Most power quality problems in distribution systems are related to voltage sags. Therefore, different solutions have been examined to compensate these sags to avoid production losses at sensitive loads. Dynamic voltage restorers (DVRs) have been proposed to provide higher power quality. Currently, a system wide integration of DVRs is hampered because of their high cost, in particular, due to the expensive DC-link energy storage devices. The cost of these DC-link capacitors remains high because the DVR requires a minimum DC-link voltage to be able to operate and to compensate a sag. As a result, only a small fraction of the energy stored in the DC-link capacitor is used, which makes it impractical for DVRs to compensate relatively long voltage sags. Present control strategies are only able to minimize the distortions at the load or to allow a better utilization of the storage system by minimizing the needed voltage amplitude. To avoid this drawback, an optimized control strategy is presented in this paper, which is able to reduce the needed injection voltage of the DVR and concurrently to mitigate the transient distortions at the load side. In the following paper, a brief introduction of the basic DVR principle will be given. Next, three standard control strategies will be compared and an optimized control strategy is developed in this paper. Finally, experimental results using a medium-voltage 10-kV DVR setup will be shown to verify and prove the functionality of the presented control strategy in both symmetrical and asymmetrical voltage sag conditions.

Optimized Control Strategy for a Medium-Voltage DVR—Theoretical Investigations and Experimental Results

This paper presents the technical aspects of designing a Dynamic Voltage Restorer to meet the stringent requirements of voltage dips mitigation with respect to the magnitude of voltage dip, fault duration, permissible line voltage deviations and response time. An introduction to the IGCT technology based on latest transparent anode GTO (I-GTO) and gate technology is given. The performance of the DVR based on digital simulations is discussed. A description of a hardware based real time simulator model for performance assessment of DVR in voltage dips mitigation is presented. The results of the real time simulator are compared with the digital simulations. Finally, control and protection concepts for the DVR are highlighted.

http://ieeexplore.ieee.org/iel4/5223/14222/00654018.pdf

Power Supply Quality ~~provement with a Dynamic Vol ge Restorer (DVR)

Voltage sags are a major problem in present distribution systems. Therefore, different solutions are examined to compensate these sags and to avoid production losses at sensitive loads. Especially, dynamic voltage restorers (DVR) are one solution to realize this goal. Presently, a system wide integration of DVRs is hampered because of costs. Voltage sag compensation requires energy storage, the amount of which depends greatly on the required voltage level (VDVR). Present control strategies are either able to minimize the needed voltage, to allow a better utilization of the storage system or to minimize the distortions at the load. To avoid this drawback an optimized control strategy is presented in this paper, which is able to reduce the voltage amplitude and causes low distortions at the load side. In the following paper, a brief introduction of the basic DVR principle is given. After this, different control strategies are briefly described, before an optimized strategy is presented. Finally, simulation results are shown to verify and prove the functionality of the presented control strategy

Optimized Control Strategy for a Medium-Voltage DVR

In this paper a dynamic voltage restorer (DVR) circuit with a bi-directional power electronic transformer (PET) structure is proposed to correct input utility voltage distortions on a sensitive load. This DVR-PET contains full-bridge DC-link inverter (FBDCI), high frequency isolating transformer (HFIT), cyclo-converter and output filter. A phase shift modulation (PSM) control scheme is used to regulate compensation voltage. The proper reference voltage is estimated to adjust the amount of battery (or capacitor) charged energy as well as minimizing energy transfer between utility power and DVR circuit. A circuit model has been simulated with PSCAD/EMTDC software and the results are presented which verify the analysis.

Bi-directional power electronic transformer based compact dynamic voltage restorer

Vector Switching Converter VeSC has been recently introduced as an alternative for controlling power flow in power system's complex interconnections by direct AC-AC conversion, and novel FACTS devices have been recently introduced based on the VeSC, such as the Xi (Ξ) and Gamma (Γ) controllers. This paper introduces the Simplified Vector Switching Converter SVeSC and the topology of the Xi (Ξ) and Gamma (Γ) controllers based on this novel simplified scheme, which reduce the number of switches, increasing the reliability and reducing the cost of implementation, while holds the operating principle. The control system of AC-link FACTS devices is simpler than the control system of the DC-link approach, and free of PLL and trigonometric calculations. Simulation results of the active power flow control between two nodes by the proposed simplified Xi (Ξ) controller are provided to prove the operating principle.

Simple topologies for AC-link flexible AC transmission systems

This paper describes a new dynamic voltage regulator (DVR) with the solid-state switched tap changer. Conventional solid-state switched tap changer was modified to minimize the total cost and the harmonic contents in the injected voltage. Proposed DVR was designed to compensate the typical power disturbance such as the voltage sag up to 50% level. It is composed of a parallel transformer, a series transformer, gate turned-off AC switches and bypass switches. Since the proposed DVR does not need any kind of energy storage devices such as capacitor or battery, we can minimize maintenance problems and maximize reliability compared to the conventional DVR, which is composed of voltage sourced inverter and energy storage capacitor. In the proposed DVR, the bypass switch shorts the secondary winding of the series transformer during the normal condition. On the other hand, AC switches are controlled to compensate the voltage sag during the voltage disturbance. Parallel transformer provides only two tap voltage levels to minimize the number of switching devices. The basic operation of the proposed DVR was explained, and new output voltage control algorithm was discussed. The proposed DVR was verified through the computer simulation.

http://ieeexplore.ieee.org/iel5/8902/28144/01259335.pdf

G.H. Kwon

Papers

Dynamic voltage regulator with solid state switched tap changer