Power Factor Correction and Harmonic Control for dc Drive Loads Danny Chretien John Tsou Mark McGranaghan Klockner-Pentaplast Hanover Engineers Electrotek Concepts ABSTRACT This paper describes the design of power factor correction and harmonic control equipment for loads at a plastic film manufacturing plant. Measurements were performed to characterize the harmonic generation and power factor requirements of the load. The electric utility supplying the new facility is requiring that the plant meet the harmonic current limits specified in the new IEEE 519. Harmonic filters which can meet the IEEE 519 guidelines for the specific load characteristics were designed. General filter design guidelines for this type of application are presented. INTRODUCTION DC drives can be a significant percentage of plant load in many industrial facilities. They are commonly used in the plastics, rubber, paper, textile, printing, oil, chemical, metal, and mining industries. These drives are still the most common type of motor speed control for applications requiring very fine control over wide speed ranges with high torques. Power factor correction is particularly important for dc drives because phasing back of the SCRs results in relatively poor power factor, especially when the motor is at reduced speeds. Additional transformer capacity is required to handle the poor power factor conditions (and the harmonics) and more utilities are charging a power factor penalty that can significantly Figure 1. Example dc Drive Current Waveform impact the total bill for the facility. and Harmonic Spectrum The dc drives also generate significant harmonic existing facility in Gordonsville, VA. Measurements currents. Figure 1 is an example current waveform performed at the Gordonsville facility are used to and harmonic spectrum for a dc drive load. The characterize these dc drive loads and additional harmonics make power factor correction more analysis is described to determine power factor complicated. Power factor correction capacitors can correction and filtering requirements for the new cause resonant conditions which magnify the facility. harmonic currents and cause excessive distortion levels [1,2]. This paper describes the design of power Klockner would like to correct the power factor to factor correction to avoid harmonic problems. 0.95 with power factor correction equipment (capacitors). However, the power factor correction must take into account the potential for resonance DESCRIPTION OF EXAMPLE SYSTEM which could magnify the harmonic currents generated by the dc drive loads. This usually means that Klockner Pentaplast manufactures heavy duty plastic harmonic filters are required. In addition, the electric film. The process uses calenders which are driven by utility supplying the new facility has required that dc motor drives. As a result, there is significant Klockner meet the proposed limits in the revised harmonic current generation and the plant power version of IEEE 519. [4] This results in a need for factor without compensation is quite low. Shunt harmonic filters to reduce the harmonic current capacitors can be added to partially correct the power components injected onto the utility system. factor but this can cause harmonic problems due to resonance conditions and transient problems during The plant electrical system consists of two sets of 480 capacitor switching by the utility [3]. Volt switch gear fed from a common 480 Volt bus. A 3750 kVA transformer steps down from a 34.5 kV Klockner Pentaplast is building a new facility in Rural distribution line for the entire facility. Figure 2 is a Retreat, VA to manufacture plastic film. This facility one line diagram illustrating the Rural Retreat facility. will include two calender lines similar to lines at their Blytheville 34.5 kV Feeder 3750 kVA 6 % Line 12 Line 11 Calender Mech. Calender Building Loads Room Loads Load Boiler Room Extruder Equipment 600 kVAr 600 kVAr Each Figure 2. One Line Diagram for Rural Retreat Facility CALCULATING POWER FACTOR The power factor requirements for the new facility are CORRECTION REQUIREMENTS calculated based on the total expected load. Table 1 calculates the power factor correction requirements at The new calender lines at Rural Retreat will be similar estimated minimum and maximum load levels. The to existing lines at the Gordonsville facility. estimated power factor of the loads is based on the Therefore, measurements at the existing facility are measurement results. Based on these estimates of used to estimate the power factor correction that will plant loading, a total compensation of 1800 kVAr be needed at Rural Retreat. Since all of the load will should be sufficient to maintain a power factor essentially be connected to the same 480 Volt bus at exceeding 0.95 for all load conditions. Rural Retreat, the important consideration is the total power factor for the two switchgear lines. Table 1. Calculation of Power Factor Correction for Total Plant Loading Measurement Results Total Load Power Factor Calculations Measurements were performed characterizing typical calender line conditions at the existing Gordonsville Bus: Total 480 Volt Bus Loading facility. There were two very important findings from these measurements: Voltage L-N 277.00 L-L 479.78 1. Displacement power factor for the calender line loads (almost exclusively