46 Non-Linear Loads?Loads which draw non-sinusoidal current from the line:Non-incandescent lightingComputersUninterruptible power suppliesTelecommunications equipmentCopy machinesBattery chargersElectronic variable speed drivesAny load with a solid state AC to DC power converter
47 Typical AC Drive Configuration M460VAC 3-phaseSimulated AC(PWM)650VDCAll AC Drives rectify AC to DC, then convert to simulated AC (PWM) to provide the motor Variable voltage and Frequency. The AC to DC conversion generates harmonics.
48 Harmonics — Definitions Non-linear loads draw current in a non-sinusoidal or distorted mannerHarmonics or harmonic content is a mathematical concept implemented to allow quantification and simplified analysis of non-linear waveformsHarmonics are typically present in both network currents and network voltagesNon-linear current draw creates non-linear voltage as it flows through the electrical networkCurrent harmonics Voltage harmonics
52 Harmonics — Why worry? Harmonic Current Distortion — Added heating in transformers and cables, reduces available capacityMay stimulate a resonance condition with Power Factor Correction CapacitorsExcessive voltageOverheating of PF correction capacitorsTripping of PF protection equipmentVoltage Distortion interfering w/ sensitive equipment. Largest Concern!
53 Harmonics, Important Terminology (Definitions per IEEE )Harmonic - A sinusoidal component of a periodic wave or quantity having a frequency that is an integral multiple of the fundamental frequency.Point of common coupling (PCC)Def. 1 - “point of common coupling (PCC) with the consumer-utility interface.” (current harmonic emphasis) This is typically the primary of the main transformer(s). This is to protect the utility.– Secondary of transformer for Voltage Distortion. This is to protect the USER.
54 PCC Example MV PWM M PCC1 (Harmonic Current Distortion) PCC2 (Harmonic LVPWMMV PWM13.8 KV4.16 KV480 VPCC1(Harmonic Current Distortion)PCC2(HarmonicVoltage Distortion)Substation TransformerTo other utility customers
55 Harmonics, Important Terminology (cont.) ISC/IL - The ratio of the short-circuit current available at the point of common coupling, to the maximum fundamental load current.Total harmonic distortion (THD) or distortion factor - The ratio of the root-mean-square of the harmonic content to the root-mean-square value of the fundamental quantity, expressed as a percent of the fundamental.Total demand distortion (TDD) - The root-sum-square harmonic current distortion, in percent of the maximum demand load current (15 or 30 min demand).
56 Effect of Short Circuit Ratio on Harmonics ISCIL~ 400ISCIL~ 8
57 Harmonics — A System Issue! Harmonics produced by an individual load are only important to the extent that they represent a significant portion of the total connected loadLinear loads help reduce system harmonic levelsTDD equals the THD of the nonlinear load multiplied by the ratio of nonlinear load to the demand load:WhereTDD = TDD of the systemTHDNL = THD of the nonlinear loadsNL = kVA of nonlinear loadDL = kVA of demand load(nonlinear + linear)
61 Reactors (Chokes)Simplest and least expensive harmonic reduction techniqueMay be included in base drive packageOften meet harmonic needs provided drive load is a small portion of total connected loadMay be implemented with AC line reactors or with DC link reactorsAC line reactors provide better input protectionDC link reactors provide load insensitive drive output voltageBoth types provide similar harmonic benefits“Swinging” choke design provides enhanced light load harmonic performance
62 Reactors, AC Line or DC Link Different design techniquesEqual harmonic reduction for same normalized % reactanceTypical full load THD (current) at drive input terminals 28% 46%
63 Hybrid Filter Installs in series with drive input May feed multiple drivesImproves power factor (may go leading)Typical full load THD (current) at filter input terminals 5% 8%Relatively unaffected by line imbalance
64 High Pulse Count Rectification Typical configurations are either 12 pulse or 18 pulsePhase shifting transformer is requiredAdditional drive input bridge(s) is neededTypical full load THD (current) at transformer primary 8% 12% (12 pulse), 4% 6% (18 pulse)Performance severely reduced by line imbalance (voltage or phase)Excellent choice if step-down transformer is already required
65 High Pulse Count Rectification (cont.) 6 pulse rectifierTransformer and cabling simpleCurrent very distortedIthd typically 45% with 3% reactorDC/AC18 pulse rectifierDC/ACTransformer and cablingcomplicatedCurrent wave form goodIthd 4% to 6% (depending on network impedance)DC/ACTransformer and cablingcomplicatedCurrent distortedIthd 8% to 12% (depending on network impedance)12 pulse rectifier
66 Active Filter Front End with LCL Filter Lineinverter(rectifier)MotorLCMLCL Filter (Sine Filter) removes high frequencies >1 kHz. (Current and voltage)Full output voltage is available with 80% input voltage (400VIn = 480VOut)Full regenerative capabilityNo transformer requiredNot affected by line imbalance
67 Harmonic Reduction Summary Remember!Even an 80% THD nonlinear load with a will result in only 8% TDD if the nonlinear load is 10% and the linear load is 90%.(80%•(10%/(10%+90%))=8%)
68 Summary – Practical Advice With a main distribution transformer, 20-30% of its load on non-linear loads will typically comply with IEEEVoltage distortion causes interference with sensitive equipment, not current distortion!5% reactors address 90+% of typical applications. They also provide protection against line transients and keep input currents low to avoid oversizing power wiring to comply with NEC.Make VFD vendor perform a harmonic distortion calculation with the submittals.
70 Peak Voltage all at 50’ of cable DrivePeak Voltage110402111031180412905135062454Peak Voltage has many Contributing FactorsInverter Rated Motors Help Minimize the IssueLess dV/dT minimizes; problems with RFI/EMI Motor Insulation & Bearing Current
71 Recommendations Keep cable length short as possible Use a NEMA MG1, Part 31 motor (not “inverter duty” or “inverter ready”Ensure that grounding is sound