Trend Tool (web) Data Resolution issue with Modbus/DNP3 data with 1sample/2s rate

Cesar · June 17, 2021, 9:25pm

I am having some issues with the Data Resolution for data with a slower sample rate like DNP3 and Modbus (1 sample every 2 seconds).

When using PMU measurements I can use the Trend/Export measurement tool with any Data Resolution without any problem. The data is displayed and retrieved very fast. However when using a signal that comes from a DNP3/Modbus connection, I can only use Data resolution as Full, otherwise nothing is displayed, or only some points when the interval is large.

It seams that the openHistorian have some trouble in aggregating the data to display in a different time resolution for DNP/Modbus.

Does anybody have any idea what could be the problem?

StephenCWills · June 17, 2021, 11:05pm

This is an unfortunate limitation of the way that downsampling is designed in openHistorian. Because the archive was designed for synchrophasor data, which is evenly sampled at a high sampling rate, the downsampling algorithm actually turns the queried time range into a collection of timestamps that are aligned to the top of each sampling interval and seeks to each timestamp in the collection. Protocols like DNP3 and Modbus do not provide a timestamp along with the data so the timestamp is derived using the openHistorian’s local clock when the data is queried with no explicit alignment to any sampling interval. As a result, when the openHistorian seeks to aligned timestamps looking for DNP3/Modbus data, it nearly always finds nothing.

Cesar · June 18, 2021, 12:11am

Hi Stephen,

Thanks for the detailed explanation! It makes sense the limitation!

Appreciate the reply!

ritchiecarroll · June 18, 2021, 7:16pm

If you use Grafana instead, we have an option called ; includePeaks which you can add to your query which should help:

github.com

GridProtectionAlliance/gsf/blob/master/Source/Documentation/GrafanaFunctions.md#special-commands

# GSF Grafana Functions

The Grafana interfaces defined in the Grid Solutions Framework allow for aggregation and operational functions on a per-series and per-group basis. The following defines the [available functions](#available-functions) and [group operations](#group-operations) that are available for a data source implementing the GSF Grafana interface, e.g., [openHistorian](https://github.com/GridProtectionAlliance/openHistorian). Note that any data source that implements the [GrafanaDataSourceBase](https://github.com/GridProtectionAlliance/gsf/blob/master/Source/Libraries/Adapters/GrafanaAdapters/GrafanaDataSourceBase.cs) class will automatically inherit this functionality.

## Series Functions

Various functions are available that can be applied to each series that come from a specified expression, see full list of [available functions](#available-functions) below. Series expressions can be an individual listing of point tag names, Guids or measurement keys separated by semi-colons - or - a [filter expression](https://github.com/GridProtectionAlliance/gsf/blob/master/Source/Documentation/FilterExpressions.md) that will select several series at once. Filter expressions and individual points, with or without functions, may be selected simultaneously when separated with semi-colons:

* Example: `PPA:15; STAT:20; SetSum(Count(PPA:8; PPA:9; PPA:10)); FILTER ActiveMeasurements WHERE SignalType IN ('IPHA', 'VPHA'); Range(PPA:99; Sum(FILTER ActiveMeasurements WHERE SignalType = 'FREQ'; STAT:12))`

Many series functions have parameters that can be required or optional &ndash; optional values will always define a default state. Parameter values must be a constant value or, where applicable, a named target available from the expression. Named targets are intended to work with group operations, i.e., [Set](#set) or [Slice](#slice), since group operations provide access to multiple series values from within a single series. The actual value used for a named target parameter will be the first encountered value for the target series &ndash; in the case of slice group operations, this will be the first value encountered in each slice. Named target parameters can optionally specify multiple fall-back series and one final default constant value each separated by a semi-colon to use when the named target series is not available, e.g.: `SliceSubtract(1, T1;T2;5.5, T1;T2;T3)`

To better understand named targets, follow these example steps:

 1. The following expression produces two unwrapped voltage phase angle series:

    [`UnwrapAngle(DOM_GPLAINS-BUS1:VH; TVA_SHELBY-BUS1:VH)`](#unwrapangle)

 2. Values from one of the series can now be substracted from values in both of the series at every 1/30 of a second slice:

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