Reference voltage and MCOV 1

[akito]

Hello,
About surge arrester, how should the rms. reference voltage relate with MCOV?

 

[dpc]

The maximum continuous operating voltage (MCOV) should be higher than the maximum expected operating voltage.

For solidly-grounded system, use the nominal phase-to-neutral voltage and add 10%. Make sure MCOV is at least this much. For ungrounded or impedance-grounded systems, use phase-to-phase voltage.

 

[tgott]

Surge arrester manufacturer's can tell you the MCOV you should use for a specified voltage. Check Hubbell/Ohio Brass catalog (page 3) at

http://www.hubbellpowersystems.com/powertest/catalog_sections/PDF_ohio/30_PVI_PVN.pdf

The rest of the catalog also gives some application explanation too. I'm sure other manufacturers have similar data.

 

[cuky2000]

Assuming that the reference voltage is the maximum rms system voltage Line-to-Line (ex E_nom/E_max: 115/121kV, 230/242, 345/362, 500/550, etc).

For Metal Oxide Surge Arresters (MOSA) the Maximum Continuous Operating Voltage (MCOV) is the maximum designated root-mean-square (rms) value of power-frequency voltage that may be applied continuously between the terminals of the surge arrester (SA).

_{Maximum System Voltage: Is the highest voltage at which a system is operated.
Note: This is generally considered to be the maximum system voltage as prescribed in ANSI C84.1-1995. Typically varies between 5% to 10% of the nominal system voltage.}

1- Grounded Systems: The minimum MCOV could be equal or greater than the maximum Phase-to-Neutral Voltage.

MCOV_min=k.Erms_LL/1.73 (5%<k<10%)​

2-Ungrounded systems or grounded through impedance: The minimum recommended MCOV would result in larger value than grounded system. The MCOV is selected basis on the corresponding duty cycle supplied by the SA manufacturer as follow:

Duty-Cycle=k.max(Erms_LL) --> MCOV on SA Table​

Several factors will such as Temporary overvoltage (TOV), COG, FOW, BIL, SWL, Energy Discharge Capability, etc. should be considered in the selection of the SA rating. Please consider review procedure for SA selection and insulation coordination on in references such as IEEE Std C62.22.

An empiric method could be use as a rule of thumb to estimate the MCOV given the Line-to-Line maximum voltage and the SA duty severity.

a- Normal Duty (<30 min) :

1.4 <(MCOV/max Erms_LL)< 2.0​

MCOV=0.835.Erms_LL – 0.077​

b- Severe Duty (<2000 hrs):

1.5 <(MCOV/max Erms_LL)< 2.5​

MCOV=0.901.Erms_LL + 2.175.​