Investigations Of Electric Power Quality In The Distribution Power System Of Republic Of Macedonia

INVESTIGATIONS OF ELECTRIC POWER QUALITY IN THE DISTRIBUTION POWER SYSTEM OF RE.PUBLIC OF MACEDONIA Ljubomir Nrkoloskr, Goran Rafajlovskr, VencislavMalrnovskr

University “Sts Cyril and Metodij”, Faculty of Electrical Engineering POB 574 9 1000 Skopje, Republic of Macedonia ABSTRACT Power quality nowadays is growing concern both in utility and consumers. It has many aspects, and it is difficult to evaluate them all at the same time. This paper focuses itself mainly on the voltage distortion measurement as a main aspect of voltage quality. Possibility of voltage sags and flicker measurement is also given. At the University “Sts Cyril and Mertodij” Skopje, Macedonia, a research program for assessing the voltage quality in power system has been started. A short description of measurement strategy and methodology for the power network at Power Company of Macedonia (ESM) is presented. They are carried out as a case studies. In this paper the measuring system is described and the results of long term harmonic measurement conducted mainly in the middle voltage network of ESM. Measurement sessions lasted 7 days and were camed out at different locations. Some measurement results and discussions are given at the end.

In the countries of the so called “transition” period (eastern European), the industry production obviously has dropped down and so some aspects of voltage quality have trend of bettering, without specially contrameasures taken. But nevertheless, the power quality research is interesting, because we almost have never made studies and measurements in previous times. We also hope for new rising trend in industry production in the near future. And the rate of increasing of non-linear loads of consumer electronics (audio visual equipment, personal computers) is even growing from year to year also in nondeveloped countries [2] The negative effects of poor quality are also present in the network of ESM. Because of lacking knowledge, sometimes the negative effects are ascribed to other reasons (e.g. poor insulation). The research project for the first time in Macedonia will deal with theory, analysis and measurement of power quality aspects.

2. THE STRATEGY OF POWER QUALITY

INVESTIGATION 1. INTRODUCTION In the last decade the harmonic levels have been significantly raised due to increasing application of power electronic converters and other non-linear loads. It is expected that this trend is going to continue or even speed up in coming years. Harmonics provoke numerous negative effects well described in the literature [ 11. Therefore, the harmonic phenomenon becomes wide spread and constant concern of electric supply authorities. Many countries have set limits on harmonics, trying to prevent further degradation of power quality. Also, supply authorities are encouraged to invest large financial resources in harmonic research projects. The areas of harmonic analysis and measurements are of special interest. The frequency of the voltage sags depends on the overall quality of management and construction of the network. The assessment of this aspect needs extensively large project for monitoring and statistical analysis. Flicker measurement also needs specially designed instruments.

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Electric Power Company of Macedonia (ESM) is the only power company in the country. Up to 1990 it was part of power system of former Yugoslavia and UCPTE, but now works as independent power system. It has about 570000 consumers of all sizes, around 6150 MWh annual delivered electric energy and almost 1400 MW installed generating power. It has 400, 200 150 and 110 kV transmission voltage levels and 110, 35, 20, 10 and 0,4 kV distribution voltage level (1 10 being both transmission and distribution voltage level). The total length of distribution lines is 22200 km 5200 of it being cable lines. Most widespread medium voltage distribution network is at 10 kV. The level 35 kV, being most important distribution level in the past, is not developed any more. The cites are mainly fed through 110110 kV substations (one, two or more depending on the city largness) and many 10104 substations. Each 10 kV feeder from 110/10 kV substations supplies several others 10/04 substations. The power quality measurement in the research project are planned as case studies at different measurement sites. The measurement sites shell be in the

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distribution voltage level mainly I OkV and 0,4kV), and also in chosen consumer in industry and in other housing office and trading buildings, or some special sites (e.g. TV studios). For the start we choosed the distribution network. The measurement sites were chosen concerning characteristics of connected loads grouped at certain area. The six typical types of consumers are distinguished: 1. Large residential area with collective settlement; 2. Large residential area with individual housing; 3. Mixed area - city centre with office buildings and shopping centre; 4. University campus; 5. Suburban area, small town and village; 6 Individual industrial loads with large power consumption. Usually 7 days period was chosen for every site which is most favourable concerning harmonic generation. The aim of measurement was to obtain overall scheme of harmonic existence in power network of ESM, their level and main sources, and also the possibilities of measurement of other voltage quality parameters, such as voltage sags and flicker. Using such knowledge, it is planned to compare the results to the already existing standards and recommendations and establishing a procedure for non-linear load connection approval in ESM.

