CN111525517B - A method and system for suppressing overvoltage of a converter valve sub-module - Google Patents

Abstract

The invention relates to a method and system for suppressing overvoltage of a converter valve sub-module. The method calculates the dynamic variance of the DC port voltage of the converter valve in the current sampling period and the previous sampling period; and judges whether or not according to the dynamic variance A serious fault occurs; if a serious fault occurs, the shutdown protection of the converter station is activated. The system reduces the energy stored in the capacitor of the sub-module by adding a serious fault judgment module to the control and protection system of the converter valve, and reduces the energy stored in the capacitor of the sub-module by quickly cutting off the continuous charging circuit of the sub-module under a serious fault. module overvoltage level to improve the safety margin of the offshore wind power system.

Description

A method and system for suppressing overvoltage of a converter valve sub-module

technical field

The invention relates to the technical field of new energy and electric power engineering, in particular to a method and system for suppressing overvoltage of a converter valve sub-module.

Background technique

With the increasing energy problems, the development and utilization of new clean energy has become a hot issue in the world. With the continuous advancement of science and technology, the capacity of offshore wind power generation is constantly expanding, and the development of offshore wind farms is of great significance for solving the energy crisis. The grid-connected operation of offshore wind power has become the most effective way to utilize wind energy on a large scale. DC power transmission is widely used in offshore wind farms because it is suitable for large-capacity and long-distance power transmission. With the increasing demand for transmission distance and greater transmission capacity, DC transmission will play an important role in the development and utilization of offshore wind farms. Compared with conventional DC transmission and DC transmission based on two-level and three-level voltage source converters, the offshore wind power flexible DC access system based on modular multi-level converters is more suitable for long-distance and large-scale offshore wind power connection. into the system.

With the continuous operation of onshore flexible HVDC transmission projects, the technology of modular multi-level converters has gradually matured. However, offshore converter stations have the characteristics of high maintenance cost, long cycle and difficulty, so their design is always large capacity. There are still many problems in the key technology of offshore wind power flexible DC transmission system. Among them, the safety and stability of the flexible DC transmission converter valve determine the reliability of the entire flexible DC transmission system. The sub-module based on the half-bridge or full-bridge topology is the smallest unit for the operation of the flexible-DC converter valve, and its stability and safety characteristics determine the operating state of the entire flexible-DC delivery system. Due to the limitation of the characteristics of the fully controlled power electronic devices, the relatively high overvoltage level and low safety margin of the flexible DC converter valve sub-module during the fault process are always the key factors affecting the safe operation of the system. For the offshore wind power flexible direct transmission system, the overvoltage problem of the converter valve sub-module is more serious.

At present, there is no offshore platform engineering example for the offshore wind power flexible direct transmission system in China, and the effect of suppressing the overvoltage level of the converter valve sub-module from the perspectives of a complete set design, insulation coordination design, and improving the voltage level of the device is limited, and The economy and engineering practicability are poor.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a method and system for suppressing the overvoltage of the converter valve sub-module, which can reduce the energy stored in the capacitor of the sub-module by quickly cutting off the continuous charging circuit of the sub-module under severe faults, and on this basis effectively reduce the The sub-module overvoltage level is used to improve the safety margin of the offshore wind power system, and the technical cost of engineering implementation is very low and has good economics.

In order to solve the above problems, one aspect of the present invention provides a method for suppressing overvoltage of a converter valve sub-module, the method comprising: calculating the dynamic variance of the DC port voltage of the converter valve in the current sampling period and the previous sampling period ;

Judging whether a serious fault occurs according to the dynamic variance;

If there is a serious fault, start the shutdown protection of the converter station, otherwise, no action will be taken.

