Microcontroller Based Power Factor Correction Using SCR

Transcription

1 Proceedings of International Conference on Innovation & Research in Technology for Sustainable Development (ICIRT 2012), November Microcontroller Based Power Factor Correction Using SCR Vishnu Prasad Pradhan, Taruna Kumawat and Ankit Soni Abstract--- Power factor correction (PFC) is a technique of counteracting the undesirable effects of electric loads that create a power factor (P.F.) that is less than 1. Power factor correction may be applied either by an electrical power transmission utility to improve the stability and efficiency of the transmission network or, correction may be installed by individual electrical customers to reduce the costs charged to them by their electricity supplier. The most practical and economical power factor correction device is the capacitor. Incorporation of a micro-controller into static capacitor device facilities a sort of an automatic control action where by the power factor is always kept a fixed value, irrespective of the load power factor conditions. It improves the power factor because the effects of capacitance are exactly opposite those of inductance. The micro-controller determines the power factor of the system at any instant of time and determines the reactive power to be supplied and the value of capacitance to be switched in to make the power factor unity. The capacitor is switched in parallel to the load through a relay controlled by the processor. The circuit is capable of correcting the power factor for any inductive load within the rating of the system. Thyristor Switching of capacitors for power factor improvement has many advantages over conventional contactor switching. Thyristor Switching is more reliable, accurate, maintenance free and especially suitable for fast variable loads, where contactor-switching systems fail to give desired results. P I. INTRODUCTION OWER factor correction (PFC) is a technique of counteracting the undesirable effects of electric loads that create a power factor (P.F.) that is unity. Power factor correction may be applied either by an electrical power transmission utility to improve the stability and efficiency of the transmission network or, correction may be installed by individual electrical customers to reduce the cost charged to them by their electricity supplier. When an electric load has a Vishnu Prasad Pradhan, VII Semester / IV Year, Electrical and Electronics Engineering, Mandsaur Institute of Technology, Mandsaur (MIT), M.P, India. vp.pradhan@mitmandsaur.info Taruna Kumawat, VII Semester / IV Year, Electrical and Electronics Engineering, Mandsaur Institute of Technology, Mandsaur (MIT), M.P, India. taruna_kumawat2010@yahoo.com Ankit Soni, VII Semester / IV Year, Electrical and Electronics Engineering, Mandsaur Institute of Technology, Mandsaur (MIT), M.P, India. ankit.soni_eee08@mitmandsaur.info power factor lower than unity, the apparent power delivered to the load is greater than the real power that the load consumes. Only the real power is capable of doing work, but the apparent power determines the amount of current that flows into the load, for a given load voltage. Energy losses in transmission lines increase with increasing current. Power companies therefore require that customers, especially those with large loads, maintain the power factors of their respective loads within specified limits or be subject to additional charges. Engineers are often interested in the power factor of a load as one of the factors that affect the efficiency of power transmission. Power factor correction returns the power factor of an electric AC power transmission system to very near unity by switching in or out banks of capacitors or inductors, which act to cancel the inductive or capacitive effects of the load. Power factor is a measure of how effectively you are using electricity. Various types of power are at work to provide us with electrical energy. Working Power is the true or real power used in all electrical appliances to perform the work of heating, lighting, motion etc. We express this as kw or Kilowatts. Common types of resistive loads are electric heating and lighting. An inductive load, like a motor, compressor or ballast, also requires Reactive Power to generate and sustain a magnetic field in order to operate. We call this non-working power kvar s, or kilovolt amperes. We determine apparent power using the formula, kva=kvxa. Going one step further, Power Factor (PF) is the ratio of working power to apparent power, or the formula PF=kW/kVA. A high PF benefits both the member and utility, while a low power factor indicates poor utilization of electrical power. Improving the power factor can maximize current carrying capacity, improve voltage to equipment, reduce power losses, and lower electric bills. The simplest way it improve power factor is to add power factor correction capacitors to the electrical system. Power factor correction capacitors act as reactive current generators. They help offset the non-working power used by inductive loads, thereby improving the power factor. Thyristor Switching of capacitors for power factor improvement has many advantages over conventional contactor switching. Thyristor Switching is more reliable, accurate, maintenance free and especially suitable for fast variable loads, where contactor-switching systems fail to give desired results. The capacitors are switched at "Zero Current Cross Over Threshold". Comparison between Fast response Thyristor Switched and Contactor Switched Capacitor APFC system.

