CHAPTER 3 DESIGN OF A PV-UPQC SYSTEM FOR VOLTAGE SAG AND SWELL COMPENSATION

Transcription

1 21 CHAPTER 3 DESIGN OF A PV-UPQC SYSTEM FOR VOLTAGE SAG AND SWELL COMPENSATION INTRODUCTION The recent increase in the use of non-linear loads creates many power quality problems such as oltage sag, swell and current disturbances. Voltage sag is one of the major power quality problems, which may cause equipment tripping, malfunction or shut down of domestic and industrial equipments. These effects could be expensie for both customers and utilities as discussed by Eldany et al (2001). Hirofumi et al (1998) has proposed a UPQC system to mitigate the sag. Wong et al (2004) uses a parallel actie filter to compensate the imbalance, reactie power, neutral current and harmonics of the source. Photooltaic energy has great potential to proide power supply with minimum impact on the enironment, since it is clean and pollution free as discussed by Kuo Liang et al (2001). When the power supply crisis started in the recent past, PV generation became a ery popular alternatie for fossil fuel electrical power generation. A large number of solar cells connected in series and parallel constitute solar arrays. One way of using photooltaic energy is, in a distributed energy system, as a peaking power source. There are many control strategies reported in the literature to determine the reference alues of the oltage and the current of UPQC. The Fuzzy Logic Control (FLC) for the control of UPQC method has been deeloped by Singh et al (1998). Bulent Irmak et al (2009) presented Power Quality Distributed-Voltage (PQD-V) using ANN controller. The drawback of the existing research work is that they address only current

2 22 and oltage harmonics problems. But, there is ery meagre discussion of other prominent power quality problems like deep sag, swell and interruption. This chapter presents the design and deelopment of the proposed PV-UPQC system. By using instantaneous p-q control theory techniques long with PI and hysteresis band controller, the mitigation of oltage sag and swell under different balance and unbalanced load conditions are simulated. The use of a PV array for maintaining constant DC link oltage is another distinguishing feature of the PV-UPQC system. With these functions, the PV-UPQC is suitable for connecting at the PCC. 3.1 PV-UPQC POWER CIRCUIT AND ITS FUNCTIONS The PV-UPQC power circuit structure is shown in figure 3.1. It consists of major components such as shunt and series inerters, DC link capacitor with PV array, the shunt and series inerter controllers, shunt and series filters and series and shunt coupling transformers. The basic function of the system is the compensation of oltage and current disturbances. The function of the series inerter is to compensate the oltage, when there are supply side disturbances, such as oltage sag, swell and unbalance. The series part of the PV-UPQC consists of a series inerter connected on the DC side to the energy storage capacitor with PV array. On the AC side, it is connected in series with the feeder through the series Low Pass Filter (LPF) and coupling transformers. The series LPF preents the switching frequency harmonics produced by the series VSI entering the system. The coupling transformers connected in series proide oltage matching and isolation between the network and the series inerter. The series inerter injects compensation oltages in series with the supply oltages, such that the load oltages are balanced and undistorted and their magnitudes are maintained at the desired leel.

3 Figure 3.1 Proposed power circuit diagram of PV-UPQC system 23

4 24 Based on the measured supply and load oltages, the control scheme of the controller generates appropriate switching signals for the series inerter switches. The series inerter is controlled in oltage-control mode, using the PWM switching technique. In order to produce the injected oltage of desired magnitude, phase shift and frequency, the desired signal is compared with a triangular signal of higher frequency and appropriate switching signals are generated. The DC link capacitor is alternately connected to the inerter outputs with positie and negatie polarity. The switching harmonics present in the output oltages of the series inerter are filtered out by the series low pass filter. The amplitude, phase shift, frequency and harmonic contents of injected oltages are controllable. The function of shunt inerter is to compensate load current harmonics, reactie power and regulate the DC-link oltage between both inerters. The shunt part of the PV-UPQC consists of a shunt inerter connected to the common DC storage capacitor with PV array on the DC side. On the AC side, it is connected in parallel with the load through the shunt interface inductor and shunt coupling transformer. The shunt interface inductors along with the shunt filter capacitor are used to filter out the switching frequency harmonics produced by the shunt inerter. The shunt coupling transformer is used for matching the shunt inerter oltage and network oltage. In order to achiee compensation objectie, the shunt inerter filter injects currents at the PCC so that the reactie and harmonic components of the load currents are cancelled and the load current unbalance is eliminated. The injection current is proided by the DC link capacitor. Based on the measured current and oltage, the control scheme generates the appropriate switching signals for the shunt inerter switches.

