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1457619088-PowerQualityAnalysisAndMitigation.rtf (Size: 4.06 MB / Downloads: 4)
Introduction
The electric power network has undergone several modifications from the time of its invention. The modern electric power network has many challenges that should be met in order to deliver qualitative power in a reliable manner. There are many factors both internal and external that affect the quality and quantity of power that is being delivered. This chapter discusses the different power quality problems, their causes and consequences.
2.2 Power Quality
The quality of electric power delivered is characterized by two factors namely- “continuity” of supply and the “quality” of voltage. As indicated by IEEE standard 1100, Power Quality is characterized as-
"The idea of controlling and establishing the touchy supplies in a manner that is suitable for the operation of the gear."
2.3 Power quality Problems
There are many reasons by which the power quality is affected. The occurrence of such problems in the power system network is almost indispensable. Therefore, to maintain the quality of power care must be taken that suitable devices are kept in operation to prevent the consequences of these problems. Here an overview of different power quality problems with their causes and consequences is presented.
2.3.1 Interruptions:
It is the failure in the continuity of supply for a period of time. Here the supply signal (voltage or current) may be close to zero. This is defined by IEC (International Electro technical Committee) as “lower than 1% of the declared value” and by the IEEE (IEEE Std. 1159:1995) as “lower than 10%”. Based on the time period of the interruption, these are classified into two types [8]-
2.3.2 Waveform Distortion:
The power system network tries to generate and transmit sinusoidal voltage and current signals. But the sinusoidal nature is not maintained and distortions occur in the signal. The cause of waveform distortions are [8]-
• DC Offset: The DC voltage which is present in the signal is known as DC offset.
Due to the presence of DC offset, the signal shifts by certain level from its actual reference level.
• Harmonics: These are voltage and current signals at frequencies which are integral multiples of the fundamental frequency. These are caused due to the presence of non-linear loads in the power system network.
• Inter Harmonics: These are the harmonics at frequencies which are not the integral multiples of fundamental frequency.
• Notching: This is a periodic disturbance caused by the transfer of current from one phase to another during the commutation of a power electronic device.
• Noise: This is caused by the presence of unwanted signals. Noise is caused due to interference with communication networks.
2.3.3 Frequency Variations:
The electric power network is designed to operate at a specified value (50 Hz) of frequency. The frequency of the framework is identified with the rotational rate of the generators in the system. The frequency variations are caused if there is any imbalance in the supply and demand. Large variations in the frequency are caused due to the failure of a generator or sudden switching of loads.
2.3.4 Transients:
The transients are the momentary changes in voltage and current signals in the power system over a short period of time. These transients are categorized into two types- impulsive, oscillatory. The impulsive transients are unidirectional whereas the oscillatory transients have swings with rapid change of polarity.
Causes:
There are many causes due to which transients are produced in the power system. They are-
Arcing between the contacts of the switches
Sudden switching of loads
Poor or loose connections
Lightening strokes
Consequences:
Electronics devices are affected and show wrong results
Motors run with higher temperature
Failure of ballasts in the fluorescent lights
Reduce the efficiency and lifetime of equipment
2.3.5 Voltage Sag:
The voltage sag is defined as the dip in the voltage level by 10% to 90% for a period of half cycle or more. The voltage signal with sag in shown in Fig. 2.2.
Causes:
The causes of voltage sag are-
Starting of an electric motor, which draws more current
Faults in the power system
Sudden increase in the load connected to the system
Consequences:
Failure of contactors and switchgear
Malfunction of Adjustable Speed Drives (ASD’s)
Voltage Unbalance:
The unbalance in the voltage is defined as the situation where the magnitudes and phase angles between the voltage signals of different phases are not equal.
Causes:
Presence of large single-phase loads
Faults arising in the system
Consequences:
Presence of harmonics
Reduced efficiency of the system
Increased power losses
Reduce the life time of the equipment
2.3.8 Voltage Fluctuation:
These are a series of a random voltage changes that exist within the specified voltage ranges. Fig. 2.4 shows the voltage fluctuations that occur in a power system.
Causes:
These are caused by the
Frequency start/ stop of electric ballasts
Oscillating loads
Electric arc furnaces
Consequences:
Flickering of lights
Unsteadiness in the visuals
Among the different power quality problems discussed, the under voltage or voltage sag is the prominent one as it occurs often and affects the power system network largely. Therefore, in this project main focus is given on voltage sag and its mitigation techniques.
2.4 VOLTAGE SAG ANALYSIS
2.4.1 Definition:
According to standard IEEE 1346-1998, Voltage Sag is defined as-
“A decrease in rms voltage or current at the power frequency for durations of 0.5 cycle to
1 min.
Typical values are 0.1 to 0.9 pu.”
2.4.2 Characteristics of Voltage Sag:
The voltage sag is characterized by its magnitude, duration and phase angle jump. Each of them is explained below in detail.
