JP2018078733A - Hybrid power conversion system, hybrid dc transmission system and method for controlling hybrid power conversion system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the problem that, as a self-excited DC transmission system used for a self-excitation and separate-excitation hybrid DC transmission system, when MMC using a bidirectional chopper circuit is applied, the bidirectional chopper circuit can output only one-way output voltage, and therefore voltage-polarity reversion facilities are required for changing a tidal-current direction of power-transmission electric power, and in a DC disconnector used for the voltage-polarity reversion facilities, since opening-closing operation cannot be performed until direct current to flow in a DC line substantially turns into zero current, time is taken for change of tidal current, and therefore acceleration of changeover time and decrease in power-transmission disabled period are required.SOLUTION: In the present invention, in a hybrid DC transmission system 1000 having a bipolar configuration which is provided with in a self-excited DC transmission system 103 and a separate-excitation DC transmission system 104, the self-excited DC transmission system includes a multilevel converter in which multiple full bridge circuits having a self arc-extinguishing element and an energy-storing element are serially connected.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a hybrid power conversion system, a hybrid DC power transmission system, and a control method for a hybrid power conversion system, and more particularly, to a hybrid power suitable for operating a self-excited converter and a separately-excited converter. The present invention relates to a conversion system, a hybrid type DC power transmission system, and a control method for a hybrid type power conversion system.

  There is a direct current power transmission system (hereinafter referred to as HVDC) as a method of transmitting generated power output from a generator to a distant demand place. As the HVDC, the separately-excited HVDC uses a separately-excited converter composed of thyristor elements, and the separately-excited HVDC has abundant operation results both in Japan and overseas. The separately excited converter is a type of AC / DC power conversion circuit that converts AC power into DC power or vice versa, and several GW class DC power transmission systems have been put into practical use. On the other hand, the self-excited HVDC is composed of a self-extinguishing power semiconductor element such as an IGBT (insulated-gate bipolar transistor). The self-excited HVDC has a feature that it can operate as a voltage source by itself because the on / off of the self-extinguishing semiconductor switching element can be controlled by a gate signal. For example, as a self-excited converter for self-excited HVDC, a modular multi-level converter (hereinafter referred to as MMC) is used, each cell is composed of a bi-directional chopper circuit, and a unit converter called this cell or submodule is provided. A plurality of arms connected in series are connected in a three-phase bridge via a buffer reactor.

  From the viewpoint of increasing the transmission capacity due to the expansion of power demand, there is a form in which HVDC is operated as a bipolar HVDC in which two HVDCs are connected by a common DC return line. In addition, since the introduction / renewal timings of the respective poles do not necessarily coincide, the system may be operated as a hybrid HVDC in which one pole of the bipolar HVDC is operated as a separately excited HVDC and the other pole is operated as a self-excited HVDC.

  Further, the HVDC is operated while switching the power flow direction to each AC system connected from the viewpoint of system stabilization and power interchange.

  In order to switch the power flow direction, in the case of separately-excited HVDC, means for determining the flow direction of the transmission power is used by switching the direct current voltage polarity and making the direct current polarity constant. Here, depending on the flow direction, the sum current of the two HVDC flows in the direct current flowing in the direct current return line. For example, if both HVDC ratings are equal and the rated power is being transmitted, this sum current will be twice the rated DC current, which may exceed the return current resistance and lead to burnout.

  If each cell of the self-excited HVDC is configured by a bidirectional chopper circuit, the polarity cannot be changed in each cell, and the direct current polarity cannot be switched as the self-excited HVDC. For this reason, a voltage polarity reversal facility consisting of a DC disconnector is installed on the DC line of the self-excited HVDC, and the circuit connection of the DC line is connected to the DC disconnector when switching the direction of power flow to each AC system. There is known a means for controlling the power flow by switching the polarity of the system DC voltage by changing the open / close state. Such a technique is described, for example, in the document J.A. P. KJAERGAARD et al. “Bipolar operation of HVDC VSC converter with an LCC converter”, Cigre Colloquium in San Francisco 2012.