5 25 The particular currents and oltages to be measured depend on the applied control strategy and the shunt inerter is controlled in current control mode. The appropriate inerter switches are turned on and off at certain time instances so that the currents injected by the shunt filter tracks some reference currents within a fixed hysteresis band according to the compensation objecties. This band refers to the constant bandwidth of the upper and lower hysteresis current. The inerter switches alternately connect the DC link capacitor to the system, either in the positie or negatie sequence. When the DC capacitor oltage is connected in the positie sequence, it is added to the supply oltage and the inerter current increases. When DC capacitor is connected in the negatie sequence, it is obsered that the oltage is opposite to the supply oltage which reduces the inerter current. Alternately increasing and decreasing the current within the hysteresis band results in generating the reference current. The DC side capacitor seres two main purposes. Primarily, it maintains the DC oltage with a small ripple in the steady state. Secondly, it seres as an energy storage element to supply the difference between the actie load and source power during the transient period. The aerage oltage across the DC link capacitor is maintained constant so that the shunt inerter filter can draw a leading current. This oltage has to be higher than the peak of the supply oltage. The proposed system can switch oer to islanding mode operation during interruption and supply the power to the load. 3.2 DESIGN OF PV-UPQC POWER CIRCUIT The design of PV-UPQC power circuit includes the following three main parameters: Shunt interface inductors DC link reference oltage

6 26 DC link capacitor The design of the shunt interface inductor and the DC link reference oltage are based on the following criteria. The first criterion is the limiting of the high frequency components of the injected currents. The second criterion is that the instantaneous rate of change of current generated by the shunt filter should be greater than the rate of change of current of the harmonic component of the load. This ensures the proper harmonic cancellation. On the other hand, a higher alue inductance is preferable for a better harmonic cancellation and reactie power compensation. Howeer, ery high alue will result in slow dynamic response of the shunt compensator. In this situation, an effectie solution has to be explored. A higher DC link reference oltage results in a higher rate of change of the shunt compensator current, better dynamic response and reactie power compensation performance. Howeer, it increases the stress in the inerter switching deices. Again, an efficient solution has to be adopted. The DC link capacitor size is selected to restrict the DC oltage ripple within reasonable limits. The DC oltage ripple is determined by both the reactie power to be compensated and the actie power supplied by DC capacitor during the interruption Design of PV-UPQC Volt Ampere rating of Shunt and Series Inerters Volt Ampere (VA) rating of series and shunt inerters of PV-UPQC determines the size of the PV-UPQC. The power loss is related to the VA loading of the PV-UPQC system. Figure 3.1(a) shows the loading calculation of shunt and series inerters of PV-UPQC with the presence of PV at its DC link. The linear load is deigned based on fundamental

7 27 frequency. Figure 3.1(a) VA rating phasor diagram of PV-UPQC where, V s1 and I s1 = supply oltage and supply current V s2 and I s2 = supply oltage and supply current during interruption V L1 and I L1 = load oltage and load current V L2 and I L2 = load oltage and load current during interruption I C1 and I C2 = compensating current of reactie components of inerters Io - output current, Z SHI shunt inerter impedance The load oltage is to be kept constant at V o Per unit (pu), irrespectie of the supply oltage ariation: V =V =V =V =V pu (3.1) The load current is assumed to be constant at the rated alue: I =I =I =I pu (3. 2) Assuming that the PV-UPQC system to be lossless, the actie power demand in the load remains constant and is drawn from the source: From phasor diagram 3.1(a) V I =V I cos (3. 3)

8 28 where V I = Input power of inerter (Dc link power) V I cos = output power of inerter (AC) In case of an interruption: V = (1-x)V s1 = V (1-x) pu (3.4) where, x is per-unit sag. To maintain constant actie power under the oltage sag condition I = ( ) = pu (3.5) Therefore series inerter VA (S sein ) rating equals to Vo I o (x cos ) Ssein VinjI s2 pu (3.6) 1- x Injected oltage through shunt inerter is V = V V = V V (1 x) (3.7) Injected current through shunt inerter is I = I + I 2I I cos (3.8) The injected current in terms of I is gien by I = I + ( ) 2I ( ) = I + ( ) 2I ( ) = I 1 + ( ) 2 ( )

9 29 = I ( ) ( ) ( ) = I ( ) { ( )} ( ) The load oltage V L2 is gien by V = V + V (3.9) = V (1-x) + V V (1 x) = V (1-x) + V (V V x) = V (1-x) + V (V 2V x + V x ) = V (1-x) + V 1 (1 x) = V [(1-x) + 1 (1 x) ] (3.10) Shunt inerter VA (S ) rating equals to S = V I + P LOSS = V I + I Subsisting the alues of V L2 and I C2 aboe equation and simplify S = V [(1-x)+ 1 (1 x) ] I 0 [ + I 1 x 2 +cos x 1 ] = ( ) {[(1-x)+ 1 (1 x) ] [ (1 x) + cos {1 2(1 x)}] } +(I ) Simplify the aboe equation = ( ) { 1 [(1-x)+ cos {1 2(1 x)]}+ (I )

