SWITCHING OVERVOLTAGES

™ There is a great variety of events that would initiate a switching surge in a power network.
™ The switching operations of greatest relevance to insulation design can be classified as follows:
1. Energization of transmission lines and cables. The following specific switching operations are some of the most common in this category:
a. Energization of a line that is open circuited at the far end
b. Energization of a line that is terminated by an unloaded transformer
c. Energization of a line through the low-voltage side of a transformer

2. Reenergization of a line. This means the energization of transmission line carrying charges trapped by previous line interruptions when high-speed reclosures are used. 

3. Load rejection. This is affected by a circuit breaker opening at the far end of the line. 

This may also be followed by opening the line at the sending end in what is called a line dropping operation. 

4. Switching on and off of equipment. All switching operations involving an element of the transmission network will produce a switching surge. 

a. Switching of high-voltage reactors 

b. Switching of transformers that are loaded by a reactor on their tertiary winding 

c. Switching of a transformer at no load 

5. Fault initiation and clearing. 

Some important switching operations which can lead to switching over-voltages

1-Line energization

2-Reclosing (energization of a line with trapped charges)

3-low voltage side Energization of a line

4- Energization a line terminated by an unloaded transformer

5- Load rejection at the receiving end of a line

6- Load rejection at the receiving end of a line followed by line dropping at the sending end

7- Interrupting lines at no-load (line dropping)

8- Switching of transformers at no-load

9- Switching reactor loaded transformers

10- Switching high voltage reactors

11- Switching at intermediate substations

12- Initiation of a single-phase to earth fault without a switching operation