Improving Pacific Intertie Stability Using Slatt Thyristor Controlled

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Improving Pacific Intertie Stability
Using Slatt Thyristor Controlled Series Compensation
Detailed summary submitted to IEEE/PES 2000 WM Panel Session, Singapore
Vaithianathan Venkatasubramanian, Member, IEEE
School of Electrical Engineering and Computer Science
Washington State University
Pullman, WA 99164-2752
This paper discusses control design issues for the
existing thyristor controlled series compensation (TCSC)
device at Slatt substation on the Slatt–Buckley 500-kV
transmission line owned by BPA. The controls were
developed at Washington State University (WSU) in a project
funded by Bonneville Power Administration (BPA). The
project has been carried out in close collaboration between
the WSU team and the BPA engineers. The controls are
primarily aimed at providing fast transient stabilizing control
during the first few swings of critical contingencies. Midterm small-signal stability enhancement is the secondary
control objective.
A comprehensive overview of TCSC control schemes
can be seen in [1]. Recent papers, [2] and [3], discuss the
control design issues of TCSC installations in China and in
Brazil respectively.
Note that the BPA TCSC installation at Slatt was the first
of its kind. Among other considerations, the site location on
the Slatt–Buckley 500-kV line was motivated by its
proximity to the thermal power plant at Boardman for
conveniently testing the subsynchronous resonance issues [4]
(see Figure 1). The Slatt–Buckley line is one of several 500kV lines that feed into the Pacific AC intertie, that is, the
California-Oregon Intertie (COI) within a complex network
of 500-kV transmission lines in the lower Columbia region.
Tradition-ally, TCSC impedance is varied for the purpose of
controlling active power flow through the line in a suitable
fashion. Owing to the existence of multiple power flow loops
in the vicinity of the Slatt TCSC, a change in active power
flow on the Slatt–Buckley line does not result in a
corresponding change in active power flow on the critical
COI tie lines. Therefore, design of controls for the Slatt
TCSC, which provide transient stability support for severe
contingencies affecting the COI tie lines, becomes a
challenging problem.
While the effect of Slatt TCSC variation on COI active
power flow is rather poor, we noted in our studies that the
Slatt TCSC can provide excellent dynamic reactive power
support for the COI lines. Therefore, the TCSC control is
formulated like a voltage control device by varying its
impedance proportional to a voltage input. Short-term overload capability of the TCSC is utilized for deriving transient
stability support during the first few swings after severe
contingencies.
A detailed description of the technical features of Slatt
TCSC can be seen in [4]. Slatt TCSC consists of six identical
modules per phase. Each module can operate either bypassed
–
Carson W. Taylor, Fellow, IEEE
Transmission Operations and Planning, TOP/Ditt2
Bonneville Power Administration
Vancouver, WA 98666-0491
or inserted. Normal rating of the TCSC is 8 ohms at 2900
amperes which is equivalent to 202 MVAr. TCSC can be
operated in either discrete mode or in continuous vernier
mode.
In discrete control mode, the TCSC ohm order can be
any one of the following: –1.2 (all caps off), 4, 8, 12, 16, 20,
or 24 ohms. In continuous ohms operation, the TCSC order
can be specified to be any value between –1.2 ohms and 24
ohms. There are nonlinear constraints on the ohm order
depending on the current through TCSC and the time
overload characteristic is shown in Figure 2. For instance, the
ohm order of 24 ohms cannot be realized under typical
operating conditions since the actual line current with TCSC
at 24 ohms would be much higher than the permissible limit
of 2000 amperes. The time-current overload capability of the
TCSC in Figure 2 has to be carefully accommodated in the
stability control design to make sure that the design is
realistic. At present, the TCSC is operated at a fixed value of
either 8 or 12 ohms, and there is no dynamic control for the
TCSC.
