Outline:
u Introduction
u A generic PMU
u The global positioning system
u Hierarchy for phasor measurement systems
u Communication options for PMUs
u File structure of “Synchrophasor” standard
u PDC files
u PMUs evolved out of the development of the “symmetrical
component distance relay”
u Orgin of PMU
u One of the most important features of the PMU technology is that
the measurements are time-stamped with high precision
u The data transmission speed is no longer a critical parameter in
making use of this data
Current/voltage signal from Instrument X’mers
Restricts bandwidth to satisfy Nyquist criterion
Analog-to-digital converter
Calculates positive-sequence estimates
communication links to higher level
- Provides 1 PPS signal - Time- tagging
u The GPS was initiated in 1978 by US Department of
Defense
u There are 30 active satellites in orbit, as of 2007
u They are arranged in six orbital planes displaced from
each other by 60°
u These planes have an inclination of about 55° w.r.t.
equatorial plane
u The satellites have an orbital radius of 16,500 miles
u They go around the earth twice during one day
u The satellites are so arranged that at least six satellites are visible at most locations
on earth
u Often as many as 10 satellites may be available for viewing
u The 1 PPS signal is most important in PMU which is provided by GPS
u This pulse as received by any receiver on earth is coincident with all other received
pulses to within 1 microsecond
u The GPS time does not take into account the earth’s rotation
u Corrections to the GPS time are made in the GPS receivers so that they provide
UTC clock time
u An architecture involving PMUs, communication links, and data concentrators must exist in order to realize the full benefit
u The local storage capacity is necessarily limited, and the stored data belonging
to an interesting power system event must be flagged for permanent storage so
that it is not overwritten when the local storage capacity is exhausted
u Typical function of the PDCs is to gather data from several PMUs, reject bad
data, align the time–stamps, and create a coherent record of simultaneously
recorded data from a wider part of the power system
u The communication links are bidirectional but the capacity for downward
communication is not as demanding as one in the upward direction
u Two aspects of data transfer are significant
u Channel capacity - measure of the data that can be sustained on the available data link
u Latency - the time lag between the time at which the data is created and when it is available for the desired application
u The communication options available for PMU data transfer are
u Leased telephone circuits
u Switched telephone circuits
u Power line carrier and microwave links
u Fiber-optic links
u Deployment
u Ground wires of transmission lines
u Separate towers for the fiber cable in the
electric utility right-of-way
u Fibers can be multimode or single mode
u They can have graded index or step-index
change in the refractive index
u Multimode fibers tend to have greater loss
because of the partial loss of energy due to
refraction at the boundary
u Synchrophasor standard defines four file types for data transmission to and from PMUs
u Header files, Configuration files and Data files are generated by PMUs
u Command File is for communicating with the PMUs from a higher level of the hierarchy
u Header file - human readable file
u Configuration and Data files - machine readable with fixed formats
u Header file - has pertinent information which the producer of data wishes to share with the user
u The data files – have phasor data which is the principal output of the PMUs
u Configuration file – has information about the data contained in the data files
For synchronization of the data transfer
Fraction of a second
Size of the total record Identifies the data
originator uniquely
Second of Century
Helps determine any errors in data transmission
Length of the Data words
u The principal functions of PDC is to
u Collate data from different PMUs with identical time-tags
u Create archival files of data for future retrieval and use
u Make data stream available to application tasks with appropriate speed and latency
u There are no industry standards for the PDC data files
u It is generally understood that PDCs will have file structures similar to those of PMUs
u There are no commercially available PDCs at this time
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