CN110739839B

CN110739839B - Charging method for extra-high voltage flexible direct-current full-bridge half-bridge hybrid converter

Info

Publication number: CN110739839B
Application number: CN201910886504.0A
Authority: CN
Inventors: 彭茂兰; 王海军; 甘宗跃; 刘航
Original assignee: Maintenance and Test Center of Extra High Voltage Power Transmission Co
Current assignee: Maintenance and Test Center of Extra High Voltage Power Transmission Co
Priority date: 2019-09-19
Filing date: 2019-09-19
Publication date: 2020-10-20
Anticipated expiration: 2039-09-19
Also published as: CN110739839A; WO2021052298A1

Abstract

The invention discloses a charging method of an extra-high voltage flexible direct-current full-bridge half-bridge hybrid converter, wherein each phase unit of the full-bridge half-bridge hybrid converter comprises an upper bridge arm unit and a lower bridge arm unit, each bridge arm unit is mainly formed by connecting a full-bridge submodule and a half-bridge submodule in series, and the method comprises the following steps: the method comprises the steps that a power device T2 or T3 in a full-bridge module is triggered in a direct-current side short-circuit charging mode, a rate device T1 or T4 in the full-bridge module is triggered in an uncontrollable charging stage in the direct-current side short-circuit charging mode, and the capacitor voltage of the full-bridge module and the half-bridge module is improved until all the modules reach rated voltage by triggering the power devices T1, T3 or T2 and T4 in the full-bridge module in the controllable charging stage. According to the method, a specific power device T2 or T3 in the full-bridge module is triggered in a direct-current side short-circuit charging mode, a module power device triggering command is issued only, measuring quantities such as alternating voltage and the like do not need to be introduced, extra criteria do not need to be added, and the full-bridge module and the half-bridge module can be electrified to normally work.

Description

Charging method for extra-high voltage flexible direct-current full-bridge half-bridge hybrid converter

Technical Field

The invention relates to the technical field of electric power, in particular to a charging method of an extra-high voltage flexible direct-current full-bridge half-bridge hybrid converter.

Background

Modular multilevel converter high voltage direct current transmission (modular multilevel converter based HVDC, MMC-HVDC) is a novel power transmission technology, and has been widely applied to large-scale wind power plant grid connection, power grid interconnection, direct current transmission and other aspects by virtue of the advantages of modularization, low harmonic content, low loss and the like. At present, a plurality of flexible direct current projects are put into operation at home and abroad, such as domestic Luxi back-to-back direct current, south Australia three-terminal flexible direct current demonstration project, American Trans Bay Cable project and the like, in recent years, the flexible direct current transmission technology is rapidly developed, and the development stage of ultrahigh voltage high-capacity flexible direct current transmission is advanced.

The MMC-HVDC system with the mixed full-bridge and half-bridge modules is shown in figure 1, each phase unit consists of an upper bridge arm unit and a lower bridge arm unit, each bridge arm unit consists of n sub-modules which are connected in series, the internal structure of the module can be a half-bridge module or a full-bridge module, the proportion of the full-bridge module and the half-bridge module is not limited, the half-bridge sub-module comprises two IGBT power devices of T1 and T2, two anti-parallel diodes of D1 and D2 which are correspondingly connected with T1 and T2 in parallel and a capacitor C, the full-bridge sub-module comprises four IGBT power devices of T1, T2, T3 and T4 which are sequentially connected in a counterclockwise mode, four anti-parallel diodes of D1, D2, D3 and D4 which are correspondingly connected with T1, T2, T3 and T4_dcIs a dc voltage.

In order to further improve the transmission capacity and the voltage grade of the flexible direct current transmission system, the extra-high voltage flexible direct current transmission project adopts a main connection mode that two valve group units are connected in series to form a one-pole symmetric bipolar system. In order to enable the converter to have direct-current fault clearing capacity and reduce the running loss of a power device, a bridge arm adopts a hybrid converter based on half-bridge and full-bridge submodules, and the hybrid converter has a wide application prospect.

