Post on 04-Apr-2018
T H E L I NE / A u g u s t 2 0 0 0 5
Fuseless Capacitor BankAdds Reliability for CERJ in Brazil
Sys tem Rel iab i li ty
by Albino Motta da Cruz, CERJ
Nelson S. Falcão F.°, Sales & Marketing Manager, Cooper Power Systems do Brazil
Marcelo Neves Martins, Application Engineer, Cooper Power Systems do Brazil
Hari Singh, Senior Powe r Systems Engineer, Systems Engineering
ompanhia de Eletricidade do Rio
de Janeiro (CERJ) is an electr ic
power distr ibution ut ility in Brazil
responsible for 23% of consumed
energy in the state of Rio de Janeiro.
Its a rea of ser vice reach es 73.3% of
the state, emb racing 66 municipal
districts, with p opulation in the order
of 4.3 million people.
The ser vice area is divided into
four regions of operation char acterized
by geographic diversity, with moun-
tains, lowlands and coa sts. One of
these regions s erves 12 municipal
districts in tourist regions along the
Atlantic coast with seasonal load
charac teristics. This means that dur ing
summer and holidays a peak load
condition occurs that can compromise
the reliability of the power system.
Recently privatized and under new
regulation from ANEEL - the federal
regulator y agency, CERJ impleme nted
a program to improve its level of
ser vice, efficiency and syst em re liabili-
ty. Since the problem during the
summer season in tourist areas was
critical, CERJ planned to h ave a s hun t
capacitor bank in its 69 kV transmis-
sion system at Porto do Carro
Substat ion, in order to have st eady
state voltage support du ring peak
load cond ition. Accor ding to CERJ
studies, the reactive power needed
to ac hieve th is was 30 Mvar at 69 kV
nominal voltage.
The challenge was to define the
project requirements for the shunt
capacitor bank at Port o do Carro
substation. Guidelines for this project
were to reduce the cos t of the project
while st ill maintaining ade qua te
reliability and availability.
Major project issues include:
s The number of stages in which
to d ivide t he 30 Mvar rea ctive
compensation.
s Choice of the technology to be
used: internally/externally fused
or fuseless bank.
s Study th e impact of capacitor
switching in the 69 kV syst em.
s Limited ar ea a vailability in Por to
do Carro subs tation to install
the capacitor bank.
The 30 Mvar shun t capacitor b ank
was divided in two s tages o f 15 Mvar to
fit within th e limited area available in
the substation and also to reduce cost.
The configuration used was unground-
ed (floating neutral) double wye with
unbalance detection scheme by neutral
current since this c onfiguration is not
sensitive to system unbalance or
triple harmonics.
Choice of Fuseless Conce pt
Several design factors influenced
the dec ision to adop t the fuseless
capacitor tec hnology in this p roject:
1) Due to the elimination of fuses,
the losses in a fuseless cap aci-
tor are t ypically 50% lower th an
a com parab le internally fused
capacitor and slightly lower
than a com parab le externally
fused capacitor.
2) Fuseless ca pacitor ban ks
have no problems associated
with nearby lightning strike or
trans ient currents, which may
cause t he ope ration of fuses in
an internally or externally fused
capacitor bank.
3) The fuseless capacitor b ank is
not prone to problems ass ociat-
ed with tran sient overvoltage
or s tored e nergy, since it doe s
not present a gap across an
open internal fuse which may
flash over under s evere
transients.
4) All dielectr ic failures in the unit
of a fuseless cap acitor bank
result in a stable, low loss
short-circuit condition. Fuse
operation in an internally fused
capacitor ban k produces a live
arc inside the unit which gener-
ates much more gas than the
actu al element failure. The
shor ting of one or two internal
series groups may not increase
the current sufficiently to oper -
ate th e fuse in an externally
fused cap acitor bank.
5) Besides the simplicity of design,
the fuseless cap acitor bank
requires about one half of the
space of a compar able external-
ly fused capacitor and may be
slightly smaller than a compar a-
- b le in te rna lly fu sed b an k u sing
similar rated units. This is
because the units of a fuseless
ban k will be s maller and lighter
than identically rated internally
fused cap acitor units.
Each of the above ad vantages
allow the fuseless cap acitor tech-
nology to cons istently meet the
established project guideline: “Reduce
costs while still maintaining adequate
reliability and availability.”
