A few days ago, Montenegro, as well as parts of several countries in the surrounding area, faced what we have, fortunately, only theoretically encountered so far: the collapse of the electric power system (EES) or blackout in modern terminology. This is also the reason for this review, with the aim of introducing the wider, lay public, to this issue, which is increasingly topical in professional circles around the world. At the same time, I do not want to prejudge in any way and draw conclusions about the real reasons and the initial point of failure, nor is this possible in the absence of information. The real reasons will be known after a detailed case study by ENTSO, the European Network of Transmission System Operators, which unites 40 system operators in 36 European countries.
In the earlier, conventional EES, electricity was generated exclusively by the use of rotary electromechanical converters - synchronous generators in hydroelectric and thermal power plants. These are machines of enormous power and enormous swing masses. Such machines are capable of overcoming short-term, even significant disruptions in the EES, usually successfully overcoming such disruptions thanks to the enormous energy accumulated in the rotating masses - a rotor of enormous mass that rotates at high speed. Their inertia gives time for other protection to react (say, switch off the faulted line) before they fall out of synchronism. If this is not the case, there is a cascading outage of several different sources of electricity in the system, which leads to a complete collapse of the system, known as the collapse of the EES or blackout. So, in short, a blackout is a situation when, over a wider geographical area, in a longer time interval, there is an interruption of the electricity supply.
Additionally, the stability of the EES of a country is significantly contributed by its connection with the EES of other countries in the surrounding area via high-voltage lines, as is the case with Montenegro.
However, with the development of modern EES, electricity is generated more often and to a greater extent from renewable sources of energy, sun and wind, although we should not forget that water is also a renewable source of energy, which is often used.
As for the electricity obtained from solar power plants, in that process there is no rotation and swinging masses, so these power plants do not in any way contribute to the stability of the EES in the aforementioned sense - transient stability, which implies system stability to significant disturbances in the network.
As far as the influence of wind generators on the transient stability of the EES is concerned, the situation is significantly more complicated and it mainly depends on the type of wind generator. Today, the current division into four types of wind generators, where two types are most often in use: a) a double-fed asynchronous generator with a static converter of limited power in the rotor circuit (also present in wind farms in Montenegro) and b) a synchronous generator connected via a static converter of full power to network. The stator winding of the first one is directly connected to the network and if the converter in the rotor circuit is inert, and to a large extent it is, it can have a short-term positive effect on the transient stability. In the second case, it is connected with the appropriate control algorithms of the converter through which the generator is connected to the grid. On the other hand, it should be kept in mind that the energy accumulated in the rotating masses of these machines is significantly less than that of conventional synchronous generators.
In other words, in modern EES, the increase in the amount of electricity generated is dominantly at the expense of solar and wind power plants, with slightly changed conditions as far as rotating masses are concerned, so the problem of transient stability of EES is becoming more and more noticeable. This topic has been very topical in recent years and is the subject of analysis in professional circles around the world.
As far as Montenegro is concerned, the influx of new, renewable energy sources is still not so great that one could expect their negative impact on the transient stability of the system. Of course, their planning in the future should definitely take this aspect into account. However, we should refer to one Montenegrin specific. It is the existing HVDC (high voltage direct current) interconnection Italy - Montenegro. This interconnection of significant capacity has its many positive sides. As for its influence on the transient stability of our system, it should be said that this connection can have a significant positive effect. However, it depends on at least two factors. The first is the question of what kind of agreement there is between Italy and Montenegro on the assumption of power in the event of such critical situations. The second is related to the first, which is what the control algorithm is, that is, how the converters on both sides of the Adriatic coast are managed in such cases.
The stability of the EES can be viewed from the other side as well. Namely, in the past, EES were planned and designed to meet the needs of industry and the population of the country they cover. Today, the situation is significantly different. With the establishment of the electricity market, high-voltage electric lines become to a large extent transit lines through which it is not possible to simply and unambiguously define power flows, so their significant overload can occur, which can result in an eventual outage.
By the way, it should be said that it is also indicative that at least part of the EES of Serbia and/or Kosovo did not collapse. As the main reason for this, I see the fact that both of these EES are still dominantly based on classic synchronous turbo-generators as primary sources of electricity, i.e. large swing masses and energy accumulated in them. Most likely, thanks to this fact, they successfully overcame the short-lived, although obviously significant disturbance.
What is up to us in the future is to learn a lesson, face reality and challenges, properly plan, invest and regularly maintain our EES at all voltage levels, keeping this aspect in mind. All this with the aim that such outbursts never happen to us again, regardless of where and what is their primary cause.
The author is a professor at the Faculty of Electrical Engineering, University of Montenegro
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