| Abstract (english) | The conventional power grid forms the foundation of modern society, ensuring a continuous supply of electricity that is essential for daily life and economic development. This grid is based on centralized electricity generation in large power plants, such as thermal power plants, hydropower plants, and nuclear power plants, from where energy is transmitted via high-voltage power lines to the distribution network and finally to end consumers. Such infrastructure, developed over the past century, allows for efficient and reliable delivery of electricity to a large number of users. However, the conventional power grid faces numerous challenges, including the need to reduce its environmental footprint, transmission losses, and the integration of an increasing share of renewable energy sources. Understanding the structure and functionality of the conventional grid is crucial for recognizing its strengths and limitations, as well as for guiding future innovations and improvements that will meet society's growing energy needs. The growing concern over climate change, environmental pollution, and the depletion of fossil fuels highlights the urgent need to transition to sustainable energy sources. Given the increasingly evident consequences of climate change, such as extreme weather events, rising sea levels, and ecosystem degradation, the need to shift to sustainable energy systems has never been greater. Renewable energy sources, such as solar energy, wind energy, hydropower, geothermal energy, and biomass, represent a key component of the global strategy to achieve energy security and sustainable development. They not only offer the potential to reduce greenhouse gas emissions but also to diversify energy resources, create new jobs, and stimulate economic growth. The development and implementation of technologies for harnessing renewable energy have significantly advanced in recent decades. In Croatia, as in many other countries, there is significant potential for utilizing renewable energy sources. The geographical location, climate conditions, and natural resources offer numerous opportunities for the development of various forms of renewable energy. Despite the obvious advantages, the integration of these technologies into existing energy systems faces numerous challenges. Among them are high initial costs, the impact on system operation management, the need for infrastructure improvements, variability in energy production, and regulatory and political issues.
With the extensive implementation of renewable energy sources into the power grid, the complexity of the electrical protection system increases. The drive of distributed sources implies permanent parallel operation with the condition of a galvanic connection to an active, i.e. rigid network. In case of loss of connection to the active network, distributed production, i.e. the power plant, has the obligation to disconnect from the rest of the network with the remaining users, which is defined as an island operation. Inadequate disconnection of the power plant from the network can lead to danger to the operating personnel, damage to the equipment both for the users of the network and in the production plant itself. For networks with an APU (automatic reconnection) function, distributed generation must be switched off before reconnection, while for networks without an APU, the IEEE 1547 standard gives a maximum islanding duration of 2 seconds. Ensuring the implementation of protection against islanding is carried out through numerical protection that operates at the interface of the production facility and the network. Although there are several types of islanding protection, the most commonly used are voltage vector angle shift protection and frequency rate of change protection. Other methods involve injecting a certain type of signal while waiting for a response in changing the electrical parameters of the system for the purpose of islanding detection. With each new connection, the distributed source goes through a trial run in which the response of the protection against island operation is tested. The effectiveness of the aforementioned islanding protection based on voltage vector angle shift and protection against the rate of frequency change as a "mainstream" detection method were investigated. The mentioned methods have been used for several years, and it is clear that their algorithms can be improved in such a way that they meet the new requirements of the electric power system.
This dissertation consists of 6 chapters. In the introductory chapter, protection against islanding in a power system with a high proportion of integrated renewable energy sources analyzed. Deficiencies of existing detection methods and algorithms have been identified and presented.
In the second chapter, a detailed overview of the impact of renewable sources on the power system is given. RES has a direct impact on the short-circuit current in the distribution network. The influence, i.e. the level of contribution to short-circuit currents, depends on its installed power type and the distance to the fault location. The occurrence of non-selective or incorrect operation of the protection in the network may be the reason for not taking into account the newly integrated RES, i.e. if the protection parameters are not adjusted according to the power plant's contribution current, which may be higher than the current threshold value. This possibility is very likely if the OIE drive machine is a synchronous or asynchronous generator or there is a large solar, i.e. concentrated set of power plants. An island or isolated drive still represents a great challenge today and is the subject of numerous researches and tests. According to the definition and legislation, RES may not supply part of the network with other users in the long term if the galvanic connection with the rest of the network has been broken. The influence on the constant periodic distortion of the mains voltage from the sinusoidal form is also called harmonic distortion. The influence factor on harmonic distortion can be caused by the production units themselves, i.e. synchronous and asynchronous generators, and especially from sources or consumers based on power electronics. Influence on automatic voltage regulation. Automatic voltage regulation in the distribution network has been and still uses traditional methods of voltage regulation by the compensation-regulation method, which includes the assumption of only one direction of electricity flow. As standard automatic voltage regulation control devices measure current without direction, the influence of large or concentrated integrated RES, in the network below the point of automatic regulation, can create an apparent false state for the control system and cause the regulation to excessively increase or decrease the operating voltage by changing the direction of the flow of electricity. The occurrence of voltage drops is most often attributed to network failures or sudden changes in the switching of large power in the system. RES is affected in the same way if its installed capacity is large enough that its sudden dropout from the network causes large voltage changes, as well as if it is suddenly connected to the network asynchronously, at nominal power.