According to an embodiment of the present invention, the dynamic variance is calculated by the following formula:

Among them, F _t1 and F _t2 are the dynamic variance of the DC port voltage in the previous sampling period and the current sampling period, respectively; A _i1 and A _i2 are the counter enable signals in the previous sampling period and the current sampling period, respectively; U _dct1 and U _dct2 are the DC port voltages in the previous sampling period and the current sampling period, respectively; U _dcN is the rated DC port voltage, and C is the non-critical fault overvoltage multiple of the rated DC port voltage U _dcN .

According to an embodiment of the present invention, the counter enable signal A _i is determined as follows:

If the DC port voltage is less than the first voltage threshold (C×U _dcN ), then A _i =0, and the counter result output is zero;

If the DC port voltage is greater than the first voltage threshold (C×U _dcN ), A _i =1, and the counter starts counting.

According to an embodiment of the present invention, judging whether a serious fault occurs according to the dynamic variance includes:

Comparing the F _t1 and the F _t2 includes: if F _t1 is greater than or equal to F _t2 , then CP=1, the counter is in a clearing state, and the counter result N _t is zero. If F _t1 is less than F _t2 and A _i =1, then CP=0, the counter starts counting, and the output of the counter counting result is N _t ;

Compare the size of the counter result N _t with the second threshold _ND , and the comparison result is output as UP, including:

If N _t is greater than or equal to N _D , then UP=1, and it is determined that the converter valve has a serious failure; if N _t is less than N _D , then UP=0, and it is determined that the converter valve has no serious failure.

According to an embodiment of the present invention, starting the shutdown protection of the converter station includes: blocking the converter valve; and simultaneously disconnecting the AC side circuit breaker of the converter valve.

Another aspect of the present invention provides a system for suppressing overvoltage of a converter valve sub-module, the system comprising:

A dynamic variance calculation module, used to calculate the dynamic variance F _{t1 of the DC port voltage of the converter valve in the previous sampling period and the dynamic variance F t2 of} the current sampling period, the dynamic variance of the sampling period and the counter enable signal A _i , the DC port voltage of the converter valve, the rated DC port voltage of the converter valve and the non-serious fault overvoltage multiple of the rated DC port voltage are related;

a serious fault judgment module, used for judging whether a serious fault occurs according to the dynamic variance;

The protection module is used for performing protection operation according to the judgment result of the serious fault judging module. If a serious fault occurs, the shutdown protection of the converter station is started, otherwise, no action is taken.

According to an embodiment of the present invention, it further includes a counter enabling module, configured to compare the magnitude of the DC port voltage of the converter valve with the first voltage threshold, and if the DC port voltage is less than the first voltage threshold, the counter enable signal A _i =0, the counter result output is zero; if the DC port voltage is greater than the first voltage threshold, A _i =1, the counter starts counting.

According to an embodiment of the present invention, the dynamic variance is calculated by the following formula:

Among them, F _t1 and F _t2 are the dynamic variance of the DC port voltage in the previous sampling period and the current sampling period, respectively; A _i1 and A _i2 are the counter enable signals in the previous sampling period and the current sampling period, respectively; U _dct1 and U _dct2 are the DC port voltages in the previous sampling period and the current sampling period, respectively; U _dcN is the rated DC port voltage, and C is the non-critical fault overvoltage multiple of the rated DC port voltage U _dcN .

According to an embodiment of the present invention, the serious fault judging module includes:

a first comparison module, configured to compare the F _t1 and the F _t2 to obtain a comparison result CP: if F _t1 is greater than or equal to F _t2 , then CP=1; if F _t1 is less than F _t2 , then CP=0;

The counter is used to output the counting result N _t according to the CP: if CP=1, the counter is in a clear state, and the counter result N _t is zero; if CP=0, and the counter enable signal A _i =1, the counter starts counting , the counter count result is output as N _t ;

The second comparison module is used to compare the size of the N _t and the second threshold _ND to obtain the comparison result UP: if N _t is greater than or equal to _ND , then UP=1; if N _t is less than _ND , then UP=0 ;

The judgment module is configured to judge whether the converter valve has a serious fault according to the UP: if UP=1, it is judged that a serious fault has occurred; if UP=0, it is judged that no serious fault has occurred.

