2 Asst Prof, Dept of EEE, G.Pullaiah College of Engineering and Technology, Kurnool, AP, India,

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1 ISSN Vol.04,Issue.04, April-2016, Pages: To Mitigate Voltage Fluctuations and Power Quality in High-Level Penetration of Distributed Generation Systems by using D-STATCOM with Positive-Sequence Admittance and Negative-Sequence Conductance TARIGOPULA SANTIRAMUDU 1, K. JAGADEESH 2 1 PG Scholar, Dept of EEE, G.Pullaiah College of Engineering and Technology, Kurnool, AP, India, santiramudu17@gmail.com. 2 Asst Prof, Dept of EEE, G.Pullaiah College of Engineering and Technology, Kurnool, AP, India, jagadeesh_smsk@yahoo.com. Abstract: Voltage fluctuations resulting from variable output power of renewable energy sources are strictly challenging power quality in distributed-generation systems. This paper presents a control method for distributed static synchronous compensator (D-STATCOM) to alleviate variation of both positive and negative-sequence voltages. The D-STATCOM simultaneously operates as fundamental positive-sequence admittance and fundamental negative-sequence conductance to restore the positivesequence voltage to the nominal value as well as reduce the negative-sequence voltage to an allowable level. Both admittance and conductance are dynamically tuned to improve voltage- regulation performances in response to load changes and power variation of renewable sources. A proportional resonant current regulator with selectively harmonic compensation is realized to control the fundamental current of the D-STATCOM as well as reduce the harmonic current, which could be an advantage in practical applications due to high voltage distortion in lowvoltage micro grids. Voltage-regulation performances are discussed for different D-STATCOM locations as well as different D-STATCOM currents. Computer simulation test validate effectiveness. Keywords: Distributed STATCOM (D-STATCOM), Micro Grid, Voltage Fluctuations, Voltage Imbalance, Reactive Power. I. INTRODUCTION Global concerns about the environment and fossil fuels continue to advance the development of renewable energy systems, such as wind turbines, photo voltaic, fuel cells, etc. The micro grid concept was proposed to intelligently coordinate various renewable energy sources (RESs) into distribution networks for both grid-connected and islanding operations. Increasing the use of RESs could help relieve network congestion, reduce system losses, and defer infrastructure in- vestments. These issues have received much attention recently, and numerous projects have been commissioned to demonstrate and evaluate functionality of micro grids by worldwide research organizations, for example, Consortium for Electric Reliability. Voltage regulation in the power system could be realized by using an on-load tap changer (OLTC) or a static VAR compensator (SVC) at substations, and a step voltage regulator or a switched capacitor on feeders. With the help of the so-called optimal or intelligent control on all devices, the voltage profile could be improved on a real-time base. Thanks to the advancement of semiconductor technologies, voltagesource- converter-based solutions, such as static synchronous compensator (STATCOM), unified power flow controller (UPFC), distributed STATCOM (D-STATCOM), and active power filter (APF), become viable in practical applications. STATCOM technology has been extensively studied and developed in transmission systems to regulate voltage by adjusting its reactive power into the power system, whereas UPFC was designed to control real- and reactive-power flows between two substations. On the other hand, D-STATCOM and APF are suitable for power quality improvement of the distributed power system, such as harmonic compensation, harmonic damping, and reactivepower compensation. Fig1. Simplified Thévenin equivalent circuit of the DG system. A D-STATCOM for compensating voltage fluctuations of load bus was presented. In this project, voltage regulation was conducted by injecting reactive current into the utility? However, regulation performances may suffer from controlling error due to either imbalanced voltage or 2016 IJIT. All rights reserved.

