With increased penetration of on-shore and off-shore wind power generation in the highly interconnected grid may lead the security, stability and reliability problems of the power system operation. In this paper, two generalized voltage collapse indices, which represent the distance to voltage collapse in terms of loading margin to maximum loading, suitable for offshore wind farms having long transmission system are derived. Generalized dynamic voltage collapse index (GDVCI) is used to initiate the preventive measures for control & protection of wind farm by employing a set of local measurements. The effectiveness of the proposed GDVCI is tested on a detailed and realistic power system having wind farms connected through long submarine/onshore cable. Dynamic responses of offshore wind farms are demonstrated. The effects of on-load tap changers (OLTC) operation on voltage collapse have also been analysed analytically. Copyright © 2009 John Wiley & Sons, Ltd.