Predictive Out-of-Step Protection and Control Scheme

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Predictive Out-of-Step Protection and Control Scheme

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Predictive Out-of-Step Protection and Control Scheme Based on Real-Time Phasor Measurement Application By Alla Deronja, P.E. Senior System Protection Engineer – PowerPoint PPT presentation

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Title: Predictive Out-of-Step Protection and Control Scheme

1

Predictive Out-of-Step Protection and Control
Scheme
Based on Real-Time
Phasor Measurement Application
By Alla Deronja, P.E.
Senior System Protection Engineer
American Transmission Company
Milwaukee, Wisconsin

This presentation is focused on demonstration of
applying
the concept of real-time measurements in a
transmission
system to perform system protection and control
functions
in a predictive - or adaptive - manner to account
for the
system operating conditions at any time.
Objectives
Real-time measurements and adaptive/predictive
protection.
Predictive out-of-step protection scheme based
on real-time measurements.
Practical application.

As a system protection engineer, I am interested
in
applying real-time phasor measurement technology
to
perform system protection, control and tripping
functions
to achieve a protection systems adaptability to
account
for all possible system operating conditions.

The concept of adaptive relaying is that many
relay
settings are dependent upon assumed conditions in
the
power system. In order to cover all possible
scenarios that
the protection system may have to face, the
actual
protection settings are often not optimal for any
particular
system state. If an optimal setting is desired
for an
existing condition on the power system network,
then it
becomes necessary for the setting to adapt itself
to the
real-time system states as the system conditions
vary
due to changing loads, network switching
operations,
or faults.

A number of relaying functions are predicated
upon an
assumed pattern of power system behavior during
transient stability oscillations and other
dynamic
conditions. An example of such functions is
out-of-step
protection.
A predictive out-of-step protection and control
scheme
based on the real-time phasor measurement
principle that
has inspired me to pursue my project proposal has
been already in operation for almost 20 years.

TOKYO ELECTRICS PREDICTIVE OUT-OF-STEP
PROTECTION SYSTEM
The predictive out-of-step protection system by
means
of observing phase differences between power
centers
and based on the realtime phasor measurement
principle similar to the one I sought to install
at my
company has been installed and successfully
operated
by Tokyo Electric Power Company (TEPCO, Japan)
since
February, 1989.

The bulk power system of TEPCO in presented in
Fig.
1. A major characteristic of this power system is
that
the power generation areas are far from the
consumption areas. The eastern, northern, and
southeastern generator groups are linked by a
bulk
power system comprising 500 kV double-circuit
transmission lines configured in duplex and
triplex
routes and forming a trunk. The western
generator
group and large local loads are thus linked to
the bulk
power system via the substations marked as A1 and
A2
in Fig. 1.

FIGURE 1. BULK POWER SYSTEM OF TEPCO

The western generator group tends to be heavily
loaded, and its own capacity cannot meet demand.
Power is received from the bulk power system to
make
up the deficiency.
When a double fault occurs along both circuits of
a
double-circuit line forming one route, the
substations at
both ends of the line are disconnected and
transmission
capability is interrupted. If a successive fault
occurs
after reclosing, a slow cyclic power swing
develops
between the western generator group and the bulk
power system.

The same situation occurs in the event of failure
of a
bus-bar protective relay to operate during a
bus-bar
fault. Over time, the phase difference of the
generator
groups thus undergoes oscillating divergence. If
this
condition is not corrected, an out-of-step
condition will
begin to occur in various parts of the power
system and
may lead to its total collapse.

In order to maintain the reliability of the power
system
should such serious rare faults occur, a
predictive
protection system that prevents total collapse of
the
power system has been developed. This protection
system utilizes online data collected before and
after
the onset of a system disturbance to determine
the characteristics of the power swing and
predict an
out-of-step condition. The system operates during
the
incubation period so that appropriate control can
be
performed before the out-of-step condition occurs.

