WO2009110648A1

WO2009110648A1 - Converter for hvdc

Info

Publication number: WO2009110648A1
Application number: PCT/KR2008/001283
Authority: WO
Inventors: Sung Geun Song; Sung Jun Park; Hae Gon Nam
Original assignee: Jun Sung Electronics Co., Ltd
Priority date: 2008-03-06
Filing date: 2008-03-06
Publication date: 2009-09-11

Abstract

The present invention relates to converters, and more particularly, to a converter for HVDC capable of enhancing input power factor, reducing ripple voltage, and improving output voltage resolution by connecting two 12-pulse bridge converters in series, and controlling firing angle thereof and transformation ratio of a voltage respectively input thereto. According to one embodiment of the present invention, the converter on HVDC comprises a main converter and a subconverter. The main converter is connected to a couple of internal converters in series. Each of input ends of the internal converters is winded by Y winding and delta winding, and a voltage transformed at a ratio of 1 :pN and 1 : p√3N is input thereto. The subconverter is connected to another couple of internal converters in series. Each of input ends of the internal converters is winded by Y winding and delta winding, and a voltage transformed at a ratio of l:sN and 1: s√3N is input thereto. The main converter is connected to the sub converter in series.

Description

Description CONVERTER FOR HVDC

Technical Field

[1] The present invention relates to converters, and more particularly, to a converter for

HVDC capable of enhancing input power factor, reducing ripple voltage, and improving output voltage resolution by connecting two 12-pulse bridge converters in series, and controlling firingangle thereof and transformation ratio of a voltage respectively input thereto.

[2]

Background Art

[3] Generally, HVDC (High Voltage Direct Current, hereinafter, simply "HVDC") is that transmits electricity electric power by converting AC power into DC power using AC/ DC converter in a sending end, and then DC power is converted into DC power using DC/ AC converter in receiving end to be provided to a load. This HVDC transmission method is recently considered as useful technique becausethey have various advantages what DC transmission method could not have.

[4] In the HVDC transmission method, since stationary-state voltage reduction of line only depends on resistance regardless of reactance or capacitor of line when DC is transmitted, long distance transmission is possible in comparison with AC transmission, and frequency and phase can be controlled by controlling converter. Thus, even if frequency connects another system, it is not necessary to synchronize frequency.

[5] For theses merits, power supply imports/exports by connecting power system between nations such as Europe, United of Sate, Canada, and so forth using HVDC. Also, load management is economically performed in consideration of time slot of production and consumption of regional disparity. In Korea, HVDC is installed and connected between Jeju and Haenam to show high economical efficiency.

[6] However, HVDC AC/DC converters produce wattless power essentially.

[7] Occurrence causes can part this wattless power by wattless power by harmonic electric current and wattless power by phase difference of fundamental wave electric current greatly.

[8] In addition, wattless power by harmonic electric current parts by method that reduce wattless power that happen in converter itself increasing pulse number and method by harmonic filter, and present 12 pulses method is used much.

[9] Wattless power by phase difference of fundamental wave electric current is generated by angle of firing that controls output voltage of converter. As the angle of firing is less, power factor improves. [10] In order to control the output voltage, angle of firing maximum and minimum values are limited, and angle of firing smallest value is determined by considering roll call failure by un-sine of receiving end voltage. [11] Accordingly, HVDC AC/DC converter urgently need researcher about composition that can make occurrence of wattless power less improving power factor controlling output voltage.

[12] FIG. 1 shows a conventional HVDC converter.

[13] Referring to FIG. 1, in the conventional HVDC converter 10, two 6-pulse converters

11 and 11' including tyristor 1 Ia are connected in series. Y winding transformer 12 and delta winding transformer 12' are used as an input end to have 30°phase difference. As a result, the conventional HVDC converter 10 takes 12 pulses finally. [14] The converters 11 and 11' are connected to a smoothing reactor 13 to decrease electric current harmonic wave of direct current line as well as to prevent un- consecutiveness of communication at light-load power factor. [15] Additionally, the conventional HVDC converter 10 includes a capacitor bank 14 for supplementing wattless power essentially absorbed by DC converter and a harmonic filter 15 for preventing harmonic wave created by controlling electric current. [16] Phase voltage of the conventional HVDC converter 10 can be expressed by the following formula 1 : [17] [Formula 1]

^[18] e _a= E _mcos(wt+60)

^[19] e _b= E_m cos(wt-60)

[20] e = E_m m cos(wt- 180)

[21] wherein E_m represents phase voltage peak value.

