Synchronized Phasor Measurements and Their Applications

- A. G. Phadke and J. S. Thorp

PART I: Phasor Measurement Techniques

Chapter 5 – Phasor Measurement Units and Phasor Data Concentrators

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

Introduction

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

A generic PMU

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

The global positioning system

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

Hierarchy for phasor measurement systems

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

Communication options for PMUs

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

File structure of “Synchrophasor” standard

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

PDC files

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

Questions ?

Thank you