Thesis Project: Power Quality Analysis at Murdoch University eng470: Engineering Honours Thesis
Most Common Power Quality Problems
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Most Common Power Quality Problems
Voltage sags/swells Harmonics Wiring/Grounding Capacitor Switching Load Interaction Other EMF/EMI Power Conditioning Figure 3: Most Common Power Quality Problems [16] Power Quality Analysis at Murdoch University 12 Voltage sags can result in crashing of data processing systems or computer systems and the tripping, or malfunctioning of motors. These may lead to loss of production and idle personnel that are costly to maintain. A UPS (Uninterruptible Power Supply) can be used to prevent the crashes, however, it will introduce harmonics into the system [20]. A series active power filter can be used for compensation of voltage sags from the AC supply and is preferred over the UPS due to its reduced economic costs and lower component ratings [18]. 2.2.2 Voltage Swells This is referred to as a voltage surge and is an increase of the RMS supply voltage within a short duration of time. Swells in voltage are usually caused by capacitor switching and faulty operation of tap changing transformer on the load [21]. Figure 5 shows the voltage swells. Figure 4: Waveform Illustrating Voltage Sags [19] Power Quality Analysis at Murdoch University 13 Short duration voltage swells may result in computer data being erased, loss of production, and shortening of lives of home appliances, as well as computers due to the stress it puts on them. Additionally, it can cause tripping of the protective circuits of a Variable Speed Drive (VSD). Voltage swells lasting for a long duration cause damage to home appliances [20]. To mitigate this, surge suppressors are typically used to protect sensitive appliances against transient voltage surges, while power conditioners can be used to shield equipment from voltage surges [18]. 2.2.3 Voltage Flicker Voltage flicker or voltage fluctuations refer to rapid short-term changes in voltage levels caused by sudden variations or switching of the load. The voltage levels increase when the load decreases, and it similarly decreases when the load increases. Based on their frequency of occurrence, flicker can be divided into four groups; cyclic flicker, cyclic low frequency, noncyclic frequent, and noncyclic infrequent. Cyclic flicker is the flicker due to periodic variations in voltage ranging between 10 every second to 2 every second. Cyclic low frequency Figure 5: Waveform Showing Voltage Swells [19] Power Quality Analysis at Murdoch University 14 has voltage variations in the range of 2 every second to 12 every minute. Noncyclic frequent has voltage variations in the range of 12 every minute to 1 every minute. Noncyclic infrequent has voltage variations in the range of 1 every minute to 3 every hour [22]. Arc welding machines, arc furnaces, oscillating loads, reciprocating compressors, pumps, spot welders both automatic and manual, drop hammers, saws, cranes, hoists, single elevators, and motor starts are the most common causes of voltage fluctuations [19]. In addition, voltage flicker decreases the lifespan of electronic equipment, lamps, and adversely affects human health since it may cause migraines and headaches due to the strain it puts on the eyes. Voltage stabilizers and motor starters can be used in correction of voltage fluctuations. Figure 6 below indicates a waveform of voltage flicker. Figure 6: Waveform Indicating Voltage Flicker [23] Power Quality Analysis at Murdoch University 15 2.2.4 Voltage Unbalance Voltage unbalance discusses the ratio of the negative sequence voltage to the positive sequence voltage. It can also be defined as a condition where there is a difference between the phase angle displacement and RMS voltage values of two consecutive phases [19]. This may occur due to unbalanced phase loads in a distribution system, unbalanced customer loads, or unbalanced network impedances. Voltage unbalance causes an increase in the temperature of motors and can result in tripping of large motors. Unless the output DC of the drive rectifier is filtered properly, AC variable speed drives may also be affected by voltage unbalance. Voltage regulators are the devices used to correct voltage unbalance [18]. For utility, voltage unbalance can be mitigated by redistribution of loads or by repairing of malfunctioning equipment. For adjustable speed drives, DC link reactors and AC line reactors can be used to decrease the effects of voltage unbalance [24]. The following Figure 7 shows an example of voltage unbalance in the phase. Figure 7: Voltage Unbalance between Three Phases [25] Power Quality Analysis at Murdoch University 16 Figure 8 below demonstrates the relationship between the increase in temperature and voltage unbalance. From Figure 8, it can be seen that temperature rises by approximately twice the square of the percentage of voltage unbalance [24]. 