CC BY
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Section 10. Technical sciences
T1 = 1,2.Tk = 0,045Н
а) б)
а) б)
Рис. 4. Зависимости перемещения фрагмента s = s(t)(мм) (кривые — а) и скорости v = s(t)(м/c) (кривые — б) от времени t(сек) при ю = 50с_1 (об/мин), о10 = 30с_1 (об/мин) и различных значений
натяжения Т(Н).
Список литературы:
1. Патент на полезную модель РУз №FAP20060452. 28.11.2006. Барабан для рыхления и очистки хлопка-сыр-ца//Патент на полезную модель Республика Узбекистан №FAP03913. 2006 г. Юнусов Р. Ф., Бородин П. Н., Хакимов Ш. Ш. [и др.].
Kholiddinov Ilkhombek Khosilzhonovich, Senior Research Fellow, External Doctorate Candidate of the Chair "Electric Power Stations, Grids and Systems" of the Abu Rayhan Beruniy Tashkent State Technical University
E-mail: [email protected]
Monitoring of the electric power quality characteristics in the low-voltage power grids
Abstract: The present article considers the issues of electric power quality, particularly unbalance of voltage and current, as well as additional losses in the low-voltage power grids. It describes the methods of measurement of
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Monitoring of the electric power quality characteristics in the low-voltage power grids
the electric power quality characteristics. For the purposes of monitoring of the separate electric power system it is suggested to use domestic devices of measurement of the electric power quality characteristics. The example of calculation of additional losses of electric power in the low-voltage grids is provided
Keywords: power grid, measuring device, methods of measurement, electric power quality characteristics, unbalance of current and voltage, additional losses.
Energy industry being one of the main economic sectors of the Republic has been developing at a growing rate for the years of independence and it not only satisfies the needs of the State in electric power, but it also allows to provide electric power export.
Mostly the efficiency of utilization of electric power is determined by means of creation of such consumption conditions which allow to provide the required quality of electric power and minimum production losses.
Delivery of electric power to the consumers is performed by the enterprises of territorial power grids via power transmission lines 0.4-110 kV of Voltage and 237.1 ths.km in length.
Development of this section until the year 2020 is established in accordance with the Decree of the President of the Republic of Uzbekistan dd. March 4, 2015 No. UP-4707 “On the Program of measures on provision of structural transformations, modernization and diversification of production for the years 2015-2019”.
Widespread industrial domestic use of electrotechnics and devices with nonlinear current-voltage char-
acteristics results in unbalanced operation modes of three-phase four-pass power grids 0.38 kV which can be explained by the fact of non-simultaneous change of the load curves and non-uniform distribution of the singlephase consumers by phases.
Numerous changes in transformer substation TP-10-6/0.4 kV have shown that power grids (PF) —
0.38 kV operate in unbalanced mode with violation of permissible values of the electric power quality characteristics (EPQC) on balance [2].
It is peculiar for the unbalanced modes of PF 3.8 kV that two EPQC are violated simultaneously: [3]:
• the value of modules of the phase-to-neutral voltage changes on the clamps of powerconsuming unit (PCU) and on the clamps of the feeding mains transformer secondary winding because of the voltage occurrence (fig. 1):
• nonlinear loads and unbalanced modes create harmonics multiple of three in the phase current and voltage [2; 3], which represent zero-phase sequence from the configuration of symmetrical components.
Fig.1. Voltage and current of the tree-phase four-pass system
The main sources of the voltage unbalance on the negative-phase sequence in the three-phase power supply grids are: arc steelmaking furnaces, induction furnaces, electroslag refining furnaces, alternative current traction
Table 1. - Power-consuming i
substations (27 kV), asymmetrically controlled converters, welding units. Some typical values of the voltage unbalance factor on the negative-phase sequence (К) created by such PCU are given in the Table 1. s, creating voltage unbalance
Type of the power-consuming unit U , kV
1 2 3
DSP-100 220 1.3
35 4.5
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Section 10. Technical sciences
1 2 3
DSP-40 110 1.4
35 4.0
Single-phase electrothermal units 10 18
Alternative current traction substation 110 4.6
Rolling mill 1700 6 1.4
10 2.0
Welding units 0.4 1-5
Normal and maximum permissible values of the voltage unbalance factor on the negative-phase sequence K2U according to the State Standard GOST 13109-97 for the grids of all nominal voltages amount correspondingly ±2 and ±4%. As you can see in the Table, K2U for many power-consuming units is either close to these values or significantly exceeds them.
