The new IEC 60255-121 standard

Authors: Andrea Bonetti, FMTP Power AB, Sweden, Murty Yalla, Beckwith Electric, USA and Stig Holst, ABB, Sweden

What is a Distance Protection Relay?

The standard defines a distance protection relay using an example block diagram as shown in Figure1. For a numerical protection relay this block diagram shows several algorithms such as signal processing, fault detection,  faulty phase or phases (phase selection elements), the fault direction (directional elements), the  impedance calculation (fault distance measurement), and distance protection characteristic and logic. 
 The standard doesn’t make any reference to the commonly used definition ”full scheme distance protection relay”; definition  that  has  created  several  misunderstandings  between  relay  manufacturers  and  users  and that if strictly followed in its implementation (”all six measuring loops must be active at the same time”) would bring to potential issues related  to  overreaching  distance  protection  relay. 
The wording “full scheme distance protection relay” could be eliminated by the technical requirements of the users, and also from the technical descriptions of relay manufacturers.
The standard clarifies also the definition of “switch onto fault” and “trip on reclose” as special conditions that are supposed to be taken care by an extra function, which could be separated from the distance protection function, or could be part of the distance relay itself.

As switch onto fault condition has been often confused as a normal operating condition for the distance protection relay; it is expected in the near future to have the end-users specify these two phenomena and relay response requirements under these conditions.
Often an instantaneous relay operation with lock- out relay activation is required for the “switch onto fault” case, while the lock-out relay should not be activated on the “trip on reclose” case, but different user applications may bring to different requirements.

Characteristic of Distance Protection
Considering the complexity of a distance protection relay, it is difficult to simplify its behavior in a characteristic shape on the so called “impedance plane”. The relay characteristic depends on the power system that is considered; for instance the resistive reach of the relay in a radial feeder situation will be higher than the reach in case of double-infeed system.
The standard defines a reference condition for the power system in evaluating the relay characteristic, which is a radial feeder system, without any superimposed load.

This simplified power system is very important even for double infeed applications, which are typical of high voltage power networks; when one breaker has opened, the second relay on the other line-end, operates with sequential trip under the mentioned radial feeder conditions graphically shown by the characteristic.
The standard requires the distance protection characteristics to be published for phase to earth, two-phase and three- phase faults.

Manufacturers are allowed to publish the distance protection characteristic in the domain that they prefer (ohm/loop, ohm/phase or combinations of both).
The accuracy of the distance protection characteristic has been defined for the entire characteristic, and in order to not confuse this accuracy with the previously known “static accuracy”, the name for this accuracy is “Basic accuracy” as shown in Figure 2.
It was not a common practice for the relay manufacturers in the past to publish the relay characteristic accuracy for the entire characteristic.

Most manufacturers only published the accuracy of the relay reach at the protected line angle.
This created misunderstandings during the commissioning phase of the distance protection relays, where different relay test set manufacturers applied their own definition of the so called “tolerance bands” for the distance protection characteristic.
It is expected in the near future that all relay test sets will be able to use the IEC 60255-121:2014 definition of “basic accuracy” when assessing the basic accuracy of the distance protection characteristic.

The characteristic tests are intended to be performed at the rated frequency and at “off-frequency” of the power system.
The test methods defined to measure the basic accuracy are clearly described by the standard as “non-realistic” from the power system simulation point of view, but are considered good enough for the purpose of measuring the basic accuracy of the distance protection relay.
The test methods defined for assessing other relay performances (operate time, transient overreach etc.) are instead based on realistic simulations of the power system.

Distance Protection Settings
Distance protection relays from different manufacturers or two different relay models from the same manufacturer may have different philosophies in the relay settings.

Often two different distance protection relays are protecting the same power line (Main1/Main2, or Sub1 / Sub2 applications) but the settings they have may be completely in different format, as well as their graphical characteristic shape, because the standard allows the manufacturer to publish the relay characteristic in the domain they prefer.
To  help  the  users  in  understanding  the  different  relay  setting  philosophies,  the  standard  requires  the  relay manufacturers to calculate and publish the relay settings for an example power system.
The example power system shown  in  Figure  3  is  described  in  the  normative  (mandatory)  annex  C:  “Setting  Example”  of  the  standard.

Operate Time and Transient Overreaching SIR Diagrams

The standard clarifies that the operate time of a distance protection relay depends on several factors such as:

  • Fault current level
  • Distance to fault
  • Source impedance ratio (SIR)
  • Magnitude and time constant of DC component
  • Type of fault

When describing the tests for measuring the operate time, the standard defines the relay operate time as the time interval from when the power system fault starts to when the relay operates.

