Application guide for the insulation coordination of Voltage Source Converter HVDC (VSC HVDC) stations

Modular Multilevel Converter Voltage Source Converter (MMC-VSC) technology is widely used for High Voltage Direct Current (HVDC) power transmission. The equipment installed in converter stations is subjected to overvoltages originating from various sources and must be adequately protected to ensure reliable operation throughout its service life. This is achieved through a combination of protective devices, such as surge arresters, and the appropriate specification of equipment withstand characteristics. The process of selecting equipment insulation levels and determining the ratings and locations of surge arresters is referred to as insulation coordination.

The purpose of this Technical Brochure (TB) is to present the general principles and arrester configurations for the insulation coordination of the most common VSC HVDC converter station topologies and configurations.

The TB first identifies the most widely used VSC HVDC system configurations, namely bipolar and symmetrical monopolar schemes, together with the predominant converter technologies based on half-bridge and full-bridge MMCs. It then examines the sources and types of overvoltages affecting VSC converters, taking into account the characteristics of the various converter topologies. Common arrester arrangements used to limit these overvoltages to acceptable levels are discussed, and the critical events governing the protective requirements of each arrester are identified. The modeling requirements, study methods, and recommended contingencies for insulation coordination studies are also presented. In addition, the TB determines the continuous operating voltages affecting surge arresters and provides recommendations for creepage distances. Finally, recommendations are given for insulation margins and withstand levels for the principal items of converter station equipment.

The document is intended for engineers involved in insulation coordination studies, including consultants, manufacturers, utilities, and project developers. It provides a better understanding of the sources of overvoltage stress, methods for evaluating their magnitude, and approaches for mitigating their impact. The TB also promotes a common understanding among the various stakeholders in a VSC HVDC project with respect to insulation coordination studies, equipment withstand requirements, and recommended protective margins.

Introduction and Objectives

Insulation coordination is the process of selecting the dielectric strength of equipment in relation to the voltages that may occur in the system for which the equipment is intended, taking into account the service environment and the characteristics of the available protective devices. This process typically includes the installation of surge arresters at critical locations within the converter station. Owing to their highly nonlinear voltage-current characteristics, surge arresters limit the voltage across their terminals to predetermined levels, thereby protecting adjacent equipment against excessive overvoltages.

Guidelines and standards for the insulation coordination of AC systems and equipment have been established over several decades. HVDC converters, however, exhibit characteristics that differ significantly from those of conventional AC equipment and therefore require specific consideration. To address this need, IEC developed IEC 60071-5, which provides guidance for the insulation coordination of line-commutated converter (LCC) HVDC stations. At the time this Technical Brochure was developed, no corresponding standard was available for VSC-based HVDC systems.