Improving small signal stability of power systems in the presence of harmonics

This thesis investigates the impact of harmonics as a power quality issue on the dynamic behaviour of the power systems. The effectiveness of the power system stabilizers in distorted conditions is also investigated. This thesis consists of three parts as follows:The first part focuses on the operation of the power system under distorted conditions. The conventional model of a synchronous generator in the dq-frame of reference is modified to include the impact of time and space harmonics. To do this, the synchronous generator is first modelled in the harmonic domain. This model helps in calculating the additional parts of the generator fundamental components due to the harmonics. Then the Park transformation is used for calculating the modified fundamental components of the synchronous generator in dq axes. The modified generator rotor angle due to the presence of harmonics is calculated and the impact of damper windings under the influence of harmonics is investigated. This model is used to study the small-signal stability of a distorted Single Machine Infinite Bus (SMIB) system. The eigenvalue analysis method is employed and the system state space equations are calculated by linearizing the differential equations around the operating point using an analytical method. The simulation results are presented for a distorted SMIB system under the influence of different harmonic levels. The impact of damper windings and also harmonics phase angles are also investigated.In the second part of the thesis, the effectiveness of the power system damping controllers under distorted conditions is studied. This investigation is done based on a distorted SMIB system installed with a Static Synchronous Series Compensator (SSSC). In the first step, the system state space equations are derived. A Power Oscillation Damping (POD) controller with a conventional structure is installed on the SSSC to improve the system dynamic behaviour. A genetic-fuzzy algorithm is proposed for tuning the POD parameters. This method along with the observability matrix is employed to design a POD controller under sinusoidal and distorted conditions. The impact of harmonics on the effectiveness of the POD controller under distorted conditions is investigated.In the last part, the steady state and dynamic operation of an actual distributed generation system under sinusoidal and distorted conditions are studied. A decoupled harmonic power flow program is employed for steady state analysis. The nonlinear loads are modelled as decoupled harmonic current sources and the nonlinear model of synchronous generator in harmonic domain is used to calculate the injected current harmonics. For the system dynamic stability study, the power system toolbox with the modified model of the synchronous generator is used. The system eigenvalues are calculated and the effectiveness of the installed Power System Stabilisers (PSS) is investigated under sinusoidal and distorted conditions. Simulation results show that in order to guarantee the effectiveness of a PSS in distorted conditions, it is necessary to consider the harmonics in tuning its parameters

Time domain analysis of a synchronous generator in distorted power systems

Calculating the synchronous generator steady state operating conditions and load angle is essential for power system studies. Time and space harmonics can cause variations on power system operation. This paper investigates the impact of time and space harmonics on the synchronous generator inductances, rotor currents, produced stator fluxes and voltages, electromagnetic torque and the steady state rotor angle. Harmonic Domain and abc-frame of reference is used for nonlinear modeling of synchronous generator. The instantaneous electromagnetic torque is calculated using rotational voltage component. Simulation results show that the presence of harmonics can affect the synchronous generator steady state operation and damper windings have considerable impact on the produces stator fluxes and voltages in the presence of harmonics. Matlab/Simulink software and Simpowersystem toolbox are used for the simulation in time domain

Impact of a SG Nonlinear Model on the Harmonic Distortion of a Distribution Generation System

In this paper the impact of a synchronous generators nonlinear model on the voltage distortion of a distributed generation system is investigated. The SGs are modelled in the harmonic domain and include the space harmonics and frequency conversion. A decoupled harmonic power flow program is used for the system harmonic study. The system nonlinear loads are modelled as decoupled harmonic current sources. Simulation results are presented for an actual 24-bus DG system including 6 distributed generators. It is shown that the SGs working under distorted conditions can inject considerable amount of current harmonics and considerably increase the level of system harmonic distortion

Impact of SG Damper Windings on Small-Signal Stability of a Distorted SMIB Including Time and Space Harmonics

This paper investigates the impact of synchronous generator’s damper windings on small-signal stability and dynamic behavior of power systems in the presence of time and space harmonics. A Distorted Single Machine Infinite Bus (DSMIB) system is used for the analysis. A nonlinear model of the synchronous generator (SG) in the Harmonic Domain (HD) is used to include space harmonics. The nonlinear effects of power system components and loads are included as voltage timeharmonics at the terminals of the infinite bus. The state space, HD and eigenvalue analysis are used to model the dynamic behavior of the distorted power system and examine its stability. Simulations are used to highlight the impact of damper windings and harmonics on the stability and power oscillations of the system. It is shown that for some harmonic spectra, power system experiences instability problems

Time Domain Analysis of a Synchronous Generator in Distorted Power Systems

Calculating the synchronous generator steady state operating conditions and load angle is essential for power system studies. Time and space harmonics can cause variations on power system operation. This paper investigates the impact of time and space harmonics on the synchronous generator inductances, rotor currents, produced stator fl uxes and voltages, electromagnetic torque, and the steady state rotor angle. Harmonic domain and abc-frame of reference is used for non-linear modelling of synchronous generators. The instantaneous electromagnetic torque is calculated using rotational voltage component. Simulation results show that the presence of harmonics can affect the synchronous generator steady state operation and damper windings have considerable impact on the produces stator fluxes and voltages in the presence of harmonics. MATLAB/Simulink software and Simpower system toolbox are used for the simulation in time domain