HP-AN1271_Improving Power System Uptime

8/14/2019 HP-AN1271_Improving Power System Uptime

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Increasing Power Transmission

System Uptime

Application Note 1271

HP 59551A GPSMeasurementsSynchronization

Module

HPSmartClo

ck

Technology


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Using wide-area synchro-nization for powersyste m monitoring and

control

As customers demand new

services and more available

power at ever-decreas ing costs,

competing power companies must

employ advanced methods to

maintain and expand power

transmission systems. Reliable

timing and measurement synchro -

nization capabilities are the basis

of power system analysis, moni-

toring, and control.

Precise timing is required for

accurat e, time-tagged sampling or

measurem ent of power line/

system status indicators such as

bus voltage magnitude, phase

angle, impedance , and a variety of

disturbances , and other critical

events. Synchronized clocks and

equipment placed throughout an

entire power system enhance the

reliability of collected data, and

provide important informationthat can only be derived through

comparison of data collected at

multiple locations.

While prec ise, accurate clocks

and s ynchronization devices have

long been available, they have

been t oo expensive and difficult

to maintain to allow generous

placement throughout a power

system. The availability of timing

signals from the Global Position-

ing System (GPS) makes itpossible to maintain consistently

accurat e t ime with significantly

lower-cost clocks and synchroniz-

ing equipment.

Devices can be adequately distrib-

uted to synchronize an entire

power system and to support

network -wide real-time m onitoringand cont rol. The expanded infor-

mation and control lets power

utilities quickly and cost-effec-

tively implement too ls and p roce-

dures that increase power system

uptime.

Transmission Line FaultLocationElectric power systems typically

cover diverse and o ften rugged

geographic terrain. The ter ritory is

usually large and includes genera-

tion sites (e.g., hydroelectric

facilities) that are significant

distances from load cente rs. Long

transmission lines and difficult

access complicate the task o f fault

location. Power com panies need

effective methods to quickly

detect and prec isely locate faults

in this challenging environment.

Search and repair crews can then

be e fficiently dispatched to

minimize costly power disruption.

Figure 1. Faultlocation system.

Traditionally, air and ground

crews have depended upon visual

inspection and fault recording

devices to locate line faults. Bothmethods have shortcomings.

Inspection crews are frequently

delayed or prevented from

traveling because of adverse

weathe r conditions the same

conditions that cause most

transmission line faults. Addition-

ally, faults not caused by icing,

downed trees, or other physical

disturbances , may not be visible

to search crews. In this case,

power companies have alterna-

tively used impedance-based faultlocation methods.

Digital fault recorders (DFRs),

effective tools in some cases ,

detect line faults by measuring the

impedance o f a line (i.e., line

voltage and current) and compar-

ing it to an un faulted line. Newer

DFRs, used in protective relaying,

allow more complex fault location

algorithms involving iterat ive

techniques for solving non-linear

equations (see F igure 1).

Bus"A"d-Y Y

Bus"B"

Communications Lines

CCVT CCVT

CT CTIS(t) IR(t)

F

GPS Receiver GPS Receiver

CB

DFR DFR

CB


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Traveling Wave Det ect ion for

Locating Faults

Traveling wave fault location,

when used in conjunction withsynchronization devices, is both

reliable and cost effective. When a

transmission line fault occu rs, an

abrupt change in voltage occurs at

the location o f the fault. The high-

frequency impulse generated at

that po int a wave traveling at

close to the s peed of light

moves along the transmission line

in both directions away from the

fault. By time-tagging the a rrival of

the wave at each end of the line,

and then comparing the timedifference, the fault can be imme-

diately detected and accurate ly

located.

The traveling wave method has

several advantages compared with

other me thods of fault location.

Because it re lies str ictly on the

accurate time-tagging of traveling

waves arriving at the end of

transmission lines, it is not af-

fected by variable load conditions

such as ground r esistance, or line

couplings.

