【正文】 10 volts. The current inputs must be converted to voltages by resistive shunts. As the normal current transformer secondary currents may be as hundreds of amperes, shunts of resistance of a few milliohms are needed to produce the desired voltage for Analog to Digital Converter (ADC). An alternative arrangement would be to use an auxiliary current transformer to reduce the current to lower level. An auxiliary current transformer serves another function: that of providing electrical isolation between the min CT secondary and the puter input system.Since the digital puter can be programmed to perform several functions as long as it has the input and output signals needed for those functions. It is simple matter to the relaying puter to do many other substation tasks, for example, measuring and monitoring flows and voltages in transformers and transmission lines, controlling the opening and closing of circuit breakers and switches, providing backup for other devices that have failed, are functions that can be taken over by the relaying puter. With the program ability and munication capability, the puterbased relaying offers yet another possible advantage that is not easily realizable in a conventional system. This is the ability to change the relay characteristics (settings) as the system conditions warrant it. With reasonable prospects of having affordable puterbased relaying which can be dedicated to single protection function, attention soon turned to the opportunities offered by puterbased relaying to integrate them into a substation, perhaps even a systemwide network. Integrated puter systems for substations which handle relaying, monitoring, and control tasks offer novel opportunities for improving overall system performance. Computer relayingThe electric power industry has been one of the earliest users of the digital puter as a fundamental aid in the various design and analysis aspects of its activity. Computerbased systems have evolved to perform such plex tasks as generation control, economic dispatch (treated in chapter 11)and loadflow analysis for planning and operation , to name just a few application areas. research efforts directed at the prospect using digital puters to perform the tasks involved in power system protection date back to the miensixties and were motivated by the emergence of processcontrol puters a great deal of research is going on in this field, which is now referred to as puter relaying. Up to the early 1980s there had been no mercially availability protection systems offering digital puterbased relays. However, the availability of microprocessor technology has provided an impetus to puter relaying.*Microprocessors used as a replace*and solid state relays non provide a number of advantages while meeting the basic protection philosophy requirement of decentralization.There are many perceived benefits of a digital relaying system:1. Economics: with the steady decrease in cost of digital hardware, coupled with the increase in cost of conventional relaying. It seems reasonable to assume that puter relaying is an attractive alternative. Software development cost can be expected to be evened out by utilizing economies of scale in producing microprocessors dedicated to basic relaying tasks. 2. Reliability: a digital system is continuously active providing a high level of a selfdiagnosis to detect accidental failures within the digital relaying system.3. Flexibility: revisions or modifications made necessary by changing operational conditions can be acmodated by utilizing the programmability features of a digital system. This would lead to reduced inventories of parts for repair and maintenance purposes4. System interaction: the availability of digital hardware that monitors continuously the system performance at remote substations can enhance the level of information available to the control center. Post fault analysis of transient data can be performed on the basis of system variables monitored by the digital relay and recorded by the peripherals. The main elements of a digital puterbased relay are indicated in Figure 959. The input signals to the relay are analog (continuous) and digital power system variables. The digital inputs are of the order of five to ten and include status changes (onoff) of contacts and changes in voltage levels in a circuit. The analog signals are the 60Hz currents and voltages. The number of analog signals needed depends on the relay function but is in the range of 3 to 30 in all cases. The analog signals are scaled down (attenuated) to acceptable puter input levels (10 volts maximum) and then converted to digital (discrete) form through analog/digital converters (ADC). These functions are performed in the block labeled “Analog Input Subsystem.”The digital output of the relay is available through the puter’s parallel output port, fivetoten digital outputs are sufficient for most applications.The analog signals are sampled at a rate between 210 Hz to about 2000 Hz. The sampled signals are entered into the scratch pad (RAM) and are stored in a secondary data file for historical recording. A digital filter removes noise effects from the sampled signals. The relay logic program determines the functional operation of the relay and uses the filtered sampled signals to arrive at a trip or no trip decision which is then municated to the system.The heart of the relay logic program is a relaying algorithm that is designed to perform the intended relay function such as over currents detection, differential protection, or distance protection, etc. It is not our intention in this introductory text to purse this involved in a relaying algorithm, we discuss next one idea for peak current detection that is the function of a digital over current relay. 譯 文繼 電 保 護(hù)微機(jī)保護(hù)電力系統(tǒng)保護(hù)領(lǐng)域中一個新發(fā)展是計算機(jī) (通常為微型機(jī)) 來進(jìn)行繼電保護(hù). 雖然計算機(jī)能提供與傳統(tǒng)繼電保護(hù)相同的保護(hù), 但它有一些優(yōu)點(diǎn). 計算機(jī)繼電保護(hù)比機(jī)