Power Quality Issues Power Electronics Group

Basic Requirements for Critical Loads Vs Is Continuous, No Break in Power Voltage Regulation Sine wave Supply Isolation Constant Frequency Power Electronics Group

Understanding Reactive Power and Power Factor Vs Is R Is Vs XL Apparent Power(S) in VA Imp. Phase Angle XL Vs Is R Reactive Power(Q) in VAR True Power(P) in Watts Power Electronics Group

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 volt-amperes. This includes fundamental and all harmonic components ” Power Electronics Group

Understanding Crest Factor Non Linear Load Vs Is Current drawn by single Phase diode rectifier Crest Factor = -------------------------- Peak Amplitude RMS Value Power Electronics Group

Understanding Harmonics Non Linear Load Vs Is 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. Power Electronics Group

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 squ ares of am plitudes o f all harm onics square of amplitude of fundame ntal x 100 Mathematic ally, THD of a voltage wa ve form ca n be defin ed as, THD = V 100 x h 2 1    Power Electronics Group

Power Quality Supply Voltage at load must The Voltage at any point in the distribution system is uniquely described by Where v,f are constants (declared Value) for all t have fundamental component only be balanced have declared magnitude under all conditions Power Electronics Group

PQ affected by Polluting Load PCC Distorted voltage Pure Sinusoidal Line Impedance ZL Power Electronics Group

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 Power Electronics Group

Harmonics-Polluting Loads Rectifiers Arc furnaces Adjustable Speed drives Power Electronic converters Power Electronics Group

Harmonics-FFT Analysis Current drawn by PC 6 Pulse Converter/Battery Charger Harmonic Spectrum of PC Current Waveform Harmonic Spectrum of 6 Pulse Converter/Battery Charger Power Electronics Group

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 Power Electronics Group

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 Power Electronics Group

Causes of Voltage sags and swells Voltage sag due to Motor Starting 1 2 3 4 5 6 7 8 9 10 11 190 195 200 205 210 215 220 RMS voltage in V Time in Cycles Causes for Sag Motor Starting Transformer Energization Transmission Faults Voltage sag due to Transformer Energization Causes for Swell Single line to ground fault Removing a large load / adding a large capacitor bank 5 10 15 20 25 10.1 10.2 10.3 10.4 10.5 10.6 10.7 10.8 10.9 11 Time in Cycles RMS voltage in kV Power Electronics Group

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 Va Vb Vc 1200 Va Vb Vc 1200 1210 1190 Balanced System Unbalanced System Causes for Unbalance Unequal system impedances Unequal distribution of single-phase loads Phase to Phase loads Unbalanced Three phase loads Power Electronics Group

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 Power Electronics Group

Effect of Voltage Unbalance AC Variable Speed drive M 3Ø Diode Rectifier 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 Power Electronics Group

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 Power Electronics Group

Voltage Flicker Causes Induction Motor drive Effect lamp flicker Human eye is most sensitive to voltage waveform modulation around a frequency of 6-8Hz. Causes Induction Motor drive Arc furnaces Arc welders Frequent motor starts Power Electronics Group

Voltage Notching Causes Adjustable Speed Drives Solid State rectifiers Power Electronics Group

Voltage Interruption Complete loss of electrical supply Cause Transmission Fault clearing time Opening / Recloser of circuit breaker Power Electronics Group

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 Power Electronics Group

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 Power Electronics Group

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 h 11 < 17 23 35 TDD 4.0 2.0 1.5 0.6 0.3 5.0 7.0 3.5 2.5 1.0 0.5 8.0 10.0 4.5 0.7 12.0 5.5 15.0 I SC / I L 20 20-50 50-100 100-1000  1000 6.0 1.4 20.0 Power Electronics Group

Reactive and Harmonic Demand PQ Mitigation Reactive and Harmonic Demand Active Filter Passive Filter Voltage sag and Swell Dynamic Voltage Restorer Tap changing transformer Power Electronics Group

PQ Mitigation Voltage Unbalance Voltage Flicker Static Power balancer Redistribution of single-phase loads equally to all phases (Utility level). Load Balancing (Plant level) Voltage Flicker Distribution static VAR compensator Power Electronics Group

Transient Disturbances PQ Mitigation Transient Disturbances Surge Arrester Isolation transformer Active/Passive Filter Power Electronics Group