Testing of IEC 61850 Sampled Value based Devices and Systems

Author:Alexander Apostolov, USA

The successful integration of these devices

  • The first component of the test system is the configuration tool that takes advantage of the standard substation or IED configuration files defined in Part 6 of IEC 61850
  • The second component is a Simulation Tool that generates the current and voltage waveforms. The specifics of each simulated test condition are determined by the complete, as well as the configured functionality of the tested device
  • The third component is the Test Evaluation Tool that includes the monitoring functions used to evaluate the performance of the tested elements of a distributed sampled analog value based system. It receives, compares and evaluates the multicast by the merging unit sampled analog values with the original current and voltage waveforms applied to the merging unit. The test system may also retrieve and compare the waveform records from the tested device to the original waveforms from the simulation tool
  • The fourth component is the Reporting Tool generating the test reports based on a user defined format and the outputs from the simulation and evaluation tools

Distributed Functions Based on Sampled Analog Values
Sampled analog values transmitted over the substation local area network are one of the key differences between the typical communication protocols and IEC 61850. They allow the elimination of copper wiring between the substation instrument transformers and the protection, control, monitoring and recording devices. As a result the user can achieve significant savings in wiring, while at the same time improve the safety of the substation environment and the functionality of the substation automation system, as well as to minimize the commissioning and maintenance costs. According to IEC 61850 all communications in the substation are based on logical interfaces. The logical interface of interest in the case of distributed sampled analog values applications is IF4: CT and VT instantaneous data exchange (especially samples) between process and bay level.

With the continuous improvements in Ethernet technology and the reduced cost of switched local area networks, multicast become the preferred choice for sampled measured values applications.
IEC 61850 defines a number of different groups of logical nodes that represent functional elements of primary or secondary substation devices. It also describes functions as tasks, which are performed by the substation automation system, and exchange data with other functions. The functions are performed by IEDs (physical devices) and may be split in parts residing in different IEDs but communicating with each other (distributed function) and with parts of other functions. In the context of this standard, the decomposition of functions or their granularity is ruled by the communication behavior only. Therefore, all functions considered in the standard, can be modeled by logical nodes that exchange data. A function is considered distributed when performed by two or more logical nodes located in different physical devices – the typical case in systems using IEC 61850 sampled values.

The above is another key characteristic of IEC 61850 that determines the functional requirements for the testing of multifunctional devices or distributed applications using sampled analog values.

Analog Signal Processing for Protection and Control
The analog input function in IEC 61850 is modeled by multiple instances of two logical nodes from the Instrument Transformers group T - TCTR and TVTR. Both use one instance per phase. These three or four instances of TCTR(n) and TVTR(n) may be allocated to different physical devices mounted within the instrument transformer per phase.

The currents and voltages from TCTR and TVTR accordingly are delivered as sampled values to the protection, measuring or recording functional elements.
In the case of a multifunctional IED all the logical nodes are located within the same physical device and the interaction between them is performed over the internal digital data bus of the device. This is not the case in a distributed function using the sampled measured values communications defined in IEC 61850. Even that the application will be modeled using the same logical nodes, they are not anymore located within the same physical device – i.e. we have a case of distributed function according to the IEC 61850 definitions and this has a significant impact on the testing.

The processing of the secondary currents and voltages represented by the TCTR and TVTR is performed in a new type of device – the Merging Unit. It is defined as an interface unit that accepts multiple analog CT/VT inputs and produces multiple time-synchronized serial unidirectional multi-drop digital point-to-point outputs.
The merging unit is synchronized using 1 PPS (Pulse per second) signal from a GPS receiver. As can be seen from the Figure 4, there is a time delay D = D1 + D2 introduced within the device. If this time delay is not compensated, it will be seen as a phase shift that will affect all functions using the sampled analog values. It is clear that such phase shift will have an impact on some protection functions. That is why it is very important that the merging unit is properly tested in order to evaluate its performance.

Distributed applications based on sampled values include protection, control, measuring, monitoring, and recording. They are different functionally but will have similarities in the requirements for functional testing.

Sampled Measured Values Test System Components

A test system designed for IEDs or distributed applications based on IEC 61850 9-2 has multiple components that are needed for the testing of the individual devices, as well as a complete application. A simplified block diagram of such a system is shown in Figure 1. The first component of the test system is the test Configuration Tool. It takes advantage of one of the key components of the IEC 61850 standard – the Substation Configuration Language. The Configuration Tool is used to create the files required for configuration of different components of the test system. It imports or exports different configuration files defined by Part 6 of IEC 61850.

