ISA Northern California Section, South Bay October 14, 2003 Craig Chidester 909 288 7990 AFDs and Their Effect on Power Quality What Kind of Power Quality Effects? • Power factor? – PF = kW / kVA – High motor content means lagging PF – 100HP motor, 460V, 93% eff, 119A • • • • (100HP x 0.746kW/HP) / 0.93 = 80.2kW 119A x 460V x 1.73 / 1000 = 94.8kVA PF = 80.2kW / 94.8kVA = 84.6% @ FL But … at actual load, more like 70% or less – PF is improved with AFDs to 90 – 95% – AFDs seen as resistive load What Kind of Power Quality Effects? • Incoming Sine Wave Notching? – – – – Arises from SCR front ends on AFD’s Forced commutation causes line notches But … modern AFDs use diode front ends Self commutating … no notching What Kind of Power Quality Effects? • Voltage sag? – – – – – – Standard motor starters allow 650% inrush “Weak” power systems are affected 500HP motor on 1000kVA, 5.75%Z Xfmr 650% X (500 / 1000) X 0.0575 = 19% sag AFD limits inrush to 110% (or 150%) 110% X (500 / 1000) X 0.0575 = 3% sag What kind of Power Quality Effects? • Harmonic Distortion – AFDs, DC Drives, UPSs, DC power supplies (computers, duplicators, fax’s) will cause current (and voltage) harmonics • Single phase – 3rd, 6th, etc (triplens) can cause transformer neutral conductor overheating • Three phase – 5th, 7th, 11th, 13th, etc can cause equipment malfunctions • Big questions – “How much?” and “How much is too much?” What are Harmonics? Definition: Harmonics are integral multiples of some fundamental frequency that, when added together, result in a distorted waveform. What are Harmonics? + sin(5x) f(x) = sin(x) f(x) = 5 The resulting wave shows a strong departure from the smooth waves comprising it: = f(x) = sin(x) + sin(5x) 5 What are Harmonics? In fact, any function may be constructed from a sine wave and some number of its harmonics: Where do they come from? The power company typically supplies a reasonably smooth sinusoidal waveform: Where do they come from? ...but nonlinear devices will draw distorted waveforms, which are comprised of harmonics of the source: Common sources of Harmonics Lighting ballasts UPS systems AC and DC drives M AC drives and Harmonics Converter DC bus & smoothing Determine the line-side harmonics Inverter Determines load-side harmonics AC drives and Harmonics EFFECTS OF LOAD-SIDE HARMONICS: Have implications for the motor insulation and windings. Inverter Essentially have zero effect on other equipment on the power system. Determines load-side harmonics AC drives and Harmonics LINE-SIDE HARMONICS CAN HAVE FAR-REACHING EFFECTS ON THE POWER SYSTEM: Distribution transformers Converter DC bus & smoothing Standby generators Communications equipment Switchgear and relays Determine the line-side harmonics Computers, computer systems Diagnostic equipment AC drives and Harmonics Typical 6-step converter waveform: Harmonic Content I5 = 22.5% I7 = 9.38% I11 = 6.10% I13 = 4.06% I17 = 2.26% I19 = 1.77% I23 = 1.12% I25 = 0.86% Harmonics and transformers Transformer overheating and potential insulation failure result from several conditions caused by harmonics: Increased skin and proximity effects Harmonics circulating in the primary winding AFC AFC Increased hysteresis losses Increased eddy current losses DC in the primary windings Harmonics and transformers Many transformers are rated by “K factor” which simply describes their ability to withstand harmonics. Transformers may also be derated to compensate for the additional heating caused by harmonics. AFC AFC Improved transformer designs have also been developed, with oversized neutral busses, special cores, and specially designed coils. Harmonics and power-correction capacitors Power correction capacitors can cause series and parallel resonance effects on a power system. If a harmonic is generated which excites a system resonance, amplification of that harmonic may occur. Calculation of the harmonic resonance frequency for the system can give an indication of potential resonance problems. Harmonics and power-correction capacitors EXAMPLE: Assume a 1500 kVA supply xfmr, with a 5.75% impedance. 1500 kVA 5.75% Also assume 600 kVA of power correction capacitors on the system 600 kVAC The harmonic resonance frequency is defined by: hr = kVAsc kVAC 1500 / 0.0575 = 6.6 = 600 Recommended limits - IEEE 519 The Institute of Electrical and Electronics Engineers (IEEE) has set recommended limits on both current and voltage distortion in IEEE 519-1992. Voltage distortion limits (@ low-voltage bus): Application class THD (voltage) Special system 3% General system 5% Dedicated system 10 % Recommended limits - IEEE 519 MAXIMUM HARMONIC CURRENT DISTORTION in percent of IL Isc/IL <20 20-50 50-100 100-1000 >1000 <11 4.0 7.0 10.0 12.0 15.0 Isc: IL: Individual harmonic number (odd harmonics) 11<h<17 17<h<23 23<h<35 2.0 1.5 0.6 3.5 2.5 1.0 4.5 4.0 1.5 5.5 5.0 2.0 7.0 6.0 2.5 Maximum short-circuit current at the Point of Common Coupling (PCC). Maximum demand load current (fundamental) at the PCC. TDD 5.0 8.0 12.0 15.0 20.0 Attenuation of Harmonics Inductive Reactance Method: Add a line reactor or isolation transformer to attenuate harmonics. Benefits: Low cost. Technically simple. Concerns: Tends to offer reductions in only higher order harmonics. Has little effect on the 5th and 7th harmonics. Because of the associated voltage drop, there are limits to the amount of reactance that may be added. Attenuation of Harmonics Passive Filters Method: Provide a low-impedance path to ground for the harmonic frequencies. Benefits: May be tuned to a frequency between two prevalent harmonics so as to help attenuate both. Concerns: Tuning the filters may be a labor-intensive process. Filters are difficult to size, because they offer a path for harmonics from any source. Quite sensitive to any future system changes. Attenuation of Harmonics Active Filters Method: Inject equal and opposite harmonics onto the power system to cancel those generated by other equipment. Benefits: Have proven very effective in reducing harmonics well below required levels. Concerns: The high performance inverter required for the harmonic injection is costly. Power transistors are exposed to conditions of the line, so reliability may be a problem. Attenuation of Harmonics 12-pulse Rectifiers Method: Two separate rectifier bridges supply a single DC bus. The two bridges are fed from phaseshifted supplies. Benefits: Very effective in the elimination of 5th and 7th harmonics. Stops harmonics at the source. Insensitive to future system changes. Concerns: May not meet the IEEE standards in every case. Does little to attenuate the 11th and 13th harmonics. Attenuation of Harmonics 18-pulse Rectifier Method: An integral phase-shift transformer and rectifier Input which draws an almost purely sinusoidal waveform from the source. Benefits: Meets the IEEE standards in every case! Attenuates all harmonics up to the 35th. Stops harmonics at the source. Insensitive to future system changes. Concerns: Can be expensive at smaller HP’s Comparison of waveforms 6-pulse converter note the level of distortion and steep current rise. 12-pulse converter the waveform appears more sinusoidal, but still not very smooth. 18-pulse converter virtually indistinguishable from the source current waveform.