Sahar Pirooz Azad

Damping Inter-Area Oscillations Based on a Model Predictive Control (MPC) HVDC Supplementary Controller

This paper introduces, formulates and evaluates an approach for damping inter-area oscillations of power systems based on supplementary current control of a line-commutated HVDC link. The proposed control is based on the model predictive control (MPC) strategy. The salient feature of the MPC as compared with other optimal control strategies, e.g., the linear quadratic Gaussian (LQG) control, is that it adjusts the control signal to achieve the objectives while explicitly respecting the plant constraints. This paper also compares the performance of the MPC with that of the LQG control. The two approaches are tested on the Western System Coordinating Council (WSCC) 9-bus system and the IEEE 14-bus system. Small-signal disturbance and large-signal disturbance stability studies are performed to demonstrate and compare the performances of the LQG and MPC methodologies. The study results show the effective and superior performance of the MPC for damping poorly damped oscillatory modes of the test systems.

Decentralized Supplementary Control of Multiple LCC-HVDC Links

This paper presents a decentralized wide-area coordinated supplementary control of multiple line-commutated converter (LCC)-HVDC links to 1) prevent interactions among the HVDC links and 2) enhance the damping of the inter-area oscillatory modes. The proposed approach is based on the sparsity-promoting optimal control. The main features of the proposed approach are 1) it requires minimal communication infrastructure to achieve the control objectives and thus reduces the impacts of communication delays and noise, 2) it entails in an optimal gain which preserves the closed-loop stability and 3) it does not require the estimates of the system states. The performance of the proposed controller is evaluated based on eigen analysis and time-domain simulation of an interconnected AC system that includes five LCC-HVDC links. Performance of the proposed controller is also compared with those of fully centralized and conventional local supplementary controllers and its merits are highlighted. The studies indicate the proposed controller, based on 13 remotely communicated signals, provides similar performance as that of a fully centralized optimal controller using 2050 communicated signals and is far superior to the conventional local supplementary controllers.

Parameter estimation of doubly fed induction generator driven by wind turbine

In order to reduce the environmental consequences of electric power generation, there has been a growing interest in the use of renewable resources for generating electricity. One way of generating electricity from renewable sources is to use wind turbines that convert the kinetic energy contained in the flowing air into electrical energy. As wind power is integrated in large scale European and North American power systems, investigating the dynamic behavior of these turbines is of great importance. Unfortunately, the parameters of the wind turbine needed to conduct dynamic analysis are frequently unknown or inaccurate. This paper analyzes the behavior of two Kalman filter based estimation techniques, the Extended Kalman Filter (EKF) and the Unscented Kalman Filter (UKF), for parameter estimation of the doubly fed induction generator (DFIG) driven by wind turbine. The performance of these two methods is evaluated from different aspects: estimation accuracy, computation time, and robustness to variation of the initial parameter estimates and filter gains. Our experiments show that the performance of the UKF is superior to that of the EKF.

A Fast Local Bus Current-Based Primary Relaying Algorithm for HVDC Grids

This paper proposes a fast, selective, reliable, and local bus primary relaying algorithm for high-voltage direct current (HVDC) grids. The proposed relaying algorithm only uses measurements obtained from current sensors to detect faults and identify their locations. This algorithm is local and requires only communication at the bus level rather than the system level. Furthermore, the algorithm is not sensitive to the system configuration and operating point and can be employed during line or converter outages with limited retuning and adjustments. The proposed algorithm is applied to a four-terminal HVDC grid. Study results show that the proposed algorithm 1) detects faults on both transmission lines and DC buses; 2) identifies the faulted bus or transmission line; 3) requires insignificant computation capacity; 4) distinguishes between faults and other system transients to prevent erroneous circuit-breaker trips; and 5) is not sensitive to measurement noise.