The western area can be islanded (Fig. 2) from
the bulk
power system at points (a) or (b) and (a) or
(b) before
out-of-step occurs and then operated
independently.
This eliminates power swing between the generator
groups of the two systems and restores stability.
The separation point is selected based on the
power
flow at pre-determined points for separation
before the
fault.

FIGURE 2. SIMPLIFIED MODEL OF TRUNK LINE
AND GENERATORS

The protection scheme is outlined in Fig. 3. The
status
of each generator group (east, north, southeast,
west)
is obtained by measuring the bus-bar voltages of
neighboring substations as representative values.
From
these, the phase differences between the western
generator group and the bulk power system are
obtained. From the phase difference values, the
corresponding values for 200 ms in the future are
predicted. If the latter exceed the setting value
(?limit),
the respective generator groups are judged to be
unstable.

FIGURE 3. OUTLINE OF PROTECTION SCHEME

The two out of three logic is employed to judge
instability of all the groups and to prevent
unwanted
operation in the event of an out-of-step of one
generator group within the bulk power system. In
order
to initiate system separation, the current swing
detection element must operate based on the
current
flowing through the transformers of the linkage
substations A1 and A2 between the bulk power
system
and the western area (Fig. 1).

When the two out of three generator groups have
been
determined to be unstable and the current swing
detection element has operated, the islanding
command
is issued from the Central Equipment to the
separation
point. The trip command is executed if the
current
swing detection element in the RTU has operated
on
the current flowing through that point. The
current
swing detection element is provided as a
fail-safe
measure for the protection calculation based on
phase
difference.

To achieve flexibility in dealing with various
operating
conditions of the western area, the present
protection
system is divided into completely separate duplex
systems System 1 and System 2. System 1 handles
separation at points (a) and (b) in Fig. 2, and
System 2
at points (a) and (b).
System 2 is nearly identical to System 1 so that
System 1 will be only described.

System 1 comprises a Central Equipment (Phasor
Data
Concentrator) at Substation A1 and Remote
Terminal
Units RTUs (Phasor Measurement Units PMUs) at
Substations B1, C, D, and E (Fig. 4).
The RTUs simultaneously sample bus-bar voltages
at
600 Hz. The sampled real-time data is transmitted
to
the Central Equipment (CE) via the data
transmission
system (a communication channel). In addition,
the
current flowing through the system separation
point (b)
at Substation B1 is measured then, the power
flow
value is calculated and transmitted to the CE.

FIGURE 4. BASIC CONFIGURATION OF SYSTEM 1
PROTECTION SYSTEM

At substation A1, the current and power flow
values
through the system separation point (a) and
through
the transformer are measured. The CE calculates
the
real-time phase difference and predicted future
phase
difference and selects the system separation
point using
this online data. The circuit breaker command is
issued
for either point (a) or (b) as selected by the CE
calculation on the measured data.

The entire system configuration is presented in
Fig. 5.
All devices are based on 16-bit microprocessors.
The
microprocessor has data transmission,
self-diagnostic
and calculating functions provided by the CE.
Data
transmission between the CE and RTUs is carried
out
synchronously via a duplex digital microwave
link. The
data transmission speed of this system is 56 kbps.

FIGURE 5. OVERALL CONFIGURATION
OF
PROTECTION SYSTEM

The data from one RTU of the C, D, and E
substations
is sent to both CE of the same system via the
data
transmission system. Each CE receives the data
for both
RTUs of each generator group, but only one set of
data
is normally employed. If an abnormality occurs,
the CE
can switch to the other RTUs data. The purpose
of this
two-fold redundancy is to decrease system
downtime
since long-distance transmission with multiple
spans is
employed.

The method of obtaining the phase difference
between
two points (western generator group and bulk
power
system) from simultaneously sampled voltage data
is as
follows.
In Fig. 6, representative voltage waveform
sampling
values for a bus-bar in the vicinity of the
northern
generator group (C in Fig. 6) and the western
generator group (B in Fig. 6) are shown.

