Automatic power system stability function relies on specific power system stability calculation results, given all possible operating modes. Special load shedding function generates trip signals for circuit-breakers of the feeders, providing power supply to the end-customers. These functions are implemented in 500 kV substations. Specific substation is chosen on the basis of available communication channels with remote substations, generation facilities and control center. Mentioned functions can be implemented using UPAE IED. System architecture is presented in Fig. 1
UPAE operation principle relies on 2-BEFORE principle. This means that at the moment of fault indication reception IED already includes prepared control actions table in case of fault indication reception it is only required to select required set of control actions. All table data is introduced at commissioning stage or received by UPAE IEDs from centralized system integrity protection over specific communication channel. If connection with these IEDs is available and all data is loaded, UPAE is functioning as a remote controller of special integrity protection system. If connection is lost, UPAE IED switches to local operating mode and is ready to operate according to the specific control actions table. Determination of the current configuration of the facility is performed on the basis of pre-fault data. Pre-fault data includes data regarding equipment maintenance: line maintenance, unit maintenance indications, etc. Power flow data is provided to UPAE as remote measurements in analog form, from specific transducers or from data transmission and acquisition system in digital form over LAN.
Standard initiation signal for automatic power system stability function is line trip identification signal. In case this initiation signal is received, pre-fault mode is considered to be over and control actions table is determined. Then specific control actions cycle is started. In this cycle all inputs are polled at high rate (one time per 1 ms), control actions table lines are identified and control actions are issued.