10.4.2 Guidelines For Setting Trend Alarms - GE Hydran 201 i Instruction Manual

• DailyTr Period: The period of time (in days) during which the daily trend is calculated.
Increasing the period decreases the normal fluctuations of the readings, but increases the
response time.
The readings of the Hydran 201i System should be monitored over a minimum period of
two months before setting these parameters; follow the same rules as for the gas High alarm
(see Section 10.4.1.1 on page 10-9).

10.4.2 Guidelines for Setting Trend Alarms

The hourly and daily trend High and High-High alarm settings cannot be described in terms
of absolute criteria. They are related to the user's field experience and the utility's standard
practice.
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, a 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.
Part 17997 Rev. 6, March 2011
Gas High and High-High Alarms
10-11