Performance of Unified Power Quality Conditioner
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Published: Wed, 21 Feb 2018
Power electronics is playing an important role in transmission and utilization of electrical power due to its capability of processing electric power in most efficient and cost-effective way. However, the nonlinear characteristics of power electronic devices give rise to two important limitations; they generate harmonics and draw lagging current from the utility. In recent years unified power quality conditioner (UPQC) is being used as a universal active power conditioning device to compensate both harmonics as well as reactive power. UPQC is an advanced version of unified power flow controller (UPFC). The performance of UPQC mainly depends upon how quickly and accurately compensation signals are derived. The UPQC mitigates harmonics and provides reactive power to the power systems network so as to improve the power factor close to unity.
The UPQC is a combination of shunt active and series active power filters connected through a dc bus. The shunt active filter of UPQC acts as a current source for injecting compensating current through a shunt transformer, whereas, the series active filter acts as a voltage source for feeding compensating voltage through a series transformer. The aim of the dissertation work is to study the control strategies of UPQC based on PI controller and fuzzy logic controller in detail.
In the case of PI controller, the dc link voltage is sensed at regular intervals and is compared with a reference value. The error signal thus derived is processed in a PI controller. A limit is put on the output of the controller to ensure that the shunt active power filter supplies active power of the load through the series active power filter.
The fuzzy logic controller is basically nonlinear and adaptive in nature. This gives a robust performance in the cases where the effects of parameter variation of controller are also taken into consideration. It is a well established fact that the fuzzy logic controller yields results that are superior to those obtained as compare to those obtained through conventional controllers such as PI and PID because of the fact that it is based on linguistic variable set theory and does not require a mathematical model. Generally, the input variables are error and rate of change of error. If the error if coarse, the fuzzy controller provide coarse tuning to the output variable and if the error is fine it provides fine tuning of the output variable.
The present thesis investigates PI controller and fuzzy logic controller as concerned to UPQC application for power quality improvement. The UPQC is studied and its advantages over conventional APFs and UPFC are discussed in detail. The relevant mathematical models and equations to explain the working of UPFC are derived for both the cases (PI controller and fuzzy logic controller).The relevant simulations are carried out using MATLAB/Simulink.
The result obtained reveals that the fuzzy logic controller gives better dynamic performance than the PI controller for power quality improvement.
The electrical power system consisting of generation, transmission and distribution system are based on alternative voltage and currents. When linear load consisting of inductances, capacitances and resistances are connected to the power system the sine wave is preserved and the system components are said to be linear. Traditionally, linear loads consume major part of electrical power. However situation has changed now as more and more electrical power are being developed using power electronic devices due to their energy efficiency and control. Power electronic devices possess inherent non linear characteristics. The nonlinear characteristics of this devices results in two important limitations, drawing of large reactive volt-amperes and injection of harmonics into the utility. Large reactive volt-amperes drawn from the utility leads to increase voltage drops at various buses. The harmonics increase the losses in transformers, generators, motors, capacitors, conductors, etc. some of the control devices interfaced with the utility starts malfunctioning due to excessive harmonic currents.
As the non linear load consists of the major portion of the total load for the last two three decades, reactive power compensation and harmonic filtering have received a great deal of attention. To restrict the consumers against excessive loading VARs and harmonics, stricter standards has been laid down by the utilities. Most popular among them is standard 519-1992 .
Static VAR compensators using thyristor switched capacitors (TSC) and thyristor control inductors (TCI) ,  have been traditionally used for reactive power compensation. As the VAR generated in these schemes are directly proportional to the energy storage capability of capacitors and inductors, there is considerable increase in the size of these elements when the VARs to be compensated are large. Moreover TSC and TCI produce additional current harmonics. Therefore shunt passive filters require filtering them out. Active power filter (APF) using voltage or current source inverter can be used for reactive power compensation and harmonic filtering together. The major advantage of using voltage source or current source inverter is that the size of the energy storing element is drastically reduced as compare to TSC or TCI.