The work of the sub-modules depends on the self-energy-taking power supply in the modules, the capacitor voltage in the modules is low in the initial charging stage, the self-energy-taking power supply cannot be started, and the modules cannot work normally, so that the reasonable charging strategy is adopted to enable all the modules to work normally, and the premise that the converter valve operates is met. The flexible direct current converter charging mode mainly comprises direct current side short circuit charging and direct current side non-short circuit charging, wherein the direct current side short circuit charging is to prepare for online input of an extra-high voltage flexible direct current valve bank before unlocking, a part of full bridge modules are cut off after the full bridge modules are powered on in an uncontrolled charging stage to enable the half bridge modules to be charged, existing literature research needs to collect instantaneous values of phase voltages of all phases and sequence the instantaneous values to find out appropriate bridge arm cutting modules, however, an alternating current voltage measurement quantity is introduced into a valve control device, and equipment operation risks are increased. The charging without short circuit at the direct current side is a common charging mode of the flexible direct current converter, a three-section charging strategy is proposed in documents, a part of modules are cut off by triggering specific power devices in the modules, so that the module voltage can be charged to a rated value, but the specific power devices are kept to be triggered for a period of time in the whole charging process, so that the loss of the power devices is increased, and the service life of the devices is influenced. In the application, the definition of excision refers to: the module is cut off so that the module capacitor is not connected in series into the charging loop for charging.

Therefore, it is necessary to deeply research the charging strategy of the half-bridge and full-bridge sub-module hybrid converter, and provide a reference for practical engineering design.

Disclosure of Invention

The invention aims to overcome the defects of the prior art and provides a charging method of an extra-high voltage flexible direct-current full-bridge half-bridge hybrid converter, which can enable a half-bridge module to be electrified and normally work by triggering specific power devices in a full-bridge module in a direct-current side short-circuit or no short-circuit charging mode, and achieves the purpose of reducing module loss by alternately triggering the specific power devices in the full-bridge module.

In order to achieve the purpose, the technical scheme of the invention is as follows:

a charging method of an extra-high voltage flexible direct current full-bridge half-bridge hybrid converter comprises A, B, C three-phase units, each phase unit comprises an upper bridge arm unit and a lower bridge arm unit, each bridge arm unit is mainly formed by connecting a full-bridge submodule and a half-bridge submodule in series, the proportion of the full-bridge module and the half-bridge module is not limited, the half-bridge submodule comprises two IGBT power devices of T1 and T2, two anti-parallel diodes of D1 and D2 on T1 and T2 correspondingly connected in parallel, and a capacitor C, the full-bridge submodule comprises four IGBT power devices of T1, T2, T3 and T4 which are connected in sequence in a counterclockwise direction, and four anti-parallel diodes of D1, D2, D3 and D4 on T1, T2, T3 and T4 correspondingly connected in parallel, and a capacitor C, the method:

the method comprises the steps that power devices T2 or T3 in a full-bridge module are triggered in an uncontrollable charging stage and a controllable charging stage under a direct-current side short-circuit charging mode, power devices T1 or T4 in the full-bridge module are triggered in the uncontrollable charging stage under the direct-current side short-circuit charging mode, and capacitor voltages of the full-bridge module and the half-bridge module are improved until all the modules reach rated voltages by triggering the power devices T1, T3, T2 and T4 in the full-bridge module in the controllable charging stage.

Further, the charging method of the extra-high voltage flexible direct current full-bridge half-bridge hybrid converter further comprises the following steps:

the power devices T2 and T3 in the full-bridge module are triggered by front and rear two cycles of an uncontrollable charging stage and a controllable charging stage in a direct-current side short-circuit charging mode in turn, the power devices T1 and T4 in the full-bridge module are triggered by front and rear two cycles of an uncontrollable charging stage in a direct-current side short-circuit charging mode in turn, and the power devices T1, T3, T2 and T4 in the full-bridge module are triggered by front and rear two cycles of a controllable charging stage in turn.

Compared with the prior art, the invention has the following beneficial effects:

(1) the specific power device T2 or T3 in the full-bridge module is triggered in a direct-current side short circuit charging mode, only a module power device triggering command needs to be issued, measuring quantities such as alternating voltage do not need to be introduced, extra criteria do not need to be added, and normal power-on work of the full-bridge module and the half-bridge module can be achieved.

