The topic of power system instability and its respective protection against such instabilities has become a great area of interest for researchers as well as engineers after recent blackouts around the globe including in Pakistan, Iran and India in 2021. There are many kinds of instability in a power plant which include (but not limited to):
- Frequency instability.
- Voltage instability
- Phase angle instability.
In this article, our concern and area of interest is Frequency instability and its protection schemes. In order to understand the protection scheme, we must understand about the frequency instability, precisely.
The frequency of a power system defines the balance of power at the generation and load end. The balance of power means the generation is equal to the consumption including all the losses during transmission and distribution. The variation at the generation and the load side causes the frequency of the system to deviate from its normal operating frequency (60Hz in case of Pakistan). The frequency goes down when there is an increase in the load or the generators trips offline. Contrary to this, frequency goes up the normal operational frequency (60Hz) when there is a sudden decrease of a total through off of the load. According to the precise conclusion of these statements, the frequency goes down when load Exceeds generation. The over frequency occurs when load through off.
Under normal conditions of power generation, transmission and distribution, the frequency changes vary from 1/1000 to 1/100 of a hertz over and down. The causes of such reduction and increment of frequency may be tripping offline of generators or increment of load as well as sudden through off load depending on the area, demography and varying power utilization. The system is brought back to normal by providing aggregate power according to the need by different protection schemes (which are not subject to our assignment and can be read in this paper ). The protection or system recovery scheme which we shall cover for the frequency protection of the plant is Under Frequency Load Shedding and Over Frequency Protection.
The power generation capacity and ability of the plants must increase and improve with the increase in the load. The compatibility or the capacity to not meet the load requirements may reduce the frequency of the system which could lead to a major collapse of the system. The other factor could be sudden loss of power generation capacity or any systematic fault isolating the total or major portion of power generation of the plant from the transmission and distribution end. Any of such imbalance between generation and load may lead to Under Frequency Load Shedding.
In this scheme, a portion of load is shed in order to maintain the system frequency. There are several stages of frequency load shedding involved in order to shed the load according to the reduction of frequency as well as not shedding the load for negligible/little fluctuations in the frequency of them system.
The Under Frequency Load Shedding schemes are designed in a manner in which the system could survive in the situation of maximum overload. Since the stability of the system including rated capacity as well as physical conditions are protected under such schemes, the maximum anticipated overload must be calculated by experts in a manner as following:
Practically it is not possible for engineers to design a protection system for loss of more than 35% of the generation without shedding a portion of load, as such massive loss of generation could lead to reduction of system frequency.
The shedding of load should be good enough to restore the frequency at least close to the standard one (60Hz for Pakistan). The frequency close to +-1Hz of standard 60Hz can be considered good enough during over load or under generation. In order to calculate the amount of load to shed, following formulae from the paper  can be utilized:
- LD is for the calculated sum of total load which the system should shed,
- L has already been calculated above as maximum anticipated load.
- F is the minimum threshold for the frequency which must not be reduced further.
- D Is the load reduction factor.
The setup of relay and steps involved in shedding the load depends on the speed of relay response and calculation on which the system is based. Normally, experts and engineers setup the triggering of first relay to shed the load of a particular area in the network when system frequency falls below 1Hz of the rated/Standard frequency (so, 50Hz for Pakistan).
The figure above is taken from the paper  and shows the setup of several relays which shed the load one after the other till the system comes to the balance with its nominal (fnom) frequency. The curve 1 shows the stable system with UFLS scheme.
The over frequency in a power plant/system could be due to the sudden through off of the load (which actually disturbs the speed controller and generator). Some fluctuations in the system frequency are always present which are overcome by system itself without any necessary protection. The disturbance caused y sudden decrease of load can damage the synchronous generators or the whole system, if not countered within seconds.
The relay scheme used for over frequency protection is different from the one used for Under Frequency Load Shedding as in this case, the fault can be refined by disconnecting the generation or isolating a portion of generation side (generators) which are no more sync with others due to changes (reduction) in Load. The typical relay as shown in the fig below is taken from paper  can be used as 2 stage protection against over frequency.
Over Frequency Protection Relay for 60Hz system The first stage of the relay can be set for 61.5Hz and triggers the breaker of grid when the frequency exceeds the value as mentioned. The second stage has a threshold Frequency of 62.5Hz and causes the generator breaker to fall and isolate the generator from the system.
As we have already discussed about the frequency deviation and abnormal frequency conditions in a power plant, our main focus was the protection schemes for each. Here under this head, we shall focus on the inertia of a generator and its effect. It must be noted that the rate of frequency deviation of a power plant depends on the inertia of a generator.
Conventional power plants have rotating synchronous generators which has kinetic energy through which, the actually add inertia. Whenever there is a frequency deviation (under or over frequency condition), the inertia of the generators’ rotating mass provides or absorbs kinetic energy to o from the grid.
The Rotational inertia has direct effect over frequency of the grid. So, the inertia minimizes the rate of change of frequency during sudden frequency deviations providing more (sufficient) time to counter the faults in the system .
It has been observed that most of the power operators around the globe are decreasing rotating inertia due to the addition of converters and more renewable power resources . The conventional power plants had more machines for rotating inertia which ae now reducing. Such massive reduction of rotating inertia has changed system dynamics.
The reduced inertia has reduced the fault overcome time and the rate of frequency deviation in the power plants has increased which, ultimately, needs modern solution or much faster relay response to counter the over and under frequency conditions.
- Mozina, C.J.. (2011). Power system instability — What relay engineers need to know. 103 – 112. 10.1109/CPRE.2011.6035609.
- Y. R. Omar, I. Z. Abidin, S. Yusof, H. Hashim and H. A. A. Rashid, “Under frequency load shedding (UFLS): Principles and implementation,” 2010 IEEE International Conference on Power and Energy, Kuala Lumpur, Malaysia, 2010, pp. 414-419.
- Chuvychin V., Sauhats A., Strelkovs V., Antonovs E. (2014) Under-Frequency Load Shedding System. In: Häger U., Rehtanz C., Voropai N. (eds) Monitoring, Control and Protection of Interconnected Power Systems. Power Systems. Springer, Berlin, Heidelberg.
- “Over Frequency Protection Working Principle -81O”, electrical4u, 2019.
- Impact of Low Rotational Inertia on Power System Stability and Operation Andreas Ulbig, Theodor S. Borsche and Göran Andersson Power Systems Laboratory, ETH Zurich ulbig | borsche |, 2014.
- Hartmann, B, Vokony, I, Táczi, I. Effects of decreasing synchronous inertia on power system dynamics—Overview of recent experiences and marketisation of services. Int Trans Electr Energ Syst. 2019.