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 

 

T H E L IN E / A u g u s t 2 0 0 0 1 5

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