife switch K and the breaker QF are turned on; 
         [0024]    2) a two-go and one-return direct current ice melting mode, that is, A and B phase lines are in parallel connection and then in series connection with a C phase line to melt ice, wherein the three-phase knife switches Sac 1  is turned off, the three-phase knife switches Sac 2  and Sac 3  are turned on, the single-phase knife switches SV 1 , SV 2 , and SV 3  are turned off, the single-phase knife switches SV 4  and SV 5  are turned on, the direct current side switching knife switches Sdc 1 , Sdc 2  and Sdc 4  are turned on, the direct current side switching knife switch Sdc 3  is turned off, and wherein the alternating current side isolation knife switch K and the breaker QF are turned on; 
         [0025]    3) an open circuit test mode, wherein the three-phase knife switch Sac 1  is turned off, the three-phase knife switches Sac 2  and Sac 3  are turned on, the single-phase knife switches SV 1 , SV 2 , and SV 3  are turned off, the single-phase knife switches SV 4  and SV 5  are turned on, the direct current side switching knife switches Sdc 1 , Sdc 2 , Sdc 3  and Sdc 4  are turned off, and wherein the alternating current side isolation knife switch K and the breaker QF are turned on; 
         [0026]    4) a zero power test mode, wherein the three-phase knife switch Sac 1  is turned off, the three-phase knife switches Sac 2  and Sac 3  are turned on, the single-phase knife switches SV 1 , SV 2 , and SV 3  are turned off, the single-phase knife switches SV 4  and SV 5  are turned on, the direct current side switching knife switches Sdc 1  and Sdc 4  are turned off, the direct current side switching knife switches Sdc 2  and Sdc 3  are turned on, and wherein the alternating current side isolation knife switch K and the breaker QF are turned on; 
         [0027]    5) a TCR mode, wherein the three-phase knife switch Sac 1  is turned on, the three-phase knife switches Sac 2  and Sac 3  are turned off, the single-phase knife switches SV 1 , SV 2 , and SV 3  are turned on, the single-phase knife switches SV 4  and SV 5  are turned off, the direct current side switching knife switches Sdc 1 , Sdc 2 , Sdc 3  and Sdc 4  are turned off, and wherein the alternating current side isolation knife switch K and the breaker QF are turned on; and 
         [0028]    6) a TSR mode, wherein the three-phase knife switch Sac 1  is turned on, the three-phase knife switches Sac 2  and Sac 3  are turned off, the single-phase knife switches SV 1 , SV 2 , and SV 3  are turned on, the single-phase knife switches SV 4  and SV 5  are turned off, the direct current side switching knife switches Sdc 1 , Sdc 2 , Sdc 3  and Sdc 4  are turned off, and wherein the alternating current side isolation knife switch K and the breaker QF are turned on. 
         [0029]    The abovementioned switching method of the multi-functional automatic switching circuit for direct current ice melting is characterized in that in either of the one-go and one-return direct current ice melting mode, the two-go and one-return direct current ice melting mode, the open circuit test mode and the zero power test mode, the reactors L 1   a , L 1   b  and L 1   c  function as commutation reactors, and the reactors L 2   a , L 2   b  and L 2   c  function as smoothing reactors. 
         [0030]    The abovementioned switching method of the multi-functional automatic switching circuit for direct current ice melting is characterized in that in the TCR mode, the reactors L 1   a , L 1   b  and L 1   c  and the reactors L 2   a , L 2   b  and L 2   c  function as phase controlled reactors, and in the TSR mode, the reactors L 1   a , L 1   b  and L 1   c  and the reactors L 2   a , L 2   b  and L 2   c  function as switched reactors. 
         [0031]    As a result of adopting the structure of the converter with the smoothing reactor, the current-through test can be completed without enabling the direct current ice melting device to be switched in the power transmission line, and the direct current ice melting device can be switched to operate in the TCR or TSR when there is no necessary to melt ice. The converter with no saturable reactor of the direct current ice melting device greatly reduces the noise during the operation of the direct current ice melting device. By providing the isolation knife switch, the present invention is able to achieve mutually automatic switching among the TCR mode, the direct current ice melting mode and equivalent test modes thereof, and other modes. The current-through test can be completed without enabling the direct current ice melting device to be switched in the power transmission line, and the problem of daily operation and maintenance can be effectively solved. The circuit able to achieve mutually automatic switching among TCR, TSR, direct current ice melting and equivalent test functions thereof, provided by present invention, is suitable for ice melting of high-voltage and ultra-high voltage power transmission lines. The multi-functional automatic switching circuit for direct current ice melting, provided by the present invention, is reasonable in design, and is convenient and practical. The switching method of the multi-functional automatic switching circuit for direct current ice melting, provided by the present invention, is simple to operate and easy to use. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]    The following will further describe the patent for invention in detail with reference to the accompanying drawing and specific embodiments. 
           [0033]      FIG. 1  shows a sub-switching circuit of a multi-functional automatic switching circuit for direct current ice melting according to the present invention. 
           [0034]      FIG. 2  shows a principle of an automatic switching circuit for six-pulse direct current ice melting comprising a sub-switching circuit according to the present invention. 
           [0035]      FIG. 3  shows a principle of a series-connection type automatic switching circuit for twelve-pulse direct current ice melting comprising two sub-switching circuits in series connection according to the present invention. 
