Software for optimal selection of places for installation of balancing devices in 0,4 kV electric power systems loaded with electric motors

. This publication considers the issues of development of the software program for designing of 0,4 kV power supply systems with motor-actuated load under voltage unsymmetry conditions (using the example of the Trans-Baikal Territory). Voltage unsymmetry is practically constant phenomenon in the electric power networks of different voltage types. Voltage unsymmetry effects significantly the electric power consumers, including the supply mains itself. It has especially negative impact on the electrical equipment operation process and its lifetime. The urgency of the problem is confirmed by multiple research on the same topic and by significant number of damages suffered by the electric power consumers staying in service (especially in the Trans-Baikal Territory and in the Far-East regions). Voltage unsymmetry causes economic loss and reduction of the electromagnetic interference value by the voltage unsymmetry coefficient in negative-phase sequence (K 2U ) gives inevitable economic effect accordingly. However, the payback period for the activities aimed at reduction of electromagnetic interference, will vary from some months to several years. The more accurate value of the payback period may be obtained using the developed software program. The developed software design program is implemented by means of the programming language C# in Microsoft Visual Studio environment, using the built-in cross-platform database SQLite. The software program shall allow making quick and accurate calculation of the power losses, to determine the economic feasibility of provision special measures for removal of the voltage unsymmetry, for determination of optimal application and location of the balancing devices. The software implementation in power systems loaded with electric motors will improve reliability and efficiency of asynchronous motors. The software is of interest for developers of projects on power supply systems for regions with non-linear loads.


Introduction
Currently, a special attention is given to the problems of electric energy quality. In modern conditions quality parameters of electric power such as voltage unbalance, unsinusoidality, become common factors which significantly reduce an efficiency of the power supply systems themselves and, accordingly, an efficiency of consumers connected to them.
A study of the main electric power quality parameters for the network equipment of 0.4, 6 and 35 kV power system in Zabaykalsky Krai showed that deviation of main quality parameters of electric power, such as voltage unbalance, voltage unsinusoidality is a common factor in Zabaikalsky power system. In some cases, unbalance level significantly (2-3 times) exceeds specified in GOST 32144 -2013 [1] value [2].
Due to unsatisfactory condition of the electric energy quality in distribution networks of Zabaikalsky power system, there is a large number of electrical failures at the consumer's facilities. Experience of electrical equipment operation shows that one of the main causes of electrical equipment failure is voltage unbalance.
A reliability and efficiency of electrical equipment depends directly on electric energy quality in distribution networks. For stable operation of electrical equipment, number of measures aimed at improving of electric energy quality, in particular, voltage unbalance elimination should be taken.
In this paper, one of possible methods for electric energy quality improvement which uses balancing devices in the electric power system both at the stage of electric network design, and in conditions of real operation is proposed.
The aim of this work is to develop an application software for optimal selection of places for installation of balancing devices in 0,4 kV electric power systems loaded with electric motors under conditions of voltage unbalance focused on engineering practice

