1
Introduction to Power Quality from Power Conditioning
Abstract
This chapter begins with an introduction to the concept of power quality, which has developed into a discipline that defines the reference parameters for assessing the suitability of the waveforms of voltage and current of an electrical system, so allowing compatible operation of all equipment that constitutes that system. Different standards set limits on the allowed disturbances of voltage and current source. Some disturbances that occur in an electrical network are due to the presence of nonlinear loads. A model based on the equivalent Norton of a load is presented, in order to develop a theoretical analysis of the system when this load type is present. Furthermore, an update of compensation equipment configurations most common in the technical literature is given, including active power filters. This equipment has proven effective in the dynamic compensation of nonlinear loads.
Keywords
power quality
shunt active power filter
series active power filter
hybrid active power filter
unified power quality conditioner
This chapter introduces the general objectives of this book. To do this, it begins with an introduction to the concept of power quality, which has become a discipline for defining the reference parameters for assessing the suitability of the waveforms of voltage and current of an electrical system. This allows to achieve compatible operation of all equipment that constitute it. This makes it possible to set limits on the disturbances in the voltage and current source which are reflected in the different standards.
An important source of the disturbances that occur in an electrical network is the presence of nonlinear loads. A model based on the Norton equivalent of a load is presented, in order to develop a theoretical analysis of the system when this load type is included.
Furthermore, an update of compensation equipment configurations most common in the technical literature that includes active power filters is also performed. This type of equipment has proven to be effective in the dynamic compensation of nonlinear loads. Different topologies are presented as an introduction. The following chapters will analyze these in depth, and will also introduce some control strategies that allow the quality of electrical power to be improved.
1.1. Introduction
In recent decades, the concept of power quality (PQ) has become more important within the field of electrical engineering, such that currently it has become a matter of great interest for producer/supplier electricity companies, equipment manufacturers and end consumers [1].
A power quality problem can be understood as a disturbance that causes the system voltage or current to differ from an ideal reference value [2]. This definition has led to specific studies that aim to obtain a detailed account of the phenomena of electromagnetic compatibility (EMC) that cause disturbances in PQ [3]. This cataloging of limiting values are detailed in various national and international standards.
A range of ways of improving power quality have been proposed that are based on passive filters, which can be understood as devices which change impedance versus frequency. Other solutions include active filters, which are able to inject harmonics that counter to the network harmonics using power-electronic converters. These filters can be connected in parallel or in series depending on the type of load that needs to be compensated. A combination of both filter types (passive and active) can also be used, and this is known as a hybrid filter. In this case the active filter can improve the frequency response of the passive filter.
In this chapter, Section 1.2 discusses the various disturbances that may be present in the voltage waveform and the limits on the distortion levels of electric current. To do this, a load classification in different classes as provided in the standard as well as its current harmonic limits are presented. Nonlinear loads generate harmonic currents that can cause disturbances and malfunction of the facilities. To limit its effects, standards set limits to the harmonic levels that loads can inject to the electric network. Section 1.3 presents these standards and summarizes these limits. In Section 1.4, a nonlinear load model that allows the analysis of systems including these loads is proposed. They are also classified according to the harmonic type generated in the system. In Section 1.5, the filter configurations most common in the literature are summarized and their main features are outlined.
1.2. Power Quality
Many social and economic activities depend on the quality and efficiency of an electrical power supply. Both industrial and commercial users are interested in guaranteeing the electrical waveform quality that supplies their different systems. The proliferation of electronic equipment increases the nonlinearity of the load and so worsens the quality of the power in the system. Harmonic current drawn from a supply by the nonlinear load results in the distortion of the supply voltage waveform at the point of common coupling (PCC) due to the source impedance. Both distorted current and voltage may cause end-user equipment to malfunction, conductors to overheat and may reduce the efficiency and life expectancy of the equipment connected at the PCC.
Nowadays, problems with power quality can be very expensive due to bad operation of sensitive load. Hence, International Standards have established limits for harmonic current emissions, power quality measurement conditions and testing techniques to apply. A summary of the disturbances and limits set by the main standards are presented here.
1.2.1. Voltage Disturbances
Electricity supplied to customers has many features that can vary and affect how it can be used. From the point of view of the consumer, it is desirable that the supply voltage has a frequency and amplitude that does not vary and that the sinusoidal waveform is not distorted. In practice, there are numerous factors that prevent this aim being achieved. Of these, the principal factor is due to using the same users make the electrical wave that disturbs their characteristics with respect to the ideal situation [4].
