Ferroresonance is a nonlinear resonance phenomenon that affects power networks. Circuit must contain at least below circuit elements as it can be defined as ferroresonance:
- Non-linear saturable inductance (example; transformer)
In a circuit consists of these elements; by changing the values of an electrical component, the values of current and voltage values on the terminals at other components change suddenly.
Ferroresonance begins some switching events such as; load rejection, energizing or de-energizing of transformer, switching of circuit breaker. The causes risk to it can be listed as follows:
- Idling or operation on low load on the systems grounded neutral point
- Insulation faults,
- Overloading or idling of voltage transformer
- Switching one or two phases
- Dissymmetry caused by connection errors
- Switching transformers or capacitors
- Connecting low load power transformers to network with short circuit load
- Long and / or capacitive cable feeding a transformer
- Streak of lightning on transmission lines
When the ferroresonance occurs some of the below symptoms are accompanied:
- Phase-to-phase or phase-to-neutral long-term over voltages
- Long – term overcurrents
- Distortion on the current and voltage waveforms
- Displacement of the neutral point voltage
- Transformer overheating (at no load)
- Load noise in voltage transformers and overheating in core and primary windings
- The failures cause of thermal effect or insulation breakdown on electrical materials (capacitor banks, current – voltage transformers etc.)
Ferroresonance On The Voltage Transformers
A good example about VT destruction by ferroresonance is that the primary winding is destroyed and secondary winding remains intact.
However, all of these symptoms are not specific to the ferroresonance. For example, displacement of the neutral point may be the consequence of a single phase to earth fault an unearthed neutral system.
Voltage transformers can be into two different types as inductive and capacitive. Inductive voltage transformers are more prone to ferroresonance. Because, they have more inductive characters and so they need more capacitance to convert ferroresonance circuit.
In ferromagnetic circuits which wirings of ferromagnetic materials such as iron, ferroresonance occurs because of the inductance. Transformers are an excellent example of ferromagnetic inductance.
Preventing Ferroresonance On Voltage Transformers
When the single pole inductive voltage transformer is used, keep in mind that ferroresonance can be occurred if the circuit closes or in the process of earthing fault damping.
Ferroresonance may cause to overheating of voltage transformers and accordingly damage or over induction. It can be damped only by lowering the voltage or connecting a fixed ohmic resistance.
Although predicting when the ferroresonance might occur, its risk can be preventing or reducing by taking precautions in advance.
There are some preventive measures for ferroresonance. However, the most practical and the most economical method is using ohmic resistance with open-delta winding on voltage transformers’ secondary wirings.
When the voltage transformers’ protection windings are used as circuit in Figure-1 (a fixed ohmic resistor is connected to ends of open-delta connection) third harmonic currents flow and thus the resonance is prevented. The advantage of this resistor doesn't affect measurement precicion and doesn't cause any loss under normal operation conditions. Open-delta circuit should be grounded from only one point shown as Figure-1. Resistor damps only unbalanced situations. On balanced situations, there is no current flow at open- delta circuit.