Phase Lock Loop Control of Matrix Converter based Dynamic Voltage Restorer for Sag Reduction

Transcription

1 Phase Lock Loop Control of Matrix Converter based Dynamic Voltage Restorer for Sag Reduction P.Nandagopal 1, R. Subramanian 2 1 College of Technology, Coimbatore 2 SNS College of Technology, Coimbatore 1 Spn.nandagopal@gmail.com, 2 deaneee@snsct.org Abstract A new series power conditioning system using a matrix converter is proposed. The proposed solution has utilized a single AC/AC power converter for interface as an conventional AC/DC/AC converter. The dynamic model is used to design a vector control system that seamlessly integrates functional of compensating load voltage and managing energy storage during voltage sag and idling modes. The purpose of matrix converter used in dynamic voltage restorer (DVR) for power quality enhancement, which is connected in between the feeder to secure the electric power from sags. The DVR is based on dqo algorithm which is discussed with matrix converter to inject external supply into the feeder by means of a series transformer. Harmonics are produced due to non-linear loads, and it is filtered out by a filter which is connected at terminal end of the system. The simulation results are carried out by MATLAB and the performance of proposed method is verified. Index Terms Dynamic Voltage Restorer (DVR), Indirect Matrix Converter (IMC), Pulse Width Modulation (PWM), Voltage Sag, Flywheel Energy Storage. I. I.INTRODUCTION The Power quality is main area should be deal in power system in modern technology for using equipment without any problems [1]. The power quality enhancement must be needed to protect the sensitive equipment from the fault which occurs at generating station and transmission lines. The power supply contains lot of power quality issues like flickers, sags, swells, harmonics, transients, and notching. In this voltage sag is main phenomena that causes vulnerable to the power system. Due to sag, the losses in machines get increases, sensitive equipment gets fail, voltage regulation will be affected, the circuit breaker trips without being overloaded, speed of motor gets vary, and makes the power system into imbalance condition. The sag causes due to lightning stroke, switching of lines, nonlinear loads like arc furnace, welding machines, weather condition, falling of trees, animal contacts, and insulation failures and short circuit fault at the distribution feeder. The EPRI contacted survey on 222 utility distributed feeders at the year between 1993 and 1995 it shows that most of the faults due to the production of sags in power system. The sag is nothing but reduction in magnitude of sinusoidal wave, which is shown in figure 1. The main aim of this paper is to reduce the sag and make the power supply into sag free. The device well known for reduction of voltage sag is dynamic voltage restorer (DVR) had been implemented as an advantage of series compensation over the shunt compensation for voltage stiff system. The mitigation of sags in sensitive equipment with high power factor is quit enhanced. The energy storage device normally required for eliminating the sag with high power factor. The series compensator, dynamic voltage restorer restore the supply voltage without sag by injecting the external source into the feeder. The injection voltage V ij must compensate both load and source voltage which is given by V ij = V load + V s (1) V Load and V s is the desired load voltage magnitude, and the source voltage during sags/swells condition

