Flicker generated by arc furnaces

In industry, many processes generate disturbances on the distribution networks of large production sites. These networks must comply with well-defined standards to guarantee the proper operation of connected equipment and the comfort of users. The networks are disturbed either by the process itself, or by the static actuators supplying the process.

A distinction must be made between the various polluters, depending on their power supply mode: some processes are fed directly from the grid, while others require converters... Rolling mills, large welding machines and certain DC arc furnaces are not connected directly to the grid, but are interfaced by thyristor or IGBT AC/DC converters. The advantage of these converters is current control, which is all the more effective the faster the actuator: for example, a three-phase thyristor Graëtz bridge can respond in one arc, i.e. 3.3 ms on a 50 Hz network. A Graëtz bridge is referred to as a direct converter, as the thyristors switch naturally from mains to DC voltage. Indirect converters comprise a Graëtz-type mains converter, a storage element, usually a capacitor, and an output converter with controllable components of the GTO or IGBT type, capable of operating at switching frequencies much higher than those of the mains, so the response time for the load is much shorter than that of a simple thyristor Graëtz bridge.

For the past ten years or so, processes with a power output of less than 10 MW have been powered by indirect IGBT converters, particularly large rolling mills known as bloomings. When direct thyristor converters were used to power them, the reactive power variations caused the bloomings to disturb the networks, but the disturbances were balanced on the three phases, hence the use of fairly simple statocompensators or SVCs. With reciprocating arc furnaces, whose power can exceed 100 MW, single-phase short-circuits occur during the melting phase, and their effects on the network are much more difficult to limit. At this power level, it is not yet realistic to envisage power supplies with indirect converters, but direct-current furnaces do exist, supplied by direct thyristor converters, based on Graëtz bridges, with a few topological evolutions designed to reduce reactive power variations, which are mainly responsible for network disturbances. That said, especially at the start of melting, even with DC furnaces, arc breaks are frequent, and because of the direct conversion, they are perceived by the networks...

The aim of this article is to assess the disturbances caused by arc furnaces on distribution networks, and to consider corrective measures.