Analysis of AC-DC Converter Based on Power Factor and THD

Transcription

1 Website: (SSN , SO 900:008 Certified Journal, Volume 3, ssue, February 03) Analysis of AC-DC Converter Based on Power Factor and THD Shiney.S.Varghese, Sincy George Department of Electrical Engineering, Fr. C.R. nstitute of Technology, Vashi. Abstract Harmonic distortion and low power factor in power systems caused by power converters have been a great concern. To overcome these problems several converters and control schemes have been proposed in recent years. This work is proposed to identify the power converters with low cost and high efficiency for three phase systems. n this paper a comparative analysis of improved quality three phase AC to DC converters has been investigated using MATLAB SMULATON. Analysis has been done to verify that the one cycle controlled AC/DC converter has improved quality with respect to power factor and THD as compared to diode rectifiers and thyristor controlled rectifiers. Simulation results of three types of AC o DC converter are presented in this paper.. NTRODUCTON Three phase rectifiers have a wide range of application like electrochemical processes, arc furnaces, adjustable speed drives etc.[]. Diode rectifiers and thyristor bridge converters were traditionally employed to obtain dc voltage from ac utility. The problems with these converters were that they used to pollute the utility with low order harmonics, which are difficult to filter []. The current spectrum of three phase converters consists of odd multiples, in pairs of 6n, with decaying amplitude for increasing order, where n is the harmonic order [3]. With thyristors, instead of the diodes, the firing angle delays the start of the conducting of the current. This will affect the active and reactive power taken from the supply, i.e. the power factor. t is observed that the power factor is low, resulting in more current being drawn from the utility. As the distortion increases, the THD (Total Harmonic Distortion) is indefinitely large. n an electric power system, a load with a low power factor draws more current for the same amount of useful power transferred. The higher currents increase the energy lost in the distribution system, and require larger wires and other equipment. Nonlinear loads create harmonic currents in addition to the original (fundamental frequency) AC current. The simplest way to control the harmonic current is to use a passive filter. This filter reduces the harmonic current, so that the non-linear device looks like a linear load. The power factor can be brought to near unity. The filters require large-value high-current inductors, which are bulky and expensive. However using PWM ac to DC converter by controlling the switching of switching device, it is possible to achieve nearly unity power and sinusoidal input current. This paper compares the various circuit of AC to DC converters and analyses the PWM bidirectional converter working on One Cycle Control technique [4]. The PWM control structure does not require a PLL, thus making its operation simple. Detailed simulation studies are carried out on the mentioned converters to verify the effectiveness of the proposed one cycle control scheme.. POWER FACTOR AND THD FOR AC-DC CONVERTER The distortion of the normal sine wave by non-linear loads is created by harmonics. Harmonics are related to the fundamental frequency and are defined as whole number multiples of the fundamental frequency. THD of a signal is a measurement of the harmonic distortion present and is defined as the ratio of the sum of all harmonic components of the voltage or current waveform compared against the fundamental component of the voltage or current wave. THD 3... N N N N () Where N is the magnitude of N th order harmonic component of current. Power factor is a measurement of how efficiently a facility uses the electrical energy and is given as: PF Distortion pf Displaceme nt pf () Where, Displaceme nt pf = Cos (3) Non-linear loads have large values of THD, and cause considerable distortion to the normal sine wave. The more the sine wave gets distorted, the lower the total power factor becomes. Usually, total power factor is associated only with the phase displacement of the voltage waveform to the current waveform, but harmonics also affect the total power factor. 350