dc drives) Min Max ranged from 0.65-0.70. This displacement power factor determines the capacitor/filter Load Range (Amps): 2400 4000 sizes required for the loads. Power Factor 0.65 0.67 2. There is significant cancellation of harmonic currents resulting from the different dc drives Load kVA 1995 3326 fed from the main bus. Whereas the total Load kW 1297 2228 harmonic distortion in the current for a single drive is approximately 30% (see Figure 1), the KVAr 1516 2469 total harmonic distortion for the total current at the bus was usually on the order of 15%, Existing Capacitors 0 0 not including the effect of resonances caused by capacitor banks. This is an important Load kVAr 1516 2469 consideration when determining the filter rating requirements and the ability to meet Load Power Factor 0.65 0.67 IEEE 519 harmonic current limits. Additional kVAr Recommended 1200 1800 Expected Power Factor 0.97 0.96 ANALYSIS OF HARMONIC DISTORTION Power Factor Calculations for the New Facility CONCERNS For the purposes of harmonic analysis, the dc drive Harmonic Distortion Levels at the Existing Facility loads can be represented as sources of harmonic currents. The system looks stiff to these loads and the Initial power factor correction procedures for the current waveform illustrated in Figure 1 is relatively Gordonsville facility involved installation of capacitor independent of the voltage distortion at the drive banks for each calender line. One or two 600 kVAr location. This assumption of a harmonic current banks were used for each line. This resulted in source permits the system response characteristics to problems with high voltage distortion levels in the be evaluated separately from the dc drive plant and also caused transient voltage magnification characteristics. The representation of the drives as when the utility company switched a higher voltage harmonic current sources is shown in Figure 3. transmission system capacitor bank. To prevent these problems, the 600 kVAr capacitor banks are being configured as harmonic filters rather than just capacitors. The configuration is shown in Figure 5. Figure 3. Representation of the DC Drives as Harmonic Current Sources Analysis of the system response is important because the system impedance vs. frequency characteristics Figure 5. Basic Configuration for a 480 Volt determine the voltage distortion that will result from Harmonic Filter Bank the dc drive harmonic currents. A simplified version of the situation is shown in Figure 4. The filter is tuned below the fifth harmonic. This limits the additional harmonic current which must be absorbed from the utility system and also allows for tolerances in the filter components. The addition of a single 600 kVAr filter significantly improved voltage distortion levels at the 480 Volt bus. Figure 6 compares the voltage harmonic spectrum with and without the filter in service. Figure 4. Voltage Distortion Caused by Harmonic Currents and System Impedance If the system is infinitely strong (no impedance), there will never be any voltage distortion. It is the harmonic currents generated by the dc drives passing through the system impedance that causes voltage distortion. Filters are the means used to control the system response. Figure 6. Comparison of Bus Voltage Distortion With and Without Harmonic Filter in Service fundamental frequency (60 Hz). 600 Volt capacitors Filter Design for the New Facility will be used for these filters to prevent overloading due to voltage rise across the reactor and harmonics The switchgear lineups at the new facility are being from the power system. In order to accomplish this, configured with 1200 Amp switchgear. For this capacitors with a nominal rating of 900 kVAr at 600 reason, the individual power factor correction steps Volts will be required. Figure 7 provides the are being limited to 600 kVAr. Based on the power specifications for the recommended filter factor correction estimates, two steps will be installed configuration. The specifications are based on two initially and a third step will be added in the future if it filters sharing the maximum load at the Rural Retreat is warranted based on actual plant loading. plant. Each 600 kVAr step will be configured as a harmonic filter tuned to approximately 4.7 times the Low Voltage Filter Calculations: Klockner Pentaplast - Rural Retreat (each 600 kVAr filter bank) SYSTEM INFORMATION: Filter Specification: 5 th Power System Frequency: 60 Hz Capacitor Bank Rating: 900 kVAr Capacitor Rating: 600 Volts 60 Hz Nominal Bus Voltage: 480 Volts Derated Capacitor: 576 kVAr Capacitor Rated Current: 866.0 Amps Total Harmonic Load: 1500 kVA Filter Tuning Harmonic: 4.7 th Filter Tuning Frequency: 282 Hz Cap Impedance (wye): 0.4000 W Cap Value (wye): 6631.5 uF Reactor Impedance: 0.0181 W Reactor Rating: 0.0480 mH Filter Full Load Current: 725.7 Amps Supplied Compensation: 603 kVAr Transformer Nameplate: 3750 kVA Utility Side Vh: 2.00 % THD (Rating and Impedance) 6.00 % (Utility Harmonic Voltage Source) Load Harmonic Current: 25.00 % Fund Load Harmonic Current: 451.1 Amps Utility Harmonic Current: 191.5 Amps Max Total Harm. Current: 642.6 Amps CAPACITOR DUTY CALCULATIONS: Filter RMS Current: 969.3 Amps Fundamental Cap Voltage: 502.8 Volts Harmonic Cap Voltage: 89.0 Volts Maximum Peak Voltage: 591.8 Volts RMS Capacitor Voltage: 510.6 Volts Maximum Peak Current: 1368.3 Amps CAPACITOR LIMITS: (IEEE Std 18-1980) FILTER CONFIGURATION: 480 Volt Bus Limit Actual Peak Voltage: 120% ←→ 99% Xl = 0.0181 Ω Current: 180% ←→ 112% KVAr: 135% ←→ 95% 900 kVAr @ RMS Voltage: 110% ←→ 85% Xc(wye) = 0.4000 Ω 600 Volts FILTER REACTOR DESIGN SPECIFICATIONS: Reactor Impedance: 0.0181 W Reactor Rating: 0.0480 mH Fundamental Current: 725.7 Amps Harmonic Current: 642.6 Amps Figure 7. 