10 30 S = ( ) (1 x) + cos {1 2(1 x)} (1 x) cos (1 2(1 x) + Io X Z 2 SHI pu (3.11) (1- x) The proposed system highlights the efficient and imperatie design of power factor and VA loading of inerter. The VA loading of inerters is calculated for the occurrence of supply oltage sag from 10 to 90% and power factor ariations from 0.6 lagging to unity. The range of supply oltage sag has been chosen so that most practical cases can be ealuated in this range. 3.3 CONTROL STRATEGY OF THE PV-UPQC SYSTEM There are many control strategies reported in the literature to determine the reference alues of the oltage and the current of UPQC. The concept of instantaneous actie power (p) and reactie power (q) and its application in shunt filter reference current generation was introduced by Akagi et al (1998).The p-q theory introduced by Tan Zhili et al (2006), has been modified into a single-phase p-q theory by Khadkikar et al (2009). The synchronous reference frame theory has been discussed by Guorong et al (2007), symmetrical component transformation has been presented by Ghosh et al (2004) and Unit Vector Template (UVT) technique by Singh et al (2010). The Fuzzy Logic Control (FLC) for the control of UPQC method has been deeloped by Singh et al (1998). Based on the aboe discussion, p-q theory with hysteresis current control mode is suitable for parallel mode operation of PV-UPQC system and p-q theory with PWM oltage control mode is suitable for interruption mode operation. The hysteresis control method is simple to implement and it has enhanced system stability, increased reliability and mitigates power quality problems.

11 31 The proposed PV-UPQC consists of two main controllers as follows: sag and swell compensation using series inerter controller Shunt inerter controller Controller strategies are designed based on two operation modes, interconnected (parallel mode) and islanding modes. In the interconnected mode, the source and the PV array jointly supply power to the load. While in the islanding mode, the PV arrays alone supply power to the load. 3.4 SAG AND SWELL COMPENSATION USING SERIES INVERTER CONTROLLER The series inerter control component of the controller injects the appropriate oltage to the load during oltage sag and swell, such that the load oltage becomes balanced, distortion free and hae the desired magnitude. Theoretically, the injected oltage can be of any arbitrary magnitude and phase. Howeer, the power flow and deice rating are important issues that hae to be considered when determining the magnitude and the phase of the injected oltage. The PV-UPQC system ensures that the phase of the injected oltage is maintained 90 in adance with respect to the supply current, so that the series compensator consumes no actie power in steady state. In the second case, the PV-UPQC injected oltage is in phase with both the supply oltage and current, so that the series compensator consumes only the actie power, which is deliered by the shunt compensator through the DC link. In the case of quadrature oltage injection, the series compensator requires additional power capacity, since the shunt compensator Volt Ampere (VA) rating is reduced as the actie power consumption of the

12 32 series compensator is minimized. It compensates part of the reactie power demand by the load. The series compensator does not compensate for any part of the reactie power demand of the load and it has to be entirely compensated by the shunt compensator. The shunt compensator must proide the actie power injected by the series compensator. It can be concluded that PV-UPQC system is the optimum solution for the actie power supply, phase angle matching and supply of reactie power. An approximate sub-optimal control strategy for the PV-UPQC system to minimize the losses in operation has been proposed. This approach of generating the reference oltage is based on the analysis of the fundamental frequency. The load oltage (V L ) is equal to the sum of the source oltages ( V ) and the oltage injected by the series filter (V f ). S VL VS Vf a jb (3.12) where, a + j b - Vector of unity magnitude of the supply current To compensate for supply oltage sag, swell and distortion, some additional component has to be added. The reference oltage (V ref ) alue is calculated using reference filter oltage (V f *), which is obtained by subtracting the positie sequence fundamental component from the disturbed source oltage. The function of the series inerter is to compensate the oltage disturbances like sag and swell on the source side, which are due to the fault in the distribution line. The series inerter control calculates the reference oltage to be injected by the series inerter. This compares the positie-sequence component with the disturbed source oltage. The reference oltage for PWM switching of the series inerter is obtained from the proportional control and the feed forward control. The control equation needed to calculate the reference oltage is gien in (3.13). Figure

13 shows the configuration of the series inerter control, which is based on this equation. Figure 3.2 Block diagram of the series inerter control V ref = [(V s * -V s ) V f ] x k + V f * (3.13) where V ref - Reference oltage, V s - Source oltage, V f - Injected oltage V f * - Reference injected oltage V s * - Positie-sequence oltage In this case, when the PV- UPQC control strategy is applied, the injected oltage is in phase with the supply oltage. Hence, the load oltage is in phase with the supply oltage and there is no need for calculating the angle of the reference load oltage. Thus, the reference load oltage is determined by multiplying the reference magnitude (which is constant) with the sinusoidal template phase-locked to the supply oltage. Then, the reference series filter oltage is obtained using the expression (3.13). Using the optimum of the different techniques for calculating the