FIGURE 6. EXAMPLE OF VOLTAGE WAVEFORM SAMPLING
DATA FOR TWO SUBSTATIONS

The phase difference, ?n, at present time n can
be
obtained from the voltage data VBn, VBn-3, VCn,
and
VCn-3 for the present time and three previous
samples as follows

Thus,
In order to simplify the calculation by replacing
X Xi
with a first-order approximation obtained via
Taylor
Series, the phase difference can be obtained by
where 0 ? X ? 1

If X ? 1, the following additional equation is
used to
perform the calculation.
If X ? 0, the following additional equation is
used to
perform the calculation.
Since the approximation error is on the order of
10-2,
accuracy is sufficient for practical use.

The phase difference ? between the two areas can
be
approximated by the following equation.
where,
?0 is initial value of phase difference ?,
? is angular frequency of ?,
? is damping constant,
A is amplitude.
This equation interprets the power swing mode as
a
sine wave that diverges or converges.

Using the phase difference values for the present
time
and previous time, the future phase difference
value
can be predicted. The predicted phase difference
? for
time TH in the future is derived from eight
pieces of
data (Fig. 7) and calculated as follows

FIGURE 7. METHOD OF PREDICTING
PHASE
DIFFERENCE

Values of 200 ms and 100 ms were selected for TH
and
TK (a time interval before the present time in
Fig. 7),
respectively, in order to predict accurately and
to
provide an acceptable operating time.
A simulation of the present prediction algorithm
calculation is given in Fig. 8. The results agree
well
with the present phase difference value and the
predicted future value at 200 ms.

FIGURE 8. SAMPLE CALCULATION OF
PHASE DIFFERENCE

When the obtained predicted phase difference
value ?
exceeds the setting value ?limit, it is judged
that the
power swing between the two generator groups is
unstable. The value for ?limit is determined by
computer
simulation under varying conditions and must
guarantee
operation when the system is unstable and prevent
operation when the system is stable.
The table in Fig. 9 shows the results of computer
simulation for several system operating patterns.
?limit is
set to 100?.

FIGURE 9. SIMULATION RESULTS

In order to guarantee fail-safe operation of the
scheme,
an input different from the voltage input, the
current
input, is used to detect the swing. Its logic
shown in
Fig. 10 comprises a rate of change detection
block to
determine whether power swing is present and a
magnitude of change detection block to detect the
size
of the current fluctuation. The element operates
on the
AND of these two blocks.

FIGURE 10. CURRENT SWING DETECTOR

The operation of the current swing detector is
presented in Fig. 11. The magnitude of change
detection block operates when the measured
current
fluctuation magnitude is greater than its
sensitivity
setting ISET during maximum power swing period
?Tmax.
The values for ISET and ?Tmax are determined by
computer simulation, and ISET is set for ?Tmax3
sec.

FIGURE 11. PRINCIPLE OF CURRENT SWING DETECTION

The element detects when the slope of the
difference of
the r.m.s. current value during the small
interval ?t
(?I/?t) is greater than constant K and continues
longer
than operation delay time T1 in order to operate
when the size of the current swing is greater
than ISET
during a sine wave that is smaller than ?Tmax. To
prevent dropout in the vicinity of extreme values
of the
sine wave, the OFF delay timer T2 (reset delay)
is set to
1 sec when ?t40 ms and T1200 ms.

A computer simulation was run for a double three-
phase-to-ground fault of a double circuit
transmission
line (1 route) in the trunk with a subsequent
failure of three-pole reclosing with synchronism
check.
The results are presented in Fig. 12.
The power swing has a tendency toward divergence
without a protection system (Fig. 12a). In this
case,
out-of-step is likely after 10 sec, and this
process would
continue to extend and eventually cause a
conventional
out-of-step relay to operate and separate the
western
area after about 13.5 sec.