Technology
When analyzing a fuseless
capacitor bank, we think in increments
of internal ser ies groups. Instead of
constr ucting each ph ase of the bank in
one or more ser ies groups of parallel-
connected c apacitors, the fuseless
capacitor bank is constru cted of one
or mo re str ings of series-connected
capacitor units.
6 T H E L IN E / A u g u s t 2 0 0 0
When o ne of the internal ser ies
groups of a capacitor unit fails to a
shor t circuit (jumping the para llel-
connected elements), the voltage on
the remaining internal series groups
in the string increases. Similar to
externally-fused capacitor banks, the
fuseless ca pacitor b ank supp lied for
CERJ was designed to notify personnel
in the event of the first internal series
group failure, and to remove t he b ank
from s ervice when voltage applied to
the remaining internal series groups
exceed s 110% of rated voltage.
Each st age of 15 Mvar from t he 30
Mvar, 69 kV, NBI 350 kV capa citor ba nk
is comp ose d of the following:
Stage Bank Rating:
Rated kV: 69/39,8
Rated kvar: 15012
Installed Units: 36
Group ing: 6 Series 1 p ara llel
Connection: Unground ed Double
Wye
Capacitor Units Rating:
Rated kV: 6.640
Rated kvar: 417
Inter nal Group ing: 3 elements
series 6 p arallel
As s hown in Figure 1, each s tring
contains 6 capacitors in series; each
capacitor cont ains 3 internal series
group, each series group is composed
of 6 parallel elements forming a bank
with 18 series group.
Capacitor Switching Study
Since switching of shunt capacitor
banks can potentially create damaging
transient voltages
and cur rents, it is
very important to
investigate the
effects of capaci-
tor switching by
proper system
modeling and
simulations.
The objective is
to ver ify the
adequacy of
existing equip-
ment as well as
new switching
and protection
equipment for the
capacitor bank,
and, if necessary,
recommend oper-
ation restr ictions
to mitigate
transients.
The engineering study for
investigating the effects of 2 x15
Mvar Porto do Carro cap acitor
ban k switching in the CERJ system
consisted of simulating and analyzing
the following phenomena:
1. Capacitor Bank Energization
a) Energizing the first 15 Mvar
capacitor step.
b) Back-to-back
energizing of
sec ond 15 Mvar
capacitor step.
c) Specify/check the
capacitor step
current-limiting
(damping) reactor
ratings
2. Capacitor Bank
Outrush Current
Effects:
Specify/check the
main cur rent -limit-
ing (damping)
reactor ratings for
69 kV close-in
faults.
3. Voltage
Magnification:
Check for resonant transient
overvoltages at b uses h aving
13.8 kV shunt capacitor(s) when
the 69 kV cap acitor b ank is
energized.
4. Transformer Phase-to-Phase
Transient Overvoltages:
Chec k for s evere t raveling wave
overvoltages at far end of
transformer -terminated lines
resulting in transformer ph ase-
to-phas e transient
overvoltages greater th an
trans former insulation
withstand capability.
5. Capacitor Bank De-energization
a) Restr ike in switching device
(breaker) for one capacitor
step.
b) Restr ike in switching device
(main breaker) for entire
capacitor bank.
c) Check for s urge arrester
ade qua cy by verifying its
energy duty.
System Mode ling and Simulations
In s witching transient studies,
deter mining the extent of the syste m to
be mod eled in detail is very impor tant
to ensu re accura te and realistic simula-
tion results. The one-line diagram of
CERJ’s 69 kV system modeled in this
study is shown in Figure 2. This model
is used for simulating the local and
remote effects of isolated (single)
capacitor bank energizing and de-
energizing. These phenomena include
the inrush current and transient
overvoltages at Porto do Carro station,
as well as the transformer p hase-to-
phase transient overvoltages and
voltage magnification at n eighboring
stations. The worst de-energization
case is due to rest rike in the main
breaker when it interrupts the total
capacitor bank current.
The inrush currents due to
back-to-back energizing, and outrush
currents due t o close-in faults are high
ProtectionRelay
CurrentTransformer
3 Phase 69kV
Capacitor unit
6,640V, 417 kvarwith 3 internalseries group and 6elements in parallel
Figure 1 CERJ’s fuseless capac itor bank schematic diagram
R. Ostras
Macae
Dimac
Buzios
P. do Carro
Cabo Frio
A. do Cabo
2 x 15 Mvar Cap Bank
IguabaS.P.
