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Electric Power Quality Tutorial Part II: Sources and Mitigation Schemes

PQ TUTORIAL: PART II. 2/44. Sources of Power Quality Problems. Nonlinear Loads Sources of Harmonics Sources of Flicker Sources of Sag Different Converter Schemes. PQ TUTORIAL: PART II. 3/44. Proliferation of Nonlinear Loads. Impact customer loads, distribution feeders and substation equi

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Electric Power Quality Tutorial Part II: Sources and Mitigation Schemes

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    1. Electric Power Quality Tutorial Part II: Sources and Mitigation Schemes by S. S. (Mani) Venkata Iowa State University Ames, Iowa

    2. PQ TUTORIAL: PART II 2/44 Sources of Power Quality Problems Nonlinear Loads Sources of Harmonics Sources of Flicker Sources of Sag Different Converter Schemes

    3. PQ TUTORIAL: PART II 3/44 Proliferation of Nonlinear Loads Impact customer loads, distribution feeders and substation equipment. Impacts individual customers’ neighbors and ultimately the source. Lack of National standards complicate the issue. Utilities, customers and suppliers need to work together.

    4. PQ TUTORIAL: PART II 4/44 Sources of Harmonics Power Electronic Devices Phase-angle regulators in lighting/heating controllers Rectifiers/Inverters Adjustable speed motor drives Interface of wind /solar power converters with the utility HVDC systems

    5. PQ TUTORIAL: PART II 5/44 Sources of Harmonics & Flicker Ferromagnetic devices Transformers (saturation non-linearity) Arcing Devices Fluorescent lamps Arc welders Arc furnaces

    6. PQ TUTORIAL: PART II 6/44 Starting of Heavy Loads Large motors Brown Outs Large loads Fault clearing times on distribution feeders: 5 to 15 cycles. Sags range from 20 to 50%. Sources of Sags

    7. PQ TUTORIAL: PART II 7/44 Effects of Sags Voltage sags not perceptible to human eye. Sensitive electronic equipment affected by voltage sags. Sags interrupt service to loads such as automated processes for many hours. Results in loss of revenue of millions of dollars.

    8. PQ TUTORIAL: PART II 8/44 Sags Need compensation devices to avoid interruption. Need to evaluate the role of protective devices. Reclosings after a fault Operating speed of circuit breakers, fuses and reclosers. Need to analyze causes for faults. e.g.: Tree falling. Multiple reclosings maybe questionable from power quality perspective. Adopt optimum tree trimming policies.

    9. PQ TUTORIAL: PART II 9/44 Phase-angle Regulators Example: Lamp dimmer Comments: Current distortion can be reduced by proper sizing of the choke. THD and radiated EMI is low for triggering angles close to 0° and 180° (i.e., full or zero brightness) THD and radiated EMI is highest for angles to 90° (half-bright).

    10. PQ TUTORIAL: PART II 10/44

    11. PQ TUTORIAL: PART II 11/44

    12. PQ TUTORIAL: PART II 12/44 Single-phase Rectifiers Examples: Computer power supplies, Battery chargers The rectifier conducts only when the line voltage magnitude exceeds the capacitor voltage. The capacitor gets charged by drawing current at the peak of the voltage cycle and gets discharges slowly into the switching regulator between the voltage peaks. Thus the circuit draws short pulses of current during line voltage peaks.

    13. PQ TUTORIAL: PART II 13/44 Current Drawn by a Computer Power Supply

    14. PQ TUTORIAL: PART II 14/44 Sequence Classification of Harmonics In AC systems, the current and voltage waveforms have rotational symmetry. even harmonics will not be present. Power system harmonics are hence predominantly the odd, i.e 3rd, 5th, 7th, etc.

    15. PQ TUTORIAL: PART II 15/44 Three-phase Rectifiers Six-pulse Rectifier Used in motor drives, traction, electrochemical plants, etc. The high inductance in the dc side causes the dc current, Id to be essentially constant.

    16. PQ TUTORIAL: PART II 16/44 Three-phase Rectifiers (cont.) Six-pulse Rectifier The Fourier series for the line current for a diode rectifier is: For symmetrical ideal triggering, only harmonics of the order 6n±1 are present in the AC side currents. The presence of source reactance and commutation effects lead to smoother current waveforms.

    17. PQ TUTORIAL: PART II 17/44

    18. PQ TUTORIAL: PART II 18/44 Twelve-pulse Rectifier (cont.) Used in high power motor drives, traction, hvdc converters, etc. The Fourier series for the line current for a twelve-pulse diode rectifier is: For symmetrical ideal triggering, only harmonics of the order 12n±1 are present in the AC side currents.

    19. PQ TUTORIAL: PART II 19/44 Cycloconverters Used in large mill drives in cement and mining industries. The characteristic harmonics generated are:

    20. PQ TUTORIAL: PART II 20/44 Integral-cycle Controllers or Pulse Burst Modulation (PBM) This technique is used in applications such as heating, ovens, furnaces, etc. Subharmonics are predominant. DC component can also be present. High frequency harmonics above 200 Hz are practically absent.

