Damping subsynchronous resonance using supplementary controls around the static synchronous series compensator.

Masters Degree. University of KwaZulu-Natal, Durban.The demand for electric power increases rapidly with the growth in human population whereas expansion of existing power transmission infrastructure is restrained by difficulties in obtaining rights of way, resource scarcity and environmental policies inter alia. This has called for better utilization of existing transmission facilities which, for many years has been achieved through series compensation of transmission lines using conventional series capacitor banks. However, during major system disturbances, these conventional series capacitors weaken the damping of torsional oscillations in the neighboring turbine-generator shafts, which may lead to the failure and damage of the shafts concerned; a phenomenon known as subsynchronous resonance (SSR). Alternative means of series compensation using high-speed semiconductor switches has been realized following introduction of Flexible AC Transmission Systems (FACTS) in power systems. This research work focuses on damping of SSR using damping controls around the second-generation series device of the FACTS family namely the static synchronous series compensator (SSSC). The SSSC is designed to inject voltage in series with the transmission line and in quadrature with line current to emulate capacitive reactance in series with the transmission line. In this research work, a model of the SSSC is developed in Power System Computer Aided Design (PSCAD) and the IEEE First Benchmark Model (FBM) is used for SSR analysis. Initially, the resonant characteristics of the SSSC compensated transmission line is studied to determine whether this device has a potential to excite SSR on its own. The results confirm earlier work by other researchers using a detailed model of the SSSC, showing that introduction of a SSSC can indeed excite SSR, although not to the same extent as conventional series capacitors. The research work then considers the addition of supplementary damping controllers to the SSSC to add positive damping to subsynchronous oscillations caused by the SSSC itself as well as by a combination of conventional series capacitors and a SSSC in the IEEE FBM. Finally, the research work considers a more complex transmission system with an additional transmission line that incorporates conventional series capacitors. Time-domain simulation results and Fast Fourier Transform analyses show that a damping controller around the SSSC can be used to mitigate SSR either due to the SSSC itself, or due to conventional series capacitors in the same line as the SSSC or due to conventional series capacitors in an adjacent line of an interconnected transmission network

Damping subsynchronous resonance oscillations using a VSC HVDC back-to-back system

A problem of interest in the power industry is the mitigation of severe torsional oscillations induced in turbine-generator shaft systems due to Subsynchronous Resonance (SSR). SSR occurs when a natural frequency of a series compensated transmission system coincides with the complement of one of the torsional modes of the turbine-generator shaft system. Under such circumstances, the turbine-generator shaft system oscillates at a frequency corresponding to the torsional mode frequency and unless corrective action is taken, the torsional oscillations can grow and may result in shaft damage in a few seconds. This thesis reports the use of a supplementary controller along with the Voltage Source Converter (VSC) HVDC back-to-back active power controller to damp all SSR torsional oscillations. In this context, investigations are conducted on a typical HVAC/DC system incorporating a large turbine-generator and a VSC HVDC back-to-back system. The generator speed deviation is used as the stabilizing signal for the supplementary controller. The results of the investigations conducted in this thesis show that the achieved control design is effective in damping all the shaft torsional torques over a wide range of compensation levels. The results and discussion presented in this thesis should provide valuable information to electric power utilities engaged in planning and operating series capacitor compensated transmission lines and VSC HVDC back-to-back systems

Integration of offshore wind farms through High Voltage Direct Current networks

The integration of offshore wind farms through Multi Terminal DC (MTDC) networks into the GB network was investigated. The ability of Voltage Source Converter (VSC) High Voltage Direct Current (HVDC) to damp Subsynchronous Resonance (SSR) and ride through onshore AC faults was studied. Due to increased levels of wind generation in Scotland, substantial onshore and offshore reinforcements to the GB transmission network are proposed. Possible inland reinforcements include the use of series compensation through fixed capacitors. This potentially can lead to SSR. Offshore reinforcements are proposed by two HVDC links. In addition to its primary functions of bulk power transmission, a HVDC link can be used to provide damping against SSR, and this function has been modelled. Simulation studies have been carried out in PSCAD. In addition, a real-time hardware-in-the-loop HVDC test rig has been used to implement and validate the proposed damping scheme on an experimental platform. When faults occur within AC onshore networks, offshore MTDC networks are vulnerable to DC overvoltages, potentially damaging the DC plant and cables. Power reduction and power dissipation control systems were investigated to ride through onshore AC faults. These methods do not require dedicated fast communication systems. Simulations and laboratory experiments are carried out to evaluate the control systems, with the results from the two platforms compared

Operation and control of voltage source converters in transmission networks for AC system stability enhancement

