Mitigation of voltage sags, imbalances and harmonics in sensitive industrial loads by means of a series power line conditioner

Power quality is a very important topic nowadays. Sensitive industrial equipment should be protected against steady-state distortions and temporary transients in the distribution lines. Typical disturbances that affect the voltage waveform quality are harmonics, imbalances and sags. This paper presents a solution to protect sensitive loads against voltage disturbances that is based on a series power line conditioner. The goal of the proposed design is that the load does not suffer considerable input voltage variations. To achieve that, a Proportional-Resonant (PR) controller and a reference generator block based on a low-gain PLL are used, which avoids a sag detection block. Furthermore, a frequency adaptation loop is included in the PR controller, which provides a proper controller operation even with utility grid frequency deviations. Finally, a hardware in the loop (HIL) test rig is used to validate the system.     

A flexible power control strategy for rotor-side converter of DFIG under unbalanced grid voltage sags

Doubly fed induction generator (DFIG) is widely applied in variable-speed wind energy conversion system. The disconnection of a substantial amount of DFIG may arouse the instability problem of power system, thus wind power generators have to remain connected during short-time grid faults. As a result, the voltage sag will lead to overcurrent and overvoltage in the rotor winding of a DFIG, moreover the unbalanced voltage sags will also cause serious fluctuations in its electromagnetic torque and output power. This paper studies the relationship of the stator instantaneous powers with the three-phase stator voltage and rotor current of a DFIG under unbalanced grid voltages. A generalized formula of current reference for the rotor-side converter of DFIG is constructed by introducing continuous adjustment coefficients. Meanwhile, the analytical equations of rotor peak current, stator active and reactive power fluctuations are derived to characterize the operating performance of DFIG. The impacts of adjustment coefficients on DFIG control performance and the feasible region of coefficients restrained by the rotor current constraint are discussed. In consideration of the rotor current limit, the flexible power control strategy for DFIG in unity power factor mode (UPFM) and reactive power supporting mode (RPSM) is presented. The correctness of proposed method is verified by simulation and experiment tests of single-DFIG and multi-DFIG systems.     

省地协调自动电压控制(AVC)实现方法的研究

对省地协调电压控制的实现方法进行了研究,基于对控制灵敏度问题求解讨论,提出了省地协调的最优潮流(OPF)模型及实现算法、协调二级电压控制(CSVC)模型与实现算法。该算法在综合考虑协调控制时约束条件之间关系的基础上给出了协调控制的设定值。给出了协调控制模式下部分中枢母线电压的计算结果及网损分析结论,结论表明电压协调控制模式有效提高了电网的经济效益和安全性。     

Lower speed range drive for sensorless vector controlled induction machines with stator voltage offset compensation method

The vector control method is widely used for induction machine drives. Recently, sensorless vector control for induction machines has been investigated and proposed. The speed range for the drives is limited to about 1:100 in industry. The main reason for this limitation is the inaccuracy of stator voltage measurement. The lower the rotor speed is, the lower the stator voltage. Therefore, it is difficult to detect the stator voltage accurately in the lower speed region, and difficult to control motor speed and motor torque precisely. This paper presents a method of improving the lower speed performance of sensorless vector controlled induction machine drives using offset compensation of stator voltage. The offset is compensated by using the fluctuation of the estimated rotor flux. The validity of the proposed method is verified by experimentation. © 2000 Scripta Technica, Electr Eng Jpn, 133(1): 79–86, 2000     

An energy optimized control scheme for a transformerless DVR

The dynamic voltage restorer (DVR) is considered as the most effective and economic solution for voltage disturbances. This paper presents an energy optimized control scheme for a transformerless DVR. The DVR structure is based on a cascaded H-bridge multilevel inverter topology to eliminate the need of insertion transformers. The proposed control algorithm maintains a balanced load-side voltage even during the compensation of unbalanced disturbances with a minimum active power injection. Moreover, the proposed scheme maximizes the ride-though capability of the DVR during voltage sags. This feature is verified by using capacitors instead of dc sources as energy storage elements for the DVR. Furthermore, the proposed minimum energy scheme prevents the rise in the dc-side voltage of the inverter when compensating voltage swells. The performance of the proposed DVR system is evaluated for compensating different types of voltage disturbances. The results validate the robustness and the accuracy of the proposed system.     