           [0036]      FIG. 4  shows a principle of a parallel-connection type automatic switching circuit for twelve-pulse direct current ice melting comprising two sub-switching circuits in parallel connection according to the present invention. 
           [0037]      FIG. 5  shows an embodiment of an automatic switching circuit for six-pulse direct current ice melting comprising a sub-switching circuit connected to the 10 kV side of a 220 kV main transformer according to the present invention. 
           [0038]      FIG. 6  shows an embodiment of an automatic switching circuit for six-pulse direct current ice melting comprising a sub-switching circuit connected to the 35 kV side of a 220 kV main transformer according to the present invention. 
           [0039]      FIG. 7  shows an embodiment of a series-connection type automatic switching circuit for twelve-pulse direct current ice melting comprising two sub-switching circuits in series connection connected to the 10 kV side of a 220 kV main transformer according to the present invention. 
           [0040]      FIG. 8  shows an embodiment of a series-connection type automatic switching circuit for twelve-pulse direct current ice melting comprising two sub-switching circuits in series connection connected to the 35 kV side of a 220 kV main transformer according to the present invention. 
           [0041]      FIG. 9  shows an embodiment of a series-connection type automatic switching circuit for twelve-pulse direct current ice melting comprising two sub-switching circuits in series connection connected to the 220 kV side of a 500 kV main transformer according to the present invention. 
           [0042]      FIG. 10  shows an embodiment of a parallel-connection type twelve-pulse automatic switching circuit for direct current ice melting comprising two sub-switching circuits in parallel connection connected to the 35 kV side of a 500 kV main transformer according to the present invention. 
       
    
    
       [0043]    In  FIG. 1  to  FIG. 10 , Uab, Ubc and Uca are three phases of voltages of the 10 kV or 35 kV or 220 kV bus. Iva, Ivb, Ivc, Iyva, Iyvb, Iyvc, Idva, Idvb, and Idvc are alternating current side currents of a converter. Iya, Iyb, Iyc, Ida, Idb, and Idc are grid side currents of rectifier transformers. Idp, Idn, and Idgn are direct current side currents. Udp and Udn are direct current side voltages. K is an alternating current side isolation knife switch. QF is an alternating current side breaker. K 1  is an alternating current side isolation knife switch. QF 1  is an alternating current side breaker. F is a filter bank. 
       DETAILED DESCRIPTION OF EMBODIMENTS 
     Embodiments 
       [0044]    The structural schematic diagrams of the present invention are shown in  FIG. 1  to  FIG. 10 . According to the present invention, a multi-functional automatic switching circuit for direct current ice melting is provided, including at least one sub-switching circuit composed of the following members, wherein the sub-circuiting circuit includes a six-pulse converter R with no saturable reactor, reactors L 1   a , L 1   b , and L 1   c , reactors L 2   a , L 2   b , and L 2   c , three-phase knife switches Sac 1 , Sac 2 , and Sac 3 , and single-phase knife switches SV 1 , SV 2 , SV 3 , SV 4 , and SV 5 . The six-pulse converter R with no saturable reactor of the sub-switching circuit is connected to the reactors L 1   a , L 1   b , and L 1   c , and connected to the reactors L 2   a , L 2   b , and L 2   c . The reactors L 2   a , L 2   b , and L 2   c  are correspondingly connected to the reactors L 1   c , L 1   b , and L 1   a  through the three-phase knife switch Sac 1 , separately. The three-phase knife switch Sac 3  is connected to one end of the six-pulse converter R with no saturable reactor at one end and short circuited at the other end. The three-phase knife switch Sac 2  is connected to the reactors L 2   a , L 2   b , and L 2   c  at one end and short circuited at the other end. The single-phase knife switch SV 1  is connected, at two ends, to a negative pole of a valve arm V 1  and a positive pole of a valve arm V 4  of the six-pulse converter R, separately. The single-phase knife switch SV 2  is connected, at two ends, to a negative pole of a valve arm V 3  and a positive pole of a valve arm V 6  of the six-pulse converter R, separately. The single-phase knife switch SV 3  is connected, at two ends, to a negative pole of a valve arm V 5  and a positive pole of a valve arm V 2  of the six-pulse converter R, separately. The single-phase knife switch SV 4  is connected between phases of the reactors L 2   b  and L 2   c , and the single-phase knife switch SV 5  is connected between phases of the reactors L 2   a  and L 2   c.    
         [0045]    The abovementioned automatic switching circuit for six-pulse direct current ice melting includes a sub-switching circuit, direct current side switching knife switches Sdc 1 , Sdc 2 , Sdc 3 , and Sdc 4 , an isolation knife switch K, a breaker QF, and a control and protection system CP. The direct current side switching knife switches Sdc 1  and Sdc 2  are in parallel connection and then connected to the short circuited end of the three-phase knife switch Sac 3  of the sub-switching circuit. The single-phase knife switches Sdc 3  and Sdc 4  are in parallel connection and then connected to the short circuited end of the three-phase knife switch Sac 2  of the sub-switching circuit. The sub-switching circuit is connected to a 35 kV or 10 kV bus of a substation through the isolation knife switch K and the breaker QF. Position signals of the knife switches Sac 1 , Sac 2 , Sac 3 , K, SV 1 , SV 2 , SV 3 , SV 4 , SV 5 , Sdc 1 , Sdc 2 , Sdc 3 , and Sdc 4  and of the breaker QF, alternating current side current signals Iva, Ivb, and Ivc, direct current side current signals Idp and Idn, and direct current side voltage signals Udp and Udn of the converter, and a monitoring signal of a six-pulse converter R are communicated to the control and protection system CP. The control and protection system CP sends on/off instructions to the knife switches and the breaker QF, and control/trigger instructions to the six-pulse converter R. 
