Application of Reactive Compensation equipment in industrial systems under aspects of harmonics and switching transients: A study of real case

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1 Application of Reactive Compensation equipment in industrial systems under aspects of harmonics and switching transients: A study of real case Patrick Roberto de Almeida BREE patrick.almeida@bree.com.br Alexandre Moriel da Silva BREE Alexander.moriel@bree.com.b r Marcelo Inácio Lemes BREE. marcelo.lemes@bree.com.br Flávio Resende Garcia BREE flavio@bree.com.br Abstract It is well known that the presence of harmonics in electrical power grid, almost of them due to the use of electronic loads at residential, commercial and industrial plants, affects the behavior of the voltage and current, therefore harming the life and functioning of equipment. In addition, the strong sensitivity of these electronic loads the sudden voltage variations generated during the transient operation of reactive compensation equipment can lead to the shutdown or the burning of these sensitive loads. This paper presents the effects and problems solutions related to harmonics and switching transient on application of equipment for power factor correction in two industrial systems where the loads are driven by frequency inverters. Index Terms Harmonics, Harmonic Resonances, Transient, Capacitors Banks, Passive Harmonics Filters. I. INTRODUCTION From the Decade of 9, non-linear loads have been given significance in residential, commercial and industrial electrical systems. In the industry, we have the equipment applied to drive electrical three phase inductions motors, such as soft-starters and frequency inverters [1]. Non-linear loads inject harmonics in power grid, decreasing the power factor and increasing levels of voltage and current harmonic distortion. In General, the first corrective measure is to use capacitors bank (BC) for power factor correction because it is a standardized parameter by ANEEL and regulated/supervised by power utilities, Another solution for power factor correction and harmonic content reduction, being one of the most effective solutions, considering both cost versus benefit, take into account the installation of shunt passive harmonic in those circuits [1]. In electric systems which require large capacitors banks, it must also evaluate transient conditions occurred during the energization of these equipment. During these switching, the transient voltages may exceed the tolerance limits of sensitive equipment such as, for example, soft-starters and frequency inverters, leading to performance of their protection or even damage them. Frequency inverters manufacturers have recommended limits of tolerability to the maximum switching transient voltage of 13% of nominal voltage. The purpose of this paper is to present the impacts produced by the application of equipment for power factor correction in two industrial plants with strong presence of sensitive electronic loads under the point of view of power surges during the energization of the same and the occurrence of harmonic resonances, as well as solutions for both problems. II. MODELING OF INDUSTRIAL SYSTEMS FOR THE ANALYSIS OF HARMONICS EFFECTS The systems under analysis consist of industrial consumers that have in their electrical grid three phase induction motors driven by frequency inverters. The Fig. 1 and Fig. 2 show the electrical diagram of the mentioned systems. Fig. 1. Electrical system unifilar diagram under study - Case 1. The electric system showed in Fig. 1, a mining industry, has three transformers that feed the loads in 13.8 kv with estimates of installation of 2 capacitors banks of 372 kvar in each transformer for the power factor correction and voltage bracket installation. The non-linear loads (frequency inverters) were modeled as well as a current source composed by harmonics orders. The electric system showed in Fig. 2, a cement industry, has 138/6.6 kv transformer feeding loads in 6, 6kV with estimates of installation of 1 automatic capacitor bank with 4 stages of 7 kvar for power factor correction and voltage bracket installation. For the non-linear loads (frequency inverters) were used current harmonic source model equivalent of all motor /16/$ IEEE

