The PMUC Real-Time Oscillograph is a new departure in monitoring equipment for power systems, containing in one product the capabilities of 1) a full feature substation digital fault recorder (oscillograph) and 2) a phasor measurement system with real-time communication capability.
The method and means of data acquisition operation is by time-synchronized measurements to precise time information for both the oscillograph and the phasor calculation. The synchronization feature is provided by a GPS clock disciplined-oscillator that generates a 60-Hz sine wave that is phase conditioned to a precise time-mark accurate to one microsecond. The oscillograph data is sampled in a method that phase-locks the sample points to the 60-Hz disciplined oscillator. By virtue of the phase lock synchronization, the data measured at all locations will be sampled synchronously. By these means the PMUC can calculate phasor information from the oscillograph data using a local “DQ0” Parkian type transformation referenced to the disciplined oscillator. This process allows the phasor calculation to be performed at the same rate as the oscillograph data, namely 120 samples per cycle.
The phasors calculated by these means allows the dissemination of phasor magnitude, phase angle, frequency, and time. It is clear that unlike the PMU’s of today the PMUC phasors are not developed by spectral manipulation and so are more suited to use in wide-area monitoring and control. The ‘DQ0’ transformation is an instantaneous process in the time-domain, which allows the data to be decimated down to the bandwidth of the data present in the signal. This process of decimation conducted in real-time, allows the data to be compressed at varying rate depending on data content without loss of information, and to be reconstructed to the equivalent rate of the originally sampled data, or at any other sample rate if required using multi-rate filtering.
The above described PMUC type monitoring, when disseminated across the power system, and interconnected across the internet to a central server, gives everyone access to wide area monitored data that is time-synchronized with signal reconstruction to the full rate of the original data collection. This process allows the review of system response at the same level as we are used to in EMTP type simulations as instantaneous volts and amperes or as phasors the way we see life in a power system stability study. Each PMU is aware of other PMU’s on line and exchanges data within its group so that each PMU can maintain a global view of the system state in real-time with wide bandwidth, allowing for coordinated control action during spreading emergency conditions.
The data multicast and central data collection can proceed at real-time rates for bandwidth up to about 10 Hz for information latency of the order of 30 msecs using Ethernet connection. This prospect provides the opportunity to perform real-time control of wide-area controllers for improved stabilization, and increased utilization of transfer capacity dynamically.
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