[22] The output voltage can be expressed by formula 2 according to formula 1 :

[23]

[24] [Formula 2]

[26] If ignore effect about electric current, mean value of output voltage is expressed by formula 3 in angle of firing (α) of converter: [27] [28] [Formula 3] ¹²⁹¹ V_d= F_rfocosα

[30] Angle of firing (α)of the conventional HVDC converter 10 is used to control elements of a sending end side. A receiving end side and the sending end side of HVDC system should have a controller that is mutually contradictory each other.

[31] In other words, when the receiving end side uses voltage controller, the sending end side (equivalent to a converter side) should compose an electric current controller. When the receiving end side (equivalent to an inverter side) uses the electric current controller, the sending end side should use the voltage controller.

[32] Last control elements of electric current control and voltage control becomes angle of firing (α), and the sending end side input power factoris changed by this angle of firing.

[33] If electric current (I₀₁) is fixed to DC transmissionline by the smoothing reactor, virtual value of line current fundamental wave ingredient of the sending end side by angle of firing (α) is as folio wings:

[34]

[35] [Formula 4]

[37] If ignore conversion loss of the converter, input/output electric power relation is expressed by formula 5: [38]

[39] [Formula 5]

[40]

^ V_mI_Ll co^= V_d I_d

[41] Relation between angle of firing (α) and power factor is expressed by formula 6 according to formula 2 to 5: [42] [43] [Formula 6] cosφ = cosα

[45] That is, we found that angle of firing (α)is the same as power factor of the sending end. [46] Meanwhile, FIG. 2 displays power factor of voltage, electric current by angle of firing (α) of the conventional HVDC converter. [47] Graph (a) of FIG. 2 displays phase voltage (e_a) of the sending end, phase current (i_a), and fundamental wave ingredient of phase current (i_at) when angle of firing (α) is 0°. Graph (b) of FIG. 2 displays phasor of graph (a) and can know that power factor of this time amounts to 1.

[48] Graph (c) of FIG. 2 displays phase voltage (e_a) of the sending end, phase current (i_a), and fundamental wave ingredient of phase current (i_at) when angle of firing (α) is 30°. Graph (d) of FIG. 2 displays phasor of graph (c). At this time, fundamental wave ingredient of phase current can know that phase voltage and phase difference appear equally with angle of firing (α) by 30°.

[49] Since angle of firing (α) becomes power factor of sending end, there is shortcomings that cause power factor declines when angle of firing (α) is enlarged.

[50]

Disclosure of Invention Technical Problem

[51] The present invention has been made in an effort to solve the above problems, and it is one object of the present invention to provide a converter capable of making minimum angle of firing limitation less and being strong in roll call failure.

[52] It is another object of the present invention is to provide a converter capable of making wattless power occurrence littleunder uniformity output voltage condition due to excellent input power factor.

[53]

Technical Solution

[54] In accordance with one aspect of the present invention for accomplishing the above objects, there is provided a converter comprising a main converter connected to a couple of internal converters in series, wherein each of input ends of the internal converters is winded by Y winding and delta winding, and a voltage transformed at a turn ratio of 1 :pN and 1 : pV3N is input thereto and an sub converter connected to another couple of internal converters in series, wherein each of input ends of the internal converters is winded by Y winding and delta winding, and a voltage transformed at a turn ratio of l:sN and 1: sV3N is input thereto. The main converter is connected to the subconverter in series.

[55] In a preferred embodiment, an output voltageof the converter is the sum of a reference voltage and a control voltage for transmitting the reference voltage. An firingangle of the main converter is fixed to output the normal reference voltage, and an firing angle of the sub converter is varied to output the control voltage.

[56] In a preferred embodiment, the main and subconverters are consisted of 12-pulse bridge converter, respectively.

[57] In a preferred embodiment, the internal converters of the main and sub converters are consisted of tyristor bridge where three pairs of two tyristors connected in series are arranged in parallel, respectively.

[58] In a preferred embodiment, the internal converters of the main converter are consisted of diode bridge where three pairs of two diodes connected in series are arranged in parallel, respectively. The internal converters of the sub converter are consisted of tyristor bridge where three pairs of two tyristors connected in series are arranged in parallel, respectively.

[59] In a preferred embodiment, the input end of the internal converter is connected to a three-phase transformer. The number of three-phase transformer is four and to be respectively connected to the output ends of the internal converters.

