Security is a key requirement that must be met when designing and operating power systems (PS). The ultimate consequence of the insecurity of the PS is manifested in its technological disintegration, or colloquially, “blackout” followed by the energy supply break. The paper presents the concept of energy and technological security of power systems in transition, structured in three levels. The first, basic level is the level of the portfolio for electricity generation. The second level is the level of individual equipment within the portfolio, and the third level is equipment maintenance and management of the electricity generation process with available equipment and with the available portfolio. The energy security of the PS is correlated with dispatchability, or the ability of the system to respond to a request for the power increase. Appropriate indicators can be used to quantitatively define security. At the first level, these are dispatchability indicators at the second level, indicators based on the rate of change of a power plant’s load, and at the third level, the performance indicators can be used. The paper provides examples of the impact of the trajectory of a hypothetical energy or technological transition on the dispatchability of power systems in nine European countries. The paper also presents the effects of the real energy transition on the dispatchability of the power system. It is concluded that the chosen transition trajectory, or the chosen final technological structure of the electricity generation portfolio, has a major impact on the numerical value of the dispatchability indicators and, through them, on the energy supply security of the power system, in terms of the risk of the system’s technological disintegration. Insufficient security achieved at the first, basic level, cannot be compensated by the security improvements at the second and third levels.