An innovative technique to evaluate network equivalent for voltage stability assessment in a widespread sub-grid system

The equivalent two-bus network models currently available are obtained by lumping all the series impedances and shunt admittances of transmission lines within a series equivalent impedance, to assess voltage stability of multi-bus power system. This paper reports the development of an equivalent pi-network model using a new methodology considering series and shunt parts of line loss separately obtained from the operational parameters of optimal power flow solution of the original multi-bus power network, which can be applied to assess the overall voltage stability status of the system accurately by developing the concept of a generalized global voltage stability margin (GVSM). Simulation results for a typical longitudinal power supply (LPS) system and a robust practical (Indian Eastern Grid) system establish that the pi-equivalent model obtained by the proposed method is highly promising for assessing voltage stability of any power system at any operating point in global scenario in a better way as compared to available series equivalent model. Continuation power flow (CPF) method has also been adopted here to verify the potential of the proposed method for voltage stability assessment. In the proposed equivalent network the generators have been modeled more accurately considering optimal operating criteria.     

Series resonant ZCS‐PFM DC‐DC power converter with high‐frequency high‐voltage transformer link for high‐power magnetron drive

This paper presents a single lossless inductive snubber‐assisted ZCS‐PFM series resonant DC‐DC power converter with a high‐frequency high‐voltage transformer link for industrial‐use high‐power magnetron drive. The current flowing through the active power switches rises gradually at a turned‐on transient state with the aid of a single lossless snubber inductor, and ZCS turn‐on commutation based on overlapping current can be achieved via the wide range pulse frequency modulation control scheme. The high‐frequency high‐voltage transformer primary side resonant current always becomes continuous operation mode, by electromagnetic loose coupling design of the high‐frequency high‐voltage transformer and the magnetizing inductance of the high‐frequency high‐voltage transformer. As a result, this high‐voltage power converter circuit for the magnetron can achieve a complete zero current soft switching under the condition of broad width gate voltage signals. Furthermore, this high‐voltage DC‐DC power converter circuit can regulate the output power from zero to full over audible frequency range via the two resonant frequency circuit design. Its operating performances are evaluated and discussed on the basis of the power loss analysis simulation and the experimental results from a practical point of view. © 2005 Wiley Periodicals, Inc. Electr Eng Jpn, 153(3): 79–87, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20126     

Conditions for existence of optimal solution in the synthesis of a multivariable control problem for power systems by using optimization problem

This paper considers the synthesis of a multivariable control problem for power systems by using the optimization method. The power system dynamics based on the usual assumptions can be formulated as a class of nonlinear dynamic systems which contain the sinusoidal functions associated with the power torque-angle curve of the ac generator. Then, an optimal control law, which retains the principal nonlinearity of the power system, is derived by using the optimization techniques in the linear optimal control theory. The feedback gain matrices are obtained by solving matrix equations which appear in the optimization procedure. In this paper, the conditions for the existence of an optimal solution of these matrix equations are derived by using Anderson's theorem. The stability criterion for the control system is discussed from the viewpoint of a Lur'ye-type Lyapunov function. Thus, the optimal control law can be applied to a class of nonlinear dynamical systems represented by the ordinary differential equations divided into linear and nonlinear terms.     

An improved method of voltage utility factor for PWM control of a five‐leg inverter in two induction motor drives

This paper presents an improved method of voltage utility factor (VUF) from 50 to 86.6% for a five‐leg inverter when two induction motors are driven in vector control and in a condition where the frequencies of the two motors are not very different, and the improved method of voltage utility is theoretically shown. Many modulation methods for the five‐leg inverter have been proposed, but the voltage utility factor is 50% in these methods. The voltage utility factor of the five‐leg inverter is lower than that of the three‐leg inverter as maximum voltage utility factor of the three‐leg inverter is 100%. © 2006 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.