Electrical current demanded by customers flowing through the distribution network produces voltage drops. This means that the supply voltage is continuously affected by these voltage drops, which in turn depend on the existing power demand at a given time. On the other hand, the system components may be subject to faults that may affect the supply voltage which could interrupt the supply to one or many consumers.
To maintain the frequency at a constant value, it is necessary to have a production capacity that can continuously adapt to simultaneous demand by all customers, although both production and demand are likely to vary in different ways. Specifically, in the case of production loss and damage to the transmission or distribution networks, the risk of an increase or decrease in frequency that will be corrected with the secondary regulation or tertiary regulation of the power system.
There are many phenomena that can disturb the normal functioning of consumer’s equipment. Some them are associated with inevitable transients or are caused by defects: maneuvers or atmospheric phenomena. Others are due to the use that is now made electrical energy, since equipment that modifies the waveform of the system voltage is connected directly to the network. This is due to the proliferation of loads that produce these effects. On the other hand, these loads include control circuits that are sensitive to these disturbances.
Therefore, the power quality in relation to some of its features depends more on client than distributor or producer, so if the goal is to obtain a certain level of quality, both client and supplier must work together to achieve this.
Either way, the standards seek to ensure on the one hand that the supply voltage presents values that guarantee the PQ within limits, [5] and on the other try to limit disturbances that are produced by customer loads [6] and so minimizing any effect on the voltage.
The standard of the voltage wave is characterized [5] by the following parameters: frequency, amplitude, shape, and symmetry. From the point of view of generation, the voltage waveform produced in large power plants can be considered to be sinusoidal. However, in the process of transmission and distribution of energy, the aforementioned parameters undergo alterations that may affect users. The origin of these abnormalities is often found in the electrical system itself as a result of maneuvers, breakdowns, natural phenomena (lightning) or in the normal operation of certain receptors such as electronic converters [7]. To check the quality of power supply in IEC 61000-4-30 [8] methods and measurement conditions are established.
1.2.1.1. Frequency Variations
In general, the network frequency is usually very stable, barring exceptional cases, because there is a high degree of interconnection of electric power systems. The frequency variation in an electrical system occurs when the balance between load and generation is altered. In an interconnected system such as that in Spain a load change of 12,000 MW is needed to vary the frequency by 0.1 Hz.
Generally, frequency variations affect the speed of rotating machines, clocks synchronized to the network, and in general any type of electronic regulation that uses the frequency as a time reference.
As established in the standard EN 50160 [5], the nominal frequency is 50 Hz under normal operating conditions, and the average value of the fundamental frequency measured in periods of 10 s should be within the following ranges:
• To networks coupled by synchronous connection to an interconnected system:
• 50 Hz ± 1% (from 49.5 Hz to 50.5 Hz) for 99.5% a year.
• 50 Hz + 4%/−6% (from 47 Hz to 52 Hz) for 100% of the time.
• For networks which have no synchronous connection to an interconnected system (existing power networks in island areas):
• 50 Hz ± 2% (from 49 Hz to 51 Hz) for 95% of the time over 1 week.
• 50 Hz ± 15% (42.5 Hz to 57.5 Hz) for 100% of the time.
1.2.1.2. Slow Voltage Variations
An important factor in determining the quality of the power supply is the amplitude of the voltage waveform and hence its root mean square (rms) value (Figure 1.1). In an electrical system the rms value of the voltage may vary with respect to its nominal value. It is considered a slow voltage variation when its duration exceeds 10 s. Many factors can cause such variations, from supply failures, mostly due to atmospheric phenomena, to variations in the impedance of the receiver (no constant energy consumption, uneven distribution areas).
Figure 1.1 Slow voltage variations.
Standard EN 50160 states that the normalized voltage for low voltage networks is:
• For four-wire, three-phase systems
Un = 230 V between phase and neutral
• For three-wire, three-phase systems
Un = 230 V between phases
With respect to voltage variations, regardless of the situations that causes the defects or interruptions, the Standard establishes that the following normal operating conditions must be in place:
• For each period of 1 week, 95% of the rms values of the supply voltage averaged over 10 min must be in the range Un ± 10%.
• For all periods of 10 min, the averaged rms values of voltage must be in the range Un + 10%/−15%.
