Application of Static VAR Compensation on
the Southern California Edison System to
Improve Transmission System Capacity and
Address Voltage Stability Issues Part 1. Planning, Design and Performance
Criteria Considerations
Janet Kowalski, Southern California Edison
Ivars Vancers, Mark Reynolds, Heinz Tyll,
Siemens Power Transmission &
Distribution, Inc.
Abstract: This paper describes the planning, specification
and design of the 500 kV Static Var System (SVS) installed
on the Southern California Edison network at Devers
Substation. The Devers SVS includes a static var
compensator (SVC) a form of flexible alternating current
transmission system (FACTS) technology and a controlled
mechanically-switched shunt capacitor bank. The Devers
SVS project is one of several planned projects to increase
power imports into the Los Angeles basin from generation
sources east of the Colorado River (EOR). A follow-up
paper is planned which will present the construction and
final commissioning of the Devers SVS at project
completion expected in September 2006.
I.
[1]. Increased import and export conditions as
studied by the California Independent System
Operators (CAISO) and Southern California Edison
(SCE) in the Southwest Transmission Expansion
Planning (STEP) process highlighted the need for
additional dynamic var support at several SCE
Network locations. The Static Var System (SVS) at
the Devers 500 kV substation was the first in a
planned series of SVC additions necessary for the
reliable operation of the SCE Transmission Network,
while maintaining a strong import/export flexibility
to proved rate payer benefits by allowing time of day
and seasonal diversity energy purchases and
exchanges. [2]
INTRODUCTION
Deregulation of electric utility industry in North
America has resulted in several new generation
interconnections. However, due to the uncertainty of
cost recovery and environmental concerns, very little
has been done in the area of transmission. Most of
this new generation is located away from major load
centers. The major load centers in the southern
California Los Angeles (Greater Metropolitan area)
have a high concentration of induction motor
arrangements and prime movers, that support critical
air conditioning loads and are heavily dependent on
local generation and or dynamic compensation to
provide the necessary reactive support. The existing
fleet of local generation presently serving the native
load customers is slowly being replaced by the new
generation located remotely from the load. The
major portion of the real power into the load center
has to be supplied from outside the area through
heavily loaded long transmission lines. These
combined impacts are the primary reasons voltage
instability problems occur at the major load centers
1­4244­0178­X/06/$20.00 ©2006 IEEE
Based on planning studies jointly conducted by
CAISO and SCE increased imports primarily from
the Palo Verde Nuclear Generation Station in
Arizona were limited by potential voltage stability
problems. The studies identified several 500 kV, 230
kV and 115 kV interconnections at the Devers and
Valley substations in the SCE system with transient
voltages outside the bounds of the WECC criteria for
N-1 contingency simulations.
Comprehensive voltage stability studies were
performed for this area of the SCE transmission
system and various reinforcement options were
evaluated. Based on technical, economical and
reliability factors, a Static VAR System (SVS)
comprised of a Static VAR Compensator (SVC) and
Mechanically switched shunt capacitor bank (MSC)
was selected as the preferred solution.
451
PSCE 2006
II. PLANNING STUDY METHODOLOGY
AND CRITERIA USED
II. STUDY RESULTS
TABLE 1: HASSAYAMPA – NORTH GILA OUTAGE
(WITHOUT DYNAMIC REACTIVE ADDITIONS)
A.
System Performance Criteria
In order to compare various alternative solutions
a standard set of performance criteria was
established. This dynamic performance criterion was
based on the Western Electric Coordinating Council
(WECC) reliability criteria [3].
This criterion
considered three main factors: 1) Voltage Dip 2)
Duration of the voltage dip, and 3) Post Transient
Voltage recovery level. The voltage dip criteria
required that the voltage at any bus should not dip
below 30% for more than 20 cycles. For post
transient voltage level criteria, the buses with voltage
below 0.92 p.u. at the end of dynamic simulation
were flagged. The primary objective of the dynamic
study was to come up with a solution which would
eliminate all violations on the buses in the areas
studied for all system disturbances analyzed.
Substation/Bus
High Desert/115 kV
Devers/500 kV
Valley Substation/115
Initial Voltage/Post
Voltage pu
0.978/0.972
1.035/0.812
1.000/0.805
B.