Materials and methods of research
In the research of the asynchronous motors, the problem of assessing the impact of unbalanced phase voltages on the motor operation is highly important.
To get a more complete picture of voltage unbalance impact on asynchronous motors it is necessary to define a dependence of asynchronous motors phase currents versus voltage unbalance coefficient for negative sequence K 2U .
The research of asynchronous motors operation modes can be efficiently carried out with by means of computer simulation using virtual model created in clear and effective simulation tool Simulink in interactive programming environment of MatLab, using a T-shaped equivalent circuit of induction motor [3][4][5].
Authors developed a simulation model ( Fig.1), which allows performing analysis of asynchronous motors operation modes, moreover, using this model, it was possible to study transient processes in the induction motor, measurements of operation and artificial mechanical characteristics, voltage, current and frequency in the power supply network [3].
Computer simulation of direct start of asynchronous motors was performed for motors of 4A, AI type at three-phase supply voltage 380 V with frequency 50 Hz. Using the developed model, the asynchronous squirrel-cage motors were studied with regard to the following parameters: -for different motor shaft loads (idle mode -120%), -for change of value of voltage unbalance coefficient for negative sequence (K 2U ) from 0 to 5%.
The simulation scheme for study of operation modes of asynchronous squirrel cage motor created by means of simulation in Matlab Simulink is shown in (Fig.1).
In the course of process simulation, an analytical curves for the phase currents versus K 2U for series 4A, AI asynchronous motors were obtained.
For each phase of asynchronous motors under consideration, curves for the phase currents versus K 2U were obtained for different values of the motor shaft load.
The general form of the current change in the most loaded phase (phase "A") for series 4A asynchronous motors at Рn = 7,5 kW, AI series asynchronous motors at Рn=11 kW, for different K 2U values and different values of the motor shaft load are shown in (Fig.2,3).
Analysis of the dependences obtained in the course of study of the asynchronous motors modes showed the following: 1) when K 2U increases, current in one of the motor phases decreases and in the other two phases the current increase is observed, 2) it can be seen from the general view of the dependencies that: the more the motor shaft load, the more the voltage unbalance impact on the operating modes of electric motors, 3) the curves general view and directions are the same, that shows that impact of voltage unbalance on different series asynchronous motors is similar.
But currents at full load of the motor are of the greatest interest. When the load is 100%, current in phase "A" of 4A series asynchronous motors with Рn = 7,5 kW exceeds permissible overload current equal to 1,1 of rated current for K 2U = 1,4% [8,9]. For AI series motor with Рn = 11 kW, the permissible current overload is exceeded at K 2U = 2 %.   This shows that the motors not only have different range of acceptable values for K 2U , but they not always meet GOST requirements. Accordingly, if the motor is operated in areas with poor electric energy quality, it is necessary to regulate the supply voltage being guided not by GOST on power quality, but by motor operation mode.
It is obvious that the obtained values of permissible K 2U values for asynchronous motors will have a key role in the selection and implementation of balancing devices in the power system loaded with electric motors.

Formulation of the problem
The work of calculation algorithm development for power supply system taking into account balancing devices and their location, which will be a base of the software for optimal selection of places for installation of balancing devices in 0,4 kV electric power systems loaded with electric motors was performed in accordance with the intended purpose. Block diagram of the calculation algorithm for power system taking into account balancing devices and their location is shown in (Fig.4).
Based on data of the experimental study for main basic quality parameters of electric power, data on power losses in the electric power system components, and based on the allowable modes of consumers operation it is possible to select balancing devices parameters and to define optimal location of these devices in the power system.
Thus, calculation of additional power losses due to voltage unbalance will make it possible to define an economic feasibility of the special measures on unbalance elimination, and in addition to determine the optimal application and location of these devices.

