To realize the connection of a Static Synchronous Compensator (STATCOM) to a high voltage system and address negative sequence compensation, you need to follow certain steps:

Determine the system requirements: Assess the voltage levels, power ratings, and control requirements of your high voltage system. Understanding the specific needs will help you design and configure the STATCOM accordingly.

Choose the STATCOM configuration: Select the appropriate configuration for the STATCOM based on your system requirements. Common configurations include Voltage Source Converter (VSC) and Current Source Converter (CSC). VSC-based STATCOMs are widely used due to their flexibility and controllability.

Design the overall system layout: Create a detailed layout and design of the STATCOM system, considering factors such as the location of the STATCOM within the high voltage network, the interface with other equipment, and the required protections.

Address high voltage connection: For high voltage connection, typically, a step-up transformer is used to match the voltage level of the STATCOM to the high voltage system. The transformer should be designed and selected based on the voltage ratio and power rating requirements.

Negative sequence compensation: Negative sequence compensation is important for maintaining balanced operation in three-phase systems. STATCOMs can help mitigate negative sequence components by injecting reactive power and voltage control. The control system of the STATCOM should be appropriately designed to detect and respond to negative sequence conditions.

Implement control algorithms: Develop and implement control algorithms for the STATCOM. The control system should be capable of detecting negative sequence components and adjusting the current injection accordingly to balance the system. Consider using advanced control techniques like vector control or model predictive control for optimal performance.

Test and commissioning: Once the STATCOM system is installed, perform thorough testing and commissioning to ensure proper operation. Test the control system, protection mechanisms, and overall performance of the STATCOM under various operating conditions.

Monitor and maintain: Regularly monitor the performance of the STATCOM system and conduct maintenance activities as per the manufacturer's recommendations. This includes periodic inspections, calibration, and addressing any faults or abnormalities promptly.

Keep in mind that implementing a STATCOM system requires expertise in power systems, high voltage engineering, and control systems. It is recommended to involve qualified professionals and consult relevant industry standards and guidelines during the design and installation process.