CN112928938B - A MMC DC Fault Current Limiting Method Based on Virtual Reactance - Google Patents

Abstract

The invention discloses a virtual reactance-based MMC DC fault current limiting method, which belongs to the technical field of DC power transmission and distribution. After a bipolar short-circuit fault occurs on the MMC DC side, the fault current rises rapidly, which endangers the safety of power electronic devices. Based on the existing MMC basic control strategy, the present invention maps the virtual reactance in the bridge arm to the controller through a feedback function, equivalently increases the bridge arm reactance, and corrects the voltage reference value of the MMC bridge arm after a fault to reduce the MMC The output DC voltage realizes the suppression of DC fault current. Specifically: First, measure the current of each phase bridge arm and calculate the average current change rate of each phase. Then, the DC voltage deviation generated by the DC fault current on the virtual reactance of the bridge arm is calculated. Finally, the relevant deviation correction value is superimposed on the reference voltage of the bridge arm of the MMC for correction. By reducing the reference value of the bridge arm voltage after the fault, the purpose of fault current suppression and DC circuit breaker breaking current can be achieved.

Description

A Method of MMC DC Fault Current Limitation Based on Virtual Reactance

technical field

The invention belongs to the field of direct current transmission and distribution, and in particular relates to an MMC direct current fault current limiting method based on virtual reactance.

Background technique

The flexible DC transmission and distribution technology based on the modular multilevel converter (MMC) has the advantages of no commutation failure, independent control of active power and reactive power, and is a solution to the long-distance collection and transmission of large-scale renewable energy. effective means. Due to the low damping coefficient of the DC power grid, once a short-circuit fault occurs on the DC side, the capacitor of the MMC sub-module will discharge rapidly, and the fault current will increase rapidly, which will endanger the safety of power electronic devices in the system.

At present, most DC power grids use DC circuit breakers to isolate faults, but high-speed and large-capacity DC circuit breakers with large breaking currents are difficult to manufacture and cost high. Therefore, reducing the DC fault current is the key to realize the large-scale application of the flexible DC distribution network. At present, most of the methods to reduce the fault current are additional current limiting devices, such as current limiting reactors, superconducting current limiters or solid state current limiters, and the installation of current limiting devices greatly increases the investment cost of the DC power grid. The current limiting reactor will seriously affect the dynamic performance of the system. Therefore, it is necessary to design a simple and reliable MMC current limiting method to suppress the DC fault current and reduce the breaking requirements of the DC circuit breaker.

Contents of the invention

Aiming at the deficiencies of the prior art, the present invention provides an MMC DC fault current limiting method based on virtual reactance. Based on the existing MMC basic control strategy, the virtual reactance in the bridge arm is mapped to the controller through a feedback function, and the equivalent increase The bridge arm reactance corrects the voltage reference value of the MMC bridge arm, and suppresses the DC fault current by reducing the output DC voltage of the MMC.

In order to solve the problems of the technologies described above, the technical solution of the present invention is:

Step 1: Use the current sensor to measure the current of the upper and lower bridge arms of each phase of the MMC in real time, denoted as i _pj and i _nj , and calculate the average current change rate of the phase according to the current of the upper and lower bridge arms of the phase. The specific calculation method It is: the bridge arm current change rate is obtained after the measured bridge arm current is differentiated, and then added and multiplied by 1/2 to obtain the average current change rate of each phase.

Step 2: Pass the average current change rate calculated above through the hysteresis comparator to obtain an action signal. When the average current change rate is greater than the action value, the output value of the comparator is 1. When the average current change rate is less than the return value, the comparator outputs The value is 0.

Step 3: Set the size of the differential coefficient K _D according to the circuit parameters of the MMC system, and use the value of K _D to reflect the size of the virtual reactance; then multiply di _j /dt with the differential coefficient K _D and the action signal of the hysteresis comparator to obtain the bridge Arm voltage deviation correction Δu _comj ^* , namely

其中,u_pj、u_nj分别为MMC基础控制下的每相上、下桥臂参考电压。Step 4: Superimpose the bridge arm voltage deviation correction amount Δu _comj ^* on the bridge arm reference voltage to correct it, and obtain the final bridge arm reference voltage for modulation.

Among them, u _pj and u _nj are the reference voltages of the upper and lower arms of each phase under the basic control of the MMC, respectively.

Through the above four steps, the present invention can effectively suppress the fault current when a short-circuit fault occurs on the DC side of the MMC system. Compared with the prior art, the beneficial effects of the present invention are: (1) when the DC system is in normal operation, the average current change rate of each phase is zero, the current limiting control link does not work, and the current rises rapidly after the fault occurs, limiting The current control link is automatically started, and the reference voltage of the bridge arm is respectively corrected to realize the active suppression of the fault current; (2) the present invention introduces a virtual reactance, not a physical reactance, and only needs to add a control link to play the role of the reactor to realize The effective suppression of fault current reduces the investment cost of MMC.

Description of drawings

Fig. 1 is the basic control method of MMC in the flexible DC grid; Fig. 2 is a block diagram of the virtual reactance current limiting control of the MMC bridge arm.

Detailed ways

A virtual reactance-based MMC DC fault current limiting method involved in the present invention will be further described in detail below in conjunction with the accompanying drawings. It should be understood that the following descriptions are only exemplary and not intended to limit the application and protection scope of the present invention.

