Power Factor Correction Input Circuit

Transcription

1 Power Factor Correction Input Circuit Written Proposal Paul Glaze, Kevin Wong, Ethan Hotchkiss, Jethro Baliao November 2, 2016 Abstract We are to design and build a circuit that will improve power factor input for a Variable Frequency Drive (VFD) provided by Lenze through a DC to DC converter. The boost converter is the DC to DC topology and a prebuilt IC (ICE3PCS01G) will be the controller. This allows for greater efficiency or more work given the same input which is important for Lenze to stay competitive as well as saving costs for their customers. The PFC circuit is to take 120VAC and get an output of 325VDC with a power factor of 0.95 with a max continuous power of 1472 watts or about 2.0 hp. We must also consider the noise induced by the switching and design a filter to eliminate it. It is important that the circuit we are designing and building is capable of handling these conditions as Lenze has stressed that they are looking for a power factor circuit to use for various projects particularly in their variable frequency drives.

2 Contents 1 Background 2 2 Theory General Theory Buck-Boost SEPIC Flyback Boost Design Specifications and Constraints Prototyping and Printed Circuit Board Solution PFC Converter Type Active vs. Passive Rectification DC/DC Conversion Control Budget Experimental Procedures Power Factor Measurement Power Efficiency Total Harmonic Distortion(THD) Preliminary Results Buck-Boost SEPIC Flyback Boost Moving Forward Fall Spring Conclusion 11 9 Personnel and Collaborators 12 1

3 1 Background Power factor is one of the most important elements in an AC electrical system. Power factor is the ratio of real power to apparent power in the circuit. The relationship can be represented by the power triangle. Here the apparent power, S can be determined by taking the vector sum of the reactive power, Q and the real power. Power Factor(PF) = cos φ = P S (1) Frequency inverters also known as variable frequency drives (VFDs) are instruments that alter the input voltage frequency and magnitude to obtain an AC output. This drive would be used to power an inductive motor. We can find VFDs in many fields ranging from large scale industrial networks to small gadgets. There are many ways to correct power factor. They all work well but they work best in different circumstances. They all are used to save power lost in inductive coils or in capacitors. Power factor correction is also important for the stability of the power grid. Utility companies will charge a fee if a company's power factor is poor. Companies are eager to buy products like this because they save money increase efficiency. 2 Theory 2.1 General Theory Power factor correction is needed to reduce the total power needed. When a load is capacitive or inductive, power is needed to create the required electrical field or magnetic field. The field must be created in one polarity, broken down, and then created again in the opposite polarity, and the cycle repeats. The power consumed in this process is not used to do any work. The ratio of power used to do work vs the total power used is the power factor. By adding technology to correct this, it can reduce the apparent power. If the power factor is very low it may destabilize the power grid, or create transients that can damage other devices on the power grid. As a result, it is imperative that industy takes advantage of power factor correction since it is much more economical and prevents any damages. 2

4 Power factor correction is in high demand in today s industrial world. Low power factor is generally caused by inductive loads, ie. motors. These inductive loads cause the current to lag behind the voltage. The current and voltage being out of phase is the driving factor in lower power factor. When apparent power rises without an increase in real power, this leads to a needless increase in current. As a result, this leads to quadratically greater losses given that conductive losses increase with the square of the current. Utility companies have always desired higher power factors because just as low power factor wastes money and resources within an industry, it is also a waste for the utility company to generate power that cannot be delivered due to conductive losses. Many utility companies will go as far as to penalize industrial accounts for low power factor. Prior to the rise of power electronics, power factor correction was limited to the use of capacitors, and/or synchronous condensers. These devices are effective at lowering the power factor, but not without their drawbacks. Using purely a capacitive power factor correction circuit is the simplest method of power factor correction, as it has the exact opposite effect of an inductor. Capacitors force the current to lead the voltage, therefore, if the capacitance and the inductance are perfectly balanced, the load will be at the unity power factor. However, capacitors are fixed devices and do not allow for adjustment and variation of load without external control of connecting and disconnecting capacitors to maintain power factor. A second option for power factor correction is the synchronous condenser. On a basic level, a synchronous condenser is a no load AC motor that uses the same characteristics that absorb the reactive power to produce reactive power in order to bring the load to unity. This practice is very effective, however it is also noisy and large. Neither of these options are feasible for installation in a small motor drive.this is where the power electronics come in. Research in power electronics began in the 1950s and over the last 60 years, have become a formidable force in the world of electricity. The use of power electronics would allow for an AC-DC rectification coupled with a power factor correction DC-DC converter that would draw power very close to the unity power factor while still fitting into a small package. While power electronics have existed for many years, their practical industrial application is a fairly recent development. At this time, Lenze has no motor drives fitted with a power correction circuit. This project will be the pioneer for integrated power factor correction. 2.2 Buck-Boost The buck-boost works with two states. When the switch is open, the inductor will increase in voltage to maintain current, and in doing so charges the capacitor. When the switch is closed the capacitor will maintain voltage while current rushed through the inductor charging it. Then by changing switching times the output voltage can be controlled. With a bit of feedback the switch can control the shape of the current correcting the power factor. The buck-boost has some unique characteristics to consider for this application. It can either increase or decrease the output voltage with a small voltage ripple and limit the inrush current. Furthermore, it can be seen that the switch is between the input power and the inductor which gives it an advantage over the other topologies. 3

