THE ANALYSIS OF MAGNIFICATION OF NEUTRAL CURRENT IN THE PRESENCE OF POWER QUALITY PROBLEMS

Transcription

1 THE ANALYSIS OF MAGNIFICATION OF NEUTRAL CURRENT IN THE PRESENCE OF POWER QUALITY PROBLEMS Alla Eldin ABD ELAZIZ Ahmed FATEHY Khalaf RUSHDY MEEDC Egypt MEEDC Egypt MEEDC-Egypt Nelly AHMED MEEDC- Egypt ABSTRACT This paper discusses the causes of the magnification of neutral current in the presence of the power quality problem such as "Triple N" that is, currents with a harmonic order that is a multiple of third harmonic. This issue is particularly important in low voltage systems where harmonic pollution by single phase loads is an increasingly serious problem. Electric distribution Companies (EDCs) are responsible for keeping power quality disturbance within the standard limits. Harmonic distortion is caused by nonlinear devices in the power system, sources of distortion are different at different voltage levels. At low voltage level all kinds of non-linear loads considered in the paper are connected, which globally are the main source of distortion. The harmonic indices are total harmonic distortion, spectrum harmonic and harmonic loss factors. The measurements and load survey were done on a set of common low voltage of different activities. The results are divided into four groups according to the value of neutral current and reasons of its magnification. At some of the results the large value of neutral current is due to the significant value of the triple harmonic and its multiple besides the presence of unbalanced loads as in the cases of study in this paper. The precautions must be taken to prevent the magnification of neutral currents and the solutions to overcome the magnification of neutral current are given and illustrated in this paper. INTRODUCTION The industrial activities were grouped in industrial sectors such as textile, chemical engineering, food, metallic and mining. Also the public utilities and commercial institutions were grouped in several sectors such as banks, hospitals, hotels, exchanges and transportations. Depending on the kind of activity if it is industrial, public or commercial loads differ upon these activities. Since the share of nonlinear loads in electrical disturbances they cause must be increasingly take into account effect of harmonics is: transformer saturation, voltage flicker and incorrect operation of voltage sensitive devices, electromagnetic interference, shorter life of insulation and malfunction of protective relays. Table(1) illustrates sources and effect of harmonics. Table(2) gives some examples of products and processing in different activities. In industrial applications, the loads that generate harmonics are adjustable speed motor drives, switched mode power supplies, converters, inverters, welding machines, power control circuit and arc furnaces. In commercial institutions, loads generating harmonics are personal computers, discharge lamps and uninterrupted power supply in fact, each industrial and commercial load harmonic is a combination between different individual load harmonics and the only accurate way to know the load harmonics values is by measuring the supply loads. Table 1: Sources & Effects of Harmonics Source Of Harmonics Supply Demand Side Side System Nonlinear loads Resonance (rectifiers, adjustable speed controls, fluorescent light, computers) Effect 1-Electronic control malfunction. 2-Nuisance tripping of circuit breakers. 3-Inconsistent meter reading. 4-Data corruption 5-Overheating of motors and transformers. 6-Computer malfunctions. THEORETICAL BACKGROUND In a star connected three phase system, the current in the neutral conductor is the vector sum of the three line currents with a balanced sinusoidal three- phase CIRED2015 1/5