2.2.5 Voltage Transients This refers to a rapid variation in frequency of the voltage in the steady state condition caused by lightning or switching operations [26]. As a result, voltage transients are classified into oscillatory and impulsive transients. Oscillatory transients are caused by capacitor switching, commutation in power devices and resonant circuits which result in a sudden change of polarity of the voltage lasting between 10 microseconds and 100 microseconds [18]. On the other hand, impulsive transients are caused by lightning and last between 1 nanosecond and 1 millisecond [19]. These are illustrated in the following Figure 9. Figure 8: Relationship between Voltage Unbalance and Increase in Temperature [24] Power Quality Analysis at Murdoch University 17 Impulsive transients can result in oscillatory transients that may lead to power line insulator damage as a result of transient overvoltage, caused by the oscillatory transients. Surge arresters are usually used to suppress the impulsive transients [20]. 2.2.6 Voltage Notching Voltage notching is a repetitive power quality disturbance that occurs during current commutation from one phase to another during normal working of power electronic devices such as a rectifier. The difference between notching and harmonics is that notching happens in steady state. Harmonic analysis starting point is the current waveform, therefore the IZ drops of harmonic currents can be used for deriving voltage notching. The primary cause of voltage notching is three phase converters or rectifiers that produce continuous DC. A short circuit occurs shortly between two phases during the current commutation from one phase to another [27]. Factors affecting depth of a notch at any point include the point being examined, isolating inductance between converter/rectifier, and the source inductance. The commutation angle is the width of the notch determined as [27]: u = cos −1 [cos a − (X s + X t )I dc ] − a (11) Figure 9: Types of Transients [19] Power Quality Analysis at Murdoch University 18 This can be simplified to the following equation: cos u = 1 − ( 2E x E dc ) (12) Where: u: is the commutation angle. 𝑎: is the delay angle. 𝐼 𝑑𝑐 : is the direct current in p.u on a converter base. 𝑋 𝑡 : is the converter transformer reactance in p.u on a converter base. 𝑋 𝑠 : is the system reactance in p.u on a converter base. 𝐸 𝑑𝑐 : is the theoretical direct voltage. 𝐸 𝑥 : is the direct voltage drop due to commutating reactance. Voltage notches introduce high non-harmonic and harmonic frequencies in the radio frequency range that result in negative effects such as overloading electromagnetic interference filters and other high frequency sensitive capacitive circuits when it is of sufficient power and introduction of signal interference into communication and logic circuits. The remedy for voltage notches involves isolating the critical and sensitive equipment from the rectifiers. Figure 10 below shows a waveform with notches. Figure 10: Voltage Notching [27] Power Quality Analysis at Murdoch University 19 2.2.7 Brownouts A brownout is a constant lower voltage condition caused by the loss of supply to one phase of a distribution transformer with the two remaining phases maintaining their supply. This results in low voltages between the phase and neutral of about 100 %, 50 %, and 50 % [28]. This can be corrected by using a voltage stabilizer [18]. 2.2.8 Short Interruptions of Power This refers to an event where there is zero voltage. Such a situation occurs for a short time of approximately 30 seconds or less. These are caused by the automatic opening and closing of the protective equipment to isolate a faulty area of the network. The leading causes of fault in a system are insulator flashover, lightning, improper grounding, and insulation failure. Short interruptions of power may result in data loss, tripping of protection devices, damage to data processing equipment, and system crashing and lock up [18]. 2.2.9 Long Interruptions of Power This is a condition lasting for more than two cycles where the voltage is zero. It is also referred to as a blackout and may be caused by utility power failure, fire, power distribution failure, human error, storms or objects destroying lines and poles and bad coordination or tripping of a circuit breaker. Long interruptions of power results in the complete shutdown of systems, data loss, and loss of control of the systems. It may also lead to damage of electrical appliances [18]. Generators can be used as power backups for this power quality problem. Nevertheless, it does not protect the equipment against power failure since the failure actually occurs before the generator is turned on. Power Quality Analysis at Murdoch University 20 2.2.10 Direct Current Equipment at the customer's end that has characteristics of a non-linear load may inject direct current in the low voltage neutral conductor of the distribution system. Failure of rectifiers that are commonly used in many AC to DC conversion technologies of modern equipment can also