Power quality, part I Introduction: The major disturbances that are introduced into the power distribution systems either from the load centre or the utility including environmental causes can be classified into: steady state and transients. For steady state, the main disturbance is harmonics. For transients, the main disturbances are: sags, dips, momentary blackouts, surges, spikes, over/under voltages, frequency variations, power lines noise (common & normal mode interference), conducted radio frequency and electromagnetic interference. Steady state disturbances: The sources of steady state disturbances like harmonics are arc furnaces, welders, saturable reactors, rectifiers, thyristor controlled loads, variable voltage/frequency drives, switching mode power supplies, UPS, magnetizing currents. Voltage notching is also produced by certain devices aforementioned like adjustable frequency drives. •Arc furnaces and welders: When the arc is stable the nonlinear resistance characteristics of the arc tends to cause a peaked current wave and a flattop voltage. The combination of the non linear arc resistance and large linear external resistance results in more apparent distortion to the supply voltage than to furnace current. The peaked current wave will have harmonics comparable to square waves but with different phase relations. A peaked or triangular wave consists of odd order harmonics. The 3rd, 7th, 11th,..etc. harmonics cross the zero reference line 180 out of phase; 5th, 9th, 13th etc cross the reference in phase with the fundamental. In the square wave, all the harmonics are odd order and cross the reference line inphase with the fundamental. When the arc within the arc furnace is unstable during the meltdown period, sudden changes in arc length would cause sudden changes of current magnitude, which in turn would cause voltage flicker. •Saturable reactors: these devices control the magnitude of current flowing to a load by means of varying the reactance of the saturable reactor. At partial loads, the resultant current wave is very similar to that of a phase controlled rectifier. The pulses are symmetric and will contain odd harmonics. At full load the harmonics problem does not exist. •Rectifiers: A broad classification of rectifiers is whether it is half wave or full wave, a further classification is the number of pulses whether 3, 6, 12, or 24. The harmonics available on a line feeding rectifiers are obtained from pn +/ 1; where p is number of pulses and n is a series of numbers: 1, 2, 3, 4, 5, 6... A full wave rectifier with a smoothing choke in series with the load will produce on the a. c. side a square wave of equal positive and negative values. This square wave is symmetric and will contain the fundamental with all the odd harmonics in phase with the fundamental with magnitudes, approximately, inversely proportional to their order. Other common configuration using three phase input with a WyeDelta rectifier transformer will have no third harmonics or its multiples, it is known as 6 pulse rectifier. Some rectifier circuits are provided with smoothing capacitors (in parallel with load) in place of a choke. In this case, the capacitor will supply energy to hold up the voltage across the load. The current will flow only during the period of time that the supply voltage is greater then the voltage across the capacitor. Thus pauses will exist between the positive and negative current pulses. The harmonics generated in this case will be of the odd order. •Thyristor controlled loads: phase controlled devices will vary the relative magnitudes of the harmonic
components as they are adjusted to control the supply to the load. A phase controlled device with 90 deg. delay may generate a wave similar to the saturable reactor at partial load. With equal phase control in both the positive and negative halfwaves, the waves are symmetrical and will,only, contain odd harmonics. If the control of each wave is independent (full wave rectifier with a phase controlled device in only one leg and a diode in the other), the presence of even harmonics become a possibility. The phase relationship between the fundamental and the harmonics vary with the phase control setting of the thyristor. •Variable voltage/frequency drives: In general each drive by itself does not present a problem, but with the application of many drives, the problem of harmonics & voltage notching may materialize. Line voltage notching and line harmonics currents result into line voltage distortion. With variable frequency drives the static power converter which is the front end of the adjustable frequency is the source of line disturbance. This converter may be a three phase full wave bridge rectifier using either diodes or SCRs. The shape of the distorted line current wave from the adjustable frequency drive will be determined by the type of converter, load and the impedance of the power source. For an inductive load, the curent wave will be close to a square wave, it is typical for SCR converters, six pulse and current source drives without d c chopper. For a capacitive load, the current wave will be non continuous, symmetric, twin half sine waves, typical for diode converters, pulse width modulation drives and six pulse drives with d c chopper. The final magnitude of the various line harmonic currents will be determined by the inductance in the power system commutating reactance, the load on the drive and if SCR are used, on the firing angle (delay angle) of the thyristor. The total harmonic current distortion that a drive produces on the a c line is given by: (sum of square of individual harmonic currents/ fundamental current) 0.5 The above expression can be applied to any harmonics generating device. Voltage notching only occurs when drives produce continuous square current waves but not with non continuous current. The notches occur when continuous line current transfers (commutates) from one phase to another in a static power converter. During commutation, two phases are connected for a very short duration through the ac source impedance (very low) by the converter, this will cause the voltage to drop near zero. The voltage notching is expressed by the following : (sum of square of all harmonics voltages /
fundamental voltage)0.5 which is equal to the voltage distortion factor. This factor is related to the area and depth of the voltage notch and the reactance of the circuit. In some cases, when SCRs are used in the converter, voltage spikes of very short duration can be generated immediately following the notch. It is a phenomenon associated with solid state devices. •Switching mode power supplies: d.c. power supplies, +/5V or +/12V for computers and other micro processor based electronic equipment is a major source of harmonic distortion and noise. •Uninteruptable power supplies (UPS): the output distortion of a UPS depends upon its design and the output impedance. Usually the distortion caused by the UPS is specified by the manufacturer. It is given as a percentage harmonic distortion and is limited to 5% total harmonic distortion. This number is based on linear loads or reactive inductive loads. In cases where the UPS supplies nonlinear loads, the harmonic distortion can be higher than with linear loads.
•Magnetizing currents: When transformers are energized, inrush currents that is rich in second harmonics will be seen by the primary of the transformer. The duration of such phenomenon can vary from 10 cycles to 3600 cycles (1 minute) function of the size of the transformer bank. After the elapse of the inrush period the exciting current which is typically 5% of the full load current of the transformer will continue to appear in the primary of the transformer and is rich in the third and fifth harmonics. If the exciting current is analyzed by Fourrier series method, it will be found to comprise a fundamental and a family of odd harmonics. The fundamental can further be resolved into two components, one in phase with the counter EMF and the other lagging by 90 deg. The fundamental in phase component accounts for the power absorbed by the hysteresis and eddy current losses in the core. It is the core loss component of the exciting current. If the core loss component is subtracted from the total exciting current, the magnetizing current can be found (the remainder). It comprises of a fundamental lagging the counter emf by 90 deg. and all the harmonics. For a typical power transformer, the third harmonics is 40% of the exciting current. Briefly, arcing faults and their effect on power quality will be discussed. In an arcing fault (to ground) on low voltage systems discontinuous current is produced. The reason for the discontinuity is the period of time before which the voltage arcs over from the line to ground. It will extinguish at the first current zero and then reignite on the succeeding wave of opposite polarity. The wave is symmetric and will have considerable odd harmonics content.