For a bus coupler or section breaker this would

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Unformatted text preview: e. • 250 • Network Protection & Automation Guide Feeder 1 Feeder 2 CT CT PU CB PU CB CT PU CB CT CB Fibre optic link Personal Computer PU Central Unit CU System Communication Network PU: Peripheral Unit CU: Central Unit Figure 15.20: Architecture for numerical protection scheme The application of numerical relay technology to busbar protection has lagged behind that of other protection functions. Static technology is still usual for such schemes, but numerical technology is now readily available. The very latest developments in the technology are included, such as extensive use of a data bus to link the various units involved, and fault tolerance against loss of a particular link by providing multiple communications paths. The development process has been very rigorous, because the requirements for busbar protection in respect of immunity to maloperation are very high. The philosophy adopted is one of distributed processing of the measured values, as shown in Figure 15.20. Feeders each have their own processing unit, which collects together information on the state of the feeder (currents, voltages, CB and isolator status, etc.) and communicates it over high-speed fibre-optic data links to a central unit. For large substations, more than one central unit may be used, while in the case of small installations, all of the units can be co-located, leading to the appearance of a traditional centralised architecture. For simple feeders, interface units at a bay may be used with the data transmitted to a single centrally located peripheral unit. The central unit performs the calculations required for the protection functions. Available protection functions are: a. protection b. backup overcurrent protection c. breaker failure Network Protection & Automation Guide d. dead zone protection In addition, monitoring functions such as CB and isolator monitoring, disturbance recording and transformer supervision are provided. Because of the distributed topology used, synchronisation of the measurements taken by the peripheral units is of vital importance. A high stability numerically-controlled oscillator is fitted in each of the central and peripheral units, with time synchronisation between them. In the event of loss of the synchronisation signal, the high stability of the oscillator in the affected feeder unit(s) enables processing of the incoming data to continue without significant errors until synchronisation can be restored. The peripheral units have responsibility for collecting the required data, such as voltages and currents, and processing it into digital form for onwards transmission to the central unit. Modelling of the CT response is included, to eliminate errors caused by effects such as CT saturation. Disturbance recording for the monitored feeder is implemented, for later download as required. Because each peripheral unit is concerned only with an individual feeder, the protection algorithms must reside in the central unit. The differential protection algorithm can be much more sophisticated than with earlier technology, due to improvements in processing power. In addition to calculating the sum of the measured currents, the algorithm can also evaluate differences between successive current samples, since a large change above a threshold may indicate a fault – the threshold being chosen such that normal load changes, apart from inrush conditions do not exceed the threshold. The same • 251 • Busbar P rotection 1 5 . 10 N U M E R I C A L B U S B A R P R OT E C T I O N SCHEMES • 15 • considerations can also be applied to the phase angles of currents, and incremental changes in them. One advantage gained from the use of numerical technology is the ability to easily re-configure the protection to cater for changes in configuration of the substation. For example, addition of an extra feeder involves the addition of an extra peripheral unit, the fibre-optic connection to the central unit and entry via the MMI of the new configuration into the central unit. Figure 15.21 illustrates the latest numerical technology employed. However, studies of the comparative reliability of conventional high impedance schemes and modern numerical schemes have shown that assessing relative reliability is not quite so simple as it might appear. The numerical scheme has two advantages over its older counterpart: a. there is a reduction in the number of external components such as switching and other auxiliary relays, many of the functions of which are performed internally within the software algorithms b. numerical schemes include sophisticated monitoring features which provide alarm facilities if the scheme is faulty. In certain cases, simulation of the scheme functions can be performed on line from the CT inputs through to the tripping outputs and thus scheme functions can be checked on a regular basis to ensure a full operational mode is available at all times 15.10.1 Reliability Considerations Busbar P rotection In considering the introduction of numerical busbar protection schemes, users have been concerned with reliability issues such as security and availability. Conventional high impedance schemes have been one of the main protection schemes used for busbar protection. The basic measuring element is simple in concept and has few components. Calculation of stability limits and other setting parameters is straightforward and scheme performance can be predicted without the need for costly testing. Practically, high impedance schemes have proved to be a very reliable form of protection. • In contrast, modern numerical schemes are more complex with a much greater range of facilities and a much high component count. Based on low impedance bias techniques, and with a greater range of facilities to set, setting calculations can also be more complex. 15 • Reliability analyses using fault tree analysis methods have examined issues of dependability (e.g. the ability to operate when required) and security (e.g. the ability not to provide spurious/indiscriminate operation). These analyses have shown that: a. dependability of numerical schemes is better than conventional high impedance schemes b. security of numerical and conventional high impedance schemes are comparable In addition, an important feature of numerical schemes is the in-built monitoring system. This considerably improves the potential availability of numerical schemes compared to conventional schemes as faults within the equipment and its operational state can be detected and alarmed. With the conventional scheme, failure to reinstate the scheme correctly after maintenance may not be detected until the scheme is required to operate. In this situation, its effective availability is zero until it is detected and repaired. 1 5 . 11 R E F E R E N C E S 15.1 The Behaviour of Current Transformers subjected to Transient Asymmetric Currents and the Effects on Associated Protective Relays. J.W. Hodgkiss. CIGRE Paper Number 329, Session 15-25 June 1960. Figure 15.21: Busbar protection relay using the latest numerical technology (MiCOM P740 range) • 252 • Network Protection & Automation Guide...
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