2 harmonic distortion. Fundamental positive- and negativesequence currents were separately controlled to improve the voltage- regulation performances of the D-STATCOM. However, negative-sequence compensation may not work properly as the imbalanced source is nearby. A harmonic damping active filter was proposed to restore the voltage swell due to distributed generators. However, discussions were limited in controlling positive-sequence voltage only. The concept of inverter-based RESs with functionality of VAR supporting was presented to accomplish voltage regulation locally. Although RESs are currently not allowed to actively regulate the voltage at the point of common coupling (PCC), this operation may be viable in the future because supplying reactive power by customers with tariff reimbursement will benefit the utility for reducing equipment investment as well. Compensating voltage fluctuations in DG systems by a D-STATCOM was presented in this project; we present extended simulations and discussions as well as experimental verification. The proposed D-STATCOM realizes positive- sequence admittance and negative-sequence conductance to regulate positive-sequence voltage as well as suppress negativesequence voltage. Both positive-sequence admittance and negative-sequence conductance are dynamically adjusted according to positive-sequence voltage deviation and imbalanced-voltage percentage. Therefore, voltage quality can be maintained at an allowable level in case of variation of DGs or loads. A proportional resonant (PR) current regulator with selective harmonic compensation is implemented to control the fundamental current of the D- STATCOM as well as reduce harmonic current due to high voltage distortion in low-voltage networks. Theoretical analysis of voltage regulation with supporting results from simulations and experiments validates the proposed approach. TARIGOPULA SANTIRAMUDU, K. JAGADEESH functions developed. In this project, the D-STATCOM is used to regulate voltage at the point of common coupling. Fig2. Basic configuration of D-STATCOM. The basic configuration of D-STATCOM as shown in fig: 3.4, it consists of three phase voltage source inverter using IGBT, DC voltage source and connected to the distribution line by coupling transformer. The D-STATCOM operates as fundamental positive-sequence admittance and negativesequence conductance as given (1) Where is the reference current, and are the fundamental positive sequence and fundamental negativesequence voltage. And are the fundamental positivesequence admittance and negative-sequence conductance are defined variable control gains to furnish regulating positivesequence voltage and suppressing imbalanced voltage. II. OPERATION PRINCIPLE OF D-STATCOM The basic diagrams of DSTATCOM system connected as a shunt compensator. DSTATCOM system consists of a standard three-phase Insulated Gate Bipolar Transistor (IGBT) based three legs VSC bridge with the input ac inductors and a dc energy storage device to obtain a selfsupporting dc bus. A three-phase ac source with line impedance feeds power to balanced/unbalanced linear and non-linear load. Suitable adjustment of the phase and magnitude of the D-STATCOM output voltages allows effective control of active and reactive power exchanges between the D-STATCOM and the ac system. The VSC connected in shunt with the ac system provides a multifunctional topology which can be used for up to three quite distinct purposes: 1. Voltage sag mitigation, voltage regulation and compensation of reactive power 2. Correction of the power factor 3. Elimination of harmonics. A dynamic model of DSTATCOM is developed in MATLAB environment to simulate its behavior. The design approach of the control system determines the priorities and Fig3. Power circuit of D-STATCOM. A. Generation of Reference current According to synchronous reference frame (SRF) transformation the control is actualized by using low-pass filter (LPF) to filter out ripple components and the positive sequence voltage is obtained. The negative-sequence voltage is determined by the combination of LPF and a band rejected filter tuned at the second order frequency. The quadrature fundamental positive-sequence voltage and negative-sequence voltage is available by applying

3 To Mitigate Voltage Fluctuations and Power Quality in High-Level Penetration of Distributed Generation Systems by using D-STATCOM with Positive-Sequence Admittance and Negative-Sequence Conductance reverse transformation. The positive-sequence current is Where and are the fundamental integral gain and equal to multiplied by and negative-sequence current is equal to multiplied by respectively. Thus, the I is generated as a current command. A dc voltage control is also designed for secure operation of the D-STATCOM. The fundamental current produced by the PI regulator which is in phase with the positive-sequence voltage to maintain the dc voltage V dc at the reference value. its frequency, respectively; represents a proportional gain; and and represent the harmonic frequency and its integral gain, respectively. The damping ratio ξ has tuned current regulator it introduce various narrow gain peaks at the harmonic frequencies to reduce current distortion and a narrow gain peak centered at the fundamental frequency for fundamental current tracking systems. Fig: 5. shows the current-loop block diagram, in which PWM and digital signal processing delay are considered. T represents a sampling period. Further discussions on current control are provided in the simulation section. Fig: 5. Current-loop block diagrams. C. Tuning Control The tuning control of both and as shown in fig.5 And are defined as by using LPF and SQRT operation they can approximately calculated, where LPF designed with cut-off frequency =10 HZ to filter out ripple components in the calculation. Fig4. Block Diagram of D-STATCOM. B. Current Control A current regulator produces the voltage command based on the current command the measured voltage E and measured current I for space vector pulse width modulation (PWM) control of the inverter. The transfer functions H h(s) and H f(s) as given as Fig6. Thévenin equivalent circuit with the D-STATCOM compensating positive-sequence current and negative-sequence current. Then, a PI regulator is actualizing to generate maintain at nominal value *. Similarly by controlling of the imbalanced voltage could be suppressed and maintained at an allowable level. The percentage of voltage unbalance factor (%VUF) is to estimate the level of imbalance voltage. %VUF is defined as the ratio of negative-sequence voltage to the positivesequence voltage and as given as (4). to (2)