1.2.1.3. Voltage Fluctuations
Voltage fluctuations (Figure 1.2) occur when there are variations in supply voltage with a duration from several milliseconds up to 10 s, and an amplitude that does not exceed ±10% of the nominal value (if the variation is less than 90% (Figure 1.3) of the rated voltage, the event is called a sag or dip). Voltage fluctuations are mainly due to load variations in customer installations or maneuvers in the network (connecting arc furnaces, welding equipment, large motors, frequency converters, compressors, laser printers, etc.).
Figure 1.2 Voltage fluctuations.
Figure 1.3 Voltage dips definition (0.10Up < ∆U < 0.99Up) and short interruptions (∆U > 0.99Up).
Such fluctuations may affect large numbers of consumers, but because the amplitude does not exceed ±10% of the nominal voltage, its effects do not usually affect the normal operation of electrical equipment. These phenomena can affect humans, however, because they cause lamps to flicker in brightness, which is generally uncomfortable for the consumer.
To quantify this phenomenon of flickering lamps, the Flicker concept has been defined. This is based on a statistical calculation which uses measurements of rapid variations in voltage:
• Pst (short-term flicker) assesses the severity of Flicker over a short time period at intervals of 10 min. This interval is valid for estimating disturbances caused by individual sources such as rolling, heat pumps or appliances. Values are typically given per unit, so that rates above 1 are considered to be visible and affect vision.
• Plt (long-term flicker) considers a measurement duration of 2 h, which is considered to be appropriate to the operating cycle of the load over which an observer can be sensitive to long term flicker. Is calculated from twelve consecutive values of Pst according to the expression:
(1.1)1.2.1.4. Voltage Sags or Dips and Short Interruptions
“Sags” or “dips” are occasional voltage drops in electric power systems. According to EN 50160, a voltage dip is defined as: “a sudden reduction of the supply voltage to a value between 90% and 1% of the declared voltage followed by a voltage recovery after a short period of time.” Conventionally the duration of a voltage dip is between 10 ms and 1 min. The depth of a voltage dip is defined as the difference between the minimum rms voltage during the voltage dip and the declared voltage. Voltage changes that do not reduce the supply voltage to less than 90% of the declared voltage are not considered to be dips. According to some classifications, a dip in the declared voltage of 1% is considered to be a short interruption.
Voltage dips and short interruptions are usually due to defects that occur in the facilities of customers, such as sudden switching of large loads, or they can occur as short circuits or malfunctions in the general network.
1.2.1.5. Voltage Imbalances
In a three-phase system a voltage imbalance occurs when the rms values of the phase voltages or the phase angles between consecutive phases are not equal. EN 50160 sets the limit on the imbalance as follows: under normal operating conditions, for each period of 1 week, 95% of the rms values averaged over 10 min from the negative sequence voltage supply must be between 0% and 2% of the positive sequence. In some areas with partly single-phase or two-phase lines, imbalances may reach 3% at supply points.
These asymmetries in the supply voltage are mostly due to the circulation of an imbalanced current system by the network.
Voltage imbalances can cause an increase of temperature in induction motors and a decrease in use level in transformers.
1.2.1.6. Harmonic Voltage
These disturbances are caused by connecting nonlinear loads to the mains, such as equipment that include a part of power electronics.
According to EN 50160, under normal operating conditions, during each period of 1 week, 95% of the rms values of each harmonic voltage averaged over 10 min intervals should not exceed the values given in Table 1.1. In addition, the rate of total harmonic distortion (THD) of the supply voltage (including all harmonics up to 40) must not exceed 8%. The THD is defined in this standard by the following relationship:
(1.2)Table 1.1
Values of Individual Harmonic Voltages at the Supply Terminals for Orders up to 25 Given in Percent of Un
| Odd Harmonics | Even Harmonics | ||||
| Nonmultiples of 3 | Multiples of 3 | ||||
| Order h | Relative Voltage (%) |
Order h | Relative Voltage (%) |
Order h | Relative Voltage (%) |
| 5 | 6 | 3 | 5 | 2 | 2 |
| 7 | 5 | 9 | 1.5 | 4 | 1 |
| 11 | 3.5 | 15 | 0.5 | 6...24 | 0.5 |
| 13 | 3 | 21 | 0.5 | ||
| 17 | 2 | ||||
| 19 | 1.5 | ||||
| 23 | 1.5 | ||||
| 25 | 1.5 | ||||
Note: No values are given for harmonics of order higher than 25, as they are usually small, but largely unpredictable due to resonance effects.