3. DESCRIPTION OF MEASURING SYSTEM The measurements were performed using specially designed computer based system which is the state of the art in power quality measurement. The system is made in the company TEAM Gmbh from Herten [3], Germany and is donated as a gift from Fachhochshule Bielefeld, Germany to faculty of El. Engineering - Skopje. Block scheme of measuring system is presented on fig. 1. -. .-..-

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12,5 kHz.It can measure high harmonic content (up to 50 -th order), unbalance and flicker. All measuring functions can be performed on 3 phases (3 voltages and 3 currents). The measuring signals are captured on the measuring transformers (voltage and current), which is convenient when me,asuring in power system on high and middle voltage. 'We have galvanic insulation through optical couplers from D/A conversion. Current signals are captured via Hall effect transducers. The instrument has A/D resolution of 16 bit, and depending on measuring task, the system has different sampling rate: For flicker measurement it uses 400 Hz,for spectral analysis 5,6 kHz, for oscilloscopic monitoring of signals .. 12,5 kHz, and for transient recorder measurements - 6,4 kHz. Calculations and evaluation of measuring signals is done with compliance of technical recommendations and standards such as IEC 1000-2-2, DIN VDE 0838, DIN EN 60555 and DIN VDE 0846 [4]. The measuring system consists of control unit (PC) measuring unit where are the circuits for conditioning of measuring signals and A/D conversion, and also of connecting accessories. The software which is used for conducting of measurement [5] [6], evaluating and presentation of the results, is installed on PC, better a Laptop type. The measurement could be separated from evaluating and presentation, which could be done "off line" in a convenient environment. The structure of the software is presented in fig. 2.

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Figure 1: Block scheme of computer based measuring system for power quality assessment. The measuring system called N€2M 111 can measure and evaluate several low frequency power quality aspects at point of common coupling (PCC) from several Hertz to

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Figure 2: The software structure of the NRM measurement system.

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There are two modes in measurement: monitoring and automatic mode. Some measuring function are possible only in monitoring mode (e.g. oscilloscope) and some are possible in both modes. In the monitoring mode it is not possible to memorise the data over the time. It is possible only to monitor the measured quantities and to memorise screens from the display. In the automatic mode, all measured quantities are stored in memory over the time in compressed form. After the measurement is over, they can be decompressed, evaluated and presented in "off line" mode. It is not possible to carry all measurements at the same time. Only the spectral analysis and flicker measurement could be done together in automatic mode, but only in consecutive time intervals. The transient recorder function could be performed only as an independent measurement alone. The function "spectral analysis" could be performer both in manual end automatic mode. There are several averaging time intervals of spectral measurement. They can be chosen 1 sec (only for the short measurements lasting to 1 hour), 1 min (most preferable time interval in 7 day measurement), 10 minutes and 15 minutes (appropriate in power maximum measurement). After evaluation, also the power over the time and unsimetry over the time could be also presented. The unsimetry is defined as a ratio of inverse to direct component of the 3 phase system. The amplitude of the harmonics could be also statistically evaluated. In the measuring function of flicker we can monitor and measure instantaneous value of flicker and statistical quantity such as short term and long term flicker. With the function of transient recorder we could record the transient phenomena such as switching or voltage sags in short time intervals of maximum 5 sec.

4. MEASUREMENT RESULTS The measurements have been carried out at 4 different locations in city of Skopje. They have lasted normally 7 days. The measuring sites could not always correspond purely to the type of consumers stated earlier. It was because we carried out our measurements at the PCC and on feeders from which many other 10/0,4 substations and also various types of consumers were fed. But nevertheless, a dominant type of consumer could always be discerned. The first measurements have been conducted at 230 V socket in a room at Faculty of El. Engineering in Skopje. The results of 3 days tracing of phase voltage total harmonic distortion (THD) is shown in fig 3. There is significant increase of harmonic distortion in evening hours. This could be explained with the fact that there are many non-linear loads as one phase rectifiers, such as PC. But it is also result from overall presence of harmonic distortion from the 10 kV PCC. They are characterised with the domination of the fifth harmonic.

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Figure 3: Three days tracing of phase voltage total harmonic distortion (THD) at low voltage 230V level 7HDIWIII

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Figure 4: THD of phase voltage, THD of current at 35 kV side of feeding transformer of substation 35/10 kV "Industriska"

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Figure 5.: THD of voltage and unsimetry of voltage in the feeder "pivara" at 10 kV bus in substation "Industriska".

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However, the level of harmonic is below limits stipulated in harmonic standards and recommendations. Some results of 7 days measurement at 35 kV level in the substation "Industriska" in Skopje are shown at fig. 4. The measurement were carried out at 35 kV side of the 8MVA feeding transformer and the whole fecding current were measured. This load is mostly industrial (approximately 75%) but there are also some residential areas of the town fed from it. The fith voltage harmonic is about 1,3% and the seventh abouth 0,35%. For the feeder "Pivara" (beer factory) at the 10 kV bus of substation "Industriska" we have purely industrial load. THD is below limits. The third voltage harmonic reaches 0,3%, the fifth up to 1,5% and the seventh 1,2%. The maximum THD of voltage is about 2%.

fig.6), third voltage harmonic beeng up to 0,5%, f i l l up to 2,2% and seventh up to 0,75%. Because the measurement

were carried out in winter time when some feeding lines were overloaded, there were many supply interruptions. Especially because the network there is mostly aerial, sensitive at moisture and fog conditions.

