Electricity: electrical systems and devices – Safety and protection of systems and devices – With specific quantity comparison means
Reexamination Certificate
1999-07-08
2001-05-29
Sherry, Michael J. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
With specific quantity comparison means
C361S091100
Reexamination Certificate
active
06239961
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to a method and apparatus for sensing a condition where voltage-sense leads of a voltage regulating power system are not properly connected to load terminals in an electric circuit, and in particular to a circuit for preventing a subsequent overvoltage condition on a voltage-regulated load.
BACKGROUND OF THE INVENTION
When electric current flows through a circuit, it encounters inherent electrical resistance or impedance as it passes through respective elements in the circuit. As established by Ohm's Law, the result of current moving through a circuit element is a voltage drop across that element. Often a circuit element can withstand only a maximum voltage drop, and may be damagedby an excessive or overvoltage condition.
Power systems that supply current or voltage to a circuit can be provided with overvoltage detectors. Such detectors can be used to sense voltage drops across circuit elements, or loads, that are at risk of encountering overvoltage conditions, but most look at voltage across terminals that are being voltage-regulated or current-regulated by the supply. Some power systems can regulate the voltage across load terminals, instead of power supply terminals, as away to achieve a more precise load voltage regulation. In order for load voltage regulation to be effective in these systems, voltage sense leads must be correctly connected to the appropriate load terminals before the load is energized. The overvoltage detector can then monitor voltage across the load terminals, preferably through the same sense leads, and if an overvoltage condition is sensed, i.e., a voltage that is greater than the threshold value, then the power system is shut off in order to prevent damaging the load. Unfortunately, if the load voltage sensors are not connected properly, for example if they are shorted together, an overvoltage condition can occur without the power system sensing it, resulting in damage to the load. Without supplemental protection, improperly connected sense leads will likely result in the load's voltage exceeding its regulation limits, and perhaps exceeding a maximum safe limit if that is within the compliance range of the power system, when it is energized. This is due to the normal behavior of gain limited negative feedback regulation employed by state of the art power systems that would act to produce greater supply terminal voltage in an attempt to increase the voltage across its shorted sense connections.
Some prior art systems with load voltage sensing capability provide supplemental overvoltage detection at the supply terminals in addition to basic detection at the load terminals. These schemes require a second voltage threshold that allows for load voltage, voltage drop along the distribution cables, component value tolerances, circuit noise and offset voltages. In some systems, the supplemental overvoltage threshold is programmable by an operator for greater precision, and in others may be fixed in hardware as determined by the system designer, providing a less precise supplemental protection.
Known prior art supplemental overvoltage protection schemes where thresholds are fixed in hardware can be incapable of providing load overvoltage protection when the load overvoltage limit is below the supplemental threshold. This condition can exist for circuits where the maximum expected cable voltage drop is significant relative to that of the load. Setting a threshold with hardware that allows both for maximum cable voltage drop and load voltage drop will also allow a supply terminal voltage to be present that could damage the load when a lesser actual cable voltage drop is present. One extreme case would exist for an unlikely condition of zero actual cable impedance. More realistic cable impedance would most likely fall between some minimum value and a maximum specified value. In this modality for ineffecfive supplemental protection in prior schemes, low cable impedance interferes with the supplemental circuit's effectiveness in protecting the load, and subsequently protection does not work if the load overvoltage limit is below the supplemental threshold less the actual total cable voltage drop. The length of the cable, size of the cable's conductor cross-section, type of conductor, temperature of the conductor, cable construction, and cable routing all contribute to determining the actual impedance of distribution cables intended for most system installations. Other modalities for failure of known prior art non-programmable protection schemes may also exist that are not described here.
For some loads, greater absolute precision may be required in enforcing a supplemental overvoltage threshold, directing design of those prior art supplemental schemes preferably toward requiring programming of the supplemental threshold value rather than having it fixed in hardware. Human programming error, if it occurs, would likely render the more precise supplemental overvoltage detection system ineffective.
Causes for failure of overvoltage load protection therefore can include human programming error, human connection error or hardware failure involving sense lead connections, and inability of known protection schemes to protect low voltage loads with possibly significant expected cable voltage drops between zero and the maximum safe load voltage.
BRIEF SUMMARY OF THE INVENTION
The inventor has found that a need exists for a detector to supplement prior art load overvoltage detection in circuits with load voltage regulating power systems, that yields fault indications in the event of a misconnection, or failed connection of the load voltage sense leads to the load terminals.
It is an object of the present invention to provide protection against an overvoltage condition across a load due to voltage supplied by a state-of-the-art load voltage or current regulating power system, for load sense lead fault conditions.
Another object of the present invention is to provide a fault detector that enables reliable protection against load overvoltage.
It is yet a further object of the present invention to provide an inventive method for sensing and responding to conditions that could produce load overvoltage.
These and other objects are achieved by providing a protection circuit that protects a load against overvoltage, the load being connected to a power source through cables, said circuit comprising a first voltage measurement circuit for measuring a total cable voltage drop across the cables; a second voltage measurement circuit for measuring current passing through the cables and for producing a measurement of voltage-limit for the cables based upon the measured current; and a controller for issuing a fault signal if the cable voltage drop measurement exceeds the total cable voltage-limit measurement by a predetermined amount.
These and other objects are also achieved by providing a protection circuit to provide protection of a load against overvoltage that results from sense lead faults, the load being connected to terminals of a power source through cables, said method comprising measuring a total voltage drop across the cables; measuring current passing through the cables; converting the measured current to a total cable voltage-limit measurement based upon maximum resistance of the cables; determining whether the cable voltage exceeds the total cable voltage limit by a predetermined amount, and if so, issuing a fault signal representing an overvoltage condition.
The subject invention may be implemented by measuring the voltage across power distribution cables in two different ways, and comparing the results. If one of the measurements is out of line with the other measurement, then the assumption can be made that a voltage sense lead is misconnected, justifying a shut off of the power source.
Further scope of applicability of the present invention will become apparent from a review of the detailed description and accompanying drawings. It should be understood that the description and examples, while ind
Elgar Electronics Corporation
Luce Forward Hamilton & Scripps LLP
Sherry Michael J.
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