Page 51 - Energize July 2021

TECHNICAL

and in some cases a dummy resistive load is added to the set to permanent magnet alternators, which are smaller and lighter than
ensure that the load never falls below this lower limit, a move which the synchronous alternator used in the fixed speed version.
affects the overall efficiency at low loads. Both load limits and There are a number of variable speed mobile power units
engine efficiency serve to restrict engine flexibility and limit hybrid available on the market, operating in the speed range of 900 to
operability. 3 2000 rpm. Most units are in the low power range, i.e., below 5MVA.
To achieve low or partial loading, conventional fixed speed diesel Separating the inverter from the diesel generator set also allows
technologies must rely on prescribed purge routines, which serve multiple units to be connected to a DC bus which feeds a separate
to increase emissions intensity and fuel consumption. Fixed speed suite of inverter units. This has application in marine power.
constraint remains the primary barrier to increased engine flexibility
and improved partial load efficiency. A solution to the problem lies Hybrid diesel/battery generator sets
in the fact that the power output of a diesel engine can be varied by Hybrid DG/battery sets are finding a number of applications in an
varying the speed as shown in Figure 4. ever-expanding market. Although such combinations have been
To use this approach requires decoupling the output of the in use for many years, older versions used separate charger/
generator from the output of the engine. In the past this has been converters for the battery storage unit. The use of a DC stage in
impractical and expensive, but the availability of power electronic the VSDG offers the possibility of adding energy storage within
devices and the proliferation of high-power inverter systems, the diesel generator system to provide a mobile hybrid power
used with variable RE sources have made this both possible and system, using a common inverter output. There are a number of
economical. The basic structure of a variable speed DG set (VSDG) hybrid system types, but all use the basic configuration shown in
is shown in Figure 5. Figure 6.
The VSDG consists of an alternator which produces a
variable frequency output, rectified to feed a DC bus. This is fed
to the inverter stage, which produces an output at the required
voltage and frequency. The inverter may be a grid-forming
inverter or a grid dependant type, subject to the application.
Where the diesel generator set is the sole source of power, a grid
forming inverter will be used. This configuration also makes it
possible to use alternator types which are more suited to variable
speed operation.
The alternator used with the VS system can differ considerably
from that used for fixed speed operation. The AC output is rectified
to DC and therefore the waveform and frequency are not as
important as for the fixed speed case. A common approach is to use
Figure 6:Hybrid VSDG set

Figure 4: Diesel engine power output vs speed 5

Figure 5: Variable speed DG set Figure 7: A typical mobile DG/Battery hybrid (GL Hybrid)

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