Possible Ferro Resonance on 33kV Powerline

[THD]

We have recently energised a section of Almond (35mm2) powerline at 33kV via single phase fuses. The feeder powerline was live when we energised. We have blown 5 surge arrestors over three days. They split open and caused all three fuses to open at the feed end (125A fuses). We were told that this seems to be ferro resonance and the problem can be solved by using a three phase fused pole mounted isolator. The surge arrestors had a MCOV of 24.4kV and the 33kV source is solidly grounded. Does the ferro resonance theory seem feasible? PS: We have had no surge arrestor failures on the sections of 33kV powerline that were energised via three phase circuit breakers.

Kevin Bosch
Rainbow Technologies

 

[Kiribanda]

THD,

Your arrester rating 24.4 kV is correct for a 33 kV solidly grounded system.

To my knowledge, Ferroresonance takes place when an inductance is dragged into saturation due to an overvoltage and then tuned with some capacitance.

In your solidly grounded system how do you get an overvoltage when one fuse is opened? It is possible in DELTA ungrounded system.

Therefore, I suspect some other reason for blown arresters.

So lets see what other members to say.

 

[magoo2]

If you're feeding an unloaded transformer with this 33 kV feeder, then it sounds like classic ferroresonance. Single pole switching is all that's needed to initiate it.

 

[jghrist]

Are the 33 kV lines overhead or underground? Are you energizing any delta primary transformers?

 

[busbar]

 

From IEEE C57.105 §7:
The use of properly adjusted 3-pole switches and fault interrupters at the source end of the cable circuit will prevent single-phasing during switching and fault conditions and consequently will prevent sustained ferroresonance. However, this is a costly solution which most utilities reject from an economic standpoint. A similar solution is to use only a 3-pole switch at the remote location and fuses only at the transformer location. This is a workable solution, but it has the drawback of causing a line lockout for faults on the cable circuit. Fortunately, the compromise of a 3-pole switch and fuses at the source end of the cable will greatly reduce the probability of ferroresonance when the primary windings are not grounded. Sustained ferroresonance will not occur during switching operations by virtue of the 3-pole switch. If a fuse blows due to a fault on the cable, the capacitance to ground on the open phase is shunted by the impedance in the fault path, and ferroresonance would not be expected. But if a fuse or conductor in the primary circuit causes an open circuit in the absence of a shunt fault, and the transformer is unloaded, ferroresonance can occur.

Fortunately, when a fuse or conductor opens, the load connected to the secondary of the transformer usually is sufficient to prevent sustained ferroresonance. However, if all of the connected secondary load is fed through a breaker or breakers which have undervoltage tripping, then the total secondary load may be disconnected during an open circuit on the primary, and ferroresonance may occur.

 

[THD]

Kiribanda:

We have blown arrestors when closing the fuses in. Usually we get an almighty bang when the second fuse goes in!!

jghrist:

The line is overhead (2km) with a couple of road crossings (cables). Both the transformers being energised when the fuses go in are delta primary and are 1000kVA.

Im almost convinced that this is typical ferroresonance...
Thanks Busbar for the IEEE extract (my weapon to get the contractor to fix up)





Kevin Bosch
Rainbow Technologies

 

[RRaghunath]

THD,

2km OHL with couple of road crossing cables do not mean much in terms of capacitance. Do you also have any capacitors connected at 33kV level!
Further, it may be useful to check the prevalent voltage at 33kV (if the voltage is not over 33kV then to that extent possibility of overfluxing in transformers and resultant reduction in transformer inductive reactance can be eliminated).

 

[jghrist]

As raghun said, there's not much capacitance with the OH line. Ferroresonance depends on capacitance being in series with a non-linear inductance. I'd say that unless there are line-ground connected capacitors on the line, that ferroresonance would be unlikely.