This paper proposes an advanced controller suitable for three-phase phase-locked loops (PLLs), which are employed in grid-connected power converters. This controller is formed of one or more lead compensators cascaded to the main proportional integral regulator. The proposed lead compensators are second order with pure imaginary roots: they have both a notch peak and a resonant peak (the notch frequency is lower than the resonant frequency). Hence, their phase versus frequency response exhibits phase wraps of ± 180°. Consequently, the parameters of each lead compensator are tuned with two objectives: to eliminate a specific frequency in the synchronous reference frame (SRF) and to enhance stability by a phase lead (phase boost). Through this technique, three-phase PLLs achieve both high bandwidth (fast transient response) and selective cancellation (filtering of unbalance and harmonics ripple in the SRF). The proposed controllers are suitable for simpler three-phase PLL schemes, such as the SRF-PLL. Therefore, big improvement is achieved without adding extra blocks and signals to basic PLL structures. Simulation and real-time implementation (dSpace DS1103) tests, emulating very demanding realistic conditions, have been performed. Key figures from these tests are shown, which prove the high performance and robustness of the proposal.

Three-Phase PLLs With Fast Postfault Retracking and Steady-State Rejection of Voltage Unbalance and Harmonics by Means of Lead Compensation

Several proposals for using of power derived from regenerative braking in railway systems are available. We present an energy recovery system with feedback to the distribution grid based on a new converter topology; the dimensioning methodology of the system is based on real data intake and validated simulations. The new converter topology presented is based on the connection in cascade of a DC/DC elevator to a 3-phase inverter. This way, the system is freed from the limitations imposed by voltage fluctuations in the grid, allowing maximum recovery at minimum catenary's voltages. For more information please visit

Kinetic energy recovery on railway systems with feedback to the grid

This paper proposes a single-phase to three-phase drive system composed of two parallel single-phase rectifiers, a three-phase inverter, and an induction motor. The proposed topology permits to reduce the rectifier switch currents, the harmonic distortion at the input converter side, and presents improvements on the fault tolerance characteristics. Even with the increase in the number of switches, the total energy loss of the proposed system may be lower than that of a conventional one. The model of the system is derived, and it is shown that the reduction of circulating current is an important objective in the system design. A suitable control strategy, including the pulsewidth modulation technique (PWM), is developed. Experimental results are presented as well.

Single-Phase to Three-Phase Drive System Using Two Parallel Single-Phase Rectifiers

Multipulse rectifier topologies based on autoconnections are increasingly applied as interface stages between mains and power electronics converters. These topologies are attractive and cost-effective solutions for meeting the requirements of low total harmonic distortion of line current and high power factor. Furthermore, as only a small fraction of the total power required by the load is processed in the magnetic core, the overall resulting volume and weight are reduced. This paper proposes a mathematical analysis based on phasor diagrams that results in a single and general expression capable of unifying all delta and wye step-up or step-down autotransformer connections for 12- and 18-pulse ac-dc converters. The expression obtained allows the choice of a wide range of input/output voltage ratio for step-up or step-down autotransformer, and this general expression is also presented in a graphical form for each converter. Moreover, it simplifies the procedure for determining turn ratios and polarities for all windings of the autotransformer. A routine for easy and fast calculations is developed and validated by a design example. Finally, experimental results are presented along with comments on a 6-kW 220-V line voltage, 400-V rectified voltage, and 18-pulse delta-autoconnected prototype.

A Family of Autoconnected Transformers for 12- and 18-Pulse Converters—Generalization for Delta and Wye Topologies

The operation of a static transfer switch is analyzed in this paper. Control system requirements for performing fast transfer between two power supplies without affecting the operation of the healthy source are explained. Maximum detection times and system conditions for fast transfer are analytically evaluated.

Static transfer switch : analysis of switching conditions and actual transfer time

In variable-speed drives systems, diode rectifiers and thyristor rectifiers are often used as the front-end circuit for AC-DC conversion. The advantages of the conventional rectifier are its simplicity and high reliability. The drawbacks include the harmonic current distortion and the lack of regeneration capability. In recent years, industries adopt the transistor-based active front-end (AFE) technologies to accomplish high power factor operation and re-generation capability. However, the cost of AFE is much higher than the conventional diode/thyristor front-end. Besides, the AFE is often less reliable than the diode/thyristor front-end under utility transients. In this paper, an IGBT-based auxiliary converter (AXC) system is proposed. The AXC operates as a shunt active filter to compensate the harmonic current of the rectifier when the load consumes power. When the dc load re-generates, the AXC system can channel the re-generation energy back into the utility system. The combination of the diode rectifier and the AXC can accomplish unity power factor operation and regeneration, but it also causes circulating current between the AXC and the rectifier, which leads to higher operational losses and higher noise level. The mechanism of the circulating current is analyzed in this paper, and solutions are also presented. Computer simulation and field test results are presented to validate the performance of the proposed AXC system.

Design of an Auxiliary Converter for the Diode Rectifier and the Analysis of the Circulating Current

In variable speed drives systems, diode rectifiers and thyristor rectifiers are often used as the front-end circuit for AC/DC conversion. The advantages of the conventional rectifier are its simplicity and high reliability. The drawbacks include the harmonic current distortion and the lack of re-generation capability. In recent years, industries adopt the transistor-based active front-end technologies to accomplish high power factor operation and re-generation capability. However, the cost of active front-end is much higher than the conventional diode/thyristor front-end. Besides, the active front-end is often less reliable than the diode/thyristor front-end under utility transients. In this paper, an IGBT-based auxiliary converter (AXC) system is proposed. The AXC operates as a shunt active filter to compensate the harmonic current of the rectifier when the load consumes power. When the DC load re-generates, the AXC system can channel the re-generation energy back into the utility system. The combination of the diode rectifier and the AXC can accomplish unity power factor operation and re-generation, but it also causes circulating current between the AXC and the rectifier, which leads to higher operational losses and higher noise level. The mechanism of the circulating current is analyzed in this paper, and solutions are also presented. Computer simulation and field test results are presented to validate the performance of the proposed AXC system.

Voltage sags have become one of the major power quality concerns in recent years. For manufacturers which have automated production lines, a few cycles of voltage sags can interrupt the entire production line and result in significant losses. For example, a typical semiconductor foundry in Taiwan loses US$ 100,000 to US$ 1,000,000 per sag event. As the high-tech manufacturers often gather closely in the industry park, the utility provides premium grid arrangement to the park, such as direct connection from the high-voltage transmission network, or independent power feeders. These approaches improve the reliability and quality of electric power. However, voltage sags still can not be eliminated. This paper discusses the use of thyristor-based solid-state transfer switches (STS) to provide ride-through during voltage sags for critical and sensitive industrial loads. A synchronous-reference-frame-based control method for identifying voltage sags is presented. Laboratory test results are presented to validate the effectiveness of the controller. Comparisons between the proposed controller and other control method proposed in previous literatures are also demonstrated to show the improvement.

Po-Tai Cheng

Papers

Design of an Auxiliary Converter for the Diode Rectifier and the Analysis of the Circulating Current

Design of an Auxiliary Converter for the Diode Rectifier and the Analysis of the Circulating Current

Design and implementation of solid-state transfer switches for power quality enhancement