Search

The objective of this work is to design a damping controller for a thyristor controlled series capacitor (TCSC) to damp robustly inter-area oscillations in power systems with an immunity against sub-synchronous resonance (SSR) oscillations which may lead to torsional oscillations. The new control strategy has two main loops; an SSR mitigation loop and a bang-bang loop, the latter is designed with the aim of damping inter-area oscillations with a settling time 8-10 sec. The appropriate selection of the bang-bang series compensation component, ∆KC, is addressed by considering the Eigen analysis of the generators’ shafts and an impedance scan of the series compensated line for different compensation levels. The SSR mitigation loop is designed with the aim of providing a fine tune control signal to be added to the main value of the inserted series compensation (KC), to damp SSR oscillations and related torsional mode of oscillations. To address this issue, a new observer-based multiple model adaptive control algorithm is designed to control a multi-stage TCSC. The SSR modelling challenges associated with the load dynamics and with the insertion of the series compensation into the transmission system are overcome by a fine tuning control loop, which adjusts the resultant series compensation (KC). Considering the integration and coordination of oscillation damping and distance protection in the transmission system, a new adaptive technique must be designed to control the distance relay (DR) to prevent its mal operation (during the damping process). The new strategy is illustrated through an 11-bus 4-machine 2-area benchmark power system. The performance and advantages of the new algorithm are validated using time domain simulation via PSCAD software.

There are significant concerns regarding the stability of weak power grids due to feeders of high impedance. This thesis investigates the stability domain of Squirrel Cage Induction Generator - Wind Turbines (SCIG-WTs) with series compensation and weak interconnections to the power grid. Detailed time-domain and state-space modeling have revealed new bifurcations and oscillatory modes for a SCIG-WT connected radially or non-radially to a weak grid through a series compensated line. The stability domain analyses are carried out by computing bifurcations in the system by analyzing eigenvalues of the linearized system. A Static Var Compensator (SVC) with an improved voltage regulator design has been proposed to improve the stability domain of the radial system with respect to series compensation. The analyses demonstrate for the first time how the degree of compensation at which the Hopf bifurcation occurs depends on the X/R ratio of the line, operating slip of the induction generator, and voltage regulator parameters as well as the time delays in measurements. A damping controller design is proposed which greatly improves the dynamic stability of the WT-SCIG and eliminates destructive Hopf bifurcations in weak grids for a wide range of series compensation. This allows for a much larger percentage series compensation than what is usually possible while avoiding instabilities and thereby maximizing power transfer capability. In addition to analyzing the stability of SCIG-WTs with radial and non-radial interconnections, the stability domains of induction motor loads fed through series compensated feeders have been analyzed. The presence of subcritical Hopf bifurcations resulting in motor stalling and reversal in the rotation of the induction motor has been shown for the first time. The influence of Automatic Voltage Regulator (AVR) parameters on the stability domain of the system with respect to series compensation has been studied. The influence of AVR gain and time constant along with the time constant of the voltage measurement transducer has been analyzed for the first time. The applications of this would help system planners to implement an appropriate level of series compensation so that safe, secure and reliable operation of the system can be achieved.

Subsynchronous oscillations (SSO) were an issue that occurred in the past with conventional generators and were studied extensively throughout the years. However, with the rise of inverter-based resources, a new form of SSO emerged under the name subsynchronous control interaction (SSCI). More specifically, a resonance case occurs between Type-3 wind turbines and series compensation that can damage equipment within the wind farm and disrupt power generation. This work explores the types of SSCI and the various analysis methods as well as mitigation of SSCI. The work expands on the concept of frequency scan to be able to use it in an on-line setting with its output data used to mitigate SSCI through the modification of wind turbine parameters. Multiple frequency scans are conducted using PSCAD/EMTDC software to build a lookup table and harmonic injection is used in a parallel configuration to obtain the impedance of the system. Once the impedance of the system is obtained then the value of the parameters is adjusted using the look-up table. Harmonic injection is optimized through phase shifts to ensure minimal disruption of the steady-state operating point and is conducted using Python programming language with PSCAD Automation Library. Simulation results demonstrate the effectiveness of this approach by ensuring oscillations do not grow exponentially in comparison to the regular operation of the wind farm.