Unbalance of the three-phase voltage system leads to occurrence of the negative-sequence currents I2U, and in the four-wire grids — zero-sequence currents I0U.
I2U currents cause additional heating of the rotating machines, creating negative rotational moment, decrease rotary speed of the rotors of asynchronous motor (AM) and performance of the mechanisms geared by them. Decrease of the rotary speed, i. e. increase of the AM slip, is followed by the increased reactive power consumption and, subsequently voltage decrease.
When the voltage unbalance amounts 2%, service life of asynchronous motors decreases by 10.8% because of additional losses of active power, service life of synchronous motors decreases by 16.2%, of transformers — by 4%, of capacitors — by 20% [4]. To prevent additional heating it is required to reduce load applied on the motor (torque on the shaft).
According to IEC 892nominal load on the motor is allowed at k2U <1%. When negative-phase sequence factor amounts 2% the load applied on the motor shall be reduced to 96%, when this factor amounts 3% — the load shall be reduced to 90%, when 4% — to 83% and to 76% when the factor amounts 5%. These figures are considered as true ones provided that the motor operates at constant load, i. e. in steady temperature mode.
Review of the electric power quality characteristics (EPQC) is performed with a use of special measuring devices — analyzers ofpower quality at the time intervals from 24 hours (minimum value) to 7 days (recommended value). Instantaneous values of the linear (phase-to-neutral) voltage in the point of common coupling of the electric power consumers are the input data for calculations.
Domestic special measuring device “MALIKA" [5] is developed at the Chair “Electric Power Stations, Grids
and Systems” of the Abu Rayhan Beruniy Tashkent State Technical University (TashSTU). This device is designed for recording of additional process losses and execution of measurement of EPQC in power grids 220-380 V of voltage. This device is applied in the process of power quality analysis and evaluation with a use of static methods including the case when list of characteristics is limited in accordance with requirements of GOST [2].
Analog part provides connection of three phases of alternative current simultaneously; connection diagram and general view of the device are shown in the figure 2.3.
Fig. 2. Connection diagram of the clamp-on ammeters
Fig. 3. General view of the “MALIKA” device
Received analog signal is transformed into digital one which calculates information on the basis of symmetrical components method. As far as is known symmetrical system of voltage can be described by three equations [4, 6, 7]:
UA =Umsin (wt+^0); UB=Umttn(wt+w0 -120°) > UC =Umsin(wt +щ0 -240°) where f0 -initial phase of voltage UA.
(1)
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Monitoring of the electric power quality characteristics in the low-voltage power grids
According to the symmetrical components method any unbalanced three-phase system can be resolved into three symmetrical components: positive phase-sequence, negative phase-sequence and zero phase-sequence which can be described by three vector systems.
U m 1 (U a + aU B + a2U c);
U a(2) = 3 (U a + a2-U в + a-U c )
Ua(o) = 3(Ua aUb +Uc).
(3)
1. л 0120° i-240° 2 j240° i-120°
where a = e = e a = e = e
Analog expressions can be also written for the unbalanced system of currents
1 A(1) = 3 (a + a ' ^ ' R )
1A(2) = 3 (( + ( ' 1 + a ' 4 ) (3)
IA(0) = 3 (IA + IB + IC );
Evaluation of unbalance of currents or voltages in the three-phase grids is performed with a help of unbalance factor of currents or voltages. Determination of the unbalance factor k2 as correlation of effective value of the negative phase-sequence current or voltage I2, U2, effective to the effective value of the positive phase-sequence current or voltage I1, U1, of the main frequency of the three phase system of currents or voltages [4].
Unbalance factor of voltages and currents is equal:
k2U = U .1000% a = a.100% (4)
U, I,
The employees of TashSTU have performed measurements in the substation TP No. 399 of the Steam-Electric-Turbine Plant Zangiatinskiy TPES in the power grids 0.38 kV with domestic and amenities loads (fig. 4,5). The curve of change of the negative phase-sequence and zero phase-sequence factors is shown in the Fig. 6; it is possible to decrease negative and zero phase-sequence factors of currents from 0.3 to 0.04 by means of the loads leveling by phases in the evening peak hours.
Power loss factor in grids determined by the currents unbalance when values of the negative phase-sequence and zero phase-sequence factors of currents K2i = K0i = 0.3 and when phase and neutral wires have
R
equal cross-sections — = 4 [8]:
R
KPK = 1 + K2,
-K2.
r Rл
\R j
= 1 + 0,32 + 0.32 ■ 4 = 1,45
i. e. excessive power losses under unbalanced load are by 45% higher than under balanced load.
