Development of Transformer Neutrals DC Current Blocking Device in High Power System

The DC current through transformer neutrals was harmful for transformer operation and influenced system stability. Among several methods of limiting or eliminating neutrals DC current, series capacitor method, composed by capacitor and fault bypass system, was an ideal way. Several criterions must be followed, they were: no or less influence to relevant relays and automations; tolerance worst fault current and keep safety of transformers and neutrals; in normal system mode must ensure neutrals in capacitor ground state; once neutrals voltage overpass threshold, bypass system should be put into operation in limited time; economical and feasible for installation, testing & maintenance, etc. Capacitor had been adopted as main component, which had been inserted between transformer neutrals and earth. Bypass system had paralleled with capacitor and composed of a full- wave diode bridge rectifier, thyristor trigger circuit, pre-test circuit before capacitor-ground, fast breaker and other auxiliary components. When transformer neutrals in direct-ground state, and DC current through transformer neutrals over setting up value, a pre -trigger process of thyristor started to check whether bypass system run correctly, if right, device transferred neutral grounding state from direct-ground to capacitor-ground; when in capacitor-ground state, if system fault event occurred and terminal voltage surpassed threshold, trigger circuit had been fired to make neutrals to direct ground state as quickly as 20 us and simultaneously fast breaker operated and ensured metal ground below 25 ms. Built in control unit and recorder unit improved device automation and reliability .By remote terminal several devices could be monitored. For engineering required, the prototype model device had been produced. Factory test including big current surge endurance and automation characteristic showed this device could satisfy power system field application.