Note for Electrical Power Quality - EPQ By UPTU Risers

Topics ( 5 )

Text from page-1

POWER QUALITY (UNIT-I) Power quality is a term which is ultimately a consumer driven issue that has different things to different people. Institute of Electrical and Electronic Engineers (IEEE) Standard IEEE1100 defines power quality as “the concept of powering and grounding sensitive electronic equipment in a manner suitable for the equipment.” A simpler and perhaps more concise definition might state: “Power quality is a set of electrical boundaries that allows a piece of equipment to function in its intended manner without significant loss of performance or life expectancy.” This definition embraces two things that we demand from an electrical device: performance and life expectancy. Any power problem manifested in voltage, current or frequency deviations that results in failure or misoperation of customer equipment will consider as power quality issue. What is Electric Power Quality? • To maintain the power distribution bus voltages to near sinusoidal waveform at rated voltage magnitude & frequency. • It is a measure of how well electric power can be utilized by customers. Need for Power Quality:  In the recent years, power quality (PQ) has become a significant issue for both power suppliers and customers.  There have been three important changes in relation to power quality. • First of all, the characteristics of load have become so complex that the voltage and current of the power line connected with these loads are easy to be distorted. • Lately, non-linear loads with power electronic interface that generate large harmonic current have been greatly increased in power system. • Next, the end-user equipments have become more sensitive to power quality than before. CLASSIFICATION OF LOADS LINEAR LOAD • The voltage and current waveforms in electrical circuits with linear loads look alike i. e. no distortion. • Example: Motors operating from sinusoidal supply mains with unsaturated magnetic circuit. • A linear element in a power system is a component in which the current is proportional to the voltage. This means that the current wave shape will be the same as the voltage as shown in fig.1. Typical examples of linear loads include motors, heaters and incandescent lamps. • AC electrical loads where the voltage and current waveforms are sinusoidal. The current at any time is proportional to voltage. Linear Loads are: power factor improvement capacitors, indecent lamps, heaters etc. Applies to those ac loads where the current is not proportional to the voltage. Foremost among loads meeting their definition is gas discharge lighting having saturated ballast coils and thyristor (SCR) controlled loads. (1)Voltage and current waveforms for linear NON-LINEAR LOAD • The current waveform does not resemble the applied voltage waveform. • Example: Motors operating from power converters • The nature of non-linear loads is to generate harmonics in the current waveform. This distortion of the current waveform leads to distortion of the voltage waveform. Under these conditions, the voltage waveform is no longer proportional to the current. Non Linear Loads are: computer, laser printers, sumps, rectifier, plc, electronic ballast, refrigerator, TV etc. The current wave shape on a non-linear load is not the same as the voltage as shown in figure. Typical examples of non-linear loads include rectifiers (power POWER QUALITY 1

Text from page-2

supplies, UPS units, discharge lighting), adjustable speed motor drives, ferromagnetic devices, DC motor drives and arcing equipment. The current drawn by non-linear loads is not sinusoidal but it is periodic, meaning that the current wave looks the same from cycle to cycle. Periodic waveforms can be described mathematically as a series of sinusoidal waveforms that have been summed together as shown in fig. The sinusoidal components are integer multiples of the fundamental where the fundamental, in the United States, is 60 Hz. The only way to measure a voltage or current that contains harmonics is to use a true-RMS reading meter. If an averaging meter is used, which is the most common type, the error can be significant. (a) Voltage and current waveforms for non-linear loads (b) Waveform with symmetrical harmonic components Each term in the series is referred to as a harmonic of the fundamental. The third harmonic would have a frequency of three times 60 Hz or 180 Hz Symmetrical waves contain only odd harmonics and un-symmetrical waves contain even and odd harmonics. A symmetrical wave is one in which the positive portion of the wave is identical to the negative portion of the wave. An un-symmetrical wave contains a DC component (or offset) or the load is such that the positive portion of the wave is different than the negative portion. An example of unsymmetrical wave would be a half wave rectifier. DEFFERANCE BETWEEN LINEAR LOADS AND NON-LINEAR LOADS Table: Difference between linear loads and non-linear loads S. No. 1 2 LINEAR LOADS Ohms law is applicable Crest Factor = √2=1.41 NONLINEAR LOADS Ohms law is not applicable Crest Factor could be 3 to 4 3 Power factor = Cos ɸ 4 Load current does not contain harmonics. Could be inductive or capacitive. Resistive, Inductive or capacitive Zero neutral current if 1 Ph. loads are equally balanced on 3Ph. Mains (Vector sum of line current) May not demand high inrush currents while starting. Power factor ≠ Cos ɸ = Displacement factor X Distortion Factor Load current contains all ODD harmonics. 5 6 7 8 Can’t be categorized. As leading or lagging Loads. Usually an equipment with Diode and Capacitor Neutral current could be 2.7 times the line current even if 1Ph. loads are equally balanced on 3 Ph. Mains Essentially very high inrush current (20 time of I Normal) is drawn while starting for approx. One cycle. Main Power Quality Problems/Issues: 1. Harmonic distortion 2. Momentary Interruptions 3. Temporary Interruptions 4. Long Term outage 5. Noise 6. Voltage Sag 7. Voltage Swell 8. Voltage Spikes 9. Undervoltages POWER QUALITY 2