The technical problem to be solved by the present invention adopts the virtual reactance control method, based on the basic control strategy of MMC, maps the newly added virtual reactance in the bridge arm to the controller through the feedback function, and increases the reactance of the bridge arm equivalently. The voltage reference value is corrected to reduce the output DC voltage of the MMC to suppress the DC fault current. The technical scheme that the present invention adopts is as follows:

在所述直流系统正常运行时，每相的上下桥臂电流相位相反，平均电流变化率为零，在所述直流系统发生故障时，平均电流变化率快速升高。Step 1: First, use the current sensor to measure the current of the upper and lower bridge arms of each phase of the MMC in real time, denoted as i _pj , i _nj (j=a, b, c), and calculate the The average current change rate di _j /dt of the phase is shown in Figure 2. Firstly, the bridge arm current change rate is obtained after the measured bridge arm current undergoes a differential link, and then added and multiplied by 1/2, then The average current change rate of each phase is obtained by:

When the DC system is in normal operation, the upper and lower bridge arm currents of each phase are in opposite phases, and the average current change rate is zero. When the DC system fails, the average current change rate increases rapidly.

Step 2: In order to prevent the false start of the virtual reactance current limiting link during the normal operation of the system and affect the normal operation of the system, the average current change rate di _j /dt of each phase is passed through the hysteresis comparator to obtain the action signal. When the average current change rate The output value of the comparator is 1 when it is greater than the action value, and the output value of the comparator is 0 when the average current change rate is less than the return value.

Step 3: Set the size of the differential coefficient K _D according to the circuit parameters of the MMC system, and use the value of K _D to reflect the size of the virtual reactance; then multiply di _j /dt with the differential coefficient K _D and the action signal of the hysteresis comparator to obtain The voltage drop of the virtual reactance under the fault current is taken as the bridge arm voltage deviation correction value Δu _comj ^* , we have

Step 4: As shown in Figure 2, subtract the bridge arm voltage deviation correction amount Δu _comj ^* from the bridge arm reference voltage to obtain the corrected bridge arm voltage reference value, that is, the bridge arm reference voltage finally used for modulation, as

Among them, u _pj and u _nj are the reference voltages of the upper and lower arms of each phase respectively, which are generated by the basic control method of MMC. The specific principle is shown in Figure 1, where U _dc is the DC side voltage of MMC, and u _j is the AC voltage The reference voltage, the bridge arm current rises rapidly after the fault, the bridge arm voltage deviation correction amount Δu _comj ^* increases, the bridge arm reference voltage used for modulation decreases, and then the DC voltage output by the MMC decreases, and the DC fault current is suppressed.

It should be noted that the above four steps are taken as the content of the invention as a whole, and the four steps are an indivisible whole.

The present invention provides an MMC DC fault current limiting method based on virtual reactance, which has the following advantages:

1. The present invention introduces a virtual reactance. When the DC system is in normal operation, the current phases of the upper and lower bridge arms of the same phase of the MMC are opposite, the average current change rate of each phase is zero, and the current limiting control will not work; After a fault, the current of the MMC bridge arm rises rapidly, and the current limiting control automatically acts to correct the reference voltage of each phase of the bridge arm. By reducing the MMC output DC voltage, the fault current can be actively suppressed, and the voltage of each phase of the bridge arm can be reduced. Achieve independent control and no need for controller switching control.

2. The virtual reactance mentioned in the present invention is not a physical reactance, and the effective suppression of the fault current can be realized only by adding a current limiting control link in the basic control of the MMC, so the volume and investment cost of the MMC device will not increase accordingly, and at the same time limit Flow control does not affect the MMC internal electrical characteristics.

The above is a specific embodiment and advantages of the present invention, but the protection scope of the present invention is not limited thereto. Those of ordinary skill in the art can make changes and modifications to the above-mentioned embodiments without departing from the technical spirit and principles described in the present invention, and these changes and modifications should also be regarded as protection of the present invention scope.

Claims (1)

1. A virtual reactance-based MMC direct-current fault current limiting method is characterized by comprising the following steps:

step 1: the current sensors are utilized to measure the current of the upper and lower bridge arms of each phase of the MMC in real time and are recorded as i _pj 、i _nj The measured bridge arm current is subjected to a differential link to obtain the bridge arm current change rate, the bridge arm current change rates are added and multiplied by 1/2 to obtain the average current change rate di of the j phase _j The specific calculation formula is as follows:

and 2, step: the average current change rate di of the j phase _j The/dt passes through a hysteresis comparator to obtain an action signal;

and step 3: setting a differential coefficient K according to MMC system circuit parameters _D Size of (2) with K _D The value of (b) reflects the magnitude of the virtual reactance; then di is mixed _j Dt and the differential coefficient K _D Multiplying action signals of the hysteresis comparator to obtain the voltage drop of the virtual reactance under the fault current, and using the voltage drop as the correction quantity of the voltage deviation of the bridge arm

And 4, step 4: correcting the deviation of bridge arm voltage

Superimposed to the bridge arm reference voltage u _pj 、u _nj To obtain the final bridge arm reference voltage for modulation, including

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