阅读说明：本技术 一种适应于混合mmc不同工况下的交流充电控制策略 (alternating current charging control strategy suitable for hybrid MMC under different working conditions ) 是由 郑星星 徐攀腾 宋述波 包威 李金安 杨学广 周登波 朱博 李建勋 严海健 焦石 于 2019-09-23 设计创作，主要内容包括：本发明公开了一种适应于混合MMC不同工况下的交流充电控制策略,包括：交流断路器合闸,全、半桥子模块进行不可控充电；待全桥子模块进入可控状态、且全桥子模块平均电压大于一定值后,导通所有全桥子模块T2开关管；当全、半桥子模块平均电压达到设定值时,投入子模块均压控制策略,对全、半桥子模块进行统一的排序,逐步切除一定个数的全、半桥子模块,将全、半桥子模块电压充电至接近额定值,充电启动完成。本发明的充电控制策略,无需MMC控制系统额外判断MMC运行方式,无需判定MMC接入状态,能够简化控制器程序设计,提高混合型MMC充电启动成功的可靠性。(The invention discloses alternating current charging control strategies suitable for different working conditions of a mixed MMC, which comprise an alternating current breaker, a full-bridge submodule and a half-bridge submodule are subjected to uncontrollable charging, after the full-bridge submodule enters a controllable state and the average voltage of the full-bridge submodule is greater than fixed value, all full-bridge submodule T2 switch tubes are conducted, when the average voltage of the full-bridge submodule and the half-bridge submodule reaches a set value, a submodule voltage-sharing control strategy is put into, the full-bridge submodule and the half-bridge submodule are subjected to systematic sequencing of , fixed number of the full-bridge submodule and half-bridge submodule are cut off step by step, the voltage of the full-bridge submodule and the half-bridge submodule is charged to be close to a rated value, and charging starting is completed.)

1, alternating current charging control strategies suitable for different working conditions of mixed MMC, which is characterized by comprising the following steps:

switching on the alternating current breaker, and uncontrollably charging the full-half bridge sub-module and the half-bridge sub-module;

after the full-bridge submodule enters a controllable state and the average voltage of the full-bridge submodule is larger than fixed value, all full-bridge submodule T2 switching tubes are conducted;

when the average voltage of the full and half-bridge submodules reaches a set value, a submodule voltage-sharing control strategy is put into use, the full and half-bridge submodules are subjected to unified sequencing, fixed number of full and half-bridge submodules are cut off step by step, the full and half-bridge submodules are charged to be close to a rated value, and charging starting is completed.

2. The AC charging control strategy adapted to different working conditions of hybrid MMC according to claim 1, wherein the set value is 0.6U_cnThe approach rated value is 0.95-1 Ucn, wherein U_cnIs the rated voltage of the sub-module.

Technical Field

The invention relates to a flexible direct current transmission technology, in particular to alternating current charging control strategies suitable for different working conditions of a hybrid MMC.

Background

Compared with the conventional direct-current transmission technology, the flexible direct-current transmission technology does not need reactive compensation and has no problem of commutation failure, active and reactive power regulation is convenient, the harmonic level is low, the flexible direct-current transmission technology is suitable for forming a multi-end direct-current system and the like, the flexible direct-current transmission related technology is rapidly developed, wherein the flexible direct-current transmission system based on an MMC (modular multilevel converter) topological structure has the most representative and technical advantages, the hybrid MMC has the outstanding advantages of direct-current side fault clearing capacity, voltage reduction operation capacity, relatively low cost and loss and the like, the flexible direct-current transmission system has broad application prospect in the field of flexible direct-current transmission, and is very suitable for the topological structure of an ultrahigh voltage flexible direct-current transmission converter valve group unit, and the ultrahigh voltage flexible direct-current transmission system based on the hybrid MMC two serial-connected valve groups mainly faces two problems in the aspect:

(1) AC charging, full, half-bridge submodule charge rate does not cause the problem under the not short circuit condition of valves direct current side:

the hybrid MMC is composed of three symmetrical phase units, each phase unit is divided into an upper bridge arm and a lower bridge arm, the bridge arms are completely symmetrical, each bridge arm is composed of N sub-modules (containing x half-bridge sub-modules and y full-bridge sub-modules) which are cascaded and series reactors, the topological structure diagram of each bridge arm is shown in figure 1, and the current marked in the figure is a specified positive direction.

The charging conditions of the full-half bridge sub-modules and the half-bridge sub-modules in different bridge arm current directions under the locking state of all the switching tubes (the controllable power devices numbered as T1, T2, T3 and T4 in FIG. 1) of the sub-modules are shown in FIG. 2.

