A control method for a composite DC unloading circuit based on a lightning arrester

Abstract

The present invention discloses a control method for a composite DC unloading circuit based on a lightning arrester, comprising: measuring the DC input power of a grid-side converter station and the AC-side output power, calculating the power difference required to be consumed by the composite DC unloading circuit; measuring the DC transmission line voltage, comparing the DC transmission line voltage with a DC transmission line reference value, and obtaining a DC unloading circuit trigger signal; forming a pulse signal for controlling all DC unloading submodules according to the power difference and the DC unloading circuit trigger signal; synchronizing the pulse signals of all DC unloading submodules to ensure the synchronous operation of all DC unloading submodules. The present invention can ensure that all DC unloading submodules can work synchronously by delaying the control signal of each group of DC unloading modules.

Description

Technical Field

[0001] The invention belongs to the technical field of direct current power transmission, and in particular relates to a control method of a composite direct current unloading circuit based on a lightning arrester. Background Art

[0002] In recent years, high-voltage direct current transmission technology has developed rapidly, and the application of offshore wind farm grid-connected flexible direct current transmission system (VSC-HVDC) has become more and more widespread. When VSC-HVDC is operating normally, the energy generated by the wind turbine when connected in an isolated manner is balanced with the energy consumed by the receiving AC power grid. When the receiving AC power grid fails, the energy it can consume is reduced and its ability to accept power is limited. Since the sending wind farm cannot directly obtain the frequency and voltage information of the AC power grid, the voltage and frequency will not change in a short period of time, resulting in energy accumulation on the DC line, and the surplus power flows into the modular multilevel converter, such as Figure 1 The power distribution diagram of the offshore wind farm when connected in an isolated way is shown in the figure. The capacitor of the modular multilevel converter (MMC) is charged, the voltage rises, and indirectly causes the DC line voltage to rise. If the receiving end converter station fails to control the DC line, the line will trip in serious cases.

[0003] To avoid failure, it is necessary to use a DC unloading circuit to consume the surplus power in an energy-consuming manner, such as Figure 2 The power distribution diagram when the DC unloading circuit is applied is shown, and the power consumption is controlled by the chopper circuit to achieve power balance. The centralized resistor chopper unloading circuit uses the switch series connection technology, and the dynamic and static voltage balancing of the switch device is difficult to achieve, and the dv / dt and di / dt are very large. For example, patent publication number CN101136582B discloses a control method for the DC side unloading circuit of a full-power converter, in which the unloading circuit is composed of a power device and an unloading resistor.

[0004] The modular design of the multi-level unloading circuit can also effectively reduce current fluctuations and achieve better dynamic and static voltage balancing. However, it uses many large-capacity semiconductor devices, and the circuit is expensive and bulky. For example, patent announcement number CN102856909B discloses an unloading system and a modular multi-level wind power converter using the unloading system. In view of the characteristics of the modular multi-level wind power converter topology structure using multiple cascaded power units, separate unloading circuits are respectively set in each cascaded power unit.

[0005] It can be seen from this that a DC unloading circuit is needed that can effectively reduce current fluctuations and achieve dynamic and static voltage balancing. Summary of the invention

[0006] In view of the above problems, the present invention discloses a control method of a composite DC unloading circuit based on a lightning arrester, comprising:

[0007] Measure the DC input power and AC output power of the grid-side converter station, and calculate the power difference that the composite DC unloading circuit needs to consume;

[0008] Measuring a DC transmission line voltage, comparing the DC transmission line voltage with a DC transmission line reference value, and obtaining a DC unloading circuit trigger signal;

[0009] Forming a pulse signal for controlling all DC unloading submodules according to the power difference and the DC unloading circuit trigger signal;

[0010] Synchronize the pulse signals of all DC unloading submodules to ensure the synchronous operation of all DC unloading submodules.

[0011] Preferably, the power difference that the composite DC unloading circuit needs to consume is the difference between the DC input power of the measuring grid-side converter station and the AC-side output power.

[0012] Preferably, the DC transmission line voltage is compared with a DC transmission line voltage reference value by a comparator to obtain the unloading circuit trigger signal.