14 34 reference oltage of the series compensator, the oltage rating of the series compensator is considerably reduced. 3.5 SHUNT INVERTER CONTROLLER The effectieness of the shunt inerter basically depends on the design characteristics of the current controller. The method implemented to generate the reference current and reference oltages is the use of instantaneous p-q control theory techniques. The control scheme of a shunt inerter calculate the current reference waeform for each phase of the inerter, maintain the DC link oltage constant and generate the inerter gating signals. The current reference circuit generates the reference currents required to compensate the load current harmonics and reactie power. The p-q control theory, firstly, transformation of the oltage and currents from the abc to 0 coordinates. The Clarke transformation and its inerse of three-phase generic oltages are gien by 0 = 2 3 1/ / 2 1/ 2 3 / 2 1/ 2 1/ 2 3 / 2 a b c (3.14) a b c = 2 3 1/ 1/ 1/ / 2 1/ / 2 3 / 2 0 (3.15) Similarly, three-phase generic instantaneous line currents, i a, i b and i c can be transformed to the 0 axes. One adantage of applying the 0 transformation is to separate zero-sequence components from the abc-phase components. The and -

15 35 axes do not contribute to zero sequence components. No zero-sequence current exists in a three-phase three-wire system, so that zero sequence current (i 0 ) can be eliminated from the aboe equations resulting in simplification. If the three-phase oltages are balanced in a three-phase three-wire system, no zero-sequence oltage is present, so that zero sequence oltage ( o ) can be eliminated. Howeer, when zero-sequence oltage and current components are present, the complete transformation has to be considered. The p-q theory defines three instantaneous powers in three phase systems, with or without neutral conductor p 0 p = q V i 0 i i (3.16) where, p o - instantaneous zero-sequence power, p - instantaneous real power, q - instantaneous imaginary power There are no zero-sequence current components in three phase threewire systems, that is, i o = 0. In this case, only the instantaneous powers defined on the -axes exist, because the product o i o is zero. Hence, in three-phase three wire systems, the instantaneous real power p represents the total energy flow per unit time, in terms of components. The instantaneous actie and reactie powers for a three-phase three-wire system are defined as:

16 36 p q - i i (3.17) where, p Actie power, q -Reactie power and -Transformation of oltages The three-phase oltages are transformed from a-b-c to and ice ersa using the following transformation relations: frame / 2 3 / 2 1/ 2 3 / 2 a b c (3.18) a b c = / 2 1/ / 2 3 / 2 (3.19) The same transformation matrices are used for the transformation of currents, from equation (3.19), the current i and i are expressed as: i i = p q (3.20) where, i and i -transformation of currents both actie power( p) and reactie power (q) defined aboe are composed of two components. p q p~ q ~ p q (3.21)

17 37 where, and represent the oscillating and aerage parts actie power and represent the oscillating and aerage parts reactie power The p-q theory has a prominent merit of allowing complete analysis and real time calculation of arious powers and respectie currents inoled in a three-phase circuit. Further, knowing the alues of undesirable currents in a circuit in real time allows us to eliminate them. For instance, if the oscillating powers were undesirable, by compensating the currents i a and i a of the load and their corresponding currents in b and c phases the compensated current drawn from the network would become sinusoidal. It can be easily shown that, i a - (i a +i a ) produces a purely sinusoidal waeform. This is one of the basic ideas of shunt inerter filtering. The shunt inerter of PV-UPQC can be controlled in two ways: The shunt inerter reference current is tracked. The shunt inerter current is used as feedback control ariable and compared with the load current and the shunt compensator reference current is calculated from it. The reference current is determined by calculating the actie fundamental component of the load current and subtracting it from the load current. This control technique inoles both the shunt inerter and load current measurements. The shunt inerter can also be controlled by tracking the supply current, when it is used as the feedback ariable. In this case, the shunt actie filter ensures that the supply reference current is tracked. Thus, the supply reference current is calculated rather than the current injected by the shunt actie filter. The supply current is often required to be sinusoidal and in phase with the supply oltage. Since the waeform and phase of the supply current is only known,

