INTERCON2017

INTERCON 2017

Paper Title: Using the DC-Link Capacitor as a Rotating Inertia in a Three Phase PV System.

Authors: Pablo Vela , Claudio Valenzuela, José Espinoza

Full paper HERE.

Abstract

This work provides the guidelines to calculate the size of the DC-link capacitor to contribute with additional energy - for a short period of time - than the photovoltaic panel can supply under normal operating conditions. First, a model of a three-phase electric generation system is used that combines photovoltaic panels and an inverter with an LCL filter at network side. In addition, a control algorithm capable of exchanging between two operating modes is implemented. Indeed, the first one is generated by the MPPT and the second one is generated when the grid demands more energy than the photovoltaic panel can generate. The results show that the PV system can provide additional energy - for a short period of time - from the dc link as emulating a large rotating inertia. The results are shown through simulations under standard test conditions (STC).

Research

Topology

The topology consists in a three phase inverter connected to a LCL filter, and to the grid.

Control Algorithm

For the control of the inverter two references are used that operate independently one from the other as shown in the Fig. The first one operates in normal mode, i.e. when the system does not demand more power than the photovoltaic panel can supply by itself and it is given by the MPPT. This control aims to obtain the maximum panel power using method P&O.

The second reference will come into operation when the system requires more power than the panel can supply and it will be given by the demanded power. Hence, the capacitor is discharged delivering additional energy for a short period of time.

It is important to emphasize that for the control the currents are measured at the inverter output and the grid voltage at each of its phases. Then, these measures are transformed to the rotating dq0 frame and these transformed variables are those that are used in the control loops.

Subsequently, the calculated modulators are converted to the stationary abc frame again.

Control Block Diagram

The following hows the complete control diagram and how the 2 current references 𝑖𝑑∗ are generated, when the generation system operates normally and when more power is required than the system can provide using only the photovoltaic panels. In contrast, the reference for the reactive power 𝑖𝑞∗ is kept at zero because the main objective is to inject more active power to the system.

A feedforward strategy is used by the internal current loops to obtain a better response than the PI controllers, besides that it acts as a decoupling term.

Simulations

During the simulation, the MPPT reference is used and in t = 2 the reference 2 is activated and reference 1 deactivated, i.e. more energy was injected to the grid. Then in the t = 2.02 the system returned to the MPPT reference. The results are shown below.

The figure shows how the capacitor gets discharged when more power its required by the grid, decreasing its voltage almost to 25 V. Naturally, the maximum power point tracking for the panel is lost.

The figure shows the power delivered to the grid. In t = 2 the power injected into the AC mains increases for nearly 20 ms.

The figure below shows the currents of the three phases at the AC network of the inverter and at time 2 (s) the reference is changed and more power is injected to the system accordingly the amplitudes increase. Then, in time 2.02 (s) the MPPT reference is returned and the currents undergo a small transient, until the capacitor is recharged.

The figure compares the active power delivered by the inverter for different dt (Time to supply extra power). As expected, while this period is shorter, the lower the power drop when returning to the MPPT, since the capacitor is less discharged.

Conclusions

It is possible to emulate an inertia using the DC-Link capacitor of an electronic converter. In addition, it is possible to design a control capable of switching between two references without having to stop the photovoltaic plant. The method developed in this work allows applying this methodology to both large and small generators.

After the capacitor gets discharged, the plant needs to recover its previous state of operation, so it is mandatory that other generators support the electric system during this period. If the capacitor discharges too much, it is reasonable to disconnect the generation plant for a while.

The proposed strategy allows to develop new methods can be developed to transform the photovoltaic generation into a more attractive and competitive form capable of not only generating energy but also collaborating in other areas such as frequency control.

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