[60] In a preferred embodiment, the input ends of the internal converter are connected to the three-phase transformer. The number of three-phase transformer is two. The internal converters of the main converter are connected to one of the three-phase transformers, and the internal converters of the subconverter are connected to another three-phase transformer.

[61] In a preferred embodiment, the output end of the internal converters are connected to at least one of the three-phase transformers.

[62]

Advantageous Effects

[63] According to the present invention, the converter has excellent input power factor to be capable of reducing wattless power as compared to the conventional HVDC converter. [64] In addition, the converter capable of using angle of firing of the sub converter from

0° to 180°four corners for controlling output voltage so that output voltage resolution can be enlarged. [65] Further, the converter is capable of reducing ripple voltage to dramatically reduce capacity of the smoothing reactor. [66]

Brief Description of Drawings [67] FIG. 1 shows a conventional HVDC converter.

[68] FIG. 2 shows power factor of voltage and electric current by angle of firing of the conventional HVDC converter.

[69] FIG. 3 shows a converter according to a first embodiment of the present invention.

[70] FIG. 4 shows a converter according to a second embodiment of the present invention.

[71] FIG. 5 shows phasor of a converter according to the second embodiment of the present invention. [72] FIG. 6 is a graph for showing characteristics of output voltage of the converter according to the first embodiment of the present invention. [73] FIG. 7 is a graph for comparing characteristics of angle of firingdepending on output voltage of the conventional HVDC converter and converter according to the first embodiment of the present invention.

[74] FIG. 8 shows power angle of the converter according to the first embodiment of the present invention.

[75] FIG. 9 shows output voltage wave of the conventional HVDC converter.

[76] FIG. 10 shows output voltage wave of the converter according to the first embodiment of the present invention.

[77] FIG. 11 is a graph for comparing current phase of the converter according to the first embodiment of the present invention and the conventional HVDC converter.

[78]

[79] <Brief explanation of essential parts of the drawings>

[80] 100, 200: Converter, 110: Main Converter,

[81] 120: Sub Converter, 13: Smoothing Reactor,

[82] 14: Capacitor Bank, 15: Harmonic Filter

[83]

Best Mode for Carrying out the Invention

[84] Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings. It should be noted that whenever possible, the same reference numerals will be used throughout the drawings and the description to refer to the same or like parts. In describing the present invention, detailedde- scriptions of related known functions or configurations are omitted in order to avoid making the essential subject of the invention unclear.

[85] As used herein, the terms "about", "substantially", etc. are intended to allow some leeway in mathematical exactness to account for tolerances that are acceptable in the trade and to prevent any unconscientious violator from unduly taking advantage of the disclosure in which exact or absolute numerical values are given so as to help understand the invention.

[86]

[87] [First Embodiment]

[88] FIG. 3 shows a converter according to a first embodiment of the present invention.

[89] Referring to FIG. 3, the converter 100 according to the first embodiment of the present invention includes a main converter 110 and an sub converter 120.

[90] The main converter 110 establishes and controls a reference voltage of output voltage of the converter 100. The sub converter 120 controls a reference voltage for transmitting it. In other words, the sub converter 120 controlselectric power transmission. [91] The input phase voltagesof the main converter 110 and the sub converter 120 are decided in accordance with voltage dimension that is required in whole HVDC system.

[92] The main converter 110 includes a couple of internal converters 111 and 112 that are connected to each other in series to output 12-pulase signal. The input ends of the internal converters 111 and 112 are winded by Y winding and delta winding, respectively.

[93] The input ends of the internal converters 111 and 112 are connected to 1,3-phase transformer 130 and 2,3-phase transformer 130a, respectively.

[94] Three-phase transformers 130 and 130a connected to the internal converters 111 and

112 of the main converter are equipped in one entity and may be connected together with the input ends of the internal converters 111 and 112.

[95] The internal converters 111 and 112 are consisted of tyristor bridge where three pairs of two tyristors connected in series are arranged in parallel to output 6 pulses.

[96] Since the internal converters 11 land 112 have phase difference by 30 each other, the main converter 110 can output 12 pulses finally.

[97] The three-phase transformers 130 and 130a connected to the main converter 110 include a transformer of turn ratio of 1 :pN and 1 : pV3N. As a result, voltage input to the input ends of the internal converters 111 and 112 is a voltage that is transformed at turn ratio of 1 :pN and 1 : pV3N.