J. et al. P. KJAERGAARD et al. , “Bipolar operation of HVDC VSC converter with an LCC converter”, Cigre Colloquium in San Francisco 2012.

  In the above prior art, in order to switch the power flow direction to each AC system to be connected, a voltage polarity reversing facility composed of a DC disconnector is installed on the DC line of the self-excited HVDC. Inversion equipment is required. The DC disconnector used in this voltage polarity reversal equipment is a mechanical part, and cannot be opened or closed until the DC current flowing in the DC line reaches a switchable current (approximately zero current). Therefore, there is a problem that the power transmission impossible period becomes long when switching the power flow direction.

  SUMMARY OF THE INVENTION An object of the present invention is to solve the above problems, and to provide a hybrid power conversion system, a hybrid DC power transmission system, and a control method for the hybrid power conversion system that can speed up the switching time in the power flow direction. It is to provide.

  In order to achieve the above object, the present invention includes a self-excited conversion device and a separately excited conversion device, and the self-excited conversion device and the separately excited conversion device are connected to a common system as an AC side, A part of the DC power transmission line of the self-excited conversion device and a part of the DC power transmission line of the separately excited conversion device are commonly used, and the self-excited conversion device includes a self-extinguishing element and a storage element. A unit unit is configured in series in multiple stages, and the separately excited conversion device is configured by a control rectifier element, and the self-excited conversion device outputs a direct current polarity by controlling the operation of the self-extinguishing element. Can be switched.

  That is, in a hybrid DC power transmission system having a bipolar configuration including a self-excited converter and a separately-excited converter, the self-excited converter is a multi-level circuit in which a plurality of full bridge circuits composed of self-extinguishing elements and energy storage elements are connected in series Consists of a converter.

  According to the present invention, it is possible to obtain an effect that the power transmission stop period is shortened when switching the power flow direction to each AC system to be connected.

Overall configuration showing a first embodiment of a hybrid DC power transmission system according to the present invention Circuit configuration of MMC single terminal used as self-excited converter of self-excited HVDC Circuit configuration of MMC arm used as self-excited converter for self-excited HVDC (U-phase upper arm) Circuit configuration used as separately excited converter of separately excited HVDC Control block diagram of hybrid HVDC control means Voltage waveform of each part when switching the power flow direction Overall configuration showing a second embodiment of a hybrid DC power transmission system according to the present invention Bipolar common control means of the second embodiment of the hybrid DC power transmission system according to the present invention Waveform of each part when system DC retrace current control is applied

  EMBODIMENT OF THE INVENTION The form (Example) for implementing this invention is demonstrated below using drawing.

  With reference to FIG. 1, the overall configuration of the first embodiment of the present invention will be described. In the following description, it is assumed that the capacities of separately excited / self-excited HVDC are equal, but the present invention is not limited to this. The hybrid HVDC 1000 according to the first embodiment of the present invention is linked to two AC power system buses 101 and 111. The two AC power system buses 101 and 111 may be an AC power system having the same power source or an AC power system having different power sources.

  The hybrid HVDC 1000 includes a self-excited HVDC 103, a separately-excited HVDC 104, and a control means 105, and two HVDCs are connected by a common direct current return line r. The DC return r may be a conductor return such as a transmission line or cable, or a ground return such as underground or seawater.

  In the self-excited HVDC 103, two self-excited converters (also referred to as self-excited converters) 106a and 106b are connected via a direct current main line p and a direct current return line r. The separately-excited HVDC 104 includes two separately-excited converters (also referred to as separately-excited converters) 107a and 107b each composed of a thyristor element (also referred to as a control rectifier element). The other end is connected to the DC return line r. For example, a 6-pulse converter or a 12-pulse converter is used as the circuit configuration of the separately excited converters 107a and 107b.

