CN-122026467-A - Method for predicting subsequent commutation failure of mixed cascade direct current system by considering MMC fault inrush current

CN122026467ACN 122026467 ACN122026467 ACN 122026467ACN-122026467-A

Abstract

The application relates to a method for predicting subsequent commutation failure of a mixed cascade direct current system taking account of MMC fault inrush current, which comprises the steps of analyzing an MMC converter transformer fault inrush current generation mechanism in the mixed cascade direct current system; the method comprises the steps of analyzing a rule of harmonic distortion of bus voltage of a grid commutation converter caused by MMC converter fault current, analyzing influence of the harmonic distortion of the bus voltage on a commutation process, and predicting subsequent commutation failure risk of the grid commutation converter at the inversion side of the hybrid cascade direct current system according to fault current amplitude, bus voltage harmonic distortion rate and commutation failure judging conditions. According to the method, the electrical coupling connection of the LCC at the inversion side and the MMC and the relevance of the fault inrush current and the subsequent commutation failure are brought into a prediction system, so that the harmonic distortion of the busbar voltage caused by the fault inrush current is completely revealed, the turn-off angle of the LCC is reduced, the chained induction mechanism of the subsequent commutation failure is triggered, the induction factors of the subsequent commutation failure are accurately positioned, and the prediction accuracy is improved.

Inventors

JI LIANGCHEN
XUE YUTING
ZHENG TAO

Assignees

华北电力大学

Dates

Publication Date: 20260512
Application Date: 20260212

Claims (10)

