This paper endeavors to present a comprehensive summary of the causes and effects of voltage unbalance and to discuss related standards, definitions, and mitigation techniques. Several causes of voltage unbalance on the power system and in industrial facilities are presented as well as the resulting adverse effects on the system and on equipment such as induction motors and power electronic converters and drives. Standards addressing voltage unbalance are discussed and clarified, and several mitigation techniques are suggested to correct voltage unbalance problems. This paper makes apparent the importance of identifying potential unbalance problems for the benefit of both the utility and customer.

Assessment of Voltage Unbalance

With more and more distributed energy resources (DERs) being connected to the grid through interface converters, the utility requires the DER generation system to remain grid-connected during voltage sags to ensure the operating stability of the AC power system. These low voltage ride-through requirements also suggest that the DER generation system injects reactive power to support grid voltages. In this paper, a positive and negative sequence reactive current injection method is proposed to meet the low voltage ride-through requirement. Its operation principle and control method are explained and analyzed. Experimental results are presented to validate the effectiveness of the proposed method. Comparisons of the proposed method and other low voltage ride-through techniques are also presented.

A low voltage ride-through technique for grid-connected converters of distributed energy resources

This work describes a novel method in improving the input current total harmonic distortion (THD) as well as the power factor of a three-phase suppressed-link rectifier-inverter circuit. This proposed method makes use of only three bi-directional low power static switches with a relatively simple gating circuit. This paper illustrates how the proposed method is superior in reducing the input current THD of a rectifier-inverter set to about 5%, which is in line with the requirements of IEEE standard 519-1992. This is accomplished without the use of any filter or complex wave shaping techniques. A delta-modulated (DM) voltage source inverter (VSI) with proportional integrator forms the output stage of the converter. It helps to provide constant volts per hertz operation without the need for additional feedback circuitry and complexity. Moreover, this novel DM technique also helps to provide a smooth transition from the pulse width modulation (PWM) to square wave, hence allowing full utilization of the DC bus voltage.

A novel suppressed-link rectifier-inverter topology with near unity power factor

In this paper, a new compensation strategy for receiving clean power of a conventional 18-pulse ac/dc converter formed by three 6-diode bridges is proposed. According to the proposed strategy, a three-phase current-controlled inverter injects the compensation currents into the three positive terminals of the three six-diode bridges. The goal of injecting currents at dc side is to improve the quality of the ac line currents. Compared to the conventional active filter deployed at the ac side, the three-phase inverter used in this paper is with lower kVA rating, and the 18-pulse converter draws nearly sinusoidal currents from the ac main by the proposed compensation strategy. The theoretical compensation command is derived in this paper, and then an approximate approach is recommended to simplify the calculation. A digital-signal processor is employed as a digital controller to calculate the compensation command, and a 3-kW prototype including the 18-pulse converter and the current-controlled inverter is built for evaluation and measurement. Moreover, the performance affected by unbalanced ac source is investigated. The experimental results demonstrate that the proposed method not only improves the line current quality but also mitigates the effects caused by unbalanced source voltages.

A DC-Side Current Injection Method for Improving AC Line Condition Applied in the 18-Pulse Converter System

The magnet power supplies provide precisely controlled current to the magnets in particle accelerators. As the research in high-energy physics progresses, the required precision of the magnet current is increasing rapidly. Existing magnet power supplies for high-current applications cannot meet the present and future demands. This paper proposes a new magnet power supply for high-current magnets. The proposed system integrates a switch-mode ripple regulator with a phase-controlled rectifier. The results from this paper indicate that the proposed system can provide a magnet current with less than 10 ppm harmonic content and can be expected to have fast dynamic response. In this paper, the general description of the power circuit and system control configuration is presented. Fourier analysis is applied to investigate the harmonic content of the magnet current. The theoretical analysis is verified experimentally. A design example is given. >

A low ripple power supply for high current magnet load

The authors analyze the effect of input voltage unbalances on the rectifier input current and on the filter kilovoltampere rating. The effect of voltage unbalance on the rectifier output voltage is examined, and a specific practical solution for the elimination of the triplen input current harmonics is proposed. The authors conclusively demonstrate that the elimination of these noncharacteristic harmonics can be accomplished by using a firing scheme that results in equal and equally spaced thyristor conduction periods, irrespective of the degree of unbalance in input voltages. However, the scheme has a drawback: the amplification of the ripple voltage at the rectifier output. These conclusions are verified experimentally using a 5 kVA setup. >

J.F. Lindsey

Papers

Problems and solutions associated with the operation of phase-controlled rectifiers under unbalanced input voltage conditions