According to an embodiment of the present invention, the protection module starting the shutdown protection of the converter station includes: blocking the converter valve; and simultaneously disconnecting the AC side circuit breaker of the converter valve.

The invention reduces the energy stored in the capacitor of the sub-module by rapidly cutting off the continuous charging circuit of the sub-module under severe fault, and effectively reduces the over-voltage level of the sub-module on this basis, thereby improving the safety margin of the offshore wind power system.

The above-mentioned technical scheme of the present invention has the following beneficial technical effects:

First of all, the criterion design is carried out through the fault judgment module, which can effectively judge the serious faults that can cause the overvoltage of the sub-module. At the same time, the fault judgment module will not be triggered in the operation modes such as non-serious faults and fault ride-through, which ensures the commutation. The overall reliability and stability of the valve work;

Secondly, the dynamic variance of the DC port voltage of the converter valve is used as the fault criterion, which can completely and accurately identify the fault and increase the accuracy of equipment fault investigation;

Thirdly, the improvement of the original control and protection system does not increase any cost, and the economy is better.

Description of drawings

Figure 1 is a schematic diagram of the power transmission mode of the offshore wind power flexible direct transmission system;

Fig. 2 is the circuit schematic diagram of the overvoltage generation process of the modular multi-level sub-module of the onshore converter station under the fault;

Fig. 3 is the circuit schematic diagram of the overvoltage generation process of the modular multi-level sub-module of the offshore converter station under fault;

4 is a schematic flowchart of a method for suppressing overvoltage of a converter valve sub-module according to an embodiment of the present invention;

5 is a schematic diagram of a severe fault criterion circuit based on the dynamic variance of the DC port voltage of the modular multilevel converter according to an embodiment of the present invention;

6 is a flowchart of a method for suppressing overvoltage of a converter valve sub-module based on a severe fault criterion according to an embodiment of the present invention;

7 is a structural block diagram of an overvoltage suppression system for a converter valve sub-module according to an embodiment of the present invention.

Detailed ways

In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the specific embodiments and the accompanying drawings. It should be understood that these descriptions are exemplary only and are not intended to limit the scope of the invention. Also, in the following description, descriptions of well-known structures and techniques are omitted to avoid unnecessarily obscuring the concepts of the present invention.

The present invention provides a method and system for suppressing overvoltage of a converter valve sub-module. The sub-module overvoltage suppression scheme is to add a critical fault criterion module to the control and protection system of a modular multi-level converter. After the fault occurs, it is concluded through the severe fault criterion module that the fault is a fault that causes serious overvoltage in the converter valve sub-module. At this time, the function signal UP of the serious fault criterion module is output as 1, which directly triggers the emergency locking of the polar control system. Shutdown protection (TAD), the modular multi-level converter is blocked, and the AC side circuit breaker is opened at the same time; when the UP output is 0, this function does not start. The method does not cause the increase of the primary system and the area of the offshore platform, the technical cost of engineering realization is very low, and it has good economy.

The overvoltage suppression scheme of the converter valve sub-module takes the severe fault criterion UP as the judgment condition for the emergency blocking and shutdown of the modular multi-level converter. According to the operation and fault characteristics of the system, a fault criterion UP, which can identify the severe faults causing sub-module overvoltage, is designed, where UP is the severe fault criterion based on the dynamic variance of the DC port voltage U _dc of the modular multilevel converter. According to the data, when the protection setting value is exceeded, UP starts, the modular multi-level converter is quickly blocked, and the AC side circuit breaker of the connected transformer is opened. The overvoltage suppression scheme of the converter valve sub-module for the offshore wind power flexible direct transmission system proposed by the present invention mainly judges the serious fault causing the over-voltage of the sub-module in advance and quickly cuts off the continuous charging circuit of the sub-module. To effectively suppress the overvoltage level of the converter valve sub-module, the reliability of the offshore wind power system can be significantly improved through this solution.

