# US 11,444,450 B2 · Adaptive fault clearing scheme for MMC VSC-HVDC grid based on source-grid coordination

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- Number: US 11,444,450 B2
- Document ID: US11444450B2
- Status: Granted
- Kind: B2
- Application number: 17/360,152
- Filed: 2021-06-28
- Published: 2022-07-14
- Granted: 2022-09-13
- Earliest claimed: 2021-01-12
- Assignees: Tianjin University
- Applicants: Tianjin University
- Inventors: Xiaolong Chen; Xiaowen Han; Yongli Li; Bin Li; Botong Li
- Web page: https://patentgrep.com/patent/US11444450B2

## CPC

- H02H 7/26: Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured (locating faults in cables G01R31/08)
- H02H 7/268: {for DC systems}
- H02H 3/066: {Reconnection being a consequence of eliminating the fault which caused disconnection}
- H02H 3/0935: {the timing being determined by numerical means}
- H02H 3/025: {Disconnection after limiting, e.g. when limiting is not sufficient or for facilitating disconnection}
- Y02E 60/60: Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]
- H02H: EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS (indicating or signalling undesired working conditions G01R, e.g. G01R31/00, G08B; locating faults along lines G01R31/08; emergency protective devices H01H)
- Y02E: REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION

## Abstract

The invention discloses an adaptive fault clearing scheme for an MMC VSC-HVDC grid based on source-grid coordination. A grid-side circuit breaker topology is disposed at each end of each DC line of the MMC VSC-HVDC grid to coordinate with a source-side control strategy when a fault occurs to the DC line so as to isolate the fault together. When a system runs stably, the voltage regulation controller does not actuate, after the fault occurs, the voltage regulation controller is started, and the output voltage regulation coefficient decreases, so that voltage at outlets of the converters drops accordingly, and coordinates with voltage of the pre-charging capacitor of the grid-side circuit breaker topology to realize fault isolation based on source-grid coordination. The adaptive fault clearing scheme provided by the invention greatly improves the speed of fault isolation.

## Claims

### Claim 1

An adaptive fault clearing scheme for a MMC VSC-HVDC grid based on source-grid coordination, wherein the MMC VSC-HVDC grid comprises two source-side converter stations MMC1 and MMC3, two grid-side converter stations MMC2 and MMC4, wherein converters of the four converter stations are connected in a square shape by double-circuit DC overhead lines, and each converter station is equipped with two converters; a grid-side circuit breaker is disposed at each end of each DC line connected with the converters to coordinate with a source-side control strategy when a fault occurs to the DC line so as to isolate the fault together; the grid-side circuit breaker topology comprises:
a steady-state low-loss branch, comprising a disconnector UFD1, an IGBT device and a disconnector UFD2 sequentially connected in series;
a transfer branch, comprising a thyristor T0 disposed in parallel, wherein one end of the thyristor T0 is connected with a front end of the disconnector UFD1, and other end is connected to a line between the IGBT device and the disconnector UFD2;
a charging circuit, disposed at a rear ends of the steady-state low-loss branch and the transfer branch and formed by sequentially connecting a pre-charging capacitor C, a switch RCB, a charging resistor RC and a charging inductor LC in series; wherein one end of the pre-charging capacitor C is connected with a rear end of the disconnector UFD2, and one end of the charging inductor LC is grounded;
a discharging branch, comprising a thyristor T1 connected in parallel with the charging circuit, wherein one end of the thyristor T1 is connected to a ground terminal of the charging inductor LC, and other end is connected to a line between the pre-charging capacitor C and the switch RCB; and
a flyback energy dissipation branch, connected in parallel with the charging circuit and disposed at a rear ends of the charging circuit and the discharging circuit, and comprising a flyback diode D and an energy dissipation resistor Re connected in series; wherein other end of the energy dissipation resistor Re is grounded, and other end of the flyback diode D is connected with the rear end of the disconnector UFD2 and a front end of a current limiting inductor Ldc disposed at a terminal;
wherein the source-side control strategy is realized by a voltage regulation controller; when a system runs stably, the voltage regulation controller does not actuate, and an output voltage regulation coefficient K is kept at 1; after a fault occurs, the voltage regulation controller is started, and the output voltage regulation coefficient K decreases within the range of [0,1], so that voltage at outlets of the converters drops accordingly, and coordinates with voltage of the pre-charging capacitor of the grid-side circuit breaker topology to realize fault isolation based on the source-grid coordination; wherein the voltage regulation controller gives corresponding voltage regulation coefficients Kat different stages after the fault occurs, and the voltage regulation controller comprises a preliminary current limiting link and an adaptive voltage regulation link; the preliminary current limiting link is put into operation until the system detects the fault, while the adaptive voltage regulation link is put into operation only after the system detects the fault; after the fault occurs, the preliminary current limiting link is started immediately to preliminarily limit the development of fault current; after monitoring the fault, a DC grid protection system switches to the adaptive voltage regulation link, and adaptively coordinates with voltage of the grid-side pre-charging capacitor to isolate the fault together.