Calculation algorithm for power supply system taking into account balancing devices and their location
Main criterion for the software application is excess of the actual value of voltage unbalance coefficient for negative sequence (K 2U ) in the particular electric power system over the permissible value K 2U specified for asynchronous motors (Table 2) installed in the same power supply system. Required K 2U value should be selected on the base of the permitted consumer operation mode. Permissible K 2U values for asynchronous motors are taken from results of the study [3]. The calculation algorithm for the power supply system taking into account balancing devices and their location can be implemented in the following sequence: 1) block 1entering of the initial data into the software.
The initial data are: -characteristic of the circuit segment (general information about the site); -parameters of the transmission lines (quantity of lines, lines numbers, length of lines; grade of wire and cable used in the line; the line loading); -parameters of electric motors connected to the power line (the line number to which the motor is connected; quantity of motors in the line, the motor type); -parameters of the transformer (quantity and type of the transformer); -value of the voltage unbalance coefficients for negative and zero sequence in the design power supply system; -cost of electric power. 2) block 2performing losses calculation for balanced operation mode in the basic elements of power supply system. Balanced mode current (current for positive sequence) is defined from the expression: (1) where Sfull load of the substation.
Losses in power lines are defined from the expression: (2) wherebalanced mode current (current for positive sequence) [10]; resistance of the phase wire. Full loading of the substation (sum of the loads due to individual lines) is defined from the expression: , where passport data for particular transformer [11]; Sfull load of the substation; nquantity of the same type transformers at the substation connected in parallel. 3) block 3transfer to condition: voltage unbalance is present in the power supply system.
At condition "NO"calculation is stopped. At condition "YES"transfer to block 4. 4) block 4transfer to special measures for unbalance elimination, definition of optimal application and location of these devices. Unbalance voltage coefficients are input for negative and zero sequences for the two variants of calculation, and also selection from the database of suitable devices for the balancing. a) variant 1considers installation of balancing device into common equipment of the lines comprising electric motors; b) variant 2considers installation of balancing device into each line of the circuit segment comprising asynchronous electric motors. 5) block 5calculation of additional losses in key elements of the supply system for unbalanced operation mode. In the course of calculation of increase in additional losses compared to the symmetric mode, presence in the network of amplitude and angular asymmetry can be accounted by using the following formula: , where , current unbalance coefficients for negative and zero sequences [10].
Based on the results of data processing for the main quality parameters of electric power in the equipment of 0,4 kV network located in Mogocha district, Zabaikalsky Krai, we take the following values of the coefficients: 5%, 4,6%, ; , resistance of neutral and phase wires. Accordingly, taking into account (6), the expression (2) can be rewritten as follows: (7) Additional loss of active power in asynchronous motor is defined from the formula: (8) where coefficient which accounts for the specific parameters of the motor (rated power, copper loss of the stator, starting current ratio); voltage unbalance coefficient for negative sequence; the motor rated active power. For industrial loads, it is generally recommended to use coefficient with value of 1,85.
When the system is operated in power transformers in the unbalanced mode for a long time, additional where the voltage unsymmetry coefficient in negative-phase sequence; loss during idle mode; losses during short-circuit mode; short-circuit voltage. Then total losses are calculated. For balanced mode, losses are defined from the following formula: (10) where quantity of the lines; the line number. For unbalanced mode, losses are defined from the following formula: (11) where jmotor number; n dv quantity of the motors in the transition line. 6) block 6calculation of the payback period for each installation of balancing devices and the least one is taken.
Additional losses without balancing devices are defined from the expression: at the initial values of the voltage unbalance coefficients for negative and zero sequences.
If balancing device is installed additional losses can be defined from the expression: (13) at the initial values of the voltage unbalance coefficients for negative and zero sequences.
Selection of the balancing device parameters is performed depending of conditions: a) variant 1power of the balancing device should be more than or equal to the sum of loads in lines outgoing from the central equipment: (14) b) variant 2power of the balancing device should be more than or equal to the sum of loads in each line comprising a motor: (15) Calculation of the total cost of balancing devices is performed by the expression: , (16) where Stotal costs of installed balancing devices; C_dcosts of particular balancing device; k_dquantity of installed balancing devices; kbalancing device number. Calculation of payback period for installation of the balancing device is performed by the formula: where PPpayback period; Stotal costs of installed balancing devices [12,13]; without the balancing devices; supplementary losses with the balancing devices; Сcost of the electric power. 7) block 7 -оutput of the resulting data. The resulting data are: -losses for balanced operation mode; -additional losses for unbalanced operation mode (without balancing devices; variant 1 -for case when balancing device is installed into common equipment of the lines comprising electric motors; variant 2 -for case when balancing device is installed into all lines comprising asynchronous electric motors); -costs of balancing devices (costs of balancing device; total costs of balancing device for variant 1; total costs of balancing device for variant 2; payback period for variant 1; payback period for variant 2).
Calculation algorithm above for electric power supply system will be a base for implementation of software for optimal selection of places for installation of balancing devices in 0,4 kV electric power systems loaded with electric motors operating under conditions of voltage unbalance.
For the software implementation and testing, let us consider segment of power supply system in village Taptugary, Mogocha district, Zabaikalsky Krai.
For this work performance, following initial data were used: 1) results of data processing for main quality parameters of electric power in the equipment of the 0,4 kV network in Mogocha district, Zabaikalsky Krai, 2) configuration of the power supply system in village Taptugary, consumers characteristics.
Power supply system, which was used for the software testing is shown in (Fig.5). The site power system consists of: 1) line 1wired with АС-25 (steel-aluminium conductor), the line length is 350 metres, feeds 7 private residence houses with furnace heating, 2) line 2cabled with GRSh 3*50+1*35 (cable with flexible core, rubber cover and poly amide silk isolation), the line length is 50 metres, feeds the twostoreyed residence house, the primary school, the kindergarten, the administration building and the library, 3) line3cabled with AVVshv 4*50 (cable with aluminium core, PVC jacket, with protective layer in the form of pressed out hose), the line length is 70 metres, feeds the boiler house with two boilers and electric motors: models 4А132S4Y3 with Рn =7,5 kW in amount of 2 pieces, smoke exhaust fan Рn=3 kW, draught fan Рn=1,4 kW, 4) line 4 -wired with SIP 4*25 (self-supporting insulated conductor), the line length is 150 metres, feeds two ban mills with motors model AI132M4 Рn=11kW.

