Overall differential protection for pump storage power plant with tapped-delta design of the unit transformer

Every pumped-storage hydroelectric power plant has at least three different operating modes of the synchronous machine which can give rise to possible challenges during the design of the plant protection scheme. These three operating modes are: the generating mode, pumping mode and machine-starting-into-pumping mode. The main differences between the generating and pumping operating modes are changes in direction of the synchronous machine rotation and change of direction (i.e. sign) of the active power flow. This rotation direction change is achieved by so-called phase reversal disconnect switches. These disconnect switches simply swap two phases in the pumping mode in order to reverse the phase sequence to the synchronous machine. However, the physical location of the phase reversal disconnect switches can be crucial for the proper design of some protection functions (e.g. overall differential protection 87O) for the generator/motor-transformer unit. For this particular installation, phase reversal disconnect switches 89G and 89P are located in-between the low-voltage bushings of the unit transformer and the synchronous machine, thus within the protection zones of the 87O overall differential relay. The machine-starting-into-pumping mode of operation is specific for every pumped storage power plant. In this particular station, the machine is started as an asynchronous motor by using the directon-line starting method with reduced voltage. Voltage reduction is obtained by a special design of the unit transformer. A so-called "tapped-delta design" of the unit transformer secondary delta winding is used.