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

Every pump storage hydro power plant has at least three different operating modes of the synchronous machine which can cause possible challenges during the design of the plant protection scheme. These three operating modes are 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 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 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 (see Figure 1 for more details), thus within the protection zones of the overall differential relay 87O. Machine starting into pumping mode of operation is specific for every pump storage power plant. In this particular station, machine is started as an asynchronous motor by using direct-on line starting method with reduced voltage. Voltage reduction is obtained by a special design of the unit transformer. So-called tapped-delta design, of the unit transformer secondary delta winding, is used (see Figure 2).