Ferroresonance Phenomena and Strategy

E 214.61

When a circuit consisting of a capa-citor and a magnetic (non-linear)device is subjected to a voltagechange, a point of discontinuitydevelops producing an abruptchange in either current or voltagedepending on series or parallel circuit connection.

A CVT without a proper ferroreson-ance damping circuit will exhibitsustained oscillatory behavior atfundamental or subharmonic frequencies. The phenomena canbe triggered by the following system conditions:

1. High speed primary reclosing

2. Variation in secondary loads (overloads and short circuit)

3. Operation of potential ground switch

Before any of these switching oper-ations, there would be a certainremnant flux in the core of the in-termediate transformer. When aswitching operation occurs, thevoltage across the primary of thetransformer will act to change thetransformer core flux. The worstcase is when the voltage polarity issuch as to further increase the coreflux and the voltage has just passedthrough zero. In this case, the peakflux in the core reaches 2.5 to 2.7times the rated peak flux in thecore. Transformers are not normally

designed to operate at that level offlux without saturation. Thus thetransformer core will go into deepsaturation, reducing its magnetiz-ing inductance by more than 100times ( to basically the air-corevalue). When the transformer in-ductance undergoes this change,at a certain point its value comesinto resonance with the circuit ca-pacitance, either at the frequencyof the system voltage or at a sub-harmonic of it. When this happensthere will be a sharply changingcurrent waveform around that in-stance of resonance and corre-sponding resonant voltage amplifi-cation across both the capacitorand the transformer primary. Sus-tained oscillations may result unlesssome means of suppression is pro-vided. The high impulse currentthen leads to excessive heating ofthe transformer primary windingand the overvoltages stress thetransformer insulation and can leadto breakdown. A typical waveformof the transformer primary voltageis given in Fig. 1. This simplified de-scription of a CVT without ferrores-onance protection illustrates thatferroresonance cannot be avoidedand it will always happen when thetransformer core goes into satura-tion. Whether this happens is de-pendent on the previous remnantflux in the core and on the phaseangle of the system voltage at theswitching operation and is there-fore statistical in nature. Some kindof device must be provided to sup-press ferroresonance oscillationsonce they are initiated.

Fig.1 Typical Subharmonic Oscillation

Trench has been successful in de-veloping a ferroresonance strate-gy which is based upon the com-bination of low flux density inthe electromagnetic unit and theuse of a saturable reactor.

A CVT with an electromagneticunit operating at low flux densityusually performs well under ferro-resonance conditions in terms ofboth recovery speed and over-voltage magnitude.

Typical Ferroresonance performance characteristic

of a Trench CVT

A saturable reactor is used byTrench as a voltage-sensing magnetic switch. Under normalvoltage, it behaves as an openswitch drawing virtually no cur-rent. When the voltage rises be-yond its knee-point, the saturablereactor will turn on a dampingresistor effectively nullifying parasitic voltage.