Patent Description:
As the most controllable transmission method at present, voltage sourced converter direct-current transmission is widely used in occasions like new energy grid connection, island power supply, and offshore platform power supply, and is also a superior solution for grid interconnection and load center power supply in urbans.

In many occasions, a cable is used as a transmission medium in voltage sourced converter direct-current transmission. A long cable has a large equivalent capacitance and will store energy while direct current passing through, it takes up to <NUM> to <NUM> minutes for the electricity accumulated on the cable to be released after voltage sourced converter direct-current transmission is out of operation, and the high voltage before discharge ends is dangerous to personal safety. This problem is particularly prominent in the fields of offshore platform power supply and offshore wind power grid connection. Offshore platforms, especially offshore oil and gas field platforms, have many devices and are extremely sensitive to heat and fire sources. When a voltage sourced converter direct-current transmission system is out of service, it is necessary to enter a valve hall timely to check whether there are any dangerous sources, so as to ensure the safety of the whole platform. However, due to the limitation of cable discharge time, people often have to wait for more than half an hour to approach the valve hall, which poses a serious safety threat.

At present, there are two methods to accelerate the discharge speed of voltage sourced converter direct-current transmission cables. The first method is to add a breaker and a resistance grounding circuit to two ends of the cable to speed up discharge, that is, adding additional complex equipment in order to realize safe and fast discharge. The second method is to accelerate discharging by means of a direct-current energy-consuming device, which cannot be realized in a system in which no energy-consuming device is designed or installed. Take offshore voltage sourced converter direct-current transmission as an example, core equipment is assembled in a dock and high-pressure testing is completed, and huge energy-consuming devices are generally not included, which leads to doubling of the test time and a significant increase in the engineering cost.

An embodiment of the application provides a discharge method for a voltage sourced converter direct-current transmission line. The method comprises: unlocking a converter valve; generating an additional control signal by an additional controller and superimposing the additional control signal on a reference voltage output by a main controller of the voltage sourced converter direct-current transmission system, so that the converter valve generates a harmonic current in a three-phase bridge arm of a converter; and locking the converter when a direct-current transmission line voltage is lower than a voltage setting or an average voltage of normal operating submodules in the converter bridge arm is lower than a submodule operating voltage setting.

According to some embodiments, before unlocking the converter, the method further comprises: charging the converter by using a residual voltage of the direct-current line, determining whether the charging can be completed, and unlocking the converter valve if yes; wherein after unlocking the converter, the method further comprises: if the charging cannot be completed, closing an AC incoming switch, charging the converter, and determining again whether the charging can be completed; and if the charging can be completed, opening the AC incoming switch and unlocking the converter valve at a same time.

According to some embodiments, the additional controller comprises an input signal generator, a first PI control module, a second PI control module, a transformation module and an inverse transformation module, wherein the input signal generator is configured to generate three-phase negative sequence currents; the transformation module is configured to transform the three-phase negative sequence currents into a first current reference component and a second current reference component; the first PI control module is configured to receive a first difference component generated by comparing the first current reference component with a measured three-phase current signal, and output a first current difference component; the second PI control module is configured to receive a second difference component generated by comparing the second current reference component with the measured three-phase current signal, and output a second current difference component; and the inverse transformation module is configured to receive the first current difference component and the second current difference component, and output the additional control signal.

According to some embodiments, the input signal generator generates three-phase negative sequence currents ia, ib, ic, the amplitude I thereof is an amplitude of a harmonic current injected into the converter, which is p1% of a rated bridge arm current, where p1 is <NUM>-<NUM> and can be set according to an actual DC transmission line voltage; a phase ωt is based on a valve side voltage Uv or a grid side voltage Us; n is an integer and n≥<NUM>; δ is a compensation angle, <NUM>°≤δ≤<NUM>°; and <MAT> <MAT> <MAT>.

According to some embodiments, the additional control signals ucira, ucirb and ucirc output by the additional controller are correspondingly superimposed on voltage control signals urefa, urefb and urefc output by a main controller of the voltage sourced converter direct-current transmission system to serve as converter valve control signals, so that the converter valve generates the harmonic current in the three-phase bridge arm.

According to some embodiments, during normal operation, the additional controller don't work and is activated when discharging is required.

An embodiment of the application provides a discharge device for a voltage sourced converter direct-current transmission line, comprising a control module, an additional controller and a converter locking module, wherein the control module is configured to unlock a converter valve; the additional controller is configured to generate an additional control signal and superimpose the additional control signal on a reference voltage output by a main controller of the voltage sourced converter direct-current transmission system, so that the converter valve generates a harmonic current in a three-phase bridge arm of a converter; and the converter locking module is configured to lock the converter when a voltage of a direct-current line is lower than a voltage setting or an average voltage of normal operating sub-modules in the bridge arm is lower than the sub-module operating voltage setting.

According to some embodiments, the control module is configured to charge the converter by using a residual voltage of the direct-current line, determines whether the charging can be completed, unlocks the converter valve if yes, closes an AC incoming switch and charges the converter if not, determines again whether the charging can be completed, and opens the AC incoming switch and unlocks the converter valve at the same time if the charging can be completed.

