The failed 40-year-old, 15 kV station-class arresters on this 13.8 kV generator present a voltage-coordination tradeoff: a lower MCOV improves the listed surge-discharge level but may conduct during a line-to-ground fault, while a higher MCOV raises that level above the generator’s approximate 40 kV BIL. The deciding quantities are the temporary voltage at each arrester during a ground fault and the complete protective voltage delivered to the generator terminals—not line-to-line voltage alone.
Voltage and insulation limits in this installation
The generator is rated 13.8 kV line-to-line and 25 MVA, with multi-turn coil windings. Its manufacturer approximates BIL near 40 kV using a rise time associated with Figure 1, Section 6.2 of IEEE 522-2004. Treat that BIL as an estimate to be confirmed against the manufacturer’s insulation data; the winding construction and specified surge shape matter when assessing turn insulation.
For a balanced system, nominal phase-to-ground voltage is calculated as 13.8 kV / √3 = 7.97 kV RMS. The case description separately calls 8.4 kV the maximum normal line-to-ground voltage. Use the actual maximum operating phase-to-ground voltage from generator/system data when checking MCOV rather than silently substituting either value. MCOV is the continuous voltage an arrester can withstand; it is not the same quantity as its duty-cycle rating or its surge-discharge voltage.
| Quantity | Value given or derived | Where it matters |
|---|---|---|
| Generator voltage | 13.8 kV line-to-line; 25 MVA | System operating point; not by itself the arrester’s phase-to-ground duty |
| Balanced phase-to-ground voltage | 13.8 / √3 = 7.97 kV RMS, derived | Compare with actual normal phase-to-ground voltage and MCOV |
| Normal maximum phase-to-ground voltage | 8.4 kV, stated in the selection discussion | Check against arrester MCOV and site operating limits |
| Generator BIL | Approximately 40 kV | Compare with the complete arrester protective level, including installation effects |
| Neutral grounding | Neutral tied to a step-down transformer with a 36 Ω resistor in parallel; described as low-impedance grounding | Determine ground-fault voltage rise and duration on unfaulted phases |
Ground-fault voltage versus arrester MCOV
When one phase faults to ground, that phase voltage can collapse toward ground while the two unfaulted phase-to-ground voltages rise. The amount and duration of the rise depend on system grounding and fault clearing. The installation description identifies a 36 Ω resistor in parallel with a step-down transformer, but that description alone does not quantify the arrester’s temporary overvoltage (TOV) duty. Obtain the grounding arrangement, ground-fault study, and clearing time, then check the candidate arrester’s TOV capability for that voltage and duration.
The selection discussion warns that a 12.7 kV MCOV arrester could see close to 13.8 kV on an unfaulted phase during a line-to-ground fault, depending on grounding. If the arrester conducts under that condition, it can contribute to a sustained fault path rather than only diverting a brief surge. That is the reason a 12.7 kV MCOV cannot be accepted solely because its listed surge-discharge voltage is below the estimated BIL. The installed arrester’s MCOV must cover continuous voltage, and its TOV capability must cover the fault condition until clearing.
A separate recommendation in the selection discussion says MCOV should be at least the rated phase-to-phase voltage when the generator neutral is not effectively grounded. Apply that as a conservative decision path only after confirming the grounding classification; do not equate “low impedance” with a specific grounding behavior without system data. The 36 Ω resistor, transformer connection, and relay clearing time determine the actual case.
Candidate arrester tradeoff
The two listed candidates differ in both continuous-voltage capability and 10 kA discharge level. The discharge values are specified for an 8/20 μs surge; they are not universal clamping voltages at every surge current or waveform.
| Candidate | MCOV / duty-cycle rating | Maximum discharge at 10 kA, 8/20 μs | Selection implication |
|---|---|---|---|
| Choice 1 | 12.7 kV MCOV / 15 kV duty cycle | 37.9 kV | Below the approximate 40 kV generator BIL by 2.1 kV, but below 13.8 kV line-to-line and potentially exposed to elevated unfaulted-phase voltage during a ground fault |
| Choice 2 | 15.3 kV MCOV / 18 kV duty cycle | 45.5 kV | Higher MCOV for the described fault concern, but listed discharge voltage is 5.5 kV above the approximate generator BIL |
| Existing unit | 15 kV rated; turn-on stated as 24.75 kV | Not provided | Its age and failure require replacement; the given turn-on value is not directly comparable to the candidates’ 10 kA discharge values |
“45.5 kV exceeds 40 kV” compares candidate residual/discharge voltage with estimated generator BIL; it does not mean the arrester has a higher BIL. Likewise, 37.9 kV being below 40 kV does not alone prove adequate protection. The listed 10 kA discharge value must be compared with the insulation withstand using the applicable insulation-coordination method and actual installation geometry. Arrester lead length and connection layout add voltage during a fast surge, so the voltage at the protected winding can exceed the arrester’s catalog discharge level.
Selection path for this generator
The reported choice was the 15.3 kV MCOV / 18 kV duty-cycle candidate because the lower-MCOV unit could conduct if unfaulted phases rise toward line-to-line voltage. That choice addresses the stated ground-fault concern, but its 45.5 kV discharge value is above the approximate 40 kV BIL. Do not resolve one side of the tradeoff by ignoring the other. The recommendation is to qualify the 15.3 kV candidate against the grounding/TOV study and ask the generator and arrester manufacturers to coordinate its protective level with the winding insulation before placing it in service. If the coordination check fails, select another arrester characteristic or system protection arrangement rather than treating the approximate BIL as “close enough.”
