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

Figure 2.12 shows on the upper plot the voltage across the switch: the envelope of the voltage
needs to go past the instant when it reaches the minimum value and then synchronization can
take place. The lower plot of Figure 2.12 shows the current in the microsource, notice also the
transient due to reclosing on an inductive network. The current drops to the lower value without
any overshoot. The upper plot of Figure 2.13 shows the power of the microsource going from the
full amount required by the load is island mode, 0.65 pu, to the value requested, 0.2 pu, never
exceeding the value that it had during island mode. The lower plot of the same figure shows the
frequency being locked to the grid value of 60Hz without first sagging towards an even smaller
value, as seen in the previous simulation, Figure 2.9
P_MS
[pu]
Frequency
[Hz]
Time [s]
Figure 2.13 Microsource Power Injection and Frequency, with Condition (ii) Met.
As already described, the synchronizing algorithm applies to the static switch that connects to the
grid as well to the contactor that connects the two microsources together. To see that it is true, in
the previous analysis one only needs to replace the grid voltage with the voltage of the unit that
has the highest frequency. Indeed, it does not matter what is the operating frequency: all it
matters is that the two voltages are rotating at different speeds. As of today, the hardware
configuration of the static switch includes a logic block that enforcers condition (i) but not
condition (ii).
The current hardware configuration correctly implements the first condition, but does not
implement the second. Figure 2.14 shows the voltage across the switch during synchronization.
The voltage diminishes in time due to the fact that the voltages at either ends of the switch rotate
at slightly different frequencies. As soon as the voltage has reached a minimum threshold the
synchronization takes place.
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