FIGURE 12. EVALUATION BY COMPUTER SIMULATION

Even after separation, the power swing would also
continue in the bulk power system.
In contrast, when the protection system is
employed
as shown in Fig. 12b, the western area is
separated
after 6.6 sec, and the power swing of the bulk
power
system starts to converge. The western area is
then
operated independently, and another control
system
such as under-frequency load shedding adjusts the
power supply and demand balance.

The present protection system also underwent a
field
test. The power system configuration and test
results
are presented in Fig. 13. A circuit breaker at
point (c) is
being closed to configure the western power
system as
a loop system.The predicted and measured phase
difference values were observed. The predicted
phase
difference values before and after closing of the
circuit
breaker agreed well with the measured values.

46
b) Example of phase difference swing detection

FIGURE 13. CONTENT AND RESULTS OF FIELD TEST

EXAMPLE OF
OUT-OF-STEP PROTECTION SCHEME
POSSIBLE UPGRADE UTILIZING
SYNCHROPHASOR MEASUREMENTS
The company I am representing - American
Transmission
Company - utilizes an out-of-step protection
scheme,
which I sought to upgrade to make it adaptive -
or
predictive - using the real-time synchrophasor
measurement principle and newest hardware
available.

The existing ATC Northern transmission system has
limited power transfer capabilities from
Michigans Upper
Peninsula (UP) generation to a key bulk power
transmission substation due to its inadequate
transmission
and generation infrastructure.
Several completed major projects have improved
the
system. However, it remains weak and still
requires the
use of a special protection scheme to block
unstable power swings by tripping UP generation
for
critical faults on the transmission system.

Two pairs of power swing relays SEL-68 are
installed at
the key bulk power transmission substation to
backup the
UP generations SPS operations. Each pair is
installed to
trip in series for redundancy to prevent a
misoperation
should one of the relays fail.
The power swing relays are time-delayed to allow
operation of the primary SPS. If the primary SPS
fails to
operate, the power swing relays are designed to
separate
the ATC power system into two islands the UP and
the
Wisconsin bulk power system.

For unstable power swings to the south of the key
bulk
power transmission substation caused by an excess
of the
UP generation, one pair of the SEL-68 relays will
trip the
three southern lines of the power transmission
corridor.
For unstable power swings to the north of the
substation caused by a deficiency of the UP
generation,
the second pair of the SEL-68 relays will trip
the three
northern lines of the power transmission corridor.

FIGURE 14. CURRENT ATC POWER SWING RELAY SCHEME

The existing out-of-step protective relays are
SEL-68,
the 1987 years vintage. Their principle of
operation is
based on comparing in the central location the
voltage
phasor angles on two buses that present two
different
systems which, if not synchronized, will have to
be split
by the out-of-step relaying.

FIGURE 15. POWER SYSTEM
MODEL

DELTA (?) is the angle between source E1 and
source
E2, and V and I are the relays voltage and
current,
respectively.
E1VjX1I
E2VjX2I
The angle DELTA between these phasors is found as
follows
AjBE1(E2)

Where A is the real part of the complex product
of E1
and the complex conjugate of E2
B is the imaginary part.
Then, DELTA inverse cotangent (A/B)
DELTA-double-dot is found from DELTA-dot by
filtered
first derivatives.

DELTA, DELTA-dot, and DELTA-double-dot are used
by
the relay SEL-68 to assess system stability by
examining
their loci in two planes (figures 16 and 17).

FIGURE 16. DELTA VS. DELTA-DOT PLANE

FIGURE 17. DELTA-DOT VS. DELTA-DOUBLE-DOT
PLANE

If tripping (or blocking) is initiated when the
swing
trajectory crosses the TRIP-BLOCK DECISION LINE
in
the DELTA vs. DELTA-dot plane (Fig. 16), then the
power circuit breaker has just the necessary time
to
complete its operation before the TRIP-BLOCK
DECISION ANGLE (TBDA) is actually reached.
In Fig. 17, the loci for several different stable
() and
unstable () swings are plotted at the moment
that the
power angle crosses the TRIP-BLOCK DECISION LINE.