Software implementation
The developed software is intended for reduction of voltage unbalance in segments of 0,4 kV power supply systems for the highest economic efficiency. The software can be used for calculation of economic efficiency of existing power systems modernization [14,15]. The software application will allow performing fast and accurate calculation of power losses to determine the economic feasibility of special measures for elimination of voltage unbalance, determining optimal use and location of balancing devices, with significant increase in productivity of the designer [16,17].
The developed software is implemented using C# programming language in Microsoft Visual Studio environment with the embedded cross-platform SQLite database [18][19][20][21].
The database contains tables for such areas: reference characteristics of the power system elements, table of the power supply system input data, table of output calculation data for power supply system. Each database table has its unique attributes and its purpose in the system.
The software consists of a database that stores all information necessary for calculation and design of 0,4 kV power supply systems loaded with electric motor; calculations module, which implements the algorithm of power losses calculation in the elements of power supply system; module for entering of input data and output of the calculation results for 0,4 kV power supply systems loaded with electric motor and operated under voltage unbalance conditions.
The initial data are data on the site of 0,4 kV power supply system loaded with electric motor.
The data is developed into database (DB) with its structure enabling easy manipulation of the data, and is more flexible, removing redundancy and nonconformity relations.
The database contains the tables of the following areas: reference parameters of the power supply system elements, tables of the power supply system input data, tables of the output data for calculation of the power supply system designing. Each BD Table has its unique attributes and its own designation in the system. Relationships of BD tables (logic model) are given in (Fig. 6).
The logic model represents relation between BD table.
The software interface is clear and simple (Fig.  7,8). The software interface includes means for entering of reference data for the individual elements of the power supply system, entering and display of the initial data for section of the power supply system, output of the calculation information for the power supply system, history of calculations.  The software interface is presented in two languages (Russian and English). There is a button for language selection in the upper right corner of the software window.
Thus, using the software we defined value of power losses in main elements of power supply system for balanced operation mode, for unbalanced operation mode without installed balancing devices, for unbalanced operation mode with installed balancing devices; we defined quantity, power and installation place for the balancing device. It can be concluded from the results of the software use for the scheme under consideration (Fig.5) that it is economically advisable to install one balancing device with a capacity of 45 kW in the common equipment of the line 3,4 loaded with electric motor, but not on each motor individually.
As a result of the software implementation, necessity of use of balancing devices for industrial consumers in the considered scheme was confirmed. The most cost-effective location of the balancing device in electric power supply system, based on the payback period of measures aimed at reducing of electromagnetic interference was defined.

Conclusion
The software is designed for specialists involved in design of 0,4 kV power supply networks and systems operating in presence of voltage unbalance. The proposed software simplifies process of designing of power supply systems; in addition, it can be widely used by project organizations for real conditions of power supply systems operation.
Use of balancing devices reduces current and voltage unbalance in 0,4 kV distribution networks significantly improving the voltage quality at the terminals of electrical consumers, so solutions for these cases are implemented in the software. This increases the operational reliability of electric motors.
Software for optimal selection of places for installation of balancing devices in 0,4 kV electric power systems loaded with electric motors operated under voltage unbalance conditions is the first developed version. Further improvement of the software is possible by expanding the functionality so that it would make it possible to display graphic images of schemes and output a graphical image of calculation diagrams showing location of all balancing devices in electric power system, rendering of the graphs of various parameters dependencies, and also import and export of input and output data of the software.