Electricity: electrical systems and devices – Safety and protection of systems and devices – Load shunting by fault responsive means
Reexamination Certificate
1999-06-02
2001-05-01
Sherry, Michael J. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
Load shunting by fault responsive means
C361S111000, C361S054000
Reexamination Certificate
active
06226162
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to apparatus for protecting a facility and its loads from surges in supply voltage, and more particularly, to a surge suppression network responsive to a rate of change in supply voltage indicative of a surge to clamp the voltage across the load to a specified voltage level.
2. Background Information
Various devices and arrangements are known for protecting loads from surges in supply voltage. Generally, two approaches are used: Series and parallel protection. In series protection, a high impedance is used in series with the load during a surge to block or limit surge current. In parallel protection, the surge current is diverted with a low impedance shunt. The present invention provides parallel protection.
It is known to use voltage clamping devices and crowbar devices to provide parallel surge protection. The voltage clamping devices clamp the voltage across the load to a specified level. A common type of clamping device is the metal oxide varistor (MOV). If used alone to provide parallel protection, the MOV must have a clamping voltage that is substantially above the nominal supply voltage so that its maximum continuous operating voltage (MCOV) is above the nominal supply voltage. Known crowbar devices include gas discharge tubes and thyristors. These devices normally have a high impedance and switch to a low impedance when a surge in the supply voltage exceeds the breakdown voltage of the gas in the case of the gas discharge tube, or is high enough to activate a trigger circuit to turn the thyristor on. A major difficulty with gas discharge tubes is that they are difficult to turn off even when the voltage goes through zero. An inherent limitation of the crowbar devices when used alone is that the low impedance that they switch into parallel with the load, depresses the load voltage below nominal.
U.S. Pat. No. 4,912,589 suggests a surge suppression network in which a clamping device such as a MOV is connected in series with a crowbar device in the form of a gas tube or a trigger device such as a silicon controlled rectifier (SCR) or a TRIAC. A triggering circuit in parallel with the clamping device switches the crowbar device when a surge exceeds a breakdown or trigger voltage. Thus, this network is responsive to the amplitude of the voltage surge.
U.S. Pat. No. 5,621,599 discloses a parallel protection circuit described as a “switched MOV device.” In this network, an MOV is connected in series with a TRIAC across the load. The TRIAC is triggered on by a voltage applied to the gate by a resistor network so that again, the network is voltage responsive.
A limitation of these known protection circuits utilizing crowbar devices, in addition to the difficulty in turning them off, is that they are relatively slow to respond. Also, the known parallel surge suppression networks which respond to voltage are intentionally triggered by harmonics and/or temporary over-voltage disturbances.
There is a need for an improved surge suppression network. This includes a need for a surge suppression network that does not respond to harmonics or temporary over-voltage disturbances.
There is also a need for an improved surge suppression network incorporating MOVs as clamping devices which does not require an MCOV well above the nominal supply voltage.
SUMMARY OF THE INVENTION
These needs and other needs are satisfied by the invention which is directed to a surge suppression network responsive to the rate of change of the amplitude rather than the amplitude of the supply voltage. This improved surge suppression network being immune to line voltage, is not triggered by electrical noise, harmonic and/or temporary overvoltage disturbances. It is ON only when a high dV/dt transient disturbance is present. In the absence of these transient disturbances, the network hibernates.
The improved surge suppression network includes: a voltage clamping means for limiting voltage thereacross to a specified clamping voltage; a crowbar means switchable from a current blocking off state to a current conducting on state in response to a trigger signal; and a trigger means connected to the supply voltage and the crowbar means to generate the trigger signal for the crowbar means in response to a specified rate of change in amplitude of the supply voltage. The crowbar means and the voltage clamping means are connected in series across the ac mains and in parallel with the facility power distribution system and load. The trigger signal switches the crowbar means to the on and off state. The crowbar means is selected from a group comprising a TRIAC, a silicon controlled rectifier (SCR), and a four-terminal thyristor. Preferably, the trigger means is a series resistor, capacitor circuit connected to the gate electrode of the crowbar means. Preferably, the voltage clamping means is selected from a group comprising a metal oxide varistor, a zener diode and a silicon avalanche diode.
For heavier duty applications, the voltage clamping means comprises a plurality of voltage clamping devices connected in parallel and the crowbar means similarly comprise a plurality of crowbar devices connected in parallel. A common RC trigger circuit is provided for all of the crowbar devices.
Where the crowbar means, such as a TRIAC, exhibits a faster turn-on characteristic in one direction, and in the case of unipolar crowbar devices such as SCRs, the surge suppression network can comprise a positive subnetwork and an anti-parallel negative subnetwork each having clamping means, crowbar means, and an associated trigger means providing a trigger signal to the crowbar means in response to the specified rate of change in the supply voltage. The positive and negative subnetworks can also have a plurality of parallel connected clamping devices, and parallel connected crowbar devices in series with the plurality of clamping devices, and each subnetwork can have its own common trigger circuit.
In the embodiments of the invention having a plurality of crowbar devices, it is preferred to have impedances that have matched inductance and minimum tolerance in series with the crowbar devices to help to equalize the current through the parallel devices. These impedances preferably introduce some inductance into series with the crowbar devices to assist in equalization of current. In an exemplary embodiment of the invention, these impedances are fuses or fuse resistors which provide some series inductance. The voltage clamping devices are shunted by a resistor having a much lower resistance than the clamping devices to reduce the voltage across these devices. This permits the surge suppression network to be used over the full range of voltages (e.g., 100 to 600 VAC) typically utilized in low voltage systems.
For multiphase supply voltages, associated voltage clamping means in series with crowbar means and trigger means for the crowbar means are provided for each of the phases of the supply voltage. Where the supply voltage also includes a neutral and a ground, associated voltage clamping means in series with crowbar means and trigger means responsive to the rate of change of voltage are also provided between the neutral and ground. As in the case of the single phase systems, these multiphase systems can have pluralities of voltage clamping devices and crowbar devices served by a common trigger circuit for each phase, and for neutral to ground if provided.
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Funke James
Ha Chi Thuong
Kladar Dalibor
Mendoza Anthony Cernan
Eaton Corporation
Huynh Kim
Moran Martin J.
Sherry Michael J.
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