2 Website: (SSN , SO 900:008 Certified Journal, Volume 3, ssue, February 03) Harmonic problems are generally caused by non linear loads such as adjustable speed drives, arcing devices, electronic ballast and switching power supplies. They can cause the nearby equipments to malfunction, voltage distortion and trigger a resonance with the utility. The relation between distortion power factor and THD is given by: Distortion pf = THD Distortion power factor takes into account the harmonic currents that do not contribute to the real work produced by the load where as displacement power factor relates to the displacement between the system current and voltage waveform. Based on these two parameters the following AC/DC converters are analyzed. a. Three phase diode rectifiers A six-pulse uncontrolled diode rectifier with a dc load, R l is shown in Fig.Three phase diode rectifiers are often used in ndustry to provide the dc input voltage for motor drives and dc-to-dc converters [5]. These rectifiers are extremely robust and present low cost, but draw nonsinusoidal currents or reactive power from the source, deteriorating the electrical power system quality [6]. Fig. Six-pulse uncontrolled diode rectifier t is seen that the fundamental component of current is in phase with the supply voltage and hence the displacement power factor is unity. Harmonic currents present in supply current consist of 3,5,7,9..order of harmonics with 3 5, 5 7 magnitude,,... Hence the distortion power 3 7 factor will be low which results in poor power factor. The harmonic current injections affect the power system by distorting the bus voltage at the point of common coupling. These aspects have a negative influence on both power factor and power quality. The current THD for diode rectifiers is usually high at 30% and the power factor is 0.954[6]. t is seen that although the displacement power factor is unity, the distortion power factor is high due to large harmonic content resulting in low power factor. (4) 35 Hence if we use diode rectifiers for high power application, filters are to be used to improve power factor so as to make input current a sinusoidal one. B. Three -phase controlled rectifier The thyristor valves are used for conversion of AC into a controlled DC and thus are the central component of any HVDC converter station. They are also used in various classes of railway rolling systems so that fine control of the traction motors can be achieved. A phase controlled rectifier is accomplished by replacing the diodes in a 6- pulse rectifier with thyristors. Since a thyristor needs a triggering pulse for transition from nonconducting to conducting state, the phase angle at which the thyristor starts to conduct can be delayed. A six pulse controlled rectifier using thyristor is shown in Fig. Fig. Six-pulse controlled bridge rectifier With firing angle =0, the input current waveform for controlled and uncontrolled rectifier will be the same. As is increased, distortion in the current waveform also increases. As it is known that displacement power factor for thyristor rectifier will be coshence total power factor will be low. As seen in three phase diode rectifier, harmonic currents present in supply current consist of 3,5,7,9..order of harmonics. Hence the distortion power factor will be high which results in poor power factor. The poor power factor causes high apparent current and the absolute harmonic currents are higher than those with a diode rectifier. Hence, in the case of controlled rectifiers, both capacitor banks and passive filters are required to make the input current nearly sinusoidal. C. PWM (pulse width modulation) AC-DC converter PWM converters shift the frequency of the dominant harmonics to a higher value where they can be easily filtered. The control structure of three-phase six switch PWM converter consists of an outer voltage control loop and an inner current control loop. The current controller senses the input current and compares it with a sinusoidal current reference. To obtain this current reference the phase information of the utility voltages or current is required.

