Updated: Feb 26, 2021
Why Frequency should be within the permissible limits?
First let us understand what is frequency and on what factors supply frequency depends.The voltage and frequency both are governed by generator at generating station. The frequency denotes the active power and voltage is reactive power for a generator. The drop in the rotational speed of the turbine decrease the frequency, and increase in the rotational speed of turbine increase the frequency. The graphical representation of frequency and load of a generating station is as shown below.
When load on the generator increase the turbine governor valve opens and supplies more steam to turbine. When load decrease the valve close and cut off the steam. Thus the frequency of the supply system is governed by controlling the main steam valve. What happens if the valve is fully open and still the frequency is at low, it means the load on the generator is more than it can supply. The lower frequency situation may further stall the turbine.
From above, it is very lucid that operation of the generating station gets affected with change in the frequency either change in frequency from load side or due to problem in generating station. To prevent the generating station stoppage on account of frequency variations, certain protection devices like over frequency, under frequency and rate of change of frequency(ROCOF) relay is used to shed the load to bring the supply frequency within the rated frequency range.
The large generator has large machine constant and generator is capable to maintain the frequency within range. A large disturbance in the power network may cause the grid frequency to change by 1 Hz, whereas the small generating station has lower machine constant and may cause lower frequency for the same perturbation. Therefore, small capacity generating stations are more vulnerable for frequency dips as compared to large rating generating station because small capacity generator is less resilient in comparison to large rating generator.
How df/dt Relay Can Prevent total Power Outage?
Let us understand the low frequency phenomenon with an example. Let three generators of 20 MVA,40 MVA,and 60 MVA are running in parallel supplying the total load 100 MW. Suppose 20 MW generator trips due to some fault, the other healthy generators may not capable to deliver the load and frequency will drop down. In this condition, if the load of certain part is not cut off the other generators will also trip one by one and this lead to blackout.
The frequency relay measures and monitor the supply frequency. There are three elements in the frequency relay- Over frequency element(81H),Under frequency element(81L) and rate of change of frequency element(81R). The frequency relay monitors the rate of change of frequency( df/dt) and if observes the rate of change of frequency is higher than the threshold limit of df/dt, the relay outputs a tripping command to breaker to shed the part of the load to bring the load less than the rated capacity of the generators in service.
If the relay further observes that the frequency has fallen below the threshold limit of the lower frequency range then again a relay trips the breaker to cut off more load from the bus. Thus, the frequency relay sheds off the part load of the bus by tripping the breaker to save the generator and ensures availability of the supply.
Operating principle and Setting of df/dt Relay:
The frequency relay measures the frequency at many points of the AC waveform in one cycle. The rate of change of the frequency is calculated by the internal circuitry of the realy.
Let the frequency is f1 at time t1 and frequency f2 at time t2. Then the rate of change of frequency is;
The minimum relay setting 0.1 Hz/Sec can be done. The setting of the rate of change of frequency is generally kept at 0.3 Hz/Sec. to 0.4 Hz/sec.