This should clarify several past misunderstandings on the definition of the relay operate time. Sometimes the operate time was considered as the time interval from when the relay starts to -when the relay operates.
The standard specifies a new method to publish the relay operate time as function of the fault position, fault type and also the source impedance ratio (SIR) which is defined as the ratio between the source impedance and the setting impedance of the relay (relay reach).

These diagrams are called SIR diagrams in the standard. Previously some relay manufacturers did publish a sort of similar diagrams, under the name of “isochronic diagrams”. The difference between the “isochronic diagrams” and the “IEC 60255-121 SIR Diagrams” is that in the isochronic diagrams the operate time is represented as function of the fault position (on the “y” axis) and the source impedance ratio (on “x” axis).
In the SIR Diagrams, the operate time is on the “y” axis, the fault position on the “x” axis and for each SIR one curve “fault position vs operate time” is plotted.

As the isochronic diagrams have often been confusing in their reading and comprehension by the users, the MT4 committee took the decision to show the same information in hopefully a more intuitive way as shown in the SIR diagram of Figure 4.
The SIR diagrams are obtained from operate times measured on the protection relay (instantaneous underreaching zone, typically zone 1) which is energized with quantities generated by the simulation of defined power network models.
These signals provide the relay under test with the more realistic voltage and current waveforms. Real time power system simulations are expected to be used for performing these tests, but new simplified software tools are appearing on the market based on the test definitions of the IEC 60255-121.

The simulated fault types are:

  • Phase to earth faults
  • Phase to phase faults
  • Phase to phase to earth faults
  • Three-phase faults

The SIR diagrams also show the transient overreach of the protection relay. In Figure 4 it is possible to see that the relay in the example does not overreach for the tests with SIR = 5, but overreaches for the tests with SIR = 10, 30 and 50.
The network models to be used for the evaluation of the SIR diagrams are shown in Figure 6. The standard specifies the line data and source impedance data for two network models denoted as “short line” and “long line”.
As far as all the other performances and tests are concerned, the standard defines the test conditions and also how the data shall be reported, but doesn’t specify any value to assess pass or fail of the relay performance; this decision can only be taken with consideration of the application.

For instance in some applications it is preferable to install a slower protection relay, with no transient overreach at , and in some other applications it may be preferable to use a faster protection relay, with higher transient overreach.
The decision is left to the user on the applicability of the protection relay for his application.

CVT-SIR Diagrams
As the Capacitive Voltage Transformers (CVT) can have an impact on the operate time (and transient overreach) of the distance protection relay, the standard also proposes a CVT model which is used in simulating CVT transients.
In order to show the impact of CVTs on the relay performances, the standard requires to run the same tests performed for the SIR diagrams (where the CTs and VTs were considered as ideal), but for the VT model the proposed CVT model shall be used for the simulations (the transient response of the model is classified as T2 according to IEC 61869-5:2011).

The diagrams obtained with these tests are called CVT-SIR Diagrams as shown in Figure 5.
By comparing the SIR Diagrams with the CVT-SIR Diagrams it is possible to see the impact of the CVT on the relay performances (operate time and transient overreach).
In the past only a few relay manufacturers used to provide data of the distance protection performance when connected to capacitive voltage transformers. This led to unfair relay performance comparison between data implicitly referred to the connection to magnetic voltage transformers, and data of some relays implicitly referred to connection to CVTs.
The intention of the standard is to differentiate the testing requirements of distance protection with capacitive voltage transformers as special attention needs to be given in the relay design to address the CVT transients.
It has always to be noted that the standard specifies the minimum requirements  for functional  and performance evaluation of distance protection relays.

This means that all relay manufacturers shall at least publish the CVT-SIR diagrams for the CVT model indicated in the normative annex K [and shown Figure 7. This is important to allow fair comparisons between different relays.  As for all other tests and performances indicated in the standard, it is expected that the relay manufacturers will perform a larger number of tests with several CVT models and/or direct field tests along with several power transmission line configurations to get a comprehensive set of CVT-SIR Diagrams.

Requirements for Current Transformers

The standard formalizes how relay manufacturers shall declare the requirements for the Current Transformer (CT) sizing, allowing the users to understand (and compare) the CT requirements for any given distance protection relay application.
Before this standard was approved, there was no formal requirement for the relay manufacturers to take the responsibility and clearly specify the CT requirements for their distance protection relays.
The manufacturers that did specify the CT requirements for their distance protection relay used different methods and formulae for the user to calculate the necessary size of the CTs.

