Abdel-Aty Edris

Enhancement of first-swing stability using a high-speed phase shifter

The static phase shifting transformer (SPST) is one of the potential options of the recently presented concept of flexible AC transmission systems (FACTS). An application of the SPST is presented for solving the transient stability problem of a generator connected to a power system. The control algorithm of the SPST is conceptualized to prevent loss of synchronism by extending the capability of a generator to produce sufficient decelerating energy to counterbalance the accelerating energy gained during a fault. The results of simulation studies confirm the validity of the theory on which the control algorithm is based. Phase shifting control not only prevents loss of synchronism but also damps the subsequent swings. >

A common modeling framework of voltage-sourced converters for load flow, sensitivity, and dispatch analysis

This paper discusses the use of injected shunt and series voltage sources to model voltage-sourced converter (VSC)-based flexible ac transmission systems (FACTS) controllers such as the unified power-flow controller (UPFC) and the interline power-flow controller (IPFC). Compared to traditional power network models including only shunt voltage injections, the inclusion of series voltage injections provides a new capability of power system models and offers a common modeling framework in which a Newton-Raphson load-flow solution and a network sensitivity analysis can be readily developed. The sensitivities can be used for optimal dispatch of FACTS controllers.

Series compensation schemes reducing the potential of subsynchronous resonance

New series compensation schemes that reduce the potential of synchronous resonance problems in series compensated electrical power systems are presented. The schemes are phase-wise compensation schemes with three different combinations of inductive and capacitive elements. The combinations chosen have frequency characteristics that give equal reactances at the power frequency and unequal reactances at the other frequencies. This implies that the subsynchronous oscillations drive unsymmetrical three-phase currents. The developed electric torque, therefore, will be lower than what it would be if the three-phase currents were symmetrical. This results in a lower coupling strength between the mechanical and electrical systems at asynchronous oscillations. Therefore, the energy exchange between the electrical and mechanical systems at subsynchronous oscillations will be suppressed, avoiding the build-up of torsional stresses on the generator shaft systems under subsynchronous resonance conditions. The validity and effectiveness of the schemes have been demonstrated on the Second IEEE Benchmark Model, by means of time simulation analysis using the EMTP program. >

Reliability Evaluation of an HVDC Transmission System Tapped by a VSC Station

Reliability evaluation of multiterminal HVDC systems has received very little attention in the past. This paper presents reliability modeling and analysis of an HVDC transmission system incorporating a voltage-sourced converter (VSC) tapping station. The use of VSC tapping stations enables the supply of power along the route to areas with comparatively little consumption. In this paper, a comprehensive detailed reliability model is developed and then converted to a manageable and computationally efficient model. Using this equivalent reliability model, various reliability indices are calculated at the load point of the system and the impacts of the VSC tapping station on these indices are illustrated. Sensitivity analyses are conducted to investigate the impacts of the load level and the location of the tapping station on the reliability indices. The conducted studies are numerically applied on a typical HVDC system and thorough discussions are presented.

Enhancement of Power System Dynamics Using a Phase Imbalanced Series Compensation Scheme

The paper presents a new series capacitive compensation concept, which enhances power system dynamics. The idea behind the concept is a series capacitive compensation which provides balanced compensation at the power frequency while it results in phase imbalance at other frequency of oscillations. The compensation scheme is a combination of a single-phase thyristor controlled series capacitor (TCSC) and fixed series capacitors in series in one phase of the compensated transmission line and fixed capacitors on the other two phases.

Coordinated Control of Wind Energy Conversion Systems for Mitigating Subsynchronous Interaction in DFIG-Based Wind Farms

The paper presents methods for mitigating subsynchronous interaction (SSI) between doubly fed induction generator (DFIG) based wind farms and series capacitor compensated transmission systems. SSI damping is achieved by introducing a supplemental control signal in the reactive power control loop of the grid side converter of DFIG and full-scale frequency converter wind turbines, as well as in the reactive power control loop of the HVDC onshore multimodule converter (MMC) of offshore wind farms. This paper also investigates the impact of the phase imbalance series capacitive compensation concept that was introduced in the 1990s as a subsynchronous resonance countermeasure on SSI damping. The validity and effectiveness of the proposed methods are demonstrated on a test benchmark through time domain simulation studies using the ElectroMagnetic Transient Program (EMTP-RV).

Damping Inter-Area Oscillations Using Phase Imbalanced Series Compensation Schemes

The recently proposed phase imbalanced series capacitive compensation concept has been shown to be effective in enhancing power system dynamics as it has the potential of damping power swing as well as subsynchronous resonance oscillations. In this paper, a comparative effectiveness of two “hybrid” series capacitive compensation schemes in damping inter-area oscillations is evaluated. A hybrid scheme is a series capacitive compensation scheme, where two phases are compensated by fixed series capacitor (C) and the third phase is compensated by either a TCSC or SSSC in series with a fixed capacitor (Cc). The effectiveness of both schemes in damping inter-area oscillations for various network conditions, namely different system faults and tie-line power flows is evaluated using the EMTP-RV time simulation program.

An SSSC-Based Hybrid Series Compensation Scheme Capable of Damping Subsynchronous Resonance

The recently proposed phase-imbalanced series capacitive compensation concept has been shown to be effective in enhancing power system dynamics since it has the potential to damp power swings as well as subsynchronous resonance oscillations. In this paper, the series capacitive compensation concept is investigated for damping subsynchronous resonance oscillations using a static synchronous series compensator (SSSC)-based hybrid series-capacitive compensation scheme. In this scheme, the series capacitive compensation in one phase is created by using a single-phase SSSC in series with a fixed capacitor , and the other two phases are compensated by the fixed series capacitors (C). The validity and effectiveness of the proposed scheme are evaluated on the IEEE first and second benchmark models for computer simulation of subsynchronous resonance by means of time-domain simulation analysis using the Electromagnetic Transients Program-RV program.

Controllable Var Compensator: A Potential Solution to Loadability Problem of Low Capacity Power Systems

The stability of the voltage at the receiving end of radial transmission links puts a limit on the amount and character of load that can be connected at that end. The paper demonstrates the potential of having a static var compensator SVC for supporting the voltage at the load end and therefore increasing the loadability of such links. Radial links of different transmission voltages and lengths as well as loads of different characters have been considered in showing the effectiveness of SVC. A general formula defining the sensitivity of receiving end voltage to shunt compensation has been developed. With the aid of this formula the regulator gain of the used SVC control model has been dimensioned and the steady state loadability limits of radial transmission links, at different load character, can be established analytically.

Subsynchronous resonance countermeasure using phase imbalance

Since the discovery in 1970 that subsynchronous resonance (SSR) was the main cause of the shaft failures at the Mohave plant in Southern Nevada, extensive research and development efforts have been devoted to the development of effective SSR mitigation measures. This paper presents a contribution to these efforts by providing a new concept for simple and reliable SSR mitigation. The basic idea is to reduce the energy exchange between the two sides of turbine-generator sets at subsynchronous oscillations by weakening the electromechanical coupling. This is attained by creating phase imbalance at subsynchronous oscillations. The imbalance diminishes the capability of the three phase currents to develop interacting electromagnetic torques and, therefore, suppresses energy exchange between the electrical and mechanical sides of turbine-generator sets. The objective of this paper is to study the influence of having the phase imbalance associated with the generator to be protected from potential torsional interaction problems. >