2 Table 2: Some products and processing. Activity Product Processing Textile Textured yarn,raw cloth, dressed Socks Chemical Mining &building material Paper, plastics, rubbers, paints, packaging material Ceramic pipes, glass shields Drawing, heating, boiling, drying, spinning, sewing Pressing, grinding, packing, drying mixing Cutting, grinding, machining, welding, sanding, painting Fig2: unbalanced 3 phase load. Engineering & electrical Tools Milling, surface grinding, turning, drilling, cutting, pressing system of currents, this summation is zero. Hence it means that the neutral current under the normal conditions usually equals zero, (Fig.1) illustrates that. In the case of a three phase power system feeding linear single phase loads, the current in the neutral conductor is rarely zero because the load in each phase is different. With an unbalanced 3 phase load the neutral current is not zero, but it is smaller than the phase current as shown in (Fig. 2). Where non linear loads are being supplied, even when the load is well balanced across the phases, there is likely to be substantial current in the neutral conductor. With non sinusoidal currents, the sum of the three line currents, even with the same r.m.s value, may be different from zero, (Fig. 3) illustrates this case with a non- linear 3 phase load, the neutral current is not zero and can also be larger than the phase current because of homopolar harmonics. Figure 3: non- linear 3 phase load. OVERVIEW OF THE EXAMINED SYSTEM Since 2000, the specialists of Middle Egypt Electricity Distribution Company (MEEDC) carried out more than 150 measurements of electrical parameters (voltage current power power factor harmonics ) in the low voltage (LV) and medium voltage(mv) end user networks. The measurements were done at low voltage bus, for some different cases types of loads. These types such as central telephone exchanges (E1, E2 and E3), One Botogas filling manufactory (B), chemical operations M, two public building (PB and SCC) and hospital building (H). The measuring sites were selected to cover a wide spread range of different facilities and load types. Power quality analyzers are used for real time monitoring, measurement of the electrical parameters and store the data for later analysis. Computer software provides methods to display the recorded data. The measurements were carried out and evaluated during one day at least. Fig1: balanced 3 phase load CIRED2015 2/5

3 Table 3: Results of cases under study Case NO. The major focus of this study is on the value of neutral current and THD values related to current for individual transformers feeding various loads. Also the analysis of the relation between the types of loads which generate more large values of the individual current harmonics which lead to great values of neutral current have been carried out, the other reasons which causes the magnification of the neutral current have been studied. The data are checked in compliance with IEEE std limit, the range for THD current is 5 % to 20 %. It is calculated from the ratio of short circuit current available and the point of common coupling to the maximum fundamental load current. The results were classified into four cases, table 3 illustrates these cases. The harmonic spectrum of third harmonic in neutral conductor and the phasor diagram of each case were illustrated in Figs.4, 5, 6, 7, 8,9,10 and 11 respectively. RESULTS The analysis of the results reveals these points:- The maximum THDi is 28.12% (IEEE standard is 20%). Customer Type of activity IEEE Limit of THDi (%) The maximum neutal to phase is 43% ( percentage current unbalance shall not exceed 10%) The maximum limit value of the odd harmonics is 12 % or 15 % depending on the value of short circuit Measured Value of THDi at Maximum Current (%) IEEE Limit Of Odd Harmonics (<11) Calculated neutral To Phase % 1 H hospital E1 Central telephone exchange 2 PB Public Building Maximum Phase current (Amper) E2 Central telephone E3 exchange M Chemical B Botogas SCC Supervisory Control Centre current calculations related to each studied transformers. Case 1 includes two cases representing a significant source of harmonic pollution, especially in terms of harmonic currents illustrated in the following harmonic bar graph, THDi percentage of maximum current are large and there is already unbalance current percentage shown in the phasor diagram of the measurements. So, this leads to amplification of neutral current amplitude and gives a large value of the calculated neutral to phase percentage. This was shown in Fig.4 and Fig.5 for two customers this case where the percentage value of current unbalance (Iunb %) from the phasor diagram is greater than 10 %. Case 2 includes three cases represents a significant source of harmonic current pollution, especially in terms of 3 rd harmonics and its multiple but there is an acceptable unbalance current percentage shown in the phasor diagram of measurements. This leads to high neutral current value and high calculated neutral to phase percentage as shown in Fig.6,Fig.7 and Fig.8 respectively. Case 3 includes two cases representing moderate source of harmonic current pollution especially in terms of 3 rd harmonics and its multiple and there is already an acceptable unbalance current percentage shown at phasor diagram of measurements. So, this leads to moderate neutral current value and the value of calculated neutral to phase percentage is less than 10%. These results are illustrated in Fig.9 and Fig.10. Case 4 includes one case study which represents moderate source of harmonic current pollution especially in terms of 3 rd harmonics and its multiple. There is a large percentage value of unbalance current shown at phasor diagram of measurements. So, the major part of CIRED2015 3/5

