Patent Application
20070063664
1. Field of the Invention
This application is in the field of remote phase identification, which is often required in three phase power distribution systems. Remote phase identification is used in balancing loads in power distribution systems and in correctly repairing systems. Remote phase identification is preferable to line tracing to determine which phase is at a given point in a distribution system. More particularly, this invention provides a more accurate measure of phase at a field unit and at a reference unit than any available commercial devices.
It is important to accurately identify each phase of a three-phase power distribution point to enable interconnection and reconnection of power lines when the path from the electrical generating station to the distribution point has passed through regions where it is impossible to physically trace each phase. This situation arises when power lines go underground, pass through transformers, or otherwise pass through regions where inadequate documentation of the connections exist. Noting that the speed of light introduces phase shifts of 1 ms in 300 kilometers, and that at 60 Hz, the phase shift between phases is 5.53 ms, it is critical that any phase measurement be much more accurate that half of that value or 2.76 ms, and have the additional ability to compensate for speed-of-light effects when the comparative reference and field unit are separated by a significant distance.
The apparatus used to determine phase must make electrical connection to both very high and low voltages. In order to extract all of the information from a sine wave, it is necessary to have a complete wave which is not cut off at the top and bottom. This requires a variable gain sensor detector, which can sense widely different line voltages and always produce a sine wave which contains all of the information. This requires a sine wave whose amplitude does not saturate voltage detector circuitry. The simplest connection is to couple through a capacitor to one of the three conductors under test, but any capacitive coupling exhibits much lower impedance to high frequencies than to low frequencies. Thus, systems using this configuration will couple transients and noise much more efficiently than the underlying 60 Hz power line frequency. This results in processing a “noisy” signal to determine the phase. All previous phase identification inventions use the so-called “zero-crossing” of the capacitively coupled noisy signal to determine the phase reference point. Those methods require measuring the absolute time delay between the point being measured and a reference zero-crossing time established at a point on the utility grid where the phase is known. This is usually accomplished using a GPS signal as the time reference. However, a zero crossing reference can provide inaccurate timing due to high-frequency transients and noise that can further cause spurious or multiple zero crossings per cycle. This introduces uncertainty in the zero crossing detection that can lead to incorrect phase identification. The problem arises because only a small portion of the captured noisy signal is used, and, in fact, only the voltages within a few dozen microseconds of the zero crossing are used while the rest of the signal is discarded. It is well known that to extract the maximum useful information from a noisy signal, as much of that signal as possible must be used, averaged, and filtered.
2. Description of the Related Art
Various zero crossing methods of phase identification are known in the art. U.S. Pat. No. 7,031,859, U.S. Pat. Nos. 6,667,610, and 6,642,700 each describe a method of phase identification which relies upon measuring the absolute time delay between the point being measured and a reference zero-crossing time established at a point on the utility grid. In these cases, a GPS signal or another very accurate time is used to provide a time reference for simultaneously measuring a field phase and reference phase.
It has also been known in the art to use phase measurement to determine power line phase. The following publications are examples, however, none of these have a feature of automatic gain control, which assures non-saturation:
Department of Energy WAMS Technology Evaluation and Demonstration, pp. 7-5 through 7-11 and 8-12 and 9-8, Jan. 27, 2001
1993 IEEE International Frequency Control Symposium Precise Timing in Electric Power Systems, Kenneth E. Martin, Bonneville Power Administration, pp. 15-22
IEEE Transactions on Power Delivery, IEEE Standard for Synchrophasers for Power Systems, K. E. Martin, et al., January 1998, Vol. 13, No. 1, pp. 73-77.
Power line phase measurement using Fourier transform techniques is also found in U.S. Pat. No. 6,236,949 to Ronald G. Hart which is for current sensors and which is entitled “Digital Sensor Apparatus and System for Protection, Control and Management of Electricity Distribution Systems.”
