Arrester MCOV - Different Values, Line And Substation

"...Continuous System Voltage When arresters are connected to an electrical system, they are continually exposed to the system operating voltage. For each arrester rating, there is a recommended limit to the magnitude of voltage that may be applied continuously. This is termed the Maximum Continuous Operating Voltage (MCOV) of the arrester. The arrester rating must be selected such that the maximum continuous power system voltage applied to the arrester is less than, or equal to, the arrester's MCOV rating. Consideration should be given to both the circuit configuration (wye or delta) and arrester connection (Line-to-Ground or Line-to-Line). In most cases the arresters are connected line-to-ground. If arresters are connected line-to-line, then phase-to-phase voltage must be considered. In addition, in determining the arrester rating, attention should be given to the grounding configuration of the system, either solidly grounded or effectively ungrounded (impedance/resistance grounded, ungrounded, or temporarily ungrounded). This is a key factor in the selection and application of an arrester. If the system grounding configuration is unknown, the reader should assume the system is ungrounded. This will result in choosing an arrester with a higher continuous system voltage and/or MCOV rating. Also, attention should be given to special arrester applications such as that on the delta tertiary winding of a transformer where one corner of the delta is permanently grounded. In this instance, the normal voltage continuously applied to the arrester will be the full phase-to-phase voltage, even though the arresters are connected line to ground..."

Quite a mouthful, isn't it? I suggest that you read the entire article, and then find someone experienced at insulation coordination studies to guide you. Anonymous strangers on the web who have no liability for their advice are no substitute for experienced engineers who have done this before, and certainly not for free.