Abstract
It is getting more common every day to install inverters that offer several grid support services in parallel. As these services are provided, a simultaneous need arises to know the combined limit of the inverter for those services. In the present paper, operational limits are addressed based on a utility scale for a real inverter scenario with an energy storage system (ESS) (1.5 MW). The paper begins by explaining how active and reactive power limits are calculated, illustrating the PQ maps depending on the converter rated current and voltage. Then, the effect of the negative sequence injection, the phase shift of compensated harmonics and the transformer de-rating are introduced step-by-step. Finally, inverter limits for active filter applications are summarized, to finally estimate active and reactive power limits along with the harmonic current injection for some example cases. The results show that while the phase shift of the injected negative sequence has a significant effect in the available inverter current, this is not the case for the phase shift of injected harmonics. However, the amplitude of the injected negative sequence and harmonics will directly impact the power capabilities of the inverter and therefore, depending on the grid-side voltage, it might be interesting to design an output transformer with a different de-rating factor to increase the power capabilities.
Highlights
With the increasing penetration of renewable energy-based generation systems, energy storage systems (ESSs) and loads such as electric cars are driving the grid to rapid changes
In order to do that, the cases are divided into two groups: when the harmonic source can be considered as a current source and when the harmonic source is approached as a voltage source
In order to define in a simple and standard manner the capacity of the harmonic active filter, this paper proposes to relate the filtering capability of inverters depending on the ZUhn /ZAFhn ratio
Summary
With the increasing penetration of renewable energy-based generation systems, energy storage systems (ESSs) and loads such as electric cars are driving the grid to rapid changes. If reactive power compensation is needed, besides harmonic compensation, inverter nominal output voltage reduction improves the current injection capability. Considering the inverter maximum current, thermally limited (and constant), the apparent power depends on the grid voltage. Depending on the phase shift between inverter current and grid voltage, the amount of active and reactive power provided by the inverter will be different. X imposes the limit of active (P) and reactive (Q) power that can be injected into the grid, as described in Equations (4): where Vinv(3). Thefrequency maximum inverterisvoltage imposes an additional restriction If another dimension is added to the diagram (grid voltage), it is possible to see active and reactive capacity limit of P and Q is described by Equation (5): power limits in the whole operational range of the inverter (Figure 3).
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