The test system Configuration Tool reads the information regarding all IEDs, communication configuration and substation description sections. This information is in a file with .SCD extension (for Substation Configuration Description) and is used to configure the set of tests to be performed. The overall functionality of any IEC 61850 compliant device is available in a file that describes its capabilities. This file has an extension .ICD for IED Capability Description.
The IED configuration tool sends to the IED information on its instantiation within a substation automation system (SAS) project. The communication section of the file contains the current address of the IED. The substation section related to this IED may be present and then shall have name values assigned according to the project specific names. This file has an extension .CID (for Configured IED Description).

The second component of such a system is a Simulation Tool that generates the current and voltage waveforms. The specifics of each simulated test condition are determined by the complete, as well as the configured functionality of the tested device or application.

The simulation tool requirements will also be different depending on the type of function being tested. For example, if the tested function is based on RMS values or phasor measurements, the simulation tool may include a sequence of steps with the analog values in each of the steps defined as phasors with their magnitude and phase angle. Based on these configuration parameters the simulation tool will generate the sine waveforms to be applied as analog signals or in a digital format to the tested components or systems.

If the tested functions are designed to detect transient conditions or operate based on sub-cycle set of samples from the waveform, an electromagnetic transients simulation will be more appropriate. Different network simulation tools allow the user to configure the specifics of the network model, type of fault, fault location, etc. that are then used to calculate the waveforms to be applied to a device or system under test.

The third part of the test system is the Virtual Merging Unit simulator. While under conventional testing the waveforms generated by the simulation tool will be applied to the tested device as current and voltage analog signals, a Virtual Merging Unit will send sampled measured values as defined in IEC 61850 over the Ethernet network used for the testing.

The Virtual Merging Unit simulator supports the defined by IEC 61850 9-2 LE 80 samples/cycle in 80 messages/cycle. Each message contains one sample of the three phase currents and voltages (WYE class).

The fourth component of the test system is the Test Evaluation Tool that includes the monitoring functions used to evaluate the performance of the tested elements within a distributed sampled analog value based system. Such evaluation tool requires multiple evaluation sub-modules that are targeted towards the specifics of the function being tested. They might be based on monitoring the sampled measured values from a tested merging unit, GOOSE messages from a tested IED, as well as reports or waveform records from the tested device.
The fifth component of the test system is the Reporting Tool that will generate the test reports based on a user defined format.

Testing of IEC 61850-9-2 Based Merging Units
Since Merging Units are an essential component of any IEC 61850 process bus based application, they have to be tested to ensure that they provide the required sampled measured values. The currents and voltages applied to the Merging Unit will be based on current and voltage waveforms produced from the network simulator in order to simulate different system conditions, such as high current faults or low current minimum load conditions.

At the same time the Test Evaluation tool will need to receive the sampled analog values from the tested merging unit and compare the individual sampled values from the Merging Unit with the samples coming from the network simulator. The testing of Merging Units will require first of all a very accurate time synchronization of both the test device and the tested MU.

It is necessary to analyze the phase (time) and magnitude differences of the individual samples and compare these to the calibration specifications of the MU. Proper documentation and reporting is required in the same manner as meter testing is performed today (Figure 2). The Merging Unit test module supports the sampling rates defined for protection applications in IEC 61850 9 – 2 LE. The merging unit test evaluation tool is used for receiving, processing, viewing, and saving Sampled Values according to the implementation guideline of the UCA International Users Group. It subscribes to the Sampled Values streams from one or multiple merging units and displays the waveforms of the primary voltages and currents in an oscilloscope view. Individual values on the traces can be looked up and compared with each other (Figure 3).

Testing of a merging unit and in particular it's time synchronization is a challenging task. The test set is time synchronized and generates analog voltages and currents, which are accurately defined in magnitude and phase. The evaluation tool captures the Sampled Values from the merging unit for a detailed assessment of the conversion performance. For a comparison of the generated and measured values, SV Scout can also subscribe to the Sampled Values of the test set and display them as a reference. RMS values and phase angles are calculated from the Sampled Values and displayed in a phasor diagram and a table. The reference for the phase angles is selectable (Figure 5). The Detail View provides additional information about a selected Samples Values stream and its individual channels. This includes the zero crossings of specific channels, values for individual samples, and the decoded quality information.

