CN105914701A - SDSM-MMC type flexible DC system DC fault reclosing method with low-current characteristic - Google Patents

Abstract

The invention discloses a DC fault reclosing method of an SDSM‑MMC type flexible DC system with low current characteristics. The DC fault current is cleared through a blocking converter, and deionization is completed; after that, all SDSM sub-modules are put into half-bridge-like Blocking mode: SDSM-MMC non-controlled rectifier bridge operation mode is realized by using SDSM half-bridge blocking mode; if the DC line re-current, it indicates that the fault still exists, it is judged as a permanent fault, and the converter is blocked immediately. Compared with the prior art, the present invention can realize reclosing judgment only by judging whether the current exists or not. When the reclosing is a permanent fault, the converter can be quickly re-blocked under the condition of a small line current, effectively avoiding It prevents the secondary overcurrent impact caused by reclosing to the system, and has the positive effect of low current hazard.

Description

A DC Fault Reclosing Method of SDSM-MMC Flexible DC System with Low Current Characteristics

technical field

The invention relates to the field of power system protection and control, in particular to a DC fault reclosing method for a flexible DC transmission system.

Background technique

Flexible DC transmission using overhead lines is one of the effective ways to realize large-capacity DC transmission in the future. In the case of overhead line transmission, the DC fault rate is greatly increased, and the use of series double sub-module (SDSM) MMC (SDSM-MMC) instead of the traditional half-bridge MMC can realize rapid fault current removal. In addition, the overhead line has a high probability of transient faults, so it is necessary to design an appropriate fault reclosing strategy. The existing reclosing strategy is mainly to unlock the converter station in zero power mode after a period of system de-ionization process, and judge whether the fault exists or not by judging whether the DC voltage can be established. However, this method uses whether the voltage is established or not as a criterion. If it coincides with a permanent fault, it will cause rapid overcurrent due to capacitor discharge, causing secondary damage to the system. In view of this problem, it is necessary to design a reclosing method that can effectively reduce the secondary overcurrent hazard of the system.

Contents of the invention

Aiming at the problem that the fault reclosing strategy of the current flexible DC system will cause the harm of the secondary overcurrent of the current, the present invention proposes a DC fault reclosing method of the SDSM-MMC type flexible DC system with low current characteristics, and realizes the DC fault reclosing method for the SDSM-MMC type Flexible direct current transmission system, fault reclosing method with low current hazard.

A low-current characteristic SDSM-MMC type flexible DC system DC fault reclosing method of the present invention, the method comprises the following steps:

Step 1. Use the SDSM converter to clear the DC fault current and complete the deionization;

Step 2. Put all SDSM converters into SDSM half-bridge blocking mode;

Step 3, using the SDSM half-bridge blocking mode to realize the SDSM-MMC uncontrolled rectifier bridge operation mode;

Step 4. Determine whether the DC line has current again to determine whether the fault still exists or has disappeared;

Step 5. If there is no more current in the DC line, it means that the fault has disappeared, then enter the system restart operation mode, and resume normal operation under the restart control strategy;

Step 6. If the DC line has a current again, it indicates that the fault still exists, and it is judged as a permanent fault, and the full-bridge sub-module converter is blocked immediately.

The SDSM class half-bridge blocking mode in the step (2) includes two implementations: the first implementation, the corresponding class half-bridge blocking mode one conducts IGBT T5, T1-T4 maintains the blocking state, and the fault current is continuously Current flow diodes D2, D4 and IGBT T5 flow; implementation method 2, the corresponding SDSM half-bridge blocking mode 2 conducts IGBT T1 and T3, T2, T4 and T5 remain in the blocking state, at this time the fault current passes through the freewheeling diode D6, IGBT T1 and T3 are in circulation.

The operation modes of the SDSM-MMC uncontrolled rectifier bridge respectively include three-phase rectifier bridges composed of two SDSM half-bridge locking modes.

Compared with the reclosing strategy based on the voltage establishment or not of the traditional direct unlocking converter, the present invention avoids capacitor discharge when reclosing on a permanent fault, and since it only needs to judge whether the current exists or not, it can be reclosed in a small In the case of line current, the converter is quickly re-blocked, which will not cause the secondary overcurrent of the system, and effectively avoid the secondary overcurrent impact of the reclosing on the system.