Calculations are also simplified by

the provision of data at two

transmission line ends. If data is

available at only a single end,

much information must be

estimated (based on known line/

wave behavior). Single-end data

gathering requires more calcula-

tions to produce accurate read-

ings. Taking readings from bothline ends helps ensure that high-

speed read ings are not missed

while the device is completing a

previous ca lculation.

The traveling wave detection

provides highly-reliable results.

With fault location frequencies of

35 kHz to 350 kHz, the wave is not

affected by series capacitor

Figure 2. Typicalsubstation layout.

banks. Using a drain coil in ser ies

with a capacitive voltage trans-

ducer (CVT) ground connectionforms a high-pass filter that

couples only to the high-frequency

traveling wave (see Figure 2 ).

Since only the leading edge of the

wave mus t be time-tagged,

programmed temporary lock-outs

can prevent succeeding edges or

reflections from appear ing as

additional faults.

GPS: Enabling Traveling Wave

Measurements

Successful traveling wave faultlocation depends on precise

timing. Devices must be accu-

rately synchronized with each

other and must de liver highly-

stable timing in order to corr ectly

time-tag wave arrivals at each end

of each transmission line. GPS

makes highly-accurat e s ynchroni-

zation possible at a cost that is

affordable for implementation

throughout a large power system.

GPS is continuously available

throughout the world even from

the r ugged geographies of large,remote power systems. Distance

or adverse weather conditions do

not hamper fault det ection. With

accuracy of 340 nanoseconds at

95-percent probability plus

additional errors, GPS has the

potential to meet the range

required by power applications.

For example, with HP SmartClock

technology, GPS accuracy can be

improved to the 110 nanosecond

level.

A-phase CVTon Circuit #1

B-phase CVTon Circuit #2

Fault Transient Interface Unit

GPS Receiver WithTime Tagging

Modem

Modem

C-phase CVTon Circuit #3

M aster Station Locatedat Control Center

Communicati ons Link


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Field-Proven Result s

Many power companies have

successfully field-tested the

traveling wave me thod of faultlocation. A Canadian power

company has for several years

used a traveling wave location

system on their 500 kV power

system. After using a variety of

other methods , including DFRs,

the Canadian company has

determined that t he traveling

wave system offers the best

results and reliability.

In its pilot program, GPS timing

receivers are placed at four

substations. The topology pro-

vides built-in redundancy for

detection of traveling waves. It

also provides for fault location

coverage even during servicing.

The system has cons istently met

performance goals, including a

fault location accuracy of within

one tower span (roughly

300 meters). The success of this

implementation is prompting

installation of GPS-based timingdevices throughout the companys

transmission system, as we ll as at

the com panys system con trol

center.

Real-Time Dispatching and

Load Scheduling. Many major

power networks are linked to-

gether by tie lines that sha re anddistribute power loads. In order to

success fully complete a pow er

transfer, systems must be synchro-

nized so that changes are intro-

duced simutaneously on all lines.

Stability Control. Synchro-

nized measurements are used to

track the status of a power system

in an attempt to control transient

swings or predict other types of

instability. Control actions are of

utmost importance immediately

after a disturbance, and therefore

require reliable measurements for

both detection and response.

Dynamic Braking. Dynamic

braking is used to pr event the

burning of excess energy from

system generators. Brakes are

applied to stabilize power s wings,

especially after detec tion of a

disturbance. Synchronized mea-

surements detect faults and areused as inputs to determine the

approp riate amount of braking to

be applied to cont rol the swing.

Remot e Terminal Units

( RTUs) of Supervisory Control

and Data Acquisition ( SCADA)

Systems. Synchronized timing

capabilities can greatly enhance

the functionality of RTUs without

increasing the cost of their place-

ment at power substations.

Additional TimingApplications for Power

Systems

In addition to traveling wave fault

location, synchronized timing is

critical to a range of applications

that in other ways improve power

system uptime. Investments in

synchronous measurement

systems can be leveraged for:

Precise Sequence of Events

( SOE) Reconst ruction. In order

to correctly analyze various

disturbance data recordedthroughout a power system, all

events must be cor rectly time-

tagged with accurately synchro-

nized clocks.

