m ite ed e t i s c microcontroller-based power factor ... · such as- capacitor bank, synchronous...

Microcontroller based Power Factor iMProveMent by using switched single caPacitor

DUET Journal 21 Volume 4, Issue 1, December 2018

Microcontroller-based Power Factor Improvement using Switched Single Capacitor

Md. Raju Ahmed*, Md. Humayan Kabir Khan, and Ashish Kumar Karmaker

Department of Electrical and Electronic Engineering, Dhaka University of Engineering & Technology, Gazipur, Bangaldesh

ABSTRACT

Low power factor due to the lagging load causes high power losses and resulting less efficiency. Although, several methods are existing for Power Factor Improvement (PFI). However, this paper presents a new technique which uses single large shunt capacitor as a replacement for a bank of switching capacitors. Bidirectional switch is used to change the voltage across capacitor & compensating capacitor current, resulting in improved power factor. Firstly, the proposed system is simulated using Proteus software and then practically implemented it using microcontroller. It is found that the proposed PFI system attains unity power factor for lagging loads regardless of load current & power factor. In addition, the proposed system will also protect the load against over current and under voltage.

*Corresponding author’s email: [email protected]

1. InTRoduCTIon

Almost all the loads used for industrial and house hold appliances are inductive in nature which leads to the power factor lagging. Low power factor increases power losses. There are lot of organizations and researchers working to improve the power factor to minimize the losses and maximize the efficiency [1]. Low power factor does not affect household appliances greatly but huge difficulty occurs in industry. As it is known that, most of the industrial loads are inductive in nature and produces reactive power which makes the power factor to be lower. Power factor improvement (PFI) unit is mainly responsible for compensating reactive power to lower the losses and to increase the efficiency. The user of the electricity has to pay the extra bill for the low lagging power factor. Reactive power compensation is necessary to improve the power quality of the ac supply systems.

There are numerous methods exist for power factor improvement. Such as- capacitor bank, synchronous condensers, phase advancers and compensators etc. Traditional PFI unit consists a number of capacitors in parallel which connected to the load through magnetic contactor [2-4]. The number of capacitors is determined by the required kVAR. By using these methods, always the power factor cannot be maintained to unity and also the magnetic contactor shows frequent technical problems [5].

A research demonstrated that, efficient use of Static VAR Compensators can improve under voltage & low power factor problems in a substation designed with ETAP

software [6]. Single-phase PFC consists of two active switches, an inductor and a small buffering capacitor can improve input power factor to 0.98 and efficiency up to 93.90% showed in a research [7]. The reactive power compensation scheme complies the MATLAB model proposed by the authors comprises thyristor-controlled SVCs which can eliminate harmonics along with low power factor improvement [8].

Another scheme proposed by a research consists of an active PFC system using a power MOSFET in boost converter demonstrate that it will perform better than a scheme with a large capacitor for improving power factor [9]. In another research performed on single phase PFC explored that, PSO based PWM control strategy is more efficient to provide unity power factor, lower level of harmonics & well-synchronized output voltage than Bang-Bang control strategy [10]. Shwehdi and Sultan [11] suggested some mathematical calculations for power factor correction and reactive power requirement of the system along with the capacitor size estimation. Celtekligil [12] discussed how the dynamic PFC and voltage regulation can be applied in a light rail transportation system. Choudhury [13] demonstrates a design and implementation of a cost-effective PFI unit for small signal low power loads. The entire process of the design segmented as modeling of small signal load, selection of appropriate capacitors & design of switching circuits. Shahid and Shabir [14] offered the design of PFI unit using PLC to achieve the desired efficiency with relatively reduced cost. The solution utilizes an algorithm to trigger switching capacitors in order to compensate excessive reactive components and thereby improved power factor. An analysis and simulation have been performed



by Sharma and Haque [15] in case of metal halide high intensity discharge lamps. In the analysis, modified boost converter using active devices was proposed along with PI controller to stabilize the control loops. Allah [16] proposed an automatic PFC based on Alienation technique designed by Alternative Transient Program (ATP) & MATLAB that can calculate the original power factor by continuous monitoring. This technique can also fix the necessary number of capacitor banks to acquire the desired power factor.