In order to introduce the technical solutions of the present invention in detail, the following description is made with reference to the accompanying drawings.

Figure 1 shows the power transmission mode of the offshore wind power flexible direct transmission system.

A typical offshore wind power flexible direct transmission system consists of an offshore wind farm, an AC collection system, an AC booster station, an offshore converter station, a DC power transmission system, and an onshore converter station, among which the converter valve and other equipment of the offshore converter station are Both are located on offshore platforms.

Each wind farm collects power through the offshore wind power collection system, which generally adopts the method of AC collection. The power of each offshore wind farm is collected to the booster station through multiple AC submarine cables, and the power of the offshore wind farm is expressed as P _wind .

After the booster station completes the power collection and voltage change, the AC power is transmitted to the offshore flexible-to-DC converter station through the AC submarine cable. The offshore flexible-to-DC converter station converts AC power into DC power P _dc , and then transmits the power to the onshore flexible-to-DC converter station through the DC submarine cable; the onshore converter station converts the DC power to AC power and transmits it to the local grid.

Due to the high cost of the offshore platform, the high failure rate of the DC submarine cable and the high price, the symmetrical single-pole modular multi-level topology is more suitable for the offshore wind power transmission system. Adopting this system scheme can effectively reduce the area of converter station equipment and offshore platform, and has good economy.

According to the characteristics of the modular multilevel converter, four-quadrant operation control can be realized during operation. The two-terminal modular multilevel converter realizes power transmission through the control of P ₁ , Q ₁ , P ₂ and Q ₂ .

The power relationship of the converter at the sending end is as follows:

In the above formula, U _S1 is the voltage amplitude of the AC system of the offshore converter station, U _MMC1 is the output voltage amplitude of the AC side of the modular multi-level converter of the offshore converter station, and X ₁ is the connection impedance of the offshore converter station system , δ ₁ is the phase angle difference between the AC system voltage of the offshore converter station and the output voltage of the AC side of the modular multilevel converter. The flexible DC converter adopts a vector control method, and the control system consists of an inner loop current controller and an outer loop power control.

During operation, the four-quadrant operation of the converter can be realized by controlling the angle δ ₁ and the voltage amplitude U _MMC1 . The modular multi-level converter of the offshore converter station mainly controls its AC side output voltage U _MMC1 to provide a stable AC voltage for the operation of the wind farm to realize power output.

When the failure of the offshore converter station or the offshore AC system causes the modular multi-level converter to lose control of the U _MMC1 or the control ability is reduced, the power transmitted from the wind farm to the modular multi-level converter is reduced, resulting in the DC system. The transmitted power is reduced.

The power relationship of the converter at the other end is as follows:

In the above formula, U _S2 is the voltage amplitude of the AC system of the onshore converter station, which is determined by the local power grid. U _MMC2 is the AC side output voltage amplitude of the modular multi-level converter of the onshore converter station, and X ₂ is the connection impedance of the onshore converter station system. δ ₂ is the phase angle difference between the AC side output voltage of the modular multilevel converter of the onshore converter station and the AC system voltage.

During operation, when the onshore converter station or the onshore AC system fails, the modular multilevel converter will lose control of the U _MMC2 or the control ability will be reduced, resulting in the reduction of the transmission power P ₂ of the grid connection point, causing flexible power surplus in a straight system.

Due to factors such as communication delay and slow fan control rate, the surplus power in the flexible straight system will cause the overvoltage of the converter valve sub-modules in the converter stations at both ends. The process of generating the overvoltage of the converter valve sub-module will be described below with reference to FIG. 2 and FIG. 3 .

Figure 2 shows the overvoltage generation process of the neutron module of the modular multilevel converter of the onshore converter station under the fault.

