Power Factor & Harmonics Andy Angrick 2014
Harmonic Distortion Harmonic problems are becoming more apparent because more equipment that produce harmonics are being applied to power systems Grounding Harmonics Surge Transient Voltage Variations
Electrical Loads of the Past
Electrical Loads Today Computers Fluorescent Lights and Ballast's Variable Frequency Drives All equipment that uses an AC to DC power supply
Which came first?.. Voltage Distortion Current Distortion In this case the Egg! Current distortion causes Voltage distortion Voltage distortion is created by pulling distorted current through an impedance Amount of voltage distortion depends on: System impedance Amount of distorted current pulled through the impedance If either increases, V THD will increase
Active Filter Harmonic Solutions Oversized Generator X s G X T 480 V Filter Low Distortion Electronic Ballast M M Blocking Filter M 12 Pulse + - K-Rated UPS w/filter Welder
Harmonics Total Harmonic Distortion (THD) (voltage or current) represents a ratio of the root-mean-square of the harmonic content to the fundamental quantity, expressed as a percent of the fundamental. 100%, 60 Hz 20%, 180 Hz % THD I I 2 2 I 2 3 I 1 I 2 4... 100% 12%, 300 Hz 4%, 420 Hz 2%, 660 Hz 2%, 780 Hz
Expected Harmonics Source Typical Harmonics* 6 Pulse Drive/Rectifier 5, 7, 11, 13, 17, 19 12 Pulse Drive /Rectifier 11, 13, 23, 25 18 Pulse Drive 17, 19, 35, 37 Switch-Mode Power Supply 3, 5, 7, 9, 11, 13 Fluorescent Lights 3, 5, 7, 9, 11, 13 Arcing Devices 2, 3, 4, 5, 7... Transformer Energization 2, 3, 4 * Generally, magnitude decreases as harmonic order increases H = NP+/-1 i.e. 6 Pulse Drive - 5, 7, 11, 13, 17, 19,
Remember Harmonics are not a problem unless they are a problem!
Harmonic Sources - Transformer Inrush
Harmonic Sources Arc Furnace
Harmonic Sources - Switch Mode Power Supplies
Harmonic Sources - Switch Mode Power Supplies Neutral of Panel
Harmonic Sources - VFD s The input side (AC) of all AC or DC drives are usually very similar. For most drives today, the input is a 6-pulse rectifier circuit. The harmonic current produced is related to the drive pulse number by the formula: h Np 1 where h = the harmonic number N = an integer multiplier beginning with 1 p = the pulse number. Theoretically, for a 6-pulse drive, the harmonic current produced would include 5th, 7th, 11th, 13th, 17th, 19th, 23rd, 25th, etc. Called the drive characteristic harmonics, and for the most part, their magnitudes decrease as their harmonic number increases No even harmonics would be generated.
Harmonic Sources - VFD s For a 12-pulse drive, the characteristic harmonic currents produced would include 11th, 13th, 23rd, 25th, 35th, 37th, etc. No even harmonics would be generated. Realistically, power electronic manufacturing tolerances, firing circuit timing variations, and component degradation or failure cause drives to produce harmonic currents in addition to the characteristic harmonics. 12-pulse drives will produce some small magnitudes of 5th, 7th, 17th, 19th, etc. harmonics. These are called noncharacteristic harmonics of the 12-pulse drive. Small magnitudes of the 2nd, 3rd, and 4th harmonics are also generated, usually 1 to 4 percent.
Harmonic Sources - VFD s
Active Filters From IEEE519A Draft
Effect of Drive Line Reactors (IEEE519A)
Total Current Fundamental = 100% Fundamental = 100% Harmonic = 20% Harmonic = 100%
Motor Heating & Vibrations Harmonic Sequence Harmonic Sequence 1 + 10 + 2-11 - 3 0 12 0 4 + 13 + 5-14 - 6 0 15 0 7 + 16 + 8-17 - 9 0 18 0
Motor Heating & Vibrations Negative Sequence Current Tries to Rotate Motor in Opposite Direction Causes Motor and Heating and Vibrations 60 Hz Rotation 5th Harmonic Rotation
Harmonic Resonance If a capacitor exists on the power system AND Harmonic producing loads are in use You MUST check for harmonic resonance (series and parallel)
Harmonic Resonance Capacitors not only supply reactive power to the loads in an electrical distribution system they also change the resonance frequency of the system. Capacitors are also a sink for harmonic currents present in a system (series resonance). When the resonance frequency of a system with PF correction capacitors is close to the frequency of a harmonic current generating load parallel resonance can occur.
Harmonic Resonance AC circuits characteristically have inductive and capacitive components and have the means to transfer energy between these components. Harmonic resonance occurs when the inductive reactance of a circuit is equal to the capacitive reactance. Resonance can be either series or parallel. Recall that inductive reactance increases as the power system frequency increases and the capacitive reactance decreases as the power system frequency increases by the following equations: and where X L X C j2 f L j L 1 1 ( j) j2 f C C XL = inductive reactance in ohms XC = capacitive reactance in ohms f = power system frequency in Hz L = component inductance in henries C = component capacitance in farads. At 60 Hz, the capacitive components have a much higher impedance than the inductive components.
