Tripping Feeder by 81R Relay

81R is used to protect generators when connected to a grid,

Usually this protection function is used to detect loss of the main system in view of opening the incoming circuit breaker to:

• protect the generators from a reconnection without checking synchronization
• avoid supplying loads outside the installation.

or in Load shedding
The "rate of change of frequency" protection function is used for load shedding in combination with the underfrequency protection to:

• either accelerate shedding in the event of a large overload
• or inhibit shedding following a sudden drop in frequency due to a problem that should not be solved by shedding.

where you use your protection relay?

so you can diagnose and estimate the correct pickup set point

regards

please note the df/dt setting is a little tricky and is linked to the system response. The setting is nearly as critical as synchro verification.This at times may also dependent on how the system is coordinated for grid island. (1)df/dt will be pretty high if the incomer breaker/(s) to the remote substation opens and much load is thrown on your generators- df/dt may be about 2-10 hz/sec falling. (2)the remote breaker or your grid incomer breaker opens df/dt may be falling.5-2 hz/sec if importing before tripping or raising if exporting before tripping.

to be accurate the df/dt relay setting should be continuously supervised either by a dedicated plc or the dcs. we have not been able to implement this in the dozens of commissioning we have done. fortunately however the normal grid swing rate(except in under developed countries)is very slow generally not more than 0.1 -0.2 hz/sec may be much lesser in developed countries 0.01-0.05 hz/sec. hence any setting clearing this(say 0.15-0.3 hz/sec) would be generally ok (we have done in almost all instances, at times by trial and error, to set at 150% of maximum grid df/dt. better set evaluation time not more than 3cycles). the delay may be 0.06-0.15 sec depending on how the plant load/governors respond- less delay if system better damped. in erratic grids we have observed df/dt up to 1 hz/sec .

if the grid disconnections with settings as above is tolerable then perhaps can better live with. if the process is critical or exporting heavily then perhaps better not save on a detailed study expense.the owners may generally be reluctant to spend but very often it is found that the such small expenses could save great great losses.

for critical situations further protections in addition to df/dt may be provided.

81R relay(Rate of change of frequency) is used for a feeder protection or bus protection and it has tripping functions also because it is a very special relay in it self calculating the % change in frequency. it is one of the main protection in the circuit for specific applications so it has tripping functions.

There are various conditions of islanding if you consider different kinds of grid protection, it mainly depends on what kind of grid you are trying to do this. Islanding can be referred for generator protection also.

As previously said why 81R relay can be referred only in generator protection?

if you are going to ask for the settings of these relay, you should conduct a transient stability study.

by then, you will know that when the load increases or the a unit of the generator trips off, then you will know what is the new system frequency at this condition. use this frequency to trip some of the loads, and notice whether the frequency gets back to normal after some loads have been tripped off or load shed..

you could also know the appropriate time settings using this study..

This is a distribution feeder relay but designed for any application, if the generator is not near the relay could we still trip on 81R?

If it is a part of load shedding programme then 81R relay may be used with care.In such special instances better to know application details before selecting settings.