[0013] Preferably, the power difference signal is processed by a proportional link and a division link to output a control signal, the control signal and the DC unloading circuit trigger signal are processed and output by a multiplication link, after being processed by the multiplication link, the control duty cycle of the DC unloading submodule is obtained by processing through a limiting link, and then the pulse signal of the DC unloading submodule is output by comparing it with a frequency-adjustable carrier signal through a comparator.

[0014] Preferably, the parameters of the proportional link and the division link are determined by the power difference and the design value of the unloading submodule.

[0015] Preferably, the number of groups of pulse signals is generated according to the number of DC unloading submodules.

[0016] Preferably, the pulse signal is processed by delay to ensure that all DC unloading submodules work synchronously, thereby reducing the dynamic voltage difference between the switch devices of the submodules.

[0017] Preferably, the delay processing includes turn-on delay processing and turn-off delay processing; the time of the turn-on delay processing is determined according to the difference between the turn-on delay of each group of DC unloading submodules and the maximum turn-on delay of all DC unloading submodules; the time of the turn-off delay processing is determined according to the difference between the turn-off delay of each group of DC unloading submodules and the maximum turn-off delay of all DC unloading submodules.

[0018] Preferably, the on-delay is the time it takes for the voltage of the switching device to drop to 10% of the rated voltage of the submodule after the switching device of the DC unloading module is turned on; the off-delay is the time it takes for the voltage to rise to 90% of the rated voltage of the DC unloading submodule when the switching device of the DC unloading module is turned off.

[0019] Preferably, the turn-on delay and turn-off delay are measured after the pulse signal is sent for the first time, and before the next pulse is sent, the difference between the turn-on delay of each group of DC unloading submodules and the maximum turn-on delay of all DC unloading submodules is calculated by the operator, and the difference between the turn-off delay of each group of DC unloading submodules and the maximum turn-off delay of all DC unloading submodules is calculated by the operator; according to the obtained difference, the pulse signal of the corresponding DC unloading submodule is delayed in the next pulse signal to ensure the synchronous operation of all DC unloading submodules.

[0020] The present invention controls the DC unloading submodule by detecting the power required to be consumed by the DC unloading circuit and the voltage of the DC transmission line, thereby ensuring the stable unloading operation of the DC unloading submodule; and by delaying the control signal of each group of DC unloading modules, it can be ensured that all DC unloading submodules can work synchronously.

[0021] Other features and advantages of the present invention will be described in the following description, and partly become apparent from the description, or understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained by the structures pointed out in the description, claims and drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0023] Figure 1 A schematic diagram of power distribution when an offshore wind farm is connected in an isolated island mode in the prior art is shown;

[0024] Figure 2 A schematic diagram of power distribution when a DC unloading circuit is applied in the prior art is shown;

[0025] Figure 3 A diagram showing the use status of the DC unloading submodule of the present invention is shown;

[0026] Figure 4 A schematic diagram of the topological structure of a DC unloading submodule SM according to a first embodiment of the present invention is shown;

[0027] Figure 5 A schematic diagram of the topological structure of a DC unloading submodule SM according to a second embodiment of the present invention is shown;

[0028] Figure 6 A schematic diagram showing the topological structure of a DC unloading submodule SM according to a third embodiment of the present invention is shown;

[0029] Figure 7 A schematic diagram showing the topological structure of a DC unloading submodule SM according to a fourth embodiment of the present invention is shown;

[0030] Figure 8 A block diagram of a control method for a composite DC unloading circuit based on a lightning arrester of the present invention is shown;

[0031] Fig. 9 A schematic diagram of submodule pulse signal control and voltage measurement of a composite DC unloading circuit based on a lightning arrester of the present invention is shown;

[0032] Fig.10 A diagram of a submodule pulse coordinated control method of a composite DC unloading circuit based on a lightning arrester of the present invention is shown. DETAILED DESCRIPTION