Load Models
For dynamic voltage stability assessment a load
model was assumed for the SCE Network that
properly captured the effect of higher levels of
induction motor load. For the study the default case
of 20 % was increased to 50 % for this area. This
higher percentage was based on the summer peak
loading caused by air conditioners. Sensitivity
studies were made with induction motor loading as
high as 80 % ; However the results at 50 %
distribution was judged to provide an adequate
representation of the motor connected load.
C.
Dynamic Performance Sensitivities
Sensitivity studies were performed on the 20062008 peak and off peak models to evaluate the
relative impacts of varying system load, generation,
and inertia and transfer levels on dynamic support
requirements. This was done to get an understanding
of the severity of the contingencies and identify the
most critical contingency from a dynamic standpoint.
This worst case scenario was identified to be a three
phase fault and tripping of the Hassayampa- North
Gila 500 kV line under off peak conditions with high
East of River (EOR) flows.
Fig 1/1A: Transient voltages in the SCE Network for
the loss of the Hassayampa-N. Gila 500 kV line.
(Simulation does not include any dynamic reactive
additions)
A typical simulation result is plotted in Figure 4
showing voltage swings outside the WECC criteria
bounding for several critical buses at both Devers and
Valley substations. Table 1 shows the maximum
impact of the most critical study cases.
A summary of the different solutions considered is
provided in Table 2.
452
TABLE 2: PROPOSED
(SOLUTIONS TO THE DYNAMIC PROBLEM)
Solution
SVC
SVC
STATCOM
Series Capacitors
Size
600 MVAR on Devers
500 kV bus
400 Mvar at Devers 500
kV and 200 Mvar on
Valley 115 kV bus
±600 MVAR at Devers
500kV bus
Increase compensation
on the Devers-Palo
Verde 500 kV line
Figure 3: V/I characteristic of Devers SVC (note the
reactive contribution from the MSC is additive to the
capacitive part of this characteristic)
A comprehensive functional specification was
developed for a Static Var System with continuous
operating range of 440 Mvar capacitive to 110 Mvar
inductive at the Devers 525 kV operating voltage.
This specified characteristic provided the primary
basis for the solicitation and technical evaluation of
bid proposals from the major SVC suppliers.
Fig 2: Plot comparing No Reactive Support (a)
(red), STATCOM (b) (green) , SVC (c) (blue)
All of the solutions proposed in Table 2
corrected the transient voltages at the Devers-Valley
area buses to levels within the WECC criteria bounds
(see Figure 2). The initial solution selected was the
installation of 400 Mvar SVC at the Devers
Substation 500 kV bus and two 100 Mvar SVCs on
the Valley Substation split 115 kV buses. Though
the dynamic study simulations did not identify a need
for a net inductive output the inductive range was
extended to 110 Mvars for improved steady-state
voltage regulation for all system conditions. An
option for continuous or vernier controlled reactive
output was selected to provide flexibility and smooth
voltage regulation of the Devers Substation 500 kV
bus which is a critical location in the SCE system.
The Devers Substation is located about 125 miles
east of Los Angeles near Palm Springs, California.
The original system selected from the over
twenty proposed configurations featured 3 – 110
Mvar Thyristor Switched Capacitor (TSC), 2 -110
Mvar Thyristor Controlled Reactor (TCR) and 2- 55
Mvar Filter branches connected to the SVC
transformer secondary bus and a 110 Mechanically
Switched Capacitor (MSC) bank on the 500 kV bus.
The third TSC branch was initially provided as a
spare branch to meet the high availability
requirements for the system. An optional equipment
upgrade of this configuration would allow the short
time use of the spare TSC branch to increase the SVC
capacitive dynamic range to 440 Mvar. An increase
in the MSC bank size from 110 Mvar to 165 Mvar
resulted in a SVS with a short-term capacitive output
of 605 Mvar. Follow-up simulations have confirmed
that this increase in dynamic range defers the
requirement for additional compensation on the
Valley Substation 115 kV system beyond the
planning study timeframe.
The basic characteristic V-I curve specified
(without the added MSC branch) is shown in Figure
3. Please note that the V/I characteristic for the
secondary side (LV) of the SVC are shown in Figure
5 after the One-Line configuration.
453
IV. SVS DESIGN
Based on the study results and in conjunction
with SCE SVS design Specifications, the contract to
design and construct the Devers Static Var System
was awarded on a turnkey basis to Siemens PT&D.
