Search

In this paper, an analytical framework for minimizing losses and harmonics is presented for the cascaded-type-based multilevel static synchronous compensator (STATCOM) with 'square-wave control.' The focus lays on low losses, low-voltage total harmonic distortion (THD), and low dc voltage ripple. All of the system parameters are studied, including buffer reactance, dc capacitance, transformer ratio, and transformer leakage inductance. The equivalent capacitance concept is used to represent the complex structure of the multilevel converter. It enables studies of the internal STATCOM resonances which have a significant influence on triplen harmonics and losses. The interactions between system parameter are investigated and analytical formulae for optimal parameters are derived. Detailed PSCAD digital simulations are used to confirm the analytical formulae. The summarized design steps are offered that are recommended to obtain optimum STATCOM system parameters. Index Terms--Harmonic distortion, multilevel converter, resonances, static synchronous compensator (STATCOM), static VAR compensators.

This paper develops a frequency-dependent analytical representation for a multilevel cascaded converter, with fundamental frequency switching. The gain and phase angle are studied with the firing angle as a parameter, where the converter is subjected to oscillatory excitation at a range of frequencies. This paper presents analytical formulae for a single cell and for a generic cascade multilevel converter where the number of cells is a parameter. Also, the formula for fundamental frequency studies is separately developed. The equivalent capacitance concept is introduced and its convergence to a constant value for high frequencies is demonstrated. The analytical formulae are tested against detailed digital simulation and good matching is reported. The formulae developed in this study will be of particular importance in studying interactions between a multilevel STATCOM and the host ac grid (similar to harmonic resonance) and in studying converter transient responses. Index Terms--Frequency response, multilevel converter, static VAR compensators.

This paper presents a novel converter configuration, based solely on conventional thyristors and aimed for the use with high voltage dc (HVDC) transmission. The converter utilizes resonant turn off and forced commutation with auxiliary thyristors to aid commutation in the converter switches. The PSCAD/EMTDC simulation confirms that the HVDC inverter is capable of operating with a wide range of firing angles, including operation with reactive power export. Further simulation proves that the system is immune to commutation failure even in the case of most severe close single-phase faults. The harmonic generation is comparable to the conventional converter and also the thyristor voltage stress is not significantly increased. The auxiliary thyristors do increase costs, but this is offset by the elimination of reactive power support and the improvements in performance. Index Terms--Commutated circuits, HVDC converters, HVDC transmission, reactive power.

This paper studies the use of a voltage-source converter (VSC) with dc capacitors as an energy storage medium for the compensation of pulsating active and reactive power of the European Organization for Nuclear Research's (CERN's) proton synchrotron (PS) particle accelerator. The PS accelerator load demands periodic, active, and reactive power pulses of about 2-s duration and a magnitude of up to 45 MW and 65 Mvar. The proposed compensator is able to control both reactive and active power exchange with the network in order to eliminate network disturbances. The controllability study reveals that the best control strategy is to use a q-axis converter input for active power and a d-axis input for reactive power regulation. An analytical system model is created to study the system dynamics and to aid the controller design. The eigenvalue study with the MATLAB model reveals that with large energy storage units and small converter losses, there is only a small interaction between the control channels. The final testing is done with a detailed nonlinear model in PSCAD/EMTDC. The simulation results show that it is possible to fully compensate the active power exchange with the network during typical accelerator cycles and, at the same time, to achieve excellent ac voltage control. Index Terms--Accelerator power supply, eigenvalues, energy storage, proton accelerators, pulsewidth-modulated power converters, root loci, state-space methods.

This paper presents an analytical, linear, state-space model of a thyristor-controlled series capacitor (TCSC). First, a simplified fundamental frequency model of TCSC is proposed and the model results are verified. Using frequency response of the nonlinear TCSC segment, a simplified nonlinear state-space model is derived, where the frequency of the dominant TCSC complex poles shows linear dependence on the firing angle. The nonlinear element is linearised and linked with the ac network model and the TCSC controller model that also includes a phase-locked-loop (PLL) model. The model is implemented in MATLAB and verified against PSCAD/EMTDC in the time and frequency domains for a range of operating conditions. The model is sufficiently accurate for most control design applications and practical stability issues in the subsynchronous range. Index Terms--Modeling, power system dynamic stability, state-space methods, thyristor converters.

