Patent Application
20160370408
This invention relates to power line sensors and methods.
Ungrounded power line sensors measure the voltage of a medium-voltage power line relative to ground through, for example, a capacitive coupling between metal plates on the outside of the sensor and ground. See U.S. Pat. No. 4,689,752 incorporated herein by this reference. Under normal conditions, accurate voltage measurements are possible. However, the accumulation of rain, snow, and/or ice on the sensor can change the capacitive coupling between the sensor and ground resulting in errors in the measurement of line voltage.
U.S. Pat. No. 4,795,973 (incorporated herein by this reference) describes a modification to a sensor with the objective of being less sensitive to snow. The entire sensor body is turned into a single, large voltage sensing plate. Such an approach may still be somewhat sensitive to snow because significant snow build-up will change the effective surface area of the sensor.
To be completely resistant to the effects of snow and ice, one typical solution is to use relatively large and heavy instrumentation transformers wired directly to each phase. A “Potential Transformer” (PT) is used to transform the line voltage down to a lower voltage that is more easily measured, typically about 120 Vrms. By measuring this lower voltage and multiplying by the turns ratio of the PT, the line-to-neutral voltage of a phase can be deduced. A “Current Transformer” (CT) is used to measure current. The line to be monitored passes once through a transformer core. A secondary with many turns is also wound around the transformer core, and the secondary is either shorted or drives a very small resistance. The secondary is isolated from the voltage on the primary, and the current on the secondary is much lower than (and proportional to) the current on the line, with the turns ratio of the transformer again being the proportionality constant. Voltage, current, power, etc., are then measured by a commercial meter attached to the PT and CT (for example, the ITRON Quantum Q1000).
Such a solution, however, can be expensive and labor intensive to install.
An ungrounded power line sensor measures the voltage of a medium-voltage power line relative to ground through a capacitive coupling between electrically conductive plates on the outside of the sensor and ground. Under normal conditions, accurate voltage measurements are possible. However, the presence of raindrops sitting on the surface of the sensor can change the capacitive coupling between the sensor and ground resulting in measurement errors in the line voltage. If two sets of voltage sensing plates are employed, one on top of the sensor and one on the bottom of the sensor, we discovered that the top plates tend to exhibit an increase in voltage in the presence of rain whereas the bottom plates tend to experience a decrease in voltage in the rain. By separately measuring the top and bottom plates, the presence of rain can be detected by the difference in the readings of the top and bottom plates. The deviation in sensor readings due to the rain can also be mitigated by computing a weighted average of the top and bottom sensor plate readings to yield a combined voltage reading that is insensitive to rain.
Featured is an ungrounded power line sensor system comprising a housing configured for coupling about a power line, at least a first voltage sensing plate supported by the housing and exposed to rain and snow, and at least a second voltage sensing plate supported by the housing and shielded from rain and snow. A processing subsystem is configured to (e.g., runs computer instructions which) measure a voltage sensed by the first voltage sensing plate, separately measure a voltage sensed by the second voltage sensing plate, and mitigate variations in said measurements due to a weather event by, for example, applying a weighted average calculation to the voltage measurements to cancel out the effects of rain on the first voltage sensing plate.
In one example, the measured voltage sensed by the first voltage sensing plate is Vtop, the measured voltage sensed by the second voltage sensing plate is Vbottom, and the weighted average calculation is Vavg=(1−c)Vtop+c Vbot where c is a constant weighting factor.
The system may further include a current sensor and then the processing subsystem preferably measures power and energy using a current measurement output by the current sensor and a measured voltage sensed only by the second voltage sensing plate. The purpose of computing power and energy using only the bottom plate is that it has been observed that voltage measurements from the top plates can be erroneously shifted in phase during snow conditions, whereas the bottom plates (which do not accumulate snow) have little or no phase shift due to snow. In some embodiments, the processing subsystem is configured to apply a scaling factor to the power and energy measurements. The scaling factor may be a function of the measured voltage sensed by the first voltage sensing plate and the measured voltage sensed by the second voltage sensing plate. In one example, the measured voltage sensed by the first voltage sensing plate is Vtop, the measured voltage sensing by the second voltage sensing plate is Vbot, and the scaling factor is (1−c)Vtop+c Vbot divided by Vbot where c is a constant weighting factor.
