Disclosed is a test system for determining a frequency response of a virtual power plant connected to a utility grid and including a plurality of distinct power plants. The test system includes a test sequence module for providing a frequency test sequence. The frequency test sequence includes a set of frequency test values; and an injection unit for injecting values of the frequency test sequence simultaneously to nodes of the virtual power plant. A node of the virtual power plant includes any of: a power production unit of a power plant, a power storage unit of a power plant, a plant controller of a power plant. Further described are a virtual power plant and a method of testing a frequency response of a virtual power plant.