Communications: electrical – Condition responsive indicating system – Specific condition
Reexamination Certificate
2000-06-15
2001-06-12
Lieu, Julie (Department: 2736)
Communications: electrical
Condition responsive indicating system
Specific condition
C340S635000, C340S660000, C340S661000, C340S664000, C340S398100, C340S008100, C340S008100, C340S008100, C340S008100
Reexamination Certificate
active
06246332
ABSTRACT:
This application is a national stage entry of PCT/US98/24780 which has internation filing date Nov. 18, 1998
FIELD OF THE INVENTION
The present invention relates in general to the field of electronic energy meters. More particularly, the present invention relates to electronic energy meters with systems for detecting voltage and current imbalances in the electrical energy supply that is provided to the energy meter.
BACKGROUND OF THE INVENTION
The recent deregulation of the utility industry has created a market for products that facilitate the efficient distribution and monitoring of electrical power. In the past, utilities have built systems that did not provide certain information that is considered necessary to adequately monitor the electrical energy that is provided to and from a substation, or multiple substations, and related feeder networks and the like. For example, existing metering systems have been used to monitor the phase voltage or phase current for each phase of a multiphase utility service system to determine whether it is within an absolute limit (i.e., between an upper and a lower threshold). Predetermined actions, such as activating an alarm or a warning, are taken when a threshold is crossed. However, such monitoring does not identify situations where the phase voltages or currents are within individual phase limits but are not proportional to each other. In other words, the phase voltages/currents are not checked relative to each other. Therefore, there can be some imbalance, although each phase is within its absolute limit.
Imbalance can exist, for example, when two phases are at the top of their current or voltage range, and one is at the bottom of its range. For example, phases A and B may be at 110% of their nominal values, and phase C may be at 90% of its nominal value. Each phase may be within individual phase limits, but the voltages/currents supplied by each phase are not equal (e.g., there may be a problem with phase C). Imbalance is not desirable, and in many applications, it is desirable to maintain closely balanced phase voltages and phase currents. Thus, in prior art systems, there is no indication of a phase voltage or phase current problem as long as each phase is within its predetermined limits, although imbalance may exist.
One reason to monitor voltage and/or current imbalance is economic, e.g., imbalance will affect equipment and processes and can result in misoperation and damage of equipment, disruption of operations, and other such anomalies. Moreover, imbalance is prohibited within some systems (e.g., imbalance determination is mandatory in some countries, such as India). Furthermore, a consumer, such as a business, may now be able to choose its utility provider, and therefore the customer may now have a need or desire to determine the quality of the power supplied by its present supplier. Similarly, the utilities have a need to monitor the power they supply to customers to ensure that they are providing power that is sufficiently balanced to retain their customers. Therefore, utilities and consumers are now in need of a metering system to detect voltage and current imbalance.
Although the art of metering systems is well developed, there remain some problems inherent in this technology, particularly the ability to detect voltage and current imbalance in multiphase service. Therefore, a need exists for a system and method for detecting imbalance that overcomes the drawbacks of the prior art.
SUMMARY OF THE INVENTION
The present invention is directed to a method for detecting a current imbalance and/or a voltage imbalance in multiphase electrical energy provided to an energy meter via a service type. The method comprises the steps of measuring a phase value for each phase of the electrical energy; determining a normalized phase value for each phase; determining a high normalized phase value responsive to the measured phase values; determining a normalized phase value ratio responsive to the normalized phase values; comparing the high normalized phase value to a first threshold; comparing the normalized phase value ratio to a second threshold; and determining that an imbalance exists when the high normalized phase value is greater than the first threshold and the normalized phase value ratio is less than the second threshold.
According to one aspect of the present invention, an imbalance indicator is activated responsive to the imbalance.
In accordance with an aspect of the present invention, the step of determining a normalized phase value for each phase comprises the steps of determining a nominal phase value for each phase, and dividing the measured phase value by the nominal value.
In accordance with a further aspect of the present invention, the step of determining the nominal phase value comprises the step of retrieving the nominal phase value for the service type from a memory.
In accordance with a further aspect of the present invention, the invention further comprises the step of determining a low normalized phase value responsive to the measured phase values, and the step of determining the normalized phase value ratio comprises the step of dividing the low normalized phase value by the high normalized phase value.
Another embodiment within the scope of this invention includes a system for detecting a current imbalance and/or a voltage imbalance in multiphase electrical energy provided to an energy meter via a service type. The system comprises means for measuring a phase value for each phase of the electrical energy; means for determining a normalized phase value for each phase; means for determining a high normalized phase value responsive to the measured phase values; means for determining a normalized phase value ratio responsive to the normalized phase values; means for comparing the high normalized phase value to a first threshold; means for comparing the normalized phase value ratio to a second threshold; and means for determining that an imbalance exists when the high normalized phase value is greater than the first threshold and the normalized phase value ratio is less than the second threshold.
According to another aspect of the present invention, means are provided for activating an imbalance indicator responsive to the imbalance. The imbalance indicator preferably comprises at least one of an LCD display, an event log, and a relay output that can be attached to an option connector.
According to another aspect of the present invention, a memory is provided for storing the nominal phase values for the service type, the first threshold, and the second threshold. The means for determining the nominal phase value comprises means for retrieving the nominal phase value for the service type from the memory.
According to other aspects of the present invention, the memory comprises a non-volatile memory, the means for determining a normalized phase value for each phase comprises an integrated circuit, and the means for determining that the imbalance exists comprises a microcontroller. The integrated circuit preferably comprises at least one analog/digital (A/D) converter and a programmable digital signal processor (DSP).
The foregoing and other aspects of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.
REFERENCES:
patent: 4119926 (1978-10-01), Frosch et al.
patent: 4319298 (1982-03-01), Davis et al
ABB Power T&D Company Inc.
Lieu Julie
Woodcock Washburn Kurtz Mackiewicz & Norris
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