The voltage references obtained by the control systems presented in the previous Subsection s (i.e. vαα∗ , vβα∗ , vαβ∗ , vββ∗ , v∗α0, vβ0∗ , v0α∗ and v0β∗ ) are transformed to the natural reference frame using the inverse αβ02 Transformation. Then, the CCV references for each cluster in the abc-rst frame are obtained (i.e. v∗ar, v∗br, vcr∗ , vas∗ , vbs∗, v∗cs, vat∗, v∗bt, vct∗).
In the natural frame, the cluster references are divided by the number of cells to obtain a voltage reference for each power cell. Here, an additional control loop is utilised to regulate at the same level the capacitor voltages within a cluster. The single-cell control scheme presented in Fig.4.7 is responsible for the capacitor voltage balancing of the capacitors within a cluster. The capacitor voltage of the ith i∈ (1, n) power cell is compared to algebraic-mean value of the corresponding CCV. The resulting error is multiplied by the sign of the cluster current (and by the gain kn) pro- ducing either an in-phase or 180◦out-of-phase voltage with respect to the cluster current, resulting in a release/absorption of power from/to each power cell. Therefore, an incremental voltage ∆Viis added to the cell reference voltage v∗xy/n.
This strategy is similar to Single-Cell balancing proposed in [89]. However, there are some incremental differences between this proposal and [89] which are listed below:
• The incremental voltage ∆Vi is produced by the sign of the cluster current instead of the cluster currents to reduce the oscillations in the control signals.
• The capacitor voltage is compared to the algebraic mean value of capacitor voltage available in the cluster instead of the desired value vc∗. By this means that the sum of ∆Viin the same cluster tends to zero and does not affect the Cluster Capacitor Voltage reference. Therefore, the Cluster Capacitor Voltage control is decoupled from the Single-Cell Control.
Finally, phase-shifted unipolar sinusoidal PWM is used to synthesise the voltage references for each power cell [89]. The single-cell voltage references of the same cluster are compared to triangular carrier signals shifted by 2π/n from each other. This modulation algorithm gives expansion for a high number of power cells, and it is simple to implement in commercial FPGA- based control platforms. Moreover, phase-shifted PWM produces power losses evenly distributed among the cells of the same cluster, and it generates an output switching frequency of 2n times the carrier frequency.
Figure 4.7: Proposed Single-Cell Balancing Control.
4.6
Summary
The Chapterhas presented a cascade control strategy for M3C based WECSs. This proposal con- siders a cascade structure, where the outer loops regulate the capacitor voltages by setting the references of the circulating currents. Moreover, they can be linked to the input and output current control loops.
Especial attention had been paid to the CCV control system. Two control proposals are in- troduced to achieve operation in LFM as well as in EFM. The first control proposal performs balancing using the Semi-controllable terms of the Power-CCV model in Double-αβ0 coordinates. Moreover, the EFM control is based on open-loop circulating current references that mitigate the voltage oscillations produced by the EF operation.
The Vector CCV Control Strategy introduces a novel dq based vector control which is especially useful for EFM operation. In this strategy, the dynamics of the CCVs is analysed in Σ∆ Double- αβ0 coordinates. When the machine frequency is not close to the unstable points, the average values of the vectors are balanced using the Semi-controllable terms of the Power-CCV model. The EFM control is performed using a closed-loop control system implemented in dq coordinates which allow operation of the converter in a broad range of frequencies, including equal frequency.
CHAPTER
5
Proposed Control Strategies for based M3C WECS -
WECS Control
5.1
Introduction
This Chapter discusses the application of the M3C to drive Multi-MW WECSs, focusing on the control structures that enable MPPT capability at the generator-side and grid code compliance at the grid-side.
The generator-side control system comprises a cascaded structure where an outer control loop performs MPPT, and an inner control loop regulates the generator current using a dq based control system suitable for PMSG based WECSs.
The grid-side control system considers LVRT control in the presence of symmetrical or asym- metrical faults. Unlike conventional LVRT algorithms in the dq frame, this proposal considers the use of Resonant Controllers to regulate the positive and negative sequence components of the grid voltage. Sequence component separation is achieved using a new fast-convergence Delayed Signal Cancellation (DSC) method which is also discussed in this chapter.
Finally, a brief explanation of the controller design criteria used to tune the cascade control systems is presented. Notice that this section includes also the control structures detailed in Chapter 4.
Figure 5.1: Proposed Control Strategy Generator-side control strategy.