In Tamilnadu Power of Attorney should be registered if you are appointing an agent to purchase or sell a property.
Regarding cancellation/revocation, it can be cancelled/revoked by mutual consent of both parties by making a revocation deed registered before the same sub-registrar where the power was registered.
If the agent is not available in station the executor of the power of attorney can cause an intimation to the agent expressing his/her interest to cancell the power of Attorney and can revoke the deed by getting registered before the same sun-registry where the power was registered. Subsequently proper public notice can be given in two news papers (one shall be vernacular news paper) locally well circulated.
With regard to power of attorney where consideration has been passed on it can not be revocked unilaterally. It can be revoked on mutual consent of the parties.
Power of Attorney is not valid when a person executed the power document is not alive.
Hope i have thrown much light on this matter.
Nanchill J Rajkumar