         [0046]    The abovementioned series-connection type automatic switching circuit for twelve-pulse direct current ice melting includes two sub-switching circuits in series connection, which are respectively a sub-switching circuit TC 1  and a sub-switching circuit TC 2 , a Y/Y coupling transformer T 1 , a Y/Δ coupling transformer T 2 , direct current side switching knife switches Sdc 1 , Sdc 2 , Sdc 3 , and Sdc 4 , an isolation knife switch K, a breaker QF, and a control and protection system CP. A middle point of a connection between the two sub-switching circuits is grounded, the sub-switching circuit TC 1  is connected to the Y/Y coupling transformer T 1 , and the sub-switching circuit TC 2  is connected to the Y/Δ coupling transformer T 2 . The six-pulse converters R with no saturable reactors of the sub-switching circuits TC 1  and TC 2  are connected through a short circuited end of the three-phase knife switch Sac 3 . The direct current side switching knife switches Sdc 1  and Sdc 2  are in parallel connection and then connected to a short circuited end of the three-phase knife switch Sac 2  of the sub-switching circuit TC 1 . The single-phase knife switches Sdc 3  and Sdc 4  are in parallel connection and then connected to a short circuited end of the three-phase knife switch Sac 2  of the sub-switching circuit TC 2 . The Y/Y coupling transformer T 1  and the Y/Δ coupling transformer T 2  are connected to a 35 kV or 10 kV or 220 kV bus of a substation through the isolation knife switch K and the breaker QF. Position signals of the knife switches Sac 1 , Sac 2 , Sac 3 , K, SV 1 , SV 2 , SV 3 , SV 4 , SV 5 , Sdc 1 , Sdc 2 , Sdc 3 , and Sdc 4  and of the breaker QF, valve side current signals Iyva, Iyvb, Iyvc, Idva, Idvb, and Idvc, grid side currents Iya, Iyb, Iyc, Ida, Idb, and Idc, direct current side current signals Idp, Idn, and Idgn, and direct current side voltage signals Udp and Udn of rectifier transformers and monitoring signals of the six-pulse converters R are communicated to the control and protection system CP. The control and protection system CP sends on/off instructions to the knife switches and the breaker QF, and control/trigger instructions to the six-pulse converters R. 
         [0047]    The abovementioned parallel-connection type automatic switching circuit for twelve-pulse direct current melting includes two sub-switching circuits in parallel connection, which are respectively a sub-switching circuit TC 1  and a sub-switching circuit TC 2 , a Y/Y coupling transformer T 1 , a Y/Δ coupling transformer T 2 , direct current side switching knife switches Sdc 1 , Sdc 2 , Sdc 3 , and Sdc 4 , an isolation knife switch K, a breaker QF, and a control and protection system CP. The sub-switching circuit TC 1  is connected to the Y/Y coupling transformer T 1 , and the sub-switching circuit TC 2  is connected to the Y/Δ coupling transformer T 2 . A short circuited end of the three-phase knife switch Sac 2  of the sub-switching circuit TC 1  is connected to a short circuited end of the three-phase knife switch Sac 3  of the sub-switching circuit TC 2 . A short circuited end of the three-phase knife switch Sac 3  of the sub-switching circuit TC 1  is connected to a short circuited end of the three-phase knife switch Sac 3  of the sub-switching circuit TC 2 . The direct current side switching knife switches Sdc 1  and Sdc 2  are in parallel connection and then connected to a short circuited end of the three-phase knife switch Sac 2  of the sub-switching circuit TC 1 . The single-phase knife switches Sdc 3  and Sdc 4  are in parallel connection and then connected to a short circuited end of the three-phase knife switch Sac 2  of the sub-switching circuit TC 2 . The Y/Y coupling transformer T 1  and the Y/Δ coupling transformer T 2  are connected to a 35 kV or 10 kV or 220 kV bus of a substation through the isolation knife switch K and the breaker QF. Position signals of the knife switches Sac 1 , Sac 2 , Sac 3 , K, SV 1 , SV 2 , SV 3 , SV 4 , SV 5 , Sdc 1 , Sdc 2 , Sdc 3 , and Sdc 4  and of the breaker QF, valve side current signals Iyva, Iyvb, Iyvc, Idva, Idvb, and Idvc and grid side currents Iya, Iyb, Iyc, Ida, Idb, and Idc, direct current side current signals Idp and Idn, and direct current side voltage signals Udp and Udn of rectifier transformers and monitoring signals of the six-pulse converters R are communicated to the control and protection system CP. The control and protection system CP sends on/off instructions to the knife switches and the breaker QF, and control/trigger instructions to the six-pulse converters R. 
       Embodiment 1 
       [0048]    Referring to  FIG. 5 , in this embodiment, the abovementioned automatic switching circuit for six-pulse direct current ice melting includes a sub-switching circuit. The sub-switching circuit is connected to the 10 kV side of a 220 kV main transformer through an alternating current side isolation knife switch K and a breaker QF. A filter bank F is used to carry out reactive compensation and harmonic suppression. The sub-switching circuit is connected to a 10 kV bus through an isolation knife switch K 1  and a breaker QF 1 . 
