DESIGN AND APPLICATION OF A CONFIGURABLE MULTIFUNCTION PROTECTION AND CONTROL UNIT

Traditionally Medium Voltage cricuit breaker protection and control schemes have been implemented using many different components, as detailed below, • Single function protection relays, either electronic or electromechanical, for detection of network faults and tripping of the circuit breaker. • Electromechanical or electronic instruments (ammeter,voltmeter, etc) for local reading of the electrical quantities. •Analogue transducers (AC/DC) for remete monitoring of analogue quantities(1,V,KM). • Electromechanical relays and timers for remotely controlling the circuit breaker (interposing trip and close) and for implementing all the panel logi functions (alarm handing,interlocking,etc) •Lamps, LEDs and mechanicl flags for local annunciation . The interface to the control system (RTU or PLC) is implemented using point to point writing using multicore cables. This approach often results in large LV cabinets of complex design with large amounts of discreate writing and terminals. The advent of powerful microprocessors has enabled the development of multifunction protection and control units eqipped with inputs and outputs, programming facilities and serial communications. The protection and control unit consists of a standard hadrware platform ( see Figure 1) which is configured with the appropr iate software modules according to the application (cable and overhead line feeders, transformer incomers,etc). The main advantages when compared with the conventional approach are: • Reduced size and complexity of the switchgear instrument cabinets • Minimal writing • Single C.T for protection and instrumentation • Ease of adaptation Multifunction protection and conrol units allow MV switchgear manufactures to standardize the circuit breaker panel designs, leading to reduced cost, improved quality and shorter delibery times. This paper discribes the approach taken by one manufacturer in the design of a such a configurable multifunction device. Particular attention is given to the practical application of ladder logic programming facilities in the implementation of complex protection schemes, such as freeder unit protection using directional blocking principle.