(2) The direct current side short circuit charging mode triggers T2 and T3 in turn in a front-back two-cycle manner, the direct current side short circuit charging mode triggers T1 and T4 in turn in a front-back two-cycle manner in an uncontrollable charging stage, the controllable charging stage triggers T1, T3, T2 and T4 in turn in a front-back two-cycle manner, and through the alternate triggering of the power devices in the front-back cycle manner, a certain power device can not be kept in a triggering and conducting state for a long time, so that the purpose of reducing the loss of the module device can be achieved.

Drawings

FIG. 1 is a schematic diagram of the composition of a MMC-HVDC system with a full-bridge and a half-bridge module mixed;

FIG. 2 is a DC side short-circuit mode charging equivalent circuit;

FIG. 3 is a current loop of a cut-off module and a charging module in a DC side short circuit charging mode;

FIG. 4 is a diagram of an equivalent charging circuit with a non-short-circuit mode on the DC side;

FIG. 5 is a current loop diagram of an uncontrollable charging cut-off module and a charging module in a DC side non-short charging mode;

fig. 6 is a current loop diagram of a controllable charging cut-off module and a charging module in a direct-current side non-short-circuit charging mode.

Detailed Description

The present invention will be described in further detail with reference to the accompanying drawings and detailed description.

Example (b):

in this embodiment, an MMC-HVDC system with a bridge arm full-bridge and a half-bridge module mixed is shown in fig. 1, and an equivalent charging loop with a short-circuit mode on a dc side is shown in fig. 2, and it is assumed that an a-phase voltage on an ac side is the highest and a B-phase voltage is the lowest, and that an uncontrolled charging phase exists in which an a-phase upper bridge arm returns through a dc short-circuit line via a B-phase lower bridge arm (arrow path No. 1) and an a-phase lower bridge arm returns through a dc short-circuit line via a B-phase upper bridge arm to two charging loops (arrow path No._sa、U_sb、U_scThe ac-side ABC three-phase voltages are shown, and P, N show the dc-side positive and negative electrodes. Because the sum of the voltage drops of the direct-current capacitors of the A-phase lower bridge arm is larger than the sum of the direct-current voltages of the A-phase upper bridge arm, the anti-parallel diodes in the A-phase lower bridge arm are turned off due to the fact that the anti-parallel diodes bear back pressure, the charging current only flows through the A-phase upper bridge arm, the branch circuit of the A-phase lower bridge arm is disconnected, namely only the module capacitor on the charging path with the arrow 1 can be charged, and the half-bridge module capacitor in the bridge arm cannot normally work. The three-phase voltage changes alternately, and the charging path analysis method is similar when other phase voltages are the highest or the lowest.

In order to enable the capacitors of the half-bridge modules to be charged, a certain number of full-bridge modules are cut off by triggering specific power devices in the full-bridge modules with the highest voltages of the capacitors of the A-phase lower bridge arm and the B-phase upper bridge arm in the uncontrolled charging stage, so that the back voltage born by anti-parallel diodes in the A-phase lower bridge arm and the B-phase upper bridge arm can be reduced, and a charging loop is switched to a No. 2 arrow charging path. As shown in fig. 3, the T2 current triggering the cut-out full-bridge module flows through T2 and D4 of the module, the T3 current triggering the cut-out module flows through T3 and D1 of the module, the current in the charged full-bridge module flows through D2 and D3, the half-bridge module triggers the T2 cut-out module, the T2 cut-out module current triggering the T2, and the charged half-bridge module current flows through D1. Through the alternate triggering of front and back two cycles of waves of T2 and T3, the current can be kept to flow through the arrow paths of the A-phase lower bridge arm and the B-phase upper bridge arm, and the alternate triggering of T2 and T3 can reduce the loss of module devices. Other bridge arm processing methods are similar, three-phase voltages are changed alternately, and charging paths have a common rule, namely charging current flows from a lower bridge arm with high phase voltage to an upper bridge arm with low phase voltage to return an alternating current system. In the controllable charging stage, more modules with higher voltage are cut off according to the sequencing result of the capacitor voltages of all the modules in the bridge arm from high to low, so that the capacitor voltages of the modules can be further improved. The full-bridge module triggers the T2 and T3 cutting modules in turn through front and back cycles; the half-bridge module cuts off the module by triggering T2, triggering T2 current to flow through T2 of the cut-off module, and charging half-bridge module current to flow through D1 until all modules reach the rated capacitance voltage value.

