21-05-2013, 12:36 PM
Detecting power grid synchronization failure on sensing out of range frequency or voltage
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Introduction
This project presents the development of a microcontroller based islanding detection for grid connected inverter with under/over voltage and under/over frequency islanding detection algorithms. The system is based on a microcontroller from Atmel 8051 family. The microcontroller monitors the under/over voltage derived from a set of comparators and under/over frequency from by the interrupt program for the utility grid and the processed value of voltage and frequency for turning ON/OFF the relay between a grid connected inverter and the utility grid. The project would alternatively use a variable frequency generator representing the inverter using 555-timer for changing the frequency while a standard variac shall be used to vary the input voltage for achieving the test conditions by a lamp load being driven from the microcontroller output as stated above. The microcontroller used in the project is of 8051 family which is of 8 bit. The power supply consists of a step down transformer 230/12V, which steps down the voltage to 12V AC. This is converted to DC using a Bridge rectifier. The ripples are removed using a capacitive filter and it is then regulated to +5V using a voltage regulator 7805 which is required for the operation of the microcontroller and other components.
MICROCONTROLLER
The microcontroller used here is a common 8 bit Atmel microcontroller AT89s8253.It is a low-power, high-performance CMOS 8-bit microcontroller with12K bytes of In-System Programmable (ISP) Flash program memory and 2K bytes of EEPROM data memory. It has 40 programmable input output lines.
ULN 2803 RELAY DRIVER
HIGH-VOLTAGE, HIGH-CURRENT DARLINGTON ARRAYS
This IC is having 8 pairs of high current Darlington array, which is required to drive the RELAYS.
FEATURES
• TTL, DTL, PMOS, or CMOS Compatible Inputs
• Output Current to 500 mA
• Output Voltage to 95 V
• Transient-Protected Outputs
• Dual In-Line Package or Wide-Body Small-Outline Package
RELAYS
Relays are electromechanical devices, which are used as a switch. Current flowing through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts. The main advantages of using relay as a switch is that, they provide electrical isolation between inputs with the help of magnetic coupling and also we can handle a large power using a relay. There are different types of relays are available and they can be categorized in various ways. For example relays can be SPST, SPDT, DPST, DPDT etc. SPDT stands for Single Pole Single Throw, DPDT stands for Double Pole Double Through and so on. Here we have used SPDT switch.
POWER SUPPLY
The power supply consists of a step down transformer 230/12V, which steps down the voltage to 12V AC. This is converted to DC using a Bridge rectifier. The ripples are removed using a capacitive filter and it is then regulated to +5V using a voltage regulator 7805 which is required for the operation of the microcontroller and other components.
555 timers
The 555 timer IC is an integrated circuit (chip) used in a variety of timer, pulse generation, and oscillator applications. The 555 can be used to provide time delays, as an oscillator, and as a flip-flop element. Derivatives provide up to four timing circuits in one package.
Conclusion:
The project is designed to develop a system to detect the synchronization failure of any external supply source to the power grid on sensing the abnormalities in frequency and voltage.
There are several power generation units connected to the grid such as hydel, thermal, solar etc to supply power to the load. These generating units need to supply power according to the rules of the grid. These rules involve maintaining a voltage variation within limits and also the frequency. If any deviation from the acceptable limit of the grid it is mandatory that the same feeder should automatically get disconnected from the grid which by effect is termed as islanding. This prevents in large scale brown out or black out of the grid power. So it is preferable to have a system which can warn the grid in advance so that alternate arrangements are kept on standby to avoid complete grid failure.