A Single Phase Power Factor Correction Using Programmable Interface Circuit

A Single Phase Power Factor Correction Using Programmable Interface Circuit

A Single Phase Power Factor Correction Using Programmable Interface Circuit

Mrs.Shamal R.Padmawar

ME student, Department of Electronics DPCOE, wagholi

Pune, India shamalrpadmawar@gmail.com

Abstract-Power factor correction (PFC) technique aim to bring the power factor closer to unity by reducing the effects of reactive power. In the great majority of cases, poor power factor is due to inductive loads which can be compensated by adding electrical devices called capacitors into the circuit and this is nothing but the PFC. The technique described is clearly apparent as a simple and cost effective power factor (PF) measurement and correction scheme by using Programmable Interface Circuit (PIC) microcontroller. The system senses the power factor and with the help of microcontroller switches required number of capacitors in the capacitor bank.

A System also includes a LCD display to show the lead- lag power factor and the number of capacitor banks that are switched on to compensate the power factor.It has widespread application because increased PF has number of advantages.

Keywords-Power factor, Reactive power, PIC microcontroller, Capacitor bank, Pulse Width Modulated wave.

1. INTRODUCTION

   Power factor (PF) is a measurement by which we can measure the efficiency of the electrical equipments as well as ac electric power system on the basis of electrical energy consumption. It determines power quality. Power factor is generally defined as the ratio of the Real power (measured in Watts) to the Apparent power (measured in VA). The classical definition of power factor is the cosine of the phase angle i.e. cos between voltage and current . Due to reactive loads, the apparent power becomes greater than the real power that increases the phase difference between voltage and current. So, the power factor drops below unity and the system becomes less efficient.

   Reducing the effects of reactive power will cause the angle

   to get closer to 0º, meaning the power factor will get closer to unity. Power factor correction techniques can achieve this. Poor power factor has negative implications for businesses because it:

   • Draws more current from the network costing more to achieve the same tasks.

   • Can incur a poor power factor penalty from the supplier, sometimes called a reactive power charge.

   • Reduces the effective capacity of the electrical supply the more reactive power that is carried,the less useful power can be carried.

   • Causes losses at transformers and other devices, leading to inefficiency and unwanted heat gains.

   • Can cause excessive voltage drops in the supply network.

   • Can reduce the life expectancy of electrical equipment in extreme cases.

   The simplest way to control the harmonic current is to use a filter: it is possible to design a filter that passes current only at line frequency (50 or 60 Hz). This filter reduces the harmonic current, which means that the non-linear device now looks like a linear load. At this point the power factor can be brought to near unity, using capacitors or inductors as required. This filter requires large-value high-current inductors, however, which are bulky and expensive. A passive PFC requires an inductor larger than the inductor in an active PFC, but costs less. This is a simple way of correcting the nonlinearity of a load by using capacitor banks.

2. IMPLEMENTATION DETAILS

   The system senses the power factor and with the help of microcontroller switches required number of capacitors in the capacitor bank. The proposed system block diagram is shown in Figure 1.

   Fig.1:Block Diagram of Proposed System

   1. Power Factor Sensing Section

      To measure displacement power factor on one phase, we need to have a square wave representative of the polarity of the voltage and current waveform on that phase. To facilitate this purpose potential transformer (PT) used to sense the voltage wave and current transformer (CT) to current wave.

      Secondary of the CT is terminated in a shunt resistor and followed by a low pass filter and amplifier to enhance the wave amplitude. A low-pass filter at the secondary of PT and CT can effectively reduce the constant or transient noise of power line produced due to fluorescent lighting tubes, solid state converters or drive systems, welding equipment etc. Generated square waves are ac signals which are then sent to clipper section to convert it to rectangular wave or pulse.

   2. PIC Microcontroller Unit

      Microchip Technologies 16 bit microcontroller PIC16F877A has several useful stand-alone features like built in Flash Program Memory, EEPROM (Electrically Erasable Programmable Read Only Memory), Data Memory, A/D (Analog-to-Digital) converter, 8 and 16 bit Timers, watchdog timer, Capture/ Compare/ PWM modules, serial and parallel communication interface etc.

