• Saravanan Balamurugan

A Protection Engineer’s Diary: Relay Pickup & Operate – A Comprehensive Clarification

As a protection engineer, we test varied relays with a varied combination of protection functions, each with its own philosophies and operation methods. Of all the commonality between them, two terms stick out and the most basics ones – pickup (start) and trip (operate).

Let’s start with the “start” condition.

Start or pick up as defined by IEC – to leave an initial condition or reset condition (Refer IEC 60050-447 or Electropedia) and Start Condition or pick-up condition is a condition when a measuring relay or protection equipment has started (picked-up).

From the Figure above, we can see the distinct differentiation between crossing a pick-up value and an actual pick-up condition. And more importantly, pickup or start has its own timing – start time or pick-up time, which is the duration between the instant a specified change is made in the value(s) of the input energizing quantity(ies) that will cause the measuring relay or protection equipment in initial condition or reset condition to start (pick up) and the instant it starts (picks up).

Now, operate as per IEC is to switch to an operate condition, where an operate condition is a specific condition when a measuring relay or protection equipment has performed its required function. Operate time is just the duration between the instant a specified change is made in the value(s) of the input energizing quantity(ies) that will cause the measuring relay or protection equipment in initial condition to operate and the instant it operates.

Please keep in mind that the term "operating" signal is wrong. “Operating time” is instead related to a circuit breaker and should not be used in relay context (Refer IEC 60050 - 448).

Now comes a very important question - For time-based relays i.e. relays with an intentional time delay, when does the relay operate? or in other words, when does the timer for the time delay actually start? Does it start after the pickup time has elapsed or does it start as soon the relay picks up?

IEC defines the relationship between the start (pickup) time and the operate time as per the figure below, where we can see that the operate time has started the moment, the relay has crossed the operate value.

A key thing to note here is - when we give an intentional operate time delay, in case of definite time relay, it is considered as an extra time delay on top of the start or pickup time. So, the operate time is equal to the start time + operate time delay setting.

Relays from some manufacturers may compensate for this additional start time, by reducing the average value from the operate time delay. For example, if the time delay setting is 500ms, and the "average" start time is 30ms, then the relay assumes a timer of setting - 500 - 30 = 470ms

Taken from IEC 60255-181:2019, the figure above clearly shows that for an under-frequency protection relay, the operate time delay setting is an intentional time delay defined by a user setting which is activated by the start signal to assert the operate signal.

This clearly means that start time or pickup time plays a very important role in the operation of our protection systems, and testing of the pickup value along with the pickup time is essential for a clear understanding of the system behaviour and can help us in preventing any erratic breaker tripping or maloperations.

Looking at various type test reports of relays, their product guides and technical manuals, the following table has been compiled:

This shows that for protections with frequency as the characteristic quantity, the pickup time as per the manufacturers' documents is as high as 220ms. Some manufacturers do not specify these values for certain relay models as well. For frequency based protections, the pickup time is based on the window size, either default or user adjustable.

But, is this even showing us the whole picture?

There are two ways of testing / checking the operate time of any protection functions -

1. inject the fault directly and measure the operate time or

2. maintain a pre-fault condition for a couple of seconds, and then inject a fault for measuring the operate time. Most of the time, we do not inject the pre-fault - time availability being the culprit for that. Another crucial test is the pickup/dropoff test - usually performed by ramping the characteristic quantity at a certain speed (dx/dt) to check the pickup value.

Can the test methodology and parameters change the results? What would be the impact of the relay measurement philosophy on the start and operate times?

Or can the philosophy of the testing equipment like the Omicron CMC, Megger SMRT or SVERKER 900 itself result in a variation?

Answer is in the next page - Coming soon !

DISCLAIMER: This article is not intended to be biased towards any manufacturer or their products. Any errors in the numbers present may be a result of outdated documents.

All images were taken from the Electropedia (http://www.electropedia.org/) and from the respective IEC Standards.

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