This paper presents a linear state-space model of a static VAR compensator. The model consists of three individual subsystem models: an ac system, a SVC model, and a controller model, linked together through d--q transformation. The issue of nonlinear susceptance-voltage term and coupling with a static frame of reference is resolved using an artificial rotating susceptance and linearizing its dependence on firing angle. The model is implemented in MATLAB and verified against PSCAD/EMTDC in the time and frequency domains. The verification demonstrates very good system gain accuracy in a wide frequency range f < 150 Hz, whereas the phase angle shows somewhat inferior matching above 25 Hz. It is concluded that the model is sufficiently accurate for many control design applications and practical stability issues. The model's use is demonstrated by analyzing the dynamic influence of the PLL gains, where the eigenvalue movement shows that reductions in gains deteriorate system stability. Index Terms--Modeling, power system dynamic stability, state space methods, static VAR compensators, thyristor converters.

This research addresses the special requirements of phase locked loops (PLLs) for a typical application with FACTS elements. A new PLL system that uses adaptation algorithms is developed with the aim of improving speed of responses, robustness to AC voltage depressions, and harmonic rejection. The adaptive PLL consists of the three control units that individually control frequency, phase angle, and voltage magnitude. The voltage controller output is used to compensate for reduced gain caused by the ac voltage magnitude depressions. The output phase angle and its derivative, the frequency signal, are controlled in two independent control systems in order to enable elimination of frequency and phase error without compromising transient responses. The simulation results are compared with a PLL available with the PSB MATLAB block-set and noticeable improvements are demonstrated. In particular, settling time and overshooting are significantly lower with conditions of reduced ac voltage magnitude. Index Terms--Frequency locked loops, modeling, phase-locked loops, power system control, thyristor circuits, tracking.

This paper presents a small signal analysis of interactions between HVAC and HVDC systems. The eigenvalue analysis, along with analysis of participation factors, is firstly described. The eigenvalue sensitivity analysis shows how the changes of AC system parameters influence the stability of the considered HVDC-HVAC system. The influence of SCR changes on both the rectifier and the inverter AC system, is studied by examining the relative movement of the system eigenvalues. The most important conclusions about AC-DC interactions are obtained by investigating the nature of inherent feedback loops between the systems. By examining the changes of all interaction variables, it is determined which of the interaction variables should be controlled and which are better be left uncontrolled. Keywords: HVDC transmission, Power system dynamic stability, State space methods, Eigenvalues/eigenfunctions.

A procedure for designing a novel HVDC current controller is presented. The proposed controller makes HVDC system performance less prone to fundamental frequency oscillations on a DC line, and thus less susceptible to the second harmonic instability. The design is based on a linearised AC/DC system model and on the pole-placement technique. Controller performance is tested against the CIGRE HVDC Benchmark model using the EMTDC/PSCAD simulation package. It is demonstrated that this controller eliminates the dominant oscillatory mode on DC side, and that it gives a significant improvement in system response even for low SCR AC systems. Keywords: HVDC transmission control, Power system dynamic stability, State space methods, Eigenvalues/eigenfunctions.

A new HVDC-HVAC analytical model is presented in this paper. The model comprises of three subsystems: AC system, PLL and DC system. The model is structured in such a manner to enable small signal analysis of HVDC-HVAC interactions and possible problems arising from these interactions. CIGRE HVDC Benchmark model is used as a test system. Model verification, performed using PSCAD/EMTDC simulations, showed good response matching for all DC and AC system variables. As an example of application of this model the influence PLL dynamics on the system stability is studied offering the important rules for tuning of PLL gains. Keywords: HVDC Transmission, Power system modelling, Phase locked loops, State space methods.

This paper presents a comparison of performance of traditionally used Synchronous Condensers (SC's) at inverter side HVDC terminal and a novel inverter control method. The novel controller has earlier demonstrated good responses with very weak inverter AC systems and widely changing AC system parameters. The aim of this research is to study if it is possible to offer adequate replacement for SC at weak HVDC -HVAC interconnection point. The simulation results on an actual HVDC system show excellent small signal responses, but the main obstacle for the application of the new control method could be increased probability of commutation failure. Subsequent tests show that in order to keep the same commutation failure probability, half of the SC must be maintained in normal operation. Further SC can also be disconnected, when the new control method is employed, however the nominal operating angle has to be somewhat increased. The cost benefit study is performed to analyze the financial benefits of the proposed project. It is concluded that, although the benefits are evident for the new HVDC links, the proposed replacement of SC may not be justifiable for the existing links. Index Terms--HVDC transmission control, power system dynamic stability.