- 1. 81 CHAPTER 6 DETUNED CAPACITORS FOR POWER QUALITY IMPROVEMENT – A CASE STUDY 6.1 INTRODUCTION Reduction of harmonic contents in loads can be done with existing load equipment. An overexcited transformer can be brought back into normal operation by lowering the applied voltage to correct range. PWM drives that charge the dc bus capacitor directly from the line without any intentional impedance are exception to this problem. Adding a line reactor or transformer in series will significantly reduce harmonics as well as provide transient protection benefits. Phase shifting transformer connections can benefit loads by significantly reducing the fifth and seventh harmonics. Delta connected transformers can block the flow of zero sequence harmonics from the line. Automatic power factor correction system (APFC) is based on fixed and predefined amount of KVAR. However, the reactive power consumes current capability of the harmonic filter and hence it is not main priority of APFC. This chapter presents a practical example of improving the power factor of the system by an appropriate design of detuned capacitor filters. The advantage of using detuned capacitors as harmonic filters over the use of plain capacitors for power factor correction is presented. The different problems originated by harmonics and how filters prevent them are reviewed, and a comparison between them is also presented.
- 2. 82 6.2 NEED FOR DETUNING CAPACITORS Non-linear loads in industry typically contain a high fifth harmonic. At the fifth harmonic, the tuned filter has a better behavior than the detuned filter. However, for proper operation, the capacitor bank must be rated to a higher voltage than the voltage level required for detuned filters. Because tuned filters absorb more harmonics, they also carry higher harmonic currents than the detuned filters. These features make tuned filters more expensive as mentioned by Francisco Ferrandis et al (2003). Tuned as well as detuned filters either absorb or reject harmonic distortion and avoid harmonic currents to flow to other equipment or the rest of the power system. A tuned filter is tuned to a frequency slightly below the filtered harmonic. On the other hand, a detuned filter is tuned to a frequency far below the filtered harmonic. 6.3 EFFECTS OF DETUNING CAPACITORS Bridgeman et al (1998) and Gagaoudakis et al (1998) installed plain capacitors on a system with a high level of harmonics, it needs to be replaced with those capacitors to detuned capacitor bank (capacitors and reactors). The first effect of the reactors is to suppress the risk of resonance with the harmonics generated by the loads and in turn to protect the capacitors. If suppressing is not done, the resonance would lead to an over sizing of active filter connected in the circuit. Secondly, although the detuned bank is not a passive filter, it will have some filtering effect and harmonic distortion will be lower than if no capacitor was installed.
- 3. 83 Calculating the minimum RMS current of the active filter to install Irms (A) = 0.013(THDi initial – THDi targeted) I1 (6.1) Based on the appropriate voltage and frequency, active filter with the RMS current directly superior to the minimum Irms can be used. Besides the filtering functionality, reactive power compensation is also possible with the active filter. Compared to traditional capacitor banks, the reactive compensation of the power quality filter is continuous, fast and smooth (no transients at switching). The compensation can be either capacitive or inductive, depending on the load type. 6.4 METHODS OF MODIFYING SYSTEM FREQUENCY RESPONSE There are number of methods to modify adverse system responses to harmonics in Dugan et al (2003). - Add a shunt filter. This shunt filter eliminates a troublesome harmonic current off the system, but it completely changes the system response. - Add a reactor to detune the system. Harmful resonances generally occur between the system inductance and shunt power factor correction capacitors. One method is to add a reactor in series with a capacitor to move the system resonance without actually tuning the capacitor to create a filter. Other way is to add reactance in the line. - Changing the capacitor size is one of least expensive options for both utilities and industrial consumers.
- 4. 84 - Move a capacitor to a point on the system with different short circuit impedance or higher losses. New bank causes telephone interference for utilities, there by moving the bank to another branch of the feeder may very well resolve the problem. This is not an option for many of industrial users because the capacitor cannot be moved far enough to make a difference. - Remove the capacitor and simply accept the higher losses, lower voltage, and power factor penalty. If technically feasible, this is occasionally the best economic choice. 6.5 DESIGN OF DETUNED CAPACITORS FOR PQ IMPROVEMENT The procedure used to convert an existing power factor correction capacitor into a harmonic filter is shown in Figure 6.1. It is being utilized for designing suitable detuned capacitor. Power factor correction capacitors may produce harmonic resonance and magnify utility capacitor switching transients. Therefore it is desirable to implement one or more capacitor banks in a facility as a harmonic filter. System parameters are described as single tuned notch filter connected to 480 V bus. The load is about 1200 KVA, power factor is 0.75 lagging. Current produced by load is of 30% harmonics in the fundamental current. Maximum harmonics are 25% of fifth harmonic current. It is supplied through transformer 1500 KVA with 6% impedance.
