Literature Review for Shunt Active Power Filters

Transcription

1 Chapter 2 Literature Review for Shunt Active Power Filters In this chapter, the in depth and extensive literature review of all the aspects related to current error space phasor based hysteresis controller for shunt active power filters has been considered. 2.1 Power quality Nowadays, as automation has become backbone for industrial growth, use of power electronic device based converters has increased tremendously to implement and control of automation. These power electronic converters act as non-linear loads that inject current harmonics in the power system and hence deteriorate quality of power. The issue has become more serious with increased use of power quality sensitive loads. Hence, for utilities, improvement in power quality has become more challenging task as the loads themselves (mostly non-linear loads) become important causes of the degradation of power quality. Hence providing adequate power quality has become a key area of concern for utilities. For a user, deterioration in power quality is a concern because it affects power quality sensitive loads such as computers, process controls, medical equipment, communication equipment and power supplies. Hence it has become difficult for electrical utilities to ensure perfect quality of power supply, as even the operation of customer equipment (e.g., power electronic converter based drives [65]) can cause power disturbances, which is beyond the control of utilities. Thus, consumers of power supplied by utility should also play an active role in the mitigation of power quality problems, especially at equipment level. Hence improvement in power quality is a joint responsibility of both utilities and consumer which is addressed by IEEE standard [1]. The philosophy conveyed by IEEE in this standard [1] is that the utility should be responsible for maintaining 25

2 Chapter 2Literature Review for Shunt Active Power Filters 26 quality of voltage waveform, and side-by-side, the consumer should be responsible for limiting harmonic currents injected onto the power system. Various standards and literatures have attempted to define power quality in different ways, but there is no universally common definition. For example, International Electrotechnical Commission (IEC) defines power quality as a set of parameters defining the properties of the power supply as delivered to the user in normal operating conditions in terms of continuity of supply and characteristics of voltage (symmetry, frequency, magnitude, and waveform) [2]. Further, Institute of Electrical and Electronics Engineers (IEEE) standard [3] defines power quality as the concept of powering and grounding electronic equipment in a manner that is suitable to the operation of that equipment and compatible with the premise wiring system and other connected equipment. Thus, power quality includes considerations regarding different aspects of reliability of electrical power supply but the main focus of power quality is concentrated to distortion in waveforms of voltage (at system level) and current (at equipment level). Power quality affects not only the consumer equipments but also has detrimental effect on the operation of the power utility. The problems caused by poor power quality like maloperation of remote controls, overheating of cables, increased eddy losses in transformers, excessive heating and failure of capacitors, the derating of neutral conductors, etc., have been reported in [1],[2],[66],[67]. A systematic approach to power quality problem solving procedures, using an expert system framework is recommended in [4]. Power quality is improved by utilizing power quality improvement equipments like passive filters, active power line conditioners, FACTS devices, etc. [4]-[19],[68]-[72]. Availability of various types of power quality improvement equipments, ranging from devices that provide less protection and are less expensive to high-end expensive devices that can provide protection against any eventualities have been reported in [4]. For example, passive filters allow basic power frequency and reject electrical noise and harmonics. Active power filters also help in mitigating harmonics. An uninterruptible power supply (UPS) offers protection against many forms of disturbances, like harmonics, electrical noise, sag, swell, impulse and at same time ensures continuity of power supply. Motor-generator sets offer protection against sags, swells, undervoltages, overvoltages, harmonics, and noise [4].

