CHAPTER 2 POWER QUALITY ISSUES

CHAPTER 2

POWER QUALITY PROBLEMS AND OVERVIEWDefinition of Power Quality:There can be completely different definitions for power quality, depending on ones frame of reference. For example, a utility may define power quality as reliability and show statistics demonstrating that its system is 99.98 percent reliable. Criteria established by regulatory agencies are usually in this vein. A manufacturer of load equipment may define power quality as those characteristics of the power supply that enable the equipment to work properly. These characteristics can be very different for different criteria [3].Power quality is ultimately a consumer-driven issue, and the end users point of reference takes precedence. Finally we can define as Any power problem manifested in voltage, current, or frequency deviations that result in failure or malfunctioning of customer equipment. Voltage quality: Voltage quality is the quality of the product delivered by the utility to the customers. It is concerned with deviations of the voltage waveform from the ideal sinusoidal waveform.Current quality: Current quality is a complementary term to the voltage quality. It is concerned with deviations of the current waveform from the ideal sinusoidal waveform. In addition to the ideal current waveform (as demanded by the utility), the current sinusoidal wave should be in phase with supplied voltage to minimize the transmitted apparent power and consequently the power system ratings. Because voltage and current are closely related, a deviation of any of them from the ideal may (with a high probability) cause the other to deviate from the ideal case [3].2.1 Power Quality Aspects of Telecom Power SuppliesElectronic power supplies (including the telecom power supplies) are interfaces between the utility and the end user. As such, they contribute to power-quality problems and, at the same time, are victims of poor power quality. The main contributions of electronic power supplies to power quality problems are Increased line current, voltage distortion (caused by the normal rectifier operation and leading to peaky line current, flat-topped line voltage, and increased neutral current)

Sags (caused by charging up the bulk capacitor at start-up), and

EMI (caused by the switching action in switching power supplies, and also by the normal rectifier operation).

The effects of poor power quality on electronic power supplies include

Failures (caused by high-energy impulses, swells, notches, and short outages),

Reduced hold-up time (during sags, also caused by flat-topping voltage distortion),

Increased output noise (caused by EMI, impulses, notches, and voltage distortion); all those effects lead to

Increased Cost (due to the need for surge suppression devices, larger bulk capacitors, and extra filtering and shielding).The relationship of telecom power supplies and power quality is similar to that of the electronic power supplies in other applications. The telecom power supply has, however, some distinctive power-quality related characteristics that set it apart from the common power supply. Those are as follows [3].

1. The telecom power supply usually operates with relatively high power level, and it has, therefore, a much greater local impact on power quality than other power supplies with smaller power levels. Even if the power level is low (as, e.g. in a remote installation), telecom power supplies constitute a substantial portion of local power consumption, and the impact on local power quality is retained.

2. The telecom power supply is fundamentally a benign load, with practically continuous operation and small variations in the load current. Its greatest effect on power quality is due to the harmonic currents and EMI injected in the line. Some earlier designs also introduce a lag in the fundamental current (power supplies with phase-angle control or with large inductive line filter).

3. Due to their long service life, in addition to power quality standards in effect at the time of commissioning, telecom power supplies are expected to comply with anticipated future standards, too.4. Availability is very important because virtually uninterrupted service is required in telecommunications. High availability requires highly reliable equipment that is resistant to line borne disturbances, and

Local energy storage (usually batteries).

In some applications, large-capacity backup batteries may prove economically impractical, and the emergency power is provided by Diesel generators. As a rule, the emergency power is of lower grade than the normal utility power. This imposes additional requirements on the telecom rectifiers and increases their cost.

5. Energy conversion efficiency is more important in telecom applications than in other applications, due to the practically continuous operation and the relatively high power levels [11].2.2 General Classes of Power Quality Problems Power quality problems encompass a wide range of disturbances that can disrupt the operation of sensitive industrial loads and cause a loss of production. In this Section, the following power quality problems are defined and briefly discussed.