18 38 its amplitude needs to be determined. The hysteresis current control technique inoles only the supply current measurement. Therefore, it has been used in the PV-UPQC simulation model. Figure 3.3 shows the configuration of shunt inerter control, which includes the current control for harmonic compensation and the output oltage control in source oltage interruption mode. In normal operation, the shunt control calculates the reference alue of the compensating current from the harmonic current and the reactie power, considering the power loss (P loss ) due to the system and inerter operation. This loss should be compensated to maintain the DC link oltage during operation of the series inerter. The reference alue of the compensating current is deried using equation (3.22). i i * * = ~ p p q loos (3.22) where, V,V - Transformed reference oltages i*, i* - compensating currents Here, the reference oltage is determined using (3.23) and (3.24). The reference oltage (V* 1 ) is expressed by the sum of the source oltage (V s) and the filter current difference I PF calculated by the PI controller. I I * I (3.23) PF PF PF V * 1 k p IPF k I IPFdt (3.24) If the shunt inerter is assumed to generate the reference oltage for each period of power frequency, the transfer function of the filter current can be deried as in (3.25).

19 39 IPF (k P / LPF)s (k I/LPF ) (3.25) * 2 I s {(k R) / L }s (k /L ) PF P PF I PF where, I PF, I* PF - filter currents kp, k I- proportional constant The oltage reference determined by using positie-sequence detector extracts the positie-sequence component from the disturbed three-phase source oltage in figure3.4. This detector deries the transformation of reference oltages V', and V' S, based on the - -0 transformation. The measured source oltage passes through the Phase- Locked Loop (PLL) and the sine wae generator to calculate the fundamental component of the transformation currents i' = sin( 1t) and i' = cos ( 1 t). The calculated actie power p s ' and reactie power q s ' includes the positie-sequence fundamental component of the source oltage V S. So, the instantaneous alue of the positie-sequence component is calculated as gien in equation (3.26). ' V ' V '2 i 1 '2 i ' i ' i ' i - i ' p ' s q ' s (3.26) The two functions of the shunt inerter are to compensate the current harmonics and to supply the actie power to the load during oltage interruption. When the oltage interruption occurs, the operation mode is changed from normal compensation mode to interruption mode. The PV array proides the actie power to maintain the load oltage constant. The shunt inerter starts to perform the oltage and current control using the PI controller.

20 40 Figure 3.3 Block diagram of shunt inerter control Figure 3.4 Block diagram of positie sequence oltage detector 3.6 AVERAGE DC LINK VOLTAGE REGULATION The regulation of DC link oltage (V dc ) is one of the tasks in PV- UPQC, since the injecting oltage (V inj ) depends on the regulated oltage of DC link capacitor. The proposed PI controller regulates the DC link oltage and reduces the harmonics in the inerter. The PI approach is used to supply reference current determination and it is based on the fact that the magnitude of the supply current depends on power balance between the

21 41 supply and the load. The DC link capacitor seres as energy storage element. If the shunt actie filter losses are neglected, in steady-state, the power supplied by the system has to be equal to the real power demand of the load and no real power flows into the DC link capacitor. The aerage DC capacitor oltage is thus maintained at reference oltage leel. If the power unbalance caused by a load change occurs, the DC link capacitor must supply the power difference between the supply and load that will result in reducing the DC capacitor oltage. To restore the aerage DC capacitor oltage to the reference leel, some actie power has to be supplied to the DC capacitor. For this propose, the supply current has to be increased. When the aerage DC capacitor oltage increases, the magnitude of the supply current has to be decreased. The amplitude of the supply current is automatically controlled by controlling the aerage oltage across the DC link capacitor. The DC oltage regulation is achieed by using a PI controller. The capacitor oltage is compared with some reference alue and a PI controller processes the oltage error. The output of the PI controller is the magnitude of the reference supply current and it is constant in steady-state. To get the source reference current, a sinusoidal template that is in phase with the supply oltage is multiplied by this magnitude. Applying this concept, the numbers of current sensors are reduced resulting in the simplification of the control circuit. Therefore, this control technique has been chosen to be used in the PV-UPQC simulation model DC Link Voltage Regulation Using Fuzzy Logic Control The oltage regulation in the UPQC DC link using fuzzy logic control has been presented by Singh et al (1998). The structure of a complete fuzzy control system is composed from the blocks, namely, fuzzification, knowledge base, inference engine and defuzzification. The fuzzification module conerts the crisp alues of the control inputs into