3 Chapter 2Literature Review for Shunt Active Power Filters Active power filters Conventionally, passive filters consisting of tuned L-C filters, have been used to suppress harmonics because of their low initial cost. However, passive filters suffer with drawbacks such as large size, parallel and series resonance that could be created with both load and utility impedances, and the filtering characteristics are strongly affected consequently by the source impedance [19]. In order to overcome these drawbacks of passive filters, active power filters (APF) were developed [6]-[9],[19],[20]. Active power filters (APF) are power electronic converter based devices that are used for power quality improvement [73],[74]. APF have many advantages compared to passive filter such as, they are less bulky, superior filtering characteristics as well as dynamic performance, flexible operation etc. [19]. Hence, APF are more preferable for harmonic compensation as compared to passive filters. Active power filters are classified into three types:- Shunt active power filter Series active power filter Hybrid active power filter Shunt active power filters are connected in parallel to non-linear load, which inject current harmonics in the system as shown in figure 2.1 [19],[20],[36],[75]-[77]. The concept of shunt active filters was initially proposed in 1971 by Sasaki and Machida [19] as a means of removing current harmonics. Figure 2.2 shows the system proposed to validate the concept of harmonic filtering by active filters. In conventional dc systems, nonlinearity of a rectifier circuit and the existence of commutation reactances induce harmonics, inject harmonics and hence introduce distortion in the secondary currents of transformer (shown in figure 2.2). This current induces the magnetic flux with a similar distorted waveform in the transformer core, and thus ac harmonic currents generated by the magnetic flux flow into ac systems. In the system reported in 2.2, the filter is used to remove fundamental component in the detected current. After filtering the fundamental frequency component, the detected signal is amplified so as to induce the same ampere-turns as those by the secondary current. When the output current of the amplifier is made to flow into the tertiary winding differentially against the secondary current, harmonic components in the magnetic flux are theoretically canceled by perfect compensation. This concept was further developed and used for shunt active power filters.

4 Chapter 2Literature Review for Shunt Active Power Filters 28 Figure 2.1: Shunt active power filter Figure 2.2: Principle of harmonic elimination by magnetic flux compensation The series active power filter is connected in series with the utility supply voltage as shown in figure 2.3 and is generally used to compensate for voltage harmonics present in the system [19]-[21],[78],[79]. Hybrid active power filter is a combination of series and shunt active power filter or active power filter and passive filter [17], [22]-[26],[80],[81]. Combination of active power filter with the passive filter helps in reducing the rating (as well as burden of compensation) of the active power filter, and also improves the dynamic performance of APF, as shown in figure 2.4. As nowadays, current harmonic elimination has become an issue of concern, the proposed research work is focussed on shunt active power filters.

5 Chapter 2Literature Review for Shunt Active Power Filters 29 Figure 2.3: Series active power filter Figure 2.4: Hybrid active power filter

6 Chapter 2Literature Review for Shunt Active Power Filters Shunt active power filters Both voltage-source converters and current-source converters (shown in figure 2.5 and 2.6) [82]- [85] are applicable to SAPF (shown in 2.1), however voltage-source converters are more preferred due to their inherent advantages of higher efficiency, lower cost, smaller size as compared to current-source converters [19],[20],[77],[86]. Application of both types of converters to shunt active power filter and their comparison from different points of view is reported in [77]. With the advancement of research in the area of SAPF, its application is not only limited to harmonic compensation, but has extended to harmonic isolation between utilities and consumers, harmonic damping throughout power distribution systems and many more [37],[73],[87]-[89], making them power conditioners in real sense. Figure 2.5: Voltage source converter Figure 2.6: Current source converter

7 Chapter 2Literature Review for Shunt Active Power Filters Reference compensating current generation strategies used and design considerations for shunt active power filters Shunt active power filters are used to eliminate current harmonics injected by the non-linear loads in the system. The shunt APF works as a current source, that injects a compensating current in order to cancel the harmonic currents. Harmonic extraction by the SAPF is performed by various mathematical computations called reference compensating current generation methods. Thus, compensation provided by the SAPF is affected by the selection of reference compensating current generation method. Hence, development of reference compensating current generation method, which can effectively extract all the harmonics has been a key area of research in the field of shunt active power filters. Many researchers have worked upon developing novel and efficient techniques for generating reference compensation current like Instantaneous reactive power theory, Synchronous reference frame method, Dc-link voltage control method, Fryze current computation technique, Notch filter technique, Sliding mode control, Predictive scheme, State feedback scheme, Fast Fourier transform method, Non-linear least-squares approach [28]-[39],[90]. Instantaneous reactive power theory also known as p-q theory, is proposed by Akagi, Nabae and Kanazawa [16],[35]. An instantaneous reactive power compensator comprising of switching devices without energy storage components which can eliminate not only the fundamental reactive power in transient states but also harmonic currents, is proposed in [16]. Instantaneous reactive power theory is very well reported and explained in [39]. IRP theory is very much popular in application for SAPF and hence researchers have applied it for three-phase system with or without neutral and also for steady state as well as transient operation of the system [91]-[100]. Effective performance of IRP theory based SAPF in compensating under sinusoidal as well as non-sinusoidal supply mains conditions and balanced as well unbalanced load conditions have been reported [93],[94]. IRP theory based compensation algorithm is used for elimination of neutral currents without using energy storage elements in three-phase, four-wire systems [97]. In synchronous reference frame method used for compensation of harmonics, threephase load currents are transformed into rotating d-q axis [14],[30],[35]. In rotating d-q axis frame, all the components associated with angular frequency ω become dc quantities and rest all (harmonics) become ac quantities [101]-[105]. In dc-link voltage regulation method, the supply currents maintain the SAPF dc-link voltage. To ob-