In the third chapter, traditionnal methods used to detect islanding are described. The methods of detection and operation of protection against islanding are basically divided into two main sections, local and remote. Local methods are further divided into active passive and hybrid methods. While remote are divided into communication, signal and methods based on artificial intelligence. The success of any islanding detection method is qualified by the value of the size of the insensitivity zone or more commonly none as NDZ-Non Detection Zone. Each method is mathematically related to the parameters by which the islanding is detected, that is, those parameters where the method is insufficient. Passive detection methods are manifested as very fast-acting protection against islanding, but also very unsafe and often ineffective in terms of incorrect and non-selective operations. These mal-operations are triggered with the occurrence of fluctuations in measured quantities caused by failures that do not result in islanding. Their zone of insensitivity is quite large, which results in lower reliability of the detection methods themselves. Active detection methods use the artificial creation of oscillations in the network by measuring the feedback response of certain electrical quantities in the distributed generation system. Artificially generated oscillations affect the change of quantities that enter into the classification of quality parameters. Compared to passive detection methods, they have a relatively small zone of insensitivity. The amplitude of artificially generated oscillations usually does not exceed the value of 1% of the nominal power of distributed generation. Hybrid detection methods use a combination of active and passive methods. Remote detection methods are installed in facilities of distributed sources and system operators, most often in distribution. These methods of detection have proven to be more reliable than local ones, but they require significant investments in the automation system and complex communication infrastructure. It is important to mention that these detection methods have almost no insensitivity zone. At the end of the chapter, a comparative section of all the mentioned islanding detection methods is presented according to the NDZ size factor, the index of wrong or non-selective operation for the RES state when it is not in island operation, and applicability per RES unit.
In the fourth chapter, the various conditions for protection against islanding are listed and described. The network rules provide the legal-technical framework of obligations and the applicability of protection against islanding together with the necessary degrees of protection for separation. Further in the chapter, conditions are described in accordance with the EU RFG regulation 631. Commission Regulation (EU) 2016/631 of April 14, 2016 on the establishment of network rules for unified requirements for connecting RES to the network. The regulation mandates the categorization of production modules from A-D according to the installed power and voltage level of the connection, which each member state has the right to define by setting the parameters within which power plants must maintain operation with the system. The basic set of requirements refers to frequency maintenance, voltage maintenance, requirements according to the level of contribution to short circuits, to the production of reactive power, to the level of management of the regime of the power plant and finally to the quality of supply. In addition, the regulation prescribes the mandatory exchange of information between the operator and the power plant (for type B, C and D, for A it is not mandatory), the communication protocol and content of the exchange telegram is defined by the transmission system operator. The most commonly used protocols for information exchange are IEC 61850 and IEC 60870.
In the fifth chapter, algorithms with improved characteristics and a newly developed algorithm are presented. The introduction to the chapter shows real oscillograms of faults in the network and when islanding occurs. The working principle of voltage vector shift detection algorithms and frequency change rate with improved characteristics is presented. Further in the chapter, the working principle of the islanding detection algorithm based on the voltage frequency differential is presented. The communication scheme used by the algorithm and the level and time resolution of the exchange of measured quantities are described. The chapter contains a graphic representation of the algorithm, a functional description of programming and implementation used in laboratory tests.
The results of the dynamic analysis of the network model and the experimental results are also presented in the fifth chapter. The working principle of algorithms with improved characteristics was carried out by parallel simulation of the results of dynamic analyses. Testing and validation of the developed voltage frequency differential algorithm was carried out by simulating real signals, using a device for laboratory testing of numerical protection that emulates oscillogram values of real faults and island drives. The obtained results indicate that the proposed algorithms really work in the detection of island work and that their zone of insensitivity is significantly reduced compared to the existing ones. The voltage frequency differential detection algorithm, according to the results of dynamic analyses, has the smallest zone of insensitivity.
The sixth chapter concludes the dissertation and provides guidelines for further research. It is indicated that the developed algorithms are applicable and that their implementation in islanding protection devices would be useful. |