From the above derivation, it can be seen that the working conditions that cause severe overvoltage in the converter valve sub-module of the offshore wind power flexible direct transmission system are mainly due to the failure of the onshore converter station, which leads to a power surplus in the system. As shown in Fig. 2, when the onshore converter station fails, there is a power surplus in the HVDC flexible transmission system, and the surplus power causes the energy surplus of the onshore converter station, which will further increase the voltage of the sub-modules.

As shown in FIG. 2 , the upper bridge arm submodule forms a charging loop of the upper bridge arm submodule _Usmup through the positive electrode U _PGL , the anti-parallel diode and the valve side AC system _uv .

The lower bridge arm forms a charging loop of the lower bridge arm sub-mode _Usmdown through the positive electrode _UNGL , the anti-parallel diode and the valve side AC system _uv .

After the onshore converter station fails, the relationship between the sub-module voltage and surplus power is as follows:

In the above formula, w _Δ is the surplus energy caused by surplus power P _Δ ; U _sm1 is the sub-module voltage value during steady-state operation, which is determined by the DC port voltage of the modular multilevel converter and the number of bridge arm sub-modules It is determined that this value is constant during normal operation; C _sm is the sub-module capacitance value.

Through the transformation of the above formula, it can be obtained that the relationship between the voltage increase of the sub-modules of the onshore converter station is as follows:

It can be seen from the above formula that when there is surplus power in the onshore converter station, the surplus power will convert the energy into electric field energy and discharge or store it in the sub-module capacitor. The greater the surplus energy, the greater the increase in the voltage of the sub-module.

When the sub-module voltage increases, it will lead to an increase in the DC port voltage U _dcL of the onshore converter station, which further causes the sub-module voltage of the offshore converter station to increase. The following describes the occurrence process of the overvoltage of the sub-module in the modular multi-level converter of the offshore converter station with reference to FIG. 3 .

Figure 3 shows the overvoltage generation process of the neutron module of the modular multilevel converter of the offshore converter station under the fault.

The increase of the DC port voltage U _dcL of the onshore converter station will lead to the increase of the port voltage U _dcH of the offshore converter station. In the process of U _dcH increasing, the DC system will affect the neutron module of the modular multi-level converter of the offshore converter station. to charge.

As shown in FIG. 3 , the upper bridge arm submodule forms a charging loop of the upper bridge arm submodule _Usmup through the positive electrode U _PG , the anti-parallel diode and the valve side AC system _uv .

The lower bridge arm forms the charging loop of the lower bridge arm sub-module _Usmdown through the positive electrode U _NG , the anti-parallel diode and the valve-side AC system _uv , which eventually increases the voltage of the sub-module in the offshore converter station.

From the above analysis, it can be seen that the occurrence of overvoltage of sub-modules in the converter station in the offshore wind power flexible direct transmission system is mainly caused by the surplus power of the system. The surplus power stores energy in the sub-module capacitors through the charging circuit, resulting in Generation of sub-module overvoltage.

One aspect of the present invention provides a method for suppressing overvoltage of a converter valve sub-module, as shown in FIG. 4 , including the following steps:

Step S100, calculating the dynamic variance of the DC port voltage of the converter valve in the current sampling period and the previous sampling period;

Step S200, judging whether a serious fault occurs according to the dynamic variance;

In step S300, if a serious fault occurs, the shutdown protection of the converter station is activated, otherwise, no action is taken.

In step S100, the sampling period of the valve control system is T, and the dynamic variance calculation is performed on the DC port voltage U _dct2 of each sampling period and the DC port voltage U _dct1 of the previous sampling period:

In the above formula, A _i1 and A _i2 are the counter enable signals in the previous sampling period and the current sampling period, respectively. F _t1 and F _t2 are the dynamic variance of the DC port voltage in the previous sampling period and the current sampling period, respectively. U _dcN is the rated DC port voltage, and C is the non-critical fault overvoltage multiple of the rated DC port voltage U _dcN .

