Introduction
The world's first commercial HVDC Light link has been in commercial operation since late September 1999. The needs and reasons for choosing this technology are reported in [1]. The project has been organized as a joint development project between the three companies ABB, GEAB and Vattenfall and has been financed by the three parties and the Swedish National Energy Administration.

This report covers three main issues of experiences:

• Verification of the technical specification
• HVDC Light Control and Network Master Control
• Validation of simulation models

Experiencies of the plant construction including all equipment design will also be reported.

Measurements
During the three months long trial operation period in summer 1999 an extensive test program was executed including various realistic operational modes. To check equipment performance as well as to verify the technical specification dedicated measurements were performed during this period within and close to the HVDC Light stations as well as in important network nodes. Among others harmonics, flicker, EMC, voltage dips, power flows and protections were recorded. Following the commisioning, power flows and control and protection actions were recorded with the permanent measurement equipment within the HVDC Light stations and in the network.

Control
During operation the control and protection coordination has been checked. The behaviour was analyzed during fault cases and for several regulation mode shifts executed manually and automatically. The wind power generation has been analyzed during faults including the possibility to trip wind power units when necessary. The unique feature of power flow controllability within the AC network is utilized by means of the network master control including set point values calculated from an overall losses minimization algorithm. The three-level voltage control including the master control set point values, the flicker control due to wind power tower shadowing around 1.5 Hz and the transient voltage control during faults were also analyzed. Finally the HVDC Light control concept to rely on local monitoring in each station without need for communication between the stations were analyzed.

Validation
Eventually a detailed EMTDC model of the HVDC Light link and a detailed SIMPOW model of the Gotland network were validated. In the EMTDC case the network model was less detailed and in the SIMPOW case the HVDC Light link was less detailed. By careful comparison between the two models good agreement with measured results was obtained in relevant parts for each model.

[1] Gotland HVDC Light Transmission - World's First Commercial Small Scale DC Transmission,
K. Eriksson et.al. To be presented at CIRED, Nice, June 1999