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

Utility
System
Event:
Transfer to
Island
F 1
P 1
4 wire
75yd
4 wire
25yd
P 2
L L
1
3
L4
L5
Figure 7.2 Series Configuration Diagram.
F 2
Maximum power corresponds to 0.8 pu in the source because of the DSP capability of handling
only quantities that are in the –1,+1 interval. To leave room for overshoot, the value of P_max
has been chosen to correspond to 0.8.
After the system transfers to island, unit 2 will be sitting at 100% of power (0.8 pu), while unit 1
will be sitting at 50% (since overall loading is 150%) of power (0.4 pu), in all of the four
configurations. What really changes, is how they get there.
Figure 7.3 and Figure 7.4 show simulation results when P1 and P2 are controlled: both units
ramp up their power output (P) at the unison. Overshoot at unit 2 is near 0.89 pu (110%).
Figure 7.5 and Figure 7.6 show that when F1 and F2 are controlled, then as P2 ramps, P1
actually backs off, after rising a little. This means that the resulting overshoot on P2 is about 1.04
pu (130%). This implies tripping of the units, since nearly above 0.9 pu is the threshold value for
power, and besides, a value larger than 1.0 is also beyond the overflow value of the bit
representation inside the DSP.
Figure 7.7 and Figure 7.8 show that when F1 and P2 are controlled, then an unacceptable
overshoot also occurs. Figure 7.9 and Figure 7.10 show that when P1 and F2 are controlled, the
overshoot is manageable again.
This implies that the choice of control configurations bears a consequence on the ratings of the
inverters that need to be available, or the gains may need to be readjusted (when possible) to
improve response. For this lab case, the above considerations implied that the test for 10% and
90% could not be carried under those cases that generated very large overshoots. To avoid
tripping of the units, the setpoint corresponding to loading of respectively 30% and 70% for unit
1 and 2 have been chosen only for the cases with larger overshoot. This problem was only
encountered during 10%, 90% setup, never during 90%, 10%, hence this latter configuration has
never been changed. Also these problems are only encountered at high load levels (importing to
grid). Therefore, during exporting no changes were necessary to 10%, 90% setup. Finally, no
problems were encountered during the parallel configuration, so no changes had to be made there
as well.
The Table 7.2 below summarizes the previous considerations.
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