DC Switch-Off Device with Segmented Commutation and Pre-Charge
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Solution Overview
Problem
Existing direct current breakers are either inefficient in terms of cost, extensibility, and breaking speed, or they incur excessive losses and are expensive due to the need for numerous fully-controlled devices to manage bidirectional current cutting, and they often cause line voltage fluctuations during reclosing operations.
Innovation Solution
A direct current switch-off device comprising an on-state current branch circuit with a mechanical switch and a current transfer module in series, connected in parallel with a current commutating and breaking unit featuring a bridge-type branch circuit and current-breaking modules with nonlinear resistors and fully-controlled devices, along with a buffering and reclosing circuit to manage voltage fluctuations and reduce device costs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a large number of fully-controlled devices are used to cut off bidirectional line currents, then the breaking capability is improved, but the equipment cost and device complexity increase significantly
Solution Approach 1:
The patent segments the current interruption function into two parts: the mechanical switch handles the main current interruption, while the fully-controlled devices only need to handle the commutation current. This segmentation allows the use of fewer, less expensive fully-controlled devices while maintaining the ability to interrupt bidirectional currents effectively.
Solution Approach 2:
The patent introduces a commutation circuit as an intermediary between the mechanical switch and the load. This commutation circuit, containing the fully-controlled devices, acts as a mediator that redirects the current through a safe path during interruption, allowing the mechanical switch to open without directly bearing the full bidirectional current interruption burden.
2Device complexity
If a mechanical switch is used for current interruption, then the equipment cost is reduced, but the breaking speed is insufficient for multi-terminal flexible direct current transmission systems
Solution Approach 1:
The patent merges the advantages of mechanical switches (low cost, low loss) with power electronic devices (fast switching speed) in a hybrid configuration. The mechanical switch provides the main current path with low conduction loss, while the power electronic devices in the commutation circuit provide fast current redirection capability, achieving both cost-effectiveness and fast breaking speed.
3Loss of time
If reclosing operation is performed without pre-charge process, then the operation speed is improved, but large switching overvoltage is produced affecting safe operation
Solution Approach 1:
The patent implements a pre-charge process as a preliminary action before the main reclosing operation. The pre-charge circuit charges the line capacitance gradually before the main switch closes, preventing the large inrush current and overvoltage that would occur with direct closing. This preliminary action ensures safe reclosing without sacrificing too much time.
Solution Approach 2:
The patent uses a pre-charge circuit that acts as a cushioning mechanism before the main reclosing operation. This circuit absorbs the energy that would otherwise cause dangerous overvoltage spikes, providing a protective buffer that allows safe reclosing operations.
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
The solution significantly reduces equipment costs, enhances extensibility, achieves high breaking speed with low loss, and minimizes line voltage fluctuations during reclosing, thereby improving the safety and reliability of direct current transmission systems.
Implementation Method 1
Each current-breaking module includes a nonlinear resistor and a valve group consisting of fully-controlled devices connected in series
Implementation Method 2
The mechanical breaker fabricated according to this principle cannot meet the requirements of a multi-terminal flexible direct current transmission system in terms of breaking time and current-breaking capability
Implementation Method 3
a pre-charge operation needs to be performed on the faulty line first
Data Source
AI summary
A direct current switch-off device comprises an on-state current branch circuit and a current commutating and breaking unit which are connected in parallel. The on-state current branch circuit has a mechanical switch (S) and a current transfer module which are connected in series. The current commutating and breaking unit comprises a bridge-type branch circuit and a current-breaking branch circuit. The current-breaking branch circuit and two bridge arms of the bridge-type branch circuit are connected in parallel. The current-breaking branch circuit comprises a nonlinear resistor (RI) and a valve group consisting of fully-controlled devices connected in series. The nonlinear resistor (RI) and the valve group are connected in parallel. Two ends of each fully-controlled device in the valve group are connected in parallel with a buffering and reclosing circuit.