22 42 fuzzy alues. Inputs to the fuzzy controller are categorized as arious linguistic ariables with their corresponding membership alues such as low, medium and high, where each is defined by a gradually arying membership function. In fuzzy set terminology, all the possible alues that a ariable can assume are named unierse of discourse and the fuzzy sets coer whole unierse of discourse. The shape of fuzzy sets can be triangular and trapezoidal. A fuzzy controller conerts a linguistic control strategy into an automatic control strategy and fuzzy rules are constructed by knowledge database. Initially, measured DC link oltage Vdc and the input reference oltage V dc-ref are the input ariables of the fuzzy logic controller. Then, the output ariable of the fuzzy logic controller is presented by the control current. The control scheme consists of three phase sine wae generator for reference current generation and generation of switching signals. The peak alue of reference currents are estimated by regulating the DC link oltage. The actual capacitor oltage is compared with a set reference alue. The error signal is then processed through a fuzzy controller, which contributes to zero steady error in tracking the reference current signal. 3.7 VOLTAGE CONTROL IN DC LINK CAPACITOR In oltage source inerter, the DC oltage has to be maintained at a certain leel to ensure the DC to AC power transfer. Because of the switching and other power losses inside PV-UPQC system, the oltage leel of the DC capacitor will be reduced if it is system is interrupted. Thus, the DC link oltage control unit is intended to keep the aerage DC bus oltage constant and equal to a gien reference alue. The DC link oltage control is achieed by adjusting the real power supply by the PV array. This real power is adjusted by changing the amplitude of the fundamental component of the reference current. The PV array source

23 43 proides some actie current to recharge the DC capacitor. Thus, in addition to supplying the reference current, the shunt actie filter has to supply some amount of actie current as compensating current. This actie compensating current flowing through the shunt actie filter regulates the DC capacitor oltage Voltage Control in DC Link Capacitor using ANN Controller The DC link capacitor oltage control by using ANN controller in Power Quality Distributed-Voltage (PQD-V) is discussed by Bulent Irmak et al (2009). The detection of the disturbance signal with the reference signal of the controller is the prime requirements for the desired compensation in case of PQD-V. The ANN is made up of interconnecting artificial neurons. It is essentially a cluster of suitably interconnected nonlinear elements of ery simple form that possess the ability to learn and adapt. It resembles the human brain in two aspects: Knowledge is acquired by the network through the learning process Interneuron connection strengths are used to store the knowledge. These networks are characterized by their topology, the way in which they communicate with their enironment, the manner in which they are trained and their ability to process information. ANN is being used to sole AI problems without necessarily creating a model of a real dynamic system. For improing the performance of a PQD-V, a multilayer feed forwardtype ANN-based controller is designed. This network is designed with three layers, the input layer with 2, the hidden layer with 21 and the output layer with 1 neuron, respectiely. The large data of the DC-link current and interals from the conentional method are collected. These data are used for training the NN. The actiation functions chosen are tan sigmoid for input and hidden layers and pure linear in the output layer, respectiely. This multilayer feed forward-type NN works as a

24 44 compensation signal generator. The ANN is shown in figure 3.5, the training algorithm used is Leenberg Marquardt back propagation (LMBP). Figure 3.5 Exploded diagram of the artificial neural network. In order to sustain the constant frequency in the utility, Singh et al (1998) hae utilized the Fuzzy Logic Controller based constant frequency UPFC. A Constant Frequency (CF) UPQC is composed of a UPQC and a matrix conerter based frequency changer. UPQC is a combination of series and shunt actie filters. The series and shunt actie filters hae been employed to compensate the oltage, current imbalance and harmonics. The Frequency Conerter (matrix conerter) has been used to control the supply frequency when it exceeds the power quality limit. To oercome aboe limitation, a PI controller is used for determining the magnitude of this compensating current from the error between the aerage oltage across the DC capacitor and the reference oltage. The PI controller has a simple structure and fast response. A simple linear control technique is applied to calculate the DC capacitor aerage oltage error by using proportional gain control and the proportional coefficient. Here, the expression used for calculation of the

25 45 proportional coefficient is obtained through integration of a first-order differential equation. Howeer, the formula deriation for the proportional coefficient is not that simple for a three-phase PV-UPQC, a residual steady-state error occurs with a proportional controller only. 3.8 DEVELOPED PV-UPQC SYSTEM SIMULATION MODELS This section discusses the proposed system model, which includes the following: Series inerter and shunt inerter models Series inerter and shunt inerter control system models PV array with control model Figure 3.6 shows the oerall PV-UPQC models implemented using Matlab /Simulink software. They consist of a series inerter and a shunt inerter models and its control models and the PV array with control model. The system is connected in distribution system. In PV-UPQC model, three phase inerters are connected to the common DC link capacitors with photooltaic array. This oltage source is an external source supplying DC oltage to the inerter for AC oltage generation. The PV-UPQC is installed in the distribution system, as shown in figure 3.6. The design considered for the deelopment of PV-UPQC system Simulink models include circuit topology, conersion efficiency, maximum load power and power quality.

