Logical Node (LN)

Basics

The logical node (LN) is the most impactful concept within the IEC 61850 (imo). In its simplest, a logical node is a PAC (Protection Automation and Control) function hosted in secondary devices in a substation. As PAC function - at least on high level - are common in substation all over the world - (you can expect overcurrent protection or circuit breaker control to be essential parts of any substation), the IEC 61850 and others outside a substation have defined a common set of logical nodes. These are defined in the IEC 61850-7-4 for substation related applications, in the IEC 61850-7-410 for wind turbines and the IEC 61850-7-420 for DERs.

A logical node is a structured variable that consists of a set of data objects (again a structured variable called Common Data Classes (CDC) ). The definitions of logical nodes primarily happens in table that look like so:

Syntax and structure

Each logical node class consists of 4 upper case ANSI characters, the first describing the PAC group (e.g. P for protection or C for Control) and the last three describing the function. The above example would then translate to: PTOC - Protection Timed overcurrent.

A logical node consists of a set of data object that are unique to the requirements of the PSC function. The Str data object for example models the start signal of the overcurrent protection. Each of the data objects itself is again a structured variable (CDC) build for a specific purpose. Take for example the data objects StrVal (setting for start value) and TmMult both are represented by a ASG (Analogue Setting Group).

SCL implementation

The structure of a logical node is modelled in the DataTypeTemplates section. For this, it uses the tLNodeType element with its DO children. An example for the upper table would look like so:

<DataTypeTemplates>
  ...
  <LNodeType lnClass="PTOC" id="...">
    ...
    <DO name="Str" type="pointer_to_ACD_specification" />
    <DO name="StrVal" type="pointer_to_ASG_specification" />
    ...
  </LNodeType>
  ...
</DataTypeTemplates>

The pointers to the structure of the data object is referring to a CDC definition and its implementation in the SCL can be found here Common Data Classes (CDC) .

A logical node class must not be the same in SCL as it is in the namespace. Most of the data object are not mandatory PresCond != M. It is up to the user - utility or vendor - to decide with of the data objects are needed in the IED or the substation communication.

The instantiation of a logical node happens in the IED section. Here the structure of the logical node is copied over-represented by the elements LN, DOI, SDI and DAI. For the above example with the StrVal instantiated to 45.7 and 34.2 would could look like so:

<IED ...>
  <AccessPoint ...>
    ...
    <Server>
      <LDevice ...>
        ...
        <LN prefix="ID_" lnClass="PTOC" inst="1" lnType="pointer_to_LNodeType">
          ...
          <DOI name="StrVal">
            <SDI name="setVal">
              <DAI name="f">
                <Val sGroup="1">45.7</Val>
                <Val sGroup="2">34.2</Val>
              </DAI>
            </SDI>
          </DOI>
        </LN>
      </LDevice>
    </Server>
  </AccessPoint>
<IED>