Where uh represents the relative amplitude of the harmonic, h, relative to the fundamental harmonic voltage.
Voltage harmonics can cause the malfunction of certain electronic equipment, nuisance tripping of the protection, etc. In the long term it can even reduce the lifetime of rotating machines, capacitors, and power transformers.
1.2.2. Harmonics
The presence of harmonics in the voltage and current signals from the mains supply is not a new phenomenon [9]. Associated problems have been a constant concern for engineers since the beginning of the electrical industry [10], meaning that harmonics have been linked to a wide variety of problems for several decades.
All nonlinear loads produce harmonics in either voltage or current. IEEE Std 519-2014 [11] establishes recommended practice for the design of electrical systems that include both linear and nonlinear loads. The voltage and current waveforms that may exist throughout the system are described, and waveform distortion goals are established for the system designer. The interface between sources and loads is described as the point of common coupling and observance of the design goals will minimize interference between different items of electrical equipment.
Furthermore, the IEC standards do not establish limits to the circulation of current harmonics through to distribution network, however, they establish harmonic levels that loads can inject to the network. Thus, IEC 61000-3-2 [6] and IEC 61000-3-4 [12] deal with the limitations of current harmonic that can be injected into the public network.
In IEC 61000-3-2 [6] the limits of harmonic current components that can be produced by equipment tested under specific conditions are specified. This standard is limited to devices with lower input current 16 A per phase.
According to the cited standards, loads are classified into three types, as is shown in Table 1.2. Based on this classification the limits shown in Tables 1.3–1.5 are established. In the case of Class B equipment, the harmonics of the input current must not exceed the absolute values given in Table 1.3, corresponding to the limits for Class A equipment multiplied by a factor of 1.5.
Table 1.2
Equipment Classification According to Standard EN 61000-3-2
| Equipment | |
| Class A |
• Balanced three-phase equipment
• Household appliances, excluding equipment identified by Class D
• Tools excluding portable tools
• Dimmers for incandescent lamps
• Audio equipment
• Everything else that is not classified as B, C, or D
|
| Class B |
• Portable tools
• Arc welding equipment which is not professional equipment
|
| Class C |
• Lighting equipment.
|
| Class D |
• Personal computers and personal computer monitors
• Television receivers
• Note: Equipment must have power level 75W up to and not exceeding 600W
|
Table 1.3
Harmonics Limit for Class A Equipment
| Order Harmonic, n | Maximum Permissible Harmonic Current (A) |
|
Odd Harmonics |
|
| 3 | 2.30 |
| 5 | 1.14 |
| 7 | 0.77 |
| 9 | 0.40 |
| 11 | 0.33 |
| 13 | 0.21 |
| 15 ≤n ≤39 | 0.15  |
|
Even Harmonics |
|
| 2 | 1.08 |
| 4 | 0.43 |
| 6 | 0.30 |
| 8 ≤n ≤40 | 0.23
|
Table 1.4
Harmonics Limit for Class D Equipment
| Order Harmonic, n | Maximum Permissible Harmonic Current per Watt (mA/W) | Maximum Permissible Harmonic Current (A) |
| 3 | 3.4 | 2.30 |
| 5 | 1.9 | 1.14 |
| 7 | 1.0 | 0.77 |
| 9 | 0.5 | 0.40 |
| 11 | 0.35 | 0.33 |
|
15 ≤n ≤39
Only odd harmonics |
|
See the table corresponding to Class A |
1.3. Nonlinear Loads Model
In the harmonic analysis of a power system, there are techniques that can be applied to linear circuits at steady state. These techniques vary depending on the required data, model complexity, the problem formulation and the computational algorithms in use. Nonlinear loads are considered as sources that inject harmonics into a linear network [13]. Depending on the type of harmonics and their behavior in the system, the nonlinear loads can be considered as current source or voltage source type loads [14–16].
Thus, in the repertoire of nonlinear loads, there will be a load type whose input current is almost invariant to changes in the source impedance, so that it can be considered as a nonlinear load that generates harmonic current sources and it is named HCS (harmonic current source). The harmonic voltage distortion at the point of common coupling that produces this type of load is usually relatively low, typically less than 5%. Figure 1.4 shows a model for this type of load, which consists of an ideal current source, representing the harmonics injected by the load.
Figure 1.4 HCS type nonlinear load model.