8 Chapter 2Literature Review for Shunt Active Power Filters 32 tain the reference currents from the dc-link voltage, is the concept of the PI controller based operation of the SAPF [37]. With dc-link voltage regulation method, SAPF provides effective compensation [38]. Though the approach gives good results, but it has drawbacks under unbalance supply voltage conditions. The performance of the PI controller based operation of the SAPF is improved by application of artificial neural network (ANN) [38] so as to get very precise solution for reference current even under unbalanced conditions of supply voltages. The method proposed in [38] is based on the recovering of the fundamental active phase currents in the load by artificial neural network. The neural network is training by Widrow-Hoff learning rule. Obtained reference compensating currents are subtracted from the total load currents to get the desired reference waveform. Fuzzy logic is used for implementing dc-link voltage regulation method, for the control of SAPF [106]. Fryze current computation method provides linearity between voltage and current while performing power compensation, even under distorted and/or unbalanced source voltages [39],[107],[108]. In the notch filter technique, load current is filtered by notch filter connected in each phase, thereby removing the fundamental and generating load harmonic current for each phase. These harmonic currents are phase shifted by 180 o [28]. The fast fourier transformation method takes the sample of load current for one period and calculates the magnitude and phase of the frequency components. Now, the fundamental component is removed by making fundamental frequency component zero and then performing inverse fast fourier transform. This extracts load harmonic currents, which are phase shifted and injected at PCC to cancel the harmonics present in nonlinear load current. In equal current synchronous detection method, the reference active source currents are calculated with an assumption that source line currents become equal after compensation. The reference active source currents are then subtracted from the sensed load currents to generate reference compensating currents. This scheme gives effective compensation even under unbalanced supply conditions and unbalanced load conditions [30]. In predictive schemes, fundamental component of ac input current or selected order of harmonic are predicted based on the relationship between the dc-link current of the bridge rectifier and its ac input current. These components or harmonics are subtracted from the non-linear load current so as to achieve the desired reference compensating currents. In sliding mode control schemes, APF is controlled by two control loops- inner current loop which actively shapes the line currents and an outer voltage control loop which regulates the magnitude of the line currents. The inner current loop uses sliding-mode control to shape the line current. The outer voltage loop regulates the average capacitor voltage by using a proportional-integral (PI) control law. In such case, the magnitude of the line current is dictated by the outer voltage control loop. For the application of sliding