Based on the above discussion, it is seen that many authors worked on power factor correction; all of them used a number of parallel capacitors to improve the power factor step by step. These methods cannot attain unity power factor for all load conditions. Moreover, the studies were performed on PFI based on capacitor bank or compensator only simulated their proposed scheme, thus it is necessary to implement the circuit practically and verify the results which are the main focus of this paper.

In the proposed system, a single switched capacitor is used instead of a number of capacitors connected in parallel. This single capacitor will always get connected to the load and thus the voltage across the capacitor will be varied to regulate the compensating current. Therefore, the expensive magnetic contactor can be omitted and continuous control of power factor can be made possible. Consequently, the proposed system will be robust and more cost-effective than the earlier methods.

The specific objectives of the proposed PFI scheme are:a. To design and implement a PFI unit which advances

the power factor of lagging loads using PWM switched capacitor.

b. To measure and display the values of load current, power factor, capacitive current, circuit input current and overall power factor in the LCD display.

c. To design a system that protects the load against over current.

The rest of the paper is sectionalized as followings: Section 2 describes the block diagram of the proposed scheme, section 3 shows the circuit diagram using Proteus software using different load condition, section 4 comprises the implemented system using microcontroller and finally, section 5 elaborates the result and discussion.

2. BloCK dIAgRAM oF THe PRoPoSed SySTeM

The wide variation of lagging loads caused the necessity for controlling reactive power which provides the desired power factor. This task is generally accomplished by using switched capacitors. In case of fluctuating loads, the KVAR of the load also varies over extensive limits. Thus,

a fixed capacitors bank might result in over-compensation otherwise under-compensation results lead to low power factor. Fig. 1 shows the active and reactive component of current. In this research, instead of using a bank of capacitors a single large capacitor is used. The switched capacitor is used in the proposed system with a low pass filter.

To compensate using capacitor, the reactive current element I sin Ø must be equal to the capacitor current, IC. Thus,

VI sin Ø= IC= ----------- (1) XC

The capacitive current, IC depends on the voltage across the capacitor, V. According to the equation (1), controlling the voltage across the capacitor, the compensating capacitor current can be controlled. The voltage across the capacitor is varied by a switch which changes its duty cycle with a PWM signal. The duty cycle of the PWM signal is changed to control reactive current and finally attains required power factor.

TONDuty Cycle, D= ---------------- ; (2) TON+TOFF

The proposed scheme detects the load power factor and the magnitude of the load current. Load current is measured using the current transformer and precision rectifier. The current & voltage waveform is digitized using logic gates. Then, the phase difference between current and voltage is estimated. The load current and phase angle of voltage and current is used as the input of a microcontroller. High frequency PWM signal is produced by PIC microcontroller for the bidirectional switch whose duty cycle varies with the product of load current and load power factor. Therefore, the voltage across the capacitor also varies and the power factor reaches to unity. Opto-coupler is used to isolate the gate signal. The block diagram for PFI unit using a single capacitor and microcontroller is shown in Fig. 2. Load voltage is reduced by using PT while power factor is measured using zero crossing detection circuit.

I cos

I sin

aV

b

o

I

Fig. 1: Active and reactive components.



LCD display is used for displaying load current and load power factor. Also, it displays the load voltage and compensating capacitive current. If there is over current or under voltage, microcontroller disconnect the load from the supply using the relay to protect the load and improve the stability of the system.

3. CuRRenT And PoweR FACToR MeASuReMenT

In the Proteus simulation, 100 µf non-polarized capacitor with low pass filter is used as single capacitor instead of bank of capacitors. To regulate the compensating capacitive current, an ac to ac buck converter is used which constantly switches on /off by a pulse width modulated (PWM) signal. The flow chart diagram for current and power factor measurement is shown below Fig. 3.

