PHASOR MEASUREMENT UNIT (PMU) AKANKSHA PACHPINDE

INTRODUCTION

OUTLINE

¡  Conventional control centers

¡  Introduction to Synchrophasors

¡  A generic PMU

¡  Applications of PMU

¡  Role of GPS

¡  Cost profile of PMU with GPS

¡  PMU with IEEE 1588

TASKS PERFORMED BY CONTROL CENTER

¡  Data is acquired from SCADA every 2s or so

¡  State estimation carried out to provide state of

system

¡  Load forecast carried out every 15mins

¡  AGC used balance power generation and load

demand

¡  Contingency analysis carried out

¡  OPF for transmission- constrained economic

dispatch

¡  Historical and forecasted data stored in storage

devices

¡  Various copies of data coordinated, synchronized

and merged in databases

¡  Control centers integrate horizontally & vertically

INTRODUCTION TO SYNCHROPHASORS

§  An AC waveform can be mathematically represented as:

§  In phasor notation it can be represented as:

where: = rms magnitude of waveform

= phase angle

A GENERIC PMU

Current/voltage signal from Instrument Transformer

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

An architecture involving the following must exist in order to realize the full benefit of the technology

¡  PMUs

¡  Communication links

¡  Data concentrators

MEASUREMENT ACCURACY REQUIRED BY SYNCHROPHASOR STANDARD

¡  The value of Total Vector Error (TVE) < 1%

¡  Possible sources of error- magnitude, angle and timing

¡  Only magnitude error < 1%

¡  Only phase error < 0.573º

¡  Only time error < 31.8µs for 50 Hz system and 26.5µs

for 60Hz system

APPLICATIONS OF PMU

Real-time operations applications

¡  Wide-area situational awareness

¡  Frequency stability monitoring and trending

¡  Power oscillation monitoring

¡  Voltage monitoring and trending

¡  Event detection and avoidance

¡  Resource integration

¡  State estimation

¡  Dynamic line ratings and congestion management

¡  Outage restoration

Planning and off-line applications

¡  Baselining power system performance

¡  Event analysis

¡  Power plant model validation

¡  Load characterization

¡  Special protection schemes and islanding

ROLE OF GPS

¡  PULSE PER SECOND (PPS) SIGNAL

¡  This pulse as received by any receiver on earth is coincident with all other received pulses to within 1 microsecond

¡  PPS signal is used for sampling the analog data

¡  TIME – STAMP

¡  The GPS time does not take into account the earth’s rotation

¡  Corrections to the GPS time are made in the GPS receivers so that they provide UTC clock time

COST PROFILE OF PMU WITH GPS

¡  Total installed cost of the technology includes cost of – device, design

and engineering, labor and material, any needed construction

¡  Cost of the device – one-quarter of the total cost

¡  Upgrades cost considerably less than installing new PMUs

¡  Projects installing a greater number of PMUs or PDCs did not have

lower average costs per device.

REASONS FOR HIGH COST

¡  GPS requirement

¡  Data storage needs

¡  Communication infrastructure requirement

¡  Changes required in substation like new busbars, additional CTs and PTs

¡  Downtime, labor cost, commissioning costs

¡  Limited experience

¡  Projects more about research, testing and demonstration

REASONS FOR HIGH COST

¡  GPS requirement

¡  Data storage needs

¡  Communication infrastructure requirement

¡  Changes required in substation like new busbars, additional CTs and PTs

¡  Downtime, labor cost, commissioning costs

¡  Limited experience

¡  Projects more about research, testing and demonstration

PMU WITH IEEE 1588

¡  Precision Time Protocol (PTP) was first defined in IEEE 1588- 2002 and upgraded in 2008

¡  It is designed for local systems requiring accuracies beyond those attainable using Network Time Protocol

¡  Designed for applications that

¡  Cannot bear the cost of a GPS receiver at each node OR

¡  For which GPS signals are inaccessible

IEEE 1588 has three types of clocks:

¡  Master clock- A  clock which is controlled ideally by a radio clock or a GPS receiver

¡  Boundary/ Transparent clock- A clock in a transmission component like an Ethernet Switch

¡  Ordinary clock-  A clock in an end device

¡  Assuming that the master-to-slave and slave-to-master propagation times are equal, the offset and propagation time can be computed as follows:

¡  Synchronization accuracies better than 1 sub-microsecond can be achieved

¡  PTP is supported by Ethernet and TCP/ IP

¡  Reallocation of time signals is done to bring the samples in their correct position

¡  The number of samples ‘N’ coming between two successive PPS edges is evaluated and the new sampling interval is calculated as inverse of ‘N’

¡  After reallocation, samples are passed to the DFT block

¡  Does not require GPS at every node

¡  Communication costs lowered as based on Ethernet

¡  Eliminates the extra cabling requirements of 1PPS to propagate highly accurate timing signals

¡  Non-recurring engineering costs – firmware development

¡  Cost of goods sold – negligible as only requires modification in Ethernet physical layer to support IEEE 1588

¡  High grade oscillators required which are expensive

¡  Lack of testing equipment supporting IEEE 1588 v2 protocol

QUESTIONS ?

THANK YOU