3AC60Hz525kV
STF2
165 Mvar
VMS
C
C
MSC
LMS
CF2
C
LF2
SN = 330MVA, uk = 11 %
3AC60Hz 17.3kV
LTC1
R2
LTS1
LTS 2
C
V1
V 1 V2
R
C
V 2 V3
R
CTS 1
LTC1
R2
CTS 3
C
C
LF1
CF1
V3
R
CTS 2
C
LTC2
R2
LTS 3
C
LTC2
R2
Figure 5:
TCR1
TSC1
TSC2
TSC3
TCR2
STF1
V/I-characteristic of the SVC at the
LV-side of the SVC-transformer
Figure 4. Devers SVS Simplified One Line
B. MSC design
The arrangement of the SVS is shown in the
simplified single line diagram in figure 4. It includes
SVC and a mechanically- switched capacitor (MSC)
bank connected to the HV bus.
The Mechanically Switched Capacitor MSC
branch of the SVS was designed to be pre-installed as
a stand alone manually switched shunt capacitor bank
to provide temporary var support before completion
of the final system. In the final installation, the
switching of the 525 kV, 165 Mvar shunt capacitor
bank will be integrated into the control algorithm of
the SVC. A 500 kV SF6 circuit breaker functions to
both switch and protect the MSC equipment.
A. SVC design
The nominal voltage of the secondary busbar of
the SVC was optimized to 17.3 kV. The 17.3 kV bus
is connected to the HV system via the SVC
transformer rated continuously for 330 MVA with a
short-term (1 hour) overload capability of 440 MVA.
For the SVC output, three TSCs together with
two filter branches provide the total capacitive
output.
For the Devers static var system, the inclusion of
a MSC branch offered several advantages. The most
obvious is the increase in capacitive output from 440
to 605 Mvars without additional transformation
capacity, power electronics and the associated
cooling requirements. Voltage stability simulations
have shown that the MSC branch can effectively
extend the dynamic range of the SVC when switched
within 8 cycles after initial fault event. This is well
within the typical circuit breaker 2-3 cycle closing
time. Other advantages include the reduction of
operating losses and the ability to provide partial
dynamic compensation even if SVC equipment is not
available as long as the control system is functional.
The major disadvantage is the additional discharge
time required between successive energizations.
The fixed filter branches are designed to provide
the required capacitive power at 60 Hz and are tuned
to limit the harmonic voltage distortion at the point of
common coupling (PCC) to within IEEE 519
guidelines. Voltage distortion typically dominant in
the 5th and 7th harmonics can result from the variation
of the TCR firing angles.
The secondary voltage V-I characteristic can be
shown below in figure 5. This V/I characteristic are
the basis for the determination of the secondary side
connected components.
454
The final arrangement of the MSC is shown in
the simplified single line diagram in figure 4. The
nominal voltage of the HV busbar is 525 kV
(1.0 p.u.). The MSC tuning reactor is located in
series with the grounded side of the capacitor and
protected by an associated arrester for lightning and
switching surges. The MSC was tuned to the 5th
harmonic so as to effectively reduce ambient network
harmonic voltages.
x
The bus voltage is below the defined steady
state operating range and above the under
voltage control block level.
x
The SVC steady-state output exceeds 165
Mvar capacitive for a defined time period.
The MSC branch will be switched off when SVC
output is at its full inductive limit (-110 Mvar) for a
defined time period.
The Devers SVS control also features a reactive
output or Q-Mode which has been modified to limit
the steady-state output to predetermined levels [5, 6].
This will maintain a “dynamic reserve” by resetting
the voltage reference to allow other equipment to be
switched in regulate the system voltage or make up
var deficits.
B TSC protection
The philosophy behind control based protection
in an SVC, for the most part, is to protect the
thyristor valves in the TSCs. Thyristor protection is
based on the maximum off-state voltage across the
thyristor valves and the maximum on-state current for
which the thyristor is designed. These quantities are
a function of the physical make-up of the thyristor
and the operating junction temperature. For transient
over currents such as those associated with misfiring,
maximum thyristor junction temperature under which
the valve can safely block becomes an important
consideration in addition to the over current value
itself. Other operating parameters like di/dt and dv/dt
that play an important role in determining the rating
and transient withstand capability of the thyristor are
accounted for in the design of the thyristor valves and
control equipment.
