Hourly And Daily Trend Computations - GE Hydran 201i Instruction Manual

Hydran* 201i Instruction Manual
This Section provides a few guidelines to optimize both the hourly and daily trend
alarms.
1. If a fast response to a sudden gas level increase is needed, it is best to rely on the
gas level alarm rather than on trend alarms. Trend alarms are ideal when it
comes to detect slow and steady changes from fluctuating readings, but are not
as good to detect abrupt changes.
2. The transformer's (or other equipment) past history and the user's field
experience are the most important criteria for alarm settings. If existing, records
of previous dissolved gas analysis (DGA) for the equipment monitored should be
used to help determine alarm settings in general and especially daily trend
settings.
3. Trend alarm delays should be set according to each trend period. For example:
• If the hourly trend period is set to four hours, the hourly trend reading may
show quite large, but short, positive and negative peaks during a day. In this
case, an hourly trend alarm delay of 50 to 75% of the period could avoid
unnecessary alarms.
• However, if the hourly trend period is set to 48 hours, daily fluctuations would
not affect the trend reading significantly. In this case, the alarm delay could
be set to 10 to 20% of the period.
4. The hourly trend period should be long enough to filter out reading fluctuations
caused by short-term changes in ambient conditions and transformer operation.
The factory default of 24 hours is a good starting value.
5. The longer the trend period, the higher the trend value in ppm is for a given
steady increase in gas level reading. For example, a constant gas level increase
of 10ppm/24 hours results in a 10ppm/period reading if the period is set to 24
hours, but in a 20ppm/period reading if the period is set to 48 hours.
6. The longer the period, the smoother the trend readings are. However, it takes
longer for the trend to stabilize ("taper off") to its final value. In the preceding
example, it would take two days to obtain the final value of 10ppm/24 hours; in
the second case, it would take four days to reach the final value of 20ppm/48
hours (for details on computations, see Section 11.4.3).

11.4.3 Hourly and Daily Trend Computations

A unique computation method is used for both the hourly and daily trends. In these
computations, the gas level reading is processed through a first- order, digital, low -pass
filter; the trend reading (slope) is then extracted using a first-order, digital, high-pass
filter. The time constants for both filters are set to 33% of the trend period for optimum
results.
Table 11-5 and Table 11-6 show two examples of trend computations.
Example 1: The gas level starts to increase at a rate of 10 ppm/24 hours. The hourly
trend period is set to 24 hours.
MA-033 Rev. 14.0, 12-Nov-18
Page 165