Harmonic Problems-Capacitors Example: A capacitor bank has reactance, measured in Ohms. Xc = 1 / 2 fc Lets say we have a Power Correction capacitor that has a reactance of 12 ohms at 60HZ. Then, 12 ohms = 1 / 2 x 3.14 x 60Hz x.00022f If this capacitor is in a 480 Volt circuit 40 Amps will flow in the capacitor (480Volts / 12 ohms = 40 Amps). If we have 3rd (180 Hz) harmonics present. Then, Reactance = 1 / 2 x 3.14 x 180Hz x.00022f Reactance = 4 ohms We will now see 120 Amps flow in the capacitor. (480 Volts / 4 ohms = 120 Amps)
Harmonic Problems-Capacitors MSB 480Y277 3000A Linear Load Example: 600A Branch feeding a 600A Busway Motor Motor Motor 480V 50 Hp 65 Amps each 160Amps of other Equipment Power Correction Capacitors - 480V, 12 Cap, 40 Amps each
Harmonic Problems-Capacitors MSB 480Y/277 3000A Non-Linear Load Example: 600A Branch feeding a 600A Busway Motor Motor Motor 480V 50 Hp 65 Amps each 120Amps each 4 ohms with 180Hz 160Amps 3rd Harmonic (180Hz) Generating Equipment Power Correction Capacitors - 480V, 12 Cap, 40 Amps each
Parallel Resonance The parallel combination of impedance is: X EQUIVALENT jx jx L L ( j) X ( j) X C C Since XL and XC have opposite signs, the denominator can equal zero if XL = XC. In reality, the only limiting factor is the difference in resistance between the capacitor and reactor. X C X L Harmonic Current Source Equivalent Parallel Resonant Circuit
Parallel Resonance This graph shows a typical frequency scan for a power system with unfiltered capacitance. This is a borderline case for which a detuned filter would typically be recommended. The scan shows parallel resonance at the 7.9 th harmonic order. The scan magnitude at the peak is approximately 615 ohms. The scan magnitude at the 7 th harmonic is about 19 ohms (which is not particularly high). However, the parallel resonant point of an unfiltered capacitor is very sensitive to both the upstream source impedance and capacitance on other substations. A source impedance change (such as being switched to a different transformer) could easily cause the parallel resonant point to shift to the 7 th harmonic or lower.
Parallel Resonance This graph shows an updated frequency scan for the same power system with the addition of a series tuning reactor immediately upstream of the capacitors. Note the series resonant point at the 4.2 nd harmonic. The addition of the series resonant point anchor has forced the parallel resonant point down to the 3.7 th harmonic. Also note that the magnitude at the parallel resonant point has been reduced to approximately 110 ohms. The chances of exciting this parallel resonant point would be very low, for a typical industrial power system. If the electrical system has specialty loads that generate large amounts of low-order harmonics, such as EAFs (Electric Arc Furnaces), custom filter design and analysis would typically be necessary. The new parallel resonant point will exhibit some shift for source impedance changes, but these shifts will generally be minimal. Typically, we want the parallel resonant point to land at approximately the 3.5 th harmonic with all steps of the PFC filter online.
Impedance in Ohms Series Resonance The series combination of impedance is: X jx ( j) X EQUIVALENT L C Since XL and XC have opposite signs, the summation can equal zero if XL = XC. In reality, the only limiting factor is the difference in resistance between the capacitor and reactor. Frequency Scan 1000 X L X C Harmonic Current Source Equivalent Series Resonant Circuit 100 10 1 0.1 60 180 300 420 540 660 780 900 1020 1140 1260 1380 1500 Frequency in Hz Frequency Scan for Series Resonant Circuit
Harmonic Modeling - Example Utility If other loads on the system are Non-Linear, consider installing a harmonic filter in lieu of an unfiltered capacitor to avoid RESONANCE Utility Bus 1000 kva 480 V Bus Capacitor Fdr LOAD CAPACITOR
Capacitor Size Harmonic Modeling - Example Parallel Resonant Frequencies for Various Capacitor Sizes 600 500 500 400 300 450 400 350 300 250 200 200 150 100 100 50 0 0 0 5 10 15 20 25 Harmonic Order
Harmonic Distortion Standards IEEE 519-1992 Maximum Harmonic Current Distortion ISC / IL TDD 1-20 5% 20-50 8% 50-100 12% 100-1000 15% 1000+ 20% ISC=Maximum short circuit current IL= Maximum demand load TDD= Total Demand Distortion
IEEE 519-1992 -Current Example: Typical Office Building 1200A 208Y/120 service 30K AIC The Maximum IEEE Harmonic distortion is: 30,000 AIC / 960 = 31 31 on the IEEE chart is 8%
IEEE 519-1992 -Current IEEE 519-1992 Maximum Harmonic Voltage Distortion Voltage TVD 69kV and below 5% TVD=Total Voltage Distortion
Troubleshooting Methods IEEE1159
Future of IEEE 519 (2006) More concise document PCC clarified New voltage range 1.0 kv and below 8% THD V 5% individual voltage harmonics
Harmonics & Generators Utility Source 2.3% THD Generator Source 5.7% THD
Transformers & Harmonic Currents Many people incorrectly assume that ALL harmonics are trapped by delta-wye transformers. The fact is: only the balanced third harmonics (and multiples of the third) circulate in the delta winding and are therefore trapped. other harmonic currents (5th and 7th, for example) and the unbalanced multiples of the third harmonics can pass through the transformer harmonic currents are inductively coupled along with the 60 Hz current by the ratio of the primary turns to the secondary turns. Higher order harmonics (> 25th) may or may not be inductively coupled through the transformer. Sometimes, higher frequency harmonics are capacitively coupled from secondary to primary (not by the turns ratio).
Power Factor & Harmonics END