         [0049]    Three phases of voltages Uab, Ubc, and Uca of the 10 kV bus, alternating current side currents Iva, Ivb, and Ivc, direct current side currents Idp and Idn, and direct current side voltages Udp and Udn of the converter, a monitoring signal of the converter, monitoring signals of the knife switches, monitoring signals of the breakers, and the like are communicated to the control and protection system CP. The control and protection system CP sends control instructions to the knife switches, the breakers, and the converter. The implementation solutions of a TCR mode, a TSR mode, a direct current ice melting mode and equivalent test modes thereof, and other modes are as follows: 
         [0050]    1) a one-go and one-return direct current ice melting mode, that is, A and B phase lines are in series connection to melt ice, wherein the three-phase knife switch Sac 1  is turned off, the three-phase knife switches Sac 2  and Sac 3  are turned on, the single-phase knife switches SV 1 , SV 2 , and SV 3  are turned off, the single-phase knife switches SV 4  and SV 5  are turned on, the direct current side switching knife switches Sdc 1  and Sdc 2  are turned on, the direct current side switching knife switches Sdc 3  and Sd 4  are turned off, and the alternating current isolation knife switch K and the breaker QF are turned on; 
         [0051]    2) a two-go and one-return direct current ice melting mode, that is, A and B phase lines are in parallel connection and then in series connection with a C phase line to melt ice, wherein the three-phase knife switch Sac 1  is turned off, the three-phase knife switches Sac 2  and Sac 3  are turned on, the single-phase knife switches SV 1 , SV 2 , and SV 3  are turned off, the single-phase knife switches SV 4  and SV 5  are turned on, the direct current side switching knife switches Sdc 1 , Sdc 2  and Sdc 4  are turned on, the direct current side switching knife switch Sdc 3  is turned off, and the alternating current side isolation knife switch K and the breaker QF are turned on; 
         [0052]    3) an open circuit test mode, wherein the three-phase knife switch Sac 1  is turned off, the three-phase knife switches Sac 2  and Sac 3  are turned on, the single-phase knife switches SV 1 , SV 2 , and SV 3  are turned off, the single-phase knife switches SV 4  and SV 5  are turned on, the direct current side switching knife switches Sdc 1 , Sdc 2 , Sdc 3  and Sdc 4  are turned off, and the alternating current side isolation knife switch K and the breaker QF are turned on; 
         [0053]    4) a zero power test mode, wherein the three-phase knife switch Sac 1  is turned off, the three-phase knife switches Sac 2  and Sac 3  are turned on, the single-phase knife switches SV 1 , SV 2 , and SV 3  are turned off, the single-phase knife switches SV 4  and SV 5  are turned on, the direct current side switching knife switches Sdc 1  and Sdc 4  are turned off, the direct current side switching knife switches Sdc 2  and Sdc 3  are turned on, and the alternating current side isolation knife switch K and the breaker QF are turned on; 
         [0054]    5) a TCR mode, wherein the three-phase knife switch Sac 1  is turned on, the three-phase knife switches Sac 2  and Sac 3  are turned off, the single-phase knife switches SV 1 , SV 2 , and SV 3  are turned on, the single-phase knife switches SV 4  and SV 5  are turned off, the direct current side switching knife switches Sdc 1 , Sdc 2 , Sdc 3  and Sdc 4  are turned off, and the alternating current side isolation knife switch K and the breaker QF are turned on; and 
         [0055]    6) a TSR mode, wherein the three-phase knife switch Sac 1  is turned on, the three-phase knife switches Sac 2  and Sac 3  are turned off, the single-phase knife switches SV 1 , SV 2 , and SV 3  are turned on, the single-phase knife switches SV 4  and SV 5  are turned off, the direct current side switching knife switches Sdc 1 , Sdc 2 , Sdc 3  and Sdc 4  are turned off, and the alternating current side isolation knife switch K and the breaker QF are turned on. 
       Embodiment 2 
       [0056]    Referring to  FIG. 6 , in this embodiment, the abovementioned automatic switching circuit for six-pulse direct current ice melting includes a sub-switching circuit. The sub-switching circuit is connected to the 35 kV side of a 500 kV main transformer through an alternating current side isolation knife switch K and a breaker QF. A filter bank F is used to carry out reactive compensation and harmonic suppression, and is connected to a 35 kV bus through an isolation knife switch K 1  and a breaker QF 1 . 