The shunt APF is the most commonly used APF. The power circuit of shunt APF is shown in Fig. 1.1. In shunt APF, a reactive volt ampere calculation estimates the real component of the load current, Ipland then determines the resistive component of the load current by subtracting Ipl from IL(Iql= IL-Ipl). If nonlinearity present in the load current, it is present in Iql as well. Since compensation current Icomp is made to follow Iql, load harmonics also get eliminated. Apart from shunt APF various other APF topologies such as series active filter, hybrid series active filter and power line conditioner have been proposed in the literature.
The series active filter as shown in Fig. 1.2 is connected in series with supply mains using a matching transformer. Its limitation is that the presence of active impedance in series with source produces voltage harmonics.
IL = Ipl +Iql
Icomp = Iql
Source side Series transformer Load side
Converter 1 converter 2
Using combine series APF and shunt APF unified power flow controller (UPFC) realized, which performs active power compensation, reactive power compensation and phase angle regulation. UPFC believed to be the most complete power conditioning device. But as the time changes, problem also changes. Now days electrical engineers facing problem regarding harmonic compensation, voltage sag and voltage flickering and UPFC is not able to overcome these problems. So a new concept based on UPFC derived called unified power quality conditioner (UPQC) as shown in Fig. 1.3, which performs all the basic functions of UPFC in addition it also compensate for current /voltage harmonics with constant voltage maintenance at load terminals.
1.2 Unified Power Quality Conditioner
The UPQC is the most versatile and complex of the FACTS devices, combining the features of the STATCOM and the SSSC. The UPQC can provide simultaneous control of all basic power system parameters, transmission voltage harmonic compensation, impedance and phase angle. It is recognized as the most sophisticated power flow controller currently, and probably the most expensive one. The basic components of the UPQC are two voltage source inverters (VSIs) sharing a common dc storage capacitor, and connected to the power system through coupling transformers. One VSI is connected to in shunt to the transmission system via a shunt transformer, while the other one is connected in series through a series transformer. A basic UPQC functional scheme is shown in Fig.1.3. The series inverter is controlled to inject a symmetrical three phase voltage system of controllable magnitude and phase angle in series with the line to control active and reactive power flows on the transmission line. So, this inverter will exchange active and reactive power with the line. The reactive power is electronically provided by the series inverter, and the active power is transmitted to the dc terminals. The shunt inverter is operated in such a way as to demand this dc terminal power (positive or negative) from the line keeping the voltage across the storage capacitor Vdc constant. So, the net real power absorbed from the line by the UPQC is equal only to the losses of the inverters and their transformers. The remaining capacity of the shunt inverter can be used to exchange reactive power with the line so to provide a voltage regulation at the connection point -.
A conventional UPQC topology is comprised of the integration of two active power filters connected back to back to a common dc link bus. A simple block diagram of a typical UPQC is shown in Fig. 1.4. The first active filter connected in series through an injection transformer is commonly termed as series filters (SF). It acts as a controlled voltage generator. It has capability of voltage imbalance compensation, voltage regulation and harmonic compensation at the utility-consumer PCC. In addition to this, it provides harmonic isolation between a sub-transmission system and a distribution system.
A UPQC consists of combination of shunt active filter and series active filter with a common dc link as shown in Fig. 1.4. The dc link capacitor allows the active power generated by the shunt active filter and active power drawn by the series filter to be same. Further dc link capacitor increases or decreases with respect to rated voltage which depends upon power generated and absorbed by both active filter can be choosen independently which gives flexibility to the power outlet.
The performance of these active filters is based on three basic design criteria. They are:
- Design of power inverter (semiconductor switches, inductances, capacitors, dc voltage);
- PWM control method (hysteresis, triangular carrier, periodical sampling);
- Method used to obtain the current reference or the control strategy used to generate the reference template.
