VTT Tarmo – a high-accuracy digitizer and a reference instrument enabling NMI level traceability for IEC 61850 compliant measurement devices

Project news

Digital substations in accordance with the IEC 61850* constitute a large paradigm shift in grid measurement technology. Most notably, the response of an instrument transformer is no longer determined as a ratio of input and output phasors of two analog signals. The output is now transmitted as a stream of digital data, better known as Sampled Values (SV) output, where the grid signal phase is referenced to a common time source and the magnitude is encoded using common rules of the IEC 61850 and IEC 61869 series of standards. We at VTT MIKES has developed a reference instrument, VTT Tarmo, that provides traceability for IEC 61850 compliant devices and can also be used as a high-accuracy general purpose digitizer. We offer this device to our customers, so do not hesitate to request a quotation.

Traceability for IEC 61850 devices using VTT Tarmo

The basis for establishing traceability for equipment with SV output or input is to generate a reference quality SV stream. This implies that both the magnitude and the phase of an input signal encoded into the stream have a known, SI traceable value. VTT’s reference device has been designed from the ground up to provide this functionality with ease. Magnitude calibration is performed in a traditional manner, for example by comparing to an AC standard. Calibrating the phase encoded into the output stream requires the determination of the absolute delay of the analog input path. VTT MIKES have developed a simple and accurate method for delay calibration, which will be performed to all inputs prior to delivery. Guidance for doing the calibration will be provided. Expanded uncertainty of the calibration is well below 30 ns.

VTT Tarmo pic 1
VTT Tarmo used as a reference device for calibrating measurement devices with Sampled Values input (e.g., digital
transformer test bridges).
Tarmo pic 2
VTT Tarmo used as a reference device for calibrating measurement devices with Sampled Values output (e.g., merging units and digital instrument transformers).

The device provides more than just IEC 61850 support

While primarily intended as a reference device for the IEC 61850 environment, VTT Tarmo can be used as a general-purpose digitizer, with accuracy surpassing most commercially available digitizers. The device constitutes of up to eight floating input channels, each with 18- or 24-bit SAR type analog-to-digital converters and linear, low-drift electronics. Two modes of operation are supported: a buffer mode and a stream mode. In buffer mode the device stores data in the internal 1M sample deep buffers for a later transfer to a host PC via USB 2.0. In stream mode the data is transferred to the PC continuously, leaving no gaps in the input signals. Sample clock can be provided from an external source, or the internal direct digital synthesis (DDS) based sample clock can be used.

VTT Tarmo pic 3
VTT Tarmo used as a sampling AC measurement standard. A separate scaling device can be used for extending the input range.

Easy to integrate into your software environment

The device is controlled through a C-type library (.h and .dll files) for easy integration into Labview, C/C++, or Python among others. A python wrapper is included for further convenience. The library enables all functionality, including device control and data transfer between the device and a PC. Support for implementing the library will be provided by VTT.

Technical specifications

VTT Tarmo reference device is intended for various AC measurement applications. The design emphasizes short- and long-term stability and linearity to reduce the impact a digitizer usually has in an uncertainty budget. 

Tarmo technical specs

The reference device can output Sampled Values data in accordance with all profiles specified in IEC 61869-9. This includes data streams used for protection, metering, and power quality measurement applications.


* IEC 61850 is an international standard defining communication protocols for intelligent electronic devices at electrical substations.

† IEC 61869 is an international standard series for instrument transformers.

Kaj Nummila
Kaj Nummila
Solution Sales Lead, National metrology institute VTT MIKES
Tapio Lehtonen
Tapio Lehtonen
Research Scientist