    21. PQ TUTORIAL: PART II 21/44 A Demonstration That a Balanced 3-Phase Load Can Result In Neutral Current

    22. PQ TUTORIAL: PART II 22/44 Mitigation Schemes Types of filters - passive and active. Passive filters harmonic analysis using driving point impedances, use of capacitors as passive filters, use of series RLC components as passive filters. Active Filters Static Var Compensators FACTS and Custom Power Devices for power quality

    23. PQ TUTORIAL: PART II 23/44 Types of Filters Passive filters provide low impedance path to ground at resonance frequency, use tuned RLC components, economical. Active filters inject harmonic currents (or voltages) out of phase with the ambient harmonics, use components such as switches and amplifiers, expensive.

    24. PQ TUTORIAL: PART II 24/44 Passive Filters Series tuned circuit offers very low impedance at resonance frequency Parallel tuned circuit offers very high impedance at resonance frequency

    25. PQ TUTORIAL: PART II 25/44 Capacitor as a Filter A shunt capacitor is the simplest form of passive filter economical, also provides reactive power (Q) compensation. Guidelines for sizing capacitive filters resonance between capacitor and circuit inductive reactance should not occur exactly at an integer multiple of fundamental frequency. sensitivity of resonant point to drift in capacitor value should be investigated, voltage and var support provided should not be excessive, IEEE Standard 18 should be consulted for sizing and placement of capacitor.

    26. PQ TUTORIAL: PART II 26/44 Active Filters - Voltage and Current Type Voltage type active filter capacitor (dc source), voltage source inverter (VSI). Current type active filter inductor (current source), current source inverter (CSI).

    27. PQ TUTORIAL: PART II 27/44

    28. PQ TUTORIAL: PART II 28/44

    29. PQ TUTORIAL: PART II 29/44

    30. PQ TUTORIAL: PART II 30/44 Active Filters - Technology Overview Active filters are also referred to as active power line conditioners. Reference [3] gives an extensive literature survey of existing and proposed line conditioning methodologies. Active filters may also be classified depending on their correction method. correction in time-domain, correction in frequency domain. Table 1 in [3] gives a summary of the various publications in the area of active filters - a few entries of which are shown here in Table 1.

    31. PQ TUTORIAL: PART II 31/44 Active Filters - Technology Overview In Table 1, ‘Type’ indicates voltage type (V) or current type (I), ‘P.E. Device’ indicates the power electronic switch used.

    32. PQ TUTORIAL: PART II 32/44 Static Var Compensator Consists of electronically switched capacitor and/or inductor. Some SVC technologies Thyristor Controlled Reactor (TCR) with fixed capacitor (FC) TCR with thyristor switched capacitor (TSC). The Adaptive Var Compensator (AVC), developed at the University of Washington, is essentially a bank of TSCs.

    33. PQ TUTORIAL: PART II 33/44 FACTS and Custom Power Devices The other families of power electronic devices, very closely related to the active filters, are Flexible AC Transmission System (FACTS) devices, Custom Power Devices. FACTS devices are intended for [4] greater control of power transmission, maximize utilization of existing transmission lines, reduction of generation reserve margin, prevention of cascading outages, damping of power system oscillations.

    34. PQ TUTORIAL: PART II 34/44 Static Condenser (STATCOM) FACTS and Custom Power Device reactive power compensation, voltage regulation (by reactive power compensation), harmonic current compensation. Behaves as a voltage source connected in shunt to the power system through an inductor.

    35. PQ TUTORIAL: PART II 35/44 Dynamic Voltage Restorer (DVR) FACTS and Custom Power Device voltage regulation (by series compensation), harmonic line voltage compensation. Behaves as a voltage source connected with the power line.

    36. PQ TUTORIAL: PART II 36/44 Solid State Breaker (SSB) Custom Power Device for instantaneous fault clearing, subcycle reclosing, zero current/voltage closing.

    37. PQ TUTORIAL: PART II 37/44 Adaptive Power Quality Compensator Decouple reactive and harmonic compensation Arrange Converters so that slow switching reactive converter bears most of the stress fast switching harmonic converter handles lower voltages and currents.

    38. PQ TUTORIAL: PART II 38/44

    39. PQ TUTORIAL: PART II 39/44

    40. PQ TUTORIAL: PART II 40/44

    41. PQ TUTORIAL: PART II 41/44

    42. PQ TUTORIAL: PART II 42/44 The Adaptive Var Compensator (AVC) No harmonics or transients are introduced by the AVC. Compensates on cycle to cycle basis power factor control, voltage control. Voltage flicker control

    43. PQ TUTORIAL: PART II 43/44 The Adaptive Var Compensator (AVC) Essentially a bank of switched capacitors. Capacitors are in a binary ratio (1:2:4). SCRs switched at zero current and zero voltage crossing.

    44. PQ TUTORIAL: PART II 44/44 References 1. S. S. Venkata, G. T. Heydt, “Proceedings of the NSF Workshop on Electric Power Quality,” Jan. 1991, Grand Canyon, AZ. J. Arrillaga, N. R. Watson, S. Chen, “Power System Quality Assessment,” John Wiley & Sons, England, 2000. R. C. Dugan, M. F. McGranaghan, H. W. Beaty, “Electrical Power Systems Quality,” McGraw-Hill, USA,1996. G. T. Heydt, “Electric Power Quality,” Stars in a Circle, USA, 1991. E. Acha, M. Madrigal, “Power Systems Harmonics: Computer Modeling and Analysis,” John Wiley & Sons, England, 2001.

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