3 Website: (SSN , SO 900:008 Certified Journal, Volume 3, ssue, February 03) This information is obtained by employing either a phase lock loop (PLL) or a current phase observer digital technique [8]. To simplify the control structure of these grids connected system, one cycle-control (OCC) based ac-to dc converters have been opted. A three phase PWM AC-DC converter with a resistive load R L is shown in Fig 3. The saw tooth waveform is compared with the a phase supply current (i sa ). At every rising edge of the clock pulse the source current increases (i sa is greater than saw tooth magnitude) switch S is on. The expression for rising slope (K ) of the source current signal is as follows []: Where, K Rs ( Vs Vo L Rs-Shunt current sensing gain, Vs-Single phase source voltage, Vo- Averaged dc link output voltage L-Boost nductors (5) Fig. 3 Three phase PWM AC-DC converter [3] D. One cycle control scheme for PWM AC to DC converter Working of a three phase PWM converter using one cycle control is explained in this section.pwm AC to DC converter can be used where we need a regulated or controlled DC output. The advantage of PWM rectifier compared to thyristor controlled rectifier is that while regulating the output voltage, it also maintains the input current nearly sinusoidal thus improving power factor near to unity. The control block diagram of one cycle controller for phase a is shown in Fig. 4. The dc link voltage Vo is sensed and compared with the desired value Vo*.The error (Vo*- Vo) is processed by a proportional integral (P) controller to generate Vm.A bipolar saw tooth waveform of amplitude Vm and having a time period of Ts is synthesized, using an integrator. Fig. 4 Control scheme for one cycle controlled converter Fig. 5 Phase currents along with saw tooth waveform for one cycle control method [3] At every rising edge of the clock pulse switches S, S4, S6 are turned on, and the source current increases. The comparator compares the inductor current with the saw tooth waveform and it determines the turning on of S, S3, and S5. Here the value of drooping source current slope is given by [] Rs ( Vs Vo K L (6) The logic of generating the switching pulses using one cycle control method is shown in Fig 5. As seen the comparator compares the source current (i A, i B, i C ) which is scaled using R s with the saw tooth to determine the switching pulses for the six switches. 35

4 Website: (SSN , SO 900:008 Certified Journal, Volume 3, ssue, February 03). SMULATON RESULTS To analyze the performance of the various AC to DC converters, detailed simulations are carried out on MATLAB platform. The converters are compared in terms of input current, power factor and THD. A. Three phase diode rectifiers The six-pulse diode rectifier shown in Fig is simulated in simulink. n this circuit, six diode switches are used to obtain a constant dc output voltage. The parameters used for simulation are as shown in Table : TABLE PARAMETERS USED FOR SMULATNG THREE PHASE DODE RECTFER Supply voltage(phase-neutral) Source voltage frequency DC link capacitor Load Resistance 30 V 50 Hz 500 F t is seen that the input source current has two peaks which is due to the high value of the capacitor. As the value of the capacitor is increased further, more distortion is observed. From the harmonic spectrum, it can be analyzed that a high amount of harmonic currents is present in the supply current consisting of 3,5,7,9..order of harmonics. TABLE MEASURED VALUES OF VAROUS PARAMETERS OF DODE RECTFER Active Power(P) Apparent Power (S) The simulation results of diode rectifier show that the input current is peaky in nature. n this case the displacement power factor is unity but due to harmonics the distortion power factor is low. The total power factor is seen to 0.98 with the THD of 06.7%. as shown in TABLE B. Three phase controlled rectifier Consider a three phase controlled rectifier shown in Fig.The circuit is same as the three phase diode rectifier where the diode are replaced by thyristor valves. For simplicity capacitors are not connected and the firing angle is assumed to be 30 degrees. The parameters used for simulation are as given in Table : TABLE PARAMETERS USED FOR SMULATNG THREE PHASE CONTROLLED RECTFER Supply voltage(phase-neutral) Source voltage frequency P.F 30 V 50 Hz Current THD % Load Resistance 30 Firing angle 30 degrees Fig.6 Waveform of source current (phase a) Fig.8 Waveform of source current (phase a) Fig.7 Harmonic spectrum of input current (phase a) 353

5 Website: (SSN , SO 900:008 Certified Journal, Volume 3, ssue, February 03) These harmonic currents are injected back into the supply system where they interact adversely with power system equipments like capacitors, transformers, motors causing additional losses, overheating and overloading. TABLE V MEASURED VALUES OF VAROUS PARAMETERS OF THREE PHASE CONTROLLED RECTFERS Active Power(P) Apparent Power (S) P.F Current THD% Fig. 9 Harmonic spectrum of input current (phase a) The simulated result of the three phase controlled rectifier shows that the nature of input source is non sinusoidal and the fundamental component is lagging the voltage. t is seen from the harmonic spectrum, that a high amount of odd multiples of harmonic current is present in the supply % Due to the presence of high amount of harmonics in supply current the distortion power factor is quite low. The total power factor is seen to 0.73 with the THD of 53.7%. Fig.0. Simulation Circuit Diagram of OCC based ac-dc Converter C. One cycle controlled ac-dc converter PWM converters are used to overcome the problem of low order harmonic as observed in three phase controlled rectifier. Consider the three phase six pulse boost converter shown in Fig 3 for simulation. The control scheme consists of P controller, clock, integrator and a flip flop. Using the dc link information and the control scheme, switching pulses are obtained for the six switches. Simulation of PWM converter with its control circuit is shown in Fig.0. The parameters used for simulation are given in TABLE V. 354