2 inverter. The system also includes the equivalent of all the capacitors bank installed with the linear loads. Fig. 2. Electrical system unifilar diagram under study - Case 2. Table I presents the short circuit levels in 138 kv for both cases. TABLE I. LEVELS OF SHORT CIRCUIT AT 138KV. Cases Voltage (kv) Three Phase Icc (ka) Three Phase Scc (MVA) Table II presents the electric power transformers data. TABLE II. POWER TRANSFORMERS ELECTRICAL DATA. Cases Busbar Voltage Power (kv) (kva) Z (%) Conection Busbar A Δ Y a 1 Busbar B Δ Y a Busbar C Δ Y a 2 Busbar A Δ Y a Table III presents the capacitors bank data. a. Wye grounded conection. TABLE III. SHUNT CAPACITORS BANK ELECTRICAL DATA. Cases Effective V b (kv) Effective Q b (kvar) Nominal V b (kv) Nominal Q b (kvar) x372 c x x x892. b. V is defined as voltage and Q is defined as reactive Power of the capacitors bank. c. 2 units per Busbar. Table IV presents the data of the loads, constant impedance model. Cases 1 2 TABLE IV. LINEAR LOADS ELECTRICAL DATA. Busbar / Load Apparent Power (kva) PF Busbar A Busbar B Busbar C MIT Loads Table V and table VI presents the data of harmonic injection by non-linear loads for modeling of harmonic source in each secondary of the transformers. TABLE V. HARMONIC INJECTION - CASE 1. Busbar A Busbar B Busbar C H Ih(A) H Ih(A) H Ih(A) TABLE VI. HARMONIC INJECTION - CASE 2. Busbar A H Ih(A) H Ih(A) The harmonic current sources inserted into each busbar of 13.8 kv and 6.6 kv, represent the harmonic current injection resulting from all the non-linear loads existing in each bar. III. RESULTS OBTAINED IN CASE 1 HARMONICS AND SWITCHING TRANSIENTS The simulations show the results of the harmonic flow, voltage harmonic distortion as well as power surges during the power-up of the BCs and harmonic filters (FH) in the case 1. Table VII shows THDv values (%) of the busbars without the presence of the BCs and FH. TABLE VII. THDV(%) OF THE BARS WITHOUT CAPACITOR BANKS AND HARMONIC FILTERS IN CASE 1 Busbar Busbar A Busbar B Busbar C THDv(%) A. Results of the Harmonic Flow using Pure Capacitors Bank- Case 1 The simulation aims to get possible resonances, when the presence of capacitors banks. The Fig. 3, Fig. 4 and Fig. show harmonic spectrum of voltage at busbar A, B and C. Fig. 3. Harmonic spectrum and voltage total harmonic distortion at A busbar.

3 Fig. 4. Harmonic spectrum and voltage total harmonic distortion at B busbar. the limits recommended by IEEE 19/214 [2], without the need of harmonic filters. In the C also occurs a voltage distortion amplification due to the presence of pure capacitors bank, which generates a resonance next to the 7th harmonic, with values above the limits recommended by IEEE 19/214 [2], therefore is recommended to install harmonic filters. B. Results of the harmonic flow with th and 7 th orders with passive harmonic filter-case 1 Figure 9, Fig. 1 and Fig. 11 show voltage harmonic spectrum at the busbars A, B and C with two harmonics fillters tuned at th and 7th order, both connected in each busbar. Fig.. Harmonic spectrum and voltage total harmonic distortion at C busbar. It is highlight that only in the bar (C), the voltage total harmonic distortion exceeds the % limit recommended by the IEEE 19/214 [2]. Figure 6, Fig. 7 and Fig. 8 show the impedance curve of A, B and C with capacitors banks installed to power factor correction. Fig. 9. Harmonic spectrum and voltage total harmonic distortion at A busbar Fig. 1. Harmonic Spectrum and voltage total harmonic distortion at B busbar Fig. 6. Impedance curve as a function of the harmonic order at A busbar. Fig. 7. Impedance curve as a function of the harmonic order at B busbar. Fig. 11. Harmonic spectrum and voltage total harmonic distortion at C busbar Figure 12, Fig. 13 and Fig. 14 show the busbars impedance curves A, B and C with the two harmonic filters tuned at th and 7th order connected in each bar. Fig. 8. Impedance Curve as a function of the harmonic order at C busbar. Note that in A and B, the voltage distortion amplification due to the presence of pure capacitors bank that generates a resonance between the th and 7th harmonic, however within Fig. 12. Impedance curve as a function of the harmonic order at A busbar.

4 Fig. 13. Impedance Curve as a function of the harmonic order at B busbar. -2,,3,6,9 1,2 [s] 1, (f ile Transitório_Chav eamento_de_bancos_barras_independentes.pl4; x-v ar t) v :BUSC_A v :BUSC_B v :BUSC_C Fig. 17. Transient voltage at busbar C during the energization of the capacitors bank at busbar C. Table VIII shows a summary of the results of the energizing of the capacitors Bank. TABLE VIII. SUMMARY OF THE VOLTAGES AND CURRENTS OF ENERGIZING Fig. 14. Impedance curve as a function of the harmonic order at C busbar. In all of them the voltage total harmonic distortion is below the threshold of %, as IEEE 19/214 [2], stressing THDv (%) at C busbar, which shows decreased 3.% approximately. In the A, B and C impedance curves reach maximum module 7 Ω, 2.2 Ω and 2.Ω respectively, with tune on th and 7th harmonic order and parallel resonances in the 4th and 6th harmonic orders, which are not harmonic characteristics and low amplitude in the system studied. Replacing capacitor banks for harmonic filters does not change the maximum impedance curves modules, but allows you to have control over the tuning frequency and the parallel resonance frequency. C. Results of the capacitor bank energization Case 1 Figure 1, Fig. 16 and Fig. 17 show the voltage behavior of A, B and C for the capacitor bank energization of A, B and C respectively ,,3,6,9 1,2 [s] 1, (f ile Transitório_Chav eamento_de_bancos_barras_independentes.pl4; x-v ar t) v :BC1A_A v :BC1A_B v :BC1A_C Fig. 1. Transient voltage at A busbar during the energization of the capacitors bank at busbar A, B and C respectively Busbar VT (kv P- FN) VT /VEFFECTIVE (PU) Current (A P) A B C The analysis of the results shows that the voltage can reach values close to 1.8 pu at 13.8 busbars and on the capacitors banks, spreading across wiring. The value measured is above of the recommended values by the manufacturers of inverters/electronic equipment (Vmax < 1.3 pu for transient). The power surges last around 1 to 2ms. This fact implies the strong possibility of improper shutdowns of sensitive loads due to overvoltage. Although current waveforms are not submitted, the higher magnitude of these at the line and at the capacitor bank amounts to about 1.8 times the rated current of the capacitor bank, far below the recommended value of 1 x In [3]. D. Results of Energization with th and 7th tuned Harmonic filters- Case 1 Figure 18, Fig. 19 and Fig. 2 show busbar voltage behavior A, B and C during the energization of the harmonics filter at busbar A, B and C ,,3,6,9 1,2 [s] 1, (f ile Transitório_Chav eamento_de_filtros_barras_independentes.pl4; x-v ar t) v :BUSA_A v :BUSA_B v :BUSA_C Fig. 18. Transient voltage at the A busbar during the energization of the harmonics filter at busbar A, B and C respectively ,,3,6,9 1,2 [s] 1, (f ile Transitório_Chav eamento_de_bancos_barras_independentes.pl4; x-v ar t) v :BUSB_A v :BUSB_B v :BUSB_C Fig. 16. Transient voltage at B busbar during the energization of the capacitors bank at busbar B and C respectively.

5 ,,3,6,9 1,2 [s] 1, (f ile Transitório_Chav eamento_de_filtros_barras_independentes.pl4; x-v ar t) v :BUSB_A v :BUSB_B v :BUSB_C Fig. 19. Transient voltage at B busbar during the energization of the harmonic filters at busbar B and C respectively. Fig. 21. Harmonic spectrum and voltage total harmonic distortion at the busbar A ,,3,6,9 1,2 [s] 1, (f ile Transitório_Chav eamento_de_filtros_barras_independentes.pl4; x-v ar t) v :BUSC_A v :BUSC_B v :BUSC_C Fig. 2. Transient voltage at C busbar during the energization of the harmonic filters at C busbar. Table IX shows a summary of the results of the energizing of harmonic filters. TABLE IX. SUMMARY OF THE VOLTAGES AND CURRENTS OF ENERGIZING. Busbar V T V T /V EFFECTIVE Current (kv P-FN) (PU) (A P) A B C The results show that the voltage can reach values close to pu on the bar, i.e. below 1.3 pu as manufacturers ' recommendation of frequency inverters. Although current waveforms are not submitted, the higher magnitude of these at the line and in the harmonic filters reaches approximately 7.1 times the rated current of the harmonic filters, far below the recommended value of 1 x In. IV. RESULTS OF CASE 2-HARMONICS AND SWITCHING TRANSIENT The simulations show the results of voltage harmonic distortion as well as power surges during the power-up of banks of capacitors and harmonic filters 2 case. The table X presents the THDv value (%) of the bar without the presence of the capacitors banks and harmonic filter. Fig. 22. Impedance curve as a function of the harmonic order at the busbar. It is highlight that the total harmonic distortion of voltage reaches 2.24% and is below the % limit recommended by the IEEE 19/214 [2]. The resonance is near the th and 7th harmonic order that can be worrying if the harmonic content in this bar is incremented in the future. At first, in