CHAPTER ONE
INTRODUCTION
1.1 Background of the Study
In this modern society, electricity has great control over the most daily activities for instance in domestic and industrial utilization of electric power for operations. Electricity can be generated from public supply to consumers in different ways including the use of water, wind or steam energy to drive the turbine as well as more recently the use of gas. Generators, solar energy and nuclear energy are also source of electricity.
Photovoltaic inverters are inverters either used for day system only or for day and night periods. The later describe the ones that have battery backup for use when the sun is down or cloudy. The simplest and least expensive photovoltaic system is the day use and consists of module wired directly to an appliance with no storage device. When sun shines on the modules, the appliance consumes the electricity generated. Higher isolation (sunshine) levels results in increased power output and greater load capacity. Hybrid describes system with battery storage. The battery backs up power to the inverter connected to it during periods of less isolation and at night. The battery is usually charged during sunny period with PV modules or alternate source to keep it ready for use during the night. Another hybrid system approach is a PV system integrated with a wind turbine. Adding a wind turbine make sense in the location where the wind blows when the sun doesn’t shine. In this case, consecutive days of cloudy weather are not a problem so long as the wind turbine is spinning [2], [5] and [6]. These forms of inverters are available in the market to drive light loads in the range of 500 to 1000watts. Higher wattages are realizable by calculating a household total electrical load and sizing the entire system (inverter, PV array and battery) to meet such loads. It is this aspect that roused our design and construction interest.
This is a fully automatic PWM technology based inverter, which provides very good performance. PWM or Pulse Width Modulationis mainly used to keep the AC supply output by the inverter to a constant 220V. In an ordinary inverter, the inverter output changes with any change in the value of the load connected at the inverter output. To solve this problem the PWM based inverter correct the output value based on the value of the load connected at the inverter output socket. In the PWM inverter this is done by changing the width of the switching frequency generated by the oscillator. In a PWM based inverter, the AC supply at the inverter output depends on the width of the oscillator frequency generated by the oscillator section.
In this inverter, a small part of the inverter output is given as reference voltage to the PWM controller 1C. Based on this reference voltage, the PWM section will increase “or reduce the width of the oscillation pulse generated by oscillator section. This change in the width will compensate any change in the inverter output, and the inverter output will always stay constant, even if there is any change in the load at the inverter output. When the inverter comes into AC mains mode, from the battery off mode, i.e. When the AC mains return after a power cut, the battery charging does not start immediately. It starts after a delay of about 8-10 seconds. – This is done to protect the MOSFET at the output section. If the charging is started immediacy, when the AC mains return, the MOSFET at the output section will receive high current and could get damaged.
Therefore, to protect the MOSFET at the output, the battery charging is delayed for 8-10 seconds after the AC mains return. This is known as Soft-Start or Mains Delay. This section informs about the availability of the AC main supply to the inverter circuit. When the battery voltage reduces from 12V to 10V, the battery is considered discharged. When the battery becomes discharged, the inverter should be switched off, otherwise the battery will go into Deep Discharge State and the battery life will get reduced.