Adaptive Relaying. Adaptive

relaying techn iques make real-

time adjustments to power

system protection functions to

best match the current system

conditions. Adaptive relaying

collects system data from peri-

odic snapshots of the powersystem. Synchronized measu re-

ments taken throughout the

system provide an accurate

picture of the current power

environment and are cr itical for

effective system adjustmen ts.

Phasor Meas urement Units

(PMUs). PMUs measure voltage

and current, and can repeatedly

calculate watts, vars, frequency,

and phas e angle during a power

line cycle. Synchronized time is

critical for collecting samples and

evaluating the wide-area system

effects and impact of distur-

bances across critical lines

including system bus es.


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Oscillator and Synchroniza-

tion Requirement sQuartz clock technology facili-

tates a ffordable solutions forapplications that require the

placement of clocks at many sites

or points on a network. The on-

going accuracy of these clocks,

however, is dependent upon the

availability of a reliable external

synchronization source .

Rubidium or cesium atomic

clocks are expensive, but can

maintain accurate t ime for long

periods after synchronization.

These clocks suit installations

where an external synchroniza-

tion source is not frequently or

easily accessible (e.g., remot e

ground locations or submarines),

or in applications that war rant the

expense o f the solution.

Traditionally, synchronization

was accomplished using traveling

clocks that were synchron ized to

a primary standard sour ce like the

USNO and physically transportedto the on-site atomic clock. This

method is prohibitively expensive

and inconvenient for many sites.

Other me thods, including sate llite

transmissions of timing signals

from VLF systems, GOES weather

sate llites, and LORAN-C transmis-

sions, are more access ible and

usually less expensive. But

transmissions are suscep tible to

unpred ictable daily variations

and weather conditions, and arenot a lways available to every

geography.

GPS and Power SystemApplications

Now fully operational, the Global

Positioning System (GPS) makes it

possible to install highly-accurate,

inexpensive quartz-based clocks

and ea sily synchronize thes e

clocks with a primary standard

time code. Developed by the

United States Departmen t of

Defense, GPS is a worldwide

satellite system that p rovides

navigation, positioning, and timing

capabilities for both military and

civilian applications.

The GPS system distributes

24 satellites that complete two

earth o rbits per day. The sate llites

usually carry two ces ium and two

rubidium clocks for synchronized

timing output that is specified to

be accur ate to within 1 microsec-

ond UTC. Acces s is free, no

authorization is required, and the

signals are cons tant and receivable

virtually anywhere on ea rth.

While GPS receivers have been

available for over ten years, newer

produc ts have improved receiver

components and are able to take

advantage of the more continuous

availability of GPS. These prod-

ucts p rovide inexpensive, but

highly-accurate timing and syn-

chronization capabilities and meet

a host o f application requirements

in power s ystem fault location,

monitoring, and control. The low

cost, small size, and high accuracyof the devices make them w ell-

suited for liberal distribution

throughout a power system.

Precision Timing

Timekeeping devices were origi-

nally based on absolute time

i.e., the time directly related to the

rotation of the earth around the

sun. Expressed in days, hours,

minutes, and seconds, the time

was derived from astronomical

observations. Because it requires

complex solutions to maintain

accuracy with this method (the

earths rotational paramet ers vary

and are affected by solar activity),

international research and

timekeeping laborato ries aroundthe world now cooperate and use

atomic clocks to perform the

measurements that provide the

basis for t he Universal Coordi-

nated Time ( UTC) scale.

To synchronize with absolute

astronomical time, UTC is periodi-

cally updated with appropriate

additions of leap seconds. The

designated UTC laborato ries, such

as the U.S. Naval Observatory

(USNO), provide primary standardtime sources o r synchronization

references.

All modern clocks keep time

based on re lative time intervals.

