UPS Inverter control using PWM techniques
Abstract
This seminar report describes the techniques used with PWM controller of UPS inverter. Since sinusoidal output are highly desirable for the UPSs, so sinusoidal PWM technique in any mode are adopted according to the type of application. There are simulated results for single phase inverter with one and two level sinusoidal PWM techniques. The report also describes design steps involved for an UPS system.
Uninterruptible power supply (UPS) systems are widely used for providing emergency power to critical loads that cannot afford utility failure. The core of a UPS is a CVCF inverter. High-quality output voltage is required for these inverters.
Introduction
Mobility and versatility have become a must for the fast-paced society today. People can no longer afford to be tied down to a fixed power source location when using their equipments. Overcoming the obstacle of fixed power has led to the invention of DC/AC power inverters. While the position of power inverter in the market is relatively well established, there are several features that can be improved upon. Aside from the differences in power wattage, cost per wattage, efficiency and harmonic contend, power inverters can be categorized into three groups: square wave, modified sine wave, and pure sine wave. A cost analysis of the different types of inverter shows that sine wave power inverter, though has the best power quality performance, has a big spike in cost per unit power. Another feature which can be improved is the efficiency of the inverter.The standard sine wave in the market has an average efficiency of 85-90% Power dissipated due to efficiency flaws will be dissipated as heat and the 10-15% power lost in the will shorten operational life span of inverters. The quality of the output power could also be improved. It is imperative that the output signal be as clean as possible. Distortion in the output signal leads to a less efficient output and in the case of a square wave , which has a lot of unwanted harmonics, it will damage some sensitive equipment. In meeting the design requirements, there are several technical challenges that must be overcome. Our single, most difficult constraint will be to produce power at a lower power per unit cost than exists. Our efficiency will be greater than 90 percent. This insures that, with a maximum load, less than 10% of power will be dissipated as heat. The total harmonic distortion will be less than 5 percent as per IEEE standard. With a total harmonic distortion this low and a pure sine wave output, we will be able to power even the most sensitive loads. The fundamental step in approaching the challenges was to examine the methods used by existing power inverters. In examining their methods, many areas were open for potential improvement. These areas include the DC/DC step up converter, the DC/AC inverter, and the feedback control system. The DC/DC step up converter in a design will use high frequency transformer, enabling it to reduce the size of the converter considerably. The use of a high frequency transformer will also enable to meet efficiency constraint. A high switching frequency will improve the efficiency of the inverter. In theory, a 100 percent efficient converter could be created. However, due to the limitations of actual device material, our efficiency will be between 90 and 100 percent. The DC/AC inverter circuit will use a microprocessor to digitally pulse the transistors. This will allow us to produce a pure sine wave output.
The feedback control system will be used to regulate the output voltage of the DC/DC converter. This is necessary since the current will vary with the load. The feedback control system will be accomplished using by sampling the output with an integrated circuit. Most of the design constraints set for the inverter were met. However, the one important constraint which the power inverter didn’t meet was the continuous power, which was probably because of the transformer and the traces on the PCB.
SPWM technique can greatly improve the output voltage, and has enjoyed extensive applications because of easy implementation, low-cost, and reliable operation. However, the main drawback of an SPWM inverter is large total harmonic distortion (THD) with nonlinear loads as well as other nonlinearities within the inverter (dead time, non ideal switching, etc). To address this, various instantaneous feedback control schemes, such as deadbeat control, multi-loop control have been used. With these methods, both high-quality output voltage and fast dynamic response can be obtained. The disadvantages are:
1) More variables need to be sensed;
2) High-speed control is required to reject the disturbance. These inevitably raise cost and make these methods less attractive to common products.
