Ferroresonance 1

[reactive]

Is it possible for ferroresonance to occur in the following scenario :

22/11kV 2MVA Dyn transformer, resistance earthed, no load on transformer. 1,1 MVAr plain capacitor bank connected at 11kV.

My understanding is that for ferrorresonance to occur there must be either an unearthed system or single phase switching. Neither condition exists here.

The PFC will not be switched simultaneously with the transformer.

 

[jbartos]

Suggestions marked ///\\reactive (Electrical) May 11, 2004
Is it possible for ferroresonance to occur in the following scenario:
22/11kV 2MVA Dyn transformer, resistance earthed, no load on transformer. 1,1 MVAr plain capacitor bank connected at 11kV.
///Yes.\\My understanding is that for ferrorresonance to occur there must be either an unearthed system or single phase switching. Neither condition exists here.
///The ferroresonance can occur where there is a nonlinear inductive characteristic with hysteresis. The capacitor load is also needed to create L-C oscillations.\\The PFC will not be switched simultaneously with the transformer.
///However, there still may be some capacitive charge in the power distribution; especially, when it comes to medium and high voltage power distribution systems.\\

 

[busbar]

At distribution voltage, shielded-cable capacitance is sufficient to incite resonant overvoltages. Besides fuse operation, “singe-phase switching” may manifest as misoperation of a ganged three-phase device or flashover in an open-contact condition, but also splice/termination breakdown or single-pole separable-connector failure.

There are a number of ANSI standards and journal papers discussing the problem, but likely not of interest given posted IEC-like numbers.

 

[alehman]

Any time you have a system with the neutral not solidly grounded, the possibility of ferroresonance exists. This provides the necessary "one degree of freedom" on the system. If the grounding resistance is low, the odds of ferroresonance is low.

See thread238-91093

 

[reactive]

From the above I must conclude that the risk of ferroresonance is acceptably low, for the following reasons:
- Earthed system.
- 3 phase switching (no grading capacitors).
- Capacitors not switched with transformer (although they could obviously be tripped with the transformer).

Must sound like a bit of a strange network (no load with PFC), just to explain - the client is reconfiguring a 22/11kV network and new PFC cannot be justified. The only option is therefore to move the existing 11kV bank to a very lightly loaded transformer to correct the 22kV incomer for tariff purposes.

Thanks.