The full on-line system shown in Fig. 5.1, now also referred to as a double
conversion module, is better illustrated in Fig. 5.13. Under normal
operation, that is, mains supply is present, the oscillator firing the
bridge circuit IGBTs will accept a signal from the mains waveform,
ensuring that the system is in synchronism with that supply. Thus, the
static switch can provide an alternative supply which is in synchronism
with the supply to the load. If, for any reason, the mains frequency is
unstable, then the oscillator control will break away and not accept
mains frequency. Clearly, mains frequency acceptance level may be set
on site and usually this is set at ±3 to 4 percent.
The system has the advantage of affording complete isolation from
mains supply and the module will operate over quite large variations in
input supply. The module is normally fitted with a maintenance by-pass,
enabling maintenance without disturbing the load. Nowadays, the module
is used on ratings of 10 kVA and above, and is comparatively expensive.
Further developments have allowed dispensation of the input
transformer reducing weight and losses. In addition, the rectifier may
now employ IBGT switching techniques, enabling a very large reduction
in the value of harmonics. Such systems now easily meet the latest
guidelines on harmonics. It should be noted that the employment of
IGBT switching systems in the rectifier does increase cost.
Another circuit described as delta conversion (Fig. 5.14a) consists of
two inverter chargers in series, battery, and a transformer in line.
Under normal operation and assuming that mains is at nominal voltage
the load is fed directly via the primary winding of the transformer.
Inverter 1 is only supporting the mains current, which clearly in this
case is equal to the load current (on the assumption that we have a linear
load). Thus, voltage from inverter 1 across the transformer is zero
and power transmission is also zero. Also under this condition, inverter
2 is idling since its inverter output voltage is equal to mains voltage.
Inverter 2 will supply any reactive or harmonic current from the load.
In Fig. 5.14b, the mains voltage is at -15 percent and the fully regulated
power to the load (±1 percent V) is obtained through inverter 2
and thus from inverter 1.
Figure 5.14c illustrates the
system reaction to a ±15 percent overvoltage
from the mains supply. In this case inverters 1 and 2 absorb the
abnormal mains voltage condition.
In Fig. 5.14d battery recharging is occurring. It is assumed that 110 percent
of power is required from the normal mains source (i.e., 100 percent
to the load and 10 percent for battery recharging). Under this condition the
battery recharging is a feedback of power from inverter 2 to the battery.
It should be explained that inverter 2 is synchronized to the mains
supply and acts as the voltage regulator to the load. Inverter 1 compensates
for variations in power factor and, although the power for battery
charging is fed from Inverter 2, the control of this function is
governed by inverter 1. Additionally, inverter 1 compensates for any
waveform variation between mains and the load.
Claimed advantages for this system are low harmonics induced onto
the mains supply and high efficiency. The high efficiency at full load is
certainly superior to a normal double conversion system but, with most
UPS systems where a load factor of 70 to 80 percent is expected, there
is little difference in overall system efficiency.
An example of a passive standby UPS module, wherein the load is
normally fed via a switch/filter/conditioner, is shown in Fig. 5.15.
Simultaneously, the battery is in charging state. On loss of mains, the
load is fed from battery/inverter/filter.
Such units are normally to be seen where ratings are low, in other
words, 2 kVA and below. Advantages of the module are low cost and
compact, lightweight. Care should be taken to establish the qualities of
the subassembly containing the switch/filter/conditioner. The switch
may be a mechanical device which may not be sensitive enough for
computer loads. Clearly the quality of filtering and conditioning of raw
mains should also be reviewed. In addition, the system depends on
mains frequency and there is no true isolation.
Line interactive UPS is shown in Fig. 5.16. Under normal operation
the load is fed directly from mains with some voltage conditioning
being provided by the inverter, but there is no isolation from mains.
Frequency is dependent on mains supply and voltage conditioning is
clearly limited. The module is competitively priced but restricted to
lower rating systems.
There are many circuits similar in respect to the above basic modules
described, and it is difficult to establish all the properties of UPS modules
particularly at low ratings.
There is a trend dictated by the market to reduce size and weight,
retaining efficiency, and above all reducing unit cost. This is particularly
critical at lower ratings as one colleague said we are entering the
“drip dry zone” (no-iron!). Transformers, so long part of the circuit, are
considered heavy and expensive compared to alternative solutions
involving simple solid-state components. Thus, circuits are appearing
at ratings up to 10 kVA as shown in Fig. 5.17. This circuit allows wide
input-voltage tolerance. The resonant converter feeding the charger
and the shown transformer are smaller in size than previous circuits.
Figure 5.18 illustrates a circuit where there is no large current-
carrying transformer and it also uses a low-voltage battery, thus reducing
considerably the cost of the battery and weight and dimensions.
Note that these circuits are applied to low-rating UPS units only.
Reliability of systems has clearly improved with development and
experience. Figures are hard to ascertain. There is no doubt that a single
module, on-line or double-conversion type, should achieve a mean
time between failure of 260,000 h. This assumes a reliable mains supply
as will be met with in Europe and the United States.
Operating modules in parallel redundancy can clearly improve these
figures (see Fig. 5.19). Each module consists of rectifier charger, battery,
inverter, and static switch. The rating of the system is n - 1, where n is
the number of modules in the system. Thus, failure of one module still
allows full load to be maintained. In addition, static switches are used for
each module. Afailure of two modules results in mains supplying the load.
Synchronism of module outputs is achieved in various ways, either
from a central master oscillator with an auxiliary or from each module
having its own natural frequency and the modules being interconnected
so the module with the highest natural frequency acts as master.
Clearly in the event of the master failing, the next available set
with comparative high frequency assumes control.
Paralleling modules needs care, and much development work has
occurred to ensure that faulty modules do not affect the continuous safe
operation of the system. Clearly, under certain conditions, a heavy fault
current could conceivably occur, and in practice the high-speed static
switches shown in Fig. 5.17 will isolate the faulty module. Sensing the
fault is of paramount concern and one chosen way is to use a voting system
to ascertain the faulty module. In this system all modules send a
constant stream of signals to a common control PCB, which in turn
ensures correct operation and isolation of the faulty module. In many
designs these interconnecting signal control cables are duplicated to
ensure guaranteed operation.
The use of a common battery is not considered good practice. There is
the possibility of circulating currents due to slight variations in charger
performance, but more significant is the effect on system reliability.
Development of more elaborate systems is illustrated in Fig. 5.20. In
panel a the three modules can be operated with either mains supply 1
or 2 and loads can be mechanically switched between the two alternative
bus bars. In panel b there are two separate parallel systems, each
with its own alternate mains supply, and the loads can choose to operate
with either system. The load transfer module is shown in Fig. 5.21.
This design ensures that there will be no break in supply to the load if
either of the two parallel redundant systems fails.
The full on-line system shown in Fig. 5.1, now also referred to as a double
conversion module, is better illustrated in Fig. 5.13. Under normal
operation, that is, mains supply is present, the oscillator firing the
bridge circuit IGBTs will accept a signal from the mains waveform,
ensuring that the system is in synchronism with that supply. Thus, the
static switch can provide an alternative supply which is in synchronism
with the supply to the load. If, for any reason, the mains frequency is
unstable, then the oscillator control will break away and not accept
mains frequency. Clearly, mains frequency acceptance level may be set
on site and usually this is set at ±3 to 4 percent.
The system has the advantage of affording complete isolation from
mains supply and the module will operate over quite large variations in
input supply. The module is normally fitted with a maintenance by-pass,
enabling maintenance without disturbing the load. Nowadays, the module
is used on ratings of 10 kVA and above, and is comparatively...
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