Use the duty-cycle rating as a separate arrester capability check, not as a substitute for MCOV. MCOV addresses continuous operating voltage; duty-cycle rating identifies a different rated-voltage withstand basis. Check both against the system’s continuous phase-to-ground voltage and documented temporary overvoltage envelope using the arrester manufacturer’s rating definitions. A duty-cycle number equal to a line-to-line system rating does not by itself establish fault-duration capability.
Replacement and coordination procedure
- Record the generator manufacturer’s insulation withstand information, including the basis for the approximate 40 kV BIL and the relevant surge rise time for the multi-turn windings. Confirm the intended insulation-coordination criteria with the generator manufacturer.
- Verify the actual grounding circuit: transformer connection, 36 Ω resistor placement and condition, grounding mode, and how the generator and step-up transformer are connected. Obtain maximum phase-to-ground voltage during a ground fault and the protection clearing time from the system study or relay records.
- For each candidate, compare MCOV with maximum continuous phase-to-ground voltage. Then check the manufacturer’s TOV curve or rating against the fault voltage for the required duration. Reject a candidate that cannot withstand the applicable continuous or temporary voltage.
- Check the arrester’s discharge voltage at the relevant current and waveform against the generator’s insulation-coordination requirement. The two candidates’ given values apply at 10 kA, 8/20 μs; get additional data if the study calls for another current or waveform. Include lead and connection effects in the voltage at the winding terminals.
- Confirm that the chosen duty-cycle rating is appropriate under the arrester manufacturer’s rating method. Do not infer adequacy from the 15 kV or 18 kV label alone; use the product’s published voltage and TOV data.
- Install one arrester per phase as described, maintaining short, direct connections and the manufacturer’s specified grounding and mounting arrangement. The existing arresters are in parallel with a 0.25 μF capacitor rated 13.8 kV; verify the capacitor’s condition, connection, and compatibility with the replacement arrangement before retaining it.
Post-installation checks and fault diagnosis
Before energizing, confirm each unit’s nameplate MCOV and duty-cycle rating against the approved selection, inspect phase identification and grounding connections, and verify the capacitor arrangement against the approved design. Record arrester model data and the coordination basis so future replacements do not rely on the old 15 kV rating or turn-on figure alone.
| Observed condition | Likely mechanism to investigate | Deciding check |
|---|---|---|
| Arrester conducts or fails during a ground fault | Unfaulted-phase TOV may exceed arrester MCOV or its voltage-duration capability | Ground-fault voltage, duration to clearing, and candidate TOV curve |
| Winding insulation is exposed to excessive surge voltage | Arrester discharge level plus lead/connection voltage may exceed the winding coordination limit | Protective-level calculation using surge current/waveform and terminal layout |
| Apparent mismatch between 24.75 kV turn-on and catalog values | Turn-on and 10 kA, 8/20 μs discharge voltage describe different operating conditions | Manufacturer’s full voltage-current characteristics and test definitions |
| Unexpected capacitor or arrester behavior | Condition, rating, or retained connection of the parallel 0.25 μF capacitor may affect the installed arrangement | Capacitor inspection and manufacturer/system design review |
Monitor for repeat arrester operation or failure after energization and after any ground-fault event; investigate using relay records and arrester inspection rather than increasing the voltage rating by guesswork. If the ground-fault TOV, winding withstand, or capacitor coordination cannot be reconciled, keep the generator out of service pending a documented review by the generator and arrester manufacturers or a qualified insulation-coordination engineer.
Frequently asked questions
What happens if I choose a 12.7 kV MCOV arrester on a 13.8 kV generator?
Its 37.9 kV discharge value at 10 kA, 8/20 μs is below the approximate 40 kV BIL, but an unfaulted phase may rise toward 13.8 kV during a ground fault. Check fault voltage and duration against MCOV and the manufacturer’s TOV capability before selecting it.
What happens if an arrester’s discharge voltage is above the generator BIL?
The stated 15.3 kV MCOV candidate has a 45.5 kV discharge level at 10 kA, 8/20 μs, versus the generator’s approximate 40 kV BIL. That flags a coordination concern; include lead effects and confirm the insulation basis with the generator and arrester manufacturers.
What happens if a ground fault raises voltage on the unfaulted phases?
The arrester experiences temporary overvoltage, which can cause conduction or failure if the voltage-duration duty exceeds its capability. Use the actual grounding circuit and relay clearing time to compare the fault condition with the arrester TOV data.
What happens if the MCOV is compared with 13.8 kV line-to-line?
That comparison alone can misstate normal phase-to-ground duty: a balanced 13.8 kV line-to-line system gives 7.97 kV phase-to-ground RMS. Ground faults can raise the unfaulted-phase voltage, so use the measured or studied phase-to-ground maximum and TOV duration, not line-to-line voltage alone.
What happens if the grounding and BIL checks do not agree?
Stop the selection and energization decision until the ground-fault TOV and generator insulation coordination are resolved. Escalate the documented system data, candidate arrester curves, and connection layout to the generator and arrester manufacturers or a qualified insulation-coordination engineer.