The processing subsystem may also mitigate variations in the voltage measurements by comparing the measured voltage sensed by the first voltage sensing plate and the measured voltage sensed by the second voltage sensing plate. The processing subsystem can be configured to report a snow event when the measured voltage sensed by the first voltage sensing plate differs from the measured voltage sensed by the second voltage sensing plate by a predetermined value.
In one version there is a set of electrically connected voltage sensing plates exposed to rain and snow and a set of electrically connected voltage sensing plates shielded from rain and snow. The system housing may have an apex between opposing outwardly sloping top voltage sensing plates exposed to rain and snow and opposing inwardly sloping bottom voltage sensing plates shielded from rain and snow. In some embodiments, the processing subsystem includes a first processor in the housing electrically connected to the first voltage sensing plate and separately electrically connected to the second voltage sensing plate. The system collector may also include a second processor in the collector. Thus, the processing subsystem can reside in the sensor, the collector, or can be distributed between those two components.
Also featured is an ungrounded power line sensing method comprising measuring a voltage sensed by a first voltage sensing plate located proximate a power line and exposed to rain and snow, separately measuring the voltage sensed by a second voltage sensing plate located proximate a power line but shielded from rain and snow, and mitigating variations in the voltage measurements due to a weather event.
In one embodiment, an ungrounded power line sensor system includes a housing configured for disposal about a power line, a current sensor associated with the housing for measuring power line current, a first voltage sensing plate supported by the housing and exposed to rain and snow, and a second voltage sensing plate supported by the housing and shielded from rain and snow. A processing subsystem is configured to measure a voltage sensed by the first voltage sensing plate, measure a voltage sensed by the second voltage sensing plate, mitigate variations in said measurements by applying a weighted average calculation to the voltage measurements to cancel out the effects of rain on the first voltage sensing plate, and measure power and energy using the power line current measurement and only the measured voltage sensed by the second voltage sensing plate.
An ungrounded power line sensing method includes measuring a voltage sensed by a first voltage sensing plate proximate a power line and exposed to rain and snow, measuring a voltage sensed by a second voltage sensing plate proximate a power line and shielded from rain and snow, measuring power line current, applying a weighted average calculation to said voltage measurements to cancel out the effects of rain on the first voltage sensing plate, and measuring power and energy using the measured current and only the measured voltage sensed by the second voltage sensing plate.
The subject invention, however, in other embodiments, need not achieve all these objectives and the claims hereof should not be limited to structures or methods capable of achieving these objectives.
Other objects, features and advantages will occur to those skilled in the art from the following description of a preferred embodiment and the accompanying drawings, in which:
Aside from the preferred embodiment or embodiments disclosed below, this invention is capable of other embodiments and of being practiced or being carried out in various ways. Thus, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of components set forth in the following description or illustrated in the drawings. If only one embodiment is described herein, the claims hereof are not to be limited to that embodiment. Moreover, the claims hereof are not to be read restrictively unless there is clear and convincing evidence manifesting a certain exclusion, restriction, or disclaimer.
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One discovery by the inventors hereof is that during a rain event the voltage measured by the top voltage sensing plate set (14a, 14b) increases and the voltage measured by the bottom sensing plate set decreases, even though the actual voltage of the line is unchanged. The top voltage sensing plate set (14a, 14b) experiences an increase in capacitance between the sensor plates and ground due to the rain droplets adding surface area to the top voltage sensing plate set (16a, 16b). This increase in capacitance causes the voltage measured by the top plates to increase. Even though there is no direct contact of rain with the lower voltage plates, the increased surface area of the top voltage plates and top sensor body due to rain causes charges to be preferentially distributed on the upper part of the sensor, ultimately reducing the voltage measured by the bottom voltage sensing plate set.
In the subject invention, microcontroller 30,
There is capacitive impedance between the sensor plates and ground, represented in
Internal to the sensor, separate sensing circuits are used to condition voltage measurements from the top and bottom plates as shown in
On sensor install, the relationship between the voltage measured by the sensor circuit and the line-to-neutral voltage must be calibrated. The sensor system automatically and separately calibrates the readings from the top and bottom voltage sensor plates 14, 16.