cause induction of direct current into an AC distribution system. Therefore, this may result in earthing systems corrosion and the introduction of unwanted current to devices working at their rated level [28]. Direct current has the effect of causing saturation in the magnetic circuits of power transformers [18]. Transformer saturation leads to increase of transformer temperature making the transformer unable to deliver full power to the load and thus creates instability in electronic load equipment due to waveform distortion. The solution to direct current is to use modular systems, which are easily replaceable by the user and by replacing the problem causing equipment that is faulty [29]. Figure 11 below illustrates a direct current offset. Figure 11: Direct Current Offset [29] Power Quality Analysis at Murdoch University 21 2.2.11 Electrical Noise This refers to a persistent, oscillatory disturbance of the waveform that has a wide frequency distribution of up to 200 kHz. Electrical noise is usually induced in the network by electromagnetic fields and can also be caused by improper grounding [21]. The following Figure 12 indicates the waveform distortion. 2.2.12 Harmonic Distortion This refers to a frequent distortion of the sine wave pattern of the supply voltage or current caused by variable speed drives and solid-state rectifiers that are loads with non-linear characteristics. Furthermore, the waveform is equal to the total of different sine waves having various phases and magnitudes and with frequencies, which are multiples of power system frequency. The causes of harmonics include, non-linear loads (power electronics equipment) such as data processing equipment, ASDs, switched mode power supplies, high-frequency lighting, DC brush motors, rectifiers, welding machines, arc furnaces, and electric machines operating above the magnetic saturation point (knee of the magnetization curve) [30]. The level of harmonic distortion is usually expressed in terms of Total Harmonic Distortion (THD) Figure 12: Electrical Noise [19] Power Quality Analysis at Murdoch University 22 [31], which refers to the sum of all harmonics of current or voltage waveform divided by the main component of the current or voltage. This can be expressed mathematically as [32]. THD = √∑ V n rms 2 ∞ n=2 V f rms (13) Where: 𝑉 𝑛𝑟𝑚𝑠 : is the RMS voltage of the nth harmonic. 𝑉 𝑓𝑟𝑚𝑠 : is the RMS voltage of fundamental frequency. Harmonic distortion may result in transformer heating and increased losses in the copper, core, and the stray-flux hence may result in erratic tripping of thermal protections and relays. Furthermore, it may cause cable losses [33], malfunctioning of traffic control or ripple control systems, increased probability of resonance occurrence, degeneration or failing of power factor correction capacitors, overload of the neutral conductors in 3 phase systems and loss of efficiency in electric machines. Finally, it may cause losses in the distribution network due to harmonic currents, a low power factor, electromagnetic interference (EMI), and communication system interference [20]. A shunt active power filter can be used to inject an opposite and equal compensating harmonic current to compensate for harmonic load current [18]. From Figure 13 below, it can be seen that a base sine waveform, its third harmonic and the distorted waveform, which is the sum of the base waveform and its third harmonic. Power Quality Analysis at Murdoch University 23 2.2.13 Interharmonics The existence of waveforms of a sinusoidal nature at frequencies that lie between 50 Hz multiples of the supply frequency is referred to as inter-harmonic interference. These can appear as a wide band spectrum or discrete frequencies. Spectral components in the quasi- steady state can be used to define harmonics and inter-harmonics over a range of frequencies. Mathematically, harmonics, inter-harmonics and sub harmonics can be described as: Harmonic: f = 0 ∗ f b where a is an integer > 0 (14) Interharmonic: f ≠ a ∗ f b (15) DC: f = a ∗ f b = 0Hz (a = 0) (16) Sub harmonic: f > 0Hz and f < f b (17) Where: 𝑓 𝑏 : is the fundamental frequency of the system. Figure 13: Harmonic Distortion [34] Power Quality Analysis at Murdoch University 24 Sub harmonic is a type of inter harmonic for frequencies less than the fundamental frequency of the system. Indeed, Interharmonics are mainly caused by cycloconverters that are widely used in static VAR generators, linear motor drives, and rolling mills. Other causes of inter- harmonics include low-frequency power line carriers, integral cycle control, induction furnaces, induction motors, and arcing loads such as arc furnaces and arc welders [35]. These impacts of inter-harmonics include vibrations in induction motors, heating effects similar to those of harmonics, malfunctioning of ripple control receivers, the flickering of light, and noise in audio appliances. Interharmonics can cause acoustic noise in power equipment, malfunctioning of television receivers, and fluorescent lamps [20]. 