Zone selective interlock for a circuit breaker system

Details Zone selective interlock for a circuit breaker system Zone selective interlock for a circuit breaker system

1999-11-05

2001-10-02

Sherry, Michael J. (Department: 2836)

Electricity: electrical systems and devices

Safety and protection of systems and devices

Feeder protection in distribution networks

C361S068000, C361S094000, C361S097000

Reexamination Certificate

active

06297939

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to electric powered trip units, such as circuit breakers and more particularly to a zone selective interlock system that cooperates with the electronic trip units.
BACKGROUND OF THE INVENTION
In a typical factory-power distribution system, power is generated by a power generation company and supplied to a factory and thereafter distributed around the factory to various equipment such as, for example, motors, welding machinery, computers, heaters, lighting, and the like.
Power distribution systems of this type are typically centrally located in switch gear rooms or substations. From there, power is divided up into branches such that each branch supplies power to a portion of the factory and/or specified loads. Frequently, transformers are disposed throughout the factory to step down the supply voltage to that required by specific pieces of equipment or portions of the factory. Therefore, a factory-power distribution system typically has a number of transformers servicing various types of equipment in various areas. Inherent with this, is the high cost of the power-distribution equipment such as transformers, as well as the cost of the equipment to which power is being supplied. Therefore, it is quite common to provide protective devices such as circuit breakers or fuses in at least each branch so that not only may each piece of equipment be protected but any problems associated with one piece of equipment does not ripple to adjacent or interconnected pieces of equipment. Further, providing fuses or circuit breakers in each branch can help minimize down time since specific loads may be energized or de-energized without affecting other loads thereby creating increased efficiencies, lower operating and manufacturing costs and the like.
Typically, when circuit breakers are utilized, they are used to detect more than just large overcurrent conditions caused by short circuit faults. In addition, they frequently detect lower level long-time overcurrent conditions and excessive ground currents. The simplest form of circuit breakers are thermally tripped as a result of heating caused by overcurrent conditions and, in this regard, are basically mechanical in nature. These mechanical-type breakers are incorporated into almost all circuit breakers, regardless of whether or not additional advanced circuitry is provided since they are extremely reliable over a long life cycle and provide a default trip-type level of protection.
Some types of circuit breakers utilize electronic circuitry to monitor the level of current passing through the branch circuits and to trip the breaker when the current exceeds a pre-defined maximum value. Electronic circuit breakers are adjustable so as to fit a particular load or condition by the end user without designing or specifying different breakers. Breakers of this type typically include a microcontroller coupled to one or more current sensors. The microcontroller continuously monitors the digitized current values using a curve which defines permissible time frames in which both low-level and high-level overcurrent conditions may exist. If an overcurrent condition is maintained for longer than its permissible time frame, the breaker is tripped.
Circuit breakers used in industrial settings and other settings using multiple circuit breakers are often arranged in a tree configuration having a plurality of levels or layers. The top of the tree is close to the power source and the base of the tree is coupled to a plurality of loads. Each circuit breaker at each different layer of the tree is coupled in series with one or more circuit breakers downstream from the power source. Utilizing such a tree configuration provides the difficulty that a circuit breaker multiple levels upstream from a load fault may be tripped when a load fault is detected thereby causing power outage to other loads, which are multiple levels downstream, that should otherwise be unaffected.
Therefore, there is a need for a system which selectively chooses which circuit breakers should be opened or tripped when a load fault or short circuit condition is detected. Also, there is a need for a circuit breaker system that allows for time delays in circuit breakers that are upstream from a load fault such that only those circuit breakers that are necessary to protect machinery coupled to the power distribution system will be tripped or opened.
SUMMARY OF THE INVENTION
One embodiment of the invention relates to a circuit breaker system. The circuit breaker system includes a plurality of circuit breakers, arranged in layers, each layer being coupled to the next layer in series and at least some of the circuit breakers including an interlock interface. The interlock interface selectively controls tripping of the circuit breaker that is coupled to the series circuit and is in the layer closest to a fault and meets a fault condition. The interlock interface selectively controls maintaining circuit breakers, coupled to the series circuit that are not in layers closest to the fault, in a closed state.
Another embodiment of the invention relates to an interlock interface configured to be coupled to a circuit having a plurality of circuit breakers arranged in layers, one layer being coupled in series to another layer. A first connection is configured to be coupled to a circuit breaker. An output is configured to provide an interlock signal. A first input is configured to be coupled to the output of another interlock interface on a different layer, and a second input is configured to be coupled to the output of another interlock interface on the same layer. The output provides an interlock signal if one of the following conditions occurs: the circuit breaker associated with the output detects a fault; the second input receives an interlock signal from another interlock interface; and the first input receives an interlock signal from another interlock interface. The interlock interfaces receiving interlock signals perform tripping delays.
Still another embodiment of the invention relates to a method of controlling a plurality of circuit breakers. The plurality of circuit breakers are arranged in layers, each layer being coupled to subsequent layers in series and the circuit breakers furthest downstream being coupled to a load. The method includes detecting a fault in a first circuit breaker that is coupled to a load. The method also includes delaying tripping the first circuit breaker until after a predetermined time has elapsed. Further, the method includes providing an interlock signal to any other interlock interfaces on the same layer as the first circuit breaker; and providing an interlock signal to interlock interfaces on the next upstream layer. Further still, the method includes delaying tripping a second circuit breaker on the next upstream layer until after a predetermined time has elapsed.


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