  The voltage detector 109 detects the system DC voltage VDCrn and inputs it to the control device 105. Current detectors 110 a and 110 b detect system DC currents IDCp and IDCn, respectively, and input them to control device 105. The control means 105 includes an active power command Pref and a reactive power command Qref transmitted between both AC systems, a connection point voltage VgVSC of the self-excited HVDC 103, a connection point current IgVSC, each arm current Iarm, a system DC current IDCp, U Cell capacitor voltage VcellupN of phase upper arm, cell capacitor voltage VcellurN of U phase lower arm, cell capacitor voltage VcellvpN of V phase upper arm, cell capacitor voltage VcellvrN of V phase lower arm, cell capacitor voltage VcellwpN, W of W phase upper arm The MMC upper arm gate signal gVSC is input with the cell capacitor voltage VcellwrN of the lower arm, the linkage point voltage VgLCC, the linkage point current IgLCC, the system DC current VDCrn, and the system DC current IDCn of the separately excited HVDC104. pN_XH, gVSCupN_XL, gVSCupN_YH, gVSCupN_YL, MMC lower-arm gate signal gVSCurN_XH, gVSCurN_XL, outputs gVSCurN_YH, gVSCurN_YL, a separately excited converter gate signal GLCC. Note that N in each symbol indicates the N-th cell in the arm, and has the same meaning in the following description.

  Referring to FIG. 2, a circuit configuration of an MMC single terminal used as self-excited converter 106a of self-excited HVDC 103 is shown. In FIG. 2, 106a is described as an example, but 106b has the same configuration. FIG. 2 shows each phase arm 202up, 202ur, 202vp, 202vr, 202wp, 202wr in which cells 201 are connected in series, buffer reactors 204up, 204ur, 204vp, 204vr, 204wp, 204wr for each phase, and each phase. Current detector 205 for detecting the arm current Iarm, voltage detector 206 for detecting the connection point voltage VgVSC and connection point current IgVSC with the AC power system bus 101, and current detector 207. The The output terminal of each phase arm is connected to points u, v, and w through buffer reactors 204 up, 204 ur, 204 vp, 204 vr, 204 wp, and 204 wr. The other output terminal of each arm is connected to the DC main line p or DC return line r, and is connected to the self-excited converter 106b at the other end.

  Next, an arm 202up used as the self-excited converter 106a of the self-excited HVDC 103 will be described with reference to FIG. In FIG. 3, 202up is described as an example, but the other arms have the same configuration. The arm 202up is configured by an MMC system in which full bridge cells 201up_1 to 201up_N configured by a full bridge circuit are connected in series. The internal configuration will be described by taking the full bridge cell 201up_1 as an example. The main circuit of the full bridge cell 201up_1 includes a circuit in which self-extinguishing semiconductor switching elements (also referred to as self-extinguishing elements) 302XH_1 and 302XL_1 connected in parallel with freewheeling diodes, and a circuit in which 302YH_1 and 302YL_1 are connected in series. , A capacitor 303_1 as an energy storage element and a voltage detector 304_1 for detecting a capacitor voltage VCellupN of each cell are connected in parallel. The self-extinguishing type semiconductor switching elements 302XH_1, 302XL_1, 302YH_1, and 302YL_1 can be applied to IGBTs, GTOs (gate turn-off thyristors), MOSFETs (metal-oxide-semiconductor field-effect transistors), etc. Is used. Further, the switching elements are operated while being switched on and off in accordance with gate signals gVSCuPN_XH, gVSCuPN_XL, gVSCuPN_YH, and gVSCupN_YL input to the self-extinguishing semiconductor switching element.

  Referring to FIG. 4, the circuit configuration of separately excited converters 107a and 107b of separately excited HVDC 104 is shown. FIG. 4A shows the circuit configuration of the separately excited converter 107a on the AC power system bus 101 side, and thyristor units 50a11, 50a12, and 50a13 are arranged in parallel between the DC return line r and the connection point 5a-in. Connected. Each thyristor unit 50a11, 50a12, 50a13 is formed of a series circuit of thyristors 50a11-1 and 50a11-2, a series circuit of thyristors 50a12-1 and 50a12-2, and a series circuit of thyristors 50a13-1 and 50a13-2. ing.

