Download BRIEFS KILOVAR FROM THE POWER SYSTEMS CAPACITOR PLANT

KILOVAR
BRIEFS
Date
FROM THE McGRAW-EDISON POWER SYSTEMS CAPACITOR PLANT
June 1989
GREENWOOD, SOUTH CAROLINA
issue 15
Capacitor Technology Advancement Enhance
D e s i g n o f L a r g e Capacitor Banks
Introduction
A few of the difficulties facing utilities today
include: All time high peak loads, little new
generation coming on-line or being planned, and
difficulty in obtaining new rights of way. To address
these problems, many utilities are turning towards the
installation of shunt capacitor banks, SVC's, and HVDC
lines. These trends and the increasingly common
practice of replacing old
capacitor banks with
fewer, consolidated installations, has resulted in an
increase in the size of many new substation banks.
Yesterday's large banks, c o m m o n l y 20 or 30 M V A R are
dwarfed by new 100 or 200 MVAR installations. Also,
with the increasing number of static var compensators
(SVC's) being installed, these large banks, once
relegated to transmission level voltages only, are now
appearing in the form of large (200+ MVA R ) banks at
voltages as low as 35 kV.
Past capacitor technology was limited by two very
important concerns; the tank rupture and energy
handling characteristics of the capacitor. Although
still valid concerns, modern power capacitors exhibit
characteristics notably superior to that of older
designs. These enhanced characteristics require the
re-evaluation of many existing application rules.
Figure 1 compares the tank rupture curves for 200 KVAR
paper-film, conventional all-film, and high energy
all-film capacitor designs.
Figure1
ComparativeTank Rupture Curves
for 200 k v a rCapacitors
Paper-Film Technology: Although most manufacturers
ceased the manufacture of paper-film capacitors in the
late 1970's, many rules of thumb are still in use today
which were based upon the limitations of paper-film
capacitor technology.
,
Post Office Box 1224. Greenwood. SC 29648
The tank rupture curve of a paper-film capacitor
exhibits two deficiencies in comparison to all-f ilm
capacitor designs:
-
The tank rupture TCC is much further to the
left, making coordination margins small.
The tank rupture TCCs were probability
curves. This was a result of the uncertainty
involved
in
the
high
resistance
characteristics of the failure location.
This resistance results in high arc voltages
being
present
which
accelerates
the
development of gas producing arcs within the
capacitor.
The latter point explicitly reveals the main drawback
in protecting paper-film capacitors; Any overcurrent
protection which coordinated with the paper-film's tank
rupture TCC still left a probability of case rupture.
This uncertainty led to many of the design rules still
in place today.
Improvements With All F i l m C apacitors
All-Film Technoloqy: The development of the all-film
capacitor allowed s o m e manufacturers to achieve
capacitor designs which adhered to a definite, not a
probability, curve. This was essentially due to the
low resistance characteristic of the failure location.
However, the tank rupture characteristics of all kvar
size and voltage rating capacitor units were not the
same, requiring a family of curves.
H i g h E n e r g y All-Film Technology: The advent of the high
energy all-film capacitor has provided for previously
unavailable tank rupture performance. The mechanically
crimped extended foil internal construction a l l o w s the
capacitors to follow one tank rupture curve
encompassing all capacitor unit ratings, with up to 10
KA fault current capability. Table I summarizes the
salient characteristics of these three designs.
Table I
Comparison of Tank Rupture Curve
Figure 2
Parallel Energy Discharge into Failed Capacitor
The amount of parallel energy discharge that a failed
capacitor can handle is a critical limiting factor in
the design of large capacitor banks. The amount of
parallel connected Kvars in each series group of a
substation bank is therefore limited by the energy
handling capabilities of the failed capacitor. For
ease of use, Table 2 converts this energy limitation
into an equivalent number of parallel kvar.