Both series voltage control and shunt current control involve use of voltage source converters. Both these inverters each consisting of six IGBTs with anti parallel diode connected with each IGBT are operated in current control mode employing PWM control technique. Capacitor is used as an interface between the two back to back connected inverters and the voltage across it acts as the dc voltage source driving the inverters
The two VSI’s can work independently of each other by separating the dc side. So in that case, the shunt inverter is operating as a STATCOM that generates or absorbs reactive power to regulate the voltage magnitude at the connection point. Instead, the series inverter is operating as SSSC that generates or absorbs reactive power to regulate the current flow, and hence the power flows on the transmission line. The UPQC has many possible operating modes. In particular, the shunt inverter is operating in such a way to inject a controllable current into the transmission line. The shunt inverter can be controlled in two different modes:
(1) VAR Control Mode:The reference input is an inductive or capacitive VAR request. The shunt inverter control translates the VAR reference into a corresponding shunt current request and adjusts gating of the inverter to establish the desired current. For this mode of control a feedback signal representing the dc bus voltage, Vdc, is also required.
(2)Automatic Voltage Control Mode:The shunt inverter reactive current is automatically regulated to maintain the transmission line voltage at the point of connection to a reference value.. The series inverter controls the magnitude and angle of the voltage injected in series with the line to influence the power flow on the line. The actual value of the injected voltage can be obtained in several ways:
Direct Voltage Injection Mode:The reference inputs are directly the magnitude and phase angle of the series voltage.
Phase Angle Shifter Emulation mode: The reference input is phase displacement between the sending end voltage and the receiving end voltage.
Line Impedance Emulation mode: The reference input is an impedance value to insert in series with the line impedance.
Automatic Power Flow Control Mode:The reference inputs are values of active and reactive power to maintain the transmission line despite system changes.
A UPQC control strategy should preferably have following attributes:
(1) Shunt converter
- Reactive power control by shunt current injection
- Real power regulation through dc link capacitor
- DC capacitor voltage regulation
- Harmonic compensation
(2) Series converter
- Real & reactive power control by series voltage injection
- Voltage control
- Phase angle regulation
- Power factor correction
1.3 Characteristics of UPQC
Basic characteristics of UPQC are same as UPFC but UPQC in addition, performs active filtering. The operation of UPQC from the standpoint of conventional power transmission based on reactive shunt compensation, series compensation and phase angle regulation, the UPQC fulfill these functions there by meet multiple control objectives by adding injected voltage with appropriate magnitude and phase angle to the terminal voltage. Using phasor representation, basic UPQC control functions explained:
(1)Terminal Voltage Regulation
The change in voltage shown in Fig.1.5 is injected in phase or anti phase. UPQC with its series voltage control detects and calculates the required terminal voltage vo to be injected in series with the line to compensate both the dip and swell in the supply voltage.
vo + vo
(2) Series Capacitive Compensation
Here, vpq = vc where vcis injected capacitive voltage in quadrature to the line current functionally it is similar to series capacitive and inductive line compensation attained by SSSC as shown in Fig. 1.6. Series inverter in combination with the insertion transformer produces the series injected voltage as calculated to mitigate the effects of the fluctuations of supply voltage by drawing the required power from the dc link.
vo vo + vc
Fig. 1.6 Series capacitive compensation
(3) Transmission Angle Regulation
Here, vpq = v (δ) is injected with an angular relationship with respect to the voltage that achieves desire phase shift without any change in the magnitude as shown in Fig. 1.7. At any given transmission angle δ, the transmitted real power demand P and reactive power demand at transmission line sending end Qs and receiving end Qr can be freely controlled by UPQC
vo + vδ
(4) Multifunction Power Flow Control
This property is executed by simultaneous terminal voltage regulation, series capacitive line compensation and phase shifting as shown in Fig.1.8. This function makes UPQC unique device that performs all power quality improvement functions.
vo + δv + vc + vδ
(e) Active Filtering
The compensating shunt currents generated contain harmonic content of the load current but with opposite polarity such that when they are injected at the point of common coupling the harmonic content of supply current is effectively reduced. As discussed earlier in this chapter.
