The International Electrotechnical Commission (IEC) is currently working on a new series of standards that covers the functional requirements of measuring relays and related equipment used to protect electrical distribution and transmission systems.

There is a common belief that these standards will only be of interest to relay manufacturers, but in reality, they also promise big benefits for the entire protection relay community.

This article was originally published in Smart Energy International 3-2019. Read the full digimag here or subscribe to receive a print copy here.

Megger’s Andrea Bonetti, who is a member of the committee developing the standards, explains more about them.

Electrical systems are becoming more and more complex and sophisticated. As a consequence the protection systems are also becoming ever more complex and, at the same time, the consequences of protection failures are becoming costlier and more disruptive. It is clear, therefore, that there is a real need for internationally recognised standards to define the functionality of the key components – the protection relays and their protection functions – that form the basis of these systems. To meet this need, the IEC is currently working on the new IEC 60255-1xx series of standards. Before looking at the benefits these can provide, however, let us consider a little background information.

The new standards are being developed and published by Maintenance Team 4 (MT4) of IEC Technical Committee 95 (TC 95) which, according to the IEC website, has as its terms of reference “Standardisation of measuring relays and protection equipment used in various fields of electrical engineering covered by IEC, taking into account combinations of devices to form schemes for power system protection including the control, monitoring and process interface equipment used with those systems.” It can be seen that these terms are very wide ranging and, as a result, there will ultimately be many standards in the series. Some of these have already been published while others are in various stages of preparation.

Examples are IEC 60255-121, which relates to distance protection; IEC 60255-151, which relates to current protection; and IEC 60255127, which relates to voltage protection.

What all of the standards have in common is that they specify minimum functional requirements, testing methodologies and methods of performance evaluation, as well as the format for publishing the test results for each function. The aim is to help users with relay selection, setting, commissioning, application and operation.

At the time of writing, the IEC 60255-1xx standards that have already been published are:

• IEC 60255-151:2009 – Functional requirements for over/under current protection.

• IEC 60255-127:2010 – Functional requirements for over/under voltage protection.

• IEC 60255-149:2013 – Functional requirements for thermal electrical relays.

• IEC 60255-121:2014 – Functional requirements for distance protection.

Work on the following standards is at an advanced stage, and they are expected to be approved and published before the end of 2019:

• IEC 60255-181 – Functional requirements for frequency protection.

• IEC 60255-187-1 – Functional requirements for restrained and unrestrained differential protection of motors, generators and transformers. These standards are in the course of preparation, and they are planned for release from 2020 onward:

• IEC 60255-187-2 – Functional requirements for busbar differential protection.

• IEC 60255-187-3 – Functional requirements for biased (percentage) differential relays for transmission lines.

In addition, the TC95/MT4 committee is looking at the application in relay protection systems of the IEC 61850 standard, which relates to communication networks and systems for power utility automation. To facilitate this work, the committee has created a special ad hoc group (AHG3) to address “the use of digital sampled values instead of analogue inputs.” It is expected that the work being carried out by this group will, at some point in the future, impact standards in the IEC 60255-1xx series.

Having explained the background for these standards and discussed their current status, let us move on to consider their applications and benefits. Many engineers and technicians who work with relay protection systems believe that the standards apply only to relay manufacturers and that as users they do not need to be aware of the contents. This is unfortunate because – although it is certainly true that the standards apply to relay manufacturers – it is equally true that users will benefit from knowing about and understanding the issues the standards address. Relay users also need to take the content of the standards into account in their work.

It is anticipated, for example, that the standards will provide an excellent reference for relay acceptance tests performed by end users. This shouldn’t be taken to imply, however, that the standards include pass/fail criteria for tests – they don’t, except in a few very particular cases. Instead users are expected to create their own “user profiles” which include acceptance criteria and requirements relating to their specific applications. In creating these profiles, however, the users will be guided by the information and definitions contained in the standards.

The standards also contain details of declarations that relay manufacturers are required to make to aid the engineering processes involved in the protection of electrical substations. Knowing that the manufacturers must provide this information will make it easier for engineers to design reliable protection systems.

As an example, particular emphasis is given by the standards to the current transformer dimensioning formulas that manufacturers are obliged to provide, which now have to be in a standardised mathematical format. In fact, gaining a consensus in the protection relay community about the importance of these formulas for relay users is, in my opinion, one of the most important goals achieved by the TC95/ MT4 committee. Relay manufacturers are required to carry out extensive testing to comply with this requirement, which will result in benefits that include easier, safer and more reliable design and engineering of protection systems.

It is worth mentioning that the standards do not include details of specific commissioning or routine tests for protection functions.

Nevertheless, there are several clauses that can be applied to these tests and doing so will help to minimise the risk of misunderstandings about validation in the field compared with the performance declared by the relay manufacturer or requested by the user. In addition, manufacturers of relay test equipment will be expected to implement the definitions and test methodologies detailed in the standards, in relation to both commissioning and maintenance tests.

In conclusion, we have seen that for relay manufacturers, the standards provide detailed requirements to which they must adhere. For users they provide clear and unambiguous guidance on what the performance relays should provide and the documentation they can expect from the manufacturers. And for manufacturers of protection relay test equipment, the standards dictate the testing methods they will be expected to support in the future. In short, standards in the new IEC 60255-1xx series are relevant and important to everyone whose work involves relay protection systems, whatever their role in this wonderful technical community. SEI

About the author

Andrea Bonetti MSEE is a Senior Application Specialist in Relay Protection and IEC 61850. He is an active member of IEC TC 95 / MT 4 ”Measuring relays and protection equipment” since 2006.
He is based at the Megger offices in Sweden.