Novel PV Solar Farm Control as STATCOM (PV-STATCOM) for SSR Mitigation in Synchronous Generators and Wind Farms

Transmission lines across the world are extensively compensated by series capacitors to increase their power transfer capacity. However, series compensation can potentially cause subsynchronous resonance (SSR) which can lead to significant damage in shafts of synchronous generators and wind farms. Dynamic reactive power compensators such as STATCOM and SVCs are typically used for mitigation of SSR, but these are quite expensive. Large scale PV solar farms are growing at a very rapid rate across the world. It is quite likely that they may get installed near synchronous generators or wind farms which are directly or indirectly interfaced with series compensated lines. For the first time, novel SSR damping controls for the patented technology of PV solar farm controlled as STATCOM (PV-STATCOM) are presented in this thesis for mitigating SSR both during night and day in synchronous generators connected to series compensated lines. The PV-STATCOM successfully mitigates SSR whether the solar farm is located at generator terminals or remotely from it. New SSR alleviation controls are also developed for PV-STATCOM for mitigating SSR in wind farms connected to series compensated networks, during night and day, irrespective of PV-STATCOM location. The SSR damping controls are based on wind farm generator speed and line current, both of which are locally available signals. As soon as a system disturbance is detected, the solar farm discontinues its real power generation and makes available its entire inverter capacity for dynamic reactive power exchange as STATCOM, for SSR mitigation. Once subsynchronous oscillations are damped the solar farm utilizes a novel ramp-up technique according to which SSR damping function is kept activated with remaining inverter capacity during power ramp-up. Such a fast ramp-up technique is not envisaged in Grid Codes. The entire process of SSR damping for all cases studied takes less than half a minute. A novel coordinated control of PV-STATCOM and DFIG based Type 3 wind farms is developed for faster and effective SSR damping, when both are connected to series compensated lines. MATLAB based eigenvalue analysis validated by nonlinear simulations through SIMULINK and PSCAD software confirm the effectiveness of proposed SSR damping controls

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

Dynamics of power systems with wind power generation and the fractional frequency transmission system

Under pressure for low carbon emissions and environmental protection, large scale wind farms are constructed and integrated into power systems to meet energy demands. On the other hand, the long distance transmission for large scale wind power and conventional power plants is another technical issue in modern power systems. These two challenges stimulate the research and development of wind energy and the fractional frequency transmission system (FFTS). Thus, the research of this thesis focuses on the dynamics of power systems with wind power generation and the FFTS. This thesis can be divided into the following three parts: Firstly, the influence of wind farms on the subsynchronous resonance (SSR) of conventional power systems is systematically examined. Both eigenvalue analysis and time domain simulations are conducted to examine the influence of wind farms. Secondly, the FFTS is proposed to deliver the energy from large scale offshore wind farms. The small signal stability of the FFTS with wind farms is studied. To improve the damping performance, a proper controller for the FFTS is also proposed. Thirdly, the FFTS is also applied in grid interconnections. The power flow controller for the FFTS is also proposed in this application

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