Compliance to the principles of free access to gas networks on a market basis, objectivity reflection of the system's cost, transparency and predictability of market participants requires changes to the tariff setting rules for transportation services. The introduction of the "entry-exit" tariff model is intended to ensure the achievement of new market requirements and the establishment of common European rules for conducting economic activities. Changing the internal model of gas transportation also results in a change in the pricing principles for services provided by the TSO. Henceforth, the charge for transportation services should be taken not for the transport distance, but for the capacity of the entrance and exit points which should be sold publicly and independently. It allows for a more objective consideration of the operating costs of servicing the system and providing greater commercial variability in ordering system services by business entities.
However, European legislation does not define a single standard for the "entry-exit" model. Depending on the technical features of the network and the priorities of the national policy, each country determines the best option for itself. In this paper we used a set of mathematical models to evaluate the implications of the introduction of three alternative variants of the tariff model in two variants of workload GTS (optimistic and pessimistic scenarios): full "entry-exit" model, two market zones, and "entry-exit" model with a long-term reservation. The results of the calculations were compared with the baseline scenario, which does not involve a change in the market model and gas transit is carried out in accordance with the terms of long-term contracts of 2009.
In general, changing the model for providing transport services in the gas market should lead to fundamental institutional transformations of contractual relations between all market actors and the principles of market functioning.

Keywords: natural gas market, transport operator, «entry-exit» model, long-term transit contracts

The desire to integrate into the European development model stipulated that Ukraine voluntarily assumed international commitments about transformation of the institutional environment according to the European model. The level of political awareness of the content of the necessary changes and how to achieve a systemic balance of reforming different sectors was often declarative in the absence of any sound estimates, including the implications for different economic actors.
However, the need to reform the energy sector was determined not only by international obligations, but also by the presence of domestic problems, which over time without the application of systemic changes would become barriers to the entire further economic development of Ukraine. The main content of the implementation of European directives in the energy sector was just intended to systematically transform market relations by changing the rules of market functioning, which was supposed to solve the main crisis problems in various areas of the energy sector.
The recent model of Ukrainian natural gas market was featured by all the shortcomings of the prereformed state of European markets: the tariff system did not promote rational consumer behavior, which exacerbated the pressure on the state budget; the delivery rules did not guarantee the proper quality of service; the absence of investment in the gas transportation sector made it impossible to expand fixed assets; the established transport and distribution rules became a barrier to free access to third-party networks and the development of competition in the supply segment.
The actual implementation of the Second and Third Energy Package standards in the functioning of the national market of the natural year began in 2015, with the adoption of the Law on the Natural Gas Market. Today, an urgent task remains to estimate the process of reforming the natural gas market and to efficiently solve those crisis phenomena that caused the distortion of the institutional environment of the natural gas market.
Since the accumulated problems of the previous model of the natural gas market were due to the inefficiency of the institutions, the article attempted to evaluate the effectiveness of the gas sector transformation process and analyze the impact of changes in certain market segments from the perspective of neoinstitutional theory and study the level of transaction costs.

Keywords:natural gas market, transaction costs, transformation of industrial organization, liberalization

The article considers the main reasons that have led to the debt crisis in the natural gas market of Ukraine due to the accumulation of debt from gas supply and gas distribution companies. The purpose of the study is to identify the main prerequisites for the formation of debt in different segments during the reform of the natural gas market through partial implementation of the requirements of European energy legislation.
Having chosen the European vector of development Ukraine is interested in meeting the requirements of European policy within the framework of its international obligations and aspirations for European integration. This necessitates adaptation of the national institutional environment to the basic requirements of the Second and Third energy packages. According to the existing international obligations, the national natural gas market is undergoing significant changes towards liberalization and development of competitive environment. However, in the segment of retail market and supply of natural gas to household consumers for a long time the necessary changes in the institutional environment did not occur or temporary decisions were made, which did not contribute to solving the sector’s crisis, but only froze the existing model of relationships between various market participants. Reluctance to implement unpopular solutions in the supply of natural gas to household consumers and attempts to shift the increase in the cost of gas supply tariffs for the needs of the household consumers to other categories of consumers led to a constant redistribution of monopolistic influence and accumulation of significant debt between market participants.
The article analyzes the causes of debt between various participants in the retail segment of the natural gas market, which makes it possible to identify the main factors of such debt. These factors include: the lack of a sufficient level of transparency in the tariff for natural gas supply and transportation services; the absence of a 100% accounting for various categories of consumers, which makes it necessary to establish consumption standards, which later become the object of significant abuse; the existing model of public service obligations, which, given the significant redistribution of natural gas, results in significant market distortions; and the imperfect approach to calculating the costs of gas distribution companies in terms of temperature coefficients in the tariff, provided that gas metering exists not in energy units, but in cubic meters. Based on the results obtained in the article, the author sets forwards various recommendations for the implementation of measures, whose main content is to eliminate the institutional prerequisites that lead to the risks of market failures, which may cause debt.

Keywords:natural gas market, household consumers, gas distribution companies, gas supply companies, arrears