Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Parameter related to the reproduction or fidelity of a...
Reexamination Certificate
1998-08-11
2001-04-10
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Parameter related to the reproduction or fidelity of a...
C324S076240, C307S106000
Reexamination Certificate
active
06215316
ABSTRACT:
RELATED APPLICATIONS
This application is related to patent application “Method for Quantifying Harmonic Pollution Sources” by Wilsun Xu, filed concurrently with the present application.
FIELD OF THE INVENTION
This invention relates generally to devices and methods for measuring harmonic pollution sources in electrical power distribution systems. More particularly, it relates to a method and apparatus for measuring the harmonic pollution generated in localized regions of a power distribution system.
BACKGROUND OF THE INVENTION
Alternating current networks and devices are typically designed to operate on sinusoidal 50 Hz or 60 Hz alternating voltages and currents. The voltage and current waveforms in modern power distribution systems are seldom sinusoidal, however, because of the increased use of nonlinear electrical loads such as variable speed motor drives. Such nonlinear loads draw a nonsinusoidal current from a sinusoidal voltage source. This results in a waveform distortion which propagates outward from the nonlinear electrical load into the network. The distorted waveform includes frequency components which are multiples of the original frequency of the electrical power (50 Hz or 60 Hz). These high frequency components are called harmonics. The harmonics are created by the nonlinear electrical load. In 60 Hz power distribution systems, harmonics at 300 HZ, 420 Hz, and 660 Hz and other odd-multiples of frequency are particularly significant. It is desirable to limit the production and propagation of harmonics because harmonics can disrupt and damage sensitive electrical devices drawing power from the power distribution system.
One problem with limiting the production of harmonics is that they are difficult to trace. Their source cannot easily be found and measured. Electric customers such as a factory create harmonic pollution which is spread by the distribution system to other customers. Other customers may experience problems due to the harmonics, but the source of the offending harmonics is difficult to identify. If harmonic current sources could be located and measured, harmonic polluters could be required to reduce their harmonic output to comply with specified limits.
There are many commercially available instruments designed to measure parameters of harmonic disturbances such as the magnitudes of the harmonic currents and voltages. Some such instruments measure the direction of harmonic energy flow, thereby helping to locate the harmonic current source. If the energy flows from the left to the right of the metering point, the harmonic power source is on the left, and vice versa. U.S. Pat. No. 4,667,152 to Hayes et. al., and U.S. Pat. No. 5,508,623 to Heydt et. al. operate according to this principle. This technique, however, has the following major shortcomings:
1) It only detects the net direction of harmonic energy flow. The method cannot determine if there are harmonic current sources on both sides of the metering point.
2) The method is qualitative. It is unable to measure the relative contributions of different harmonic current sources if the sources are located on opposite sides of the metering point.
3) The method detects direction based on net harmonic power. Intervening impedances can result in misleading information about the locations of the harmonic current sources. For example, it is possible for the right side harmonic current source to produce more harmonic energy, but for the net harmonic energy flow to be from left to right.
What is needed is a technique for measuring a harmonic energy flow which does not suffer from the above disadvantages and which can accurately identify producers of harmonic pollution.
OBJECTS AND ADVANTAGES OF THE INVENTION
Accordingly, it is a primary object of the present invention to provide a method and apparatus for characterizing and locating harmonic pollution sources that:
1) is able to quantitatively measure the strength of a single harmonic pollution source even when there are multiple harmonic current sources connected to the power distribution network; and
2) yields correct measurements of a harmonic current source when there exist unknown impedances between the measurement point and the harmonic current source.
These and other objects and advantages will be apparent upon reading the following description and accompanying drawings.
SUMMARY OF THE INVENTION
The above objects and advantages are attained by an apparatus for determining a characteristic of a harmonic pollution source of interest connected to a power line. The harmonic pollution source is connected to the power line at a connection point. The harmonic pollution source of interest and power line comprise part of a power distribution system. The apparatus has a harmonic perturbation device for changing the harmonic energy present in the power distribution system. The perturbation device can increase, decrease or change the phase of harmonic energy in the power distribution system. The apparatus also has a means for connecting the perturbation device to the distribution system, a means for measuring a harmonic voltage at the connection point, and a means for measuring a harmonic current flowing between the connection point and the harmonic current source. The measurements from the current and voltage meters with and without the perturbation of the harmonic energy in the power distribution system allow one to determine characteristics of the harmonic current source.
The harmonic perturbation device can generate harmonic current, absorb harmonic current, or change the phase of harmonic current at the frequency of interest. In the preferred embodiment, the perturbation device is a harmonic shunt connected in parallel with the harmonic pollution source of interest. Alternatively, the perturbation device is a frequency generator which generates harmonic current at the frequencies of interest.
Also preferably, the harmonic perturbation device is connected at the connection point between the power line and harmonic pollution source of interest.
The present apparatus is capable of determining many characteristics of the harmonic pollution source of interest. Examples of characteristics which can be determined include the magnitude of harmonic current produced by the harmonic pollution source of interest, the phase of the harmonic current, and an impedance associated with the harmonic pollution source of interest. Of course, the phase, current magnitude, and impedance are typically different at each harmonic frequency. The apparatus may include a computer to perform the calculations necessary to determine characteristics of the harmonic pollution source from the current and voltage measurements.
The present invention can also include a switch for selectively connecting between the different power lines in a 3-phase power system.
The present invention also includes an embodiment without a harmonic perturbation device. Instead, the apparatus has a means for monitoring the harmonic voltage and harmonic current and a means for storing the voltage and current measurements. This embodiment relies upon spontaneous temporal fluctuations in the harmonic pollution source (fluctuations in harmonic voltage or harmonic current). When a fluctuation is detected, voltage and current measurements before and after the fluctuation are used to calculate characteristics of the harmonic pollution source. A larger fluctuation results in a more accurate determination of the harmonic pollution source characteristics.
REFERENCES:
patent: 4439823 (1984-03-01), Gyugyi et al.
patent: 4667152 (1987-05-01), Hayes et al.
patent: 5351178 (1994-09-01), Brennen et al.
patent: 5355025 (1994-10-01), Moran et al.
patent: 5508623 (1996-04-01), Heydt et al.
patent: 5731965 (1998-03-01), Cheng et al.
Chazottes, B. et al., Estimating the true customers contribution to the power system harmonic disturbances, Source Unknown, 9 pages, EDF-DER Clamart-France. No month/year available.
Yang, H., Assessing the harmonic emission level from one particular customer, Source Unknown, 8 pages, University of Liege-Belg
Lumen Intellectual Property Services Inc.
Metjahic Safet
Nguyen Vincent Q.
The Governor of the University of Alberta
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