Electricity: electrical systems and devices – Safety and protection of systems and devices – With specific current responsive fault sensor
Reexamination Certificate
2000-03-15
2001-04-24
Sherry, Michael J. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
With specific current responsive fault sensor
C361S062000, C361S093800, C700S292000
Reexamination Certificate
active
06222714
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to protective relays used in electric power system applications and, in particular, to the automatic and/or semi-automatic control of these protective relays.
As shown in
FIG. 1
, in the transmission and distribution of electrical power within an electrical power system operated by an electric utility, there exists multiple substations (A, B, C, D) which control the switching and distribution of electric power from a generating source (main dispatch center) to various distribution points and loads within the power system. These substations normally include a plurality of power transformers whose primary windings are connected to input lines and whose secondary windings are connected to output feeders/distribution lines. The substations also include a plurality of circuit breakers which are coupled to the incoming transmission lines, the power transformers and the outgoing feeders to switch (and hence protect) the respective lines and/or piece of equipment when a fault (e.g., an overload) condition is sensed. The switching (opening and closing) of the circuit breakers within a substation is controlled by protective relays which, by controlling the opening and closure of the circuit breakers, protect the incoming transmission lines, the power transformers, the outgoing feeders and the circuit breakers contained within the substation. The protective relays may be of various configurations, including electromechanical, non-programmable static, and programmable microprocessor based relays. Each protective relay is designed such that when it senses, or responds to, a fault condition, it causes its associated circuit breaker(s) to interrupt the power to, or out of, the device the relay is designed to protect.
In general, the protective relays are designed to sense and/or respond to a fault condition based on a plurality of settings maintained in, or applied to, each relay. Those protective relays which include a microprocessor/controller normally also include memory storage in which these settings are stored. The stored settings may be compared to various signals and preset conditions to enable a relay to sense the presence of faults based on these different signals and preset conditions. Further, these microprocessor based relays have means for selecting one of the stored setting groups when a signal is presented.
Typically, the power ratings of pieces of equipment such as the power transformers, the power distribution lines and the circuit breakers change as a function of temperature. By way of example, a particular power transformer may be rated to safely carry 15,000 amperes at minus (−) 35 degrees centigrade and only 10,000 amperes at plus (+) 85 degrees centigrade. In general, the resistance of the wiring and material used to form the transformers and distribution lines increases with increasing temperature resulting in a decrease in the power rating of these pieces of equipment with increasing temperature. For the example of the particular power transformer, above, a protective relay associated therewith would be, for example, set to “trip” an associated circuit breaker when the current through the transformer would exceed 12-to-13,000 amperes in the winter and 8-to-9,000 amperes in the summer. Therefore, it is a normal operating practice for electric power utilities to change the settings of protective relays at certain times during the course of a year to take into account different seasonal ambient conditions such as temperature. It is also accepted practice in the industry to send a highly skilled technician to each substation to reprogram each protective relay or manually set a new setting group via a rotary switch in the substation. Because of the number of substations, the distance between them, and the time to re-set and test the various protective relays, this is both expensive and impractical.
Also, on a particular day, the ambient temperature in a substation can be substantially higher (or lower) than expected, which either leaves the settings of the various protection relays too sensitive (e.g., too low) or not sensitive enough (e.g., too high). This can lead to mis-operation of these protection devices which results in unwanted blackouts, or to a catastrophic failure of a power transformer or circuit breaker.
Thus, the fixed settings applied to the relays does not solve the problem resulting from the fact that the temperature within the substation can vary over a wide range at any instant of time. This gives rise to a basic problem since the settings of the relays within the substation and their set points is based on preset or preprogrammed information and not on the actual value of the temperature and load conditions. Because of the significant change in the power rating of the transformers and circuit breakers (and other pieces of equipment) as a function of temperature, undervoltage and overvoltage conditions can occur. Also, either there may be a failure to recognize a fault condition or a false error condition may be sensed resulting in unnecessary disruption of power to customers.
SUMMARY OF THE INVENTION
Applicant's invention resides, in part, in the recognition that temperature changes within a substation can occur relatively quickly and unpredictably, whereby even where the relay settings are changed seasonally, they may not be appropriately set at any particular point in time. Therefore, it is highly desirable to continuously sense and monitor ambient conditions, such as temperature, within each substation and to continuously and automatically operate (open and close) the circuit breakers designed to protect the power distribution system. Where protective relays are used to “trip” (open and close) the circuit breakers, a microprocessor (microcontroller) may be used to continuously and automatically signal and control the protective relays for the temperature conditions currently existing within the substation, as well as the load conditions.
A substation containing power transformers and other pieces of equipment for distributing power to outgoing feeders may be subjected to a wide range of ambient and temperature conditions which affect the power transformers and the distribution equipment. Circuit breakers to protect the transformers and the other pieces of equipment are coupled to various points of the transformers and the pieces of equipment. The circuit breakers are operated by means of protective relays which are designed to sense and respond to various fault conditions. In systems embodying the invention, certain of the protective relays and their operation are made responsive to the ambient temperature of the substation.
In a particular embodiment, a temperature probe sensing the temperature of the substation is coupled to the input of a microcontroller whose output controls the settings of protective relays coupled to circuit breakers which, in turn, control the distribution of electric power into and out of the substation.
Protective relays for use in practicing the invention may be controlled by microprocessors and may be used in distribution substations for the purpose of determining when to change a preprogrammed group of settings stored in these protective relays and their associated microprocessors. Typical microprocessor based protective relays include unit and line protection devices with multiple setting groups. This includes, but is not limited to, transformer protection, transmission line protection, and feeder protection relays.
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patent: 582265 (1897-05-01), Campbell
patent: 4972290 (1990-11-01), Sun et al.
patent: 5181026 (1993-01-01), Granville
patent: 5224011 (1993-06-01), Yalla et al.
patent: 5513061 (1996-04-01), Gelbien et al.
patent: 5644463 (1997-07-01), El-Sharkawi et al.
patent: 5822165 (1998-10-01), Moran
patent: 6014304 (2000-01-01), Burnus et al.
Huynh Kim
Schanzer Henry I.
Sherry Michael J.
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