4 TARIGOPULA SANTIRAMUDU, K. JAGADEESH (3) %VUF= (4) Basically, three control loops in the proposed system. The bandwidth of the current control loop, which is depends on the switching frequency of the inverter. The current command is generated by the tuning of both admittance and conductance to improve the power quality. so their band widths are lower than that of current loop. To control both admittance and conductance by the tuning of PI parameters with suitable transient response and zero steady state error. Due to inverter loses the voltage on the dc capacitor will fluctuate and imbalanced voltage suppressed by conductance. Lower the dc capacitance, larger fluctuations will happen. Generally, the bandwidth of dc voltage control is lowest due to large capacitance in the system. Fig7. Tuning control of and. D. Phasor Analysis: In this section, D-STATCOM operation will be discussed based on phasor analysis. Fig8. Positive & Negative sequence phasor diagrams. Fig6 show the Thévenin equivalent circuit with the proposed D-STATCOM compensating positive-sequence current as well as negative-sequence current. Before the D- STATCOM starts operation, positive sequence voltage is swelled up, as shown in the red vectors of Fig: 8. is obviously larger than. This results from the voltage drop on line impedance Z = R + j when is injected into the grid. On the other hand, the negative- sequence current flowing on the line impedance Z causes the negativesequence voltage drop, as shown in the red vectors. When the D-STATCOM draws =., the blue vectors of Fig: 8. shows that could be restored to the nominal value (the dashed line) by due to being 90 0 lagging with respect to. In addition, phase leading of after compensation is dependent on the line impedance Z and realpower injection Similarly, the D-STATCOM performs negative-sequence conductance reciprocal of resistance, to provide low impedance for negative-sequence current, thus reducing negative-sequence voltage. As shown in the blue vectors of Fig: 8, the D-STATCOM draws =. To mitigate negative-sequence voltage by =.Z accordingly, could be maintained at an acceptable value by variable. The acceptable value is represented by a- dotted circle. As a consequence, we conclude that positivesequence voltage could be restored by introducing an active admittance (or inductance) and that negative-sequence voltage could be suppressed by emulating an active conductance (or resistance). III. SIMULATION STUDIES A radial line rated at 23 kv and 100 MVA in Fig: 9. are established by using the so-called alternative transient program to illustrate Voltage Fluctuations and verify the

