Page 52 - Energize July 2021

TECHNICAL

The latest development incorporates all the equipment,
including rectifier and battery, in the diesel generator sets canopy.
This provides a transportable solution where the components are
designed to interwork efficiently. Where the load is primarily AC, the
inverter will be sized to supply the full load.

There are several variations in the way that the battery is used:
• No-break standby
• Fixed cyclic operation
• Power boosting (step loads)
• Controlled low load cyclic

In the most basic operation, a relatively low-capacity battery is
provided to bridge the start-up period of the diesel engine, providing
an effective no-break standby plant. The battery may be charged Figure10: Low load cycling of DG/battery hybrid set (Powr2)
via a separate charger fed from the grid or by the DG during routine
maintenance runs. The same cyclic operation can be used with mobile hybrid
Then the batteries can be used to “ride through” an AC failure, sets, but with both DC and AC outputs or a combination, the
without any need to start the generator. The generator only needs to cycle period depends on the sizing of the DG, battery and
start for longer AC failures. load. This arrangement allows the DG to be run at optimum
Combinations of diesel generators and batteries in cyclical load, and is very useful when low load periods occur regularly,
operations have been used for many years to power remote such as at night.
sites such as telecommunications base stations. In the classical This application finds use not only in sites with a constant
solution, a conventional diesel generator set, running at a fixed load, such as telecom base stations, but particularly in
speed, is used to generate AC which is rectified by conventional situations where the load varies considerably, such as a
telco rectifiers, and charge a 48 V battery. The battery powers site with a night-time load consisting mainly of lighting. The
the site. The unit runs on a cycle of charge and discharge of the arrangement is also useful when the DG set is used as a
battery, as shown in Figure 9. When the battery is fully charged, temporary power supply, as the configuration allows different
the DG shuts down and the site is powered from the battery. When loads to be accommodated by varying the charge/discharge
the battery reaches a set level of discharge, the DG starts up and cycle parameters.
charges the battery and supplies the load. A typical cycle would In the power boosting mode the battery is used to boost the
consist of DG and battery power. output to help cope with step load changes and transient high
loads such as motor startup.
A further application uses the DG as the main power source
and relies on the batteries to supply power under low load
conditions, obviating the need to run the diesel at low loads
(Figure 10). In this case the cycle is controlled by the load level,
and the level of the battery. If the battery reaches a low state of
charge, the DG starts up and recharges it.
The combination of a VSDG with batteries provides a very
flexible and efficient mobile power unit with an ever increasing
range of applications. n

References
Figure 8: No-break standby (Eaton)
1. N Ulwar: “The future of diesel-generated power”, Cummins
power generation.
2. T Overton: “Diesel gensets aim at the future”, Powermag,
October 2015.
3. J Hamilton: “The potential of variable speed diesel application
in increasing renewable energy source penetration”,
ICEP2018, 13 - 15 December 2018.
4. Atlas Copco: “QAS 60/35 VSG, the game changer”.
5. J Leuchter: “Dynamic Behaviour of Mobile Generator Set with
Variable Speed and Diesel Engine”, PESC 2007.

Figure 9:Cyclic hybrid/DG battery system operation (Eaton) Send your comments to rogerl@nowmedia.co.za

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