-
[show abstract]
[hide abstract]
ABSTRACT: This paper deals with the coordinated control of rotor- and grid-side converters in wind turbines with doubly fed induction generators (DFIGs) to improve the low-voltage ride-through capability. The rotor-side converter control and additional equipment, called stator damping resistor, are used to limit the rotor inrush current and to reduce the oscillations and settling time of DFIG transient response during the voltage dip. Also, the grid-side converter is controlled to limit the dc-link overvoltage during the voltage drop. It is found that the dynamics of the grid-side converter and dc-link voltage exhibit nonminimum phase behavior, and thus there is an inherent limitation on the achievable dynamic response during the fault. Since the dc-link dynamics is nonlinear, the linear control scheme cannot properly limit the dc-link voltage under large voltage dips. Thus, a nonlinear control scheme applied to the grid-side converter is proposed, which stabilizes the internal dynamics and limits the dc-link voltage fluctuations during the fault. The proposed ride-through approaches limit the peak values of rotor current and dc-link voltage at the instants of occurring and clearing the fault. They also limit the oscillations of electromagnetic torque, and consequently, improve the DFIG voltage dip behavior.
IEEE Transactions on Energy Conversion 10/2010; · 2.27 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: This paper first discusses dynamic characteristics of wind turbines with doubly fed induction generator (DFIG). Rotor back electromotive force (EMF) voltages in DFIG reflect the effects of stator dynamics on rotor current dynamics, and have an important role on rotor inrush current during the generator voltage dip. Compensation of these voltages can improve DFIG ride-through capability and limit the rotor current transients. It is found that the electrical dynamics of the DFIG are in nonminimum phase for certain operating conditions. Also, it is shown that the dynamics of DFIG, under compensation of rotor back EMF and grid voltages, behave as a partially linearizable system containing internal and external dynamics. The internal and external dynamics of DFIG include stator and rotor dynamics, respectively. It is found that under certain operating conditions, the internal dynamics, and thus, the entire DFIG system becomes unstable. This phenomenon deteriorates the DFIG postfault behavior. Since the DFIG electrical dynamics are nonlinear; the linear control scheme cannot properly work under large voltage dips. We address this problem by means of a nonlinear controller. The proposed approach stabilizes the internal dynamics through rotor voltage control, and improves the dynamic behavior of the DFIG after clearing the fault.
IEEE Transactions on Energy Conversion 07/2010; · 2.27 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: As the penetration of wind power in electrical power systems increases, it is required that wind turbines remain connected to the grid and actively contribute to the system stability during and after grid faults. This study proposes an efficient control strategy to improve the low-voltage ride-through (LVRT) capability in doubly fed induction generators (DFIGs). The proposed scheme consists of passive and active LVRT compensators. The passive compensator is based on a new crowbar arrangement located in series with stator windings. It considerably reduces the rotor inrush current at the instants of occurring and clearing the fault. The active LVRT compensator is realised through rotor voltage control and reduces the oscillations of electrical torque, rotor instantaneous power and DFIG transient response during the voltage dip. The proposed LVRT control not only reduces the peak rotor fault current, but also minimises the rotor instantaneous power oscillations, and consequently limits the dc-link voltage fluctuations. At the end, results of theoretical analyses are verified by time-domain simulations.
IET Renewable Power Generation 06/2010; · 1.74 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Closeness to the nose point of P-V curve, or loadability limit (LL), and voltage instability is one of the main concerns of modern power systems. Hence, online evaluation of LL has attracted the interest of researchers in recent years. This letter introduces a simple local index for online estimation of closeness to LL, using two consecutive scalar local measurements. This index is achieved based on the fact that variation of the load apparent power is zero at LL. The index is suitable for implementation in numerical relays to stimulate local control actions to prevent voltage instability, and in the system control center for system-wide coordinated analysis and control measures. Simulation results on the IEEE 30-bus test system show the suitability of the proposed index.
IEEE Transactions on Power Systems 03/2010; · 2.68 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Voltage stability is the ability of a power system to maintain acceptable voltages at all buses in the system under normal conditions and after being subjected to a disturbance. This paper employs the management of reactive power generation to improve the voltage stability margin (VSM), in the framework of optimal reactive power dispatch (ORD) problem. Scheduling of VAR sources is performed using the concept of relative value of VAR sources. The proposed methodology has been tested on the IEEE 14-bus test system. Simulation results show that after the optimal reactive power scheduling, VSM of the systems is increased considerably. In addition, it is observed that reactive power reserve is increased and active and reactive power losses are decreased.
Power & Energy Society General Meeting, 2009. PES '09. IEEE; 08/2009
-
[show abstract]
[hide abstract]
ABSTRACT: Different analysis methods have been used for voltage stability assessment. In comparison with static analysis methods, little work has been done on dynamic analysis of large interconnected power systems. Voltage instability can be studied effectively with a combination of static approaches and time simulations. This paper discusses voltage stability assessment using mixed static and dynamic techniques. Using static methods, a voltage stability based ranking is carried out to specify faint buses, generators and links in power system. The system is analyzed for most severe conditions. Then, time domain simulation is performed for the conditions determined by voltage instability ranking. The mixed approach benefits from advantages of both static and dynamic analyses. The New England (IEEE 39 bus) system was used as a test system
Transmission and Distribution Conference and Exhibition: Asia and Pacific, 2005 IEEE/PES; 02/2005