2 Proceedings of International Conference on Innovation & Research in Technology for Sustainable Development (ICIRT 2012), November II. POWER Power is the time rate at which work is done or energy is transferred. In calculus terms, power is the derivative of work with respect to time. The SI unit of power is the watt (W) or joule per second (J/s). Horsepower is a unit of power in the British system of measurement. 2.1 Types of Power There are three types of power: a. Active power b. Reactive power c. Apparent power 2.2 Power Factor AC power flow has the three components: real power (P), measured in watts (W); apparent power (S), measured in voltamperes (VA); and reactive power (Q), measured in reactive volt-amperes (VAr). The power factor is defined as: Power factor = Real Power Apparent Power For a DC circuit the power is P=VI and this relationship also holds for the instantaneous power in an AC circuit. However, the average power in an AC circuit expressed in terms of the rms voltage and current is P avg = VI COSΦ Where φ is the phase angle between the voltage and current. The additional term is call the power factor. Power factor = COS φ = R / Z 2.3 Causes of Low Power Factor The important inductive loads responsible for low power factor are as follows- 1. Most of the Ac motors are induction type. Threephase induction motor operates at a power factor about 0.8 lagging at full load. At light load these motors works at a very small power factor of the order of 0.2 to 0.3 lagging. Single phase motors operate at power factor of around A transformer draws magnetizing current from the supply. At a normal load, this current does not affect the power factor much but at light loads the primary current power factor is low. 3. Arc lamps, electric discharge lamps, industrial heating furnace, welding equipment operate at low lagging power factor. 2.4 Disadvantage of Low Power Factor The important disadvantages of low power factor are: 1. The first is that transmission lines and other power circuit elements are usually more reactive than resistive. Reactive components of current produce larger voltage drop than resistive components, and add to the total IZ = (I(R + LX)) drop, therefore, the system voltage regulation suffers more and additional voltage- regulating equipment may be required for satisfactory operation of the equipment using power. 2. The second disadvantage is the inefficient utilization of the transmission equipment since more current flow per unit of real power transmitted is necessary due to the reactive power also carried in the power lines. If the current necessary to satisfy reactive power could be reduced, more useful power could be transmitted through the present system. 3. The third disadvantage is the cost of the increased power loss in transmission lines. The increased power loss is due to the unnecessary reactive power, which is in the system. The reactive power losses vary as the square of the reactive current or as the inverse of the power factor squared. 4. Higher currents give rise to higher copper loss in the system and therefore the efficiency of the system is reduced. Also the cost of the energy loss in the system is increased. 5. Higher current gives larger voltage drop in cables and other apparatus. This result in poor voltage regulation. 2.5 Power Factor Correction Many loads are highly inductive, such a lightly loaded motors and illumination transformers and ballasts. You may want to correct the power factor by adding parallel capacitors. You can also add series capacitors to "remove" the effect of leakage inductance that limits the output current. When a load draws reactive power from the supply, its power factor is said to be lagging, the phase angle between voltages and current is 90º with voltage leading. When the reactive power is exported to the supply, its power factor is said to be leading, the phase angle between voltages and current is 90º with current leading. This is reference to the phase of the load current with respect to the supply voltage. 2.6 For Linear Loads Figure 2.1: Wave Form Power factor correction brings the power factor of an AC power circuit closer to 1 by supplying reactive power of opposite sign, adding capacitors or inductors which act to cancel the inductive or capacitive effects of the load, respectively. For example, the inductive effect of motor loads