9 Chapter 2Literature Review for Shunt Active Power Filters 33 mode control theory to the active power filter, the sliding surfaces or the reference trajectories of currents are defined and to ensure proper line current shape, system state should always be on the sliding surface [34]. Non-linear least-squares approach has been used for harmonic estimation [90]. The non-linear least-squares approach is used for estimating the grid frequency and harmonic magnitudes and phases. [37] presents control algorithm of a shunt active filter for ac voltage regulation at load terminals (at PCC), harmonic elimination, powerfactor correction and load balancing of nonlinear loads. Three-phase voltages at PCC along with dc bus voltage of the APF are used for implementation of control scheme. A self-supporting dc bus of the APF is realized using a PI controller on the sensed and reference values of dc bus voltage of the APF. The PI voltage controller on the dc bus voltage of the APF provides the amplitude of in-phase components of reference supply currents. The three-phase unit current vectors are derived in-phase with the supply voltages. Another PI controller is used over the reference and sensed values of peak supply voltage. The output of this PI controller is considered as an amplitude of quadrature components of reference supply currents. The three-phase quadrature unit current vectors are derived from in-phase unit current vectors. The multiplication of in-phase amplitude with in-phase unit current vectors results in the in-phase components of three-phase reference supply currents. Similarly, multiplication of quadrature amplitude with quadrature unit current vectors results in the quadrature components of three-phase reference supply currents. Algebraic sum of in-phase and quadrature components results in the three-phase reference supply currents. For regulation of voltage at PCC the three-phase reference supply currents have two components. The first component is in-phase with the voltage at PCC to feed active power to the load and the losses of the APF. The second component is at quadrature with the voltage at PCC to feed reactive power of load and to compensate the line voltage drop by reactive power injection at the PCC. For power-factor correction to the unity, harmonic elimination and balancing of nonlinear load, the quadrature component of reference supply currents is set to zero. For the voltage regulation at PCC, the supply currents should lead the supply voltages while for the power factor control to the unity, the supply currents should be in phase with the supply voltages. As, voltage regulation at PCC and power-factor control to unity can not be achieved simultaneously, therefore, the control algorithm of the APF is made flexible to achieve either voltage regulation, harmonics compensation, load balancing or power-factor correction to unity, harmonics compensation, load balancing.thus, with three-phase supply voltages and dc bus voltage as feedback signals, the control algorithm of the APF provides the three-phase reference supply currents as output signals.

10 Chapter 2Literature Review for Shunt Active Power Filters 34 Proper design of SAPF inductor, dc-link capacitor and choice of dc-link voltage is very much crucial for appropriate performance of SAPF. Main consideration in design of SAPF inductor is that it should allow harmonics (which are to be compensated) to flow through it, but at the same time should restrict high frequency switching harmonics from propagating to supply mains (PCC). While, dc-link capacitor and dc-link voltage selection should be such that appropriate compensation is provided by the SAPF. Many researchers have tried to address the design considerations for selection of the components of SAPF [109]-[118]. 2.5 Current Controllers used for shunt active power filters Current controllers force the actual compensating current to track the reference compensating current and hence selection of controller significantly affects the performance of SAPF. Different types of current controllers are used in Shunt APFs, among which hysteresis controllers [119]-[120] are widely used due to their inherent simplicity and fast dynamic response. Hysteresis controllers keep the actual compensating current within the hysteresis band by the switching action of the SAPF. Conventional hysteresis current controller (HCC) scheme used in APFs uses three independent hysteresis controllers one for each phase (shown in figure 2.7) which compare the actual compensating current with the reference compensating current. The output signals of these hysteresis controllers are used to trigger the individual switches of SAPF. This scheme suffers from lack of coordination between the three individual HCCs, resulting in random switching. They also suffer from draw backs like limit cycle oscillations, overshoot in current error and generation of sub-harmonic components in current. Efforts have been made to reduce the overshoot in current error, torque pulsations [122] limit cycle oscillations [40], inverter switching frequency variations [41]-[43],[123] and to achieve fast transient response [44],[45], for the applications of hysteresis controllers in induction motor drives. Apart from hysteresis control technique, deadbeat control and linear rotating frame control is also used for active filters [59]. The conventional linear current controller performs a sine-triangle PWM voltage modulation of the power converter using the current error filtered by a proportional integral (PI) regulator as the modulating signal.the analog implementation of linear current control technique though being quite simple, provides unsatisfactory performance as far as active filter applications are concerned. This is due to the limitation of the achievable regulator bandwidth which is implied by the necessity of sufficiently filtering the ripple in the modulating signal. This necessity compels the loop gain

