PHD Measurement Data Logical Node Data Object Common
Data Class
Tap changer Tap changer position ATCC TapChg BSC
Accessories Oil level
Air moisture SIML Lev
H2OAir
MV MV Cooling Equipment Inlet temperature CCGR OilTempin MV
Active Part Oil temperature SPTR TopTmp MV
Table 5.1:Small partial list of Logical Nodes for condition monitoring according to IEC 61850 [2].
least two logical nodes: LPHD and LLN0. They are obligatory and are part of the system logical nodes group. The LPHD logical node is used to model common issues for phys-ical devices and contains physphys-ical device information. The LLN0 logphys-ical node is used to address common issues for logical devices. Apart from those, there is the possibility for engineering work to be done with respect to what logical nodes that can be configured, as the engineer is free to choose which logical nodes to be used.
For condition monitoring data acquisition, power transformer temperatures and oil-related attributes can be modeled for example by using the logical nodes SPTR and SIML respec-tively. An example of this is shown in Figure 5.3.
5.2 Communication Methods
The data model define a standardised data access method for describing power system devices that enables them to present data using identical structures that are directly related to their power system function. The data model is abstract and separates services from coding. Therefore it is referred to as the Abstract Communication Service Interface (ACSI) [40]. Different communication methods are supported by ACSI services and is separated into two types:
• Client - Server Services
• Real-time Services (Multicast)
Figure 5.3:Example of condition monitoring logical nodes.
Client / Server
A server contains everything that is defined to be visible and accessible from the commu-nication network. A client is any physical device that uses the ACSI services to exchange data with a server. Clients request services from a server and receive confirmations from the server with the data it asked for. It may also receive unsolicited reports from a server.
This is different from master-slave communication, as the slave does not possess the ability to send unsolicited reports to the master. Client-server communication is non-time-critical, meaning data transfer is delayed. Figure 5.4 illustrates a client-server session.
Real-Time (Multicast)
For a multicast communication, two new roles are defined, the publisher and the sub-scriber. A multicast information exchange is provided between one publisher, and one or
5.2 Communication Methods
Figure 5.4:Client-server communication [41].
many subscribers, as illustrated in Figure 5.5. Multicast communication is used for real-time and real-time-critical applications, meaning data transfer delay is low compared to the client-server communication. It is also unconfirmed communication, as the publisher does not receive any response from the subscriber that the information was received.
Figure 5.5:Multicast communication [41].
5.2.1 Mapping to Protocols
IEC 61850 defines a set of services and the responses to those services that enable all devices to behave identically from the network perspective. These services and responses are abstract and not related to any specific protocol. ACSI ensures that the standard can
continuously be extended to new communication protocols and technologies, and therefore IEC 61850 is said to be future proof [37].
IEC 61850-8-1 maps the application layer services to the protocol Manufacturing Message Specification (MMS), of ISO 9506. The mapping is based on a service mapping where a specific MMS service/services are chosen as the means to implement the various services of ACSI. The object models of IEC 61850 are mapped to specific MMS objects, for in-stance logical device object is mapped to an MMS domain. Table 8.2 in the appendix C shows some ACSI services mapped to MMS [41].
In addition to the mapping to the application layer, part 8-1 defines communication meth-ods, that are dependent on the services. There are three main communication methods included:
• Sampled Values (SV)
• Generic Object Oriented Substation Event (GOOSE)
• MMS Messages
SV and GOOSE are mapped directly into the Ethernet data frame thereby eliminating processing of any middle OSI layers. This makes them real-time and fast, avoiding the delays associated with processing of the header and trailer in a data transfer. A drawback is that they cannot be routed over network. MMS messages are mapped to the middle layers using TCP/IP, meaning they are routable over network. However, this makes MMS messages slower. Figure 5.6 shows the communication methods linked to the respective OSI layers.
GOOSE works on the publisher-subscriber mechanism on multicast or broadcast MAC ad-dresses. It is designed for peer-to-peer communication. GOOSE uses VLAN and priority tagging to have separate virtual networks within the same physical network and sets appro-priate message priority level. GOOSE also features enhanced retransmission mechanism.
The same GOOSE message is retransmitted with varying and increasing re-transmission intervals. A new event occurring will result in the existing GOOSE retransmission sage being stopped. A state number identifies whether a GOOSE message is a new mes-sage or a retransmitted mesmes-sage.The GOOSE has also been extended to communicate over network with the Routable GOOSE (R-GOOSE) [42].
SV data are streams of data acquired by the voltage and current transformers which are sent to a merging unit (MU). The MU samples the signals at an configured and synchronized rate. In this manner, any IED can input data from multiple MUs and automatically align
5.2 Communication Methods and process the data. SV streams consist of 4 voltages and 4 currents which are sampled at 4 kHz, or 80 samples per cycle according to IEC 61850-9-2 LE. In addition to SV, the ability to acquire status information as well as set output controls is desirable, and therefore IEC 61850 has implemented Sampled Measured Values (SMV) services, defined in part 9-1 and 9-2 [37].
MMS messages works on the client-server mechanism using IP addresses. As mentioned, MMS defines a set of standard objects which must be implemented in devices to execute services such as read, write, set, get, etc. Virtual manufacturing device (VMD) is the main object and all other objects like variables, domains, journals, files etc. comes under VMD.
An advantage with MMS messages is that they are routable over a network.
To summarise, Figure 5.6 shows the mapping of the types of messages to the OSI layers.
Figure 5.6:IEC 61850 communication mappings.
5.2.2 System Configuration
To configure a system based on IEC 61850, a ”language” called substation configuration language (SCL) has been created. It is based on the extensible markup language (XML), which is amarkup languagethat defines a set of rules for encoding that is both human-readable and machine-human-readable. SCL specifies a hierarchy of configuration files that enable multiple levels of the system to be described in XML files. The various SCL files include System Specification Description (SSD), IED Capability Description (ICD), substation configuration description (SCD), and configured IED description (CID) files [40]. They
are based on the same method and formats, but have different scopes:
• The System Specification Description (SSD) filedescribes the single line diagram of the substation, existing voltage levels, primary equipment and required logical nodes for implementing the substation functions. The standard defines as optional the import of this data in the system configurator1.
• The IED Capability Description (ICD) filedescribes the functional capabilities of an IED type. Each IED type has its related ICD file. It contains the IED logical nodes, data and supported services.
• The Substation configuration description (SCD) filecontains all configured IEDs, the communication configuration and the complete substation description. This file is generated by the system configurator during the engineering process and is an important part of the project documentation.
• The Configured IED description (CID) fileis optional and vendor specific. It con-tains a subset of the SCD file with all information related to one specific IED. If used, one CID file has to be generated for each existing IED in the project for load-ing the configured data into the IED.
Figure 5.7:The IEC 61850 system files.
1The IEC 61850 system configurator is a software that runs on computers from which devices can be config-ured.