## 2 Synchrophasor Technology and State of the Art Applications  
### Application of Synchrophasors in Modern Control Centers  
Conventional supervisory control and data acquisition (SCADA) systems do not provide the ability to monitor transient events [7; 8]. Even though the connected remote terminal units (RTU) can communicate cyclically, the measurements samples are typically not time aligned when arriving at the control center [9; 10]. Thus, a state estimator is required [9]. Unless a general interrogation is inquired, traditional SCADA protocols (e.g., IEC 60870-5-101 /-104 [11] and DNP3 [12]) only transmit spontaneous messages in case of measurement value changes or if a switch status message has been received. This requires a state estimation (SE) to generate a valid steady-state power flow image of the system state. This state estimate is used by most secondary energy management system (EMS) functions and applications (i.e., contingency analysis, optimization, etc. ); hence the SE module is often referred to as the cornerstone of an EMS. Dynamic stability assessment (DSA) systems can enhance the situational awareness in transmission control centers significantly [13]. DSA systems run time-domain simulations to evaluate possible contingencies as well as suitable mitigation strategies. These results can also be used for the creation of training sets for machine learning based analysis modules. Additionally, phasor measurement unit (PMU) based wide area monitoring (WAM) systems increase the dynamic observability by recording transient events by time synchronized samples. The synchronization accuracy of modern GPS linked PMUs is below 1\(\upmu\)s [14]. Modern phasor data concentrators (PDC) can acquire data from several hundred PMUs [15; 16; 17]. An IEEE C37.118 [18] conform interface to IEC 61850 [19] is available, which increases the flexibility of PMU data processing [20]. The synthetic patterns created by DSA systems and the recorded patterns from WAMS can be used to improve the training set as well as the DSA time-domain model. A promising application is the HVDC based RAS, an automated control strategy to exploit the flexibility of the VSC technology to substitute the expensive generator-based redispatch [21]. Today's synchrophasor applications typically run in parallel to existing SCADA based control room applications (see Figure 1). According to [22], common PMU applications can be distinguished into real-time operation tasks (e.g., frequency stability monitoring, power oscillation monitoring, phasor-only state estimation, dynamic line rating) and planning tasks (e.g., model calibration, primary frequency response analysis, post-mortem analysis). Further applications as well as possible enhancements of SCADA based monitoring and control applications were also investigated in [23; 24]. A recent study identified the following hindering issues for a wider application of synchrophasor technology in today's system operation [25]:* A clear determination of accurate, dynamic operating limits (i.e., phase-angle differences, or oscillations) is not available, which diminishes the value of the information gain for the system operation. * The data quality still strongly depends on the quality of the instrument transformers, as well as the ICT infrastructure. * The rising observability due to a newly installed WAMS can lead to an exposition of new issues, which can lead to a commitment of valuable personnel to investigate the problems. * Unless some TSOs exchange data of strategic important PMUs, the data exchange is often subject to cyber security issues or other sensitivities. * A separate development of EMS and WAMS lead to challenges for human operators, who prefer a single unified user interface to support a smoother workflow and a clear decision-making. * A general evaluation scheme of system dynamics and correlating actions still needs to be defined.