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

with different size. This control is somewhat universal because of its ability to be used with
different hardware configurations without having to change anything internally. Every block will
be expanded to show the operations on the variables inside.
The active power is regulated to a desired value during operation in parallel with the grid. During
transfer to island operation, the frequency of the network will be allowed to sag slightly,
adopting the active power-frequency droop. The characteristic will ensure that all the units will
immediately ramp up their output power to match the missing quota from the grid, without the
usage of an explicit network of communication between the several units. This P versus
frequency block generates the angle that will be tracked by the gate pulse generator.
Each of the blocks that appear in Figure 3.2 will be examined in detail: the only block that it is
left out is the gate pulse generator, that will be described in great detail in Section 6.4.
3.1.1 P and Q Calculation
The blocks that calculate the values of active and reactive power will use the knowledge of
instantaneous values of line to line voltages and line currents. These are exactly the quantities
that are brought in from the sensing equipment. Since there is no ground to refer to, the voltages
are always measured across the phases. The count of the sensing equipment is kept to a
minimum by measuring only two of the line to line voltages and calculating the third one from
the fact that the sum of the three delta voltages must equal zero, in balanced as well under
unbalanced conditions. Only two currents are measured and the third one is calculated assuming
their overall sum to be zero, which is correct only under balanced conditions.
P and Q calculation
Inverter Current
i
inv a,
b,
c
Load Voltage
Measure e
a, b,
c
Q
Calculation
Q
Inverter or Line
Current
i
a,
b,
c
P
Calculation
P
Figure 3.3 P and Q Calculation Blocks.
Figure 3.3 shows the input-output layout for the P and Q calculation. Notice that the measure of
the voltage at the microgrid side is passed to both blocks, while the current can either be the
inverter current or the feeder current, depending respectively if the output power of the
microsource or the power flow on the feeder is controlled.
The equations used are:
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