5 There are a few challenges with the buck-boost as well. It is hard to get to 325 volts needed for this task. Also the output voltage is inverted making because of the orientation of the switch. It also cost more being less efficient and larger components then the buck-boost. 2.3 SEPIC The single-ended primary-inductor converter, or SEPIC is another topology available to us to use as a PFC circuit. It includes two inductors or a coupled inductor, a MOSFET, two capacitors and a diode. It is also one of the more complex topologies investigated in regards to loop control. Like the Buck-Boost, it is able to increase or decrease the input voltage values but its output is the same polarity as the input and due to the inductor being on the input side, it will limit the slope of the current. However, because it has two inductors, it is larger than the other topologies. Additionally, since it transfers all its energy through series capacitors, large capacitors are needed. As a team, we decided that SEPIC was not the optimal choice in this project due to its size and its complexity. 2.4 Flyback The Flyback converter is one of the possible topologies to correct power factor. Basically a derivative of the Buck-Boost converter, what makes this topology stand out above the rest is the lack of an additional inductor and the use of a Flyback Transformer. The topology consists of just a single MOSFET, a diode, a mutual inductor and a capacitor. It carries all the positives of a Buck-Boost converter with additional benefits. A Flyback can isolate voltages, it is very simple to design, a low cost option and has a small footprint. However, it shares drawbacks similar to a Boost Converter where it is difficult to control due to right hand plane (RHP) issues. Additionally, the Flyback is not optimal for high power applications due to the high current stress on components and the need for a larger transformer. 2.5 Boost The boost converter is a step-up power converter. This means that the output voltage is higher than the input voltage. When the switch is closed, current will be drawn into the inductor, charging it. When the switch is open, the current from the inductor flows through the diode to charge the capacitor and powers the load. When the switch closes again, the capacitor maintains the output power to the load while the inductor charges again. This converter is advantageous because it is the simplest topology that can correct the power factor and raise the voltage. Because of this simplicity, it offers the smallest footprint and is less expensive than the other topologies. Despite this simplicity, the boost converter provides a power factor far over the requirement of 0.95 at a large range of power levels. The boost converter, however, does have a few drawbacks. The most pertinent drawback of the boost converter is that it is difficult to stabilize due to a right half plane zero. This will be further addressed in the control section of the report. The other drawbacks are a high inrush current and high output voltage ripple due to the pulsating input. 4

6 3 Design 3.1 Specifications and Constraints Input Voltage Range = V single phase AC AC Line Frequency Range = 48-62Hz Input power factor 0.95 above 50% load Inrush Current >40A Hold up time 10ms Note: Pout = 1472W Vout 228VDC Mean Output Voltage = 325 VDC Output Voltage Ripple = 20Vpk-pk at full load Mean Output Current = 4.5 A Maximum Continuous Output Power = 1472 W Peak output power = 200% load for 3 seconds, 150% load for 1 minute 3.2 Prototyping and Printed Circuit Board We first began our design by following equations in the Infineon PFC Boost Converter Guide. Using the calculated values from that, we built a model in MATLAB Simulink for our preliminary simulations using PI Control. Moving forward, we will add non-idealities to our simulation for further analysis. We are also going to model our circuit with the Infineon PFC Boost Controller IC chip. From there, we will design a PCB. While designing and waiting for the PCB to be made, we will hand wire a prototype for initial testing. 4 Solution 4.1 PFC Converter Type Through our research, we analyzed various PFC techniques such as capacitor banks, synchronous condensers, and power electronics converters. It quickly became apparent that neither the capacitor bank nor the synchronous condenser were suitable for our purposes. The capacitor banks are not a viable option due to adjustability constraints and the synchronous condenser is not an option due to size and noise constraints. As a result, power electronics was the chosen method as they were small and generate little to no noise. 4.2 Active vs. Passive Rectification The three stages for PFC using power electronics are the rectification stage, the DC/DC power stage, and the control stage. The rectification is the simplest stage of the three as there are only two major options: passive and active rectification. The advantages of using passive rectification are simplicity and cost. Passive rectification is very simple in that a full wave diode-bridge rectifier only has 4 components. Due to the simplicity, the cost is very low. The advantages of active rectification are a lower voltage drop resulting in higher efficiencies and the option of bi-directional current flow. Team 1703 and Lenze agreed that 5

