Hi Johan,
1). Can we keep the Network Key same all the time on every power on? Or it is generated randomly on every Power on?
The Network key will be the same if the device does not make a factory reset.
2). Does the Trust center will send the Network key encrypted with respective Install code when Any node comes for the association. Is this process handled?
The Trust Center will encrypt the transport key using the install code, so if the device doesn't know this key, it would not decrypt the packet.
3). What if TC Decide to change the Network Key at any time?
It doesn't change any time, or What do you mean?
4). Any changes required in the Code to make security centralized?
Network security is centralized.
For a better reference, could you please look at the Maximizing Security in ZigBee Networks
Regards,
Mario