600 kVAr Filter Specifications Figure 8 shows the frequency response of the system conditions to share the harmonic loading. The filters looking from the 480 Volt bus when there are two prevent any resonances near the characteristic 600 kVAr filters in service. There should always be harmonics of the dc drive (5th and above). at least two filters in service during normal load System Frequency Response Characteristic 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 0 300 600 900 1200 1500 Frequency (Hz) Figure 8. Frequency Response Looking from 480 Volt Bus with Two 600 kVAr Filters in Service evaluation is based on an "average maximum demand Evaluation of Current Limits in IEEE 519 current" defined as the average of the monthly maximum demand values for twelve months. For a The newest version of IEEE 519, "Recommended new plant this must be estimated. 3000 kVA was Practice for Harmonic Control in Electric Power used for this evaluation. Systems", provides recommended harmonic current limits for individual customers at the point of common Figure 9 evaluates the expected current distortion coupling with the electric utility. The utility supplying levels with respect to the IEEE 519 limits for the case the new facility has specified these harmonic current without compensation or harmonic filters. The limits limits in the contract with Klockner-Pentaplast. are exceeded at almost every individual harmonic Therefore, it is important to make sure that the frequency and for the total demand distortion (TDD). specified harmonic filters will adequately limit the harmonic currents injected onto the utility system. Figure 10 illustrates the effect of the proposed 600 kVAr filters on the expected harmonic current levels A few assumptions are required to make this being injected onto the utility system. These values evaluation. A short circuit capacity at the transformer were obtained from a simulation of the system high side of 23 MVA is assumed. This is relatively response. The limits are not exceeded at any low because the plant is supplied from a long 34.5 kV individual frequency or for the total demand feeder circuit. Worst case harmonic generation levels distortion. There should be no problem with the are assumed which do not include significant IEEE 519 limits for the proposed filter configuration. cancellation from the different drives. The IEEE 519 System Data: KLOCKNER-PENTAPLAST - Rural Retreat Conditions: No Filters Transformer = 3750 kVA Impedance = 6.00 % Total Z = 0.0137 Ohms Source Strength = 23 MVA Low Side Strength = 16.81 MVA Max. Avg. Demand kVA = 3000 kVA Short Circuit Ratio = 5.60 Max. Avg. Dem. Current= 3613 Amps Harmonic Frequency Load Harmonics IEEE Current IEEE Current Exceeds Number (Hz) (Amps) Limit (%) Limit (Amps) Limit 1 60 3613 N/A N/A N/A 3 180 4.00% 145 OK 5 300 995 4.00% 145 Exceeds 7 420 198 4.00% 145 Exceeds 11 660 274 2.00% 72 Exceeds 13 780 78 2.00% 72 Exceeds 17 1020 76 1.50% 54 Exceeds 19 1140 38 1.50% 54 OK TDD: 29.26% 5.00% Exceeds >>>>>>>>>>>>>>>>>>>>> Figure 9. IEEE 519 Evaluation Without Filters System Data: KLOCKNER-PENTAPLAST - Rural Retreat Conditions: Two 600 kVAr Filters Transformer = 3750 kVA Impedance = 6.00 % Total Z = 0.0137 Ohms Source Strength = 23 MVA Low Side Strength = 16.81 MVA Max. Avg. Demand kVA = 3000 kVA Short Circuit Ratio = 5.60 Max. Avg. Dem. Current= 3613 Amps Harmonic Frequency Load Harmonics IEEE Current IEEE Current Exceeds Number (Hz) (Amps) Limit (%) Limit (Amps) Limit 1 60 3613 N/A N/A N/A 3 180 4.00% 145 OK 5 300 47 4.00% 145 OK 7 420 38 4.00% 145 OK 11 660 72 2.00% 72 OK 13 780 22 2.00% 72 OK 17 1020 22 1.50% 54 OK 19 1140 11 1.50% 54 OK TDD: 2.76% 5.00% OK >>>>>>>>>>>>>>>>>>>>> Figure 10. IEEE 519 Evaluation With Two 600 kVAr Filters SUMMARY DC Drive loads can have a low displacement power factor, resulting in a need for power factor correction. The power factor correction can be sized based on the displacement power factor of the load but all of the compensation should be installed as harmonic filters to avoid harmonic resonance problems and excessive voltage distortion levels. Filters tuned below the fifth harmonic will usually be adequate to keep voltage distortion levels below 5% and current harmonics injected onto the utility system below the levels specified in IEEE 519. REFERENCES [1] T.E. Grebe, M.F. McGranaghan, and M. Samotyj, "Solving Harmonic Problems in Industrial Plants and Harmonic Mitigation Techniques for Adjustable Speed Drives," Electrotech 92 Proceedings, Montreal, June 14-18, 1992. [2] T. Grebe, "Why Power Factor Correction Capacitors May Upset Adjustable Speed Drives," Power Quality, May/June, 1991. [3] M.F. McGranaghan, R.M. Zavadil, G. Hensley, T. Singh, M. Samotyj, "Impact of Utility Switched Capacitors on Customer Systems - Magnification at Low Voltage Capacitors," Presented at the 1991 IEEE T&D Show, Dallas, TX, September, 1991. [4] Revised Standard IEEE 519, submitted for approval Spring, 1992.