A Local Backup Protection Algorithm for HVDC Grids

DC faults in HVDC grids lead to quickly increasing currents which should be interrupted sufficiently fast to prevent damage to power-electronics components. Although several primary relaying algorithms for HVDC grids have been proposed, fast backup relaying algorithms are needed to ensure system reliability when primary protection fails. This paper proposes a local backup relaying algorithm for HVDC grids, which leads to a short delay between primary and backup protective actions. The proposed algorithm, consisting of breaker and relay failure subsystems, uses classifiers which detect primary protection malfunctions based on the voltage and current waveforms associated with dc breaker operation. The algorithm initiates the detection of uncleared faults during primary protection operation, which results in accelerated actions by the backup protection after primary protection failure. The proposed algorithm is applied to a four-terminal HVDC grid. Study results show that the proposed algorithm accurately detects uncleared faults, identifies the source of primary protection malfunction, and expedites backup protective actions by operating during the fault current interruption interval of the primary protection.

Dynamic Stability Enhancement of a DC-Segmented AC Power System Via HVDC Operating-Point Adjustment

Hopf bifurcation phenomenon of a power system results in oscillatory dynamics which can lead to instabilities in the system. Therefore, it is desirable to operate the system such that a sufficient margin to Hopf bifurcation is ensured. This paper presents a methodology based on the adjustment of the setpoint values of the HVDC link controllers, to prevent instability or increase the stability margin of the system subject to Hopf bifurcation. In this paper, the first-order sensitivities of the Hopf stability margin to the setpoint values of the HVDC links are presented. These sensitivities identify the optimum direction to change the HVDC setpoints to steer the system away from instability, increase the stability margin, and improve the damping of oscillatory modes. The proposed method is evaluated on various system configurations subject to Hopf bifurcation phenomena caused by a variety of events, such as load and line impedance variations. Simulation results show that at the optimum operating point, for a variety of Hopf bifurcations, the stability margin and damping of the oscillatory modes improve.

Stability Enhancement of a DC-Segmented AC Power System

This paper proposes and investigates a new line-commutated current-sourced converter (LCC)-high voltage dc current (HVDC) global supplementary control (GSC) strategy for stabilizing and enhancing the dynamic performance of a large ac system that is segmented by LCC-HVDC links. The GSC is designed based on a linear quadratic Gaussian (LQG) method and enables coordinated supplementary control action of multiple HVDC links that participate in segmentation. The GSC can stabilize the ac system while either minimizing the propagation of oscillatory dynamics from one segment to other segments (GSC1) or enabling their controlled transfer from a disturbed segment to other segments (GSC2). The study results show that in a fully-dc-segmented system: 1) GSC1 and GSC2 are able to stabilize the system; 2) under GSC1, each segment can experience major disturbances without causing adjacent segments to experience the disturbances with the same degree of severity; and 3) GSC2 enables controlled transfer of the oscillations among the segments and, depending on the system configuration, can further reduce the magnitude and duration of oscillatory dynamics. The studies are conducted on a three-segment ac system including three interconnecting LCC-HVDC links.

Damping low-frequency oscillations by tuning the operating point of a dc-segmented ac system

This paper introduces and evaluates a new method for the small-signal stability enhancement of large ac power systems based on segmentation through line-commutated HVDC links. The proposed method is based on the fact that the oscillatory modes of the system may vary by changing the system operating point. In this study, the system operating point is varied by rerouting the flow of power in the ac transmission lines. The flexibility of the HVDC lines in controlling the flow of power in a dc-segmented ac system is used to control the power transmission in the ac lines. An optimization problem is proposed to determine the optimum set-points of the HVDC lines to increase the damping ratio of the underdamped oscillatory modes. Simulation results show that at the optimum operating point, the low-frequency oscillatory modes are damped out significantly and the system net oscillation is less than that of the system at the operating point obtained from a standard economic dispatch. Simulation results also show that there is a trade-off between the generation cost and damping ratio and the proposed method effectively enhances the small-signal stability for a variety of system configurations.