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

Chapter 3. Microsource Details
This chapter gives the details of the system that composes a microsource. Figure 3.1 shows the
microsource layout implementation. The controller sends the gate pulses to the inverter that
generates a three phase 480V line to line voltage. This waveform is rich in harmonic content at
the switching frequency, 4kHz. To filter out these harmonics there is a low pass LC filter
immediately connected at the inverter terminals. Then there is the series of the coupling
inductance and transformer. The sensed quantities are the voltages at the load bus and the
inverter currents. From these quantities it is possible to extract the load voltage magnitude and
the active and reactive power injected by the unit. If the unit controls the feeder power flow, then
the measures of the currents flowing on the feeder from the side that connects to the grid are also
passed to the controller to enable the calculation of this active power flow.
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)
Figure 3.1 Microsource Diagram.
Towards
Grid
3.1 Microsource Controller
This section gives the details of the final form that the control assumes during the
implementation. Hardware realization of the control has been plagued by issues of noise
propagation from the analog to the digital word inside the controller. The fundamental frequency
selective filter is a first tool to handle some of the higher harmonics of the noise, but the
calculated values of active and reactive power, as well as the voltage magnitude still suffered
from oscillations determined by random spikes in the measured quantities.
The complete control of the microsource is shown in Figure 3.2 [4]. The inputs are either
measurements (like the voltages and currents) or setpoints (for voltage, power and the nominal
grid frequency). The outputs are the gate pulses that dictate when and for how long the power
electronic devices are going to conduct. The inverter voltage and current along with the load
voltage are measured. The voltage magnitude at the load bus and the active power injected are
then calculated. When controlling active power, there is a choice of regulating the power coming
from the unit or the power flowing in the feeder where the source is connected. If the power in
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