The circuit diagram for the measurement of current and power factor is shown in Fig. 4. In this circuit, CT and PT are used. CT is used to step down the input current while precision rectifier is used to convert the AC signal to equivalent DC signal. The DC voltage is measured by the microcontroller. For the measurement of power factor, zero crossing detector circuit is used after CT and PT. Microcontroller takes load current and phase angle of voltage & current as input. The high frequency PWM signal is produced by the microcontroller for bidirectional switch whose duty cycle needs to vary. The

220v, 50Hz

C.T-1

C.T-2

P.T Switch LPF IGBT

LCD

Gate DriverCircuitMicrocontroller

PIC16F877A

MicrocontrollerPIC16F72

PrecisionRectifier

PrecisionRectifier

InductiveLoad

ZCD-1

ZCD-V ZCD-1

Opto-coupler

LED PWMOutput

SingleCapacitor

Fig. 2: Block diagram of proposed power factor correction system.

Start

Initializing of System

Measure Irms

RB2 = = 1

RB4 = = 1

Start Timer0

Stop Timer0

Measure engle between Vand I through uc

Display Load Current &Load Power Factor.

Fig. 3: Flow chart diagram for current and power factor measurement.

Fig. 4: Circuit diagram for current and power factor measurement.



microcontroller uses an algorithm for producing PWM signal with different duty cycle depending on load current and power factor.

The circuit is simulated in Proteus at different load conditions. If there is no load, LCD display will show “NO LOAD”. When the load is not connected to the system, the load current is zero, therefore the duty cycle of PWM signal is zero which is shown in Fig. 5. The inductive loads (50+j63) Ω, (50+j79) Ω and (50+j47) Ω are connected with the circuit in Proteus software, it provides the load current, power factor, voltage and current waveform and also, the PWM signal produced by microcontroller.

After simulation, different loads are assumed for demonstrating the power factor correction system with the proposed switched single capacitor.

When an (50+j63) Ω inductive load is connected, there is a phase difference between current and voltage signals as shown in Fig. 6. Depending on the load current and the load power factor, microcontroller changes the duty cycle of the PWM signal to attain unity power factor as shown in Fig. 7. The Input voltage and current waveforms after power factor correction and the voltage across the switched single capacitor are shown in Fig. 8 and Fig. 9 respectively. When three (50+j79) Ω inductive loads are connected, there is a phase delay between current and voltage signals as shown in Fig. 10.

Fig. 6: Voltage and current waveforms with one (50+j63) Ω inductive load.

Fig. 7: PWM signal is produced by microcontroller to attain unity power factor for one (50+j63) Ω inductive

load.

Fig. 8: Input voltage and current waveforms after power factor correction.

Fig. 9: Voltage across the shunt capacitor (100µF) was 123.16 V (peak).

Microcontroller changes the duty cycle of PWM signal to attain unity power factor as shown in Fig. 11. The Input voltage and current waveforms after power factor correction and voltage across the switched single capacitor are shown in Fig. 12 and Fig. 13 respectively.

Fig. 5: PWM signal with zero duty cycle without load.



Fig. 10: Voltage and current waveforms with three (50+j79) Ω inductive loads.

Fig. 11: PWM signal is produced by microcontroller to attain unity power factor for three (50+j79) Ω inductive

loads.

Fig. 12: Input voltage and current waveforms after power factor correction

Fig. 13: Voltage across the shunt capacitor (100µf) was 302.60 V (peak).

When an (50+j47)Ω inductive load is ON, there is a phase delay in between current and voltage signals as shown in Fig. 14. The microcontroller changes the duty cycle of PWM signal to attain unity power factor as shown in Fig. 15. The Input voltage and current waveforms after power factor correction and voltage across the switched single capacitor are shown in Fig. 16 and Fig. 17 respectively.

4. exPeRIMenTAl SeTuP

After getting the suitable results from Proteus simulation, the implementation of the PIC microcontroller based automatic power factor correction project is carried out.

Fig. 14: Voltage and current waveforms with one (50+j47) Ω inductive load.

Fig. 15: PWM signal is produced by microcontroller to attain unity power factor for one (50+j47) Ω inductive

load

Fig. 16: Input voltage and current waveforms after power factor correction.



Fig. 17: Voltage across the shunt capacitor (100µf) was 126.69 V (peak).

Ahmed and Alam recommended switched single capacitor method to increase the power factor continuously to unity which is simulated only assuming various inductive loads [17]. However, in this research, the overall system for PFI unit using single switched capacitor is developed experimentally.

