VIEWS: 65 PAGES: 29 CATEGORY: Business POSTED ON: 12/3/2010 Public Domain
Understanding of Harmonics in Power Distribution System Dr. Adel. M. Sharaf Department of Electrical & Computer Engineering University of New Brunswick Outline Power System Harmonics? Why Harmonics are Troublesome? Nonlinear Loads Producing Harmonic Currents Harmonic Distortion? Negative Effects of Sustained Harmonics Mitigation of the Effects of Harmonics Evaluation of AC Power System Harmonics? Conclusions References 2/23/2006 EE 6633 Seminar 1 2 What are Power System Harmonics? Harmonic: a mathematical definition, generally used when talking about Integral orders of Fundamental frequencies Power system harmonics: currents or voltages with frequencies that are integer multiples (h=0,1,2,…N) of the fundamental power frequency [1] 1st harmonic: 60Hz 2nd harmonic: 120Hz 3rd harmonic: 180Hz Figure: 1 [2] 2/23/2006 EE 6633 Seminar 1 3 How are Harmonics Produced ? Power system harmonics: presenting deviations from a perfect sinusoidal-waveform (voltage or current waveform). The distortion comes from a Nonlinearity caused by saturation, electronic-switching and nonlinear electric loads, Inrush/Temporal/Arc/Converter/Limiter/Threshold Type Loads. Figure: 2 [1] 2/23/2006 EE 6633 Seminar 1 4 Why Bother about Harmonics? 50-60% of all electrical Ac Systems in North America operate with non-linear type loads Power-Quality-PQ Issues & Problems Damage to Power Factor Correction capacitors Waveform Distortion can create SAG/SWELL/NOTCHING/RINGING/… All can cause damage effects to consumer loads and power systems due to Over-Current/Over- Voltage or Waveform Distortion Additional Power/Energy Losses 2/23/2006 EE 6633 Seminar 1 5 Loads Producing Harmonic Currents Electronic lighting ballasts/Controls Adjustable speed Motor-Drives Electric Arc Welding Equipment Solid state Industrial Rectifiers Industrial Process Control Systems Uninterruptible Power Supplies ( UPS )systems Saturated Inductors/Transformers LAN/Computer Networks 2/23/2006 EE 6633 Seminar 1 6 Current vs. Voltage Harmonics Harmonic current flowing through the AC Power System impedance result in harmonic voltage- drop at the load bus and along the Feeder!! Figure: 3 [3] 2/23/2006 EE 6633 Seminar 1 7 How to Quantify Harmonic Distortion? Total Harmonic Distortion-THD: the contribution of all harmonic frequency Currents/Voltages to the fundamental current. [3] The level of THD-for Current or Voltage is directly related to the frequencies and amplitudes of the Offending Quasi-Steady State persistent Harmonics. Individual Distortion Factor-(DF)-h quantify Distortion at h –harmonic-order 2/23/2006 EE 6633 Seminar 1 8 Calculation of THD THD: Ratio of the RMS of the harmonic content to the RMS of the Fundamental [3] (Eq-1) Current THD-I (Eq-2) Voltage THD-V (Eq-3) 2/23/2006 EE 6633 Seminar 1 9 Negative Effects of Harmonics Overheating and premature failure of distribution transformers [1] Increasing iron and copper losses or eddy currents due to stray flux losses Overheating and mechanical oscillations in the motor-load system [1] Producing rotating magnitude field, which is opposite to the fundamental magnitude field. Overheating and damage of neutral ground conductors [2] Trouble sustained type Harmonics: 3rd, 9th, 15th … A 3-phase 4-wire system: single phase harmonic will add rather than cancel on the neutral conductor Malfunction/Mal-Operation of Sensitive Tele-control and Protection Relaying 2/23/2006 EE 6633 Seminar 1 10 Negative Effects of Harmonics (cont’ d) False or spurious Relay operations and trips of circuit breakers [2] Failure of the Firing/Commutation circuits, found in DC motor-drives and AC drives with silicon controlled rectifiers (SCR-Thyristor) [1] Mal-Operation instability of voltage regulator [1] Power factor correction capacitor failure [1] Reactance (impedance)-Zc of a capacitor bank decreases as the frequency increases. Capacitor bank acts as a sink for higher harmonic currents. The System-Series and parallel Resonance can cause dielectric failure or rupture the power factor correction capacitor failure due to Over-Voltages & Over-Currents. 