Text from page-3

Voltage based Power Quality Problems: • Voltage sag • Voltage swell • Voltage Interruption • Under/over Voltage • Voltage Flicker • Harmonic Distortion • Voltage Notching • Transient Disturbance • Outage and frequency variation Current based Power Quality Problems: • Reactive Power Compensation • Voltage Regulation • Current Harmonic Compensation • Load Unbalancing (for 3-phase systems) • Neutral Current Compensation (for 3phase 4-wire systems) Sinusoidal Voltage Sources of Power Quality Problems: • Power electronic devices • IT and office equipments • Arching devices • Load switching • Large motor starting • Embedded generation • Sensitive Equipment • Storm and environmental related damage Distorted Voltage (Voltage Drop)   Distorted Load Current TRANSIENTS • A transient can be unidirectional impulse of either polarity or a domped oscillatory wave with first peak occuring in either polarity. • Other definitions in common use are broad in scope and simply state that a transient is “that part of the change in a variable that disappears during transition from one steady state operating condition to another.” • Transients can be classified into two categories: impulsive and oscillatory. These terms reflect the waveshape of a current or voltage transient. Impulsive transient An impulsive transient is a sudden, non–power frequency change in the steady-state condition of voltage, current, or both that is unidirectional in polarity (primarily either positive or negative). • Impulsive transients are normally characterized by their rise and decay times, which can also be revealed by their spectral content. The most common cause of impulsive transients is lightning. Oscillatory transient • An oscillatory transient is a sudden, non–power frequency change in the steady-state condition of voltage, current, or both, that includes both positive and negative polarity values. • An oscillatory transient consists of a voltage or current whose instantaneous value changes polarity rapidly. • Oscillatory transients with a primary frequency component greater than 500 kHz and a typical duration measured in microseconds (or several cycles of the principal frequency) are considered high-frequency transients. These transients are often the result of a local system response to an impulsive transient. POWER QUALITY 3

Text from page-4

• A transient with a primary frequency component between 5 and 500 kHz with duration measured in the tens of microseconds (or several cycles of the principal frequency) is termed a medium-frequency transient. • A transient with a primary frequency component less than 5 kHz, and a duration from 0.3 to 50 ms, is considered a low-frequency transient. Long-Duration Voltage Variations • Long-duration variations encompass root-mean-square (rms) deviations at power frequencies for longer than 1 min. • Long-duration variations can be either overvoltages or undervoltages. • Overvoltages and undervoltages generally are not the result of system faults, but are caused by load variations on the system and system switching operations. Such variations are typically displayed as plots of rms voltage versus time. Short-Duration Voltage Variations: • Short-duration voltage variations are: voltage dips and short interruptions. Each type of variation can be designated as instantaneous, momentary, or temporary, depending on its duration as shown below: Instantaneous Interruption 0.5-30 cycles <0.1p.u Sag (dip) 0.5-30 cycles 0.1-0.9p.u Swell 0.5-30 cycles 1.1-1.4p.u Interruption 30cycles-3s <0.1p.u Sag (dip) 30cycles-3s 0.1-0.9p.u Swell 30cycles-3s 1.1-1.4p.u Interruption 3s-1min <0.1p.u Sag (dip) 3s-1min 0.1-0.9p.u Swell 3s-1min 1.1-1.4p.u Momentary Temporary Short-duration voltage variations are caused by fault conditions, the energization of large loads which require high starting currents, or intermittent loose connections in power wiring. Depending on the fault location and the system conditions, the fault can cause either temporary voltage drops (sags), voltage rises (swells), or a complete loss of voltage (interruptions). Interruption An interruption occurs when the supply voltage or load current decreases to less than 0.1 pu for a period of time not exceeding 1 min. Interruptions can be the result of power system faults, equipment failures, and control malfunctions. Sags (dips) A sag is a decrease to between 0.1 and 0.9 p.u in rms voltage or current at the power frequency for durations from 0.5 cycle to 1 min. Voltage sags are usually associated with system faults but can also be caused by energization of heavy loads or starting of large motors. Sags starve a machine of the electricity it needs to function, causing computer crashes or equipment lock-ups. POWER QUALITY 4