One example of load with this type of behavior is a rectifier with a high enough inductance to obtain a practically constant dc side current. This inductance is much larger than the source impedance, so that its variations do not significantly affect the load current. Figure 1.5 shows the voltage and current for a load of this type. The figure shows how the harmonic distortion of the current is much higher than the voltage distortion at the load.
Figure 1.5 Waveforms of a HCS load type: (a) voltage at the point of common coupling; (b) load current.
For a steady state analysis this simple model is usually sufficient for most applications. However, in certain situations this model is not adequate, and it is necessary to perform a frequency analysis or a transient analysis [17,18]. In such cases, a model in which the nonlinear load is represented by a Norton or Thevenin equivalent can be used [19,20]; that is, by a real current source or a real voltage source, as shown in Figure 1.6. In such cases the value of the equivalent impedance should be determined using field measurements or detailed simulation models of the nonlinear load.
Figure 1.6 Norton and Thevenin models of a nonlinear load.
The parameter values of the elements that constitute the mentioned models can be determined by measuring the voltage and current harmonics of the load under two different operating conditions, [21,22]. Thus, in the circuit of Figure 1.7, it can be seen that connecting or disconnecting the switch k1 causes the load voltage Vh, the harmonic current Ih and the current IZN,h to have different values. The values of the current Ih,1 and the voltage Vh,1 before closing k1, along with the values of voltage and current, Ih,2, Vh,2, once the switch is closed allows us to solve the equations:
(1.3)
(1.4)
Figure 1.7 Determination of the Norton equivalent of a nonlinear load.
These equations can be used to calculate the Norton current source and the equivalent impedance for each harmonic. We must keep in mind that equations (1.3) and (1.4) are complex, so that not only measures of the rms values of voltage and current but also measurements of phase angles are necessary. It is also important that the measurements carried out for the two sets of operating conditions must be referred to a phase angle of a common variable that does not change with the condition of the system. In Figure 1.7 this common reference is the voltage VS,h.
Similar equations can be used when considering the connection and disconnection of the switch k2 in the circuit of Figure 1.7.
This load model allows better accuracy in a wider range of operating conditions than the ideal model with current source [9]. Another important feature of the proposed method to obtain the Norton and Thevenin equivalent parameters is that it is not necessary to know the system fully [23].
There are others type of loads where the current drawn by them is strongly affected by the value of the source impedance. However, the voltage at the PCC remains virtually unchanged with reasonable changes in the impedance of the supply side. It can be considered that this load behaves like a harmonic voltage source (HVS) connected to the network, Figure 1.8.
Figure 1.8 HVS type nonlinear load model.
A typical example of a HVS load is a rectifier with a large capacitor to eliminate the ripple and to achieve a substantially constant voltage at the dc side. Figure 1.9 shows the current and voltage waveforms at the PCC for this rectifier type load. The current and voltage THD at the point of common coupling are 35.74% and 6.66%, respectively. Variations in the source impedance maintain the voltage at the PCC substantially constant, so in this case the ideal voltage source model in Figure 1.8 could be used.
Figure 1.9 Waveforms of a HVS type load: (a) current; (b) voltage at the point of common coupling.
Like with the HCS type loads, a more complete model can be obtained, formed by a real voltage or current source as shown in Figure 1.7.
Example 1.1
The purpose of this case study is to obtain the Norton model of a nonlinear load. Figure 1.10 shows the circuit diagram consisting of an uncontrolled three-phase rectifier with a resistor and 50/3 Ω and an inductance of 55 mH connected in series at its dc side. This load is connected to a three-phase sinusoidal source with a phase voltage of 400 V and frequency of 50 Hz, with a source impedance modeled by an inductance of 2.8 mH and a resistance of 1.8 Ω. Another RL resistance and inductance branch with and inductance of 13 mH and a resistance of 50 Ω has been included to modify the network conditions through the switch k.
Figure 1.10 Scheme to model a HCS type load.
Voltage and current harmonics vh, ih are measured before and after closing the switch. Table 1.6 shows the rms values and phase angles for the most significant harmonics.
