Mumbai University > Electronics Engineering > Sem7 > Power Electronics 2

Marks: 7M

Year: May15, Dec14, Dec11, Dec10

Definition of UPS: A Uninterruptible Power Supply [UPS] is a device that maintains a continuous supply of electric power to the equipment by supplying power from a separate source when the main power supply is not available. The UPS is normally inserted between the commercial utility mains and the critical loads. When a power failure occurs, the UPS will immediately switch from utility power to its own power source.

Various techniques exist to enable the correct selection of batteries for UPS applications.

a) The selection of the physical [[batteries | battery]] (cells) is dependent on several factors:

• type of battery (sealed, vented, lead acid, NiCad, etc.)
• expected life of the battery
• usage of the batter (number of charge/discharge cycles)
• dimensions and weight of the battery
• construction materials
• ambient environment and conditions
• maintenance requirements and seismic characteristics

b) Battery Sizing:

Manufacturers provide sizing information for their batteries. Generally this information assumes a room temperature of 25°C. Batteries which will be operated at different temperatures continuously should be calculated specifically for that temperature.

Batteries are generally sized using [Watts] per cell or on [Ampere] per cell.

i. Watts per cell method: Normally information supplied for lead acid batteries designed for short discharge times (5-120 minutes) is in the form of kilowatts per cell tabulated for various back-up times. The required [Watts] per cell are given by:

$watt/cell=\frac{VA \times pf}{ η \times N}+A_1$

Where: VA = VA of the load

pf = power factor

η = efficiency of the UPS

N = Number of cells

Al = any addition load connected to the batteries

ii. Amperes per cell :

Long term discharge lead acid batteries and most nickel cadmium The batteries are sized using charts expressed in available amps for specified periods of time. Required [Amperes] per cell is:

$Amperes=\frac{VA \times pf}{ V_{dc}}+A_1$

Where:

VA = VA of the load

pf = power factor

η = efficiency of the UPS inverter (dc to ac)

$V_{dc}$ = Average Discharge Voltage

Al = any addition load connected to the batteries (in A)

c) Battery Charger Sizing :

In general, a short term discharge battery can be recharged to 85% capacity in 8-10 times the discharge time. A long term discharge battery can be recharged to 85% capacity in a minimum of 8 hours provided the charger is sized properly.

Assuming the UPS is float charging, the following charging current Ic is required:

$I_b=\frac{VA \times pf}{η/V_{dc}}$

And

$I_i=\frac{VA \times pf}{η/(float voltage)} \\ I_c=I_bT_dkT_r+l_i+I_a $

Ic = Charging current

IB = Battery current required

Td = Battery discharge (run) time

k = Safety factor (typically 1.5)

Tr = Battery recharge time

Ii = Inverter current required

Ia = Any additional load in (A)