Understanding of Harmonics in Power Distribution System By Wei Wu Instructor: Dr. Adel. M. Sharaf Department of Electrical & Computer Engineering University of New Brunswick

Outline What are Power System Harmonics? Why Bother about Harmonics? Loads Producing Harmonic Currents How to Quantify Harmonic Distortion? Negative Effects of Harmonics Mitigation the Effects of Harmonics When to Evaluate System Harmonics? Conclusion Reference 2/23/2006 EE 6633 Seminar 1

What are Power System Harmonics? Harmonic: a mathematical definition, generally used when talking about frequencies Power system harmonics: currents or voltages with frequencies that are integer multiples 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

How are Harmonics Produced ? Power system harmonics: presenting deviation from a perfect sine waveform (voltage or current waveform). The distortion comes from electronic and nonlinear devices, principally loads. Figure: 2 [1] 2/23/2006 EE 6633 Seminar 1

Why Bother about Harmonics? 75% of all electrical devices in North American operate with non-linear current draw Important aspect of power quality Affecting power factor correction capacitors Combining with the fundamental frequency to create distortion Causing damage effects to consumer loads and power system 2/23/2006 EE 6633 Seminar 1

Loads Producing Harmonic Currents Electronic lighting ballasts Adjustable speed drives Electric welding equipment Solid state rectifiers Industrial process controls Uninterruptible Power Supplies ( UPS )systems Saturated transformers Computer system 2/23/2006 EE 6633 Seminar 1

Current vs. Voltage Harmonics Harmonic currents flowing through the system impedance result in harmonic voltages at the load Figure: 3 [3] 2/23/2006 EE 6633 Seminar 1

How to Quantify Harmonic Distortion? Total Harmonic Distortion: the contribution of all harmonic frequency currents to the fundamental current. [3] The level of distortion: directly related to the frequencies and amplitudes of the harmonic current. 2/23/2006 EE 6633 Seminar 1

Calculation of THD (Eq-1) THD: Ratio of the RMS of the harmonic content to the RMS of the Fundamental [3] (Eq-1) Current THD (Eq-2) Voltage THD (Eq-3) 2/23/2006 EE 6633 Seminar 1

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 conductors [2] Trouble Harmonics: 3rd, 9th, 15th … A 3-phase 4-wire system: single phase harmonic will add rather than cancel on the neutral conductor 2/23/2006 EE 6633 Seminar 1

Negative Effects of Harmonics (cont’ d) False or spurious operations and trips of circuit breakers [2] Failure of the commutation circuits, found in DC drives and AC drives with silicon controlled rectifiers (SCRs) [1] Interference and operation instability of voltage regulator [1] Power factor correction capacitor failure [1] Reactance (impedance) of a capacitor bank decreases as the frequency increases. Capacitors bank acts as a sink for higher harmonic currents. The overvoltage and resonance cause dielectric failure or rupture the power factor correction capacitor failure. 2/23/2006 EE 6633 Seminar 1

Harmonics and Parallel Resonance Circuit Harmonic currents produced by variable speed drives: amplified up to 10-15 times in parallel resonance circuit formed by the capacitance bank and network inductance [5] Amplified 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

Harmonics and Series Resonance Circuit The voltage of upstream network is distorted the series resonance circuit, formed by capacitance of the capacitor bank and short circuit inductance of the supplying transformer: drawing high harmonic currents through the capacitors [5] leading to high voltage distortion level at low voltage side of the transformer Figure 5: Series resonance circuit and its equivalent circuit [5] 2/23/2006 EE 6633 Seminar 1

Measure Equipments of Harmonics Digital Oscilloscope: Wave shape, THD and Amplitude of each harmonic “True RMS” Multimeter: Giving correct readings for distortion-free sine waves and typically reading low when the current waveform is distorted Figure 6: “True RMS” Multimeter [3] 2/23/2006 EE 6633 Seminar 1

Standard of Harmonics Limitation 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 Iscc / Iload Harmonic odd numbers (<11) numbers (>35) THD < 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

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 as (%) of Fundamental THDv (%) <= 69Kv 3.0 5.0 69 - 161Kv 1.5 2.5 >= 161 Kv 1.0 2/23/2006 EE 6633 Seminar 1

Mitigation the Effects of Harmonics [1] Ranging from variable frequency drive designs to the addition of auxiliary equipments Power System Design: Limiting the non-linear load 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. Determining if resonant condition on the distribution could occur: (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

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

Mitigation the Effects of Harmonics [1] (cont’d) Isolation 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 SCRs as bridge rectifiers Line Reactors More commonly used for size and cost Adding reactor in series with capacitor bank The best suitable to AC drives that use diode bridge rectifier front ends 2/23/2006 EE 6633 Seminar 1

Mitigation the Effects of Harmonics [1] (cont’d) Harmonic 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

Harmonic Trap Filters (cont’d) Tuned to a specific harmonic such as the 5th, 7th, 11th, etc to meet requirements of IEEE 519-1992 Standard The number of turned branches depends on the harmonics to be absorbed and on required reactive power to be compensated Figure 9: Typical Filter Capacitor Bank [5] 2/23/2006 EE 6633 Seminar 1

Harmonics Filter Types [6] Isolating harmonic current to protect electrical equipment from damage due to harmonic voltage distortion Passtive Filter: are built-up by combinations of capacitors, inductors (reactors) and resistors most common and available for all voltage levels   Active Filter: Inserting negative harmonics into the network, thus eliminating the undesirable harmonics on the network. mainly for low voltage networks 2/23/2006 EE 6633 Seminar 1

Harmonics Filter Types (cont’d) [7] Unified Switched Capacitor Compensator: 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 or SMPS-computer network loads. Figure 10 [7] 2/23/2006 EE 6633 Seminar 1

Harmonics Filter Types (cont’d) [7] The USCS is a switched/modulated capacitor bank using a pulse-width modulated (F'WM) strategy. The switching device uses either solid state switch (IGBT or GTO). Figure 11 [7] 2/23/2006 EE 6633 Seminar 1

When 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. During the design stage of a facility composed of capacitor banks and harmonic generating equipment. 2/23/2006 EE 6633 Seminar 1

When to Evaluate System Harmonics? [1] (cont’d) In facilities where restrictive power company requirements limit the harmonic injection back into their system to very small magnitudes. Plant expansions that add significant harmonic generating equipment operating in conjunction with capacitor banks. When coordinating and planning to add an emergency standby generator as an alternate power source in an industrial facility. 2/23/2006 EE 6633 Seminar 1

Conclusion The harmonics distortion principally comes from nonlinear loads. The application of power electronics is causing increased level of harmonics. Harmonics distortion can cause serious problem for uses of electric power. Harmonics are important aspect of power quality. Oversizing and Filtering methods are commonly used to limit effects of harmonics. 2/23/2006 EE 6633 Seminar 1

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_Wireless_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

Question ? 2/23/2006 EE 6633 Seminar 1