bonitron M3534B, Customer Reference Manual

Page: 39 / 48

User’s Manual
39
5.
Estimate the losses for each capacitor.
6.
Determine the minimum number of capacitors per series string required for the
minimum voltage.
7.
Add capacitors in series for outage energy.
8.
Determine if parallel strings are required to provide outage energy.
These are the definitions of the variables we will use in the following equations:
𝐶
𝑒𝑜𝑙
- Capacitance at end of life
𝐶
𝑡𝑜𝑡
- Total capacitance of the entire cap bank
𝐸𝑠𝑟
𝑡𝑜𝑡
- Total equivalent ESR for the entire cap bank
𝐸𝑠𝑟
- Internal resistance at end of life
𝐼
𝑝𝑒𝑎𝑘
- Peak current into the M3534 during the discharge cycle
𝐽
𝐴𝑣𝑎𝑖𝑙𝑎𝑏𝑙𝑒
- Total energy in charged capacitor bank prior to discharge
𝐽
𝐿𝑜𝑠𝑠𝑇𝑜𝑡
- Total losses during discharge
𝐽
𝑜𝑢𝑡
- Energy required to hold up the system during the outage
𝑛
𝑠𝑒𝑟𝑖𝑒𝑠
- Number of capacitors in each series string
𝑃
𝑠𝑦𝑠
- System power in kilowatts
𝑇
𝑜𝑢𝑡
- Time outage in seconds
𝑉
𝐶ℎ𝑎𝑟𝑔𝑒
- Charge voltage at beginning of discharge
𝑉
𝐶𝑎𝑝𝐸𝑛𝑑
- The capacitor string voltage at the end of the discharge
𝑉
𝐶𝑎𝑝𝑀𝑎𝑥
- Maximum charge voltage for the capacitor
𝑉
𝑒𝑛𝑑
- Lowest voltage the capacitor bank can reach
𝑉
𝑇𝑒𝑟𝑚𝐸𝑛𝑑
- Terminal voltage of an individual capacitor during discharge
7.4.1. E
NERGY
R
EQUIRED FOR
O
UTAGE
The total energy required for the event must be calculated first.
Using the following equation, the total number of joules required for the outage
can be calculated:
J
Out
= P
sys
∗ T
out
We will use a specification of 100 hp. for a 2 second outage for this example.
For instance, a 100 hp drive outage for 2 seconds would be
P
sys
= 100hp ∗ .746 kwatts horsepower
⁄
= 75kW
The total number of joules required is:
J
Out
= 75kW ∗ 2s = 150kJ
7.4.2. M
INIMUM
C
APACITOR
B
ANK
V
OLTAGE
There is a minimum voltage level that must be maintained at the end of the
discharge during backup. An M3534R for a 460VAC nominal system has a
minimum input voltage of 320VDC. Therefore, the final discharge voltage of
the capacitor bank (
𝑉
𝑒𝑛𝑑
) should be 320 VDC.
These data are available in the specifications for the specific M3534R by
nominal system voltage.