Page 157 - Uninterruptible Power Supplies

Static Uninterruptible Power Supplies
Static Uninterruptible Power Supplies 155

diagrams and comments on some these circuits are given in the follow-
ing section.

Designs Now Available
The full on-line system shown in Fig. 5.1, now also referred to as a dou-
ble 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 mod-
ule is used on ratings of 10 kVA and above, and is comparatively expen-
sive. 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 volt-
age 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 lin-
ear 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 regu-
lated 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 over-
voltage 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 per-
cent of power is required from the normal mains source (i.e., 100 percent

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