Conclusion

For protection of EHV lines generally directional overcurrent and earth fault relays are used. In case of directional relay it is necessary to set correct MTA for over current as well as earth fault relay. Maximum Torque Angle depends upon factors such as source impedance and method adopted for earthing of generators and transformers. In transmission system generally solidly earthed transformers are used. Hence it is common to use following settings for MTA

    a.   For Overcurrent Relay MTA = +45⁰ or 45⁰ leading
b.   For Earth Fault Relay MTA = -45⁰ or 45⁰ lagging