  Thyristor units 50a21, 50a22, 50a23 are connected in parallel between the connection point 5a-in and the DC main line n. Each thyristor unit 50a21, 50a22, 50a23 is formed of a series circuit of thyristors 50a21-1 and 50a21-2, a series circuit of thyristors 50a22-1 and 50a22-2, and a series circuit of thyristors 50a23-1 and 50a23-2. ing.

  Here, each thyristor is configured such that the DC return line r side is the cathode and the DC main line n side is the anode side.

  On the other hand, the transformer 501a (three-winding transformer) includes a delta winding 501a1, a Y winding 501a2, and a delta winding 501a3. The delta winding 501a1, the Y winding 501a2, and the delta winding 501a3 are magnetically coupled to each other. Each of the u-phase, v-phase, and w-phase of AC power system bus 101 is connected to delta winding 501a1. Further, the connection point between the thyristors 50a11-1 and 50a11-2, the connection point between the thyristors 50a12-1 and 50a12-2, and the connection point between the thyristors 50a13-1 and 50a13-2 are connected to the Y winding 501a2. A connection point between the thyristors 50a21-1 and 50a21-2, a connection point between the thyristors 50a22-1 and 50a22-2, and a connection point between the thyristors 50a23-1 and 50a22-2 are connected to the delta winding 501a3.

  It comprises a voltage detector 506 and a current detector 507 for detecting a connection point voltage VgLCC and a connection point current IgLCC with the AC power system bus 101.

  The circuit configuration of the separately excited converter 107b is shown in FIG. In the separately excited converter 107a, each thyristor is configured by connecting the DC return line r side to the cathode and the DC main line n side to the anode side, whereas in the separately excited converter 107b, each thyristor includes: The difference is that the DC return line r side is configured as the anode and the DC main line n side is configured as the cathode side. Since other configurations are substantially the same, description thereof is omitted.

  In this way, the separately excited converters 107a and 107b use 12-phase thyristor rectifiers in which two 6-phase thyristor rectifiers and transformers 501a and 501b are combined as thyristor converters. The transformers 501a and 501b flow into the AC system by using a star connection for the transformer winding connected to one 6-phase thyristor rectifier and a delta connection for the transformer winding connected to the other 6-phase thyristor rectifier. Harmonic current can be suppressed. Separately excited converters 107a and 107b are configured such that the directions of the internal thyristor elements are opposite to each other. By sequentially turning on / off the thyristor elements in the separately excited converters 107a and 107b, power conversion can be performed, and the polarity of the DC return line r side and the DC main line n side can be changed.

  Next, referring to FIG. 5, a control method executed by the control means 105 is shown. The control means 105 is composed of, for example, an electronic computer, and each means functionally described in the drawings can be executed by operating software. FIG. 5 shows active power command distribution means (bipolar common control means) 401 for controlling two HVDCs, self-excited HVDC control means 402 for controlling self-excited HVDC 103, and separately-excited HVDC control means 403 for controlling separately-excited HVDC 104. Consists of.

  Bipolar common control means (active power command distribution means) 401 is composed of a function of distributing active power command Pref to each HVDC and a function of determining reactive power command Qref output by hybrid HVDC 1000.

  Self-excited HVDC control means 402 includes system DC current control means 420, MMC cell capacitor voltage control means 404, active / reactive power control means 405, system voltage phase detection means 407, AC current control means 408, circulating current control means 409. , Power command Pref polarity detection means 411, PWM means 416, pulse distribution means 417, adders 406, 410, 413, 414, 415, and multiplier 412.

