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    Determining the Location of a Disturbance Direction

    Issue
    To determine the direction of a disturbance relative to the meter.

    Product Line
    ION meters with Disturbance Direction Detection module.

    Environment
    Firmware in ION meter.

    Cause
    When a disturbance occurs in a power system due to switching or a fault the location of the cause of the event relative to the metering location can be determined by examining the change to voltage and current when the event occurs. Analyzing direction information from multiple meter locations in a power system allows the location of the cause of the disturbance to be determined. This article describes the method used for determining the direction of a disturbance event at the individual meter.

    Resolution
    The location of the cause of a disturbance relative to the metering location can be determined by examining the change to the voltage and current when the event occurs. For example, if a load is energized downstream from the metering location, the meter can measure the resulting current increase and the corresponding voltage decrease and determine that the event was downstream. Under ideal conditions, with pre-event waveforms that are unchanging, this determination can be made easily and reliably by simply finding the point on the waveform that is different from the pre-event waveforms. However, when the waveforms are varying and noisy, the sample of pre-event waveforms is small, and the magnitude of the event was small, it becomes much more difficult to locate the event in the waveform data and to correctly determine its direction.

    A unique approach is used in the Circuit Monitor to solve this problem. It has also been demonstrated that the same analysis technique can be used in software to analyze stored waveform capture data. When a disturbance event occurs, a waveshape alarm or a sag/swell alarm is activated which triggers a waveform capture. Since the evaluation is based on changes that occur when the event occurs, the waveform capture must contain at least 3 pre-event cycles. The average of the first 2 cycles of the waveform capture is used to establish the pre-event conditions.

    When the waveform capture is triggered, the raw data for the entire capture is collected in a memory array. After all of the data is collected, the raw data is processed, residual channels are computed, and the data is sent to a file on the Disk-On-Chip (DOC) one segment at a time. A segment is a fixed-size block of data which contains a variable number of cycles depending on the resolution of the waveform capture and the number of channels being recorded. This new direction detection function has been inserted into this process, processing a segment at a time before it is sent to the DOC. 

    The analysis is based on the changes to voltage, current, and power when the event occurs. The waveform for the metering channel that triggered the alarm is examined at 3 time scales; (1) point-by-point, (2) window-by-window [a window being a fraction of a cycle], and (3) cycle-by-cycle. The location of the event is determined by computing the difference of the point, window and cycle under test with the reference cycles. The difference is divided by the average difference of the 2 reference cycles. This ratio, which can be thought of as a signal to noise ratio, is then compared with setpoints for the appropriate time scale. If the setpoint is exceeded, the location of the event and coincident data on the other channels is captured for later analysis.

    After the event is located on the alarm channel, a scoring system is applied to the data collected and the direction is determined. All 3 time scales are examined to see if they agree in the direction and in the location in the waveform capture. The magnitude of the event compared to the pre-event "noise" is also used to establish the confidence. The determination of the direction is straightforward and follows the following table. The uncertainty comes in the inability to detect a small magnitude event in a variable "noisy" waveform.




    Conditions and their effect on confidence:
       event detected on alarm channel:                                             0 – 5 points
       event detected on coincident channels:                                   0 – 10 points
       location of voltage, current and power events agree:            0 – 6 points
       ratio of min to max or max to min greater than 2:                    0 – 9 points
       direction of 3-phase voltage agrees with alarm channel:     0 – 5 points
       direction of 3-phase current agrees with alarm channel:      0 – 5 points
       direction of 3-phase power agrees with alarm channel:        0 – 5 points
    Total possible number of points                                                    0 – 45 points

    Each event is assigned a score which expresses the direction of the event and the confidence that can be placed on that analysis. The final score is expressed as an integer percentage of the number of points assigned. Positive values are downstream, negative values are upstream

    For complete information, refer to the attachment.

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