[98] The sub converter 120 includes a couple of internal converters 121 and 122 which are connected in series to output 12-pulse signal. The input ends of another couple of the internal converters 121 and 122 are winded by Y winding and delta winding, respectively.

[99] The input ends of another couple of the internal converters 121 and 122 are connected to 3,3-phase transformer 140 and 4,3-phase transformer 140a, respectively

[100] The three-phase transformers 140 and 140a connected to the input ends of the internal converters 121 and 122 of the sub converter 120 are equipped in one entity and may be connected together with the input ends of the internal converters 121 and 122 of the subconverter 120.

[101] Meanwhile, all input ends of four internal converters 111, 112, 121, and 122 included in the main and sub converters 110 and 120 may be connected to the one three-phase transformer.

[102] The internal converters 111, 112, 121, and 122 of the main converter 110 and the subconverter 120 are consisted of 6-pulse converter in which three pairs of two tyristors connected in series are arranged in parallel.

[103] The converter 100 according to the first embodiment of the present invention is consisted of four 6-pulase converters that are consisted of tyristor 11a are connected in series. [104] Therefore, the converter 100 according to the first embodiment of the present invention employs the tyristor device 1 Ia to be applicable in HVDC system for both- direction electric power transmission.

[105] The three-phase transformers 140 and 140a connected to the sub converter 120 include a transformer of turn ratio of l:sN and 1: sV3N. As a result, the turn ratio of the input ends of another couple of the internal converters 121 and 122 becomes l:pN and l: pV3N.

[106] The ratio of output voltage of the main converter 110 and the sub converter 120 is s:p.

[107] Under the assumption that turn number of the transformers 12 and 12 of the conventional HVDC converter 10 is N, p and s, voltage ratio of the main converter 110 and the sub converter 120, decides so that the following formula may be satisfied.

[108]

[109] [Formula 7]

^[110] pN+sN=N

[111] In order to control output voltage of the converter 100 according to the first embodiment of the present invention, two angle of firing orders that is angle of the main converter 110 and angle of firing of the sub converter 120 are required. Mean valueof the output voltage by angle of firing of the converter 100 is as following formula 8.

[112]

[113] [Formula 8]

^[114] V_d= V _dop∞s a_p+ V _do cos a_s

^[115] V_do =p V_do, V_do =s V_do

P s

[116] wherein α_p represents firing angle of the main converter 110, and α_s represents firing angle of the sub converter 120. [117] As described in formula 8, there are a lot of angle of firing solution of the main converter 110 and the sub converter 120 so as to produce specification output voltage. [118] In the first embodiment of the present invention, angle of firing of the main converter

110 does by given value, and chooses angle of firing mode of control of the sub converter 120. [119] The fundamental wave ingredient of the sending end phase current in angle of firing

(α) for controlling output voltage of the conventional HVDC converter 10 is expressed as following formula 9. [120] [121] [Formula 9]

[123] At this time, maximum value and minimum value of angle of firing (α) of the conventional HVDC converter 10 are established by roll call failure prevention and input power factor, and is acted inside the following formula 10.

[124]

[125] [Formula 10]

[126] _{< <}

^ min — ^ — ^ max

[127] Fundamental wave ingredient of the sending end phase current is given by the following formula 11 by angle of firingof the main and sub converters 110 and 120 and turn ratio of two transformers of the converter 100 according to the first embodiment of the present invention.

[128]

[129] [Formula 11]

[131] The control extent of angle of firing of the main and sub converters 110 and 120 is expressed as following formula 12. [132]

[133] [Formula 12] [134] a_p _-K^, a_min < _^ a_s< , π

[135] That is, the converter 100 according to the first embodiment of the present invention can use angle of firing of the sub converter 120 from 0°to 180° [136] FIG. 6 is a graph for showing characteristics of output voltage of the converter according to the first embodiment of the present invention. [137] Referring to FIG. 6, the main converter 110 forms schedule reference voltage (V_dp) by given angle of firing (α_p) and forms control voltage (V_ds) by angle of firing (α) of the sub converter 120. [138] Therefore, last output voltage is the sum of the reference voltage (V_dp) of the main converter 110 and the control voltage (V_ds) of the sub converter 120. By angle of firing

(α_s)of the sub converter 120, last output voltage is acted ranging from V_dmmto V_dmax.

Also, last output voltage of the converter 100 is depended on direct-current voltage that required in whole HVDC system. [139] The direct-current voltage required in whole HVDC system is the sum of voltage input to the main and subconverters 110 and 120.

