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Generator Loading, Harmonics Monitoring and Mitigating Analysis in a Water Treatment Plant Eddie Jones, PE; Larry Ray, PE; Tim Shuter, PE; Square D Engineering Services Abstract Water treatment plants are often equipped with variable-frequency drives, ozone generators, and other loads that produce harmonic distortion. These facilities are also equipped, in many cases, with emergency standby generators required to operate critical electrical loads during extended electric utility outages. One such facility, a reverseosmosis desalination plant located on the United States coast, was concerned about the ability of its 938-kVA standby generator to operate reliably in the presence of harmonic distortion injected by large VFD’s serving pump motors. Subsequent harmonic measurements compared distortion levels on both utility and generator sources, and engineering analysis offered harmonic mitigating techniques indicated by the measured harmonic levels. Electric System Onsite Genset 760 kW synch 938 kVA Standby rated Utility source 2000 kVA 6% Z Open transition transfer switch R1, R2: Line reactors, 3% Z at 60 hp Circuit Monitor VFD1-3: 250-hp 6-pulse PWM 2000 A 3-section QED Switchboard T1 T2 T3 Key: T1-3: 270 kVA dry-type isolation transformers; 460/460-V, 5.3% Z at 170 C R1 R2 VFDTP1, VFDTP2: 60-hp, 6-pulse PWM MF1-3: Drive output (motor) filters VFD1 VFD2 VFD3 MF1 MF2 MF3 HPP1 HPP2 HPP3 VFDTP1 VFDTP2 HPP1-3: 250-hp vertical suction water pumps TP1, TP2: 60-hp pumps TP1 TP2 Summary of Results Load measurements indicated that the onsite standby generator was loaded to about 53% of its rated capacity under the load conditions that existed at the time. Load levels peaked at 403 kW and 431 kVA during the test. Key power system parameters, including voltage regulation and unbalance, and current unbalance, were within acceptable limits during the test on both electric utility and generator sources. Harmonic distortion levels measured during the test did not cause noticeable operating problems for any of the plant equipment. Harmonic voltage distortion at the 480-V bus peaked at about 6.3% at highest load on the electric utility, and about 10.5% at highest load on the generator. Square D Engineering Services 1 Recommendations While harmonic distortion levels did not cause noticeable operating problems for the facility loads, harmonic reduction techniques should be considered in order to maximize equipment life and system reliability. IEEE Standard 519-1992 harmonic limits are being exceeded when operating on the standby generator. Bypassing one of the existing isolation transformers serving a 250-hp drive may achieve some beneficial effects due to increased harmonic cancellation of 5th and 7th current components, but this measure should be considered temporary. The better solution is to replace one delta/wye isolation transformer with a delta/zig-zag transformer to improve cancellation while maintaining attenuation effects. The best solution is to install an active harmonic filter at the service entrance. Onsite power monitoring should be installed to track performance of desalination facility in harmonic distortion, voltage quality, disturbances, and costs. Monitoring Results Summary Testing was performed at the line (source) side of the Square D QED switchboard serving the desalination facility. Testing was performed with a PowerLogic CM4000T Portable Circuit Monitor, capable of measuring over 200 power system parameters. Harmonic distortion measurements were taken at a sampling rate of 512 points per cycle, to provide accuracy to the 250th harmonic. Average Current During Load Test 700 Average 3 Phase RMS Current Three 250 hp, two 60 hp 600 Three 250 hp, one 60 hp 500 Three 250 hp VFD Three 250 hp, two 60 hp Three 250 hp, one 60 hp Three 250 hp VFD Two 250 hp VFD 400 Two 250 hp VFD One 250 hp VFD 300 One 250 hp VFD 200 Lighting and HVAC 100 Utility Source Generator Source Lighting Only; Transfer to Generator 0 13:30 14:00 14:30 15:00 15:30 Average RMS current during the test shows the effects of various machines operating. Current unbalance was acceptable during the test, about 20% without the drives operating (lighting and HVAC only), and less than 5% with drives operating. Square D Engineering Services 2 RMS Voltage During Load Test 495 Three 250 hp, two 60 hp Three 250 hp, two 60 hp VOLTAGE (PHASE TO PHASE) 490 485 Lighting and HVAC Lighting Only; Transfer to Generator 480 Utility Source 475 470 13:30 Generator Source 14:00 14:30 15:00 15:30 Voltage levels were slightly lower on the generator source, but both utility and generator sources were able to maintain acceptable