Upgrading Outdated Rectifier Control Systems - Needs and Challenges

Rectifier control systems supplied in the 1970´s and 80´s were based upon analog technology. These systems typically have failures which result in longer down times since components are either not available, are obsolete or need to be repaired. Tolerance degradation of these components contributes to problems such as loss of regulation accuracy or the malfunctioning gate firing circuits leading to potentially heavy production losses. This makes it difficult to run the plant economically and is unsafe for operating people. Modern control systems are quite comprehensive, which poses a great challenge when considering an upgrade for old control systems. An upgrade must result in more reliable and efficient operation. Time available for designing, manufacturing and execution is typically very short. Necessary logic, interlocks, etc. to match current operation philosophy has to be achieved. When conventional maintenance procedures are not adequate to improve the availability, safety, productivity and efficiency of a plant beyond a certain level, then a technical assessment of the plant needs to be carried out to determine ways for upgrade or replacement. This paper presents a case study for control replacements for a thyristor plant and for a diode plant, illustrating needs and challenges, including: incorporating centralized control; improvement in measurement accuracy to increase production; monitoring and recording of important parameters such as current, flows, temperatures, etc.; high speed response capabilities to protect the system; suitable firing circuitry for thyristor plants; event recording, listing and printing all events that occurred, hence providing a fault diagnostics tool; elimination of possible human errors in operation and maintenance; high immunity against electro magnetic noise; and suitability for existing plant topology