   3. Capacitor Bank

   When power factor improvement capacitor banks are designed and arranged properly, the PF correction scheme becomes efficient. The capacitor bank is comprised of individual capacitor elements. The individual capacitors for PF correction applications may be metal-enclosed oil-filled or dry units (epoxy filled in a plastic case), and should be capable of working over the temperature range of up to

   +700C. Some salient features of PF correction capacitor banks are: extremely high reliability with self-healing capabilities; capable of controlling the requirement of kVARs to achieve PF as close as unity; compact, efficient and long service life; protected against over-voltage, over-current, over temperature, switching surges and harmonics .The actual capacitor values in farads of a capacitor bank can be calculated using the following equation .

   Where VAR=capacitor units VAR rating; C=Capacitor (Farads); f= frequency (Hz or Cycles/Second); VR=Capacitor units rated voltage. Standard capacitor sizes are 50, 100, 150,200, 300 and 400kVAR

   Switching drivers may consist of transistors and relays. On receiving output signal from the microcontroller port, the transistor energizes the relay coils and the relay then switches the capacitors parallel to the power line. A 2×16 line LCD is used to display value of power factor, lead/lag and label of switched capacitor bank.

3. GENERATION OF PWM WAVE

   Applying pulses to an AND gate, a Pulse Width Modulated (PWM) wave is obtained where the displacement angle between voltage and current wave determines the pulse width as shown in Figure 2.

   Fig.2: Generation of PWM waves

   Applying pulses to an AND gate, a Pulse Width Modulated (PWM) wave is obtained where the displacement angle between voltage and current wave determines the pulse width as shown in Fig.3

   (a)

   (b)

   (c)

   Fig.3: Generation of PWM wave from equivalent voltage and current pulse, (a) at Power Factor=1, (b) at Power Factor=0.5, (c) at Power Factor=0.

4. SOFTWARE DEVELOPMENT

   Simplified flowcharts for the developed software are shown in Figure 4. MikroC compiler was used to code and compile the program and WinPIC to load HEX code to microcontroller unit. The main program, Figure 4 a, initializes and configures the LCD and I/O ports.

   Fig. 4: Program flow charts, (a) main program (b) interrupt service routine.

5. LABORATORY TESTING & EVALUATION

   A prototype of the power factor measurement and correction scheme was constructed and tested under different load coditions by the help of Electromechanical Training System (Model: 8001) manufactured by Lab-Volt, Canada. Figure 5 shows the experimental setup to observe the system performance. 220/380V, 50Hz power supply was used during test. Output waves at different stages of the system are shown in Figure 6.

   Fig. 5 :Operation of laboratory prototype

   Fig. 6: Oscilloscope records of system operation

   Different combination of static loads (resistors, inductors and capacitors) were used to produce different values of power factor. For evaluating system efficiency in power factor measurement, experimental reading is further compared with the theoretically calculated PF and the reading of Lab-Volt power factor meter. Comparative studies of results are shown in Figure 7 and 8.

   Fig.7: Comparison of results on measured values of lagging power factor. (X- axis: no. of observation, Y-axis: power factor, blue ones are theoretical values, red ones measured by Lab-Volt pf meter, green ones measure by proposed system)

   Fig. 8: Comparison of results on measured values of leading power factor.(X- axis: no. of observation, Y-axis: power factor, blue ones are theoretical values, red ones measured by Lab-Volt pf meter, green ones measure by proposed system)

   PF correction performance can be determined from Figure 9.

   Fig 9. :Comparison of power factor (PF) before and after correction.(X axis: no. of observation, Y-axis: power factor, blue ones are theoretically calculated PF before correction, red ones are after correction)

   Theoretical PF was measured from the load values. The test result indicates that accuracy of the proposed power factor correction scheme is acceptable and comparable to similar equipments currently available in the market.

6. CONCLUSION

The system has the ability to sense power factor effectively and by using proper algorithm sufficient capacitors are switched on in order to compensate the reactive power. PWM based power factor measurement and correction unit can improve the power factor close to unity in an automatic way and can remove the capacitor banks when the power factor is leading.

The system gives reliable and economic solution for industries and multi-storied building applications. We can improve its efficiency in abnormal situations as well as we can include protective relaying facility to protect the system from hazards.

REFERENCES

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