Table 1.6
Voltages and Currents Measured at Two Different Operating Conditions
| Harmonic | k, on | k, off | ||||||
| Current | Voltage | Current | Voltage | |||||
| rms (A) | fase | rms (V) | fase | rms (A) | fase | rms (V) | fase | |
| 1° | 20.47 | −12.55 | 190.4 | −2.99 | 19.88 | −12.64 | 184.6 | −3.67 |
| 5° | 3.52 | 114.6 | 16.75 | 2.32 | 3.46 | 114.3 | 15.44 | −1.36 |
| 7° | 2.01 | 90.95 | 12.89 | −15.34 | 1.99 | 90.74 | 11.84 | −19.64 |
| 11° | 0.76 | −152.6 | 7.44 | 106.9 | 0.78 | −151.2 | 6.86 | 103.7 |
| 13° | 0.46 | 172.5 | 5.29 | 73.54 | 0.48 | 175.2 | 4.89 | 71.75 |
| 17° | 0.21 | −118.90 | 3.15 | 144.30 | 0.2 | −111.6 | 2.58 | 147.3 |
The application of expressions (3) and (4) enable to calculate ZN,h e INh. Table 1.7 shows the values of the Norton currents and impedances for the regarded harmonics from the data of Table 1.6.
Table 1.7
Impedance and Current Source Values of the Norton Model for Example 2.1
| Harmonic | 5 | 7 | 11 | 13 | 17 |
| ZN,h | 26.59∠−92.60° | 65.75∠−86.98° | 25.64∠68.50 | 14.42∠52.55° | 1.78∠−74.72° |
| IN,h | 2.99∠−120.56° | 2.19∠89.25° | 0.48∠−159.20° | 0.22∠120.70° | 1.96∠−138.67° |
1.4. Active Power Line Conditioners
Various compensation methods and circuit topologies have been proposed for harmonic mitigation and load compensation. So, in the 1970s [17,19] basic compensation principles of active power filters (APFs) were proposed. APFs are inverter circuits, comprising of active devices, i.e., semiconductor switches that can be controlled to act as harmonic current o voltage generators. However, there was almost no advance in active power line conditioners beyond a laboratory testing stage, because circuit technology was too poor to practically implement the compensation principle in the 1970s. Over the last 5–10 years, remarkable progress of fast switching devices such as bipolar junction transistors and static induction thyristors has spurred interest in the study of shunt and series active power filter for reactive power and harmonic compensation. In 1982, a shunt active conditioner of 800 kVA, which consisted of current source pulse width modulation (PWM) inverters using gate turn-off (GTO) thyristors, was put into practical use for harmonic compensation for the first time in the world.
Active Power filters, or APFs, can be used as a practical solution to solve the problems caused by the lack of electric power quality. The merging technology of power-electronic devices and the new developments in digital processing (DSP) have made their use practical [24,25]. These power filters can fully compensate the nonlinear loads of electrical power systems: harmonics, reactive power, unbalances, etc. So, they can be called active power line conditioners (APLCs).
Since there is a large number of possible APLC configurations, this section will present in a necessarily summarized form, the characteristics of those commonly used, in view of the number of published papers where they are referred as well as to their industrial use.
1.4.1. Shunt Active Power Filters
The operation principle of the shunt active filter (Figure 1.11) is based on injecting into the network the harmonic current consumed by the load but in opposite phase, so that the current source presents a pure sinusoidal waveform [26,27]. Unlike passive filters, the impedance of the system rarely affects in the filtering features, so they avoid one of the problems presented by passive filters. This configuration provides good compensation characteristics although their practical application has some drawbacks among which can be summarized as follows:
• It is difficult to build a high power PWM converter with fast response and low losses.
• Active filters have a high initial cost compared with passive filters.
• The injected current to the network by the active filter can flow through other passive filters and capacitors connected in the system.
Figure 1.11 Basic scheme of an active filter with shunt connection.
1.4.2. Series Active Filters
In this configuration the active filter is connected in series with the load (see Figure 1.12). This arrangement allows disturbances in the voltage signal (unbalances, harmonics, etc.) to be eliminated [28–30]. According to the type of control to be established by the active filter it is possible on the one hand to eliminate the distortion in the waveform voltage produced by the load and on the other hand, to regulate the voltage at the terminals of the load regardless of the voltage drop, overvoltage, distortion, or unbalances in the supply voltages. This is carried out with a configuration that is known as a dynamic voltage restorer or DVR [31].
Figure 1.12 Basic scheme of an active filter with series connection.
When the series active filter is designed to compensate the voltage harmonics generated by the load, it generates the appropriate waveform to neutralize the voltage distortion caused by the load, so that this is not transmitted to the supply voltage. The operating principle of DVR [32] is based on generating a voltage waveform of the appropriate frequency and magnitude to neutralize imbalances or distortions in the supply voltage so that the voltage at the terminals of the load has the appropriated waveform.
