Converting AC Distribution Lines to DC to Increase Transfer Capacities and DG Penetration

Abstract

Transfer capacities of urban distribution networks need to be increased to fulfill the increasing load demands and to accommodate distributed generation (DG). However, there are limited spaces to build new substations and lines, and curtailment of DGs may happen due to voltage violation during DG outputs fluctuation. This paper proposes and analyzes various methods to convert some existing ac MV lines to dc lines in order to form a hybrid ac/dc distribution network, based on which transfer capacities of lines can be increased, and flexible power shift can be achieved through a voltage source converter between two lines. The increases of transfer capacities are quantified. Optimal operation to fully utilize the increased capacities is achieved, in which losses are minimized in day-ahead scheduling, and node voltages are regulated real-time within security ranges based on limited measurement. Not only reactive power but also real power optimization are designed to maximize load supply and DG accommodation. A sensitivity method is proposed considering relatively large r/x ratio of an MV distribution network, which is effective for the real-time voltage regulation. Simulations are performed to verify the proposed method.

Reliability Evaluation of Voltage Source Converters for MVDC Applications

“…It can be seen that there is only one SP in each arm of the 2L-VSC and two SPs in each arm of the 3L-NPC VSC. The number of IGBT modules in each SP can be calculated through (3). Then, the reliability Rsp(t) of each SP is calculated as below…”

“…For instance, the congestion of distribution networks and unbalance conditions of three-phase AC network. These issues may result in system instability and force distribution network operators (DNOs) to expand and reinforce their grid infrastructures [3].…”

Enhancing Line Capacity Utilization in Power Transmission System Using Active MVDC Link

“…However, the power flow in the converter-based DC line can be manipulated by the operator. Therefore, research has been conducted to delay the grid expansion, increase the flexibility of the power system and operate the grid more efficiently by using the converter and DC link-based facility which is capable of power flow control [7][8][9][10][11]. In [7], research has been conducted into delays in the expansion of new facilities by connecting each distribution feeder with an MVDC link as the back-to-back (BTB) type in urban areas where the installation of a new line and transformer is difficult due to high load density and a complicated network.…”

“…In addition, the application of an MVDC link at the normally open point (NOP) for use as soft open point (SOP) can be used to reduce power losses in the distribution network and balance loads between feeders [8][9][10]. There has also been a study focused on maximizing the hosting capacity of REG in distribution networks by converting the conventional AC line into a DC line so as to use the advantage of a voltage source converter (VSC) that can quickly change the output power [11].…”

Dynamic Average Converter Model for MVDC Link Harmonic Analysis

“…At present, voltage source converter (VSC) based MVDC technology is widely used in traction applications, shipboard power systems and is identified as a feasible option for offshore wind power collection systems and micro-grid applications [11]- [13]. Besides these, an MVDC system for the conversion of an existing AC line to DC operation has been considered to enhance the utilization of existing assets [4]- [6]. For instance, the "ANGLE DC" project which is the first MVDC link in Europe.…”