what could be the rating of lightening arrestor for 24 KV to 220 KV stepping up? how rating of LA decided?

Answer 1

Think of the lightning arresters as an insurance policy to protect your expensive transformer.

MOV arresters come in several classes. Here is a list of the classes in order of increased protection and cost:

Distribution; Riser; Intermediate; and Station class

In your application you'll want to use "Station Class" MOV arresters. Arresters have a maximum continuous over-voltage (MCOV) rating. You need to select an arrester with an MCOV value higher than what you'll ever have on the system.

The proper arrester voltage rating is also dependent on whether this is a 3-wire or 4-wire circuit and the BIL of the system.

I recommend using arresters with a polymer housings instead of porcelain.

I'm not familiar with the 220 kV equipment, but the 24 kV equipment is readily available from several manufacturers. In the US, these include:

Cooper Power Systems
Ohio Brass (a.k.a. Hubble)
Joslyn (a.k.a. Thomas & Betts)

Answer 2

Lightning Arresters
In telegraphy and telephony a lightning arrester is placed where wires enter a structure, preventing damage to electronic instruments within and ensuring the safety of individuals near them. Lightning arresters, also called surge protectors,are devices which are connected between each electrical conductor in a power and communications systems and the earth. These provide a short circuit to the ground that is interrupted by a non-conductor over which lightning jumps. Its purpose is to limit the rise in voltage when a communications or power line is struck by lightning.

The non-conducting material may consist of a semi-conducting material like silicon carbide or zinc oxide, or a spark gap. Primitive varieties of such spark gaps are simply open to the air, but more modern varieties are filled with dry gas and provided with a small amount of radioactive material to encourage the gas to ionize when the voltage across the gap reaches a specified level. Other designs of lightning arresters use a glow-discharge tube (essentially like a neon glow lamp) connected between the protected conductor and ground, or any one of a myriad of voltage-activated solid-state switches called varistors or MOV's. Lightning arresters built for substation use are impressive devices, consisting of a porcelain tube several feet in length and several inches in diameter, filled with disks of zinc oxide. A safety port is supplied on the side of the device to vent the occasional internal explosion without shattering the porcelain cylinder.

The ZA Varistor Series has a Wide Operating Voltage Range VM(AC)RMS of 4V to 460V, DC Voltage Ratings of 5.5V to 615V.

Typical parameters for a V220ZA05 Metal oxide varistor:

5mm dia disc
220VDC nominal (198-253V @ 1mA)
6 joules for a 10/1000 microsecond pulse
360VDC max clamp @ 5 Amp
400Amp max transient surge
180VDC max continuous
140VAC RMS max continuous
0.2W avg power dissipation
90 pf capacitance