         [0057]    Three phases of voltages Uab, Ubc, and Uca of the 35 kV bus, and alternating current side currents Iva, Ivb, and Ivc, direct current side currents Idp and Idn, and direct current side voltages Udp and Udn of the converter, a monitoring signal of the converter, monitoring signals of the knife switches, and monitoring signals of the breakers, and the like are communicated to a control and protection system CP. The control and protection system CP sends control instructions to the knife switches, the breakers, and the converter. The implementation solutions of a TCR mode, a TSR mode, a direct current ice melting mode and equivalent test modes thereof, and other modes are as follows: 
         [0058]    1) a one-go and one-return direct current ice melting mode, that is, A and B phase lines are in series connection to melt ice, wherein the three-phase knife switch Sac 1  is turned off, the three-phase knife switches Sac 2  and Sac 3  are turned on, the single-phase knife switches SV 1 , SV 2 , and SV 3  are turned off, the single-phase knife switches SV 4  and SV 5  are turned on, the direct current side switching knife switches Sdc 1  and Sdc 2  are turned on, the direct current side switching knife switches Sdc 3  and Sd 4  are turned off, and the alternating current side isolation knife switch K and the breaker QF are turned on; 
         [0059]    2) a two-go and one-return direct current ice melting mode, that is, A and B phase lines are in parallel connection and then in series connection with a C phase line to melt ice, wherein the three-phase knife switch Sac 1  is turned off, the three-phase knife switches Sac 2  and Sac 3  are turned on, the single-phase knife switches SV 1 , SV 2 , and SV 3  are turned off, the single-phase knife switches SV 4  and SV 5  are turned on, the direct current side switching knife switches Sdc 1 , Sdc 2  and Sdc 4  are turned on, the direct current side switching knife switch Sdc 3  is turned off, and the alternating current side isolation knife switch K and the breaker QF are turned on; 
         [0060]    3) an open circuit test mode, wherein the three-phase knife switch Sac 1  is turned off, the three-phase knife switches Sac 2  and Sac 3  are turned on, the single-phase knife switches SV 1 , SV 2 , and SV 3  are turned off, the single-phase knife switches SV 4  and SV 5  are turned on, the direct current side switching knife switches Sdc 1 , Sdc 2 , Sdc 3  and Sdc 4  are turned off, and the alternating current side isolation knife switch K and the breaker QF are turned on; 
         [0061]    4) a zero power test mode, wherein the three-phase knife switch Sac 1  is turned off, the three-phase knife switches Sac 2  and Sac 3  are turned on, the single-phase knife switches SV 1 , SV 2 , and SV 3  are turned off, the single-phase knife switches SV 4  and SV 5  are turned on, the direct current side switching knife switches Sdc 1  and Sdc 4  are turned off, the direct current side switching knife switches Sdc 2  and Sdc 3  are turned on, and the alternating current side isolation knife switch K and the breaker QF are turned on; 
         [0062]    5) a TCR mode, wherein the three-phase knife switch Sac 1  is turned on, the three-phase knife switches Sac 2  and Sac 3  are turned off, the single-phase knife switches SV 1 , SV 2 , and SV 3  are turned on, the single-phase knife switches SV 4  and SV 5  are turned off, the direct current side switching knife switches Sdc 1 , Sdc 2 , Sdc 3  and Sdc 4  are turned off, and the alternating current side isolation knife switch K and the breaker QF are turned on; and 
         [0063]    6) a TSR mode, wherein the three-phase knife switch Sac 1  is turned on, the three-phase knife switches Sac 2  and Sac 3  are turned off, the single-phase knife switches SV 1 , SV 2 , and SV 3  are turned on, the single-phase knife switches SV 4  and SV 5  are turned off, the direct current side switching knife switches Sdc 1 , Sdc 2 , Sdc 3  and Sdc 4  are turned off, and the alternating current side isolation knife switch K and the breaker QF are turned on. 
       Embodiment 3 
       [0064]    Referring to  FIG. 7 , in this embodiment, the abovementioned series-connection type automatic switching circuit for twelve-pulse direct current ice melting includes two sub-switching circuits which are respectively a sub-switching circuit TC 1  and a sub-switching circuit TC 2 . The sub-switching circuit TC 1  and the sub-switching circuit TC 2  are connected to the 10 kV side of a 220 kV main transformer through three-phase double-winding rectifier transformers T 1  and T 2 , an isolation knife switch K and a breaker QF. A filter bank F is used to carry out reactive compensation and harmonic suppression, and is connected to a 10 kV bus through an isolation knife switch K 1  and a breaker QF 1 . 
         [0065]    Three phases of voltages Uab, Ubc, and Uca of the 10 kV bus, valve side currents Iyva, Iyvb, Iyvc, Idva, Idvb, and Idvc, grid side currents Iya, Iyb, Iyc, Ida, Idb, and Idc, direct current side currents Idp, Idn, and Idgn, and direct current side voltages Udp and Udn of rectifier transformers, a monitoring signal of the converter, monitoring signals of knife switches, monitoring signals of the breakers, and the like are communicated to a control and protection system CP. The control and protection system CP sends control instructions to the knife switches, the breakers, and the converter. The implementation solutions of a TCR mode, a TSR mode, a direct current ice melting mode and equivalent test modes thereof, and other modes are as follows: 
         [0066]    1) a one-go and one-return direct current ice melting mode, that is, A and B phase lines are in series connection to melt ice, wherein the three-phase knife switch Sac 1  is turned off, the three-phase knife switches Sac 2  and Sac 3  are turned on, the single-phase knife switches SV 1 , SV 2 , and SV 3  are turned off, the single-phase knife switches SV 4  and SV 5  are turned on, the direct current side switching knife switches Sdc 1  and Sdc 2  are turned on, the direct current side switching knife switches Sdc 3  and Sd 4  are turned off, and the alternating current side isolation knife switch K and the breaker QF are turned on; 
         [0067]    2) a two-go and one-return direct current ice melting mode, that is, A and B phase lines are in parallel connection and then in series connection with a C phase line to melt ice, wherein the three-phase knife switch Sac 1  is turned off, the three-phase knife switches Sac 2  and Sac 3  are turned on, the single-phase knife switches SV 1 , SV 2 , and SV 3  are turned off, the single-phase knife switches SV 4  and SV 5  are turned on, the direct current side switching knife switches Sdc 1 , Sdc 2  and Sdc 4  are turned on, the direct current side switching knife switch Sdc 3  is turned off, and the alternating current side isolation knife switch K and the breaker QF are turned on; 
