1.1 General These Guidelines and Parameter Lists to Functional Specifications describe specific functional requirements for HVDC Grid Systems. The terminology \'HVDC Grid Systems\' is used here describing HVDC systems for power transmission having more than two converter stations connected to a common d.c. circuit. While this document focuses on requirements, that are specific for HVDC Grid Systems, some requirements are considered applicable to all HVDC systems in general, i.e. including point-to-point HVDC systems. Existing IEC, Cigré or other documents relevant have been used for reference as far as possible. Corresponding to electric power transmission applications, this document is applicable to high voltage systems, i.e. .only nominal d.c. voltages equal or higher than 50 kV with respect to earth are considered in this document. NOTE While the physical principles of d.c. networks are basically voltage independent, the technical options for designing equipment get much wider with lower d.c. voltage levels, e.g. in case of converters or switchgear. Both parts have the same outline and headlines to aid the reader. 1.2 About the Present Release The present release of the Guidelines and Parameter Lists for Functional Specifications describes technical guidelines and specifications for HVDC Grid Systems which are characterized by having exactly one single connection between two converter stations, often referred to as radial systems. When developing the requirements for radial systems, care is taken not to build up potential show-stoppers for meshed systems. Meshed HVDC Grid Systems can be included into this specification at a later point in time. The Guidelines and Parameter List to the Functional Specification of HVDC Grid Systems cover technical aspects of - Coordination of HVDC Grid and a.c. Systems - HVDC Grid System Characteristics - HVDC Grid System Control - HVDC Grid System Protection - Models and Validation - Beyond the present scope, the following aspects are proposed for future work: - AC/DC converter stations - HVDC Grid System Equipment - HVDC Grid System Integration Tests