### Claim 2 (depends on 1)

The adaptive fault clearing scheme for the MMC VSC-HVDC grid based on the source-grid coordination according to claim 1, wherein the pre-charging capacitor C needs to be charged during normal operation of the DC grid, so that the pre-charging capacitor C has a rated DC voltage after charging to lower an output voltage of the voltage regulation controller by adjusting a number of sub-modules for the converters in a fault isolation stage, and coordinate with the voltage of the pre-charging capacitor C to decrease and turn off the fault current.

### Claim 3 (depends on 1)

The adaptive fault clearing scheme for the MMC VSC-HVDC grid based on the source-grid coordination according to claim 1, wherein a voltage attenuation speed of the pre-charging capacitor C is different at different transition resistance when a fault occurs to the DC grid, and the voltage regulation coefficient K is matched with different transition resistance to adaptively adjust a number of removed sub-modules to cope with both metallic short-circuit faults and high-resistance short-circuit faults.

### Claim 4 (depends on 3)

The adaptive fault clearing scheme for the MMC VSC-HVDC grid based on the source-grid coordination according to claim 3, wherein when the fault occurs to the DC grid, fault clearing is achieved by the following steps: at time t0, the fault occurs, and the fault current flows from the converter stations to a fault point through the steady-state low-loss branch, in the meantime, the preliminary current limiting link is started immediately to preliminarily limit the rise of the fault current, and an instruction is given to block the IGBT device and turn on the thyristor T0 to allow the fault current on the steady-state low-loss branch to be quickly transferred to the transfer branch; and the disconnector UFD1 is turned off when a current flowing through the disconnector UFD1 drops to 0;
at time t1, the fault is detected, the disconnector UFD1 is turned off, and the thyristor T1 is turned on, the pre-charging capacitor C is connected to the discharge branch, and a number of control sub-modules of a source-side converters is reduced, so that the sum of voltage of sub-modules for upper and lower bridge arms of the converters is less than a voltage uCflt of the pre-charging capacitor; in the process, the fault current in the transfer branch drops to zero at a large rate to turn off the thyristor T0, and the fault point is isolated after the thyristor T0 is turned off;
at time t2, the thyristor T0 is turned off, energy of the pre-charging capacitor C is transferred to the current limiting inductor Ldc, and the disconnector UFD2 is turned off to isolate a fault line; after the fault is isolated, a MMC switches from an adaptive voltage regulation control mode to a conventional control mode to restore normal operation of the DC grid;
at time t3, the pre-charging capacitor C is reversely charged, and a current iC of the charging circuit and a current iL of the current limiting inductor Ldc start to decrease, in the meantime, the diode D in the flyback energy dissipation branch is turned on, a current iD of the diode D starts to rise from zero, energy of the current limiting inductor Ldc is released to the flyback energy dissipation branch, and the pre-charging capacitor C is reversely charged;
at time t4, the current iC of the charging circuit drops to zero, the thyristor T1 is turned off, and the current iD of the diode D reaches a maximum value and starts to decrease; and residual energy of the current limiting inductor Ldc is gradually released to the flyback energy dissipation branch; and
at time t5, energy dissipation is over and the fault clearing is completed.