11 Chapter 2Literature Review for Shunt Active Power Filters 35 Figure 2.7: Hysteresis current controller for SAPF crossover frequency to be kept well below the modulation frequency. This causes poor rejection of the disturbances injected into the current control loop, mainly due to the ac line voltage at the fundamental frequency. To overcome this limitation d-q rotating frame is applied to linear current control. Control variables are in the rotating frame as shown in 2.8. Now, inorder to d-q transformation, information of the instantaneous phase angle of the sinusoidal waveforms is not required. Thus the fundamental harmonic components of voltage and current signals appear constant to the current regulator. Hence the line voltage, which is almost sinusoidal, is seen by the current regulator as a constant quantity. As a result of this the rejection of this disturbance is much more effective. But the bandwidth limitation of the PI regulators still persisits and hence introduces significant errors in the tracking of the high-order harmonic components of the reference compensating current. For APF, it affects the compensation characteristics adversely [59]. The schematic of a digital deadbeat controller used for the control of APF is shown in 2.9. Here, the deadbeat control algorithm estimates instantaneous line voltage values, so as to make the compensating current reach its reference by the end of the following modulation period. The calculations are performed in αβ frame and thus the space-vector modulation (SVM) strategy is applied for the switching of APF. The advantage of this technique is that it does not require the line voltage measurement in order to generate the current reference. However, the delay due to the calculations is a drawback for this controller. Harmonics will be compensated if the movement of current error (generated due to the difference between APF compensating current and reference compensating current) is restricted within a specified boundary. Application of current error space phasor based hysteresis controller for control of induction motor drives is studied and reported [40]-[55]. Current error space phasor based hysteresis controllers [40],[41],[44],[46]-[48] allow the current error space phasor to move within a specified boundary. Different strategies have been proposed for induction motor drives, to keep the current error space phasor within the boundary [40]-[44], [49]-[50]. Researchers have worked upon Hexagonal, circular, rectangular and parabolic shapes of current error space phasor

12 Chapter 2Literature Review for Shunt Active Power Filters 36 Figure 2.8: Scheme of linear current regulator Figure 2.9: Scheme of digital deadbeat current regulator boundary [40],[41],[44],[46]-[48],[51], [52],[124]-[127]. In order to decrease the number of switching and in turn the switching frequency, space vector modulation technique is applied to current hysteresis controller which enables the use of zero switching vector along with non zero vectors [53]-[55]. Researchers have worked upon application of space vector modulation based hysteresis current controllers for control of SAPF [56]-[59]. Research has been conducted on application of space vector current controls based on stationary αβ-coordinates and rotating xy-cordinates. In literature related to application of space vector control for SAPF, sector selection is done by sensing the point of common coupling voltages [56]. The voltage space phasor structure used for space vector modulation technique implementation for control of two-level SAPF is shown in figure Here V dc is the dc-link voltage of SAPF and V 1 to V 6 are active vectors and V z is zero vector generated by various switching states as shown in figure 2.10 [121]. In the proposed research work application of current error space phasor based hysteresis controller to Shunt APF is investigated for power quality improvement.

13 Chapter 2Literature Review for Shunt Active Power Filters 37 Figure 2.10: Voltage space phasor structure with switching states for two-level SAPF Advent in semiconducting technology and development of power semiconducting devices at high power level have lead to the development of multi-level inverters [60]-[62]. Figure 2.11 shows one leg for different types of multi-level inverters. Multi-level inverters overcome the difficulties in application of conventional two-level inverters at medium voltage and high power level and hence they have found popularity in application to shunt active power filters [128]-[132]. Voltage space phasor structure used for the control of three-level SAPF is shown in figure Here V dc is the dc-link voltage of SAPF and V 1 to V 18 areactivevectorsandv 0 is zero vector generated by various switching states as shown in figure 2.12 [50]. Research has been reported on application of space vector current controls based on stationary αβ-coordinates and rotating xy-cordinates and use of point of common coupling voltage information for necessary sector selection [56]. Reduction in number of switching events with rotating xy-cordinates space vector control as compared to that achieved by stationary αβcoordinates is reported. In literature related to application of space vector control for SAPF, sector selection is done by sensing the point of common coupling voltages [56].With the advent in embedded systems and availability of fast DSPs [133], effective implementation of various control strategies and current controllers for the control of SAPF is possible [134]-[137].

14 Chapter 2Literature Review for Shunt Active Power Filters 38 In the proposed research work, current space phasor based hysteresis controller for two-level shunt active power filter is implemented using DSP TMS320LF2407A [138]- [148]. The reference compensating current generation and current controller algorithm is developed in assembly language [142],[143],[146]. For providing voltages and currents to DSP, voltage and current sensors [149],[150] as well as offset circuit. Figure 2.11: One leg of different multi-level inverters Figure 2.12: Voltage space phasor structure with switching states for three-level SAPF