5 To Mitigate Voltage Fluctuations and Power Quality in High-Level Penetration of Distributed Generation Systems by using D-STATCOM with Positive-Sequence Admittance and Negative-Sequence Conductance effectiveness of the proposed D-STATCOM. Since the grid voltage at the end of a radial line is most sensitive to injection of both real and reactive powers based on load low analysis, the D-STATCOM is proposed to be installed at the end of the line. Table I and II list line and load data respectively. The D-STATCOM parameters are given as follows. 1. PWM frequency: 10 khz. 2. The reference fundamental positive-sequence voltage and the reference voltage imbalance factor are set as * = 1.0 p.u. and %VUF* =2.0%, respectively. 3. Current controller: = = 40 (for h =5, 7, 11, and 13), =25, and ξ = Tuning controller: PI parameters for * ( = 0.001, = ) and for %VUF( =10and = 0.05). 5. Voltage base: 23 kv, current base: 2510 A, and impedance base: 5.29 ohm. B. Frequency-Domain Analysis Fig: 11. Show frequency responses of the current control, including open and closed loop gains. Fundamental current tracking capability is assured by a resonant gain at the fundamental frequency. Various resonant gains at the 5th, 7th, 11th, and 13th frequencies are introduced to reduce harmonic current. The phase margin of the designed current loop approaches 70. Fig: 11(f) shows that D-STATCOM currents are almost maintained as sinusoidal waveforms. This could confirm the functionality of harmonic reduction because the nonlinear load at Bus 4 results in severely distorted line voltages (THDa = 3%, THDb = 2.5%, and THDc = 3.7%). Note that the inverter-based DG is assumed to be installed at the end of the bus, and also all single-phase loads are connected between phases a and b to generate severe voltage variation as well as voltage imbalance. The power of the DG is controlled by a PI regulator in the SRF to produce the current command. Similar to the current control of the D- STATCOM, resonant current control is realized to regulate the output current of the DG. The control of the DG has been sufficiently studied in other publications, so we will not repetitively discuss this issue in this project. A. Steady State Operation Before the D-STATCOM starts operation, Fig: 9. shows that bus voltages are significantly swelled and imbalanced due to the DG and single-phase loads. Voltage fluctuation is getting worse toward the end of the line. For example, = 1.06 p.u. and %VUF = 5.1% at Bus 5. Table I summarizes and %VUF for all buses. When the D-STATCOM is initiated with compensation of the positivesequence voltage only ( = 0), on each bus could be restored to the nominal value, as listed in Table II. At this time, the D-STATCOM is operated at = 0.37 p.u. with rms currents = = = 0.37 p.u. However, Fig: 11(b) shows that voltage fluctuation is still significant due to imbalanced voltage. After imbalance suppression is activated, Fig: 11(c) shows that bus voltages are clearly recovered from fluctuation. Table III illustrates that and %VUF could be maintained below the presetting level (1.0 p.u. and 2%) on all buses. As shown in Fig: 11(f), the D-STATCOM consumes imbalanced currents = 0.52 p.u., = 0.25 p.u., and = 0.35 p.u. with = 0.37 p.u. and = 9.6 p.u., respectively. Fig9. Simulation circuit. Fig10. Block Diagram of Simulation.

6 TARIGOPULA SANTIRAMUDU, K. JAGADEESH (a) three phase AC voltage. (d) Current at D-STATCOM off. (b) Voltage at D-STATCOM off. (e) Current at D-STATCOM on. (c) Voltage at D-STATCOM on. (f) Positive & Negative sequence Voltages and Current at D-STATCOM on.

7 To Mitigate Voltage Fluctuations and Power Quality in High-Level Penetration of Distributed Generation Systems by using D-STATCOM with Positive-Sequence Admittance and Negative-Sequence Conductance TABLE: I: BUS VOLTAGES BEFORE THE D- STATCOM IS STARTED TABLE: II. BUS VOLTAGES AFTER THE D-STATCOM ONLY COMPENSATES POSITIVE-SEQUENCE VOLTAGE (g) Reactive Power Q at D-STATCOM on. TABLE: III. BUS VOLTAGES AFTER THE D- STATCOM COMPENSATES BOTH POSITIVE AND NEGATIVE-SEQUENCE VOLTAGES (h) DC voltage at D-STATCOM on. IV. DISCUSSIONS A. D-STATCOM Location In this section, voltage-regulation performances are evaluated considering the D-STATCOM at different locations. Fig: 9. shows and when the D- STATCOM is deployed at Buses 2, 3, 4, and 5, respectively. At the installation point, and can be clearly maintained at 1.0 p.u. and 2%, respectively. Regulating performances on the left side of the installation point are better than those on the right side. Installing the D- STATCOM at the end of the line provides the best performances of voltage regulation on the entire line, while voltage fluctuations could not receive much improvement if the D STATCOM is closed to the voltage source. This result absolutely complies with other studies. Note that imbalance suppression is no longer needed ( = 0) when the D- STATCOM is located at Bus 2 due to 2%. lower than (i) Reference Voltage Fig11. Simulation Test Results. at D-STATCOM on. B. D-STATCOM Current We will concentrate on the required D-STATCOM current for various levels of and. For convenience, percentage positive-sequence voltage derivation is defined. Thus, both positive-

