Conclusions

The proposed reinforcements for the transmission networks for many countries use series compensation to increase the power transfer capability. The effect of the wind farms on the occurrence of Sub-Synchronous Resonance (SSR) in the steam turbine shafts was simulated and investigated to resemble the operation of the mainland networks for many countries such as the UK in 2020.

7.1.1 The effect of FSIG-WTs on SSR

The influence of Fixed Speed Induction Generator-Based Wind Turbines (FSIG- WTs) on SSR in a series compensated transmission line was investigated at a wide range of series compensation levels and different power ratings of FSIG-WTs. Eigenvalue analysis and time domain simulations were carried out using MATLAB and PSCAD.

The results reveal that FSIG-WTs have an adverse effect on SSR occurring at steam turbine shafts. That means connecting FSIG-WTs to the FBM increases the range of series compensation levels at which SSR can occur.

The frequency of the subsynchronous currents over the series compensated transmission line affects FSIG-WTs. Therefore, FSIG-WTs have two slips; subsynchronous slip and the rated slip. The subsynchronous slip changes the

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equivalent impedance of the FSIG-WTs which, in turn, causes a reduction of the natural frequency of the series compensated transmission line by changing the value of the equivalent impedance causing it.

7.1.2 Damping SSR using FRC-WTs

The possibility of utilizing the grid side converters of the FRC-WTs to damp SSR was investigated. This investigation was presented by designing an auxiliary SSR damping control within the grid side converters of the FRC-WTs connected to IEEE First Benchmark Model (FBM).

The primary control scheme for the grid side converter of FRC-WTs was designed to control the active and reactive power. An SSR damping controller was designed as an auxiliary controller by using two different types of controller. Eigenvalue analysis and time domain simulations have been carried out using MATLAB to show the capability of FRC-WTs to damp SSR in steam turbine shafts using an auxiliary controller.

a- An optimal controller based on Linear Quadratic Regulator (LQR) was designed as an auxiliary controller within the grid side converter of the FRC- WTs. A full order observer was designed to estimate the unmeasured state variables to enable LQR to feedback all the state variables. The extensive eigenvalue analysis and time domain simulation over wide varying levels of series compensation revealed that the FRC-WT with a LQR controller was able to mitigate the SSR in the steam turbine shafts if the system is fully visible.

b- A classical controller based on the Lead/lag controller to damp SSR in the steam turbine shafts. Eigenvalue sensitivity was studied to choose the most suitable feedback signal and to design an SSR damping controller for the grid side converter of a FRC-WT. The synchronous generator speed deviation was the most suitable control input signal for the Lead/lag

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controller to damp SSR in the steam turbine shafts. Eigenvalue results and time domain simulations show that the auxiliary lead/lag controller was able to damp SSR at steam turbine shafts at 50% series compensation.

7.1.3 Contribution of the thesis

The following objectives have been set and achieved:

- To investigate and study FBM using mathematical analysis. FBM was modelled by using MATLAB and PSCAD. The eigenvalue analysis was used to validate the results of the modelled FBM. Furthermore, FBM was modified by integrating different types of wind farms, FSIG-WTs and FRC- WTs, with it.

- To evaluate the effect of the FSIG-WTs on the SSR at steam turbine because of fixed series compensation.

- To design an optimal controller as an auxiliary controller within the grid side converters of FRC-WTs to damp SSR in the steam turbine shafts. The results revealed that if the system is fully visible, LQR controller is an effective controller to damp SSR over a wide range of series compensation.

- To design classical controller as an auxiliary controller within the grid side converters of FRC-WTs to damp SSR in the steam turbine shafts. Synchronous generator speed deviation was the most suitable feedback control signal to damp SSR by using eigenvalue sensitivity method. Lead/lag compensation was designed as an auxiliary controller to damp SSR occurrence at the steam turbine shafts.

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7.1.4 Achievements of research

The outcomes of this research were written up in a journal and two conference papers as:

(4) A. Ewais, C. Ugalde-Loo, J. Liang, J. Ekanayake, N. Jenkins, “The Influence of the Fixed Speed Induction Generator-Based Wind Turbines on Subsynchronous Resonance,” University Power Engineering Conference (UPEC 2011), Soest, Germany, Sept. 2011.

(5) A. Ewais, C. J. Liang, J. Ekanayake, N. Jenkins, “Influence of the Fully Rated Converter-Based Wind Turbines on SSR,” IEEE PES Innovative Smart Grid Technologies (ISGT-ASIA 2012), Tianjin, China, May 21-24, 2012.

(6) A. Ewais, J. Liang, N. Jenkins, “The Impact of the Fixed Speed Induction Generator-Based Wind Turbines on Power system stability,” will be submitted to IET journal.