[0033] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0034] The present invention provides a composite DC unloading circuit based on a lightning arrester for use in DC power transmission. At least one unloading branch is provided between the positive line and the negative line of the DC power transmission line. The unloading branch includes one or more DC unloading submodules. The multiple DC unloading submodules are connected in series between the positive line and the negative line of the DC power transmission line. A composite DC unloading circuit is formed by connecting a lightning arrester in parallel to a traditional DC unloading circuit. The composite DC unloading circuit and the lightning arrester are modularized to form a DC unloading submodule. Several groups of DC unloading submodules are connected in series. By increasing the number of DC unloading submodules connected in series, the dv / dt (voltage rise rate) and di / dt (current rise rate) of the entire circuit are reduced, so that the power regulation characteristics are smoother. A composite DC unloading circuit based on a lightning arrester includes n DC unloading submodules SM: DC unloading submodules SM1,..., DC unloading submodules SM i ,···,DC unloading submodule SMn , n is any positive integer, i is any positive integer greater than or equal to 1 and less than or equal to n, the DC unloading submodule SM includes a positive input terminal and a negative output terminal, and the positive and negative electrodes of the n DC unloading submodules SM are connected in series in the positive and negative lines of the DC line. By flexibly using the number of DC unloading submodules SM and reasonably selecting the withstand voltage and current capacity of the turn-off device, a composite DC unloading circuit for different occasions can be designed. The more the number of cascaded DC unloading submodules SM, the smaller the dv / dt and di / dt of the DC circuit, and the smoother the power regulation characteristics.

[0035] The DC unloading submodule includes a positive input terminal and a negative output terminal, and a chopper circuit and an arrester are connected between the positive input terminal and the negative input terminal. The arrester can be of many types, including metal oxide arresters, line-type metal oxide arresters, gapless line-type metal oxide arresters, fully insulated composite jacket metal oxide arresters, and removable arresters. The chopper circuit is connected in parallel with a buffer circuit to provide buffer protection for the chopper circuit. The buffer circuit includes a buffer resistor-capacitor circuit, which includes a resistor and a capacitor connected in series with the resistor. The resistor is connected to the positive input terminal, and the capacitor is connected to the negative output terminal. A bypass switch is connected between the positive input terminal and the negative output terminal. A diode is connected between the positive input terminal and the negative output terminal, the positive electrode of the diode is connected to the negative output terminal, and the negative electrode of the diode is connected to the positive input terminal. The chopper circuit includes at least one group of switch devices and at least one group of energy-consuming devices, and the switch device and the energy-consuming device are connected in series between the positive input terminal and the negative output terminal. The energy dissipation device comprises a load-discharging resistor, and the switch device is connected in reverse parallel with a freewheeling diode.

[0036] For the convenience of description, only the application of the DC unloading submodule SM in an offshore wind farm is used as an example, and only one unloading branch is used in the DC transmission line of the offshore wind farm. It should be noted that the DC unloading submodule SM of the present invention is not only applicable to offshore wind farms, but also to other DC transmission lines, and the number of unloading branches can also be multiple groups, and the number can be installed and set as needed.

[0037] For example, Figure 3 The diagram shows the use status of the DC unloading submodule SM. Multiple DC unloading submodules SM are connected in series between the positive line and the negative line of the DC transmission line of the offshore wind farm. The positive input terminal of the first DC unloading submodule SM1 is connected to the positive line of the DC transmission line of the offshore wind farm; the i-1th DC unloading submodule SM i-1 The negative output terminal is connected to the i-th DC unloading submodule SM i The positive output terminal of the nth DC unloading submodule SM nThe negative output end is connected to the negative line of the DC transmission line of the offshore wind farm.

[0038] As a first embodiment of the present invention, Figure 4 The topological structure diagram of the DC unloading submodule SM shown in FIG. 1 is a schematic diagram of the topological structure of the DC unloading submodule SM, wherein the DC unloading submodule SM comprises a switch tube T and an unloading resistor R connected in series. i , through the unloading resistor R i To consume surplus power, the switch tube T is used as a switching device. The switch tube T can be a fully controlled power electronic device such as IGBT, IGCT, etc. The collector of the switch tube T is connected to the positive input terminal, and the emitter of the switch tube T is connected to the unloading resistor R i , unloading resistor R i Connected to the negative input terminal, a lightning arrester MOV (metal oxide lightning arrester) is connected between the positive input terminal and the negative output terminal. The lightning arrester MOV is used to strictly limit overvoltage and ensure the voltage equalization effect; the lightning arrester MOV and the buffer RC circuit are used to achieve dynamic and static voltage equalization and overvoltage and overcurrent protection of the switch tube T under transient conditions, thereby improving the reliability of the switch tube T.