8 TARIGOPULA SANTIRAMUDU, K. JAGADEESH sequence current and negative sequence current could STATCOM and SVC can be integrated together to reduce be displayed in the same figure the required power rating of the D-STATCOM. (5) D-STATCOM currents for and in the range of 0% 5% larger and are required to comply with the stricter standards of and. This result could help estimate the required current rating of the D-STATCOM to reduce voltage fluctuations being up to a certain level for a given feeder. Fig: 12. D-STATCOM currents with respect to and. C. Cooperative Control of D-STATCOM Since the proposed D-STATCOM is intended for voltage regulation in the distributed power system, it is worth discussing on the cooperative control with other equipment, such as OLTC and SVC. First, the coordination of the D- STATCOM with them is presented, and then, the operation of multiple D-STATCOMs is considered. OLTC: OLTC is usually installed at the substation to regulate the grid voltage by moving the transformer tap. Its response is too slow to cope with voltage fluctuations resulting from power variation of DGs. On the contrary, the proposed D-STATCOM is able to inject reactive power to maintain the grid voltage at an acceptable level with faster response time than OLTC. Accordingly, a low-frequency communication between OLTC and D-STATCOM can be established to reduce the power rating of the D-STATCOM after OLTC starting operation. SVC: SVC is constructed by Thyristor switches, capacitor banks, and inductors. The compensation of reactive power can only be adjusted in a stepped manner. On the other hand, the proposed D-STATCOM can change reactive-power compensation continuously. Therefore, both the D- Multiple D-STATCOMs: When multiple D-STATCOMs are installed together, the PI-based voltage control may not work properly. Instead, the droop control method needs to be developed to adjust the voltage command. Generally, the voltage command is designed to droop according to the rated KVA capacity of the D-STATCOM. Thus, various D- STATCOMs can evenly share reactive power. V. CONCLUSION This project has presented a control method of the D- STATCOM to alleviate voltage fluctuations in high-level penetration of DG systems. Together with positivesequence admittance to recover the positive-sequence voltage, negative-sequence conductance is implemented to cooperatively improve imbalanced voltage. A tuning control is designed to dynamically adjust admittance as well as conductance commands to maintain both positiveand negative-sequence voltages at an allowable level in response to power variation of DGs or loads. The voltage regulation performances of the D-STATCOM deployed at different locations have also been investigated. The termination installation D-STATCOM is the best option to suppress voltage fluctuations. For example, large clusters of current-controlled DGs are usually connected at the end of the lateral in the distributed power system. Finally, the cooperative control of the D-STATCOM has been discussed. By establishing a low frequency communication, the D- ST AT COM can work together with both OLTC and SVC to regulate the grid voltage. Thus, the rated kilo volt ampere capacity of the D-STATCOM can be significantly reduced. In addition, multiple D-STATCOMs are able to cooperatively provide reactive power compensation under the help of the so called droop control. Various D-STATCOMs can evenly share workload according to their kilo volt ampere (KVA) rating. VI. REFERENCES [1] R. Lasseter, Micro grids, in Proc. IEEE Power Eng. Soc. Winter Meeting, 2002, pp [2] F. Katiraei, R. Iravani, N. Hatziargyriou, and A. Dimeas, Micro grids management, IEEE Power Energy Mag., vol.6, no.3, pp , May/Jun [3] Consortium for Electric Reliability Technology Solutions (CERTS), US2010.[Online]. Available: [4] Department of the New Energy and Industrial Technology Development Organization (NEDO), Japan, [Online] Available: lish/index.html. [5] C. L. Masters, Voltage rise: The big issue when connecting embedded generation to long 11 kv overhead lines, Inst. Elect. Eng. Power Eng. J., vol. 16, no. 1, pp. 5 12, Feb

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