At the initial stage after the alternating current circuit breaker is switched on, the energy-taking power supply of the submodule is not powered, all switch tubes are in a turn-off state, and current can only flow through the anti-parallel diodes of the switch tubes, and the stage is called an uncontrollable charging stage. The half-bridge sub-module can be charged only when the bridge arm current is in a positive direction, and the half-bridge sub-module is in a bypass state and cannot be charged when the bridge arm current is in a negative direction; the full-bridge submodule can charge the capacitor under the condition of positive or negative bridge arm current.

An equivalent circuit diagram of the hybrid MMC under the ac side closing charging can be drawn by combining fig. 1 and fig. 2 as shown in fig. 3. Under the uncontrollable charging state, the direction of bridge arm current depends on the three-phase voltage U at the MMC alternating side at the current moment_va、U_vb、U_vcThe instantaneous values are phase with the highest phase voltage, the upper bridge arm of the phase flows negative current and the lower bridge arm of the phase flows positive current, and phase with the lowest phase voltage, the upper bridge arm of the phase flows positive current and the lower bridge arm of the phase flows negative currentThe arm is flowing a negative current. The MMC alternating-current side three-phase voltage is a regular three-phase sine wave with the phase difference of 120 degrees and is 0-axis symmetry, the bridge arm current also changes periodically along with the MMC alternating-current side voltage in positive and negative half cycles, the charging opportunities of 6 bridge arms are equal, but the full-bridge submodule can be charged in the positive and negative half cycles of the bridge arm current, and the half-bridge submodule can be charged only in the positive half cycle, so that the charging rate of the full-bridge submodule is far higher than that of the half-bridge submodule, and the serious voltage unbalance between the full-bridge submodule and the half-bridge submodule is caused.

At present, Chinese patent with application publication number CN106787087A discloses starting charging methods, after a full-bridge submodule is charged to an energy-obtaining power supply to work, a T4 switch tube of the full-bridge submodule is immediately triggered and conducted, a schematic diagram is shown in FIG. 4. after the T4 switch tube is turned on, the full-bridge submodule is not charged by a bypass under the condition that bridge arm current is negative, so that the full-bridge submodule is forced to be charged only in a positive half cycle of the bridge arm current, and the difference between voltages of the full-bridge submodule and the half-bridge submodule in the charging process is reduced.

(2) AC charging, the half-bridge submodule can not get the electric work problem under the short circuit condition of the direct current side of the valve group:

compared with a conventional high-voltage flexible single-valve-group direct-current transmission system, the extra-high-voltage flexible series-connection two-valve-group direct-current transmission system also needs to meet basic requirements, namely the on-line investment of valve groups, namely the requirement that the flexible converter valve group needs to have the charging starting and unlocking operation capacity under the condition of direct-current side short circuit besides the charging starting and unlocking operation capacity of the conventional flexible valve group_vaHighest, U_vbThe lowest example (i.e., the a-phase upper bridge arm current is negative, the lower bridge arm current is positive, and the b-phase upper bridge arm current is positive, and the lower bridge arm current is negative) illustrates the sub-module charging situation under the condition of short circuit at the direct current side, and the schematic diagram of the equivalent circuit is shown in fig. 5.

In the initial charging stage, the upper and lower bridge arms of phase a are charged, the current of the upper bridge arm is negative, only the full-bridge submodule is charged, the current of the lower bridge arm is positive, the full-bridge submodule and the half-bridge submodule are charged, as the charging is carried out, the direct current voltage of the lower bridge arm (the voltage of the full-bridge submodule + the voltage of the half-bridge submodule) is higher than that of the upper bridge arm (the voltage of the full-bridge submodule only), and the diode of the lower bridge arm is turned off; the upper bridge arm and the lower bridge arm of the phase b are the same, so that only the full-bridge submodule is charged, and the half-bridge submodule cannot be charged and started normally.

Aiming at the problem, the research content of a hybrid MMC starting charging strategy [ J ] in the process of putting an extra-high voltage flexible direct-current valve bank into operation and power system automation 2018 and 42(24) discloses starting methods, wherein a part of a full-bridge submodule in a bridge arm is cut off by measuring the current of the bridge arm of the MMC and the instantaneous voltage value of a three-phase valve side when the current of the bridge arm is positive, so that the current of the bridge arm is forced to flow through the bridge arm containing the half-bridge submodule to charge the half-bridge submodule, but the method is only suitable for the charging starting of the hybrid MMC under the condition that the direct-current side of an alternating-current valve bank.

