Ambra Sannino

Low-Voltage DC Distribution System for Commercial Power Systems With Sensitive Electronic Loads

In this paper, the use of DC power system to supply sensitive electronic loads is treated. First, general design issues regarding DC power systems are discussed, and then the measurement results from a scaled laboratory setup are presented. The results show that it is possible to supply sensitive electronic loads through an AC/DC interface, and to keep them online during grid transients. The use of a DC power system to supply sensitive electronic loads will have lower losses compared with a conventional AC uninterruptible power-supply solution due to fewer power conversion steps.

Protection of Low-Voltage DC Microgrids

In this paper, a low-voltage (LV) DC microgrid protection system design is proposed. The LV DC microgrid is used to interconnect distributed resources and sensitive electronic loads. When designing an LV DC microgrid protection system, knowledge from existing DC power systems can be used. However, in most cases, these systems use grid-connected rectifiers with current-limiting capability during DC faults. In contrast, an LV DC microgrid must be connected to an AC grid through converters with bidirectional power flow and, therefore, a different protection-system design is needed. In this paper, the operating principles and technical data of LV DC protection devices, both available and in the research stage, are presented. Furthermore, different fault-detection and grounding methods are discussed. The influence of the selected protection devices and grounding method on an LV DC microgrid is studied through simulations. The results show that it is possible to use available devices to protect such a system. Problems may arise with high-impedance ground faults which can be difficult to detect.

An Adaptive Control System for a Dc Microgrid for Data Centers

In this paper, an adaptive control system for a dc microgrid for data centers is proposed. Data centers call for electric power with high availability, and a possibility to reduce the electric losses and, consequently, the need for cooling. High reliability can be achieved by using local energy sources, and by using a dc power system, the number of conversion steps, and therefore also the losses, can be reduced. The dc microgrid can also supply closely located sensitive ac loads during outages in the ac grid. The proposed dc microgrid can be operated in eight different operation modes described here, resulting in 23 transitions. The control system coordinates the operation of converters, sources, and switches used in the dc microgrid. The control system is tested in the simulation software package PSCAD/EMTDC, and the results of the most interesting transitions are presented. The results show that it is possible to use the proposed dc microgrid to supply sensitive electronic loads and also, during ac-grid outages, supply closely located sensitive ac loads. To reduce the current transients experienced by grid-connected ac/dc converters, fast grid-outage detection and fast switches are required.

Efficiency analysis of low- and medium- voltage DC distribution systems

In this paper, sustainability aspects connected to the use of dc for power delivery in low- and medium-voltage distribution systems are treated. The efficiency of an example AC system, a DC system and a mixed AC-DC system are calculated and compared. It is shown that, under the assumption of a substantial reduction in semiconductor losses, the total system losses decrease using DC. This means that for the same energy delivered, less energy must be produced from the available sources, thereby indirectly reducing the environmental impact of energy production. Moreover, the DC system seems to lead to better utilization of the HV/MV transformer, so that the same system can be expanded to supply a higher load without changing the transformer.

Practical Implementation of Delayed Signal Cancellation Method for Phase-Sequence Separation

In this paper, the use of a method for online detection of positive- and negative-sequence components of three-phase quantities, named the delayed signal cancellation (DSC) method, is investigated. Problems that arise in practical implementation of the DSC method in computer-controlled systems are investigated. Expressions of the detection error due to nonideal discretization are derived and calculations are verified experimentally. Two methods for reducing the detection error are presented and verified. It is also shown that the given expressions and proposed methods for reducing the detection errors can also be applied to the case of grid frequency variations

Feasibility of a DC network for commercial facilities

This paper analyzes the feasibility of direct current for the supply of offices and commercial facilities. This is done by analyzing a case study, i.e. the supply to a university department. Voltage drop calculations have been carried out for different voltage levels. A back-up system for reliable power supply is designed based on commercially available batteries. Finally, an economic evaluation of AC vs. DC is performed.

A New Control Strategy of a VSC–HVDC System for High-Quality Supply of Industrial Plants

This paper proposes a control strategy for a VSC-HVDC for improving the quality of power supply to industrial plants. The new idea of this control strategy is to give high priority to keep up the ac voltage and slightly decrease the frequency during disturbances. The motivation for choosing this strategy is that the sensitive processing industries are much more sensitive to voltage drops as compared with frequency deviations. In order to test this control strategy, the dynamic performance of a VSC-HVDC supplied industrial plant is investigated during sudden load disturbances, balanced and unbalanced faults from the grid. Simulation results obtained using PSCAD/EMTDC show that with the use of the proposed control strategy, the system can ride through disturbances such as motor starting and faults by slightly decreasing the frequency. The current limit of the VSC-HVDC converters has a significant influence on the dynamics of the system. An increase of the current limit significantly improves the power quality of the system. The comparison of the behavior between a pure ac supplied industrial plant and a VSC-HVDC supplied industrial plant clearly shows the capability of the VSC-HVDC to mitigate voltage dips during faults.

Power-electronic solutions to power quality problems

Abstract In this paper, an overview of power-electronic based devices for mitigation of power quality phenomena is given. The concept of custom power is highlighted. Both devices for mitigation of interruptions and voltage dips (sags) and devices for compensation of unbalance, flicker and harmonics are treated. The attention is focused on medium-voltage applications. Details about field experience are given and recent research results are reported. It is shown that custom power devices provide in many cases higher performance compared with traditional mitigation methods. However, the choice of the most suitable solution depends on the characteristics of the supply at the point of connection, the requirements of the load and economics.

Transient performance of voltage source converter under unbalanced voltage dips

Robust operation of voltage source converters under unbalanced voltage dips can be obtained by controlling the positive and negative-sequence currents separately with a dual vector current controller (DVCC). In this paper, the performance of the DVCC is investigated under all voltage dips that can affect the converter. Two different methods for taking into account the oscillating powers dissipated in the filter are presented, tested and compared. The effect of the phase-angle jump of the dip is also studied.

Protection Scheme for Meshed Distribution Systems with High Penetration of Distributed Generation

This paper identifies the impact of high DG penetration on protection coordination and proposes a protection scheme to mitigate the identified problems, in a network with high penetration of DG. The scheme emphasizes on keeping most DGs on line to supply loads during the fault, without putting DG or any part of the distribution network in islanding operation, whilst ensuring that the conventional overcurrent protection devices (breakers with overcurrent relays - reclosers - fuses) do not lose their functions and their proper coordination