Patent ID: 8545189

Claim:
A method for controlling a pump station, that comprises at least two pumps, each of the at least two pumps being arranged to transfer liquid from or into a tank and being controlled by an electric drive comprising a frequency converter, the method comprising operating the at least two pumps in turns through the steps of: measuring a liquid surface level in the tank by means of a sensor, controlling activation of each of the two pumps on the basis of the measured liquid surface level, selecting a first value of the liquid surface level, selecting as a first value of pump rotation speed substantially a predetermined value at which an amount of transferred liquid relative to consumed energy is at maximum, detecting a first moment when the liquid surface level reaches said first value of the liquid surface level from a predetermined direction, controlling, as a consequence of the detection of the first moment, the pump rotation speed of one of said at least two pumps to said first value of the pump rotation speed, running the one of said at least two pumps at the first value of the pump rotation speed in order to move the liquid surface level in a direction opposite to the predetermined direction, detecting a second moment when the liquid surface level reaches, from the direction opposite to the predetermined direction, another value of the liquid surface level that is later in the direction opposite to the predetermined direction than the first value of the liquid surface level, and controlling, as a consequence of the detection of the second moment, the one of said at least two pumps to be stopped so as to keep the surface level between said first value and the other value of the liquid surface level, the change of amount of liquid in the tank when the surface level changes between said first value and the other value of the liquid surface level being smaller than the whole volume of the tank, wherein said at least two pumps are controlled at the pump station in such a way that said at least two pumps are alternated in operating turns in which the pump rotation speed is said first value of the pump rotation speed and monitoring of the liquid surface level and the controlling of the pump rotation speeds are performed in the frequency converter.