         [0068]    3) an open circuit test mode, wherein the three-phase knife switch Sac 1  is turned off, the three-phase knife switches Sac 2  and Sac 3  are turned on, the single-phase knife switches SV 1 , SV 2 , and SV 3  are turned off, the single-phase knife switches SV 4  and SV 5  are turned on, the direct current side switching knife switches Sdc 1 , Sdc 2 , Sdc 3  and Sdc 4  are turned off, and the alternating current side isolation knife switch K and the breaker QF are turned on; 
         [0069]    4) a zero power test mode, wherein the three-phase knife switch Sac 1  is turned off, the three-phase knife switches Sac 2  and Sac 3  are turned on, the single-phase knife switches SV 1 , SV 2 , and SV 3  are turned off, the single-phase knife switches SV 4  and SV 5  are turned on, the direct current side switching knife switches Sdc 1  and Sdc 4  are turned off, the direct current side switching knife switches Sdc 2  and Sdc 3  are turned on, and the alternating current side isolation knife switch K and the breaker QF are turned on; 
         [0070]    5) a TCR mode, wherein the three-phase knife switch Sac 1  is turned on, the three-phase knife switches Sac 2  and Sac 3  are turned off, the single-phase knife switches SV 1 , SV 2 , and SV 3  are turned on, the single-phase knife switches SV 4  and SV 5  are turned off, the direct current side switching knife switches Sdc 1 , Sdc 2 , Sdc 3  and Sdc 4  are turned off, and the alternating current side isolation knife switch K and the breaker QF are turned on; and 
         [0071]    6) a TSR mode, wherein the three-phase knife switch Sac 1  is turned on, the three-phase knife switches Sac 2  and Sac 3  are turned off, the single-phase knife switches SV 1 , SV 2 , and SV 3  are turned on, the single-phase knife switches SV 4  and SV 5  are turned off, the direct current side switching knife switches Sdc 1 , Sdc 2 , Sdc 3  and Sdc 4  are turned off, and the alternating current side isolation knife switch K and the breaker QF are turned on. 
       Embodiment 4 
       [0072]    Referring to  FIG. 8 , in this embodiment, the abovementioned series-connection type automatic switching circuit for twelve-pulse direct current ice melting includes two sub-switching circuits which are respectively a sub-switching circuit TC 1  and a sub-switching circuit TC 2 . The sub-switching circuit TC 1  and the sub-switching circuit TC 2  are connected to the 35 kV side of a 500 kV main transformer through three-phase double-winding rectifier transformers T 1  and T 2 , an isolation knife switch K and a breaker QF. A filter bank F is used to carry out reactive compensation and harmonic suppression and is connected to a 35 kV bus through an isolation knife switch K 1  and a breaker QF 1 . 
         [0073]    Three phases of voltages Uab, Ubc, and Uca of the 35 kV bus, valve side currents Iyva, Iyvb, Iyvc, Idva, Idvb, and Idvc, grid side currents Iya, Iyb, Iyc, Ida, Idb, and Idc, direct current side currents Idp, Idn, and Idgn, and direct current side voltage signals Udp and Udn of the rectifier transformers, a monitoring signal of the converter, monitoring signals of knife switches, monitoring signals of the breakers, and the like are communicated to a control and protection system CP. The control and protection system CP sends control instructions to the knife switches, the breakers, and the converter. The implementation solutions of a TCR mode, a TSR mode, a direct current ice melting mode and equivalent test modes thereof, and other modes are as follows: 
         [0074]    1) a one-go and one-return direct current ice melting mode, that is, A and B phase lines are in series connection to melt ice, wherein the three-phase knife switch Sac 1  is turned off, the three-phase knife switches Sac 2  and Sac 3  are turned on, the single-phase knife switches SV 1 , SV 2 , and SV 3  are turned off, the single-phase knife switches SV 4  and SV 5  are turned on, the direct current side switching knife switches Sdc 1  and Sdc 2  are turned on, the direct current side switching knife switches Sdc 3  and Sd 4  are turned off, and the alternating current side isolation knife switch K and the breaker QF are turned on; 
         [0075]    2) a two-go and one-return direct current ice melting mode, that is, A and B phase lines are in parallel connection and then in series connection with a C phase line to melt ice, wherein the three-phase knife switch Sac 1  is turned off, the three-phase knife switches Sac 2  and Sac 3  are turned on, the single-phase knife switches SV 1 , SV 2 , and SV 3  are turned off, the single-phase knife switches SV 4  and SV 5  are turned on, the direct current side switching knife switches Sdc 1 , Sdc 2  and Sdc 4  are turned on, the direct current side switching knife switch Sdc 3  is turned off, and the alternating current side isolation knife switch K and the breaker QF are turned on; 
         [0076]    3) an open circuit test mode, wherein the three-phase knife switch Sac 1  is turned off, the three-phase knife switches Sac 2  and Sac 3  are turned on, the single-phase knife switches SV 1 , SV 2 , and SV 3  are turned off, the single-phase knife switches SV 4  and SV 5  are turned on, the direct current side switching knife switches Sdc 1 , Sdc 2 , Sdc 3  and Sdc 4  are turned off, and the alternating current side isolation knife switch K and the breaker QF are turned on; 
         [0077]    4) a zero power test mode, wherein the three-phase knife switch Sac 1  is turned off, the three-phase knife switches Sac 2  and Sac 3  are turned on, the single-phase knife switches SV 1 , SV 2 , and SV 3  are turned off, the single-phase knife switches SV 4  and SV 5  are turned on, the direct current side switching knife switches Sdc 1  and Sdc 4  are turned off, the direct current side switching knife switches Sdc 2  and Sdc 3  are turned on, and the alternating current side isolation knife switch K and the breaker QF are turned on; 
         [0078]    5) a TCR mode, wherein the three-phase knife switch Sac 1  is turned on, the three-phase knife switches Sac 2  and Sac 3  are turned off, the single-phase knife switches SV 1 , SV 2 , and SV 3  are turned on, the single-phase knife switches SV 4  and SV 5  are turned off, the direct current side switching knife switches Sdc 1 , Sdc 2 , Sdc 3  and Sdc 4  are turned off, and the alternating current side isolation knife switch K and the breaker QF are turned on; and 
         [0079]    6) a TSR mode, wherein the three-phase knife switch Sac 1  is turned on, the three-phase knife switches Sac 2  and Sac 3  are turned off, the single-phase knife switches SV 1 , SV 2 , and SV 3  are turned on, the single-phase knife switches SV 4  and SV 5  are turned off, the direct current side switching knife switches Sdc 1 , Sdc 2 , Sdc 3  and Sdc 4  are turned off, and the alternating current side isolation knife switch K and the breaker QF are turned on. 