Above all there were no specification about minimum fault positions (forward and reverse close-in faults, zone 1 underreach and overreach faults) that should be considered when deciding the CT requirements.

Nor the degree of DC offset and levels of CT remaining flux that should be considered were specified. All together the CT requirements specified by different relay manufacturers were not comparable for the users.
Several relay manufacturers did not specify any requirement for the CT sizing.

The standard requires that for correct operations of the distance protection relay a minimum level of saturation voltage from the connected current transformers must be ensured.

The CTs must have a minimum rated equivalent limiting secondary e.m.f. Eal according to IEC 61869-2 (Instrument transformers Part 2: Additional requirements for current transformers).

The required rated equivalent limiting secondary e.m.f. Ealreq depends on the design of the relay and on the application. This Ealreq is given in the following equation:

Ealreq = If / Ipr * Ktot * ISR (Rct + Rba)                              (1)

If              is the maximum primary fault current for the considered fault case.    
Ipr            is the CT rated primary current
Isr            is the CT rated secondary current
Ktot          is the total over-dimensioning factor (including the transient dimensioning factor and the remanence dimensioning factor)
Rct           is the CT secondary winding resistance
Rba          is the total resistive burden, including the secondary wires and all relays in the circuit

The Ktot factors are specified and provided by the relay manufacturer for all fault positions or cases specified in the standard. The factors normally depend on the primary time constant and shall be reported for the complete intervals of primary time constants specified in the standard.

The values of If, Rba, Isr and Ipr depend on the application and on short circuit studies. They are determined by the user.
The relay manufacturer needs to perform comprehensive tests to determine the necessary over-dimensioning factor Ktot for the different cases. The practical execution of these tests depends on the available test environment. Therefore there is no mandatory test method specified and the standard does not state in details which CT models shall be used or the amount of necessary and required tests to decide the CT requirements.

As the relay manufacturer has detailed information about analog input circuits, the measuring algorithms and the relay protection algorithms it is left to the relay manufacturer to perform the necessary amount of tests to fulfill the criteria and conditions stated in the standard.
However, the standard provides an informative guide describing example test procedure to determine the CT sizing requirements.  


Andrea Bonetti has a MSEE degree from Universitá La Sapienza of Rome, Italy. Andrea worked as high voltage protection relay engineer at ABB Substation Automation Products in Västerås, Sweden, with main focus to line protection relays and applications.  From 2008 to 2012 Andrea worked at Programma/Megger in Stockholm, as product manager and technical specialist for relay test equipment. After working at STRI AB as technical manager for the Substation Automation Unit, Andrea works now at FMTP AB as technical manager. He holds a patent in the area of IEC 61850 testing tools and algorithms for protection and control applications. Andrea is member of the IEC TC95/ MT4; was sub-group leader for the development of the IEC 60255 -121 standard and received the IEC 1906 Award for the contribution to the development of the IEC 60255-121 standard in 2013.


Murty V.V.S. Yalla has been with Beckwith Electric Co. since 1989 and presently is the President. He has a BSEE and MSEE from India and a Ph.D. in EE from the University of New Brunswick, Canada. From 1988 to 1989, he taught and conducted research at Memorial University in Newfoundland, Canada. He holds five U.S. patents. He is the chair of IEC TC95 and convener of TC95 MT4. He was a U.S. delegate to the CIGRÈ WGs on Modern Techniques for Protecting and Monitoring Generating Plants and Power Transformers. He was a member of the NERC System P&C Subcommittee. Dr. Yalla is IEEE Fellow. He is the Chair of the Rotating Machinery Protection Subcommittee of the IEEE PES PSRC. He was the chairman of the WG which developed IEEE Standard C37.102-2006." He co-authored an IEEE PES tutorial on the "Protection of Synchronous Generators." Dr. Yalla received the IEC 1906 Award in 2010 which honors the IEC experts worldwide.


Stig Holst is Application Senior Specialist at ABB AB, in Vasteras, Sweden.  He received his MSEE degree from Chalmers University of Technology in Gothenburg, Sweden. Stig joined ABB in 1996 working in the area of application of protection for lines, busbars, transformers and generators. He is also a specialist regarding transient performance of CTs and CT requirements of protection. From 1973 to 1996 he worked in different leading positions at the Swedish utility Sydkraft (today a part of E.ON). He was project manager for international consultant services. He participated in international WGs and was the convenor of a CIGRE WG regarding co-ordination of protection relays and conventional CTs. He has also published several articles in the protection system field. Stig is a member of Cigré and from 2004 to
2012 he was the Swedish regular member of SC B5.  He is a member of IEC TC95/MT4 and received the IEC 1906 Award in 2015.

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