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Figure 7: THD of voltage and current (all three phases) at 110 kV level in 110/10kV substation "Glavinov" in Skopje The current is measured at one of the two 35MVA feeding transformlxs. Figure 6: THD of voltage and current of residential area (high housing) in the substation 110/10kV "Aerodrom" at the feeder No 6 (10 kV) I

Fig 6 shows THD of voltage and current in 10 kV feeder which supplies residential area in Skopje, all of them high residential buildings. There is dominant 5-th harmonic in voltage, characteristic for non-linear one phase rectifiers. The third voltage harmonic reaches 0,6%, the fifth up to 3,4% and the seventh 1.4%. The maximum THD of voltage is about 4%. In fig.7 the THD of phase voltage and current on 110 kV side of feeding transformer of substation 110/10 kV "Glavinov" in Skopje is shown. The fed area is mostly residential, but there is approximately 1/3 of it as an industrial load. The dominant voltage harmonics are the fifth (up to 1,1%), the third (up to 0,6%) and seventh (up to 0,5%). The results shown in fig. 8 are recorded at feeder No 4 (10 kV) at substation "Glavinov". The feeder supplies residential area with individual housing, mainly villages in Skopje surrounding. But also on this feeder is approximately 1/3 industrial 1oad.Tlie harmonic distortion is lower then in residential dense populated area (compare

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Figure 8: Voltage and current THD for feeder No.4 at 10 kV level in substation "Glavinov" Skopje The next measuring period we connected the instrument on the same feeder in transient recorder mode, and we captured some transients and voltage sags, as

shown in fig.9.

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harmonics are generated from power electronic, converters, mainly from three phase rectifiers, either controlled for DC motor drives or uncontrolled for AC motor drives (rectifier inverter sets). However, in all cases, voltage harmonics were below limits indicating that harmonic capacity of the network have not been filled. Voltage THD limits of 5% were applied as reference, which is widely accepted in many country's standards. The results of measurement that such limiting values are also acceptable in case of power network of Macedonia. Voltage sags measurements is difficult to measure in large scale. For this we need many measuring devices located in different sites of the network. With only one instrument we could carry only case study, but also with difficulties, because for statistical evaluation the time interval for measurement can be very long. For flicker evaluation we use the instrument in combined measurement, spectral and flicker measurement in consecutive time intervals in our case studies of Power Quality measurements.

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Acknowledgments

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The authors gratefully acknowledge the help from Prof. Dr-Ing. J. Schlabbach from Fachhochschule Bielefeld, Germany and Dr-Ing. D. Blume from TEAM Herten Germany for steering and helping this work. This paper is in part sponsored by Ministry of Science and by Electric Power Company ESM of Macedonia.

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Figure 9: Voltage sag and transients at switching on of the feeder No. 4 (10 kV) in substation 110/10 "Glavinov" in Skopje 5. CONCLUSIONS

The presented results show that with this instrument we can carry out case studies and evaluate different aspects of power quality. The results of harmonic measurements show presence of voltage and current harmonics, the fifth being dominant in residential areas, while the fifth and the seventh in industrial loads. It is interesting to note that in residential

areas the level of the TKO is higher than in industrial. This could be ascribed to lowering of industrial production in the last years. The main cause of it in residential areas as well in mixed (industrial and residential and commercial) are found to be TV sets and the other appliances with single phase rectifiers inside( PC, radio sets, audio and video equipment, video games and similar). In industry, the

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6. REFERENCES [l] J Arillaga, D. Bradley, P. Bodger: "Power System Harmonics", J. Wily & Sons, Chichester, 1985. [2] V. Katic: Computer based harmonic measurement systems: Discussion and realisation, IEEE - Int. Conf. on Harmonics in Power Systems, ICHPS, V, Atlanta, Sept. 1992, pp. 16-22. [3] Dirk Blume: Informationtechnische Analyse Leitungsgebundener Netzruckwirkungen, Dissertation Uni Dortmunt 1994. [4] Standards: IEC 1000-2-2; DIN VDE 0838; DIN VDE 0846; VDE 0839; IEEE 519. [SI Ljubomir Nikoloski, Goran Rafajlovski: Voltage Quality - First Experiences in Republic of Macedonia, 2nd International Conference on Electrical Equipment Diagnosis, Electrical Energy Quality, Pula, Sept. 1997, Croatia [6] J. Schlabbach: Expert system measures harmonics and EMC. IEEE - CAP (Computer Applications in Power), Bd. 7 No3, July 1994, pp 26-29.

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