Smart grid SCADA (Supervisory Control and Data Acquisition) systems are key drivers to monitor, control and manage critical processes for the delivery and transmission of electricity in smart grids. Security attacks to such systems can have devastating effects on the functionality of the smart grids leading to electrical blackouts, economic losses or even fatalities. This paper presents an analytical game theoretic approach to analyzing security of SCADA smart grids by constructing a model of sequential, nonzero sum, two-player game between an attacker and a security administrator. The distinction of our work is the proposed development of game payoff formulae. A decision analysis can then be obtained by applying backward induction technique on the game tree derived from the proposed payoffs. The paper describes the development of the game payoffs and illustrates its analysis on a real-world scenario of Sybil and node compromised attacks at the sensor level of the smart grid SCADA systems.