       Embodiment 5 
       [0080]    Referring to  FIG. 9 , in this embodiment, the abovementioned series-connection type automatic switching circuit for twelve-pulse direct current ice melting includes two sub-switching circuits which are respectively a sub-switching circuit TC 1  and a sub-switching circuit TC 2 . The sub-switching circuit TC 1  and the sub-switching circuit TC 2  are connected to the 220 kV side of a 500 kV main transformer through three-phase three-winding rectifier transformers T 1  and T 2 , an isolation knife switch K and a breaker QF. A filter bank F is used to carry out reactive compensation and harmonic suppression and is connected to a third winding of the rectifier transformers though an isolation knife switch K 1  and a breaker QF 1 . 
         [0081]    Three phases of voltages Uab, Ubc, and Uca of the 220 kV bus, valve side currents Iyva, Iyvb, Iyvc, Idva, Idvb, and Idvc, grid side currents Iya, Iyb, Iyc, Ida, Idb, and Idc, direct current side currents Idp, Idn, and Idgn, and direct current side voltages Udp and Udn of the rectifier transformers, a monitoring signal of the converter, monitoring signals of knife switches, monitoring signals of the breakers, and the like are communicated to a control and protection system CP. The control and protection system CP sends control instructions to the knife switches, the breakers, and the converter. The implementation solutions of a TCR mode, a TSR mode, a direct current ice melting mode and equivalent test modes thereof, and other modes are as follows: 
         [0082]    1) a one-go and one-return direct current ice melting mode, that is, A and B phase lines are in series connection to melt line, wherein the three-phase knife switch Sac 1  is turned off, the three-phase knife switches Sac 2  and Sac 3  are turned on, the single-phase knife switches SV 1 , SV 2 , and SV 3  are turned off, the single-phase knife switches SV 4  and SV 5  are turned on, the direct current side switching knife switches Sdc 1  and Sdc 2  are turned on, the direct current side switching knife switches Sdc 3  and Sd 4  are turned off, and the alternating current side isolation knife switch K and the breaker QF are turned on; 
         [0083]    2) a two-go and one-return direct current ice melting mode, that is A and B phase lines are in parallel connection and then in series connection with a C phase line to melt ice, wherein the three-phase knife switch Sac 1  is turned off, the three-phase knife switches Sac 2  and Sac 3  are turned on, the single-phase knife switches SV 1 , SV 2 , and SV 3  are turned off, the single-phase knife switches SV 4  and SV 5  are turned on, the direct current side switching knife switches Sdc 1 , Sdc 2  and Sdc 4  are turned on, the direct current side switching knife switch Sdc 3  is turned off, and the alternating current side isolation knife switch K and the breaker QF are turned on; 
         [0084]    3) an open circuit test mode, wherein the three-phase knife switch Sac 1  is turned off, the three-phase knife switches Sac 2  and Sac 3  are turned on, the single-phase knife switches SV 1 , SV 2 , and SV 3  are turned off, the single-phase knife switches SV 4  and SV 5  are turned on, the direct current side switching knife switches Sdc 1 , Sdc 2 , Sdc 3  and Sdc 4  are turned off, and the alternating current side isolation knife switch K and the breaker QF are turned on; 
         [0085]    4) a zero power test mode, wherein the three-phase knife switch Sac 1  is turned off, the three-phase knife switches Sac 2  and Sac 3  are turned on, the single-phase knife switches SV 1 , SV 2 , and SV 3  are turned off, the single-phase knife switches SV 4  and SV 5  are turned on, the direct current side switching knife switches Sdc 1  and Sdc 4  are turned off, the direct current side switching knife switches Sdc 2  and Sdc 3  are turned on, and the alternating current side isolation knife switch K and the breaker QF are turned on; 
         [0086]    5) a TCR mode, wherein the three-phase knife switch Sac 1  is turned on, the three-phase knife switches Sac 2  and Sac 3  are turned off, the single-phase knife switches SV 1 , SV 2 , and SV 3  are turned on, the single-phase knife switches SV 4  and SV 5  are turned off, the direct current side switching knife switches Sdc 1 , Sdc 2 , Sdc 3  and Sdc 4  are turned off, and the alternating current side isolation knife switch K and the breaker QF are turned on; and 
         [0087]    6) a TSR mode, wherein the three-phase knife switch Sac 1  is turned on, the three-phase knife switches Sac 2  and Sac 3  are turned off, the single-phase knife switches SV 1 , SV 2 , and SV 3  are turned on, the single-phase knife switches SV 4  and SV 5  are turned off, the direct current side switching knife switches Sdc 1 , Sdc 2 , Sdc 3  and Sdc 4  are turned off, and the alternating current side isolation knife switch K and the breaker QF are turned on. 