Fig. 18 shows the circuit implemented in the laboratory for power factor correction. Fig. 19 shows the practically implemented power factor improvement circuit under running condition. The different components of the practically implemented circuit are shown in Fig. 20. The implemented power factor correction circuit can be divided into three sections- i.e. power circuit, display circuit and control circuit.

Fig. 18: Circuit implementation of the proposed circuit.

Fig. 19: Practically implemented proposed project in working mode.

Fig. 20: Different components of the current and power factor measurement circuit.

In the control circuit, high frequency PWM signal is generated with respect to load current and load power factor to attain unity power factor. In this circuit, one opto-coupler is used for generating one isolated gate signal for driving the IGBT and T section LC low pass filter for smoothing chopped ac voltage.

This proposed system will incorporate the facility to monitor the load current, voltage, power factor, compensating capacitive current and protection of load



against over current and under voltage. The proposed scheme facilitates the power factor correction as well as provides a protection against under voltage and over current to make the system stable & efficient.

5. exPeRIMenTAl ReSulTS & dISCuSSIon

The proposed method of improving power factor is found efficient for lagging loads. In addition, the simulation and the experimental results are showing that the method will perform better owing to the properties as user friendly, cost-effective and technologically sound. Table 1 shows the experimental results of the proposed PFI unit before and after power factor correction. Table 1 indicates that further varying voltage across the capacitor using PWM signals the overall power factor is necessarily improved to unity or near about unity. The current is reciprocal to the power factor. Thus, when the load current decreases with the increasing the capacitor voltage, the overall power factor increases. For various lagging loads, the proposed system attains unity power factor which is demonstrated in Table 1.

The proposed design attains unity power factor for all load conditions by continuous monitoring of load current & load power factor. The corresponding PWM signal is generated to vary the duty cycle for controlling voltage across the capacitor and thereby compensating the load current. And finally, the power factor reaches about unity.

Table 1: Experimental results before and after power factor correction

Before After Voltage AcrossSingle

Capacitor (V)

Load Current

Load P.F

(Lag.)

ResultantCurrent

Overall P.F

0.11 0.6814 0.08 0.9994 650.11 0.6931 0.08 0.9994 580.11 0.3704 0.04 1.0000 780.11 0.4001 0.04 1.0000 820.16 0.7271 0.11 0.9895 900.16 0.3853 0.08 1.0000 1180.20 0.8702 0.16 0.9895 790.20 0.3853 0.08 1.0000 1360.23 0.8454 0.20 0.9994 1060.23 0.4148 0.16 1.0000 1510.23 0.4001 0.16 1.0000 1690.23 0.7997 0.11 0.9988 1140.28 0.4148 0.16 1.0000 1870.32 0.4148 0.23 1.0000 1760.40 0.7488 0.36 0.9535 1760.44 0.8702 0.36 0.9953 1430.44 0.4294 0.32 1.0000 2020.44 0.5553 0.32 1.0000 1860.52 0.6329 0.44 1.0000 2030.59 0.4148 0.47 1.0000 2180.64 0.8367 0.59 0.9070 1890.68 0.6453 0.59 1.0000 216

In this case, the loss will be minimized and efficiency of the system will be increased. The voltage across the capacitor varies between 65 V and 216 V where load current varies in the range of 0.11 A to 0.68 A. Single switch capacitor used for power factor correction requires less cost, easy maintenance. After using the single switched capacitor, the power factor approximately attains unity which is an important finding of the proposed research.

6. ConCluSIon

Reactive power compensation is known as a crucial factor in the design and operation of any power system. In this paper, a single capacitor is used for PFI which replace the use of bank of capacitors. The proposed system also omits the use of expensive magnetic contactor required for switching. Therefore, the proposed method will be more cost effective and robust method of power factor improvement of varying inductive loads. Another point is that, it will reduce the losses due to the high inductive current. Due to compensating capacitor current control, the proposed system can maintain unity power factor sleeplessly at all load condition. The proposed approach of PFI is implemented at the laboratory and tested by varying the load current and load power factor. Although switching loss is ignored in the proposed project, it will be the future works to determine the switching loss and the ways to minimize it. Even though, the implemented project is made only for single phase load, hence in future, it can be implemented for three- phase load. Use of the proposed method, can be able to disconnect the load from supply during over current and under voltage.