In summary, the current ac charging method for the hybrid MMC can only be effective under specific operating conditions, and two or more methods must be combined to realize normal charging start of the hybrid MMC under two conditions of short circuit and no short circuit on the dc side, meanwhile, the implementation of the methods needs to depend on a control system to judge the wiring mode (direct current side short circuit starting or direct current side non-short circuit starting) of the current valve group in advance, and select different charging starting control strategies according to the current operation mode, therefore, a control system is required to access a switch position signal (such as a valve bank bypass switch position signal and the like) for judging the state of the MMC, program valve bank state judgment logic and charging mode selection logic are designed, complexity and design difficulty of a control program are increased, and risk of failure of MMC charging starting caused by misjudgment of a charging starting mode exists.

Disclosure of Invention

In view of the above reasons, the present invention provides ac charging control strategies adapted to different working conditions of the hybrid MMC, which are used for charging the hybrid MMC under two working conditions of short circuit and no short circuit at the dc side, without determining the MMC access state, thereby simplifying the controller program design, reducing the external switch position signal access controller, and improving the reliability of successful charging start of the hybrid MMC.

In order to realize the purpose, the invention adopts the technical scheme that:

AC charging control strategy suitable for mixed MMC under different working conditions comprises:

switching on the alternating current breaker, and uncontrollably charging the full-half bridge sub-module and the half-bridge sub-module;

after the full-bridge submodule enters a controllable state and the average voltage of the full-bridge submodule is larger than fixed value, all full-bridge submodule T2 switching tubes are conducted;

when the average voltage of the full and half-bridge submodules reaches a set value, a submodule voltage-sharing control strategy is put into use, the full and half-bridge submodules are subjected to unified sequencing, fixed number of full and half-bridge submodules are cut off step by step, the full and half-bridge submodules are charged to be close to a rated value, and charging starting is completed.

, the set value is 0.6U_cnThe approach rated value is 0.95-1 Ucn, wherein U_cnIs the rated voltage of the sub-module.

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

the charging control strategy of the invention does not need an MMC control system to additionally judge the MMC operation mode and judge the MMC access state, can simplify the program design of the controller, reduce the access of an external switch position signal to the controller, eliminate the risk of failure of MMC charging start caused by misjudgment of the charging start mode due to the reasons of error, disappearance, jitter and the like of an external switch position signal, and improve the reliability of successful charging start of the mixed type MMC.

Drawings

FIG. 1 is a schematic diagram of a hybrid MMC topology and full and half bridge sub-modules;

FIG. 2 is a schematic diagram of charging full-bridge and half-bridge sub-modules in different bridge arm current directions; (ii) a

FIG. 3 is an equivalent circuit diagram of AC charging without short circuit on the DC side of the hybrid MMC;

FIG. 4 is a schematic diagram of a prior art path where bridge arm current is negative after a T4 switching tube is triggered;

FIG. 5 is an equivalent circuit diagram of AC charging under the condition of short circuit on the DC side of the hybrid MMC;

FIG. 6 is a schematic diagram illustrating a charging process of the charging control strategy according to the present invention;

FIG. 7 is a schematic diagram illustrating the variation of the number of sub-modules in the controllable charging phase according to the present invention;

FIG. 8 is a schematic diagram of the current path after full bridge submodule T2 of the present invention is turned on;

FIG. 9 is a schematic diagram of a charging path of the full-bridge submodule T2 according to the present invention that is not shorted at the DC side after being turned on;

fig. 10 is a schematic diagram of the dc-side short-circuited charging path after the full-bridge sub-module T2 is turned on.

Detailed Description

In order to make the aforementioned objects, features and advantages of the present invention comprehensible, a more detailed description is provided below in conjunction with the accompanying drawings and the detailed description.

As shown in fig. 6 and 7, ac charging control strategies adapted to different operating conditions of the hybrid MMC in the present invention include:

(1) after the MMC control system receives a charging instruction, the full-bridge submodule and the half-bridge submodule are subjected to uncontrollable charging (the full-bridge charging speed is high, and the voltage is about 2 times of that of the half-bridge) after the alternating-current circuit breaker is switched on;

(2) the full-bridge submodule is firstly charged to the minimum working voltage, the full-bridge submodule is communicated with the MMC control system to enter a controllable state (at the moment, the half-bridge submodule is still not charged to the minimum working voltage and is in the uncontrollable state), and when the average voltage of the full-bridge submodule is larger than , the constant value U is obtained_y1After the system parameters are set, the MMC control system issues switching-on trigger signals of all full-bridge submodule T2 switching tubes, and controls all full-bridge submodule T2 switching tubes in six bridge arms to be in a switching-on state;

(3) along with the charging, the half-bridge sub-module is also charged to the minimum working voltage and establishes communication with the MMC control system to enter a controllable state; when the average voltage of the full and half-bridge sub-modules reaches 0.6Ucn or the setting is calculated according to the actual condition of the system (Ucn is the rated voltage of the sub-module), the sub-modules are all put intoThe voltage control strategy is to carry out systematic sequencing on the full and half-bridge submodules, and a fixed number N is put into the initial stage_kRapidly balancing the voltage among the submodules with lower voltage, then reducing the input number of the submodules according to the fixed slope of , gradually controlling the average voltage value of the submodules to be 0.90-1 Ucn, and finally fixing the number N_s(the fixed number can be calculated according to the condition of the MMC system, the average voltage of the submodule after controllable charging is ensured to be 0.90-1 Ucn), the charging starting is completed, and an MMC unlocking operation signal is waited.

