Upgrading Power System Protection to Improve Safety, Monitoring, Protection, and Control

摘要

One large Midwestern paper mill is resolving an arc-flash hazard (AFH) problem by installing microprocessor-based (μP) bus differential protection on medium-voltage switchgear and selectively replacing electromechanical (EM) over current relays with μP relays. In addition to providing critical bus differential protection, the μP relays will provide analog and digital communications for operator monitoring and control via the power plant data and control system (DCS) and will ultimately be used as the backbone to replace an aging hardwired load-shedding system. The low-impedance bus differential protection scheme was installed with existing current transformers (CTs), using a novel approach that only required monitoring current on two of the three phases. The bus differential relay provides fast fault clearing to reduce the AFH condition and also detects other problems outside the bus differential zone that could indicate a possible problem with switchgear breaker performance. Using the μP bus differential relay's math functionality, the current data from each feeder and source position were combined with bus voltage data also monitored by the relay to provide real-time watt and VAR power flow information. This paper discusses the design of the bus differential protection scheme, the studies required to verify that the existing CTs were adequate for the bus differential application, the design of end-zone protection, and the math computations used to provide real-time power flow data. The paper also discusses how the analog and digital information from this scheme, and others like it, will be concentrated and processed to provide an overall plant power management system.