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1、 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 necessar
2、y. 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. Intro
3、duction 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 int
4、egration 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 d
5、evices, 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 t
6、ransducers, 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 de
7、rived 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 inform
8、ation 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 an
9、d 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 cos
10、ts 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 t
11、o 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 process
12、ing 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 availabl
13、e 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.
14、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, di
15、gital 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 se
16、cond. 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
