PREVENTION AND CORRECTION OF VOLTAGE SAG

There are so many Technical methods for the prevention and correction of voltage sag a few of them are described below.

1. Tap Changing Transformers
2. Saturable Reactor Regulators
3. Static Voltage Regulator (SVR)
4. Motorized Variacs
5. Phase Controlled Regulators
6. Electronic Voltage Regulators
7. Soft Switching Line Conditioners
8. Ferro resonant Transformers (CVT)
9. Magnetic Synthesizers
10. Flywheels and Motor-Generator (M-G) Sets
11. Uninterruptible Power Supplies (UPS); Battery Storage) 
12. Superconducting Magnetic Energy Storage (SMES)
13. Compressed Air Energy Storage (CAES)
14. Dynamic Voltage Restorer (DVR)
15. Energy storage using capacitors. Super capacitors
16. Dynamic Sag Corrector (DySC)

So these were the technical methods used inside the industrial plants to improve the power quality of the system against voltage sag, short interruptions and for their correction. If we want to study each technique, it is a wide field and for ideas about the techniques mentioned in the list you can go through reference number.