1.4 Aim of Work
This work deals with UPQC, which aims at the integration of series-active and shunt-active power filters. Fig. 1.3 shows the basic system configuration of such a UPQC. In this system, the power supply is assumed to be a three-phase, three-wire system. The two active power filters are composed of two 3-leg voltage source (VSI). The main purpose of the series-APF is harmonic isolation between a sub transmission system and a distribution system. In addition, the series-APF has the capability of voltage imbalance compensation as well as voltage regulation and harmonic compensation at the utility-consumer point of common coupling (PCC). Atthe same time, the main purpose of the shunt- APF is to absorb current harmonics, compensate for active power and reactive power injected by the load. Also, the voltage of the DC link capacitor is controlled to a desired value by the shunt-APF.
The aim of the dissertation is to design different control strategies for (UPQC), which is one of the major custom power solutions capable of mitigating the effect of supply voltage sag, swell, flicker and spikes at the load end or at the Point of Common Coupling (PCC). It also prevents load current harmonics from entering the utility and corrects the input power factor of the load. Further, the main aim of the dissertation is to implement a control strategy for UPQC, modeling of UPQC using simulink and to analyze the control strategy to use the series voltage injection and shunt current injection for UPQC control The control strategies used here are based on PI controller, fuzzy controller. The relative performance of the two controls is also studied.
The present work discusses the compensation principle and different control strategies (PI, Fuzzy) of the UPQC in detail -. The control strategies are modeled using MATLAB/Simulink. The performance of UPQC is examined by considering, a diode rectifier feeding an RL load (non linear load) that acts as a source of harmonics, to the system of concern. The performance is also observed by switching the extra RL load. The simulation results are listed in comparison of different control strategies and for the verification of result -.
1.5 Organization of the Report
The report of the work done is organized as follows:
Chapter 2 gives brief overview of control strategy of UPQC. In this chapter introduction to dq theory, compensation strategy, basic control function and modeling of UPQC using PI controller discussed with results. Chapter 3 discusses about fuzzy logic controller and implementation in UPQC. Membership functions, rule base table and surface viewer also discussed in this chapter. Chapter 4 gives comparison studied between fuzzy logic controller and PI controller. Simulation results of both are discussed in detail with the help of table and graphs. The last chapter 5 presents important conclusions and future work. Adequate references provided at the end of the chapter.
CONTROL STRATAGEY FOR UNIFIED POWER QUALITY CONDITIONER
Control strategy plays vital role in overall performance of power conditioner. Control strategy includes features like rapid detection of harmonic signals by maintaining higher accuracy, fast processing, and faster dynamic response of the controller. The control strategy can be realized using discrete analog and digital devices or advanced programmable devices, such as single chip micro computers, DSPs etc.
The control strategy determined by the appropriate switching pattern or signal obtained by compensating gate signal compared obtained by comparing with its reference
value. Since derivation of reference signal plays an important role in control strategy, many theories and techniques were proposed in recent years. There are number of control strategies were proposed among them dq method is used in the present work and discussed below:
2.2 dq Transformation
It is established that the active filter flows from leading voltage to lagging voltage and reactive power flows from higher voltage to lower voltage. Therefore both active and reactive power can be controlled by controlling the phase and the magnitude of the fundamental component of the converter voltage with respect to line voltage. dq theory provides an independent control of active reactive power by controlling phase and the magnitude of the fundamental component with respect to converter voltage
According to the dq control theory three-phase line voltages and line currents are converted in to its equivalent two-phase system called stationary reference frame. These quantities further transformed into reference frame called synchronous reference frame. In synchronous reference frame, the components of current corresponding to active and reactive power are controlled in an independent manner. This three-phase dq transformation and dq to three-phase transformation are discussed in detail in this chapter. The outer loop controls the dc bus voltage and the inner loop controls the line currents. The instantaneous real power at any point on line can be defined by:
p =vRIR + vBIb + vCIc (2.1)
And we can define instantaneous reactive voltage conceptually as a part of three phase voltage set that could be eliminated at any instant without altering p.
