Hybrid signal analysis in protection and automation systems

Modern protection and automation systems for electrical power systems are increasingly using communication protocols to transfer information. In the mid-term, more and more hybrid systems and systems distributed across large distances will be implemented.
 In this context, hybrid means that protection and automation systems will operate simultaneously with classical secondary quantities and conventionally wired binary status signals, as well as with communication protocols such as those according to IEC 61850. Measuring and evaluating all of these different signals within such hybrid systems requires measurement systems that are also distributed and operate in a hybrid way. The following article provides an overview of the current status of developments, describes the challenges currently being faced, and presents potential solutions.
Hybrid systemsBesides classical secondary quantities and conventionally wired binary status signals, more and more of the mission critical information of the automation systems is transported across communication networks. In systems utilising IEC 61850 communication, the transmission of binary status information via GOOSE messages is an established practice. The next step will be to replace the secondary quantities with Sample Values, which will then transmit digitally coded current and voltage values across the communication network. But even if data transfer within automation systems will be primarily performed via communication protocols in the future, classical actuators and quantities within the process will still need to be acquired by conventional means.
Distributed systemsOnce it is digitised and wrapped in data packets, the transmission of information across large distances is essentially simplified. Whereas the installation of wire loops between individual binary outputs and inputs quickly becomes too costly or susceptible to interference, transmission via noise immune network connections is comparatively easy and allows large volumes of information to be transmitted. This enables the use of applications which previously could not be implemented effectively without digital communication technology. These include bridging distances within a substation across local networks, as well as between substations across wide area networks. Depending on the bandwidth and signal propagation delay (latency) of the network connections, additional new deployments have become possible. While formerly it was only possible to transfer a few bits using modulators and pilot wires, and even then often with significant delays, channels with bandwidths of multiple Mbit/s and low latency are frequently available now. Concepts such as remote interlocking or remote tripping of circuit breakers are increasingly being applied today.
Distributed hybrid measuring systemSupervising and evaluating the function of distributed systems requires a measuring solution which is capable of registering data and signals at all the relevant measurement locations of the distributed system. The system itself must also be hybrid in order to record both the classical signals and the network communication at the same time. The new DANEO 400 signal analyser from OMICRON offers these options. Its central software manages all the acquisition devices deployed at various measuring locations as a single measurement system. A precondition for such a system is that all recording devices can be accessed via a network connection. It is then no longer necessary to manually transfer individual files from the acquisition devices and compile all of the data. The measured data is actually compiled for the user in a transparent process and refers to the observed event – the subsequent analysis is then based directly on the related data. It is also possible to export the measured data in generic formats (COMTRADE, PCAP) in order to perform special analyses with external tools.
Time synchronisation and triggersA precise time reference is required in order to correlate data from multiple acquisition devices accurately. For example, to assess the propagation times for messages within local networks, the error of the time synchronisation must not exceed one microsecond. If the highly accurate Precision Time Protocol as per IEEE 1588 is being used in the respective network, the recording device can be synchronised optimally. Alternatively, the acquisition device can also receive the precise time from a GPS receiver. This ensures that the acquired data can be aligned correctly via individual time stamps. A reference for a common trigger is not necessary, since it is generally not possible for all acquisition devices to detect a trigger pattern at precisely the same time in distributed systems. The corresponding uncertainty needs to be compensated via a suitable pre-trigger time to ensure that the recordings from all acquisition devices cover the relevant time period. If only certain devices detect the trigger, they must then inform the other acquisition devices that a trigger has occurred. So-called post-trigger actions can be used to establish such a trigger chain.
Factory acceptance testTo provide evidence that a representative part of the automation system is working correctly, it is necessary to measure and document all relevant signals. On systems utilising IEC 61850 communication, the description of the communication system in standardised SCL format forms the basis for a comparison between the configuration and the GOOSE and Sampled Values streams actually present in the network. The reference to the signals from the process provides an indication for the correct coordination of procedures, for example the timed response of an interlocking function and its margins for secure operation. With the analysis of propagation times of data packets in the local network, it is possible to observe heavily loaded sections within the network infrastructure and highlight critical conditions which may possibly occur. In addition to this, analysis of the data volume on the communication network provides insights whether something like a suspected GOOSE avalanche is actually occurring and whether any effects on the reaction of the automation system should be anticipated.
Start-up and system acceptance testThe description of the communication system in standardised SCL format also forms the basis for this test when working with a system that utilises IEC 61850 communication. Since individual devices are put in operation one by one, verification of the anticipated configuration and the actual configuration can be performed repetitively – whereby the individual communication nodes do not need to be active at the same time. If a function stretches across multiple stations at different geographical locations and the data transfer is performed using a wide area communication connection, the time response of the signal transmission and coordination of the procedures can be captured using multiple, distributed acquisition devices. The signals used during the system acceptance test are documented and the archived measured data can then be used for analyses at a later date.
TroubleshootingIn order to track down sporadic malfunctions, acquisition devices can also be operated on site in an unattended mode. The acquisition devices only begin capturing signals when the configured trigger condition occurs and then they re-arm themselves again for further recordings as necessary. Depending on the type of data being captured and the duration of the intended operation, very large data volumes are to be expected in certain circumstances. In addition to the built-in mass storage (SSD), high-capacity external storage media can be connected to the DANEO 400. If a network connection is in place, remote access allows one to determine whether any recordings have taken place. If the bandwidth allows it, the recordings can also be downloaded via the network for analysis.
SummaryThere are many potential use cases for testing in distributed, hybrid automation systems. Whether the goal is purely to assess data transmission in a wide area network or simply to measure the classical signals of a secondary system – a hybrid measurement system, which can be used at distributed measurement locations with precise time synchronisation, can cover all of these scenarios. The standardised SCL data for an IEC 61850 system forms the comparative basis between configured messages and ones that are actually occurring in a communication network. The analysis of the captured data ultimately uncovers the relationship between classical signals and communication. New analyses can be performed at any time using the archived data.

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