It is necessary to note that normative losses of electric power established for the power grids amount — 10%.
Electric power losses in the power grids 0.38 kV equal 10% are determined for the low-voltage grids under the load by formula:
AWK=0.1-WK kW-h
where WK — consumed power with domestic and amenities load.
Power losses factor at K2i = K0i = 0,04 :
KPH = 1-
K2 + K:
v Ri
= 1 + 0.042 + 4 ■ 0.042 = 1,008.
Therefore, when the negative phase-sequence and zero phase-sequence factors of currents decrease from 0.3 to 0.04, the power loss increase factor decreases from 1.45 to 1.008.
During the calculation period from August 27 to 28, 2015, the following amount of electric power had passed through the feeder [9]:
Wa = 195 kW-h — active electric power,
On the basis of these data we can find the following values:
AWK=0.1-195=19.5 kW-h Excessive losses during the day:
f ТА \
SW . =(w -AWa)■
a excessive \ a /
1 - Kph
V
k
pk J
кВт-ч
1.008
= (195 -19.5 )* I 1 —=-1 = 59.9
1.53
Total cost of active electric power lost due to the currents unbalance at the feeder 3 of TP No. 399:
С . w = SW x Cost of 1 kW/h of electric power = 66787,38 kW/h x 155 soms = 9 286 soms As you can see from the given calculations, excessive losses in case of the currents unbalance at the feeder 3 of TP No. 399 during 1 day result in additional costs amounting 9 286 (nine thousand two hundred and eighty six) soms, during 1 month amounts 278 595 (two hundred and seventy eight thousand five hundred and ninety five) soms, and during 1 year amounts 3 389 571 (three million three hundred and eighty nine thousand five hundred and seventy one) soms.
Consequently, unbalance of currents and voltages in the 0.38 kV grid can be eliminated with a help of organizational actions: replacement of the transformers with a connection pattern Y/YN and Y/ZN, rearrangement of the single-phase electric power consumers, etc.
To eliminate unbalance ofthe currents and voltages in the 0.38 kV grid it is also required to use balancing devices.
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Section 10. Technical sciences
Fig. 4. Daily schedule of change of the phase-to-neutral voltages
Fig. 5. Daily schedule of change of the phase currents and current in the neutral wire
Fig. 6. Daily schedule of change of the unbalance factors of negative phase-sequence and zero phase-sequence currents
Essential condition of the effective power quality power with consideration of its quality, which is required
control is utilization of the suggested domestic device for periodic (summer, winter) and constant measuring. which implements the algorithm of payment for electric
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Monitoring of the electric power quality characteristics in the low-voltage power grids
2. State Standard GOST 32144-2013. Electric energy. Electromagnetic compatibility of technical equipment. Power quality limits in the public power supply systems.
3. Golovkin P. I. Electric Power Systems and Consumers of Electric Power. - Moscow: Publishing house “Energoatomizdat” 1984.
4. Electrical Power Quality Control/I. I. Kartashev, V. N. Tulskiy, R. G. Shamonov and others; under the editorship ofYu. V. Sharov. - Moscow: Publishing house MEI, 2006. - 320 pages.: il.
5. Allaev K. R., Kholiddinov I. Kh., Shaismatov S. A., Patent on the Utility Model RUz. No. FAP 20150075 Recording device for additional losses of electric power at the loads unbalance in the low-voltage power grids.//Intellectual Property Agency RUz.
6. Kholiddinov I. Kh. Algorithm of evaluation of the voltages unbalance level//Collected works of the Eighth International Scientific and Technical Conference//“Electric-Power Industry: Control, Quality and Efficiency of the Power Resources Utilisation”.- Blagoveshchensk: 2015. P. 196-201.
7. Kholiddinov I. Kh. The application of an information and measurement device for the calculation of level nonsymmetry of voltages and current in low-voltage networks/European Applied Sciences, Stuttgart, Germany, #7-2015. P. 55-59.
8. Kosoukhov F. D. Methods of Calculation and Analysis of Current and Voltage Unbalance in Countryside Distribution Grids: Teaching Guide. In the collection of scientific works LSHI, 1984. 42 p.
9. Report on the Commercial Contract No. 9/15 concluded with Tashkent TPES, JSC, “Development of the Methods, Software and Hardware for Electric Power Quality Provision in the 0.4 kV Distribution Grids”, 2015. -65 p.
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