POWER CONDITIONERS AND UNINTERRUPTABLE POWER SUPPLIES (UPS) Part 2
(Continuation of part 1)
1.
BATTERY BANK – supplies power to the inverter during line outage; lead-acid batteries are commonly
used for UPS systems.
BATTERY BANK CHARGING SCHEME
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In the normal mode of operation when the line voltage is present, the battery is TRICKLE CHARGED to
offset the slight self-discharge by the battery. This requires that a constant trickle charge voltage be
applied across the battery, and the battery continuously draws a small amount of current, thus
maintaining itself in a fully charged state.
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In the event of a line voltage, the battery supplies the power to the load.
(The capacity of a battery is expressed in ampere-hours, which is the product of a constant
discharge current and the duration beyond which the battery voltage falls below a voltage level
called the FINAL DISCHARGE VOLTAGE. The battery voltage should not be allowed to fall below
the final discharge voltage level; otherwise the battery life is shortened. Typically, a 10-h current
is defined as the current in amperes that causes the fully charged battery to discharge in 10
hours to its final voltage level. Discharge currents in excess of the 10-h current cause the final
discharge voltage to be reached sooner than their magnitudes would suggest. Therefore, the
higher discharge currents reduce the effective battery capacity.)
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Once the line voltage is restored, the battery bank in a UPS is brought back to its fully charged state. This
is accomplished by initially charging the battery at a constant charging current rate as shown below. This
causes the battery terminal voltage to increase to its trickle charge voltage level. Once the trickle charge
voltage is reached, the voltage applied is kept constant, as shown below, and the charging current finally
decreases to the trickle charge current and stays at that level.
2.
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Figure 1
INVERTER – produces either a single-phase or a three-phase sinusoidal waveform (depending on the
application) to be supplied to the load.
The filtered output of the inverter is normally specified to contain very little harmonic distortion, even
though most loads are highly nonlinear and, hence inject larger harmonic currents into the UPS.
Therefore, the inverter must allow almost instantaneous control over its output ac waveform.
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The output voltage harmonic content is specified by means of a term called TOTAL HARMONIC
DISTORTION (THD)
Where: V1 is the fundamental-frequency rms value of the output voltage and Vh is the rms magnitude at
the harmonic order h. Typically, THD is specified to be less than 5%; each harmonic voltage as a ratio of V 1
is specified to be less than 3%.
INVERTER ARRANGEMENTS:
1.
Modern UPSs normally use the PWM (Pulse Width Modulation) dc to ac inverters with either a single
phase or three phase ac output. An isolation transformer is generally used at the output as shown below
Figure 2
2.
Large UPSs may employ a scheme where the outputs of two or more inverters are paralleled through
transformers with phase shift as shown below. This allows the inverters to operate at a relatively lower
switching frequency.
Figure 3
3.
It is also possible to use resonant converters, high frequency isolation transformers and the integral-halfcycle frequency converter as shown below.
Figure 4
INVERTER WITH FEEDBACK CONTROL:
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It is important to minimize the harmonics content of the inverter output. This decreases the filter size,
which not only results in cost savings but also results in an improved dynamic response of the UPS as the
load changes. A feedback control is shown below, where the actual output waveform is compared with
the sinusoidal reference. The error is used to modify the inverter switching. A control loop with a fast
response is needed for a good dynamic performance.
Figure 5
3.
UPS WITH SEVERAL LOADS
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Above a few kilowatts, most UPS provide power to several loads connected in parallel. As shown below,
each load is supplied through a fuse. In the event of a short circuit in one of the loads, it is important for
the UPS to blow that particular fuse and to keep on supplying the rest of the loads. Therefore the current
rating of the UPS under a sustained short circuit condition should be sufficient to blow the fuse of the
faulted load.
Figure 6
4.
STATIC TRANSFER SWITCH
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For additional reliability, the power line itself is used as a backup to the UPS, and a static transfer switch
transfers the load from the UPS to the power line as shown below. As an alternative, in the normal mode
the load is supplied by the power line. In the event of a line outage, the static transfer switch transfers the
load to the UPS. This arrangement is usually referred to as the STANDBY POWER SUPPLY. When a static
transfer switched is used, the inverter output should be synchronized to the line voltage. Therefore,
transferring the load from one source to the other results in the least amount of disturbance seen by the
load.
Figure 7
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