Enrique Acha

Advanced SVC models for Newton-Raphson load flow and Newton optimal power flow studies

Advanced load flow models for the static VAr compensator (SVC) are presented in this paper. The models are incorporated into existing load flow (LF) and optimal power flow (OPF) Newton algorithms. Unlike SVC models available in open literature the new models depart from the generator representation of the SVC and are based instead on the variable shunt susceptance concept. In particular, a SVC model which uses the firing angle as the state variable provides key information for cases when the load flow solution is used to initialize other power system applications, e.g., harmonic analysis. The SVC state variables are combined with the nodal voltage magnitudes and angles of the network in a single frame-of-reference for a unified, iterative solution through Newton methods. Both algorithms, the LF and the OPF exhibit very strong convergence characteristics, regardless of network size and the number of controllable devices. Results are presented which demonstrate the prowess of the new SVC models.

A comprehensive Newton-Raphson UPFC model for the quadratic power flow solution of practical power networks

A new and comprehensive load flow model for the unified power flow controller (UPFC) is presented in this paper. The model is incorporated into an existing Newton-Raphson load flow algorithm. Unlike existing UPFC models available in open literature, it can be set to control active and reactive powers and voltage magnitude in any combination or to control none of them. A set of analytical equations has been derived to provide good UPFC initial conditions. Hence, the algorithm exhibits quadratic or near-quadratic convergence characteristics. Suitable guidelines are suggested for an effective control coordination of two or more UPFCs operating in series or parallel arrangements. Test results are presented which demonstrate the effectiveness of the new model.

A thyristor controlled series compensator model for the power flow solution of practical power networks

A new and comprehensive load flow model for the thyristor controlled series compensator (TCSC) is presented in this paper. In this model the state variable is the TCSCs firing angle, which is combined with the nodal voltage magnitudes and angles of the entire network in a single frame-of-reference for a unified iterative solution through a Newton-Raphson method. Unlike TCSC models available in the open literature, this model takes account of the loop current that exists in the TCSC under both partial and full conduction operating modes. Also, the model takes proper care of the resonant points exhibited by the TCSC fundamental frequency impedance. The Newton-Raphson algorithm exhibits quadratic or near-quadratic convergence characteristics, regardless of the size of the network and the number of TCSC devices.

Modeling of VSC-Based HVDC Systems for a Newton-Raphson OPF Algorithm

The paper presents the model of a voltage source converter -high voltage direct current (VSC-HVDC) suitable for optimal power flow (OPF) solutions using Newtons algorithm. The VSC-HVDCs ability to provide independent control of the converters ac voltage magnitudes and phase angles relative to the system voltage, which allows the use of separate active and reactive power control loops for system regulation, is well represented by the model. In this new development in Newton OPF, the VSC-HVDC system equations are incorporated directly into the matrix W for a unified optimal solution in a single frame-of-reference. The multipliers method is used to handle all inequality constraints of variables, leading to highly efficient OPF solutions of constrained power networks. The solution approach does not require structural changes in the linearised system of equations during the iterative process using Newtons method. The effectiveness of the VSC-HVDC model and its proposed implementation in Newton OPF is demonstrated by means of two sample systems.

A New VSC-HVDC Model for Power Flows Using the Newton-Raphson Method

The paper presents a new model of the VSC-HVDC aimed at power flow solutions using the Newton-Raphson method. Each converter station is made up of the series connection of a voltage source converter (VSC) and its connecting transformer which is assumed to be a tap-changing (LTC) transformer. The new model represents a paradigm shift in the way the fundamental frequency, positive sequence modeling of VSC-HVDC links are represented, where the VSCs are not treated as idealized, controllable voltage sources but rather as compound transformer devices to which certain control properties of PWM-based inverters may be linked - just as DC-to-DC converters have been linked, conceptually speaking, to step-up and step-down transformers. The VSC model, and by extension that of the VSC-HVDC, takes into account, in aggregated form, the phase-shifting and scaling nature of the PWM control. It also takes into account the VSC inductive and capacitive reactive power design limits, switching losses and ohmic losses.

A New STATCOM Model for Power Flows Using the Newton–Raphson Method

The paper presents a new model of the STATCOM aimed at power flow solutions using the Newton-Raphson method. The STATCOM is made up of the series connection of a voltage-source converter (VSC) and its connecting transformer. The VSC is represented in this paper by a complex tap-changing transformer whose primary and secondary windings correspond, notionally speaking, to the VSCs ac and dc buses, respectively. The magnitude and phase angle of the complex tap changer are said to be the amplitude modulation index and the phase shift that would exist in a PWM inverter to enable either reactive power generation or absorption purely by electronic processing of the voltage and current waveforms within the VSC. The new STATCOM model allows for a comprehensive representation of its ac and dc circuits-this is in contrast to current practice where the STATCOM is represented by an equivalent variable voltage source, which is not amenable to a proper representation of the STATCOMs dc circuit. One key characteristic of the new VSC model is that no special provisions within a conventional ac power flow solution algorithm is required to represent the dc circuit, since the complex tap-changing transformer of the VSC gives rise to the customary ac circuit and a notional dc circuit. The latter includes the dc capacitor, which in steady-state draws no current, and a current-dependent conductance to represent switching losses. The ensuing STATCOM model possesses unparalleled control capabilities in the operational parameters of both the ac and dc sides of the converter. The prowess of the new STATCOM power flow model is demonstrated by numerical examples where the quadratic convergence characteristics of the Newton-Raphson method are preserved.