[140] FIG. 7 is a graph for comparing characteristics of angle of firing depending on output voltage of the conventional HVDC converter and converter according to the first embodiment of the present invention.

[141] The following formula 13 shows relationship between output voltage (V_dM) and maximum angle of firing (αj of the conventional HVDC converter 10.

[142]

[143] [Formula 13]

[144]

- Y _f- nun -^

« max⁼ COS ( — )

[145] The following formula 14 shows interrelationship between angle of firing and turn ratio of transformer for generating the same output voltage as the output voltage (V_dM) of the conventional HVDC converter 10.

[146]

[147] [Formula 14]

[148] _Λ .

- 1 , S- I +COS(X _x α,= cos ( )

S

[149] To form equal output voltage (V_d), angle of firing (α) of the conventional HVDC converter 10 is less than angle of firing (α_s) of sub converter 120.

[150] In addition, angle of firing (α) of the conventional HVDC converter 10 becomes power factor directly, but angle of firing (α_s) of the sub converter 120 according to the embodiment of the present invention affects in power factor but the effect appears little because turn ratio of transformer is small.

[151] When output voltage (V_dπώl) is extremely, power angle of the conventional HVDC converter 10 is imposed power angle (X_1112x amount to 0 in the converter 100 according to the first embodiment of the present invention.

[152] The following formula 15 shows formula 11 by phasor to investigate power factor for fundamental wave ingredient of the sending end phase current by angle of firingof the converter 100 according to the first embodiment of the present invention.

[153]

[154] [Formula 15]

[156] FIG. 8 shows power angle of the converter according to the first embodiment of the present invention. [157] In other words, FIG. 8 is a graph for showing power angle depending on output voltage from the formula 14 and 15.

[158] Referringto FIG. 8, when turn ratio of three-phase transformers connected to the converter 100 according to the first embodiment of the present invention is 0.8 in case of p and 0.2 in case of s, power factor is appeared excellently by 0.96 (a), and power factor of the conventional HVDC converter 10 is appeared by 0.60 (b) in uniformity output voltage drive extent.

[159] Thus, the transformer turn ratio of the main converter 110 and the subconverter 120 of the converter 100 according to the first embodiment of the present invention is established in 8:2.

[160] There is an advantage in that input power factor of the converter 100 according to the first embodiment of the present invention is excellent in comparison with that of the conventional HVDC converter 10.

[161] FIG. 9 shows output voltage wave of the conventional HVDC converter.

[162] Referring to FIG. 9, in the event that output voltage order is established 80V, we can know that output voltage (V_γ) of the converter 11 where input end is winded by Y winding, output voltage (V_Δ) of the converter 11 where input end is winded by delta winding,and last output voltage (V_d). Also, we found that ripple (c) of last output voltage (Vd) is appeared by 50V.

[163] FIG. 10 shows output voltage wave of the converter according to the first embodiment of the present invention.

[164] Referringto FIG. 10, when output voltage order of the converter 100 according to the first embodiment of the present invention is established in 80V, ripple (d) becomes 10V. That is, ripple (d) reduces about quintuple rather than ripple (c) of the conventional HVDC converter 10.

[165] The capacity of the smoothing reactor (not shown) of the converter 100 according to the first embodiment of the present invention can be dramatically reduced.

[166] FIG. 11 is a graph for comparing current phase of the converter according to the first embodiment of the present invention and the conventional HVDC converter.

[167] FIG. 11 is result that compares electric current status (e) of the converter according to the first embodiment of the present inventionwith electric current status (f) of the conventional HVDC converter.

[168] Referring to FIG. 11, the power factor of the converter according to the first embodiment of the present invention is considerably better than that of the conventional HVDC converter.

[169]

Mode for the Invention [170] [Second Embodiment]

[171] FIG. 4 shows a converter according to a second embodiment of the present invention.

[172] Hereinafter, the same configuration as the converter 100 according to the first embodiment of the present invention will be omitted to avoid duplicate description.

[173] Referring to FIG. 4, a converter 200 according to the second embodiment of the present invention includes a main converter 210 and an sub converter 220, which are connected in series.

[174] The main converter 200 establishes a reference voltage of the converter 200, and the subconverter 220 controls a control voltage for electric power transmission. These configurations are the same as the converter 100 according to the first embodiment of the present invention.

