DC Network Fault Clearance Using Active Impedance Control
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Solution Overview
Problem
Current methods for fault clearance in DC electrical networks are either costly, complex, or result in significant downtime due to the need for large and expensive circuit breakers, complex communication systems, or increased component ratings, which can lead to prolonged loss of transmission capability.
Innovation Solution
Incorporating active power electronic devices that can rapidly reduce current to zero in faulty DC transmission paths independently of converter operation, using a current flow controller with switching elements and energy storage devices to inject voltage drops and vary impedance, allowing for fast and efficient fault clearance without requiring all components to be rated for twice the nominal voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If DC circuit breakers are used to interrupt current flow in faulty DC transmission paths, then fault isolation is achieved, but the circuit breakers are large, bulky, and expensive
Solution Approach 1:
The patent introduces current limiting reactors as intermediary devices inserted into DC transmission paths. These reactors limit fault current flow without requiring large, expensive circuit breakers. The reactors work in conjunction with existing circuit breakers to achieve fault isolation while using smaller, more cost-effective switching equipment.
Solution Approach 2:
The patent changes the electrical parameters of the DC transmission path by inserting reactors that modify the impedance characteristics. This parameter change limits the fault current magnitude, allowing standard-sized circuit breakers to handle faults effectively without requiring oversized equipment.
2Reliability
If power flow is blocked from external sources by operating converters or opening circuit breakers, then current is reduced to zero for disconnection, but transmission capability is lost for several hundred milliseconds
Solution Approach 1:
The patent applies preliminary action by pre-inserting current limiting reactors into the DC transmission paths before faults occur. This preliminary configuration allows circuit breakers to rapidly clear faults without requiring extended current decay time, reducing transmission downtime while maintaining safe disconnection capabilities.
Solution Approach 2:
The patent enables rapid fault clearance by using the current limiting reactors to suppress fault current magnitude, allowing circuit breakers to interrupt current quickly without waiting for natural current decay. This 'rushing through' the fault clearance process minimizes transmission downtime.
3Reliability
If DC electrical network is configured to permit voltage shift from symmetric ±1 p.u. to asymmetric 2 p.u. and 0 p.u., then current can be forced to zero, but all components must be rated for twice the nominal voltage
Solution Approach 1:
The patent changes the impedance parameter of the DC transmission path by inserting current limiting reactors. This parameter change suppresses fault current without requiring voltage shifts or component overrating, maintaining standard voltage ratings while achieving effective fault current limitation.
4Reliability
If converter control action is used to force current to zero in faulty DC transmission paths, then fault clearance is achieved, but complex communication systems are required for converter coordination
Solution Approach 1:
The patent introduces current limiting reactors as passive intermediary devices that automatically limit fault current without requiring active converter control or complex communication systems. The reactors provide inherent fault current suppression that simplifies the overall protection system architecture.
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
This solution minimizes downtime and costs by enabling rapid fault clearance with small, low-cost power electronic devices, reducing the need for complex communication systems and component overrating, while maintaining normal network operation.
Implementation Method 1
a switching element, the switching element being connected to a capacitor to selectively provide a voltage source
Data Source
Figure 1
Figure 2~3
Figure 4~5
AI summary
A DC electrical network (200) comprises: a plurality of DC terminals (202), each DC terminal (202) being operatively connectable to a converter; and a plurality of DC transmission paths (204), each DC transmission path (204) being arranged to interconnect two or more of the plurality of DC terminals (202), each DC transmission path (204) including a DC power transmission medium (16,18,19), each DC transmission path (204) further including a switching apparatus (212) operable to selectively block current from flowing in the corresponding DC transmission path (204), the DC electrical network (200) further including: at least one active power electronic device (10) connected in at least one of the plurality of DC transmission paths (204), the or each active power electronic device (10) being configured to be operable to selectively vary an apparent impedance of the or each corresponding DC transmission path (204); a detector (214) configured to detect one or more faults occurring in the plurality of DC transmission paths (204); and a control unit (216) programmed to operate the or each active power electronic device (10) to vary an apparent impedance of a faulty corresponding DC transmission path (204) so as to force a current flowing in the faulty corresponding DC transmission path (204) to reduce to a target current level, the control unit (216) being further programmed to operate the or each switching apparatus (212) to block current from flowing in the faulty corresponding DC transmission path (204) when the current flowing in the faulty corresponding DC transmission path (204) is reduced to the target current level, wherein the target current level corresponds to a current threshold at or below which the switching apparatus (212) can be operated to block current from flowing in the corresponding DC transmission path (204).