In industrial environments, DVR has been proposed as a way of reducing the impact that voltage disturbances produce in some loads. However, most of the time these devices are simply waiting for some disturbance to occur, and are usually operating well below their capacity [29,33,34]. In addition, these devices introduce an extra impedance in the network (because it will be connected to the network by coupling transformers), with the consequent losses. Therefore, it is usual to include additional functionality in these devices in addition to their operation as a DVR.
1.4.3. Hybrid Active Filters
In order to take advantage of the various strengths of each of the filtering methods, different combinations of filter topologies have been proposed, [14,35]. Twenty-two of these have so far been proposed. This section briefly presents some of those most cited in the technical literature.
Figure 1.13 shows the combination of a passive filter and an active filter connected in parallel with the load [36,37]. The passive filter removes the most significant harmonics, reducing the power of the active filter. This topology has the same drawbacks as those associated with passive filters with parallel connection.
Figure 1.13 Parallel combination of an active and a passive filter.
Figure 1.14 shows a hybrid topology composed of an active filter in series with the source and a passive filter in parallel with the load [38,39]. Various control strategies on the active filter have been tested in order to improve the filtering characteristics of passive filter [40,41]. Other strategies have also included controlling the voltage at the load [42]. For the latter purpose a common strategy is to generate a voltage in the active filter proportional to the harmonic of the supply current. Here, the active filter behaves as a high impedance barrier to the harmonics of the load current. This improves the performance of the passive filter, first by avoiding possible resonances and second by making the filtering feature of the passive filter independent of the source impedance. Furthermore, this can be achieved with a reduced nominal power active filter.
Figure 1.14 Hybrid topology with series active filter and passive filter in parallel with the load.
Finally, Figure 1.15 shows another hybrid filter topology used for eliminating current harmonics [43]. An active and a passive filter are connected in series, and the set is connected in parallel with the load. The control strategy for the active filter is based on the generation of a voltage proportional to the harmonics of the source current. This configuration improves the performance of the passive filter, with an active filter of reduced power, compared to the power required for an active filter connected in parallel with the load. This configuration has proved an enhanced behavior for the mitigation of harmonic currents and reactive power compensation of the load.
Figure 1.15 Combined topology of series active and passive filters, in parallel with the load.
1.4.4. Unified Power Quality Conditioners
Another combination is shown in Figure 1.16, which shows a series active filter and a parallel active filter [48]. Thus the active filter connected in series eliminates the voltage perturbances and the shunt active filter acts to eliminate disturbances in current. From the functional point of view this configuration can act dynamically both on voltage and current, so it has been called universal active filter or unified power quality conditioner [36,49,50]. The main drawback of this configuration is its comparatively high cost.
Figure 1.16 Combined topology of a series active filter and a parallel active filter.
1.5. Summary
This chapter provides an introduction to the concept of electric power quality. The disturbances that can appear in the voltage and current of the network are presented. With respect to the voltage, the different types of disturbances are defined, and the conditions for complying with the standards are given. In this case the standard that is referred is EN 50160. This establishes limits for:
• Frequency variations.
• Slow voltage variations.
• Voltage fluctuations.
• Voltage dips and short interruptions.
• Voltage imbalances.
• Voltage harmonics.
Standard IEEE Std. 519-2014 covers current distortions, and establishes recommendations in the design of electrical systems that include nonlinear loads. On the other hand, the IEC 61000 standard does not set limits to the circulation of harmonic currents in the distribution network, however it sets the harmonics levels that the loads can inject to the network.
Suitable models are necessary for the analysis of circuits where nonlinear loads are present. Simple or complex load models can be used, depending on the accuracy required in the analysis. In this chapter, a load model based on the Norton equivalent is proposed, and this will be used in later chapters.
A range of different topologies of active conditioners has been presented, and a brief introduction has been given of these conditioners and their usefulness from the point of view of improving the power quality. Thus, shunt active conditioners, which are connected in parallel with the load, have shown to be useful for compensating for current distortion. Series active filters, connected between supply and load, eliminate the disturbances caused by the supply voltages. Hybrid filters include LC (inductance and capacitor) branches tuned to specific frequencies and they can also improve the power quality with active filters of reduced power. Finally, the unified power quality conditioners (UPQC) are presented. They include a series active filter and a shunt active filter, allowing them to act simultaneously against voltage and current disturbances.
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