The controls are developed in our work based on
heuristic analysis of simulation results. All the simulations
were carried out on detailed large-scale representations of the
WSCC western power system, which typically contain about
6500 buses and 900 generators. The simulations were mostly
carried out using the Extended Transient Midterm Stability
Program (ETMSP) developed by Electric Power Research
Institute (EPRI) and the BPA power flow program. The EPRI
small-signal stability program PEALS was also used for
screening of bad data, and for studying the effects of controls
on damping of the modes. User-defined detailed dynamic
models were developed for the power electronic devices in
Pacific Northwest including the Slatt TCSC. Detailed models
for the HVDC interties, developed previously by BPA
engineers, were incorporated into the seasonal base cases of
the entire WSCC system.
The proposed controls were tested for several
contingencies including: 1) Simultaneous tripping of two
Palo Verde nuclear units in the southwest, which is a
generation loss of about 2700 MW and, 2) Tripping of two
500-kV transmission lines from Paul to Allston that typically
carry about 2200 MW. Seasonal WSCC base cases for 1997
summer peak, 1997 summer off-peak, 1998 Spring peak were
used, together with validated cases for the WSCC
disturbances on July 2, 1996 and August 10, 1996. The
controls are expected to improve transient stability performance for specific critical contingencies for the various
seasonal cases as stated above. Moreover, they are required
not to degrade the stability performance for all credible
contingencies that were studied. The control benefits were
measured by the “MW margin,” that is, the change in COI
active power flow of the critically stable cases for specified
severe contingencies with and without the proposed controls.
from Ingledow (BC Hydro)
Chief
Joseph
Custer
Monroe
Grand
Coulee
N
Maple
Valley
W
Schultz
Raver
E
S
Vantage
Paul
Hanford
Allston
Little
Goose
Ashe
Lower
Lower
Granite
Monumental
Keeler
Big Eddy
Pearl
John
Day
McNary
Ostrander
Marion
Slatt
Coyote
Springs
Buckley
Boardman
Lane
Grizzly
Alvey
Fort Rock
to Midpoint
Capt. Jack
Meridian
to Olinda
Summer
Lake
Malin
to Round Mt.
DC to Sylmar
Figure 1. One-line diagram of the 500-kV transmission network in
Pacific Northwest.
Figure 2. Reactance capability of TCSC [4].
In a traditional sense, TCSC is quite effective for
controlling the active power flow through a line.
Accordingly, traditional phase-lead/phase-lag designs were
studied first using several inputs: 1) local 500-kV voltage, 2)
local frequency, 3) Slatt–Buckley line current, 4) COI line
current, and 5) COI active power flow. The common problem
with all the controllers was that they did not provide robust
transient support. When low control gains were used, the
TCSC controller did not have much effect on the intertie
stability. On the other hand, when high gains were used, the
controller could be tuned to provide good performance for a
specific contingency. However, the high gains interacted
negatively with inter-area modes for other contingencies,
which is undesirable.
As stated earlier, while the active power flow through the
Slatt–Buckley line could be varied very well with the TCSC,
this did not translate into controlling the COI power flow
because of the topology of the parallel 500-kV transmission
lines that feed the COI lines (see Figure 1). Therefore, an
alternate approach of operating the TCSC as a dynamic
reactive device based on 500-kV local voltage input, just like
a SVC, was studied next.
Among local voltage-based Slatt TCSC controllers, we
noted that a simple proportional control design provides
excellent transient/damping support, whereas phase-lead and
phase-lag designs may not be robust. In fact, the proportional
control can be seen to provide consistent and robust
transient/small-signal stability support for a variety of
contingencies, and for several different seasonal operating
conditions in the simulations. Therefore, the proportional
TCSC control based on local voltage input is recommended
as an effective candidate for the TCSC controller.
Since the Slatt TCSC by design is a discrete control
system (consisting of six 4-ohm capacitor banks), the
proportional control can be implemented quite easily in the
form of discrete control logic as shown in the next section.
The discrete controller provides “dead-bands” around different ohm settings so that the TCSC switching can be reduced
drastically during system transients. The “dead-bands” in the
discrete controller would also prevent the TCSC from
interacting with local modes and small-amplitude interarea
oscillations of distant areas such as other California or
Alberta/British Columbia modes.