3 Proceedings of International Conference on Innovation & Research in Technology for Sustainable Development (ICIRT 2012), November may be offset by locally connected capacitors. If a load had a capacitive value, inductors (also known as reactors in this context) are connected to correct the power factor. In the electricity industry, inductors are said to consume reactive power and capacitors are said to supply it, even though the reactive power is actually just moving back and forth on each AC cycle. 2.7 For Non-Linear Load A non-linear load on a power system is typically a rectifier (such as used in a power supply), or some kind of discharge device such as a fluorescent lamp, electric welding machine, or arc furnace. Because current in these systems is interrupted by a switching arc action, the current contains frequency components that are multiples of the power system frequency. Non-linear loads change the shape of the current waveform from a sine wave to some other form. Non-linear loads create harmonic currents in addition to the original (fundamental frequency) AC current. Addition of linear components such as capacitors and inductors cannot cancel these harmonic currents, so other methods such as filters or active power factor correction are required to smooth out their current demand over each cycle of alternating current and so reduce the generated harmonic currents. There are two types of power factor controller used for the non-linear loads: 1. Passive PFC 2. Active PFC III. WHY CORRECT THE POWER FACTOR? The current through the reactive component (I reactive) dissipates no power, and neither does it register on the watthour meter. However, the reactive current does dissipate power when flowing through other resistive components in the system, like the wires, the switches, and the loss part of a transformer (R line). Switches have to interrupt the total current, not just the active component. Wires have to be big enough to carry the entire current, etc. Correcting the power factor reduces the amount of over sizing necessary. 3.1 Power Factor Improvement Methods Normally the power factor of the whole load on large generating stations is in the region of 0.8 to 0.9. However sometimes it is lower in such cases it is generally desirable to take special steps to improve the power factor. This can be achieved by the following methods: By using Static Capacitor The power factor can be improved by connecting capacitors in parallel with the equipment operating at lagging power factor. The capacitor (generally known as static capacitor) draws a leading current and partly or completely neutralizes the lagging power factor component of load current. This raises the power factor of load. For three phase load, the capacitor can be connected in delta or star. Static capacitors are invariably used for power factor improvement in factories. Hence by connecting a capacitor in parallel with an inductive load, the power factor is improved and the current taken from the supply is reduced without altering either the current or power taken by the load. Figure 3.1: Static Capacitor By Synchronous Condenser A synchronous condenser is a synchronous motor operating at no load. It is a property of such a motor that it takes lagging kva, when the field current is below a certain value and a leading kva when the field current is above this value. The efficiency of this machine is very high. The real power it takes will be small, just its losses. For simplicity, let us consider a line having resistance R and inductive reactance X, and work in terms of voltage to neutral Ep, which may be assumed to be the same at the two ends of the line. The use of rotating synchronous condensers, common through the 1950s, is now making a comeback as an alternative to capacitors for power factor correction. Figure 3.2: Synchronous Condenser 3.2 Drawbacks of Low Power Factor The current for a given load supplied at constant voltage will be higher at a lower power factor and lower at higher power factor. The higher current due to poor power factor affects the system and results in following disadvantages. 1. Ratings of generators and transformers are proportional to their output current. Hence inversely proportional to power factor, therefore, large generators and transformers are required to deliver same load but at low power factor.