this case, there is no need for harmonic filtering. B. Results of the Harmonic Flow with passive Filter 4.2nd- Case 2 Figure 23 and Fig. 24 show harmonic spectrum of voltage and impedance curve at busbar A, respectively. Fig. 23. Harmonic spectrum and voltage total harmonic distortion at busbar A TABLE X. THDV (%) OF THE BAR WITHOUT CAPACITORS BANK AND HARMONIC FILTER IN CASE 2. Local THDv(%) Busbar A 1,68 A. Results of the Harmonic Flow with capacitor Bank-Case 2 Figure 21 and Fig. 22 feature harmonic spectrum of voltage and the impedance of the busbar, respectively. Fig. 24. Impedance curve as a function of the harmonic order at busbar A. In Fig. 23 observed the detuned harmonic filter, reducing somewhat the breadth of th harmonic order. Fig. 24 shows the impedance curve of the bar, which shows the tuning frequency,

6 the parallel resonance between the 3rd and 4th harmonic order and the maximum module of 2.2 Ω. C. Results of the capacitor bank Energization-Case 2 Figure 2 shows the behavior voltage bar during the energization of the capacitor bank stages [s] (file VCim_EqBCMT.pl4; x-var t) v:x4a v:x4b v:x4c Fig. 2. Transient voltage at busbar during the energization of the capacitors banks stages. It is observed that during the first stage there is switching overvoltage of 78%. For the second stage, the surge reaches 38.2%. During the third stage the energy surge is 23.39% and for energizing the room stage over-voltage 16.9% of the nominal voltage reaches. In this way, the overvoltage that occurs during the switching stages of capacitor bank can cause problems in operation of sensitive equipment installation. The amendment of the capacitor bank into harmonic filters has as aim, in general, the replacement of the inrush reactors for reactors with higher inductance (microhenry order to milihenry), i.e. reactors that tune the filter on a given frequency designed for the system. D. Results of Energization with Detuned Filter of th Harmonic 4.2-Case 2 Figure 26 shows voltage behavior at busbar during the energization of the harmonics filter stages. 6 [V] [s] (file VCim_EqFHMT.pl4; x-var t) v:x4a v:x4b v:x4c Fig. 26. Transient voltage at bubar during the energization of the harmonics filters. recommended by IEEE 19/214, requiring the application of harmonic filters. In the others busbars "A" and "B", although the resonance, THDv levels (%) are below the limit referred above. In case 2 the predominant harmonic are the th and 11th order. The presence of capacitors banks produces resonance near th and 7th harmonic, which can cause severe damage when the entry of new loads in the system, whether or not consumer, that have non-linear feature with injection of harmonic orders. Despite the resonance, THDv levels (%) are below the limit of IEEE 19/214 not being necessary the application of harmonic filters. With respect to the transient energizing of the reactive compensation equipment in both case (1 and 2), despite them having inrush reactors that limit switching transient currents, it is highlight that the power surges present peak values up to 1.8 pu. This value is above the limits of tolerability of inverters and other sensitive electronic equipment, which are the recommendation by manufacturers support up to 1, 3pu. In this way, it is necessary to change the capacitors for harmonic filters with the replacement of the inrush reactors for reactors with higher inductance (microhenry order to milihenry), choosing such inductance for tuning a frequency generated by charges for the same filtering. The presence of reactors with higher inductance in the system increases the damping factor, decreasing the power surges during switchings. The solution chosen is to install harmonic filters, because despite the voltage total harmonic distortion does not exceed the limits recommended by IEEE 19/214 at the busbars, the transients during the switching may impair the operation of sensitive loads. Thus, the passive harmonics filters have in these studied cases, drain the system harmonics, avoid dangerous resonances, limit the overvoltage generated during switching and provide the necessary reactive energy for power factor correction. REFERENCES [1] GARCIA, f. R, Handout "Harmonics in Electric Power Systems", IESA/capacitors Inepar. [2] IEEE 19, "IEEE Recommended Practice and Requirements for Harmonic Control in Electric Power Systems", 214. [3] NBR 282, "power capacitors in derivation for rated voltages above 1V- Brazilian standard ", It is observed that there is no excessive voltage during the switching of the stages, since the replacement of the inrush reactors of the stages of the capacitors banks for reactors with higher impedance that composes the harmonics filter stages. V. CONCLUSION The results of harmonic flow show that case 1, C shows most critical results, where the THDv (%) is above the limits