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

cable of the 4 wire system inside the microgrid. Due to the connectivity of the transformer
secondary, any kind of load can be connected on the feeders of the microgrid: three phase loads
connected at delta or wye and single phase loads between a live phase and the neutral such as
commercial appliances.
The transformer provides electrical insulation between the voltages on the feeder and the
voltages of the inverter side through the magnetic coupling of the coils. The choice of the delta to
star connectivity provides also a filter for the zero sequence currents that may flow in the feeder
due to a fault: this provides the power electronic devices of the inverter with a level of protection
from faults.
(a)
(b)
Figure 4.12 Transformer Current During Fault on a) Wye Side, b) Delta Side.
Figure 4.12 shows the line currents flowing on the wye and delta side as a consequence of a
single line to ground fault on the feeder side without the intervention of protections to open the
circuit. For ease of comparison, the transformer has a unitary turn ratio resulting in same line
current magnitudes on either side. Normal loading conditions determine a current of about 50A,
but as soon as the fault is applied the currents on the wye side reach over 400A in the faulted
phase, while on the neutral cable that provides the return path for the sum of the three line
currents, the faulted condition determines almost 900A, Figure 4.12a. On the delta side, the line
currents corresponding to the currents flowing in the inverter legs can be seen to barely exceed
the 200A value in Figure 4.12(b). The voltage of the faulted phase on the feeder side will be zero
due to the fault to ground, but due to the wye-delta connectivity two of the voltages on the delta
side will be 88 percent of the original values while the third line to line voltage will be 33
percent of the nominal value [7]. With the introduction of the transformer a voltage sag on the
inverter side will never be lower than 33 percent of the nominal value with any kind of single
line to ground faults.
The transformer creates a free variable out of the DC bus since any voltage at the inverter
terminals can be interfaced with the feeder voltage by an appropriate choice of the turn ratios.
The ratings of the transformer must be enough to transfer all the active and reactive power that is
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