  The system DC current control means 420 has a PI control calculation function for controlling the DC current IDCp flowing in the DC main line p to the command value IDCref. Capacitor voltage control means 404 has a PI control calculation function for controlling the total average value of DC voltage VCellN of capacitor 303 of cell 201 of MMC to command value VCellref. The active / reactive power control means 405 calculates the active / reactive power output from the self-excited HVDC from the connection point voltage VgVSC and the connection point current IgVSC, and from the active power command distribution means (bipolar common control means) 401. PI control calculation means for calculating an effective current command value and a reactive current command value so as to match the obtained PrefVSC and QrefVSC. The system voltage phase detection means 407 includes means for detecting the phase θ of the system voltage VgVSC of the AC power system buses 101 and 111 that are interconnected. The AC current control means 408 performs PI control calculation for controlling the d-axis coordinate Id and the q-axis current Iq after the dq coordinate conversion flowing in the AC power system buses 101 and 111 to coincide with the command value Idref and the q-axis current Iqref. Means functions are provided. The circulating current control means 409 has a function of stabilizing the DC voltage VCellN of the capacitor 303 of each cell by controlling the circulating current Iz flowing between the arms of each phase by PI control calculation. The polarity detection unit 411 includes a unit that determines the polarity of the PrefVSC obtained from the active power command distribution unit (bipolar common control unit) 401 and changes the system DC voltage polarity. The PWM means 416 compares Vref_p, which is the voltage command value for the upper arm, Vref_l, which is the voltage command value for the lower arm, and the carrier carrier, and inputs a gate pulse that is input to the self-extinguishing semiconductor switching element in the full bridge cell 301. Means for generating. The pulse distribution means 417 includes means for distributing the gate pulse to the four semiconductor switching elements 302XH_N, 302XL_N, 302YH_N, and 302YL_N in the full bridge cell 201.

  The separately excited HVDC control means 403 includes a separately excited converter control means 418 and a pulse distribution means 419. The separately excited converter control means 418 has a function of controlling the firing angle α of the thyristor elements in the separately excited converters 107a and 107b so that the separately excited HVDC 104 can output the active power command PrefLCC. The pulse distribution means 420 generates a gate pulse gLCC for commutating the thyristor element at each firing α.

  With reference to FIG. 6, each waveform at the time of tidal current inversion is shown. The reason why the voltage polarity can be reversed by using the full bridge cell 301_N will be described below. FIG. 6 shows, from the top, the arm 202up, 202vp, 202wp and the arm end-to-end voltage Vuparm, Vvparm, Vwparm, arm 202ur, 202vr, 202wr, and arm end-to-end voltage Vwarm between the buffer reactors 204up, 204vp, 204wp. , Vvrarm, Vwarm, system DC voltage VDCpr, and system DC voltage VDCrn.

  In general, there will be a need for tidal switching. For example, when power is interchanged between the system power bus 101 and the system power bus 111, the supply and demand balance of each system fluctuates depending on the season and the like, and therefore it is necessary to reverse the power flow direction in order to stabilize the system. In order to stabilize the system, for example, it is necessary to maintain the frequency and voltage within specified values.

  At time t1, in the self-excited HVDC control means 402, the system DC voltage VDCpr starts up to a positive rated voltage. At this time, each arm voltage includes a DC voltage that is ½ of the system DC voltage VDCpr. The voltage across the arm generates a stepped pulse waveform by combining the cell output voltages output from the cells 301_1 to 301_N, and approaches the sine wave. In the case of the U phase, the system DC voltage VDCpr can be expressed by (1).

[Equation 1]
Vuparm + Vurarm = VDCpr (1)
For example, in the case of a bidirectional chopper cell, since the cell output voltage can be output only in one direction in principle, the polarity of the system DC voltage VDCpr / 2 contained in the voltage across the arm cannot be switched by the self-excited converter itself. However, by configuring the cell with a full bridge circuit as shown in FIG. 3, the cell output voltage can be selected as the positive voltage or the negative voltage can be selected by the on / off operation of the semiconductor switching element. As described above, since the voltage across the arm is generated by combining the output voltages of the full bridge cells 301_1 to 301_N, the polarity of the system DC voltage VDCpr can be switched.

  On the other hand, at time t1, the separately excited HVDC control means 403 starts up the system DC voltage VDCpn up to a positive rated voltage.

