Frequency-selective circuit protection arrangements

Details Frequency-selective circuit protection arrangements Frequency-selective circuit protection arrangements

2000-03-28

2002-08-20

Huynh, Kim (Department: 2836)

Electricity: electrical systems and devices

Safety and protection of systems and devices

Ground fault protection

C361S113000

Reexamination Certificate

active

06437955

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to electrical circuit protection.
2. Introduction to the Invention
Ground fault interrupters (GFIs) are widely used to provide protection from the harmful effects which can result when there is leakage from an electrical circuit to ground. GFIs compare the current flowing at two different locations in a circuit, and interrupt the circuit if the currents differ by more than a predetermined value, e.g. as the result of a ground fault between the locations. GFIs do not, however, protect against faults which do not result in such a current imbalance, e.g. an overcurrent resulting from a short within the load, an overvoltage arising from lighting, electrostatic discharge, switching of reactive loads, etc., or a current or voltage having a frequency different from, e.g. higher than, the excitation frequency (e.g., 50 Hz or 60 Hz) of the power source of the circuit.
There are many circumstances in which a circuit may be subject to a current or voltage which has a frequency other than the excitation frequency and which is potentially harmful. For example, devices such as motors, phase controllers, non-linear loads and DC power supplies can inject or reflect currents at harmonics (i.e., multiples) of the excitation frequency. Other devices, such as inverters, non-linear loads and switching power supplies can cause currents at high frequencies which are not necessarily harmonics of the excitation frequency. In some circumstances it is desirable to protect a load from potential harmful effects of such high or low frequency currents. In some circumstances it is desirable to protect the power source, particularly, for example, from harmonics generated by reactive loads which result in a reduction of the power factor.
In addition, the presence of even modest currents at high frequencies can be indicative of an unusual condition which should be corrected. For example, some self regulating heating cables containing conductive polymers, if improperly installed or operated, can generate arcs which have a broad frequency spectrum.
Therefore, there is a need to protect electrical circuits from currents or voltages at frequencies different from the excitation frequencies of the circuits.
SUMMARY OF THE INVENTION
We have been investigating the use of GFIs in arrangements which provide overcurrent and/or overvoltage protection in electrical circuits in addition to protection from ground faults. A number of such arrangements are disclosed in the earlier applications incorporated by reference herein. We have discovered, in accordance with the present invention, that very useful frequency-selective protection systems can be produced using GFIs. Such frequency-selective systems can be configured to protect from currents or voltages at frequencies above a selected frequency (referred to in this specification as high-pass), and/or from currents or voltages at frequencies below a selected frequency (refereed to in this specification as low-pass) and/or from currents or voltages at frequencies within a selected frequency range (referred to in this specification as band-pass). As used in this specification: (i) “frequency-selective protection” refers to protection from currents and/or voltages in one or more selected high-pass, low-pass or band-pass ranges; and (ii) “frequency-selective current” and “frequency-selective voltage” refer respectively to a current or voltage having a frequency in one or more selected high-pass, low-pass or band-pass ranges.
As used herein, the frequency response of a GFI circuit refers to the response of the GFI circuit to a current imbalance as a function of frequency, as compared with the response of the GFI circuit to a current imbalance at the excitation frequency of the power supply. As used herein, the frequency response range of a GFI circuit refers to the frequency range, above and/or below the excitation frequency of the power supply, within which the GFI circuit will trip on a specified current imbalance level. As used herein, a specified current imbalance level refers to a current imbalance level at which it is intended for a particular frequency-selective protection arrangement to cause the GFI circuit in that arrangement to trip.
In one embodiment of the invention, frequency-selective current protection is provided by connecting a control element in series with the line path (or return path) of a GFI, and a bypass element in parallel with the combination of the control element and the line path (or return path) of the GFI. Under normal current conditions, little or no current flows through the bypass element. However, the frequency response characteristics of the control and bypass elements are such that a frequency-selective component (i.e. high-pass, low-pass or band-pass) of the current in the circuit is diverted through the bypass element. Therefore, when the magnitude of such a frequency-selective component reaches a predetermined value, a resulting current imbalance in the GFI causes the GFI to trip. Circuit protection systems in accordance with this first embodiment are particularly useful to protect against currents at frequencies which are within the frequency response range of typical GFI circuits, e.g. as high as 1 kHz, preferably as high as 10 kHz, especially as high as 25 kHz This facilitates the use of GFI circuits, without modification, in systems of the invention.
In a second embodiment of the invention, frequency-selective voltage protection is provided by connecting a bypass element: 1) from the line path input, through a GFI transformer, to the return path output of the GFI; 2) from the line path input to the return path input of a GFI; 3) from the line path output through a GFI transformer, to the return path input of the GFI; or 4) from the line path output to the return path output of a GFI. Under normal conditions, little or no current flows through the bypass element However, when a frequency-selective voltage above a predetermined voltage value is present across the bypass element, an increased current passes through the bypass element creating a current imbalance in the GFI and causing the GFI to trip. Circuit protection systems in accordance with this second embodiment are particularly useful to protect against voltages at frequencies which are within the frequency response range of typical GFI circuits, again facilitating the use of GFI circuits, without modification, in systems of the invention.
In a third embodiment of the invention, frequency-selective current protection is provided by connecting (i) a bypass element: 1) from the line path input, through a GFI transformer, to the return path output of the GFI; 2) from the line path input to the return path input of a GFI; 3) from the line path output through a GFI transformer, to the return path input of the GFI; or 4) from the line path output to the return path output of a GFI; and (ii) a control element in the line (or return line) of the circuit. Under normal conditions, little or no current flows through the bypass element. However, when a frequency-selective current above a predetermined current value is present, the control element causes the bypass element to pass a portion of the circuit current, thereby creating a current imbalance in the GFI and causing the GFI to trip. Circuit protection systems in accordance with this third embodiment can protect against currents at frequencies within the frequency response range of typical GFI circuits. In addition, however, they are also useful to protect. against currents at frequencies which are outside the frequency response range of typical GFI circuits, again facilitating the use of GFI circuits, without modification, in systems of the invention. This feature is facilitated by the use of a control element which is capable of detecting a current at a frequency outside the frequency response range of the GFI circuit When the control element detects a current within a frequency-selective range, and above a predetermined current value, the control eleme

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