Internal to the microcontroller, the RMS voltage of the top plate 14 is calculated (denoted Vtop). The RMS voltage of the bottom plate 16 is also calculated (Vbottom) along with real and reactive power and energy which are a combination of voltage and current measurements. The power and energy quantities are preferably calculated only using the bottom voltage sensor plate 16 for the voltage input to the microcontroller because the bottom sensor plate, shielded by the sensor body, experiences little or no phase shift in snow conditions.
Microcontroller 30 is thus configured to measure the voltage Vtop sensed by the top voltage sensing plate(s) 14 and to separately measure the voltage Vbottom sensed by the bottom voltage sensing plate(s) 16 and to mitigate variations between Vtop and Vbottom.
Preferably, when Vtop differs from Vbottom by a predetermined amount (e.g., a −1% to +4% difference between top and bottom plate voltages), microcontroller 30 outputs a signal transmitted wirelessly by sensor transmitter 36 to radio 64 of collector 33,
The sensor 10′,
Collector 33 is preferably powered from transformer 35,
The collector supply voltage 37 is fed into a voltage conditioning circuit 42. This circuit preferably including a voltage divider and an op amp buffer reduces the voltage from the 120V supply voltage to a low voltage in the range of a few volts for measurement with an Analog-to-Digital Converter (ADC). In the initial reduction to practice, a circuit based on the LTC 1992 differential Op Amp was employed. The signal output by the circuit 42 is then repeatedly measured by an ADC built into microcontroller chip 44 of the collector. In the one prototype device, a TI MSP-430 class microcontroller samples an associated 16-bit ADC at a rate of 2048 Hz. A True RMS-type filter (in the prototype implementation, taking the RMS by squaring the sensed signal, applying a low-pass, and taking the square-root of the result) is then applied in software operated on the microcontroller 44.
The microcontroller 44 also communicates with the sensors via a 2.4 GHz Industrial, Scientific, and Medical (ISM) band radio module 64 obtaining measurements of voltage, current, power, and energy from the sensors. The microcontroller 44 passes both sensor and the collector supply voltage measurement to microprocessor 62 running embedded Linux. Software on the microprocessor 62 applies scaling factors determined during calibration to the phase voltage measurement from the sensors. Collector calibration factors, also determined during calibration, may be applied to the collector supply voltage measurement to produce an alternative voltage for each phase. The software may then compares the alternative and phase voltages to determine if there is a snow condition and logs and/or corrects various measurements for the snow condition. The microprocessor 62 may use a Secure Digital (SD) Memory Card 72 to locally store the collected data may use an Ethernet module 66, a 900 MHz mesh radio 68, or a WiFi Radio 70 to transmit the collected data to end consumers of the data (e.g. SCADA systems). The voltage measurement circuit configured to measure the collector's supply voltage, however, could be implemented in other ways. Only one preferred embodiment includes voltage conditioning circuit 42, microcontroller 44, and microprocessor 62. See U.S. patent application Ser. No. 14/621,696 incorporated herein by this reference.
In addition to simply detecting the presence of rain, the sensor system can correct for the presence of rain. Once per minute, a snapshot of all of a sensor's registers (i.e. voltages, current, power, energy, etc.) is taken and sent to the collector. At that time, the deviation in sensor readings due to the rain can also be mitigated by computing and reporting a weighted average of the top and bottom voltage sensor plate(s) readings to yield a combined voltage reading that is insensitive to rain. The weighted average is thus:
Vavg=(1−c)Vtop+cVbot; (1)
where c is a constant weighting factor that is selected based on experimental measurement of the sensor's performance in the rain. Collector 33 radio 70 transmits this computation to end users.
Power and energy values, which are computed using exclusively voltage measurements from the bottom plate, can then be adjusted by multiplying by a scaling factor.
For example, the measured real power is the instantaneous voltage multiplied by the instantaneous current. Here, the instantaneous voltage is the voltage measured only by bottom sensor plate 16,
Vavg/Vbot. (2)
Measured reactive power, incremental volt-hours, incremental real energy, and incremental reactive energy and the like are similarly adjusted by the same scaling factor.
