Due to the mission-critical nature of energy management, smart power grids are prime targets for cyber-attacks. A key security objective in the smart grid is to protect the integrity of synchronized real-time measurements taken by phasor measurement units (PMUs). The current communication protocol in substation automation allows the transmission of PMU data in absence of integrity protection for applications that strictly require low communication latency. This leaves the PMU data vulnerable to man-in-the-middle attacks. In this paper, a lightweight and secure integrity protection algorithm has been proposed to maintain the integrity of PMU data, which fills the missing integrity protection in the IEC 61850-90-5 standard, when the MAC identifier is declared 0. The rigorous security analysis proves the security of the proposed integrity protection method against ciphertext-only attacks and known/chosen plaintext attacks. A comparison with existing integrity protection methods shows that our method is much faster, and is also the only integrity protection scheme that meets the strict timing requirement. Not only the proposed method can be used in power protection applications, but it also can be used in emerging anomaly detection scenarios, where a fast integrity check coupled with low latency communications is used for multiple rounds of message exchanges.