2 Fig.1. Block diagram of DVR, and sags in the supply. The active power injection at steady state when using inphase voltage injection is [8] P ij = 3(V 0 V s ) I COSФ = 3 VI COS Ф (2) To inject the voltage into the grid the DVR needed energy. The energy is taken from two type of sources, the storage source or the energy from same feeder. The energy storage system could improve the power factor over a wide range for sensitive loads rather than the supply taken from same feeder [3]. The flywheel has more advantage over super capacitor, batteries and fuel cells, which results in minimum cost, efficiency and eco-friendly. II. SYSTEM ARCHITECTURE The system architecture based on the operational conditions: which three requirements have been proposed in the fig 2. 1) To mitigate the voltage sag and recover storage energy, bidirectional power flow is requiring. 2) According to the nature of the load the system should operate properly, so it must contain wide range of power factor at converter end. 3) The converter must have capability to operate in buck mode, when the grid injection voltage is very low during fully charged flywheel. Fig. 2. System level requirements for dvr with ac/ac converter. Comparing to the existing DVR method herein the proposed method, DC link capacitor and passive components get eliminated. The current stiff method is followed by connecting a LC filter and interface the grid and converter section. A. Modeling of matrix converter The matrix converter [8] is necessary to operate as bi-directional converter or switch. The proposed matrix converter is indirect matrix converter (IMC) in which it maintained DC link voltage positive and allowing the current to be either in positive or negative. The current source inverter is connected to the source and voltage source inverter is connected to the grid. The matrix converter should operate according to the flywheel supply from rotation. To maintain the DC link voltage positive, displacement angle must be limited to (-л/6, л/6),it is normally done by an algorithm. The average dclink current (i p ) and dc-link voltage (v p ) are related to the stiff the current i u, i v, and i w and the stiff voltage v a, v b, v c by the following: (3) (i p ) = ½(m u i u + m v i v + m w i w ) (v n ) = (m a v a + m b v b + m c v c ) Where m a, m b, m c and m u, m v, m w are modulation parameters for three phase legs for the current source 1369

3 bridge and the voltage source bridge. The average input current and output voltage is given by given by i i = (3/4)m i (m 0 i 0 ) (4) (i a ) = m a (i p ) (i b ) = m b (i p ) (i c ) = m c (i p ) This phase voltage variables should be transfer to the abc into a synchronously rotating dqo pharos coordinate. As a first step the phase coordinates is transfer to dq reference frame on voltage source bridge side is given by M 0 = 2/3(m u +α m v + α 2 m w )e -jw0t I o = 2/3(i u + α i v + α 2 -jw0t (5) i w )e Where w0 is output fundamental frequency on the voltage source bridge side α = e j2л/3 In a same manner, the modulation function of space function and the voltage on the current source bridge side are given by v 0 = (3/4)m 0 (m i v i ) (9) The absence of passive components in matrix converter the input and output are related to equation 9. B. Injection Voltage on Reference Generator The DVR is connected at the point of common coupling (PCC), when the voltage sag occurs, the reference voltage v ij is set in phase with the point of common coupling v pcc is measured [5]. The sag occurred in the grid is maximized, compensation voltage range must be high, and flywheel should run in high speed. During idling condition the flywheel should not be disturbed, there by grid voltage will compensate for the load voltage. By utilizing the reactive power for load, the real power is normally at the distribution system and take real power from the system. The load voltage v load is being kept balance condition with the voltage at point of common coupling v pcc is shown in fig 3. M i = (2/3)(m a + αm b + α 2 m c ) e -jw0t V i = (2/3)(v a + αv b + α 2 v c ) e -jw0t ( 6) Where wi is the fundamental frequency on the current source bridge side, with the assumption of v a + v b + v c = 0. (7) M i v i = (2/3)(m a v a + m b v b + m c v c ) To attain space vector in term of average dc link voltage and current by substituting (6) and (8) into (3) (i p ) = (3/4)m 0 i o (v p ) = (3/2)m i v i (8) The input current and output voltage space vectors is Fig. 3. Phasor diagram of the injection voltage reference vector during idling mode. The amount of power drawn from the system is calculated by q-axis component (v q ij1) of the injection voltage v ij1. The injected reference voltage is used for sag compensation. The v q ij1 is derive from speed of the flywheel and v q ij1 is related to The.لا voltage injection to the system from the energy storage must have minimum disturbance to the load voltage magnitude. The total injection voltage reference v ij is compare with the energy 1370

