Reactive Power Management in VSC-HVDC Multi-infeed Systems

Abstract

The increasing penetration of power electronics (PE) in the transmission system is one of the greatest changes that the power network has ever experienced. Most of the increase in PE is due to the growth of renewable sources and the need to transfer power over long distances by implementing technologies such as High Voltage Direct Current (HVDC). This also increases the number of Multi-infeed (MI) systems as they are formed when two or more HVDC links are connected in the same area. The growth in renewable energy and HVDC links is reducing the number of synchronous generators and the power system is being operated near its limits to reduce costs. This generates power system stability concerns due to the synchronous generators having traditionally provided reactive power, which supports voltage stability. Voltage Source Converters (VSC) offer several advantages for providing power system stability due to their flexibility and ability to provide ancillary services such as reactive power control, frequency control and ramp capability. These capabilities are highly influenced by the control schemes implemented in the VSCs. VSCs have some limitations such as that they cannot be overloaded, unlike traditional synchronous generators. This means that they have to be sized accordingly, which brings associated costs. VSC-HVDC link owners generate most of their revenue by trading active power and the current schemes that remunerate reactive power are not very attractive. Therefore, they have an interest in being capable of trading as much active power as possible, which is compromised when providing reactive power support. The levels of reactive power provided by each VSC in a MI system may differ depending on aspects such as controllers, topologies, line lengths and loads. This produces reactive power inequity between VSCs and consequently the VSCs that provide less reactive power can trade higher levels of active power. These challenges create opportunities for researching reactive power sharing in MI systems to measure how equally reactive power is shared amongst VSCs. This thesis has developed a method to quantify power sharing amongst VSCs. A variety of commonly-proposed control schemes have been implemented in a VSC and tested in a two bus system to analyse their capabilities for manging reactive power. Once the controllers’ performance is analysed in a two bus system, controllers are implemented in a MI four bus test system developed in this thesis and in a modified New England IEEE 39 Bus System to analyse reactive power sharing amongst VSCs. This enables the comparison of reactive power sharing depending on aspects such as controllers, line length, topology, and load distribution. Reactive power sharing analysis conducted in this research indicates that reactive power sharing does not always compromise volage stability. Therefore, equal reactive power sharing scenarios bring significant advantages such as each VSC being able to trade the same amount of active power, provide the same level of reactive power and have the same degree of aging due to the current flow.

Date of Award	1 Aug 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Robin Preece (Supervisor) & Ognjen Marjanovic (Supervisor)