According to some embodiments, the additional controller comprises an input signal generator, a transformation module, a first PI control module, a second PI control module and an inverse transformation module, wherein the input signal generator generates three-phase negative sequence currents; the transformation module transforms the three-phase negative sequence currents into a first current reference component and a second current reference component; the first PI control module receives a first difference component generated by comparing the first current reference component with a measured three-phase current signal, and outputs a first current difference component; the second PI control module receives a second difference component generated by comparing the second current reference component with the measured three-phase current signal, and outputs a second current difference component; and the inverse transformation module receives the first current difference component and the second current difference component, and outputs the additional control signal.

According to some embodiments, the application further provides a computer-readable storage medium on which a processor program is stored, wherein the processor program is used to execute the method as mentioned above.

According to the technical scheme provided by the embodiments of the application, the fast discharge method for the voltage sourced converter direct-current transmission cable does not need any additional equipment, so the economical performance is good. Further, the method is controllable in discharge time and excellent in flexibility, and can be used reliably in any situation, thus having great adaptability.

In order to explain the technical solution in the embodiments of the application more clearly, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings in the following description only illustrate some embodiments of the application, and for those of ordinary technicians in the field, other drawings can be obtained according to these drawings without paying creative effort.

The technical solution in the embodiments of the application will be described clearly and completely with reference to the drawings in the embodiments of the application. Obviously, the described embodiments are part of the embodiments of the application, not all of them. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of the application.

It should be understood that the terms "first" and "second" in the claims, specification and drawings of the application are used to distinguish different objects, but not to describe a specific order. As used in the specification and claims of the application, the terms "comprise" and "include" indicate the presence of the described features, integers, steps, operations, elements and/or components, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components and/or groups thereof.

<FIG> is a circuit diagram of a discharge method for a voltage sourced converter direct-current transmission line provided by an embodiment of the application.

A converter <NUM> is controlled to turn on, so that a direct-current line <NUM> and a direct-current line <NUM> form a circuit through the converter valve <NUM>, and accumulated charges in the direct-current line are dissipated through equivalent resistance and reactance in the converter <NUM>, thus realizing rapid discharge of the direct-current line.

<FIG> is a flowchart of a discharge method for a voltage sourced converter direct-current transmission line provided by an embodiment of the application.

When the converter valve is unlocked, accumulated charges in direct-current line are dissipated through equivalent resistance and reactance in the converter.

In S03, an additional control signal is generated from an additional controller and superimposed on a reference voltage output by a main controller of the voltage sourced converter direct-current transmission system, so that the converter valve generates a harmonic current in a three-phase bridge arm of the converter, thus accelerating the process of the discharging in the direct-current line through the equivalent resistance and reactance in the converter.

In S05, when a voltage of the direct-current line is lower than a voltage setting or an average voltage of normal operating submodules in the bridge arm of converter is lower than a submodule operating voltage setting, the converter is locked, thereby finishing the whole accelerated discharge process.

<FIG> is a flowchart of another discharge method for a voltage sourced converter direct-current transmission line provided by an embodiment of the application.

In S01, when a voltage sourced converter direct-current transmission system is shut down and an AC incoming switch is opened, whether a voltage of a sub-module is higher than an unlocking threshold and meets the unlocking conditions is determined, and the control process is ended if not.

In S031, after the converter valve is unlocked, an additional control signal is generated from an additional controller and superimposed on a reference voltage output by a main controller of the voltage sourced converter direct-current transmission system, so that the converter valve generates a harmonic current in a three-phase bridge arm of a converter, thus accelerating the discharging in the direct-current line through the equivalent resistance and reactance in the converter.

In S05, when a voltage of the direct-current line is lower than a voltage setting or an average voltage of normal operating submodules in the bridge arm of the converter is lower than a submodule operating voltage setting, the converter is locked, thereby finishing the whole discharge process accelerated.

<FIG> is a flowchart of yet another discharge method for a voltage sourced converter direct-current transmission line provided by an embodiment of the application.

In S01, after a voltage sourced converter direct-current transmission system is shut down and the AC incoming switch is opened, a converter is charged by using a residual voltage of a direct-current line, and whether the charging can be completed is determined.

In S02, if the charging can be completed, the converter valve is unlocked.

In S03, if the charging cannot be completed, the AC incoming switch is closed, the converter is charged, and whether the charging can be completed is determined again.

In S04, if the charging can be completed, the AC incoming switch is opened and the converter valve is unlocked.

When the converter valve is unlocked, accumulated charges in a direct-current line are dissipated through equivalent resistance and reactance in the converter.

In S05, when a voltage of the direct-current line is lower than a voltage setting or an average voltage of normal operating submodules in the bridge arm of the converter is lower than a submodule operating voltage setting, the converter is locked.

After the residual voltage of the direct-current line is reduced below a specified value or the average voltage of the modules in a normal operating state in the bridge arm is reduced to a specified value, the converter is locked. Then the whole discharge process is ended.

<FIG> is a flowchart of still another discharge method for a voltage sourced converter direct-current transmission line provided by an embodiment of the application.