steady state voltage (100% to 103% of 480 V) for the entire duration of the testing. Voltage unbalance was likewise within acceptable limits, less than 1%. Real and Total Power Demand During Load Test 500 Three 250 hp, two 60 hp 450 Real (kW) & Total (kVA) Demand Three 250 hp, two 60 hp 400 Total Power, kVA 350 300 250 Lighting and HVAC 200 Real Power, kW 150 100 Utility Source 50 0 13:30 Generator Source Lighting Only; Transfer to Generator 14:00 14:30 15:00 15:30 Real and total power demand trends on one-minute intervals during the test. Note that the 938-kVA-rated generator is about 50% loaded during peak operation. Square D Engineering Services 3 Power Factor During Load Test 1 0.9 Three 250 hp, two 60 hp Three 250 hp, two 60 hp 0.8 Lighting and HVAC Power Factor (Lagging) 0.7 0.6 Lighting Only; Transfer to Generator 0.5 0.4 Utility Source Generator Source 0.3 0.2 0.1 0 13:30 14:00 14:30 15:00 15:30 Power factor during the load test. Note that power factor is lowest during times of lightest load and is high when all drives are operating. This makes it difficult to utilize conventional harmonic filters to reduce harmonics on this circuit due to their inherent power factor improvement characteristics. Harmonic Distortion During Load Test 20 Voltage THD or Current TDD, % 18 Three 250 hp, one 60 hp 16 14 Three 250 hp, two 60 hp Three 250 hp, one 60 hp Three 250 hp, two 60 hp Three 250 hp VFD Three 250 hp VFD 12 10 Current Distortion 8 6 Lighting Only; Transfer to Generator Lighting and HVAC 4 Voltage Distortion 2 Utility Source 0 13:30 14:00 Generator Source 14:30 15:00 15:30 Harmonic distortion levels during testing. Voltage distortion peaked at about 6.5% on the utility source and about 10% on the generator source. Note that current distortion drops when 60 hp drives are operated with 250 hp drives. This is due to the cancellation effects of the combination of delta-wye transformers and line reactors. Current Total Demand Distortion (TDD) is based on IL of 900 A, or 80% of the generator rating of 1129 A. Square D Engineering Services 4 Harmonic Analysis Summary Square D performed harmonic simulations to estimate the reduction in harmonic current that could be achieved by the following modifications. Note that the “Existing Harmonic Cancellation” section shows that some harmonic cancellation is already being achieved when 250-hp and 60-hp drives are operated at the same time. The other techniques that were considered include: ♦ Bypass one existing delta-wye isolation transformer serving a 250-hp drive to increase cancellation of 5th and 7th harmonics (short-term measure); ♦ Replace one existing delta-wye isolation transformer serving a 250-hp drive with a delta/zig-zag-wound transformer to increase cancellation of 5th and 7th currents, and to maintain the beneficial effects of harmonic attenuation; ♦ Install a passive harmonic filter at the 480-V main to remove harmonics injected by the drives. ♦ Install an active harmonic filter at the 480-V main. Existing Harmonic Cancellation Harmonic Current Shows Effects of Cancellation 600 500 Despite increase in total rms current when all drives are operating... Amperes, RMS 400 ...5th and 7th harmonic currents show effects of cancellation from mix of delta-wye transformers (250 hp drives) and line reactors (60 hp drives) 300 Base Case Worst Case All Drives 200 100 0 Total 5th 7th 11th 13th Harmonic Note that existing system of delta-wye transformers and line reactors serving the facility’s drives benefits from some cancellation of 5th and 7th harmonic currents. The system required about 27% and 16% less current at 5th and 7th harmonics, respectively, despite a 19% increase in total rms current. “Worst Case” occurs when only the 250-hp drives are operating. Square D Engineering Services 5 Harmonic Reduction/Mitigation Analysis Replace One Isolation Transformer Bypassing one of the existing three isolation transformers serving a 250-hp drive seems to have a beneficial effect on the amount of harmonic current distortion at the facility. As shown in the following summary, cancellation effects can be improved to further reduce 5th and 7th harmonic currents by bypassing one transformer and allowing its 5th and 7th harmonic currents to be unchanged. Once these currents are added to the other one or two 250-hp drive currents, additional cancellation occurs. This technique, however, should be considered only for temporary improvement until more effective means are installed. Harmonic: <11 11-17 17-23 Measured Worst Case All Drives 10.6% 7.9% 3.4% 3.4% 1.5% 1.5% Simulated w/o isol xfmr 6.5% 2.0% 1.1% Current Total Demand Distortion, ITDD, based on IL=900 A (80% of generator rating of 1129 