Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Polyphase
Reexamination Certificate
1998-07-06
2002-04-16
Nguyen, Vinh P. (Department: 2858)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Polyphase
C127S070000
Reexamination Certificate
active
06373238
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a three-phase electrical power measurement system. More specifically the present invention relates to a three-phase electrical power sub-metering system including three transformers where the measurement device is enclosed within a housing surrounding one of the transformers.
Electrical power is provided to many devices, such as large motors, by three separate cables, each of which supplies a single phase of three phase-power. In an ideal system, each of the phases within the respective cable has a phase angle which is generally 120 degrees apart from the other phases. Accordingly, the total power flowing through the three cables to a three-phase load (or from a three-phase generator) is:
p
(
t
)
3−phase
=V
p
I
p
*[cos(2wt+
a+&bgr;
)+cos(2wt+
a+&bgr;−
240)+cos(2wt+
a+&bgr;P−
240)+cos(2wt+
a+&bgr;−
480)]+3
*V
p
I
p
*cos&phgr;,
where V
p
and I
p
represent the root-mean-square values of the phase voltages and phase currents.
Although there exists numerous devices suitable to measure the power flowing through a single conductor, these devices are not suitable to measure power flowing through multiple conductors. For example, one device for measuring power through a single conductor is produced by Veris Industries, Inc. of Portland, Ore. Veris Industries, Inc. markets a single phase power measurement device under the name KT 6300 that includes a split core transformer that encircles a cable to sense the current flowing therein. The KT 6300 also includes multiple wire leads that are connected to the one or more cables to sense the voltage therein. A measuring circuit enclosed within the housing of the transformer calculates the power flowing through the cable. Unfortunately, the KT 6300 is not capable of measuring the power usage of three-phase power systems.
An electrical power utility measures the power usage of each of its customers using a power revenue meter (normally on the exterior of each customer's building). The power revenue meter electrically interconnects the secondary service of the utility with the primary service of the customer. The electrical power used by the customer is measured by the power revenue meter and the customer is billed periodically.
The power revenue meter is normally a glass meter with a spinning disc that rotates proportionally to power usage. To install such a power revenue meter in new construction the customer routes a first three-phase cable (three separate conductors, each of which carries a single phase) from a customer's power box to a power revenue meter base for power returned from the customer to the utility. The power box is normally located within a customer's building and encloses a panel with circuit breakers for distribution of the electrical power to different electrical loads of the customer. Such loads may, for example, include lighting, motors, air conditioning systems, and pumps. A power revenue meter base is installed on the exterior of the building and the first cable is connected thereto. A second three-phase cable is connected to and routed from the power revenue meter base to the power box for supplying power used by the customer. After the two three-phase cables are properly installed, a power revenue meter is installed in the power revenue meter base. The installation of the three-phase cables and the power revenue meter (including its base) is labor intensive and incurs substantial expense.
The expense associated with installing the power revenue meter in new construction is normally included in the total construction cost.
Installation of the power revenue meter in an existing building is substantially more expensive than installation of the power revenue meter in new construction. In existing buildings, at least one hole needs to be drilled though the wall and conduit routed between the power box on the interior of the building and the power revenue meter on the exterior of the building. Also, suitable interior wall space must be located to mount the power box in a location near the power revenue meter. If suitable wall space is not available nearby then an excessive length of conduit must be installed or devices located on the nearby suitable portion of the wall need to be relocated, both of which are time intensive and expensive.
There are numerous occasions in which a customer may wish to install additional power revenue meters. For example, customers may wish to monitor power usage of particular loads using additional power revenue meters. Many companies desire to allocate their electrical power usage based on power usage by individual departments. By using multiple power revenue meters the expense for electrical power usage can be allocated and monitored at the department level. In this manner each department is responsible for payment of their own electrical power usage.
Shopping malls and marinas are examples of customers that often need to install additional power revenue meters for tenant sub-metering. In these cases, each tenant's individual power usage is individually billed to that particular tenant, as opposed to merely guessing what portion of the total power usage is attributable to each tenant. Tenant sub-metering is important when there are significant differences between the amounts of power usage by different tenants.
Another example of a customer that may need to install additional power revenue meters are universities or other multiple building institutions that desire to determine where electrical power is being wasted because many buildings have antiquated electrical systems. Monitoring power usage on an individual building basis permits the customer to renovate those portions of the institution where the resulting cost savings will pay for, at least in part, the renovations.
Sometimes multiple power revenue meters are used to isolate use of particular systems. For example, cooling systems use a substantial amount of electrical power so there is a need for installing additional power revenue meters for optimizing the cooling systems to reduce electrical power usage. When redesigning cooling systems and adjusting electrical usage of different portions of existing systems there is a trade off between the electrical power consumed by the pumps which vary the fluid flow and the electrical power consumed by the fans. A proper balance between the electrical power usage of the pumps and fans may reduce the overall power usage.
Unfortunately for most systems, such as departmental billing, tenant sub-metering, multiple building institutions, and cooling systems, the expense associated with installing additional power revenue meters does not outweigh the potential benefits to be derived therefrom.
In contrast to installing additional utility power revenue meters, a power sub-metering system may be used to provide sub-metering capability. Sub-metering involves measuring the power delivered from a customer's power box to a particular device. As opposed to installation of additional power revenue meters that are monitored by the utility company, a customer using sub-metering receives a single bill from the utility for each power revenue meter but is able to allocate the utility bill from each power revenue meter among its different uses using information provided by the sub-metering system.
A sub-metering system generally includes a separate transformer installed on each respective cable of the three cables of a three-phase system within a customer's power box containing the electrical panel. Each transformer senses a changing current within a respective cable and produces an output voltage or current proportional to the changing current. A measuring circuit is electrically connected to the three transformers and receives each of the transformer output voltages or currents. The measuring circuit is also electrically connected to the three cables by voltage “taps” to measure the voltage therein. The voltage “tap” measu
Lewis Jim
Stoffregen Paul
Kevin L. Russell of Chernoff, Vilhauer, McClung & Stenzel, LLP
Nguyen Vinh P.
Veris Industries, LLC
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