Selecting Lightning (Surge) Arrester 1

[stockman1]

Sorry if this is being resent. The previous thread was accidently removed.

Problem: The existing 40 year old porcelain station class lightning arresters that are protecting a generator have failed and need replacing (one for each phase).

Generator info: I have a 13.8KV Hydro Generator (25MVA) multi-turn coil windings. The manufacture for the generator approximates the BIL of the generator to be close to 40KV with a rise time defined by figure 1 section 6.2 of IEEE 522 - 2004. The neutral of the generator is tied to a step-down transformer with a 36ohm resistor in parallel (low impedance grounding).

Arrester info: The existing lightning arresters are rated at 15KV and turn on at 24.75KV. The lightning arresters are in parallel with a 0.25uF capacitor (rated voltage 13.8KV). The 13.8KV lines go through a step-up transformer and then connect to the transmission lines.


Questions:
What is the rule of thumb for selecting a station class lightning arrester in this application?
When making a selection, how is the Duty Cycle voltage and the MCOV (max continuous operating voltage) considered in reference to the operating Line-to-Line 13.8KV?
Appreciate any input.

 

[RRaghunath]

I think your present arrester rating is OK, considering that the generator neutral might have been earthed non-effectively, the arrester rated MCOV shall be atleast equal to the rated generator phase-to-phase voltage. Turn-on should show atleast 25% margin with the BIL of the equipment to be protected.

 

[stockman1]

My decision...

There were two different choices available that were close to the line-to-line voltage of 13.8KV.

Choice 1: 12.7KV MCOV (15KV duty cycle), with max discharge voltage at 10KA (8/20us surge) of 37.9KV.
Advantage: 37.9KV < 40KV (BIL) thus providing plenty of protection for the generator.
Disadvantage: 12.7KV MCOV < 13.8KV (line-to-line). Normal line to ground voltage is 8.4KV max. However, when a fault occurs from line-to-ground, the voltage on the faulted phase drops close to 0V, thus shifting the phase of the two remaining unfaulted phases close to 13.8KV (depending on grounding). With a 12.7KV MVOC lightning arrester installed, and the voltage of the unfaulted phases close to 13.8KV, the arresters potentially could begin conducting and suddenly there would exist a line-to-line-to-ground fault.

Choice 2: 15.3KV MCOV (18KV duty cycle), with max discharge voltage at 10KA (8/20us surge) of 45.5KV.
Advantage: 15.3KV MCOV will not unitentionally conduct if there is a line-to-ground fault.
Disadvantage: 45.5KV > 40KV (BIL); this lightning arrester has a higher BIL than the generator being protected. However, the BIL of the generator is approximated graphically, so I decided they are close enough.

Final decision:
I chose to use the 15.3KV MCOV lightning arresters.

Thanks for the assistance.