In this invention Applicant in the field unit and reference unit utilizes discrete Fourier transform analysis to compute Fourier transforms of phase values. In order to provide accurate data for the computation of the phase values, it is necessary to capture all of the phase information available in a sine wave, which represents a voltage which has been sensed and detected. The magnitude of the detected sine wave is not important. It is necessary to adjust the voltage to a voltage detector which is a digitizer. If the voltage to the detector is above the saturation level of the circuitry, data will be lost. The loss of data is caused by the cutting off of the top and bottom of a sine wave presented to the voltage detector. In order prevent this condition, the voltage presented to the voltage detector must be reduced to a level where the digitizing circuits are not saturated.
In this invention, a hot stick is used to sense voltage on a power line. The hot stick is a long pole which can be held by a lineman on the ground and which can hold a sensor, detector, and RF transmitter on its end. Power lines, however, vary widely in the voltage present, and it is, therefore, necessary to adjust the voltage to the digitizer on the hot stick in order to prevent saturation no matter what the power line voltage may be. In addition, capacitive coupling varies with the relative humidity, requiring an adaptive circuit to accommodate the variances.
This invention provides an electronics system that can accommodate widely varying signal levels without “saturation” in order to use all the available data contained in a capacitively coupled AC signal. If saturation occurs, the information in the waveform will be lost. To meet this goal, the Applicants have invented a two-stage automatic gain control for the hot stick that uses a microprocessor to switch the gain-determining elements of an adjustable gain amplifier for coarse gain switching, and a fully fed-back integrator and mixer that makes fine, continuous gain changes. This system works as follows:
1. Gain initializes at maximum and a “precision rectifier” circuit rectifies the amplified output. This output is then fed to an integrator that “averages” the rectified signal for a time long compared to the period of the waveform. The resulting DC signal is used to determine whether gain adjustments need to be made. If so, then the gain is switched by a discrete amount by the microprocessor, for example, reduced by a factor of ten. If the precision rectifier signal is now below saturation then further discrete gain switching is terminated by the microprocessor. If the circuits are still saturated, gain is again reduced in this manner.
2. Once the gain is within about a factor of ten of the desired gain, the precision rectifier output voltage is multiplied with the signal voltage via a mixer to provide continuous fed-back gain control. This process is independent of the microprocessor. Gain is adjusted until the precision rectifier signal equals a user-selectable value, indicating correct gain. The response time of this feedback loop is made to be much longer than the period of the waveform.
3. Once the gain is optimum, as detected by the microprocessor, digitization of the amplified signal is initiated. Using a sampling rate of 10 kHz provides adequate over-sampling to ensure accurate reproduction of the 60 Hz component of the waveform. All the previous analog processes for amplification of the signal should be bandwidth limited to the Nyquist frequency of the digitizer. For example, if the digitizer operates at 3.6 kilo-samples/second then all the electronics should have an upper frequency pass band of about 1.8 kHz. By implementing such a pass-band in the analog gain amplifiers, no information is lost at 60 Hz.
4. Transmitting the digitized data from the hot stick transmitter to the main computational package located at the field unit requires care due to the high voltages involved. One method is to use an FM modulated RF link. Most commercially available links of reasonable cost are bandwidth limited to above 20 Hz. This is inadequate because a 22 Hz lower limit will shift the phase measurably at 60 Hz. Therefore, the Applicants have implemented a pulse-width-modulated system whereby digitization of the analog signal is accomplished with a microprocessor that generates a pulse-width-modulated digital signal which is transmitted over a standard RF link, and which is easily reconstructed by the receiver at the field unit.
This invention provides an apparatus for detecting power line AC voltage comprises in combination, a capacitor voltage sensor having an output proportional to a power line voltage, a digitizer voltage detector, an automatic gain control for adjusting voltage input to the voltage detector to a level which prevents saturation of the voltage detector such that all available data in the AC voltage is detected.
The apparatus for detecting power line voltage also comprises a gain control, an adjustable gain amplifier which is connected to said voltage sensor, a rectifier circuit connected to said adjustable gain amplifier which rectifies an output signal of said amplifier, a CPU connected to the output of the rectifier circuit which determines if the rectifier output signal is above saturation, and a CPU that provides a discrete gain adjustment signal to the adjustable gain amplifier when the averaged rectifier output is above a saturation level.