4 Proceedings of International Conference on Innovation & Research in Technology for Sustainable Development (ICIRT 2012), November The cross sectional area of the bus-bar and the contact surface of the switch gear is required to be enlarged for the same power to be delivered, but at low power factor. 3. For the same power to be transmitted, but at low power factor, the transmission line or distributor or cable has to carry more current. The size of the conductor will have to be increased if current density in the line is to be kept constant. Thus more conductor material is required for transmission lines, distributors and cables to deliver the same load but at low power factor. 4. Energy losses are proportional to the square of the current, hence inversely proportional to the square of the power factor i.e., more energy losses incur at low power factor, which results in poor efficiency. 5. Low lagging power factor results in large voltage drop in generators, transformers, transmission lines and distributors, which results in poor regulation. Hence extra regulating equipments is required to keep the voltage drop within permissible limits. 6. Low lagging power factor reduces the handling capacity of all the elements of the system. IV. MICROCONTROLLER BASED PFC Static capacitors used for power factor correction have many advantages such as low cost, low space consumption, very low losses, extremely high efficiency, fast control, easy availability and safe handling. Despite these merits, power factor correction using static capacitors have not become the final word in power factor correction due to its inherent drawbacks such as need of complicated maintenance, higher 4.1 Block Diagram cost of the capacitors for high output and its uncontrollable nature. Of this, the uncontrollable nature or power capacitors have imposed the limit on universal acceptance of static capacitors for power factor correction. That is static capacitors currently used for power factor correction is available as bank of static capacitors which can be connected across the load either in star or delta. The problem with such banks is that they always draw the same amount of leading reactive power irrespective of the actual lagging reactive power drawn by the load. Thus capacitor banks are insensitive to power factor of the load and changes in power factor of the load. Some kind of controllable capacitor banks have been developed, but its not automatic, i.e. it needs an operator to determine the power factor of load, then determine the value of capacitance to be switched and then switches it from the bank. Thus features have declined the popularity of static capacitors in the field of power factor correction. This project aims at incorporating a sort of an automatic control of power factor of the load. This makes the device capable of sensing the power factor and the changes in power factor of the load and apply a correction as and when needed in the correct amount. Thus the power factor can always be kept at a fixed value or even unity. Thus effort is made to make an ordinary capacitor bank an automatic and controllable power factor correcting device. Thus the static capacitors can revert some of the most serious inherent drawbacks, thus increasing its popularity in power factor correction in electrical equipments and distributions and transmission networks. AC 230 V Zero Crossing Detectors SCR Step Down X-mer 9V-0-9V, 500mA Microcontroller AT89C51 Opto-Coupled Firing Circuit Capacitor Bank Regulated 5V DC ADC 0804 CT Load Figure 4.1: Block Diagram of Microcontroller Based PFC

5 Proceedings of International Conference on Innovation & Research in Technology for Sustainable Development (ICIRT 2012), November Circuit Diagram 4.3 Zero Crossing Detector Figure 4.2: Circuit Diagram of Microcontroller Based Power Factor Correction Using SCR 4.4 Regulated +5V Power Supply Figure 4.3: Zero Crossing Detectors Figure 4.4: Regulated +5V Power Supply

6 Proceedings of International Conference on Innovation & Research in Technology for Sustainable Development (ICIRT 2012), November Analog-to-Digital Converter & Current Transformer 4.6 Microcontroller & Display Figure 4.5: Analog-to-Digital Converter & Current Transformer 4.7 Thyristor Switch & PFC (Capacitor Bank Figure 4.6: Microcontroller & Display Figure 4.7: Thyristor Switch & PFC (Capacitor Bank)

7 Proceedings of International Conference on Innovation & Research in Technology for Sustainable Development (ICIRT 2012), November Component Layout 4.9 PCB Layout Figure 4.8: Component Layout Figure 4.9: PCB Layout

8 Proceedings of International Conference on Innovation & Research in Technology for Sustainable Development (ICIRT 2012), November V. CONCLUSION This project work is an attempt to design and implement the power factor controller using micro controller. In this work there is a provision to define the own current minimum range and power factor minimum and maximum range and then according to the lagging power factor it takes the control action. This project gives more reliable and user friendly power factor controller. This project makes possible to store the real time action taken by the microcontroller. This project also facilitates to show the power factor changes on LCD in real time. REFERENCES [1] Principles of power system by V. K. Mehta [2] Power Electronics by Dr. P. S. Bhimbhra [3] Electrical power system by Ashfaq Husain [4] Electrical power system by C. L. Wadhwa [5] Advanced Microprocessors and peripherals by A.K. Ray and K.M. Bhurchandi [6] [7] [8] [9] [10]