bonitron M3534B, Customer Reference Manual

Page: 40 / 48

M3534R 20A
40
7.4.3. P
EAK
C
URRENT
The peak current from the capacitor bank will occur at the minimum voltage.
This can be estimated from the equation
I
peak
=
P
sys
V
end
For our example,
I
peak
=
75kW
320Vdc
= 235A
7.4.4. C
APACITOR
S
PECIFICATIONS
At this point, a specific capacitor's characteristics can be used. It is best to
use the values that are listed at end of life for the capacitor to make sure that
the storage system is sized for the eventual degradation of performance over
time. The critical points to use are
𝐸𝑠𝑟 = Internal resistance at end of life, typically 150-200% of the initial value.
𝐶
𝑒𝑜𝑙
= Capacitance at end of life
𝑉
𝐶𝑎𝑝𝑀𝑎𝑥
=
Maximum charge voltage for the capacitor. (A general rule is to use
95% of the value listed on the datasheet for a reasonable margin)
For our example, we will use the following values:
𝐸𝑠𝑟
= .01Ω
𝐶
𝑒𝑜𝑙
= 132F
𝑉
𝐶𝑎𝑝𝑀𝑎𝑥
= 46V
7.4.5. M
INIMUM
S
ERIES
S
TRING
From
𝑉
𝑐𝑎𝑝𝑀𝑎𝑥
, we can calculate the minimum series string of capacitors that
will be required by the voltage rating. Below this voltage, there is an amount
of stored energy that cannot be used, and will remain in the capacitor bank.
Since there can be significant terminal voltage drop at the end of the discharge
cycle due to
𝐸𝑠𝑟
, it is best to use the terminal voltage of the capacitors
(
𝑉
𝑇𝑒𝑟𝑚𝐸𝑛𝑑
)
for this calculation.
V
TermEnd
= V
CapMax
− (I
peak
∗ Esr)
V
TermEnd
= 46V − (235A ∗ 0.01Ω) = 43.7V
For our example, the minimum number of caps in a series string would be:
n
series
=
V
end
V
TermEnd
=
320V
43.7V
= 8