4 calculated neutral to phase percentage is resulted due to unbalance current, moreover load survey has been done, so we found non symmetrical load distribution at three phase power line, Fig. 11. Due to the variety of loads in each customer of the previous cases, the generation of THD in current and 3 rd harmonics values are changed. This leads to an appreciable effect in the value of neutral current. Also, the nonsymmetriacl phase current causes significant value of neutral current. Typically, the power electronic devices generate large value of THDi in terms of 3 rd harmonics. It is clear in case 1 and case2. Where, there are various loads such as communication devices, computers and printers at customers were named E1,E2 and E3 which type of activity is central telephone exchange. The same thing with customers were named (H,Pb) which kind of activity is hospital and public building respectively. Their loads are medical instruments, computers, fluorescent lamps and other office devices. Fig 6: case 2(E3) Fig.4: case 1(E1) Fig 7: case 2(E2) Fig 5: case1 ( H) Fig 8: case 2(Pb) CIRED2015 4/5

5 Fig 9: case3(b) Fig 10: case3( M) Fig 11: case4( SCC) CONCLUSION In fact, third harmonic component (and all other harmonics where the order is a multiple of three the sixth, ninth, etc.) of the line currents are all in phase with each other (i.e. they are homopolar components), so the sum arithmetically rather than cancelling by vector addition. The neutral current amplitude may exceed the phase current in amplitude at the supply frequency due to the third harmonic. These cause some bad effects such as:- i. Addition power losses cause excessive stress due to heat in the neutral conductors. ii. Reduced forward operating torque and overheating of induction motors iii. Excessive electromagnetic interference (EMI) to sensitive equipment in buildings. iv. Additional error in power measurement system. So, it can be recommend that :- 1- Using derating transformers or use K- factor transformers. 2- Oversize all neutral components 1.73 times rated full load current. 3- Use separate neutral conductors for nonlinear loads and avoid shared neutral conductors where practical. 4- Use neutral over current sensors to trip phase conductors. REFERENCES [1] IEEE std , " IEEE Recommended Practices and Requirements for harmonics Control in Electric Power System". [2] Mohammad Alzoubi and Mohammad Obeidat, 2013, "Harmonic Penetration Assessment in Residential Areas", Proceeding of CIRED 22 nd international conference, [3] Hafez El salmawy, Kamelia Youssef, Shereen Abdulla and Iman Ahmed, 2012, " The Effects of Harmonic Distortions On Transformers", Proceeding of 8 th international conference on electrical engineering (ICEENG), 1 7. [4] K.Youssef, A.Khodeir,M.Tantawi and A.Elsaeed, 2004, "Overview Of Electrical Anomalies", Proceeding of 1 st international conference on electrical engineering (ICEENG), [5] Prof.Jan Desmetand and Prof.Anglo Baggini, 2003, "Harmonics Neutral Sizing in Harmonic Rich Installations", Power Quality Application Guide, Copper Development Association. [6] Dr.Prasad Enjeti, 2001, "Harmonics in low voltage Three-Phase Four-Wire Electric Distribution Systems and Filtering Solutions", PSERC Online Seminar. [7] A.ElMofty,2000, "Highlight and Guideline for Commercial And Industrial Load Harmonics Borders", Proceeding of 7 th international MIDDLE- EAST Power Systems Conference( MEPCON'2000), [8] Attia,M.Tantawy and K.youssef, 2000, " Characterstics Of Electrical Loads For Banks ", Proceeding of 7 th international MIDDLE-EAST Power Systems Conference ( MEPCON'2000), CIRED2015 5/5