Figure 6 Shows the various reactive branch MVA
contributions during the operation of the SVC
(without showing the additional 165 MVAR MSC
contribution)
V.
CONTROL AND PROTECTION
A. SVS Control Strategy
The primary control mode of the Devers Static
Var System is the regulation of the 500 kV bus
voltage by either supplying or absorbing reactive
power. This is accomplished by coordinating the
firing angle of the reactor branch thyristors and the
electronic switching of the capacitor branches.
Figure 5 shows the operating and transition points for
each reactive exchange level through the -110 to
+440 MVAR SVC operating range (without the
MSC). It is important to note the hysteresis between
operating levels which ensures smooth transitions
and eliminates the possibility of undefined operating
points.
1). Over-voltage protection
The over-voltage strategy employed for TSC
legs is usually the quickest protection for blocking
the valves during overvoltages. Only the SVC
coupling transformer high side voltages are
monitored. For higher and sustained over-voltages,
the strategy is to trip the SVC. TSC capacitor banks
and the thyristor valves. The TSC valve together
with the associated MOV arrestor is configured to
block all significant and sustained overvoltage
conditions.
The MSC branch will be switched on during the
following conditions
455
2)
permitted for a second misfire within a predetermined
time period.
Under-voltage protection
Transformer energization and geomagnetically
induced currents can cause harmonic inrush which
might induce the control system to initiate false
protection. So, proper signal processing and filtering
of the voltage and current signals is done before they
are seen by the control system. Usually, notch filters
added at the 2nd, 3rd 5th, 6th, and 7th harmonics
filter the voltage and current signals.
The most commonly used strategy for dealing
with under voltage conditions is to block firing of
TSC valves because the valves do not have sufficient
forward voltage to safely turn on [6,7]. In addition,
blocking the TSC valves prevents the occurrence of
large transients that would occur if the voltage
happens to recover at that instant. The stability
controller (the gain-supervision control or the deadband control) is also usually blocked during the under
voltage condition. After the system voltage recovers,
the stability controller is deblocked after a suitable
time delay.
3)
In order to accomplish the junction temperature
misfiring protection mentioned above, a thermal
replica is also incorporated in the SVC control
system which evaluates the thyristor valve junction
temperature. The thermal replica takes into account
the hot-spot effects and junction switching losses.
Based on the differential resistance of the thyristor
wafer (which is a function of time) and based on the
thyristor currents in each phase, the thermal loss in
each phase is computed.
Over-current protection
Transient over-currents caused by valve
misfiring and thyristor junction heating caused by
increased switching and conduction losses at
maximum continuous current are the two main
concerns while developing the strategy for controlbased TSC over-current protection. A misfire event
combined with maximum valve blocking voltage
(under worst system over voltage) with a
precondition of maximum steady state junction
temperature caused the most severe stresses to the
valve. When the thyristor valve current exceeds an
over-current threshold (usually around 4 p.u. rated
current), a misfire is assumed to have occurred. The
misfire over-current protection strategy is then
decided by the particular application.
From these losses and from the junction to case
and case to heat sink thermal resistances, the
temperature rise due to junction heating is computed
in the thermal replica for each phase and current
direction. Using this temperature rise and the
ambient temperature of the coolant, the junction
temperature, each worst case thyristor level can be
identified, and the tripping point can be determined.
If the junction temperature as computed by the
thermal replica to be over 120 degrees C, then TSC
valve pulse blocking is disabled because thyristors
lose blocking capability. If however, protective
firing occurs, an SVC trip is simultaneously initiated.
In any case, if the junction temperature exceeds
148 degrees C, an instantaneous SVC trip is issued.
If it is desired that the SVC survive a misfire and
still be available, then the thyristor valves have to be,
in effect, designed to withstand the stresses of a
second misfire.
In such a situation, the
corresponding TSC valve is blocked for a period of
time (usually around 0. 5 seconds). Misfire detection
is then disabled for a quarter of a cycle to avoid
retriggering due to the second half-wave of the 4th
harmonic misfire current. If a second misfire is then
detected, a block inhibit signal is sent to the valves,
triggering the valves continuously and then tripping
the SVC because the valves may have lost their
blocking capability.
4).
