IEEE Std 1250-2018 pdf download – IEEE Guide for Identifying and Improving Voltage Quality in Power Systems.
3.4.4.1 Large generation/load imbalances When large generators or large loads, sized at several hundred megawatts, suddenly trip offline, the interconnected power system is left with an imbalance of active power. It compensates for this by adjusting its frequency. For example, in the interconnected Western United States power system, this frequency deviation has been measured during such disturbances and found not to exceed 0.10 Hz and to last for no more than a few seconds. Corrective action is taken by generator governors and usually last no more than a few minutes. Such an event occurs somewhere in a large interconnected power system several times per month. 3.4.4.2 Major blackouts When a large portion of an interconnected power system trips offline, the balance of the system may be left with an enormous active power imbalance. Protection schemes are put in place usually to prevent such a disturbance from blacking out the entire interconnected system. When this happens, the portion of the power system that remains with power can deviate in frequency by as much as 0.75 Hz. This kind of disturbance is rare in developed countries. 3.5 Conclusions There is a wide range of power quality variations that can be important to a facility engineer. Understanding the power quality that can be expected from the supply system is a critical part of developing the best design for equipment specifications and for the facility protection. Electrical power is a product that is generated, delivered, and used practically instantaneously. As a result, there are some unique challenges when it comes to quality control. The use of the product, the sine wave of voltage, is ultimately affected by both how it is delivered and how it is used. Understanding the normal variability of the steady-state parameters and the characteristics of disturbances is important when trying to identify power quality issues for particular customers.