IEEE Electrification Magazine - June 2017 - 44

Keating Nanogrid Controller

Control Entities
Battery

Load

Figure 16. The energy management system at the Keating Nanogrid.

Current/Power

utilization of solar energy. The charge-discharge current of the
battery is strategically regulated by power converters using the
terminal voltage signal. In the Keating Nanogrid, a Xantrex
XW MPPT 80 600 buck converter and Conext XW+ 6848 NA
bidirectional converter, manufactured by Schneider Electric,
provide the charge control signal for the batteries installed in
dc and ac subsystems, respectively.
In the charge mode, a battery module will store the surplus power generated by PV arrays while maintaining the
bus voltage within a safe range. A three-stage process,
illustrated in Figure 18, is commonly adopted to charge a
battery module. This battery-charging process, which is
more efficient than setting up a fixed time for charging, is
also advantageous in reducing battery gassing and electrolyte loss. During the bulk stage, the power converter operates in the constant current mode and delivers the
maximum possible current to charge the battery module.
When the battery module's voltage reaches the absorption
voltage in Figure 18, the power converter will operate in
the constant voltage mode, and the charging current falls
gradually as the SOC is restored to its full level. The power
converter will transit to the float stage if either the charging current falls below the exit current threshold (e.g., 2%

r
we
Po
Current

Voltage

Figure 17. The MPP at the Keating Nanogrid.

I E E E E l e c t r i f i c ati o n M agaz ine / j un e 2017

Demand-Side Management at the Keating Nanogrid
The additional ZigBee module embedded in the LED light
fixtures forms a scalable mesh network for wireless communication in the Keating Nanogrid. Figure 21 shows the

Bulk Stage

Absorption Stage
Voltage

MPP

Charge Profile

PV Array

Configure
Optimize
Device Settings Power Exchange

Request
Monitor
Operating State Power Exchange

IIT Microgrid Controller

of the capacity) for a certain period or the absorption stage
lasts longer than the allowable time. During the float stage,
when the charge current is negligible, the module's voltage
is held constant at the predefined float voltage.
In the discharge mode, a battery module will function
as a voltage source for supplying the load when the power
output of PV arrays is inadequate. Aquion Energy's AHI batteries can be discharged to 0% SOC, which provides additional flexibility for balancing the power in the Keating
Nanogrid. The maximum discharge current and the maximum discharge power will decrease, while the SOC decrement will force the voltage at the battery module to drop.
In the Keating Nanogrid, the battery voltage is largely
dependent on the bus voltage in either the ac or the dc subsystem. Meanwhile, the subsystem bus voltage shows
whether any surplus power is available to feed the nanogrid
load. Accordingly, nanogrid batteries are controlled using the
bus voltages. Figure 19 shows the battery-control logics
applied to the Keating Nanogrid. The power converter monitors the bus voltage and regulates the corresponding battery
power. When the PV generation is larger than the load, the
bus voltage will be higher than its nominal value, and the
batteries will be charged. However, when the bus voltage is
lower than its nominal value, the PV generation cannot fully
feed the load, and thus batteries will be discharged. In
charge mode, batteries residing in the float stage will transit
back to the bulk stage as the bus voltage drops below the
predefined recharging voltage. In discharge mode, a low
voltage (i.e., lower than the low-level voltage) will result in
the power import (e.g., from the other subsystem via the
bidirectional ac-dc converter) for maintaining secure
nanogrid operations. The voltage settings corresponding to
Figure 19 are configured by the Keating Nanogrid controller.
Figure 20 shows an example of voltage settings for controlling the batteries in the dc subsystem.

Current

Time
Figure 18. The three-stage charging process.

Float Stage

Table of Contents for the Digital Edition of IEEE Electrification Magazine - June 2017