2/23/2006 EE 6633 Seminar 1 11 Harmonics and Parallel Resonance Circuit Harmonic currents produced by variable speed motor-drives: can be amplified up to 10-15 times in parallel resonance circuit formed by the capacitance bank and network inductance [5] Amplified/intensified harmonic currents: leading to internal overheating of the capacitor unit. Higher frequency currents: causing more losses than 60hz currents having same amplitude Figure 4: Parallel resonance circuit and its equivalent circuit [5] 2/23/2006 EE 6633 Seminar 1 12 Harmonics and Series Resonance Circuit The voltage of upstream AC Network can be also distorted due to series/parallel resonance formed by capacitance of the capacitor bank and System/load inductance : Ca cause high harmonic current circulation through the capacitors [5] Parallel Resonance can also lead to high voltage distortion. Figure 5: Series resonance circuit and its equivalent circuit [5] 2/23/2006 EE 6633 Seminar 1 13 Measure Equipments of Harmonics Digital Oscilloscope: Wave shape, THD and Amplitude of each harmonic “True RMS” Multi-Meter: Giving correct readings for distortion-free sine waves and typically reading low when the current waveform is distorted Use of Harmonic Meters-Single Phase or three Phase Figure 6: “True RMS” Multi-Meter [3] 2/23/2006 EE 6633 Seminar 1 14 Standards for Harmonics Limitation IEEE/IEC IEEE 519-1992 Standard: Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems (Current Distortion Limits for 120v-69kv DS) Table 1: Current Harmonic Limits [4] Ratio Harmonic odd Harmonic odd THD-i Iscc / Iload numbers (<11) numbers (>35) < 20 4.0 % 0.3 % 5.0 % 20 - 50 7.0 % 0.5 % 8.0 % 50 - 100 10.0 % 0.7 % 12.0 % >1000 15.0 % 1.4 % 20.0 % 2/23/2006 EE 6633 Seminar 1 15 Standard of Harmonics Limitation (cont’d) IEEE 519-1992 Standard: Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems (Voltage Distortion Limits) Table 2: Voltage Harmonic Limits [4] Bus Voltage Voltage Harmonic limit THD-v (%) as (%) of Fundamental <= 69Kv 3.0 5.0 69 - 161Kv 1.5 2.5 >= 161 Kv 1.0 1.5 2/23/2006 EE 6633 Seminar 1 16 Mitigation Of Harmonics [1] Ranging from variable frequency motor- drive to other nonlinear loads and equipments Power System Design: Limiting the non-linear load penetration to 30% of the maximum transformer’s capacity Limiting non-linear loads to 15% of the transformer’s capacity, when power factor correction capacitors are installed. Avoiding/Detuning resonant conditions on the AC System: (Eq-4) hr = resonant frequency as a multiple of the fundamental frequency kVAsc = short circuit current as the point of study kVARc = capacitor rating at the system voltage 2/23/2006 EE 6633 Seminar 1 17 Mitigation the Effects of Harmonics [1] (cont’d) Delta-Delta and Delta-Wye Transformers Using two separate utility feed transformers with equal non-linear loads Shifting the phase relationship to various six-pulse converters through cancellation techniques Figure 7: Delta-Delta and Delta-Wye Transformers [1] 2/23/2006 EE 6633 Seminar 1 18 Mitigation the Effects of Harmonics [1] (cont’d) Isolation-Interface Transformers The potential to “voltage match” by stepping up or stepping down the system voltage, and by providing a neutral ground reference for nuisance ground faults The best solution when utilizing AC or DC drives that use SCR/GTO/SSR.. as bridge rectifiers Line Isolation-Reactors More commonly used for their low cost Adding a small reactor in series with capacitor bank forms a Blocking series Filter. Use diode bridge rectifier as a front end to avoid severe harmonic power quality problems 2/23/2006 EE 6633 Seminar 1 19 Mitigation the Effects of Harmonics [1] (cont’d) Harmonic-Shunt or Trap Filters: Used in applications with a high non-linear ratio to system to eliminate harmonic currents