  From time t2 to t3, the self-excited HVDC control means 402 switches the system DC voltage polarity to switch the flow direction of the active power. Specific means of the voltage polarity switching method can be realized by using the polarity detection means 411 shown in FIG. The active power command PrefVSC of the self-excited HVDC 103 is normalized to +1 in the case of positive rated power (power transmission to other terminals) and -1 in the case of negative rated power (power received from other terminals), depending on the power command value A value from +1 to −1 is output as Pdir. By multiplying this by the system DC voltage feedforward term VDCpr_rated at the subsequent stage and generating a DC voltage command VDCref, the polarity of VDCpr / 2 included in each arm is switched, and the voltage polarity of the system DC voltage VDCpr is also switched.

  On the other hand, the voltage polarity of the system DC voltage VDCpn is also switched by controlling the firing angle applied to each thyristor element in the separately excited HVDC control means 403 from time t2 to t3.

  According to the present embodiment, the output DC voltage polarity of the self-excited HVDC can be continuously switched, so that an effect that the power transmission stop period is shortened can be obtained.

  With reference to FIG. 7, the overall configuration of the second embodiment of the present invention will be described. However, description of the same or corresponding parts as in the first embodiment is omitted, and only different parts are described.

  The hybrid HVDC 1000 includes a DC current sensor 601 that detects the system DC return current IDCr in addition to the components of the first embodiment, and the detected value is input to the control means 105.

  Next, the active power command distribution means (bipolar common control means) in the control means 105 in this embodiment will be described with reference to FIG. FIG. 8 shows system DC retrace current control means 701, distributor 702 that distributes Pref to self-excited HVDC 103 and separately-excited HVDC 104, distributed active power command, and correction power of system DC retrace current control means 701. It comprises a subtracter 703 for subtracting the command Pc or an adder 704 for adding. The system DC retrace current control means 701 outputs a correction power command Pc that causes the system DC retrace current IDCr to follow the DC retrace current command value IDCrref by using the PI control calculation means.

  Next, with reference to FIG. 9, each waveform when the system DC retrace current control means 701 of the present embodiment is introduced will be described. Note that FIG. 9 uses an operation waveform when the hybrid HVDC 103 is in steady operation as an example, but this embodiment is not limited to only steady operation, and power flow reversal operation, HVDC start and stop, etc. Similar effects can be obtained in transient operation. FIG. 9 shows PrefVSC of self-excited HVDC, PrerefLCC of separately-excited HVDC, correction power command Pc, and system DC retrace current IDCr flowing in the DC retrace r of FIG.

  Assume that the active power commands PrefVSC and PrefLC of each HVDC are given by Pref / 2 at time t1. When both active power commands are the same value, ideally, the system DC currents IDCp and IDCn flowing through the respective DC main lines p and n are the same, so IDCr flowing through the DC return line r is canceled and becomes zero. However, due to differences in the control algorithms of the self-excited HVDC control means 402 and the separately-excited HVDC control means 403, differences in each control response, transmission delay of the control line, etc., both currents are not canceled by the IDCr. Is likely to flow. When the system DC retrace current control means 701 is operated at time t2, Pc is added to or subtracted from Pref / 2 so that IDCr becomes IDCrref (IDCrref = zero current in FIG. 9), and PrefVSC and PrefLCC are generated. At time t3, the system direct current output from each HVDC is canceled and IDCr is controlled to zero.

  According to the present embodiment, the same effects as in the first embodiment can be obtained, and unlike the first embodiment, the hybrid HVDC can be operated while suppressing the steady and transient increase of the system DC retrace current IDCr. it can.