[0039] As a second embodiment of the present invention, Figure 5 The topological structure diagram of the DC unloading submodule SM shown in FIG. 1 is a schematic diagram of the topological structure of the DC unloading submodule SM, wherein the DC unloading submodule SM comprises a switch tube T and an unloading resistor R connected in series. i , through the unloading resistor R i To consume surplus power, the switch tube T is used as a switching device. The switch tube T can be a fully controlled power electronic device such as IGBT, IGCT, etc. The collector of the switch tube T is connected to the positive input terminal, and the emitter of the switch tube T is connected to the unloading resistor R i , unloading resistor R i Connected to the negative input terminal, switch tube T and unloading resistor R i A buffer resistor-capacitor circuit is connected in series and then in parallel. The buffer resistor-capacitor circuit includes a resistor R S and capacitor C S , resistor R S Connected to the positive input terminal, capacitor C S Connected to the negative output terminal, the buffer RC circuit can slow down the voltage rise speed of the switch tube T when the switch tube T is turned off, and reduce the dynamic voltage difference. The buffer RC circuit is connected in parallel with the MOV (metal oxide arrester), and the MOV is used to strictly limit the overvoltage to ensure the voltage balancing effect; the MOV and the buffer RC circuit are used to achieve dynamic and static voltage balancing and overvoltage and overcurrent protection of the switch tube T under transient conditions, thereby improving the reliability of the switch tube T.

[0040] As a third embodiment of the present invention, Figure 6 The topological structure diagram of the DC unloading submodule SM shown in FIG. 1 is a schematic diagram of the topological structure of the DC unloading submodule SM, wherein the DC unloading submodule SM comprises a switch tube T and an unloading resistor R connected in series.i , through the unloading resistor R i To consume surplus power, the switch tube T is used as a switching device. The switch tube T can be a fully controlled power electronic device such as IGBT, IGCT, etc. The collector of the switch tube T is connected to the positive input terminal, and the emitter of the switch tube T is connected to the unloading resistor R i , unloading resistor R i Connected to the negative input terminal, switch tube T and unloading resistor R i A buffer resistor-capacitor circuit is connected in series and then in parallel. The buffer resistor-capacitor circuit includes a resistor R S and capacitor C S , resistor R S Connected to the positive input terminal, capacitor C S Connected to the negative output terminal, the buffer RC circuit can slow down the voltage rise speed of the switch tube T when the switch tube T is turned off, and reduce the dynamic voltage difference. The buffer RC circuit is connected in parallel with the arrester MOV (metal oxide arrester), and the arrester MOV is used to strictly limit the overvoltage to ensure the voltage equalization effect; the arrester MOV and the buffer RC circuit are used to achieve dynamic and static voltage equalization and overvoltage and overcurrent protection of the switch tube T under transient conditions, thereby improving the reliability of the switch tube T. The RC circuit is also connected in parallel with the switch S, and the bypass switch S plays a bypass function when the unloading submodule fails. The RC circuit is also connected in parallel with the bypass switch S and the diode D. D is a diode. In order to prevent the unloading submodule from being reversely connected and being damaged by reverse voltage, the cathode of the diode D is connected to the positive input terminal, and the cathode of the diode D is connected to the negative output terminal.

