DC Offset Problem in Power Systems Relaying

This code explains how the transient of synchronous generators affects the protection relay's instantaneous function leading to malfunction

Introduction

The problem in instantaneous overcurrent relay (50) protection is the transient and subtransient current and DC components. This dc offset can trip other relays which is not intended for this particular zone. The relay is then said to be overreached. Because it is instantaneous, it will catch the transient period.

The transient is characterized by:

• System impedance
• DC offset

The solution was to make the setting at a value different from the remote end (minimum) SC value, typically at 75-80% of the line.

Code

The code explains how the moment of the fault affects the DC current offset and shows the ratio between DC to steady state current and the relay overreach percentage.

the ratio between DC to steady-state current has a minimum value of 0 and a maximum value of 1. The max value indicates that there is a full offset of the fault current to the steady state value. While the minimum value indicates that the dc offset does not exist.

By changing the moment at which the fault occurs, which is the angle (a) in the code, we realize that;

• at a=0 there will be a fully positive offset and we see that from the plotting and also from the ratio between DC to steady state current= 1.00 which appears in the command window this results in Relay overreach= 22.39% this can be treated as a measurement error. This is why the solution mentioned earlier is introduced and also uses inverse time or definite time to protect the other 20% and also to provide backup.
• The last results are repeated at a=180 degrees but this time with a fully negative offset
• At a=90 degrees we see that there is no offset and this is the lucky moment where we don't have ant relat overreach as Maximum DC offset Fault Current= 35 A relatively lower than Maximum Steady State Fault Current= 705 A giving a ratio of 0.05. and relay overreach of 0.16% these results can be neglected giving 0 and 0.00% respectively
• At another intermediate moment of 45 degrees the ratio between DC to steady state current= 0.67 and Relay overreach= 12.64%.

We conclude then that any deviation from 90 degrees leads to an overreach of the protection relay.

This is due to the transient behavior of the synchronous generator during faults. Which involves the effect of armature reaction and damper windings.