Reference frame theory based d-q model of shunt active filter is presented in this section. While dealing with instantaneous voltages and currents in three phase circuits mathematically, it is adequate to express their quantities as the instantaneous space vectors . Vector representation of instantaneous three phase quantities R, Y and B which are displaced by an angle 2π/3 from each other is shown in Fig.2.1 .
90o R α
The instantaneous current and voltage space vectors are expressed in terms of instantaneous voltages and currents as:
v= [vRvYvB] I = [IR IY IB] (2.2)
Instantaneous voltages and currents on the RYB co ordinates can be transformed into the quadrature α, β coordinates by Clarke Transformation as follows:
Where Transformation matrix
Since in a balanced three-phase three-wire system neutral current is zero, the zero sequence current does not exist and zero sequence current can also be eliminated using star delta transformer.
These voltages in α-β reference frame can further be transformed into rotating d- q reference frame as Fig. 2.2.
Where ωr is the angular velocity of the d- q reference frame as shown in Fig. 2.2.
The current components in the d- q reference frame can be similarly obtained using the α-β to d-q transformation matrix T1. The unit vector required for this transformation is generated using the grid voltage
2.3 Compensation Strategy
As shown in Fig. 2.3,vs is the supply voltage. vc, Ic are the series compensation voltage, shunt compensation current and vL, iL are the load voltage and current respectively. The source voltage may contain negative, zero as well as harmonic components. The per phase voltage of the system can be expressed as:
va=v1pm+sinωtsinθ+valn+k=2∞Vaksin kωt + θka (2.8)
Where v1pa is the fundamental frequency positive sequence components, v1naand v10a are negative and zero sequence components respectively. The last term of equation represents the harmonic content in the voltage. In order for the load voltage to be perfectly sinusoidal and balanced, the series filter should produce a voltage of:
vah=v1an+v10a+ k=2∞vka sin kωt + θka 2.9
In the latter section, it will be shown how the series-APF can be designed to operate as a controlled voltage source whose output voltage would be automatically controlled according to the above equation.
The functions of the shunt active filter is to provide compensation of the load harmonic current, load reactive power demand and also to maintain dc link current constant. To provide load reactive power demand and compensation of the load harmonic and negative sequence currents, the shunt-APF acts as a controlled current source and its output components should include harmonic, reactive and negative-sequence components in order to compensate these quantities in the load current . The per phase load current of shunt active filter is expressed as:
Ial=I1pmcos ωt – θ1 + Taln+k=2∞Ialk (2.10)
=I1pmcosωt cosθ1 + I1pmsin ωt sin θ1 k=2∞Ialk (2.11)
In order to compensate harmonic current and reactive power demand the shunt active filter should produce a current of:
Iah=I1pm+sin ωt sin θ1 +Ialn+k=2∞Iak (2.12)
Then the harmonic, reactive and negative-sequence current will not flow into power source. Hence, the current from the source terminal will be:
Ias=Ial-Iah=Ipmcos ωt – θ1 + Taln+k=2∞Ialk (2.13)
This is a perfect harmonic free sinusoidal current in phase with voltage.
2.4 Basic Control Function
It is evident from above discussion that UPQC should separate out the fundamental frequency positive sequence components first from the other components. Then it is required to control both series and shunt active filter to give output as shown in equations (2.9) and (2.18) respectively. The control strategy uses a PLL based unit vector template for extraction of reference signal from the distorted input supply. The block diagram of extraction of unit vector template is as given in Fig. 2.4.
The input source voltage at point of common coupling contains fundamental and distorted component. To get unit vector templates of voltage, the input voltage is sensed and multiplied by gain equal to 1/vm, where vm is peak amplitude of fundamental input voltage. These unit vector templates are then passed through a PLL for synchronization of signals. The unit vector templates for different phases are obtained as follows:
vb=sin (ωt-1200) (2.14)
2.5 Shunt Converter Control
The unit vector template of voltage is used to generate the reference signal for shunt APF. The control block diagram of shunt active filter is given in Fig. 2.5. As indicated earlier, the shunt APF compensates current harmonics in addition to maintaining the dc link current at a constant level. To achieve this, dc link current of the UPQC is compared with a constant reference current of magnitude equal to peak of harmonic current [10.]. The error between measured dc link current and reference current is processed in a PI controller.