A. Magnitude of Sag:
A sag magnitude is defined as the minimum voltage remaining during the event. The magnitude can be defined in a number of ways. The most common approach is to use the rms voltage. The other alternatives are to use fundamental rms voltage or peak voltage. Thus, sag is considered as the residual or remaining voltage during the event. In
case of three-phase system where the dip in voltage is not same in all phases, the phase with lowest dip is used to characterize sag.
The magnitude of voltage sag at a certain point depend on-
Type of fault
Fault impedance
System Configuration
Distance of the fault from the point of consideration
The duration of sag is the time for which the voltage is below a threshold value. It is determined by the fault clearing time. In a three phase system all the three rms voltages should be considered to calculate the duration of the sag. A sag starts when one of the phase rms voltage is less than the threshold and continues until all the three phase voltages are recovered above the threshold value. Based on the duration of sag, the voltage sags are classified as shown in Table-I.
C. Phase-Angle Jump:
The short circuits in power system not only cause a dip in voltage, but also change the phase angle of the system. The change of phase angle is called as “Phase-Angle Jump”. It causes the shift in zero crossing of the instantaneous voltage. This phenomenon affects the power electronic converters which use phase angle information for their firing.
D. Point-on-Wave:
To perfectly characterize sag, the point-on-wave where the sag starts and where it ends should be found with high precession. The point-on-wave is nothing but the phase angle at which the sag occurs. These values are generally expressed in radians or degrees.
2.4.3 Voltage Sag Mitigation Analysis:
To prevent the occurrence of voltage sag preventive measures can be taken at different stages. They are-
A. During the Production of Equipment:
The basic and economical solution is to strengthen the sensitive devices to the power quality problems. This prevents the damage of these devices to the abnormalities in the power system. The device manufacturers use a specific curve like ITIC (Information Technology Industry Council) curve during manufacturing. This curve specifies the withstanding capability of sensitive devices like computers, PLC’s, ASD’s during voltage imbalance occurring in the system. Based on this curve the design is improved so that the damage of these devices is prevented.
B. Analysis of the Causes:
The second basic way to prevent the occurrence of voltage sag is to analyze the causes that lead to voltage imbalance. Improving the poor wiring and weak grounding systems can prevent the damage of the sensitive equipment. The medium which causes power quality problems should be avoided to the extent possible.
C. Power Conditioning Equipment:
The use of power conditioning equipment is the most common solution to protect the power system network from these problems. Most of the power conditioning equipment is voltage monitoring devices as most of the faults that occur in power system are voltage imbalance faults. These devices may be connected at the source side or in the transmission network, or at the load end. In general, these devices are connected at the point of common coupling (PCC) where the load is connected to the supply. This is done
as the cost of the power conditioning device increases from load end to source side. There are different power conditioning devices like-
i. Line-voltage regulators: These are special transformers connected in series with the transmission line designed to regulate the voltage in accordance with the changes in the system. Examples of line voltage regulators are- tap changing transformers, CVT’s, buck-boost regulators etc.
ii. M-G Sets (Motor-generator Sets): These M-G sets are installed at the load side in order to supply power to critical loads during the interruptions from the power supply company. In this maintenance and safety are main concern.
iii. Magnetic Synthesizers: These employ resonant circuits made of inductors and capacitors. They are used to filter the harmonics from affecting the loads. But these are bulky and noisy.
iv. SVC (Static VAR Compensators): These also use passive elements like inductors and capacitors. But the use of solid state switches to control the voltage injection increases their efficiency. The switches are controlled such that correct magnitude of voltage is injected at correct point of time so that voltage fluctuations are reduced. But these are expensive.
v. UPS (Uninterruptible Power Supplies): It provides a constant voltage during both voltage sags and outages from a battery or super conducting material. The main parts of an UPS are battery, rectifier and an inverter.
vi. SMES (Superconducting magnetic energy storage): SMES stores electrical energy within a superconducting magnet. It provides a large amount of power (750 KVA to 500 MVA) within a short time.
vii. Custom Power Devices: All the above mentioned conventional devices are not suitable to mitigate voltage disturbances effectively. Therefore, there is a need to use new type of devices known as Custom Power Devices. These are power electronic equipment aimed to help in mitigating power quality problems. These are of many types like- Dynamic Voltage Regulator (DVR), D-STATCOM, auto transformer, UPQC etc. In this project a study of these devices is carried out for improving the power quality.
Mitigation
The nonlinear characteristics of many industrial and commercial loads such as power converters, fluorescent lamps, computers, light dimmers, and variable speed motor drives (VSDs) used in conjunction with industrial pumps, fans, and compressors and also in air-conditioning equipment have made the harmonic distortion a common occurrence in electrical power networks. Harmonic currents injected by some of these loads are usually too small to cause a significant distortion in distribution networks. However, when operating in large numbers, the cumulative effect has the capability of causing serious harmonic distortion levels. These do not usually upset the end-user electronic equipment as much as they overload neutral conductors and transformers and, in general, cause additional losses and reduced power factor [1–5]. Large industrial converters and variable speed drives on the other hand are capable of generating significant levels of distortion at the point of common coupling (PCC), where other users are connected to the network [6, 7].