- 5. 85 Figure 6.1 Design procedure of detuned capacitors for PQ improvement 6.6 SIMULATION RESULTS This section presents the results of installing detuned filters in an industrial plant with a significant amount of non-linear loads. For studying the effect of capacitor on harmonic resonance, various sizes of capacitor with varied load condition are simulated by MATLAB software.
- 6. 86 The source voltage, current at the PCC and the current at load bus are analyzed using waveforms. The harmonic distortion in the voltage and current waveforms are compared for the cases of compensation. From the figure, the cases to be considered are: Case 1: With fixed capacitor compensation The magnitude of harmonic currents in an individual non-linear load depends greatly on the total effective input reactance, which is comprised of the source reactance plus added line reactance. In the case of non-linear load, we can predict the resultant input current harmonic spectrum based on the input reactance. The value of source reactance and its harmonic content are inversely proportional. The voltage and current harmonic waveforms with fixed capacitor compensation are presented. Figure 6.2 Voltage and current waveforms with fixed capacitor compensation Time in seconds Voltage (V), Current (A) waveforms of source, bus 1 and bus 2
- 7. 87 Figure 6.3 THD of current at PCC (With fixed capacitor compensation) Case 2: Compensation with detuned capacitor To avoid these resonances, a reactor is connected in series with the capacitor, in such a manner that the fundamental reactive power is compensated but the harmonics are not amplified. Figure 6.4 Voltage and current waveforms with detuned capacitor compensation Time in seconds Voltage (V), Current (A) waveforms of source, bus 1 and bus 2
- 8. 88 The fifth order voltage and current harmonics are 4.1% and 33.3%. The detuned capacitor bank of 75 KVAR, 525 V with 7% reactor tuned for 5th order which has given output of 53.76 KVAR @ 415 V. Figure 6.5 THD of current at PCC (With detuned capacitor compensation) Table 6.1 Comparison of total harmonic distortions in different compensation Total harmonic distortion (THD) in % Without capacitor With fixed capacitor Detuned capacitor Source voltage 17.05 15.63 1.54 Current at PCC 31.82 12.64 8.91 Current at load bus 33.82 10.15 8.96
- 9. 89 6.7 EXPERIMENTAL RESULTS – INDUSTRIAL CASE STUDY This section presents the results of installing detuned filters in an industrial plant with a significant amount of non-linear loads. The detuned capacitor filters installed in Adwaith Textiles Private Limited at Coimbatore, India. The filters where connected on the point of common coupling (PCC). The capacity of plant is 1750 KVA / 1100 KW. There are three transformers with the capacity of 750 KVA, 1000 KVA and 1600 KVA supplying energy to linear and nonlinear loads. It is connected with 2 nos. of 25 KVAR fixed capacitors. By replacing the fixed capacitors by detuned capacitor bank of 75 KVAR, 525 V with 7 % reactors, the reactors will give an output of 50 KVAR @ 415 V. Hence there will be a definite reduction in the KW, since the harmonic currents are reduced and the compensation will be adequate. Parameters of industrial plant are listed below. Capacity of the industry : 24,000 spindles. Transformer capacity : 1) 1000 KVA with impedance : 5.49% 2) 750 KVA with impedance : 4.90% 3) 1600 KVA with impedance : 5.99% Total sanctioned demand load : 1750 KVA / 1100 KW Total utilized load 60% @ 40% power cut : 1064.2 KVA Average power factor : 0.75 Type of loading : Balanced Load Total installed load of machines : 3265 KW Normal loads : 2000 KW Non linear loads (with drives) : 525 KW Unutilized loads : 740 KW
- 10. 90 Non linear loads are listed below. Ring spinning frames : 20 Nos. Auto-coner frames : 06 Nos. Carding machines : 23 Nos. (with drives) Simplex frames : 08 Nos. Linear loads are listed below. Draw frames : 04 Nos. (without drives) Comber frames : 15 Nos. Compressor : 03 Nos. Blow room machines : 01 No. (without drives) Humidification plant : 01 No. HT Panel meter readings Voltage harmonics THD % R – Phase = 6 % , Y – Phase = 6 % , B – Phase = 5 % Current harmonics THD % R – Phase = 14 %, Y – Phase = 14 % , B – Phase = 13 % At transformer no : 1 1000 kVA at secondary side of the meter at the main PCC Panel. Current harmonics : PHASE 3rd [Peak] 5th [Peak] 7 th [Peak] R 18.8 18.8 18.7 Y 17.6 17.3 17.4 B 21.3 21.4 17.2