Ia Ib I
vavbvc Iar Ibr Icr
The output of PI controller is added to real power loss component to derive reference source current given as:
vαvβ = 1/2 -1/2-1/203/2 -3/2 vavbvc (2.15)
IαIβ =1/2 -1/2-1/203/2 -3/2IaIbIc (2.16)
In matrix form it is given as:
pq = vαvβ-vβvα IαIβ (2.18)
From equation 2.18 the values of p and q can be expressed in terms of dc components plus the ac components as follows:
p is the dc component of the instantaneous power p, and is related to the fundamental active current.
p is the ac component of the imaginary power p, and is related to the harmonic current caused by the ac component of the instantaneous real power
q is the dc component of the imaginary instantaneous power q, and is related to the reactive power generated by the fundamental components of voltage and current
qis the ac component of the instantaneous imaginary power q, and is related to the harmonic current caused by the ac component of instantaneous reactive power.
To compute harmonic free unity power factor, three-phase currents, compensating powers pc and qc are selected as:
pc = pldc + ploss (2.20)
qc = 0
Where, plossis the instantaneous active power corresponding to the switching loss and resistive loss of UPQC. The total instantaneous active power is calculated by adding real power loss due to switching as shown in Fig.2.5. The orthogonal components of the fundamental current are obtained as follows:
IαIβ = vαvβ-vβvα pcqc (2.21)
The a-b-c components of fundamental reference current are obtained as follows:
i*sai*sbi*sc =2/30-1/31/3-1/31/3IαIβ (2.22)
The reference currents are then; compared with actual source current in a hystresis controller band to derive the switching signals to shunt inverter.
2.6 Series Converter Control
In order for the load voltage to be perfectly sinusoidal and balanced, the series filter should produce a voltage equal to equation (2.9). The reference load voltages are obtained by multiplying the unit vector templates with a constant equal to peak amplitude of fundamental input voltage. The compensation signals for series filter are thus obtained by comparing these reference load voltages with actual source voltage using equation (2.23).
v*fa=vsa-vmva v*fa=vsb-vmvb v*fa=vsc-vmvc (2.23)
The control of the series-active power filter is given in Fig. 2.6. The series-APF should behave as a controlled voltage source and its output should follow the pattern of voltage given in equation (2.9). This compensating voltage signal can be obtained by comparing the actual load terminal voltage with the desired value. These compensation signals are compared with actual signals at the terminals of series filter and the error is taken to hystresis controller to generate the required gating signal for series filter as shown in Fig. 2.6.
vlb vfa vfb vfc
Fig. 2.6 Control block diagram of series-APF
2.7 Modeling of UPQC
The three-phase system shown in Fig. 2.7 is considered for verifying the performance of UPQC. Three-phase source feeding this system at one end. For the best performance, UPQC is placed at the midpoint of the system as shown in Fig. 2.7. UPQC is placed between two sections B1and B2 of the transmission line. The complete system parameters are given in Table 2.1.
The STATCOM model in UPQC is connected in shunt with transmission line using step down transformer. the voltage can be regulated to improve the voltage stability of the power system. Thus the main function of the STATCOM is to regulate key bus voltage magnitude by dynamically absorbing or generating power to the ac transmission line.
The SSSC which is connected by series transformer with transmission line generates three-phase voltage of controllable magnitude and phase angle. This voltage injection in series with the transmission line is almost in quadrature with the line current and hence emulates an equivalent inductive or capacitive reactance in series with the transmission line. A small part of this injected voltage is in phase with the transmission line current supplying the required losses in the Inverter Bridge and transformer.
Three-phase AC source
Transmission line parameters
Resistance of the line