A) A better technique is to replace one isolation transformer – instead of bypassing it – with a harmonic mitigating transformer of similar impedance. This modification would provide both cancellation effects (harmonic currents are not phase-shifted through delta/zig-zag-wound transformers) and attenuation effects. At this point, however, the simple computer model breaks down, perhaps due to a resonance between the drive capacitance and the line reactor that may or may not be real. The model predicts a further reduction in 5th and 11th currents to about 5.8%, but shows an increase in 11th and 13th currents over the “w/o isol xfmr” case that cannot be fully explained. Other Techniques Two other options were considered. The first option that was considered but is not recommended is the addition of a 5th harmonic filter at the 480-V main. This option is not practical because passive harmonic filters of this type also increase fundamental power factor. Since power factor at the facility is already high (94%) at full load, the system cannot tolerate much parasitic capacitance without putting the facility into a leading power factor. The best option that should be considered if further harmonic current reduction is desired is an active filter. Active filters measure the amount of harmonic current required by the load and inject current 180° out of phase with that current. This technique reduces the harmonic distortion levels considerably and is often used where stringent harmonic limits are applied. How Much Harmonic Distortion Is Too Much? IEEE Guidelines Drives and other facility loads operated without disruption during the May 7 testing, so why consider harmonic reduction? Since the adverse effects of harmonics may be manifested over an extended time period, it is difficult to determine the economic attractiveness of reducing harmonic distortion. The Institute of Electrical and Electronics Engineers developed the Recommended Practices and Requirements for Harmonic Control in Electric Power Systems, IEEE Standard 519-1992, to offer guidelines on this question. While this standard was developed primarily as a recommended practice for electric utilities and their customers, not as an equipment or facility standard, it can be used to develop some guidelines. Square D Engineering Services 6 Harmonic: IEEE "Limit" <11 4.0% 11-17 2.0% 17-23 1.5% 3.4% 3.4% 1.5% 1.5% Measured Worst Case All Drives 11% 7.9% Simulated 6.5% 2.0% 1.1% w/o isol xfmr IEEE 519-1992 is often used as a guide for assessing existing harmonic current levels inside a facility even though that is not its primary purpose. This table shows the harmonic limits from Table 10-3 of the Standard typically imposed on power generation equipment. Note that these limits were exceeded during the onsite measurements. As before, “Measured” and “Simulated” ITDD figures on based on 80% of the generator full-load amperage. Square D Engineering Services 7 Voltage and Current Waveforms Baseline – No Drives Operating Utility Source Phase A-N Voltage Phase A Current 512 Points / Cycle 400 300 200 100 0 -100 -200 -300 -400 Current Voltage 512 Points / Cycle 0.0 2.5 5.0 7.5 10.0 Milliseconds 12.5 200 150 100 50 0 -50 -100 -150 15.0 0.0 2.5 5.0 7.5 10.0 Milliseconds 12.5 15.0 12.5 15.0 12.5 15.0 12.5 15.0 Generator Source Phase A-N Voltage Phase A Current 511 Points / Cycle 400 300 200 100 0 -100 -200 -300 -400 150 100 50 Current Voltage 511 Points / Cycle 0 -50 -100 -150 0.0 2.5 5.0 7.5 10.0 Milliseconds 12.5 15.0 0.0 2.5 5.0 7.5 10.0 Milliseconds Worst Case – Three 250-hp Drives Operating at 83% of Rated Load Utility Source Phase A-N Voltage Phase A Current 512 Points / Cycle 400 300 200 100 0 -100 -200 -300 -400 750 500 250 Current Voltage 512 Points / Cycle 0 -250 -500 -750 0.0 2.5 5.0 7.5 10.0 Milliseconds 12.5 15.0 0.0 2.5 5.0 7.5 10.0 Milliseconds Generator Source Phase A-N Voltage Phase A Current 511 Points / Cycle 400 300 200 100 0 -100 -200 -300 -400 750 500 250 Current Voltage 511 Points / Cycle -0 -250 -500 -750 0.0 2.5 5.0 7.5 10.0 Milliseconds Square D Engineering Services 12.5 15.0 0.0 2.5 5.0 7.5 10.0 Milliseconds 8 All Drives Operating (Three 250 hp, Two 60 hp) Utility Source Phase A-N Voltage Phase A Current 512 Points / Cycle 400 300 200 100 0 -100 -200 -300 -400 Current Voltage 512 Points / Cycle 0.0 2.5 5.0 7.5 10.0 Milliseconds 12.5 750 500 250 0 -250 -500 -750 15.0 0.0 2.5 5.0 7.5 10.0 Milliseconds 12.5 15.0 12.5 15.0 Generator Source Phase A-N Voltage Phase A Current 511 Points / Cycle 400 300 200 100 0 -100 -200 -300 -400 Current Voltage 511 Points / Cycle 0.0 2.5 5.0 7.5 10.0 Milliseconds Square D Engineering Services 12.5 15.0 1000 750 500 250 0 -250 -500 -750 0.0 2.5 5.0 7.5 10.0 Milliseconds 9 Harmonic Distortion Fundamentals Introduction Electronic or “non-linear” loads, such as variable frequency and dc motor drives, computers, and automation equipment, draw current from the power system that is not sinusoidal. The resulting non-linear current may be expressed as a series of sinusoidal waves with frequencies equal to integer multiples of the 60 Hz system frequency. The individual high-frequency waves are referred to as harmonics—the 3rd harmonic is equal to 3 times the system frequency (i.e., 180 Hz), the 5th harmonic has a frequency of 5 x 60 Hz = 300 Hz, etc. Harmonic currents are undesirable because they are an additional source of thermal stress on various types of electric equipment, reducing the current-carrying capability of conductors and transformers. Harmonic currents also interact with line impedance, causing the system voltage to become distorted. When equipment such as capacitors or motors are served with a distorted system voltage, significant harmonic currents may flow, effectively reducing the lifetime of such equipment. When power factor correction capacitors are installed in a facility that contains harmonic-producing loads, the plant electrical system should be evaluated to ensure that no resonant conditions are created that could cause excessive harmonic voltage or current levels within the facility. Parallel resonance occurs when the installed capacitance interacts with the existing power system to create a high impedance at some resonant frequency greater than the 60 Hz system frequency. As a result, harmonic currents near the resonant frequency are amplified, in turn producing harmonic voltage levels higher than would normally be expected. Cancellation Effects Harmonic cancellation is an effective means of reducing harmonic distortion, especially among three-phase drives. Cancellation works best when pairs of drives of similar size are operating at similar loading on the same bus. Cancellation is achieved by serving one of the drives from a delta-wye transformer, and the other by a delta-delta transformer or simple line reactor. Some cancellation can also be achieved by using a combination of delta-wye transformer and no inductive device, as has been modeled for Dare County. Cancellation is achieved because 5th and 7th harmonic frequency current components are phase-shifted by 30° in a delta-wye transformer. When these phase-shifted currents are combined with the unchanged currents passing through line reactors or delta-delta transformers, significant cancellation can occur. 480 Volt Bus ASD ASD Harmonic cancellation can be achieved on similar size adjustable-speed drives (ASD) operating at similar load levels. Source: IEEE Standard 519a, Draft Application Guide. Attenuation Effects Harmonic distortion can be reduced by adding impedance, usually in the form of an isolation transformer or line reactor, between the harmonic load and its source. Inductive impedance serves to limit the amount of higher Square D Engineering Services 10 frequency currents that the harmonic load can inject into the system. Of course, there is a limit to the amount of impedance that can be inserted into the system without affecting performance and operation of the harmonic load. Input Current Distortion as a Function of Choke Inductor Size 80.0% Input Current Distortion (%) 70.0% 60.0% 50.0% 40.0% 30.0% 20.0% 10.0% 0.0% 0% 1% 2% 3% 4% 5% Choke Inductor (% on Drive Base) Reduction in harmonic currents can also be achieved by attenuation. This chart shows the typical drop in current distortion for line reactors (inductors) of various impedances. Harmonic Mitigating Transformers Harmonic Mitigating Transformers, or HMT’s, use phase shifting, electromagnetic flux, and source impedance to help reduce the effects of harmonics in electrical distribution systems. These devices provide the beneficial effects of harmonic cancellation and harmonic attenuation, as described above, in order to improve reliability of power systems serving variable-frequency drives. These transformers can be supplied with either delta/wye windings that provide 30° phase shift for 5th and 7th harmonics, or delta/zig-zag windings that do not provide phase shifting. System Modeling A computer simulation of the power system was performed using the SKM Hi_Wave computer program. The model of the system, complied from data collected by the Power Logic monitoring along with information gained from inspection of equipment at the site, depicts the major loads and sources. Information about the utility system was derived based on load measurements taken during the onsite visit. Square D Engineering Services 11