Current Converter Unit Asymmetrical Fault Management
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Solution Overview
Problem
Asymmetrical faults in current converters, particularly those affecting one or two poles of a three-phase AC voltage system, hinder rapid fault clearance due to the absence of zero-crossings in the AC current, leading to potential thermal overloading and irreversible damage in modular multi-level power converters.
Innovation Solution
A current converter unit with a current-damping electrical component connects a connection point to ground potential, ensuring primary side alternating currents experience zero-crossings during asymmetrical faults, allowing for secure AC power circuit-breaker operation, and a direct current switch bypasses the damping component during normal operation to prevent electrical losses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a short-circuiting unit is employed to convert asymmetrical faults into symmetrical faults, then zero-crossings occur in the AC current enabling circuit breaker opening, but the AC grid system and connected components are substantially loaded
Solution Approach 1:
The invention divides the fault management function into two independent parts: the existing short-circuiting unit for converting asymmetrical faults, and a new current-damping unit specifically for damping fault current. This segmentation allows the short-circuiting unit to maintain its fault conversion function while the dedicated current-damping unit handles the energy dissipation, preventing excessive loading of the entire AC grid system.
Solution Approach 2:
The current-damping unit acts as an intermediary component between the asymmetrical fault and the AC grid system. It introduces a damping effect that limits the transmission of fault current to the grid, thereby protecting the grid system and connected components from substantial loading while still enabling fault clearance.
2Reliability
If a tertiary winding of the transformer is short-circuited by a short-circuiting unit, then zero-crossings are achieved in the AC current, but a high-capacity tertiary winding is required which is complex and expensive
Solution Approach 1:
The invention extracts the current damping function from the transformer structure itself. Instead of relying on a complex high-capacity tertiary winding within the transformer, the damping function is taken out and implemented by a separate current-damping unit connected to the transformer, thereby simplifying the transformer structure while maintaining fault clearance capability.
Solution Approach 2:
The current-damping unit serves multiple functions: it damps fault current to enable zero-crossings for circuit breaker opening, and it can be integrated with the existing transformer structure without requiring a separate high-capacity tertiary winding. This multi-functional approach reduces overall system complexity.
3Productivity
If the connection point is directly connected to ground potential, then high-voltage direct current transmission is efficient, but asymmetrical faults cause absence of zero-crossings preventing rapid fault clearance
Solution Approach 1:
The invention makes the ground connection dynamic by introducing a controllable switch that can change the connection state based on fault conditions. During normal operation, the connection point is directly grounded for efficient HVDC transmission. During asymmetrical faults, the switch adjusts the connection to enable current damping, thereby achieving zero-crossings for rapid fault clearance.
Solution Approach 2:
The invention changes the electrical parameters of the ground connection based on operating conditions. In normal operation, the connection has zero impedance for maximum efficiency. During asymmetrical faults, the connection parameters are modified by the current-damping unit to introduce appropriate damping, enabling zero-crossings while maintaining the ability to restore efficient transmission after fault clearance.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enables rapid and secure fault clearance in current converters by ensuring zero-crossings in primary side alternating currents during asymmetrical faults without the need for additional transformer windings or short-circuiting switches, maintaining efficient high-voltage direct current transmission.
Implementation Method 1
ensuring primary side alternating currents experience zero-crossings during asymmetrical faults
Implementation Method 2
a direct current switch bypasses the damping component during normal operation to prevent electrical losses
Implementation Method 3
the AC flowing in the AC power circuit-breaker no longer features any current zero-crossings (absence of zero-crossings in the AC). This absence of zero-crossings in the AC, further to fault detection, prevents a rapid opening of the AC power circuit-breaker, as the arc which is generated upon the opening of the AC power circuit-breaker is only securely quenched upon the occurrence of zero-crossings in the AC
Data Source
AI summary
A current converter unit for a high-voltage direct-current (HVDC) transmission contains a first converter and a second converter. A first direct-current terminal of the first converter is connected to a first conductor terminal point for a first HVDC transmission conductor. A second direct-current terminal of the first converter is connected to a first direct-current terminal of the second converter such that a connection point is formed. A second direct-current terminal of the second converter is connected to a second conductor terminal point for a second HVDC transmission conductor. The connection point is connected to ground potential by a current-damping electric component, and the connection point is connected to the ground potential by a direct-current switch.