DETECTING POWER GRID SYNCHRONISATION FAILURE ON SENSING FREQUENCY OR VOLTAGE BEYOND ACCEPTABLE RANGE (Electronics project)

Detecting Power Grid Synchronisation Failure on Sensing Frequency or Voltage Beyond Acceptable Range

Project Synopsis

In power distribution systems, the power grid station gets supply from different feeder stations like a thermal power station, a wind power station, a solar power station etc. For feasible transmission, the frequency and voltage of the AC supply should be within the limits as decided by the grid, depending upon the demand of the power supply. In case these limits are exceeded and the demand for power is more than the demand for supply, it results in grid failure. In such situations, the feeder unit is completely disconnected from the grid, causing islanding situation. Thus synchronization is needed between the grid and the feeder unit.

This project defines a way to detect the variations in frequency and voltage of the power supply from the feeder unit to determine the synchronization failure. Here a frequency variation detection system and a voltage variation detection system are used. The frequency variation is achieved using an external timer and detection is achieved using the internal timer of the microcontroller. Incase of the frequency limits exceeding or being too low, the lamp is accordingly switched on. The voltage variation is achieved using a VARIAC and detection is achieved using a comparator arrangement and any variation in voltage is detected by the microcontroller. Incase of any voltage variations, the lamp is switched on.

Block Diagram Explanation

The whole block diagram of the system can be divided into 4 sections – The DC power supply section, the voltage detection section, the frequency detection section, the control section and the output section.

The DC power supply section consists of the step down transformer to step down the AC supply voltage, the bridge rectifier to convert AC to pulsating DC, the regulator IC to produce a regulating output.

The voltage detection section consists of an OPAMP IC with two comparators to sense any change in low or high voltage. For both the comparators, the varied input voltage is compared with a threshold reference indicating low and high limit of the voltage respectively.

The frequency detection section consists of a zero voltage detector which gives the input at the normal frequency, a timer which gives the input at varied frequency.

The control section consists of a microcontroller which receives input from the voltage sensing or the current sensing circuit and accordingly operates the relay driver, which is a transistor in this case.

The output section consists of the relay driven by the relay driver which in turn switches on or off the lamp, which is the load in this case.

Circuit Explanation

The DC power supply part consists of the AC supply being stepped down using a step down transformer. A bridge rectifier consisting of diodes converts this AC to pulsating DC voltage, which is regulated using a regulator IC. This DC supply is then given to the Microcontroller and the other parts of the circuit.

Voltage Sensing Part

The microcontroller is connected to the zero voltage sensing circuit to ensure the frequency of the supply is at normal frequency of 50Hz. A VARIAC is used to get variable voltage.

Initially both the presets are adjusted such that both the output pins of the OPAMP IC are at normal low and normal high level. The VARIAC is adjusted so as to get the input AC voltage more than the normal value. Now the normally high pin of the OPAMP IC will go low, giving an interruption pulse to the pin of the microcontroller. The microcontroller accordingly sends a high logic pulse to switch on the relay driver, which in turn energizes the relay and the lamp glows as it gets the AC power supply. Similarly when the VARIAC is adjusted so as to get input AC voltage less than the normal value, at some point, the normally low pin of the OPAMP IC goes high and the microcontroller on receiving this interruption, sends a high logic signal to the relay driver to switch on the relay and hence the lamp which starts glowing.

Frequency Sensing Part

The VARIAC is adjusted such that the AC input voltage is at its normal value. The microcontroller pin is connected to the output of the timer through a PNP transistor. The timer works in astable mode to produce signals at frequencies which can be adjusted using the variable frequency. This output is connected to the internal timer of the microcontroller which accordingly calculates the frequency of the pulses and when the frequency of the pulses goes beyond the normal frequency or less than the normal frequency, the relay driver is triggered, which in turn energizes the relay and the AC supply is given to the lamp which starts to glow.

Output Video Explanation

Initially the normal frequency input is given to the microcontroller pin through a zero voltage detector circuit. There are two presets- one representing the low voltage set point (LVSP) and other representing the high voltage set point (HVSP). When the LVSP preset is adjusted such that the input voltage is less than the threshold low voltage, the lamp starts glowing. Similarly when the HVSP preset is adjusted so that the input voltage is higher than the threshold upper voltage, the lamp starts glowing. In both the cases, the glowing of lamp indicates the voltage limits have been exceeded or going beyond.

Now the voltage is set at stable point and the slide switch is connected such that the microcontroller now receives input through a timer. The variable resistor of the timer is adjusted such that the frequency of the output signals from the timer changes. Incase the frequency deviates from the normal limit, the lamp starts glowing, which indicates islanding or isolating of the grid from the feeder point has taken place due to deviation in frequency.

Conclusion

This project gives a demonstration of the system to detect the power grid synchronization failure. Any deviation in the supply voltage from the lower and threshold limits or any changes from the set frequency is detected and accordingly the grid is disconnected from the supply source, which is indicated by the glowing of lamp in this case.

Future Scope

This project can be improved further by using power electronic devices which can sense the deviation for each cycle of the AC signal and accordingly isolate the grid. This can provide a more sophisticated means of detection.