101, 111 ... AC power system bus 1000 ... Hybrid HVDC
103 ... Self-excited HVDC
104 ... Separately-excited HVDC
105 ... Control means 106a, 106b ... Self-excited converters 107a, 107b ... Separately excited converters 108a, 108b ... DC reactors 109, 206, 304_1 to 304_N ... Voltage detectors 110a, 110b , 205, 207, 601 ... current detectors 201p, 201r ... arm converters 202up, 202vp, 202wp, 202ur, 202vr, 202wr ... arms 204up, 204vp, 204wp, 204ur, 204vr, 204wr ... Buffer reactor 301up_1-301up_N, 301vp_1-301vp_N, 301wp_1-301wp_N, 301ur_1-301ur_N, 301vr_1-301vr_N, 301wr_1-301wr_N: Full bridge cell 3 02XH_1 to 302XH_N, 302XL_1 to 302XL_N, 302YH_1 to 302YH_N, 302YL_1 to 302YL_N: Self-extinguishing semiconductor switching elements in which free-wheeling diodes are connected in parallel 303_1 to 303_N ... Capacitor 401 ... Bipolar common control means 402 ... Self-excited HVDC control means 403 .. Separately excited HVDC control means 404... Capacitor voltage control means 405... Active / reactive power control means 406, 410, 413, 414, 415, 704. ... system voltage phase detection means 408 ... AC current control means 409 ... circulating current control means 411 ... polarity detection means 412 ... multiplier 416 ... PWM means 417 ... self-excited conversion Pulse distribution means 418, etc. Wherein the converter control means 419 ... other commutated converters pulse distribution means 420 ... system DC return current control means 701 ... system DC return current control means 702 ... distributor 703 ... subtractor

Claims (8)

  A self-excited converter, and a separately-excited converter, the self-excited converter and the separately-excited converter are connected to a common system as an AC side, and a part of a DC transmission line of the self-excited converter A part of the DC power transmission line of the separately excited conversion device is commonly used, and the self-excited conversion device is configured by serially connecting unit units each including a self-extinguishing element and a storage element, The separately excited converter includes a control rectifier element, and the self-excited converter is configured to be capable of switching the polarity of a direct current output by controlling the operation of the self-extinguishing element. Hybrid power conversion system.   2. The hybrid power conversion system according to claim 1, wherein the unit unit has a self-extinguishing element formed of a full bridge circuit.   3. The hybrid power conversion system according to claim 1, wherein the separately-excited conversion device switches a DC current polarity by controlling an ignition angle. 4.   2. The hybrid power conversion system according to claim 1, wherein the self-extinguishing element is operated by PWM conversion according to a voltage command based on a system DC current command and a voltage command based on a polarity command. Hybrid power conversion system.   The hybrid power conversion system according to claim 1, further comprising a control unit that continuously changes a voltage polarity of a system DC voltage command value of the self-excited conversion device according to a flow direction of active power. A hybrid power conversion system.   3. The hybrid power conversion system according to claim 2, wherein a current detector for detecting a return current flowing in a DC return and a return current detected by the return current detector are made to follow a current command value. A functioning return current control unit, a distribution unit that distributes an active power command value to the self-excited conversion device and the separately excited conversion device, and a corrected power command obtained from the return current control unit are transmitted by the distributor. A hybrid power conversion system comprising a calculation unit that adds to or subtracts from an active power command of each distributed DC power transmission system.   There are two power converters, and at least one of the power converters has a self-excited converter and a separately-excited converter, and the self-excited converter and the separately-excited converter are on the AC side. A part of the DC power transmission line of the self-excited conversion device and a part of the DC power transmission line of the separately excited conversion device are commonly used. A unit unit composed of an element and a storage element is connected in series, and the separately excited converter is composed of a control rectifier, and the self-excited converter controls the operation of the self-extinguishing element. Thus, the hybrid DC power transmission system is configured to be capable of switching the polarity of the DC current to be output, and performs DC power transmission from one of the two power converters to the other of the two power converters. A self-excited conversion device and a separately-excited conversion device, wherein the self-excited conversion device and the separately-excited conversion device are connected to a common system as an AC side, and a part of a DC transmission line of the self-excited conversion device and the A part of the DC power transmission line of the separately excited conversion device is commonly used, and the self-excited conversion device is configured by serially connecting unit units each including a self-extinguishing element and a storage element. An excitation converter is a control method for a hybrid power conversion system including control rectifier elements, and the self-excitation converter has a direct current polarity output by controlling the operation of the self-extinguishing element. A control method for a hybrid power conversion system that converts power between alternating current and direct current in response to switching.

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