Fig. 4 shows an equivalent charging circuit in an uncontrolled charging stage in a direct-current non-short-circuit charging mode, and taking an AB-phase charging circuit as an example, two charging circuits, namely, a path with an arrow 1 returned by an upper bridge arm in an a-phase through an upper bridge arm in a B-phase and a path with an arrow 2 returned by a lower bridge arm in the a-phase through a lower bridge arm in the B-phase exist, where U is U_sa、U_sb、U_scThe ac-side ABC three-phase voltages are shown, and P, N are the dc-side positive and negative electrodes. Because the full-bridge module charge time is twice of the half-bridge module charge time, the full-bridge module capacitor voltage is twice of the half-bridge module. To increase the half-bridge module capacitor voltage, the specific power devices that can trigger all full-bridge modules are shown in fig. 5. Triggering the full-bridge module T1 current to flow through the T1 and D3 of the cut-off module, the charged full-bridge module current to flow through D1 and D4; the trigger full-bridge module T4 current flows through T4 and D2 of the cutout module, and the charged full-bridge module current flows through D1 and D4. Through the alternate triggering of front and back two cycles of T1 and T4, the charging time of the full-bridge module and the half-bridge module is consistent, the capacitor voltage of the half-bridge module is increased, and the alternate triggering of T2 and T3 can reduce the loss of the module device.

In order to further improve the capacitor voltage of the modules, according to the sequencing result of the capacitor voltages of all the modules in the bridge arm from high to low, more modules with higher voltages are cut off, the full-bridge module alternately triggers the T1, the T3, the T2 and the T4 to cut off the modules through front and back two cycles, as shown in FIG. 6, the currents for triggering the T1 and the T3 to be cut off the modules flow through the T1 and the D3, the currents for triggering the T2 and the T4 to be cut off the modules flow through the D2 and the T4, and the current loop of the charging module is the same as that in. The half-bridge module cuts off the module by triggering T2, triggering T2 current to flow through T2 of the cut-off module, and charging half-bridge module current to flow through D1 until all modules reach the rated capacitance voltage value.

The above embodiments are only for illustrating the technical concept and features of the present invention, and the purpose thereof is to enable those skilled in the art to understand the contents of the present invention and implement the present invention accordingly, and not to limit the protection scope of the present invention accordingly. All equivalent changes or modifications made in accordance with the spirit of the present disclosure are intended to be covered by the scope of the present disclosure.

Claims

1. A charging method of an extra-high voltage flexible direct current full-bridge half-bridge hybrid converter comprises A, B, C three-phase units, each phase unit comprises an upper bridge arm unit and a lower bridge arm unit, each bridge arm unit is mainly formed by connecting a full-bridge submodule and a half-bridge submodule in series, the half-bridge submodule comprises T1 and T2 IGBT power devices, two anti-parallel diodes and a capacitor C, the D1 and the D2 anti-parallel diodes are correspondingly connected with the T1 and the T2 in parallel, the full-bridge submodule comprises four T1, T2, T3 and T4 IGBT power devices which are sequentially connected in a counterclockwise mode, and four anti-parallel diodes and a capacitor C are correspondingly connected with the T1, the T2, the T3 and the D1, the D2, the D3 and the D4 anti-parallel diodes and the capacitor C on the T4:

the controllable charging and uncontrollable charging stages in the direct current side short circuit charging mode trigger a power device T2 or T3 in the full-bridge module, the uncontrollable charging stages in the direct current side short circuit charging mode trigger a power device T1 or T4 in the full-bridge module, and the controllable charging stages trigger the power devices T1, T3, T2 and T4 in the full-bridge module to improve the capacitor voltage of the full-bridge module and the half-bridge module until all the modules reach rated voltage;

the method further comprises the following steps:

the power devices T2 and T3 in the full-bridge module are triggered in turn by front and rear two cycles of controllable charging and uncontrollable charging in a direct-current side short-circuit charging mode, the power devices T1 and T4 in the full-bridge module are triggered in turn by front and rear two cycles of uncontrollable charging in the direct-current side short-circuit charging mode, and the power devices T1, T3, T2 and T4 in the full-bridge module are triggered in turn by front and rear two cycles of controllable charging in the controllable charging mode.