Measure, calculate, store – ring buffer was yesterday!
As described in detail in the previous article, our latest generation measuring devices are equipped with highly powerful signal processors (DSP), which enable the constant and seamless determination of current and voltage, as well as the calculation of every conceivable parameter. How does this take place in detail, what is the measuring process sequence, in what form are the measured values made available, where are they saved?
Modern measuring devices such as our UMGs can essentially be considered as PCs.The average elements are the CPU (DSP), RAM, hard drive (flash memory) and communication ports (RS485, RJ45).
It is fundamentally possible to distinguish between the following measured value groups:
Online values are determined over a measurement interval of 200 ms or as a mean value of the full wave effective values over 10 periods. Online values are all values that are constantly determined and evaluated by the measuring device. Depending on the measuring device this can be up to 2,000 values available for all measuring channels per 200 ms.The significant values can be read out directly from the UMG displays. Using the GridVis® software and working in the topology screen it is possible to view the complete scope of measured values.
All measured values are constantly available in defined Modbus memory registers for external access via suitable third party software.
Historical values are generated using the online values. For this purpose one or more recording configurations are predefined in the device configuration. For the purpose of the respective recording a period is stipulated for the generation of a mean value, e.g. 15-minute mean value for the recording of load curves, 1-hour mean value for energy, etc. The time frames can lie between 200 ms and multiple days, depending on the type of device. In order to conduct power quality measurements per EN 50160, EN 61000-2-4 or EN 50160, IEEE519, pre- defined recording configurations are available and these can be activated at the click of a mouse button.
Historical values are generally initially stored in a measuring device on internal flash memory.This was formerly referred to as a ring buffer. Each stored value is assigned a time stamp. Using the GridVis® software the values are read out manually or automatically (Service). The measured value and time stamp are stored in a database. Using GridVis® or external database tools it is possible to evaluate these values on a tabular or graphical basis.
Fig.: Online values, value tree UMG 605
Events are under- and overvoltages as well as overcurrents. The basis is 20- ms full wave effective values with UMG 604 and UMG 508 or 10-ms half wave effective values with UMG 605 and UMG 511. With an exceeding or undercutting of the stipulated tolerance limits the event is stored on the flash memory. Additionally, a pre- and post event period are defined, so that network incidents can be analysed directly before and after the event occurs. As such, all voltage and current channels are graphically shown as a maximum across the specified time frame.
In order to record transients the full performance of the UMGs is required. With a sampling rate of 20 kHz it is possible to capture transients from 50 μs. Similarly to with the recording of events, threshold values as well as pre- and post periods can be defined. Likewise, it is also possible to stipulate which channels are written to a graph in waveform at the time that the transients occur.
Flags are used to mark and save irregularities in measurements and recordings, in accordance with IEC 61000-4-30. In this way it is possible to recognise the causes of gaps in recordings for example.
|LostWindow||200 ms measurement window has been lost|
|LostPLL||The device has lost the grid synchronisation|
|Firmware upgrade||Firmware upgrade|
All recordings of historical data, events, transients and flags run constantly, independently of each other and in parallel in the measuring device.
All saved data is historically sorted for storage. If the flash memory is full then the oldest data historically is overwritten.Through the regular reading out of the data to a database, values that are overwritten on the measuring device will already have been saved to the server, meaning that no measured values are lost.