Table 2
Energy Handling Limitations of Various Capacitors
Capacitor
Technology
Maximum
Parallel
Energy
Equivalent
Parallel
Connected Kvar
Per Series Group
3100 Kvar
Paper film
10 kJ
Conventional
All Film
15 kJ
Mechanically
Crimped Extended
Foil (EX-7)
4650 Kvar
30 kj
9300 Kvar
Electrical Parameters
Capacitor
Technology
Max*
# of Curves
Fault
Curve
Current
Characteristics
Needed
Probability
1 to 4
4,000A
All Film
Definite*
4
5,500A
High Energy
Mechanically
Crimped
Extended Foil
(Type EX-7 )
Definite
1
10,000A
Paper Film
*By some manufacturers
* * For a 200 kvar, 7200 V unit at 1.2 cycles
Parallel Stored Energy
In substation applications, multiple capacitor units
are connected in parallel to form each series group in
a bank. When a capacitor unit fails, it fails to a
short circuit. Therefore, before the fuse operates
when a failure occurs, the energy stored in the
parallel connected good capacitors will discharge
through the failed unit. This situation is depicted in
Figure 2.
The ability of the mechanically crimped extended foil
capacitors to be applied safely with up to 9,300 kvar
of parallel connected capacitors per series g m u p is a
significant breakthrough in relation to past practice.
One rule frequently cited by utility personnel is that
contained within the NEMA Standard CP-1 issued in 1976.
This standard limits parallel connected kvar to 3,100.
When use is made of the 9300 parallel kvar l i m i t
available today, significant economic benefits may
result. These benefits are best illustrated through use
of an example.
S a m p le Bank Design Using High Energy All-Film Capacitors
For our sample installation, a 43,200 kvar, 69 kV L-L,
grounded wye bank was chosen. Utilizing the high energy
all-film capacitor design l i m i t of 9,300 k v a r , the
following design is achieved.
= 43,200 / 3 = 14,400 Kvar
One phase parer
Number of series groups = 2 (using 19.92 kV
capacitor units)
Power per Series Group = 14,400 / 2 = 7,200 Kvar
= 400 kvar
Power per Capacitor
Number of Parallel Units = 7,200 / 400 = 18
Total number of Capacitors
per bank = 3 x 2 x 18 = 108
(1)
(2)
(3)
(4)
(5)
Figure
3
illustrates
the
normal
mechanical
configuration of this bank. Note that this bank fits
entirely into one, three-phase assembly.
Power per Split wye
= 43,200 / 2 = 21,600 kvar (7)
= 21,600 / 3 = 7,200 kvar (8)
One phase power
Number of series groups = 2 (using 19.92 kV
capacitor units)
(9)
Power per Series Group
= 7,200 / 2 = 3,600 MVAr
(10)
= 400 kvar
(11)
Power per Capacitor
Number of Parallel Units = 3,600 / 400 = 9
(12)
Total number of Capacitors
per bank = 2 x 3 x 2,x 9 = 108
Figure
4
illustrates
the
normal
mechanical
configuration of this bank. Note that this bank
consists of
two
structures,
each
similar
in
configuration to that required by the single wye bank.
One Three Phase Stack
Figure 3
Layout of 43.2 Mvar Bank Using
High Energy All-Film Capacitors
The Current-Limitinq F u s e Alternate
Redesigning the bank to utilize conventional all-film
capacitors presents a problem. From equation (3), it is
apparent that the 7,200 kvar in parallel per series
group is in excess of conventional capacitor's
capability. The most direct solution is to replace the
expulsion fuses with current-limiting fuses. This
allows the bank configuration to remain the same.
However, current-limiting fuses have two inherent
drawbacks:
-
They are significantly more expensive.
Typical costs for original or replacement
current-limiting fuses are roughly 5 times
the cost of the initial expulsion fuse
assembly.
Current-limiting fuses exhibit much higher
power losses. The 2OT expulsion fuse link
typically
used
in
this
application
contributes a negligible amount of watts per
kvar of losses. The 25 a m p current-limiting
fuse contributes losses roughly equivalent to
those generated by the capacitors themselves.
This effect almost doubles the power losses
of the installation.
The initial expense of the current-limiting fuses adds
approximately 10% to the price of the bank. The cost o f
the additional losses due to the fuses must also be
evaluated. In this case, they would add an additional
5% to 7% to the cost.
