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Functional Testing of IEC 61850 Based Protection Relays
A. Apostolov (OMICRON electronics), B. Vandiver (OMICRON electronics)
I. INTRODUCTION
IEC 61850 is a new international standard for communications in substations that is creating
opportunities for a revolution in the way we design and implement the protection, automation and
control of electric power systems. It represents the next step in integration of multifunctional Intelligent
Electronic Devices (IEDs) based on the development of advanced distributed protection and control
applications.
IEDs are the standard in new or upgraded integrated substation protection, monitoring and control
systems. Protective IEDs are sophisticated multifunctional devices designed to protect substation
equipment and the electric power system from the effects of different abnormal system conditions. Since
fault conditions are very rare in the system, to take advantage of their data acquisition and processing
capabilities they also include multiple non-protection functions like metering, disturbance and event
recording, as well as some built in fault analysis tools. This makes them the typical device at the lower
level of the substation automation system.
Control, power quality monitoring and disturbance recording devices may complement the protection
IEDs by providing some specific functionality that may not be available within the relays. This allows
the optimization of the integrated substation automation system, while at the same time meets the strict
requirements for reliability and security.
The operation of all above described devices is based on currents and voltages from the secondary
windings of the instrument transformers or equivalent sources.
Tripping of the breakers in case of fault or exchange of signals between IEDs in a protection or
control scheme is performed using hard wiring between the different devices.
Methods for testing of such conventional microprocessor based IEDs and schemes are well defined
and software and hardware tools with advanced functionality are available to the testing specialists.
IEC 61850 offers significant advantages compared to the previously used substation communications
protocols in the sense that it allows the replacement of analog and binary signals with communications
messages. This reduces the cost of the system and improves the flexibility and functionality. But it also
imposes new requirements for the testing of IEC 61850 based devices.
II. MULTIFUNCTIONAL PROTECTION IEDS IN IEC 61850 BASED SYSTEM
The proper testing of any protection IED that can be used in a substation automation system is
possible only when there is good understanding of its functionality. This includes not only the basic
protection functions of the device, but also built-in and user programmable schemes.
When we talk about multifunctional protection IEDs in a conventional substation automation system
we need to consider also their non-protection functions, such as:
• Measurements
• Control
• Monitoring
• Disturbance recording
• Event recorders
Conventional protection IEDs have a hardware architecture that includes analog and binary (opto)
input modules, as well as relay outputs. The analog input modules convert the secondary currents and
voltages into digital samples transferred over an internal to the IED data bus to the processing module of
the IED. The different sampled or calculated values are then used by the individual functional elements
or schemes that may lead to the operation of the device and activation of one or more relay outputs.
The Station Bus is used for IED to IED communications in a peer-to-peer mode, as well as for
interface between the IEDs and the substation level HMI and other applications in a Client – Server
mode. It allows the replacement of copper control cables with fiber and leads to improvements in the
functionality and reduction of engineering, commissioning and maintenance costs.
IEC 61850 defines also the use of another type of device related to protection or non-protection
functions – the Merging Unit (MU). It is an interface unit that accepts multiple analogues CT/VT and
binary inputs and produces multiple time-synchronized serial unidirectional multi-drop digital point to
point outputs to provide data communication via the logical interfaces 4 and 5.
Existing Merging Units have the following functionality:
• Signal processing of all sensors – conventional or non-conventional
• Synchronization of all measurements – 3 currents and 3 voltages
• Analogue interface – high and low level signals
• Digital interface – IEC 60044-8 or IEC 61850-9-2
It is important to be able to interface with conventional and non-conventional sensors in order to allow
the implementation of the system in different substation environments. A simplified diagram with the
communications architecture of an IEC 61850 based substation automation system is shown in Fig. 1.
Ethernet Switch
Rou ter
SCADA Master
WAN
Su b st a t i o n
Computer
Su b s t a t i o n
HMI
MU IOU MU
IED IED IED IED
Ethernet Switch Process Bus
Substation Bus
IOU
Fig. 1. Simplified communications architecture
Interoperability between merging units and protection, control, monitoring or recording devices is
ensured through a document providing implementation guidelines. Two modes of sending sampled
values between a merging unit and a device that uses the data are defined. For protection applications
the merging units send 80 samples/cycle in 80 messages/cycle; i.e each Ethernet frame has the MAC
Client Data containing a single set of V and I samples. For power quality monitoring and waveform
recording applications such sampling rate may not be sufficient. That is why 256 samples/cycle can be
sent in groups of 8 sets of samples per Ethernet frame sent 32 times/cycle.