To provide effective dynamic reactive support at Slatt
TCSC, a simple discrete controller was designed to vary the
Slatt TCSC capacitance proportional to the voltage input
signal. The thermal overload capability of the device is
programmed as part of the control design. Slatt voltage is
nominally maintained around 542 kV in this design. The
following voltage based controller has provided excellent
transient support in our studies.
TCSC nominal
=> 8 Ω.
Slatt voltage > 545 kV => TCSC all caps off (-1.2 Ω).
Slatt voltage > 540 kV => TCSC to 4 Ω.
Slatt voltage < 530 kV => TCSC to minimum of (16 Ω,
IL based 10 sec. overload limit) for Timer < 6 sec.
=> TCSC to minimum of
(12 Ω, IL based 30 min. overload limit) for Timer > 6
sec.
Here, Timer counts TCSC operation at the 10 second
overload limit (i.e., when voltage < 530 kV). During swings,
whenever the Slatt voltage goes below 530 kV this controller
operates the TCSC at its 10-second capacitive limit for up to
6 seconds. After 6 seconds (that is, when 60% of overload
capacity is reached), the TCSC is kept at 12 ohms or at the 30
minute overload limit. In our simulations, 12 ohms is well
within the 30 minute overload limit of the TCSC. Also, the
controller would switch out capacitors whenever the Slatt
voltage swings above 545 kV as stated. Essentially, the
controller provides dynamic reactive support to COI lines
based on a local voltage input. We have also included an
upper limit on the number of switchings over a specified time
period to prevent the possibility of hunting between different
controller settings.
When compared with fixed TCSC operation at 8 ohms,
the MW margins from the proposed discrete controller for the
1997 WSCC summer peak case for the critical contingencies
are stated below:
• Double Palo Verde: +200 MW, and
• Double Paul-Allston: +200 MW.
The MW margin for the critical contingency of the 1997
WSCC summer off-peak case is:
• Double Palo Verde: +100 MW.
The MW margin for the 1998 spring peak case is:
• Double Palo Verde: +100 MW.
Overall, the discrete Slatt controller provides excellent
transient stability support to COI lines, and also provides
significant damping enhancement to interarea oscillations in
all our studies. As an enhancement, the TCSC could be
designed to operate in a continuous vernier mode during
nominal voltage conditions as a small-signal stabilizing
controller. For instance, when Slatt voltage is between 540
kV and 545 kV in the design stated above, the TCSC
impedance can be varied by 4 to 8 ohms about the nominal
setting to provide small-signal damping support. Other
control formulations, which utilize wide-area remote measurements, should also be pursued.
Acknowledgements. This work was supported by Bonneville
Power Administration under grants 97FC33945 and
98FC03310. The authors thank BPA engineers Dmitry
Kosterev, Ravi K. Aggarwal, and Bill Mittelstadt for helping
us with the WSCC data and on the modeling of thyristor
controls. The authors are also grateful to Karl W. Schneider,
now at National Systems and Research Inc., for his immense
help in carrying out the WSCC simulations while he was a
graduate student at WSU.
References
[1] X. Zhou and J. Liang, “Overview of control schemes for
TCSC to enhance the stability of power systems”, IEE
Proc.-Gener. Trans. Distrib., Vol. 146, No. 2, March
1999, pp. 125–130.
[2] X. Zhou, et al., “Analysis and control of Yimin-Fengtun
500-kV TCSC system,” Electric Power Systems
Research (Elsevier), 46, 1998, pp. 157–168.
[3] C. Gama, “Brazilian north-south interconnection –
Control application and operating experience with a
TCSC,” Proceedings of the 1999 IEEE PES Summer
Meetings, Edmonton, Alberta, 1999.
[4] J. Urbanek, R. J. Piwko, E. V. Larsen, B. L. Damsky, J.
D. Eden, B. C. Furumasu, W. Mittelstadt, “Thyristor
controlled series compensation prototype installation at
the Slatt 500-kV substation,” IEEE Trans. Power
Delivery, Vol. 8, No. 3, July 1993, pp. 1460–1469.
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