Miscellaneous TSC protection
Valve base electronics (VBE) also are tasked to
perform other important protective duties. The
grading circuit, or snubber circuit, consisting of
resistances and capacitors, ensures uniform voltage
across thyristor pairs and provides a shunt for
transient over voltages. Break-over diodes (BODs)
are incorporated into each thyristor, to provide overvoltage protection of the valves if the voltage exceeds
the thyristor breakover voltage. This BODs function
could be external, as in electrically triggered
thyristors, or incorporated into the thyristor wafer as
in the case of direct light triggered (LTT) thyristors
as applied to this Project.
If, on the other hand, the misfire protection
philosophy is to ride through only one misfire, the
TSC valves are usually continuously fired followed
by an SVC trip. This prevents thyristor valve
blocking following the high current condition.
The Devers substation SVC is specified to
experience one misfire without tripping. Tripping is
456
D.
The VBE also monitors the voltage across each
thyristor pair; when a block is issued, the VBE
determines the number of thyristor pairs supporting
forward voltage. After a delay, if a sufficient number
of levels indicate that they are not supporting forward
voltage, the TSC is preventively fired and the SVC is
tripped. When the monitored anode-cathode voltage
in the thyristor valves is greater than a voltage
threshold (usually 10 – 50 V), the forward thyristors
are fired.
When the monitored anode-cathode
voltage is less than the reverse pair threshold (usually
between -10 and -50 V), the reverse pair is fired. Fire
latches are usually set for longer than 180 degrees.
Additionally, an interlock is provided which prevents
fire latches being set if the forward voltage of the
respective string exceeds a safe value. The VBE
transmit data back to the control system regularly,
whether the thyristor pair is energized or not. This
allows detection of gate circuit faults prior to
energization. If failure to deblock is detected (failure
to detect current in a deblocked state), the TSC
thyristors are protectively blocked. If failure to block
condition is detected (detection of current when in
blocking state), the TSC thyristors are protectively
fired and the SVC tripped.
TCR protection
1) TCR Valves and Reactors
The TCR valve is equipped with integrated BOD
protection on each thyristor. This results in a
minimum valve protection level of 60 kV when the
valve is not conducting.
During conduction period, the current surges will
be applied across the reactor. Each reactor is
designed with an insulation level of 150 kV terminal
to terminal and terminal to ground. This assures a
straightforward main-circuit protection arrangement
with the minimum of complexity.
Design simulations were made to complete the
protection arrangement adequacy. In Figure 7 below
the effect of trapped charges and worst case
conditions are demonstrated after a worst case fault
sequence.
6000
5.21 kA
[A]
4000
2000
The valves in the TSC branches are protected by
MOV arresters. The VR-arrester (valve and reactor)
is connected across the TSC valve and the reactor. A
V-arrester (valve) is connected across the TSC valve.
The arrestors are coordinated together with the
minimum blocking voltage capability to ensure that
BOD valve firing will not occur.
0
-2000
-4000
-6000
0.00
0.05
0.10
(file DCODE_01.pl4; x-var t) c:TCR1RS-TCRI1R
Another important protective control employed
in TSCs is current supervision. This function
supervises correlation between thyristor pulses and
thyristor main current and is used to detect major
faults. Capacitor bank current is calculated from the
low-side voltage and TSC susceptance. This value is
compared to the measured TSC branch current. If the
difference in the two is more than a certain threshold
value for more than a time period, an SVC trip is
issued.
Transient events like transformer
energization may cause this protection to
malfunction. Harmonic distortion of the voltage and
current waveform during ac network events may
cause the TSC to extinguish when normally the
current is expected to be at a maximum level. If this
condition persists for an extended period of time, the
TSC current supervision control may incorrectly trip
the SVC. However, this condition was tested and it
was verified that the control system did not
incorrectly trip the SVCs.
0.15
c:TCR1ST-TCRI1S
0.20
0.25
[s]
0.30
c:TCR1TR-TCRI1T
Figure 7 Shows the trapped current levels after a
three phase fault (at the worst possible set of
conditions)
2) DC Trapped Current on a TCR Valve
The TCR valves will be stressed with a DC
trapped current when a three phase system fault
occurs at the peak of one TCR current.