Description of drawings

Fig. 1 is the circuit diagram of SDSM class half-bridge blocking mode, (a), realization one; (b), realization two;

Figure 2 is a schematic diagram of the uncontrolled rectification operation mode of SDSM-MMC, (a), the SDSM-MMC uncontrolled rectification operation mode 1 based on the implementation mode 1 of the half-bridge-like blocking mode; (b), the implementation mode based on the half-bridge-like locking mode Two constitute SDSM-MMC uncontrolled rectification operation mode two;

Fig. 3 is an overall flowchart of a DC fault reclosing method for SDSM-MMC flexible DC transmission system according to the present invention.

detailed description

Aiming at the characteristics of the SDSM topology, the present invention proposes a new type of sub-module operation mode and the corresponding uncontrolled rectification operation mode of the converter, and accordingly designs a fault reclosing strategy with low current hazards, specifically content include:

Aiming at the characteristics of SDSM topological structure, a corresponding new sub-module operating mode is proposed, and the present invention refers to the operating mode as a half-bridge-like blocking mode. As shown in Figure 1(a), it is the first implementation of the half-bridge-like locking mode. Turn on the IGBT T5 in Figure 1(a), and T1~T4 remain in the locked state. At this time, the fault current can only pass through the freewheeling diodes D2 and D4. And IGBTT5 circulation, its physical characteristics are the same as the half-bridge sub-block in the locked state, so it is named as the half-bridge-like locking mode; as shown in Figure 1(b), it is the second implementation of the half-bridge-like locking mode, and the conduction is shown in Figure 1 IGBT T1 and T3, T2, T4 and T5 in (b) remain in the locked state. At this time, the fault current can only flow through the freewheeling diode D6, IGBT T1 and T3, and the physical characteristics are also the same as the half-bridge module in the locked state. same.

According to the two implementation modes of SDSM half-bridge blocking mode, two SDSM-MMC uncontrolled rectification operation modes as shown in Figure 2 are designed accordingly. In this operating mode, the converter is equivalent to an uncontrolled rectifier bridge.

As shown in Figure 3, it is an overall process of a DC fault reclosing method for SDSM-MMC flexible DC transmission system proposed by the present invention, including the following steps:

Use SDSM-MMC to clear the DC fault current and complete the deionization process, step 1; put all SDSMs into the half-bridge-like locking mode, step 2; realize the SDSM-MMC uncontrolled rectifier bridge operation mode in the half-bridge-like locking mode, step 3 ; Judging whether the fault still exists or disappears by judging whether the current in the DC line appears again (that is, under the situation of not considering the measurement error, judge whether the current I _dc of the DC line is equal to 0), step 4; if the current I _dc =0 , indicating that the DC line no longer has current, the fault has disappeared, the system enters the restart operation mode, and resumes normal operation under the restart control strategy, step 5; otherwise, if the current I _d c ≠ 0, it indicates that the DC line has current again, If the fault still exists, it is judged as a permanent fault, and the converter is blocked immediately, step 6.

Claims (3)

1. a SDSM-MMC type flexible direct current system direct current fault reclosing method for low current characteristic, its feature Being, the method comprises the following steps:

Step (1), utilize SDSM inverter to remove direct fault current, and complete to dissociate；

Step (2), all SDSM inverters are put into SDSM class half-bridge non-blocking mode；

Step (3), SDSM class half-bridge non-blocking mode is utilized to realize the SDSM-MMC uncontrollable rectifier bridge method of operation；

Step (4), judge that DC line occurs that electric current carrys out failure judgement and yet suffers from or disappeared the most again；

Step (5) if electric current no longer occurs in DC line, then shows that fault has disappeared, then enter system reboot Operational mode, recovers properly functioning under restarting control strategy；

Step (6) if DC line occurs electric current again, then shows that fault yet suffers from, it is determined that for permanent fault, Locking full-bridge submodule inverter immediately.

2. the SDSM-MMC type flexible direct current system direct current fault of low current characteristic as claimed in claim 1 overlaps Lock method, it is characterised in that the SDSM class half-bridge non-blocking mode in described step (2) includes two kinds of implementations: Implementation one, corresponding class half-bridge non-blocking mode one turns on IGBT T5, T1～T4 and keeps blocking, now fault Electric current is through sustained diode 2, D4 and IGBT T5 circulation；Implementation two, corresponding SDSM class half-bridge locking Pattern two turns on IGBT T1 and T3, and T2, T4 and T5 keep blocking, and now fault current is through fly-wheel diode D6, IGBT T1 and T3 circulates.

3. the SDSM-MMC type flexible direct current system direct current fault of low current characteristic as claimed in claim 1 overlaps Lock method, it is characterised in that the described SDSM-MMC uncontrollable rectifier bridge method of operation includes two kinds of SDSM classes respectively The three-phase commutation bridge that half-bridge non-blocking mode implementation is constituted.