In S041, after the converter valve is unlocked, an additional control signal is generated from an additional controller and superimposed on a reference voltage output by a main controller of the voltage sourced converter direct-current transmission system, so that the converter valve generates a harmonic current in a three-phase bridge arm of the converter, thus accelerating the process of dissipating the accumulated charges in the direct-current line through the equivalent resistance and reactance in the converter.

<FIG> is a diagram of an additional controller provided by an embodiment of the application.

As shown in <FIG>, the additional controller C01 comprises an input signal generator C02, a first PI control module C04 and a second PI control module C05, a dq transformation module C03, and a dq inverse transformation module C06.

The input signal generator C02 generates three-phase negative sequence currents ia, ib, ic, and the amplitude I thereof is an amplitude of a harmonic current injected into the converter, which is p1% of a rated bridge arm current, where p1 is <NUM>-<NUM> and can be set according to an actual DC transmission line voltage; a phase ωt is based on a valve side voltage Uv or a grid side voltage Us; n is an integer and n≥<NUM>; δ is a compensation angle, <NUM>°≤δ≤<NUM>°. <MAT> <MAT> <MAT>.

The three-phase negative sequence currents ia, ib and ic generate current reference components <MAT> and <MAT> through the dq transformation module C03, and difference components generated by comparing current reference components <MAT> and <MAT> with dq components ifnd and ifnq of a measured three-phase current signal are input into the first PI control module C04 and the second PI control module C05 to obtain id<NUM> and iq<NUM>. Here, the difference components: <MAT>.

The dq axis components of the additional control signal of the additional controller are: ucird and ucirq : <MAT> <MAT>.

In C06, the additional control signals ucira, ucirb and ucirc of the additional controller are obtained through dq inverse transformation.

As shown in <FIG>, in one possible implementation of the method, the additional control signals ucira, ucirb and ucirc output by the additional controller are superimposed on voltage control signals urefa, urefb and urefc output by a main controller B01 of the voltage sourced converter direct-current transmission system as converter valve control signals, so that the converter valve generates a harmonic current in a three-phase bridge arm of the converter. As shown in <FIG>, the converter valve generates a harmonic current <NUM> in the three-phase bridge arm of the converter valve.

When the voltage sourced converter direct-current transmission system is in normal operation, the additional controller is not activated. The additional controller is put into operation when the system is shut down and accelerated discharge is required.

<FIG> is a functional block diagram of a discharge device for a voltage sourced converter direct-current transmission line provided by an embodiment of the application, comprising a control module <NUM>, a converter locking module <NUM>, and an additional controller <NUM>.

The control module <NUM> unlocks a converter valve by using a residual voltage of a direct-current line.

The additional controller <NUM> generates an additional control signal and superimposes the additional control signal on a reference voltage output by a main controller of the voltage sourced converter direct-current transmission system so that the converter valve generates a harmonic current in a three-phase bridge arm of a converter.

The converter locking module <NUM> locks the converter when a direct-current transmission line voltage is lower than a voltage setting or an average voltage of normal operating submodules in the bridge arm of the converter is lower than a submodule operating voltage setting.

Optionally, the control module <NUM> charges the converter by using a residual voltage of the direct-current line; determines whether the charging can be completed; unlocks the converter valve if yes; closes an AC incoming switch and charges the converter if not, determines again whether the charging can be completed; and opens the AC incoming switch and unlocks the converter valve at the same time if the charging can be completed.

The additional controller <NUM> may comprise an input signal generator, a transformation module, a first PI control module, a second PI control module and an inverse transformation module.

The input signal generator generates three-phase negative sequence currents; the transformation module transforms the three-phase negative sequence currents into a first current reference component and a second current reference component; the first PI control module receives a first difference component generated by comparing the first current reference component with a measured three-phase current signal, and outputs a first current difference component; the second PI control module receives a second difference component generated by comparing the second current reference component with a measured three-phase current signal, and outputs a second current difference component; and the inverse transformation module receives the first current difference component and the second current difference component, and outputs the additional control signal.

It should be understood that the above-mentioned device embodiments are only schematic. For example, the division of units/modules described in the above embodiments is only a logical function division, and there may be another division mode in actual implementation. For example, multiple units, modules, or components may be combined, or integrated into another system, or some features may be omitted or not performed. If the integrated units/modules are implemented in the form of a software module and sold or used as an independent product, it can be stored in a computer-readable memory. The memory includes a USB flash drive, Read-Only Memory (ROM), Random Access Memory (RAM), removable hard disk, magnetic disk or optical disk, and other media that can store program codes.

Claim 1:
A discharge method for a voltage sourced converter direct-current transmission line, comprising:
unlocking a converter valve;
generating an additional control signal by an additional controller and superimposing the additional control signal on a reference voltage output by a main controller of a voltage sourced converter direct-current transmission system, so that the converter valve generates a harmonic current in a three-phase bridge arm of a converter; and
locking the converter when a direct-current transmission line voltage is lower than a voltage setting or an average voltage of normal operating submodules in the bridge arm of the converter is lower than a submodule operating voltage setting.