Series compensated lines are protected from overvoltage by metal-oxide-varistors (MOVs) connected in parallel with the capacitor bank. The nonlinear characteristics of MOV devices add complexity to fault analysis and distance protection operation. During faults, the impedance of the line is modified by an equivalent impedance of the parallel MOV/capacitor circuit, which affects the distance protection. The intermittent wind generation introduces additional complexity to the system performance and distance protection. Wind variation affects the fault current level and equivalent MOV/capacitor impedance during a fault, and hence the distance relay operation. This thesis studies the impact of the intermittent wind power generation on the operation of MOV during faults. For the purpose of simulation, an equivalent wind farm model is proposed to generate a wind generation profile using wind farm generation from California independent system operator (ISO) as a guide for wind power variation to perform the study. The IEEE 12-bus test system is modified to include MOV-protected series capacitor and the equivalent wind farm model. The modified test system is simulated in the MATLAB/Simulink environment. The study has been achieved considering three phase and single line to ground (SLG) faults on the series compensated line to show the effect of wind variation on the MOV operation. This thesis proposes an adaptive setting method for the mho relay distance protection of series compensated line considering effects of wind power variation and MOV operation. The distributed parameters of a transmission line are taken into account to avoid overreaching and underreaching of distance relays. The study shows that variable wind power affects system power flow and fault current in the compensated line during a fault which affects the operation of MOVs for different fault conditions. The equivalent per-phase impedance of the MOV/capacitor circuit has an effect on the system operation and line protection. Distance protection study is also performed with variable wind power, different line compensation levels, and other system conditions. Results show that variable wind power affects apparent impedance calculation of distance relay through the variable equivalent MOV/capacitor impedance. Underreaching and overreaching issues of the distance relay are discussed. Based on the results, a variable distance relay setting is proposed to mitigate the negative impact. Both fixed and variable distance relay settings are presented and compared to each other. The results demonstrate the ability of the proposed adaptive setting method to resetting the distance relays to adapt to various system conditions, including three wind generation and different compensation levels.

This thesis deals with the dynamic responses of fixed-speed and variable-speed wind turbines (WTs) in terms of voltage stability and power quality issues using series compensation in both a transmission and distribution network. The main power quality aspects, such as voltage sag, flicker and harmonics, due to uneven power production by intermittent wind sources are presented, with static and dynamic voltage stability analyses under normal and contingency periods conducted. The latest regulations enforced by different utilities on wind power plants are studied and the impacts of power system modelling on stability analysed. Compensating devices, wind farms and loads are considered harmonics generating sources and are modeled according to the international standards. Both short-term and long-term flicker emissions of fixed-speed wind turbine (FSWT) and doubly-fed induction generator (DFIG) based wind farms are presented using series compensation while voltage sag analysis is carried out using failure data. To enhance transmission efficiency series compensation is implemented to utilise full transmission assets by reducing voltage drops in a long transmission line and feeder as most WTs are remotely located to obtain suitable wind conditions, and loss in a transmission line degrades the voltage profile of the network. Like the shunt compensation, series compensation can increase transient voltage stability and, to some extent, contribute to local voltage regulation. The case studies presented in this thesis illustrate the contributions of wind power plants and voltage source converter (VSC) based series compensation on the fault levels of symmetrical and asymmetrical faults. From stability and power quality analyses, the cases studied show that large-scale wind power penetration substantially degrades the voltage stability but that series compensation can enhance the collapse margin and reduce flicker and the sensitivity of bus voltage to reactive power, as well as improve both dynamic and transient voltage stability, including providing effective fault ride-through (FRT) support.