Frequency Rate of Change

By looking at the maximum rate of change of frequency specified in the equipment connected downstream of it, and setting the trip setting to that figure or lower.

1) Is your system is isolated or interconnected?

2) Where is the relay is located, at an interconnection tie-line, nearer to generation or the load?

3) What are you trying to protect against, system separation and/or equipment damage?

4) What are the inertia constants(H) of the system components?

Combining the above with information from the systems' load-flow, steady-state, and dynamic stability studies will provide some guidance to selecting the proper ROCOF relay and its settings. If this is for an interconnected system, you may find that the relevant grid-codes provide some useful limits as a starting point.

Hello,

1. The system is interconnected

2. The relay is located on the interconnecting tie line

3. I am trying to protect the system from collapsing due to over load in case of loss of big generators in the grid our system is connected to

4. Do not know all inertial constants of system components at the moment

1) Are you radially fed or do you have multiple tie-lines?

2)How long is tie-line?

3) What type of generation do you have?

4) What type of loads do you have?

5) What is the size of your system relative to the generation on the other side of the tie-line(s)?

6) Do you ever export power over the tie-line(s); if so, how often?

Depending upon your location there may be a grid-code authority that can provide some guidance or mandated settings; after all, they want to ensure that your system is cut free long before their system is in impacted by your loss of capacity.

Your relay should be part of a system, and in that system this relay has a function...it was purchased for that specific purpose and you should have an operator's manual, or perhaps you can find it online...The setting parameters and recommendations will be in that manual...

https://etrical.blogspot.com/2016/12/dfdt-relay-load-shedding.html

You should not be determining this attribute. The circuitry relying on this protection relay determines what you should set this to trip at. We don't know what that circuitry or system is so we cannot help you.

Undefined: <...We...>.

Is non-subscription to <...We...> an option?

Are you implying that you do know what circuitry or system is connected. Please, explain the system for us. If you cannot because you don't know the system then you already subscribe to this union of we.

One could always read the original equipment manufacturer's [OEM] instructions that came with the thing for guidance. If they aren't to hand, a telephone call to the OEM may well precipitate a replacement. However,

• The OEM can't see to what the thing is connected.
• Neither can this forum.

The manuals generally provide information on how to install, program, and set the necessary operating parameters of the relay. Unfortunately, the determination of those parameters is left as an exercise to the experienced user, simply because the manufacturer cannot know how their equipment will be deployed, as can be seen here.

You can also look at pages 225-233 in this GE/Alstom manual, which provides sample settings, but absolutely no information on how to determine these settings. Instead they are left as an exercise to the experienced purchaser. From the manual:

"TARGET AUDIENCE

This manual is aimed towards all professionals charged with installing, commissioning, maintaining, troubleshooting, or operating any of the products within the specified product range. This includes installation and commissioning personnel as well as engineers who will be responsible for operating the product.

The level at which this manual is written assumes that installation and commissioning engineers have knowledge of handling electronic equipment. Also, system and protection engineers have a thorough knowledge of protection systems and associated equipment."

Richard Matsiko, df/dt is an attribute for the power system and hence the grid authority stipulates the setting.

If you are speaking of an industry drawing power from the grid, you won't be able to decide on your own. It needs modelling of the entire grid and running various scenario to arrive at appropriate setting.

If you set sensitive you run the risk of unnecessary islanding and if you set it liberal, you run the risk of not islanding when it is required to save your system (when the external grid is in distress).