VSC-HVDC Converter Adaptive Switching for DC Fault Current Limiting
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Solution Overview
Problem
Existing VSC-HVDC systems face challenges in effectively limiting DC fault currents and suppressing bridge-arm currents, leading to potential interruptions in power transmission during faults, and require additional costly equipment to achieve this.
Innovation Solution
A method that involves calculating an adaptive switching coefficient and incorporating bridge-arm current suppression control to limit DC fault currents, reducing the need for increased DC circuit breaker capacity and preventing converter blockage, by dynamically adjusting the converter's switching coefficient and adding bridge-arm current suppression in the inner loop control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If additional fault current suppression devices (such as DC current limiter, flexible DC converter, or improved sub-module topology with blocking IGBT) are installed, then the DC fault current suppression capability is improved, but the engineering investment costs and device complexity increase
Solution Approach 1:
The converter station itself provides fault current suppression capability through its existing components (sub-module capacitors and IGBTs) by dynamically adjusting the switching coefficient, eliminating the need for separate fault current suppression devices. The system uses its own resources (capacitor discharge energy) to limit fault current, making the system self-sufficient for protection.
Solution Approach 2:
The invention changes the operating parameters of the existing converter by dynamically adjusting the switching coefficient (ratio of full-bridge to half-bridge sub-modules) in response to fault conditions. This parameter adjustment transforms the converter's impedance characteristics to limit fault current without requiring additional hardware.
2Reliability
If DC circuit breaker capacity is increased to handle higher fault currents, then the fault current breaking capability is improved, but the equipment investment costs increase
Solution Approach 1:
The converter station proactively limits the fault current at its output端 before the current reaches the DC circuit breaker. By dynamically adjusting the switching coefficient to increase output impedance during fault conditions, the system prevents excessive current from developing, thereby protecting the DC circuit breaker from requiring excessive breaking capacity.
3Reliability
If conventional fault current suppression methods are used, then the DC fault current is limited, but the bridge-arm current is not suppressed, causing converter blockage and power transmission interruption
Solution Approach 1:
The invention dynamically adjusts the switching coefficient based on real-time detection of fault conditions (when DC current rising rate exceeds threshold). This dynamic adaptation allows the system to optimize between fault current limitation and bridge-arm current suppression, maintaining power transmission capability while protecting against faults.
Solution Approach 2:
The system continuously monitors the DC current rising rate and provides feedback to the switching coefficient adjustment mechanism. When the rising rate exceeds the threshold (UdcN/2Ldc), the system activates the adaptive switching coefficient mode to suppress both DC fault current and bridge-arm current, ensuring continuous operation.
Data Source
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AI summary
The present invention provides a fault current limiting method in combination with a bridge-arm current suppression for VSC-HVDC, including: extracting an equivalent resistance, an equivalent inductance and an equivalent capacitance of a converter, an inductance value of a DC smoothing reactor, a rated DC voltage, a proportional parameter and an integral parameter of a current inner loop proportional-integral (PI) controller; setting a corner frequency of the DC current suppression control according to a fault current suppression requirement, and calculating a switching coefficient of the converter; in an inner loop current control stage, adding bridge-arm current suppression control; unlocking a converter station; setting a DC pole-to-pole fault to occur; when a rising rate of the DC current is detected to exceed UdcN/2Ldc, triggering the DC suppression control, and changing the initial switching coefficient of the converter to the calculated adaptive switching coefficient Kc, and meanwhile, adding the bridge-arm current suppression control. The present invention realizes the purpose of limiting the DC fault current, and reducing the requirement for the breaking capacity of the DC circuit breaker, while also preventing the converter from being blocked before the action of the DC circuit breaker to ensure that the transmission of the active power will not be interrupted during the fault.