Testing of IEC 61850-9-2 Based IEDs

The testing of different functions in IEDs that are based on sampled measured values can be achieved in a couple of different ways depending on the requirements of the specific test. One approach is acceptable when testing the IED only, while another can be used if the testing includes the complete MU/IED system. The difference is that in the first case there is no hard wiring between the test device and the tested IED – i.e. the test system can be communications based only.

The key component of this module is the Merging Unit simulator described earlier. It will have to take the waveforms generated from the Network Simulator and then format them in the required 80 samples/cycle and multicast the individual sampled values to the LAN 80 times per cycle (e.g. 80 messages/cycle) (Figure 6(a)).

The testing of different types of functions available in the IED will be similar to the testing of conventional hardwired or hybrid devices. The only difference is that an Ethernet link is used instead of a hardwired interface between the test device and the IED under test. This applies to both the configuration and analysis modules of the test system (See page 46).
In order to gain confidence that the performance of IEC 61850 Sampled values applications is similar to the performance of conventional hardwired devices of the same model the test system should be able to simultaneously apply the simulated analog signals to the hardwired IED and the corresponding sampled values to the IED with the SV interface. The GOOSE messages from both IEDs are received by the test devices and used to evaluate their performance (Figure 6(b)).

If we are testing the protection IED that is interfacing with a merging unit as a system, the test device will apply the simulated analog signals to the merging unit which will then publish the sampled values. The IED subscribes to these sampled values and publishes the GOOSE messages when it operates. These GOOSE messages are then used by the testing tool to evaluate the performance of the tested protection function based on the expected operating time..

The test system needs to subscribe to and monitor the GOOSE messages received from the tested IED that represent the operation of the tested functional elements in order to determine if the devices operated as required. If the tested device has relay outputs as well, they will have to be wired into the test device and their operation will be compared with the received GOOSE messages to determine if the performance of communications based solutions is similar to the hard-wired case.
The test system may also retrieve the waveform records from the tested device and again compare them with the original waveforms from the simulation tool. Figure 6(c) shows the system configuration for hybrid testing of IEDs that have relay outputs and at the same time support GOOSE messages.

Testing of IEC 61850 Digital Substation
The testing of distributed bay and substation level functions that are based on communications only – IEC 61850 8 – 1 or 9 – 2 – will be similar functionally to the testing of an individual IEDs. The main difference is that in this case there will be multiple test devices with virtual simulators or analog outputs. The simulation of the substation and system environment required for the functional testing of bay and system level functions will require the simulation of multiple merging units (IEC 61850 9-2) and other IEDs (IEC 61850 8-1).

The simulation tool will also be different, because first of all it will require a multi-node system simulator. Once the results from the simulation are available, the results from the Network System Simulator are distributed between the individual test devices that perform the simulations, as well as to make them available as sampled measured values from the virtual merging units that participate in the test.

The evaluation of the performance of the distributed functions in this case will be based on the subscription of the test system components to the GOOSE messages from the different IEDs participating in the tested distributed applications. At the final stage of the site acceptance testing when the test devices are injecting analog signals to the merging units in the substation yard and capturing the relay outputs operation that trip the breakers, all test devices need to be accurately time synchronized in order to perform the tests. A simplified block diagram of this test system is shown in the Figure

Biography

Alexander Apostolov received a MS degree in Electrical Engineering, MS in Applied Mathematics and Ph.D. from the Technical University in Sofia, Bulgaria. He has more than thirty years of experience in the field of electric power systems protection, control and communications. He is presently Principal Engineer for OMICRON electronics in Los Angeles, CA, USA. He is an IEEE Fellow and Member of the Power Systems Relaying Committee and Substations C0 Subcommittee. He is the past Chairman of the Relay Communications Subcommittee, serves on several IEEE PES Working Groups and is Chairman of Working Group D21: Supporting of IEC Standard for Distance Relay Characteristics. He is member of IEC TC57 and Convener of CIGRE WG B5.27 and member of several other CIGRE B5 working groups. He is Chairman of the Technical Publications Subcommittee of the UCA International Users Group. He holds three patents and has authored and presented more than 350 technical papers. He is IEEE PES Distinguished Lecturer, Adjunct professor at CPUT, Cape Town, S. Africa, and Editor-in-Chief of the PAC World magazine.