The criterion for causing a serious fault of the sub-module overvoltage will be described below with reference to FIG. 5 , that is, step S200 will be described in detail.

Figure 5 shows the schematic diagram of the severe fault criterion circuit based on the dynamic variance of the DC port voltage of the modular multilevel converter.

During the charging process of the sub-module after a fault occurs, the DC port voltage will increase accordingly. The more serious the fault, the more surplus energy, the more the sub-module voltage increases, and the more the DC port voltage of the modular multilevel converter increases.

Under stable operation and partial fault conditions, the DC port voltages U _dcL and U _dcH of the converter stations at both ends are basically constant, or the fluctuation range is limited. After extensive analysis and simulation of the system, the DC port voltage protection setting (C×U _dcN ) is obtained, that is, the first voltage threshold and the counting setting N _D , where C is the non-serious fault overvoltage of the rated DC port voltage U _dcN . The pressure multiple is generally between 1.15 and 1.25. As shown in Figure 5, the DC port voltage enters the control and protection system through the sampling system, and the enable signal of the counter is A _i . If the DC port voltage is less than the protection fixed value, A _i =0 The counter does not start, and the counting result outputs zero. If the DC port voltage is greater than the protection fixed value, A _i = 1 counter starts and has the counting function.

Then F _t1 and F _t2 are compared, and the output result CP of the comparator is used as the clearing signal of the counter. When F _t1 is greater than or equal to F _t2 , CP=1, the counter is in the state of clearing, the counting result N _t is zero, less than N _D , and the output result UP of the critical fault criterion is 0.

When F _t1 is less than F _t2 , CP=0, the counter has a counting function, when the enable signal A _i2 =1, it starts counting, and the counting output result is N _t ; when N _t is greater than N _D , the critical fault criterion outputs the result UP is 1.

The overvoltage suppression operation of the converter valve sub-module is performed according to the failure criterion UP that can cause the serious fault of the converter valve sub-module over-voltage as the action command. The sub-module overvoltage suppression scheme will be described below with reference to FIG. 6 .

Fig. 6 shows the overvoltage suppression scheme of the converter valve sub-module based on the severe fault criterion.

As shown in FIG. 6 , after the fault occurs, it is judged according to UP whether it is a fault that causes severe overvoltage of the sub-module. As shown in Figure 5, when the onshore converter station has a serious failure, the flexible direct system will have a relatively large surplus power, and the modular multi-level port voltage U _dc of the converter stations at both ends will continue to increase for a period of time. , the dynamic variance protection of the port voltage U _dc will be triggered quickly to start the fast shutdown protection TAD. The continuous charging time of the sub-modules of the converter stations at both ends is greatly reduced, thereby effectively reducing the overvoltage level of the sub-modules.

When UP is 1, the converter station shutdown protection TAD is started immediately, and the opening command is issued to the AC side circuit breaker of the converter station, and the blocking command is issued to the modular multi-level converter at the same time. Since the opening time of the AC circuit breaker is relatively long, the modular multi-level converter has completed the blocking during the opening process of the AC circuit breaker.

When UP is 0, it means that the system needs to carry out fault ride-through, or the failure of the system will not cause serious sub-module overvoltage phenomenon to the converter valve, and the control and protection system will judge and operate according to the conventional processing method.

Another aspect of the present invention provides an overvoltage suppression system for a converter valve sub-module, as shown in FIG. 7 , comprising: a dynamic variance calculation module for calculating the DC port voltage of the converter valve in a previous sampling period The dynamic variance F _t1 of the current sampling period and the dynamic variance F _t2 of the current sampling period, the dynamic variance of the sampling period is related to the counter enable signal A _i , the DC port voltage of the converter valve, the rated DC port voltage of the converter valve and the rated DC port The voltage is related to the non-critical fault overvoltage multiple;

a serious fault judgment module, used for judging whether a serious fault occurs according to the dynamic variance;

The protection module is used for performing protection operation according to the judgment result of the serious fault judging module. If a serious fault occurs, the shutdown protection of the converter station is started, otherwise, no action is taken.

