Thursday, August 25, 2016

How to Exchange a Voltage Measurement with IEC 61850?

As discussed before you will find a reasonable example to learn the benefits of applying IEC 61850.
Let's look at a voltage measurement:

According to IEC Electropedia we find many names for the same semantic: voltage, Spannung, spenning, ... Ok. These help humans to understand what we are talking about. But what about machines (controllers, SCADA systems, ..)?
They have to use a data type (int16, int32, float32, ...) and a reference (address ...) for a specific protocol like Modbus. Each vendor will likely use different types and addresses.
What's about the scale in applications that use integer? Is the scale known when you read the value of a voltage? Do you know the offset or the multiplier (V, kV, mV, ...)?
How do you know where the measurement is taken in the electrical system (location in the single line diagram)?
Answers to these questions may be found in a set of documents sitting on a shelf or on someones computer - hard to find out if the owner is on vacation.
With IEC 61850 we have a model that could be implemented so that all these details are always accessible online from each device that is a source of measurements:

Phase A voltage has a standard name "MMXU.PhV.phsA" with the value, quality, timestamp, units, and scale. These names are used all over in any IEC 61850 device.

IEC 61850 services allow to retrieve the MMXU model and read the values:

The device has all information to interpret the voltage value for phase A.

Finally we need to know where the value is measured in the single line diagram. IEC 61850-6 (SCL) provides the solution specified as an SCL file (simplified SSD - Substation Specification Description):

The above voltage could be designated as follows:

The value is located in the device "BayController". The device is communication wise identified by an IP Address.

This information really exposes all information needed to interpret a measurement. 
Note that this name needs not to be communicated when the value is reported cyclically or issued by a limit change.The report message could only carry the value, quality and timestamp.
The SCL file has all information to configure the whole system and the devices.
Any question?
Hope you have learned this: IEC 61850 goes very far beyond a protocol! We only need the protocol when we retrieve the selfdescription or read out or report the values.
And: the nice thing is that any device that implements the standard uses the same model, configuration, and services. What else do we need?

If we would apply just a protocol like Modbus then most of the information exposed (directly from the device) through the standard IEC 61850 would have to be stored in paper docs or excel sheets ... 

No comments: