电气类外文翻译--基于继电保护的电力系统监控设备.doc

外文翻译（原文） Power System Monitoring and Control Facilities on Protective Relays Abstract: It is now possible to consider integrating the functions of the power system protection systems with those of the local and remote data collection and control systems. A structured approach to this integration is necessary. However, if the full benefits are to be realized. A solution which will solve many of the problems previously associated with this integration is presented together with an example of how it might in future be applied in a typical substation. Keywords: Digital communications, Integration 1. Introduction The current practice in power system transmission and distribution environments is to separate the functions of the local control, protection and supervisory control and data acquisition (SCADA) systems. One reason for this has been the technical constraint that has limited the amount of integration which can be reliably achieved. Local control facilities have consisted of hardwired panels taking up much control room space. Control logic has been provided by hardwired contacts or programmable logic controllers. Until recently much of the protection equipment has consisted of analogue devices, again taking up much space. Most modern protection devices using electronic and microprocessor technologies have so far concentrated on reducing the space taken to implement traditional protection functions. Generally, SCADA systems have been added more recently and have supplied their own transducers, interface units and wiring. These have grown up in parallel with the local control and protection systems despite the fact that this often resulted in much functional duplication. Where information concerning the protection operation has been required by the SCADA system this has been derived in a secondary fashion, for example, feeding the protection outputs back into SCADA digital input units. Recent technology advances have led to the realization that this degree of duplication is becoming less and less necessary. Given infinite computing power it could be argued that the information necessary to perform protection functions is all available or can be made available on the SCADA network. It is conceivable then that the SCADA system could perform its own protection algorithms and issue its own trip signals through its control network. In practice reliability requirements and the need for rapid fault clearances have limited this approach to a few specialized instances such as long time thermal overload protection. A far more viable approach is to make the information and control facilities within the relay available to the SCADA network. If this is done many of the costs associated with the SCADA analogue and digital I/O systems can be reduced. Additionally, if control facilities provided within the protection equipment are utilized, a central substation computer can also be used to replace much of the local control system. One reason for the failure of systems to integrate protection functions within an overall control package is the sheer amount of processing required. Modern digital protection relays use state of the art microprocessors to provide complex protection functions. When many of these are spread around a substation it is clear that the processing power required to absorb their functions at a central point is formidable. On the other hand the analogue and digital transducers used by the SCADA system are relatively simple devices as are the digital output units. Their equivalents already exist within the protection and making them available to the SCADA system often requires little more than the addition of communications facilities. The ability of the protection equipment to replace much of the local control and SCADA I/O systems hinges on the ability of the protection equipment to communicate in a structured and deterministic way. It is essential that the protection performance is not compromised whilst at the same time the requirements of the local control and the SCADA systems are still met. From the local control and SCADA point of view the principal requirements are for analogue inputs for measurement and data logging, digital input data for annunciators and alarms, and digital outputs for controlling plant. Most measurement data is used for general indication purposes where an accuracy off 5% is sufficient. Analogue and digital data used for visual indications and data logging require scan rates of around once a second. Where sequence of event recording is required relative accuracy across the system is more important than absolute accuracy. Generally a resolution of 1ms should be aimed for. Control response times should be around 200ms. 2. Protective Relay Communications 2.1 Communications Philosophy The protective relays prime function remains the protection of the power system. It is essential therefore that the relays protection performance is not compromised by the requirements of data monitoring and control. For this reason it is considered necessary to provide monitoring and control communications separate from any communications requirements of the protection. Thus in a blocking scheme for example, blocking signals would be transmitted over their own protection signaling link e.g. pilot wires, and not over the monitoring and control communications link. In this way the deterministic behavior of the protection is maintained. Also, there remains those users who do not yet need some or all of the features available .It is important for these users that the operation of the relay does not depend on the monitoring and control communications link and that the full protection capabilities can still be realized when such links have not been installed. The full benefits of relay communications will only be achieved if they can be installed at all the relevant points on a utilitys power system. This will not happen overnight and it is therefore very important that any chosen system can be installed on a piecemeal basis across a system as it becomes required. One of the major factors influencing the take up of relay communications will be the cost to the user. This cost consists not just of the additional cost of the hardware on the relay but also wiring costs, set-up and configuration costs and on-going operational costs. It is important therefore that steps are taken to control all of these cost areas. Set against these costs should be the savings on the SCADA system and the operational savings which result from the increase in system data available. 2.2 Communications Topology It is possible to connect the SCADA system and the protective relays using a number of different communications topologies. The choice of topology is important as it has a direct bearing on the communications efficiency of the system. Figure 1: Simple Protection/SCADA Topology A simple form of connection is to connect each relay separately to remote terminal units (RTUs) fitted with digital communications facilities. These RTUs in turn connect to the SCADA network -see Figure 1. These RTUs act as network switches, the main SCADA system being responsible for the actual polling of information. In this topology the protective relays have effectively become intelligent transducers. There is a saving for the SCADA system in terms of the transducers that have been replaced but this may be offset by the more complex RTUs. Even at this simple level however, there are benefits to the SCADA system in the amount of additional data that is available from the relays. Unfortunately it is this same increased amount of data which ultimately limits the performance of such systems. Figure 2: Use of Multidrop Connections An improved communications topology is illustrated in Figure 2. Several relays are connected to a single RTU on a single communications spur. This relies on the protective relays being fitted with a communications link capable of multidrop connection. In this scheme the RTU is now responsible for the polling of all units attached. In this way information can be pre-processed and overall data rates can be reduced. This requires a more complex RTU, however a single RTU can handle more relays so fewer may be required and wiring complexity reduced. In theory this principle could be extended to the entire substation, using just one RTU to communicate with all protective relays. In practice this is not possible due to data rate considerations and is also undesirable from a reliability point of view. The number of devices which can be continuously monitored/controlled on a single channel is dependent on both the baud rate used and the amount of data to be transferred. Figure 3: Use of Substation Central Computer A more sophisticated topology is shown in Figure 3. This topology utilizes an IBM PC compatible computer as a substation computer. Where reliability is thought to be a problem, a second slave computer is added in parallel with the first. The substation computer replaces the RTUs described above and gives a number of advantages to the user. Firstly there is now a local control point within the substation in addition to the remote control facilities of the SCADA network. This can take the form of a mimic diagram program, complete with panel metering, annunciators,.etc., freeing much of the local control system panel space if required. The substation computer is responsible for the continuous monitoring of all the connected devices and carries out substantial data pre- processing for the main SCADA system. In particular the PC performs local data logging to its own disk, relieving the SCADA system of a substantial processing burden, especially during and immediately following fault situations. This data is subsequently available both locally at the substation PC and remotely on the SCADA system, as required. The substation computer can also be used as a single access point to all relays at commissioning time. Separate communications spurs are likely to be taken to each substation section, each capable of supporting32 relays. Up to eight spurs can be provided by a single PC giving a theoretical capacity of 256 relays. On such a system it is still possible for a modern PC to poll and extract data from each relay at a rate greater than once a second. In the unlikely event that this number of relays is insufficient further substation computers may be added. These may be independently connected into the SCADA system. Alternatively, an optional additional level of substation computer with the same control facilities, may be added, as in Figure 4. Note now that each substation computer may be physically remote. It is also worth noting that this final topology has in effect become a mini SCADA system in its own right. For many smaller utilities this solution may offer all the remote facilities required.Figure 4: Multi Level Topology 2.3 Communications Hardware Hardware for digital communications can take many forms, most of which are not suitable for use in power system environments. The first choice to be made is between parallel and serial systems. Parallel systems involve the transmission of several bits of information concurrently over several separate wires (typically eight or sixteen). Such systems offer faster data transfer rates than serial systems but involve far higher wiring costs. For this reason they are not suitable as a universal solution for power system monitoring and control. Serial communications involve the transmission of streams of data one bit at a time over a single pair of wires. Clearly wiring costs are reduced at the expense of overall data transmission rates which are proportionally lower. For monitoring and control applications the slower data rates remain acceptable and serial type communications are used almost exclusively. The communications hardware most commonly used by protective relays at present conforms to the EIAs RS232 standard. This takes the form of the familiar 25 or 9 way D connector. This has usually been used to connect the relay to a personal computer (sometimes indirectly, via a modem) allowing the relay to be setup and allowing post fault information to be extracted. RS232 connections are convenient because of their almost universal availability. RS232 connections do have a number of limitations which make them less suitable in monitoring and control applications. The most serious of these is that RS232 is designed for point to point systems. A single device can only communicate with one other device over a given link.If communications with more devices are required, as they are for data monitoring and control within substations, separate links must be provided. Alternatively, multiplexers or code switches could be added though this would generally impose some sort of limitation on how the links are operated. RS232 also imposes a limit on the physical length of the communications link of just over 15, and a maximum data rate of 19.2kbaud. This can also be overcome but again requires additional equipment. Finally RS232 does not offer any significant level of isolation. Optically isolated RS232 ports can be created but these are expensive. A more suitable communications standard is RS485. This allows for a multidrop system with up to 32 nodes on a single spur, sufficient to connect at least a single bay of relays. RS485 specifies a maximum transmission distance of 1200 metres and a maximum data rate over this distance of l00kbaud, significantly further and faster than RS232. It uses a balanced driver and differential signaling which is less susceptible to interference than the unbalanced driver referenced to ground as used in RS232 systems. RS485 requires a single shielded twisted pair cable which is low cost and easy to terminate. Within the electrical industry in general this has typically been terminated in either 25 or 9 way D connectors similar to those used by RS232. In a substation environment these connectors are not really suitable and a pair of conventional terminals is preferred. The use of optical fibres to connect directly between relays remains expensive for most users, especially at distribution voltage levels. Fibres are however suitable for connecting the local network of relays to remote master stations where distances exceed 1200 metres or where the risk of interference is high. In such cases modems are used to interface a group of relays to an optical fibre. As with electrical based communications, a number of different solutions are available. For distances of up to four or five kilometers, 850nm multimode fibres are quite adequate and offer relatively low complexity. For greater distances up to around 25 kilometres, 1300nm single mode fibres can be used. These require more complex transmitters and receivers, however their widespread use in the telecommunications industry means that they may well become more economic than 850nm fibres even over short distances. Serial data communications may be classified as either asynchronous or synchronous. RS232 communications ports on protectivc relays are invariably asynchronous. In asynchronous systems timing or synchronization information is transmitted together with each character. In synchronous systems either a separate clock is transmitted or the receiver derives the clock information from the data itself. Synchronous systems are more complex than asynchronous systems but roughly 20% more efficient. More importantly intercharacter gaps on synchronous systems are fixed. This allows frequency modulated (FM) coding methods to be used which result in no DC component in the signal. Hence the problems of isolation in RS232 systems can easily be overcome using simple transformer isolation. A second benefit of FM coding is that the two signal wires are non-polarized and may be freely interchanged throughout a system. Modern serial control chips allow the extra complexity of synchronous transmission to be absorbed at no extra cost. In summary, of the common communications interfaces, multidrop synchronous RS485 transmission using some form of FM encoding is currently the most suitable for use in power system data measurement and control applications. This can provide fast economic communications with electrical interference immunity sufficient for power system environments. 2.4 Communications Language Successful digital communications depends not just on compatible communications hardware but also on the communications language and protocol that are used. Traditionally relay manufacturers (in common with those in other fields) have developed their own languages. This has been less important in the past when there has been no need to integrate the relays into control systems. When the relays have been integrated a bespoke solution has been necessary with custom programming for each different relay. The costs of this have been uneconomic for most users. Until now no language suitable for use by all protective relays has been proposed. The major drawback with most languages is that they assume the master station must have an intimate knowledge of the relay. If a particular piece of data has been required it has been asked for using its memory location in the relay or some device specific code. This address must be explicitly coded into the master station software. Moreover the relay has typically responded with raw unformatted data. The master station must assume a scale for the reply and convert it accordingly. Where a large amount of data must be extracted from a range of relays the problems are further compounded. Even when communications are on a one to one basis errors can still occur if the master station software version doesnt match that of the relay. The language presented here overcomes these problems. It is suitable for use by all