Page 43 - Energize July 2021

TECHNICAL

• Dynamic high efficiency mode. The rectifier is switched off in
this mode.
• High efficiency mode. Supply is switched through directly to the
load. The inverter module is switched on, but harmonic distortion
correction is not operative.
• Normal mode. In normal mode the supply is switched through
directly to the load and the activated inverter module corrects
current harmonic distortion from the load. High efficiency mode
of operation.
• Disturbance mode. If any disturbance is detected on the incoming Figure 8: Comparison of right-sized and oversized UPS (Schneider).
supply, the UPs switches to DC mode, and “clean” quality power
is supplied via the converter/inverter combination. In this mode Multiple modular control
losses occur through both the inverter and converter. The modularity principal can be further extended for UPS systems
• Supply failure mode. Upon failure of the supply the inverter with variable loads. Most modular systems are designed for equal
supplies power to the load using the battery as an input supply. load sharing, where each module will carry the same load. The
Load losses in the inverter are incurred. problem that arises here is that with varying loads, the modules
will not always be operating at the highest capacity, and with load
The multimode inverter mode achieves a higher efficiency than sharing, efficiency drops as the load decreases. This is compounded
the DC mode while retaining the power quality properties of the with N+n redundancy, where even at full load, the individual modules
DC mode. are operating at less than full load.
An innovative approach to this problem is to control the system so
Modularity that only the number of modules required to carry the load at full load
Efficiency improvements can be achieved by making use of modular are in service, and the remaining modules are in hot standby mode.
UPS configurations which boost scalability as well as redundancy. This would ensure that even at very low loads, the unit is operating
Efficiency losses often occur due to oversized and redundant close to maximum efficiency. This is illustrated in Figure 9 which
systems and multi-module approaches can reduce this. A modular shows how modular management can be applied to a system of three
approach allows flexibility in the growth of the UPS and also allows full systems versus the modular approach of three modular systems.
for better efficiency, as the UPS can be sized to match the maximum Multi-module control can be implemented on individual multi-
anticipated load on installation and extended in modules to match module UPS systems so that in each system only the required
increased growth. number of modules is used. This gives a high granularity in control
Matching modules with the load allows operation at a and allows the maximum efficiency principle to be applied down to a
higher utilisation point, and gives a further advantage when N+1 very low load level. n
redundancy is used, as a single module can be used for redundancy.
This is illustrated in Figure 8. Configuration “A” is a scalable on-line References
UPS that is right-sized with two modules that can carry 40% of 1. R Sawyer: “Making large UPS systems more efficient”, Schneider
white paper 108.
the maximum capacity. Configuration “B” is the same exact UPS
2. J Samstead: “Technical comparison of on-line vs. line-interactive
but sized to the maximum capacity with five modules. The graph
UPS designs”, Schneider white paper 79.
illustrates the two points on the curve where this comparison takes 3. Active power white paper: “High efficiency UPS systems for a power-
place (75% load and 30% load for configuration A & B respectively). hungry world”, Active power, 2011.
These two points correspond to efficiencies of 96,9% and 94,9%
respectively. Send your comments to rogerl@nowmedia.co.za

Figure 9: Multi-module control.

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