Transcript Slide 1

DEVELOPMENT OF WIDE-AREA MEASUREMENTS
IN MODERN POWER SYSTEMS
A.G. Phadke
Lecture outline:
• History of wide-area measurements
• Power system state estimation
• Control with feed-back
• Adaptive relaying
• Remedial Action Schemes
• Measurement Errors
• Future prospects
History of Wide-Area Measurements
•
Wide-area measurements in power systems have been used
in EMS functions for a long time. Economic Dispatch, tie line bias
control etc. all require wide area measurements.
•
These measurements are quasi-steady-state, non-simultaneous
and relatively few in number. They are used to provide very specific
information for very specific tasks.
EMC
•
However, the birth of modern wide-area measurement systems
can be traced back to a very significant event which took place
in 1965.
• Control of power system frequency
• The primary control is through the speed governor
(steady-state analysis)
Reset control
Steam
Valve
Speed
governor
f
HP
f0
Supplementary
(reset) control
load
LP
Generator
• Control of power system frequency
• Tie-line bias control (steady-state)
T0
Area Control Error (ACE) = D T - B Df0
For each area, ACE is calculated, and if non-zero,
generation is adjusted. This keeps the interconnection
frequency on schedule.
Time error and inadvertent interchange.
• Control of power system frequency
• Tie-line bias control (steady-state)
f
f0
Pre-disturbance balance
ACE
T0
Tie-line flow
Wide Area Measurements for tie line bias
control and economic dispatch
• Measurements
performed every
few minutes
• Measurements
at common
points on tie
lines
• Frequency
measured
at control center
• Permissive
generation change
commands issued
Df S
-BDf DT
ACE
DG
Econ.
Dispatch
Control Center
History of Wide-Area Measurements
• This is the famous 1965
Blackout.
• Actually, the story of
how this blackout
occurred is interesting.
Sir
Adam
Beck #2
History of Wide-Area Measurements
•
Following the blackout, federal commission appointed.
•
Fault found with utility companies: no real-time knowledge
of the state of the power system.
•
Recommendation made: establish a real-time measurement
system and develop computer based operational and
management tools.
•
This launched the activities which led to state estimation
and real-time contingency analysis.
•
Pioneering work was done by Stagg, Dopazo, Schweppe, DyLiacco
and others, which culminated in current crop of EMS software.
•
The starting point is wide-area measurements and state
estimation.
Introduction to State Estimation
Operators supplied with operating guides from planning
studies. Unexpected events produced emergency calls
to run more cases and modify the operating guides.
Time lag in response.
Solution: Operator Load Flow installed in the control
center.
Operators could manually enter data to check for new
situations. The problem was that it was difficult to match
existing conditions with the result of the load flow.
Introduction to State Estimation
Problem caused by



Insufficient data
Data in non-uniform type (voltages, currents, flows)
Errors in data, not necessarily gross error, but
within accuracy limits of meters
 Modeling errors – unknown switching on the
system, unknown tap positions, wrong bus
configurations, and even wrong line data.
It was important that the operators load flow be
accurate so that succeeding studies (outage and relief)
be accurate. What was needed was a way of processing
available measurements to make an estimate of the
existing state of the system in real time.
• The Birth of the PMUs
• Computer Relaying developments in 1960-70s.
• Symmetrical Component Distance Relay Development.
• Significance of positive sequence measurements.
• Importance of synchronized measurements.
• Development of first PMUs at Virginia Tech ~ 1982-1992
• Development funded by AEP, DOE, BPA, and later NYPA
• First prototype units assembled at Va Tech and installed
on the BPA, AEP, NYPA systems.
GPS
receiver
PMU
Signal
conditioning
unit
User
Interface
(b)
• Introduction to phasors
q
Imaginary
Phasors are used to represent fundamental
frequency components of voltages and currents.
They are central to all power system calculations.
q
Real
t=0
sines
cosines
Input signal
Data
samples
• Introduction to phasors
sin and cos
functions
xn
xn-1
.
.
x1
t
Phasor X =
2
--S
x (coskq - j sinkq)
N
k
• Synchronization
Substation A
Substation B
At different locations
By synchronizing the sampling processes for
different signals - which may be hundreds of miles
apart, it is possible to put their phasors on the same
phasor diagram. This is essential for carrying out
all power system calculations.
• Synchronization
• GPS
• A phasor measurement unit
GPS
receiver
Analog
Inputs
Anti-aliasing
filters
Phase-locked
oscillator
16-bit
A/D conv
Modems
Phasor
microprocessor
Except for synchronization, the hardware is the same
as that of a digital fault recorder or a digital relay.
Power system state estimation
Present practice
Measurements
are scanned
and are NOT
simultaneous
Measurements
are primarily
P, Q, |E| = [z]
Control
Center
State is the vector
of positive sequence
voltages at all
network buses [E]
Phasor measurement based state estimation offers
many advantages as will be seen later.
Power system state estimation
Estimation with phasors
Control
Center
Since the
currents and
voltages are
linearly related
to the state vector,
The estimator
equations are
linear, and no
iterations are
required.
[Z] = [A] [E] , and once again the weighted least
square solution is obtained with a constant
gain matrix.
Control with feed-back
ADVANCED CONTROL FUNCTIONS
Present system: model based controls
Controller
Measurements
Controlled
Device
Control with feed-back
ADVANCED CONTROL FUNCTIONS
Phasor based: Feedback based control
Controller
Measurements
Controlled
Device
Adaptive Relaying
Definition
Adaptive protection is a protection
philosophy which permits and seeks
to make adjustments in various
protection functions automatically
in order to make them more
attuned to prevailing power system
conditions.
Adaptive Relaying
Controlled Security & Dependability
System State
Protection
No 3
Or
And
Vote
Breakers
To Circuit
Protection
No 2
Logic
Arbitration
Protection
No 1
Remedial Action Schemes
• Wide Area Protection Schemes
SPS or RAS
• System Protection Schemes came into being as
control functions which required protection system
interventions.
• Often the inputs may be derived from protection
systems, and certainly the outputs of protection systems
are used to execute the controls.
Wide Area Protections
• The role is to prevent major system failures.
• Voltage instability, angular instability are the main
driving considerations.
Remedial Action Schemes
• Wide Area Protection Schemes
Large System
Example:
If Pt > P0
And
If Pg > P0’
Pt
S1 , S2 , S3
And
If S1 OR S2 OR S3 open
Pg
Then
Drop load L
L
F-NET measurement system
(a wide area measurement system)
Originator: Prof. Yilu (Ellen) Liu of Virginia Tech.
GPS
receiver
Ethernet port
household
120 volt input
Low cost, versatile frequency measuring unit with
GPS synchronization and ethernet access. Idea is to
install these in various locations in large numbers.
SoftSwitching® power quality unit:
Inexpensive
units, but
the main
draw-back
is the lack
of precise
time stamping.
August-2003 disturbance frequency record collected
by i-grid units.
Uses of wide area frequency measurements:
(1) Feed-back to post mortem analyses. However, for
this to be useful, the measurements would have
to be precisely synchronized as in f-net system.
(2) Integrating frequency deviations, one could
obtain CHANGES in phase angles.
(3) Frequency or changes in angles could be used
as feed-back signals in some control systems.
Future prospects
• Wide area measurement systems are going to be more important in future.
• Wide area communication systems are an essential element of this trend.
• Prospect of improved protection and control are on the horizon.
• All of these activities will lead to evolutionary changes.
• It is very important that all interested parties take part in Standard
development. Examples: IEEE 1344, IEC 61850 etc.