       Embodiment 6 
       [0088]    Referring to  FIG. 10 , in this embodiment, the abovementioned parallel-connection type automatic switching circuit for twelve-pulse direct current ice melting includes two sub-switching circuits which are respectively a sub-switching circuit TC 1  and a sub-switching circuit TC 2 . The sub-switching circuit TC 1  and the sub-switching circuit TC 2  are connected to the 35 kV side of a 500 kV main transformer through three-phase three-winding rectifier transformers T 1  and T 2 , an isolation knife switch K and a breaker QF. A filter bank F is used to carry out reactive compensation and harmonic suppression, and is connected to a third winding of the rectifier transformers through an isolation knife switch K 1  and a breaker QF 1 . 
         [0089]    Three phases of voltages Uab, Ubc, and Uca of the 35 kV bus, valve side currents Iyva, Iyvb, Iyvc, Idva, Idvb, and Idvc, grid side currents Iya, Iyb, Iyc, Ida, Idb, and Idc, direct current side currents Idp and Idn, direct current side voltages Udp and Udn of the rectifier transformers, a monitoring signal of a converter, monitoring signals of knife switches, monitoring signals of the breakers, and the like are communicated to a control and protection system CP. The control and protection system CP sends control instructions to the knife switches, the breakers, and the converter. The implementation solutions of a TCR mode, a TSR mode, a direct current ice melting mode and equivalent test modes thereof, and other modes are as follows: 
         [0090]    1) a one-go and one-return direct current ice melting mode, that is, A and B phase lines are in series connection to melt ice, wherein the three-phase knife switch Sac 1  is turned off, the three-phase knife switches Sac 2  and Sac 3  are turned on, the single switches SV 1 , SV 2 , and SV 3  are turned off, SV 4  and SV 5  are turned on, the direct current side switching knife switches Sdc 1  and Sdc 2  are turned on, the direct current side switching knife switches Sdc 3  and Sd 4  are turned off, and the alternating current side isolation knife switch K and the breaker QF are turned on; 
         [0091]    2) a two-go and one-return direct current ice melting mode, that is, A and B phase lines are in parallel connection and then in series connection with a C phase line to melt ice, wherein the three-phase knife switch Sac 1  is turned off, the three-phase knife switches Sac 2  and Sac 3  are turned on, the single-phase knife switches SV 1 , SV 2 , and SV 3  are turned off, the single-phase knife switches SV 4  and SV 5  are turned on, the direct current side switching knife switches Sdc 1 , Sdc 2  and Sdc 4  are turned on, the direct current side switching knife switch Sdc 3  is turned off, and the alternating current side isolation knife switch K and the breaker QF are turned on; 
         [0092]    3) an open circuit test mode, wherein the three-phase knife switch Sac 1  is turned off, the three-phase knife switches Sac 2  and Sac 3  are turned on, the single-phase knife switches SV 1 , SV 2 , and SV 3  are turned off, the single-phase knife switches SV 4  and SV 5  are turned on, the direct current side switching knife switches Sdc 1 , Sdc 2 , Sdc 3  and Sdc 4  are turned off, and the alternating current side isolation knife switch K and the breaker QF are turned on; 
         [0093]    4) a zero power test mode, wherein the three-phase knife switch Sac 1  is turned off, the three-phase knife switches Sac 2  and Sac 3  are turned on, the single-phase knife switches SV 1 , SV 2 , and SV 3  are turned off, the single-phase knife switches SV 4  and SV 5  are turned on, the direct current side switching knife switches Sdc 1  and Sdc 4  are turned off, the direct current side switching knife switches Sdc 2  and Sdc 3  are turned on, and the alternating current side isolation knife switch K and the breaker QF are turned on; 
         [0094]    5) a TCR mode, wherein the three-phase knife switch Sac 1  is turned on, the three-phase knife switches Sac 2  and Sac 3  are turned off, the single-phase knife switches SV 1 , SV 2 , and SV 3  are turned on, the single-phase knife switches SV 4  and SV 5  are turned off, the direct current side switching knife switches Sdc 1 , Sdc 2 , Sdc 3  and Sdc 4  are turned off, and the alternating current side isolation knife switch K and the breaker QF are turned on; and 
         [0095]    6) a TSR mode, wherein the three-phase knife switch Sac 1  is turned on, the three-phase knife switches Sac 2  and Sac 3  are turned off, the single-phase knife switches SV 1 , SV 2 , and SV 3  are turned on, the single-phase knife switches SV 4  and SV 5  are turned off, the direct current side switching knife switches Sdc 1 , Sdc 2 , Sdc 3  and Sdc 4  are turned off, and the alternating current side isolation knife switch K and the breaker QF are turned on.