The rapid expansion in power transmission for the integration of large-scale renewables is foreseen in the future. This will be complemented by infrastructure reinforcements in the form of series compensation and high-voltage direct current (HVDC) links. These changes will bring forth new operability challenges to grid operators. The stability issues pertained to such reinforcements: potential threat of subsynchronous resonance (SSR) and frequency regulation will be investigated in this thesis. Utilising the existing and future voltage source converters (VSC) based HVDC links to support the AC system by proving ancillary services will be of significant importance in the coming decades. The research work presented in this thesis is aimed to address these challenges, in particular, the technical barriers associated with AC/DC interaction and to propose measures to avoid any potential instability. The main contributions of this research work comprise of four parts, namely, (1) analysis of interactions in-terms of SSR in AC/DC grids, (2) design of SSR damping (SSRD) controllers, (3) experimental demonstration of SSRD schemes, and (4) assessment and improvement of frequency regulation in a wind-thermal bundled AC/DC grid. An VSC-HVDC connected series-compensated AC system resembling the Great Britain (GB) power system has been used as the test network to evaluate the operability challenges pertained to the reinforcements. A state-space representation has been formulated and an eigenvalue analysis has been performed to assess the impact of VSC-HVDC on the torsional modes of nearby connected thermal generation plants. This is followed by damping torque investigation for SSR screening with the results compared against time-domain simulations for testing the accuracy of the small-signal models for SSR studies. A series of SSRD schemes is presented which have been integrated with the VSC-HVDC to damp SSR in the series-compensated GB power system. In addition, this thesis proposes an adaptive SSRD method based on the real-time estimation of the subsynchronous frequency v Abstract component present in series-compensated transmission lines–key information for the optimal design of HVDC subsynchronous damping controllers. Furthermore, the combined AC/DC GB network has been implemented in a real-time digital simulator and connected to a VSCHVDC scaled-down test-rig to performhardware-in-the-loop tests. The efficacy and operational performance of the AC/DC network while providing SSR damping is tested through a series of experiments. In order to provide frequency support in a wind-thermal bundled AC/DC system a dualdroop controlmethod is presented. The scheme binds the system frequency with the DC voltage of an HVDC network. For completeness, the performance of the proposed method is compared to conventional frequency regulation schemes. Sensitivity studies and eigenvalue analyses are conducted to assess the impact that wind penetration and changes in the dual-droop coefficient have on grid stability. Experimental validation is performed using a real-time hardware-inthe- loop test-rig, with simulation and experimental results showing a good agreement and evidencing the superior performance of the proposed frequency support scheme

Stability analysis of VSCs connected to an AC grid

As a typical renewable energy resource, wind power has been extensively exploited in the past years. With the continuous growth of wind power installed capacity, power electronic devices are widely used due to their good control performances. However, the interaction between power electronic devices and the power grid will lead to power system stability problems. Subsynchronous interaction (SSI) between wind farms and AC grids, as one of the most severe power system stability problems, has aroused great concerns. Different from the SSI between wind turbine generator (WTG) controllers and fixed series compensation or between generators and high voltage direct current (HVDC) controllers, recently, a new type of SSI is detected which is caused by the interactions between power electronic devices of WTGs and weak AC grid. The work in this thesis focuses on the mechanism and characteristics of this new type of SSI. A simplified system model with permanent magnet synchronous motors (PMSGs) connected to weak AC grids is established to investigate the new SSI. The linear impedance model of the studied system is conducted. The correctness of the proposed impedance model is validated by the comparison between the analysis in MATLAB and time-domain simulations in PSCAD/EMTDC. In the model of a single VSC connected to an AC grid, the mechanism of the SSI is investigated. The characteristics of the studied system under various conditions are analysed. The effects on system stability of different factors have been studied including the number of connected WTGs line reactance, Phase-locked loop (PLL), feed-forward voltage low-pass filter, current loop and outer loop of the VSCs. Time-domain simulation results verify the correctness of the analysis. An equivalent model of multiple VSCs connected to an AC grid is presented to investigate the characteristics when VSCs with different control systems and control parameters are connected to an AC grid. A new approach based on the Generalised Nyquist Criterion (GNC) is proposed to analyse the system stability. Compared to the existing traditional criteria, the new criterion has the advantages of better accuracy and simplicity. The new approach is validated in time-domain simulation. The study of this research work contributes to the stability analysis of power systems in the subsynchronous frequenc

The effect of wind turbines on subsynchronous resonance

With the rapid growth of the penetration of wind power into the power system, fixed series compensation is considered as an economic solution to increase power transfer capability. This will render the power system vulnerable to Sub-Synchronous Resonance (SSR). This thesis conducts research on the effect of wind turbines represented by Fixed Speed Induction Generator-Based Wind Turbines (FSIG-WTs) and Fully Rated Converter-Based Wind Turbines (FRC-WTs) on damping SSR. Firstly, SSR is investigated through mathematically modelling IEEE First Benchmark Model (FBM) using MATLAB package. Modal analysis is used to study SSR over a wide range of series compensation percentages. Secondly, the effect of incorporating FSIG-WTs into FBM on SSR is studied over a wide range of series compensation percentage and different power size of FSIG-WTs. Furthermore, the ability of the grid-side converters of the FRC-WTs connected with the FBM to damp SSR occurrence in the steam turbine shafts is evaluated using two different types of control. An optimal controller based on a Linear Quadratic regulator (LQR) has been designed as an auxiliary controller of the grid-side converter of FRC-WTs. A full-order observer was designed to estimate the unmeasured state variables to enable a vii full-state feedback. Finally, eigenvalue sensitivity was studied to choose the most suitable feedback signal for an SSR damping controller. Lead/Lag compensation controller based on the residue method is designed as an auxiliary controller within the grid-side converters of FRC-WTs. Eigenvalue analysis and time domain simulations over widely varying levels of series compensation have been carried out. The simulation studies were carried out in MATLAB and PSCAD. Connecting FSIG-WTs to the FBM increases the range of series compensation level at which SSR can occur. Therefore, it was shown that FSIG-WTs have an adverse effect on the SSR occurring at the multi-mass synchronous generator. If the system is visible, LQR as an auxiliary damping controller within the grid-side converters of FRC-WTs is an effective controller to damp SSR over a wide range of series compensation percentages. Based on eigenvalue sensitivity technique, synchronous generator speed deviation is the most suitable feedback signal for damping SSR occurrence in the steam turbine shafts