The information exchange for sampled values is based on a publisher/subscriber mechanism. The
publisher writes the values in a local buffer at the sending side, while the subscriber reads the values
from a local buffer at the receiving side. A time stamp is added to the values, so that the subscriber can
check the timeliness of the values and use them to align the samples for further processing. The
communication system shall be responsible to update the local buffers of the subscribers. A sampled
value control (SVC) in the publisher is used to control the communication procedure.
The receiving devices then process the data, make decisions and take action based on their
functionality. The action of protection and control devices in this case will be to operate their relay
outputs or to send a high-speed peer-to-peer communication GOOSE message to other IEDs in order to
trip a breaker or initiate some other control action.
The Process Bus offers significant advantages and can be successfully used especially in the
refurbishment of old distribution substations that require replacement of copper cables with failing
insulation. The functions in an IEC 61850 based integrated protection and control system can be local to
a specific primary device (distribution feeder, transformer, etc.) or distributed and based on
communications between two or more IEDs over the substation local area network. This will have an
impact on the testing of the individual devices used in the substation.
Fig. 2. Hybrid IEC 61850 compliant IED
The implementation of IEC 61850 in the substation IEDs can be defined as partial, hybrid and
complete. We consider an IED as one with partial implementation when it supports only station bus
communications, while the analog signals are based on conventional hard wiring.
Hybrid implementation defines the case when the IED has process and station bus interface, but the
execution of the trip function is based on hard wires between the relay outputs and the breaker trip coil.
A hybrid implementation solution block diagram is shown in Figure 2.
Full implementation is when an IED has communications based interface only. The difference in the
case compared to the hybrid IED is that here the tripping is based on a GOOSE message sent to the
breaker control device that performs the actual breaker tripping. Figure 3 shows the back of an IED with
full implementation of IEC 61850.
Fig. 3 IED with full implementation of IEC 61850
The testing of devices with partial, hybrid and full implementation of IEC 61850 require some
additional functions in the testing tools and devices.
III. FUNCTIONAL TESTING OF IEC 61850-BASED PROTECTION IEDS
The new concept for the execution of protection and control functions introduced by IEC 61850
results in requirements for a different approach and technology for testing as well.
The testing of conventional protection devices has some similarities and some differences with the
IEC 61850 communications based solutions. As can be seen from Figure 4, in the case of conventional
testing the test device has to simulate the substation process through hard-wired interface between the
analog and binary outputs of the test device and the analog and binary inputs of the test object. At the
same time the test device has to monitor the closing of relay outputs of the tested device in order to
detect the operation of the IED and analyze it to determine if the performance meets the specification.
The operating time usually is measured from the simulated by the test device process change of state
that has to trigger the tested function until the moment when it will detect the operation of the IED relay
output controlled by the tested function.
Multifunctional
IED
Laptop
Computer
Fig. 4 Conventional IED functional testing
V ITrip
52a
Test
Device
Figure 5 shows the testing configuration for a partial implementation of IEC 61850 communications
in the tested IED. In this case the multifunctional IED interfaces with the process in a similar way to the
conventional method described above, i.e. hard wired analog signals between the test object and the test
device.
The communications based distributed functions in this case use IEC 61850 GSSE or GOOSE
messages. All devices with communications interface have to be connected to the substation network
switch as shown in Fig. 5.
Fig. 5 IEC 61850 GOOSE based IED functional testing
Since the expected communications based performance should be similar to the conventional hard-
wired interface, it is a good idea to compare the operation of a relay output and a GSSE message driven
by the same functional element in the IED logic.
Another difference between the conventional testing and the IEC 61850 GSSE or GOOSE based
functional testing is that the state-of-change part of the process simulation is achieved by a GSE
messages sent from the test device to the test object. An example is to indicate the opening of the
auxiliary contact 52a of the circuit breaker monitored by the tested IED.
Figure 6 shows the simulated waveforms from a functional test of an IEC 61850 GOOSE based device
and the captured operation of the relay output and the received communications message. As can be
seen from the recording, the operation of the captured hardwired trip (HWT) output and the GOOSE
based Trip output look identical and are with a range of less than 100 microseconds for this IED.
The test device has to be able to simulate more than one IEC 61850 IED since the test case may
require interface of the test object with several devices. For example if a permissive overreaching
scheme implementation and configuration in a distance relay is being tested, including single pole
autoreclosing, the test object will need to receive:
• GOOSE message from a communications IED indicating that it received a permissive signal from
the remote end of the line
• GOOSE message from the breaker IED indicating the change of state of each phase of the breaker
IEC 61850
Based IED
V I
Ether net
Ether net
Switch Laptop
Computer
GOOSE
or GSSE
GOOSE
or GSSE
Trip
Ether net IEC 61850
Based Test
Device
The above requires the ability to configure the test device through the testing software to publish two
different GOOSE messages - one representing the communications IED, the second representing the
breaker IED.