Since the power and energy measurements were computed using exclusively the bottom plate, the power factor and therefore the power and energy computations will be accurate in snow conditions where snow effects are corrected as described in U.S. patent application Ser. No. 14/621,696 incorporated herein by this reference. The adjustment by the weighted average voltage yields robustness to the influence of rain as well.
Note that the computation of Vavg and the associated scaling operation could be performed either in the sensor prior to sending measurements to the collector or the computation could be performed in the collector itself. In the presently implemented version, the computation of Vavg and the adjustment of power and energy values takes place in the sensor.
Also note that the scaling factor could be computed using either the instantaneous top and bottom voltages at the time at which a snapshot is taken or by using the average voltage over the entire, nominally one-minute, reporting period. Initial implementations used the instantaneous voltages to perform the adjustment. Later implementations use the average top and bottom voltage over the reporting period to provide greater robustness to special situations e.g. where the rain begins part way through the reporting period.
Although a simple linear combination of Vtop and Vbot was used in the initial implementation, other more elaborate combinations of Vtop and Vbot might be used in the future, on the basis of future field testing and experimental results. For example, some nonlinear blend of the two voltages may yield improved performance. Alternately, the combination of plates could be the result of a real time adaptive and dynamic ratio that results from analysis of top and bottom plate voltages and predictive modeling, i.e. incorporating information from previous values of Vtop and Vbot to yield more accurate adjusted values.
In practice, the weighted average scheme provides acceptable rain accuracy. For example, a pilot test site in Mission, BC measured the voltages as pictured in
The separate sensing of top and bottom sensors allows partial mitigation of snow effects in instances where other adjustments are not applicable (i.e. when there is no direct access to supply voltage or in cases where the supply voltage has no correlation to the voltage of the medium voltage line). When snow conditions occur, the difference in voltage between the top and bottom plates exceeds the difference that is normally expected during rainy conditions.
If the discrepancy between top and bottom plates is large enough to indicate snow conditions, the sensors can then communicate this condition to external equipment that is monitoring the sensor's measurements. For example, the sensors might communicate this information digitally, e.g. via a field in the sensor's DNP3 messaging interface. A more elaborate implementation of snow reporting could incorporate the temperature reading of the sensor and use additional logic to infer actionable weather information which would then be transmitted back to the utility, e.g. indicating the presence of potentially damaging freezing rain conditions versus more benign snowfall, etc.
An indication of a snow condition can also be conveyed in an analog form. For one of implementations, the sensor is used as a voltage input to a capacitor bank controller. The collector communicates with the capacitor bank controller by producing an AC analog output voltage proportional to the voltage measured by the line-mounted sensor. The capacitor bank controller then measures the analog signal from the collector as an indication of the line voltage (i.e. as if the sensor were an electronic voltage transformer). If snow conditions are detected by the system, the collector generates a low voltage, specifically chosen to be below the capacitor bank controller's “inhibit voltage”, the voltage below which capacitor switching functionality is disabled by the capacitor bank controller. In this way, the capacitor bank controller will not switch during a snow event.
The strategy for identifying and indicating snow conditions in the capacitor bank sensor scenario can be outlined as follows:
Although specific features of the invention are shown in some drawings and not in others, this is for convenience only as each feature may be combined with any or all of the other features in accordance with the invention. The words “including”, “comprising”, “having”, and “with” as used herein are to be interpreted broadly and comprehensively and are not limited to any physical interconnection. Moreover, any embodiments disclosed in the subject application are not to be taken as the only possible embodiments.
In addition, any amendment presented during the prosecution of the patent application for this patent is not a disclaimer of any claim element presented in the application as filed: those skilled in the art cannot reasonably be expected to draft a claim that would literally encompass all possible equivalents, many equivalents will be unforeseeable at the time of the amendment and are beyond a fair interpretation of what is to be surrendered (if anything), the rationale underlying the amendment may bear no more than a tangential relation to many equivalents, and/or there are many other reasons the applicant can not be expected to describe certain insubstantial substitutes for any claim element amended.
Other embodiments will occur to those skilled in the art and are within the following claims.