Further, it also includes a counter enable module for comparing the DC port voltage of the converter valve with the first voltage threshold, if the DC port voltage is less than the first voltage threshold, the counter enable signal A _i =0, the counter result The output is zero; if the DC port voltage is greater than the first voltage threshold, A _i =1, and the counter starts counting.

Further, the serious fault judging module includes:

a first comparison module, configured to compare the F _t1 and the F _t2 to obtain a comparison result CP: if F _t1 is greater than or equal to F _t2 , then CP=1; if F _t1 is less than F _t2 , then CP=0;

The counter is used to output the counting result N _t according to the CP: if CP=1, the counter is in a clear state, and the counter result N _t is zero; if CP=0, and the counter enable signal A _i =1, the counter starts counting , the counter count result is output as N _t ;

The second comparison module is used to compare the size of the N _t and the second threshold _ND to obtain the comparison result UP: if N _t is greater than or equal to _ND , then UP=1; if N _t is less than _ND , then UP=0 ;

The judgment module is configured to judge whether the converter valve has a serious fault according to the UP: if UP=1, it is judged that a serious fault has occurred; if UP=0, it is judged that no serious fault has occurred.

Further, starting the shutdown protection of the converter station by the protection module includes: blocking the converter valve; and simultaneously disconnecting the AC side circuit breaker of the converter valve.

Aiming at the characteristics of the offshore wind flexible direct delivery system, the present invention proposes a scheme for suppressing the overvoltage of the converter valve sub-module. This protection function will not be triggered in the state of , and when a fault that does not cause a severe overvoltage of the sub-module occurs.

After adopting the overvoltage suppression scheme of the converter valve sub-module of the offshore wind power flexible direct transmission system proposed by the present invention, the voltage stress level of the modular multi-level converter sub-module can be effectively reduced, and the safety of the equipment and the system can be improved. Facilitate the design of the diverter valve. In addition, the scheme does not add any primary equipment and measuring equipment during the implementation process, the implementation process is simple, and the operation of the actual converter station has good economy.

First of all, the criterion design is carried out through the fault judgment module, which can effectively judge the serious faults that can cause the overvoltage of the sub-module. At the same time, the fault judgment module will not be triggered in the operation modes such as non-serious faults and fault ride-through, which ensures the commutation. The overall reliability and stability of the valve work;

Secondly, the dynamic variance of the DC port voltage of the converter valve is used as the fault criterion, which can completely and accurately identify the fault and increase the accuracy of equipment fault investigation;

Thirdly, the improvement of the original control and protection system does not increase any cost, and the economy is better.

It should be understood that the above-mentioned specific embodiments of the present invention are only used to illustrate or explain the principle of the present invention, but not to limit the present invention. Therefore, any modifications, equivalent replacements, improvements, etc. made without departing from the spirit and scope of the present invention should be included within the protection scope of the present invention. Furthermore, the appended claims of this invention are intended to cover all changes and modifications that fall within the scope and boundaries of the appended claims, or the equivalents of such scope and boundaries.

Claims (8)

1. A method for suppressing an overvoltage in a converter valve submodule, the method comprising:

calculating the dynamic variance of the voltage of the direct-current port of the converter valve in the current sampling period and the previous sampling period; the dynamic variance is calculated by:

wherein, F_t1And F_t2Dynamic variances of the direct current port voltage in the previous sampling period and the current sampling period are respectively; a. the_i1And A_i2Respectively enabling signals of a counter in a previous sampling period and a counter in a current sampling period; u shape_dct1And U_dct2The DC port voltages in the previous sampling period and the current sampling period are respectively; u shape_dcNIs rated DC port voltage, C is rated DC port voltage U_dcNNon-critical fault overvoltage multiples of (1);

judging whether serious faults occur according to the dynamic variance;

and if a serious fault occurs, starting the shutdown protection of the converter station, otherwise, not acting.