7 it would be best to pursue passive rectification given that cost reduction is paramount for this design. 4.3 DC/DC Conversion The second stage of PFC design is the DC/DC conversion. In most applications of DC/DC converters, the input is a clean DC wave. In our case, the input is a pulsating DC waveform ranging from 0 to 170V. This means that we are forced to use a larger inductor and capacitor in order to maintain a clean and consistent output waveform regardless of which topology is used. With that in mind, the boost, buck-boost, flyback and SEPIC were chosen to further investigate. With each topology, we looked at the pros and cons as well as the waveforms to compare the results and ultimately decide on which topology to focus our efforts into. By looking at the strengths in simplicity, size, cost, power level, and voltage regulation of the various topologies, the Boost Converter was determined to be the most appropriate. Team 1703 and Lenze have decided to continue forward with the Boost Converter design and implement it in between the AC/DC rectification circuit and the VFD input. 4.4 Control The third stage of the PFC circuit is the control circuit. This is the most complex part of the circuit due to the range of options for control. The three major options are analog control, designing and building our own feedback circuit using a microcontroller, and a prebuilt IC chip. Each of these options have their own advantages and disadvantages. Analog control excels in its speed, and cost effectivity. Despite that, analog control falls short in stability over a range of conditions and adjustability. Another option is designing and building a feedback circuit using a microcontroller. This option excels in its adjustability as we could optimize it specifically to each application. Where this option falls short is cost, complexity and development time. The last option is using a pre-built IC control chip. This option excels in cost, stability, and size. The only downfalls of this are the dependence upon a 3rd party chip, which Lenze has no problems with given that they have other products that rely on 3rd party chips, and that we are limited on adjustability. The PFC Control chip that we have in mind, Infineon (ICE3PCS01G) is less than $2.00 per chip and has been tested at power levels very similar to ours. This chip allows adjustability of output voltage and switching frequency from 21kHz to 250kHz. 6

8 4.5 Budget Lenze has stated the budget to be $2000 which is very lenient. Additionally, since we are planning on the boost topology, further costs are minimized as boost is the simplest topology of the four we investigated. Lenze has stated that they will purchase parts and PCBs. 5 Experimental Procedures 5.1 Power Factor Measurement In simulation, we measure power factor using a voltage and a current probe to determine real and reactive power and use those to calculate power factor. With hardware, we will use a power meter that measures the input voltage and input current and computes the real, apparent, and reactive power, and displays the power factor. 5.2 Power Efficiency In order to calculate efficiency of the converter, we will use a power meter on the input and output of the circuit and compare them using the following equation: 5.3 Total Harmonic Distortion(THD) In simulation, we measure THD using a Spectrum Analyzer. With hardware, we will use an oscilloscope. 7

9 6 Preliminary Results 6.1 Buck-Boost 6.2 SEPIC 8

10 6.3 Flyback 6.4 Boost 9

11 7 Moving Forward 7.1 Fall The basic timeline for this project is as shown consisting of our milestones and tasks. We plan on having a prototype completed by November 27, 2016 and continue testing which may include optimization of components and trying different ICs to achieve a power factor greater than Then, we plan on discussing our results with Lenze to determine where to go from there. If given the time, designing a 3-Phase Circuit or active rectification instead of passive rectification are possibilities. With a working prototype, testing will take place over Winter Break in December going into next semester. 7.2 Spring During the Spring Semester, we should already have our hard wired prototype working as well as data obtained during the break. The plan ahead mostly consists of testing and weekly deliverables. Over the course of the three months various experiments will take place. During this time, optimization of the the design will take place as well as finalizing it. 10

12 8 Conclusion This project is focusing on designing a PFC circuit for Lenze that would work with one of their variable frequency drives. Working with a 120VAC input we must design a circuit that can output 325VDC and a power factor of 0.95 or greater. Looking at various topologies based on their size, simplicity, power level and voltage regulations, the boost converter was the most suitable for the design. Using the Simulink software in MATLAB we obtained some preliminary results for all the converters. Moving forward, we plan to work with the PFC control chip (ICE3PCS01G) by Infineon to complete our closed loop control. Additionally looking to the future, we expect to have a working prototype by the November and start testing moving into next semester. As an extra goal, we plan on looking into a three-phase design as that is much more optimal versus single phase due to industy demand. 11

13 9 Personnel and Collaborators Ethan Hotchkiss Senior Engineering Student Kevin Wong Senior Engineering Student Paul Glaze Senior Engineering Student Jethro Baliao Senior Engineering Student Program Manager: Christopher Johnson Faculty Advisor: Ali Bazzi University of Connecticut Storrs, CT Lenze Americas Corporation Uxbridge, MA

14 References [1] H. Wei, and I. Batarseh. Comparison of Basic Converter Topologies For Power Factor Correction [2] J.W. Kolar and T. Friedli. The Essence of Three-Phase PFC Rectifier Systems-Part I. IEEE Transactions on Power Electronics, Vol. 28, No.1,pp [3] S. Abdel-Rahman, F. Stuckler, K. Siu. PFC boost converter design guide [4] P.T. Krein. Elements of Power Electronics 2nd Edition