26 46

27 SIMULATION PARAMETERS The following circuit parameters used in the simulation is gien in Table 3.1 Table 3.1 PV-UPQC simulation parameters Source Voltage 230Vrms(325Vpeak) 50Hz Impedance R=0.04, L=0.4mH DC-Link Capacitor C1=2000µf Reference Voltage 400V Shunt Inerter Filter L, C mh,20µf Switching Frequency. 10kHz Series Inerter Switching Frequency. 10kHz Filter L, C mh, 140µF PV Array Power 10kW 3.10 RESULTS AND DISCUSSION The performance of PV-UPQC is ealuated in terms of load balancing, unbalancing and mitigation of oltage sag and swell under different load conditions. The simulation results of the Simulink model (from figure 3.6) are represented in figures 3.7 to 3.11, which indicate the performance of the proposed system for seen different cases. In case 1, the system is in normal operation, three-phase oltages and currents are sinusoidal and balanced. In case 2, three-phase balanced oltage sag results in 30% decrease from the nominal alue.

28 48 While in case 3, single-phase sag results in 30% decrease from nominal alue. Sag eent occurs in phase A. As the other phases are not affected, both phases B and C keep their nominal alue. In case 4, the three-phase unbalanced sag in phase A is 30% and it is 20% in phase C resulting in the decrease from their nominal alues. As the phase B is left in normal operation, nominal alue of the phase B remains unchanged. In case 5, three-phase balanced oltage swell results in 15% increase from nominal alue. In case 6, single-phase swell occurs resulting in 15 % increase from nominal alue. While swell eent occurs in phase A, other phases are not affected and both phase B and C keep their nominal alue. In case 7, with the unbalanced swell mode, phase A oltage increases by 15% and phase C oltage increases by 10% from their nominal alues. As the phase B is left in normal operation, nominal alue of the phase B remains change. Case 1: Normal Operation Balanced Supply with Linear Loads In case 1, normal operation mode is simulated. Under normal conditions, three-phase oltages are sinusoidal under balanced condition and the simulation result is illustrated. The peak load oltage measured is 325V and it is depicted in figure 3.7(a) and the load current is shown in figure 3.7(b).

29 49 Figure 3.7 (a) Load oltage during normal operation Figure 3.7 (b) Load current during normal operation Case 2: Balanced Voltage Sag In case 2, the simulation is conducted considering balanced oltage sag with and without PV-UPQC. The occurrence of a three-phase fault results in 30% of sag and the oltage decreases from its nominal alue between the period 0.1s and 0.2s in all phases. The simulation is illustrated through figures 3.8(a) to 3.8 (d). The output load oltage is kept constant

30 50 (325V) by using PV-UPQC. Figure 3.8 (e) load oltage sag compensation with PQD-V using ANN controller. Figure 3.8 (a) Source oltage with sag Figure 3.8 (b) Load oltage sag compensation with PV-UPQC using PI

31 51 Figure 3.8 (c) source current with sag Figure 3.8 (d) Load current with PV-UPQC Figure 3.8 (e) Load oltage sag compensation with PQD-V using ANN

32 52 Figure 3.8 (f) Load oltage sag compensation with UPQC using FLC Case 3: Single Phase Voltage Sag In case 3, the single-phase oltage sag is ealuated with and without PV-UPQC. The occurrence of a single-phase fault results in 30% of sag and the oltage decreases from its nominal alue. Sag eent occurs only in phase A. Consequently, as other phases are not affected, both phase B and C keep their nominal alue, as show in figures 3.9 (a) to 3.9 (d). The output load oltage is kept constant (325V) with PV-UPQC. Figure 3.9 (a) Single-phase source oltage with sag

33 53 Figure 3.9 (b) load oltage with PV-UPQC Figure 3.9 (c) Single-phase source current with sag

34 54 Figure 3.9 (d) Load current with PV-UPQC Case 4: Unbalanced Voltage Sag In case 4, the simulation is conducted considering three phases unbalanced sag with and without PV-UPQC. In this case, the three phase unbalanced sag in phase A is 30% and it is 20% in phase C resulting in the decrease from their nominal alues between the period 0.1s and 0.2s as shown in figures 3.10 (a) to 3.10 (d). Figure 3.10 (a) unbalanced sag source oltage

35 55 Figure 3.10 (b) Load oltages with PV-UPQC Figure 3.10 (c) Source current with unbalanced Sag

36 56 Figure 3.10 (d) Load current with PV-UPQC Case 5: Balanced Voltage Swells In case 5, the simulation is conducted considering balanced oltage swell with and without PV-UPQC. The occurrence of a three-phase fault results in 15% of swell and the oltage increases from its nominal alue between the period 0.1s and 0.2s in all the phases. The simulation is illustrated through figures 3.11 (a) to 3.11 (d). The output load oltage is kept constant. Figure 3.11 (a) Source oltage with swell

37 57 Figure 3.11 (b) Load oltages with PV-UPQC Figure 3.11 (c) Source current with swell Figure 3.11 (d) Load current with PV-UPQC

