Three-Phase GFCI Zero-Cross Trip Timing for AC Contactors
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Solution Overview
Problem
Existing GFCI devices struggle to meet the stringent performance requirements set by standards such as UL943(C), particularly in terms of trip time, harmonic attenuation, and continuous monitoring, while using AC contactors with inconsistent opening times.
Innovation Solution
The implementation of an AC contactor with delayed trip decision based on a calculated optimum interval relative to the VAC line voltage zero-cross point, combined with a low-computational complexity half-wave DFT filter for harmonic attenuation, continuous auto-monitoring, and contactor status detection using a contactor coil current monitoring circuit.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If an AC contactor is used in the GFCI device, then the device can be manufactured with standard components, but the contactor opening time becomes inconsistent and may exceed the 20 ms trip time requirement
Solution Approach 1:
The patent applies preliminary action by delaying the trip decision until after the VAC line voltage zero-cross point is detected. This ensures that the contactor is commanded to open at a known optimal time, allowing the system to compensate for the inherent inconsistency in contactor mechanical response times. By waiting for the zero-cross event before initiating the trip sequence, the system establishes a predictable reference point that improves timing consistency across different contactor units.
Solution Approach 2:
The patent changes the timing parameter of the trip decision by introducing a delay relative to the VAC line voltage zero-cross point. Instead of using a fixed absolute timing, the system dynamically adjusts the trip command timing based on the detected zero-cross event. This parameter change allows the system to accommodate variations in contactor opening times while maintaining compliance with the 20 ms maximum trip time requirement.
2Speed
If the trip decision is made immediately upon detecting ground fault, then the response is fast, but the contactor opening time varies and may not meet the 20 ms requirement
Solution Approach 1:
The system performs preliminary detection of the VAC line voltage zero-cross point before issuing the trip command. This preliminary action establishes a known reference timing that allows the contactor to be commanded to open at the optimal moment in the AC cycle, ensuring both fast response and consistent timing that meets the 20 ms requirement.
Solution Approach 2:
The system uses feedback from the VAC line voltage zero-cross detection to adjust the trip decision timing. By monitoring the actual voltage waveform and detecting the zero-cross point, the system dynamically adjusts when to command the contactor opening, ensuring that the trip occurs at the most favorable time for achieving consistent and compliant opening times.
3Manufacturing precision
If a fast DC contactor with consistent opening time is used, then the trip time requirement is met, but the device complexity and cost increase
Solution Approach 1:
The patent changes the timing parameters of the trip decision by introducing a delay relative to the VAC line voltage zero-cross point. This parameter adjustment allows standard AC contactors to achieve consistent and compliant opening times, eliminating the need for specialized fast DC contactors and reducing device complexity and cost.
4Reliability
If continuous monitoring is implemented, then ground fault detection reliability improves, but the computational load and complexity increase
Solution Approach 1:
The patent extracts and eliminates odd harmonics from the monitoring signal using a half-wave DFT filter. By removing these harmonics, the system reduces noise and interference in the ground fault detection process, improving reliability while keeping the computational requirements manageable. The filter focuses only on the fundamental frequency components, reducing the complexity of the overall monitoring system.
Solution Approach 2:
The system changes the computational approach by using a half-wave DFT filter that processes only the necessary frequency components. This parameter change in the signal processing method allows continuous monitoring with improved reliability while maintaining acceptable computational load and system complexity.
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 reduces average contactor opening time, effectively meets UL943(C) trip time requirements, attenuates odd harmonics, and enables continuous non-destructive auto-monitoring without compromising ground fault response.
Implementation Method 1
A low-computational complexity half-wave DFT filter for harmonic attenuation
Implementation Method 2
a current transformer (CT) 110 having a window through which the conductor lines 102, 103, and 104 are passed and configured to generate a current proportional to a vector sum of primary currents flowing through the conductor lines 102, 103, and 104
Data Source
Figure 1
Figure 2
Figure 3A
AI summary
Devices herein include conductor lines (102-104) connected between a power supply (101) and load (106), each of the conductor lines coupled to an AC contactor (112) and a contactor control circuit (122), wherein the contactor control circuit is operable to open and close one or more contactors of the AC contactor. The devices may further include a current transformer (110) coupled to the conductor lines, the current transformer operable to output a secondary current corresponding to a primary current magnitude of an electrical current not flowing to the load, wherein the AC contactor is connected between the power supply and the load. Devices may further include a zero cross detection circuit (129) operable to generate an interrupt at each of a plurality of zero crossings for a microprocessor (133), and determine whether to open the one or more contactors of the AC contactor in a predetermined optimum interval calculated with respect to the zero crossing points.