IEEE Power & Energy Magazine - July/August 2017 - 46

Voltage (kV)

Voltage
Signal (Bool)

The microgrid controller is interfaced to the RTDS through
a data manager that performs protocol-translation and dataconcentrator functions.
One of the battery energy storage systems (BESSs) is represented in power hardware by a 480-V/540-kW three-phase
battery inverter. An ac power amplifier-a controllable, bidirectional ac source-is used to create the interface between
the power hardware and virtual parts of the model. The RTDS
controls the ac power amplifier to follow the (scaled) voltage
on the simulated bus to which the BESS is connected. The
current output of the BESS is measured and reflected in the
RTDS model as a (scaled) current source, closing the PHIL
loop. A controllable, bidirectional dc supply is controlled to
act as a battery.
The PHIL interfaces include digital-to-analog (and analog-to-digital) conversions and appropriately designed compensators to ensure that the time delays introduced by these
conversions do not result in an unstable experiment. They
also include compensators to ensure accuracy.
Figure 4 shows the simulation results during a microgrid
planned islanding event, which may be performed prior to a

2.0
1.5
1.0
0.5
0.0

Microgrid Controller Disconnect Signal

2.16 2.18 2.20 2.22 2.24 2.26 2.28
(a)

40
20
0
-20
-40

Microgrid Voltage

Current (A)

2.16 2.18 2.20 2.22 2.24 2.26 2.28
(b)
2
1
0
-1
-2

Battery IA
Battery IB
Battery IC

Current (A)

2.16 2.18 2.20 2.22 2.24 2.26 2.28
(c)
10
5
0
-5
-10

Battery IA
Battery IB
Battery IC
2.16 2.18 2.20 2.22 2.24 2.26 2.28
Time (s)
(d)

figure 4. PHIL and CHIL test results of the Spirae Wave
microgrid controller for a planned islanding event: (a) control signal, (b) microgrid voltage, (c) circuit breaker current,
and (d) current output of battery inverter.
46

ieee power & energy magazine

scheduled grid-outage event or in response to degrading electric grid power quality. In Figure 4, (a) shows the disconnect
signal issued by the microgrid controller, (b) is the voltage on
the microgrid side of the breaker at the POI, (c) shows the current through the POI breaker, and (d) is the output current of
the hardware battery inverter. The POI breaker current is low
prior to islanding because the microgrid controller reduces the
power flow across the POI to nearly zero to ensure a smooth
transition. The microgrid voltage and battery inverter current
waveforms confirm that a smooth transition occurred.
NREL is also supporting a distinct project funded by the
California Energy Commission (CEC) with PHIL and CHIL
testing to evaluate microgrid controller technology. Borrego
Springs, California, is a desert community served by one
30-mi radial transmission line that extends across mountains
and deserts, where the circuit is susceptible to severe weather
and fires. The CEC awarded a project grant to San Diego Gas
& Electric to expand an existing DOE-funded microgrid demonstration project to cover the entire Borrego Springs community of 2,800 metered customers. The goals are to provide
customers with greater reliability and resiliency-and to leverage more local renewable energy to power the community. The
expanded microgrid includes a large PV plant that enables the
community to operate during the day, solely on renewable
energy. NREL's test setup includes a microgrid controller and
two genset controllers as the CHIL, and it includes a representative battery inverter and the actual PV inverter that is used in
the field as the PHIL. The CEC requires evaluation of similar
functional requirements as those in the funding opportunity
announcement Microgrid Research Development, and System
Design as well as additional tests.

Microgrid Controller Integration
with DERs and Distribution
Management Systems
In the future, microgrid controllers are expected to play a
significant role in the management of electric distribution
systems, especially where DERs connected to a distribution
system are located within microgrids. An integrated approach
is required to allow a microgrid controller to work in harmony
with a DMS to effectively manage the electric distribution
system. Over the past few years, industry activities to create
standards for DERs have focused on the behaviors of individual DER units and open communication protocols over
field networks that connect directly to these end devices. To
better integrate and manage increasingly diverse distributed
resources, EPRI has been developing functional requirements and communication protocols for DERMS operating
in a grid-tied mode. Management of the electrical distribution
grid has occurred without the expansion of DMS functions,
accounting for the proliferation of DERs and microgrids on
existing electrical distribution systems. Currently, the DMS,
DERs, DERMS, and microgrid controllers are managed as
separate entities with minimal communication, let alone
coordination, among them.
july/august 2017

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - July/August 2017