This invention discloses a DC/AC power electronic converter system that takes part in the regulation of grid voltage and frequency without affecting the normal operation of the DC port, which is either connected to a load when operated as a rectifier or to a DC source when operated as an inverter. In addition to the normal DC and AC ports of a DC/AC converter, the disclosed converter has an extra port for the connection of storage units. Its controller consists of a storage voltage controller to make sure that the storage voltage VDC does not exceed the specified range, a power controller to interact with the grid, and an inner-loop controller that takes the grid voltage and/or the grid current as the feedback. This turns the disclosed converter into a continuous reserve. It enables rectifier-fed loads to provide continuous demand response and inverter-fed sources to provide continuous frequency response. Possible applications include any field that adopts a DC/AC converter, e.g. in wind power, solar power, storage systems, home appliances, IT equipment, motor drives, electric vehicles, and LED lights.