Application of New DC Circuit Breakers in Short-Circuit Fault Protection
I. Introduction
With the rapid advancement of modern information technology, intelligence has become a major trend in the development of industrial equipment. In the field of high-voltage switching, intelligent circuit breakers—as critical control components in power systems—form the foundation for automation and intelligence in power systems. This study focuses on an intelligent DC circuit breaker based on single-chip microcomputer (SCM) technology, emphasizing its practical application in real-time current monitoring and fault interruption within shipboard DC power supply systems. In addition to a conventional arc-extinguishing chamber, this circuit breaker incorporates an intelligent operating system, a fault current detection unit, and a signal processing unit, enabling it to effectively address the special requirements of DC system fault protection.
II. Current Transfer Principle of DC Circuit Breakers
The core challenge for circuit breakers in DC systems lies in arc extinction. According to arc theory, extinguishing an arc requires a current zero-crossing point. However, DC systems lack a natural current zero point, making arc extinction exceptionally difficult.
Solution – Current Transfer Principle:
By introducing a reverse current into the circuit, an artificial current zero point is created, providing the necessary condition for arc extinction. The specific principle is as follows:
|
Circuit State |
Component Operation |
Current Change and Arc Extinction Process |
|
Normal State |
Circuit breaker QF is closed. |
High-voltage DC power supplies the load through QF, ensuring stable circuit operation. |
|
Fault State (A–B short) |
1. Current increases rapidly (rate depends on L₁, L₂). |
1. Discharge current I₂ opposes the original current I₁. |
III. System Design
(1) Monitoring Module
The monitoring module serves as the control signal source for the electronic operating system, enabling real-time monitoring of circuit current changes and providing timely, accurate responses to current abnormalities.
Signal Processing Flow:
- Signal Acquisition: Current signals are collected via a shunt with a grounded low-voltage terminal (to prevent high-voltage pulse interference) and non-inductive resistance (to preserve current amplitude and waveform).
- Signal Processing: Acquired voltage signals (small amplitude with high-frequency noise) → Filter circuit (noise removal) → Isolation amplification circuit (using high-precision linear optocoupler HCNR201, primary-side op-amp LM324, secondary-side op-amp OP07, acting as a DC transformer) → Sample and hold → A/D conversion → Sent to SCM.
- Fault Response: If the current exceeds allowable limits, the SCM issues a trip command and triggers a buzzer alarm.
(2) Data Processing by SCM
Fault Judgment Criteria:
- Normal operation: Current rise rate Kᵢ ≤ Kₘₐₓ, current value I ≤ Iₘₐₓ.
- Short-circuit fault: Kᵢ > Kₘₐₓ, and I may exceed Iₘₐₓ rapidly.
Mathematical Model and Simplified Calculation:
From ΔU = ΔI · Rբ (shunt resistance),
Kᵥ = ΔU/Δt = Kᵢ · Rբ → Kᵢ = ΔU/(Δt · Rբ).
Advantage: After fixing Δt, only ΔU between two moments is needed to compute Kᵢ, avoiding floating-point operations and significantly reducing response time.
Fault Criterion: The SCM judges a fault when Uᵢₙ > Uₘₐₓ or ΔUᵢₙ > ΔUₘₐₓ.
(3) Anti-Interference Measures
Due to the high-voltage, high-current environment with strong electromagnetic interference, multi-dimensional anti-interference design is adopted:
|
Anti-Interference Dimension |
Specific Measures |
Purpose |
|
Input Signal |
Isolation via linear optocoupler HCNR201 |
Isolates control system from high-power circuits; suppresses interference and enhances safety. |
|
Signal Output |
SCM controls optocoupler switches to drive thyristors in discharge circuit |
Ensures only signal connection; prevents high-current effects on control system. |
|
Signal Pre-Channel |
Low-pass filter circuit |
Blocks RF, power frequency, and pulse interference; improves reliability. |
|
Software Level |
1. Composite digital filtering (median + moving average) |
Filters data noise, ensures command accuracy, and prevents program runaway. |
(4) Overall Structural Design
Operating Mechanism – Bistable Permanent Magnet Mechanism:
- Composition: Closing/opening coils, permanent magnets, moving iron core (dashed), housing.
- Operating Circuit: Coils series-connected with pre-charged capacitors (energy source) and thyristors form discharge circuits.
- Action Process: SCM signal → amplified by transistors → controls thyristor gates → during fault, SMC sends opening signal → thyristor conducts → capacitor discharges through opening coil → iron core moves → QF opens. Closing is manually controlled via a switch.
Current Transfer Circuit (Improved Structure):
- Improvement: Replaces spark gap switches with vacuum switches (QF₂), reducing time dispersion.
- Structural Parameters: QF₁ and QF₂ equidistant from pivot O; arm lengths determined based on specific parameters.
- Fault Action: Permanent magnet mechanism energizes → iron core moves down → QF₁ opens, QF₂ closes → capacitor C discharges → arc current in QF₁ crosses zero → arc extinguishes.
IV. System Experiment
- Environment: Synthetic Circuit Laboratory, Institute of Power Electronics, Dalian University of Technology.
- Method: Low-frequency AC current simulates DC short-circuit rise; reverse current introduced at peak current.
- Results:
- Current waveform through QF₁ shows reverse current precisely introduced at t₀.
- Reverse current forces zero-crossing, achieves arc extinction, and successfully interrupts short-circuit current.
V. Conclusion
Experiments demonstrate that the new DC circuit breaker with the electronic operating system successfully interrupts short-circuit currents in DC power supply systems, with satisfactory results. This solution can be widely applied in short-circuit protection for DC systems such as ships, subways, DC electrolysis, and electric furnaces.
Core System Features:
- Real-Time Performance: SCM-based acquisition enables real-time monitoring with strong controllability and minimal time dispersion.
- Rapid Response: Simplified algorithms avoid floating-point operations, reducing response time for quick fault detection.
- Reliability: Bistable permanent magnet mechanism reduces mechanical failures and shortens opening time; improved structure ensures synchronization between interruption and transfer operations.
The intelligent DC circuit breaker solution presented in this study offers high practical value and promising application prospects, meeting the urgent demand for intelligent protection equipment in modern DC power systems.