The apparatus further comprises an amplifier which is connected to said voltage sensor, said amplifier having an output, an analog multiplier connected to said amplifier output, a rectifier circuit connected to an output of said analog multiplier and which rectifies an output of said analog multiplier, an integrator connected to an output of the rectifier circuit, wherein the integrator averages the rectifier output signal, and wherein the integrator has an output, a CPU connected to the output of the integrator circuit which determines if the rectifier output signal is above saturation, and a CPU that provides a discrete gain adjustment signal to the adjustable gain amplifier when the integrator output signal is above a saturation level.
The apparatus further comprises an amplifier which is connected to said voltage sensor, said amplifier having an output, a rectifier circuit connected to an output of said analog multiplier and which rectifies an output of said analog multiplier, an integrator connected to an output of the rectifier circuit, wherein the integrator averages the rectifier output signal, and wherein the integrator has an output, wherein the integrator output is connected to an input of the analog multiplier, and wherein the analog multiplier multiplies a voltage from said amplifier by said integrator output and provides an input to said voltage detector.
A system for measuring phase angle difference between two conductors comprises in combination a hot stick having a voltage sensor having an output proportional to a power line voltage, a voltage detector which is a first digitizer for digitizing of the voltage signal, an automatic gain control for adjusting voltage input to the voltage detector to a level which prevents saturation of the voltage detector, wherein prevention of saturation of the voltage detector enables detection of all available phase information contained in the voltage sensor output, a hot stick computer which generates a pulse-width modulated signal, and a radio frequency transmitter for transmitting a pulse-width modulated wave;
A field unit having a radio frequency receiver for receiving said pulse width modulated wave and a converter for generating a sine wave from the pulse width modulated RF wave, a second digitizer having an output for generating a digitized output of the reference voltage, which is initiated by a GPS pulse, and a first computer for computing by a Fourier transform a power line phase value of a fundamental frequency of said reference voltage from the second digitizer output.
A reference unit having a reference voltage sensor, a reference voltage detector which is not saturated by a voltage from the reference voltage sensor, a third digitizer having an output for generating a digitized output of the reference voltage, which is initiated by said GPS pulse, a second computer for computing by a Fourier transform a reference phase value of a fundamental frequency of said reference voltage from the third digitizer output, and a computer for determining a difference between the reference phase value and the power line phase value where the computer is located at the field unit or the reference unit.
In
The automatic gain control of this invention includes the step adjustable gain amplifier 16, the analog multiplier 18, a precision rectifier 22, an integrating error amplifier 24, and a blocking capacitor 26. These components operate with the CPU 20 to produce a course discrete gain adjustment loop which includes the step adjustable gain amplifier 16 and the CPU 20.
Next, a fine gain control is provided by a loop which comprises a precision rectifier 22, an integrating error amplifier 24, and the analog multiplier 18.
In
In
In both
As shown in
In
It is important that both digitizer 56 and digitizer 68 be initiated at the same time as determined by the GPS clock in order that the Fourier transform calculations begin at the same time. In the reference unit, a CPU is used for computing the Fourier transform of the reference phase value. This occurs at block 70. The reference unit also includes a receiver/transmitter 72.
As shown in
In this invention, Applicant utilizes an automatic gain control in order to adjust the voltage input to a CPU/digitizer. The voltage to the CPU/digitizer must be less than the saturation voltage of the CPU/digitizer in order for all information in the sine wave to be detected. As is well known in the art, a phase represents voltages in power systems. However, in this invention, the magnitude of the phase is not important. Instead the significant information is the angle of the phase, which represents the phase of the voltage at the point of measurement.
