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vacon • 8
SPECIAL CHARACTERISTICS AFFECTING THE
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3.
SPECIAL CHARACTERISTICS AFFECTING THE
SELECTION
Different chemistry causes different behavior in cell voltage as a function of charge/discharge and
SOC (State of Charge). This creates "voltage window" requirement similar to the solar inverter.
Galvanic isolation requirement is different from many industrial drive application. This is due to the
fact that the battery system should not be predisposed for common mode voltage.
For the Battery Management System (BMS) to be able to reset the SOC calculation, it is necessary
to charge the battery to 100% SOC. This ensures that BMS is able to calculate SOC accurately and
maintain the battery in safe operating area. For this, a balance charger or a maintenance charger is
needed in some cases.
3.1
Voltage window
For both the DC/DC converter and the GTC (Grid Tie Converter) the first dimensioning question
comes from energy storage (battery) voltage dimensioning. It is important to define the “voltage
window" for empty and full battery cell voltage. Depending on battery chemistry the ratio can be full/
empty = 1,2… 2… (meaning, for example, full being 1000 Vdc, and empty being from 800 Vdc to 500
Vdc) and for super capacitors even bigger. Especially for GTC this is a limiting factor. The limitations
come from minimum tolerable DC-link voltage to maintain controllable grid voltage and from
maximum allowed voltage to maintain within design criterion of the hardware.
The behavior of voltage stretch in a battery can be illustrated with a spring being pulled or pushed.
Figure 5. Spring analogy of the battery voltage change
120
100
80
0
20
40
60
80
100
UDC [%]
SOC [%]
Charging of battery
Discharging of battery
1C
2C
3C
6C
9C
Summary of Contents for NX6
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