The Split W y e Alternate
To avoid the added expense and high losses of current
limiting fuses, one common approach in conventional
all film applications is to arrange the bank into a
split wye, rather than a single wye, configuration. The
following calculations would be used to arrive at this
configuration
.
One o f Two
Three Phase Stacks
Figure 4
Layout of 43.2 M V A r Bank Using
Conventional All-Film Capacitors
In a Split Wye Configuration
Depending on bank layout, the total substation area
requirement increases by up to 15%. The cost of the
additional substation land area required must be
factored into the final price. Also, the extra
structure
and
insulators
required
for
this
configuration would add approximately 10% to the price
of the single wye configuration.
Alternatives Using Additional Series Groups
As the 4,650 kvar limitation is based on a per series
group basis, another solution to the problem would be
to leave the bank as a single wye, but increase the
number of series groups per phase. The new design would
be arrived at as follows:
One phase power
= 43,200
3 = 14,400 kvar (14)
Max Power per
= 4,650 kvar
(15)
Series Group
Number of series groups = 14,400 + 4,650 = 3.1
(16)
As three series groups would result in 4,800 kvar in
parallel, the next larger number of series groups must
be used.
Number of
Power per
Power per
Number of
Series groups =
Series Group =
Capacitor
=
Parallel Units=
4 (using 9.96 kV units)
14,400 + 4 = 3,600 Kvar (17)
400 kvar
(18)
3,600
400 = 9
(19)
Total number of Capacitors
per bank = 3 x 4 x 9 = 108
As each block, or frame sub-assembly, of a bank can
electrically contain up to two series groups of
capacitors, two blocks must be used per phase.
Installing all six blocks in one bank assembly would
result in a bank which would stand 35 feet high. This
height is undesirable due to maintenance and mechanical
stability concerns. Common practice is to limit
capacitor banks to three or four blocks per structure.
For banks using two blocks per phase, the most typical
configuration is shown in Figure 5. Note that the bank
now consists of three structures, one per phase.
Table 3
Summaryof Alternative Bank Designs
Required for Conventional All-Film Capacitors
Conventional All-Film
Type EX-7
Bank
Single Wye Single Wye Double Wye Single Wye
Charac- 2 Series 2 Series
2 Series 4 Series
Groups
teristics Groups
Groups
Groups
Unit Kvar
400
400
400
400
Unit
Voltage
19,920
19,920
19,920
9,960
Parallel
Kvar
7,200
7,200
3,600
3,600
Total #
of Units
108
108
108
108
Number of
Structure
1
1
2
3
Current
Limiting
Expulsion
Expulsion
F u s eType Expulsion
Figure 5
Layout of 43.2 Mvar Bank Using
Conventional All-Film Capacitors
In a 4 Series Group Configuration
The added structural material required to assemble this
bank will again add approximately 10% to the price of
the bank. Once more, an enlarged substation area will
be required for its installation. Table 3 summarizes
the salient characteristics of the three bank design.
Addition
Required
Substat.
Area (1)
0%
0%
0-15%
15-75%
Addition
cost of
Losses
Due to
C/L Fuses
0%
5-7%
0%
0%
%Increase
Initial
Price of
0%
5-10%
10%
10-15%
Bank
The recent development of advanced dielectric systems
and internal construction methodologies utilized in
high energy all-film capacitor designs, such as
McGraw-Edison's Type EX-7, has allowed for many
improvements in the application of power capacitors.
These include:
1.
The replacement of probability tank rupture
curves with definite TCCs.
2.
The use of one tank rupture curve to
represent all unit kvar sizes and voltage
ratings, eliminating the need for a family of
curves.
.
3.
Enhanced safety characteristics due to a
significant increase in the coordination
margins between the tank rupture c u r v e and
fuse.
4.
Ability to connect up to 9300 kvar in
parallel per series g r o u p while being able to
retain the use of inexpensive expulsion
fuses.
5.
A savings of 5 to 10% in the initial cost of
substation banks utilizing designs which take
advantage of
the 9300
parallel
kvar
capability.