Sized to withstand the RMS current as well as the value of current for the harmonics Providing true distortion power factor correction Figure 8: Typical Harmonic Trap Filter [1] 2/23/2006 EE 6633 Seminar 1 20 Harmonic Trap Filters (cont’d) Tuned to a specific harmonic order such as the 5th, 7th, 11th,… etc to meet requirements of IEEE 519-1992 Standard The number of tuned filter-branches depends on the offending steady-state harmonics to be absorbed and on required reactive power level to be compensated Figure 9: Typical Filter Capacitor Bank [5] 2/23/2006 EE 6633 Seminar 1 21 Harmonics Filter Types [6] Isolating harmonic current to protect electrical equipment from damage due to harmonic voltage distortion Passive Filter-Low cost: Built-up by combinations of capacitors, inductors (reactors) and resistors most common and available for all voltage levels Active Power Filter APF: Inserting negative phase compensating harmonics into the AC-Network, thus eliminating the undesirable harmonics on the AC Power Network. APF-Used only for for low voltage networks 2/23/2006 EE 6633 Seminar 1 22 Harmonic Filter Types (cont’d) [7] Unified Switched Capacitor Compensator USCS: The single line diagram (SLD) of the utilization (single- phase) or (three-phase- 4-wire) feeder and the connection of the Unified Switched- Capacitor Compensator (USCS) to the Nonlinear-Temporal Inrush /Arc type Loads or SMPS-Computer/LAN-Network loads. Figure 10 [7] 2/23/2006 EE 6633 Seminar 1 23 Harmonics Filter Types (cont’d) [7] The USCS is a switched/modulated capacitor bank using a pulse-width modulated (PWM/SPWM) Switching Strategy. The switching device uses either solid state switch SSR-(IGBT or GTO). Figure 11 [7] 2/23/2006 EE 6633 Seminar 1 24 Need To Evaluate System Harmonics? [1] The application of capacitor banks in systems where 20% or more of the load includes other harmonic generating equipment. The facility has a history of harmonic related problems, including excessive capacitor fuse operation or damage to sensitive metering/relaying/control equipment. During the Planning/Design stage of any facility comprising capacitor banks and nonlinear harmonic generating equipment. 2/23/2006 EE 6633 Seminar 1 25 When to Evaluate System Harmonics? [1] (cont’d) In facilities where restrictive Electric Power Utility Company Standards/Guidelines limit the harmonic injection back into their system to very small magnitudes. Industrial/Commercial Plant expansions that add significant harmonic generating nonlinear type equipment operating in conjunction with capacitor banks. When coordinating and planning to add any emergency standby generator as an alternate/renewable power source 2/23/2006 EE 6633 Seminar 1 26 Conclusions The harmonic distortion principally comes from Nonlinear-Type Loads. The application of power electronics is causing increased level of harmonics due to Switching!! Harmonic distortion can cause serious Failure/Damage problems. Harmonics are important aspect of power operation that requires Mitigation!! Over-Sizing and Power Filtering methods are commonly used to limit Overheating Effects of Sustained Harmonics. 2/23/2006 EE 6633 Seminar 1 27 References [1] www-ppd.fnal.gov/EEDOffice-w/Projects/CMS/LVPS/mg/8803PD9402.pdf [2] www.pge.com/docs/pdfs/biz/power_quality/power_quality_notes/harmonics.pdf [3] www.metersandinstruments.com/images/power_meas.pdf [4]http://engr.calvin.edu/PRibeiro_WEBPAGE/IEEE/ieee_cd/chapters/CHAP_9/c9toc/c9_frame.htm [5] www.nokiancapacitors.com.es/.../EN-TH04-11_ 2004- Harmonics_and_Reactive_Power_Compensation_in_Practice.pdf [6]http://rfcomponents.globalspec.com/LearnMore/Communications_Networking/RF_Microwave_Wir eless_Components/Harmonic_Filters [7] A.M. Sharaf & Pierre Kreidi, POWERQ UALITYE NHANCEMEUNSTI NGA UNIFIEDSW ITCHED CAPACITOCRO MPENSATOR, CCECE 2003 - CCGEI 2003, Montreal, Mayimai 2003 0-7803-7781-8/03/$17.00 0 2003 IEEE 2/23/2006 EE 6633 Seminar 1 28 Question 2/23/2006 EE 6633 Seminar 1 29