Aldeia
E. Araruama
Rocha Leao
VilaVerde
Tamoio
Figure 2 One-line diagram of CERJ 69 kV system model
continued on page 15
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frequency pheno mena that are local
to the Por to do Carro subs tation. For
such simulations it is sufficient to
represent only the 69 kV ungrounded
capacitor bank by its high-frequency
mod el, as sho wn in Figure 3.
Impact of Simulation Results
The energization simulations
showed th at the phas e-to-ground
transient overvoltage peaks at Buzios
and Arraial do Cabo stations a re high-
er than a t any other location in the
system. The corresp onding phase-to-
phas e transient overvoltage peaks at
the 69/13.8 kV transformer primary are
as h igh as 3.5 pu in the worst-case
energization (Figure 4). This is du e to
the traveling wave effect on the radial
lines from Porto d o Carro to thes e
adjoining stations (This effect is not
seen at Cabo Frio due to the s hor t
length o f both tr ansformer-terminated
lines .) The level of syst em load ing
(hea vy or light) had negligible effect
on these overvoltage transients, but it
was found that the 13.8 kV capacitors
at Porto do Carro, Buzios and A. do
Cabo stations were influential in reduc-
ing the overvoltage magnitude and
also providing increased damping.
This sensitivity analysis from several
simulation cases was crucial in recom-
mending the preferred en ergization
pra ctice to CERJ in order to minimize
the risk of transformer insulation fail-
ure at Buzios and A. do Cabo stations.
The energization simulations also
indicated t hat th ere was no voltage
magnification pr oblem at 13.8 kV
capacitor buses around Porto do
Carro.
The back-to-
back energization
simulation and
the close-in fault
outrush simula-
tion (using the
high-frequency
model of Figure
3) demonstrated
some results
typical to an
ungrounded
capacitor bank.
It was noted th at
the breaker pole
closing times
(seque ntial or
simultaneous)
influence the peak
magnitude bu t not the frequency of
the inrush c urrent transient. Also, it
was noted that the characteristic
outrush current transient has high
peak magnitude and fre-
quency only for th e
three-phase-to-ground
close-in fault; the line-to-
ground fault doe s not
produce severe outrush
currents for ungrounded
capacitor banks.
Study Conclusions
Due to the high
phas e-to-phas e tra nsient
overvoltages produced
at buses with trans-
former-terminated
radial lines, use of
synchronous c losing
breakers is the most
effective way to control
Porto d o Carro capacitor switching
tra nsient s in th e CERJ 69 kV syst em.
Meanwhile, simulations show that
trans ient overvoltages at Buzios an d
A. do Cabo are minimized if the cap ac-
itor bank is energized when the 13.8
kV capacitors a t Porto do Carro are
on-line, but th ose at Buzios and A. do
Cabo are not in ser vice. This is th e
recommend ed energization practice.
Simulation results also sh ow that the
main and step reactor s in the capaci-
tor b ank are adequ ately rated to limit
the back-to-back energization inrush
currents and close-in fault outrus h
currents within the subst ation breaker
cap ability limits. The de-energization
simulations show that in the unlikely
case of any capacitor bank brea ker
Fuseless Capac itor Banks from pag e 6
Lo = 500µH
Li = 90µH
2 x 15 Mvar
69 kV System Equivalent
69 kV
Figure 3 Ungrounded cap acitor bank at Porto do Carro
(high frequency model)
restr ike, the e nergy withsta nd
cap ability of MOV surge ar res ter s
will not be exceeded.
Enginee ring Solution
Cooper Power Systems designed
and installed a shun t capacitor bank
which was within the limited footprint
area available in Porto do Carro s ub-
sta tion, while mainta ining all requ ired
safety clearances. This was a challenge
becaus e this capa citor bank installa-
tion included several other pieces of
equipment such as circuit breakers,
current-limiting reactors, grounding
and d isconnecting switches , and
current trans former s. The installation
also included t he cont rol cabinet
which provided all protection, control
and interlocks within the capacitor
bank. Cooper was also resp onsible
for th e overall system des ign and
specifications a s es tablished by
CERJ spe cifications an d s ystem
study recommendations.
The project was succes sfully
managed locally by Cooper Power
Systems do Brazil, which was responsi-
ble for coordination between different
operations – Capacitors, in Greenwood,
South Carolina, USA, for fuse less
capacitor units and bank conception;
System Engineering, in Franksville,
Wisconsin, USA, for system reliability
stu dy; local Brazilian sup pliers of
equipment, and its own engineering
department which provided the design,
erection and star t-up ser vices. T H E L I N E
Figure 4 Phase-to-phase transient overvoltages at Buzios transformer