What is Frequency Relay?

In every power generating station, Frequency Relays are used to keep on eye on the standard operating frequency i.e. 50 Hz or 60 Hz. Since operating at a standard frequency is very crucial in any power system therefore, whenever frequency is disturbed due many reasons, the frequency relay operates and isolates the generator which is running under or over standard frequency.

Why keeping a constant frequency is important?

To understand why we need to maintain a constant frequency in every power station, we need to understand what is a Power Grid. Basically, a power grid is formed when several generating stations are synchronized together. For example, here in Pakistan, there are different power stations that are coal fired, thermal, hydro and solar. All of these different power stations are inter-connected through long and medium transmission lines because all power stations are located at different locations within the country. Now, to inter-connect different generating stations, we need to match electrical parameters of RMS Voltages, Phase Sequences and most importantly Frequencies of all the power stations. 

Analogy

To understand the power grid, think of a car with four tyres. Now, to make the car move straight and smooth, all the tyres are required to run at the same speed so that the whole car body can move stably and straight. Here, the car is analogous to power grid and the tyres are analogous to different power stations. Imagine one of the four tyres of the car starts to rotate at a different speed than all the rest of the tyres. The speed can be less or greater than the other three tyres. What would happen? Obviously, due to that tyre, an unwanted pressure will be exerted on other three tyres which were rotating normally. Therefore, the whole car will become unstable and will crash. 

Why we need Frequency Relays?

Now as of above analogy, the same thing happens in a power grid. All the power stations or generating units that are interconnected with the power grid need to run a same speed. Well this is something amazing that all the generators in a power grid need to run at a same speed provided that they have same number of poles.

Have a look at this equation:

Here,

N = Synchronous Speed (Speed of the Rotor)

f = Frequency Generated

P = Number of Poles in the generator.

The above equations shows that the output frequency is directly related to the speed of the rotor provided that the number of poles is constant.

The change is frequency in any power station is detected by Frequency relays. (Under frequency Relays and Over Frequency Relays). Read further to understand the whole concept.

Why Frequency Changes in a Power System?

The main thing is that the Generation and Demand must be equal. If the load demand is greater than the generation, the frequency drops under the standard frequency because overloading causes the rotors of every generator in grid slow down hence decreasing the frequency according to above equation. Similarly, When demand is less than generation, the frequency of the whole power grid increases. Therefore, power control departments take care of Generation and Demand of power and they regulate the power in the grid. 

Now, there are many factors that can cause increase or decrease in Frequency of a power grid or power system. The main factors are discussed below.

1. Sudden Load Shedding

The sudden shedding of load connected to power grid increases the overall frequency of power system because each and every generator is connected together through synchronization.

2. Loss of a Power from a Generation Unit

Due to certain reasons, whenever a power generating unit or station is shut down, the whole power demand is tackled by remaining generators within the power grid. This causes the frequency of whole power grid to decrease. Now to tackle this, either a certain load is shed or a Diesel Powered Generator is synchronized with the grid until the faulty or out of service power station.

3. Short Circuit in a Transmission Line

A short circuit in a high voltage transmission line, when not isolated, can cause the frequency of power grid fall below the standard frequency.

4. Opening of a Transmission Line

When a high voltage, high power transmission line is opened due to many unwanted reasons, the frequency of power grid decreases.

Solution

When change in frequency in a power grid is not detected, the tripping cascades because the load remains constant while the generators continue to trip. Now to solve this, frequency relays are used which are of two types.

Under Frequency Relay

Under Frequency Relays are used to timely detect decrease in frequency. When frequency falls below a certain threshold value, the under frequency relay sends a signal to circuit breakers to isolate the generator from further electrical or mechanical damage. 

For example, settings are performed to feed the threshold frequency. Let's say the standard frequency is 50 Hz and we want to operate the isolating circuit breakers when frequency falls to 49.5 Hz. To achieve this, we will set the percentage at 99 %. 

This will give us 99 % of 50 Hz equals 49.5 Hz so setting for 49.5 Hz threshold will be on 99%.

Over Frequency Relay

Whenever the frequency increases than the standard frequency of 50 Hz, the over frequency relay operates and sends a signal to circuit breakers to isolate the generator. 

For instance, we want Over Frequency relay to operate at 50.5 Hz, the setting will be on 101 % of 50 Hz. Therefore, the over frequency relay will operate at 50.5 Hz

Note:

These are just example. In reality, even lesser threshold values are set.

Watch this video to see the simulation of above said things.