Large integration of wind power in modern power systems sets new challenges in power system operation raising serious concerns on issues related to dynamic security, stability and reliability of the system. In addition to this, replacement of conventional power plants by wind power plants of similar size increases the risk of system failures. Nowadays, these new challenges in the power systems operation have resulted in continuous revision of the grid codes for wind power plants by the system operators in order to facilitate high penetration levels of wind power in a secure and reliable way. This paper presents an overview of the main technical issues related to the interconnection of large wind power into the grid as well as the main requirements adopted by the system operators in modern grid codes in order to allow large integration of wind power into the power system while ensuring the security and the reliability of the power system. Emphasis in the paper is on the grid requirements in Denmark and China regarding regulation of active power, regulation of reactive power and low voltage ride through capability.