Electric service load limiter

Details Electric service load limiter Electric service load limiter

2000-03-14

2002-04-16

Tso, Edward H. (Department: 2838)

Electrical transmission or interconnection systems

Plural load circuit systems

Selectively connected or controlled load circuits

Reexamination Certificate

active

06373150

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to electric service load limiters, and more particularly, to an electric service load limiter for limiting the amount of electricity that is allowed to pass from a power line and through an electrical power leg supplying electricity to a consumer.
In the electric utility industry, household consumers pay for electric service according to the amount of energy consumed. Therefore, utility companies normally bill these customers in proportion to the total amount of electrical current drawn during a particular billing period. When such a customer becomes seriously delinquent in paying a bill, the utility company has the option of shutting off electric service until the bill is paid. While this usually is an effective incentive to cause a household consumer to make payments that are past due, it also presents potential regulatory problems. Local statutes often prohibit utilities from totally discontinuing service to the extent that a consumer is unable to operate essential appliances such as a furnace, a refrigerator, or a water pump. Regulations of this type typically prohibit discontinuing electrical service in the winter when operation of a furnace can be essential. For this reason, a utility company wishing to restrict a delinquent consumer's electricity consumption must be able to do so without seriously disrupting the consumer's essential electricity requirements.
Previous attempts to limit electrical consumption by delinquent household consumers have included mechanical circuit breaker-type load limiters. These limiters characteristically incorporate thermal or magnetically-triggered mechanisms that trip or automatically disconnect electrical power legs leading into a consumer's building. The purely mechanical functioning of these designs often means that their proper operation requires constant, non-fluctuating electrical, physical or environmental conditions. For example, a transient current, such as is typically present in a home electrical circuit during the start of a major appliance, is sometimes sufficient to cause a mechanical load limiter to automatically disconnect, even though the total combined draw of all working electrical devices on the circuit when they are at equilibrium is well under the load limiter's set point, or maximum allowed current draw. Excessive environmental heat can similarly affect the operation of thermally-triggered mechanical units. In a purely mechanical load limiter, the load limiter's set point is also susceptible to change as the mechanism continues to wear over time, reducing the unit's overall reliability.
To allow delinquent customers to continue drawing essential electricity requirements, mechanical service load limiters typically allow for customer resetting after automatic disconnection or tripping. A customer must manually restore service by resetting mechanical circuit breakers in the load limiters after an excessive, nonessential load has been removed. When a thermally actuated mechanical load limiter trips due to a customer's excessive current draw, it is often difficult or impossible for a customer to immediately reset the load limiter because the overheated breaker mechanism remains hot for awhile. Thus, the consumer's home may be without electricity for awhile. Additionally, due to their design, mechanical units are highly susceptible to customer tampering. For example, customers sometimes bypass a mechanical load limiter by simply taping or jamming a circuit breaker reset button into the reset position. In addition to rendering the unit inoperable, such tampering can also lead to overheating and damage, significantly reducing a load limiter's service life.
A further disadvantage of existing mechanical load limiters has been the lack of selectable set point current levels on a single unit. Since previous load limiters depend on specific mechanical conditions being met before tripping, they are normally limited to a single set point level. As a result, a utility company must purchase and stock separate units for each different set point level it uses. It follows that a different load limiter must be installed for each individual customer according to the customer's essential requirements. A utility company must also incur the added expense of installing a new unit when a new customer with different essential electric service requirements occupies the home of a previous customer or whenever an existing customer's essential electricity requirements change. This also further precludes the possibility of setting a customer's set point level remotely from a central location.
Previous attempts to limit electrical loads have also included the incorporation of solid state electronic monitoring and logic circuits for removing discrete loads from a customer's power circuit after consumption surpasses a set-point maximum. U.S. Patent to Hedges (U.S. Pat. No. 4,211,933) discloses a method and U.S. Patent to Leyde (U.S. Pat. No. 4,034,233) discloses an apparatus and method for automatically limiting total current in a power circuit to a set-point maximum by sequentially disconnecting individual loads from a power circuit until total current drawn ceases to surpass the set-point maximum. Individual loads remain disconnected only as long as total current flowing through the power circuit continues to surpass the set-point maximum. Thereafter, disconnected loads are automatically reconnected to the power circuit, and no manual resetting by the consumer is required. At no time is all current on the building's power circuit entirely disconnected from a utility power line. This substantially reduces a circuit's effectiveness as a negative incentive for a consumer to make past due utility payments. Moreover, such circuits require a consumer to preset the priority in which loads are disconnected to selectively determine which loads are first terminated during an overload condition. In the event that load priority is improperly set or programmed, such an arrangement could effectively serve to terminate essential services while failing to terminate nonessential services or electrical conveniences to the consumer, thereby achieving the opposite of the desired result.
Other solid state devices specifically limit the amount of current that may travel through a consumer's power circuit during a set interval. For example, U.S. Patent to Summe (U.S. Pat. No. 5,289,109) discloses a circuit including a current sensing resistor, a current-controlling P-channel field effect transistor and an NPN bipolar transistor. When the magnitude of a load current flowing through the circuit reaches a predetermined current limit value, the conduction of the current-controlling P-channel transistor is controlled to limit the load current flowing through the circuit. Current flowing through a power circuit is never permitted to exceed the set point maximum, regardless of the number of loads placed on a circuit. There is no trip mechanism and no means to automatically disconnect excessive individual loads. Thus, even in the event of an excessive total load, the design neither terminates nor automatically removes individual loads from the power circuit. While such devices may be suitable for imposing a strict limit on the amount of current that is permitted to be drawn through a power circuit, they are inherently unsuited for use as utility load limiters. In the absence of a trip or disconnect mechanism, such devices are alone incapable of signaling overload conditions. Rather than disconnecting loads in excess of a set-point maximum, such circuits merely distribute the limited current among the various loads that remain connected to the circuit, resulting in insufficient power to adequately maintain all connected loads. As a result, even essential appliances could be adversely affected, contrary to the desired effect of limiting the amount of power supplied to a consumer. Moreover, if a consumer becomes content with the substandard but b

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