[0041] As a fourth embodiment of the present invention, Figure 7 The topological structure diagram of the DC unloading submodule SM shown in FIG. 1 is a schematic diagram of the topological structure of the DC unloading submodule SM, wherein the DC unloading submodule SM comprises a switch tube T and an unloading resistor R connected in series. i , through the unloading resistor R i To consume surplus power, the switch tube T is used as a switching device. The switch tube T can be a fully controlled power electronic device such as IGBT, IGCT, etc. The collector of the switch tube T is connected to the positive input terminal, and the emitter of the switch tube T is connected to the unloading resistor R i , unloading resistor R i Connected to the negative input terminal, switch tube T and unloading resistor R i A buffer resistor-capacitor circuit is connected in series and then in parallel. The buffer resistor-capacitor circuit includes a resistor R S and capacitor C S , resistor R S Connected to the positive input terminal, capacitor C SConnected to the negative output terminal, the buffer RC circuit can slow down the voltage rise speed of the switch tube T when the switch tube T is turned off, and reduce the dynamic pressure difference. The buffer RC circuit is connected in parallel with the arrester MOV (metal oxide arrester), and the arrester MOV is used to strictly limit the overvoltage to ensure the voltage balancing effect; the arrester MOV and the buffer RC circuit are used to realize the dynamic and static voltage balancing and the overvoltage and overcurrent protection of the switch tube T under transient conditions, thereby improving the reliability of the switch tube T. The RC circuit is also connected in parallel with the switch S, and the bypass switch S plays a bypass function when the unloading submodule fails. The RC circuit is also connected in parallel with the bypass switch S and the diode D. D is a diode. To prevent the unloading submodule from being reversely connected and being damaged by reverse voltage, the cathode of the diode D is connected to the positive input terminal, and the cathode of the diode D is connected to the negative output terminal. D acts as a diode to prevent the DC unloading submodule SM from being reversely connected and being damaged by reverse voltage. The switch tube is reversely connected in parallel with the freewheeling diode, and the freewheeling diode is used to protect the switch tube.

[0042] The composite DC unloading circuit based on the arrester of the technical solution can realize the dynamic and static voltage balancing of the switch device, and the modular design reduces the dv / dt and di / dt of the circuit. By removing the bulky unloading capacitor, the volume of a single unloading submodule is reduced by more than 30%. In addition, the cost is greatly reduced, a reverse resistance diode and unloading capacitor are omitted, and the capacity of the unloading resistor and the switch device is reduced.

[0043] The technical solution also provides a control method for a composite DC unloading circuit based on a lightning arrester. When the composite DC unloading circuit based on a lightning arrester is in operation, the positive line of the DC transmission line voltage is monitored in real time. When the voltage of the positive line exceeds a set threshold, the composite DC unloading module is triggered.

[0044] like Figure 8 The control method of the composite DC unloading circuit based on the arrester is shown in the block diagram, and the control method includes detecting the DC input power (P in_G ) and AC side output power (P out_G ), calculate (P out_G -P in_G )Grid-side converter station DC input power (P in_G ) and AC side output power (P out_G ) to obtain the power difference (△P) that the composite DC unloading circuit based on the lightning arrester should consume.

[0045] The power difference signal is processed through a proportional link and a division link to obtain a control signal of the unloading circuit. The proportional link and the division link are determined by the per-unit value of the power difference and the design value of the unloading submodule.

[0046] Measuring the voltage value of the DC transmission line E dcand the DC transmission line voltage reference value E dc_ref Specifically, the DC transmission line voltage measurement value E dc The DC transmission line voltage reference value E dc_ref By comparing with the comparator, when the DC transmission line voltage measurement value E dc Higher than the DC transmission line voltage reference value E dc_ref When the comparator outputs a high level signal, the DC transmission line voltage measurement value E dc The DC transmission line voltage reference value E dc_ref By comparing with the comparator, when the DC transmission line voltage measurement value E dc Not higher than the DC transmission line voltage reference value E dc_ref When , the comparator outputs a low level signal.

[0047] The unloading circuit control signal and the unloading circuit trigger signal are multiplied by a multiplication link, and the multiplied control signal is limited by a limiting link to obtain a duty cycle of the control switch device. The limiting link is used to avoid damage to the switch device caused by excessive signal amplitude.

[0048] A frequency-adjustable triangular carrier signal is obtained from a signal generator or other signal generating device, and according to the duty cycle, a pulse signal of corresponding frequency and duty cycle can be obtained through a comparator to control the chopping of the switching device.

[0049] For example, when the control of the DC line by the grid-side converter station of the offshore wind farm fails, the DC input power of the grid-side converter station is detected to be P in_G And the AC side output power P out_G , calculate the DC input power P of the grid-side converter station in_G And the AC side output power P out_G The power difference is P out_G -P in_G =△P, and the power difference that the composite DC unloading circuit based on the arrester should consume is obtained. The power difference signal is processed through the proportional link and the division link to obtain the control signal of the unloading circuit. The proportional link and the division link are determined by the per-unit value of the power difference and the design value of the unloading submodule.

