You can also use dQ/dt> detection since the most DG work with power factor (cos phi=1) or close to 1 . If  the generator is opened disconnect from network with full balance between the production and consumption Pg=PL you will still have consumption of reactive power for plant own motors , trafo etc. Then power factor decrease to 0,95 or less and you have deviation of Q-reactive power which can be detected. Such practical solution we use and it works on one biomass power plant..

Srinivasa,

In fact, if you wish to use a relay that would detect islanding, the logic suggested by Srete is likely to be a more effective and faster.

The power imbalance would take time to detect.

-Sanjay 

ROCOF> Rate of Chane of Frequency and VVS Voltage vector shift> are classical protection for anti-islanding protection but can not help in case of the balance between generator production and consumption in the tie-line of PCC. The rate of change of angle can be efective (but you will not have the chane of angle in perfect balance) If the value P=0.00 kW or close to this value these protection function can not detect. If  you have AR- Automatic Reclosing on overhead lines very offen with fast opening in a cycle of tie-line breaker (about 300-400 ms is pause time in Croatia) ROCOF and VVS can not detect it. Also other passive protection  f>> or f can not be used, Only the chagne of the  reactive power maye be detected since almost all RES work with cos phi close to 1 beacause of feeding tariff pay only for active power P,  and if the tie-line breker is open generator need to suppy its own reactive consumption in power plant and that can be detected with high probability and fast tripping.