1. The method for predicting the subsequent commutation failure of the mixed cascade direct current system considering MMC fault inrush current is characterized in that a rectifying side of the mixed cascade direct current system adopts a power grid commutation converter, a high-end valve group of an inversion side of the mixed cascade direct current system adopts the power grid commutation converter, and a low-end valve group adopts a plurality of modularized multi-level converters to be connected in parallel and then connected with the high-end valve group in series on the direct current side, and the method comprises the following steps: obtaining control strategies of converters corresponding to a rectifying side and an inverting side according to the topological structure of the hybrid cascade direct current system; analyzing a fault inrush current generation mechanism of the MMC converter transformer in the mixed cascading direct current system according to a control strategy of the converter, and determining the association between a fault inrush current amplitude and the MMC converter transformer saturation characteristic; According to the electric coupling relation between the high-end valve group and the low-end valve group at the inversion side, analyzing the rule that the MMC converter fault inrush current causes harmonic distortion of the bus voltage of the power grid converter at the inversion side, and establishing the relation between the MMC converter fault inrush current amplitude and the harmonic distortion rate of the bus voltage of the power grid converter; Analyzing the influence of the harmonic distortion of the bus voltage of the grid commutation converter on the commutation process of the grid commutation converter, and determining the judgment condition of the commutation failure of the hybrid cascade direct current system; and predicting the subsequent commutation failure risk of the grid commutation converter at the inversion side of the hybrid cascade direct current system according to the fault inrush current amplitude of the MMC converter under different saturation characteristics, the harmonic distortion rate of the busbar voltage of the grid commutation converter and the judgment condition of commutation failure.
2. The method for predicting the subsequent commutation failure of the mixed cascade direct current system considering MMC fault inrush current according to claim 1, wherein the mechanism for generating the MMC fault inrush current in the mixed cascade direct current system comprises the steps of after a single-phase grounding fault occurs on a valve side of an MMC converter transformer, suddenly increasing bridge arm current, enabling an IGBT to be blocked due to overcurrent, enabling fault phase current to flow into the MMC converter transformer valve side due to the fact that a large amount of direct current component is contained in fault phase current, and enabling an iron core working point to enter a saturation region due to continuous accumulation of an MMC converter transformer iron core flux linkage, so that the fault inrush current is generated.
3. The method for predicting the subsequent commutation failure of the hybrid cascade direct current system taking the MMC fault inrush current into consideration according to claim 1, wherein when a control strategy of the converter corresponding to the rectifying side and the inverting side is obtained, a UMEC model is further required, and different saturation characteristics of the MMC converter are simulated by setting different volt-ampere characteristic curves.
4. The method for predicting the subsequent commutation failure of a hybrid cascade direct current system taking into account MMC fault inrush current according to claim 1, wherein when the MMC fault inrush current generation mechanism in the hybrid cascade direct current system is analyzed according to a control strategy of a converter, the higher the saturation level of the MMC fault inrush current is, the larger the amplitude of the fault inrush current is.
5. The method for predicting the subsequent commutation failure of the hybrid cascade direct current system taking account of MMC fault current as claimed in claim 1, wherein the fault current comprises 2-7 times of harmonic components, the amplitude of the 2 times of harmonic components is highest, the fault current amplitude is positively correlated with the harmonic distortion rate of the bus voltage of the grid commutation converter, and the higher the fault current amplitude is, the higher the harmonic distortion rate of the bus voltage of the grid commutation converter is, and the more serious the voltage waveform distortion is.
6. The method for predicting a subsequent commutation failure of a hybrid cascaded direct current system taking into account MMC fault current as defined in claim 1, wherein the influence of the harmonic distortion of the bus voltage of the grid commutated converter on the grid commutated converter commutation process comprises: The harmonic distortion of the bus voltage of the power grid commutation converter can cause the zero crossing point of the commutation voltage waveform of the power grid commutation converter to move forward and the voltage amplitude to be reduced, so that the commutation overlapping angle of the power grid commutation converter is increased, the turn-off angle is reduced, the time area provided by the commutation voltage can not meet the requirements of the commutation process of the power grid commutation converter, and the risk of commutation failure is increased.
7. The method for predicting the subsequent commutation failure of the hybrid cascade direct current system taking account of MMC fault inrush current of claim 1, wherein when analyzing the rule of harmonic distortion of bus voltage of a grid commutation converter at the inversion side caused by MMC fault inrush current, dividing a process from fault occurrence to stabilization of each electrical quantity change into four phases according to the size of an off angle, namely a first commutation failure phase, a recovery phase after the first commutation failure, a subsequent commutation failure phase and a recovery phase after the subsequent commutation failure; And analyzing the influence rule of the fault inrush current on the harmonic distortion of the bus voltage by combining the electrical quantity characteristics of each stage, wherein the fault inrush current has the most obvious influence on the harmonic distortion of the bus voltage of the power grid commutation converter in the subsequent commutation failure stage, the harmonic distortion rate of the bus voltage reaches a peak value in the stage, the harmonic distortion influence in the first commutation failure stage and the recovery stage is dominated by the fault voltage drop, the amplitude of the fault inrush current in the recovery stage after the subsequent commutation failure is attenuated, and the harmonic distortion rate of the bus voltage is gradually reduced.
8. The method for predicting the subsequent commutation failure of the hybrid cascade direct current system taking account of MMC fault inrush current of claim 1, wherein the determination conditions of the commutation failure of the hybrid cascade direct current system include: And judging whether the actual turn-off angle in the phase change process of the power grid phase change converter is smaller than 7 degrees, wherein when the actual turn-off angle is smaller than 7 degrees, the power grid phase change converter at the inversion side is judged to have the risk of phase change failure, and when the actual turn-off angle is larger than or equal to 7 degrees, the power grid phase change converter at the inversion side is judged not to have the risk of phase change failure.
9. The method for predicting the subsequent commutation failure of the hybrid cascade direct current system taking account of MMC fault inrush current of claim 1, wherein when predicting the risk of the subsequent commutation failure of the grid commutation converter at the inversion side of the hybrid cascade direct current system, the system electric quantity change of the MMC converter in an ideal saturation-free state is required to be used as a reference, if the grid converter at the inversion side of the hybrid cascade direct current system has no subsequent commutation failure in the ideal saturation-free state and the grid converter meets the commutation failure judgment condition in the saturation state, the subsequent commutation failure risk of the grid converter at the inversion side of the hybrid cascade direct current system is judged, and the fault inrush current is an induction factor of the subsequent commutation failure.
10. The method for predicting the subsequent commutation failure of the hybrid cascade direct current system considering MMC fault inrush current according to claim 1, wherein the control strategy of the converter at the rectifying side adopts the fixed current control strategy of the grid commutation converter, the control strategy of the converter at the middle-high-end valve bank at the inverting side adopts the fixed voltage control strategy or the fixed turn-off angle control strategy of the grid commutation converter, and the control strategy of the converter at the low-end valve bank adopts the combination of the fixed direct current voltage control strategy of the modularized multi-level converter and the fixed active power control strategy of the modularized multi-level converter.