The clocks measure the t ime

between two events, such as the

width of a pulse, and depend on an

oscillator that produces consistent

pulses or time intervals. To

maintain on-going accuracy, mos t

clocks used in precision app lica-

tions have to be periodically

synchronized with a primary

standard time source such as an

atomic clock . On-going timing

accuracy requires a high-quality

oscillator and an available syn-

chronization source.


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Hewlet t-Packard GPSSynchronizationSolutions

The HP 59551A GPS Measure-

ments Synchronization Module is

Hewlett-Packards first precision

timing product based on advanced

GPS technology. It provides a low-

cost synchr onization foundation

for monitoring wide-area t rans-

mission systems, or for real-time

monitoring and control.

The time base for the HP 59551A

is the HP 10811D Quartz Oscilla-tor, a h ighly-reliable crysta l

component charact erized by low

sensitivity to temperat ure

changes, low phase noise and

well-underst ood aging charact er-

istics. Integrated with the quartz

osc illator, HPs SmartClock

algorithm boosts the performance

of the HP 59551A, making it

approach the performance of a

rubidium-based solution with

accuracy of 110 nanoseconds at

95-percent probability.

The HP SmartClock algorithm

compare s the oscillator frequency

with the GPS reference signal.

By learning the aging behavior

and the environmental effects on

the os cillator over time,

HP SmartClock ad justs the

oscillator ou tput frequency

accordingly and significantly

improves accuracy.

A holdover mode ensure s accu-

rate synchr onization in the

unlikely event of satellite

signal loss or interruption.

HP SmartClock will continue to

maintain time and frequency with

less than 8.6 microseconds loss in

accuracy for up to 24 hours of

GPS signal loss. See Figure 3.

Figure 3.HP SmartClockholdover.

The low cost of theHP 59551A makesmonitoring wide-area transmissionsystems affordable.

714600

714500

714400

714300

714200

714100

714000

713900

7138000 1 2 3 4 5 6 7

Time (days)

EFCCommand

Ac tua l EFC Comma nd Pre dicte d EFC Comma nd

3 Day Learning Period

Start ofAccum.AvgDayTime ErrorFreq4 1.74S 2.04e-11

5 4.032.63e-1167.504.07e-11

HPSmartCloc

Technology


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Figure 3 gives typica l data

illustrating the effect of

HP SmartClock . All of the data

are taken with the unit lockedto GPS. During the first th ree

days, HP SmartClock us ed the

GPS reference to learn the

aging and tempe rature behav-

ior of the quartz oscillator. The

light, shaded line is the actual

digital steering command sent

to the o scillator to keep it

synchronized with GPS time.

The heavy solid line is the

computed performance of the

quartz oscillator starting with

Day 4 using the cor rections

provided by HP SmartClock.

The dominant effect during the

predicting period are changes

due to external temperature. If

the GPS signal had not been

present , the oscillator wou ld

have been steer ed in holdover

by HP SmartClock using the

corrections tht it had deter-

mined.

Holdover performance can be

computed from the difference

between the actual steered

performance, and the predicted

performance. The data for the

first thr ee days in holdover are

shown in Figure 3.

Time tagging, included with the

HP 59551A, facilitates a variety

of applications including fault

location, network disturbanceanalysis, and detailed sequence-

of-events analysis. Events can be

time-stamped (with a

100-nanosecond resolution),

recorded, and down-loaded to a

compute r system for review.

Input/output capabilities of the

HP 59551A allow its u se with a

variety of existing event and fault

recorders. Standard functions

include time tagging of

conditioned TTL input signals,

IRIG-B output, and an Alarm

BITE output tha t indicates a

system fault or loss of sat ellite

lock. Hewlett-Packar d a lso

provides fiber optic distribution

systems (the HP 59552A Fiber

Optic Distribution Amplifier and

HP the 59553A Fiber Optic

Receiver) that transmit signals

and timecodes through noise

immune filters to measurement

and control instruments.