Because of the strict requirement of power quality at the input AC mains, various harmonic standards and engineering recommendations such as IEC 1000-3-2, IEEE 519 (USA), AS 2279, D.A.CH.CZ, EN 61000-3-2/EN 61000-3-12, and ER G5/4 (UK) are employed to limit the level of distortion at the PCC. To comply with these harmonic standards, installations utilizing power electronic and nonlinear loads often use one of the growing numbers of harmonic mitigation techniques [8]. Because of the number and variety of available methods, the selection of the best-suited technique for a particular application is not always an easy or straightforward process. Many options are available, including active and passive methods. Some of the most technically advanced solutions offer guaranteed results and have little or no adverse effect on the isolated power system, while the performance of other simple methods may be largely dependent on system conditions. This paper presents a comprehensive survey on harmonic mitigation techniques in which a large number of technical publications have been reviewed and used to classify harmonic mitigation techniques into three categories: passive techniques, active techniques, and hybrid harmonic reduction techniques using a combination of active and passive methods. A brief description of the electrical characteristics of each method is presented with the aim of providing the designer and site engineer with a more informed choice regarding their available options when dealing with the effects and consequences of the presence of these harmonics in the distribution network.
Passive Harmonic Mitigation Techniques
Many passive techniques are available to reduce the level of harmonic pollution in an electrical network, including the connection of series line reactors, tuned harmonic filters, and the use of higher pulse number converter circuits such as 12-pulse, 18-pulse, and 24-pulse rectifiers. In these methods, the undesirable harmonic currents may be prevented from flowing into the system by either installing a high series impedance to block their flow or diverting the flow of harmonic currents by means of a low-impedance parallel path [9].
Harmonic mitigation techniques used for supply power factor correction and harmonics mitigation in two ways to qualify the products performance. One is to put a limit on the PF for loads above a specified minimum power. Utility companies often place limits on acceptable power factors for loads (e.g., <0.8 leading and >0.75 lagging). A second way to measure or specify a product is to define absolute maximum limits for current harmonic distortion. This is usually expressed as limits for odd harmonics (e.g., 1st, 3rd, 5th, 7th, etc.). This approach does not need any qualifying minimum percentage load and is more relevant to the electric utility.
Harmonic regulations or guidelines are currently applied to keep current and voltage harmonic levels in check. As an example, the current distortion limits in Japan illustrated in Tables 1 and 2 represent the maximum and minimum values of total harmonic distortion (THD) in voltage and the most dominant fifth harmonic voltage in a typical power system
Effect of Source Reactance
Typical AC current waveforms in single-phase and three-phase rectifiers are far from a sinusoid. The power factor is also very poor because of the high harmonic contents of the line current waveform. In rectifier with a small source reactance, the input current is highly discontinuous, and, as a consequence, the power is drawn from the utility source at a very poor power factor.
The magnitude of harmonic currents in some nonlinear loads depends greatly on the total effective input reactance, comprised of the source reactance plus any added line reactance. For example, given a 6-pulse diode rectifier feeding a DC bus capacitor and operating with discontinuous DC current, the level of the resultant input current harmonic spectrum is largely dependent on the value of AC source reactance and an added series line reactance; the lower the reactance, the higher the harmonic content [1–3].
Other nonlinear loads, such as a 6-pulse diode rectifier feeding a highly inductive DC load and operating with continuous DC current, act as harmonic current sources. In such cases, the amount of voltage distortion at the PCC is dependant on the total supply impedance, including the effects of any power factor correction capacitors, with higher impedances producing higher distortion levels [7, 11].
2.2. Series Line Reactors
The use of series AC line reactors is a common and economical means of increasing the source impedance relative to an individual load, for example, the input rectifier used as part of a motor drive system. The harmonic mitigation performance of series reactors is a function of the load; however, their effective impedance reduces proportionality as the current through them is decreased [12].
2.3. Tuned Harmonic Filters
Passive harmonic filters (PHF) involve the series or parallel connection of a tuned LC and high-pass filter circuit to form a low-impedance path for a specific harmonic frequency. The filter is connected in parallel or series with the nonlinear load to divert the tuned frequency harmonic current away from the power supply. Unlike series line reactors, harmonic filters do not attenuate all harmonic frequencies but eliminate a single harmonic frequency from the supply current waveform. Eliminating harmonics at their source has been shown to be the most effective method to reduce harmonic losses in the isolated power system. However, the increased first cost entailed presents a barrier to this approach. If the parallel-connected filter is connected further upstream in the power network, higher day-to-day costs will accumulate due to losses in the conductors and other plant items that carry the harmonic currents. Conversely, for series-connected filter at the load, there are increased losses in the filter itself. These losses are simply the result of the higher series impedance, which blocks the flow of harmonics but increases the line loss as a result of the flow of the remaining components of the load current [12, 13]. The quality factor of the filter inductor affects the actual value of the low-impedance path for each filter. Usually, a value of ranges between 20 and 100 [14]. Many types of harmonic filters are commonly employed, including the following: