Advantages and disadvantages of microprocessor relay protection

Modern relay protection and automation devices are built on a microprocessor-based elemental base (we denote them as MP device). When discussing the advantages and disadvantages of MP device, they often do this in comparison with electromechanical protection devices (EM device).

This is probably due to the fact that so far the overwhelming number of relay protection and automation devices in operation at Russian energy facilities is precisely EM relay protection. However, in a historical context, it is not entirely appropriate to compare the MP device with EM device. It is obvious that the relay protection and automation equipment was born as an alternative to its previous generation of relay protection, built on a microelectronic base (ME device). Therefore, in comparisons below, for greater objectivity, we will consider both ME device and EM device.

The main advantages and disadvantages of MP device are summarized in table. one.

Tab. 1. The main advantages and disadvantages of MP device

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multifunctionality high price
compactness reliable operating current required
observability needs electromagnetic compatibility
may be part of industrial control system needs information security
self diagnosis narrow operating temperature range
resistance to mechanical stress
simpler selection of settings
high accuracy and stability of measurements
low power consumption
high sensitivity
low load on current transformers
The disadvantages of MP Device
  • High price – the first industrial designs of MP device came to Russia from Western countries, they were very expensive and were used more as a tribute to fashion. However, in the very west, the price of MP device was already comparable to EM device . The fact is that, unlike the USSR, EM device was manufactured there according to high, one might say, space, technologies. But the high price of such an EM relay protection was compensated by the savings on maintenance, because relay protection engineers in the west are highly paid personnel. At present, the price of MP device is gradually decreasing, while more and more budgetary solutions appear on the market. In addition, due to the development of information technologies in the electric power industry, there is a tendency towards a significant reduction in the cost of maintaining the relay protection and automation equipment, and towards a transition to maintenance as it is.
  • The need for reliable sources of operating current – this is the disadvantage of MP device only in comparison with EM device . The previous generation, ME device , in the same way needs operational current, as well as MP device . It should be noted that recently the need for operational power supply of MP device has ceased to be strict. Development applications are found in which the power supply is integrated inside the microprocessor terminal, and the power of both the terminal itself and the disconnection circuits is provided from secondary current or voltage circuits.
  • The need for electromagnetic compatibility is also a disadvantage of MP device, which can only be compared with EM device. ME device is also subject to the influence of external electromagnetic fields, and to a greater extent than MP device. To make sure the last statement is correct, it is enough to recall that digital technology is more noise-protected than analog.
  • The problem of cyberthreats is associated exclusively with MP device. For none of the previous generations of relay protection, it did not exist. However, with a sober look, it is clear that information security is needed only for additional functions that are laid in microprocessor terminals in addition to the relay protection function itself. If MP deviceremove all connections with the external digital world, then this problem will cease to exist for it in the same way as for any other relay protection system of previous generations. Without prejudice to additional functionality, this could be done by placing protections in the microprocessor terminal on a separate board, informationally isolated from the outside world and from other boards.
Advantages of MP Device
  • Multifunctionality – functionally one microprocessor relay terminal is able to incorporate a whole set of all kinds of relays, a complex logic circuit and many output relays (actuators). Filling with protection can be so large that we can already talk about centralized protection, covering the whole switchgear. In many ways, this is an undoubted benefit, but multifunctionality reduces the reliability of the protection system. One way to increase reliability to the proper level can be duplication. A good option for duplication is to implement it within one terminal of MP device. In this case, two independent from each other instances should not only present the relay protection functionality, but also the current and power circuits.
  • Compactness – often one panel of MP device is able to replace several panels of EM device, but if installed in cells, the terminals of MP device also significantly save space. The terminals of MP device can become especially compact if the IEC 61850 standard (or its new analogues) finds wide application, because massive analog input cards and multiple discrete I / O cards will become unnecessary. At the same time, in the relay protection and automation panels, the sizes of terminal rows and cable ties will also dramatically decrease.
  • Observability, self-diagnostics and integrability in automatic process control systems – all this can significantly increase the reliability of the relay protection system (potentially): careful monitoring of the state of relay protection devices allows timely detection of its malfunction; Using a comprehensive analysis of the emergency mode and protection actions, vulnerabilities can be detected in the functional part of relay protection and automation equipment.
  • Resistance to mechanical stresses is an advantage of MP device in comparison with EM device, it is also inherent in all static relays. This advantage is used where relay protection experiences great mechanical disturbances, for example, in submarines.
  • Low load on current transformers – unlike EM device, when calculating the load on current transformers, the resistance of the current input of the terminal of MP device can be ignored – usually it does not exceed 0.1 Ohm. The resistance of the wires remains, which, as a rule, is much greater than this value. But the application of the IEC 61850-9-2 standard practically nullifies it, especially if optical current transformers are used. Of course, when implementing IEC 61850-9-2, you can encounter many nuances, but this is a separate issue.
  • High sensitivity – both in ME device and MP device, the sensitivity coefficients of measuring organs are much closer to unity than in EM device. Also, the sensitivity is increased due to more complex forms of relay response characteristics with two input values, which is easiest to implement in relay protection and automation equipment. Here it could be noted as an advantage of MP device the use of measuring bodies for emergency components. However, emergency components have long been actively used by EM device, in so-called filter shields, even if it may not be as effective as in MP device.

Thus, in almost all respects, MP device surpasses the previous generation – ME device. Compared to EM device, it is not so simple, especially in terms of electromagnetic compatibility. For example, this aspect can be fatal in terms of national security in military conflicts.

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