Fig. 6 IEC 61850 GSSE based IED functional testing results
The test device also needs to be configured to subscribe to the GOOSE messages from the test object
as shown in Figure 7. Considering the fact that the GOOSE configuration of all IEDs in an IEC 61850
based substation automation system is achieved using the different files defined in the Substation
Configuration Language (SCL).
Fig. 7 IEC 61850 test device GOOSE subscription
The subscription of the test device to the GOOSE messages from the tested IED will be based on the
extracted information from the Substation Configuration Description (SCD) file.
The SCD file should also be used as the source of information to configure the publishing of GOOSE
messages simulating other IEDs included in a tested distributed application such as the breaker and
communication IEDs in the example described earlier. A simplified block diagram of the IEC 61850
based device testing system is shown in Figure 8.
When the test object is a partial or hybrid implementation of IEC 61850, the test device will also have
to monitor the change of state of the relay outputs of the tested IED. This means that the configuration of
the binary inputs of the test device will represent a mixture of physical (opto) inputs and virtual
(GOOSE) inputs. All of them are treated in a similar way by the processing modules of the test device
and time stamped for analysis of the behavior of the tested relay – as shown in Figure 6).
Fig. 8 IEC 61850 test configuration process
Figure 8 also shows the Merging Units simulators required in the case of testing of IEDs with a hybrid
implementation of IEC 61850.
A network simulator, a state sequence simulator or scheme testing tool can be used to produce the
sampled values with a sampling rate of 80 samples/cycle or 256 samples/cycle as required by the type of
device or function being tested. The test device formats the Ethernet message according to IEC 61850 9-
2 LE and publishes the sampled analog values over the network for testing.
Testing of devices with hybrid or full implementation can be combined with the testing of a merging
unit. In this case the analog signals from the test device will be hardwired to the Merging Unit. The
process bus based function will be performed by the IEC 61850 based IED that will send a GOOSE
message to an IO Unit that will operate a relay output to control the process, for example to trip the
breaker.
Fig. 9 IEC 61850 process and station bus test setup
V I
Ethern et
Switch
Laptop
Computer
Ether net
GOOSE
or GSSE
GOOSE
or GSSE
Trip
IEC 61850 Based
Test Device
IEC
61850
Based
MU
IEC
61850
Based
IOU
IEC
61850
Based
IED
The test device monitors different elements of the distributed function and can analyze their
performance, as well as the overall function operating time.
When the multifunctional IED with the tested function operates, it will send a GOOSE message to the
interface unit that will control the process. The test device will subscribe and capture this message and
also detect the operation of the binary output of the interface unit.
The difference between these two times can be used to calculate the required time to send a GOOSE
message over the network, process it in the interface unit and operate the binary output. The test setup
will look like the block diagram shown in Figure 9.
If the tested IEC 61850 based IED also has a binary output, the test device can monitor it as well. This
can provide valuable information in the overall performance evaluation process.
The testing of the merging unit (MU) in this case will require comparison between the analog signal
waveform applied to it and the IEC 61850 9-2 LE messages sent by the merging unit. The test device
needs to subscribe to these messages and perform the comparison and evaluation. It should include not
only the accuracy of representation of the waveform, but also any phase shift that may be the result of
the processing of analog signals in the MU The time stamps of the sampled values will be used for this
purpose. Accurate time synchronization of both the test device and test object is essential for the testing
of the merging unit.
IV. CONCLUSIONS
The development and implementation of IEC 61850 based devices and substation automation systems
requires a new generation of specialized test devices and methods for functional testing of different
components of the system.
Simulation of the substation process and monitoring of the operation of the tested devices or
distributed control and protection functions is quite different. The conventional testing requires hard
wiring between the test device and the test object, while in the case of IEC 61850 based applications it
may be partially or completely communications based.
Comparison between the performance of hard-wired and communications based solutions is required
to confirm that IEC 61850 can be successfully used to replace conventional substation control and
protection systems without any degradation in the overall performance of the system.
Different testing configurations are analyzed and show that state-of-the-art test systems can be
successfully implemented for the functional testing of IEC 61850 based control and protection functions
in different types of IEDs and different levels of implementation of the standard.
V. REFERENCES
[1] IEC 61850-1 Communication Networks and Systems in Substations, Part 6: Substation
Configuration Language
[2] IEC 61850-9-2 LE: Implementation Guideline for Digital Interface to Instrument Transformers
Using IEC 61850 9-2
