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Case Study - Filter Bank Performance & Power Quality Study This case study summarizes the results of the power system study that was conducted by Northeast Power Systems, Inc. (NEPSI) at a facility in New England. The primary purpose of the study was to test the performance of an existing 600 kvar 480 volt twelve step automatic harmonic filter bank and to determine if it was creating power quality problems within our customers facility.. System Measurements Current and Voltage Distortion measurements were taken at the service entrance of the facility while the filter bank was energized as well as de-energized to determine if the filter bank was having a negative impact on power quality. Table 1 below summarizes the distortion measurements while the bank was de-energized. The Total Harmonic Voltage Distortion (Vthd) reached 1.6% during the measurement period. The corresponding Total Harmonic Current Distortion (Ithd) reached 5.4%. The majority of the distortion came from the 3rd, 5th, and 7th harmonics as shown in Table 1. These recorded harmonics current values are typical of single phase and three phase electric furnaces that were present at the site. Table 2 summarizes the distortion measurements that were recorded while the filter bank was energized. The total harmonic voltage distortion, as would be expected, dropped from 1.6% to 1.4% due to the energization of the harmonic filter. A sizable drop in voltage distortion was not seen because the majority of voltage distortion present at the customer site is from the third harmonic where the filter has very little filtering capability. Figure 1 provides a comparison chart to help illustrate the performance of the harmonic filter bank. The filter bank reduced the voltage harmonics at the 5th, 7th as would be expected. The increase in voltage distortion at the 4th is also normal. This increase is due to the resonant point that exists below the filter bank tuning frequency. The increase in voltage distortion at the 11th may be a result of load variation during the measurement period or a resonance on the medium voltage system from distribution capacitors. The increase, however, is not of concern. Figure 1 – Bar Chart Showing Effect of Filter Bank on System Voltage Distortion Figure 2 shows how the current distortion on the service entrance conductors is effected by the energization of the harmonic filter bank. As can be seen in the figure, the 5th harmonic current drops as expected. The increase in the third and forth harmonic current is normal due to the resonance that exist below the turning frequency of the harmonic filter bank. The values measured are within acceptable limits and should not cause any harmonic related power quality problems at the customer site. During testing it was noted that no transient or transient magnification was occurring during filter bank capacitor stage energization. In addition, problems at the electric furnaces were not coincident with the switching of the filter bank stages. Figure 2 - Bar Chart Showing Effect of Filter Bank on Current Distortion at Service Entrance Filter Bank Test ResultsAs part of the system measurements, NEPSI completely tested the 12 stages of the automatic harmonic filter bank. The following items were checked.
Table 3 shows the filter bank measurement data taken during the site visit. Since the capacitor bank is connected in Delta, phase-to-phase values were measured and are presented in this report. Table 4 provides the phase-to-phase tuning points of each stage as well as the three-phase kvar, and phase-to-phase kvar of each stage. Tuning points above 5.0 in Table 4 are indicated with red text. These stages (stages 1, 2, 7, & 12) are not tuned correctly due to capacitor failure (Unlike medium voltage capacitors, low voltage capacitors lose capacitance over time). It is strongly recommended that these stages be repaired. The individual capacitors in each stage can be removed and tested with a digital multi-meter that measures capacitance. Capacitors with low values will indicate immanent capacitor failure and should be replaced. Conclusion & RecommendationThe power quality problems that have been occurring with the electric furnaces do not appear to be a result of any malfunctioning of the harmonic filter bank. The harmonic filter bank decreases the system voltage distortion and does not magnify or create in sizable transients that are of concern. Steady state voltage rise is also minimal and not of concern. During the system measurements, it was seen that a very low amount of system voltage variation (less than 1/2%) resulted in a sizable variation in the furnace output. The source of the voltage variation could not be located, but could be from within the facility or outside of it (from utility). Due to the small magnitude, however, the utility would not be responsible for a solution. In regards to problems associated with the voltage variation, NEPSI recommends the following: Continue the search to find the source of voltage variation within the plant. Contact your utility and have power quality meters installed at the service entrance. This metering data may help determine if the voltage variation is from outside your plant. It is important to time stamp problems within the plant so that they can be compared with the measurement data (which also needs to be time stamped). Change the control system on the electric furnace to be less responsive to voltage variations. In addition to the above the following should be noted. It is likely that installing line reactors in front of the electric furnaces will not solve the problems associated with the voltage variations. Although transient voltage surge suppression (TVSS) will not solve the voltage variation problems, they will help protect your equipment from larger transients associated with lightning, faults, and switching operations on the utility system, as well as on facilities power system. The cost of TVSS products are low when the consideration of loss production and equipment damage are considered.
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