38 58 Case 6: Single Phase Voltage Swells In case 6, the single-phase oltage swell is ealuated with and without PV-UPQC. The occurrence of a single-phase fault results in 15% of swell and the oltage increases from its nominal alue. Swell eent occurs only in phase A. Consequently, as other phases are not affected, both phase B and C keep their nominal alue, as show in the figures 3.12(a) to 3.12(d). The output load oltage is kept constant. Figure 3.12 (a) Single-phase source oltages with swell Figure 3.12 (b) Load oltages with PV-UPQC

39 59 Figure 3.12 (c) Single-phase source current with swell Figure 3.12 (d) Load current with PV-UPQC Case 7: Unbalanced oltages swell In case 7, the simulation is conducted considering three phases unbalanced swell with and without PV-UPQC. In this case, the three phase unbalanced swell in phase A is 15% and it is 10% in phase C resulting in the decrease from their nominal alues between the period 0.1s and 0.2s as shown in figures 3.13(a) to 3.13 (d). The output load oltage is kept constant.

40 60 Figure 3.13 (a) unbalanced source oltage with swell Figure 3.13 (b) Load oltages with PV-UPQC Figure 3.13 (c) Unbalanced source current with swell

41 61 Figure 3.13 (d) Load current with PV-UPQC Table 3.2 Comparison of percentage load oltage compensation for Power Quality Distributed-Voltage (PQD-V) by Bulent Irmak et al (2009) and proposed PV-UPQC. Percentage of load Percentage of load Case No Eents oltage compensation for oltage compensation for PQD-V with ANN PV-UPQC with PI 1 30% sag 3 phase with balanced 2 30% sag 3 phase with unbalanced 3 15% swell 3 phase with balanced 4 15% swell 3 phase with un balanced

42 62 For comparatie analysis, 30% sag and 15% swell are considered for both PQD-V with ANN system and PV-UPQC system. As shown in table 3.2, the maximum percentage load oltage compensation achieed by the PV-UPQC system is 3.5% higher than that of the PQD-V with ANN system. It has been shown that the proposed system has better performance in comparison with the PQD-V. The comparisons are also shown in figures 3.14 and Figure 3.14 Percentage load oltage compensation of PQD-V and PV- UPQC with sag Figure 3.15 Percentage of load oltage compensation of PQD-V and PV-UPQC with swell

43 63 Table 3.3 Comparison of source oltages and load oltages with and without PV-UPQC Case No Eents Phase A Phase B Phase C With out PV- UPQC source oltages [V rms](olts) Ph A Ph B Ph C With PV-UPQC Load oltages [V rms](olts) 1 Normal Normal Normal % sag 30%sag 30% sag % sag Normal Normal % sag Normal 20% sag %swell 15%swell 15%swell %swell Normal Normal %swell Normal 10%swell Ph A Ph B Ph C Table 3.3 shows oltage sag and swells with balanced and unbalanced supply with and without PV-UPQC. The system restores the load oltage to normal alues. A comparison of the ealuation conducted considering oltage sag and swells is shown with and without PV-UPQC. In addition, figures 3.16 and 3.17 highlight the accomplished results of enhancement of power quality by the proposed system. Figure 3.16 Load oltages with and without PV-UPQC with sag

44 64 Figure 3.17 Load oltages with and without PV-UPQC with swell On the basis of the research work presented in this chapter, a paper entitled Improed Power Quality Using Photooltaic Unified static Compensation Techniques has been published in the Asian Power Electronics Journal. Vol.4 No.2, pp Aug On the basis of the research work presented in this chapter, a paper entitled Implementation of UPQC for Voltage Sag Mitigation, - International Journal of computer communication and information system, Vol.2, No.1, pp , Dec CONCLUSION In this chapter, the power circuit structure, design considerations and the controller design is discussed. It can be obsered that the aerage DC oltage regulation method turned out to be the most feasible control solution that accomplishes two tasks simultaneously. It controls the oltage across the dc link capacitor and also determines the amplitude of the supply current. Among the all control techniques, the hysteresis current control technique is the most preferable for the shunt inerter controller. The hysteresis control method leads to enhanced system stability, compared to

45 65 ANN and FLC controllers The PV-UPQC design helps in the deelopment of the simulation model and controllers. With this controller, the DC link oltage will be maintained stiff by controlling PV array DC-DC conerter. The PV-UPQC system has been simulated in different sag and swells conditions. The results obtained show that the proposed system has the ability to compensate oltage sag and oltage swell. The effectieness of the simulation results has been erified under balanced and unbalanced load conditions. By comparing PQD-V system with respect to IEEE and IEC standards the proposed PV-UPQC system gies much improed performance. The simulation results show that the proposed system gies more efficiency and better dynamic response. The main adantage of the proposed system is that the control strategy enables it to produce balanced oltage waeforms.