Table 2.1 Power Disturbances DISTURBANCE TYPE DESCRIPTION CAUSES

Narrow pulse with fast rise and exponential or damped oscillatory decay; 50 V to 6 kV amplitude, 0.5 s to 2 ms durationLoad switching, fuse clearing, utility switching, arcing contacts, lightning

Repetitive low energy disturbances in the 10 KHz to 1 GHz band, with 100 V to 100 V amplitudeNormal equipment operation (switching power supplies, motor speed controllers, etc.), carrier power-line communication, wireless broadcasting

Low voltage (typ. less than 80%), for more than one period HighStarting heavy load, utility switching, ground fault

High voltage (typ. more than 1l0 % ), for more than one periodLoad reduction, utility switching

Small repetitive fluctuations in the voltage levelPulsating load

Repetitive dips in the line voltage, with short durationsCurrent commutation in controlled or uncontrolled three-phase rectifier circuits

Deviation from ideal sine wave due to the presence of harmonics or inter harmonicsRectifiers, phase-angle controllers, other nonlinear and/or intermittent loads

Deviation of the frequency from the nominal valuePoorly regulated utility equipment, emergency power generator

Zero-voltage condition of a single phase or several phases in a multiphase system, for more than a half periodLoad equipment failure, ground fault, utility equipment failure, accidents, lightning, acts of nature

2.3 Transients

The term transients have long been used in the analysis of power system variations to denote an event that is undesirable and momentary in nature. Another word in common usage that is often considered synonymous with transient is surge. A utility engineer may think of a surge as the transient resulting from a lightning stroke for which a surge arrester is used for protection. Broadly speaking, transients can be classified into two categories, impulsive and oscillatory. These terms reflect the wave shape of a current or voltage transient. 2.4 Long-Duration Voltage VariationsLong-duration variations encompass root-mean-square (rms) deviations at power frequencies for longer than 1 min. Long-duration variations can be either over voltages or under voltages. Over voltages and under voltages generally are not the results 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.2.4.1 OvervoltageAn overvoltage is an increase in the rms ac voltage greater than 110 percent at the power frequency for duration longer than 1 min. Over voltages are usually the results of load switching (e.g., switching off a large load or energizing a capacitor bank). The over voltages result because either the system is too weak for the desired voltage regulation or voltage controls are inadequate. Incorrect tap settings on transformers can also result in system over voltages.

2.4.2 UndervoltageAn under voltage is a decrease in the rms ac voltage to less than 90 percent at the power frequency for a duration longer than 1 min. Undervoltage are the result of switching events that are the opposite of the events that cause overvoltages. A load switching on or a capacitor bank switching off can cause an under voltage until voltage regulation equipment on the system can bring the voltage back to within tolerances. Overloaded circuits can result in under voltage also. The term brownout is often used to describe sustained periods of under voltage initiated as a specific utility dispatch strategy to reduce power demand. Because there is no formal definition for brownout and it is not as clear as the term under voltage when trying to characterize a disturbance, the term brownout should be avoided [6].

2.5 Short-Duration Voltage Variations

This category encompasses the IEC category of voltage dips and short interruptions. Each type of variation can be designated as instantaneous momentary, or temporary, depending on its duration. 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). The fault condition can be close to or remote from the point of interest. In either case, the impact on the voltage during the actual fault condition is of the short-duration variation until protective devices operate to clear the fault.2.6 InterruptionsAn interruption occurs when the supply voltage or load current decreases to less than 0.1 p.u. for a period of time not exceeding 1 min. Interruptions can be the result of power system faults, equipment failures, and control malfunctions. The interruptions are measured by their duration since the voltage magnitude is always less than 10 percent of nominal. The duration of an interruption due to a fault on the utility system is determined by the operating time of utility protective devices. Instantaneous reclosing generally will limit the interruption caused by a nonpermanent fault to less than 30 cycles. Delayed reclosing of the protective device may cause a momentary or temporary interruption. The duration of an interruption due to equipment malfunctions or loose connections can be irregular [11].2.7 Sags (dips)

