RATIONAL DISTRIBUTION OF EXCESS REGENERATIVE ENERGY IN ELECTRIC TRANSPORT SYSTEMS ON THE BASIS OF FUZZY LOGIC APPLICATION

Purpose. To develop the fuzzy model of distribution of excess regenerative energy in traction and external power supply systems allowing to exercise effective operation of the power equipment. Relevance. For effective distribution of excess regenerative energy in systems of electric transport, as well as for systems equipped with stationary stores of energy and inverting traction substations with regulators of output voltage it is necessary to solve a number of the problems with high degree of uncertainty demanding taking into account a set of random factors such as the modes of power lines and traction loadings. These factors have to be considered in operation for choosing the rational modes of energy stores, inverters and regulators of voltage on buses of substations to provide the rational conditions for energy regeneration on electric transport. Scientific novelty. The control system of energy storage devices, inverters and traction substation output voltage regulators, designed on the basis of fuzzy logic, can provide the necessary conditions for the regeneration on electric transport on sections with a shortage of traction power consumption and allows to optimize the distribution of excess braking energy of transport vehicles. It is achieved by determining the rational relationship between the components of the excess current of regeneration in real time, which can ensure a minimum of power losses of regenerative energy in traction and external power supply systems. Practical importance. The use of the developed approach is effective under the conditions of incomplete information received by measurement systems and on the basis of additional studies it can allow to minimize rated capacity of stores, inverters and traction substation output voltage regulators that could reduce the costs of modernization of existing sections and electrification of new electric transport systems.


Introduction
The perspective pillar of development of nonautonomous electric transport systems is transition to the distributed traction power supply, introduction of the energy store device (SD) and intellectual train dispatching systems allowing to solve successfully power and environmental problems on transport.Meanwhile, one of the most effective method of energy saving on electric transport still remain the effective use of regenerative energy, which in the case of the rational modes of the vehicles' movement allows to reduce the power consumption for transportation process in different modes of movement by 15...30% (Shevlyugin, 2007;Sulim et al., 2012).However, owing to specifics of traction power consumption modes the average value of regenerated energy, for example in systems of the main transport, does not exceed currently 2...3% (Shcherbak and Nerubackiy, 2011;Sergienko, 2010;Sopov, 2012).It is mainly related with a time spread of processes of energy consumption and energy generation by the vehicles which are on a section in the traction and regenerative modes that is especially noticeable in the case of small traffic.Energy quality during regeneration on the buses of substations must also be taken into account (Sychenko et al., 2015).Complex application of the mentioned technologies can improve regenerative modes of the electric power on non autonomous electric transport.

Review of literature and problem statement
For providing the regenerative mode of energy the voltage on a pantograph of the regenerating train has to exceed the voltage on a contact line (Getman, 2011) and not exceed the maximal admissible value (GOST 6962-75, 1975;EN 50163, 1996;Red, Pawlik, 2015).Very frequently (especially in the case of small traffic) there is a problem of consumption of the regenerated energy.Under the conditions of decreased traction power consumption or lack of it on a regenerative zone the voltage on pantograph of the regenerating trains reaches the maximal admissible value max U (fig. 1) and return of energy to a traction line becomes impossible.Fig. 2 shows the realizations of voltage and current of a suburban electric DC train EPL2T on some section.Here it is possible to observe frequent unsuccessful acts of regenerative braking of the train, which termination is caused by achievement of voltage on a pantograph the maximal admissible value (4,0 kV) owing to absence in a regenerative zone of trains in the traction mode.In addition, the effectiveness of the modes of regeneration depends on the voltage level at the busbars of traction substations (Sychenko et al., 2015).In this case the energy saving effect from regeneration decreases as the generated energy is utilized by brake mechanisms of the vehicle.
The solution of problem of consumption of excess regenerative energy can be carried out in several directions (Shchurov, 2002): 1) transmission of energy from a traction line to external power supply system; 2) optimization of train schedules; 3) using of energy store devices; expansion of a regenerative zone by regulation (decreasing) of voltage on buses of the traction substations (TS).At stopping braking of vehicles takes place a shortterm energy generation, which can be characterized by the indicators given in (Sablin, 2014).Considering the fact that transmission of braking energy of trains from a traction line through the traction substations (TS) to the external power supply system (on DC sectionthrough the inverting TS) is possible under a condition when its reception by a line is coordinated both in technical, and commercial aspects, such as (Shchurov, 2002): the maximal admissible short-term power accepted by a line; the acceptable level of harmonics; readiness of the power supplying company for refinancing.The measurements made on mountain section with inverting DC TS (Sablin et al., 2016) show that efficiency of consumption of excess regenerative energy by external power supply system (on 35, 110, 220 kV) depends of the modes of not traction loadings in knots of TS connection, especially during the periods when the power supply system is underloaded.Lack of necessary value of not traction loadings in a power line increases the voltage on TS inputs in the regenerative mode and leads to the termination of energy transmission from a traction line, and to the termination of the regeneration on a rolling stock on a condition of the maximal admissible voltage on pantograph.Besides, it is experimentally established that in the case of connection of adjacent inverting TS to lines at different voltage regeneration of energy near one TS (connected to 35 kV) cannot lead to its transition to the invertor mode, and transfer of excess braking energy of the train occurs through the remote TS (connected to 110 kV).In this case there is unequal distribution of loading of the inverting TS in the recovery mode that leads to increased regeneration energy losses in traction line.Thus, for realization of effective distribution of regeneration energy in systems of electric transport (as well as in the perspective power supply system equipped with energy store devices, reversible TS with smooth regulation of output voltage) it is necessary to solve a number of the problems with high degree of uncertainty demanding the accounting of a set of random factors such as the modes of power lines and traction loadings.These factors have to be considered in dispatching of energy store devices modes , inverters and voltage regulators on TS buses allowing to provide rational conditions of energy regeneration.