[0050] In addition, the measured voltage value of the DC transmission line of the offshore wind farm is E dc and the DC transmission line voltage reference value E dc_ref Specifically, the DC transmission line voltage measurement value E dc The DC transmission line voltage reference value E dc_ref By comparing with the comparator, it is found that the DC transmission line voltage measurement value Edc Higher than the DC transmission line voltage reference value E dc_ref When the comparator outputs a high level signal, the DC transmission line voltage measurement value E dc The DC transmission line voltage reference value E dc_ref By comparing with the comparator, it is found that the DC transmission line voltage measurement value E dc Not higher than the DC transmission line voltage reference value E dc_ref When , the comparator outputs a low-level signal. Among them, the high-level signal of the unloading circuit trigger signal is 1, and the low-level signal is 0. The unloading circuit trigger signal and the unloading circuit control signal are processed through the multiplication link, and the multiplication-processed control signal is limited through the limit link to obtain the duty cycle of the control switch device. The limit link is used to avoid damage to the switch device caused by excessive signal amplitude, so as to obtain the duty cycle of the control switch device in the DC transmission line voltage measurement value E. dc Higher than the DC transmission line voltage reference value E dc_ref All switch devices of the unloading circuit submodule are turned on at the same time, and the unloading work is performed through the unloading circuit submodule.

[0051] It should be noted that when a serious fault occurs in the grid-side converter station of an offshore wind farm, the duty cycle of the control switch device is always controlled to be 1, and the switch devices of all unloading circuit submodules are always in the open state to perform the unloading work of the DC transmission line; when a general fault occurs in the grid-side converter station of an offshore wind farm, the duty cycle of the control switch device is controlled to be 0-1, and the size of the duty cycle is inversely related to the circuit operating frequency. According to the severity of the fault and the voltage requirements of the DC system, the control method parameters of the composite DC unloading circuit can be designed by collaboratively controlling the operating frequency and the duty cycle.

[0052] like Fig.10 The diagram of the sub-module pulse coordinated control method of the composite DC unloading circuit based on the arrester is shown. In order to ensure the synchronous control of all the DC unloading sub-modules in series, the turn-off delay and turn-on delay of the switch devices of each group of DC unloading sub-modules are measured, and the control signal of the switch device is delayed. Among them, when the switch device is turned on, the voltage drops, and the time when the voltage drops to 10% of the rated voltage of the sub-module is considered to be the turn-on delay of the switch device; when the switch device is turned off, the voltage rises, and the time when the voltage rises to 90% of the rated voltage of the sub-module is considered to be the turn-off delay of the switch device. Traverse the turn-on delay and turn-off delay of all DC unloading circuit modules, and select the maximum turn-on delay t on and the maximum turn-off delay time t off .

[0053] According to the number of DC unloading submodules, a corresponding number of pulse signals are generated. Fig. 9The sub-module pulse signal control schematic diagram and voltage measurement schematic diagram of the composite DC unloading circuit based on the lightning arrester are shown, and the pulse signal includes an output pulse signal and a feedback pulse signal.

[0054] After the pulse is sent, the voltage of the switch device of the DC unloading submodule is detected by a voltage sensor.

[0055] Before the next pulse is sent, the opening delay and the maximum opening delay t of each submodule are calculated by the operator. off The difference △t on , the closing delay and the maximum closing delay t of each submodule are calculated by the operator on The difference △t off .

[0056] According to the difference △t on and difference △t off The next group of pulse signals corresponding to the DC unloading circuit module is delayed.

[0057] Each group of pulse signals after delay processing is supplied to the switch device of each DC unloading circuit module, which can ensure the synchronous operation of all the switch devices.