This explains how information from the two planes
(figures 16 and 17) is utilized by the SEL-68
relay to
arrive at a TRIP-BLOCK (or, equivalently, STABLE-
UNSTABLE) decision. Examining the swing loci in
the
DELTA vs. DELTA-dot plane permits the relay to
determine when the power angle has reached a pre-
determined value (TBDA), and plotting the loci in
the
DELTA-dot vs. DELTA-double-dot plane at that
instant
permits the stable/unstable decision.

The relays are designed to support four system
operating conditions, one normal and three
alternative,
each based on pre-calculated system impedances.
It is clear that the power system can have more
than
four operating conditions, and pre-calculated
impedance
values, even updated on a regular basis, will
never
provide the most accurate representation of the
real
system configuration. A precedence for potential
misoperation or insecurity of the protection
system has
been set.

Upon detection of an out-of-step condition, the
next
step of the protection system is to permit
selective
tripping for clearly unstable cases so that the
power
system is separated in the islands with a
reasonable
match between load and generation in each island.
At
present, the places where tripping is permitted
are pre-
determined based upon simulations performed
during
system stability studies. Eventually, a more
appropriate
procedure may be developed to determine both the
nature of an in-progress swing as well as
desirable
locations for separation in real-time.

63
FIGURE 18. PREDICTIVE OUT-OF-STEP
PROTECTION SYSTEM
BASED ON REAL-TIME PHASOR MEASUREMENTS
64

Implementation of this scheme will require
substantial
communication channel capacity. Phasors from the
two
key places must be communicated to a central
location
where stability swing evaluation and prediction
are to
be carried out. After the prediction phase,
direct trip
or block commands must be communicated locally
and
to one remote substation. It seems almost certain
that
fiber optic communication channels will be
necessary
for adaptive features of this category to be
implemented.

REFERENCES
Y. Ohura, M. Suzuki, K. Yanagihashi, M. Yamaura,
K. Omata, T. Nakamura, A predictive Out-of-Step
Protection System Based on Observation of the
Phase Difference Between Substations, IEEE
Transactions on Power Delivery, Vol. 5, No. 4,
November 1990.
2. A. G. Phadke and J. S. Thorp, Computer
Relaying for Power
Systems, Research Studies Press Ltd.
ISBN 0 86380 074 2.
3. G. Benmouyal, E. O. Schweitzer III, A.
Gusman, Synchronized Phasor Measurement in
Protective Relays for Protection, Control, and
Analysis of Electric Power Systems.

REFERENCES (cont.)
4. Ph. Denys, C. Counan, L. Hossenlopp, C.
Holweck, Measurement of Voltage Phase for the
French Future Defense Plan Against Losses of
Synchronism, IEEE Transactions on Power
Delivery, Vol. 7, No. 1, January 1992.
V. Centeno, J. De La Ree, A. G. Phadke, G.
Mitchell, J. Murphy, R. Burnett, Adaptive
Out-of-Step Relaying Using Phasor Measurement
Techniques, IEEE Computer Applications in Power,
October 1993.
6. S. H. Horowitz and A. G. Phadke, Boosting
Immunity to Blackouts, IEEE Power Energy
magazine, September/October 2003.

REFERENCES (cont.)
Schweitzer Engineering Laboratories, SEL-68 Out
of Step Blocking/Tripping Relay Swing Recorder.
Instructional Manual, April 1986
8. E. O. Scweitzer III, T. T. Newton, R. A.
Baker, Power Swing Relay Also Records
Disturbances, 13th Annual Western Protective
Relay Conference, Spokane, WA, October 21-23, 1986

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