- 11. 91 Voltage harmonics : PHASE 3rd [Peak] 5th [Peak] 7th [Peak] R—Y 3.1 @ 6.2 V 3.2 @ 12 V 3.2 @ 4.6 V Y—B 3.1 @ 1.1 V 3.0 @ 11.4 V 3.1 @ 5.6 V B—R 3.2 @ 0.8 V 3.3 @ 12.5 V 3.3 @ 5.5 V Comments : Heavy duty capacitors would be fine. 525 V to be suitably derated. Fixed Capacitor Bank + 7 % Reactor connected in series can be erected at the power house itself. At transformer no : 2 750 kVA reading could not be taken because of No Load. At transformer no : 3 1600 kVA at secondary side of the meter at the main PCC Panel. Current harmonics : PHASE 3rd [Peak] 5th [ Peak ] 7 th [ Peak ] R 9.8 10.0 10.4 Y 9.7 10.4 10.9 B 10.3 10.4 10.4 Voltage harmonics : PHASE 3rd [Peak] 5th [Peak] 7th [Peak] R—Y 2.4 @ 0.6 V 2.3 @ 8.9 V 2.5 @ 4.0 V Y—B 2.4 @ 0.8 V 2.4 @ 9.4 V 2.3 @ 3.6 V B—R 2.4 @ 0.7 V 2.5 @ 9.4 V 2.4 @ 3.4 V
- 12. 92 It is connected with 2 nos. of 25 KVAR fixed capacitors. By replacing the fixed capacitors by detuned capacitor bank of 75 KVAR, 525 V with 7 % reactors, the reactors will give an output of 50 KVAR @ 415 V. Hence there will be a definite reduction in the KW, since the harmonic currents are reduced and the compensation will be adequate. Based on the industrial load, the measurements in installing the filters before and after compensation are presented together with waveforms. The comparison is made in Table 6.2. Figure 6.6 THD of three phase voltage waveform Figure 6.7 Spectrum of 5th order voltage harmonics values Equivalent line voltage in volts Harmonic order
- 13. 93 Figure 6.8 Spectrum of 5th order current harmonics values Figure 6.9 voltage and current waveforms after detuned compensation Figure 6.10 Harmonic spectrum after detuned compensation Time in seconds Harmonic order Harmonic order
- 14. 94 Power factor is calculated based on the capacitor rating with respect to reactive power multiplier factors. The power factor before and after compensation was 0.75 and 0.99. The voltage THD satisfies IEEE limit and the appropriate reduction in current THD from 57.5 to 48.8. With effect of detuning capacitors, there is a significant reduction in power consumed about 20.16%. 6.7.1 Comparison of Results Table 6.2 Industrial system parameters of fixed and detuned capacitors System parameters With detuned capacitor bank [off] With detuned capacitor bank [on] Total RMS current demand 177 Amps 142 Amps Average kW required 94.5 kW 94.5 kW Average kVA required 126 kVA 101 kVA Average power factor 0.75 0.93 5th order voltage harmonics 10.5 4.27 5th order current harmonics 53.2 40.1 Voltage THD 18.2 4.7 Current THD 57.5 48.8 Table 6.3 Reduction in industrial system parameters Reduction in system parameters 35 amps 19.77% 25 kVA 19.84% By implementing the detuned capacitor banks in the above system, there is a reduction in average current consumption from 172 A to 142 A. Similarly the average kVA required is reduced from 126 kVA to 101 kVA.
- 15. 95 The reduction in system parameters shows potential improvement in terms of efficiency as well as economical savings. 6.8 CONCLUSION This chapter presented an industrial case study. The plant is having a significant number of non linear loads. The installation of detuned filters under harmonic conditions shows improvement in power factor. It establishes a practical and economical way to recover p.f. It is found that the voltage THD satisfies IEEE limit but the current THD reduces from 57.5 to 48.8. With effect of detuning capacitors, there is a significant reduction in kVA required about 19.84%. The power factor was increased from 75% to 99% after installing detuned filters. The individual THD increases as TDD decreases. The overall conclusion is that detuned filter ensures less power requirement, but the effect in harmonic reduction is not so significant.