HVDC Converter Full-Bridge Cells DC Fault Isolation
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Solution Overview
Problem
Voltage source converters in HVDC power transmission networks are vulnerable to DC side faults, leading to high fault currents that can damage components and result in prolonged power interruptions due to the need for additional components and specialized equipment to manage these faults.
Innovation Solution
The integration of auxiliary units and limb portions forming three-phase static synchronous compensators allows for controlled reactive power exchange with the AC network, isolating DC side faults without disrupting power conversion, and enabling quick fault recovery by minimizing fault current risks and preventing power arcs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional voltage source converters are used with anti-parallel diodes, then the converter can operate normally, but fault current cannot be interrupted and components are damaged during DC side short circuits
Solution Approach 1:
The converter bridge is divided into multiple independently controllable full-bridge cells. Each cell can be individually switched off during a fault, segmenting the fault current path and isolating the fault from the control system without requiring the entire converter to shut down.
Solution Approach 2:
The converter uses dynamically controllable IGBT switches instead of fixed diode paths. During normal operation, the IGBTs are switched to control power flow. During a fault, the switching is adjusted to block fault current while maintaining normal operation capability, providing dynamic response to fault conditions.
2Reliability
If additional inductive components are added to limit fault current, then converter tolerance to fault current increases, but converter size, weight, and cost increase
Solution Approach 1:
The converter uses its own switching elements (IGBTs and anti-parallel diodes) to limit fault current by controlling the switching state during a fault. The existing components serve dual purposes: normal power conversion and fault current limitation, eliminating the need for additional dedicated fault protection components.
Solution Approach 2:
The IGBT switches and anti-parallel diodes perform multiple functions: they control normal power flow during operation and simultaneously limit fault current during abnormalities. This multi-functionality eliminates the need for separate fault protection components, reducing overall converter size and weight.
3Ease of repair
If circuit breakers are used to interrupt DC fault current, then fault isolation is achieved, but specialized expensive DC circuit breaking equipment is required and power interruptions occur
Solution Approach 1:
The converter bridge is segmented into independently controllable full-bridge cells. During a DC fault, only the affected cell or phase is switched off, isolating the fault locally while maintaining operation of other cells. This eliminates the need for complete converter shutdown and enables continuous power transmission through healthy phases.
Solution Approach 2:
The converter maintains continuous power transmission capability during DC faults by switching off only the faulty phase or cell while keeping other phases operational. This ensures uninterrupted power flow through healthy paths, eliminating the downtime associated with traditional circuit breaker interruption.
4Reliability
If the converter is designed to tolerate high fault current, then sufficient time is available for fault detection, but converter size and component capacity must be increased
Solution Approach 1:
The converter uses its existing switching elements to actively limit fault current through controlled switching, rather than relying on passive tolerance through oversized components. The IGBTs and anti-parallel diodes self-regulate the fault current by adjusting their switching state, providing fault protection without requiring increased component capacity.
Solution Approach 2:
The converter changes its operating parameters dynamically: during normal operation, it operates at full power capacity; during a fault, it changes switching states to limit current to safe levels. This parameter change allows the same components to handle both normal high-power operation and fault conditions without requiring oversized rating.
Data Source
Figure 1~2
Figure 3~4
Figure 5~6
AI summary
A power electronic converter (40) for use in high voltage direct current power transmission and reactive power compensation comprises three converter limbs (42), each converter limb (42) including first and second DC terminals (46,48) for connection in use to a DC network (52) and an AC terminal (50) for connection in use to a respective phase of a three- phase AC network (54), each converter limb (42) defining first and second limb portions (56,58) being connected in series between the respective AC terminal (60) and a respective one of the first and second DC terminals (46,48), each limb portion (56,58) including at least one switching element (62,66) being controllable in use to facilitate power conversion between the AC and DC networks (52,54), the power electronic converter (40) further including a plurality of auxiliary units (44), each auxiliary unit (44) being operably associated with the respective phase of the AC network (54), each auxiliary unit (44) including at least one module (60) including a voltage source, the limb portions (56,58) being controllable in use to define at least one three-phase static synchronous compensator including at least one of the plurality of auxiliary units (44) in each branch of a star configuration, each of the first and/or second DC terminals (46,48) defining the neutral point of the respective star configuration.