Elaboración de la Matriz de Riesgos en cuanto a Procesos Operativos

To investigate the real-time operational characteristics of the dual-droop scheme in a mixed AC/DC grid, experimental tests have been carried out on the multi-platform network shown in Figure 6.11. The tests aim to validate the effectiveness of the proposed frequency support controller against conventional ones and to validate the controller implementation and its impact on the converter performance. The data for real-time hardware-in-the-loop (RT-HiL) tests is acquired in RSCAD and the dSPACE controller, with results plotted in MATLAB.

6.7.1 RT-HiL Experimental Test Set-Up

The RT-HiL platform consists of a real-time digital simulator (RTDS), a grid simulator (GS), and an HVDC test-rig. The connection of different components to form the HiL configuration is depicted in Figure 6.11(a). To perform the RT-HiL tests, the system shown in Figure 6.1 was modelled using the RSCAD software of the RTDS. This network can be easily expanded to represent generators, loads, transformers and transmission lines of more complex AC systems. High voltages were converted to a low voltage suitable for the GS and the HVDC test-rig through analogue output (GTAO) cards of the RTDS firmware. The technical parameters of the RTDS used in this work are included in Table 6.2.

6.7. Experimental Validation

The GS produces a three-phase mains supply voltage from the GTAO cards of the RTDS. This is achieved by using a four-quadrant amplifier rated at 2 kVA and 270 V (line-to-ground rms). To close the loop between the RTDS and the HVDC rig, the three-phase line current is tapped and fed to the analogue input (GTAI) card of the RTDS (see Figure 6.11(a)).

The HVDC test-rig is formed by three VSCs, three transformers, a DC network cabinet and a dSPACE DS1105 controller. The VSCs are operated at a rated power of 1.5 kW, 140 V AC and ± 125 V DC (see Table 6.1). Through the GS, a conversion ratio of 400 kV / 140 V is achieved, which means that an AC voltage of 140 V in the test-rig represents 400 kV of the high voltage system. The output voltage of the GS is controlled using an autotransformer. The dSPACE platform acquires data and monitors system states of the test-rig and controls each VSC. The hardware set-up is shown in Figure 6.11(b) and the parameters for the VSC HVDC rig can be found in Table 6.2.

6.7.2 Experimental Results

Experimental tests were carried out for Cases 1 to 4, with results shown in Figure 6.12. The wind power profile employed for the experiments is slightly different from that in the simulations. In this case, step changes in wind are considered as opposed to ramp changes (see Figure 6.12(a)). The frequency of Grid 1 is shown in Figure 6.12(b) and the power injection through VSC1 in Figure 6.12(c). The injected power into Grids 2 and 3 and the DC link voltage variations are plotted in Figures. 6.12(d) and 6.12(e). DC voltage traces show a deviation of less than 0.05 p.u. while providing frequency support when the dual-droop scheme is implemented in VSC1. This variation in DC voltage is a result of the additional 0.5 p.u. of power injection into the MTDC grid (see Figure 6.12(c)) to accommodate the power variation in the wind-thermal bundled Grid 1. These deviations are accommodated by the power injection of Grids 2 and 3 (see Figure 6.12(d)). This agrees well with the simulation studies, with results well within operational limits for the satisfactory operation of the MTDC grid.

The experimental results show that the dual-droop scheme outperforms the conventional frequency support schemes, which validates its effectiveness to accommodate high wind pene- tration without violating AC and DC operational limits.

6.7. Experimental Validation

Figure 6.12: RT-HiL results. (a) Wind power injection; (b) Frequency profile of Grid 1; (c) Injected power of Grid 1; (d) Absorbed power by Grid 2 and 3; (e) DC voltage.

6.8 Summary

The effect of high wind penetration on power system frequency regulation and stability has been studied in this chapter using a wind-thermal MTDC bundled network. To this end, a control scheme named dual-droop that binds system frequency with the DC voltage of a VSC-based MTDC grid has been proposed. The dual-droop scheme is capable of providing frequency support, generation balancing, and reduction of wind power curtailment.

To show the effectiveness of the dual-droop control scheme, the stability issues arising from wind power fluctuations and the relatively slow primary frequency response of thermal power plants have been analyzed. The sensitivity studies and eigenvalue analysis performed in this work have elucidated the capabilities of the proposed scheme to accommodate large-scale wind power penetration and enhance system stability.

Simulation and experimental results have shown that the frequency stability issues caused by wind power variations can be addressed and, more importantly, mitigated when the proposed method is employed. Results have shown that the dual-droop scheme outperforms conventional frequency regulation schemes while keeping the DC voltage within a narrow range regardless of significant power flow changes.This enables an enhanced performance of the wind-thermal bundled AC system without compromising the DC grid operational constraints.

7

Conclusions

7.1 General Conclusions

Increased application of VSC based power electronic devices is foreseen in the future transmis- sion grids, which will operate in parity with the existing compensated or uncompensated AC lines. Power converters based on VSC technology provides a cost-effective and reliable solution for transmission network reinforcements and sustainable energy integration. Utilizing VSC’s to provide AC system operation and stability support can add more value proposition to the asset. To achieve these services the technical barriers associated with the VSC’s integration into the existing AC transmission asset base have to be addressed.

To this end, this thesis investigated the contributions of VSC based HVDC connections to AC system stability enhancement, through their control flexibility and fast power injection capability. In particular, how the inclusion of the proposed VSC HVDC link in the GB system contributes to SSR damping and frequency management.