Description

Method for predicting subsequent commutation failure of mixed cascade direct current system by considering MMC fault inrush current Technical Field The application relates to the technical field of relay protection of power systems, in particular to a method for predicting a subsequent commutation failure of a mixed cascade direct current system by considering MMC fault inrush current. Background Commutation failure is a typical fault in a dc power transmission system, and is mainly characterized by instantaneous drop of dc voltage and abrupt rise of dc current, which may cause power imbalance of the ac systems at the transmitting end and the receiving end. Under severe alternating current faults, the first commutation failure of the direct current transmission system is difficult to inhibit, but the influence of the first commutation failure on the power grid is limited, and the subsequent commutation failure can seriously threaten the safe and stable operation of the power grid, even can cause direct current blocking, and causes interruption of the transmission power, thereby having great harm to the safe and stable operation of the system. The hybrid cascade direct current system combines the advantages of conventional direct current and flexible direct current, and becomes a new mode for realizing long-distance large-capacity power transmission. Because the inversion side adopts a structure of cascading a power grid commutation converter (line commutated converter, LCC) and a modularized multi-level converter (modular multilevel converter, MMC), complex electric coupling connection exists between the LCC and the MMC, and fault inrush current caused by ground fault of a MMC converter valve side can cause subsequent commutation failure of the LCC, so that safe and stable operation of the system is endangered. But at present, partial researches only exist for a fault-induced current surge mechanism and the influence thereof on the converter differential protection of a converter transformer grounding fault of a hybrid cascade direct current transmission system on the subsequent converter failure of a line-commutated converter based high-voltage direct-current (LCC-HVDC) converter of a power grid. The subsequent commutation failure of the hybrid cascade direct current transmission system caused by the inversion side MMC commutation fault inrush current is not researched. Therefore, in view of the above-mentioned problems, there is a need for an LCC subsequent commutation failure analysis method that considers complex coupling connection between different converters at the inverter side and the influence of MMC commutation failure inrush current for a hybrid cascading dc power transmission system, so as to accurately predict LCC subsequent commutation failure risk of the hybrid cascading dc power transmission system. Disclosure of Invention In order to overcome the defects in the prior art, the application provides a method for predicting the subsequent commutation failure of a mixed cascade direct current system by considering MMC fault inrush current, which specifically adopts the following technical scheme: The method for predicting the subsequent commutation failure of the mixed cascade direct current system considering MMC fault inrush current comprises the steps that a power grid commutation converter is adopted on a rectifying side of the mixed cascade direct current system, a power grid commutation converter is adopted by a high-end valve group on an inverting side of the mixed cascade direct current system, and a plurality of modularized multi-level converters are connected in parallel and then connected with the high-end valve group in series on a direct current side, wherein the method comprises the following steps: obtaining control strategies of converters corresponding to a rectifying side and an inverting side according to the topological structure of the hybrid cascade direct current system; analyzing a fault inrush current generation mechanism of the MMC converter transformer in the mixed cascading direct current system according to a control strategy of the converter, and determining the association between a fault inrush current amplitude and the MMC converter transformer saturation characteristic; According to the electric coupling relation between the high-end valve group and the low-end valve group at the inversion side, analyzing the rule that the MMC converter fault inrush current causes harmonic distortion of the bus voltage of the power grid converter at the inversion side, and establishing the relation between the MMC converter fault inrush current amplitude and the harmonic distortion rate of the bus voltage of the power grid converter; Analyzing the influence of the harmonic distortion of the bus voltage of the grid commutation converter on the commutation process of the grid commutation converter, and determining the judgment condition of the commutation failure of the hybrid ca