The low cost of the HP synchro-

nizing devices makes it feasible

to place one at each of the

critical points in a power s ystem.

By synchronizing multiple points

in the distribution network,

reliable and meaningful field data

can be collected, and crucial

system performance and operat-

ing characteristics can be

extracted.


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H

For more information on Hewlett-PackardTest and Measurement products,application or services please call yourlocal Hewlett-Packard sales ofices. A

curr ent listing is available via Webthrough AccessHP at http://www.hp.com.

If you do not have acc ess to the internetplease contact one of the HP centerslisted be low and the y will direct you toyour nearest HP representa tive.

United States :Hewlett-Packard CompanyTest and Measurem ent Organization5301 Stevens Creek Blvd.Bldg. 51L-SCSant a Clar a, CA 95052-80591 800 452 4844

Canada:Hewlett-Packard Canada Ltd.

5150 Spectru m WayMississauga, OntarioL4W 5G1(905) 206-4725

Europe:Hewlett-Packard

European Marketing CentreP.O. Box 9991180 AZ AmstelveenThe Netherlands

Japan:Yokogawa-Hewlett-Packard Ltd.Measurem ent Assistance Center9-1, Taka kura -Cho, Hachioji-Shi,

Tokyo 192, Jap an(81) 426 48 3860

Latin America:Hewlett-PackardLatin American Region Headquarters5200 Blue Lagoon Drive9th Floor

Miami, Florida 33126U.S.A.(305) 267 4245/4220

Australia/New Zealand:Hewlett-Packard Australia Ltd.

31-41 Joseph StreetBlackburn , Victoria 3130Australia

131 347 ext 2902

Asia Pacific:Hewlett-Packard Asia Pacific Ltd.17-21/F She ll Tower, Time Square,1 Matherson Street, Causeway Bay,Hong Kong(852) 2599 7070

References

Precision Timing

Gary Smith, A Laymans Guide to

Precision Timing, EW Design Engineers

Handbook, Horizon House Microwave

Inc., 1987.

Synchronization Applications

Wayne R. Block, Samuel C. Sciacca,

Advanc ed SCADA Concepts . IEEE

Computer Applications in Power, January

1995.

Robert O. Burnett , Jr., Marc M. Butts,

Patrick S. Sterlina, Power System

Applications for Phasor Measurement

Units. IEEE Computer Applications in

Power, J anuar y 1994.

Virgilio Cent eno , Jaime DeLaRee, A.G.

Phadke, Gary Michel, J. Murphy, R.

Burnett, Adaptive Out-of-Step Relaying

Using Phasor Measurement Techniques.

IEEE Computer Applications in Power,

October, 1993.

D.A. Coleman, J. Esztergalyos, K.E.

Martin, J.M. Nordstrom,

The Application of Pre cise Time

Synchronization for Real Time Control

and Operation of Electric Power Systems.

Presented at American Power Conference,

January 1992.

Mladen Kezunovic, Branislava Per unicic,Synchronized Samp ling Improves Fault

Location. IEEE Computer Applications in

Power, April 1995.

Working Group H-7 of the Relaying

Channels Subcommittee of the IEEE

Power System Relaying Committee,

Synchronized Sampling and Pha sor

Measur ement s for Relaying and Control.

IEEE Transactions on Power Delivery,

Vol. 9, No.1, January 1994.

Traveling Wave

H. Lee, Development of an Accurate

Traveling Wave Fa ult Locator Using the

Global Positioning Satellites. Prese ntedat Canadian Elect ric Association Annual

Engineering and Operating Technology

Conference, March 1993.

Data Subject to Change

Printed in U.S.A. Jun e 1995

Hewlett -Packard CompanyCopyrigh t 1995

5964-0398E

For more information:HP 59551A GPS MeasurementsSynchronization Module Brochu re.

HP 59551A Technical Specifications.

HP 59551A Price List.

HP-AN1271_Improving Power System Uptime

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