4 storage feedback v ij1 and the energy feedback v ij2 and reference voltage is produce. The injected voltage reference under two different operating mode are actually adopted to merge together for band with requirement for the speed regulation is much more slower than the voltage sag compensation mode. Vij1 = 2(vload) cos( -δ)-cosφ (10) Fig.4. Control algorithm for matrix converter. The simulation is based on dqo algorithm in which system is compare with reference voltage and error voltage is produced, by converting into abc to dqo algorithm as shown in fig 4. According to the error value, pulses are generated to control the matrix converter to inject the external source into the system. The overall simulation diagram is shown in figure 6. δ = - acos (cos + sin Vij (11) n=1 VLoad ) The angle δ is solved from (18) and assumption from speed regulation Vij1 = Vij1 tan( + л+δ 2! ) (12) III. CONTROL CIRCIT DISCRIPTION The phase lock loop is implemented to compare two non idling signals and gives mean signal for protection of pulse with modulation (PWM). The PWM is control signal to control IGBT is shown in fig 4. Fig.5. Proposed architecture of the DVR system. 1371

5 Fig.6. Overall simulation diagram of DVR V, it is adequate to validate the power conditioning concept and the proposed control algorithm. The DVR is proposed [4] with filter to protect the system from sensitive load is shown in fig 5. The matrix converter is constructed by IGBT and power supply with the rating of 1.5MV it has capable to produce three phase balance voltage sag that emulate [6] the voltage sag at the point of common coupling (PCC), the wave form of grid and reference is shown in fig 7 and 8. The energy storage used in the experiment is flywheel that has coupled with induction motor and produces the source voltage. The flywheel rotate 1500 RPM/min and supply the energy of 1372

6 compensated, but there will be harmonics in the system, it may causes power quality issues. For elimination of harmonics the filters were connected in delta along the line, and harmonics were filtered out.. Fig.7. Grid voltage with sags. Fig.8. Injection voltage at matrix converter terminal. The harmonic spectra of supply voltage is analysed and level of harmonics is reduced to 1.58% which is shown in fig 12 and the FFT analysis of teminal voltage of DVR was accounted in fig 12. Fig.8. Grid reference voltage without any sags. As in simulation 4000V has to be injected to the grid by means of series transformer and the injection voltage by matrix converter wave is shown in figure 9. As per equation 1 both source and injection voltage makes equal of load voltage and it is shown in figure 1. The load which has been connected is non linear load, so nonlinear load itself causes the sag in transmission line. The DVR must be capable of eliminate such a sags in transmission line. After compensation the DVR produce an output and it was measured and shown in figure 10 [7]. The terminal end of wave form shows that the sag get completely Fig.10. Compensated voltage without any sags at point of common coupling. 1373

7 4 Transformer resistance Rs 10 KR 5 Transformer inductance Ls 12 mh 6 Line impedace mh 7 Load inductance 8 mh 8 Load resistor 0.15 ohm The time domain perfomance of the system is analyse and verified in clear manner in simulation using a simulink model. The wave forms are shows the sag period and compensation by abc reference frame shown in simulation fig 6. By the wave form it could be observe that load voltage remain undistrub during the 60% voltage sag occures at the PCC. The sag reduction is effectivly reduce by the control algorithm. The key parameters used in simulation are listed in table 1. Fig.11. Analysis of FFT. V CONCLUSION The method of DVR proposed in this paper introduces matrix converter, which neglects DC link based topology as used in back to back converter. The implementation of phase lock loop which maintains the pulse width modulation for control the matrix convertem is propsed. The closed loop method propsed, to improve the dynamic performance of the system. The modeling and control circuit is based on dqo algorithm, in which sags are effictivily reduced by the DVR. It will quikily and provides excellent voltage regulation in power supply. The balance and unbalance voltage level gets regulated by approprite voltage is injecting and compensate the power supply. By this method, grid supply is maintained constant for all time. The advantages of DVR are low cost, simple control circuit and it can mitigate long duration voltage sags efficently. The simulation of DVR using MATLAB/SIMULINK has been presented and verified. Fig.12. THD Analysis of harmonic. IV. NUMERICAL VERIFICATION Table I-List of Key Parameters Used in Simulation 1 Filter capacitor 30 micro farad 2 Filter inductance 1.2 milli farad 3 Line impedance mh 1374

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