6 Website: (SSN , SO 900:008 Certified Journal, Volume 3, ssue, February 03) TABLE V PARAMETERS USED FOR SMULATNG THE ONE CYLE CONTROLLER BASED AC-DC CONVERTER Supply voltage(phase-neutral) Source voltage frequency nductance of boost reactor DC link capacitor Switching frequency One cycle controller proportional gain(kp) One cycle controller proportional gain(ki) 30 V 50Hz mh 500 F 0kHz Fig. 3 Harmonic spectrum of input current (phase a) As seen from the harmonic spectrum of input current the current THD is very low as compared to diode and thyristor rectifiers. TABLE V MEASURED VALUES OF VAROUS PARAMETERS OF ONE CYCLE CONTROLLED AC TO DC CONVERTER Active Power(P) Apparent Power (S) P.F Current THD 400 nput voltage and current waveform(phase a) % n PWM controlled AC-DC converter (by one cycle control), it is seen that the supply voltage and input current are in phase. The voltage and current being in phase the displacement power factor is unity. Due to presence of low harmonics (8.6%), the distortion power factor is high. As seen the resultant power factor is high at Fig. Waveform of nput voltage and current (phase a) Current waveforms in phases a, b, and c Fig. Current waveforms in phases a, b, and c The input voltage and current waveform (Fig. ) shows that one cycle controlled converter draws a near sinusoidal input current while providing a regulated output dc voltage. V. CONCLUSON PWM converters are employed to overcome the problem of low order harmonics as observed in thyristor bridge converter, which pollute the utility. By using one-cycle controlled method for ac-to-dc converter, the control structure can be simplified as this control technique does not require the service of the PLL. n this scheme the switching frequency of the power semiconductor devices is held constant, which is an added advantage for medium and high power applications. The simulation results prove the effectiveness of control technique where the usage of filters can be eliminated. REFERENCES [] Rohit Gupta,Ruchika, A Study of AC/DC Converter with mproved Power Factor and Low Harmonic Distortion nternational Journal on Computer Science and Engineering (JCSE) [] Dharmraj.V.Ghodke,Kishore Chatterji,B.G Fernandez Modified One cycle controlled Bidirectional High Power Factor AC-to DC, EEE Transaction on ndustrial Electronics,Vol 55,No-6,June

7 Website: (SSN , SO 900:008 Certified Journal, Volume 3, ssue, February 03) [3] Keyue MSmedley,Slbodan Cuk One cycle controlled of switching Converters, EEE Transaction on Power Electronics,Vol 0,No- 6,November 995 [4] K. L. Lian Member, B. K. Perkins, and P. W. Lehn, Harmonic Analysis of a Three-Phase Diode Bridge NSERC,University of Toronto [5] ABB drives Technical guide No. 6,Guide to harmonics with AC drives [6] Shweta Srivastava, Sanjiv Kumar, Comparative analysis of improved quality three phase ac/dc converter nternational Journal of Emerging Technology and Advanced Engineering (SSN , Volume, ssue9, September 0 [7] Sreeraj.E.S, Kishore Chatterjee Power Factor mprovement in One Cycle Controlled Converter EEE SE 006, July 9-, 006, Montreal, Quebec, Canada [8] Rajesh Ghosh, G. Narayanan, Control of Three-Phase, Four-Wire PWM Rectifier, EEE Transaction on Power Electronics,Vol 3,No-,January