2.2.14 Electromagnetic Interference (EMI) The interaction between magnetic and electric fields and sensitive electronic devices and circuits is a high-frequency phenomenon referred to as EMI [2]. The interaction between radio frequency fields and sensitive communication or data equipment is known as Radio Frequency Interference (RFI). EMI and RFI result in inductance creation on data carrying systems. The voltage that is in excess of the operating data levels can create data opposite or data that is not the same as that travelling in the data line since data line signals are digital signals in the form of zeros and ones. Therefore, EMI and RFI introduce noise to the system hence affecting the power quality. The solution to this power quality issue is to employ shielding of the data devices and their cables or to relocate the data cables and devices from the EMI/RFI source [29]. Power Quality Analysis at Murdoch University 25 2.3 Power Quality Standards Due to the massive costs associated with poor power quality, various standards have been put in place to give guidelines, recommendations, and limits to assist in ensuring the user equipment is compatible with the system where it is to be used [39]. These standards include IEEE 519, IEEE 1459, ANSI C84.1, EN 50160, IEC 61000, and AS/NZS 61000.3.2:2003. 2.3.1 IEEE 519 Standard IEEE 519 is an international standard drafted by the Institute of Electrical and Electronics Engineers (IEEE) that gives recommendations to control harmonic distortion levels on the overall power system. Other IEEE standards on power quality disturbances include IEEE 1100 (Powering and Grounding Sensitive Equipment) and IEEE 1159 (Monitoring and Definition of Electric Power Quality) [39]. 2.3.2 IEEE 1459 Standard IEEE 1459 is a standard that gives definitions for the measurement of electric quantities under sinusoidal and non-sinusoidal, single phase and polyphase and balanced and unbalanced situations. This standard proposes the concepts and the definitions that are essential for evaluating the quality of transmission of electrical energy. This is for the purposes of designing measurement instrumentation, for billing, and development of measurement algorithms [40]. 2.3.3 ANSI C84.1 Standard ANSI refers to American National Standards Institute and gives guidelines on power quality in the United States based on the IEEE standards and other standards generated by the institute Power Quality Analysis at Murdoch University 26 itself. The most common ANSI standard (ANSI C84.1) provides recommendations for steady state voltage ratings [39]. 2.3.4 EN 50160 Standard The EN 50160 (European Standard) deals with supply quality necessities for European services. This standard outlines the particular levels of voltage characteristics that must be adhered to by the utilities and the techniques for assessing compliance. Moreover, the EN 50160 limits itself to voltage characteristics at the PCC (Point of Common Coupling), and consequently does not stipulate requirements for power quality within the supply, or the client's facilities. Regarding the limits in EN 50160, it is apparent that these are very similar to the IEC 61000-3-6 compatibility levels for harmonic voltages with respect to the corresponding medium and low voltage systems, with the absence of higher order harmonic limits in EN 50160 being the exclusion [41]. 2.3.5 IEC 61000 Standard The IEC 61000 (Electromagnetic Compatibility (EMC) Standards) categorizes power quality issues into six categories. Part 1 deals with terminology and definitions and part 2 describes the environment where the equipment will be used. Part 3 recommends the limits of disturbances that can be caused by equipment in the power system, while part 4 details the guidelines and procedures for tests and measurement devices, and tests to confirm compliance with the standards. Guidelines on how to install equipment that is to be used for mitigating power quality disturbances are detailed in part 5. Finally, part 6 of the standard deals with the standards for specific equipment, so that they can be immune to the disturbance levels of the power system they are to be used in [39]. Power Quality Analysis at Murdoch University 27 2.3.6 AS/NZS 61000.3.2:2003 Standard AS/NZS 61000 is an Australian/ New Zealand standard that is compatible with IEEE 519, and it is similar to IEC 61000. This standard is made up of six parts that prescribe the limits for harmonics and voltage disturbances produced by equipment whose input current is less than, equal to or greater than 16 A per phase. It provides the limits for voltage disturbances in equipment in low voltage systems that are rated less than or equal to 75 A. The standard also provides the methods to confirm compliance with the set standards [42]. The following Table 1 is a summary of the power quality standards. Table 1: Power Quality Standards Download 1.28 Mb. Do'stlaringiz bilan baham: 
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