Purpose and research task
The purpose of this article is creation of indistinct model of regeneration energy distribution in traction systems and in external power supply system on electric transport, taking into account the prospects of using on TS inverters, energy stores and regulators of output voltage.For rational distribution of regenerative energy in this article the problem of dispatching of the modes of energy store devices, inverters and regulators of output voltage on TS is solved on the basis of fuzzylogic.

Formulation of the problem of rational distribution of excess regenerative energy
For providing rational conditions for regeneration on electric transport in traction power supply system equipped with stationary operated energy store device (SD) and reversible TS with smooth regulation of the output voltage (fig.3) it is necessary to solve a number of problems with high degree of uncertainty.It involves taking into account many random factors, such as the modes of power lines and traction loadings, which directly influence to optimum algorithms of dispatching of SD, inverters and regulators of output voltage on TS.The scheme of regeneration current decomposition on electric transport reg I is given at fig. 3 , where excess current is shown as a part of reg I , which cannot be directly consumed on traction by passing (opposite) trains that are in a recovery zone, i.e.  elements of traction and external power supply system it is necessary to provide the minimum possible distance to the potential consumers (the trains, SD, external power system) taking into account their efficiency.The problem of rational distribution of excess regenerative energy of trains can be defined as finding out the ratio between values of current' components (1) in real time that minimize the criterion function (fig. 1, 3).
The requirements of modeling accuracy demand to take into account many factors defining rational distribution of regenerative energy of train, development of difficult mathematical models and measurement methods, which realization demands big expenses.The experts assessment can serve as a basis for decision-making process (Passino, 1998; Wang, 1997).
Considering the low level of support information in traction power supply systems, for decision-making according to (2) the principles of fuzzy dispatching can be used.Fuzzy regulation laws of the modes of traction power supply system provide maintenance of voltage on pantograph of the regenerating trains in admissible limits under the condition of insufficient traction power consumption in a regeneration zone.It is realized by making decisions about the level of regenerative energy that could be consumed by store devices (depending on their charge), returned to the external power supply system (depending on its loading), and transferred to the adjacent zones between TS to the remote trains (depending on their situation on a section).It are given at fig. 4. The linear form of accessory functions and its parameters are chosen according to the expert opinion.However, other form of accessory function can be chosen (for example, exponential) on the basis of comparison of the solution of this task by determined and fuzzy methods.The proximity of the received results will correspond to the more adequate accessory function.The mentioned accessory functions can be presented as tab. 3 and 4.    The model of rational distribution of excess regenerative energy is developed on the basis of five blocks of rules, which structure is given in fig. 5.Each block of rules uses Mamdani's method for an fuzzy conclusions (Piegat, 2001).Blocks are connected in the form of sequence for ensuring stepby-step decision-making, on the set priorities.

Fig. 5. Structure of the model of regenerative energy distribution
The output of the first block serves as an input for the following one that allows to determine the need of distribution of the rest of energy according the directions with less priority.The inputs for the last block of a output are all previous decisions that allow to define a conclusion only if the decision was not made yet.If there is deficiency or lack of a traction power consumption a on a section of regeneration ( 1 1max UU  , fig.3), it is necessary to make a number of commutation of the power equipment on TS for providing the required conditions for regeneration.Rules for making decisions about distribution of current exc reg I , elaborated by the expert, represent a sequence of steps on regulation of the power equipment on TS.
Rule 1.To switch SD in a charge mode: .

Fig. 1 .
Fig. 1.Distribution of voltage along a contact line during regeneration of the electric power of current of regeneration consumed on traction by passing (opposite) trains; reg2 I the part of current of regeneration utilized in brake rheostats; reg3 I the part of current of regeneration, which can be transmitted to adjacent zones to remote trains by regulation of voltage on transit TS buses; reg4 I the part of current of regeneration, which is consumed by SD; reg5 I the part of current of regeneration transmitted to the external power supply system (via TS inverters).

Fig. 3 .
Fig. 3. Distribution of current of regeneration in traction and external power supply system For reduction of regenerative energy losses reg P  in

Fig. 4 .
Fig. 4. Accessory functions for input parametertraction loading in a regeneration zone (a) and output parametera part of regenerative current consumed by brake rheostats (b)

Fig
Fig. 6a.Decision-making area for equations

Table 3 .
Accessory function for traction load on a regeneration zone

Table 4 .
Accessory function of the part of the regenerative current consumed in brake rheostats To switch on TS to the inverting mode (if voltage of a power line