Handbook of Electrical Engineering

PROTECTIVE RELAY COORDINATION 321

Conversion factor for referring the generator reactances to theCTandVTsecondary circuits:-

Xsec=

Xgen%×Vgen^2 ×CTratio

100 ×Sgen×VTratio

=

Xgen%× 66002 × 800 × 110

100 × 750 , 0000 ×1 6600

=

Xgen%

100

× 5. 808 × 13. 333

Xsec=Xgen%×0.7744 ohms

X′dsec= 25. 0 × 0. 7744 =19.36 ohms

Xdsec= 250. 0 × 0. 7744 =193.6 ohms

Choose an offset of 0.75X′d, a circle diameter of 0.5Xd. This will allow the generator to run
in the leading power factor zone with a large transient rotor angle (up to 120 degrees).

Relay offset= 0. 75 × 19. 36 =14.52 ohms

rounded to 15.0 ohms

Relay circle diameter= 0. 5 × 193. 6 =96.8 ohms

rounded to 100.0 ohms

Relay time delay, choose 4 seconds.

These results are shown in Figure 12.8.

12.2.5 Reverse active power relay

Reverse active power protection (32) is required to prevent the prime mover from being driven by
the generator. This can occur during transient disturbances when a generator is lightly loaded, the

Figure 12.8 Loss of excitation characteristic of an admittance relay. The diagram is drawn in the impedance
domain.