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Power Quality Issues Basic Requirements for Critical Loads Is Vs Load Continuous, No Break in Power Voltage Regulation Sine wave Supply Isolation Constant Frequency Understanding Reactive Power and Power Factor Is Is R Vs Vs XL XL Vs R Imp. Phase Angle True Power(P) in Watts Reactive Power(Q) in VAR Is Displacement and True Power Factor Displacement Power Factor: “Ratio of the active power of the fundamental, in watts, to the apparent Power of the fundamental wave, in volt-amperes” True Power Factor: “Ratio of the total power, in watts, to the total voltamperes. This includes fundamental and all harmonic components ” Understanding Crest Factor Is Vs Non Linear Load Current drawn by single Phase diode rectifier Peak Amplitude Crest Factor = -------------------------RMS Value Understanding Harmonics Is Vs Non Linear Load Deviation from a perfect sine wave can be represented by Harmonics. Sinusoidal Component having a frequency that is an integral multiple of the Fundamental frequency. Total Harmonic Distortion Defines the total harmonic content of current or voltage Ratio of the RMS of the harmonic content to the RMS of the Fundamental, as % of Fundamental THD = sum of squares of amplitudes of all harmonics x 100 square of amplitude of fundamental Mathematically,THD of a voltage wave form can be defined as, 2 h Vh THD = x 100 2 h2 V 1 Power Quality Supply Voltage at load must have fundamental component only be balanced have declared magnitude under all conditions The Voltage at any point in the distribution system is uniquely described by V(t)ft 2 VSin ( 2 ) Where v,f are constants (declared Value) for all t PQ affected by Polluting Load PCC A Polluting Load Pure Sinusoidal Line Impedance ZL B Distorted voltage Common PQ Disturbances Reactive Power Demand Harmonic Distortion Voltage sags and swells Undervoltages and overvoltages Voltage Unbalance Voltage Flicker Voltage Notching Voltage Interruption Transient Disturbances Frequency variations Harmonics-Polluting Loads Rectifiers Arc furnaces Adjustable Speed drives Power Electronic converters Harmonics-FFT Analysis Current drawn by PC Harmonic Spectrum of PC Current Waveform 6 Pulse Converter/Battery Charger Harmonic Spectrum of 6 Pulse Converter/Battery Charger Implication of Reactive and Harmonics currents Oversize of all installation equipments to transmit Reactive and Harmonic currents namely Transformer Cables Circuit breakers & distribution switch boards Neutral overloads Increase in Transmission & Distribution loss Reduction in voltage stability margin Overheating and loss of life & equipments Major Increase in cost Voltage sags and swells • Reduction in the ac voltage, at the power frequency, for durations from a half-cycle to a few seconds. • Voltage Sag is Characterized by two parameters – Magnitude and Duration • Power Electronics Loads are Sensitive to Voltage Sags Causes of Voltage sags and swells Voltage sag due to Motor Starting 220 Causes for Sag Transmission Faults RMS voltage in V Motor Starting Transformer Energization 215 210 205 200 195 190 0 1 2 3 4 5 6 7 Time in Cycles 8 9 10 11 Causes for Swell Single line to ground fault Removing a large load / adding a large capacitor bank RMS voltage in kV Voltage sag due to Transformer Energization 11 10.9 10.8 10.7 10.6 10.5 10.4 10.3 10.2 10.1 10 0 5 10 15 20 Time in Cycles 25 Voltage Unbalance Definition In a balanced sinusoidal supply system the three line-neutral voltages are equal in magnitude and are phase displaced from each other by 120 degrees V V a 1200 Vc a Balanced System 1200 1200 1200 Vb Unbalanced System 1190 1210 Vc Causes for Unbalance Unequal system impedances Unequal distribution of single-phase loads Phase to Phase loads Unbalanced Three phase loads Vb Effect of Voltage Unbalance Induction Motor drive • • • • Overheating and loss of Insulation life Reduced Motor Efficiency Noisy in their operation due Torque and speed pulsation Motor derating factor NEMA Induction motor derating curve Effect of Voltage Unbalance AC Variable Speed drive 3Ø Diode Rectifier M Line Current of 3Ø diode Rectifier for Balanced Input Line Current of 3Ø diode Rectifier for 5%unbalanced Input Line Current of 3Ø diode Rectifier for 15%unbalanced Input • Draws uncharacteristic triplen harmonics • Triplen harmonic current can lead to undesirable harmonic problems • Excessive thermal stress on diodes Voltage Flicker Definition Repetitive or random variations of the voltage envelope modulated at frequencies less than 25 Hz, which the human eye can detect as a variation in the lamp intensity of a standard bulb due to sudden changes in the real and reactive Power drawn by a load Voltage waveform showing flicker created by an arc furnace Voltage Flicker Causes Induction Motor drive • Arc furnaces • Arc welders • Frequent motor starts Effect lamp flicker Human eye is most sensitive to voltage waveform modulation around a frequency of 6-8Hz. Voltage Notching Causes Adjustable Speed Drives Solid State rectifiers Voltage Interruption Complete loss of electrical supply Cause Transmission Fault clearing time • Opening / Recloser of circuit breaker Transient Disturbances Transient disturbances are caused by the injection of energy by switching or by lightning Causes Lightning Capacitor Switching Load switching Oscillatory transient waveform caused by capacitor energizing Need for Power Quality Business Problems: • Momentary disturbance can cause scrambled data, interrupted communications, system crashes and equipment failure • Lost productivity and idle people and equipment • Overtime required to make up for lost work time • Revenue and accounting problems such as invoices not prepared, payments held up etc According to Electric Light and Power Magazine, 30 to 40 percent of all business downtime is related to Power Quality Problems IEEE 519 Harmonic Standard IEEE 519 “Recommended Practices and Requirements for Harmonic Control in Electric Power Systems” Specifies load current harmonic limits at PCC Specifies supply voltage harmonic limits at PCC HARMONIC CURRENT DISTORTION LIMITS IN % OF IL V 69 kV I SC/ I L 20 20-50 50-100 100-1000 1000 h <11 11 h 17 17h 23 23 h 35 35h TDD 4.0 7.0 10.0 12.0 15.0 5.0 8.0 12.0 15.0 20.0 2.0 3.5 4.5 5.5 7.0 1.5 2.5 4.0 5.0 6.0 0.6 1.0 1.5 2.0 2.5 0.3 0.5 0.7 1.0 1.4 PQ Mitigation Reactive and Harmonic Demand Active Filter Passive Filter Voltage sag and Swell Dynamic Voltage Restorer Tap changing transformer PQ Mitigation Voltage Unbalance Static Power balancer Redistribution of single-phase loads equally to all phases (Utility level). Load Balancing (Plant level) Voltage Flicker Distribution static VAR compensator PQ Mitigation Transient Disturbances Surge Arrester Isolation transformer Active/Passive Filter