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

inverter ratings limits (in kVA) can only result in shutdown and is the topic of protection
analysis.
This report is about preventing the inverter from injecting a steady state output power that is
larger than maximum prime mover output. If that were to occur, the storage would deplete all its
energy. This suggests that as long as the inverter is within its apparent power ratings (kVA) it is
possible to overshoot the maximum power limit (kW) without compromising equipment safety.
Some action must be taken to avoid that this overshoot is sustained in steady state, by enforcing
the behavior of the unit to belong to some particular characteristic.
This report is also about preventing the inverter from injecting a steady state output power with a
negative sign: if that were to occur, the storage would overshoot its maximum capacity. This
limit is defined by the fact that the prime mover cannot behave like a load. For instance,
microturbines are thermodynamic machines that convert chemical energy of the fuel into
mechanical power. This machine will not be able to perform the opposite task of using
mechanical power to convert it to some other form. It follows that the prime mover has the value
of P = 0kW as the lower limit of power. The inverter does not have such an issue. It can legally
operate in all four quadrants of the P,Q plane as long as its ratings are not exceeded. In practice,
the voltage and current flowing in the inverter could have a phase displacement so that the
overall active power injection is negative. This power cannot be transferred to the prime mover,
an would be stored in the battery. A steady state operation in this condition would overfill the
energy storage. To avoid this situation there is the need to prevent the inverter from ever
exceeding minimum power during steady state, acknowledging the fact that it would be legal to
overshoot this limit for a short period of time.
3.2.1 Steady State Characteristics with Output Power Control
This section describes the configuration with the control regulating the voltage and the power
injected by the unit at the local point of connection with the feeder to a desired amount. To this
end, the active power injected and the feeder voltage are measured and passed back to the control
loop. Figure 3.13 shows the diagram of this configuration: the measure of power injected by the
unit, P, is calculated from the current injected by the source and the voltage E measured at the
point where the unit is installed.
This chapter shows the steady state characteristics on the power vs. frequency plane that allow to
enforce limits for the output active power and frequency. This chapter describes the
characteristics to be used when the units are configured to regulate their output active power.
Figure 3.13 shows that when connected to the grid, load changes are matched by a corresponding
power injection from the utility. This is because the unit holds its injection constant. During
island mode all the units participate in matching the power demand as loads change.
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