Control and Design of Microgrid Components - Power Systems ...

Microsources are connected to the local feeder with a transformer in series to an inductance. The
connection is at delta on the inverter side and at wye on the feeder side with the center star
connected to the neutral wire of the feeder cable. The voltage level of the feeder is 208V, while
the inverter operates with voltages of 480V. In summary, operations are on a three wire
environment on the inverter side of the transformer, while they are on a four wire environment
on the microgrid.
Figure 6.2 shows the basic equipment that appears in a microsource: the inverter is connected to
an ideal DC source and creates the AC voltage at its three phase terminals. The inverter is
controlled by the gate signals that are generated by the control blocks. The controller uses the
measures of voltage and current at the feeder, where the unit is installed. When the unit is
controlling the feeder power flow, then the current from the feeder on the side leading to the grid
are also fed back to the control. The microsource is connected to the feeder with a transformer to
lower the voltage from 480V to 208V. The inverter side is connected to delta, while the lower
voltage side is star connected, with the neutral of the transformer carried on in the feeder to allow
for single phase loads to be connected.
v abc
X (t)
F
480 V 208 V
+
V DC
Inverter
Local
Feeder
Gate
Signals
X
F
C F
X
e
i
abc
abc
(t)
(t)
n
Controller
Feeder Currents i abc
(t)
Towards
Grid
Figure 6.2 Single Phase Diagram of Inverter Connection to the Feeder.
6.2 Description of the Laboratory System
This section describes in detail the constituent parts present in the hardware circuit. The overall
single phase circuit diagram is represented in Figure 6.3. Somewhere in the building there is a
connection that can be considered a stiff voltage source, representing the utility system. From
that connection, there is a three wire cable (Z1) that reaches the transformer T1, located in the
laboratory. This transformer has the neutral connected to the fourth wire of the system that runs
in parallel to every feeder. On the microgrid side of the transformer there is the static switch to
connect and disconnect from the grid. Then there is a short 4 wire cable (Z2) that connects to the
first microsource (MS1) and load center (L1). From there a 75 yd, 4 wire cable (Z3) reaches an
intermediate load center (L2). The last 25 yd of cable (Z4) reach the second microsource (MS2)
and the last load center (L3).
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