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. The power quality community has used the term sag for many years to describe a short-duration voltage decrease. Although the term has not been formally defined, it has been increasingly accepted and used by utilities, manufacturers, and end users. The IEC definition for this phenomenon is dip. Figure 2.1 shows typical voltage sag that can be associated with a single-line-to-ground (SLG) fault on another feeder from the same substation. 80 percent sag exists for about 3 cycles until the substation breaker is able to interrupt the fault current. Typical fault clearing times range from 3 to 30 cycles, depending on the fault current magnitude and the type of over current protection [3]. Figure 2.1 Voltage sag caused by an SLG fault. RMS waveform for Voltage Sag event. 2.8 SwellsA swell is defined as an increase in values between 1.1 and 1.8 p.u. in rms voltage or current at the power frequency for durations from 0.5 cycles to 1 min. As with sags, swells are usually associated with system fault conditions, but they are not as common as voltage sags. One way that a swell can occur is from the temporary voltage rise on the unfaulted phases during an SLG fault [4]. Figure 2.2 illustrates a voltage swell caused by an SLG fault. Swells can also be caused by switching off a large load or energizing a large capacitor bank. Swells are characterized by their magnitude (rms value) and duration. The severity of a voltage swell during a fault condition is a function of the fault location, system impedance, and grounding. On an ungrounded system, with infinite zero-sequence impedance, the line-to-ground voltages on the ungrounded phases will be 1.73 p.u during an SLG fault condition. Close to the substation on a grounded system, there will be little or no voltage rise on the unfaulted phases because the substation transformer is usually connected delta-star, providing a low-impedance zero-sequence path for the fault current. Faults at different points along four-wire, multigrounded feeders will have varying degrees of voltage swells on the unfaulted phases [4].

Figure 2.2 Instantaneous Voltage Swell caused by an SLG fault. 2.9 Waveform Distortion

Waveform distortion is defined as a steady-state deviation from an ideal sine wave of power frequency principally characterized by the spectral content of the deviation. There are five Primary types of waveform distortion [3]:

DC offset

Harmonics

Inter-harmonics

Notching

Noise

2.10 DC offsetThe presence of a dc voltage or current in an ac power system is termed dc offset. This can occur as the result of a geomagnetic disturbance or asymmetry of electronic power converters [5]. Incandescent light bulb life extenders, for example, may consist of diodes that reduce the rms voltage supplied to the light bulb by half-wave rectification. Direct current in ac networks can have a detrimental effect by biasing transformer cores so they saturate in normal operation. This causes additional heating and loss of transformer life. Direct current may also cause the electrolytic erosion of grounding electrodes and other connectors.

2.11 HarmonicsHarmonics are sinusoidal voltages or currents having frequencies that are integer multiples of the frequency at which the supply system is designed to operate (termed the fundamental frequency; usually 50 Hz). Periodically distorted waveforms can be decomposed into a sum of the fundamental frequency and the harmonics (3rd harmonic frequency=3 times of fundamental frequency). Harmonic distortion originates in the nonlinear characteristics of devices and loads on the power system. Harmonic distortion levels are described by the complete harmonic spectrum with magnitudes and phase angles of each individual harmonic component. It is also common to use a single quantity, the total harmonic distortion (THD), as a measure of the effective value of harmonic distortion [7].

Figure 2.3 Additive Third Harmonics 2.12 InterharmonicsVoltages or currents having frequency components that are not integer multiples of the frequency at which the supply system is designed to operate (e.g., 50 or 60 Hz) are called inter-harmonics. They can appear as discrete frequencies or as a wideband spectrum. Interharmonics can be found in networks of all voltage classes. The main sources of Interharmonics waveform distortion are static frequency converters, cyclo-converters, induction furnaces, and arcing devices. Power line carrier signals can also be considered as Inter harmonics.2.13 Notching

Notching is a periodic voltage disturbance caused by the normal operation of power electronic devices when current is commutated from one phase to another. Since notching occurs continuously, it can be characterized through the harmonic spectrum of the affected voltage. However, it is generally treated as a special case. The frequency components associated with notching can be quite high and may not be readily characterized with measurement equipment normally used for harmonic analysis. Figure 2.4 shows an example of voltage notching from a three-phase converter that produces continuous dc current. The notches occur when the current commutates from one phase to another. During this period, there is a momentary short circuit between two phases, pulling the voltage as close to zero as permitted by system impedances.

Figure 2.4 Voltage Notching caused by Three-Phase Converter2.14 NoiseNoise is defined as unwanted electrical signals with broadband spectral content lower than 200 kHz superimposed upon the power system voltage or current in phase conductors, or found on neutral conductors or signal lines. Noise in power systems can be caused by power electronic devices, control circuits, arcing equipment, loads with solid-state rectifiers, and switching power supplies [6]. Noise problems are often exacerbated by improper grounding that fails to conduct noise away from the power system. Basically, noise consists of any unwanted distortion of the power signal that cannot be classified as harmonic distortion or transients. Noise disturbs electronic devices such as microcomputer and programmable controllers. The problem can be mitigated by using filters, isolation transformers, and line conditioners.