Method for protecting two static converters with direct-current link from overcurrents

Abstract

Abstract The field windings of large generators of hydroelectric power stations which are continuously connected to a three-phase system and are designed to produce large capacitive powers, are fed from a "positive" main static converter, designed for 8 kA, via a field circuit. An anti-parallel "negative" auxiliary static converter designed for 600 A maintains a link current (i Z ) of 200 A in a direct-current link with a link choke. To prevent overcurrents, which can be harmful for the link choke and the auxiliary static converter, in the direct-current link due to disturbances such as, for example, a short-circuit in the three-phase system or connection or disconnection of capacitor banks, a free-wheeling residual current is initiated in the direct-current link when the link current (i Z ) exceeds a predeterminable first current limit value (i G1 ), corresponding to 120% of the nominal link current, during a minimum period of time of 0.5 ms. The free-wheeling condition is terminated when the link current drops below a second current limit value (i G2 ), corresponding to 10% of the first current limit value. To initiate the free-wheeling condition, normal control signals of a firing pulse regulator for firing angle devices are blocked and a firing pulse generator is started which emits a continuous firing pulse or a chain of short-time firing pulses to the control electrode of that thyristor (T4, T4') of the negative bridge half of the particular static converter which is connected to the same alternating-current phase (U, V, W) as the thyristor (T1, T1') of the positive bridge half which happens to be conducting.

Inventors

• 👤 Rolf Bertschi

Bibliographic Details