The principle that the charging control strategy of the present invention can be applied to the ac charging of the hybrid MMC under the conditions of short circuit and no short circuit on the dc side is explained as follows:

, and principle analysis suitable for the condition that the direct current side is not short-circuited

After the full-bridge submodule is charged to the minimum working voltage and enters a controllable state, the T2 switch tube is triggered, the current circulation condition in the positive and negative periods of the bridge arm current is shown in figure 8, it can be known from the figure that when the full-bridge submodule T2 switch tube is switched on, the full-bridge submodule degenerates to be charged only in the period that the bridge arm current is negative, the MMC operation characteristic can be known that the bridge arm current is 0-axis symmetric periodically-changed regular sine wave in the charging state, namely the charging effect of the positive and negative half periods is the same, therefore, after the full-bridge submodule T2 switch tube is switched on, the charging opportunities of the full-bridge submodule and the half-bridge submodule are equal, and the problems that the charging rate of the full-bridge submodule and the half-bridge submodule is not under the condition that the direct current side is.

Under the condition that a direct current side is not short-circuited, the analysis of the charging effect of the full-bridge submodule T2 after being turned on is carried out, the case that the phase a is the highest and the phase b is the lowest is still taken as an example, an MMC equivalent circuit diagram is shown in FIG. 9, currents simultaneously flow through an upper bridge arm and a lower bridge arm of the phases a and b, wherein currents flow through all full-bridge submodules of the upper bridge arm of the phase a + all half-bridge submodules of the upper bridge arm of the phase b, currents flow through all half-bridge submodules of the lower bridge arm of the phase a + all full-bridge submodules of the lower bridge arm of the phase b, current path parameters are completely the same, the characteristics of natural charging current balancing are provided, the charging rates of the full-bridge submodule and the half-bridge submodules are completely , a voltage balancing.

Principle analysis suitable for direct current side short circuit condition

Similarly, after the alternating current circuit breaker is switched on, all the full-bridge submodules are charged to the minimum working voltage firstly, and after the full-bridge submodules enter a controllable state, the T2 switching tube is triggered. As can be seen from the foregoing analysis and by referring to fig. 8, the full-bridge sub-modules with positive bridge arm currents are all in the cut-off state, and can be charged only when the bridge arm currents are negative, and the equivalent schematic diagram is shown in fig. 10, which still takes the example that the a-phase voltage is the highest and the b-phase is the lowest.

As can be seen from fig. 10, at this time, all full-bridge sub-modules (the number is y) of the phase a upper bridge arm are charged, all half-bridge sub-modules (the number is x) of the phase b lower bridge arm are charged, and y is required to be greater than x in order to fully exert the advantages of the hybrid MMC in the engineering, so that the direct current voltage (the sum of all the full-bridge sub-module voltages) of the phase a upper bridge arm is higher than that of the phase b lower bridge arm (only the half-bridge sub-module voltage), and the diodes of; b, the upper bridge arm and the lower bridge arm are the same, so that charging current can be forced to flow through the half-bridge submodule to charge the half-bridge submodule, the half-bridge submodule is also charged to the minimum working voltage along with the charging, communication is established between the half-bridge submodule and the MMC control system, after the half-bridge submodule enters a controllable state, a voltage-sharing strategy can be put into when the average voltage of the full-bridge submodule and the half-bridge submodule reaches 0.6Ucn, the voltage of the full-bridge submodule and the half-bridge submodule is uniformly charged to be close to a rated value, the charging starting is completed.

Therefore, the charging control strategy of the invention does not need an MMC control system to additionally judge the MMC operation mode and judge the MMC access state, can simplify the program design of the controller, reduces the access of external switch position signals to the controller, eliminates the risk of failure of MMC charging start caused by misjudgment of the charging start mode due to the reasons of external switch position signal error, disappearance, jitter and the like, and improves the reliability of the success of hybrid MMC charging start.

The above detailed description is specific to possible embodiments of the present invention, and the embodiments are not intended to limit the scope of the present invention, and all equivalent implementations or modifications that do not depart from the scope of the present invention are intended to be included within the scope of the present invention.