IEEE Electrification Magazine - December 2013 - 32

System Efficiency (JP8 Based)

30
25
20
15
10
5
0

6
4
5
Load (kW)

Figure 1. The efficiency of a 10-kW generator versus load.

Generator 1

Generator 2
Vsb

Vsa

Battery Bank
+ -

EMS

Critical Loads

Noncritical Loads

Figure 2. The EMS provides an interface between loads, power
sources, and energy storage elements.

Step 1:
Source A Disconnected
LsA
RsA
VsA

Step 2:
Source B Connected
RsB
LsB

IIsA

IIsB

VsB
+
Vcfil
-

660 µH
+
Vbatt

990 µF

EMS

1.95 mF

Figure 3. A schematic of the EMS laboratory setup.

I E E E E l e c t r i f i c ati o n M agaz ine / december 2013

Lfil
IEMS

to 200 V, creating a dc bus for the H-bridge converter. a thyristor switch, Crydom Cwd2410-10, is used as the disconnect/connect switch. The lab power supply is used to
represent both voltage sources VsA and VsB since they
operate at separate times.
a photo of the laboratory experiment is shown in
figure 4. The batteries are shown in the top left corner
while the electronics, including three pCBs, are visible on
the front. Two of the three pCBs are custom while the third
one is a Xilinx fpGa development board.
The handoff from one voltage source to the other is
demonstrated by the experimental setup shown in figure 3
in two steps. in step 1, the eMs disconnects from VsA by
turning off a thyristor switch. once disconnected from
VsA, the eMs draws power from the batteries as it waits
for Vcfil to synchronize with VsB. once synchronized, the
eMs connects to VsB at the next Vcfil zero crossing, as
shown in step 2.
The experimental voltages and currents produced
when disconnecting the eMs from VsA are shown in
figure 5, and the corresponding waveforms for the connection to VsB are shown in figure 6. in these figures, Vcfil
is the output bus voltage seen by the load, IeMs is the current injected from the H-bridge inverter, Iload is the current through the load resistor, and IlsA and IlsB are the
source currents from VsA and VsB, respectively. The two
sets of experimental plots demonstrate that the load
does not experience any disturbance when a generator
handoff occurs.
note that the voltage Vcfil is slightly lower when the
generators are disconnected and the eMs alone provides
voltage to the load, as shown in figure 5. This was intentionally done to visualize the transition in the laboratory
experiment. The eMs was programmed to supply a voltage slightly lower than the main
power source. since the H-bridge is
controlled as a voltage source inverter, the voltage can be controlled very
precisely.
from figure 5, it can be noted that
just before VsA is turned off, there is a
Iload
moment when current flows
between the main power source and
Ccfil
the eMs, thus producing a spike in
the IlsA and IeMs waveforms because
the main power and the eMs are
both trying to control the ac bus voltage. a small angle or phase difference between the eMs output voltage
and the main power supply voltage
cause a discontinuity to occur when
VsB gets connected, as shown in figure 6. These glitches do not affect the
load; however, they will be addressed
in the future as the eMs digital controller gets further refined.

Table of Contents for the Digital Edition of IEEE Electrification Magazine - December 2013