[175] Comparing with the converter 100 according to the first embodiment of the pre- sentinvention, there is a difference in that the converter 200 according to the second embodiment of the present invention includes internal converters of the main converter 210 is consisted of a diode bridge in which three pairs of two diodes are arranged in parallel.

[176] The converter 200 according to the second embodiment of the present invention is capable of providing electric power supply in one direction.

[177] The above-mentionedconverter 200 is applicable to HVDC system not requiring two- direction electric power transmission such as long-distance electric power transmission, so that it is possible to low manufacturing cost as.

[178] FIG. 5 shows phasor of a converter according to the second embodiment of the present invention.

[179] Referring to FIG. 5, (a) indicates voltage (e_a), electric current (i_ad), waveforms (E_a), and phase (I_adt).

[180] At this time, power factorof the main converter 210 providing reference voltage is always 1, power supply can be stably provided without roll call failure.

[181] (b) of FIG. 5 indicates voltage (e_a), electric current (i_ac), waveforms (E_a), and phase (I act)-

[182] The phase control angle of the sub converter 220 is from 0°to 360°to control whole area. [183] Meanwhile, power factor of the sub converter 220 becomes the phase control angle, but power output by the sub converter 220 is small, so that output of whole converter

200 is little influenced thereby. [184] (c) of FIG. 5 indicates voltage (e_a), electric current (i_a), waveforms (E_a), and phase (I_at

)•

[185] As known in (c) of FIG. 5, the main converter 210 establishes the reference voltage of the converter 200 and outputs it. The subconverter 220 outputs the control voltage for electric power transmission.

[186] In addition, the output characteristicof the HVDC converter according to the first embodiment of the present invention described in FIGs. 6 to 11 is the same as that of HVDC converter according to the second embodiment of the present invention, and thus the detailed description thereof will be omitted.

[187]

Industrial Applicability

[188] The present invention relates to converters, and more particularly, to a converter for HVDC capable of enhancing input power factor, reducing ripple voltage, and improving output voltage resolution by connecting two 12-pulse bridge converters in series, and controlling firing angle thereof and transformation ratio of a voltage respectively input thereto. Therefore, the converter according to the present invention is useful in the field of high voltage direct current.

[189] Although the present invention has been described herein with reference to the foregoing embodiments and the accompanying drawings, the scope of the present invention is defined by the claims that follow. Accordingly, those skilled in the art will appreciate that various substitutions, modifications and changes are possible, without departing from the spirit of the present invention as disclosed in the accompanying claims. It is to be understood that such substitutions, modifications and changes are within the scope of the present invention.

[190]

Claims

[ 1 ] A converter comprising : a main converter connected to a couple of internal converters in series, wherein each of input ends of the internal converters is winded by Y winding and delta winding, and a voltage transformed at a turn ratio of 1 :pN and 1 : pV3N is input thereto; and an sub converter connected to another couple of internal converters in series, wherein each of input ends of the internal converters is windedby Y winding and delta winding, and a voltage transformed at a turn ratio of l:sN and 1: sV3N is input thereto, wherein the main converter is connected to the subconverter in series.

[2] The converter according to claim 1, wherein an output voltage of the converter is the sum of a reference voltage and a control voltage for transmitting the reference voltage, and wherein an firing angle of the main converter is fixed to output the reference voltage, and an firing angle of the sub converter is varied to output the control voltage.

[3] The converter according to claim 1 or claim 2, wherein the main and sub- converters are consisted of 12-pulse bridge converter, respectively.

[4] The converter according to claim 3, wherein the internal converters of the main and sub converters are consisted of tyristor bridge where three pairs of two tyristors connected in series are arranged in parallel, respectively.

[5] The converter according to claim 3, wherein the internal converters of the main converter are consisted of diode bridge where three pairs of two diodes connected in series are arranged in parallel, respectively, and wherein the internal converters of the sub converter are consisted of tyristor bridge where three pairs of two tyristors connected in series are arranged in parallel, respectively.

[6] The converter according to claim 1, wherein the input end of the internal converter is connected to a three-phase transformer, and wherein the number of three-phase transformer is four and to be respectively connected to the output ends of the internal converters.

[7] The converter according to claim 1, wherein the input ends of the internal converter are connected to the three-phase transformer, and wherein the number of three-phase transformer is two, and wherein the internal converters of the main converter are connected to one of the three-phase transformers, and the internal converters of the sub converter are connected to another three-phase transformer.

[8] The converter according to claim 1, wherein the output end of the internal converters are connected to at least one of the three-phase transformers.