We are developing the next-generation AC voltage standard systems based on the Josephson effect. These systems will enable direct calibration for AC voltage with quantum accuracy, drastically improving the measurement uncertainty in our conventional systems. They are also expected to open new metrological and physical fields such as waveform standard, Johnson noise thermometry, etc. Up to now, many types of methods have been proposed for realizing quantum AC voltage standards. It is important to carefully understand both advantages and disadvantages of each method and put the right method in the right application. This article focuses on some of them and reviews basic mechanisms and characteristics of the methods. The present status and the prospects of our researches in AIST are also reported.