ReFeRenCeS

[1] G. R. P. Lakshmi, “Power factor improvement of canonical switch cell converter fed BLDC motor drive”. In Computation of Power, Energy Information and Communication (ICCPEIC), 2016 International Conference on, pp. 624-627. IEEE, 2016.

[2] M. Ates, “PIC Kullanarak Güç Katsayısı Ölçüm Devresi Tasarımıve Simülasyonu” (Design and simulation of Power Factor Measurement Circuit by using PIC), Master Thesis, Yuzuncu Yıl University, Science Institute, 2009.

[3] C. A. Heger, P. K. Sen, and A. Morroni, “Power factor correction; A fresh look into today’s electrical systems,” 2012 IEEE-IAS/PCA 53rd Cement Industry Technical Conference, pp: 1 – 13, 2012.

[4] H. Y. Kanaan, C. Somers, and K. Al-Haddad, “Power Factor Correction with a Modified Sheppard-Taylor Topology Operating in Discontinuous Capacitor



Voltage Mode and Low Output Voltage,” IEEE Journal of Emerging and Selected Topics in Power Electronics, pp (99): 1–1, 2014.

[5] A. R. Ghanbari, E. Adib and H. Farzanehfard, “Single-stage single-switch power factor correction converter based on discontinuous capacitor voltage mode buck and fly back converters,” IET Power Electronics, pp(1): 146–152, 2013.

[6] Usman, H. Fuad, and M. F. Zia. “Undervoltage and Power Factor Improvement of Substation Using Static Var Compensators.” Journal of Electrical and Electronics Engineering 10, no. 1 (2017): 79.

[7] Li, Sinan, Wenlong Qi, Siew-Chong Tan, and S. Y. Ron Hui. “A single-stage two-switch PFC rectifier with wide output voltage range and automatic AC ripple power decoupling.” IEEE Transactions on Power Electronics 32, no. 9 (2017): 6971-6982.

[8] S. Das, D. Chatterjee, and S. K. Goswami. “A GSA-Based Modified SVC Switching Scheme for Load Balancing and Source Power Factor Improvement.” IEEE Transactions on Power Delivery 31, no. 5 (2016): 2072-2082.

[9] Umesh, Suma, L. Venkatesha, and A. Usha, “Active power factor correction technique for single phase full bridge rectifier.” In Advances in Energy Conversion Technologies (ICAECT), 2014 International Conference on, pp. 130-135. IEEE, 2014.

[10] S. Durgadevi and M. G. Umamaheswari. “Analysis and Design of Single Phase Power Factor Correction using DC-DC SEPIC Converter with Bang-Bang and PSO based Fixed PWM Techniques.” Energy Procedia 117 (2017): 79-86.

[11] H. H. Shwehdi and M. R. Sultan, “Power Factor Correction Capacitors; Essential and Cautions”, IEEE Power Engineering Society Summer Meeting, 2000, pp. 1317-1322.

[12] U. Celtekligil, “Capacitive Power Factor and Power Quality Correction of a Light Rail Transportation System”, 50th International Symposium of ELMAR, 2008, pp. 415-418.

[13] S. M. Choudhury, “Design and Implementation of a low-cost Power Factor Improvement Device”, IEEE Region 10 Conference TENCON, 2008, pp.1-4.

[14] A. Shahid and A. Shabir, “Microchip based Embedded System Design for Achievement of High Power Factor in Electrical Power Systems”, Power and Energy Engineering Conference APPEEC, 2013, pp.1-5.

[15] R. Sharma and A. Haque, “Simulation and Analysis of Power Factor Correction in Electric Control System for Metal Halide High Intensity Discharge Lamps”, Advance in Electronic and Electric Engineering, vol. 2, 2014, pp. 185-192.

[16] R. A. Allah, “Automatic Power Factor Correction Based on Alienation Technique”, International Journal of Engineering and Advanced Technology, 2014, pp. 194-202.

[17] M. R. Ahmed and M. J. Alam, “Power Factor Improvement by Pulse Width Modulated Switched Single Capacitor”, International Journal of Engineering Studies, vol. 2, no. 1, pp. 75-80, 2014.

m ite ed e t i s c microcontroller-based power factor ... · such as- capacitor bank, synchronous...

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