An Advanced STATCOM Model for Optimal Power Flows Using Newton's Method

This paper presents the optimal power flow (OPF) formulation of a recent power flow STATCOM model . The new model puts forward an alternative, insightful interpretation of the fundamental frequency operation of the PWM-controlled voltage source converter (VSC), in an optimal fashion. The new model makes provisions for the explicit representation of the converters internal ohmic and switching losses which in the context of an OPF formulation, yields an optimum operating point at which these power losses are at a minimum. The STATCOM model possesses unparalleled control capabilities in the operational parameters of both the AC and DC sides of the converter. Such control modeling flexibility is at its best when expressed in the context of an OPF solution using Newtons method. The STATCOM equations are incorporated into the OPF formulation using Lagrangian functions in quite a natural manner for efficient optimal solutions using a single frame-of-reference. The inequality constraint set of variables is handled equally well using the multipliers method. The prowess of the new model is demonstrated using two sample systems.

A Novel STATCOM Model for Dynamic Power System Simulations

This paper introduces an advanced model of the STATCOM suitable for steady-state and dynamic simulations of large-scale power systems. It allows for a comprehensive representation of the STATCOMs AC and DC circuits-this is in contrast to current practice where the STATCOM is represented using an equivalent variable voltage source which is not amenable to a proper representation of its DC circuit. The new STATCOM model comprises a voltage source converter (VSC) in series with an LTC transformer. The former is represented by a complex tap-changing transformer whose primary and secondary windings would correspond, in a notional sense, to the VSCs AC and DC buses, respectively. The magnitude and phase angle of the complex tap changer correspond to the amplitude modulation index and the phase shift that would exist in a PWM inverter to enable either reactive power generation or absorption purely by electronic processing of the voltage and current waveforms within the VSC. The numerical technique employed to solve the STATCOM model is the Newton-Raphson method for both operating regimes, the steady-state and the dynamic-state. The latter involves discretization of the STATCOMs and synchronous generators differential equations so that the nonlinear algebraic equations and the discretized differential equations are linearized around a base operating point and assembled together in a unified frame-of-reference for robust iterative solutions.

Fundamental analysis of the static VAr compensator performance using individual channel analysis and design

In this paper the performance of a synchronous generator x96 SVC system is evaluated using Individual Channel Analysis and Design (ICAD), a control-oriented framework suitable for small-signal stability assessments. The SVC is already a mature piece of technology, which has become very popular for providing fast-acting reactive power support. The great benefits of ICAD in control system design tasks are elucidated. Fundamental analysis is carried out explaining the generator dynamic behavior as affected by the SVC. A multivariable control system design for the system is presented, with particular emphasis in the closed-loop performance and stability and structural robustness assessment. It is formally shown in the paper that although the addition of the SVC with no damping control loop does not improve the dynamic of the system, its inclusion is very effective in enhancing voltage stability. Moreover, ICAD analysis shows that with the use of the SVC the dynamical structure of the system is preserved and no considerable coupling or adverse dynamics are added to the plant.

Transmission line model with frequency dependency and propagation effects: A model order reduction and state-space approach

A new and comprehensive transmission line model for the study of power systems electro-magnetic transients based on a highly effective model order reduction, is presented in this paper. The model takes the form of a state-space representation and differs fundamentally from the well-known family of transmission line model based on the wave-travelling concept. The core principles of the model order reduction procedure are the application of singular approximations, balancing-free square-roots and the computation of projection matrices. Grammian factors in combination with a Cholesky factorization are applied to the solution of the Lyapunov equations and, hence, to the calculation of the projection matrices. Commensurate with state-of-the-art transmission line models for the study of electromagnetic transients, the new model includes full frequency dependency, propagation effects and geometric imbalances.While its frequency dependency characteristic is modelled using a set of parallel RL branches, the long-line effects are incorporated by discretizing its distributed parameters with respect to the line distance. Newtonpsilas method is used to fit the set of parallel RL branches and Singular Value Decomposition is employed to solve cases where the Jacobian becomes ill-conditioned. The effectiveness of the proposed transmission line model is demonstrated using a non-transposed version of the Jaguara-Taquaril three-phase transmission line system.