INTRODUCTION TO UPS

INTRODUCTION

The concept of Uninterruptible Power Supply (UPS) comes to mind because of the challenging problems we encounter in the Nigeria power system.
In Nigeria where we experience unpredicted power cut, voltage fluctuation, low voltage, etc, efforts has been made to substitute for existing power with an alternative power which is usually generator. This ensures that businessmen and other business organizations will be able to meet their customer's day to day needs and requirements in order to efficiently maximize profit.
However, the use of private generators has posed more problems to business men because of the inherent disadvantages associated with maintenance cost, repair due to wear of machine parts and cost of fueling the generator. This makes it exorbitantly expensive to run. Moreover, the noise associated with its operation is a nuisance to the neighborhood, a major source of pollution. Another more dangerous pollutant is the fume released to the atmosphere when the generator is in normal working operation. This is hazardous to the environment and society at large.
Consequently, because of the aforementioned disadvantages associated with generators own by individuals, the inverter provides a clean, noiseless, maintenance free power supply to run our domestic and commercial loads when there is utility failure. By incorporating automatic switching mechanism between utility and inverter, the UPS is defined. The UPS ensures that there is constancy of power to the system in use even if there is utility failure and supplies rated voltage and frequency to load even if utility voltage is low.
The inverter converts DC battery power to standard AC power. They allow us to run regular 230Vac appliances, including TVs, computers, microwave and power tools. Inverter can be used any time of the day and night.
Inverters are rated according to the continuous power that they can produce.. Moreover, for the purpose of this project, a 1KVA uninterruptible power supply (UPS) will be designed and constructed.
The UPS consist of the following incorporated systems:
1. Inverter system
2. Automatic battery charger with controller
3. battery monitor( to automatically disconnect load when battery is low)
4. Automatic voltage control(AVC)
5. Indicators/display to signal 1,2,&3
6. Auto transfer between utility power and inverter.

1.1 THE INVERTER SYSTEM
This converts DC battery voltage to alternating voltage AC. The AC voltage is an approximation of sine wave; it is the pulse width modulation type.

1.2 AUTOMATIC BATTERY CHARGER
This charges the battery immediately when utility power is restored. The charger is a two stage charger which provides a constant current until the battery reaches its rated capacity and then switches to a float voltage. The current then reduces as necessary to maintain the battery at the float voltage (trickle current).

1.3 THE BATTERY MONITOR
This is achieved by use of comparator. The comparator compares the output battery voltage with the input and when battery voltage goes below the set point, it automatically disconnects the load from battery so that battery is not completely flat.

1.4 INDICATOR/DISPLAY
It indicates normal working condition of the above features.

1.5 AUTO TRANSFER SWITCH
These are relay operated switches to automatically switch on/off load from utility to inverter or vice versa. When there is utility failure it switches automatically to inverter and when utility power is restored, it automatically switches from inverter to utility power.

1.6 AUTOMATIC VOLTAGE CONTROL (AVC)
This functions like a voltage regulator. It ensures that there is no much voltage variation from the preset desired value of 230V when load of its rated capacity is impressed on it. This makes the UPS unique.
Moreover, the design of the UPS requires a careful selection of semiconductor parts from the electronic data sheet, the power switching semiconductor device so chosen is the MOSFET. The power MOSFET has a positive temperature coefficient for resistance; hence, paralleling of the device is relatively simple. The popular IRF 150N is selected because it is more versatile, rugged and has a maximum drain current rated 40A, Vds 100V maximum. This component will be carefully mounted on an aluminium heat sink for heat dissipation during working condition.