Tomas Larsson

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.

High-frequency impacts in a converter-based back-to-back tie; the Eagle Pass installation

Voltage sourced converter-based transmission controllers, which use gate turn off switching components, introduce a new type of high voltage stress on insulation systems and materials. Fast switching components like IGBTs may generate or cause high-frequency voltages, which are superimposed upon the power frequency (50/60 Hz) voltage. Insulation within the station may thereby be continuously exposed to a new type of voltage stress, for which it has not been designed or tested. The high-frequency voltages may, under special circumstances, cause insulation damages at magnitude levels, which are well below the dielectric voltage withstand at power frequency. Compact insulation systems with resistive/refractive grading of the electrical field are particularly critical. Several compact type cable terminations rated 24 kV with resistive/refractive stressgrading have failed in the voltage soured converter-based back-to-back installation at Eagle Pass substation in the State of Texas. Through field measurements, laboratory tests, and simulations, the root cause of the failures has been identified. The compact cable terminations have been replaced by another type of termination, which is not sensitive to high-frequency voltages. The voltage sourced converter-based back-to-back installation in Eagle Pass is a joint ABB, EPRI, and AEP field demonstration project, which has been commissioned.

Short circuit capacity estimation for HVDC control application

This paper studies various operational aspects of recursive least square algorithm as a potential solution for the estimation of short circuit capacity in HVDC application. In this work, low computational requirement, minor operational complication and acceptable accuracy of the estimated grid parameters have been considered as the performance metrics for the selection of the suitable estimation algorithm. The chosen algorithm forms a regression problem using at least two algebraic complex equations based on two different operating points. The appropriate selection of the second operating point which fulfills the minimum required accuracy and convergence rate is of great importance, particularly in the practical implementation. This paper proposes and analyzes several choices of providing the second operating point for the estimation algorithm adopted to HVDC application. Furthermore, a sensitivity analysis has been carried out to assess the impact of different parameters and operational conditions such as execution time on the performance of the estimation algorithm for the HVDC control purposes.

DC grid system behavior: A real-time case study

The overall system behavior of a three-terminal DC grid with symmetric monopole configuration is the focus of this paper. The modeling of the DC grid test system which includes converter stations, hybrid DC breakers, cables, etc. is carried out in real-time digital simulator (RTDS) platform. For control and protection, ABBs MACH system is used. Some important system aspects of DC grid, e.g. AC fault-ride-through, clearing of DC faults through hybrid DC breakers, post-fault re-dispatch of power through master control and elimination of DC voltage unbalance are demonstrated in this paper through real-time hardware-in-the-loop simulations.

Dynamic energy storage - Field operation experience

For intermittent power sources like wind and solar, the challenge is to connect and integrate this type of generation while still meeting the required grid stability and reliability, particularly at a low reserve capacity level. This requires viable solutions of electrical energy storage both for distributed and bulk power applications. UK Power Networks has installed a dynamic energy storage system at a site in Norfolk in England in collaboration with ABB, and Durham University. The system is located in an 11 kV network with some penetration of wind power. The paper highlights the grid characteristics, offers salient design features of the energy storage and gives some results from commissioning and the first power exchanges carried out. It also discusses uses of the installation in the 11 kV grid with the aim of harnessing wind power more efficiently than was possible before.