In the field unit, when the sine wave is received from the hot stick, the method to insure utilization of all information is as follows:
1. The received analog signal is digitized by the analysis microcomputer and then digitally multiplied by a synthesized sine wave and a synthesized cosine wave whose absolute phase is determined by synchronization with a time reference such as WWV or GPS clock, and whose frequency is the line frequency, for example 60 Hz in the US. The synchronization is done by mathematical fitting routines that compare Asin(ωt+Φ1), where A is the amplitude, ω is 2πƒ and ƒ frequency is 60 Hz in the USA and 50 Hz in many other locations, t the time reference and Φ1 the reference phase. The same math is applied at the test point by determining Asin(ωt+Φ2), where Φ2 is the phase detected at the test location.
2. The multiplied results are averaged over the course of the measurement interval which might be 10 cycles of 60 Hz.
3. The absolute phase is then simply the arc tangent of the sine-multiplied average divided by the cosine-multiplied average. The result uses all the information, providing an exceptionally accurate value for the absolute phase of only the 60 Hz frequency component in the received signal, and is immune to noise and high-frequency spurious components.
The resulting field unit phase angle is compared by a computer to one similarly obtained at the reference location where the phase is known, and which can be corrected for speed of light effects (if desired) between the reference point and the measurement point. From this, the phase of each of the three transmission lines is now known to accuracy not heretofore possible with zero crossing methods.
The hot stick sensor and electronics and the field unit work together to acquire a bandwidth-limited (this means that high-frequency noise is low-pass filtered out) accurate sine wave from the phase to which the hot stick is connected, transmit it to the field unit and produce a level-shifted sine wave at a receive AC pulse terminal on the field unit board Receiver analog Pulse RAP.
1. The analog pulse at the hot stick main board is a sine wave of approximately 2.5V peak-to-peak amplitude, level shifted so that it has a DC component of 2.5V as shown in
In the hot stick there may be an indicator light block (LED) and an auto-shutdown block (ASB) as well. The pulse width modulation (PWM) frequency is set to 5 kHz in software.
2. In addition to the RAP hot stick signal, the receiver located at the field unit generates a Receiver Analog Strength signal that indicates signal strength. Because the RAP signal will look like hash or be zero if no good sine wave is sent, and because the hot stick will not transmit until a good sine wave is present, it is not necessary to use this signal.
Operation of Hot Stick AGC
1. The phase voltage from the capacitor coupling to the power line is divided by the hotstick itself down to manageable but unknown levels and processed by the AGC block 40. The amplitude of the sine wave is converted to a 0-3.5Vdc signal automatic gain control voltage (AGCV) by the precision rectifier and is also fed to a very-slow-response closed-loop continuously-variable integrating error amplifier 24 gain control that is in turn connected to the analog multiplier 18. No programming is required for this—the fine gain control is closed loop. AGCV is the rectified amplitude of the actual final sine wave to be sent to the transmitter and must be near 2.5V for a properly acquired sine wave and the control CPU 20 tests for this. After about 4 seconds, AGCV will stabilize.
a) If AGCV is above about 2.5 volts, then the sine wave to be digitized is too high and must be decreased. If AGCV is below 2.5 volt it is too low.
b) The first gain stage provides step-control of gain by control from CPU 20 to amplifier 16. It will take about 4 seconds for AGCV to stabilize. This stage will provide about a factor of 100 change in gain, while the fine gain AGC block is good for another factor of 10 or so and is not under programming control.
c) When AGCV is 2.5V, correct gain has been achieved. If this cannot be achieved, then no useful sine wave is present. On correct AGCV detection the control CPU 20 will indicate that the PWM can be started and can enable the transmitter.
d) A possible mode is to enable the transmitter hot stick right away. If the PWM is not yet running, this transmits a dc voltage to the receiver. Thus, instead of hash, a stable voltage is present, easily detected by the main board CPU at the field unit as an incorrect signal.
This way, Applicants can use a signature of the received signal (a sine wave is transmitted only if EVERYTHING is ok) for the main board to know it has a good sine wave.
a) The hot stick control CPU detects some sort of idle state (no sine wave for 10 minutes) and disconnects all power from the system, shutting it down.
b) A manual push of a switch for (one second) will do a hard restart.
This application claims the priority of U.S. provisional application 60/719,209 filed on Sep. 22, 2005, which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60719209 | Sep 2005 | US |