Different cases of 3-phase faults have been
regarded. The highest stresses occur during a fault
sequence starting with inductive operation with both
TCR and without the MSC in operation at increased
system voltage of 1.1 pu followed by the 3-phase
fault and fault clearing. Only little damping is present
due to the high quality factor (X/R = 250) of the TCR
branches. Depending on the time of fault clearing an
additional peak in thyristor current will occur
[7,8].Clearing the fault after about 90 ms the TCR
current reaches a peak value of approximately 5.21
kA.
457
E.
MSC protection
Anticipated completion of total project including the
SVC branch is mid-September 2006.
The MSC protection is based on standard Shunt
Capacitor protection philosophy, configured with a
main and back-up protection arrangement.
The next SVC will be installed and
commissioned on the SCE system at the 230 kV
Rector substation in the Northern Region of SCE’s
Network system near Visalia, California.
The
continuous output range of this SVC is +200 Mvar
capacitive to -120 Mvar inductive. Completion of the
Rector substation SVC is anticipated to be in service
mid June 2007.
The power circuit breaker used for this MSC was
protected by PCB failure protection, along with two
coordinated and overlapping differential protection
arrangements.
This protection was further
coordinated with the 500 kV bus differential
protection scheme
REFERENCES
The capacitor bank is protected first by timeovercurrent relays, phase unbalance detection, and
time-overcurrent overload protection for the series
connected air-core reactors. The reactor protection
scheme is essentially based on the temperature rise
characteristics of the reactor coils.
[1] C.W.Taylor, Power System Voltage Stability,
McGraw-Hill Inc, 1992
[2] I. Green , CAISO Grid Planning, “Dynamic
Stability Analysis for the Short-Term Upgrades
of the STEP Project,, July 2004.
[3] WECC Reliability Criteria Document, Voltage
Performance Parameters.
[4] I. Green , CAISO Grid Planning, “Dynamic
Stability Sensitivities for the Short-Term
Upgrades of the STEP Project,, September 2004
[5] D. Dickmander, B. Thorvaldsson, G. Stromberg,
D. Osborn, et al, “Control System Design and
Performance Verification for the Chester, Maine
Static VAr Compensator,” IEEE Power
Engineering Society, Summer Meeting 1991
[6] H. Tyll, K. Leowald, F. Labrenz, D. Mader,
“Special Features of the Control System of the
Brushy Hill SVC,” CEA HVDC and SVC
control committee, Power System Planning and
Operating Section, March 1989.
[7] S.R.. Chano et al., “Static VAr Compensator
Protection,”
IEEE Transactions on Power
Delivery, Vol. 10, No. 3, July 1995.
[8] G. Thamm, H. Tyll, “A closer look at thyristors
in SVC applications”, Energy and Automation
Vol X No 1, 1989
All of these protection setpoints are calculated
and arranged so that any temporary overvoltage
stresses on the capacitors are coordinated within the
inherent overvoltage vs. time limit ratings determined
by the capacitor unit supplier.
VI. CONTROL AND PROTECTION
SUMMARY
In addition to voltage control and reactive power
control, the control system of the SVC plays an
important role in the protection of the various SVC
components. The control based protection schemes
employed in the SVCs augment the conventional
protective relays.
As part of the SVC
commissioning, the real-time simulator tests proved
to be particularly helpful in studying and
understanding the control and protection systems of
the SVC.
AUTHORS:
VII. CONCLUSION
Janet Kowalski is a Senior Engineer in the
Apparatus Engineering Department at Southern
California Edison Company, Rosemead, California,
USA (e-mail: Janet.Kowalski@sce.com)
Ivars Vancers is with Siemens Power
Transmission & Distribution, Wendell, North
Carolina (e-mail: ivars.vancers@siemens.com)
Mark Reynolds is with Siemens Power
Transmission & Distribution, Lake Oswego, Oregon
(e-mail:mark.reynolds@ptd.siemens.com)
Heinz Tyll is with Siemens Power Transmission
& Distribution AG, Erlangen, Germany (email:heinz.tyll@siemens.com)
Based on system studies performed by SCE and
selected consultants, it was recommended to install a
Static Var System in SCE Devers substation located
near Palm Springs, California. The primary purpose
of Static Var System was to address dynamic voltage
stability issues and increase import capability from
generation sources east of the Colorado River (EOR).
It will also provide steady-state voltage regulation of
the Devers Substation 500 kV bus during the
extremes of load and import conditions.
The MSC portion of the Devers SVS was
commissioned into service by July of 2005.
458