2. The method of claim 1, wherein the counter enable signal A_iThe following were determined:

if the DC port voltage is not greater than the first voltage threshold (CxU)_dcN) Then A is_iWhen the counter value is equal to 0, the counter result output is zero;

if the DC port voltage is greater than the first voltage threshold (C U)_dcN) Then A is_iThe counter starts counting at 1.

3. The method of claim 2, wherein said determining whether a catastrophic failure has occurred based on said dynamic variance comprises:

comparing said F_t1And said F_t2The method comprises the following steps: if F_t1Greater than or equal to F_t2If CP is equal to 1, the counter is in the zero clearing state, and the counter result N is obtained_tIs zero; if F_t1Is less than F_t2If CP is equal to 0, and A_iWhen the counter counts 1, the counter starts counting, and the result of the counter is N_t；

Comparison counter nodeFruit N_tAnd a second threshold value N_DThe comparison result is output as UP, which includes:

if N is present_tIs greater than or equal to N_DIf the UP is equal to 1, judging that the converter valve has serious faults; if N is present_tLess than N_DAnd if the UP is equal to 0, judging that the converter valve has no serious fault.

4. A method according to any of claims 1-3, characterized in that said initiating converter station shutdown protection comprises: locking the converter valve; and simultaneously disconnecting the converter valve AC side breaker.

5. A converter valve sub-module overvoltage suppression system, characterized in that the system comprises:

a dynamic variance calculation module for calculating the dynamic variance F of the DC port voltage of the converter valve in the previous sampling period_t1And the dynamic variance F of the current sampling period_t2The dynamic variance is calculated by:

wherein, F_t1And F_t2Dynamic variances of the DC port voltage in the previous sampling period and the current sampling period are respectively; a. the_i1And A_i2Respectively enabling signals of a counter in a previous sampling period and a counter in a current sampling period; u shape_dct1And U_dct2The DC port voltages in the previous sampling period and the current sampling period are respectively; u shape_dcNIs rated DC port voltage, C is rated DC port voltage U_dcNNon-critical fault overvoltage multiples of (1);

the serious fault judgment module is used for judging whether a serious fault occurs according to the dynamic variance;

and the protection module is used for performing protection operation according to the judgment result of the serious fault judgment module, starting the converter station to perform shutdown protection if a serious fault occurs, and otherwise, stopping the operation.

6. The system of claim 5, further comprising a counter enable module configured to compare the DC port voltage of the converter valve to a first voltage threshold, wherein the counter enable signal A is asserted if the DC port voltage is less than the first voltage threshold_iWhen the value is equal to 0, the result output of the counter is zero; if the DC port voltage is greater than the first voltage threshold, then A_iThe counter starts counting as 1.

7. The system of claim 5, wherein the critical failure determination module comprises:

a first comparing module for comparing F_t1And said F_t2To obtain a comparison result CP: if F_t1Is greater than or equal to F_t2If CP is 1; if F_t1Less than F_t2If CP is 0;

a counter for outputting a counting result N according to the CP_t: if CP is equal to 1, the counter is in zero state, and the result N of the counter_tIs zero; if CP is equal to 0, and the counter enables signal A_iWhen the counting result is equal to 1, the counter starts to count, and the counting result output of the counter is N_t；

A second comparison module for comparing the N_tAnd a second threshold value N_DTo obtain a comparison result UP: if N is present_tGreater than or equal to N_DIf UP is equal to 1; if N is present_tLess than N_DIf UP is 0;

a judging module, configured to judge whether the converter valve has a serious fault according to the UP: if UP is 1, judging that a serious fault occurs; if UP is 0, it is determined that no serious failure has occurred.

8. The system according to any one of claims 5-7, wherein said protection module initiating converter station shutdown protection comprises: shutting off the converter valve; and simultaneously disconnecting the converter valve AC side breaker.

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