[0058] Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent substitutions for some of the technical features therein; and these modifications or substitutions do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A control method for a composite DC unloading circuit based on a lightning arrester, characterized in that: include: Measure the DC input power and AC output power of the grid-side converter station, and calculate the power difference that the composite DC unloading circuit needs to consume; Measuring a DC transmission line voltage, comparing the DC transmission line voltage with a DC transmission line reference value, and obtaining a DC unloading circuit trigger signal; Forming a pulse signal for controlling all DC unloading submodules according to the power difference and the DC unloading circuit trigger signal; Synchronize the pulse signals of all DC unloading submodules to ensure the synchronous operation of all DC unloading submodules; The pulse signal is processed by delay to ensure that all DC unloading submodules work synchronously and reduce the dynamic voltage difference between the switch devices of the submodules; The delay processing includes an on-delay processing and an off-delay processing; The time of the on-delay processing is determined according to the difference between the on-delay of each group of DC unloading submodules and the maximum on-delay of all DC unloading submodules; the time of the off-delay processing is determined according to the difference between the off-delay of each group of DC unloading submodules and the maximum off-delay of all DC unloading submodules; The on-delay and off-delay are measured after the pulse signal is sent for the first time, and before the next pulse is sent, the difference between the on-delay of each group of DC unloading submodules and the maximum value of the on-delay of all DC unloading submodules is calculated by the operator, and the difference between the off-delay of each group of DC unloading submodules and the maximum value of the off-delay of all DC unloading submodules is calculated by the operator; according to the obtained difference, the pulse signal of the corresponding DC unloading submodule is delayed in the next pulse signal to ensure the synchronous operation of all DC unloading submodules; The DC unloading submodule SM includes a switch tube T and an unloading resistor R connected in series. i , through the unloading resistor R i Consume surplus power; the collector of the switch tube T is connected to the positive input terminal, and the emitter of the switch tube T is connected to the unloading resistor R i , unloading resistor R i Connected to the negative input terminal, switch tube T and unloading resistor R i A buffer resistor-capacitor circuit is connected in series and then in parallel. The buffer resistor-capacitor circuit includes a resistor R S and capacitor C S , resistor R S Connected to the positive input terminal, capacitor C S Connected to the negative output end, the voltage rise speed of the switch tube T is slowed down by the buffer RC circuit when the switch tube T is turned off, and the dynamic pressure difference is reduced; the buffer RC circuit is connected in parallel with the lightning arrester MOV, and the overvoltage is limited by the lightning arrester MOV to ensure the voltage balancing effect; the lightning arrester MOV and the buffer RC circuit are used to realize dynamic and static voltage balancing and overvoltage and overcurrent protection of the switch tube T under transient conditions; the RC circuit is also connected in parallel with a bypass switch S, and the bypass switch S plays a bypass function when the unloading submodule fails. The RC circuit is also connected in parallel with a diode D, and the diode D is used to prevent the unloading submodule from being reversely connected and being damaged by reverse voltage. The cathode of the diode D is connected to the positive input end, and the cathode of the diode D is connected to the negative output end; the switch tube is reversely connected in parallel with a freewheeling diode, and the switch tube is protected by the freewheeling diode; The DC transmission line voltage is compared with a DC transmission line voltage reference value by a comparator to obtain the unloading circuit trigger signal; The power difference signal is processed by the proportional link and the division link to output the control signal. The control signal and the DC unloading circuit trigger signal are processed by the multiplication link and output. After being processed by the multiplication link, the control duty cycle of the DC unloading submodule is obtained by the limiting link. After that, the pulse signal of the DC unloading submodule is output by comparing it with the frequency-adjustable carrier signal through the comparator.

2. The control method of the composite DC unloading circuit based on the arrester according to claim 1 is characterized in that: The power difference that the composite DC unloading circuit needs to consume is the difference between the DC input power of the measuring grid-side converter station and the AC-side output power.

3. The control method of the composite DC unloading circuit based on the arrester according to claim 1 is characterized in that: The parameters of the proportional link and the division link are determined by the power difference and the design value of the unloading submodule.

4. The control method of the composite DC unloading circuit based on the arrester according to claim 1 is characterized in that: The number of groups of pulse signals is generated according to the number of DC unloading submodules.

5. The control method of the composite DC unloading circuit based on the arrester according to claim 1 is characterized in that: The on-delay is the time it takes for the voltage of the switching device to drop to 10% of the rated voltage of the submodule after the switching device of the DC unloading module is turned on; the off-delay is the time it takes for the voltage to rise to 90% of the rated voltage of the DC unloading submodule when the switching device of the DC unloading module is turned off.

Images